Cool Tips for Cutting Titanium

8/7/2019 Cool Tips for Cutting Titanium

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Because of its high strength andlight weight, titanium is a favorite

with aircraft engine designers.(Photo courtesy Pratt & Whitney.)

Cool Tips for Cutting TitaniumJames Benes12/21/2007

It is not difficult to understand why automotive, medical,chemical, micro-component and, especially, aerospacedesigners love titanium. Its density is only about half of steel,so titanium parts weigh roughly half as much as steel parts.

But its high strength 80,000 psi for pure titanium and180,000 psi-plus for its alloys is far greater than the strengthof many alloy steels giving it an extremely high strength-to-

weight ratio.

Titanium has twice the elasticity of steel, mak ing it an idealchoice for applications that require flexible materials that don’tcrack or rupture. Also, titanium alloys resist corrosion andoxidation better than stainless steels.

Many of the same qualities that enhance titanium’s appeal formost applications also contribute to its being one of the mostdifficult to machine materials.

However, shops that understand this material’s peculiaritiescan machine them successfully and cost-effectively.

Most titanium alloys are poor thermal conductors. Heat generated during cutting doesn’t dissipatethrough the part and machine structure, but concentrates in the cutting area. The hightemperatures that can be reached 2,000°F in some cases can lead to cutting edge chipping

and deformation, and dull edges on tools generate even more heat and further reduce tool life.Cutting temperatures can get so high that titanium chips sometimes burst into flames.

The high temperature generated during the cutting process also causes a work hardeningphenomenon that affects the surface integrity of titanium, and could lead to geometric inaccuraciesin the part and severe reduction in its fatigue strength.

Titanium alloys’ elasticity, which is beneficial and desirable for finished parts, encourages deflectionand vibration during heavy machining cuts. Under cutting pressures, the “springy” material movesaway from the tool. C onsequently, the cutting edges rub rather than cut, particularly in light cuts.This rubbing process also generates heat, compounding problems caused by the material’s poorthermal conductivity.

Machining a thin-wall part or ring common operations withanything but a positive-rake tool will push and deflect the partrather than cut it. This makes it difficult to cut to size. Insteadof cutting the part, the wrong tool pushes it, straining the

material. As the material moves away from the cutting edge itdeforms plastically, instead of elastically, and that increasesthe material’s strength and its hardness at the point of cut. Asthe alloy gets harder and stronger, cutting speeds that wereappropriate at the start of the cut become excessive, and thetool wears dramatically.

The alloy the workpiece is made from determines the cuttingspeed needed to cut it. Unalloyed titanium can be machined atspeeds to 180 sfm, while tougher beta alloys require speeds aslow as 30 sfm. In general, the more vanadium and chromiumin a particular alloy, the lower the cutting speed that is calledfor. In all cases, titanium alloys demand heavy chip loads toovercome the problem of rubbing and the work-hardening thatresults.

The magnitude of cutting forces generated when machining

Tools:

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The 4E turning geometry from ATIStellram, that has a micrograincarbide substrate and a superhard Nano TiAlN PVD coating,

provides positive cutting action toreduce built-up edge in high-

temperature titanium machining.

A complicated-shaped jet enginediffuser required ultistage

groove-turn operations thatremoved a large amount of

material from solid-ring shapedworkpiece. Photo courtesy of Iscar

Metals Inc.

titanium is only slightly higher than those developed whencutting steels with an equivalent hardness, even thoughmachining titanium appears to be more difficult and complex.

Flank wear, notching and built-up edge are the common typesof tool wear when cutting titanium. Edge notching appears as alocalized abrasive wear on both the flank and rake face, alongthe line corresponding with the depth-of-cut parameter. Thiswear is caused partially by the presence of a hardened layerthat typically is formed by previous casting, forging, heattreating, or prior machining operations.

Chemical reaction between the cutting tool material and the

workpiece a lso could lead to a notching-wear mechanism. Thisoccurs when machining temperatures exceed 800° C., andinduce diffusion between the tool and the workpiece.

In contrast, during the machining process, deposits of titaniumwork materials tend to accumulate on the rake face of the

insert. The high pressure developed in this area can weld these particles to the cutting edge,forming a built-up edge phenomenon. These particles, over successive ly shorter intervals, areinclined to peel off the cutting edge, pulling some carbide content from the cutting insert away withit.

The best tool substrate and coating for machining titanium alloys and super alloys is a submicronsubstrate that is combined with a physical vapor deposition (PVD) TiAlN coating. The thin, smoothsurface of the PVD coating, together with sufficient residual stress, enhances tool resistance tochipping and notching wear, so PVD coatings provide enhanced wear resistance, chemical stabilityand resistance to built-up edge. Machining problems that were seen in the past that arose fromearlier coatings, no longer exist with PVD coatings because of the improved adhesion techniques

and the uniformity of the coatings.Titanium and its alloys Titanium alloys are available in four varieties: alpha, alpha/ beta, beta and the newer titaniumaluminide. Because more alloying elements are being added to the particular grades, these alloysare progressively more difficult to machine.

The Alpha phase of titanium is pure titanium, relatively soft and can be machined at high speeds.

This material presents no significant machining problems.However, the material lacks the beneficial properties of theother alloys, primarily strength and flexibility, so its uses arelimited.

Alpha/beta alloys are the most common titanium alloys, and Ti-6A1-4V (6% aluminum, 4% vanadium) is used extensively inthe aerospace industry, particularly for jet engines. Ti-6A-4V is

used to a lesser extent in the medical and chemical industries.These alloys are moderately difficult to machine, and relativelyshort tool life can be a problem because alpha/beta chips aredifficult to break and are abrasive.

Beta phase titanium alloys do not have the toughness of thealpha/betas, but they are harder and more brittle. They alsoare more difficult to machine because of the higherpercentages of vanadium, molybdenum and chromium withwhich they are made. Beta phase alloys of titanium arebecoming more common, and present serious machiningchallenges.

Titanium aluminides are very difficult to machine, but they areextremely lightweight and strong. Earlier, a lack of toughnesslimited their application. However, material science research

has addressed their lack of toughness, and applications are beginning to be developed in autoracing engines, where they are used for push rods and valve stems, and in components for jetengines.

A strategy for success in cutting Ti

• Use positive cutting geometries to minimize cutting forces, heat generation and part deflection.• Use constant feed to prevent work hardening of the workpiece. Never stop feeding while the toolis in the cut.• Use large volumes of coolant to preserve thermal stability and to prevent temperature build-upthat can lead to subsurface irregularities and possible tool failure.•Keep tools sharp. Dull tools accentuate heat build-up, and cause galling and seizing that lead totool failure.• Machine titanium alloys in the softest state possible. Because many alloys are age hardenable –they get harder when heat is applied – they become stronger and more abrasive as second-phase particles form.





• Use a large tool nose radius or round inserts whenever possible to put more of the tool into thecut. This decreases the cutting force at any one point and prevents localized damage.

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Cool Tips for Cutting Titanium