Continuous 3D printing of iron and cobalt alloys: Heat treatment and transformation mechanisms
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Date
2024-08
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University of New Brunswick
Abstract
The high strength and hardness of maraging steel can be functionally integrated with the thermal stability and toughness of CoCrMo alloy through multi-material additive manufacturing via laser powder bed fusion. To broaden the potential applications of this combination, this work proposes different heat treatment strategies that mutually enhance the mechanical properties of both alloys. Comparative analysis of texture development, precipitation sequence, and mechanical properties of as-built and heat-treated dual structures at different scales has been conducted. The results reveal that discontinuous martensitic transformation occurs in the interface after printing due to the combined effects of solute segregation, local stress distribution, and intermixed cellular growth. Of all the heat treatment attempts, exposure to 1150 °C for 2 hours, 940 °C for 5 hours, and 490 °C for 6 hours achieved the highest tensile strength through cooperative strengthening of intragranular γ-ε transformation in CoCrMo and Ni3Ti precipitation in maraging steel. Initial evidence of local grain variant selection has been revealed in as-built samples due to thermal cycling, which generates residual stresses, recoil forces, and convective flow. Surprisingly, the missing variants can also be inherited after heat treatment with insufficient solution temperatures. For the CoCrMo alloy, a higher ε-phase fraction is obtained after direct aging compared to solution treatment due to band-like isothermal transformation, coherent grain face massive transformation, and recrystallization. For the maraging steel, over-aging treatment revealed that austenite reversion starts at cellular boundaries that have disintegrated over time due to chemical diffusion. The formation of austenite islands during reversion closely resembles the morphological patterns of prior cellular and melt pool boundaries. Based on overall results, it can be concluded that the CoCrMo alloy forms a compatible interface with maraging steel, and suitable heat treatment schedules can provide a synergistic balance between strength and ductility for both alloys.