【作者】 郭志宏；

【导师】 马春茂；

【作者基本信息】 中北大学 ， 机械设计及理论， 2014， 博士

【摘要】 随着世界工业的飞速发展,镁合金铸件的生产在不断扩大,人们对镁铸件的质量和生产效率、环保提出了更高的要求,挤压铸造技术是适应这一新的要求的有效手段。目前,国际上广泛地将挤压铸造应用于镁合金的铸造生产,其中也包括镁基复合材料,并取得了巨大的经济效益,而我国的铸造业中挤压铸造技术还只停留在试验阶段,距离发达国家的技术水平还有相当的距离,因此进行镁合金挤压铸造工艺及复合材料研究,具有非常重要的意义。挤压铸造可获得优质高性能零件,并且十分适合于镁合金的成形。镁合金的综合性能与其它金属相比具有明显优势。为了寻找镁合金挤压铸造工艺参数的适用性特点及制备高强韧镁基复合材料,本文以AZ80镁合金为研究对象,通过软件模拟挤压铸造力学性能,用形态学矩阵优化了挤压铸造工艺参数,对镁合金挤压铸造工艺及性能进行了系统的研究,并采用Y+SiC+SC制备了具有优良力学性能的挤压铸件,为镁合金挤压铸造工艺优化设计及制备高强韧镁基复合材料提供理论基础和实验依据。主要研究成果如下：(1)在软件平台上,确定了AZ80镁合金试样合理的挤压铸造工艺方案。通过对镁合金试样的挤压铸造凝固过程及力学性能模拟结果的分析,验证了镁合金挤压铸造的工艺特点,预测了缺陷的位置,同时寻找到了最佳的参数组合,从而进一步优化了工艺方案。(2)通过正交试验表以及模拟的分析,得出了一组最佳的工艺参数。影响因素由高到低依次为挤压压力、浇注温度和保压时间。当浇注温度700℃,挤压压力100MPa,保压时间15s时,实验的挤压铸造AZ80镁合金力学性能可稳定地达到最佳性能。此时,抗拉强度为271.4Mpa、伸长率为7.4%、洛氏硬度为98.2HRC,这与模拟的结果基本一致。挤压压力提高了镁合金过冷度、热传导率、形核率,使晶粒细化,进而提高力学性能。(3)运用形态学矩阵对AZ80镁合金挤压铸造工艺参数进行优化,采用Lg(33)正交方法中的正交列对不同挤压压力,模具预热温度和压力持续时间进行组合。一个三水平正交阵列用来确定S/N率,用方差分析确定了影响力学性能最重要的工艺参数,并利用多变量线性回归分析对拉伸强度、延伸率和硬度进行了确定。结果获得最佳的挤压铸造工艺参数,因此,此方法可以用来寻找最优条件,得到较好的力学性能。(4)Y的加入使挤压铸造AZ80镁合金固／液界面前沿的成分过冷增大,使铸态组织得到细化,β-Mg17Al12相由网状分布变为断网分布,从而提高了铸件的力学性能；加Y提高AZ80镁合金力学性能的主要原因是Y的固溶强化作用以及A1-Y相的弥散强化作用,而且Y与A1的结合减少了形成热稳定性较低的Mg17Al12目的数量。这些主要弥散分布的A12Y颗粒比AZ80合金晶界上Mg17Al12的热稳定性高得多,在高温条件下能对相邻晶粒的移动起到钉扎作用,有效阻碍了高温下晶界和位错的移动。(5)挤压铸造对镁基复合材料的内部组织晶粒有细化的效果,并且发现经过挤压铸造后很多SiC颗粒被基体合金晶粒包裹在里面,从而提高了SiCp/AZ80镁基复合材料的综合性能,使得SiCp/AZ80镁基复合材料的力学性能有很大的提高。把稀土Y和SiC同时加入AZ80镁合金挤压铸造可以显著提高性能,组织更加致密。这种方法可用来制备高强韧镁基复合材料,组织中观察到Al2Y+SiCp混合物。稀土的最佳加入量为3%,SiC的最佳加入量为1%,加入多了也和Y一样会形成SiC团聚物,降低了SiC颗粒相在组织中的弥散分布程度,削弱了其在高温下的弥散强化作用,并造成合金显微组织和成分的不均匀,容易引起应力集中,引起合金高温性能的下降。本研究通过对AAZ80镁合金挤压铸造工艺参数进行优化,提出了用形态学矩阵优化参数的方法和思路；通过对镁合金挤压铸造进行模拟及实验验证,节约了生产成本、提高了效率,并制备了高强韧AZ80镁基复合材料,为进一步研究镁合金挤压铸造奠定了基础。更多还原

【Abstract】 With the rapid development of world industry, the production of magnesium alloy casting is increased rapidly, people put forward the higher request for the quality of magnesium casting pieces, production efficiency and environmental protection. The squeeze casting have been applied broadly in international and received great economy benefits, including magnesium matrix composites; the squeeze casting’s application in domestic now is only in the experiment stage and has a long distance to the developed countries yet. It is significant to study processing and composite of magnesium alloy squeeze casting.Squeeze casting can get high quality parts, which can be applied on manufacturing magnesium alloys parts. Magnesium alloys possess more multipurpose properties than other materials. In order to explore the applicability of the characteristics and laws of magnesium alloy squeeze casting process parameters and prepare high toughness magnesium matrix composites, magnesium alloy squeezing casting process and properties were studied through simulation of squeeze casting mechanical properties, optimization of squeeze casting technological parameters with morphological matrix based on the AZ80magnesium alloy as the research object, and squeezing casting pieces of excellent mechanical properties were prepared using Y+SiC+SC.A theoretical basis and experimental evidence were provided for process optimization and preparation of high toughness magnesium matrix composites of magnesium alloy squeezing casting. The main results can be summarized as follows:(1) Using the software, AZ80was studied to determine the magnesium alloy wheels and sample reasonable squeeze casting process. Squeeze casting of magnesium alloy process features were verified by analyzing the simulation results of the filling and solidifying process of squeeze casting for magnesium alloy wheels and sample, and also predicted the position of defects. As the same time the best combination of parameters was found so as to the processing plan was optimized.(2) A set of optimal process parameters were obtained through analysising the orthogonal test and simulation of AZ80magnesium alloy, which are extrusion pressure of100Mpa, mould preheating temperature of200℃and pouring temperature of700℃. The influences of squeeze casting process parameters on mechanical properties of AZ80magnesium alloy cast extrusion pressure, pouring temperature and holding time in the simulation from strong to weak. It was found the squeeze casting can be achieved very good mechanical properties:σ b=271.4Mpa,δ=7.4%,HRC=98.2in the pouring temperature is700℃, holding time for the15S, the extrusion pressure is100MPa.This is basically consistent with minimum defects of simulation results. Extrusion pressure makes magnesium alloy solidification cooling rate improved and the heat transfer coefficient improved, so an increase of nucleation rate was obtained, which makes grains refined to improve the mechanical properties.(3) Squeeze cast process parameters of AZ80magnesium alloy by morphological matrix were studied. Experiments were conducted by varying squeeze pressure, die pre-heat temperature and pressure duration using L9(33) orthogonal array of Taguchi method. In Taguchi method, a three level orthogonal array has been used to determine the S/N ratio. Analysis of variance is used to determine the most significant process parameters affecting the mechanical properties. The mechanical properties such as ultimate tensile strength, elongation and hardness of the components have been ascertained using multi variable linear regression analysis. Optimal squeeze cast process parameters were obtained. Thus, this method is used to search for optimal squeeze cast process parameters to obtain better mechanical properties of the components.(4) The AZ80squeeze casting magnesium alloy solid/liquid interface front composition undercooling was increased, thus as-cast group was refined, β-Mg17Al12phase network distribution was broken, so as to the mechanical properties of the casting were improved because of the addition of Y; The main reason of AZ80magnesium alloy mechanical properties improvement is that the solid solution strengthening effect of Y and Al-Y phase dispersion strengthening effect, and the combination of Y and Al reduces the formation of the amount of thermal stability lower Mg17Al12phase. Thermal stability of these main diffuse distribution Al2Y particles much higher of Mg17Al12of AZ80alloy on the grain boundary, neighboring grain movement pinning was effectd at high temperatures, the grain boundary and dislocation movement under high temperature were effectively prevented.(5) The internal organization grains of squeeze casting magnesium matrix composites were refined, and a lot of SiC particles were parceled inside alloy grain after squeeze casting, the comprehensive performance of SiCp/AZ80magnesium matrix composite materials were greatly improved. The performance of AZ80magnesium alloy squeeze casting was significantly improved because of addittione of rare earth Y and SiC, organization was more compacted, Al2Y+SiCp mixture were observed in the organizations. This method can be used for the preparation of high-toughness magnesium matrix composites. The best addition amount of rare earth is3%, the best addition amount of SiC is1%, SiC reunion was formed when the more amount of SiC was added, this also like Y, the degree of SiC particle phase dispersion in tissue distribution was reduced, the dispersion strengthening effect at high temperature was weaked, and the uneven of alloy microstructure and composition was caused, easy to stress concentration was caused, high temperature alloy performance degradation was caused.Magnesium alloy squeeze casting process parameters were optimized on the research. The method and train of thought was proposed by using the morphological matrix parameter optimization. Through simulation and experimental verification of magnesium alloy squeeze casting, production costs were saved, efficiency was improved, and AZ80magnesium matrix composites were prepared,so as to the foundation was laided for the magnesium alloy squeeze casting technology further study.更多还原