Mechanical performance of 316 L stainless steel by hybrid directed energy deposition and thermal milling process

Due to its excellent temperature resistance, 316 L stainless steel (SS) is widely used in integral impellers; however, as the engine performance requirements have increased, the shape of integral impellers has become increasingly complex. Directed energy deposition (DED), with a high production rate and low cost, is used to directly fabricate complex structural geometries. Nevertheless, the surface quality is lower than that achieved by conventional methods, i.e., milling. To overcome this problem, a novel hybrid approach with DED followed by a subtractive milling process within a single workstation is developed. This method can directly produce internal and highly complex structural parts with ideal dimensional accuracy. However, the process parameters and mechanical properties of DED and subtractive thermal milling (starting milling temperature of 200-300 degrees C) after each deposition of a set of layers have rarely been evaluated. The purpose of this study is to address these research limitations. The densification, phase composition, microstructure and mechanical behaviour are studied, and a correlation between process parameters and performance is newly established. The results indicate that the nearly fully dense 316 L SS specimens exhibit high microhardness and tensile strength under the optimum process parameters, which is attributed to the high density and fine microstructure. Moreover, the highest tensile strength (683.3 MPa) among all tensile samples is obtained with v = 8 mm/s. The tensile strength values for wrought (hot work-annealed), wrought (cold-worked), cast samples, and for the industry requirement for 316 L SS are 480, 574, 552 and 450 MPa, which are 42.97 %, 19.56 %, 24.33 %, and 52.51 % lower, respectively, than that for the hybrid DED and thermal milling process. The test results and a comparison analysis show that the components from the hybrid DED and thermal milling process can satisfy the industry requirements for 316 L SS.