Improved surface integrity from cryogenic machining of Al 7050-T7451 alloy with ultrafine-grained structure

Materials with ultrafine-grained (UFG) structure become the prioritised selections for specific applications where high strength, hardness, wear/corrosion resistance, etc. are sought. Machining materials with UFG structure requires further research as their mechanical and material properties are different from the ones with regular coarse grains. In this study, Al 7050-T7451 aerospace alloy was first processed by friction stir processing (FSP) with UFG structure for use in subsequent machining experiments. Cryogenic machining using liquid nitrogen as the coolant was applied in the experimental work of machining Al 7050-T7451 alloy with UFG structure. Surface integrity characteristics including microstructure, grain size and hardness were investigated in order to understand the differences of these properties induced by dry and cryogenic machining. Smaller grains with the grain size of around 1.6 μm were observed in the machined surface generated by cryogenic machining compared to the grains of bulk materials and the ones acquired by dry machining. On the other hand, higher hardness measured from the surface and subsurface regions of the machined samples show a harder layer formation by cryogenic machining. Forces and temperatures were measured during dry and cryogenic machining. Cutting and thrust force components for both dry and cryogenic condition were found to be almost identical due to the homogeneous UFG structure achieved by the previous FSP while cryogenic machining generated lower cutting temperature. Also, microstructure and hardness of the chips formed by machining were investigated to understand the effects of cryogenic machining on materials with UFG structure from the dynamic recrystallisation and grain growth points of view. Grains with approximately 1 μm grain size existed in the machined chips, and the hardness of the chips was about 42% higher than the bulk material. Overall, the experimental findings demonstrate that the proposed approaches were beneficial to enhance surface integrity characteristics of this material.