Microstructure and tensile properties of bulk AISI D2 tool steel fabricated by direct energy deposition

This study investigated the microstructure and tensile properties of AISI D2 tool steel manufactured by direct energy deposition (DED). The effect of standard post-heat treatment on the mechanical properties and defor-mation behavior of DED AISI D2 was also examined. The as-built AISI D2 manufactured through the DED process showed the eutectic structure in the interdendrite area in addition to dendrites about 4-5 mu m wide. After heat treatment, the carbides in eutectic lamellar structure were transformed into the network and needle type sec-ondary carbides. The DED-built material was interestingly composed of austenite, which transformed into martensite after heat treatment. Hardness value for DED-built material was 486.2 +/- 10.6 HV, while for heat -treated material the value was 720.1 +/- 10.0 HV. Furthermore, post heat treatment led to tremendous in-creases in yield strength (784 +/- 7.7 MPa to 1480 +/- 43.0 MPa) and tensile strength (1054 +/- 2 MPa to 1794 +/- 69.0 MPa). The hardness value, yield and tensile strengths for DED-built AISI D2 were similar to those of heat -treated conventional wrought AISI D2. In addition, brittle fractures were observed in the tensile fracture surfaces regardless of whether heat treatment was applied. In particular, tensile deformed specimen before heat treatment showed deformation-induced martensite transformation in the areas close to the eutectic structures, while after heat treatment interdendritic cracking as well as transdendritic cracking within the matrix were mainly detected. Based on the above results, the deformation behavior and fracture mechanism of AISI D2 manufactured by direct energy deposition were also discussed.