高强铝合金7075半固态流变制浆技术及装置的研发

半固态流变成形技术作为一种复杂形状零部件的净近成形工艺，具有短流程、低成本、产品性能好等优势，近年来备受关注。流变成形技术投入大规模工业应用的前提与关键在于高效、稳定的流变制浆工艺。高强铝合金7075具有优越的力学性能，已广泛运用在飞机、汽车等领域。传统的塑性成形方法生产效率低、成本高，流变成形方法有望以较低的成本生产出具有同等水平优异力学性能的7075铝合金零部件。但现有的制浆技术无法制备出液相含量均匀且具有中等固相分数（约40%）的7075铝合金半固态浆料，这也限制了其进一步的发展和应用。本文针对高强铝合金7075，提出一种新型半固态流变制浆技术并完成装置设计，主要成果如下： （1）采用差示扫描量热法与数值模拟法获得不同降温速率下7075铝合金的液相线温度、固相线温度及液相分数对温度的敏感系数等参数，并与铝硅系319s合金对比。结果显示7075铝合金的半固态温度区间随降温速率增加而增加。当降温速率为1~20℃/min时，7075铝合金半固态温度区间均大于150℃，大于319s铝合金的半固态温度区间。但7075铝合金的液相分数曲线不像319s铝合金一样在中等固相分数区间出现拐点。在较小的降温速率下（1~10℃/min），在中等固相分数温度区间，7075铝合金液相分数对温度的敏感性大于0.03℃-1。在实际制浆过程中，通常存在半固态浆料内外温度不均匀的情况，较高的液相分数对温度的敏感性加剧了液相分数的不均匀程度，使浆料性质不均匀，这是高强铝合金7075流变制浆的最大难题。 （2）针对7075铝合金半固态浆料液相分数不均匀的问题，提出一种新型流变制浆方法——热焓补偿法，即在合金凝固过程中施加外场加热表层浆料，达到消除合金温度梯度、提高液相分数均匀性的目的。使用热焓补偿法制备浆料过程中温度场表达式为： T=(b_c T_c+b_s T_p)/(b_c+b_s )+(〖(b_c T_c+b_s T_p)/(b_c+b_s )-T〗_c )erf(x/(2√αt)) 使用ANSYS Workbench软件模拟了热焓补偿法制备7075铝合金浆料过程中温度场变化情况，结果表明在无热焓补偿的制浆过程中浆料内部温差始终大于60℃，而通过热焓补偿法可将温差降低到6.8℃。当感应电流密度为30×106A/m2，频率为1kHz的交流电加热时长为40~45s时能确保有足够的热焓补偿。热焓补偿足够时，在加热结束之前浆料温差会有升高的现象。 （3）基于热焓补偿法，设计开发出一套半固态流变制浆装置，其主要部件包括感应电源、变压机柜、感应线圈、贯通式坩埚等。使用热焓补偿法能制备出温度均匀且具有中等固相分数的7075铝合金半固态浆料，在制浆过程中5s~50s时间段使用加热功率为13.5kW，电流频率为1kHz的感应加热辅助制备浆料时，在57s浆料内部的温度为618.6±2.1℃。其中，浆料在径向与竖直方向上的最大温差分别为1.4℃与2.4℃。

As a significant near net-shape forming method for the metallic materials, rheoforming has attracted increasing interest of scientists and engineers due to its short process, low cost and high performance of parts. A key success of rheoforming is having an efficient and powerful process to prepare semi-solid slurry. Owing to excellent mechanical properties, 7075 aluminum alloy has been widely used in aerospace, automobile and other fields. Nevertheless, it is difficult for conventional plastic forming to produce parts with low cost and good mechanical properties while SSP is expected to grapple with this problem. It should be stated, however, that there is no rheoforming process to date by far can produce good semi-solid slurry with medium solid fraction for 7075 aluminum alloy, which limits its further application. In this paper, a novel slurry making process and equipment of 7075 aluminum alloy are developed. The main results are as follows: (1) The liquidus, solidus and liquid fraction vs. temperature curve of 7075 aluminum alloy, which were compared with 319s aluminum alloy, were obtained by differential scanning calorimetry (DSC) under different kinetic conditions and numerical simulation. The semi-solid temperature interval increases with the increase of cooling rate. The solidification temperature range of 7075 aluminum alloy measured is more than 150℃. However, the liquid fraction curve of 7075 aluminum alloy did not show the inflection point in the medium solid fraction range as 319s alloy did. In addition, when the liquid fraction is over 50% and the cooling rate is relatively small (1~10 ℃/min), the sensitivity to temperature is almost greater than 0.03 ℃-1. In the actual slurry making process, there is usually uneven temperature inside and outside the semi-solid slurry. The high sensitivity of liquid fraction to temperature aggravates the degree of uneven liquid fraction and makes the slurry properties uneven. This is the biggest problem in slurry making process of 7075 aluminum alloy. (2) In order to grapple with the problem, the Enthalpy Compensation Process for preparing semi-solid slurry of 7075 aluminum alloy was developed. And the expression of the temperature field of the semi-solid slurry is as follows: T=(b_c T_c+b_s T_p)/(b_c+b_s )+(〖(b_c T_c+b_s T_p)/(b_c+b_s )-T〗_c )erf(x/(2√αt)) The temperature field of the semi-solid slurry prepared by Enthalpy Compensation Process was simulated by ANSYS Workbench software. The results indicate that the temperature difference in the slurry is always more than 60℃ without induction heating process while being reduced to 6.8℃ using Enthalpy Compensation Process. There is sufficient enthalpy compensation when the induction current density is 30×106A/m2, the frequency is 10kHz, and the heating time is 40~45s. When the enthalpy compensation is enough, there will be an increase of temperature difference before the end of heating. (3) Based on the Enthalpy Compensation Process, a set of devices were designed and developed for Enthalpy Compensation Process, which chiefly consist of induction power supply, pressure converter cabinet, induction coil, crucible, temperature measuring frame and other components. Semi-solid slurry with uniform temperature and medium solid fraction can be prepared by Enthalpy Compensation Process with heating power of 13.5 kW and current frequency of 1 kHz as well as heating duration of 5s~50s. The temperature field is relatively uniform and the temperature of the slurry is 618.6±2.1℃. The maximum temperature difference in the radial and vertical direction of the slurry is 1.4℃ and 2.4℃, respectively.