简介概要

Microstructural evolution of spray-formed Al-11.5Zn-2.0Mg-1.6Cu alloy during hot-extrusion and heat-treatment

来源期刊:中国有色金属学报(英文版)2009年增刊第2期

论文作者:郭舒 宁志良 曹福洋 孙剑飞

文章页码:343 - 348

Key words:Al-Zn-Mg-Cu alloy; spray forming; microstructure; hot extrusion; aging

Abstract: Al-11.5Zn-2.0Mg-1.6Cu alloy was synthesized by spray forming (SF) technique followed by hot extrusion and heat-treatment. Its microstructural evolution was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) in the whole process. The curve of hardness as a function of aging time was obtained at 120 ℃. Test results indicate that the grain morphology is equiaxed and uniform in the central region of the spray-formed billet, but fine and irregular in the bottom and top regions. Both the grain boundary and the intragranular phases are identified as MgZn2 intermetallics. Hot extrusion promotes the refinement of the microstructure. Grain boundary phase disappears, meanwhile, η-phase (MgZn2) and Al3Zr particles precipitate from the matrix after extrusion. The alloy reaches its maximum hardness after being aged at 120 ℃ for 14 h, associated with a massive precipitation of intermediate η′-phase (MgZn) in the matrix.



详情信息展示

Microstructural evolution of spray-formed Al-11.5Zn-2.0Mg-1.6Cu alloy during hot-extrusion and heat-treatment

GUO Shu(郭 舒), NING Zhi-liang(宁志良), CAO Fu-yang(曹福洋), SUN Jian-fei(孙剑飞)

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

Received 10 June 2009; accepted 15 August 2009

                                                                                                

Abstract:  Al-11.5Zn-2.0Mg-1.6Cu alloy was synthesized by spray forming (SF) technique followed by hot extrusion and heat-treatment. Its microstructural evolution was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) in the whole process. The curve of hardness as a function of aging time was obtained at 120 ℃. Test results indicate that the grain morphology is equiaxed and uniform in the central region of the spray-formed billet, but fine and irregular in the bottom and top regions. Both the grain boundary and the intragranular phases are identified as MgZn2 intermetallics. Hot extrusion promotes the refinement of the microstructure. Grain boundary phase disappears, meanwhile, η-phase (MgZn2) and Al3Zr particles precipitate from the matrix after extrusion. The alloy reaches its maximum hardness after being aged at 120 ℃ for 14 h, associated with a massive precipitation of intermediate η′-phase (MgZn) in the matrix.

Key words:  Al-Zn-Mg-Cu alloy; spray forming; microstructure; hot extrusion; aging

                                                                                                           


1 Introduction

Al-Zn-Mg-Cu series alloys, commercially trademarked as 7××× Al-alloys, have well-known applications in the field of aircraft industry for their high levels of strength, high fracture toughness and relatively low densities[1]. For this series of alloys, the precipitation-hardening arising from the structural particles, such as GP zones, η′(MgZn) and η(MgZn2), formed at aging stage after solid solution treatment plays a key role in the improvement of their strength performance[2-3]. Thus, it is an effective method to improve their mechanical properties by increasing the numbers of precipitated particles in the matrix of the alloys. Many investigations[2-4] have shown that the aging properties of 7××× Al alloys can be obviously improved by increasing the zinc content, due to the fact that it enhances the extent of solute supersaturation, then promotes precipitation-hardening. However, if the mass fraction of zinc is over 8%, low cooling rate of ingot metallurgy (IM) process results in several serious foundry defects, such as cracking, macro-segregation and coarse microstructure in these alloys. So, high solute alloys can be produced only through rapid solidification processes[5].

In the last decade, more interest focused on using spray forming (SF) technique to improve the strength level of 7××× Al alloys. SF technique, which was firstly introduced by Singer and developed by Osprey Metals Ltd., combines melt atomization and deposition into one single-step process[6]. It shows great advantages in many aspects, for instance, eliminating macro-segregation, promoting the formation of refined grains and extending the solubility of alloy elements[6-8]. Thereby, it is much suitable for the preparation of high solute Al-Zn-Mg-Cu series alloys.

In the present study, a new Al-Zn-Mg-Cu alloy with zinc of 11.5% (mass fraction) was synthesized using the SF process, followed by hot extrusion and heat treatment to obtain end-product performance. The main work was done on characterization of its microstructural evolution in the process. The variation of aging hardness with one-step aging time was also investigated.

2 Experimental

The master alloy with nominal composition of 11.5% Zn, 2.0% Mg, 1.6% Cu, 0.2% Zr and balance Al was prepared from industrial grade materials. The SFexperiment was carried out on a vacuum SF equipment with a close-type atomization nozzle. A rotating deposition substrate was placed under the nozzle. The main parameters used in the experiment are listed in Table 1. During the experiment, the substrate was continuously moved in the opposite direction of billet growing to enable the atomized droplets to fly over a constant distance. The as-deposited billet was machined off to 100 mm in diameter, and subsequently hot- extruded at (400±10) ℃ into round rods at an extrusion ratio of 28?1. The purposes of hot deformation are to eliminate the porosity in deposition and refine grains and the second phases. The extruded rods were treated in salt bath at 480 ℃ for 1.5 h, then immediately quenched in water to room temperature. The one-step temper process was chosen which involved aging at 120 ℃ for up to  80 h.

Table 1 Primary spay forming parameters

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