Spot Welding of Light Alloys

Posted By Tom Cropper on 14 May 2014

Posted in The Vintage Machinery Almanac

This article first appeared in Practical Engineering 1940 Vol1 No5. The information that appears in the article is accurate as of 1940. This article describes new technological methods employed in Engineering at the time.

Background

Light alloys are in general good conductors of heat and electricity. Their melting point is low and close above the temperature at which they soften and can be welded. Most of them are easily corroded at normal temperatures, and more so when heated. With some light alloys the structure alters when they are raised above a certain temperature, and in this case special heat treatment is required to restore them.

After long exposure to air, the surface of light alloys becomes covered with a thin film of oxide, which is very hard and adherent. While forming a good protection for the metal, it has the disadvantage from a welding point of view that the resistance of this film is of very variable value. All these peculiarities make the welding of light alloys a much more difficult and delicate task than in the case of steels.

Current Density

The variations of contact resistance due to the oxide film can be very considerable, and to ensure regularity in welding it is essential to get rid of this film. This should be done not too long before the welding operation by “pickling" the metal in a suitable acid. When the metal is thus prepared reliable contact can be obtained between the two sheets.

The heat produced by the passage of current through the alloy is low, and the waste of heat by conduction high. These conditions lead to the use of considerable electrical power. While sheet steel is usually welded with a current density of 80 to 200 amps per sq. mm., light alloys require a current density of the order of 2,000 to 3,000 amps, per sq. mm.

Results of Research

To lessen the power necessary for the welding, research has been carried out to bring to a minimum the contact pressure between the pieces, so increasing the resistance. By this means the speed of the weld has been increased considerably, but a small variation in pressure is then liable to cause a big variation in contact resistance, and therefore also in the regularity of the welds. To overcome this, means of control of the electrodes have been devised, to ensure a very constant pressure, and results have been greatly improved.

From experience with certain steels it was at first thought necessary to weld aluminium alloys using very short current times. While in the case of stainless steel this has the important advantage of avoiding carbon precipitation, no similar advantage is given with light alloys, even when used in an annealed state. Moreover, examination of the shape of the metal affected by the heating shows that this is flatter when the speed of welding is higher.

The Weld Core

It does not seem possible to say as yet with absolute certainty what is the most favourable shape for the core of metal affected by the welding current, to give the best tensile strength and resistance to fatigue. It can be asserted, however, that this core should never extend to the outer surface of the sheet, as there is then a tendency to electrolytic corrosion between the core and the parent metal.

If on account of too high a contact resistance the welding time is prolonged, this may cause fine cracks to appear in the welded area, allowing air to penetrate to the core. In this case trouble with corrosion may also be experienced.

It is generally taken that the weld core should not exceed two-thirds of the thickness of the metal. A weld showing cracks on the surface can, however, clearly be considered defective. It often happens that small metal projections are produced between the sheets of metal. These do not seem of great importance and do not affect the strength of the weld, but they do, however, seem to lessen the resistance to corrosion of, the unprotected metal.

Properties of The Weld Core

The metal in the region of the welding spot forming the core of the weld, having been raised to a temperature near to the melting point, is likely, on cooling, to have mechanical properties differing from those of the surrounding metal. This will be particularly noticeable in the case of a heat-treated alloy. In the case of steels, this difficulty is overcome by applying a high pressure at the end of the welding period in order to harden the heated metal. During the actual weld, however, a lighter pressure is desirable to avoid reducing the contact resistance, with consequent increase in power required. At the beginning of the welding period a high pressure is also used momentarily to ensure that the two pieces meet properly.

Pressure Variation

This double pressure variation, which gives good results in the case of steels, is not of much practical utility for light alloys. Owing to the vastly increased speed of the welding it is impossible to synchronise the pressure variation with the passage of the current. The pressure depends on the operation of a valve and the filling of a compressed air cylinder, and the time of these operations is much longer than the welding times used for light alloys on modern machines. In fact, the time variation of these operations is actually comparable with the welding time.

A slight improvement of the structure of the metal can be obtained by applying a high pressure during the weld, that is to say, a pressure nearing the ultimate strength of the metal. By this means also, the melted projections between the sheets can be almost entirely avoided. The disadvantage of increasing the pressure is, as already stated, that the power necessary for the weld is considerably increased, but on the other hand, the regularity of the welds is improved, and any possibility of burning or perforating the metal is removed completely.

Control of The Welding Current

The small difference between the softening and melting temperatures of light alloys makes it essential to control with great accuracy the amount of heat supplied to the weld. This control becomes more difficult as the power gets higher.

Good results depend entirely on the application of the proper pressure and electrical power, and in maintaining the absolute regularity of these factors. A good automatic timer on mechanical principles operating a contactor which is kept in a perfect state of cleanliness will give good results with most alloys but only at the cost of high maintenance. To avoid this, electronic timers have been successfully developed, and by this means absolutely accurate repetition is ensured.

This type of apparatus which is, of course, high in first cost, has been much used in the United States and gives very good results. A high production speed can be kept up without difficulties, and maintenance costs are comparatively low, though time is required, especially in connection with, the starting up of the apparatus.

Energy Storage Applied to Spot Welding

To weld aluminium alloys at the high electrode pressures shown to be desirable requires, as already stated, considerable instantaneous power, for example, 200kVa to weld 2 x 16 S.W.G. and 350kVa to weld 2 x 12 S.W.G. A load of this nature may cause considerable voltage fluctuation in the mains, and is looked upon with little favour by supply authorities, who may impose very onerous conditions. As a rule, only in large factories with a load capacity of several thousand kVa can such large load variations be tolerated.

To overcome the disadvantage of high peak loads being thrown back on the mains, various methods have been tried whereby the energy is drawn comparatively slowly from the mains and stored by any of several well-known methods. The energy so stored is then partly or wholly released to perform the weld.      

One such method is to use a flywheel convenor. A suitable machine is very costly, and its efficiency is comparatively poor. Furthermore, it does not make any easier the control of the current time. Much more convenient and less costly are methods whereby the energy is stored in an inductance or a condenser.

High Current Discharge

Both these systems can be made to give a high current discharge in a very short time, that is, of the order of hundredths or thousandths of a second. The preliminary energising of these systems can easily take place a hundred times more slowly, and the peak load on the mains is thus reduced to an insignificant value.

It is of particular interest that the condenser storage system, which, incidentally, has a much higher efficiency than the inductive system, requires only 20kVa to weld 12 S.W.G. sheet under the best conditions already mentioned. This method has the further advantage that it is possible to control with great accuracy the amount of heat generated at the weld. The electrical energy stored in the condenser is discharged through the welding transformer primary, and thus it is supplied also to the welding electrodes without any possible cause of irregularity.