Innovative Consolidation of Titanium and Titanium Alloy Powders by Direct Rolling

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1 Manuscript refereed by Dr Erich Neubauer, Austrian Institute of Technology GmbH Innovative Consolidation of Titanium and Titanium Alloy Powders by Direct Rolling G. M. D. Cantin 1, P.L. Kean 1, N. A. Stone 1, R. Wilson 1, M. A. Gibson 1, D. Ritchie 1 and R. Rajakumar 2. 1 CSIRO Process Science and Engineering; 2 CSIRO Light Metals Flagship P.O. Box 33, Clayton South, Melbourne, Victoria 3169, Australia. Abstract R&D efforts at CSIRO, Australia, into the production of lower cost powders are complemented by a strong, downstream powder metallurgy program. One of these efforts has focused on the direct powder rolling of commercially pure (CP) titanium powder with a view to the continuous production of fully dense strip. Considerable research is also being undertaken to produce titanium alloy strip, initially from the Ti-6Al-4V alloy, using this process. An experimental design approach has been employed to establish key parameters, maximise the process window and meet property specifications. Demonstration of a proof-ofsystem at pilot scale is well advanced and the focus is now shifting to seeking industrial engagement with a view to collaboration, technology transfer and commercialisation of the technology. The paper will present the current status of the technology including aspects of the associated market trends and commercial feasibility. Introduction The drive to lower the cost of components made from titanium and titanium alloys has led to the development of innovative processes for the production of powders [1,2,3], some examples of these activities include the CSIRO TiRO process and the ITP-Armstrong process. In addition, this has stimulated the development of alternative downstream technologies for the manufacture of semi-finished flat products based on powder metallurgy techniques [4,5,6]. The CSIRO is working on the development of a continuous process for the manufacture of titanium and titanium alloy strip by a powder metallurgy route [4]; in this process powders are compacted by cold rolling to form a green strip, this strip is then fed to a preheating station where it is rapidly heated in an argon atmosphere for a period of up to a few minutes then transferred in a continuous manner to a hot rolling station. The strip exits from the rolls directly into a cooling chamber which is also purged with argon to minimise the pick-up of atmospheric gases. This technique is capable of producing strip from CP titanium powders, pre-alloyed powders, and blended elemental powders. Throughout this paper the terms direct powder rolling (DPR) and hot roll densification (HRD) have been used to describe the green rolling and hot rolling stages of the process, respectively. A significant aspect of this direct powder rolling combined with the hot roll densification technique is the short preheating time prior to hot rolling [4]; this factor directly impacts on productivity. Another important advantage of this process is that both CP grade and titanium alloy strip can be manufactured to thin gauges (<1 mm) in one or two hot rolling passes [4], which significantly reduces material waste, especially the yield losses resulting from surface removal treatments. The intensification of effort in this project has resulted in a recent up-scaling of the facilities including the installation of a new pilot scale hot roll mill scheduled for June 2010, with commissioning anticipated during for the third quarter of the year. This facility consists of a four-high roll system, which will significantly increase the capacity of the process, and is designed for the production of 400 mm wide and nominally mm thick strip. CSIRO is

2 currently actively seeking industry engagement to expand the process from it s current pilot plant status to a production operation. A flow diagram of the process is shown in Figure 1. The stages within the combined DPR and HRD sections have been highlighted by the frames. Figure 1: Flow diagram showing the basic sequence of processes for manufacture of strip by direct powder rolling and hot roll densification. The production of CP titanium strip by this process has been previously described [4,7], and some key results are included here. This paper describes work now in progress to investigate the procedures required to produce Ti-6Al-4V alloy strip using a blended elemental (BE) powder metallurgy approach. The results obtained to date are presented and future work in this area is discussed. Finally, this paper examines the commercial feasibility for manufacture of titanium and titanium alloy strip using DPR/HRD. Experimental Work The manufacture of CP Ti strip using the DPR/HRD process has been discussed in a previous article [7]. In summary, strips with a density greater than 99.5% were fabricated from powders with different morphologies and properties such as angular hydride-dehydride and sponge-like titanium powders. The pick-up of oxygen during the DPR/HRD process was measured to be low (<200 ppm). The mechanical properties of the consolidated and annealed strips were found to be dependent on the oxygen content of the strips, which was in turn influenced by the oxygen content of the powder feedstock. The tensile properties of strips made from powders with an oxygen content of approximately 0.35% and rolled to a percentage reduction of the thickness greater than 40% approach those of the ASTM B265 Grade 3 material.

3 Recent work has been undertaken to produce titanium alloy strip from blended elemental (BE) powders using the DPR/HRD process. The Ti-6Al-4V alloy has been chosen initially as it is a widely used alloy and as such a wealth of data exists on microstructural and mechanical properties. It is expected, however, that other titanium alloys will be able to be made using this process, for example α+β alloys that are more easily cold-workable and have been targeted for the manufacture of thin sheet, strip, foil and tubing by cold processing [8]. For the BE powder metallurgy approach, where titanium powder is blended with an aluminium-vanadium master alloy powder, there is a requirement for the material to be chemically homogenised during the consolidation process. This means that the sequence of thermo-mechanical processes outlined above for the production of CP Ti strip would have to be altered to an extent. One of the primary objectives of this study was to determine the degree to which the process would have to be changed in terms of process parameters, sequence of processing steps and the need for additional stages. The basic DPR/HRD process consists of numerous stages as shown earlier in Figure 1, further, each stage is characterised by a number parameters each of which is expected to have an influence on the material. As such, it was decided to employ an experimental design methodology to determine the influence of material, processing and heat treatment variables on the microstructure and mechanical properties of the Ti-6Al-4V strip. The following considerations were taken into account when designing the matrix of experiments: The finished material should be completely homogenised, while the productivity of the overall process should be maintained. It should be possible to manipulate by thermo-mechanical processing the microstructure of the finished strip in order to achieve similar microstructures and mechanical properties as those obtained for wrought Ti-6Al-4V products. As a starting point, the levels for the parameters used at each step of the process were based on those typically used to manufacture Ti-6Al-4V products by ingot metallurgy. The details of these parameters as well as the details of the starting powders, blending and DPR procedures, and properties of the green strips have been published previously [9]. A brief summary of the relevant aspects is given here. The batch of BE Ti-6Al-4V powder was made by mixing -100 mesh hydride-dehydride powder with both Al-V master alloy powder and Al powder. The oxygen contents of the titanium and master alloy powders were 0.24% and 0.19%, respectively, and the nitrogen content was 0.02% for both powders. The blended powder was rolled into green strips with thicknesses of about 2.2 mm. In that work, because of the relatively large number of experimental conditions, and therefore specimens to be produced, a small-scale facility operating in a continuous manner as described above for the full scale process was used for the preheating and hot rolling stages. As a result the green strips were sectioned to produce smaller specimens with dimensions 115 mm long by 25 mm wide for hot rolling. The green density of the specimens varied between 84% and 90%. In the previous study [9] it was reported that a homogeneous Ti-6Al-4V strip and with a density of 99.6% could be produced at the end of the annealing treatments. The annealing temperature, however, had to be above the β transus,1030 C-1040 C, which resulted in a microstructure consisting of coarse α+β lamellae as shown in Figure 2(a) and (c). Tensile data were obtained from specimens cut in the direction of rolling of this strip and with an average oxygen content of 0.23%; the ultimate tensile strength was 1061 MPa, the yield strength was 970 MPa and the elongation to failure was 8.8 %. The dimensions of the rolled strips did not allow the properties transverse to the rolling direction to be determined, these will be obtained when larger scale strips are made. Further work has now been undertaken to improve the properties of Ti-6Al-4V strip made by this process and amongst others has included investigating the influence of a reduction in the particle size distribution (PSD) of the master alloy powder. The d 50 of the distribution has been decreased from 42 µm (used in previous work [9] and referred to here as case A) to less than 30 µm (referred to here as case B). In case B, -100 mesh hydride-dehydride

4 titanium powder was blended with the Al-V master alloy powder and Al powder. The oxygen contents of the titanium and master alloy powders were 0.24% and 0.20%, respectively, and the nitrogen content was 0.02% for both powders. The results of this investigation are discussed here. The same processing sequence was employed for the production of strips in both case A and case B. In both cases, the strips made by DPR were hot rolled twice and achieved a similar level of percentage reduction of the thickness. For the first hot rolling pass a preheat temperature high in the β phase field, but below 1320 C to avoid localised melting of the master alloy particles, was employed. The preheat temperature prior to the second hot rolling pass was, in case A, also in the β phase field, whereas it was within the α+β region in case B. Following hot rolling the strips were annealed at close to 1040 C for case A and 995 C for case B. Furnace cooling was employed in both variants. The objective of using lower temperatures for preheating and annealing in case B was in an attempt to refine the microstructure. In addition these experiments were carried out as part of larger experimental matrices, aimed to determine parameter windows for the production of this alloy using DPR and HRD, and the effects of these parameters on the mechanical properties and microstructure. (a) (b) (c) (d) Figure 2: Optical micrographs of longitudinal sections of the Ti-6Al-4V strips showing the microstructure of the specimen obtained in (a) and (c) Case A [9], and (b) and (d) Case B. A study of the microstructure of the material from the case B experiments has been undertaken, optical micrographs of the longitudinal section of the strip are shown in Figure 2(b) and (d). Following the trend observed in case A, the degree of homogeneity of the

5 microstructure significantly increased only after employing an annealing heat treatment at a temperature near the β transus. The microstructure of the strip from case B (Figure 2(b) and (d)) appears to consist of bands of grains with equiaxed and lamellar morphologies. The equiaxed grains are approximately 9 to 20 µm in size. It is possible that this banded structure reflects the as-rolled microstructure; the possible causes of which are being investigated. These could include varying levels of strain through the strip thickness, the effects of temperature gradients and the process of homogenisation itself. So although it seems that a finer microstructure can be achieved in the Ti-6Al-4V alloy strip, further work is required to optimise the conditions that will fully homogenise and recrystallise the microstructure. This will be followed by a determination of the mechanical properties of the finished strips. Titanium market trends, commercial feasibility and application of DPR/HRD technology The simplified processing operation of DPR/HRD, with strip production at near final gauge thickness and correspondingly high yield all contribute to a low cost production route. The strip produced via this process is expected to demonstrate equivalent properties to conventional material potentially at a lower cost and therefore could target replacement of stainless steel. The driver for substitution in this case would be purely based on economics and so minimisation of process steps and low cost feedstock are both essential. The ITA reported in November 2008 that titanium applications in the Plate Heat Exchanger (PHE) market only represent about 30% of the market. This is an industry with good growth prospects, limited volatility and is estimated to be worth in the order of $4B globally. Factors that have been identified as limiting titanium usage in these applications are cost and lead time. Similarly, welded tube applications offer significant potential for replacing stainless steel. In addition to targeting stainless steel replacement, the ability to manufacture strip from either pre-alloyed powder or blended elemental powders means that new, novel alloys and even functionally graded materials can be contemplated that would not be possible via traditional melt routes. Techno-economic modelling of the DPR/HRD process has indicated that a facility producing ~3,500 tpa would have a direct conversion cost of <$4/kg. Clearly, a major factor in the production of DPR/HRD strip at reduced cost is the cost of the powder feedstock. Whilst the economies of scale are yet to be fully demonstrated for these lower cost powders, it is worth noting that DPR/HRD also plays a complementary role to conventional production technologies and is capable of adding to the overall growth of the titanium industry. To more clearly understand how DPR/HRD could fit into the titanium production landscape, it is useful to look at the significant volatility seen in the industry in the last 5-10 years. This volatility has been attributed to a coincidence of various supply side and demand side drivers [10]. For example, on the supply side, the surge in China s steel consumption in the early 2000 s increased prices for ferrotitanium, resulting in increased demand for scrap and sponge as a substitute. However the low aircraft production rates around 2003 meant that there was a shortage of scrap. By 2005, the Defence Logistics Agency (DLA) stockpile had been sold down and the surge in aircraft orders during resulted in intensifying the shortages. On the demand side, commercial aircraft orders were running at record levels through , these new generation aircraft also had significantly higher titanium content. In the military area, full time production of the F22 had commenced and there was an increased requirement for military armour. In addition to this, industrial demand was also running at significant levels.

6 Having just emerged from several lean years in the late 90 s and early 2000 s and with a new sponge facility requiring $ m of investment and 3 years to construct, the industry could not respond fast enough to the demand. Understandably, where industry is considering such significant capital investment and lead times to increase capacity, there is also a reluctance to move too soon in case the demand is not sustained. More recently, delays in the Boeing Dreamliner and the GFC of 2008/9 have also played their part in contributing to the overall volatility. Aerospace companies are familiar with the volatility of the titanium industry and are willing to enter into long term contracts in an attempt to mitigate some of these fluctuations. However, unexpected demand pressures, limited supply, capital constraints on adding new capacity, all result in significant spot price fluctuations and extended product delivery times. For many industrial applications, this level of volatility in spot price and delivery lead time is a major disincentive to use titanium. As a result, substitution with other lesser performing, but cheaper or more easily obtainable materials, can often occur. Sustainable long term growth in the titanium industry requires, apart from just a reduction in the cost of alloys, the development of an industry that is more responsive to market changes and less dependent on the significant economies of volume to justify very large capital investments. To look at other industry models in this context can be enlightening. Today for example, steel strip is made in two completely different types of mills the differences are not just in size but also in their approach to the market and product requirements. Conventional integrated steel mills usually have a capacity of around 3-5 million tonnes per annum. These mills are oriented to large customers with high quality and tonnage requirements and their location is not bound to their customers geographically. The numerous operations allow for the rectification of some initial defects and thus production of high quality strip. However, the capital and labour intensive nature of the process requires throughputs of several millions tonnes to be profitable. At the other end of the scale, mini mills have production capacity of million tonnes. They are based on the concept of near net shape production, for customers who are not in need of large tonnage or aerospace-certified materials. The process is dependant on a different feedstock, is compact, easy to control and does not require large investment in either capital equipment or labour. Whilst at one level, the integrated mill and the mini mill may be seen as competing for market share, it can also be demonstrated that the mini mill concept can be successfully incorporated into a conventional integrated mill operation as a relatively low cost capacity upgrade. In the context of the titanium industry, the relatively low capital cost and simplified processing in a DPR/HRD facility means that installation can be modular as the market increases and is analogous to the mini mill concept. In this context, it can be seen that a DPR/HRD installation is ideally suited to: 1. Offering greater manufacturing flexibility in terms of lead times and material grades and alloys. 2. Mature markets where incremental increases in production capacity are required. For example, the ability to add incremental capacity for industrial applications, thus freeing up more mill time for premium-qualified aerospace applications. This would represent a significant alternative for capacity upgrade and flexibility for conventional mills. 3. Developing markets where there is a demand for relatively small quantities of a wide range of alloys.

7 Conclusions CSIRO is developing a process for the manufacture of titanium and titanium alloy strip products using a powder metallurgy based approach. The process combines, in a continuous manner, direct powder rolling (DPR) of powders with hot roll densification (HRD) to produce a consolidated strip. Commercially pure (CP) titanium strip made using this process has a final density greater than 99.5%. It has been found that contamination of the strip with atmospheric gases during processing is low. The tensile properties, therefore, of the material after annealing tend to be dependent on the oxygen content of the feedstock titanium powder. The work being undertaken to produce Ti-6Al-4V alloy strip by this process is focussed on expanding the window of parameters that will result in a homogeneous material and with the tensile properties required for the ASTM B265 Grade 5 alloy. A homogeneous but coarse microstructure is achieved after the hot rolled strip has been annealed at temperatures above the β transus. It has been found, however, that the annealed microstructure may be refined by reducing the particle size of the master alloy powder, by using a lower preheat temperature prior to the second hot rolling pass, and by decreasing the temperature for annealing closer to the β transus temperature. The CSIRO DPR/HDR process offers: 1. Low cost production of strip via simplified processes. 2. The ability to manufacture new and novel alloys not possible via conventional melt processing. 3. Enhanced production flexibility, with lower capital and operating costs compared to traditional production routes Efforts at CSIRO to expand its activities in this area have led to the up-scaling of the HRD facility. A new four-high hot roll mill capable of processing strip designed for 400 mm wide and <3 mm thick is being installed in the second half of This mill a will be set up in-line with the existing powder rolling mill and preheating furnace. References 1. G. A. Wellwood, C. Doblin, C. Dobrin and G. A. Villewood, WO A1, J. Haidar, S. Gnanarajan and J. B. Dunlop, WO A1, D. R. Armstrong, S. S. Borys and R. P. Anderson, US A1, N. A. Stone, R. Wilson, Y. Merchant and M. A. Gibson, PCT WO 2008/ A1, V. S. Moxson and V. A. Duz, US Patent 7,311,873 B2, D. Eylon and F. H. Froes, US Patent 4,917,858, N. A. Stone, D. Cantin, M. Gibson, S. Lathabai, D. Ritchie, R. Wilson, M. Yousuff, R. Rajakumar and K. Rogers, A Continuous Process for Production of CP Titanium Sheet by Direct Powder Rolling, Materials Science Forum, , 2009, J. J. Hebda, R. Hickman and R. A. Graham, Processing of Titanium-Aluminium- Vanadium Alloys and Products Made Thereby, PCT WO 2004/ A1, G. M. D. Cantin, N. A. Stone, D. Alexander, M. A. Gibson, D. Ritchie, R. Wilson, M. Yousuff, R. Rajakumar, and K. Rogers, Production of Ti-6Al-4V Strip by Direct Rolling of Blended Elemental Powder, PRICM7, Materials Science Forum, , J. S. Seong, O. Younossi, B. W. Goldsmith, Titanium: Industrial Base, Price Trends, and Technology Initiatives, RAND Corporation, 2009.

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