Influence of overlap passes on the microstructure and properties of friction-stir surface processed cold-work steel

Friction stir processing (FSP) is a solid-state surface modification technique widely used for engineering alloys. However, the effects of multi-pass FSP on the large-area treatment of AISI D2 cold-work steel remain unclear. In this study, double-pass FSP with varying overlap rates was applied to annealed D2 to evaluate its microstructural changes, mechanical and corrosion properties. The results indicated that, due to dynamic recrystallization and mechanical fragmentation, the grain size in the stir zone was significantly reduced from 8.5 to 2.2 μm. The microstructure presented a refined martensitic matrix, retained austenite, and dispersed and refined (Cr, Fe) 7 C 3 after FSP. Notably, in the tempered zone near the heat-affected zone of the second pass, metastable quenched martensite transformed into tempered martensite, resulting in an increase in hardness from 500 to 648−720 HV 0.2 . However, wear rate increased from 0.25×10 -5 to 0.65–0.75×10 -5 mm 3 /N·m, with abrasive wear and adhesive wear observed in TZ of double-pass specimens due to less retained austenite and more brittle martensite. Additionally, carbide precipitation depleted the Cr content, which impaired passivation and reduced corrosion resistance. Consequently, the corrosion potential increased from 358.0 mV SCE to between 392.9 and 459.9 mV SCE , while the current density rose from 9.5 μA/cm 2 to between 10.8 and 13.2 μA/cm 2 .