TITANIUM MATERIALS

The Strategies for Machining Titanium.

As more job shops gravitate toward higher-end work, they must deal with the challenge of cutting titanium a hard-to-machine metal. Here are several ways to boost productivity:
As they take on higher-end work, more and more U.S. job shops will have to master how to mill titanium, a lightweight metal that’s hard to machine. That’s because high-end parts, such as aircraft components and medical devices, are often made of this corrosion-resistant material. In fact, titanium and its alloys have already claimed a wide range of aerospace, industrial, marine and commercial applications. In addition, learning how to handle titanium is important because it provides insight into how shops can boost productivity without having to increase cutting speed.

Raising cutting speed is a big no-no when milling titanium because of two reasons. First, even a small increase in cutting speed can significantly exacerbate edge wear. And second, it can cause heat to build up quickly because of the metal’s low thermal conductivity. In fact, excessively fast milling may even result in combustion.

But rest assured, says cutting tool supplier Kennametal, you can still increase the speed of production without boosting cutting speed. To increase your metal removal rate while keeping the cutting speed steady, Brian Hoefler, the company’s product manager for milling, recommends selecting tooling with two important traits. First, it must be able to fully utilize the power of the current machine, and second, it must be able to offset any limitations the machine may have in terms of rigidity.

To choose the right tool, the first thing you must do is to consider the cutting tool material, says Hoefler. Carbide often a shop’s go-to material when it comes to difficult jobs is not necessarily the best choice. Newer generation high-speed steel can be a more suitable selection. That’s because carbide’s superior wear resistance comes at the cost of bulk toughness. In other words, carbide is not very good at resisting fracturing and chipping both of which can result in tool failure in titanium milling. A tougher tool such as one made of high-speed steel can allow deeper cuts to be taken without the edges chipping. This more tolerant tool material especially on a less rigid machine tool will enable a shop to reach a higher metal removal rate through cut depth as opposed to cutting speed.

But carbide should not be ruled out entirely in milling titanium. It can be used for low-radial-immersion cuts, for example, in which cuts have a relatively light depth to control heat. In such applications, Hoefler recommends using a coated carbide tool. In particular, a carbide tool coated with titanium aluminum nitride (TiAlN) is effective because it excels in maintaining its integrity and properties as the temperature in the cut rises. Heat actually activates its protective mechanism; the energy produced during machining frees the aluminum, which aids in the formation of a protective layer of aluminum oxide. Coated carbide tools could also be used when making heavier cuts. In such cases, a stronger coating such as titanium carbo-nitride (TiCN) can be utilized. This coating can resist micro-chipping.

Another effective strategy in milling titanium, says Hoefler, is increasing the number of effective edges to boost the metal removal rate. You can do this by selecting tools with very fine pitch or trying an approach called “plunge roughing,” in which a shell mill or another appropriate milling tool, is fed into the work vertically.
Additionally, job shops can also increase the metal removal rate by minimizing chatter. This can be accomplished in three ways, says Hoefler. First, you have to make sure that both the interface between the tool and the toolholder and that between the toolholder and the spindle are kept as stiff as possible. Second, you should consider a tool with an eccentric relief or a “margin.” This can provide process damping, which prevents chatter. And third, you can space cutting edges unevenly so they cannot hit the work with uniform frequency, thereby warding off chatter.

In short, with the correct tooling and a sound approach, milling a hard-to-machine metal such as titanium can be accomplished productively and cost-effectively.

Milling Titanium Alloys.

Milling titanium is like milling other hard-to-machine metals in that a small increase in cutting speed can lead to a big increase in edge wear.

Milling titanium is different from other metals because of the risk of heat build-up. Thanks to the metal’s low thermal conductivity, overly aggressive milling may even pose a risk of combustion. With titanium, in other words, there may be more than one reason why the cutting speed can’t be increased.

And yet the speed of production still can be increased. A shop milling titanium can raise its metal removal rate even while the cutting speed stays constant. Accomplishing this does not have to involve a more powerful or higher-end machine tool, but it does require tooling that can take advantage of the power of the existing machine. It also requires tooling that can compensate for any shortcomings the machine may have when it comes to rigidity.

One company that has studied titanium milling is cutting tool supplier Kennametal. And one advisor in this company who has consulted on many titanium milling applications is Brian Hoefler, product manager for milling. This article is based on his experience and recommendations.

Why is milling titanium worth the attention? There are at least two reasons. First, the material is used for high-end parts–not just components used in an aircraft’s frame and engine, but also medical parts, for example. Shops able to thrive in the United States increasingly will migrate toward higher-end work, meaning that a growing percentage of U.S. shops will encounter this material.

That’s one reason. An other, broader reason for covering titanium milling relates to the procedures for machining effectively when the material is difficult to cut or the available speed is low. Not every shop has access to high spindle speeds and feed rates. What do you do to achieve higher productivity when raising the cutting speed is not option?

Weigh Wear Resistance Against Toughness:

The fundamental choice of cutting tool material should be the first consideration, Mr. Hoefler says. Carbide might be the right choice. But shops are often so accustomed to viewing carbide as the superior cutting tool material that they routinely choose it for all difficult jobs. With titanium, newer generation high speed steel can be the better alternative.

The wear resistance that allows carbide to reach a high cutting speed comes at a price. That price is paid in “bulk toughness,” or the ability of the tool to resist fracturing and chipping. Carbide in general is more brittle than high speed steel.

This is significant in titanium milling, because it is generally not edge wear that causes the tool to fail in this material. Rather, it’s chipping or breakage that leads to failure. In addition, heat build-up may make it impossible to take advantage of the cutting speed that carbide makes available. These two factors both suggest that the trade-off in toughness may not be worth making. A tougher tool–a high speed steel tool, that is–can take a deeper cut without fear that shocks will cause the edges to chip. Particularly on a less rigid machine tool, this more forgiving tool material can let the shop realize a higher metal removal rate through depth of cut instead of through speed.

But oven this material presents a range of choices, Too few shops realize that there is more than one kind of high speed steel. While commodity high speed steel tools are made through a process that involves heat treatment, the alternative–powder metallurgy tooling–can be manufactured so that the steel has a more uniform structure with more closely controlled properties. Powder metallurgy tools are more expensive, but they generally offer better performance.

Heat Resistance

Sometimes carbide is needed. Low-radial-immersion cuts, for example, can allow a surprisingly high speed (see shaded box at left). In cuts such as these, it’s not just wear resistance but wear resistance at high temperatures that’s important. That requirement suggests a coated carbide tool.

Mr. Hoefler says titanium aluminum nitride (TiA1N) coated carbide is usually the best choice for machining titanium. Out of the handful of basic cutting tool coating types, TiA1N is clearly the best at maintaining its integrity and properties as the temperature in the cut gets hot. In fact, heat actually drives this coating’s protection. Aluminum that is liberated from the coating through the energy of machining helps to form a protective layer of aluminum oxide. This layer reduces both thermal transfer and chemical diffusion between the tool and the workpiece. Coatings coming soon add even more aluminum to encourage this reaction. (See the shaded box below.)
When TiA1N is not the right choice, the reason why relates to vibration. Titanium carbo-nitride (TiCN) is a stronger coating that offers better resistance to micro-chipping. “When you’re using an indexable insert, and you’re taking a heavier cut on a less rigid machine, try TiCN–this may be the better choice,” Mr. Hoefler says.

Number Of Effective Edges:

Even when the speed, the chip load and the depths of cut are all fixed, productivity can still be improved. To raise the metal removal rate, increase the number of effective edges.

On a helical mill, for example, choose the tool with the finest pitch possible. (A corncob tool may also work.) Counting edges in this way creates one more reason to consider high speed steel, because high speed steel generally can offer more cutting edges than a comparable tool that uses carbide.

Another way to achieve a higher number of effective edges is to take milling in a different direction. Through “plunge roughing,” a shell mill, or another suitable milling tool, is fed into the work along the Z axis as if it were a drill. The parallel plunges are programmed to overlap, so the cutter is never completely surrounded by material and the chips have room to escape.

This approach can only be used for roughing, because the adjacent passes leave scallops between them that have to be milled away later. But because plunge roughing engages a larger number of the tool’s cutting edges, the feed rate in inches per minute can be increased while the chip load remains constant. Feeding in Z also takes advantage of the machine’s stiffness, because the various connections along the spindle that would tend to deflect along X or Y (such as the toolholder interface) are compressed in the Z direction. Along Z, the machine is more rigid. That means it may be possible even to increase the chip load.

Mr. Hoefler says, “Plunge roughing can be a very productive approach to material removal in high-strength metals. I don’t think enough shops today are taking advantage of this.”

Vibration Elimination:

Potential for deflection is also important because of another, more serious problem–chatter. Where chatter is concerned, milling titanium seems to offer the worst of both worlds. On the one hand, high forces are involved, making significant chatter more likely. On the other hand, high spindle speeds generally are not involved, making it impossible to find some “sweet spot” rpm value that can tune the chatter away.

Chatter, in fact, will decide the productivity of most titanium milling applications. The maximum achievable metal removal rate will occur not at the point where the horsepower is maxed out, but at the point where significant chatter begins. That’s why it’s important to construct the process so that it impedes chatter as much as possible.
Mr. Hoefler suggests any and all of these considerations:

1. Stiffness. The interface between the tool and the toolholder, and the interface between the toolholder and the spindle, both need to be made as stiff as possible. For the tool interface, shrink fit offers a solution. For the spindle, an HSK interface can offer better stiffness than a conventional conical taper.

2. Damping. A tool with an eccentric relief, or a “margin,” can offer process damping that wards off chatter. When the tool deflects, this eccentric relief comes in contact with the workpiece and rubs. Not all materials take well to the rubbing; aluminum tends to adhere. But in titanium, the margin can make for an effective shock absorber.

3. Variable cutting edge spacing. This is an approach to tool design and chatter prevention that many shops may be unfamiliar with. Chatter results from the oscillation caused by the cutting edges hitting the work with a regular frequency. Some milling cutters use unequally spaced flutes to disrupt this regularity. Two cutting edges may be 72 degrees apart, while the distance to the next is 68 degrees and the distance to the edge after that is 75 degrees. The irregular spacing aims to avoid chatter by preventing a steady frequency from taking hold. Another option, patented by Kennametal, exploits a varying axial rake angle to achieve a similar vibration-disturbing effect.

10 Percent Radial Depth? Double The Speed

Shops cutting titanium are familiar with the practice of using low radial immersion to control heat. In a low-radial-immersion pass, the radial depth of cut is much less than the radius of the tool. As a result, every cutting edge spends more time out of the cut than in it, giving each edge relatively little time to heat up and much longer to cool down.

This practice works so well at controlling heat, says Kennametal’s Brian Hoefler, that many users fail to realize how much extra speed they may be able to realize. The light depth of cut precludes a high metal removal rate, but the shop making finishing passes using this method can partially compensate by leaving the recommended speeds behind.

Mr. Hoefler suggests these rules of thumb:

* When radial depth is less than 25 percent of diameter, increase the sfm by 50 percent (over the nominal speed used for heavier cuts).

* When radial depth is less than 10 percent of diameter, increase the sfm by 100 percent.

Coming Soon: High-Aluminum Coating

The “A1” in TiA1N is where much of the effectiveness of this tool coating comes from. The aluminum in the coating helps to form a protective layer of aluminum oxide. More aluminum in the coating would make this mechanism even more effective.

Now, thanks to improved evaporation techniques used in manufacturing the coating, TiA1N with higher aluminum content will soon be available. The coating offers better hot hardness than previous versions of TiA1N without compromising toughness, Mr. Hoefler says. Kennametal hopes to introduce the new TiA1N during the first half of this year.

General Machining Tips: Many of titanium’s material and component design characteristics make it expensive to machine. A considerable amount of stock must be removed from primary forms such as forgings, plates, bars, etc. In some instance, as much as 50 to 90% of the primary form’s weight ends up as chips. (The complexity of some finished parts, such as bulkhead, makes difficult the use of near-net-shape methods that would minimize chip forming.) Maximum machining efficiency for titanium alloys is required to minimize the costs of stock removal.
Historically, titanium has been perceived as a material that is difficult to machine. Due to titanium’s growing acceptance in many industries, along with the experience gained by progressive fabricators, a broad base of titanium machining knowledge now exists. Manufacturers now know that, with proper procedures, titanium can be fabricated using techniques no more difficult than those used for machining 316 stainless steel.

Stories about problems encountered when machining titanium have usually originated in shops working with aircraft alloys. The fact is that commercially pure grades of titanium (ASTM B, Grades 1, 2, 3, and 4) with tensile strengths of 241 to 552 MPa (35 to 80 ksi) machine much easier than aircraft alloys (i.e. ASTM B, Grade 5: Ti-6AL-4V).

With higher alloy content and hardness, the machinability of titanium alloys by traditional chip-making methods generally decreases. (This is true of most other metals.) At a hardness level over 38 RC (350 BHN) increased difficulty in operations such as drilling tapping, milling, and broaching can be expected. In general, however, if the particular characteristics of titanium are taken into account, the machining of titanium and its alloys should not present undue problems.

Machining of titanium alloys requires cutting forces only slightly higher than those needed to machine steels, but these alloys have metallurgical characteristics that make them somewhat more difficult to machine than steels of equivalent hardness. The beta alloys are the most difficult titanium alloys to machine. When machining conditions are selected properly for a specific alloy composition and processing sequence, reasonable production rates of machining can be achieved at acceptable cost levels.

Care must be exercised to avoid loss of surface integrity, especially during grinding; otherwise a dramatic loss in mechanical behavior such as fatigue can result. To date, techniques such as high-speed machining have not improved the machinability of titanium. A breakthrough appears to require the development of new tool materials.

The fact that titanium sometimes is classified as difficult to machine by traditional methods in part can be explained by the physical, chemical, and mechanical properties of the metal. For example:

Titanium is a poor conductor of heat. Heat, generated by the cutting action, does not dissipate quickly. Therefore, most of the heat is concentrated on the cutting edge and the tool face.

Titanium has a strong alloying tendency or chemical reactivity with materials in the cutting tools at tool operating temperatures. This causes galling, welding, and smearing along with rapid destruction of the cutting tool.

Titanium has a relatively low modulus of elasticity, thereby having more “springiness” than steel. Work has a tendency to move away from the cutting tool unless heavy cuts are maintained or proper backup is employed. Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing, and tolerance problems. Rigidity of the entire system is consequently very important, as is the use of sharp, properly shaped cutting tools.

Titanium’s fatigue properties are strongly influenced by a tendency to surface damage if certain machining techniques are used. Care must be exercised to avoid the loss of surface integrity, especially during grinding. (This characteristic is described in greater detail below.)

Titanium’s work-hardening characteristics are such that titanium alloys demonstrate a complete absence of “built-up edge.” 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. Furthermore, the combination of high bearing forces and heat produces cratering action close to the cutting edge, resulting in rapid tool breakdown.

With respect to titanium’s fatigue properties, briefly noted in the above list, the following details are of interest.

As stated, loss of surface integrity must be avoided. If this precaution is not observed, a dramatic loss of mechanical behavior (such as fatigue) can result. Even proper grinding practices using conventional parameters (wheel speed, downfeed, etc.) may result in appreciably lower fatigue strength due to surface damage. The basic fatigue properties of many titanium alloys rely on a favorable compressive surface stress induced by tool action during machining. Electromechanical removal of material, producing a stress-free surface, can cause a debit from the customary design fatigue strength properties. (These results are similar when mechanical processes such as grinding are involved, although the reasons are different.)

General

The term “machining” has broad application and refers to all types of metal removal and cutting processes. These include turning, boring, milling, drilling, reaming, tapping, both sawing and gas cutting, broaching, planing, gear hobbing, shaping, shaving, and grinding.

The technology supporting the machining of titanium alloys basically is very similar to that for other alloy materials. Efficient metal machining requires access to data relating the machining parameters of a cutting tool to the work material for the given operation. The important parameters include:

• Tool life
• Forces
• Power requirements
• Cutting tools and fluids

Subsequent paragraphs discuss these parameters in general terms.

Tool Life

Tool-life data have been developed experimentally for a wide variety of titanium alloys. A common way of representing such data is Showing tool life (as time) against cutting speed (fpm) for a given cutting tool material at a constant feed and depth in relation to Ti-6Al-4V. It can be seen that at a high cutting speed, tool life is extremely short. As the cutting speed decreases, tool life dramatically increases.

Titanium alloys are very sensitive to changes in feed and speed. Machining community generally operates machine tools at cutting speeds & feeds which are provided by tool manufacturers with focus to longer tool life. Curve fitting of tool life to feed, speed, and other machining parameters is commonly being done by means of computer techniques. However, in cases where no data base exists, certain rules of thumb should be recognized. For example, when cutting titanium, a high shear angle is produced between the workpiece and chip, resulting in a thin chip flowing at high velocity over the tool face. High temperatures develop, and, since titanium has low thermal conductivity, the chips have a tendency to gall and weld themselves to the tool cutting edges. This speeds up tool wear and failure. When dealing with high-fixed-cost machine tools production output may be much more important than a cutting tool’s life! It thus may be wise to work a tool at its maximum capacity, and then replace it as soon as its cutting efficiency starts to drop off noticeably, ereby maintaining uptime as much as possible.

When machining titanium in circumstances in which production costs are not of paramount concern, it is still unsound practice to allow tools to run to destruction. The other extreme, premature tool changing, may result in a low number of pieces per tool grind, but the lower the tool wear, the less expensive the regrinding.

Ideally, a tool should be permitted to continue cutting as long as possible without risking damage to the tool or the work but with the retention of surface integrity. The only way to find a safe stopping point is to check a few runs by counting the pieces produced and inspecting the surface finish, dimensions, and surface integrity. In this manner it can be established how many acceptable pieces can be produced before the tool fails.