FAST-forge of novel Ti-6Al-4V/Ti-6Al-2Sn-4Zr-2Mo bonded, near net shape forgings from surplus AM powder

Titanium alloys are used extensively in the aerospace sector due to the good combina on of high strength-to-weight ra o and corrosive resistance. Many aerospace components are exposed to extreme service stress states and temperatures, which in some applica ons could compromise the component’s performance if a single tanium alloy is used. A poten al solu on to this issue could be the combina on of dissimilar tanium alloys in subcomponent regions, achieved through consolida ng powders via field assisted sintering technology (FAST-DB) and subsequent hot forging (FAST-forge). In this paper, near net shape taniumtanium alloy demonstrator components are produced from oversized AM powders in just two hybrid solid-state steps; FAST-DB and hot forging.


Introduction
Titanium has a high specific strength combined with an excellent corrosion resistance which makes it a key material in the aerospace sector. However, some of the applica ons in which tanium is used can suffer from a combina on of failure mechanisms during service. For example, some components may require high creep resistance in some regions and high fa gue resistance in others. Therefore, the ideal solu on would be to have different alloys in the sub regions of the component. Nevertheless, there are two main challenges with this solu on; the first one is to find a technique that can produce a reliable mul material component. The second one is to make sure that the component does not fail at the bond line between the dissimilar alloys.
Powder metallurgy is one way to create mul material components because during the layout, the powders can be placed in defined arrays. Recently, spark plasma sintering (SPS) also known as field assisted sintering technology (FAST) has been exploited to join tanium alloys. Guillon et al. [3] discusses the several advantages of FAST over HIP. For example, the opera onal mode and the control over the sintering process is simpler for FAST. Moreover, it is an alterna ve method to fully densify powder, it has high reproducibility and the moulds can be recycled a�er each run. Suarez et al. [4] pointed out that FAST can increase densifica on without coarsening the microstructure due to the high hea ng rates produced in the hea ng step.
He et al. [5] were some of the first authors to use the FAST process to bond tanium, specifically, bonded two solid blocks of tanium, as opposed to powder. The results of the mechanical test performed in the join showed that failure occurred in areas near the bond. A similar experiment was performed by Miriyev et al. [6] in which Ti-6Al-4V was bonded to AISI4330 steel. The bond between the two materials failed by bri�le fracture due to the forma on of tanium carbides. Vincente [7] used FAST to bond the tanium alloy CP-Ti grade 2 with Co-28Cr-6Mo and observed that the roughness of the interface is influenced by the hardness of the two materials. The effect of the temperature, pressure and me in the mechanical proper es when joining pre-sintered billets of Ti-45Al-7Nb-0.3W was studied by Zhao et al. [8]. It was observed that at higher temperatures the material failed in the base material while at higher pressure the material failed at the bond interface. Mar n et al. [9] used an innova ve approach to bond two tanium alloys using FAST: The method consisted of bonding a Ti-6A-4V 3D structure made with by electron beam mel ng (EBM) to CP-Ti grade 2 powder. This method generated a fully consolidated component with 99.5% of density. Recently, Pope et al. [10] studied the integrity of the diffusion bond (DB) of dissimilar alloy powders such as Ti-5553, Ti-6Al-4V and CP-Ti grade 2 through FAST, termed FAST-DB. Such FAST bonds of dissimilar alloys displayed excellent mechanical integrity under tensile tes ng, with failure occurring in the base material of the lowest strength alloy, as opposed to the bond region.
The current challenge with FAST is to directly obtain microstructures and shapes required for aerospace applica ons without further processing. For example, Weston and Jackson [11] developed and proved the concept of FAST-forge which consists of combining a component made via FAST with a forging step in order to provide enhanced mechanical performance. This process enables near net shape component to be produced from powder in only two solid-state steps. The effec veness of FAST-forge for the high strength alloy Ti-5553 was demonstrated by Calvert et al. [12] where the microstructure of the conven onal (+40 stage) processing route was similar to the two step FAST-forge.
The aim of this paper is to demonstrate the FAST-forge processing route on dissimilar FAST-DB tanium alloy preforms of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo. A schema c of the FAST-forge process for a FAST-DB component is shown in Figure 1.

Powder feedstock
The two tanium spherical powders used in this study are Ti-6Al-4V (Ti-64) produced by the plasma rota ng electrode process (PREP) and Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) produced by the gas atomisa on process (GA). The Ti-6242 was sourced from LPW Carpenter and the Ti-64 from ASM Starmet. The powder was surplus to requirements for addi ve manufacturing (AM) technologies. The par cle size distribu on (PSD) was measured under wet condi ons with a Malvern Mastersizer 3000 laser par cle size analyser. The PSD of the powder used is shown in Table 1 were the standard devia on is taken from a total of 20 measurements.
The powder's aspect ra o, roundness and porosity were also characterised using the op cal microscope Olympus Bx51 with the so�ware Clemex Vision PE image analysis system. The data showed that Ti-64 powder had higher roundness and less porosity than Ti-6242 powder.

Processing of the FAST-DB Ti-64/Ti-6242 disks from powder
The first step was to create a FAST-DB disk from the Ti-64 and Ti-6242 powder with defined bond lines. The FAST was carried out with an FCT Systeme GmbH Spark Plasma Sintering Furnace type H-HP D 250 located at Kennametal Manufacturing (UK) Ltd., Newport, shown in Figure 2 (a), which is capable of producing 250 mm diameter consolidated disks. With a view of crea ng dissimilar tanium alloy forging billets with defined bond lines, it was necessary to set up dividers inside the graphite mould, as shown in Figure 2 (b). Each box of the grid created with the dividers was filled with either Ti-64 or Ti6242 powder (NB: that the powders were never blended together in this study). Once, the mould was filled with powder, the dividers were removed carefully to keep a straight line in the interface (and Figure  2 (c) s ll retains the grid pa�ern). Finally, a�er inser ng the graphite paper and the top punch in the mould, the graphite mould is placed in the FAST furnace. The processing temperature below the β trans with a dwell and pressure.
In Figure 2 (d) a schema c of the FAST furnace arrangement applied to fully consolidate the powder is shown. The sintering mechanism is accelerated through the applica on of a pulsed electrical current to heat the powder in combina on with uniaxial mechanical pressure. All this process is done in a vacuum chamber to avoid oxida on of the powder and the temperature is controlled by a pyrometer situated very close to the sample. In the case of metal powders, such as tanium and nickel superalloys, the la�er stages of the FAST process cycle are very similar to a slow strain rate isothermal forging process, as the powder has fully consolidated and is behaving as wrought material at high temperature.

Machining of the forging billets from the FAST-DB disk.
The next step is to prepare the forging billets. First, it is necessary to remove the graphite paper adhered in the disk by grit blas ng, see Figure 3 (a). Then, the preform billets were machined parallel to the bond, as shown in Figure 3 (c). A total of 5 off 22 mm diameter forging FAST-DB billets were machined, in addi on, to a solely Ti-6242 forging billet. In Figure 3 (c) the distribu on of the two tanium alloys in the forging billets is presented.

Closed-die forging of the FAST-DB Ti-64/Ti-6242 billets
The forging was conducted in collabora on with W.H. Tildesley, Wolverhampton, UK, one of the leading (and oldest) UK closed die forging specialists. A Massey 1.1 MSC drop hammer forge with foot pedal control and 11 kJ blow energy was used to forge the billets is shown in Figure 4 (a). The samples were heated in a gas furnace, shown in Figure 4 (b) at approximately 950°C for 10 minutes. The temperature of the billets was measured with two type K thermocouples inserted in a dummy sample with two holes. The temperature of the top die was 235°C and the bo�om one was 320°C.
The final near neat shape forged component profile was a legacy motorcycle rocker arm which had a good degree of complexity in order to understand and demonstrate the advantages of the FAST-DB and subsequent closed-die forging technology.
There were three main steps in the forging process. The first one was to remove the billet from the furnace with tongs and transfer it to the drop hammer bo�om die. The second one was to forge the sample with 4 to 5 hammer blows and the final step was to crimp off the flash and quench the component in water. The component has a similar degree of complexity to a compressor blade and is of a similar size. The process required two specialist forgers doing the three steps, therefore, the me between each step varied for each sample. Consequently, three cameras were set up with the aim to know the me of each step during the forging process for all the samples.

Characterisation
The samples were prepared following a standard metallographic procedure which included silicon carbide abrasive paper discs from P800 to P2500. The final polish was done with 0.06 μm colloidal silica mixed with 10% concentra on hydrogen peroxide. The microstructure was observed under cross-polarized light with a Reichert Jung Polyvar Met light microscope.
Heat n ng was used to differen ate the bond between the two alloys a�er sec oning the forged components. This technique consists in polishing the sample to mirror finish and oxidize the polished surface in a furnace at 540°C for 50 min. The two tanium alloys have a different oxida on rates and oxide scales, for example Ti-64 has a brown colour as a result of the processing condi ons, and therefore the exact loca on of bond line was easily dis nguishable a�er quenching.

Results and Discussion
A total of 13 rocker arms were forged from the original 6 forging billets. A video s ll image of a hot billet before and a�er the first hammer blow is shown in Figures  5(a) and 5(b). It is important to note that the billet has a bond through middle of the billet, yet there is no nega ve indica on of the bond during the closed-die forging step. The form of the final rocker arm shape is already visible in Figure 5(b), with three subsequent hammer blows to generate defini on in the high strain sec ons. In fact, the forging engineers commented on how easy the FAST-DB billets were to forge compared to their experience with steel and even aluminium.
In Figure 5 (c) the final near net shape rocker arm a�er crimping and quenching is shown in Figure 5(c). As stated earlier, the different oxide colours make it easy to locate the evolu on of the final bond line from top to bo�om (red do�ed line). The posi on of the bond was different for each of the 13 components, this was due to the way the ini al forging billets were made, as discussed in sec on 2.3, and the human factor involved in the forging process (ie. loca ng the forging billet on the bo�om die). Standard forging NDE dye penetrant inspec on was carried out which further confirmed the structural integrity of the bond line post forging. Regardless of the posi on of the bond and the amount of bonds in the sample, no cracks were observed in any of the rocker arm components.

Figure 5. A video s ll of the FAST-DB forging billet (a) before the first drop hammer blow, and (b) frac ons of a second a�er the first hammer blow. (c) A photograph of a final rocker arm near net shape component.
A select few of the rocker arm components have been sec oned, polished and then heat nted to determine the posi on and the microstructure of the bond region, an example is shown in Figure 6 (a). The sec oned component shows that the bond line has experienced high levels of strain and severely plas cally deformed, yet importantly the bond region is defect free.
The microstructure of the final components had a martensi c structure with parent beta grains, typical of super transus forging followed by water quenching, as shown in Figure 6 (b). Although it was planned to forge at subtransus temperatures (with respect to both tanium alloys), the gas furnace clearly went above 1100°C for short period, as shown in Figure 6(c).

Conclusions
FAST-forge, a two-step hybrid solid-state processing route, has successfully been demonstrated to convert two dissimilar alloy powders into one complex near net shape component. In step 1, Ti-64 and Ti-6242 spherical powders were fully consolidated into a disk with bonds using field assisted sintering technology (FAST-DB). In step 2, 22 mm diameter FAST-DB forging billets were closed-die forged using a tradi onal drop hammer press into rocker arm components. No flaws were found at any stage of the processing, even though the bond region was under severe plas c deforma on during the forging stage.
The technology in this paper demonstrates that small-to-medium sized forgings can be produced in two simple steps from powder. Secondly, different tanium alloys with slightly different proper es can be used in different regions of the part. FAST-forge of dissimilar alloys provides an opportunity for engineers and designers to be�er u lise alloys in specific loca ons.