Equipment and power sources for resistance welding
TWI Bulletin, May/June 1990
Steve Westgate is the section leader for Resistance Welding Processes in the Forge and Resistance Processes Department. He gained an honours degree in metallurgy at the University of Birmingham and joined TWI in 1972. Having worked in all the resistance welding processes for about 15 years, Steve has also become involved with adhesives and weldbonding, particularly in the field of sheet material joining. Activities in the section currently include instrumentation and monitoring of resistance welding processes and the durability testing of adhesively-bonded coated steel.
Steve has published a number of reports and papers, some of which have been presented in German, in which he has gained London Chamber of Commerce qualifications.
Resistance welding is the most widely-used welding process for joining sheet metal components in the mass production industries. It lends itself to advanced automation and, with continuing developments, competes well with alternative joining technologies. In this article, based on a paper presented to the Harrogate conference on Advances in welding processes 1989, Steve Westgate reviews the state of the resistance-welding art.
Automation of the welding process
The controlling factors in the choice of automation for resistance welding are:
- Initial capital cost;
- Flexibility, ie component variants to be welded;
- Volume and rate of production;
- Projected life of equipment and product.
There are three main categories of automated assembly, namely multi-welders, conventional six-axis robots and dedicated two- to three-axis simple robots.
The design of large multi-welders has been improved to allow easier and more rapid machine maintenance, tool changing and electrode maintenance procedures. More efficient package transformers and cables have been developed which allow a compact and effective machine design. Figure 1 shows a typical multi-welder in an automotive application.
Fig. 1. Multi-weld equipment for automotive application (Courtesy Ford Motor Co)
Multi-spot-welding machines, which are suitable for small volume production of various sub-assemblies because they allow a greater degree of flexibility in the product mix, have also been developed. More sophisticated electronic control equipment has made it possible to programme the complex sequences these machines demand; furthermore, such equipment makes it easier to control the sequencing of the weld initiation for each set of welds, thereby minimising the peak current demand from the mains during the operation.
Compared with manual gun welding, robotic welding enables gun positioning to be more accurate and reproducible, thus helping to improve the reliability of weld quality. In comparison with multi-welders, robots also provide a greater degree of flexibility. The dedicated robot ( Figure 2) provides a cheaper alternative to the conventional six-axis robot and allows heavier welding equipment to be manipulated. This is a particularly cost-effective way of handling components of simple geometry, when welds are to be made in one plane.
Fig. 2. Lightweight guns provided with x-y movement in a dedicated cell for bonnet components (Equipment manufactured by Volvo Olofström Plant Sweden)
In the past, spot-welding guns used with robots were of the separate transformer type, ie provided with only the gun attached to the wrist and the transformer located remotely. However, robot guns with integral transformers have now become more common ( Figure 3). Their advantages include a much reduced power demand because of the lower secondary impedance. The supply cables are therefore much smaller, carrying only primary current, and no secondary cables are required; manoeuvrability of the gun is thus increased. The disadvantages, however, are that the robot must carry a greater weight, and protection from the mains voltage must be provided at the gun.
Fig. 3. Body-side components welded with an integral transformer gun mounted on a robot (Courtesy Volvo Olofström Plant Sweden)
Significant efforts have been made to reduce the weight of the gun and transformer by using alternative materials in the gun itself and by increasing the efficiency of the transformer using higher-frequency supplies. Lightweight guns include a stainless steel stiffened gun already used in production ( Figure 2), and a new fibre-reinforced composite stiffened gun recently exhibited at the 1989 Essen Welding Fair.
Advances in power supplies
Most spot welding guns and machines currently in production run at mains frequency (50Hz in the UK) single-phase AC secondary welding current. The disadvantages of such machines are their low power factor and the unbalanced loads they impose on the electrical supply.
Three-phase DC supplies
The power factor of spot-welding machines can be improved by using equipment running on a DC secondary welding current. Both frequency converter (primary rectified) and secondary rectified systems are available; the latter now predominate.
The increase in the use of DC power supplies has tended to be in the larger capacity machines where demand on the mains supply would be unacceptably high for single-phase AC. The DC provides a balanced three-phase demand with a much reduced primary current. The high welding currents available - in excess of 500kA for the largest machines - enable large annular projection welds or large numbers of embossed projections to be made in one shot.
Such an approach, incorporating automatic feed, has offered an economic alternative to multi-spot-welding or robot-spot-welding of sub-components ( Figure 4), where the minimal inductive losses in the secondary circuit and uniform current distribution can contribute to savings in power demand.
Fig. 4. Large secondary rectified DC equipment suitable for multi-projection welding of components with automatic carousel feed (Courtesy Schlatter Ltd)
Although three-phase secondary rectified power supplies also improve weldability in some spot-welding applications, including zinc-coated steels, the higher cost of such equipment limits its application for smaller machines.
Capacitor discharge
Industry is regaining interest in high-power capacitor discharge welding equipment. It appears to be competing with large DC equipment where the advantages are high production rates and a low mains power demand. High electrode forces and secondary currents are required but weld times are of the order of milliseconds.
Special provision is therefore made in the welding head for the very fast follow-up characteristics required. The equipment needs tighter control of tolerances and alignment of components than that using conventional AC and DC supplies, but the heat input is relatively low. Machines range from those suitable for small projections to large annular projection applications, with peak currents up to about 300kA.
Transistorised DC
The need for fine control of the current waveform in the field of miniature resistance welding applications has led to the development of transistorised power supplies, which give accurate control of current pulsing and the shape of the current envelope. Such equipments are fitted with current and voltage feedback control to enable constant current, constant voltage or constant power to be used. By choosing the appropriate control methods, welds can be made in difficult materials which may also have a surface oxide. Weld reliability is increased, overheating or splash can be avoided, and electrode life may be increased.
High-frequency supplies
Substantial weight savings can be achieved in the welding transformer by using higher-frequency supplies to increase its efficiency. Frequencies between 420 and 1200Hz have been used for integral transformers, with the lower frequencies suitable for AC secondary current. However, secondary rectification is required for the higher frequencies to avoid excessive inductive losses. A 23kVA transformer for 420Hz AC has been produced weighing only 5kg, and a commercially available 58kVA transformer, operating at 1000Hz, weighs 15kg including the secondary rectifying diode assembly. These are less than half the weight of equivalent 50Hz AC transformers.
High-frequency power supplies normally embody a three-phase transformer and rectifier to produce 500-600V DC. This DC voltage is then switched, normally by transistors, to produce the high-frequency AC (420-1200Hz) providing the primary supply to the lightweight welding transformer ( Figure 5).
Fig. 5. HF supply showing mains rectifier/filter, transistorised DC-to-AC inverter and welding transformer with an AC secondary current
The use of 420Hz AC equipment will be limited to guns with small throats but reduced energy demand; high power factor and reduced weld times are claimed compared with conventional 50Hz supplies. The greatest use of the higher-frequency supplies is for the range 600 to 1200Hz, where secondary rectification is provided. This rectification is achieved using diodes built into the secondary side of the transformer ( Figure 6). The machines currently produced have a limitation of about 21kA secondary current, since available transistors are unable to handle power demands in excess of the 300A equivalent primary current.
Fig. 6. A welding transformer with rectified secondary current as supplied by high frequency from the inverter
HF-DC systems require much less energy to produce a given weld size. The mains load is balanced and it is claimed that much closer control of the current is possible. Corrections to the current can be achieved every half cycle of the current waveform, typically 20 times as fast as with 50Hz supplies, which also offers greater potential for adaptive control of weld quality.
In practice HF-DC has been supplied to both gun welders ( Figure 7) and pedestal-type machines. The former are particularly suited for robotic applications and are beginning to find widespread application in automotive body-in-white lines. Over 250 units were manufactured in Japan in 1988. Despite higher capital cost than conventional AC equipment, substantial savings may be possible by using robots of lower capacity, and a steep increase in production is expected.
Fig. 7. High-frequency (600-1000Hz) power supply and integral transformer gun with secondary rectified DC
Summary
The advances in resistance welding process equipment, power supplies and control provide a tremendous choice to allow reliable, cost-effective production over a broad range of applications. It is important to note, however, that the advantages offered by the sophisticated automation and advanced power supplies may only be fully realised when applied to suitable equipment and components. Factors such as head follow-up, machine rigidity, electrode design, and set-up and component handling must be suitable for the job and properly maintained.
In addition to the developments in power sources discussed in this review, parallel developments are also taking place in electronic control, monitoring and in-process feedback systems, all of which are aimed at ensuring 100% reliability coupled with the minimum of weld testing.
