A novel low-cost selective laser-melted 718HH plastic mold steel with an excellent combination of strength and toughness

Selective laser melting (SLM) has become a critical technique for manufacturing molds with conformal cooling channels to achieve high cooling efficiencies. A novel selective laser-melted 718HH plastic mold steel with an excellent combination of strength and toughness was investigated. After SLM fabrication, quenching and tempering are conducted as post-printing heat treatment (PPHT) to improve the mechanical properties of the as-built samples. Both the microstructure and the corresponding mechanical properties were systematically studied. The results show that PPHT facilitates the complete martensite transformation. Meanwhile, the retained austenite (γ) phase was still found in the as-built samples. And high-density dislocations were dispersively distributed within the martensite matrix for both as-built and as-PPHTed samples. After PPHT, due to the recovery and recrystallization of martensite, reduced dislocation density and increased high-angle grain boundaries, the microhardness of the as-built samples decreased from 498.8 ± 16.7 to 382.1 ± 5.0 HV_0.3. Correspondingly, the strength, including the ultimate tensile strength and yield strength, of the as-built and as-PPHTed samples also decreased from 1411.3 ± 17.8 to 1208.7 ± 3.2 MPa and 1267.3 ± 11.7 to 1084.7 ± 5.1 MPa, respectively. On the contrary, the value of impact energy significantly increased from 15.3 ± 1.2 J (as-built) to 39.7 ± 1.2 J (as-PPHTed). Notably, the mechanical properties of SLMed 718HH samples are significantly better than those of corresponding wrought samples.