Article: TOOLING UP FOR TITANIUM

TOOLING UP FOR TITANIUM

END-MILL GEOMETRY ADVANCES HELP MACHINISTS PUSH SURFACE FEET PER MINUTE IN AN UNFORGIVING MATERIAL.

In the not-too-distant past, before carbide became so pervasive, a high-speed-steel tool with a 30-degree helix angle could have been called an “across the board” solution. Today, specialized tools serve specific needs, and carbide has machined what was in the past thought to be unmachinable—titanium included.

Tool-geometry considerations can be boiled down to helix and rake angle of the tooth, designed specifically to cut into a specific material as efficiently as possible. And for titanium, this evolution has resulted in a unique geometry to handle some major challenges. Known for its high strength to weight ratio, titanium on a metallurgical level really can’t be said to be “stronger” than steel. Most titanium falls in the 20 to 30 Rockwell range. (For comparison Inconel 718 is about 45 to 50 Rockwell).

Titanium’s memory makes it a bear in the machining center. With a low elastic modulus, it “remembers” its shape, and when a cutting tool applies pressure, a small amount of that material springs back—hence its “gummy” characteristics, the worry about deflection during machining and the need to make relatively light, fast cuts.

Nevertheless, recent tooling advancements hope to push the depth-of-cut limits in titanium. With the medical and aerospace markets soaring, demand for tooling that can give that productivity edge, however slight, has hit the footlights.

THE GEOMETRY ADVANTAGE

“The evolution of [tool geometries for titanium] has involved higher helix angles—50 and 60 degrees—along with positive rake angles,” says Stephen Jean, milling products manager for Emuge Corp., West Boylston, Mass.

That high helix angle results in good chip evacuation for titanium. A straight, zero-degree flute would take significant bites out of the metal simultaneously, producing among other things a chip traffic jam in the work area. For softer, gummy materials like titanium, the long, thin chips (analogous to stainless) “get a ride” up that flute, which propels the chip, and the heat it contains, away from the work zone. Such an angle means it takes less rotation of the end mill to “eject” the chips away from the cutting zone. “The higher the helix angle,” Jean explains, “the more benefit is created to chip removal and evacuation.”

The positive rake angle makes the cutting edge sharp, cutting through gummy titanium like a knife through butter, to use a rough analogy. Contrast this with hardened steel, which requires a negative rake, closer to perpendicular to the actual cut; this makes the cutting edge less sharp but much stronger, able to bludgeon the brittle material with great force and produce the characteristically small chips.

To push titanium (and similar materials) a bit further, tooling designers have introduced staggered tooth geometries. Here, no longer are flutes set off consistently from each other—for instance, by 90 degrees in a four-flute cutter. Instead, one flute may be set at 87 degrees, another at 93 degrees and so on. This geometry, done primarily with flat-ended end mills, “makes for smooth cutting and eliminates the vibration that comes from repetitive motion,” Jean explains.

Consider a slotting operation using a four-flute cutter, with those flutes offset 90 degrees from one another. Looking from overhead, one tooth on the east side wall would be just coming out of the cut, while the tooth on the west side wall would be just entering the cut. Meanwhile, the tooth to the north would be the only one fully engaged. This pattern would repeat multiple times a second, and it is this repetition, Jean explains, that produces harmonics that result in vibration and chatter.

With a staggered geometry, on the other hand, the “north” tooth would never be fully engaged by itself without other teeth engaged to some degree, “balancing” the motion and reducing the effect of the repetitive motion. Balancing those cutting forces helps reduce the vibration, avoid chatter and assist in pushing depths-of-cuts slightly further.

SUBSTRATE AND COATING

For tooling substrates, there has been the traditional trade-offs between hardness and durability. The harder a material is the more brittle it is. For more durability, you must reduce hardness. Regarding titanium, “you can trade off some of the hardness that you don’t need for cutting material like titanium, and have the result be more durability that allows a higher rake-angle edge to hold up better.”

Fortunately, the new micro-grain and sub-micro-grain geometries available today makes that trade-off between hardness and durability not quite so stark. A micro-grain can boast high durability and relatively high hardness—which makes for a more flexible tool-geometry design, including high rake angles.

Also adding to durability has been coating advances. With end mills TiAlN coating has remained the most popular, and new coating combinations continually appear on the horizon, Jean says, adding that chromium nitride—CrN—exhibits real potential for titanium applications.

GAINING A COMPETITIVE EDGE

The combination of geometry advancements, micro-grains in the substrate and coatings, according to Jean, has pushed the titanium milling from between 150 and 200 surface feet per minute to about 400 sfm—still slow compared to hardened steel, which typically sees 900 sfm or more. But in percentage terms, that represents significant productivity gains.

Jean groups titanium with materials like stainless and Inconels that he calls “the great equalizers. Our company, for instance, recently introduced a tool that just eats through steel. But for titanium, which has a relatively low overall cutting speed, the actual gain is relatively low. You unfortunately can’t change the physical characteristics of the material. It’s harder to stand out.”

More work has swayed away from commercially pure (CP) titanium and toward materials like Ti-6Al-4V, which add a dose of vanadium to make life easier for the machinist—but not that much easier. Within the machining center, spindle, toolholder and workholding, technology has taken rigidity to new levels, and titanium has seen the benefit.

But not as much benefit as many would like. Indeed, make any major productivity improvement in milling titanium represents a big accomplishment. Baby-step increases in production can give any metal manufacturer a major competitive edge.

Editor’s Note: Photo courtesy of Emuge Corp., www.emuge.com.

author: By Tim Heston

Descaling and Surface Treatment of Titanium and its Alloys

Descaling and Surface Treatment

When titanium and its alloys are heated in air, absorption of oxygen and, to a lesser extent, nitrogen, results in the formation of an outer layer of oxide and nitride and an underlying thin layer into which oxygen and nitrogen have diffused. Removal of this hardened metal layer is essential for optimum mechanical properties, and an integral part of any descaling process.

All types of scale can be removed in fused caustic soda, but use of an unmodified bath leads to hydrogen contamination and poor surface quality. The sodium hydride process results in good surfaces and efficient scale removal but, again, hydrogen contamination occurs. Consequently, neither process is suitable for thin sections.

Caustic soda with about 10% oxidizing additions can be used for slightly thicker material, descaling conditions being 20-30 minutes immersion (longer for very heavy scale) at 425°C. Reaction between titanium and any fused caustic soda bath may lead to a dangerous build-up of heat if a stack of thin sheet is descaled. Thin-gauge material should, therefore, be handled in small batches, at a temperature not exceeding 425°C.

Anti-galling Treatments. The tendency for titanium to gall when in sliding contact with itself or with other materials can be reduced by some form of surface treatment. This is particularly desirable for bearing surfaces and for threads of bolts. Both anodizing and `Sulfinuz` treatments reduce the galling tendency, while adherent nickel and chromium deposits provide good wear resistant surfaces. Cadmium plating or the use of anti-galling paints are effective in preventing seizure of bolt threads. Details of electro deposition and anodizing procedures are given in the following paragraphs.

Electrodeposition. Adherent metallic coatings can only be electrodeposited on to titanium if the surface is suitably prepared. A procedure which has been found successful for depositing nickel, chromium, zinc and cadmium on to some titanium alloys uses a pretreatment comprising: (1) Vapour degrease, (2) Hydrochloric acid etch, 5 min in concentrated HCl at 90-110°C, (3) Cold water rinse, (4) Nickel strike for 3 min, (5) Cold water rinse.

Anodizing. Surface properties of titanium and its alloys can be modified by anodic oxidation treatment, which covers the entire surface with a thin but compact oxide film. Almost any aqueous solution can be used, but immersion in a solution of 80% phosphoric acid, 10% sulphuric acid and 10% water gives a particularly coherent film. A potential increasing from 0 to 110 volts over ten minutes should be applied.

Anodized titanium has no affinity for dyestuffs, but the film itself shows interference colors, determined by the final anodizing potential.

Machining and Grinding Titanium and Titanium Alloys

Machining and Grinding

Titanium and its alloys can be machined successfully on conventional machine tools provided that certain requirements are satisfied. In all machining operations rigidity of both work piece and cutting tool is desirable. Best results will be obtained if the cutting tools have a good surface finish. If the cutting tools are in good condition, it is no more difficult to machine titanium than an alloy steel of equivalent strength.

Titanium has a tendency to gall or smear on to other metals. Sliding contact between the work piece and its support should be avoided, and the use of roller steadies and running centres is recommended.

Turning. In general, cutting speeds should be low and feeds as coarse as practicable. A good surface finish can be obtained with very coarse feeds by using suitably shaped tools with a large nose radius. This will, however, be limited by work piece rigidity as a large nose radius causes increased tool loads and work piece deflection. Due to the lower elastic modulus of titanium, these deflections are greater than would occur on steel workplaces.

Tool materials may be high-speed steel, cast alloy, or tungsten carbide. The “super” grades of high-speed steel are satisfactory, giving good results in turning where large feeds can be employed, and particularly where the surface is rough or the cut intermittent. Tungsten carbide may be necessary for heavy work on certain harder alloys or for intermittent cutting, but in general its use is confined to lighter, more continuous cuts. For economic use of carbide tools it is essential to regrind before wear becomes excessive, and mechanically clamped tips are an obvious advantage.

Threading. Single-point screw cutting is preferable to threading with a die. Conventional methods of screw-cutting can be used, but success can also be achieved when increments of cut of 0.25-0.50 mm are applied at right angles to the axis of the component. Cuts of less than 0.13 mm should be avoided. Machine tapping with cutting speeds up to 6 m/min is preferable to hand manipulation. Tapping of full threads should be avoided: a thread of 80% depth is much easier to tap and loses little strength.

Planing. Shaping and planing of titanium are not difficult, provided that the foregoing requirements of rigidity, speed and feed are satisfied. Tungsten carbide tools with a large radius, producing a broad and relatively thin chip, are most successful. As in all cutting operations, it is essential to use sharp tools and replace them before appreciable wear occurs. For planing, clamped circular buttons of tungsten carbide have obvious advantages.

Milling. In milling, the chief problem arises from chips welding on to the teeth, resulting in cutter chipping and breakage. This is minimized with climb milling, in which the tooth finishes its cutting stroke when moving parallel to the feed. Absolute rigidity is necessary to avoid chatter, but the chip is only attached to the tooth by a thin sliver which is easily broken off.

Drilling. Titanium may be drilled with short high-speed-steel drills; the holes should be as shallow as possible. A 140o point is best for sizes below 6-5 mm and a 90° or double-angle point for larger sizes. For holes of a depth greater than five diameters, it is helpful to retract the drill at intervals and clear the swarf. Flood lubrication with a heavily chlorinated cutting oil reduces frictional troubles.

Grinding. A reduction in wheel speed to a half or a third of the conventional speed, together with the use of a suitable coolant, will usually achieve an acceptable grinding ratio. Water-base soluble oils result in poor wheel life, but some chlorinated or sulphurised grinding oils, and solutions of vapour-phase rust inhibitors of the nitrite-amine type, are satisfactory.

Polishing. Titanium can be mechanically polished by techniques similar to those used for stainless steel; reductions in wheel or mop speeds are often beneficial. If a high polish is required, light pressures are necessary during the final operations. Good results have been obtained with a canvas wheel coated with 240E1 `Alundum` grit, which can be blended with stearic acid for a finer finish.

Heat Treating Titanium

Heat Treatment

With heating in conventional furnaces there is always some surface contamination and a risk of hydrogen absorption. Vacuum treatment, though ideal, is rarely practicable, so it is customary to use ordinary electric furnaces; hydrogen pick-up is not usually excessive. Fuel-fired furnaces should be avoided if at all possible; titanium rapidly absorbs free or combined hydrogen from the surrounding atmosphere, and this can be serious, particularly with thin sections.

Superficial hardening by oxygen diffusion is almost inevitable at the higher annealing and preheating temperatures suggested for some titanium alloys. The hardening effect is insignificant at low annealing temperatures but above 600°C may lead to surface embrittlement. Both the oxide film and the underlying oxygen-rich layer should therefore be removed by one of the methods of surface treatment; this is particularly important for high-strength alloys.

Forming Titanium

Forming

Annealed and solution treated sheet can be pressed, stretch-formed, spun and dimpled, but maximum deformation depends upon the load being applied slowly. Good results are achieved with hydraulic presses, the rubber-pad method being useful for forming light-gauge parts. Drop hammer forming, with heated blanks, is widely used for sheet metal parts of complex contour. Punch presses, which should be slowed down to half or one-third their normal speed, can also be used.

Blanks may be prepared for forming by shearing, sawing, nibbling or blanking, using slow cutting speeds. Edge condition is important, and edge cracking may be minimized by keeping the guillotine blade sharp and close fitting or by heating metal before shearing. All burrs must be removed and, for more difficult forming operations, cut edges may need filing or polishing.

Simple shapes can be formed at room temperature, deformation being limited by the strength and springiness of the material. Solid lubricants such as soap, molybdenum disulphide or graphite are preferred to mineral oils and greases. ICI “Trilac” coating and polythene sheeting have been found to effect considerable improvement in difficult pressing operations.

For more complicated designs, the work piece and, where possible, the dies should be heated to facilitate forming. The use of heat in forming increases ductility, which is reflected in lower minimum bend radii and reduces both the load required to effect deformation and subsequent spring-back, thus ensuring greater accuracy.

Furthermore, at elevated temperatures, the spread between yield and ultimate strengths is increased, which also aids forming. The temperature to select for hot forming depends upon the alloy and the severity of the shape to be produced. Good results can be expected using temperatures of about 200-300oC for commercially pure titanium and IMI Titanium 230, and 550-650oC for IMI Titanium 317 and 318.