Built-up edge formation and flank wear evolution in conventional machining of AZ91 magnesium alloy

Abstract

Magnesium alloys are largely desirable in numerous industries today because of their intrinsically attractive properties. Low density and high strength coupled with significant biocompatibility are the main advantages. Machining of magnesium alloys is challenging as a result of the persistent chip ignition risk as well as material build-up on the tool due to their softness. Therefore, magnesium alloys’ machinability has not been fully assessed in the available literature. The present work aims to address this aperture by assessing flank wear progression and the prevalence of adhesion-driven built-up edge (BUE) and built-up layer (BUL) in different tool coating materials during the dry short-interval conventional orthogonal turning of AZ91 magnesium alloy. A higher and lower combination of feed rates and cutting speeds were utilized for all tools to compare the possible range of maximum flank wear land width, average flank wear land width, and chip contact length while capturing and assessing said parameters through digital and optical microscopy. The data presented a large prevalence of adhesion wear across all tool coating types which is portrayed by the cyclic behaviour of the flank wear land width plots, which resulted from the repeated accumulation and wear of deposited workpiece material. The TiCN-coated tool presented the least amounts of BUL and BUE throughout both levels of testing, while the TiN-coated tool presented the largest amounts of average flank wear land width due to recurrent aberrative local deposition accumulation and extended streak line depositions. TiN-coated, Al₂O₃-coated, and uncoated tools presented higher average flank wear land width than the TiCN-coated tool by 8.5%, 3.1%, and 27.2% during level 1 cutting conditions and 84.8%, 21.3%, and 37.2% during level 2 cutting conditions. This implies the TiCN coating's increased resistance to elevated cutting temperatures, and in turn, reduced deposition of material through adhesion.