Thixoforming 7075 aluminium alloys

Abstract

Commercially extruded 7075 alloy (extrusion ratio of 16:1) has been used as a feedstock for thixoforming in order to investigate thixoformability of a high performance aluminium alloy. The microstructure in the semi-solid state consists of fine spheroidal solid grains surrounded by liquid. The results of thixoforming with one step, two-step and three-step induction heating regimes are presented. Typical defects in poorly thixoformed material (e.g. liquid segregation, impedance of flow by unrecrystallised grains and porosity) are shown alongside successfully thixoformed material (thixoforming temperature of between 615 and 618 °C with a three-step induction heating regime). The highest yield strength and elongation obtained for material thixoformed into a simple graphite die and heat treated to the T6 condition is 478 MPa and 6.9% elongation. For thixoforming at 615 °C into a tool steel die heated to 250 °C, the highest yield strength and elongation obtained are 474 MPa and 4.7% (ram velocity 2000 mm/s). These values (particularly for strength) are approaching those of 7075 in the wrought heat treated condition (505 MPa and 11% elongation).

Introduction

Thixoforming is a semi-solid metal (SSM) processing route. The process has been established as a relatively new technology for metal forming [1]. For the alloy to be shaped in the semi-solid state it must have a non-dendritic structure. It then behaves as a ‘thixotropic’ slurry in which viscosity decreases with increasing shear rate and at constant shear rate the viscosity decreases with increasing time. Such alloy slurries flow in a laminar manner, which allows for uniform die filling as opposed to the turbulent flow associated with the fully liquid state (casting) forming processes. Furthermore, the process has high capability for near net shaping because there is less solidification shrinkage than for fully liquid casting. Components have high mechanical integrity with little porosity, making the replacement of heavier materials such as steel for safety critical components technologically feasible. In addition, sections can be made thinner and hence lighter. The main disadvantage of thixoforming is the cost premium for the starting material, which must be in such a condition that when it is reheated into the semi-solid state the required microstructure is obtained. For commercial thixoforming, magnetohydrodynamic (MHD) stirring is usually used to produce the starting material. Various other routes to creating the starting material are available including recrystallisation and partial melting (RAP) [2]. In this route, material is warm worked, below the recrystallisation temperature. When it is then reheated and recrystallisation occurs, liquid penetrates the recrystallised boundaries to give spheroids surrounded by liquid. It is this route which is utilised in the work described here.

In thixoforming, a cylindrical slug of the appropriate size is cut from a bar of starting material (in this case in the extruded state). The slug is then reheated (usually with induction heating) into the semi-solid condition with approximately 30–50% fraction liquid. The temperature of the slug must be carefully controlled in order to obtain a homogeneous (in the sense of uniform volume fraction liquid and uniform spheroid size) microstructure prior to forming. Finally, the slug is forced into the die. Thixoforming is in commercial use but only with the casting alloys such as A356 and A357. These alloys give strength between 220–260 MPa and 8–13% elongation [3]. Therefore one of the major challenges is to develop thixoforming for the higher performance alloys which are normally wrought e.g. 2000 series, 6000 series and 7000 series. The difficulties in thixoforming these alloys centre around the wide interval over which solidification occurs, which can lead to hot tearing [4], and the steep slope for the fraction liquid versus temperature curve in the region of 40% liquid, which leads to narrow processing windows [5]. For example, for 6061 the temperature window for processing between 30 and 50% liquid is only a few K [6].

This study is focused on high strength 7075 wrought aluminium alloy. It is typically used for aerospace applications and is heat treatable to obtain a yield strength of 505 MPa and 11% elongation [7]. The precipitation hardening phase is MgZn₂, provided the ageing temperature is below 200 °C [8]. 7075, with more than 1% Cu also precipitates CuMgAl₂ [8]. The hardening precipitates are up to 0.01 μm in size. Dispersoid particles are also present, based on the transition elements Cr, Zr and Mn. These include Al₁₂Mg₂Cr [9] and Al₁₈Mg₃Cr₂ (E-phase) [10]. These range in size from 0.5–2 μm and play an important role in grain and subgrain boundary pinning. The E-phase particles are of sufficient size and volume fraction to make the alloy difficult to recrystallise. Fe and Si are present in the alloy as impurities. They give rise to constituent phase particles, which are detrimental to most of the mechanical properties of the alloy [9] and are resistant to dissolution. They range in size from 1 to 30 μm.

7075 Alloy has been shown to have the potential for thixoforming [11], [12], [13], [14]. Here results are presented on: microstructural evolution in the semi-solid state; thixoforming with different die materials and ram velocities; mechanical properties of thixoformed material in the T6 condition.