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.
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.