Resistance welding and mechanical joining are the main joining technologies in the automotive industry for the assembly of the car body, or the Body-in-White (BIW). Nearly all steel sheet components are assembled by spot welding. Most aluminium components are assembled by self-piercing-riveting(SPR) and clinching as well as spot welding, whereas projection welding is used for fastening weld nuts to sheets.
With increasing demands on lightweight vehicles for reducing CO2 emissions and increasing strength for safety, the automotive industry uses advanced high strength steels (AHSS), aluminium alloys, and new surface coatings. These materials have largely complicated the welding processes and engineers meet the new challenges of finding new process parameters before setting up the production as well as during production maintenance.
SORPAS® has the largest user group in the automotive industry with the most customers at:
- R&D departments for weldability evaluations of new materials, and for innovations of new joining processes,
- Planning departments for welding process planning and optimizations, and for setting up new welding procedures, and
- Assembly plants for product 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 the 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 a 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 modeling the electrodes with worn conditions and applying changed material properties to the electrode tips. You can check the changes in weld qualities and also prepare solutions by using the process windows. To read more, please visit the post Electrode degradation.
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 a splash.
- Electrode contact: small contact area, misalignment
- Surface conditions: dirt, oxide, or anything that 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 the 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.