Improved Cross-tension Strength of a Friction-element-welding Joint by Tempering Treatment Using Electric Heating

Abstract

Resistance spot welding is widely used in automobile assembly, but reduction of the cross-tension strength (CTS) is inevitable when using high-strength steel sheets. Therefore, we focused on joining high-strength steel sheets using friction element welding (FEW), which is mainly used to join steel sheets and aluminum alloy sheets, to improve the CTS. However, even when using FEW, the CTS decreases at the joint, when a high-carbon-content steel sheet is used as the lower sheet. This CTS decrease is presumed to be due to the low local ductility caused by the large hardness difference (ΔH_n) between martensite (M) and ferrite (α) in the inter-critically annealed and quenched area during joining. The local ductility of the material having complex structure with hard and soft phases is affected by the structural morphology and ΔH_n. Therefore, we attempted to improve the CTS by creating joints whose α of the inter-critically annealed and quenched area was equiaxed or acicular, and performed tempering treatment using electric heating to reduce ΔH_n of this part. The results showed that ΔH_n decreased at the annealed joint and the CTS improved. Furthermore, the highest CTS was observed at the joint having acicular α when tempering was performed. It was inferred that the reduction of ΔH_n using electric heating improved the local ductility and CTS of the area. Furthermore, it was considered that the local ductility and CTS are also influenced by the α morphology.