1. Introduction to hard materials machining:
Hard
materials machining is machining of parts with a hardness of above 45 HRC,
although most frequently the process concerns hardnesses of 58 to 68 HRC. The work
piece materials involved include various hardened alloy steels, tool steels,
case-hardened steels, super alloys, nitrided irons and hard-chrome coated
steels, and heat-treated powder metallurgical parts. It is mainly a finishing
or semi-finishing process where high dimensional, form, and surface finish
accuracy have to be achieved. Some common hard materials used in machining of
aerospace components are Titanium, Steel, Stainless steel, maraging steels.
Steel: Steel is an alloy of iron
and a small amount of carbon. Carbon is the primary alloying element, and its
content in the steel is between 0.002% and 2.1% by weight. Additional elements
may also present in steel: manganese, phosphorus, sulfur, silicon, and traces
of oxygen, nitrogen and aluminum.
Stainless steel: Stainless steel does
not readily corrode, rust or stain with water as ordinary steel does, but despite
the name it is not fully stain-proof, most notably under low oxygen, high
salinity, or poor circulation environments. There are different grades and
surface finishes of stainless steel to suit the environment the alloy must
endure. Stainless steel is used where both the properties of steel and
resistance to corrosion are required. Improved stainless steel materials, such
as Custom 465 and Project 70+15C5-5Ni are being used in structural components
such as flap tracks, slat tracks, engine mounts, and landing gear hardware
Titanium:
Symbol: Ti; Atomic number: 22; Atomic weight: 47.9;
Titanium and Titanium Alloys (110–450 HB) (≤48 HRC)
Pure:
Ti98.8, Ti99.9
Alloyed:
Ti5Al2.5Sn, Ti6Al4V, Ti4Al2Sn4Zr2Mo, Ti3Al8V6Cr4Mo4Zr,
Ti10V2Fe
3Al, Ti13V11Cr3Al, Ti5Al5Mo5V3Cr
1.1. Challenges in machining advanced materials:
New advanced materials impose
demanding challenges on current machining systems, particularly on the cutting
tools. Titanium brings tremendous properties to aircraft design and
manufacturing – light weight, great stiffness, strength, heat and
fatigue-resistance. In the new generation aircraft, the amount of titanium is
increasing in high-strength, complex, monolithic components that join composite
sections, high-stress landing gear, and high-temperature engine applications.
With the anticipated demand, it is estimated that production will require more
titanium-machining capacity than currently exists worldwide! Therefore, there
is a critical need to improve machining efficiency.
Milling
titanium is different from other metals because of the risk of heat build-up.
Titanium is generally machined at slow speeds and low feed rates with less than
30% radial and low axial depths-of-cut. As a result, the cost of machining
titanium can be as high as 10X that of conventional machining of aluminum.
Thermal Conductivity: Titanium is a very poor
heat conductor. As the pie charts indicate, thermal conductivity
characteristics between steel and titanium differ greatly when comparing the
heat imparted to the workpiece versus the chip.
With steel-based components, more
than 75% of the heat generated by the cutting process is transferred to the
evacuated chip, whereas with titanium-based parts, only 25% of the heat is transferred
to the chip, thus creating a greater heat concentration on the cutting edge of
the tool. This condition of course leads to more rapid tool failure or
diminished productivity due to the slower cutting speeds required to address
the heat generation problem. The solution is heat resistant solid carbide and
effective coolant.
Low Modulus of Elasticity: Low modulus of elasticity leads to a
“springiness” characteristic whereby titanium parts may move under the force of
the cutting edge—and then spring back. This condition can lead to accuracy
problems. The solution lies in proper tool geometry (primary and secondary
relief angles and helix angle) and cutting tool edge preparation.
Work Hardening Tendency:
Work hardening tendency in forged
parts impose challenges with very hard, at times non-homogeneous surfaces,
resulting in high cutting pressures and excessive heat generated. The resulting
high cutting pressure, heat generation and undesirable thin, fanfold chips. The
solution is machine tool and software capability that eliminates tool dwelling.
High Chemical Reactivity:
High chemical reactivity can lead
to chemical reaction and an undesirable diffusion reaction at the tool/work
piece interface leading to premature tool wear. The solution is the use of thin
film coatings that act as a heat barrier.
1.2. Material Characteristics of Titanium:
•
Relatively poor tool life, even at low cutting speeds.
•
High chemical reactivity causes chips to gall and weld to cutting edges.
•
Low thermal conductivity increases cutting temperatures.
•
Usually produces abrasive, tough, and stringy chips.
•
Take precautionary measures when machining a reactive (combustible) metal.
•
Low elastic modulus easily causes deflection of work piece.
•
Easy work hardening.
1.3. Primary Attributes of Titanium Alloys:
· Elevated Strength-to-Density
Ratio (high structural efficiency)
· Low Density (roughly half the
weight of steel, nickel and copper alloys)
· Exceptional Corrosion Resistance
(superior resistance to chlorides, seawater and sour and oxidizing acidic
media)
· Excellent Elevated Temperature
Properties (up to 600°C (1100°F))
1.4. Why select Titanium alloys?
· Exceptional erosion and erosion
corrosion resistance
· High fatigue strength in air and
chloride environments
· High fracture toughness in air
and chloride environments
· Low modulus of elasticity
· Low thermal expansion coefficient
· High melting point
· Essentially nonmagnetic
· High intrinsic shock resistance
· High ballistic
resistance-to-density ratio
· Nontoxic, non allergenic and
fully biocompatible
· Very short radioactive half-life
· Excellent cryogenic properties
1.5. Titanium and Titanium Alloys Characteristics:
1.6. Why titanium machining is difficult?
Problems
in machining titanium originate from three basic sources: high cutting, temperatures,
chemical reactions with tools, and a relatively low modulus of elasticity.
Unlike steel, titanium does not form a built-up edge on tools, and this
behavior accounts for the characteristically good surface finishes obtained
even at low cutting speeds. Unfortunately, the lack of a built-up edge also
increases the abrading and alloying action of the thin chip which literally
races over a small tool-chip contact area under high pressures. This
combination of characteristics and the relatively poor thermal conductivity of
titanium results in unusually high tool-tip temperatures. Good tool life and
successful machining of titanium alloys can be assured if the following
guidelines are observed:
• Maintain sharp tools to minimize
heat buildup and galling
• Use rigid setups between tool and
work piece to counter work piece flexure
• Use a generous quantity of
cutting fluids to maximize heat removal
• Utilize lower cutting speeds
• Maintain high feed rates
• Avoid interruptions in feed
(positive feed)
• Regularly remove turnings from machines.
The machinability of commercially
pure grades of titanium has been compared by veteran shop men to that of 18-8
stainless steel, with the alloy grades of titanium being somewhat harder to
machine.
1. Milling of Titanium:
The milling of titanium
is a more difficult operation than that of turning. The cutter mills only part
of each revolution and chips tend to adhere to the teeth during that portion of
the revolution that each tooth does not cut. On the next contact, when the chip
is knocked off, the tooth may be damaged. This problem can be alleviated to a
great extent by employing climb milling, instead of conventional milling. In
this type of milling, the cutter is in contact with the thinnest portion of the
chip as it leaves the cut, minimizing chip “welding”. For slab milling, the
work should move in the same direction as the cutting teeth; and for face
milling, the teeth should emerge from the cut in the same direction as the work
is fed. In milling titanium, when the cutting edge fails, it is usually because
of chipping. Thus, the results with carbide tools are often less satisfactory
than with high speed steel. Consequently, it is advisable to try both high
speed steel and carbide tools to determine the better of the two for each
milling job. The use of a water-base coolant is recommended.
2.1. Two other factors influencing machining operations:
1. Because of the
lack of a stationary mass of metal (built-up edge) ahead of the cutting tool, a
high shearing angle is formed. This causes a thin chip to contact a relatively
small area on the cutting tool face and results in high bearing loads per unit
area. The high bearing force, combined with the friction developed by the chip
as it rushes over the bearing area, results in a great increase in heat on every
localized portion of the cutting tool.
2. Further, the
combination of high bearing forces and heat produces catering action close to
the cutting edge, resulting in rapid tool breakdown. The basic machining properties
of titanium cannot be altered; however the following basic rules have been
developed in machining Titanium.
1. Use low cutting
speeds. A change of 20 surface feet per minute to 150 surface feet per minute
using carbide tools results in a temperature change from 800 to 1700 F.
2. Maintain high feed
rates. Temperature is not affected by feed rate so much as by speed, and the
highest feed rates consistent with good machining practice should be used.
3. Use copious amounts of cutting fluid.
4. Use sharp tools
and replace them at first signs of wear. Tool failure occurs quickly after a
small initial amount of wear.
5. Never stop feeding
while tool and work are in moving contact. Allowing a tool to dwell in moving
contact causes work hardening and promotes smearing, galling, seizing and tool
breakdown.
The productivity factor
between typically used cutting tools can easily be 4-to-1 in many cases. Older
tools can be replaced by today’s tools if the entire system is modified where
needed and accounted for where it is unalterable. Tool life can be increased by
the same factor simply by changing from flood to through-tool coolant delivery
and utilizing our newest technology, coolant delivered directly at the cutting
edge. Don’t ask more of your machine than it can deliver. Most machines cannot
constantly cut at a rate of 30 cubic inches (492cc) per minute. There are many
usual failures or weak points in every system. They include, but are not
limited to, drive axis motors, adapter interface, and a weak joint, torque
available to the spindle, machine frame in one or more axes, or compound angles
relevant to machine stability and system dampening.
2.2. General Machining requirements:
The difficulties inherent in
machining titanium can be minimized considerably providing the proper cutting environment. The basic requirements
include rugged machine tools in good condition; vibration-free, rigid setups;
high-quality cutting tools; and suitable speeds, feeds, and cutting fluids
Machine Tools:
Machine tools used for machining operations on titanium need certain
minimum characteristics to insure rigid, vibration-free operation. They are:
·
Dynamic balance of
rotating elements
·
True running spindles
·
Snug bearings
·
Rigid frames
·
Wide speed/feed ranges
·
Ample power to maintain
speed
·
Easy accessibility for
maintenance.
Heavy-duty milling machines produce
the best results in milling titanium (Ref. 32). Horizontal or vertical
knee-and-column milling machines, as well as fixed-bed milling machines, are
used on various face- and end-milling operations. Numerically controlled or
tracer controlled milling machines are used for profile- and pocket-milling
operations. Generally speaking, 10 to
15 horsepower is usually sufficient for milling titanium. This means, for
example, a Number 2 heavy-duty or a Number 3 standard knee-and-column milling
machine. However, the machines needed to accommodate large parts may have as
much as 25 to 50 horsepower available.
Vibration Effects: Vibration-free operation can be obtained by
eliminating excessive play in power transmissions, slides, or screws of machine
tools. Undersized or underpowered machines should be avoided. Certain aisle
locations of machines near or adjacent to heavy traffic also can induce
undesirable vibration and chatter during machining. Last, but not least,
insufficient cutter rigidity and improper tool geometry can contribute to
vibration.
Rigidity Considerations: Rigidity is achieved
by using stiff tool-tool holder systems, and adequate clamps or fixtures to
minimize deflection of the workpiece and tool during machining. In milling
operations, large-diameter arbors with double arm supports; short, strong
tools; rigid holding fixtures; frequent clamping; and adequate support of thin
walls and delicate workpieces are desirable Rigidity in Milling is achieved by
machining close to the spindle, gripping the work firmly in the collets, using
a short tool overhang, and providing steady or follow rests for slender parts.
Drilling, tapping, and reaming require short tools, positive clamping, and
backup plates on through holes.
•
If vertical spindles are employed, your fixturing is still an important aspect.
•
Rigidity is paramount.
•
Keep work low and secure.
•
Keep work as close as possible to spindle/quill.
•
Use gravity to your advantage.
•
Keep work closest to strongest points of fixture.
•
Keep work as close as possible to spindle/quill.
•
Know the power curve of your machine.
•
Ensure sufficient axis drive motors for power cuts.
•
Every setup has a weak link — find it!
•
Horizontal spindles enable chips to fall away from your work.
•
Horizontal fixturing necessitates use of “tombstones” or angle plates.
•
High-pressure, high-volume, through-spindle coolant delivery will increase tool
life tremendously (>4x).
•
Try to keep work close to the strongest points of the fixture to help avoid the
effects of harmonics.
•
Rigidity will make or break your objectives:
—
Look for weak parts of machine structure and avoid moves that may compromise
the rigidity.
— Tool adaptation must fit
the work
— Check for backlash in the
machine’s spindle.
— Identify your drawbar’s
pull-back force.
— Watch your adapter for
fretting and premature wear
—
Signs of overloading your cutting tool and damaging your spindle and bearings
over time.
Cutting-Tool Requirements: High-quality cutting
tools are needed for all machining operations. They should be properly ground
and finished. The face of the tool should be smooth and the cutting edges sharp
and free of burrs. Milling cutters, drills, and taps should be mounted to run
true. In a multiple-tooth cutter like a mill or a drill, all teeth should cut
the same amount of material.
2.3. Tool Materials For maching Titanium:
Carbide,
cast alloy, and high-speed steel cutting tools are used. Carbide tools require
heavy-duty, amply powered, vibration free machine tools and rigid tool-work
setups to prevent chipping. If these two basic conditions cannot be met, then
high-speed steel tools give better results the choice depends on seven basic
factors including:
·
The condition of the
machine tool
·
The over-all rigidity
situation
·
The type of cut to be
made
·
the amount of metal to
be removed
·
The skill of the
operator.
Carbide Tools: Carbide cutting tools are normally
used for high-production items, extensive metal-removal operations, and scale
removal. The so-called nonferrous or cast iron grades of carbides are used for
titanium. These have been identified as CISC Grades C-I to C-4 inclusive by the
Carbide Industry Standardization Committee.
High-Speed Steel: High-speed steel tools
can be used at low production rates. Tool life is low by ordinary standards.
Both the tungsten and molybdenum types of high-speed steel have been used. The
hot hardness of tungsten high-speed steels results from a reluctance of the
dissolved tungsten carbide in tempered martensite to precipitate and coalesce
at elevated temperatures, a phenomenon which causes softening of hardened
steel. Molybdenum carbides, as found in molybdenum high-speed steel, dissolve
more readily in austenite than do tungsten carbides, and at lower solution temperatures.
However, molybdenum carbides show somewhat greater tendencies to precipitate at
tempering temperatures. Cobalt is often added to both tungsten and molybdenum high-speed
steels to increase their red hardness above 1000 F. Ordinary high-speed steels
become too soft to: Lut effectively
much in excess of this temperature.
·
Cast Alloy: A number of cast-alloy cutting-tool materials
have been developed; these nonferrous alloys contain cobalt, chromium, and
tungsten and are particularly effective in penetrating the hard skin on cast
iron and retaining their cutting ability even when red hot.
2.4. Tool Wear in Milling Hard
materials:
On
general lines, those aspects commented upon above for turning are also valid
for milling. The standard ISO 8688 [7] describes the main wear patterns and
localizations, shown in Figure 2.15.
•
Flank wear (VB): the
loss of particles along the cutting edge, that is, in the intersection of the
clearance and rake faces, being observed and measured on the clearance face of
end milling tools. Three different measurements are possible:
−
Uniform flank wear (VB1):
the mean wear along the axial depth of cut.
−
Non-uniform flank
wear (VB2): irregular wear in several zones of the cutting edge.
−
Localized flank wear (VB3):
wear usually found in specific points. One type is that placed just in the
depth of cut line, the notch wear (VBN), typical of materials susceptible
to mechanical hardening.
• Wear on the rake face (KT): this is located on the
internal flutes of end mills. The most typical is the crater wear (KT1),
a progressive development of a crater oriented parallel to the major cutting
edge.
• Chipping (CH): irregular flaking of the cutting edge, at
random points (see Figures 2.16 and 2.17). It is very difficult to measure and
prevent. It consists of small tool portions breaking away from the cutting edge
due to the mechanical impact and transient thermal stresses due to cycled
heating and cooling in interrupted machining operations.
• Uniform chipping (CH1): small edge breaks of
approximately equal size along the cutting edge engaged on material.
• Non-uniform chipping (CH2): random chipping
located at some points of the cutting edge, but with no consistency from one
edge to another.
• Flaking (FL): loss of tool fragments, especially observed
in the case of coated tools.
• Catastrophic failure (CF): rapid degradation of tool and
breakage.
Mean flank wear size is
the usual tool life criterion, due to it implying a significant variation of
tool dimensions and therefore in the dimension of the machined part. Values of
0.3–0.5 mm are the maximum accepted, the former for finishing and the latter
for roughing. Chipping greater than 0.5 mm is also a tool life criterion. In
low machinability alloys several wear types appear simultaneously, adding and
multiplying their negative effects [8] (see Figure 2.18).
2.5. Parameters considered for maching Titanium:
Cutting Speed: Gutting speed is the most critical
variable affecting metal-removal operations on titanium. Cutting speed has a
pronounced effect on the tool-chip temperature. Since excessive speeds cause
overheating and poor tool life, cutting speeds should be limited to relatively
low levels unless the cutting site is properly cooled. Rotating cutters or
workpieces should be at the desired speed when cutting starts. The increase in cutting speeds of 20-30% which is possible
with carbide tools compared with high speed steel tools does not always
compensate for the additional tool grinding costs.
Feed: All machining operations on titanium
require a positive, uniform feed. The cutting tool should never dwell or ride
in the cut without removing metal. As an added precaution, all cutters should
be retracted when they are returned across the work. Feed rates for milling
titanium are usually limited to the range of 0. 002 to 0.008 inch per tooth
(ipt) to avoid overloading the cutters, fixtures, and milling machine. It is
important to maintain a positive feed. Cutters must not dwell or stop in the
cut. Climb milling is preferred for carbide and cast alloy tools except for scale
removal operations. Conventional milling is more suitable for high-speed steel
tools and for removing scale.
Depth of Cut: The selection of cut depth depends
on setup rigidity, part rigidity, the dimensions and tolerances required, and
the type of milling operation undertaken. For skin-milling 25 operations, light
cuts (0.010 to 0.020 inch) seem
to cause less warping than deeper cuts (0.04 to 0.06 inch). When cleaning up
and sizing extrusions, a 0.05-inch depth is usually allowed. Depths of cut of
up to 0.15 inch can be used, however, if sufficient power is available. When
forging scale is present, the nose of each tooth must be kept below the hard
skin to avoid rapid tool wear.
Cutting Fluids: Cutting fluids are
used on titanium to increase tool life, to improve surface finish, to minimize
welding of titanium to the tool, and to reduce residual stresses in the part.
Soluble oil-water emulsions, water-soluble waxes, and chemical coolants are
usually employed at the higher cutting speeds where cooling is important.
Low-viscosity sulfurized oils, chlorinated oils, and sufochlorinated oils are
used at lower cutting speeds to reduce tool-chip friction and to minimize
welding to the tool.
A low coefficient of friction
is developed by using proper coolant delivery. This results in lower
temperature so the workpiece doesn’t get soft and tool life is extended. Under
pressure and direction, the coolant knocks chips off the cutting edges and
provides anti-corrosive benefits for machine tool and work. There is a high
correlation between the amount of coolant delivered and the metal removal rate.
First, the cutting edge and
workpiece are kept as cool as possible. Second, the cutting edge and workpiece
are also lubricated for a minimum coefficient of friction. Finally, the coolant
stream effectively forces the cut chips away from the cutting edge, thereby
eliminating the possibility of recut chips. Do not use multi-coolant lines. Use
one line with 100% of the flow capacity to evacuate the chips from the work
area.
Use synthetic or
semi-synthetic at proper volume, pressure, and concentration. A 10% to 12%
coolant concentration is mandatory. Through-coolant for spindle and tool can
extend the tool life by four times. Maximize flow to the cutting edges for best
results. At least 3 gal/min (13 liter/min) is recommended, and at least 500 psi
(35 bar) is recommended for through-tool-flow.