On a Testing Methodology for the Mechanical Property Assessment of a New Low-Cost Titanium Alloy Derived from Synthetic Rutile
On a Testing Methodology for the Mechanical Property Assessment of a New Low-Cost Titanium Alloy Derived from Synthetic Rutile
L.L. BENSON 0
L.A. BENSON MARSHALL 0
N.S. WESTON 0
I. MELLOR 0
M. JACKSON 0
0 L.L. BENSON, L.A. BENSON MARSHALL, N.S. WESTON, and M. JACKSON are with the Department of Material Science and Engineering, The University Of Sheffield , Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD , UK. Contact
Mechanical property data of a low-cost titanium alloy derived directly from synthetic rutile is reported. A small-scale testing approach comprising consolidation via field-assisted sintering technology, followed by axisymmetric compression testing, has been designed to yield mechanical property data from small quantities of titanium alloy powder. To validate this approach and provide a benchmark, Ti-6Al-4V powder has been processed using the same methodology and compared with material property data generated from thermo-physical simulation software. Compressive yield strength and strain to failure of the synthetic rutile-derived titanium alloy were revealed to be similar to that of Ti-6Al-4V.
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Widespread use of titanium alloys is mainly inhibited
by the high cost of the production of titanium alloy
components. This costly upstream extraction and
multistage processing route have resulted in the restriction
of high strength titanium alloys mainly to the aerospace
sector.[
1
] Titanium’s unique blend of properties such as
high strength-to-weight ratio, corrosion resistance, and
biocompatibility make it an attractive material for many
commercial applications. However, without a step
change in the economics of titanium production, the
super-metal will be confined to the aerospace industry
and niche applications in markets such as the defence
and automotive industries.
One long-term solution is the production of titanium
metal components entirely in the solid state via the
combination of electrochemical extraction (Metalysis
FFC process)[
2
] to directly produce a titanium alloy
powder and subsequent consolidation via near net
shaping technologies. Solid-state consolidation
techniques such as the use of field assisted sintering
technology (FAST) in conjunction with hot forging are
capable of producing shaped metal components with full
densities and wrought properties from a powder
feedstock.[
3
]
Titanium is currently extracted via the Kroll process,
a discontinuous metallothermic reduction process,
which involves the reduction of TiCl4 by Mg to produce
a titanium metal sponge. Master alloys are added to the
Kroll sponge, before compaction and welding into an
electrode for melting. Vacuum arc melting requires
multiple re-melts to produce homogeneous ingots,
particularly in the case of alloying additions such as
Fe or Mn, which are prone to segregation.[
4
] After
melting, ingots are subject to multistep hot forging and
heat treatments to refine the grain structure and
homogenize the chemistry in the billet. Finally,
significant wastage is endured during expensive machining of
titanium alloys, with some critical aerospace titanium
alloy parts having a reported buy-to-fly ratio of
40-to-1.[
5
]
Although powder can be produced from Kroll sponge
via additional procedures such as hydride dehydride
processing, plasma rotating electrode process (PREP) or
gas atomization (GA), these are expensive powder
production routes that reduce the cost effectiveness of
using near net shape powder metallurgy (PM).
Hence, producing titanium alloy powder directly via
the solid-state FFC extraction process, followed by
downstream solid-state consolidation using FAST and
hot forging (‘‘FAST-forge’’[
3
]) to near net shape, will
significantly reduce the cost of titanium alloy
components. Cost reductions are achieved by directly
producing an alloy powder, reducing the number of multistep
forging and heat treatment steps, and minimizing both
wastage and machining. Further, as the entire
production route is conducted in the solid state, melting
procedures can be eliminated entirely. It is generally the
melting stage in which most defects in titanium alloys
originate and so its removal has additional benefit.[
6
]
Further cost advantages can be made by utilizing
synthetic rutile (SR) as a feedstock to the FFC process.
SR is derived from the iron-rich titanium ore, ilmenite
(FeTiO3), and as such contains a range of alloying
elements, principally iron. Following reduction of SR an
a + b type titanium alloy is produced, without the cost
of alloying additions.[
7
] Hence, the use of SR as a
feedstock is notably cost-effective, as not only are
feedstock costs reduced, but the dependency on master
alloy additions further downstream is reduced or
eliminated.
As the utilization of a synthetic rutile feedstock within
a production route entirely in the solid state is a
particularly lucrative operation, this paper assesses the
mechanical properties of a synthetic rutile-derived
titanium alloy (3.9 w (...truncated)