| The Al-Si-Cu-Ni-Mg alloy formed by adding Cu、Ni、Mg to eutectic Al-Si a lloy is wide ly used in the area of industrial lightweight to produce all kinds of piston and cylinder by its great mechanical properties under both room and high temperature. Currently the research of Al-Si-Cu-Ni-Mg alloy focuses on the composition and the function of the elements, no systematic research on the treatment process exists, namely the design of the temperature and time of solid solution and aging. By using the phase calculation software Pandat and heating DSC curve, the alloy phase composition and transformation are studied to determine the reasonable T6 technology, which could be used as a general method for the series Al alloy of Al-Si-Cu-Ni-Mg to determine the heat treatment process. Further study is carried on the change of microstructure and mechanical properties of the alloy added with rare earth elements Nd, V, Zr, the corresponding heat treatment process, and high temperature stability after thermal exposure under different temperature and time on the basis of optima l heat treatment condition.The casting A l-11.9Si-3.5Cu-1.7Ni-0.8Mg is formed by α(Al) matrix、primary Si phase 、eutectic Si phase、Q phase(Al5Cu2Mg8Si6)、Al3CuNi、Al7Cu4Ni and θ phase(Al2Cu). The equilibrium solidification sequence is calculated by Pandat and the solid- liquid transition temperature is 509℃.From the DSC heating curve, the first endothermic peak appears at 502.7℃, which is corresponded to the Al2 Cu transition temperature, in order to prevent the negative impact of overheating of Al2 Cu, the initial solid solution temperature should not exceed 502.7 ℃, the single step solid solution(495 ℃ ×6 h, 10 h, 14 h) plan are designed.The lowest two endothermic peak disappear on DSC curve after solid solution at 495℃ for 2h, the beginning of the lowest endothermic peak temperature increases to 522.1 ℃, as we know, the higher the temperature the greater the solubility, so the two-step solution((495 ℃×2 h) +(515 ℃×4 h, 8 h, 12 h)) heat treatment is designed.Artificia l aging at 200℃ for all the sample after the completion of solution treatment to get the aging curves, tensile mechanical properties was tested under the peak aged condition, the results show that two step solution(495℃×2 h)+(515℃×8 h) with aging at 200℃ for 4 h could get the optimum mechanical properties, the room temperature ultimate tensile strength was 367 MPa compared with 212 MPa under cast condition, and the elongation was 0.66% compared with 0.44%. The high temperature ultimate tensile strength of 300℃ was 160 MPa compared with 110 MPa under cast condition, and the elongation was 3.0% compared with 5.0%.This article explores the function of e lements Nd, V, Zr on the changes of microstructure and mechanical properties of alloy, and find that the temperature distribution of the endothermic peak in DSC curve changes after adding elements. Compared with DSC curve of base alloy, the first two endothermic peak of Al-11.9Si-3.5Cu-1.7Ni-0.8Mg- 0.2Nd/0.44 Nd disappear, so only one step solid solution is needed; the DSC curves of Al-11.9Si-3.5Cu-1.7Ni-0.8Mg-0.2V/0.15 Zr are similar with the base alloy, so two step solid solution is still needed only the temperature of first step will increase from 495℃ to 500℃. The elevated temperature properties of all the alloys with element are higher than the base alloy, but the room temperature properties are lower. The block-like rich Nd/V/ Zr mesophase will be produced after adding elements, which has better elevated temperature stability to improve the e levated temperature mechanical properties, but decrease the room temperature mechanical properties due to bad influence of making it easier for the crack initiation.The changes of organization and mechanical properties of the base alloy after thermal exposure under high temperature are studied. The alloy is put into the vacuum tube furnace after heat treatment with the best T6 technology under the temperature of 200 ℃, 300 ℃ and 300 ℃ respectively with time of 0 h, 25, 50 h h, 75 h, 100 h, 150 h, 250 h, 500 h respective ly. With the increase of heat exposure time, the tensile strength decreases, elongation increases, and tends to be stable in the end. The higher the temperature, the faster the decreasing speed of tensile strength and the increasing speed of elongation. The residual tensile strength after thermal exposure for 500 h decreases first and then increases with the increasing of exposure temperature, to be the lowest under 300℃ of both the room and high temperature tensile strength, the tensile strength of base alloy under 400℃ of thermal exposure decreases quickest but easiest to be stable at a high level. With the increase of heat exposure time, block-like primary silicon phase won’t change, and the eutectic silicon phase changes to be smaller, the mesophase changes to fragment, spheroidize and coarsen. The way of fracture of base alloy is the mixed mode of accurate cleavage and cleavage, with the increase of heat exposure time, the tear stare blankly begins to increase, the proportion of cleavage plane begins to decrease, the number of toughening nest significantly increases, and the size of the toughening nest depth are rising and elongation of alloy increases gradually, so as the increase of the plastic. |
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