Effects of Surface Treatment and PVD ta-C Coated Tool on Properties and Performance Evaluation during Dry Turning of Al 7075-T6 Alloy

Abstract

Al 7075-T6 alloy characterized by high ultimate tensile strength, tensile yield strength and fatigue strength has extensive applications in marine, automobile and aviation industries. However, the same alloy suffers from poor machinability owing to its elevated strength and high chemical affinity towards tool materials. Although CVD diamond coating has been traditionally used for machining aluminium alloys, it is essential to develop a similar carbon-based coating using more environment-friendly PVD route with the potential to deposit at much lower temperature and capability to impart smoother coating microstructure. The present research has therefore been undertaken to deposit amorphous carbon coating with sufficiently high hardness using cathodic arc evaporation. Micro blasting as a surface treatment technique was utilized both prior to as well as after coating deposition. The purpose of substrate micro blasting is to enhance coatingsubstrate adhesion while micro basting of coated surface is expected to improve surface finish and wear resistance properties of the coated tool. Analysis of Raman spectra indicated the deposited coating layer comprised of carbon atoms in the amorphous form called tetrahedral amorphous carbon (ta-C) with unstructured mixture of sp3 (hard phase diamond) and sp2 (soft phase graphite) hybridised carbon atoms. The ratio of I (D)/I (G) was proportional to sp3/sp2 ratio. The hardness of the pre and post treated ta-C coated tool was 3% higher than that of only pre-treated coated tool. Post deposition micro blasting was beneficial in minimising formation of micro droplets which is a typical feature of cathodic arc evaporation. Coating with prior micro blasting resulted in higher surface roughness than the one with pre and post treatment.
Low pressure bombardment of coated surface with Al2O3 particles was found to be effective in improving surface finish. Under lower range of cutting speed, PVD ta-C coating did not significantly contribute in reducing cutting forces, while the same coated tools clearly outperformed their uncoated counterpart in improving surface finish in the entire range of cutting speed. Under the condition of low to medium cutting speed (100-300 m/min.), superior performance of only pretreated ta-C coated tool in terms of lower cutting force, machined surface roughness and work piece material adhesion is related to enhancement of coatingsubstrate adhesion which is essentially required to counter higher contact stress and drag force under the same cutting condition. Enhanced resistance to wear under higher cutting speed (300-500 m/min.) could be successfully realised with the pre and post treated ta-C coated tool which resulted in lower cutting force, surface roughness and flank wear. There was a gradual transition from long ribbon-like continuous chips at low cutting of 100 m/min. to small broken chips under the maximum cutting speed of 500 m/min. Spiral and spring-like chip, connected arc type chips and snarled type chips were obtained in between. Increase in cutting speed also caused a decrease in chip curl radius. Therefore, taC (ta-C) coating with prior micro blasting is recommended for machining Al 7075-T6 alloy in the low to medium range of cutting speed i.e. 100-300 m/min., while pre- and post-treated ta-C coating is more suitable for high speed machining of the same aluminium alloy.