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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
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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 micromachining.
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