Effect of foam copper interlayer on the mechanical properties and fretting wear of sandwich clinched joints

Fretting wear has a considerable effect on the fatigue properties of clinched joints and is the most important cause of fatigue failure in them. Foam metal is a new type of structural material that can be sandwiched into a clinched joint to improve its mechanical properties with respect to fretting wear. In the work reported here, clinched single-lap and cross-lap joints with and without foam copper layers were prepared and subjected to static load testing. Fatigue tests were conducted on the single-lap joints to analyse the fatigue properties and failure modes of the aluminium-alloy clinched and foam metal sandwich clinched joints. The damage caused in single-lap joints by fretting wear was assessed by examining joint cross-sections before completely fatigue broken. Scanning electron microscopy and energy spectrometry were used to examine the failure fractures of the two kinds of joints and to investigate the fretting wear failure mechanism. The incorporation of the foam copper layer increased the shear strength of the clinched joint by 5% but had little effect on the peeling strength. It also increased the energy absorption capacity of the joint under a shear force by 33%, but reduced it under a peeling load by 27%. The fretting wear of the clinched joint occurred mainly at the contact surface of the two sheets in the lap area and at the clinched neck. Fretting wear generated oxide wear debris and caused micro-cracks on the sheet surface and these micro-cracks then continuously expanded until sheet fracture failure occurred. The foam copper layer effectively reduced the fretting wear between the upper and lower sheets and greatly enhanced the fatigue life of the aluminium-alloy clinched joint.