Laser powder bed fusion of high-strength maraging steel with concurrently enhanced strength and ductility after heat treatments

Abstract

Laser powder bed fusion (LPBF) has been demonstrated as a metal additive manufacturing technique that can manufacture high-performance metal components with complex shapes directly from raw powder materials. Although 18Ni-300 maraging steel has been extensively employed in this process, a significant trade-off has always existed between strength and ductility when using traditional solution and ageing heat treatments. This study addresses this problem by introducing a heat treatment combination that includes hot isostatic pressing (HIP) before the conventional ageing treatment. The experimental results demonstrated an enhancement of the relative density from 99.6% in the as-built conditions to 99.96% after HIP, although a small number of residual gas pores remained. The subsequent ageing treatment after HIP had a negligible effect on the reopening of these closed defects. Microstructure observations revealed that a fully martensitic structure can be obtained after HIP, and austenite reversion was less sensitive with the HIP + ageing treatment (HAT) compared to direct ageing treatment (DAT). Grain growth and alloying homogenisation also occurred during HIP. Massive nanoscale precipitations were formed both after DAT and HAT; quantitative analysis based on TEM observation indicated that the size of these precipitates was larger after subjecting to HAT. The room-temperature ultimate tensile strength and elongation to failure after HAT were approximately 6% and 100% higher than DAT, confirming the simultaneous enhancement of the strength and ductility after HAT. The findings of the present study offer insights into heat treatment approaches that can overcome the strength-ductility trade-off of LPBF-manufactured 18Ni-300 maraging steel, while also providing an alternative strategy for obtaining the simultaneous improvement of strength and ductility through heat treatments.