Solidification Behavior Of Spray Deposited Al-Zn-Mg-Cu Alloys And Their Microstructural Evolution During Thermal Processing

Solid-liquid interface stability and the evolution of crystal growing morphology, during the rapid solidification (RS) and deep undercooling processes, have been the focus of much attention in the field of solidification. And, the refinement of as-solidified microstructure based on RS technique offers an effective approach to improve the mechanical properties of materials. Although the planar front growth in the absolutely stable state was already predicted by theoretical calculation, few experimental phenomena can be observed to support it to date. In this dissertation, the experimental proof of the transition from microcellular growth to rapid planar front growth was obtained by characterizing the microstructures of the gas-atomized Al alloys powder particles using electron microscopy. The evolution of the solidifed morphologies of powders with various particle-sizes has been discussed as well. Subsequently, the effect of the gas/melt ratio (G/M) on the microstructures of the spray deposited (SD) Al-Zn-Mg-Cu alloys was analyzed and illuminated in detail. Hot deformation behavior of the alloys prepared was investigated using the uniaxial hot-compression test, as well as the microstructural change. On the based of the hot-processing maps, thin-wall tubes made of the as-deposited alloy were produced by hot extrusion. In order to determine the optimal heat treatment process, the isothermal aging-hardening feature of the Al-Zn-Mg-Cu alloys was studied systematically by means of detailed HREM examinations on the matrix precipitates and the analysis for aging dynamics. As a result, a high tensile strength was yielded in combination with good ductility for the alloy.In the characterization of the solidified microstructure of the Al-Zn-Mg-Cu alloy powder, it has been demonstrated that the solidified morphology ofα-Al changes from microdendritic-cellular crystal to planar crystal with the decrease in powder particle-size. In the sample with particle-size of d<1μm, a full planar crystal microstructure accompanied with complete solute-entrapping can be observed. Similar phenomenon was also discovered in the atomized Al-Fe-V-Si alloy powders. The results above confirm the existence of the planar front growth of solid-liquid interface in absolutely stable state. The theoretical solidification velocity v of a droplet with d=0.5μm is 3721ms^-1, which is about the same magnitude of the critical velocity vab controlled by heat diffusion. At the same time, metastable i-Mg₃₂(AlZn)₄₉ icosahedral quasicrystal was generated intergranularly due to RS, and a new hexagonal structured crystalline phase formed by decomposition of supersaturatedα-Al matrix has been specified through TEM diffraction technique (a=0.281nm, c=0.459nm,and nominal EDS composition of Al_81.29Zn_10.99Mg_5.69Cu_2.03). In the microstructural study of the SD Al-Zn-Mg-Cu alloys in the condition of high gas/melt ratio (G/M=3.5), broken dentrite fragments and i-Mg₃₂(AlZn)₄₉ quasicrystal phase inherited from the original powders can be observed embedded in the alloy matrix. Medium gas/melt ratio (G/M=2.3) can form an ideal microstructure consisting of tinyα-Al equixed grains and discontinuous MgZn₂ secondary phase. In the condition of low gas/melt ratio (G/M=1.4), theα-Al equiaxed grains were observed coarsened together with bcc-Mg₃₂(AlZn)₄₉ eutectic phase which appears intergranularly. Microalloying elements can affect the solidified structure by forming various intermetallic phases in the as-deposited alloy. For instance, trace amount of Sc and Zr formed L1₂-ordered Al₃(ScZr) primary phase, which can cause the refinement of eutectic phases. Trace amount of Fe formed the needle-shaped Al₂₃CuFe₄ primary phase with the long axis in [001] direction. A new base-centered orthorhombic structured phase enriched in Cr and Mn was detected (a=4.434nm, b=2.542nm and c=1.270nm, and nominal EDS composition of Al_68.07Mg_13.07Zn_9.22Cr_3.93Mn_3.76Cu_1.95).Analysis of the experimental data derived from the hot compression tests of the as-deposited Al-Zn-Mg-Cu alloy indicates that the relationship of flow stress, deformation temperature and the strain rate obeys the Arrhenius law. Under high Zener-Hollomon parameter (Z value) condtions, the softening process is mainly dominated by dynamic recovery (DRV) mechanism. The models on Z parameter of DRV grain-boundary length fraction fDRV and subgrain size dDRV are ln f_DRV = 1.190+0.114lnZand d_DRV^-1 =0.0613lnZ-0.628, respectively. Under low Z value conditions, the softening process is dominated by dynamic recrystallization (DRX) mechanism, including continuous and discontinuous dynamic recrystallizations; the corresponding DRX models are ln f_DRX = 11.671-0.410lnZ(Z>8×108) and ln f_DRX = 5.4580-0.0761lnZ (Z≤8×10⁸). The hot-processing maps were established under the condition of 0.7 ture stain. In combination with microstructural verification, the ideal processing parameters for the alloy's deformation are determined: temperature ranging from 400℃to 460℃and stain rate of 0.05s^-10.007s^-1, also temperature ranging from 450℃to 460℃and stain rate of 1s^-10.05s^-1. The thin-wall tubes of SD alloy was successfully prepared according to the processing maps.The maximum hardness of the deposited Al-10.83Zn-3.39Mg-1.22Cu alloy appears after 12 hours′aging at 120℃following solid-solutionizing treatment, whereη' phase is the dominant strengthening phase. Compared with conventional alloys, the as-deposited alloy possesses much higher solid solubility and defects density, which can effectively shorten the aging time before reaching the maximum of hardness. The studies of aging dynamics lead to the conclusions that: during the growing stage of precipitation, the half disc-thickness of the precipitate phase h is proportional to the square root of aging-time t, following the model h² = 0.216+0.0337t; during the coarsening stage, the h is proportional to the cubic root of t, following the model h³ = -2.863+0.133t. The volume fraction of the precipitate phase changes upon time following the Johnson-Mehl-Avrami (JMA) law: X_t = 0. 0815[1-exp(?0.423t^0.323)]. Considering all the factors above-listed, the optimized aging time of the spray deposited Al-10.83Zn-3.39Mg-1.22Cu alloy have been determined as: solid solutionizing at 470℃/1h+490℃/1h followed by aging at 120℃/12h. The performance of as-prepared alloy has been evaluated at room temperature: 816MPa (Ultimate Tensile Strength—UTS), elongation of more than 8%, electric conductivity of 27.8%IACS. Better corrosion-resistance performance can be obtained by T7 two-step aging (120℃/12h +160℃/8h): UTS is 758MPa along with the electric conductivity exceeding 36.7%IACS.