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

Abstract

Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. Such 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 270 to 980 MPa. The weld nugget diameter was varied from three to five square root t in each material. A paint baking thermal cycle (170 °C for 20 min) was applied to the welded joints before the tensile tests were carried out. The paint baking treatment does not cause a change in the shear strength of joints in any of the conditions (materials and nugget diameter). On the other hand, in the case of materials of 590 MPa grade or higher is used, the paint baking treatment can cause an increase in peel strength of the 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 specimens would be more practical data, representing the actual joint performance in automotive bodies.

Keywords:

• Resistance spot welding
• joint strength
• paint baking
• high strength steel sheet
• peel tensile test