Experimental and numerical studies of laser riveting for dissimilar materials joining

Abstract

In the automotive and aerospace sectors, there is an increasing interest in lightweight structures by dissimilar joining, which enables the reduction of weights and enhances the frame performance by selecting the appropriate materials. Currently, for dissimilar joining, the widely used mature methods are mechanical fastening (riveting and bolting) and adhesive bonding, but they still have certain limitations in specific conditions. Therefore, laser riveting is designed and developed as an innovative technique, aims to improve the joining flexibility, applicability and productivity compared to traditional methods. Laser riveting consists of joining two heterogeneous materials by additively manufacturing a rivet to interlock them, the jointed rivet is produced by laser-based metal wire cylindrical deposition (LMwcD) as a building method. A cylindrical feature was successfully built by LMwcD onto a Ti6Al4V substrate on a small scale (1mm-4mm), which proves the positive feasibility for laser riveting, followed by parametric optimization has allowed increased productivity almost 8 times. The cylindrical deposition showed a positive correlation with heat accumulation, and primary grain size with micro-hardness was increased with higher energy input. LMwcD was applied for joining for Ti6Al4V substrate to AA6061 sheets in laser riveting (LR) processing, then a high-frequency laser washing is implemented as the additional post-process. The post-wash procedure significantly improves the wetting condition and the quality in the welding area. The rivet crown and welding areas improved by the post-wash process were directly reflected in the micro-hardness and shear tests, increased by up to 95% and 180% respectively, compared with those in unwashed rivet. For the further exploration of Ti6Al4V/ CFRP joining, compared to the direct LMwcD-LR, the post-processed depositions can maximally avoid contamination from CFRP degradation regarding optimized energy accumulation in process. In addition, the experimental analysis has been combined with a finite element analysis for LMwcD and LR process. The calculated thermal-historical results show a maximum 10% related error by calibration through corresponding experiments. The results indicate that the models have certain reliability to support understanding the fundamental experimental principle, and accurately predicting the thermal behaviour of the processing in the future research.