Investigating the High-Speed Micro – Machinability of Aerospace Alloys

Abstract

Micro technologies including Micro-Machining have become essential part of the various industries of modern era. Micro-machining is contributing to almost all fields of industries like aerospace, automobiles, telecommunication, electronics, and medical sectors etc. Therefore, 3D products in the industry require quality product and dimensional accuracy even at micron level. Micro-milling, the most versatile micro-machining technique, has gained importance in mass production of 3D components. Micro mechanical tools have great potential for economical manufacturing of miniatured products from variety of materials, in micro-milling operations. However, the previous research work shows some critical issues in direct application knowledge of macro machining domain into micro domain with help of simple analysis of parts dimensions. Thus, research work focuses on areas which require scientific knowledge to be developed for further implementation at micro scale level to improve the quality of final product. Mechanical micro-milling of aerospace alloys remained focus of many researchers in past because of having excellent mechanical properties, at extreme temperatures, that are suitable for different sectors like aviation, automotive, nuclear, marine, and biomedical applications. These are also considered a better option for applications where stresses need to be minimised. Inconel alloy (Nickel-chromium based) and Monel alloy (Nickel-copper based) being part of aerospace alloy, possesses greater strength and excellent corrosion resistance, work hardening properties at elevated temperature. In aerospace sector, parts like discs, some critical jet engine parts are also being manufactured with these alloys. Nevertheless, Nickle alloys are hard to machine materials and have inherited poor machinability because of low thermal conductivity. Low thermal conductivity results in significant increase of temperature at the cutting zone which results in decreased life of cutting tool. Available research knowledge of vital process parameters (feed per tooth, depth of cut, cutting speed, etc.) in relation to micro-milling of Monel and Inconel alloys at high-speed micro-machining (HSM) under various cooling environments and with multiple micro tool coatings present a research gap. There is need to cover this gap by analysing effects of multiple cutting parameters with wide range on responses at high-speed micro-machining of these alloys. This research focuses on investigation of high-speed micro machinability of aerospace alloys including Inconel 600, Monel 400 under multiple cooling environments and various cutting x conditions. Machining related key input parameters like cutting speed (m/min), feed per tooth (µm/tooth), depth of cut (µm), various micro tool coating (TiAlN, TiSiN and nACo) and multiple cooling conditions (Cryogenic, wet and dry) were taken into considerations for analysis and their effect on responses like tool wear, surface roughness, and burr formations. To have a thorough insight into micro machinability of aerospace alloys, Feed per tooth, was selected above and below the cutting-edge radius of the micro tools. Due consideration was also given to burr formation at up milling side and down milling side. All experiments were categorized into three sets, where first set of experiments were carried out with key process parameters under various cooling conditions (Cryogenic, wet and dry). Second set of experiment was conducted with input parameters using multiple micro tool coatings (TiAlN, TiSiN, nACo) in addition to un-coated micro tool. Third set included validation experiments of all categories. Investigation, into the micro machinability of Inconel and Monel alloys, was carried out through statistical analysis and multi objective optimization (MOO) with various cooling environments and multiple cutting conditions. As process parameters and response parameters are independent and different in nature therefore multi objective optimization was essentially required to optimize the machining output. Grey Rational Analysis (GRA) ranked each experiment. Regression model analysis of multi objective function, identified optimum process parameters in both categories. Outcome of this research work provides in-depth and significant knowledge on utility and importance of making manufacturing system more productive with quality and accuracy required for 3D parts at micro-machining level. Results show that proper selection of tool coatings and cooling environment produce significant improvements in performance compared to conventional tools and cooling environments, in field of micro machining. As a result of ANOVA, ‘Cooling condition’ figured out the most significant factor with contribution ratio (29%) towards surface roughness followed by cutting speed with contribution ratio (26%). It was also influential factor for tool wear with contribution ratio 26%. Feed per tooth figured out as most significant factor for their effect on burr formation in both cases i.e., up, and down milling case for Inconel 600 alloy with contribution ratios, Burr width - down milling case (66%), Burr height- down milling case (44%), Burr width- up milling case (35%). xi Feed per tooth was the most significant factor for its effects on surface roughness including burr formation in both modes i.e., up and down milling side, in micro-milling of Monel 400 alloy using multiple tool coatings, with contribution ratios, Surface roughness (28%), Top Burr width (up and down milling side) 56%, 57%, Top Burr height (up and down milling side) 24%, 28%, respectively. Depth of cut is the most significant factor for tool wear with contribution ratio 34%. nACo coated tool showed least tool wear in micro-milling of Monel 400 alloy whereas uncoated tool showed worst tool wear. Cutting condition, with TiAlN coated tool, has a positive intercept gain of 4%, 8% and 1.03% on uncoated micro tool, TiSiN micro coated tool and nACo micro coated tool conditions, respectively