Heat treating of aluminum and aluminum alloys

Heat treating processes for aluminum are precision processes. They must be carried out in furnaces properly designed and built to provide the thermal conditions required, and adequately equipped with control instruments to insure the desired continuity and uniformity of temperature-time cycles. To insure the final desired characteristics, process details must be established and controlled carefully for each type of product.

The general types of heat treatments applied to aluminum and its alloys are:

  • Preheating or homogenizing, to reduce chemical segregation of cast structures and to improve their workability
  • Annealing, to soften strain-hardened (work-hardened) and heat treated alloy structures, to relieve stresses, and to stabilize properties and dimensions
  • Solution heat treatments, to effect solid solution of alloying constituents and improve mechanical properties
  • Precipitation heat treatments, to provide hardening by precipitation of constituents from solid solution.

INGOT PREHEATING TREATMENTS (HOMOGENIZING)

The initial thermal operation applied to ingots prior to hot working is referred to as “ingot preheating”, which has one or more purposes depending upon the alloy, product, and fabricating process involved. One of the principal objectives is improved workability. The microstructure of most alloys in the as-cast condition is quite heterogeneous. This is true for alloys that form solid solutions under equilibrium conditions, and even for relatively dilute alloys

ANNEALING

The distorted, dislocated structure resulting from cold working of aluminum is less stable than the strain-free, annealed state, to which it tends to revert. Lower-purity aluminum and commercial aluminum alloys undergo these structural changes only with annealing at elevated temperatures. Accompanying the structural reversion are changes in the various properties affected by cold working. These changes occur in several stages, according to temperature or time, and have led to the concept of different annealing mechanisms or processes.

Recovery. The reduction in the number of dislocations is greatest at the center of the grain fragments, producing a subgrain structure with networks or groups of dislocations at the subgrain boundaries. With increasing time and temperature of heating, polygonization becomes more nearly perfect and the subgrain size gradually increases. In this stage, many of the subgrains appear to have boundaries that are free of dislocation tangles and concentrations.

Recovery annealing is also accompanied by changes in other properties of cold worked aluminum. Complete recovery from the effects of cold working is obtained only with recrystallization.

Recrystallization is characterized by the gradual formation and appearance of a microscopically resolvable grain structure. The new structure is largely strain-free-there are few if any dislocations within the grains and no concentrations at the grain boundaries.

Grain Growth After Recrystallization. Heating after recrystallization may produce grain coarsening. This can take one of several forms.

PRECIPITATION HARDENING

General Principles of Precipitation Hardening. The heat treatable alloys contain amounts of soluble alloying elements that exceed the equilibrium solid solubility limit at room and moderately higher temperatures. The amount present may be less or more than the maximum that is soluble at the eutectic temperature.

Nature of Precipitates and Sources of Hardening. Intensive research during the past forty years has resulted in a progressive accumulation of knowledge concerning the atomic and crystallographic structural changes that occur in supersaturated solid solutions during precipitation and the mechanisms through which the structures form and alter alloy properties. In most precipitation-hardenable systems, a complex sequence of time-dependent and temperature-dependent changes is involved.

Kinetics of Solution and Precipitation. The relative rates at which solution and precipitation reactions occur with different solutes depend upon the respective diffusion rates, in addition to solubilities and alloy contents. Bulk diffusion coefficients for several of the commercially important alloying elements in aluminum were determined by various experimental methods.

Nucleation. The formation of zones can occur in an essentially continuous crystal lattice by a process of homogeneous nucleation. Recent investigations provide evidence that a critical vacancy concentration is required for this process and that a nucleation model involving vacancy-solute atom clusters is consistent with certain effects of solution temperature and quenching rate.

The nucleation of a new phase is greatly influenced by the existence of discontinuities in the lattice. Since in polycrystalline alloys grain boundaries, subgrain boundaries, dislocations, and interphase boundaries are locations of greater disorder and higher energy than the solid-solution matrix, they are preferred sites for nucleation of precipitates.

Quenching

Quenching is in many ways the most critical step in the sequence of heat treating operations. The objective of quenching is to preserve as nearly intact as possible the solid solution formed at the solution heat treating temperature, by rapidly cooling to some lower temperature, usually near room temperature.

Critical Temperature Range. The fundamentals involved in quenching precipitation-hardenable alloys are based on nucleation theory applied to diffusion-controlled solid state reactions. The effects of temperature on the kinetics of isothermal precipitation depend principally upon degree of supersaturation and rate of diffusion.

Quenching Medium. Water is not only the most widely used quenching medium but also the most effective. It is apparent that in immersion quenching, cooling rates can be reduced by increasing water temperature. Conditions that increase the stability of a vapor film around the part decrease the cooling rate; various additions to water that lower surface tension have the same effect.

Aging at Room Temperature (Natural Aging)

Most of the heat treatable alloys exhibit age hardening at room temperature after quenching, the rate and extent of such hardening varying from one alloy to another. No discernible microstructural changes accompany the room-temperature aging, since the hardening effects are attributable solely to the formation of zone structure within the solid solution.

Since the alloys are softer and more ductile immediately after quenching than after aging, straightening or forming operations may be performed more readily in the freshly quenched condition.

Precipitation Heat Treating (Artificial Aging)

The effects of precipitation on mechanical properties are greatly accelerated, and usually accentuated, by reheating the quenched material to about 100 to 200°C. The effects are not entirely attributable to a changed reaction rate; as mentioned previously, the structural changes occurring at the elevated temperatures differ in fundamental ways from those occurring at room temperature. These differences are reflected in the mechanical characteristics and some physical properties. A characteristic feature of elevated-temperature aging effects on tensile properties is that the increase in yield strength is more pronounced than the increase in tensile strength. Also ductility, as measured by percentage elongation, decreases. Thus, an alloy in the T6 temper has higher strength but lower ductility than the same alloy in the T4 temper.

Precipitation Heat Treating Without Prior SoIution Heat Treatment

Certain alloys that are relatively insensitive to cooling rate during quenching can be either air cooled or water quenched directly from a final hot working operation. In either condition, these alloys will respond strongly to precipitation heat treatment.

Precipitation Heat Treating Cast Products

The mechanical properties of permanent mold, sand, and plaster castings of most alloys are greatly improved by solution heat treating, quenching, and precipitation heat treating, using practices analogous to those employed for wrought products.

Welding of Aluminum Alloys

Aluminum and its alloys can be joined by more methods than any other metal, but aluminum has several chemical and physical properties that need to be understood when using the various joining processes.

The specific properties that affect welding are its oxide characteristics, its thermal, electrical, and nonmagnetic characteristics, lack of color change when heated, and wide range of mechanical properties and melting temperatures that result from alloying with other metals.

Oxide. Aluminum oxide melts at about 2050 oC which is much higher than the melting point of the base alloy. If the oxide is not removed or displaced, the result is incomplete fusion. In some joining processes, chlorides and fluorides are used in order to remove the oxide contain. Chlorides and fluorides must be removed after the joining operation to avoid a possible corrosion problem in service.

Hydrogen Solubility. Hydrogen dissolves very rapidly in molten aluminum. However, hydrogen has almost no solubility in solid aluminum and it has been determined to be the primary cause of porosity in aluminum welds. High temperatures of the weld pool allow a large amount of hydrogen to be absorbed, and as the pool solidifies, the solubility of hydrogen is greatly reduced. Hydrogen that exceeds the effective solubility limit forms gas porosity, if it does not escape from the solidifying weld.

Electrical Conductivity. For arc welding, it is important that aluminum alloys possess high electrical conductivity — pure aluminum has 62% that of pure copper. High electrical conductivity permits the use of long contact tubes guns, because resistance heating of the electrode does not occur, as is experienced with ferrous electrodes.

Thermal Characteristics. The thermal conductivity of aluminum is about 6 times that of steel. Although the melting temperature of aluminum alloys is substantially bellow that of ferrous alloys, higher heat inputs are required to weld aluminum because of its high specific heat.
High thermal conductivity makes aluminum very sensitive to fluctuations in heat input by the welding process.

Forms of Aluminum. Most forms of aluminum can be welded. All the wrought forms (sheet, plate, extrusions, forgings, rod, bar and impact extrusions), as well as sand and permanent mold castings, can be welded. Welding on conventional die-castings produces excessive porosity in both the weld and the base metal adjacent to the weld because of internal gas. Vacuum die-castings, however, have been welded with excellent results. Powder metallurgy (P/M) parts also may suffer from porosity during welding because of internal gas.
The alloy composition is a much more significant factor than the form in determining the weldability of an aluminum alloy.

Filler Alloy Selection Criteria

When choosing the optimum filler alloy, the application (end use) of the welded part and its desired performance must be prime considerations. Many alloys and alloy combinations can be joined using any one of several filler alloys, but only one filler may be optimal for a specific application.

The primary factors commonly considered when selecting a welding filler alloy are:

  • Ease of welding
  • Tensile or shear strength of the weld
  • Weld ductility
  • Service temperature
  • Corrosion resistance
  • Color match between the weld and the base alloy after anodizing
  • Sensitivity to Weld Cracking.

Ease of welding is the first consideration for most welding applications. In general, the non-heat-treatable aluminum alloys can be welded with a filler alloy of the same basic composition as the base alloy.

The heat-treatable aluminum alloys are somewhat more metallurgically complex and more sensitive to “hot short” cracking, which results from heat – affected zone (HAZ) liquidation during the welding operation. Generally, a dissimilar alloy filler having higher levels of solute (for example, copper or silicon) is used in this case.

  • The high-purity 1xxx series alloys and 3003 are easy to weld with a base alloy filler, 1100 alloy, or an aluminum – silicon alloy filler, such as 4043.
  • Alloy 2219 exhibits the best weldability of the 2xxx series base alloys and is easily welded with 2319, 4043 and 4145 fillers.
  • Aluminum-silicon-copper filler alloy 4145 provides the least susceptibility to weld cracking with 2xxx series wrought copper bearing alloys, as well as aluminum-copper and aluminum-silicon-copper aluminum alloy castings
  • The cracking of aluminum-magnesium alloy welds decreases as the magnesium content of the weld increases above 2%.
  • The 6xxx series base alloys are most easily welded with the aluminum-silicon type filler alloys, such as 4043 and 4047. However, the aluminum-magnesium type filler alloys can also be employed satisfactorily with the low-copper bearing 6xxx alloys when higher shear strength and weld metal ductility are required.
  • The 7xxx series (aluminum-zinc-magnesium) alloys exhibit a wide range of crack sensitivity during the welding. Alloys 7005 and 7039, with a low copper content (<0.1%), have a narrow melting range and can be readily joined with the high magnesium filler alloys 5356, 5183 and 5556. The 7xxx series alloys that possess a substantial amount of copper, such as 7975 and 7178, have a very wide melting range with a low solidus temperature and are extremely sensitive to weld cracking when are welded.

Welding Processes

The GTAW (gas-metal arc welding) process has been used to weld thicknesses from 0,25 to 150 mm and can be used in all welding positions. Because it is relatively slow, it is highly maneuverable for welding tubing, piping and variable shapes. It permits excellent penetration control and can produce welds of excellent soundness. Weld termination craters can be filled easily as the current is tapered down by a foot pedal or electronic control.

The ac – GTAW process provides an arc cleaning action to remove the surface oxide during the positive electrode half of the cycle and a penetrating arc when the electrode is operated at negative polarity.

The dc – GTAW Process. Negative electrode polarity direct current can be used to weld aluminum by manual and mechanized means.

Other arc welding processes include shielded metal arc welding (SMAW), as well as electroslag and electrogas welding (ESW, EGW). SMAW with flux-coated rods has been replaced to a very substantial degree by the GMAW process.

The oxyfuel gas welding (OFW) process uses a flux and either an oxyacetylene or oxyhydrogen gas flame. When the oxyacetylene flame is used, a slightly reduced flame is required, which causes a carbonaceous deposit that obscures the weld and slows the travel speed.

Electron – beam welding (EBW) in a vacuum chamber produces a very deep, narrow penetration at high welding speeds. The low overall heat input produces the highest as-welded strengths in the heat treatable alloys. The high thermal gradient from the weld into the base metal creates very limited metallurgical modifications and is least likely to cause intergranular cracking in butt joints when no filler is added.

Laser-beam welding (LBW) is now considered to be a viable fusion joining process for aluminum with the advent of commercially available, stable, high-power laser systems. Because of aluminum`s high reflectivity, effective coupling of the laser beam and aluminum requires a relatively high power density.

Practical Aspects of Modern Inert Gas Welding Of Aluminum

Metallurgical aspects

The specific demands made on welded constructions require the application of the most suitable alloy. Pure aluminum and the Al-Mn alloy are still in use for the construction of containers and chemical appliances owing to their favorable prices and good corrosion resistance, wherever the demands made on the mechanical resistance are not excessive.

Typical alloys for the manufacture of vessels and containers, and indeed as welding materials, are the non-heat-treatable alloys Al-Mg or Al-Mg-Mn, which still have a medium resistance in the soft state. The alloys Al-Mg-Mn particularly are characterized by their good resistance after welding. This explains the soft zone in the vicinity of the weld area of hard or medium hard alloys as the welding temperature is above 660°C. The quickest welding process, involving the smallest energy inflow, will result in the narrowest soft zone.

For the construction of road and rail vehicles, which makes extensive use of the high versatility of the extruded sections, the heat-treated alloys Al-Mg-Si as well as the self-hardening alloy Al-Zn-Mg are mainly used.

In the soft state the heat-treatable alloys Al-Mg-Si possess mechanical characteristics which are hardly higher than those of pure aluminum and Al-Mn alloy. Appropriate use of the high heat induced by inert-gas welding, by speedy welding and adequate fixtures for heat evacuation will lead to a narrow annealing zone. In addition, a heat-treating effect will take place which will partly reduce the softening of the material. A special place is occupied by the self-hardening alloy Al-Zn-Mg. This alloy will reach its full original characteristics in the annealed welding zone by natural ageing after a few weeks in room temperature is an alloy which is widely used for welded structures.

Aluminum alloys with large melting and solidification intervals presenting at the same time a lack of eutectic parts tend to weld cracking. To weld the alloys Al-Mg-Si it is therefore usual to use filler containing 5% silicon, whereas the welded alloy content is only 0.5-1% silicon.

For the self-hardening Al-Zn-Mg alloy it would be desirable to have a similar self-hardening filler material. Unfortunately such filler belongs to the range of weld cracking material and furthermore zinc fumes prevent reliable weldings of the MIG arc process.

Mechanical properties

The static mechanical properties were systematically tested according to various welding methods, many combinations of fillers and alloys including castings. Basic calculation methods were set up which allow accurate dimensioning of welded structures. The values of the endurance fatigue established on test specimens cannot be used readily for the calculations.

Welding processes

Within the last 30 years after the introduction of the inert-gas welding processes, great technical progress has been achieved in the development of welding equipment. Welding methods have become more reliable. Experience in technology has led to the construction of welding sets which are specially adapted to the welding of aluminum. The use of various types of inert-gas power sources with special characteristics such as pulsed current and other types have largely extended the scope of inert-gas welding.

TIG welding with alternating current is similar in execution to oxyacetylene welding. With its soft arc under argon gas shielding between a tungsten electrode and the work piece, TIG welding makes it possible to carry out one side butt and corner joints between 1 and 8 mm thick work pieces without a backing bar. The process is particularly favorable for seams with numerous direction changes, intermittent breaks and restarts.

Once more the danger caused by the so-called oxide notches should be mentioned. Due to their high inciting temperatures the oxide films which cannot be reached and removed by the arc have to slip out mechanically on the back of the bead. If this procedure is prevented, dangerous oxide folds may form which will inadmissibly reduce the weld properties.

Dual welder method

The simultaneous method is still often used, especially by bulk container makers. Two welders, one inside, the other one outside, weld upwards simultaneously with normal TIG torches. The welds obtained are of good appearance and high quality. The welding costs are somewhat higher than those of the other processes and a well trained pair of operatives is necessary.

The TIG helium direct current is a modern and promising process which is, however, limited to special applications due to its severe technical contingencies. It was used, among others, for the welding of the Saturn rocket.

In the case of inert gas welding it is known that the oxide film with its melting point of over 2000°C is eliminated only when the electrode is positive. With this reversed polarity, considerable heat is produced causing the melting of the tip of the tungsten electrode even with low currents. With helium straight polarity welding, the elimination of the oxide film by the arc is intentionally relinquished: the considerable heat is concentrated in the welding pool, thus obtaining speedy and narrow seams with reduced shrinkage less construction and smaller heat affected zone. With thin sections often welded without filler material, filler wire can be automatically fed into the weld pool.

MIG welding

The greatest part of aluminum welding is now carried out by the MIG process. The heavy energized direct current arc with positive electrode and argon or helium gas protection allows adequate welding of good quality. It can be operated with either manual or automatic torch advance.

The choice of an appropriate welding equipment, optimum welding conditions, filler, edges preparation, etc., imply appropriate knowledge of the method. The answer to the questions of the material thicknesses, the type and the size of the work piece, will dictate the choice of the type of MIG equipment to be purchased.

MIG pulsed-arc welding

The “Impulse” welding or “Pulsarc” process has gained increased recognition in the last few years. Thin material can be welded with relatively large-diameter electrodes. In case of work pieces of various thicknesses there is no need to change the diameter of the welding wire.

The process is operated with a welding current too low for a spray arc, no drop could bedetached from the wire. Superimposed higher surge impulse current with 50-100 frequencies per second, according to circumstances, will ensure a drop detachment from the wire.

The most important advantages of the pulse-current welding are: the possibility of obtaining butt welds with material up to a thickness of about 5 mm without backup plate, with regular and deep penetration; the use of thicker thus cheaper welding wire; and, in case of wire filler with zinc content, for welding of Al-Zn-Mg alloy, the possibility of keeping down zinc fumes within reasonable limits.

Difficulties are encountered with the regulation of the arc conditions which require skilled operators. The argument put forward for a long time that pulse-current welding ensures particular porosity freedom has so far not been proved. In case of wire speed changes, electric self-adjustment takes place to keep the arc′s length practically constant. This system, therefore, renders manual welding practically free from any disturbance as small wire feeding hindrances do not revert at once to burn backs to the gun.

MIG high current density – large diameter filler wire welding

These are two new similar procedures for the automatic welding of heavy gauge materials. Both methods may make use of the high ionizing arc density of helium in order to produce with helium or with a mixture of argon and helium greater arc energy.

The high current density method, as its name implies, makes use of high specific current density for filler wire of conventional diameters. Large diameter wire reaches the necessary welding heat flow by thicknesses up to 6 mm in diameter.

The development of these processes has stagnated at present owing to equipment complications. On the one hand mastering gas protection on large fusion weld pools as well as transferring high density current to the filler wire is difficult. On the other hand is the fact that for thick gauge sheets applications positional welds are generally used which are not possible with these methods.

The normal MIG process is characterized by its high welding speed. Especially in the case of fillet and overlap joints sound welds with good penetration are obtained. The rapid speed incurred by the transfer of the droplets to the weld pool impedes the regular penetration in the case of butt welds unless the root of the bead is supported by so-called “backup bars”. These may be made of a grooved section in steel, copper or aluminum or a refractory textile strip. The molten aluminum does not bind with aluminum backup bars as long as the arc has not perforated and destroyed the oxide film of the backup bar. Aluminum backup bars can usually be used indefinitely.

Brief heating steps of aluminum material

Heat treatment is the aluminum material on a certain medium, heated to a suitable temperature, holding at this temperature for a period of time in the future, and the use of different speeds for a cooling process, the aluminum material is the most important process steps. Today aluminum factory here on a brief introduction of aluminum material which the heating step.

An annealing. Refers to the workpiece is heated to an appropriate temperature, depending on the size of the workpiece material and the different incubation times, and then slowly cooled, its main purpose is to reduce the hardness of the material, to improve the plasticity, in order to facilitate subsequent processing, reduce the residual stress and improve homogenization organization and composition. annealing divided according to different purposes recrystallization annealing, stress relieving back fireball annealing, fully annealed and so on.

Second, normalizing. Means after the workpiece is heated to a suitable temperature of the cooling air, the same effect as normalizing annealing similar, but finer tissue obtained, generally used to improve the cutting performance of the material, is sometimes used for a number of less demanding parts the final heat treatment.

Third, the tempering. Refers to the material after quenching unbalanced tissue is generally hard and brittle, requires a temperature above room temperature for a long time to heat, and then cooled.

Fourth, quenching. Refers to the workpiece after the heat insulation, water, oil, or other inorganic salt, an organic aqueous medium, quenching rapidly cooled.

Aluminum sheet material in the heating process, the heating rate is mainly provided by the holding time and cooling rate of the material the material is aluminum, heat treatment process is the most important process steps. Its characteristic is to improve the intrinsic quality of the work, which is generally not the naked eye can observe. Today’s aluminum material is heated knowledge for everyone to stop here, and if you want to know more knowledge of the aluminum plate, welcome to contact the major aluminum manufacturers.

Corrosion of Aluminum and Aluminum Alloys

Aluminum owes its excellent corrosion resistance and its usage as one of the primary metals of commerce to the barrier oxide film that is bonded strongly to its surface and, that if damaged, re-forms immediately in most environments.
On a surface freshly abraded and then exposed to air, the barrier oxide film is only 1 nm thick but is highly effective in protecting the aluminum from corrosion.

The natural film can be visualized as the result of a dynamic equilibrium between opposing forces-those tending to form compact barrier layer and those tending to break it down.

If the destructive forces are absent, as in dry air, the natural film will consist only of the barrier layer and will form rapidly to the limiting thickness. If the destructive forces are too strong, the oxide will be hydrated faster than it is formed, and little barrier will remain.

Between these extremes, where the opposing forces reach a reasonable balance, relatively thick (20 to 200 nm) natural films are formed.

Pitting corrosion

Corrosion of aluminum in the passive range is localized, usually manifested by random formation of pits. The pitting potential principle establishes the conditions under which metals in the passive state are subject to corrosion by pitting.

For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl ) is the most frequently encountered in service. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential.

Generally, aluminum does not develop pitting in aerated solutions of most nonhalide salts because its pitting potential in these solutions is considerably more noble (cathodic) than in halide solutions and it is not polarized to these potentials in normal service.

Solution Potentials

Because of the electrochemical nature of most corrosion processes, relationships among solution potentials of different aluminum alloys, as well as between potentials of aluminum alloys and those of other metals, are of considerable importance.

Furthermore, the solution potential relationships among the microstructural constituents of a particular alloy significantly affect its corrosion behavior. Compositions of solid solutions and additional phases, as well as amounts and spatial distributions of the additional phases may affect both the type and extent of corrosion.

The solution potential of an aluminum alloy is primarily determined by the composition of the aluminum rich solid solution, which constitutes the predominant volume fraction and area fraction of the alloy microstructure. Solution potential is not affected significantly by second phase particles of microscopic size, but because these particles frequently have solution potentials differing from that of the solid solution matrix in which they occur, localized galvanic cells may be formed between them and the matrix.

Since most of the commercial aluminum alloys contain additions of more than one of these elements; effects of multiple elements in solid solution on solution potential are approximately additive. The amounts retained in solid solution, particularly for more highly alloyed compositions, depend highly on fabrication and thermal processing so that the heat treatment and other processing variables influence the final electrode potential of the product.

Solution potential measurements are useful for the investigation of heat treating, quenching, and aging practices, and they are applied principally to alloys containing copper, magnesium, or zinc.

In aluminum-copper and aluminum-copper-magnesium (2xxx) alloys, potential measurement can determine
the effectiveness of solution heat treatment by measuring the amount of copper in solid solution. Also, by measuring
the potentials of grain boundaries and grain bodies separately, the difference in potential responsible for intergranular corrosion, exfoliation, and stress corrosion cracking (SCC) can be quantified. Solution-potential measurement of alloys containing copper also show the progress of artificial aging as increased amounts of precipitates are formed and
the matrix is depleted of copper.

Potential measurement are valuable with zinc-containing (7xxx) alloys for evaluating the effectiveness of the solution heat treatment, for following the aging process, and for differentiating among the various artificially aged tempers.
These factors can affect corrosion behavior.

In the magnesium containing (5xxx) alloys, potential measurements can detect low-temperature precipitation and are useful in qualitatively evaluating stress-corrosion behavior. Potential measurement can also be used to follow the diffusion of zinc or copper in alclad products, thus determining whether the sacrificial cladding can continue to protect the core alloy.

Atmospheric Corrosion

Most aluminum alloys have excellent resistance to atmospheric corrosion (often called weathering), and in many outdoor applications, such alloys do not require shelter, protective coatings or maintenance.

Corrosion in Waters

Aluminum alloys of the 1xxx, 3xxx, 5xxx and 6xxx series are resistant to corrosion by many natural waters. The more important factors controlling the corrosivity of natural waters to aluminum include water temperature, pH, and conductivity, availability of cathodic reactant, presence or absence of heavy metals, and the corrosion potentials and pitting potentials of the specific alloys.

Effects of Composition and Microstructure on Corrosion

1xxx Wrought Alloys. Wrought aluminums of the 1xxx series conform to composition specifications that set maximum individual, combined, and total contents for several elements present as natural impurities in the smelter – grade or refined aluminum used to produce these products.

Corrosion resistance of all 1xxx compositions is very high, but under many conditions, it decreases slightly with increasing alloy content. Iron, silicon and copper are the elements present in the largest percentages. The copper and part of the silicon are in solid solution.

2xxx wrought alloys and 2xxx casting alloys, in which copper is the mayor alloying element, are less resistant to corrosion than alloys of other series, which contain much lower amounts of copper.

Alloys of this type were the first heat-treatable high-strength aluminum base materials and have been used for more than 75 years in structural applications, particularly in aircraft and aerospace applications. Much of the thin sheet made of these alloys is produced as an alclad composite, but thicker sheet and other products in many applications require no protective cladding.

Electrochemical effects on corrosion can be stronger in these alloys than in alloys of many other types because of two factors: greater change in electrode potential with variations in amount of copper in solid solution and, under some conditions, the presence of no uniformities in solid solution concentration. However, that general resistance to corrosion decreases with increasing copper content is not primarily attributable to these solid-solution or second phase solution-potential relationships, but to galvanic cells created by formation of minute copper particles or films deposited on the alloy surface as a result of corrosion.

2xxx Wrought Alloys Containing Lithium. Lithium additions decrease the density and increase the elastic modulus of aluminum alloys, making aluminum-lithium alloys good candidates for replacing the existing high-strength alloys, primarily in aerospace applications.

3xxx Wrought Alloys. Wrought alloys of the 3xxx series (aluminum-manganese and aluminum-manganese-magnesium) have very high resistance to corrosion. The manganese is present in the aluminum solid solution, in submicroscopic particles of precipitate and in larger particles of Al6(Mn,Fe) or Al12(Mn,Fe)3Si phases, both of which have solution potentials almost the same as that of the solid solution matrix.

4xxx Wrought Alloys and 3xx.x and 4xx.x Casting Alloys. Elemental silicon is present as second-phase constituent particles in wrought alloys of the 4xxx series, in brazing and welding alloys, and in casting alloys of 3xx.x and 4xx.x series.

Corrosion resistance of 3xx.x castings alloys is strongly affected by copper content, which can be as high as 5% in some compositions, and by impurity levels. Modifications of certain basics alloys have more restrictive limits on impurities, which benefit corrosion resistance and mechanical properties.

5xxx Wrought Alloys and 5xx.x Casting Alloys. Wrought Alloys of the 5xxx series (aluminum-magnesium-manganese, aluminum-magnesium-chromium, and aluminum-magnesium-manganese-chromium) and casting alloys of the 5xx.x series (aluminum-magnesium) have high resistance to corrosion, and this accounts in part for their use in a wide variety of building products and chemical-processing and food-handling eguipment, as well as applications involving exposure to seawater.

6xxx Wrought Alloys. Moderately high strength and very good resistance to corrosion make the heat-treatable wrought alloys of the 6xxx series (aluminum-magnesium-silicon) highly suitable in various structural, building, marine machinery, and process-equipment applications.

7xxx Wrought Alloys and 7xx.x casting alloys contain major additions of zinc along with magnesium or magnesium plus copper in combinations that develop various levels of strength. Those containing copper have the highest strengths and have been used as constructional materials, primarily in aircraft applications, for more than 40 years.

The copper-free alloys of the series have many desirable characteristics: moderate-to-high strength, excellent toughness, and good workability, formability, and weldability. Use of these copper-free alloys has increased in recent years and now includes automotive applications, structural members and armor plate for military vehicles, and components of other transportation equipment.

The 7xxx wrought and 7xx.x casting alloys, because of their zinc contents, are anodic to 1xxx wrought aluminums and to other aluminum alloys. They are among the aluminum alloys most susceptible to SCC.

Resistance to general corrosion of the copper-free wrought 7xxx alloys is good, approaching that of the wrought 3xxx, 5xxx and 6xxx alloys. The copper-containing alloys of the 7xxx series, such as 7049, 7050, 7075, and 7178 have lower resistance to general corrosion than those of the same series that do not contain copper. All 7xxx alloys are more resistant to general corrosion than 2xxx alloys, but less resistant than wrought alloys of other groups.

Although the copper in both wrought and cast alloys of the aluminum-zinc-magnesium-copper type reduces resistance to general corrosion, it is beneficial from the standpoint of resistance to SCC.

Aluminum Extrusion Industry

Extruded Aluminium SectionExtruded Aluminium Sections

Aluminum extrusions are linear aluminum products highly valued in a wide spectrum of structural applications due to aluminum’s high strength-to-weight ratio and the cost effectiveness of the metal extrusion process. Like other types of metal extrusions, extruded aluminum is either hot extruded or cold extruded through a die, shaping aluminum stock into various types of extruded aluminum shapes, such as angles and beams, aluminum channels, aluminum profiles or aluminum extruded tubing.

Extruded aluminum products like aluminum channels, shapes and profiles are both strong and lightweight, making them perfect for structural applications such as light poles, building and window frames, lighting fixtures, car bumpers, hardware joints, trim, and many other uses in construction, industrial and automotive industries. Shapes and channels can be extruded into complex, precision tolerance shapes to interlock with other aluminum channels or structures, or they may be extruded into heat sinks for cooling electronics, refrigerators and heat engines. Because aluminum is strong, rust and temperature resistant, easily fabricated and 100% recyclable, aluminum and aluminum alloy extrusions are often the first choice in building or structural materials.

Aluminum Profiles

The number of industries which use aluminum extrusions is both extensive and diverse since a wide number of shapes are achievable through the extrusion process. For example, extruded aluminum channels make great components for automotive and transportation construction, as it is light and corrosion resistant; aluminum channels and profiles are used in vehicles such as trains, SUVs, semi trucks and cars for parts and components including panels, window panes, runners and bumpers. In addition, machinery and industrial equipment such as scaffolding, process and mining equipment use extruded aluminum tubing, shapes and profiles as lightweight, durable equipment components, while many types of office and hospital furniture use aluminum tubing and channels in their construction. The building, architectural and construction industries use aluminum profiles extensively, whether it be practical application such as for structural and ceiling beams or for aesthetic applications such as decorative trim and window paneling. Capable of being extruded through complex dies into close-tolerance shapes, small extruded aluminum shapes are frequently fabricated into medical and electronics components such as heat-absorbing and dissipating heat sinks.

Thermomechanical Treatments of Aluminum Alloys

Thermomechanical processes are defined in the broadest sense to include any combination of thermal or deformation processes that give rise to interactive microstructural features. The varieties of mechanisms involving the creation, rearrangement and elimination of dislocations and grain boundaries are reviewed to show the range of possibilities in microstructure and property production.

The interactions of dislocations and grain boundaries with solutes and second phase particles are examined, and the opportunities for synergistic combinations are discussed. The primary concern is for aluminum alloys, but attention is paid to contrasting and comparative alloy systems. The processes result in improvements in yield strength, toughness and resistance to stress corrosion cracking, fatigue and creep.

Traditionally the thermal treatment and deformation processing of metals were kept separate in both practice, and theory. This arose on one hand because austenitizing completely eradicated any worked structure and on the other because the martensitic steel vas very difficult to work.

Non-ferrous alloys were largely not heat-treatable and the introduction of ones that were did not immediately change preconceived notions. The shaping of metals, especially hot working, was considered solely as a mean of changing shape and metallurgical effort was directed at improving size capability, rate of processing and yield. Cold working and annealing were recognized as means of controlling the strength and the grain size but were considered techniques reserved for metals that were not heat treatable.

As knowledge grew of properties, microstructures, substructures, and mechanisms active during heat treatment and during deformation, and as the concept of strengthening mechanisms developed, the idea sprang up of superimposing mechanistic effects by combining processes. The term “thermomechanical processing” was coined to cover manufacturing in which a heat treatment and deformation were combined so that microstructural changes wrought by each interacted.

For the purposes of this paper, thermomechanical processing includes all combinations of thermal and mechanical treatments, irrespective of their order, which produce microstructural changes that do not obliterate each other. Another aspect which distinguishes such processing from simple forming operations is that the primary goal is product microstructure and properties, with the shape production and flow stresses being secondary.

The microstructures produced by forming processes over a broad range of temperatures are examined. This includes the effects on dynamic restoration mechanisms of solute, particle dispersions and massive second phase.

Cold and Warm Working. Deformation at room temperature is a simple technique which can be carried out with great precision and good surface finish. Even with good lubrication, very high forces are developed at high strains and, depending on the process, there are limits to ductility. When the rate of working is increased, adiabatic heating may raise the temperature so that warm working ensues. The limits of warm preheating for forging or impact extrusion are, set by the lubricant stability.

Hot Working – Dynamic Recovery. Deformation at temperatures above 0.5 Tm, above which dynamic recovery is dependent on climb and vacancy migration, gives rise to a polygonized substructure. During the initial strain-hardening phase, dislocations accumulate in regular tangles and sub-boundaries. In distinction from cold or warm working, stable equiaxed subgrains form and persist through steady state deformation (temperature, strain rate and stress constant) without change in size or wall density.

As the temperature is raised or the strain rate lowered, the steady state subgrains become larger and more perfect. As the degree of dynamic recovery increases, the hot flow stress decreases and the ductility increases.

Hot Working – Dynamic Recrystallization. The degree of high temperature dynamic recovery depends upon the stacking fault energy of the metal much as in cold working. Thus, for metals such as α-Fe and Zr, the substructures are very similar to those of Al. On the other hand, metals such as Ni and Cuhave much smaller less perfect subgrains which have a higher dislocation density.

The recrystallization results in work softening to a new steady state in which the microstructure consists of equiaxed grains with a distribution of sizes. The larger grains are those which nucleated the longer time and strain before the point of observation and consequently have developed a high density dynamically recovered substructure. The preferred orientation after dynamic recrystallization appears to be the same as that developed with only dynamic recovery because of the continuing deformation.

The yield strength at room temperature follows the normal Petch relationship for grain size, but is stronger than recrystallized material of the same grain size because of the substructure.

Hot Working-Effects of Alloying. Solid solution alloying is likely to decrease the amount of dynamic recovery as it lowers the stacking fault energy as observed in Cu-Zn and Zr-Sn alloys. The effect on microstructure is not nearly so clear in other cases where a well defined substructure is produced, e.g. Al-Mg alloys, ferritic Fe-Si, Fe-Cr and Fe-Ni alloys, austenitic stainless steels, and nickel superalloys.

In the case of Al-Mg alloys the substructure depends on the absence or presence of dynamic strain aging. If the alloy composition, thermal history and deformation temperature and strain rate are such that the impurity atoms interact dynamically with the dislocations, they inhibit subgrain formation and change the activation energy. Somewhat similar effects are observed in Al-Cu alloys and in α-Fe -C-N alloys. Solute addition frequently delays dynamic recrystallization to higher strains since it inhibits grain boundary migration.

Fine, well-distributed particles can interact with the dislocations increasing their density, pinning the cell walls and in some cases may define the substructure. In hot working, the effect is more noticeable than in cold working, since the metal, without the particles, would have much larger subgrains. Particles which shear may prevent the formation of a substructure. Dynamic precipitation may also occur with one or other of the effects already mentioned. Fine particles on the sub-boundaries stabilize them slowing static recovery, and delaying or preventing static or dynamic recrystallization.

Static Recovery and Substructure Strengthening. If a metal containing a substructure is heated, the stored energy gives rise to static restoration processes. Recovery involves individual dislocation motions within the existing grains whereas recrystallization involves displacements of grain boundaries which eliminate the deformed structure entirely. Recovery usually always occurs first to some degree but can be the sole mechanism if a critical stored energy dependent on the annealing temperature is not reached. This critical stored energy is higher for metals which have dynamically recovered more at higher temperatures.

Product Strengthening by Recovered Substructure. Cold deformation followed by static recovery is a thermomechanical process used in industry mainly for stress relief anneals in which extensive recovery is not the object. It does appear, however, that in preparation for deep drawing of aluminum cans, annealing to polygonize the cold work substructure is employed to raise the strain hardening coefficient. Hot working is almost always followed by static recovery during the period after deformation until the temperature is too low for vacancy migration. In Al-alloys and ?-Fe alloys, the dynamically recovered structure is such that moderate rates of cooling can avoid recrystallization and permit static recovery that is limited to annihilation of the dislocations within the sub-grains.

Deformation Substructures and Precipitates. Precipitates or dispersed particles can be present at the start of deformation and help to define the substructure. The powder metallurgy pressing and sintering of Ni-ThO2 alloys and the internal oxidation of Cu-Al2O3 alloys are the initial phases of thermomechanical treatments. To improve high temperature stability, CuAl2has been precipitated in Al-Cu alloys and Fe-Al6 and (FeCo)Al9 have been included in aluminum prior to working and annealing. However, as an example of a different possible phenomenon, the Cu precipitates in Fe-Cu alloys prevent the formation of a substructure.

Precipitates may form on a substructure created in the first step of a thermomehanical treatment, e.g. Cu precipitates on the dislocations of a cold worked and recovered substructure in Fe. The prior deformation usually accelerates the precipitation by providing more sites for heterogeneous nucleation, e.g. in austenite the rate of precipitation of Nb is increased an order of magnitude.

Thermomechanical Treatments for High Strength Aluminum Alloys. These processes have been called ITMT, intermediate thermomechanical treatments. The research in this area also indicates that improved properties can be achieved by extending structure control to earlier stages of processing.

Rapid cooling produces very fine dendrite arm spacing and thus finer inclusions and constituent particles which do not reduce the toughness as do coarser distributions. Powder metallurgy fabrication also reduces the problems of inclusions and large dendrite spacing. High purity alloys with reduced inclusion contents exhibit improved ductility, toughness, and resistance to fatigue and stress corrosion cracking.

For many years, a simple press heat treatment has been practiced, in which the hot working served as the solution treatment and the hot worked product had to be rapidly quenched. This gave fairly satisfactory results for Al-Cu (2000 series) and Al-Mg-Si alloys (6000 series) but was in general inappropriate for the Al-Zn-Mg-Cu alloys (7000 series with exception possibly of 7005 and 7039). The problems arise from three areas:

  1. the hot working is not of adequate duration and precision for a solution treatment;
  2. the quenching rates, because of the forming process are not rapid enough and
  3. the heterogeneous nucleation on dislocations supplants the very fine uniform, partially-coherent precipitate needed for highest strength.

The high strength aluminum alloys of the Zn-Mg-Cu and Zn-Mg classes can have their toughness and resistance to fatigue and stress corrosion cracking improved by what is known as FTMT, final thermomechanical treatment. This consists of solution, preaging, deformation and final aging.

The preaging plays a very important role: carried out near or slightly below the temperature and time for normal aging, it provides a set of uniformly distributed nuclei which guarantees that homogeneous nucleation can compete with heterogeneous nucleation on dislocations during final aging.

Such preaging also provides for more uniform deformation in contrast to the heavy narrow slip bands which occur in slightly aged material. The deformations utilized have been in the range 10-50% and may be cold or warm. The substructure produced by the working should be uniform with slight cellularity being acceptable. The optimum appears to be deformation at or slightly above the aging temperature to strains of about 20%. Without the final aging the strength would be superior to simple aged material with ductility and toughness the same.

The second aging is frequently an overaging which lowers the strength to the normal value but increases the resistance to stress-corrosion cracking thus being somewhat similar to the second in a double aging treatment. Similar treatments have been worked out for Al-Mg-Si (6000 series), with similar restrictions on acceptable deformation structures. The Al-Cu alloys can usually be improved by working after the solution treatment but the conditions are not as restrictive as for the alloys with Zn and Mg.

In examining potential thermomechanical treatments, it is useful to include processing which involves only dislocation accumulation and restoration. Through manipulation of many variables, they are capable of yielding a spectrum of product microstructures either as complete processes or in conjunction with phase transformations.

Some of these dislocation and grain boundary manipulating treatments have been applied to Al-alloys, the most important being:

  • preservation of the hot worked substructure to strengthen non heat-treatable alloys,
  • use of a particle stabilized subgrain structure to influence the evolution of the substructure during cold drawing and annealing to produce electrical wire with improved resistance to thermal softening,
  • the establishment through control of ingot homogenization, rolling and annealing of an equiaxed microstructure with well distributed constituent particles which on further rolling to plate gives improved short transverse properties.

The high strength aluminum alloys can have their ductility, toughness and resistance to stress corrosion cracking greatly improved by a special preaging, warm working and post aging which does not detract from the normal precipitate distribution.

Aluminum Alloys – Effects of Alloying Elements

The important alloying elements and impurities are listed here alphabetically as a concise review of major effects. Some of the effects, particularly with respect to impurities, are not well documented and are specific to particular alloys or conditions.

Antimony is present in trace amounts (0.01 to 0.1 ppm) primary in commercial-grade aluminum. Antimony has a very small solid solubility in aluminum (<0.01%). Some bearing alloys contain up to 4 to 6% Sb. Antimony can be used instead of bismuth to counteract hot cracking in aluminum-magnesium alloys.

Arsenic. The compound AsAl is a semiconductor. Arsenic is very toxic (as AsO3) and must be controlled to very low limits where aluminum is used as foil for food packaging.

Beryllium is used in aluminum alloys containing magnesium to reduce oxidation at elevated temperatures. Up to 0.1% Be is used in aluminizing baths for steel to improve adhesion of the aluminum film and restrict the formation of the deleterious iron-aluminum complex.

Bismuth. The low-melting-point metals such as bismuth, lead, tin, and cadmium are added to aluminum to make free-machining alloys. These elements have a restricted solubility in solid aluminum and form a soft, low-melting phase that promotes chip breaking and helps to lubricate the cutting tool. An advantage of bismuth is that its expansion on solidification compensates for the shrinkage of lead. A 1-to-1 lead-bismuth ratio is used in the aluminum-copper alloy, 2011, and in the aluminum-Mg-Si alloy, 6262. Small additions of bismuth (20 to 200 ppm) can be added to aluminum-magnesium alloys to counteract the detrimental effect of sodium on hot cracking.

Boron is used in aluminum and its alloys as a grain refiner and to improve conductivity by precipitating vanadium, titanium, chromium, and molybdenum. Boron can be used alone (at levels of 0.005 to 0.1%) as a grain refiner during solidification, but becomes more effective when used with an excess of titanium. Commercial grain refiners commonly contain titanium and boron in a 5-to-l ratio.

Cadmium is a relatively low-melting element that finds limited use in aluminum. Up to 0.3% Cd may be added to aluminum-copper alloys to accelerate the rate of age hardening, increase strength, and increase corrosion resistance. At levels of 0.005 to 0.5%, it has been used to reduce the time of aging of aluminum-zinc-magnesium alloys.

Calcium has very low solubility in aluminum and forms the intermetallic CaAl4. An interesting group of alloys containing about 5% Ca and 5% Zn have superplastic properties. Calcium combines with silicon to form CaSi2, which is almost insoluble in aluminum and therefore will increase the conductivity of commercial-grade metal slightly. In aluminum-magnesium-silicon alloys, calcium will decrease age hardening. Its effect on aluminum-silicon alloys is to increase strength and decrease elongation, but it does not make these alloys heat treatable.

Carbon may occur infrequently as an impurity in aluminum in the form of oxycarbides and carbides, of which the most common is A14C3, but carbide formation with other impurities such as titanium is possible. A14C3 decomposes in the presence of water and water vapor, and this may lead to surface pitting.

Cerium, mostly in the form of mischmetal (rare earths with 50 to 60% Ce), has been added experimentally to casting alloys to increase fluidity and reduce die sticking.

Chromium occurs as a minor impurity in commercial-purity aluminum (5 to 50 ppm). It has a large effect on electrical resistivity. Chromium is a common addition to many alloys of the aluminum-magnesium, aluminum-magnesium-silicon, and aluminum-magnesium-zinc groups, in which it is added in amounts generally not exceeding 0.35%. In excess of these limits, it tends to form very coarse constituents with other impurities or additions such as manganese, iron, and titanium. Chromium has a slow diffusion rate and forms fine dispersed phases in wrought products. These dispersed phases inhibit nucleation and grain growth. Chromium is used to control grain structure, to prevent grain growth in aluminum-magnesium alloys, and to prevent recrystallization in aluminum-magnesium-silicon or aluminum-magnesium-zinc alloys during hot working or heat treatment.

Cobalt is not a common addition to aluminum alloys. It has been added to some aluminum-silicon alloys containing iron, where it transforms the acicular ß (aluminum-iron-silicon) into a more rounded aluminum-cobalt-iron phase, thus improving strength and elongation. Aluminum-zinc-magnesium-copper alloys containing 0.2 to 1.9% Co are produced by powder metallurgy.

Copper. Aluminum-copper alloys containing 2 to 10% Cu, generally with other additions, form important families of alloys. Both cast and wrought aluminum-copper alloys respond to solution heat treatment and subsequent aging with an increase in strength and hardness and a decrease in elongation. The strengthening is maximum between 4 and 6% Cu, depending upon the influence of other constituents present.

Copper-magnesium. The main benefit of adding magnesium to aluminum-copper alloys is the increased strength possible following solution heat treatment and quenching. In wrought material of certain alloys of this type, an increase in strength accompanied by high ductility occurs on aging at room temperature. On artificial aging, a further increase in strength, especially in yield strength can be obtained, but at a substantial sacrifice in tensile elongation.

Copper-magnesium plus other elements. The cast aluminum-copper-magnesium alloys containing iron are characterized by dimensional stability and improved bearing characteristics, as well as by high strength and hardness at elevated temperatures. However, in a wrought Al-4%Cu-0.5%Mg alloy, iron in concentrations as low as 0.5% lowers the tensile properties in the heat-treated condition, if the silicon content is less than that required to tie up the iron as the aFeSi constituent.

Gallium is an impurity in aluminum and is usually present at levels of 0.001 to 0.02%. At these levels its effect on mechanical properties is quite small. At the 0.2% level, gallium has been found to affect the corrosion characteristics and the response to etching and brightening of some alloys.

Hydrogen has a higher solubility in the liquid state at the melting point than in the solid at the same temperature. Because of this, gas porosity can form during solidification. Hydrogen is produced by the reduction of water vapor in the atmosphere by aluminum and by the decomposition of hydrocarbons. In addition to causing primary porosity in casting, hydrogen causes secondary porosity, blistering, and high-temperature deterioration (advanced internal gas precipitation) during heat treating. It probably plays a role in grain-boundary decohesion during stress-corrosion cracking. Its level in melts is controlled by fluxing with hydrogen-free gases or by vacuum degassing.

Indium. Small amounts (0.05 to 0.2%) of indium have a marked influence on the age hardening of aluminum-copper alloys, particularly at low copper contents (2 to 3% Cu).

Iron is the most common impurity found in aluminum. It has a high solubility in molten aluminum and is therefore easily dissolved at all molten stages of production. The solubility of iron in the solid state is very low (~0.04%) and therefore, most of the iron present in aluminum over this amount appears as an intermetallic second phase in combination with aluminum and often other elements.

Lead. Normally present only as a trace element in commercial-purity aluminum, lead is added at about the 0.5% level with the same amount as bismuth in some alloys (2011 and 6262) to improve machinability.

Lithium. The impurity level of lithium is of the order of a few ppm, but at a level of less than 5 ppm it can promote the discoloration (blue corrosion) of aluminum foil under humid conditions. Traces of lithium greatly increase the oxidation rate of molten aluminum and alter the surface characteristics of wrought products.

Magnesium is the major alloying element in the 5xxx series of alloys. Its maximum solid solubility in aluminum is 17.4%, but the magnesium content in current wrought alloys does not exceed 5.5%. The addition of magnesium markedly increases the strength of aluminum without unduly decreasing the ductility. Corrosion resistance and weldability are good.

Magnesium-Manganese. In wrought alloys, this system has high strength in the work-hardened condition, high resistance to corrosion, and good welding characteristics. Increasing amounts of either magnesium or manganese intensify the difficulty of fabrication and increase the tendency toward cracking during hot rolling, particularly if traces of sodium are present.

Magnesium-Silicon. Wrought alloys of the 6xxx group contain up to 1.5% each of magnesium and silicon in the approximate ratio to form Mg2Si, that is, 1.73:1. The maximum solubility of Mg2Si is 1.85%, and this decreases with temperature.
Precipitation upon age hardening occurs by formation of Guinier-Preston zones and a very fine precipitate. Both confer an increase in strength to these alloys, though not as great as in the case of the 2xxx or the 7xxx alloys.

Manganese is a common impurity in primary aluminum, in which its concentration normally ranges from 5 to 50 ppm. It decreases resistivity. Manganese increases strength either in solid solution or as a finely precipitated intermetallic phase. It has no adverse effect on corrosion resistance. Manganese has a very limited solid solubility in aluminum in the presence of normal impurities but remains in solution when chill cast so that most of the manganese added is substantially retained in solution, even in large ingots.

Mercury has been used at the level of 0.05% in sacrificial anodes used to protect steel structures. Other than for this use, mercury in aluminum or in contact with it as a metal or a salt will cause rapid corrosion of most aluminum alloys.

Molybdenum is a very low level (0.1 to 1.0 ppm) impurity in aluminum. It has been used at a concentration of 0.3% as a grain refiner, because the aluminum end of the equilibrium diagram is peritectic, and also as a modifier for the iron constituents, but it is not in current use for these purposes.

Nickel. The solid solubility of nickel in aluminum does not exceed 0.04%. Over this amount, it is present as an insoluble intermetallic, usually in combination with iron. Nickel (up to 2%) increases the strength of high-purity aluminum but reduces ductility. Binary aluminum-nickel alloys are no longer in use but nickel is added to aluminum-copper and to aluminum-silicon alloys to improve hardness and strength at elevated temperatures and to reduce the coefficient of expansion.

Niobium. As with other elements forming a peritectic reaction, niobium would be expected to have a grain refining effect on casting. It has been used for this purpose, but the effect is not marked.

Phosphorus is a minor impurity (1 to 10 ppm) in commercial-grade aluminum. Its solubility in molten aluminum is very low (~0.01% at 660oC) and considerably smaller in the solid.

Silicon, after iron, is the highest impurity level in electrolytic commercial aluminum (0.01 to 0.15%). In wrought alloys, silicon is used with magnesium at levels up to 1.5% to produce Mg2Si in the 6xxx series of heat-treatable alloys.

Vanadium. There is usually 10 to 200 ppm V in commercial-grade aluminum, and because it lowers conductivity, it generally is precipitated from electrical conductor alloys with boron.

Zinc. The aluminum-zinc alloys have been known for many years, but hot cracking of the casting alloys and the susceptibility to stress-corrosion cracking of the wrought alloys curtailed their use. Aluminum-zinc alloys containing other elements offer the highest combination of tensile properties in wrought aluminum alloys.

Zinc-Magnesium. The addition of magnesium to the aluminum-zinc alloys develops the strength potential of this alloy system, especially in the range of 3 to 7,5% Zn. Magnesium and zinc form MgZn2, which produces a far greater response to heat treatment than occurs in the binary aluminum-zinc system. The strength of the wrought aluminum-zinc alloys also is substantially improved by the addition of magnesium. Increasing the MgZn2, concentration from 0.5 to 12% in cold-water quenched 1.6 mm sheet continuously increases the tensile and yield strengths. The addition of magnesium in excess (100 and 200%) of that required to form MgZn2 further increases tensile strength.

Zinc-Magnesium-Copper. The addition of copper to the aluminum-zinc-magnesium system, together with small but important amounts of chromium and manganese, results in the highest-strength aluminum-base alloys commercially available. In this alloy system, zinc and magnesium control the aging process. The effect of copper is to increase the aging rate by increasing the degree of supersaturation and perhaps through nucleation of the CuMgAl2 phase. Copper also increases quench sensitivity upon heat treatment. In general, copper reduces the resistance to general corrosion of aluminum-zinc-magnesium alloys, but increases the resistance to stress corrosion. The minor alloy additions, such as chromium and zirconium, have a marked effect on mechanical properties and corrosion resistance.

Zirconium additions in the range 0.1 to 0.3% are used to form a fine intermetallic precipitate that inhibits recovery and recrystallization. An increasing number of alloys, particularly in the aluminum-zinc-magnesium family, use zirconium additions to increase the recrystallization temperature and to control the grain structure in wrought products.

Aluminum and Aluminum Alloys Casting Problems

Aluminum castings have played an integral role in the growth of the aluminum industry since its inception in the late 19th century. The first commercial aluminum products were castings, such as cooking utensils and decorative parts, which exploited the novelty and utility of the new metal. Those early applications rapidly expanded to address the requirements of a wide range of engineering specifications.

Alloy development and characterization of physical and mechanical characteristics provided the basis for new product development through the decades that followed. Casting processes were developed to extend the capabilities of foundries in new commercial and technical applications. The technology of molten metal processing, solidification, and property development has been advanced to assist the foundry man with the means of economical and reliable production of parts that consistently meet specified requirements.

Today, aluminum alloy castings are produced in hundreds of compositions by all commercial casting processes, including green sand, dry sand, composite mold, plaster mold, investment casting, permanent mold, counter-gravity tow-pressure casting, and pressure die casting.

Alloys can also be divided into two groups: those most suitable for gravity casting by any process and those used in pressure die casting. A finer distinction is made between alloys suitable for permanent mold application and those for other gravity processes.

Material constraints that formerly limited the design engineer’s alloy choice once a casting process had been selected are increasingly being blurred by advances in foundry technique. In the same way, process selection is also less restricted today. For example, many alloys thought to be unusable in permanent molds because of casting characteristics are in production by that process.

Melting and Metal Treatment

Aluminum and aluminum alloys can be melted in a variety of ways. Coreless and channel induction furnaces, crucible and open-hearth reverberatory furnaces fired by natural gas or fuel oil, and electric resistance and electric radiation furnaces are all in routine use. The nature of the furnace charge is as varied and important as the choice of furnace type for metal casting operations. The furnace charge may range from prealloyed ingot of high quality to charges made up exclusively from low-grade scrap.

Even under optimum melting and melt-holding conditions, molten aluminum is susceptible to three types of degradation:

  • With time at temperature, adsorption of hydrogen results in increased dissolved hydrogen content up to an equilibrium value for the specific composition and temperature
  • With time at temperature, oxidation of the melt occurs; in alloys containing magnesium, oxidation losses and the formation of complex oxides may not be self-limiting
  • Transient elements characterized by low vapor pressure and high reactivity are reduced as a function of time at temperature; magnesium, sodium, calcium, and strontium, upon which mechanical properties directly or indirectly rely, are examples of elements that display transient characteristics.

Turbulence or agitation of the melt and increased holding temperature significantly increase the rate of hydrogen solution, oxidation, and transient element loss. The mechanical properties of aluminum alloys depend on casting soundness, which is strongly influenced by hydrogen porosity and entrained nonmetallic inclusions.

Hydrogen influence on aluminum

Hydrogen is the only gas that is appreciably soluble in aluminum and its alloys. Its solubility varies directly with temperature and the square root of pressure. During the cooling and solidification of molten aluminum, dissolved hydrogen in excess of the extremely low solid solubility may precipitate in molecular form, resulting in the formation of primary and/or secondary voids.

Drossing fluxes are designed to promote separation of the aluminum oxide (Al2O3) dross layer that forms on the surface of the melt from the molten metal. Drosses and liquid or solid metal are usually intermingled in the dross layer. The drossing fluxes are designed to react with Al2O3 in the slag or dross layer and to recover metal. The fluorides wet and dissolve thin oxide films according to the general reaction.

Hydrogen Sources. There are numerous sources of hydrogen in aluminum. Moisture in the atmosphere dissociates at the molten metal surface, offering a concentration of atomic hydrogen capable of diffusing into the melt. The barrier oxide of aluminum resists hydrogen solution by this mechanism, but disturbances of the melt surface that break the oxide barrier result in rapid hydrogen dissolution. Alloying elements, especially magnesium, may also affect hydrogen absorption by forming oxidation reaction products that offer reduced resistance to the diffusion of hydrogen into the melt and by altering liquid solubility.

Hydrogen Porosity. Two types or forms of hydrogen porosity may occur in cast aluminum. Of greater importance is inter-dendritic porosity, which is encountered when hydrogen contents are sufficiently high that hydrogen rejected at the solidification front results in solution pressures above atmospheric. Secondary (micron-size) porosity occurs when dissolved hydrogen contents are low, and void formation is characteristically subcritical.

Finely distributed hydrogen porosity may not always be undesirable. Hydrogen precipitation may alter the form and distribution of shrinkage porosity in poorly fed parts or part sections. Shrinkage is generally more harmful to casting properties. In isolated cases, hydrogen may actually be intentionally introduced and controlled in specific concentrations compatible with the application requirements of the casting in order to promote superficial soundness.

Hydrogen in Solid Solution. The disposition of hydrogen in a solidified structure depends on the dissolved hydrogen level and the conditions under which solidification occurs. Because the presence of hydrogen porosity is a result of diffusion-controlled nucleation and growth, decreasing the hydrogen concentration and increasing the rate of solidification act to suppress void formation and growth. For this reason, castings made in expendable mold processes are more susceptible to hydrogen-related defects than parts produced by permanent mold or pressure die casting.

Hydrogen Removal. Dissolved hydrogen levels can be reduced by a number of methods, the most important of which is fluxing with dry, chemically pure nitrogen, argon, chlorine, and freon. Compounds such as hexachloroethane are in common use; these compounds dissociate at molten metal temperatures to provide the generation of fluxing gas.

Gas fluxing reduces the dissolved hydrogen content of molten aluminum by partial pressure diffusion. The use of reactive gases such as chlorine improves the rate of degassing by altering the gas/metal interface to improve diffusion kinetics. Holding the melt undisturbed for long periods of time at or near the liquidus also reduces hydrogen content to a level no greater than that defined for the alloy as the temperature-dependent liquid solubility.

Oxidation

Oxide Formation. Aluminum and its alloys oxidize readily in both the solid and molten states to provide a continuous self-limiting film. The rate of oxidation increases with temperature and is substantially greater in molten than in solid aluminum. The reactive elements contained in alloys such as magnesium, strontium, sodium, calcium, beryllium, and titanium are also factors in oxide formation. In both the molten and solid states, oxide formed at the surface offers benefits in self-limitation and as a barrier to hydrogen diffusion and solution. Induced turbulence, however, results in the entrainment of oxide particles, which resist gravity separation because their density is similar to that of molten aluminum.

Oxides are formed by direct oxidation in air, by reaction with water vapor, or by aluminothermic reaction with oxides of other metals, such as iron or silicon, contained in tools and refractories. Aluminum oxide is polymorphic, but at molten metal temperature the common forms of oxide encountered are crystalline and of a variety of types depending on exposure, temperature, and time. Some crystallographic oxide forms affect the appearance and coloration of castings, without other significant effects.

Oxide Separation and Removal. It is usually necessary to treat melts of aluminum and its alloys to remove suspended nonmetallics. This is normally accomplished by using either solid or chemically active gaseous fluxes containing chlorine, fluorine, chlorides, and/or fluorides. In each case, the objective is the dewetting of the oxide/melt interface to provide effective separation of oxides and other included matter and the flotation of these nonmetallics by attachment to either solid or gaseous elements or compounds introduced or formed during flux treatment.

Fluxes can also be used to minimize oxide formation. For this reason, melts containing magnesium are often protected by the use of salts that form liquid layers, most often of magnesium chloride, on the melt surface. These fluxes, termed covering fluxes, must be periodically removed and replaced. Carbon, graphite, and boron powder also effectively retard oxidation when applied to the melt surface.

Effects of Inclusions. In addition to oxides, a number of additional compounds can be considered inclusions in cast structures. All aluminum contains aluminum carbide (Al4C3) formed during reduction. Borides may also be present. By agglomeration, borides can assume sufficient size to represent a significant factor in the metal structure, with especially adverse effects in machining.

Under all conditions, inclusions whether in film or particle form are damaging to mechanical properties. The gross effect of inclusions is to reduce the effective cross section of metal under load. The more devastating effect on properties is that of stress concentration when inclusions appear at or near the surface of parts or specimens. Fatigue performance is reduced under the latter condition by the notch effect. Ultimate and yield strengths are typically lower, and ductility may be substantially reduced when inclusions are present.

Hard particle inclusions are frequently found in association with film-type oxides. Borides, carbides, oxides, and nonmetallic particles in the melt are scavenged and then concentrated in localized regions within the cast structure.

Structure Control

A number of factors define the metallurgical structure in aluminum castings. Of primary importance are dendrite cell size or dendrite arm spacing, the form and distribution of microstructural phases, and grain size. The foundryman can control the fineness of dendrite structure by controlling the rate of solidification.

Microstructural features such as the size and distribution of primary and intermetallic phases are considerably more complex dendrite cell size measurements are becoming increasingly important.

Grain Structure

A fine, equiaxed grain structure is normally desired in aluminum castings. The type and size of grains formed are determined by alloy composition, solidification rate, and the addition of master alloys (grain refiners) containing intermetallic phase particles, which provide sites for heterogeneous grain nucleation.

Grain Refinement Effects. A finer grain size promotes improved casting soundness by minimizing shrinkage, hot cracking, and hydrogen porosity. The advantages of effective grain refinement are:

  • Improved feeding characteristics
  • Increased tear resistance
  • Improved mechanical properties
  • Increased pressure tightness
  • Improved response to thermal treatment
  • Improved appearance following chemical, electrochemical, and mechanical finishing

Under normal solidification conditions spanning the full range of commercial casting processes, aluminum alloys without grain refiners exhibit coarse columnar and/or coarse equiaxed structures.

A fine grain structure also minimizes the effects on castability and properties associated with the size and distribution of normally occurring intermetallics. Large, insoluble intermetallic particles that are present or form in the temperature range between liquidus and solidus reduce feeding. A fine grain size promotes the formation of finer, more evenly distributed intermetallic particles with corresponding improvements in feeding characteristics. Because most of these more brittle phases precipitate late in the solidification process, their preferential formation at grain boundaries also profoundly affects tear resistance and mechanical properties in coarse-grain structures.

Porosity, if present, is of smaller discrete void size in fine-grain parts. The size of interdendritic shrinkage voids is directly influenced by grain size.

The finer distribution of soluble intermetallics throughout grain-refined castings results in faster and more complete response to thermal treatment. More consistent mechanical properties can be expected following thermal treatment.

Grain Refinement. All aluminum alloys can be made to solidify with a fully equiaxed, fine grain structure through the use of suitable grain-refining additions. The most widely used grain refiners are master alloys of titanium, or of titanium and boron, in aluminum. Aluminum-titanium refiners generally contain from 3 to 10% Ti. The same range of titanium concentrations is used in Al-Ti-B refiners with boron contents from 0.2 to 1% and titanium-to-boron ratios ranging from about 5 to 50. Although grain refiners of these types can be considered conventional hardeners or master alloys, they differ from master alloys added to the melt for alloying purposes alone.

To be effective, grain refiners must introduce controlled, predictable, and operative quantities of aluminides (and borides) in the correct form, size, and distribution for grain nucleation. Wrought refiner in rod form, developed for the continuous treatment of aluminum in primary operations, is available in sheared lengths for foundry use.

Aluminum Alloys: Terms, Definitions and Products

Aluminum alloys have numerous technical advantages that made them one of the dominant structural material families of the 20th century. Aluminum has low density (2.71 g/cm3) compared with competitive metallic alloy systems. It also has good inherent corrosion resistance because of the continuous, protective oxide film that forms very quickly in the air, and good workability that enables aluminum and its alloys to be economically rolled, extruded, or forged into useful shapes. In this article standardized terms and products made from aluminum alloys are described.

Profiles

Profile, section, shape: Wrought product, usually extruded, of uniform cross-section along its whole length and with a cross-section other than rod/bar, wire, tube, sheet or strip. They are usually supplied in straight lengths but sometimes in coiled form. Depending on the form of the cross-section, it can be called solid profile or hollow profile.

Extruded profile: Profile brought to final dimensions by extrusion.

Solid profile, solid section, solid shape: Profile in which the cross-section does not include any enclosed void.

Hollow profile, hollow section, hollow shape: Profile in which the cross-section includes either one enclosed void, provided that the cross-section is other than a tube, or more than one enclosed void.

Precision profile: Profile, which fulfils special requirements concerning tolerances on form and dimensions.

Rods/bars

Rod/bar: Solid wrought product of uniform cross-section along its whole length, supplied in straight lengths. Cross-sections are in the shape of circles, oval, squares, rectangles, equilateral triangles or regular polygons. Products with a square, rectangular, triangular or polygonal cross-section can have corners rounded along their whole length.

Extruded rod / bar: Rod or bar brought to final dimensions by extrusion.

Cold-drawn rod / bar: Rod or bar brought to final dimensions by cold drawing.

Brazing rod: Rod made of a low melting temperature alloy for use as filler metal in brazing.

Filler rod, welding rod: Rod for use as filler metal in joining by welding.

Square rod/bar: Rod or bar of a square cross- section.

Rectangular rod/bar: Rod or bar of a rectangular cross-section. The thickness of these rods exceeds one-tenth of the width. The term “rectangular rod/bar” includes “flattened circles” and “modified rectangles” of which two opposite sides are convex arcs, the other two sides being straight, of equal length and parallel.

Hexagonal rod/bar: rod or bar having the cross-section of a regular hexagon.

Wire

Wire: Wrought product of uniform cross-section along its whole length, supplied in coiled form. Cross-sections are in the shape of circles, ovals, squares, rectangles, equilateral triangles or regular polygons. Products with a square, rectangular, triangular, or polygonal cross-section can have corners rounded along their whole length.

The thickness of rectangular wires exceeds one-tenth of the width. Wire in the upper thickness range is often called “coiled rod”. The term “rectangular wire” includes “flattened circles” and “modified rectangles”, of which two opposite sides are convex arcs, the other two sides being straight, of equal length and parallel.

Drawing stock: Semi-finished solid wrought product of uniform cross-section along its whole length supplied in coils of a quality suitable to drawing into wire. Cross-sections are approximately round, triangular or regular polygonal with dimensions usually exceeding 7 mm.

Conductor wire: Wire possessing the requisite electrical and mechanical properties for use as an electrical conductor.

Filler wire, welding wire: Wire for use as filler metal in joining by welding.

Flattened wire: Wire produced by flattening round wire between rolls or by drawing through a die with flat opening.

Brazing wire: Wire of a low melting temperature alloy for use as filler metal in brazing.

Tubes

Tube: Hollow wrought product of uniform cross-section with only one enclosed void along its whole length, and with a uniform wall thickness, supplied in straight lengths or in coiled form. Cross-sections are in the shape of circles, ovals, squares, rectangles, equilateral triangles or regular polygons and can have corners rounded along their whole length, provided the inner and outer cross-sections are concentric and have the same form and orientation. Tubes can also be formed by piercing and by forming and joining sheet or strip.

Extruded tube: Tube brought to final dimensions by extrusions.

Drawn tube: Tube brought to final dimensions by cold drawing.

Porthole/bridge tube: Tube produced by extrusion of a solid billet through a porthole or bridge die. This tube is characterized by one or more seams formed by longitudinal bonding of two or more edges under pressure.

Seamless tube: Tube in which longitudinal bonding of edges is made by pressure, fusion or mechanical interlocking.

Welded tube: Tube formed from plate, sheet or strip with the abutting edges automatically welded.

Seam welded tube: Welded tube fabricated using filler wire.

H.F. seam welded tube: Welded tube fabricated from strip by use of H.F. current without filler wire.

Tube stock: Semi-finished tube suitable for the production of drawn tube.