Microstructural evolution during isothermal annealing of a cold-rolled Al-Mn-Fe-Si alloy with different microchemistry states

In this paper, investigation of the softening behaviour of a supersaturated Al-Mn-Fe-Si alloy during annealing after cold rolling has been carried out. Two different homogenization conditions were considered, of which one gives a condition of a large amount of small pre-existing dispersoids, i.e. providing a significant static Zener drag, while the other gives a condition where both concurrent precipitation and dispersoid drag effects are limited. The homogenized samples with different microchemistry states were then cold-rolled to different strains before subsequent annealing at 300 o C. The softening and concurrent precipitation behaviours have been monitored by hardness and electrical conductivity measurements respectively, and the microstructural evolution has been characterized by EBSD. It is clearly demonstrated that the actual microchemistry state, i.e. amount of solutes and second-phase particle structures as determined by the homogenization procedure strongly influence the softening behaviour where a fine dispersion of pre-existing dispersoids together with concurrent precipitation slow down the recrystallization kinetics considerably and give a very coarse and elongated grain structure. Introduction AA3xxx alloys, which contain large Al-Mn-Fe-Si constituent particles and/or finely dispersed particles (dispersoids), are widely used in automobile industry, architecture and packaging industry. It is widely accepted that second-phase particles are of the utmost importance for the recrystallization of alloys containing such particles [1-4]. In general, large particles can act as nucleation sites to promote nucleation while fine dispersoids can inhibit nucleation process by pinning boundaries at the initial stage of the recrystallization process [5]. The microchemistry state of the alloys, i.e. amount of solutes and second-phase particle structures, determined by the chemical composition and homogenization procedures of the alloys is thus an important aspect in studying the softening behaviour of deformed AA3xxx alloys. The softening behaviour of deformed AA3xxx aluminium alloys during isothermal heat treatments, in terms of recrystallization kinetics, final microstructure and texture, have been extensively investigated in several papers [6-8], some of which already focused on the interactions between dispersoids/precipitation. Less focus, however, has been paid to the temporal microstructure evolution during annealing of AA3xxx alloys [9]. In this paper, the effect of microchemistry states on the softening behaviour of cold-rolled Al-Mn-Fe-Si alloys is analysed during isothermal annealing treatments at 300 o C. Different microchemistry states were obtained by varying the homogenization conditions. The softening behaviours of the deformed samples with different microchemistry states have been monitored by hardness and electrical conductivity measurements, and the microstructural evolution has been characterized by EBSD. Experimental details Alloy conditions and annealing treatment. For the current study, the investigated materials were commercial DC-cast AA3xxx extrusion billets, supplied by Hydro Aluminium. The as-received Materials Science Forum Vols. 794-796 (2014) pp 1163-1168 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.794-796.1163 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 TTP, www.ttp.net. (ID: 129.241.170.145-03/06/14,17:19:11) materials were in as-cast state, with the chemical compositions listed in Table1. The cast materials have an equiaxed grain structure with an average grain size of ~140μm, and constituent particles are mostly decorated in the interdendritic regions and grain boundaries [10]. Table 1 Chemical composition of the AA 3xxx model alloys, in wt. % Alloy Si Fe Mn Others C1 0.152 0.530 0.390 <0.01 The received material (denoted as C1) was subsequently homogenized at two different conditions to get different microchemistries in terms of solutes and second-phase particles. The homogenization treatments were conducted in an air circulation furnace with a temperature accuracy of ± 2 K, starting from room temperature (about 20 °C). One set of the samples were heated at 50 o C/h to 450 o C and kept for 4 hours, refereed as C1-2. Another set of the samples were subjected to a two-stage homogenization treatment. First, the samples were heated at 50 o C/h to 600 o C for 4 hours, then these samples were cooled at 25 o C/h to 500 o C where they were kept for another 4 hours, giving the C1-3 condition. Materials were water quenched to room temperature at the end of the homogenization procedure to freeze the state of supersaturation/precipitation state. The variant homogenized at the lower temperature, i.e. C1-2, has obviously more but finer dispersoids than its counterpart C1-3, as illustrated in Fig.1, noticing that the magnification of the two graphs is different. The sizes of the constituent particles for C1-2 and C1-3 are 0.96 μm and 1.1 μm, respectively. It turned out that the C1-2 variant has higher solid solution level of Mn (0.16 wt.%) than C1-3 (0.11 wt.%), which provides a larger potential for concurrent precipitation, for which more details can be found in reference [8]. Fig.1 BSE-SEM images of the dispersoids for the two homogenized states. a) C1-2; b) C1-3 The homogenized materials were then cold rolled to three different strains ε = 0.7, 1.6 and 3.0. The rolled sheets were then isothermally back-annealed in a pre-heated salt bath at 300 °C and with different holding times in the range of 5-10 5 s, followed by quick water quenching. Microstructure characterization. The softening and precipitation behaviours during annealing were followed by Vickers hardness (VHN) and electrical conductivity (EC) measurements performed on the RD-TD plane of sheets. Each reported value was obtained by averaging eight measurements. EC value was measured by a Sigmascope EX 8 at room temperature of about 293 K (20 °C). Metallographic examinations of constituent particles and dispersoids were performed by backscatter electron channelling contrast imaging in a Zeiss Ultra 55 field emission gun scanning electron microscope (FEG-SEM). Images were captured electronically and analysed using standard image a) b) 1164 Aluminium Alloys 2014 ICAA14