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Effect of cryogenic cooling on texture and microstructural evolution of ZK60 magnesium alloy during machining

Producing ultra-fine grains on a material’s surface is highly pronounced nowadays, which has more influence on the surface integrity properties of the finished component. In this report, severe plastic deformation (SPD) was employed through cryogenic machining to form fine grains on the machined surface of the ZK60 magnesium alloy. Using liquid nitrogen (LN2) as a coolant during machining of ZK60 alloy induced deformation twins in the grains located on the deformation bands due to severe plastic deformation. Nucleation for the fine grain formation occurred at the deformation shear bands. Evolution of strong basal texture with deformation twins during cryogenic machining increased the fraction of submicron grains from <8% to ≈36%. The average diameter of fine grains was measured as ≈1.2 μm on the machined surface. Moreover, high lattice strain ensured the slip system that necessary for the occurrence of low temperature dynamic recrystallization (DRX). Pole figures resulted from EBSD analysis showed that the refined grains were oriented on the basal plane. The transformation of low angle grain boundaries (LAGB) to high angle grain boundaries (HAGB) about ≈61% under cryogenic machining which was ≈29% in as-received material confirmed the occurrence of grain refinement. Higher magnitude of compressive residual stress was also noticed below the machined surface under cryogenic machining.