High-temperature stability and mechanical property optimization of laser powder bed fusion 316L steel after controlled annealing

Abstract

The research demonstrated that laser powder bed fusion (LPBF) coupled with controlled annealing at 1200 °C, could significantly increase the proportion of coincidence site lattice (CSL) grain boundary, thereby achieving an outstanding synergy of enhanced strength and exceptional ductility. The plastic deformation behavior, strain hardening behavior, and fracture behavior of LPBF 316L steel annealing at 1200 ℃ for 20 h were studied through quasi-in-situ tensile process. It was found that LPBF 316L steel formed a certain proportion of deformation twins during the tensile process, and the formation of twins changed the crystal orientation, thus promoting further slip and crystal deformation. The synergistic effect of slip and twin promoted higher plasticity. LPBF process coupled with controlled annealing at 1200 °C for 20 h leads to a ultimate tensile strength of 613 MPa and total elongation of 73.8%.

1. Introduction

In metals or metal alloys, physical or mechanical properties can be improved by controlling special types of grain boundaries, such as twin boundaries and low-energy boundaries [1−3]. Traditional methods for grain boundary control usually require multiple cycles of annealing of the sheet, followed by processing or forming the final product [4]. Heat treatment is used to reduce residual stress, homogenize the microstructure, or adjust phase proportions [5]. It also affects grain boundary distribution, making it one of the common processes for improving microstructure and mechanical properties [6]. During the solidification process of LPBF 316L steel, the extremely fast cooling rate within the micro-melting pool, due to the interaction between the laser and the powder, results in a hierarchical structure at macroscopic, microscopic, and nanoscale levels [7], including melting pools, columnar grains, low-angle grain boundaries (LAGBs), cellular dislocation structures [8], and nano-sized oxide inclusions [9]. This results in a good combination of high strength and good elongation through dislocation strengthening and twin-induced plasticity (TWIP) effects during plastic deformation [7]. This unique microstructure endows 316L stainless steel with good strength and ductility, which cannot be achieved through traditional manufacturing routes. However, this microstructure may be unstable [10, 11], and different heat treatment conditions for LPBF 316L, apart from eliminating residual stresses [6], can change or even eliminate some of the unique microstructures (i.e., melting pools, cellular structures, etc.) [12], control precipitation [13], or cause phase transformations [14−16], thus revealing the effect of certain microstructures on mechanical properties [9].

CHAO et al [17] conducted heat treatments on LPBF 316L stainless steel samples at different temperatures and durations, finding that heating at 1100 ℃ for 5 min released nearly 90% of the residual stress and induced recrystallization. Increasing the holding time to 8 h resulted in the coarsening of intergranular inclusions, grain growth, and the transformation of most columnar grains into equiaxed grains. LIU et al [18] performed annealing and solution treatments on LPBF 316L stainless steel samples, discovering that heat treatment could refine grains and densify the microstructure. Interestingly, after tensile testing of the heat-treated samples, they observed improvements in tensile strength, yield strength, and elongation, which is contrary to the common trend of mechanical properties after heat treatment [8, 19]. Solution treatment can induce recrystallization, transforming most columnar grains into equiaxed grains. High-temperature heat treatments generally reduce substructures [17] and decrease dislocation entanglement [8, 17, 20]. Furthermore, it has been suggested that increasing the recrystallization time can reduce anisotropy [20], offering potential for the application of LPBF in industry. However, there is still limited research on the microstructure stability and mechanical behavior of LPBF 316L after annealing for different durations under high temperature conditions.