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.
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.
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.
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.