Recent Advances of Titanium Alloy Powder Production by Ceramic-free Inert Gas Atomization

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1 Titanium2008 International Titanium Association September 21-24, 2008 CAESARS PALACE LAS VEGAS, NEVADA USA Recent Advances of Titanium Alloy Powder Production by Ceramic-free Inert Gas Atomization Henrik Franz 1, Laurenz Plöchl 1, Dr.Frank-Peter Schimansky 2 1 ALD Vacuum Technologies GmbH, Wilhelm-Rohn-Str.35, D Hanau, Germany 2 GKSS Research Center, Max-Planck-Str. 1, D Geestacht, Germany ABSTRACT CP-Ti and γ-tial barsticks have been atomized by the ceramic-free Electrode Induction-melt Inert Gas Atomization (EIGA) technique. To date, the EIGA technique had been limited to relatively small feedstock dimensions (ca. Ø50-60mm) and relatively low melt flow rates (ca kg/h). In this work, the feedstock dimensions and melt flow rates were significantly increased (ca. triplicated). Steady-state process conditions have been achieved at melt flow rates of up to 90 kg/h with feedstock dimensions of up to Ø140mm. These achievements enable to utilize titanium alloy VAR electrodes as feedstock for the EIGA and to atomize titanium powder at significantly lower specific gas consumption. INTRODUCTION Clean, spherical titanium alloy powder is used for various novel powder metallurgical processing routes such as Metal Injection Molding (MIM), rapid prototyping by laser sintering [1,2] as well as Hot Isostatic Pressing (HIP) and subsequent hot forming in the medical [3,4] and aerospace industries [5,6,7]. Typical alloy grades are CP-Ti, TiAl6V4 and also intermetallic γ-tial. Due to the reactivity and high melting point of these alloys, only cold crucible or crucible-free melting techniques in combination with argon atomization can be applied. One technique capable of delivering the required powder quality, fine powder yield and cost efficiency is Electrode Induction-melt Inert Gas Atomization (EIGA) [8]. Melt flow rates of up to 50 kg/h with electrode diameters up to 60mm have been demonstrated with titanium [9]. The present work aimed at further and significant increase of the melt flow rate and electrode diameter with the ultimate objective of making the EIGA technique suitable for the utilization of commercially available titanium alloy VAR (Vacuum Arc Remelting) electrode feedstock, which is available at significantly lower cost than hot-forged titanium rods but only in diameters 150mm (6 in.). Furthermore, the increase of the melt-flow rate will lead to a proportional decrease of the specific inert gas consumption during atomization. DESCRIPTION OF EIGA TECHNIQUE Electrode Induction-melt Inert Gas Atomization (EIGA) is a technique for powder manufacturing by gas atomization. The process can be conducted ceramic-free and is therefore especially suited for reactive and refractory metals/alloys (e.g. TiAl6V4, γ-tial). The EIGA technique [8] is schematically shown in fig. 1. The prealloyed electrode is immersed into a conical induction coil. By induction of a high-frequency electro-magnetic field into the electrode tip, the latter is heated up to melting temperature. The liquid metal flows downward along the surface of the heated cone and falls into a gas nozzle, were it is atomized using Ar gas. The melt droplets solidify during free fall in the atomization tower, are separated from the Ar gas in the downstream cyclone and collected under Ar atmosphere in a vacuum-tight powder can. The self-consuming electrode is continuously fed downwards into the induction coil by an electric drive system. Recent developments [10,11] allow the utilization of a bare, non-insulated copper coil without the occurrence of spark discharge between the coil and the electrode, thus providing a ceramic-free atomization technique resulting in contamination-free powder. EIGA process parameters which determine the powder cost are: page 1 of 4

2 Titanium2008 International Titanium Association September 21-24, 2008 CAESARS PALACE LAS VEGAS, NEVADA USA fig.1 EIGA schematic, showing conical induction coil, inert gas nozzle, atomization tower and powder collection system As a result of the present work, not only the atomization conversion cost is further reduced by further, significant increase of the melt flow rate, but also the titanium feedstock cost itself is addressed. ATOMIZATION EXPERIMENTS For up-scaling atomization experiments were carried out with CP-Ti and γ-tial feedstock, ranging between 60 and 150mm in diameter. For each barstick diameter a dedicated, conical, non-insulated copper induction coil was manufactured. Fig.2 illustrates well the size-step taken in the present work, showing the induction coils for 40 and 120mm diameter, respectively. fig.2 Conical induction coil for 40mm (left) and 150mm (right) electrode feedstock. Large coil has non-insulated windings. cost of the feedstock material; melt flow rate [kg/h], [lb/min]; specific Ar gas consumption (gas flow per mass unit powder [Sm 3 /kg], [scf/lb]); powder yield of the useful fraction; amount of satellite formation (in most cases well expressed by the powder tap density) Since the Ar gas flow itself cannot be reduced below a technical limit which is determined by the adjustment of the gas nozzle gap and the required gas nozzle aspiration pressure, recent work [9] has focused on the increase of the melt flow rate by design optimization of the resonance circuit and induction coil, leading to a directly proportional decrease of the specific gas consumption without significant change of the powder particle size distribution. The LC-oscillating circuit of the EIGA equipment was tuned to a frequency in the range of khz and in such a way as to run the trials just below the maximum admissible voltage of the capacitor bank. After switching on the HF-power, the melt started to drop off the electrode tip usually within 1-2 min. For compensation of alignment tolerances between the electrode and the coil as well as of manufacturing tolerances of the coil itself, the electrode was rotated at a slow speed (ca. 5 rpm). In order to achieve a continuous, steady-state melt-flow, the vertical feed rate of the electrode was gradually increased such as to keep the immersion of the electrode tip into the coil constant. page 2 of 4

3 Titanium2008 International Titanium Association September 21-24, 2008 CAESARS PALACE LAS VEGAS, NEVADA USA In all trials, the atomization gas pressure was adjusted to 25 bar. Argon was used as atomization gas. For each electrode a quantity of g powder was atomized and representative powder samples of g were obtained by a sample divider. The powder samples were characterized by sieve analysis and the fine powder fraction (<45µm) was determined. RESULTS Table1 provides a summary of the atomization trial conditions and results. The larger annular slit diameter for trials no.4-8 was chosen in order to provide the alignment tolerances for the larger induction coil diameter. Atomization of γ-tial electrodes resulted in a better fine powder yield. In trial no.8, steady-state melt-flow was not achieved due to EIGA equipment power limits. trial no. alloygrade electrode diameter [mm] steady-state melt flow rate [kg/h] 1 CP-Ti CP-Ti γ-tial CP-Ti CP-Ti CP-Ti γ-tial CP-Ti 150 (60) trial no. annular slit dia. [mm] fine powder yield <45µm [wt%] remark better fine powder yield with TiAl better fine powder yield with TiAl steady-state not reached table 1 atomization trial results A continuous melt flow at the electrode tip could be established in all trials, also with larger electrode diameters. The fine powder yield is generally lower for the larger electrode diameters 100 mm because of the utilization of a larger gas nozzle annular slit diameter. fig.3 CP-Ti EIGA powder A SEM image of the fine powder fraction of CP- Ti is shown in fig.3. CONCLUSION / OUTLOOK Melt flow rates of up to 90 kg/h have been demonstrated with the EIGA technique, using electrode diameters up to 150mm of CP-Ti and γ-tial. A continuous, steady-state melt flow was achieved. This represents approximately a tripling of the melt flow rate and corresponding cutting in three of the specific Ar consumption compared to the previous status. With the other atomization parameters being the same, better fine powder yields could be obtained with γ-tial compared to CP-Ti. This is probably caused by different physical properties of both alloy melts (such as melting point, surface tension, melt viscosity). page 3 of 4

4 Titanium2008 International Titanium Association September 21-24, 2008 CAESARS PALACE LAS VEGAS, NEVADA USA The utilization of mm electrode diameter represents a qualitative breakthrough, allowing for the first time to use titanium alloy VAR electrode feedstock directly (instead of hot-forged rod material), which is available more readily and at lower cost. REFERENCES [1] N. Calder, M. Hedges, Near Net Shape Rapid Manufacture and Repair by LENS, AVT-139 Specialists Meeting on Cost Effective Manufacturing via Net-Shape Processing, Amsterdam, The Netherlands, May 2006 [2] M. Hedges, R.Grylls, Euro-uRapid 2007 Conference Proceedings [3] W. Limberg, E. Aust, T. Ebel, R. Gerling and B. Oger, Euro PM 2004 Conference Proceedings, Eds. H. Danninger, R. Ratzi, EPMA, Shrewsbury, UK, 2004, Vol. 4, p [4] C. Over, W. Meiners, K. Wissenbach and R. Poprawe, Ti 2003, Science and Technology, Eds. G. Lütjering and J. Albrecht,WILEY- VCH, Weinheim 2004, Vol. I, p [5] D. Furrer and R. Boyer, Ti 2003, Science and Technology, Eds. G. Lütjering and J. Albrecht,WILEY- VCH, Weinheim 2004, Vol. I, p [6] R. Gerling, A. Bartels, H. Clemens, H. Kestler, F. P. Schimansky, Intermetallics, Vol. 12 (2004), p [7] S. Bystrzanowski, A. Bartels, H. Clemens, R. Gerling, F.P. Schimansky, G. Dehm, M. Weller and H. Kestler, Intermetallics Vol. 13 (2005), p [8] M. Hohmann and N. Ludwig, German Patent DE A1,1991. [9] R. Gerling, M. Hohmann, F. P. Schimansky, Thermec 2006, Materials Science Forum Vols (2007) pp [10] S. Pleier, M. Hohmann, W. Goy and B. Schaub, Euro PM 2004 Conference Proceedings, Eds. H. Danninger, R. Ratzi, EPMA, Shrewsbury, UK, 2004, Vol. 1, p. 89. [11] R. Gerling and F. P. Schimansky, Euro PM 2004 Conference Proceedings, Eds. H. Danninger, R. Ratzi, EPMA, Shrewsbury, UK, 2004, Vol. 1, p. 77. [12] G. Wegmann, R. Gerling, F.P. Schimansky, Acta Materialia, Vol.51 (2003),pp CONTACT Henrik Franz ALD Vacuum Technologies GmbH Wilhelm-Rohn-Str.35 D Hanau / Germany Tel.+49 (6181) henrik.franz@ald-vt.de Laurenz Plöchl ALD Vacuum Technologies GmbH Wilhelm-Rohn-Str.35 D Hanau Tel.+49 (6181) laurenz.ploechl@ald-vt.de Dr.Frank-Peter Schimansky GKSS Research Center Max-Planck-Str. 1 D Geestacht / Germany Tel. +49 (4152) Frank-Peter.Schimansky@gkss.de page 4 of 4

5 Henrik Franz 1,a, Laurenz Plöchl 1,b, Dr.Frank-Peter Schimansky 2,c 1 ALD Vacuum Technologies GmbH, Wilhelm-Rohn-Str.35, D Hanau, Germany 2 GKSS Research Center, Max-Planck-Str. 1, D Geestacht, Germany a henrik.franz@ald-vt.de, de b laurenz.ploechl@ald-vt.de, de c frank-peter.schimansky@gkss.de

6 Presentation Contents Description of EIGA Atomization Process Previous Status and Limits - Large EIGA Development Objectives Atomization Experiments Results Conclusion and Outlook

7 Description of EIGA Atomization Process Frequency Voltage Current Rotational Speed Vertical Feed Coil Immersion Depth of Melt Power of Electrode to match Physical Properties Drip/Melt Flow Behavior of Feedstock Material (Electrode) HF EM-field (ca.200 khz) electrode tip temperature field drip melting electrode tip

8 Description of EIGA Atomization Process EIGA: Electrode Induction-melt Inert Gas Atomization

9 EIGA Characteristics suitable for all metallic materials and especially for reactive, refractory and precious metal alloys (Ti, Zr, Hf, V, Cr, Nb, Mo, Pt) d 50 usually > 60µm (free-fall fall atomizing system) spherical powder super-clean powder, no ceramic inclusions from melting process 100 Batch process with quick electrode change-over 90 (few minutes) Cum m. Volume Percen ntage [%] Description of EIGA Atomization Process Example Particle Size Distributions of EIGA powder Stainless Steel, Ø 45/65 mm Ti-Alloy, Ø 60 mm Ti-Alloy, Ø 45 mm Niobium-Alloy, Ø 35 mm Particle Diameter [μ m] TiAl6V4 EIGA powder (fraction <45µm)

10 Previous Status and Limits atomization rate 50 kg/h batch (electrode) weight ranging from 5 to 50kg, depending on alloy mass density specific inert gas consumption approx. 10 Sm 3 /kg due to limited coil size and electrode diameter Large EIGA Development Objectives Increase significantly the electrode diameter Thereby increase significantly the metal flow rate Thereby significantly decrease the specific inert gas consumption 40mm Ti electrode 120mm Ti electrode

11 Atomization Experiments Fabrication of large diameter conical induction coils (non-insulated copper coils ceramic completely eliminated) Set proper process parameters (U, I, electrode tip immersion into coil) depending on electrode diameter for melt-flow start-up Achieve and uphold steady-state state melt-flow (steady-state state is characterized by stabilization/constant values of U, I and electrode tip immersion in dynamic equilibrium with vertical electrode feed rate) Atomization of Ø60, Ø80, Ø100, Ø120, Ø150 mm CP-Ti electrodes (For each electrode diameter a dedicated conical induction coil was fabricated.) Atomization of Ø80, Ø140 mm γ-tial electrodes Production of g powder per atomization batch Division into representative samples of g 120g Sieve analysis determination of fine powder fraction <45µm >> Movie Clip Ø100mm CP-Ti

12 Results trial electrode alloy gas (Ar) steady-state melt gas nozzle fine powder remark no. Ø [mm] pressure flow rate [kg/h] annular slit fraction [bar] diam. [mm] <45µm (wt%) 1 60 CP-Ti CP-Ti γ-tial better fine powder yield for TiAl CP-Ti CP-Ti CP-Ti γ-tial better fine powder yield for TiAl CP-Ti 25 (60) steady-state not reached due to equipment power limit

13 Results A continuous melt flow at the electrode tip could be established in all trials, also with larger electrode diameters. The fine powder yield is generally lower for the larger electrode diameters 100 mm because of the utilization of a larger gas nozzle annular slit diameter. The larger annular slit diameter for trials no.4-8 was chosen in order to provide the alignment tolerances for the larger induction coil diameter. Atomization of γ-tial electrodes resulted in a better fine powder yield. In trial no.8, steady-state melt-flow was not achieved due to EIGA equipment power limits. CP-Ti EIGA powder

14 Conclusion and Outlook Melt flow rates of up to 90 kg/h have been demonstrated with the EIGA technique, using electrode diameters up to 150mm of CP-Ti and γ-tial. A continuous, steady-state melt flow was achieved. This represents approximately a tripling of the melt flow rate and corresponding cutting in three of the specific Ar consumption compared to the previous status. With the other atomization parameters being the same, better fine powder yields could be obtained with γ-tial compared to CP-Ti. This is probably caused by different physical properties p of both alloy melts (such as melting point, surface tension, melt viscosity). The utilization of mm electrode diameter represents a qualitative breakthrough, allowing for the first time to use titanium alloy VAR electrode feedstock directly (instead of hot-forged rod material), which is available more readily and at lower cost.