The welding of aluminium and its alloys

Spot welding

9.4.1 Spot welding principles and parameters

Spot welding is by far the most widely used variant of the resistance welding process. The basic principles of the technique are illustrated in Fig. 9.1. As many as five overlapping sheets of aluminium may be welded together in a single operation. The weld nugget extends through the sheets but without melting the surfaces of the outer plates. The main welding parameters are current, pressure and time - typical parameters are given in Tables 9.1 and 9.2. It is recommended that when developing a welding procedure the elec­trode sizes, the welding time and the welding force should be selected first and the welding current increased until the desired nugget size is achieved. Minimum recommended nugget sizes for the thinner of two sheets being welded are as given in Table 9.3.

The welding force required by three phase frequency converter equip­ment is some 2 to 5 times that of the single phase AC units and for three phase secondary rectified machines somewhere in the region of 0.5 to 2 times. Note that excessive forging force will result in indentation of the sheets, increased distortion and sheet separation. Too low a forging force

results in metal expulsion, surface burning because of poor contact, tip pick­up or contamination and internal defects of porosity and cracking.

If a forging force is required to assist in consolidating the weld, particu­larly for the crack-sensitive alloys this should be in the region of 2.5 to 3 times the welding force. Welding current for three phase frequency con­verter units should be some 30% higher than for the single phase AC units.

A controlled up-slope on the welding current, say over two or three cycles, enables the electrodes to seat on the surface reducing metal expul­sion and surface overheating. A down-slope or current decay reduces the rate of solidification and assists in consolidation of the weld nugget if a post­weld forge is used.

9.4.2 Welding head requirements

The welding head may be mounted on a pedestal, a bench, a dedicated machine, a manually operated boom or a robot. The simplest machine is the manually operated pedestal machine but even this may be supplemented with automatic feed and ejection. The pedestal machines are capable of pro­viding the highest power output, with capacities ranging from 5 to 400 kVA. The portable guns such as those used on robots in the automo­tive industry generally range in capacity from 10 to 150kV A. The design of the welding head is important in reducing electrode tip wear, assisting in reducing porosity and cracking and enabling high production rates to be achieved. The head characteristics that affect electrode wear comprise the speed at which the head approaches the job - larger equipment may have a two-speed head that applies the full force only after the initial electrode contact is made. Although tip life is extended the slow approach speed will increase the weld cycle time and reduce production rates. The inertia of the head affects the speed of acceleration and deceleration and ideally the head should be designed to be as light as possible consistent with rigidity. Too much flexure of the arms will result in accelerated electrode wear due to movement between the electrode and the workpiece and unacceptable electrode alignment.

Low inertia is also required during weld pool solidification. As the molten weld metal cools and solidifies, the weld nugget contracts. The electrode must be able to respond rapidly and be capable of following this slight deformation if sound and high-quality welds are to be produced. A ‘squeeze’ is therefore often applied, which assists in consolidating the weld, reducing shrinkage porosity and hot cracking.

9.4.3 Welding electrodes

The bulk of the cost of a spot weld is the cost of dressing or replacing the electrode, the life being defined as the number of spot welds that can be made with a pair of electrodes while maintaining a minimum weld nugget diameter. Pick-up of aluminium on to the tip and rapid wear are the two main reasons for the short life of spot welding electrodes. High welding currents, surface finish and electrode forces further assist in shortening the electrode life. It is not uncommon in very high-quality applications such as aerospace for the electrode to require cleaning after as few as 20 spot welds.

Electrode life may be extended by the use of replaceable caps on the electrode tips or, it is claimed, by the use of copper alloys with increased hardness which reduces mushrooming of the tip. Increases in hardness can be achieved by alloying with zirconium or cadmium-chromium and dis­persion hardened with aluminium oxide. Of these the 1% Cd-Cu are used for the softer alloys with the harder 1% Cr-Cu or 21% Cr-Zr-Cu alloys for the welding of the cold-worked or age-hardened alloys.

The profile of the electrode tip is important with respect to both the tip life and weld quality. Tips may be conical, truncated conical, flat, domed or cylindrical. Of these types the truncated cone and the dome predominate. The most commonly recommended shape is the domed tip, the shape of which is more easily maintained in production than the truncated cone. Alignment is also less of an issue and is favoured particularly for portable equipment. The truncated cone tends to be used for commercial quality applications, mainly because electrode alignment is more critical and diffi­cult to maintain consistently in production. Tip life, however, is markedly better, by a factor of two to three, than can be achieved with the domed electrode. Cone angles vary from 60° to 150° including an angle with a slight radius on the tip which aids in alignment and reduces marking of the sheet. The tip profile may be maintained by grinding, filing or by the use of abrasive cloth in a shaped former. While this dressing operation may be performed manually it is difficult to maintain the correct tip shape and electrode alignment. The use of automatic tip dressing tools or spe­cially designed hand-held manual or pneumatic tip dressers is strongly recommended.

Efficient electrode cooling is also necessary to maintain tip life. Large diameter electrodes will provide a greater heat sink but efficient water cooling is imperative. The cooling channel should be carried as close to the tip as possible, a distance of between 12 and 20 mm being usual with water flow rates of 5-10 litres/min. Water inlet temperature should be in the region of 20 °C and the outlet temperature in the region of 30 °C.

9.4.4 Quality control

There is no specification available for the quality control of aluminium spot welds within the UK although ASME IX includes rules for procedure approval of spot welds. There is, however, a British Specification for the spot welding of steel, BS 1140, which contains a multitude of recommendations that may also be applied to aluminium. The main method of demonstrat­ing acceptable quality is by means of the peel test, a simple, inexpensive test, but this may be replaced or supplemented by a tension shear test or a twist test. The test piece for all three tests is essentially the same - two overlapping plates welded together with a single spot. An acceptable result in the peel test is when the weld nugget is pulled out of the parent plate.

Monitoring of the welding parameters is an effective method of assuring quality during production welding. This monitoring may be very simple, detecting only the absence of a weld, or may be a sophisticated electroni­cally based monitor which will both monitor and record current, number of cycles, pressure and time. This recording can be augmented by audible or visual warning of out-of-range welding parameters. Finally, one of the most effective quality control methods is visual inspection where surface melting, adhesion of the electrodes, pits, cracks, asymmetry of the weld spot and surface indentation can be readily identified. These features may be used to assess the quality of production batches.