Forming of Titanium and Titanium Alloys: Part Two

요약:

The commercial production of titanium plate, sheet, strips, and bars is carried out using hot and cold rolling mills to achieve the necessary reductions and desired shapes. Rolling may be defined as the reduction of the cross-sectional area of a piece by compressive forces applied through rolls. Cold rolling is carried out at temperatures below which the rate of strain hardening is greater than the rate of recrystallization. When reduction is carried out above such a temperature, the process is termed hot rolling. The major quantity of titanium plate, sheet, strips and bars is processed using hot rolling techniques.

The commercial production of titanium plate, sheet, strips, and bars is carried out using hot and cold rolling mills to achieve the necessary reductions and desired shapes. Rolling may be defined as the reduction of the cross-sectional area of a piece by compressive forces applied through rolls.

Cold rolling is carried out at temperatures below which the rate of strain hardening is greater than the rate of recrystallization. When reduction is carried out above such a temperature, the process is termed hot rolling. The major quantity of titanium plate, sheet, strips and bars is processed using hot rolling techniques.

The forged billets, whose surfaces have been descaled, are rolled between 1350 and 1500°F (730 and 815°C). This temperature is approximately 200°F (110°C) lower than the forging temperature. Titanium can be continuously rolled at temperatures as low as 1100°F (595°C).

As the thickness of the material to be rolled is decreased, the temperature of the piece must be considerably lowered to minimize surface contamination. A careful choice of pass sequences to obtain a certain reduction must be made when rolling titanium. Pass sequence refers to the number of reductions taken and percentage reduction of the piece per pass.

Continuous sheet and strip are best cold- or hot rolled with the application of back and forward tensions to reduce the friction in the roll gap. In cold rolling thin sheet, extremely tight roll settings are required to produce uniform cross section.

Extrusion

Extrusion is the shaping of metal into a chosen continuous form by forcing it through a die of the desired shape. Titanium can be extruded to produce rounds, squares, tubes, and other simple shapes. Typical extrusion temperatures range between 1800 and 1900°F (980 and 1040°C).

Titanium metal has been observed to have better flow characteristics than steel. It more readily fills the die, causes less die wear, and maintains closer tolerances than do steels.

Tube and Wire Drawing

Titanium tube and wire have been successfully cold drawn. Although unalloyed and alloyed titanium wire has been produced, only the unalloyed grades of tubing are currently commercially available.

Seamless titanium tubing is formed from hot extruded billets which are pierced and drawn cold over a stationary or moving mandrel. Intermittent vacuum anneals are necessary to produce tubes with smooth surfaces and close tolerances. To form welded tubing, cold-rolled titanium strip is rolled into tubular form and inert-arc-welded prior to the drawing operation.

Titanium wire, both unalloyed and alloyed, has been cold drawn to 0.007 inch (0.18 mm) diameter. The wire is cold drawn through dies of approximately 14 degrees with phosphate coatings or copper as a primary lubricant. Repeated applications of these coatings are usually necessary during the reduction process. Vacuum annealing is frequently carried out intermittently between two or three successive drawing operations.

Spinning

Spinning is used to form metal parts having a rotational symmetry. The operation is performed by placing a circular blank or a conical piece in a lathe, rotating it around its axis, and forcing it against a preformed chuck. The advantage of the partially formed or conical shape is that it reduces the work hardening encountered in spinning of circular blanks.

The spinning of titanium differs from conventional spinning operations in that elevated temperatures, approximately 1000 to 1300°F (540 to 705°C), are required to reduce work hardening effects.

Deep Drawing-Hydropress-Punch Press

Deep drawing of unalloyed and alloyed titanium requires no specialized equipment. Unalloyed grades can be reduced from 20% to 40% employing pressures of 3500 psi (24 MPa) or greater and with slow rates of deformation. In addition to these requirements, alloy grades require a temperature of 800 to 1000°F (425 to 540°C). Parts can be partially preformed by drop hammer techniques and then finished by deep drawing.

Piercing and blanking operations for punch pressing can be carried out cold on unalloyed titanium, but elevated temperatures are required when working with the alloy grades.

Bending

The satisfactory forming of titanium sheet, strip, and tubing by bending methods has been carried out both hot and cold. Titanium sheet and strip are usually bent-formed by the use of a V-block or a power brake.

Stretch Forming

Depending upon the shape to be formed, stretch forming of titanium and its alloys is carried out either by wrapping the material around a die, clamping the material at its ends, and then pressing a die against its surface, or by the application of uneven tension along the edges.

Various techniques have been applied to the forming of the metal. In general, where a stretch less than 5 % is required, room temperature forming proves adequate. Where the stretch required is greater than 5%, elevated temperatures are usually necessary. For the unalloyed titanium, the temperatures employed are from 400 to 600°F (205 to 315°C); whereas alloy grades require a range from 800 to 1000°F (425 to 540°C). Also, interstage annealing has proved advantageous in some cases in conjunction with final stress relieving.

Dimpling

Dimpling is a method of forming whereby the material is cupped so that rivet heads will be flush with the surface of the metal. An undersized hole is drilled in the material where the rivet is to be positioned before dimpling. The material is deformed by heated hemispherical dies. After the dimpling operation has been carried out, the hole is reamed to size.

This process has been successfully carried out using pneumatic coin dimplers. For the unalloyed titanium, these dies are heated to 600 to 800°F (315 to 425°C), and from 800 to 1000°F (425 to 540°C) for the alloy grades, with dwell times of the order of two seconds.

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