Influence of cutting speed and tool wear on the surface integrity of the titanium alloy Ti-1023 during milling
Yang Houchuan
Chen Zhitong
Zhou ZiTong
0
) Department of Aviation Repair Engineering, The First Aeronautical Institute of Air Force
,
Xinyang, Henan Province 464000
,
China
This study focuses on the machined surface integrity of the titanium alloy Ti-10V-2Fe-3Al (Ti-1023) during face milling. Surface roughness, machining defect, microhardness, and microstructure variations are investigated at different cutting speeds and tool average flank wear (VB) values. Experimental results show that surface roughness increases when cutting speed is increased from 40 to 100 m min1 and decreases when cutting speed is increased from 100 to 300 m min1 by using a new tool. Moreover, surface roughness values are stable when cutting speed is increased by using a worn tool at VB = 0.2 mm and increases when VB is increased. As for defects, the machining defect is determined to be dependent on cutting speed and tool wear directions. Microhardness is decreased to 35 m beneath the machined surface at different cutting speeds by using a new tool. The influence of tool wear on hardening is significant, with the depth of hardening being less than 55 m by using a worn tool at VB=0.2 mm and reaching 130 m at VB=0.35 mm. No significant phase transformation or evident deformation can be observed at different cutting speeds by using a new tool (initial wear). Severe plastic deformation and deep alteration of the microstructure in the machined surface are produced with VB from initial wear to VB = 0.35 mm. VB is best maintained at less than 0.2 mm for finish machining of Ti1023.
1 Introduction
The titanium alloy Ti-10V-2Fe-3Al (Ti-1023) has been
extensively used to manufacture critical components, such as
aircraft fuselage, wing, landing gear, and helicopter rotor parts,
employed in the aviation and aerospace industries because of
its high strength-to-weight ratio, excellent corrosion and
fatigue resistance, high fracture toughness, good casting
properties, and hardenability [1]. These critical components are
frequently used in extreme environments. The processing
quality of these components directly affects the performance
of the entire aircraft structure [2]. Finishing is still needed to
satisfy tolerance requirements or achieve improved surface
integrity. However, similar to other titanium alloys, Ti-1023
cannot be easily machined because of its high strength,
chemical reactivity, and low thermal conductivity. Numerous
investigations have confirmed that the performance, longevity, and
reliability of machined components during their service life
are considerably dependent on their surface quality [3].
Surface integrity requirements (e.g., surface roughness,
machining defect, microhardness, microstructure, and residual stress)
must be satisfied to eliminate severe failures produced by
fatigue, creep, and stress corrosion cracking generated on the
surface of components [46]. Hence, the quality of a
machined surface is important to satisfy the increasing demands
for sophisticated component performance.
Many studies have been conducted to investigate the
surface integrity of titanium alloys, including Ti-6Al-4V,
Ti6242S, and Ti-6246 [713]. Sun and Guo [8] reported that
roughness is obviously influenced by cutting speed and tool
wear. As cutting speed increases, roughness value decreases.
Consequently, the machined surface tends to become smooth
toward the end of tool life at a cutting speed of 60 m min1.
Other researchers have reported that cutting speed [7, 911,
15] is effective to a certain degree in increasing surface
roughness. As cutting speed is increased, surface roughness
increases for Ti-6246 and Ti-64 [7, 8, 13, 14]. Moreover, tool
parameters, such as tool insert shape, tool average flank wear
(VB), and the selected coolant, affect surface roughness values
[16]. Tool wear close to tool half-life results in a slight
decrease in surface roughness values than that in the previous
period. When half-life is reached, the tool fits the workpiece
and surface roughness is decreased. After half-life, however,
the tool starts to wear severely and causes anomalies in the
toolworkpiece contact surface, which increases surface
roughness significantly [13]. Many forms of surface defects
have been reported in literature [12, 15, 1720]. The main
forms include surface drag, material pullout/cracking, feed
marks, adhered material particles, tearing surface, chip layer
formation, microchip debris, surface plucking, deformed
grains, surface cavities, slip zones, laps (materials folded onto
the surface), and lay patterns [12, 15, 1720]. Cutting speed
can affect the amount of microchip debris on the surface [12,
17]. The most commonly reported surface damages are plastic
deformation of the grains in the direction of cutting and
surface cavities. Jeelani and Ramakrishnan [21, 22] reported
that surface cavities are formed by chip fragmentation on the
workpiece surface or by welding between the titanium
adhering to the tool and the workpiece. The adher (...truncated)