Cold working effects on mechanical properties in sheet metal joining for automotive applications

by 1981- Blake, David Matthew

by David Matthew Blake, M.S. Washington State University December 2006 Chair: Dae-Wook (Dave) Kim With increasing demands from government agencies and consumers to increase fuel economy, automotive industries are having to research and produce lighter vehicles yet maintain safety and durability. Sheet metal is often the focal point for improvement, with the joints being a critical aspect, for automotive applications. RSW (resistance spot welding) is a fusion based joint where a high electrical and thermal energy creates localized melting of sheet metal creating a weld nugget. SPR (self-piercing rivet) is a mechanical joint where a rivet is driven in to the sheet metal with an opposing die causing interlocking between the sheets. In a typical vehicle there are 2000-5000 RSW and in some class 8 trucks there are approximately 1600 SPR joints making these two joining techniques the most common used single point fasteners. Residual stresses are created during the RSW process in both steel and aluminum and are often the cause for failure. A recently developed cold working process for improving the mechanical properties for drilled holes has been applied to RSWs. This process has proven an increase in fatigue strengths for adequately sized steel RSW but this research studies the iv post-weld cold working effect on small sized RSWs. This research also develops the optimum cold working parameters for Al RSW. Metallography, microhardness, tensileshear strength, fatigue strength, and FEA tool LS-DYNA are all used to study cold working on RSWs. In SPR it is unknown what the effect of stamped aluminum sheet metal has on the process force and mechanical properties of the joint. To accomplish the study of cold worked sheets used for SPR, the following analyses were examined: process force, tensile-shear strength, fatigue strength and metallography. It was found for inadequate sized steel RSWs the cold working process does not improve fatigue life. For Al RSWs the fatigue life can be improved by up to 80 times with the correct cold working parameters. The cold working parameters used on the sheet metal prior to the SPR process resulted in an increase of the riveting process force and no effect on mechanical properties of the joint. v