Please cite this article in press as: R. Lapovok, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.01.075 ARTICLE IN PRESS +Model MSA-23951; No. of Pages 10 Materials Science and Engineering A xxx (2008) xxx–xxx Low-temperature compaction of Ti–6Al–4V powder using equal channel angular extrusion with back pressure R. Lapovok ∗ , D. Tomus, B.C. Muddle ARC Centre of Excellence for Design in Light Metals, Department of Materials Engineering, Monash University, 3800 Victoria, Australia Received 1 August 2007; received in revised form 3 January 2008; accepted 4 January 2008 Abstract Equal channel angular extrusion (ECAE), with simultaneous application of back pressure, has been applied to the consolidation of 10 mm diameter billets of pre-alloyed, hydride–dehydride Ti–6Al–4V powder at temperatures ≤400 ◦ C. The upper limit to processing temperature was chosen to minimise the potential for contamination with gaseous constituents potentially harmful to properties of consolidated product. It has been demonstrated that the application of ECAE with imposed hydrostatic pressure permits consolidation to in excess of 96% relative density at temperatures in the range 100–400 ◦ C, and in excess of 98% at 400 ◦ C with applied back pressure ≥175 MPa. ECAE compaction at 20 ◦ C (back pressure = 262 MPa) produced billet with 95.6% relative density, but minimal green strength. At an extrusion temperature of 400 ◦ C, the relative density increased to 98.3%, for similar processing conditions, and the green strength increased to a maximum 750 MPa. The relative density of compacts produced at 400 ◦ C increased from 96.8 to 98.6% with increase in applied back pressure from 20 to 480 MPa, while Vickers hardness increased from 360 to 412HV. The key to the effective low-temperature compaction achieved is the severe shear deformation experienced during ECAE, combined with the superimposed hydrostatic pressure. © 2008 Elsevier B.V. All rights reserved. Keywords: Ti–6Al–4V alloy; Powder compaction; Equal channel angular extrusion; Back pressure; Pores; Density 1. Introduction Alloys of Ti can have highly attractive and, in some cases, unique combinations of mechanical properties, achieved at a density approximately half that of steels. However, the produc- tion of both Ti metal and wrought Ti alloys is highly energy consuming and the resulting high cost of conventional solid- ification processing of Ti billets currently severely limits the potentially wider application of Ti alloys. The recent emergence of extractive processes promising lower cost Ti metal in powder form has stimulated a resurgence of interest in alternative pow- der metallurgy processes which might have significantly lower energy requirements and thus lower costs of production. An ideal secondary processing path might see direct net-shape forming of powder to manufactured product, most likely utilising pre- alloyed (PA) powder stock to ensure compacts with uniform composition and a high level of mechanical properties [1]. ∗ Corresponding author. E-mail address: Rimma.Lapovok@eng.monash.edu.au (R. Lapovok). The most common of the various compaction processes for consolidating PA powder is hot isostatic pressing (HIP), in which a container filled with PA powder is hermetically sealed after an evacuation cycle and subjected to hydrostatic pressure at high temperature in a pressure vessel. The extent of sintering result- ing from HIP depends on the packing density of the powder, which depends in turn on the shape and size distribution of the particles [2]. For Ti–6 wt.% Al–4 wt.% V powder, HIP is typi- cally performed at temperatures of 845–955 ◦ C and a pressure of ∼105 MPa for 2–4 h. However, the presence of residual porosity after sintering [3,4] remains a common problem, which influ- ences significantly the quality and mechanical properties of the compact. The potential for contamination of powder and com- pact with gaseous impurity constituents at temperatures above 400 ◦ C [12] is also quite high, which again can be harmful to resultant properties. A report [5] of the effects of HIP processing conditions, including temperatures in the range 20–850 ◦ C, pressures between 10 and 60 MPa, and compaction times up to 1 h, on the consolidation of Ti–6Al–4V powder have shown that rel- ative densities in the range 98–100% can only be obtained at processing temperatures in excess of 800 ◦ C. For such pressing 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.01.075