Inhomogeneity of microstructure and mechanical properties in the interlayer regions for narrow gap laser wire filling welding of 316L stainless steel

Abstract

A systematic and accurate analysis of microstructural evolution in the interlayer regions of multilayer welded joint is of great importance for in-depth understanding of influence mechanism of uneven microstructure on mechanical properties. Herein, the interlayer microstructure acted on different thermal cycle curves in the upper layer (UL), middle layer (ML) and bottom layer (BL) of narrow gap laser wire filling welding (NGLWFW) of 316L stainless steel with a thickness of 40 mm were compared and investigated through experiment and numerical simulation. Results indicated that the microstructures of components created by NGLWFW exhibit distinct inhomogeneity with multiple rapid thermal cycles caused by repeated weld filling process, which provides a great deal of effect when considering mechanical properties of welded components. The molten pool size and the peak temperature in the UL acquired by simulation are higher than that in other layers ascribed to the high heat accumulation effect. The remelting zone forms between layers owing to the repeatedly heating, cooling and solidification process, which leads to the variation of microstructure from the formed layer to the latter layer. The maximum orientation distribution density and average size of grain in different layers are characterized by distinction presenting a monotonically upward trend from BL to UL. The δ-ferrite phase content in the BL is higher than that in other layers due to higher cooling speed, which results in higher hardness of the BL.