Surface integrity enhancement of LA103Z Mg-Li alloy by regulating microstructure and texture evolution in cutting metamorphic layer

The high level of tensile stress and high roughness induced by cutting are the main causes of low surface integrity of Mg-Li alloy components. Their values depend on the evolution mechanism of the cutting metamorphic layer. This research was aimed to analyze and compare the microstructure evolution mechanisms of the metamorphic layer under dry and cryogenic milling and reveal their action mechanism on the surface integrity for the duplex LA103Z Mg-Li alloy based on the microstructure characterization and mechanical properties measurement experiments. It was revealed that the metamorphic layers under dry and cryogenic milling both consists of second phase particles accumulation layer and featureless layer with less visible grain boundaries. The compositions and degree of the phase transition and the texture evolution of the cutting metamorphic layer both change dramatically from dry to cryogenic milling. The deformation modes of dry milling is non-basal slip, and appears the dynamic recrystallization (DRX) related prismatic fiber texture {11 2 0} <0001> in alpha-Mg phase and gamma fiber {111} <112> in the beta-Li phase. Whereas cryogenic milling is basal slip accompanied by prismatic slip, and appears a dynamic recovery (DRV) related alpha fiber {111} <110> in beta-Li phase. Moreover, cryogenic milling exhibits a more pronounced phase transformation characteristics owing to the transient pressurization heating and extremely cold conditions. Compared with dry milling, cryogenic milling has significant advantages in improving surface hardness, reducing surface tensile stress and improving surface finish by increasing the hard phase, forming the strong basal texture, and strengthening the material physical parameters on the milled surface. In general, the cryogenic regulation method is found to be more suitable for machining high-quality Mg-Li alloy components.