Applications of Resistance Welding in Automotive Industry

Resistance welding is an efficient welding process that is widely used in automotive manufacturing. It is mainly used for welding assemblies for body-in-white, air reservoirs, oil pans, shock absorbers, etc. For example, there are 5,892 resistance welding spots on a vehicle body.

Currently, automotive manufacturers of resistance welder secondary output mainly to industrial frequency AC and DC, its rated power is generally above 63KVA, up to 400KVA or higher,and the power consumption is larger.

During spot welding, the following factors that influence the quality of the spot welds: welding current, welding pressure, shape of the electrode's end faces,ferromagnetic material penetrating the electrode and shunt. Especially in welding workshops with a lot of resistance welders, the welders working simultaneously induct each other and effect the power grid.In severe cases,it will affect the trigger of the controller, resulting in poor stability and consistency of the welding quality.

In automotive manufacturing,the welding quality is the common focus of manufacturers and users, the welding quality is mainly dependent on welders to ensure.For the vehicle body spot welding, the controller mainly ensure that the set welding parameters within a certain fluctuation range, so as to obtain stable welding spots quality. Domestic controllers with constant current control method can basically meet the needs of mild steel and galvanized steel welding, but its control accuracy still needs to be further improved, the welding current’s control is still open loop control.As working conditions change, the consistency of the spot weld quality is difficult to ensure.

In order to obtain reliable welding quality, welding inspection is essential, which in vehicle body manufacturing involves non-destructive spot checks of the spot welds, because if the spot welds are not inspected, product quality will be seriously compromised. If we can ensure that 100% of the spot welds are qualified, each vehicle body can reduce the welding spots’ number about 200 points, which can save costs of about 80 RMB. Therefore, online inspection and monitoring of spot (projection) welding quality is very important to ensure the reliability and consistency of spot (projection) welding quality, and this technology has been widely concerned and researched from domestic and foreign scholars. There are many real-time monitoring methods, but it is not common that they can be applied in actual production.

Trends in spot welding quality control technology:

(1) Control mode, has evolved from mode control to multi-mode control, dynamic resistance monitoring, dynamic electrode displacement monitoring are all to realize this multi-control integrated mode, dynamic resistance difference and dynamic resistance rate of change integrated; maximum displacement and displacement speed integrated.

(2) Control methods, has evolved from one monitoring method to multiple monitoring methods to make decisions on the spot welding process and the trend of quality control.

(3) Adjustment parameters, evolved from the initial univariate regulation (welding time or welding current) to multivariate regulation, welding current, welding time and welding pressure are regulated simultaneously during the welding.

In terms of control decision-making, it has evolved from the traditional methods of control decision-making (from the deviation value of the controlled parameters to the determination of the level of control parameters by the checklist) to the decision-making methods of artificial intelligence (neural networks, fuzzy logic, etc.) 

The service life of spot welding and seam welding electrodes has a direct iimpact on manufacturing costs and welding quality in the automotive industry,which is particularly important when welding coated steel sheets. Spot welding electrode materials mainly include: chrome zirconium copper, diffusion strengthened copper, beryllium bronze,etc. Diffusion strengthened copper alloy material, its normal temperature and high temperature hardness, electrical conductivity are better than chromium zirconium copper material. The use of diffusion strengthened copper electrodes has 2-4 times longer sevice life than chromium zirconium copper electrodes. However, the cost is higher than that of  chromium zirconium copper material. In the domestic automotive manufacturers mainly use chromium zirconium copper electrode, there are a small number of joint venture companies using diffusion strengthened copper electrode. However, they mainly rely on imports. In order to improve the service life of the electrode, domestic and foreign scholars are studying the application of coated electrodes, and it is reported that compared with the commonly used chromium zirconium copper electrodes,their service life can be increased by 5-6 times, and the cost is also lower.

Currently, resistance welders widely use AC 50Hz single-phase AC power supply, with large capacity and low power factor. The development of three-phase low frequency resistance welders, three-phase secondary rectifier welders (already used in ordinary spot welders, seam welders, projection welders) and IGBT inverter resistance welders can solve the imbalance of the power grid and improve the power factor. At the same time,it can further save electricity, facilitate the realization of microcomputer control of parameters, can be more suitable for welding aluminum alloys, stainless steel and other difficult to weld metal welding. In addition, the weight of the equipment can be further reduced.