Forming titanium alloys

The alpha and alpha-beta titanium alloys can only be formed to a limited extent at room temperature because their high yield point to tensile strength ratios (usually over 90%) and low uniform elongation limit plastic formability to a very narrow range. At temperatures above 500°C both the yield point to tensile strength ratio and the elastic modulus to yield point ratio improve. This is why alloyed titanium plates are mainly hot-formed. The beta titanium alloys have similar cold forming properties to the commercially pure grade 3.7035, but this is not dealt with here.

In all heat treatments of titanium materials consideration should be given to the fact that titanium and titanium alloys are highly reactive with hydrogen, oxygen and nitrogen. For this reason the heat treatment facilities must be designed so as to prevent reactions with the material. This particularly applies to hydrogen pickup from the furnace atmosphere. Heating should take place in a neutral or slightly oxidizing atmosphere and the heating cycle kept as short as possible.

Hydrogen pickup takes place already at temperatures of 500°C. Above 700°C oxygen and nitrogen result in increased scale formation and simultaneous diffusion of oxygen into the part surface (diffusion zone). These chemical reactions reduce the toughness and thermal stability of titanium materials.

Unlike with oxygen and nitrogen, the diffused hydrogen can be largely removed by vacuum annealing under suitable conditions.

For heat treatment, electric or gas-fired air furnaces can be used as well as controlled-atmosphere (noble gases only) and vacuum furnaces. With gas-fired furnaces a 10-15% air surplus should be set and direct contact with the gas flame should be avoided due to the risk of hydrogen pickup and local overheating.

Further details on heat treatment equipment are contained in DIN 65084.

Any steel fixturing that could come into contact with the titanium should be largely scale-free due to the risk of chemical reactions between titanium and iron oxide. In addition, titanium forms low-melting phases with certain metals such as nickel and copper and at the temperatures used in heat treatment equipment these can result in local melting. Contact with these metals through charging racks etc. should be avoided.

Hot forming takes place above the recrystallization temperature in a range of temperatures defined by microstructural requirements, forming resistance and susceptibility to cracking. Forming temperatures should be chosen so as to achieve optimum mechanical properties and microstructural conditions for the particular titanium alloy.

In addition, consideration must be given to the lower thermal expansion coefficient and lower thermal conductivity of titanium in relation to steel, for example.

Suitable lubricants for hot forming are glass mixtures with selected softening ranges or grease/graphite/MoS2 lubricants.

All forming operations on titanium and titanium alloys are usually followed by soft annealing which for all titanium materials takes place at temperatures between 650 and 805°C. The annealing times required depend on the previous forming and on the cross section of the part. As a general rule the holding time should be around 2 minutes per mm wall thickness and a minimum of 30 minutes for most alloys (approx. 2 hours for larger forgings).

Any stress relieving that may be needed can be carried out at lower temperatures. Unalloyed titanium is stress relieved between 500 and 600 °C, alpha-beta alloys between 600 and 675°C and beta alloys at temperatures between 700 and 750°C.

More details on temperatures, holding times and cooling rates for soft annealing and stress relieving of the various titanium alloys are likewise contained in DIN 65084.

Before every heating operation parts must be cleaned of adhering contaminants to prevent diffusion of decomposition products into the material.

Small-section parts are often annealed in a high vacuum. This treatment can also be used to reduce the hydrogen content. Before high-vacuum annealing however the oxide scale must be removed to prevent oxygen from diffusing from the surface inside the material.

Any scale arising as a result of heat treatment must be removed mechanically by sand blasting, grinding or brushing followed by pickling. Any slight discoloration can be removed by pickling alone. For pickling, an aqueous solution consisting of

20 vol.-% HNO3 (65% nitric acid) and
2 vol.-% HF (40% hydrofluoric acid)

has proved successful. It is important in this treatment to ensure that not only the oxide layer on the surface but also the oxygen-enriched diffusion zone underneath is removed, as this adversely affects machinability, for example, i.e. the service lives of turning and milling tools.

After the removal of scale and oxide layers, the hydrogen content of the material should always be checked as annealing and pickling are associated with the risk of hydrogen pickup.

A common forming process used in the aerospace sector on sheets and thin-walled tubes made of titanium alloy TiAl6V4 (TIKRUTAN LT 31) is superplastic forming Â also frequently in conjunction with diffusion welding.

Details on this and further information on forming titanium alloys can be found in the following publications:

H. Bühler, H.W. Wagener "Umformeigenschaften von Titan und Titanlegierungen" Bänder, Bleche, Rohre 6 (1965) Teil 1: H. 11, S. 625/30 und 667 Teil 2: H. 12, S. 677/84
U. Zwicker "Titan und Titanlegierungen" Kap. 16.1 - Warmumformung Springer-Verlag 1974, S. 469/80
A.-A. Hegazy "Untersuchungen zur Warmumformbarkeit der Titanlegierung TiAl6V4" Aluminium 59 (1983), H. 6, S. 451/55
R. Kopp, L. Scheffer "EinfluÃŸ des mehrmaligen Erwärmens beim partiellen Schmieden von Werkstücken aus TiAl6V4" Metall 37 (1983), H. 4, S. 345/49
D. Dunst, H. Mecking "Analysis of Experimental and Theoretical Rolling Textures of Two-phase Titanium Alloys" Zeitschrift für Metallkunde (1996), H. 6, S. 498/507
D.M. Ward "Superplastic forming of titanium alloys" Metals and Materials, September 1986, S. 560/63

Deutsche Titan, Nov. 2000

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