Novel WC-reinforced iron-based composites with excellent mechanical properties synthesized by laser additive manufacturing: Underlying role of reinforcement weight fraction

Laser powder bed fusion (LPBF) process was utilized to prepare high-performance steel matrix composites (SMCs) consisting of a tool steel (1.2767 L) matrix with various contents of submicron-sized WC reinforcing particles. The influence of the content of WC reinforcing particles on constitutional phase, microstructural evolution, densification rate and mechanical properties of SMCs was investigated. It shows that the microstructures and mechanical properties of SMC are highly sensitive to the WC-content. A higher weight fraction of WC leads to a reduced martensite start temperature (M-s) since more W and C atoms are dissolved within the iron matrix during the LPBF-process. The microstructure consists of more retained austenite using a higher WC-content. The addition of 2 wt% WC particles enables a significant grain refinement of the iron-based matrix, due to the formation of a (Fe,W)(6)C carbidic network that restricts the growth of the sub-grain boundaries of the parent austenite. This refinement-effect is less pronounced at higher WC content due to the reduced self-diffusion activation energy of the Fe atoms in the parent austenite. Increasing WC-content also increases the thickness of the carbidic network, leading to a reduced directionality in the texture of the composite-microstructure. Compared with the unreinforced steel parts, the composites reinforced with 2 wt% WC show a synergetic reinforcing effect in compressive strength of similar to 3210 MPa and fracture strain up to similar to 30.2 % and ultimate tensile strength of similar to 1677 MPa and elongation of similar to 8.5 %. The improved mechanical properties result from the combined effect of transformation-induced plasticity (TRIP) effect, grain refinement, non-equilibrium grain-boundary strengthening, and nano-scaled precipitation.