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Forging of Titanium & alloys

Forging of Titanium & Alloys (1) - General

There are three general types of titanium-base alloys: (1) α-alloys, (2) α, β-alloys, and (3) β-alloys. Transformation of those phases goes at approximately 1625F from α to β during heating. The α-phase is a hexagonal close-packed structure and β is a body-centered cubic structure. Alloying elements are added to stabilize one or the other of these phases by either raising or lowering the transformation temperatures. Forging behavior and forging practice are controlled by the α, β relationship of each alloy type.

Forging temperatures and their determination

Due to the α-β transformation and the great difference of the mechanical properties of the α- and β-phase, forging temperature plays a vital important role for receiving optimal microstructure and mechanical property. Forging temperature for each alloy will be covered in detail in the next three parts of this paper series on the Forging of Titanium & alloys.

Very important in the control of these properties is the control of microstructure, which, in turn, is controlled by forging procedure, primarily the forging temperature, and subsequent heat treatment. The beta transformation temperature establishes the maximum temperature to which an alloy can be heated without developing a new microstructure that may result in reduced final properties.

Other Forging Parameters

There are a number of factors that make titanium alloys more difficult to forge than steels. Besides forging temperature, major factors that need to be carefully handled for the titanium alloy forging are as follows:

  • Forging pressure requirements,
  • sensitivity to strain rate; and
  • susceptibility to cracking.
  • The forging pressure for each of the three types of titanium alloys increases more rapidly with decreasing temperature than does that of alloy steels. As examples, the dependence of forging pressure for three titanium alloys Ti-13V-11Cr-3Al, Ti-8Al-1Mo-1V and Ti-6Al-4V are much stronger that that of steel 4340. Thus, in ordinary impression die forging operations, cooling of the workpiece has a more critical effect on forging pressures for titanium than for steels.

    The forging pressure for the titanium alloys is very sensitive to strain rate. Approximately 50% more energy is needed for a hammer forging than for pressure forging. For the alpha, near-alpha, and alpha-beta alloys, microstructure can also be an important consideration, particularly at the slower strain rates. At strain rates in the range of 0.001 to 0.1 per second, a 2.5 X increase in the primary alpha grain size could double the flow stress.

    Forging Processes

    Conventional Forging

    Conventional forging of titanium alloys employs similar machinery and forging practices to those of alloy steels, except that the forging pressures are higher. Attentions need to be paid on the higher degrees of dependency of flow stress on temperature and strain rate. So in relatively low-speed machines (i.e., in hydraulic presses), alloys are not allowed to lose much temperature during forging. In high-rate forging (i.e., in hammers), overheating due to heat generated by deformation should be avoided. As a matter of fact, in selecting initial stock temperatures, the forging machine being used should also be considered as an influence factor. Thus, conventional forging of titanium alloys requires much closer temperature and process control than are necessary in forging of alloy steels.

    Isothermal and Hot-Die Forging

    In forging titanium alloys, it is desirable to employ isothermal forging in which the dies are kept at the same temperature throughout the forging, or hot-die forging in which dies are pre-heated at a temperature slightly below forging temperature. With the isothermal or hot-die forging, the part can be forged at low speeds to reduce flow stress and forging pressure, without much danger of die chilling. However, those methods require expensive die-heating systems and die materials that can maintain strength at high temperatures. In addition, they Impose high requirement for lubrication and billet coating. Heat-resisting alloys such as Waspaloy, Udimet 700, Astroloy IN-100, and Inconel 713C, etc. are selected used to make dies. Various glass mixtures are used for coating the billets. In addition, it is often necessary to apply an additional "die-separation agent" to the stock prior to forging so that the forged part can be removed from the die without distortion.

    Design Considerations

    Since virtually all the titanium forging alloys are double or triple melted, they rarely contain segregations of other materials that might cause wide variations in forgeability. Initial breakdown forging of titanium alloys can be done at temperatures above the β transus because the body-centered cubic structure is more ductile and forging pressure requirements are generally lower.

    Design principles for α- and α-β titanium alloys fall between those for alloy steels and hot-cold work alloys. The designs should provide an adequate amount of deformation during forging, but the reductions need not be confined to narrow limits. Because the ß-alloy requires precise control of reductions, the designs are limited to generously contoured shapes.

    Due to shrinkage die sinking allowance, limited contour fill, great forging force and high requirement on surface finish, forging of titanium alloys requires the use of specially designed dies. Production of precision titanium alloy forgings with close tolerance has proved to be technically feasible. However, owing to such factors as excessive die wear, the need for expensive tooling, problems of microstructure control, and contamination, the cost of close tolerance forging is usually prohibitive. Successful precision forging, therefore, is confined to small forgings that do not have complex flow patterns.

    Heating temperature should be controlled within narrow limits. Although heating in vacuum or in a suitably inert atmosphere is feasible, billets are often precoated with a glass-type coating material and heated in the usual manner. Prolonged heating should be avoided for all titanium alloys. If an unscheduled delay such as a press breakdown occurs, stock should be removed from the furnace, and then it should be reheated when forging is resumed. Particular attention need to be paid throughout the processing cycle to avoid contamination by oxygen, nitrogen, carbon, and/or hydrogen, which can severely impair ductility and toughness properties and overall quality of a forged part. 

    References

    [156] James F. Young & Robert S. Shane: Materials and Processes. Part B: Processes. Marcel Dekker, Inc. 1985. ISBN 0-8247-7198-2
    [173] ASM: Metals Handbooks, Desk Edition. ASM 1998. ISBN 0-87170-654-7.

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