Mg-Al系压铸镁合金显微组织对力学性能和腐蚀性能影响的研究

发布时间：2018-05-23 17:27

本文选题：压铸镁合金 + 力学性能　；参考：《吉林大学》2017年博士论文

【摘要】：镁合金是目前汽车工业上具有广阔发展前景的轻量化工程材料,其具有低密度、高比强度、切削加工性能好、铸造性能佳、可以回收利用等优异性能。而压铸镁合金是镁合金中使用最为广泛的一种,目前有许多企业正在研发由压铸镁合金制造的方向盘骨架、座椅骨架、仪表板骨架等零部件。然而,压铸镁合金的力学性能和腐蚀性能较差,限制了此类材料的发展。因此,针对压铸镁合金使用性能提高的研究,成为国内外轻金属领域研究的重点和难点。本工作主要针对Mg-Al系压铸AZ91镁合金和压铸AM50镁合金,采用无损探伤、拉伸、浸泡腐蚀、三维显微组织重构、金相观察、扫描电镜等多种测试和观察手段,对合金的显微组织及其内部缺陷特征、合金的力学性能以及腐蚀性能展开系统地研究,并对其对应关系进行讨论和模拟,从而掌握Mg-Al系压铸镁合金显微组织对力学性能和腐蚀性能影响的规律,为改善合金的性能提供一定的理论依据。具体研究成果包括:1、研究了压铸AZ91镁合金的显微缺陷对力学性能的影响规律。压铸AZ91镁合金显微组织主要由α-Mg相、β-Mg17Al12相组成,并含有一定数量的气孔和缩松缺陷。各相以及显微缺陷的尺寸以及分布在空间上均存在不均匀的特点:自铸件表面至中心区域,α相尺寸逐渐增大、β相数量减少、气孔及缩松尺寸和数量均增高。根据三维显微重构模型进行统计,气孔尺寸范围为4~505μm左右,其中绝大多数气孔尺寸为100μm左右。结合拉伸实验和有限元模拟结果可知,气孔的尺寸、分布、横截面上气孔面积百分比最大值(MPAF)等因素对合金的抗拉强度和延伸率有明显不利影响。随着材料内部最大气孔尺寸增大,合金的抗拉强度和延伸率降低;垂直于样品受力方向分布的气孔对力学性能的影响比平行于受力方向分布的气孔影响严重;随MPAF增大,合金的抗拉强度和延伸率降低:MPAF每增加1%,合金的延伸率降低大约0.37%,抗拉强度降低大约12.02MPa。2、研究了压铸AM50镁合金的显微组织对力学性能的影响规律。压铸AM50镁合金显微组织主要由α-Mg相、β-Mg17Al12相和少量的Al Mn相组成,并含有一定数量的显微缺陷。相对于压铸AZ91镁合金,压铸AM50镁合金中显微缺陷的数量及尺寸均较小,气孔的尺寸普遍小于100μm,有极少量较大尺寸的气孔,合金力学性能较好。结合拉伸试验和有限元模拟结果,气孔对于hpdcam50镁合金的力学性能有不利影响,其中气孔的尺寸和在横截面上面积比的最大值(mpaf)是两个主要影响因素。随着hpdcam50镁合金内气孔尺寸的增大,样品的抗拉强度和延伸率均下降。同时,随着气孔在横截面上面积比的最大值的数值增大,样品的抗拉强度和延伸率也均近似线性下降。mpaf每升高1%,hpdcam50镁合金的抗拉强度降低大约12.28mpa,而延伸率降低1.96%。冷隔、裂纹等缺陷对合金力学性能影响严重,会导致在拉伸过程中超出屈服强度后过早失效。3、合金凝固过程中不同的冷速会形成不同的显微组织。不同厚度的hpdcam50镁合金由于具有不同的显微组织,因而其耐腐蚀性能有所差别。对于厚度从3.0mm至4.5mm的样品而言,耐腐蚀性能先减弱再增强,在3.5mm厚度左右其耐腐蚀性能最差。腐蚀速度与α-mg晶粒尺寸、β相分布以及孔隙存在情况有关:α-mg晶粒尺寸的增大、枝晶数目的增多、β相连续性的减弱,或孔隙含量增高,均可能导致合金的腐蚀速度增加。此外,在利用光学显微镜观察腐蚀的合金样品表面形貌时,暗场像(darkfieldimage,dfi)相对于通常的显微观察中常用的明场像(brightfieldimage,bfi)具有显著的优势。使用暗场像能够较为容易的观察到腐蚀区域的细微变化。4、改善压铸镁合金的显微组织主要有三个途径:(1)优化压铸过程工艺参数;(2)采取全新压铸方式;(3)添加能够改善显微组织的合金元素。本研究中以改善合金显微组织为目的,使用procast软件对压铸am50镁合金的三个工艺参数进行测试和优化,获得的相对较好的参数搭配为:浇注温度670℃,模具温度200℃,压射速度2.0m/s。5、针对在本研究中遇到的超声波无损探伤过程中的实际性问题,设计了两种自动超声波探伤装置,分别用于回转体试样和不规则形状试样的超声波无损探伤。这两项设计通过使用万能夹具、多方向运动机构、自动喷涂设备等,主要解决了探伤过程中样品夹持不便、探头运行轨迹不规律、耦合剂喷涂困难等问题,实现了超声波无损探伤的自动化、三维化和高效化。6、针对金相图像采集在获取铸造缺陷形貌特征中的片面性和随机性问题,运用三维显微组织重构方法,实现了对组织内部缺陷的重建,达到使缺陷尺寸、形状、分布等特征直观、准确的目标。
[Abstract]:Magnesium alloy is a lightweight engineering material with broad development prospects in the automobile industry. It has excellent properties such as low density, high specific strength, good cutting performance, good casting performance, and can be recycled. And die-casting magnesium alloys are the most widely used in magnesium alloys. At present, many enterprises are developing die cast magnesium alloys. However, the mechanical properties and corrosion properties of the die cast magnesium alloys are poor, which limits the development of such materials. Therefore, the research on the improvement of the performance of the die casting magnesium alloys has become the key and difficult point in the field of light metal research at home and abroad. This work is mainly aimed at the Mg-Al system pressure. AZ91 magnesium alloy and die-casting AM50 magnesium alloy are made by nondestructive testing, tensile, immersion corrosion, three-dimensional microstructure reconstruction, metallographic observation, scanning electron microscopy and other testing and observation methods. The microstructure and internal defects of the alloy, the mechanical properties and corrosion properties of the alloys are systematically studied, and their corresponding relations are discussed. The influence of microstructure of Mg-Al die-casting magnesium alloy on mechanical properties and corrosion properties is mastered to provide a theoretical basis for improving the properties of the alloy. The specific research results include: 1, the influence of the micro defects on the mechanical properties of the die casting AZ91 magnesium alloy is studied. The microstructure of the die cast AZ91 magnesium alloy is mainly composed of the microstructure of the die casting alloy. The alpha -Mg phase and beta -Mg17Al12 phase consist of a certain number of stomata and shrinkage defects. The size and distribution of the phases and microdefects are uneven in space: the size of the alpha phase increases gradually from the casting surface to the central region, the number of beta phase decreases, the size and quantity of stomata and shrinkage are all increased. The size range of the stomata is about 4~505 mu m, and most of the pores are about 100 mu m. The size and distribution of the pores and the maximum percentage of the pore area on the cross section (MPAF) have obvious adverse effects on the tensile strength and elongation of the alloy. The tensile strength and elongation of the alloy increased, the tensile strength and elongation of the alloy decreased, and the influence of the air hole perpendicular to the direction of force distribution on the mechanical properties was more serious than that of the air hole parallel to the distribution of the force. The tensile strength and elongation of the alloy decreased with the increase of MPAF: the elongation of the alloy decreased by about 0.37% per increase of 1%. The tensile strength decreased by about 12.02MPa.2. The influence of microstructure on the mechanical properties of the die cast AM50 magnesium alloy was studied. The microstructure of the die cast AM50 magnesium alloy consists mainly of alpha -Mg phase, beta -Mg17Al12 phase and a small amount of Al Mn, and contains a certain number of microscopic defects. The size and size are smaller, the size of the pores is generally less than 100 m, with a very small size of the pores, and the mechanical properties of the alloy are better. In combination with the tensile test and the finite element simulation results, the porosity has a negative effect on the mechanical properties of the hpdcam50 magnesium alloy, and the size of the pores and the maximum value of the area ratio on the cross section (mpaf) are two. The tensile strength and elongation of the sample decreased with the increase of the pore size in the hpdcam50 magnesium alloy. At the same time, the tensile strength and elongation of the sample decreased approximately linearly with the increase of the maximum value of the area ratio on the cross section, and the tensile strength of the hpdcam50 magnesium alloy decreased by about 1% per liter, and the tensile strength of the hpdcam50 magnesium alloy decreased approximately. 12.28mpa, while the elongation reduction of 1.96%. cold septum, crack and other defects have a serious effect on the mechanical properties of the alloy, which will lead to premature failure of.3 after the tensile process exceeding the yield strength. The different cooling rates of the alloy during the solidification process will form different microstructure. The different thickness of the hpdcam50 magnesium alloy is resistant to the microstructure, thus it is resistant to the microstructure. Corrosion resistance is different. For samples with thickness from 3.0mm to 4.5mm, corrosion resistance is first weakened and re enhanced, and its corrosion resistance is the worst at the thickness of 3.5mm. The corrosion rate is related to alpha -mg grain size, beta phase distribution and pore existence: increasing the size of alpha -mg grain, increasing dendrite number, decreasing the continuity of beta phase, or In addition, the dark field image (darkfieldimage, DFI) has a significant advantage over common microscopic observations (brightfieldimage, BFI), when using optical microscopy to observe the surface morphology of corroded alloy samples. There are three main ways to improve the microstructure of the die cast magnesium alloy: (1) to optimize the process parameters of the die casting process; (2) to adopt a new die-casting method and (3) to add alloy elements that can improve the microstructure of the alloy. The purpose of this study is to improve the microstructure of the alloy and use the software of ProCAST to die casting AM50 magnesium alloy. Three process parameters are tested and optimized, and the relatively good parameters are as follows: pouring temperature 670, die temperature 200, and injection speed 2.0m/s.5. Aiming at the practical problems in the process of ultrasonic nondestructive testing encountered in this study, two kinds of automatic hyper acoustic detection devices are designed to be used in rotary body samples and unconventional. The two designs, through the use of universal jig, multi direction moving mechanism and automatic spraying equipment, mainly solve the problems of the inconvenience of sample holding, the irregular track of the probe and the difficulty of the coupling agent spraying, which have realized the automation of ultrasonic nondestructive testing, three-dimensional and efficient.6, In view of the one-sided and randomicity of metallographic image acquisition in the feature of casting defects, three-dimensional microstructure reconstruction is used to reconstruct the internal defects of the tissue, which can make the defect size, shape, distribution and so on intuitionistic and accurate.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG249.2;TG146.22

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