行波磁场作用下ZL205A合金凝固组织及缺陷形成规律
发布时间：2018-08-17 17:11
【摘要】：ZL205A是强度高、综合性能较好的铸造铝合金,同时还具有良好机械加工性能,是航空航天以及汽车工业领域的重要结构材料。但是ZL205A合金铸造性能较差,大型铸件生产过程中易形成疏松、偏析等缺陷,疏松与偏析成为影响该合金大型铸件性能的主要缺陷。开发或者完善现有的铸造工艺,避免疏松和偏析缺陷的发生,对ZL205A合金在大型铸件上的应用具有重要的意义。本文利用行波磁场在合金熔体中产生的特殊作用力,对合金溶质分布以及凝固补缩产生作用,达到消除比重偏析、增加凝固补缩的目的。通过数值模拟,分析了励磁电流强度、励磁电流频率等行波磁场控制参数和铸件位置、铸件壁厚等铸件的结构参数对铸件内各位置的电磁力和平均电磁力密度的影响规律。各位置处的电磁力及铸件的平均电磁力密度与励磁电流强度的平方呈线性关系,随着励磁电流强度的增加,各位置处的电磁力及铸件的平均电磁力密度迅速增加。随着励磁电流频率增加,各位置处的电磁力及铸件的平均电磁力密度均存在极限值,其数值先增加后减小。随着铸件与行波磁场发生器相对距离和铸件壁厚的增加,各位置处的电磁力及铸件的平均电磁力密度均明显地减小,分布趋向平缓。以衰减系数表示电磁力在合金熔体内衰减速度,衰减系数越大,合金熔体内感应产生的电磁力衰减速度越快。建立了与衰减系数相关的电磁力空间分布函数,并基于此函数建立了行波磁场对合金熔体压力梯度和流场的影响模型。研究了平均电磁力密度、衰减系数表示的电磁力在不同壁厚铸件的合金熔体流动的速率分布以及临界零速率位置的影响规律。随着平均电磁力密度的增加,行波磁场引起的合金熔体的速率呈线性增加;随着衰减系数的增加,合金熔体的形成环流的流动速率先增加后减小,也就是流动速率存在极值,随着铸件壁厚以及电磁力衰减系数的增加,临界零速率位置逐渐向行波磁场发生器一侧偏移。ZL205A合金是多组元多相合金,其凝固过程中出现的各相比重存在差异,很容易由于各相受力不均形成相的偏聚,甚至形成偏析缺陷。各相的电导率也存在较大差异,在电磁场作用下也会出现类似的现象。偏析现象主要表现为溶质和相分布不均匀,本文先以ZL205A合金中的溶质Cu为研究对象,通过实验研究发现,行波磁场的励磁电流强度的越大,成分分布的越均匀。进一步对容易形成比重偏析的相进行研究发现,行波磁场的励磁电流强度越大,相分布得越均匀,相应形成的晶粒尺寸也比较均匀。通过分析发现这是由于行波磁场在合金熔体内形成的电磁力诱发了合金熔体的流动,随着熔体流动速率的增加,相间粘滞力超过各相之间电磁力和比重的差异,使得ZL205A合金铸件的凝固组织,随着流动速率的增加各相的分布也更加均匀。对不同壁厚铸件在凝固过程中施加行波磁场,研究了行波磁场对ZL205A合金铸件密度、凝固组织、抗拉强度和延伸率以及断口形貌等方面的影响规律。研究发现,行波磁场可以明显地促进合金的凝固补缩过程;其补缩效果与铸件壁厚、行波磁场方向有关,合金熔体内感应产生的电磁力与合金熔体补缩方向相同时可以促进补缩过程进行;对于一定壁厚的铸件,存在最佳的励磁电流强度,超过这个强度后,行波磁场对凝固补缩的促进作用逐渐减弱;当铸件壁厚增加,行波磁场作用下合金熔体内更容易形成环形流动,这种流动通过阻碍枝晶和晶粒搭接减小补缩阻力,但是随着环流速率的增加,促进了铸件成分均匀化的过程中,使温度分布均匀化,从而增加了糊状区的厚度,增加了补缩阻力,因此行波磁场对ZL205A合金凝固补缩效果存在极值,即存在最佳作用效果。
[Abstract]:ZL205A is a kind of cast aluminum alloy with high strength and good comprehensive properties. It is also an important structural material in aerospace and automotive industries. However, the casting properties of ZL205A alloy are poor, and it is easy to form defects such as porosity and segregation during the production of large castings. Major defects in the properties of ZL205A castings. Developing or perfecting the existing casting technology to avoid porosity and segregation defects is of great significance to the application of ZL205A alloy in large castings. The effects of traveling wave magnetic field control parameters such as excitation current intensity, excitation current frequency, casting position and casting wall thickness on electromagnetic force and average electromagnetic force density in each position of the casting were analyzed by numerical simulation. The average electromagnetic force density is linear with the square of the excitation current intensity. With the increase of the excitation current intensity, the electromagnetic force at each position and the average electromagnetic force density of the casting increase rapidly. With the increase of the relative distance between the casting and the traveling wave magnetic field generator and the wall thickness of the casting, the electromagnetic force and the average electromagnetic force density of the casting at each position decrease obviously and the distribution tends to be gentle. The faster the velocity is, the spatial distribution function of electromagnetic force related to attenuation coefficient is established. Based on this function, the influence model of traveling wave magnetic field on pressure gradient and flow field of alloy melt is established. With the increase of the average electromagnetic force density, the velocity of alloy melt caused by traveling wave magnetic field increases linearly; with the increase of attenuation coefficient, the flow rate of alloy melt forming circulation first increases and then decreases, that is, the flow rate has an extreme value, with the increase of casting wall thickness and electromagnetic force attenuation coefficient. ZL205A alloy is a multi-component multi-phase alloy, and its specific gravity varies during solidification. It is easy to form phase segregation or even segregation defects due to uneven force on each phase. The conductivity of each phase is also quite different, and also under the action of electromagnetic field. Similar phenomena may occur. Segregation is mainly manifested by the inhomogeneous distribution of solutes and phases. In this paper, the solute Cu in ZL205A alloy is taken as the research object. It is found that the greater the excitation current intensity of traveling wave magnetic field, the more uniform the composition distribution. The larger the excitation current intensity, the more homogeneous the phase distribution and the corresponding grain size are. It is found that the electromagnetic force formed by traveling wave magnetic field in the alloy melt induces the flow of alloy melt. With the increase of melt flow rate, the phase viscosity force exceeds the difference of electromagnetic force and specific gravity between the phases. The effect of traveling wave magnetic field on density, solidification structure, tensile strength, elongation and fracture morphology of ZL205A alloy castings with different wall thickness was studied. Traveling wave magnetic field can obviously promote the solidification and feeding process of the alloy; its feeding effect is related to the thickness of the casting wall and the direction of traveling wave magnetic field; the electromagnetic force produced in the alloy melt can promote the feeding process when the feeding direction is the same as that of the alloy melt; for the casting with a certain wall thickness, there is an optimum excitation current intensity, which exceeds this one. The effect of traveling wave magnetic field on solidification and feeding is gradually weakened after strength, and the annular flow is easier to form in the melt of the alloy when the wall thickness of the casting increases. This flow reduces feeding resistance by hindering the overlap of dendrites and grains, but with the increase of circulating velocity, the homogenization of the casting composition is promoted and the temperature is increased. The uniform degree distribution increases the thickness of paste zone and the feeding resistance, so the traveling wave magnetic field has an extreme value on the solidification feeding effect of ZL205A alloy, that is, there is an optimum effect.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.21
[Abstract]:ZL205A is a kind of cast aluminum alloy with high strength and good comprehensive properties. It is also an important structural material in aerospace and automotive industries. However, the casting properties of ZL205A alloy are poor, and it is easy to form defects such as porosity and segregation during the production of large castings. Major defects in the properties of ZL205A castings. Developing or perfecting the existing casting technology to avoid porosity and segregation defects is of great significance to the application of ZL205A alloy in large castings. The effects of traveling wave magnetic field control parameters such as excitation current intensity, excitation current frequency, casting position and casting wall thickness on electromagnetic force and average electromagnetic force density in each position of the casting were analyzed by numerical simulation. The average electromagnetic force density is linear with the square of the excitation current intensity. With the increase of the excitation current intensity, the electromagnetic force at each position and the average electromagnetic force density of the casting increase rapidly. With the increase of the relative distance between the casting and the traveling wave magnetic field generator and the wall thickness of the casting, the electromagnetic force and the average electromagnetic force density of the casting at each position decrease obviously and the distribution tends to be gentle. The faster the velocity is, the spatial distribution function of electromagnetic force related to attenuation coefficient is established. Based on this function, the influence model of traveling wave magnetic field on pressure gradient and flow field of alloy melt is established. With the increase of the average electromagnetic force density, the velocity of alloy melt caused by traveling wave magnetic field increases linearly; with the increase of attenuation coefficient, the flow rate of alloy melt forming circulation first increases and then decreases, that is, the flow rate has an extreme value, with the increase of casting wall thickness and electromagnetic force attenuation coefficient. ZL205A alloy is a multi-component multi-phase alloy, and its specific gravity varies during solidification. It is easy to form phase segregation or even segregation defects due to uneven force on each phase. The conductivity of each phase is also quite different, and also under the action of electromagnetic field. Similar phenomena may occur. Segregation is mainly manifested by the inhomogeneous distribution of solutes and phases. In this paper, the solute Cu in ZL205A alloy is taken as the research object. It is found that the greater the excitation current intensity of traveling wave magnetic field, the more uniform the composition distribution. The larger the excitation current intensity, the more homogeneous the phase distribution and the corresponding grain size are. It is found that the electromagnetic force formed by traveling wave magnetic field in the alloy melt induces the flow of alloy melt. With the increase of melt flow rate, the phase viscosity force exceeds the difference of electromagnetic force and specific gravity between the phases. The effect of traveling wave magnetic field on density, solidification structure, tensile strength, elongation and fracture morphology of ZL205A alloy castings with different wall thickness was studied. Traveling wave magnetic field can obviously promote the solidification and feeding process of the alloy; its feeding effect is related to the thickness of the casting wall and the direction of traveling wave magnetic field; the electromagnetic force produced in the alloy melt can promote the feeding process when the feeding direction is the same as that of the alloy melt; for the casting with a certain wall thickness, there is an optimum excitation current intensity, which exceeds this one. The effect of traveling wave magnetic field on solidification and feeding is gradually weakened after strength, and the annular flow is easier to form in the melt of the alloy when the wall thickness of the casting increases. This flow reduces feeding resistance by hindering the overlap of dendrites and grains, but with the increase of circulating velocity, the homogenization of the casting composition is promoted and the temperature is increased. The uniform degree distribution increases the thickness of paste zone and the feeding resistance, so the traveling wave magnetic field has an extreme value on the solidification feeding effect of ZL205A alloy, that is, there is an optimum effect.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG146.21
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