Machining Titanium Alloys By Dr. H.E. Trucks

Structural titanium alloys are coming in for increased use because they are light, ductile and have good fatigue and corrosion-resistance properties As a result, more manufacturing engineers are learning that machining these alloys can be a tricky job due to their unique physical and chemical properties. The problems that arise in drilling, turning, and grinding of titanium can be better understood if we look at these properties. They hold the key to successful machining operations. The specific weight of titanium is about two thirds that of steel and about 60 percent higher than that of aluminum. In tensile and sheet stiffness, titanium falls between steel and aluminum. But titanium's strength (80,000 PSI for pure titanium and 150,000 PSI and above for its alloys) is far greater than that of many alloy steels, giving it the highest strength-to-weight ratio of any of today's structural metals.

Thermal properties are another matter. Titanium alloys have high melting points, which is usually a sign of excellent temperature stability. However the strengths of titanium alloys fall off rapidly at temperatures above 800 degrees F, and their coefficients of expansion are even less than that for steels. These unusually poor thermal properties account, to a large extent, for the difficulties in machining titanium.

A CLOSE LOOK

Titanium alloys have a hexagonal closed-packed (HCP) lattice structure similar to magnesium alloys, however, at about 1625 degrees F, titanium undergoes an allotropic transformation, changing from HCP to a body- centered cubic (BCC) structure. These allotropic forms of titanium are known as alpha and beta respectively. Alloying elements favor one or the other For example, a 6-percent aluminum addition stabilizes the alpha phase, resulting in an increase in the alpha + beta and raising the beta transformation temperature to about 1820 degrees F (i25 degrees F) It also increases the metal's elevated temperature strength level Chromium, iron, molybdenum, manganese and vanadium lower the transformation temperature, thereby making the beta phase stable at a lower temperature.

Titanium alloys fall into three classes, depending on the structures present. In addition to the alpha and beta phases described in the preceding paragraph, there is also an alpha-beta phase that includes most of the titanium alloys now in use.

Ti-6Al-4V, an alloy introduced in 1954, comes as close to being a general-purpose grade as possible in titanium. In fact, it's considered the workhorse titanium alloy and is available in all product forms. Its density is 0.160 pound per cubic inch. It can be heat-treated to ultimate strengths in excess of 170,000 PSI and responds to heat-treatment in sections up to 1/2 inches This alloy is stable at temperatures ranging from 423 degrees F to over 1000 degrees F.

T1-6Al-6V-2SN, an extension of the aluminum vanadium-titanium system, is the most highly beta stabilized grade of the alpha-beta the alpha phase and increases the hot-workability range by raising the beta transus temperature to approximately 1735 degrees F. (The beta transus is the temperature level above which the alpha phase in the structure transforms completely into the beta phase in an equilibrium condition . )

The alloying elements used in Tl-6Al-6V-2SN permit heat-treatment of the alloy to high strength levels by solution treatment and aging. Due to the deep hardening capability of this alloy, it is recommended for high-tensile-strength forgings.

KEEP TOOLS SHARP

Titanium has a tendency to gall, and its chips can weld to the cutting edges of the tool. This is particularly so once tool wear begins. Sharp tools should be employed at all times and should be replaced before they dull. The feed should not be stopped while the tool and work piece are in moving contact.

Titanium's low modulus of elasticity can cause slender work pieces to deflect more than comparable pieces of steel. This can create problems of chatter, tool contact and holding tolerances.

The machining characteristics of titanium alloys change significantly at hardness levels of 38 Rockwell (C scale). Above this hardness level machining operations that normally employ high-speed-steel tools such as broaching, drilling, end milling and tapping can present problems. In such cases, carbide tooling may be required. Suggested feeds and speeds for turning, milling, drilling and grinding of titanium and its alloys are provided in the tables on pages 5-6. For turning and milling, speeds and feeds are provided for carbide as well as high-speed-steel tooling.

High -speed steels are widely used for machining titanium because of their flexibility and lower cost than cemented carbides. When it comes to true tool economy, do not equate least expensive tooling with the most economical tooling; often the tooling that costs least to buy ends up being the most expensive on a cost-per-cut basis. For best tool economy, the cutting tool should be matched to the material being machined.

The machinability of materials can best be defined in terms of tool life power requirements and surface finish. Of these factors, tool life is usually the most important. In production operations, tool life is usually expressed as the number of pieces machined per tool grind. In general, the aim of the manufacturing engineer is to achieve the optimum combination of tool life, production rate, power input and surface finish for a given machining operation. This optimum condition results in an increase in production rate and a reduction in the cost of performing the operation. In order to determine the most economical cutting-tool material for given machining operation. An analysis should be made as to the break-even quantity of the cost of the cutting-tool material being evaluated.

In conclusion, while titanium presents a unique set of machining problems, many of those problems can be alleviated or eliminated by adhering to the following set of guidelines

• Use the recommended cutting speeds and feeds
• Use large volumes of recommended cutting fluids.
• Use the abrasion and heat-resisting cutting tools recommended in the tables.
• Replace cutting tools at the first sign of wear.
• Never stop feeding while the cutting tool and work piece are in moving contact.

It should be noted that these recommendations should be used as a guide and may vary slightly with various machines and material input.

(NOTE. Portions of this article have been reprinted from the Machine and Tool Blue Book Vol. 82 NO 1 with permission granted by Dr H. E. Trucks and Machine & Tool Blue Book. )

GRINDING OF TITANIUM

In grinding, the difference between titanium and other metals is the activity of titanium at high temperatures. At the localized points of wheel contact titanium can react chemically with the wheel material. The most important facts to consider in order to prevent this and ensure successful grinding are:

1. Effective use of coolants. Water based soluble oils can be used but, in general, result in poor wheel life. Solutions of vapor-phase rust inhibitors of the nitrite amine type give good results wrth aluminum oxide wheels.

2. Correct wheel speeds. A good guide is to use one half to one-third of conventional operating wheel speeds to get the best result with titanium.

3. Selection of proper wheels. Silicon carbide wheels can be used at 4000-6000 surface feet per minute to give optimum surface finish at minimum wheel wear but the high speeds essential with these wheels produce intense sparking which can cause a fire hazard unless the work is flooded with coolant. However, vitrified bond A60 wheels, hardness J-M have been successfully used at speeds of 1500 to 2000 surface feet per minute while removing as much as 0.08 cubic inches of metal per minute.

JOINING OF TITANIUM

Titanium and titanium alloys can be readily joined by normal mechanical fastener techniques With the exception of brazing and friction welding, these methods are the only satisfactory means of making joints between two nonweldable titanium alloys or between titanium and dissimilar materials.

Fusion, resistance, flash butt, electron beam, diffusion bonding and pressure welding techniques are available and are widely practiced to produce joints in titanium and titanium alloys.

Production of joints by fusion welding is restricted to commercially pure titanium or weldable titanium alloys. The D.C. argon-arc process (electrode negative) is recommended using titanium wire or tungsten electrodes with titanium filler rods.

HINTS FOR MACHINING TITANIUM

Titanium can be fabricated. using techniques which are no more diflicult than those used to machine Type 316 stainless steel. Commercially pure grades of titanium with tensile strengths of 35,000 to 80,000 psi machine fabricate far easier than the aircraft alloys (i.e.) 6Al-4V with tensile strengths up to 200,000 psi

Titanium's work hardening rate is less than austenitic stainless steels, and about equivalent to 0.20 carbon steel. Titanium requires low shearing forces, has an absence of "built-up edge' and is not notch sensitive. Titanium has been classified as difficult to machine due to its physical properties. Heat caused by the cutting action does not dissipate quickly because titanium is a poor heat conductor. Titanium has a strong alloying tendency or chemical reactivity with material in the cutting tools which cause galling welding, smearing and rapid destruction of the cutting tool. Due to its relatively low modulus titanium has a tendency to move away from the cutting tool unless heavy cuts are maintained or proper back-up is employed.

Two other factors influence 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 a very localized portion of the cutting tool.

2. The combination of high bearing forces and heat produces cratering action 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:

• Use low cutting speeds.
• A change of 20 surface feet per minute to 160 surface feet per minute using carbide tools results in a temperature change from 800 to 1700 F.
• 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 should be used.
• Use copious amounts of cutting fluid.
• Use sharp tools and replace them at the first sign of wear. Tool failure occurs quickly after a small initial amount of wear.
• 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

Working with Titanium: Titanium is highly reactive and will react with its environment at relatively low temperatures. When it is heated in air, a self-protective, titanium-oxide film, which is very adherent, will form on its exposed surfaces. In many corrosive environments, the film becomes a barrier and, in the absence of abrasion will decrease the corrosion rate. If titanium is heated in the presence of hydrogen, the titanium readily absorbs the hydrogen. Upon cooling, titanium hydrides form and may seriously impair ductility.

Forming of Titanium: Titanium can be formed into various shapes by bending, shearing, pressing, deepdrawing, expanding, fluid pressure bulging, etc. However, when designing, it is necessary to take into consideration titanium's strong spring-back characteristics. Forming high-yield strength alloy titanium is difficult at room temperature -- a 392 to 752 degrees F temperature range is recommended.

Annealing of Titanium: Residual stress can be removed by annealing the titanium at a temperature between 932 and 1112 degrees F. Full annealing is accomplished at about 1292 degrees F. Heating of narrow or thin items must be done in a vacuum or inert-gas atmosphere. Atmospheric annealing is sufficient for forging, thick plate, etc. However, it must be done in an oxidizing atmosphere. The titanium can be left in the furnace until it reaches room temperature.

Descaling of Titanium: Scales formed during atmospheric annealing (under 1112 degrees F) can easily be eliminated by pickling in a solution of 2% hydrofluoric acid and 20% nitric acid. However scales formed by full annealing under normal atmosphere (greater than 1292 degrees F) are difficult to remove by pickling alone. These thick scales deteriorate corrosion resistant properties and must be removed mechanically or by pickling by the above mentioned mixture of acids. ) (Note: "Hints For Machining Titanium" has been reprinted from OREMET Titanium technical data.)