A unified approach to assessing the mechanical performance of resistance spot welds.

Abstract

Resistance spot welding is the primary joining technique used in automotive assembly. In this study a unified approach is proposed to link weld performance and quality to weld attributes and properties. Efforts have also been made to obtain optimal and consistent methods for static and impact testing of spot welds. Three major aspects of spot weld quality are discussed: static tensile-shear testing, impact testing, and relationship between weld quality and weldment properties. The main conclusions are summarized below. (1) Tensile-shear testing results depend on testing procedures and specimen configuration. Specifically, the specimen size should meet a certain requirement to obtain consistent and comparable testing results. Specimen width is critical in choosing optimal specimen dimensions. By Design of Experiment (DOE) of computer simulation, an analytical solution for critical specimen width is obtained, which falls between the requirements of International Institute of Welding (IIW) and American Welding Society (AWS). (2) Impact strength of spot welds is a very important quality index in the automotive industry. In this study, the impact strength of steel spot welds was measured using a newly designed impact tester. Detailed impact process characteristics of this tester were analyzed, which may be important to understanding a weld's impact performance for vehicle design and analysis. Initial tests using this impact tester show that it can be used as a routine test for spot welds as well as a large variety of other joints (laser-welded, brazed, mechanical fasteners and adhesion, etc). (3) Weld quality and performance are associated with weld attributes. The relationships between weld attributes and weld quality have been studied. It is learned that the weld is one of the weakest parts in a structure due to stress concentration, and failure usually initiates around a weld. Nugget diameter, sheet thickness, HAZ, and yield strength of base metals are the critical parameters in determining spot weld quality. Based on statistical analyses, quantitative relationships between weld strength and weld attributes have been established. This study provides a basic understanding of deformation of spot-welded specimens and weld-quality definition. The results of this study will help the adoption of new advanced materials in sheet metal assembly, and the methodologies can be extended to other systems or processes, such as mechanical fastening and adhesion.