【作者】 陈云；

【导师】 陈定方；

【作者基本信息】 武汉理工大学 ， 机械制造及其自动化， 2013， 博士

【摘要】 镁合金具有低密度,高的比强度、比刚度,好的减震性、阻尼性,优良的铸造性能、切削加工性能,已经广泛应用于汽车、航空航天、通信等领域,被誉为21世纪“绿色”工程材料。挤压铸造是一种实现铸锻相结合的先进成形工艺,由此工艺生产的铸件晶粒细小、致密度高、缩松小、力学性能优良,已成功应用于铝合金、铜合金等合金的实际生产中。近年来,挤压铸造理论与应用的研究受到越来越多的学者和企业所重视,但关于镁合金挤压铸造成形机理方面的研究确很少,已严重制约了镁合金挤压铸造技术的推广。同时,我国是镁资源大国,也是镁生产大国,镁合金使用具有重要的战略意义。因此,研究镁合金挤压铸造成形机理对推广高性能镁合金铸件的使用具有显著意义和紧迫性。论文利用正交试验方法研究了挤压铸造工艺参数中浇注温度、模具温度、挤压速度对不同壁厚镁合金流动性的影响规律。正交试验结果表明,对壁厚4mm以下试样的流动性影响最大的因素是浇注温度,其次是模具温度,挤压速度；对4mm试样的流动性影响最大的因素是模具温度,其次是浇注温度,挤压速度。在镁合金挤压铸造实际生产过程中,浇注温度在700℃到750℃变化时,增加浇注温度对提高AZ91D镁合金的流动性是有利的；对厚壁铸件(3mm和4mm)通过提高模具温度而增加镁合金的充型能力是非常有效；增加挤压速度,合金流动性也有所增加。对镁合金熔液充型停止流动机理进行分析,并采用一种半定量的计算方法来表述镁合金的充型能力,得出镁合金充型长度的半定量数学表达式。对金属液充型过程的流体力学分析表明,当金属液以设定的充型速度充型时,充型特性受流体特性(粘度、表面张力)、流道特性(铸件壁厚和内浇道形状)、充型流动距离等因素的影响。数值模拟与实验研究表明,对6mm厚平板,低的充型速度(<186.8mm/s)容易造成金属液流前沿波动较大,充型过程不平稳,同时也降低了合金的流动性,在铸件最后充型部位容易形成浇不足、冷隔等缺陷；高的充型速度(>933.8mm/s)虽然提高了生产效率,但容易形成喷射流,造成卷气缺陷。对12mm平板,在相同的充型速度下,采用扇形内浇道的充型过程比采用矩形内浇道充型平稳,同时扇形内浇道也能以不高的速度完成快速充型,从而避免喷射流的出现。对挤压铸造AZ91D镁合金在不同状态下的显微组织、力学性能以及断裂行为进行了测试和分析,得出挤压铸造AZ91D镁合金铸态、T4态和T6态的硬度、屈服强度、抗拉强度力学性能指标随着试样壁厚的增加而减小,而延伸率随着试样壁厚的增加而增加。以8mm厚试样为分析对象,T4处理后的抗拉强度和延伸率分别达到205MPa、7.0%,其中与铸态相比较,延伸率提升幅度较大,硬度和屈服强度略有下降：T6处理后的硬度和屈服强度显著提高,其中屈服强度与T4态相比较,提升幅度为44.2%,抗拉强度也出现小幅提高,但是延伸率则有所下降。铸态AZ91D镁合金断口以脆性断裂为主,断口比较平整,断口出现的撕裂韧窝和撕裂棱数量比较少。经固溶处理处理后,AZ91D镁合金断裂模式为准解理断裂,断口出现了发达的河流花样和由撕裂棱连接的较大解理刻面,而在撕裂棱周围也出现了一些深浅不一的韧窝。经固溶时效处理后,AZ91D镁合金断口中也分布了一些细小的河流花样及少量较浅的韧窝,但断口整体表现为为沿晶断裂。通过理论分析与微观组织分析,研究了镁合金间接挤压铸造中冷夹层的形成机理。显微组织分析发现,冷夹层像一层保温膜,将铸件分割为两部分,导致夹层两侧组织明显不同；对冷夹层进行EDS能谱分析表明,冷夹层中高亮质点处主要存在O、Mg、Si三种元素,说明冷夹层中形成了高熔点的氧化物以及冷夹层表面存在污染,冷夹层不易重熔。进一步分析指出消除间接挤压铸造铸件中冷夹层的必须保证浇入到压室内的镁合金液具有一定的过热度,并使冷凝层进入模具型腔时被破碎与重熔。从理论上分析了冷凝层重熔的温度条件并给出了相应的计算公式。适当提高压室温度和浇注温度,可减少压室内冷凝层的厚度,有利于冷凝层的破碎和重熔；在压室内壁均匀涂上一层合适的涂料,降低了压室的导热能力,也可减少冷凝层的厚度；减少镁合金熔液在压室中的停留时间和采用机械方法也可以减少或消除铸件中的冷夹层。更多还原

【Abstract】 Magnesium alloy is very attractive in such applications as automobile, aerospace and communication industries due to its low density, high specific strength, stiffness, good absorption of vibration and damping, excellent castability and machinability. It is praised as the green engineering material in21st century. Squeeze casting, an advanced forming technology with the combination of casting and forging, has been successfully used in the practical production of aluminum alloy and copper alloy etc. The process has the capability of producing castings which are fine grains, high density, pore free and excellent mechanical property. In recent years, more and more scholars and enterprises pay more attention to the study of theory and application of squeeze casting. But limit researches have been done about the forming mechanism of magnesium alloy squeeze casting. The promotion of the process is seriously restricted. Meantime, magnesium resource and production is very abundant in our country, and the using of magnesium alloy has a very important strategic meaning. Therefore, it is necessary to discuss the forming mechanism of magnesium alloy squeeze casting for the promotion of high-quality castings.The influence of pouring temperature, mould temperature and squeeze velocity on the fluidity of magnesium alloy with different wall thickness was investigated using orthogonal test. The experimental results show that the most important factor influencing the fluidity of specimens with1,2and3mm wall thickness is pouring temperature and then mould temperature and squeeze velocity, but for specimen with4mm thickness mould temperature at first, and then pouring temperature and squeeze velocity. In practical production, it is very effective to increase pouring temperature for better filling ability of squeeze cast AZ91D magnesium alloy between700to750℃, but it is not suitable above750℃. But the filling length of magnesium alloy in the thick section (3and4mm) increases remarkably because of the longer filling time. The fluidity of magnesium alloy increases with the increase of squeeze velocity. Based on the stopped-flow mechanism of magnesium alloy, semi-quantitative mathematical expression of the filling length is educed.Fluid mechanics analysis on the filling of molten metal shows that the filling characteristic is affected by liquid characteristic (viscosity and surface tension), runner characteristic (wall thickness and shape of ingate) and filling length etc. Numerical simulation and experimental study shows that low filling velocity (<186.8mm/s) can easily cause the great fluctuation of the fluid level, the instability of filling process and the decrease of fluidity for6mm thickness plate. Misrun and cold lap appear in the last filling position. The defects of air entrainment are caused by the jetting of the molten metal at high filling velocity (>933.8mm/s). The filling process is smoother using fanned gate than straight gate at same filling velocity for12mm thickness plate. The fan ingate helps accomplish a rapid fill without high velocities and avoids jetting effects.Microstructure, mechanical properties and fracture characteristics of squeeze casting AZ91D alloy under different states are measured and analyzed The results show that hardness, yield strength and tensile strength decrease but elongation increases with the increase of wall thickness under as-cast, T4heat-treating and T6heat-treating. Taking8mm thickness sample as analysis object, tensile strength reaches205MPa, and elongation reaches7.0%in T4heat-treating. Compare with as-cast, elongation increases in large amplitude, but hardness and yield strength drop slightly. Hardness and yield strength significantly increases, tensile strength increases slowly, and elongation drops slightly in T6heat-treating. Compare with T4heat-treating, yield strength increases by44.2%. The fracture mode of squeeze cast AZ91D specimens in as-cast is mainly brittle fracture. A few tearing ridges and dimples with small size are observed on fracture surfaces. The fracture mode of AZ91D specimens in T4heat-treating is a combined fracture of quasi-cleavage and a few dimples. The cleavage surfaces, river markings and dimples are observed on the fracture surfaces. The fracture mode of AZ91D specimens in T6heat-treating is mainly intergranular fracture, and a few shallow dimples and tiny tearing ridges are observed on the fracture surfaces.The forming mechanism of cold clamp in squeeze cast AZ91D is discussed by theory analysis and microstructure analysis. The casting is divided into two parts by cold clamp which is like a layer of insulation. Therefore, both sides structures of cold clamp are obviously different. EDS analysis results indicate that high light particles in the cold clamp involve O, Mg and Si three elements. The particles consist of high melting point oxides and contamination from cold clamp surface. Further analysis shows that molten metal must contain superheat, and solidification layer in the pressure chamber must be broken up and remelted when filling cavity in order to eliminate cold clamp. The calculating formula for solidification layer remelting is deduced in the paper. The thickness of solidification layer decreases with the increase of the temperature of pressure chamber and pouring temperature. Thin solidification layer is easy to break up and remelt. A layer of suitable coating is evenly coated in the inner wall of pressure chamber, which can reduce the capacity of heat transmission of pressure chamber and decrease the thickness of solidification layer. Reducing the resident time of molten metal in the pressure chamber and using mechanical method can decrease or eliminate cold clamp.更多还原