Effect of direct aging on the microstructure and sustainable milling performance of L-PBF 17-4 PH stainless steel

Abstract

This study investigates the influence of direct aging (DA) and cooling/ lubrication strategies on the milling performance of 17–4 PH stainless steel (SS) fabricated via laser powder bed fusion with a comparative assessment against commercially obtained 17–4 PH SS. Direct aging was performed at 580 °C for 1, 2, and 3 h to induce precipitation hardening, and its effect on both microstructural and machinability was systematically evaluated. Cutting performance was evaluated in terms of cutting temperature, acceleration amplitude and surface roughness, while microstructural features and microhardness were analyzed to establish process–structure–property relationships. The results indicate that DA promotes the formation of Cu-rich precipitates, which contributes to a slight increase in microhardness. Compared to the as-received sample, the directly aged 17–4 PH steel exhibited reductions of approximately 23.63% in cutting temperature, 38.7% in acceleration amplitude, and 45% in surface roughness. Furthermore, across all samples, minimum quantity lubrication assisted machining consistently reduced cutting temperature, vibration amplitude, surface roughness, and tool wear due to its enhanced lubrication and cooling capabilities, as confirmed by scanning electron microscopy observations revealing markedly lower tool wear.