Effect Of Aging And Pre-strain On The Strength Of Clinched Galvanized SAE1004Steel-to-aluminum AA6111Joints

Automotive lightweighting has become an inevitable trend in the development ofautomotive industry. In order to meet the requirements of automobile lightweight,aluminum and steel dissimilar metals are widely used in car manufacturing industry.Mechanical clinching method is a feasible alternative to spot welding for joiningdissimilar materials. Recently, a majority of studies have focused on the processoptimization. It is rarely to study the effect of material properties of the workpieces onthe strength of the clinched joints. Therefore, there is an urgent need to understand theeffects of aged aluminum and pre-strain on the strength of the clinchedsteel-aluminum joints. In this paper,1.0mm thick AA6111aluminum was firstlysolution heat-treated at550℃for30minutes followed by natural aging treatment forvarious times. And then the aged and pre-strained aluminum sheets were clinchedwith0.7mm thick galvanized SAE1004steel with SKB die. The effect of naturalaging and pre-strain of the AA6111aluminum on the quality of the clinched joint wasstudied, respectively. The electrical resistance of clinching joints were measured andthe possibility of electrical resistance to be used to inspect joint quality of the clinchedaluminum treated with various natural aging and pre-strain was explored. The methodof preheating was conducted to improve joint quality of the clinched pre-strainedaluminum. The experimental results are as follows:Natural aging of aluminum AA6111at ambient temperature increased the tensileand yield strengths and decreased the elongation of aluminum, which resulted in adecrease in undercut and an increase in bottom thickness of the joints. With theincreasing the strength of aged AA6111aluminum, the strength of clinched joint isincreased, even though it decreased the undercut of the joints.The pre-strain affected the mechanical properties of aged AA6111aluminum.With the increase of pre-strain, the strength and hardness increased while the ductilityand the deformation ability of materials decreased prior to the clinching. Therefore,the joints fabricated pre-strained aged aluminum workpieces had smaller undercut,larger neck thickness and bottom thickness as well as had higher cracking probabilitycompared with that fabricated by no pre-strained aged aluminum workpieces. Thepre-strain of aged AA6111workpieces prior to clinching increased the joint strengthfor low strength AA6111aluminum workpieces (natural aged0to5days), but decreased the joint strength for high strength AA6111aluminum workpieces (naturalaged10days). Pre-heating pre-strained AA6111-T4aluminum can improve itsductility effectively and decrease cracking probability. Pre-heating is beneficial toincrease the undercut, clinchability and the strength of the clinched joint. Theoptimum preheating temperature is120℃.An analytical model has been developed to estimate the strength of the clinchedSAE1004steel-to-aluminum AA6111-T4joints. Based on the model, the influence ofaged aluminum AA6111on the joint strength was primarily attributed to thestrengthening of aluminum, which required large force to make the undercut in thelower AA6111aluminum workpiece during the clinching. Therefore, large residualpress stresses existed on the surface of the lower workpiece to lock the upperworkpiece after clinching. By introducing a damage coefficient, an improvedanalytical model can well explain the effect of pre-strain on the strength of clinchedSAE1004steel-AA6111aluminum joints, the modeling results and the experimentalresults are basically identical.Natural aging and pre-strain of AA6111prior to clinching caused the increase inelectrical resistance of clinching joints. Therefore, the electrical resistance of joints issensitive to the natural aging and pre-strain of aluminum prior to clinching. Thoughthe electrical resistance can’t monitor the effect of natural aging on the pre-strainedAA6111workpieces, there is good relation between the increment of electricalresistance and joint strength of clinched aged aluminum. The increment of electricalresistance and joint strength have the same trend with natural aging time both foraluminum without pre-strain and with pre-strain. The electrical resistance incrementmay supply an alternative monitor method to the quality of the clinched joints.