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.
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, 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.
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 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.
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.
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.