Forming of the Titanium and Titanium Alloys: Part One

요약:

Hot and cold working or shaping of the titanium alloys involves forging, rolling, extrusion, drawing, spinning, and other such operations.
Operations such as forging and rolling, in which the basic ingot is processed into standard forms of billets, sheet, plate, rod, and wire, will be referred to as primary forming operations. Bending, extrusion, drawing, spinning, in which these standard forms are further fabricated, will be referred to as secondary operations.

Hot and cold working or shaping of the titanium alloys involves forging, rolling, extrusion, drawing, spinning, and other such operations.

Operations such as forging and rolling, in which the basic ingot is processed into standard forms of billets, sheet, plate, rod, and wire, will be referred to as primary forming operations. Bending, extrusion, drawing, spinning, in which these standard forms are further fabricated, will be referred to as secondary operations.

For primary forming, conventional equipment used for steel has been found applicable to titanium. Titanium, of course, requires a modification of the forming techniques used for other metals. These involve such important considerations as time, temperature, and pressure. Likewise, the secondary forming operations require similar modifications.

In general, the shaping of titanium by the various conventional techniques can be successfully accomplished. Several simple rules might apply to all forming procedures. Form cold wherever possible. Where hot forming is required, heat can be applied to the metal or forming dies. In all instances, forming should be done with slow steady pressures. In cases where severe cold forming is carried out, post-heat treatment for the relief of residual stresses is usually preferable.

Forging

Forging is a method of shaping whereby the metal is deformed by the action of repeated blows. Open die and closed die are the two basic methods of forging titanium. Here the force is applied by a hammer, with gravitational force being utilized either alone or in combination with steam pressure, air pressure, or some supplementary force. The forging operation must be carried out with the metal kept at an elevated temperature to facilitate the flow of the titanium.

Open die forging is usually employed to rough-shape the material. To accomplish this, flat dies V-dies, and swage die are used. Flat dies are primarily used to forge flat material or to forge rounds into polygonal shapes. V-dies may also accomplish this on round or square stock. Swage or curved dies are used to reduce the diameter of round stock or to produce rounds from polygonal stock.

Titanium has been successfully forged by the open die method using conventional equipment designed for steel, but with lower temperatures and increased pressures. The lower temperature is required in the forging of titanium since this will, first, limit the resultant surface contamination and, second, prevent excessive grain growth, both of which reduce ductility in the forged part. Because of the lower temperature, it becomes necessary to use higher pressures to deform the metal.

Forging temperatures usually range between 1450°F (790°C) and 1950°F (1065°C). Lower temperatures up to 1650°F (900°C) are applicable to the unalloyed titanium, while higher temperatures are used with alloyed titanium. Lower temperatures than those stated can be used if the equipment is capable of delivering the force necessary to deform the metal.

It has been recommended that soaking temperatures are around 1200 to 1300°F (650 to 700°C) and that the material be heated to the forging temperature just prior to the operation. Forging can be carried out as the temperature drops. To obtain good ductility, it has been found preferable to finish-forge at a temperature not below 1550°F (840°C).

Closed die forging differs from open die in that a finished shape is produced. The metal is forced into a preshaped die so that the exact desired contours are obtained. In closed die forging, temperatures and pressures for heavy parts are comparable to those employed for open die forging. Light gauge material is formed at 800 to 1000°F (425 to 540°C) with slightly lower pressures required and a slower deformation rate.

Swaging

Swaging may be defined as the tapering or reduction of the end of a rod, polygonal bar, or tube. This is not to be confused with the swage dies used in open die forging. This process employs either two or four dies held on a revolving spindle, which have on their face the impressions to be shaped. This operation has been successfully carried out on titanium at temperatures between 500 and 700°F (260 and 370°C).

Upsetting

Upsetting is used to increase the diameter of the end of a bar, rod, or tube. In this process, a uniformly round piece is held between two preformed dies, and pressure is applied intermittently in rapid blows against the end, forcing the metal against the dies. This forces the ends to spread laterally and to be compressed longitudinally.

Any desired shapes which are to be formed on the upset portion of the metal must be preformed into the dies. With titanium the process has been best carried out at temperatures from 800 to 1000°F (425 to 540°C).

Cold Heading

For forging rivets and bolts and similar types of fasteners, cold heading has proved an adequate method. Cold heading involves the forming of somewhat intricate shapes on the end of wire or small diameter rod by upsetting the end of the rods and forcing the metal to cold-flow into a die.

Unalloyed titanium has been cold-headed on either single or double rivet or screw heading machines. On forming large-headed fasteners, it has been found preferable to preheat the stock to 500°F (260°C) to prevent cracking in the upset portion.

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