Effect of paint baking thermal cycle on joint strength of spot welds

Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. Such a use requires higher joint strength of the resistance spot weld in AHSS. In laboratory tests, the joint strength of specimens is evaluated in as-welded condition. The effect of paint baking cycles is usually not considered, so the actual joint strength in the automotive body is not clear. In this study, we focus on the effect of the thermal cycle corresponding to paint baking treatment on the joint strength of spot welds. Two types of tensile test, shear tensile test and L-shaped peel tensile test were carried out. The material strength grades used in this test were from 270MPa to 980MPa. The weld nugget diameter was varied from three to five square root t in each material. A paint baking thermal cycles (170 degrees centigrade for 20 minutes) was applied to the welded joints before the tensile tests were carried out. The paint baking treatment doesn't cause change in shear strength of joints in any of the conditions (materials and nugget diameter). On the other hand, in case materials of 590MPa grade or higher are used, the paint baking treatment can cause an increase in peel strength of joint. Especially, the increase of joint strength by the paint baking treatment is most obvious in smaller nugget conditions. The results of this study suggest that the improvement in peel strength by paint baking treatment is caused by a tempering effect in the weld nugget. From these results, we concluded that the joint strength of paint baked specimen would be more practical data, representing the actual joint performance in automotive bodies.