Resistance welding and mechanical joining are the predominant joining technologies applied in automotive industry for assembly of the car body, or the Body-in-White (BIW).
Nearly all sheet metal components of steels are assembled by spot welding, and most sheet metal components of aluminium are assembled by self-piercing-riveting and clinching as well as spot welding, whereas projection welding is used for fastening weld nuts to sheets.
With increasing demands on the light-weight vehicle for reducing CO2 emission and increasing strengths and crashing performance for improving safety measures, more and more new materials including advanced high strength steels (AHSS), aluminium alloys, and new surface coatings are introduced into the automotive production. These materials have largely complicated the welding processes and made the existing knowledge in welding of the conventional materials invalid. More welding tests and process optimizations are needed before setting up the welding production as well as during production maintenance.
SORPAS® has the largest user group in automotive industry with most customers:
- at the R&D departments for weldability evaluations of new materials, and for innovations of new joining processes,
- at the Planning departments for welding process planning and optimizations, and for setting up new welding procedures, and
- at the Assembly plants for production launching and maintenance to improve weld quality and production stability.
The severe conditions of high current and pressure during resistance welding expose the electrode tips at a high risk of degradation. The photo shows a comparison of the new and used electrode tips in spot welding of galvanized steel sheets. With increasing number of welds, there will be two major changes in the electrode tips:
1. Geometric changes
Geometric changes happen typically in resistance welding for steel sheets. Due to the high melting temperature and higher strength of steel than strength of electrode, the electrode tip diameter will increase due to deformation and wear, such as mushrooming, pitting or local material removal by picking up.
2. Metallurgical changes
The material properties near the tip surface will also change during resistance welding such as alloying with sheet and coating materials, and recrystallization and softening by overheating. Metallurgical changes in electrodes happen rapidly in spot welding of aluminium sheets due to sheet metal sticking, where more frequent electrode cleaning is needed to maintain stable production conditions.
SORPAS® can be used to simulate both geometric and metallurgical changes in electrodes by modelling the electrodes with worn conditions and applying changed material properties to the electrode tips, to predict the resulted changes in weld qualities and also the resulted changes in the weld lobes or welding process windows. To read more, please visit the page Electrode degradation and Tip dressing.
To ensure good quality welds, the welding process parameters are often set close to a splash condition. But due to dynamic condition changes in the production, it can lead to expulsion/splash/spatter of molten metal from the weld nugget. The main causes of splashes are:
- Welding process parameters: Too high welding current, too long welding time, or low force can cause splash.
- Electrode contact: small contact area, misalignment
- Surface conditions: dirt, oxide or anything can lead to a high resistance surface, which generates excessive or localized heat.
SORPAS® can simulate and predict the splashes in spot welding of steels and aluminium alloys. With also simulation and prediction of the weld nugget sizes, SORPAS® 2D.welding can be used to simulate and predict the weldability lobes and welding process windows. Many customers in automotive industry are using these functions for welding process planning and optimizations.
There are different methods for inspections and evaluations of spot weld qualities. Although non-destructive testing with ultrasonic or x-rays scanning is desired for inspection of spot welding quality, the destructive strength testing is still one of the most commonly used method for evaluation of the weld strengths by manual chisel test or mechanized tests. Below are the standard mechanized testing methods:
- Tensile Shear test
- Cross Tension test
- Peel test
SORPAS®3D.testing can be used to simulate the weld strength testing processes and predict the load-elongation curve and the maximum breaking load. With the new module of SORPAS®2D3D.link, it is also possible to simulate the strength of mechanical joining such as SPR based on the results of SORPAS®.2D.joining simulations.