Materials Science Forum ISSN: 1662-9752, Vols. 618-619, pp 513-516 doi:10.4028/www.scientific.net/MSF.618-619.513 © 2009 Trans Tech Publications, Switzerland
Online: 2009-04-17
Consolidation of Titanium, and Ti-6Al-4V Alloy Powders by Powder Compact Forging Deliang Zhang1,a, Stiliana Raynova1, Vijay Nadakuduru1, Peng Cao1, Brian Gabbitas1 and Barry Robinson2 1
Waikato Centre for Advanced Materials (WaiCAM), Department of Engineering, University of Waikato, Private Bag 3105, Hamilton, New Zealand 2 South Auckland Forging Engineering Ltd (SAFE), Auckland, New Zealand a
[email protected]
Keywords: titanium alloy, powder compact forging, powder metallurgy, microstructure, mechanical properties
Abstract Consolidation of titanium and titanium alloy powders using thermomechanical powder metallurgy (TPM) processes (powder compact forging, extrusion and rolling) is one way that can lead to costeffective production of high value-added consolidated titanium and titanium alloy products such as near-net shaped components, tubes and plates. This paper provides an overview of the quality, microstructure (to limited depth), porosity level and mechanical properties of disks produced using open die forging of powder compacts of CP titanium and Ti-6Al-4V alloy powders. The general materials science principles underlying the relationships between processing conditions, microstructure and the mechanical properties of the disks made by using the powder compact forging are discussed. Introduction With the development and commercialisation of processes for cost-effective production of titanium and titanium alloy powders, powder metallurgy (PM) processes become increasingly important for turning titanium and titanium alloy powders into products of different forms for a variety of applications for which the mechanical, physical and chemical properties of titanium and titanium alloys are highly desirable. The forms of titanium alloy PM products include near-net shaped components such as connecting rods for engines and semi-shaped products such as rolled plates, extruded bars, and forged blocks. Up to now, the most widely used PM processes for consolidation of titanium alloy powders are die-pressing plus sintering [e.g., 1-3], powder injection moulding (PIM) plus sintering [e.g., 4-6] and hot isostatic pressing [e.g., 7, 8], while thermomechanical powder metallurgy (TPM) processes such as powder compact forging, extrusion and rolling have recently attracted researchers and engineers’ interests. In TPM processes, which involve plastic deformation of particles in the powder compact, the deformation of powder particles have two beneficial effects: changing the shapes of the particles to better fill the gaps between particles leading to more effective densification; and creating new particle surfaces that can assist with joining the particles. Therefore, TPM processes that involve plastic deformation are more capable of achieving fully densified and fully bonded PM materials with controlled microstructures such as ultrafine grains than powder compact sintering. This study focuses on powder compact forging of titanium and Ti-6Al-4V and aims to achieve a good understanding of the effects of processing conditions on the quality and microstructure of the forged disks produced. Experimental Procedure For powder compact forging of the titanium powder, a -200mesh titanium powder produced by using the hydride-dehydride (UDH) process was used. For the powder compact forging of Ti6wt%Al-4wt%V (Ti-6-4) alloy powders, both atomised and HDH powders were used. The compacts of the HDH titanium and Ti-6-4 alloy powders were produced by die-pressing at room All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 130.216.12.50, University of Auckland, Auckland, New Zealand-03/05/16,23:24:55)
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temperature followed by cold isostatic pressing at a pressure of 200MPa. The green densities were ~ 68% for the titanium powder compacts and 64% for the Ti-6-4 alloy powder compacts. The compacts of the atomised Ti-6-4 alloy powder was produced by warm pressing of a mixture of 200mesh powder and -100 mesh powder at a ratio of 20:80 at 500oC, and had a green density of around 64. The dimensions of the cylindrical powder compacts were 40mm in diameter and 3842mm in height. For the forging experiments, the powder compacts were heated to a temperature around 1200oC using an induction furnace under argon, after forging disks with a thickness of around 6-8mm were produced. The microstructures of the forged disks were characterised using optical and scanning electron microscopy while their mechanical properties were measured by conducting tensile testing using the specimens cut from disks using an electric discharge machining (EDM) cutter. The tensile testing specimens had cross sectional dimensions 2mm x 2.5mm and a gauge length of 20mm. The strain rate used in the tensile testing was 1 x 10-4s-1. The oxygen contents of the powders and forged disks were determined using the LECO inert gas combustion method. Results and Discussion Figure 1 shows the images of typical forged titanium and Ti-6-4 alloy disks produced by powder compact forging. The forged disks made using the HDH titanium powder showed some shallow cracks on the side surface, but did not show any visible cracks on the top and bottom surfaces. The circular mark on the top surface shown in the image was due to the fast top surface cooling of the powder compact during its contact with the top ram for a few seconds before the pressing started. The forged disks made using the atomised Ti-6-4 alloy powder showed large cracks on the side surface and the top and bottom surfaces, while the forged disks made using the HDH Ti-6-4 alloy powder showed large cracks on the side surface and in the narrow region near the perimeter of the disks. This suggests that the compacts made using HDH titanium and Ti-6-4 powders behaved more favourably during forging. The oxygen contents of the titanium forged disk and the Ti-6-4 forged disks produced using the atomised and HDH powders were 0.52, 0.33 and 1.11wt%, respectively, all being clearly higher than the oxygen contents of the starting powders (0.3wt% for HDH powders and 0.2wt% for atomised Ti-6-4 powder). This result shows that there was some oxygen pick up during the consolidation process, either due to the oxygen present in the atmosphere or due to the moisture adsorbed on the particle surfaces in the powder compacts. Further investigation is needed in order to clarify what is the main cause of the oxygen pick up. (a)
(b)
(c)
Figure 1. Disks produced by the powder compact forging: (a) titanium; (b) Ti-6-4 alloy produced using the atomized powder; (c) Ti-6-4 disk produced using the HDH powder. Figure 2 shows the optical micrographs of the forged disks produced using HDH titanium powder and atomised and HDH Ti-6-4 alloy powders respectively. The microstructures of both titanium and Ti-6-4 alloy forged disks showed Widmanstatten patterns, which are a typical feature of the microstructures of titanium containing oxygen (near-alpha alloy) and Ti-6-4 alloy (apha/beta alloy). The cross sections of the central parts of the disks did not show any internal pores or cracks (see Fig. 3(a)), suggesting that the large amount of plastic deformation (~80%) of the particles in the
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compacts was sufficient to fully densify the compacts. However, the inter-particle boundaries were still visible in the Ti-6-4 alloy forged disks as shown by the arrows in Figs. 2(b) and 2(c). It was found that the narrow region (about 1mm wide) near the edge of the forged disks still contained a substantial fraction of pores, as shown in Fig. 3(b). This can be attributed to the lack of a large amount of plastic deformation of the particles due to their free movement in the transverse direction. It is believed that to eliminate the pores, the forged disks need to fill the die cavity completely, so that the pores in the region near the edge can be closed in the last stage of the forging process. (a)
(b)
(c)
Figure 2. Microstructures of the forged disks: (a) titanium; (b) Ti-6-4 alloy produced using the atomized powder; (c) Ti-6-4 produced using the HDH powder. (a)
(b)
Figure 3. SEM micrographs of (a) the central region and (b) the region near the edge of the forged disk produced by powder compact forging of the atomized Ti-6-4 powder. Figure 4 shows the typical engineering stress-strain curves of the specimens cut from the titanium forged disks and Ti-6-4 alloy forged disks made using the HDH powder. The engineering stressstrain curves show that the titanium forged disks have a yield strength of approximately 700MPa, a tensile strength of 800MPa and an elongation to fracture of 2.3%. The yield strength of the samples is higher than that of pure titanium, but their elongation to fracture is far smaller than that of pure titanium. There are several possible reasons for this: the high oxygen content that makes titanium stronger but more brittle; the microstructure caused by the fairly rapid cooling during forging; and some of the inter-particle boundaries being lack of full atomic bonding. Further investigation is needed to clarify which reasons play the dominant role. The engineering stress-strain curves of the Ti-6-4 alloy forged disk made using the HDH powder show an average fracture strength of around 500MPa, and no ductility. The absence of yielding and the fact that the fracture strength of the tensile testing specimens cut from the Ti-6-4 alloy forged disk is so much lower than the yield strength expected of a good quality solid Ti-6-4 alloy made by casting of powder metallurgy (>900MPa) suggest that the inter-particle boundaries are far from being fully atomically bonded. The exact fraction of the inter-particle boundaries that are not fully bonded are not known at this
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point of the investigation. The main reason why the full extent of inter-particle atomic bonding was not achieved while the powder compact was fully densified might be that the temperature of the powder compact was too low to allow a full extent of rapid diffusion bonding between particles when it was forged. Ti
Engineering Stress (MPa)
800
600
Ti64 400
200
0 0.000
0.005
0.010
0.015
0.020
0.025
Strain
Figure 4. Typical tensile engineering stress-strain curves of the titanium (Ti) and Ti-6-4 (Ti64) alloy forged disks produced using the HDH powders. Conclusions Powder compact forging to titanium and Ti-6-4 alloy powders were performed with various degrees of success. For titanium powder, powder compact forging was successful in producing true solid disks with nearly full density and an almost full extent of inter-particle atomic bonding, while for Ti-6-4 alloy powders, it was more difficult to achieve a full extent of inter-particle atomic bonding using powder compact forging. Further research is needed to define the process conditions that are essential for achieving the full or almost full extent of inter-particle atomic bonding during powder compact forging. Acknowledgements The authors would like to thank the Foundation for Research, Science and Technology (FRST), New Zealand, for funding to this work. References [1] Froes, F. H. (2002) Developments in titanium PM: lightweight heavyweight, Metal Powder Report, April, 14-24. [2] Froes, F.H., Mashl, S.J., Moxson, V.S., Hebeisen, J.C. and Duz V.A. (2004) The technologies of titanium powder metallurgy, JOM, November, 46-48. [3] Low, R.J., Robertson, I.M. and Schaffer, G.B. (2007) Excessive porosity after liquid-phase sintering of elemental titanium powder blends, Scripta Materialia 56, 895–898. [4] Froes, F. H. (2006) Getting better: big boost for titanium MIM prospects, Metal Powder Report, December, 20-23. [5] Weil K. S., Nyberg, E., and Simmons K. (2006) A new binder for powder injection molding titanium and other reactive metals, Journal of Materials Processing Technology 176, 205–209. [6] Guo, S., Qu, X., He, X., Zhou, T. and Duan, B.(2006) Powder injection molding of Ti–6Al– 4V alloy, Journal of Materials Processing Technology 173, 310–314. [7] Delo, D.P. and Piehler, H.R. (1999) Early stage consolidation mechanisms during hot isostatic pressing of Ti-6Al-4v powder compacts”, Acta Mater. 47, 2841-2852. [8] Mentz, J. Bram, M., Buchkremer, H. P. and Stöver, D. (2006) Improvement of mechanical properties of powder metallurgical NiTi shape memory alloys, Advanced Engineering Materials 8, 247-252.
Light Metals Technology 2009 10.4028/www.scientific.net/MSF.618-619
Consolidation of Titanium, and Ti-6Al-4V Alloy Powders by Powder Compact Forging 10.4028/www.scientific.net/MSF.618-619.513 DOI References [2] Froes, F.H., Mashl, S.J., Moxson, V.S., Hebeisen, J.C. and Duz V.A. (2004) The technologies of titanium powder metallurgy, JOM, November, 46-48. 10.1007/s11837-004-0252-x [3] Low, R.J., Robertson, I.M. and Schaffer, G.B. (2007) Excessive porosity after liquid-phase sintering of elemental titanium powder blends, Scripta Materialia 56, 895–898. 10.1016/j.scriptamat.2007.01.040 [4] Froes, F. H. (2006) Getting better: big boost for titanium MIM prospects, Metal Powder Report, December, 20-23. 10.1016/S0026-0657(06)70762-7 [5] Weil K. S., Nyberg, E., and Simmons K. (2006) A new binder for powder injection molding titanium and other reactive metals, Journal of Materials Processing Technology 176, 205–209. 10.1016/j.jmatprotec.2006.03.154 [6] Guo, S., Qu, X., He, X., Zhou, T. and Duan, B.(2006) Powder injection molding of Ti–6Al–4V alloy, Journal of Materials Processing Technology 173, 310–314. 10.1016/j.jmatprotec.2005.12.001 [7] Delo, D.P. and Piehler, H.R. (1999) Early stage consolidation mechanisms during hot isostatic pressing of Ti-6Al-4v powder compacts”, Acta Mater. 47, 2841-2852. 10.1016/S1359-6454(99)00132-9 [8] Mentz, J. Bram, M., Buchkremer, H. P. and Stöver, D. (2006) Improvement of mechanical properties of powder metallurgical NiTi shape memory alloys, Advanced Engineering Materials 8, 247-252. 10.1002/adem.200500258
