Microstructure Evolution and Recrystallization Resistance of a 7055 Alloy Fabricated by Spray Forming Technology and by Conventional Ingot Metallurgy

Microstructure Evolution and Recrystallization Resistance of a 7055 Alloy Fabricated by Spray Forming Technology and by Conventional Ingot Metallurgy ZHIQIANG XIE, ZHIHONG JIA, KAIYUN XIANG, YAPING KONG, ZHENGUO LI, XI FAN, WANTAI MA, HAO ZHANG, LIN LIN, KNUT MARTHINSEN, and QING LIU The effect of different fabricating processes (spray forming and conventional casting) and homogenization treatment on the microstructure of an 7055 alloy was investigated by optical microscopy (OM), scanning electron microscopy (SEM), electron probe X-ray micro-analyzer (EPMA), and transmission electron microscopy (TEM). It was found that the grain size of the as-deposited (spray formed) 7055 alloy had half the size as that of the as-cast 7055 alloy and there was no Al2CuMg phase that embedded in the coarse Mg(Zn, Cu, Al)2 phase distributed along the grain boundaries in the as-deposited 7055 alloy. No segregation of zirconium was observed in the as-deposited 7055 alloy. After homogenization heating at 350 C/5 hours + 470 C/24 hours, Al3Zr dispersoids were inhomogeneously distributed within grains in the traditionally cast 7055 alloy, while more homogeneously distributed within grains in the spray-formed 7055 alloy. Compared with the traditional cast 7055 alloy, the uniform distribution of Al3Zr dispersoids in the spray-formed 7055 alloy retards recrystallization more effectively. This investigation highlights the advantage of spray forming technology on improving microstructure of a 7055 alloy. https://doi.org/10.1007/s11661-020-05931-w  The Minerals, Metals & Materials Society and ASM International 2020 I. ZHIQIANG XIE, KAIYUN XIANG, YAPING KONG, and ZHENGUO LI are with the International Joint Laboratory for Light Alloys (Ministry of Education), College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. ZHIHONG JIA is with the International Joint Laboratory for Light Alloys (Ministry of Education), College of Materials Science and Engineering, Chongqing University and also with the Electron Microscopy Center of Chongqing University, Chongqing 400044, China and also with the Key Laboratory for Light-Weight Materials, Nanjing Tech University, Nanjing 210009, China. Contact e-mail: XI FAN and HAO ZHANG are with the Jiangsu Haoran Spray Forming Alloy Co., Ltd., Zhenjiang 212009, China. WANTAI MA is with the College of Mechanical & Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China. LIN LIN is with the Southwest Aluminium (Group) Co., Ltd., Chongqing 401326, China. KNUT MARTHINSEN is with the Department of Materials Science and Engineering, NTNU - Norwegian University of Science and Technology, Alfred Getz vei 2b, 7491 Trondheim, Norway. QING LIU is with the International Joint Laboratory for Light Alloys (Ministry of Education), College of Materials Science and Engineering, Chongqing University and also with the Key Laboratory for LightWeight Materials, Nanjing Tech University. Manuscript submitted March 8, 2020. METALLURGICAL AND MATERIALS TRANSACTIONS A INTRODUCTION 7055 Al-Zn-Mg-Cu alloy is an important member of the 7xxx series alloys and is extensively used in the aerospace industry owing to its light weight and high mechanical performance.[1] Many attempts have been made to further improve the mechanical properties of the 7055 aluminum alloy. Improvement in the performance of most aluminum alloys can be attributed to modifications of chemical composition and/or employing new production methods.[2–5] The spray forming technology is an advanced fabrication technique, which is based on rapid solidification and powder metallurgy.[6] The success of spray deposition can possibly also be exploited to enhance the performance of 7055 aluminum alloys. Compared with conventional ingot metallurgy, the spray forming technology provides possibilities to avoid many unfavorable factors that generally are detrimental to the performance of aluminum alloys, such as large grains, serious compositional segregation, and severe casting defects due to low solidification rate. Most importantly, a uniform distribution of chemical composition and microstructure can be obtained because of the rapid solidification that is an inherent feature of the spray deposition process.[7–11] Therefore, the spray forming technology possibly provides an alternative route to develop high-strength aluminum alloys. A homogenization heat treatment is an indispensable process for traditional casting alloys, aiming at dissolving large size eutectic phases, redistributing the solute, eliminating intragranular segregations, i.e., level out compositional variations, reducing internal stresses and removing other casting defects.[12,13] Besides, in 7xxx aluminum alloys with small additions of zirconium, coherent Al3Zr dispersoids are precipitated during homogenization, which may have a significant effect on inhibiting recrystallization so that alloys maintain their deformed microstructure during possibly subsequent high temperature exposure.[14–16] Hence, alloys may obtain excellent mechanical properties via the combination of their stable deformed substructure and Al3Zr precipitation hardening. However, the segregation of zirconium during casting of zirconium-containing aluminum alloys is well known, which may result in large variations in the Al3Zr distribution within single grains.[17–19] Generally, Al3Zr dispersoids concentrate in the center of dendrite grains while precipitate-free zones (PFZ) result at the dendrite grain boundaries. At the same time, the effectiveness of preventing recrystallization is closely related to the size, number density, and spatial distribution of Al3Zr.[14,19,20] Many studies have focused on precipitation of the MgZn2 strengthening phase in 7xxx alloys and the associated aging behavior, while less studies have focused on the optimal homogenization conditions as means to control the precipitation of Al3Zr dispersoids. Even less studies have reported on the effect of homogenization on sprayformed Al-Zn-Mg-Cu alloys. In particular, the precipitation and distribution of Al3Zr dispersoids during the homogenization process and the resulting effects in terms of recrystallization resistance have not been studied in spray-formed Al-Zn-Mg-Cu alloys. A clear difference in microstructure is expected to be found between an as-cast and an as-deposited sprayformed 7055 alloy. Although many unwanted features related to conventional casting, e.g., strong micro-segregations, are expected to be nearly non-existing in a spray-formed alloy, it does not mean that the homogenization is no longer needed, as a large number of Al3Zr dispersoids are formed during the homogenization process. Understanding the recrystallization resistance of 7055 alloys requires a comprehensive analysis of the precipitation behavior of Al3Zr dispersoids, in terms of their spatial distribution, size and number density, and possible precipitate-free zones (PFZ) formed at th (...truncated)