Feasibility study on application of hot forming quenching to patchwork blanks using two-stage refilled friction stir spot welding

The objective of this study is to apply hot forming quenching (HFQ) to patchwork blanks using two-stage refilled friction stir spot welding (TFSSW). TFSSW, which consists of two stages, is a joining method that was developed to improve the joint strength achieved with conventional friction stir spot welding (FSSW). The first stage is a conventional FSSW process, and the second stage is a process for refilling keyholes. Design of experiments was used to optimize the TFSSW process parameters. When the optimum process parameters were based on TFSSW, the joint strengths were 2.97 and 4.97 kN in the first and second stages, respectively. To compensate for the disadvantages of TFSSW, HFQ, one of hot forming processes, was applied. A hat-shape forming test was performed using a patchwork blank manufactured with optimal process parameters to determine the feasibility of applying HFQ. The HFQ hat-shaped part was sequentially heat-treated under an artificial aging (T6) condition. The hardness of the weld zone was measured to confirm the degradation in mechanical properties compared to that of the conventional forming method. After performing HFQ, the hardness of the base metal was found to be approximately 105 Hv, and the hardness curve exhibits a “W” shape. The application of HFQ to patchwork blanks using TFSSW was thus validated. To investigate the advantages obtained with the application of HFQ to patchwork blanks using TFSSW, the die surface pressure during forming was analyzed and a three-point bending test was performed; the results of this test were compared with those of the HFQ patchwork blank using conventional FSSW. The die surface pressure was lower when HFQ was applied, and the three-point bending test was higher when HFQ was applied. The results showed that applying a patchwork blank using TFSSW to HFQ improves die life and improves the strength of the forming part.