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New tool coating strategies can make
it easier to deal with difficult-to-machine materials.
By Dr. Dennis T. Quinto, Surface Engineering
Consultant, and Fred Teeter, Teeter Marketing Services LLC.
continuous development of new workpiece materials aimed at making
products lighter yet stronger often pose machining problems. Ongoing
development work in chemical vapor deposition and physical vapor
deposition wear-resistant thin-film coatings—driven by the
need to continuously improve productivity— is directed at
these new machining applications. This article examines tool coating
options and strategies for difficult applications, such as milling
new grades of titanium and machining composites for aerospace parts.
There are several reasons why aerospace materials are typically
more difficult to machine than more common materials, such as the
steels and cast iron alloys used in the auto industry. Metal removal
involves chip formation where the workpiece material is plastically
deformed until it is fractured, creating a chip that is separated
from the workpiece to a given DOC. Metallurgists have a standard
tensile test that generates a stressstrain diagram, which defines
the amount of plastic deformation (total elongation) prior to reaching
the material’s breaking point or fracture strength (related
to hardness). These values are given in standard materials handbooks.
For example, various steels with differing levels of strength/hardness
and elongation/ductility, according to composition and heat treatment,
are shown in Figure 1. They can be categorized broadly as plain
carbon, nonhardened alloy and hardened alloy steels.
A plain carbon steel with low hardness but high elongation to fracture
generates a long chip because it deforms quite a bit before breaking;
conversely, a high-alloy, high-strength steel quickly breaks once
the loading exceeds its fracture strength, producing short chips.
The specific fracture energy is proportional to the
product of fracture stress times the fracture strain; this is known
as the plastic deformation energy that is converted to heat during
chip formation. The second source of heat is friction generated
as the chip rubs on the tool’s rake face. The higher the workpiece’s
ductility, the higher its contact length with the tool rake, which
produces more frictional heat.
A plot of the envelopes of hardness and ductility
properties of the steels along with different workpiece materials
is shown in Figure 2. Machinability improves with short-chipping
metals but declines as longer chips are generated (due to ductility)
and higher cutting forces are needed (due to strength or hardness).
Other metallurgical factors affecting machinability and tool life—second
phases, inclusions, strain hardening—are also indicated.
Easily machinable workpieces can be machined at high
ing speeds; conversely, speeds must be lowered for workpieces with
poor machinability. Aerospace-grade titanium alloys and nickel-base
superalloys, such as Inconel, with excellent strength and ductility,
pose the biggest metalcutting challenges because they generate high
plastic deformation energy and frictional heat at the cutting edge.
Their low thermal conductivity further shortens tool life because
heat is not rapidly conducted into the flowing chip but remains
and is concentrated at the tool/chip interface.
Hard coatings for cutting tools combat the high stress
and temperatures imposed on the cutting edge during chip formation.
The tool life difference between uncoated and coated tools today
can vary by a factor of two to 10, depending on the application.
A snapshot of proven successful coatings in cutting the various
workpiece materials discussed previously is also provided in Figure
Coatings are matched to workpiece chip formation characteristics
and cutting conditions. The early generation of PVD TiN and TiCN
coatings is still effective at normal machining conditions for carbon
and alloy steels, but as speed is increased, TiAlN-type coatings
become more suitable because they have enhanced stability at higher
Difficult-to-machine aerospace alloys demand cutting
tools with optimal cutting edge design, including controlled microgeometry—the
parameters defining cutting edge sharpness, such as an edge chamfer
or edge hone radius. The hone may be fixed or variable along the
length of the cutting edge. These microscopic features— hone
radius is typically 0.001"—lower stress concentrations
at the cutting edge, which is further protected by the PVD coating.
Materials scientists have altered the compositions
of new PVD coatings— such as adding Si to TiN to form TiSiN,
increasing levels of Al in AlTiN or substituting Cr for Ti in AlCrN—to
form nanostructures that yield high hardness, plasticity and oxidation
resistance at the high temperatures generated when cutting difficult-to-machine
materials. The coatings enable higher speeds and increased tool
life while keeping other cutting conditions the same as with uncoated
tools. Several studies have postulated various mechanisms of improvement
for different cutting applications, e.g. better abrasive wear resistance,
microcrack suppression, lowered contact friction, formation of stabilizing
AlCr oxide surface films or improved heat barriers due to hard coatings.
These new coatings are being applied in many demanding applications,
such as those encountered in the aerospace industry. This industry
expects substantial growth over the next 20 years and is using several
difficult-to-machine, high-performance materials, such as composites,
titanium and Inconel. These materials provide greater strength at
lighter weights, which are needed to produce more fuel-efficient
airplanes, but the materials require new metalcutting strategies.
For example, the new midsize Boeing 787 Dreamliner,
with deliveries due to begin in 2008, is expected to consume 20
percent less fuel than traditional midsize jets by using lightweight,
high-strength-to-weight-ratio materials. About half of the 787’s
primary structure is made from composite materials and another 15
percent from titanium. In addition to being lighter than aluminum,
both titanium and composite materials resist corrosion and require
less maintenance than aluminum. Titanium is also more compatible
with composites than aluminum, which produces corrosive galvanic
reactions with carbon fibers in the composite structure.
Titanium, however, poses several machining challenges.
It generally requires slow cutting speeds and low feed rates and
requires radial DOCs on the order of 100 percent and axial DOCs
on the order of 30 percent to avoid heat build-up. As a result,
machining titanium can cost 10 times more than machining aluminum.
A new grade of titanium, Ti-5553 (Ti-5Al-5V-5Mo-3Cr) is now preferred
for key components over Ti-6Al-4V because it exhibits excellent
hardenability and has high cycle fatigue behavior properties. Unfortunately,
it is even more difficult to machine than traditional titanium alloys.
Several cutting tool manufacturers are working on
this problem. One of those companies, Niagara Cutter Inc., Amherst,
N.Y., is working with aerospace companies to develop optimal tooling
and operating parameters to machine Ti-5553. Testing is underway
at NCI’s Reynoldsville, Pa., laboratory and in the field to
explore the role of tool geometries, edge preparation techniques
and the use of coatings to act as a heat barrier in solving these
The company has developed guidelines for rough, semifinish
and finish milling of Ti-5553. NCI recommends an M-42 cobalt-HSS
or P/M cobalt- HSS fine-pitch roughing endmill with a TiCN or TiAlN
coating and a cutting speed from 40 to 60 sfm at a feed rate of
3 to 5 ipm. For semifinish milling, a 4-flute solid-carbide endmill
with a TiAlN coating, a 70- to 100-sfm speed and 5- to 7-ipm feed
is recommended, and for finish milling, a 6- to 8-flute solid-carbide
endmill with a TiAlN coating, operating at 400 sfm and 5 to 7 ipm.
Sherwood Bollier, president of NCI, said a new tool
geometry that NCI refers to as a variable-face profile, in combination
with coatings and proper operating parameters, yields improved chip
flow characteristics and eliminates problems associated with chip
packing, also known as a bird’s nest.
Composite materials are being used in an ever-growing list of products,
most notably in aircraft. A composite material is typically made
up of resin, reinforcing fibers and one or more fillers or additives.
High strength-toweight ratios and enhanced mechanical and thermal
properties make composites a viable choice to replace metals.
According to Roger Bollier, president of Diamond Tool
Coating, North Tonawanda, N.Y., CVD PCD coatings are the best choice
for machining many composites. DTC supplied coatings for development
work by the National Center for Defense Manufacturing and Machining,
Latrobe, Pa., for Lockheed Martin Aeronautics, which machines advanced
composite wing-skin material for the F-35 Lightning II stealth fighter.
Initial machining of the wing-skin material produced
short tool life and excessive delamination. The use of 20µm-thick
CVD PCD coatings, in combination with optimized tool geometry, increased
tool life more than six times and reduced the workpiece delamination
Machining performance improvements have always relied
on the synergies in designing tool materials, cutting edge geometries
and coatings. In recent years, PVD coating developments in particular
have lowered the traditional barriers to metalcutting productivity
in the so-called difficultto- machine aerospace materials.
About the Authors
Dr. Dennis T. Quinto, with 25 years of experience in cutting tool
and coating technologies, recently retired from Oerlikon Balzers
Coating USA Inc. as technical director and was formerly advanced
technology director at Kennametal Inc. He is now an independent
surface engineering consultant. Contact him by e-mail at den email@example.com.
Fred Teeter is managing director of the Surface Engineering Coating
Association, Amherst, N.Y., and president of Teeter Marketing Services
LLC, Niagara Falls, N.Y. Contact him at (716) 791-8100 or firstname.lastname@example.org
or visit www.surfaceengineering.org.