STUDIES ON WELD-BONDED JOINTS OF METALLIC SHEETS

Abstract

Weld-bonding is a combination of adhesive bonding and resistance welding. The adhesive is pre-placed in the joint and a spot weld made through the adhesive. The strength of weld-bonded specimens is attributed to the strength of the weld nugget and adhesive strength. The existence of an insulating epoxy adhesive layer causes a rise in contact resistance and current density during the welding stage, and thus enhances the heat input. The heat build-up in the contact area in the first four to six cycles of current flow is greater with weld-bonding than resistance spot welding. An initial joint strength was developed in the period of four to six cycles as a result of solid phase welding across the interface with localized melting on weld-bonding. In the present study first various significant parameters of weld-bonding namely surface roughness, curing time, welding current, weld time and electrode pressure and their ranges were identified by conducting pilot experiments for both the adhesive bonding and resistance spot welding. The adherend material used for the experimental tests was an aluminium alloy 6061 T651 in the form of sheet thickness 2 mm, and the adhesive used was a high strength plain epoxy. The tensile shear resistance of the adhesive joint and resistance spot welds was evaluated using uni-axial load on two ends of the lap joints and applying tensile shear load and the results were obtained in the terms of ultimate shear tensile load (USTL). Process parameters for maximum USTL for adhesive joining and spot welding were identified. The maximum shear load carrying capacity 10.3 kN was found in case of adhesive joints produced using surface roughness with 220 emery grade and cured at temperature 120°C for 60 min. A maximum USTL 2.76 kN was obtained in case of spot weld joints produced using welding current 20 kA, weld pressure 5 kg/cm2 and weld time 8 cycles. The optimized parameters of spot welding and adhesive joining were used to develop the weld-bond. A maximum USTL 16.2 kN was obtained in case of weld-bond joint produced using welding current 16 kA, weld pressure 4 kg/cm2, weld time 6 cycles and cured at temperature 120°C for 60 min. The ultimate shear tensile load bearing capacity of the resistance spot weld and weld-bonded joints are compared and correlated with welding parameters. The strength of adhesive joint was found to be more than the resistance spot weld joint individually..