Cryogenic and conventional milling of AZ91 magnesium alloy

• Published in: Journal of magnesium and alloys 2024-06, Vol.12 (6), p.2503-2519
• Main Authors: Marakini, Vikas, Pai P, Srinivasa, Bolar, Gururaj, Achar, Bhaskara P
• Format: Article
• Language: English
• Subjects: Cryogenic machining; Magnesium alloy; Microhardness; Microstructure; Multi criteria decision making; Residual stress; Roughness
• ISSN: 2213-9567; 2213-9567
• DOI: 10.1016/j.jma.2024.06.004

•Improvement of surface integrity of AZ91 magnesium alloy using liquid nitrogen assisted milling and comparison with conventional dry milling.•Cryogenic milling minimized the machining temperature by about 29%, minimized the alloy surface roughness by about 28% and maximized the microhardness by about 23%.•Compressive residual stresses are generated in cryogenic milled surfaces.•No significant microstructural damages observed from both cryogenic and conventional milling operations.•The optimal process parameters for milling AZ91 alloy under both dry and cryogenic conditions have been identified. Use of magnesium is the need of the hour due to its low density as well as its high strength-to-weight and stiffness-to-weight ratio etc. This study focuses on the effectiveness of liquid nitrogen (LN2) assisted cryogenic machining on the surface integrity (SI) characteristics of AZ91 magnesium alloy. Face milling using uncoated carbide inserts have been performed under liquid nitrogen (LN2) assisted cryogenic condition and compared with conventional (dry) milling. Experiments are performed using machining parameters in terms of cutting speeds of 325, 475, 625 m/min, feed rates of 0.05, 0.1, 0.15 mm/teeth and depth of cuts of 0.5, 1, 1.5 mm respectively. Most significant surface integrity characteristics such as surface roughness, microhardness, microstructure, and residual stresses have been investigated. Behaviour of SI characteristics with respect to milling parameters have been identified using statistical technique such as ANOVA and signal-to-noise (S/N) ratio plots. Additionally, the multi criteria decision making (MCDM) techniques such as additive ratio assessment method (ARAS) and complex proportional assessment (COPRAS) have been utilized to identify the optimal conditions for milling AZ91 magnesium alloy under both dry and cryogenic conditions. Use of LN2 during machining, resulted in reduction in machining temperature by upto 29% with a temperature drop from 251.2 °C under dry condition to 178.5 °C in cryogenic condition. Results showed the advantage of performing cryogenic milling in improving the surface integrity to a significant extent. Cryogenic machining considerably minimized the roughness by upto 28% and maximised the microhardness by upto 23%, when compared to dry machining. Cutting speed has caused significant impact on surface roughness (95.33% – dry, 92.92% – cryogenic) and surface microhardness (80.33% – dry, 82.15% – cryogenic). Due to the