The Effect of Thread Throat Diameter of Welding Bolt on Weld Defect Formation and its Optimization in Projection Welding

The welding process plays a critical role in the joining of sheet metal parts in the automotive industry, as in other manufacturing sectors. In this study, a welding defect observed in M6x21 weld bolts joined by projection welding—utilizing the protrusion geometry on the fasteners was investigated. The joining was carried out using 3 mm thick WSS-M1A365-A22 sheet steel and 8.8 grade M6x21 weld bolts; however, insufficient weld strength and low rupture loads were detected. Analytical evaluations revealed that the root cause of the defect was the contact between the bolt geometry and the sheet sur-face, which caused the welding current to dissipate over a wide area. To eliminate this issue, an M6x26 weld bolt with a smaller thread root diameter and longer thread length was used in the welding process. In order to ensure adequate rupture strength in projection welding with the M6x26 bolt, welding process parameters were investigated. The Taguchi L9 orthogonal array was used to evaluate three different welding currents (26, 28, 30 kA) and three different welding forces (400, 500, 600 daN), with the cycle time kept constant. The rupture load results were analyzed using the “larger-the-better” crite-rion, and the optimum welding parameters were determined as 600 daN welding force and 30 kA welding current. However, considering energy efficiency in the production environment, it was concluded that a current of 28 kA and a force of 600 daN also provided sufficient weld strength. Scanning Electron Microscope (SEM) analyses enabled microstructural evaluation of weld region defects.