High-Strength D2 tool steel via material extrusion additive manufacturing and post-heat treatment

• AISI D2 tool steel was made via M−MEX and post-heat treated to control properties. • The MEX D2 had near-spherical carbides (2–5 μm), unlike coarser ones in wrought D2. • After heat treatment, MEX and wrought D2 formed a fully lath martensitic structure. • After heat treatment, both showed sub-micron-scale carbide precipitation. • MEX D2 had higher tensile strength (2.09 GPa) and elongation (1.27%) than wrought D2. AISI D2 tool steel was fabricated using metal-materials extrusion additive manufacturing (M−MEX), followed by heat treatment to optimize its microstructure and mechanical properties. These were compared with conventional wrought D2 materials. The as-sintered M−MEX D2 showed a pearlite matrix with a higher fraction of low-angle boundaries and developed substructures, in contrast to the fully ferrite matrix of the as-fabricated wrought D2. MEX D2 also retained near-spherical carbides (2–5 μm), evenly distributed as in the starting powder, whereas wrought D2 contained coarse carbides exceeding 5 μm, often reaching tens of μm. After heat treatment, both materials transformed into fully lath martensite with sub-micron carbide precipitation. Heat-treated MEX D2 achieved a tensile strength of ∼ 2.09 GPa and elongation of 1.27 %, representing a 15.5 % increase in strength and 170 % increase in elongation compared to heat-treated wrought D2 (1.81 GPa and 0.47 %). Fractography revealed extensive carbide fracture in wrought D2, whereas MEX D2 exhibited limited carbide cracking, with matrix substructure dominating the fracture surface. These results demonstrate that the M−MEX process, combined with heat treatment, yields finer carbides, reduced carbide cracking, and significantly improved mechanical performance, highlighting its potential as a superior manufacturing route for high-strength D2 tool steel.