The sensitivity analysis of microstructure and mechanical properties to welding parameters for linear friction welded rail steel joints

In this study, high carbon rail steel (U71Mn, Chinese grade) was welded by linear friction welding (LFW) under different welding parameters. The microstructure and mechanical properties of the joints were characterized to investigate the connection mechanism of the joint. In order to fully yield the metal at the welding interface, sufficient heat input must be provided with friction to fully offset the heat loss. Therefore, welding parameters have a decisive influence on joint quality. The joint can be divided into the weld center zone (WCZ), the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ) and the base material (BM). The BM have a mixed microstructure of the pearlite and upper bainite, and the TMAZ consists of pearlite and a small amount of martensite. In WCZ, the martensite content increases further due to the higher cooling rate. During LFW, dynamic recrystallization occurs in the weld, which results in a significant change in the ratio of the deformed grain, sub-structure grain and recrystallized grain compared with that in BM. In the tensile test, all samples have fractured in WCZ. The highest tensile strength reached of the joint is 78.5% of the BM strength. In addition, there is a large amount of martensite in WCZ, which increases the microhardness while reduces the toughness of the weld.