Three-Stage Homogenization of Al-Zn-Mg-Cu Alloys Containing Trace Zr
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I.
INTRODUCTION
AL-ZN-MG-CU (7xxx series) alloy materials are widely used in the aeronautical and astronautic industries because of their excellent mechanical properties and low densities.[1,2] However, recrystallized grains that emerge during the processes of hot-rolling and solution treatment can seriously deteriorate the overall properties of the as-aged materials.[3,4] Two ways to obtain an unrecrystallized alloy containing a large number of subgrains are (1) enforcing strict control of the hot-rolling and heat treatment, and (2) the addition of small quantities of a trace element, such as zirconium. The coherent and metastable Al3Zr dispersoids are capable of exerting a strong Zener pinning effect on the grain boundaries during hot-rolling and solution treatment, improving the overall properties.[5–10] However, the zirconium segregates toward the center of the grains, resulting in a precipitate-free zone (PFZ) of Al3Zr dispersoids in the grain boundary areas during homogenization treatment.[6,8] Although the width of the PFZ might be lower for slow heating or a two-stage homogenization process, the sizes of the Al3Zr dispersoids near the grain boundaries were relatively large, with a heterogeneous distribution.[11] In the current study, the effect of the pre-precipitated MgZn2 particles on the precipitation of Al3Zr dispersoids during the homogenization process in an Al7.8Zn-1.62Mg-1.81Cu (AA7085) alloy was investigated. A three-stage homogenization process was designed to YUN-LAI DENG, Professor, YUNYA ZHANG, and LI WAN, Graduate Students, and XINMING ZHANG, Professor, are with the School of Materials Science and Engineering, Central South University, Room 114, Te Ye Building, Changsha, Hunan 410083, P.R. China. Contact e-mail: [email protected] ALF A. ZHU, Senior Scientist, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904. Manuscript submitted October 1, 2012. Article published online February 12, 2013 2470—VOLUME 44A, JUNE 2013
enhance the homogenous precipitation of Al3Zr dispersoids inside and along the grain boundaries.
II.
EXPERIMENTAL PROCEDURE
The materials in the current study were prepared by ingot metallurgy in the laboratory. The raw materials were high-purity Al (99.998 pct), high-purity Zn (99.98 pct), high-purity Mg (99.98 pct), Al-3 pct Zr, Al-30 pct Cu, and Al-5 pct Ti-B (wt pct). The alloy to be tested was melted in a graphite crucible and was heated using an electrical resistance furnace. During melting, environmental conditions were considered and strictly controlled to sustain the purity of the alloy. The liquid metal was poured into an iron mold to produce a 30 9 80 9 120 mm3 ingot. Three specimens each with dimension of 30 9 80 9 40 mm3 were cut from the ingot and committed to homogenization. The chemical composition of the ingot after homogenization is shown in Table I. The homogenization process included three stages: the first [723 K (450 C)/12 hours] and third stages [753 K (480C)/12 hours] were the same for all samples, whereas t
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