Titanium can be economically machined on a routine production basis if shop procedures are set up to allow for the physical characteristics common to the metal. The factors which must be given consideration are not complex, but they are vital to successfully machining titanium.The different grades of titanium, i.e., commercially pure and various alloys, do not have identical machining characteristics, any more than all steels, or all aluminum grades have identical characteristics. Like stainless steel, the low thermal conductivity of titanium inhibits dissipation of heat within the workpiece itself, thus requiring proper application of coolants.

 

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 workpiece to counter workpiece 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. Specific information on machining, grinding and cutting titanium.

 

Commercially pure and alloyed titanium can be turned with little difficulty. Carbide tools should be used wherever possible for turning and boring since they offer higher production rates and longer tool life. Where high speed steels are used, the super high speeds are recommended. Tool deflection should be avoided and a heavy and constant stream of cutting fluid applied at the cutting surface. Live centers must be used since titanium will seize on a dead center. 

 

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. 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. 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.

 

Successful drilling is accomplished by using sharp drills of proper geometry and by maintaining maximum drilling force to ensure continuous cutting. It is important to avoid having the drill ride the titanium surface since the resultant work hardening makes it difficult to reestablish the cut.

 

Another important factor in drilling titanium is the length of the unsupported section of the drill. This portion of the drill should be no longer than necessary to drill the required depth of hole and still allow the chips to flow unhampered through the flutes and out of the hole. This permits application of maximum cutting pressure, as well as rapid drill removal to clear chips and drill re-engagement without breakage. An adequate supply of cutting fluid to the cutting zone is also important. High speed steel drills are satisfactory for lower hardness alloys and for commercially pure titanium but carbide drills are best for most titanium alloys and for deep hole drilling.

 

Percentage depth of thread has a definite influence on success in tapping titanium and best results in terms of tool life have been obtained with a 65% thread. Chip removal is a problem which makes tapping one of the more difficult machining operations. However, in tapping through-holes, this problem can be simplified by using a gun-type tap with which chips are pushed ahead of the tap. Another problem is the smear of titanium on the land of the tap, which can result in the tap freezing, or binding in the hole. An activated cutting oil such as a sulfurized and chlorinated oil is helpful in avoiding this problem.

 

Titanium is successfully ground by selecting the proper combination of grinding fluid, abrasive wheel, and wheel speeds. Both aluminum oxide and silicon carbide wheels are used. Considerably lower wheel speeds than in conventional grinding of steels are recommended. Feeds should be light and particular attention paid to the coolant. A water-sodium nitrite coolant mixture gives good results with aluminum oxide wheels. Silicon carbide wheels operate best with sulfochlorinated oils, but these can present a fire hazard, and it is important to flood the work when using these oilbase coolants.

 

Two common methods of sawing titanium are band sawing and power hacksawing. As with titanium machining operations, standard practices for sawing titanium are established. Rigid, high quality equipment should be used incorporating automatic, positive feeding. High speed steel blades are effective but for specific blade recommendations and cutting practices the blade manufacturers should be consulted. Cutting fluids are required. Abrasive sawing is also commonly employed with titanium. Rubber bonded silicon carbide cutoff wheels are successfully used with water-base coolants flooding the cutting area.

 

Water jet cutting is a recent innovation in cutting titanium. A high speed jet containing entrained abrasive is very effective for high cutting speeds and for producing smooth burr-free edges. Sections up to three inches have been cut and the process is relatively unaffected by differences in hardness of the titanium workpiece. Electric Discharge Machining Though not common, complex titanium components with fine detail can be produced via EDM. The dielectric fluid often consists of various hydrocarbons (various oils) and even polar compounds, such as deionized water. Care must be taken to avoid or remove any subtle surface contamination in fatigue sensitive components.

 

Chem milling has been used extensively to shape, machine or blank fairly complex titanium components, especially for aerospace applications (e.g., jet engine housings). These aqueous etching solutions typically consist of HNO3-HF or dilute HF acids, with the HNO3 content adjusted to minimize hydrogen absorption depending on the specific alloy.

 

Titanium and its alloys can be cold and hot formed on standard equipment using techniques similar to those of stainless steels. However, titanium possesses certain unique characteristics that affect formability, and these must be considered when undertaking titanium forming operations. The room temperature ductility of titanium and its alloys is generally less than that of the common structural metals including stainless steels. This necessitates more generous bend radii and less allowance for stretch formability when cold forming. Titanium has a relatively low modulus of elasticity, about half that of stainless steel. 

This results in greater springback during forming and requires compensation either during bending or in post-bend treatment.Titanium in contact with itself or other metals exhibits a greater tendency to gall than does stainless steel. Thus, sliding contact with tooling surfaces during forming calls for the use of lubricants. Effective lubricants generally include grease, heavy oil and/or waxy types, which may contain graphite or moly disulfide additives for cold forming; and solid film lubricants (graphite, moly disulfide) or glassy coatings for higher temperature forming.The following is basic information on forming titanium. A great deal of published information exists on titanium forming practices in the common commercial forming processes.The reader is urged to consult the references in the back of this booklet and other qualified sources before undertaking a titanium forming operation for the first time.

 

Before titanium sheet is formed it should be clean and free of surface defects such as nicks, scratches or grinding marks. All scratches deeper than the finish produced by 180-grit emery should be removed by sanding. To prevent edge cracking, burred and sharp edges should be radiused. Surface oxides can lead to cracking during cold forming and should be removed by mechanical or chemical methods. Plate products should be free of gross stress raisers, very 

rough, irregular surface finishes, visible oxide scale and brittle alpha case (diffused-in oxygen layers) to achieve reasonable cold or warm formability. Experience has shown that pickled plate often exhibits enhanced formability (e.g., in brake bending and dish forming) compared to plate with as-grit blasted and/or asground surface finishes.

 

Commercially pure titanium, the ductile, low-alloy alpha and unaged beta titanium alloys can be cold formed within certain limits. The amount of cold forming either in bending or stretching is a function of the tensile elongation of the material. Tensile elongation and bend data for the various grades of titanium sheet and plate can be found in ASTM Specification B265. Heating titanium increases its formability, reduces springback, and permits maximum deformation with minimum annealing between forming operations. Mild warm forming of most grades of titanium is carried out at 204-316°C (400-600°F) while more severe forming is done at 482-788°C (900-1450°F). Heated forming dies or radiant heaters are occasionally used for low temperature forming while electric furnaces with air atmospheres are the most suitable for heating to higher temperatures. Gas fired furnaces are acceptable if flame impingement is avoided and the atmosphere is slightly oxidizing. Any hot forming and/or annealing of titanium products in air at temperatures above approximately 590-620°C (1100- 1150°F) produces a visible surface oxide scale and diffused-in oxygen layer (alpha case) that may require removal on fatigue- and/or fracturecritical components. Oxide scale removal can be achieved mechanically (i.e., grit-blasting or grinding) or by chemical descale treatment (i.e., molten hot alkaline salt descale). This is generally followed by pickling in HF-HNO3 acid solutions, machining or grinding to ensure total alpha case removal, where required. These acid pickle solutions are typically maintained in the 5:1 to 10:1 volume % HNO3 to HF ratio (as stock acids) to minimize hydrogen pickup depending on alloy type.

 

Cold forming and straightening operations produce residual stresses in titanium that sometimes require removal for reasons of dimensional stability and restoration of properties. Stress relieving can also serve as an intermediate heat treatment between stages of cold forming. The temperatures employed lie below the annealing ranges for titanium alloys. They generally fall within 482-649°C (900-1200°F) with times ranging from 30 to 60 minutes depending on the workpiece configuration and degree of stress relief desired. Hot sizing is often used for correcting springback and inaccuracies in shape and dimensions of preformed parts. The part is suitably fixtured such that controlled pressure is applied to certain areas of the part during heating. This fixtured unit is placed in a furnace and heated at temperatures and times sufficient to cause the metal to creep until it conforms to the desired shape. Creep forming is used in a variety of ways with titanium, often in conjunction with compression forming using heated dies.

 

Following are descriptions of several typical forming operations performed on titanium. They are representative of operations in which bending and stretching of titanium occur. The forming can be done cold, warm or hot. The choice is governed by a number of factors among which are workpiece section thickness, the intended degree of bending or stretching, the speed of forming (metal strain rate), and alloy/ product type.

 

In this operation, bending is employed to form angles, z-sections, channels and circular cross sections including pipe. The tooling consists of unheated dies or heated female and male dies.

 

Stretch forming has been used on titanium sheet primarily to form contoured angles, hat sections, Zsections and channels, and to form skins to special contours. This type of forming is accomplished by gripping the sheet blank in knurled jaws, loading it until plastic deformation begins, then wrapping the part around a male die. Stretch forming can be done cold using inexpensive tooling or, more often, it is done warm by using heated tooling and preheating the sheet blank by the tooling.

 

These cold, warm or hot processes shape titanium sheet or plate metal into seamless hollow parts (e.g., cylinders, cones, hemispheres) using pressure on a rotating workpiece. Spinning produces only minor thickness changes in the sheet, whereas shear-forming involves significant plastic deformation and wall thinning.

 

SPF of titanium alloys is commonly used in aircraft part fabrication, allowing production of complex structural efficient, lightweight and cost-effective component configurations. This High temperature sheet forming process (typically 870-925C°(1660-1700°F)) is often performed simultaneously with diffusion bonding (solid-state joining) in argon gas-pressurized chambers, eliminating the need for welding, brazing, sizing or stress relief in complex parts. Titanium sheet alloys that are commonly superplastically-formed include the Ti- 6Al-4V and Ti SP-700 alpha-beta alloys.

 

 

Titanium alloy sheet and plate products are often formed cold, warm or hot in gravity hammer and pneumatic drop hammer presses involving progressive deformation with repeated blows in matched dies. Drop hammer forming is best suited to the less high strain ratesensitive alpha and leaner alpha-beta titanium alloys. Hot closed-die and even isothermal press forging is commonly used to produce near-net shape components from titanium alloys. Trapped-rubber forming of titanium sheet in cold or warm (540°C (1000°F) max.) pressing operations can be less expensive than that utilizing conventional mating "hard die" tooling. Even explosive forming has been successfully employed to form complex titanium alloy sheet/plate components.

 

The lower strength, more ductile titanium alloys can be roll-formed cold as sheet strip to produce long lengths of shaped products, including welded tubing and pipe. Welded or seamless tubing can be bent cold on conventional mandrel tube benders. Seam-welded unalloyed titanium piping can also be bent cold or warm on standard equipment utilizing internal mandrels to minimize buckling, whereas higher strength alloy seamless piping can be successfully bent in steps via hot induction bending.

 

 

This is a process involving titanium bending and stretching in which deep recessed parts, often closed cylindrical pieces or flanged hat-sections, are made by pulling a sheet blank over a radius and into a die. During this operation buckling and tensile tearing must be avoided. It is therefore necessary to consider the compressive and tensile yield strengths of the titanium when designing the part and the tooling. The sheet blank is often preheated as is the tooling.The softer, highly ductile grades of unalloyed titanium are often cold pressed or stamped in sheet strip form to produce heat exchanger plates, anodes or other complex components for industrial service.

 

 

Commercially pure titanium and most titanium alloys are readily welded by a number of welding processes being used today. The most common method of joining titanium is the gas Tungstenarc (GTAW) process and, secondarily, the gas metal-arc (GMAW) process. Others include electron beam and more recently laser welding as well as solid state processes such as friction welding and diffusion bonding. Titanium and its alloys also can be joined by resistance welding and by brazing. The techniques for welding titanium resemble those employed with nickel alloys and stainless steels. To achieve sound welds with titanium, primary emphasis is placed on surface cleanliness and the correct use of inert gas shielding. Molten titanium reacts readily with oxygen, nitrogen and hydrogen and exposure to these elements in air or in surface contaminants during welding can adversely affect titanium weld metal properties. As a consequence, certain welding processes such as shielded metal arc, flux cored arc and submerged arc are unsuitable for welding titanium. In addition, titanium cannot be welded 

to most other metals because of formation of embrittling metallic compounds that lead to weld cracking.

 

While chamber or glove box welding of titanium is still in use today, the vast majority of welding is done in air using inert gas shielding. Argon is the preferred shielding gas although argonhelium mixtures occasionally are used if more heat and greater weld penetration are desired. Conventional welding power supplies are used both for gas tungsten arc and for gas metal arc welding. Tungsten arc welding is done using DC straight polarity (DCSP) while reverse polarity (DCRP) is used with the metallic arc.

 

An essential requirement for successfully arc welding titanium is proper gas shielding. Care must be taken to ensure that inert atmosphere protection is maintained until the weld metal temperature cools below 426°C (800°F). This is accomplished by maintaining three separate gas streams during welding. The first or primary shield gas stream issues from the torch and shields the molten puddle and adjacent surfaces. The secondary or trailing gas shield protects the solidified weld metal and heat-affected zone during cooling. The third or backup shield protects the weld underside during welding and cooling. Various techniques are used to provide these trailing and backup shields and one example of a typical torch trailing shield construction is shown below. The backup shield can take many forms. One commonly used for straight seam welds is a copper backing bar with gas ports serving as a heat sink and shielding gas source. Complex workpiece configurations and certain shop and field circumstances call for some resourcefulness in creating the means for backup shielding. This often takes the form of plastic or aluminum foil enclosures or “tents” taped to the backside of the weld and flooded with inert gas.

 

Titanium weld joint designs are similar to those for other metals, and the edge preparation is commonly done by machining or grinding. Before welding, it is essential that the weld joint surfaces be free of any contamination and that they remain clean during the entire welding operation. The same requirements apply to welding wire used as filler metal. Contaminants such as oil, grease and fingerprints should be removed with detergent cleaners or non-chlorinated solvents. Light surface oxides can be removed by acid pickling while heavier oxides may require grit blasting followed by pickling.

 

A good measure of weld quality is weld color. Bright silver welds are an indication that the weld shielding is satisfactory and that proper weld interpass temperatures have been observed. Any weld discoloration should be cause for stopping the welding operation and correcting the problem. Light straw-colored weld discoloration can be removed by wire brushing with a clean stainless steel brush, and the welding can be continued. Dark blue oxide or white powdery oxide on the weld is an indication of a seriously deficient purge.The welding should be stopped, the cause determined and the oxide covered weld should be completely removed and rewelded. For the finished weld, non-destructive examination by liquid penetrant, radiography and/or ultrasound are normally employed in accordance with a suitable welding specification. At the outset of welding it is advisable to evaluate weld quality by mechanical testing. This often takes the form of weld bend testing, sometimes accompanied by tensile tests.

 

Spot and seam welding procedures for titanium are similar to those used for other metals. The inert-gas shielding required in arc welding is generally not required here. Satisfactory welds are possible with a number of combinations of current, weld time and electrode force. A good rule to follow is to start with the welding conditions that have been established for similar thicknesses of stainless steels and adjust the current, time or force as needed. As with arc welding, the surfaces to be joined must be clean. Before beginning a production run of spot or seam welding, weld quality should be evaluated by tension shear testing of the first welds.