Studies on the 316/NiTi functionally gradient ultra-thick coatings fabricated with directed energy deposition: Microstructure, crystallography and wear mechanism

Abstract

316 and NiTi alloy powders were selected as raw materials to prepare functional gradient materials (FGM) by Directed energy deposition (DED) on 316 substrates due to the wide range of engineering and industrial applications. The 316/NiTi FGM with NiTi composition gradually transformation along the building direction was prepared, and the relationship between microstructure and wear properties was revealed through a series of microstructure and performance experiments. The phase composition of 316/NiTi FGM mainly consists of intermetallic compounds composed of Fe, Ni, and Ti, while the bonding morphology between the gradient layers exhibits typical dendrite growth. The overall chemical composition distribution of 316/NiTi FGM showed a gradient transformation without abrupt variation confirmed by qualitative and quantitative analysis. From the bottom to the top, the preferred orientation of FGM grains changes from 〈001〉 to random orientation, benefiting from the cooling effect of the matrix and surrounding environment on FGM leading to the transformation of the maximum temperature gradient. The FGM demonstrates better wear resistance at both low- and high-speeds compared to 316 substrates, which is due to the superelasticity and high hardness of intermetallic compounds resulting in slighter oxidative and abrasive wear.