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Ti-6Al-4V is the workhorse amongst titanium alloys for its all-round strength
properties and user's confidence. The commonly practiced hot working temperature for
this alloy is in the range of 1,173-1,223 K (900-950
°C). Since the alloy is used for superplastic forming/isothermal forging of aerospace parts, the need to use
expensive superalloy die materials cannot be avoided. If, however, the working temperature
can be brought down by refining the grain size, it is possible to use inexpensive
die materials, e.g., die steel, stainless steel, etc. In an earlier study, Salischev et al. (1994) reported grain refinement up to 0.06
mm in Ti-6Al-3.2Mo alloy through large forging deformation at a low temperature, followed by annealing. Later,
Salischev et al. (1997 and 2002) again reported grain refinement up to 0.06
mm in some other titanium alloys by multiple forging. However, the scheme of forging for
achieving large deformations has not been revealed in detail. In the meantime, Dutta
and Venugopal (2000) could reduce the forging temperature of a difficult-to-forge
titanium aluminide alloy of grade Ti-48Al-2Cr-2Nb from 1,373 K (1,100
°C) to 1,123K
(850 °C), by performing 3 sets of multiaxial forgings. In a similar alloy, Salischev et al. (2000) brought down the superplastic forming temperature by 200-400
°C, by multiple forging and dynamic recrystallization. Goloborodko et al. (2006) studied Multidirectional Forging (MDF) on 7,475 aluminium alloy and found that the
grain size can be refined to 5.5  m at a temperature of 490
°C, under a strain rate of
3 x 10–2 s–1. Kobayashi et al. (2007) multidirectionally forged copper at 195 K
and obtained a grain size of 0.16 m at a large cumulative strain of more than 5. A
Ni-Fe alloy was multidirectionally forged by
Miura et al. (2009) at 873 K and they
exhibited a homogeneous evolution of equiaxed fine grains of about 0.8
mm at a cumulative strain of only 2.4. It has been observed that most of these studies were carried
out with alloys other than the workhorse titanium alloy Ti-6Al-4V.
Therefore in the present investigation, a slightly innovative method of
multiaxial compression has been attempted to refine the grain size of the titanium alloy
Ti-6Al-4V. In order to select the suitable temperatures for forging, strain rate of
deformation, prior compression tests have been carried out at 1,023 K, 1,073 K, 1,123 K
and 1,173 K (750 °C, 800 °C, 850 °C and 900
°C) with strain rates of 10–3 s–1, 10–2 s–1,
10–1 s–1 and 100 s–1. For subsequent multiaxial compression, the strain rate
chosen was 10–2 s–1 keeping in view the limitation of load at higher strain rates. In
multiaxial compression, cuboidal samples were pressed from each side, sequentially at 1,123
K (850 °C). The same set of experiments was repeated at 1,073 K (800
°C) and
1,023 K (750 °C). |
