High-speed face milling of AZ91 Mg alloy: Surface integrity investigations

Abstract

Magnesium (Mg) alloys are popular in the aerospace and automotive sector owing to their light-weight aspects. Amongst various Mg alloys, AZ91 alloy behaviour under machining has been trending and needs to be completely explored. The selection of optimal machining parameters is an important decision making process to achieve highest quality along with reduced cost and time. In this regard, this article describes experimental investigations to evaluate the performance of face milling operations on the surface characteristics of AZ91 magnesium alloy. The experiments were carried out with uncoated and PVD (Physical Vapour Deposition) coated carbide inserts at three levels of cutting speed (500, 700 and 900 m/min), feed rate (0.1, 0.2 and 0.3 mm/teeth) and depth of cut (0.5, 1.0 and 1.5 mm) under dry machining conditions. Major surface integrity indicators, such as roughness, hardness, residual stresses and microstructure are analysed. Chip morphology is also analysed and the correlation between chips and machined surface roughness is established. Face milling operation significantly improved surface roughness and microhardness of this alloy. Roughness improvement up to 85% (0.067 μm) and hardness improvement up to 33% (91.8 HV) is observed from the use of uncoated carbide inserts. Whereas, from PVD coated inserts, roughness improvement up to 81% (0.083 μm) and hardness improvement up to 60% (111.2 HV) is achieved. A similarity in behaviour between the two types of insert conditions are observed with increase in roughness from feed increase and decrease in hardness from cutting speed increase. Microstructural analysis showed PVD coated insert conditions producing surface with no defects, when compared to the crack observed in the surface from the use of uncoated carbide inserts. Marginally higher compressive residual stresses are detected at the surfaces from use of the uncoated inserts. Overall, due to no surface defect and the significant improvement in hardness and roughness from the PVD coated inserts, they are recommended for use in face milling operation for the cutting conditions investigated in this study.