866Wrought Product Processing

This section addresses the primary processing of wrought (ingot) product to mill products. The following section will address the fabrication of these mill products into final components. Mill products include billet, bar, plate, sheet, strip, foil, extrusions, tubing, and wire. Besides the reduction of section size, and shaping, the other objective of primary processing is control (generally refinement) of the microstructure to optimize final mechanical property combinations [8,12]. In general terms as processing proceeds the temperature of the processing is decreased. The P-transus temperature, below which the a phase can be present, is the critical temperature for control of the microstructure. In many cases titanium is processed on the same equipment used for steel, with appropriate special auxiliary equipment [8].

Other concerns with the processing of titanium alloys include the high reactivity of titanium at elevated temperatures and the strain rate sensitivity; especially for the P alloys, strength decreases as the strain rate is reduced.

Billet product from an ingot starts above the P-transus temperature and proceeds at progressively decreasing temperatures. In some cases the P-grain size is reduced by a recrystallization treatment well above the P-transus temperature. However, minimization of grain boundary a, control of the morphology of the a phase, and refinement of the transgranular a can necessitate working below the P-transus temperature.

Bar, plate, sheet, and foil products are produced on a relatively routine basis. Generally, the processing is done hot, although the very high ductility of the metastable P alloys allows finishing of strip and foil by cold rolling.

Forging is a very common method for producing titanium alloy components. It allows both control of the part shape and manipulation of the microstructure and hence mechanical properties. Generally, titanium alloys are considerably more difficult to forge than aluminum alloys and alloy steels, particularly when processing at temperatures below the P-transus temperature is required.

Extrusions, tubing, and wire titanium products are also produced routinely with the same caveats regarding microstructural control as for the product forms discussed above.

0 0

Post a comment