Machining, Forming and Welding Technology

Why Select Titanium Alloys
September 26, 2016
GRADE 5 (R56400) Ti-6Al-4V
September 26, 2016

Machining, Forming and Welding Technology

Machining Titanium

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 do not have identical machining characteristics. Like stainless steel, the low thermal conductivity of titanium inhibits dissipation of heat within the workpiece itself, thus requiring proper application of coolants.


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 turning.The cutter mills only part of each revolution. 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. 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 resulting work hardening makes it difficult to re-establish the cut.


Percentage depth of thread has a definite influence on success in tapping titanium and best results 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 in 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 sulfo-chlorinated oils, but these can present a fire hazard, and it is important to flood the work when using these oil based 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

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.

Chem Milling

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.

Forming Titanium

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.

Surface Preparation

Before titanium sheet is formed it should be cleaned and free of surface defects such as nicks, scratches or grinding marks. All scratches deeper than the finished product 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.

Cold Versus Hot Forming

Commercially pure titanium, the ductile, low-alloy alpha and un aged 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 spring back, 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 fracture-critical 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.

Stress Relief and Hot Sizing

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 spring back and inaccuracies in shape and dimensions of preformed parts. The part is suitably fixed such that controlled pressure is applied to certain areas of the part during heating. This fixed 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.

Typical Forming Operations

The 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.

Brake Forming

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

Stretch forming has been used on titanium sheet primarily to form contoured angles, hat sections, Z-sections 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.

Spinning and Shear-Forming

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.

Superplastic Forming (SPF)

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 super-plastically formed include the Ti- 6Al-4V and Ti SP-700 alpha-beta alloys.

Other Forming Processes

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 less high strain rate sensitive 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.

Deep Drawing

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.

Welding Titanium

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 is 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. 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.

Welding Environment

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.

Inert Gas Shielding

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.

Weld Joint Preparation

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.

Weld Quality Evaluation

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.

Resistance Welding

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.

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