Methods to Control Curing Induced Distortion in Hybrid Joining of Dissimilar Metals

In lightweight structures with dissimilar metal designs, structural adhesive joining is a potential joining method. Adhesives help in reducing galvanic corrosion by minimizing physical contact between two dissimilar metals. Along with adhesives, fasteners are often used as a secondary joining method to hold the assembly together during adhesive curing. Therefore, a hybrid joint which is a combination of adhesives and mechanical fasteners is potential joining method to join dissimilar metals. However, when two dissimilar metals such as aluminum to steel are joined with hybrid joint by adhesive curing at elevated temperature, the distortion of assembly is observed when cooled at room temperature. This is due to the mismatch between coefficients of thermal expansion of aluminum vs steel. The adhesive may also experience residual stress and fracture. In this study, adhesive curing induced distortion is studied using 1.1 meter-long specimens of aluminum to steel hybrid joint assembly. The base materials consist of 4.8 mm thick aluminum 6061 T6 joined to 1 mm thick UHSS with 30 mm adhesive overlap. One-part heat cured structural epoxy adhesive was used in hybrid joint. Variables which may have effect on distortion are identified. The variables included are adhesive layer thickness, adhesive material type, fastener type (rivet vs. screw), fastener spacings, fastener clearance hole size and adhesive curing temperature. Coordinate Measuring Machine (CMM) was used to measure the assembly distortion. It is shown that, adhesive curing induced distortion can be controlled using variables chosen. Among all the variables, adhesive layer thickness, fastener spacing and fastener clearance hole size have significant impact on distortion. Fasteners act as constraint and affect the distortion. Higher fastener spacing and higher clearance hole size increased distortion of assembly considerably. Adhesive material type, adhesive curing temperature has minimal effect on assembly distortion