Effect of aluminum pre-straining on strength of clinched galvanized SAE1004 steel-to-AA6111-T4 aluminum

Abstract

Normally, the sheet metal may be pressed or stamped prior to mechanical clinching. The pre-strain from the pressing or stamping may have an influence on the quality of the clinched joints. To understand how the pre-strained sheets materials affect the joint quality, we conducted the study to understand the effect of pre-straining aluminum on the static strength of the clinched aluminum-to-steel joint. The section parameters (i.e. undercut, neck thickness, and bottom thickness) and joint strength were measured. It was found that the work hardening resulting from pre-straining decreased the ductility of aluminum AA6111-T4 and induced some ductile damage on the clinched aluminum workpieces. A 5% pre-strain on AA6111-T4 caused a significant decrease (about 20%) in joint strength. Though the bottom thickness is a good indicator for detecting the strength variation for the clinched as-received and pre-strained aluminum–steel joints, it barely detected the effect of ductile damage in the pre-strained aluminum on the joint strength. The application of the similar “X” parameter (i.e., bottom thickness of the clinched joint) from the as-received aluminum to monitor the quality of the clinched steel-pre-strained aluminum joint resulted in overestimates of the joint strength. Therefore, the optimum clinching tools and process variables for clinching of the pre-strained AA6111-T4-to-steel cannot be derived from the experiments using the as-received aluminum workpieces. To obtain the desired strength of the joints made with dissimilar materials, it is important to achieve a good balance between the undercut and neck thickness by optimizing the clinching tools and process parameters. The variation of electrical resistance of the joints during the clinching process can differentiate the difference of the damage between the as-received and pre-strained clinched joints. Therefore, a method of monitoring the variation of electrical resistance is proposed to inspect the quality of the clinched steel-to-pre-strained aluminum joints.