稀土镁合金NZ30K激光焊接头组织演变与力学性能研究

发布时间：2018-04-14 05:30

本文选题：NZ30K + 激光焊接　；参考：《江苏科技大学》2017年硕士论文

【摘要】：目前镁合金在航空航天、机械制造以及电子产品等行业中有着广泛的应用,但其强度低、耐腐蚀性差严重制约了镁合金的发展。为了提高镁合金的性能、推广镁合金的应用领域,近年来含稀土元素的新型镁合金及其连接技术得到了广泛的关注。采用传统熔焊方法焊接镁合金会造成焊接变形大、热影响区性能变化大等缺点。激光焊作为高能束焊接方法之一是解决上述问题的途径之一。本文针对4mm的NZ30K稀土镁合金,采用激光自熔焊的方法进行焊接,研究了不同焊接工艺参数对焊缝成形、接头组织和性能的影响规律,通过OM、SEM、EDS、XRD等测试方法观察和分析了接头不同部位的组织演变行为和特征,确定了焊缝中析出相的种类;通过接头的拉伸试验测试分析了接头的力学性能,从而阐明了焊接工艺参数和接头组织特征、接头性能的关系;对比分析了AZ31镁合金和NZ30K稀土镁合金在高温条件下(200°C)的拉伸性能,同时研究了常温下不同接头腐蚀性能的差异。对NZ30K镁合金试验所得接头进行了力学性能测试,并对接头的高温(200°C)力学性能进行测试。通过利用金相显微镜、SEM、EDS、XRD等手段进行分析,观察焊缝显微组织以及合金元素分布;研究结果表明:当采用激光功率3kW、焊接速度3m/min、离焦量-5mm时焊缝接头抗拉强度达到228MPa,是母材抗拉强度的90%左右。接头高温抗拉强度达到162MPa,是接头常温抗拉强度70%左右。激光焊接后接头力学性能良好。焊缝组织主要由大量的等轴晶组成,Nd元素主要集聚在晶界处,晶界处主要成分为Mg12Nd,激光焊接后焊接接头没有明显的软化现象。对AZ31激光焊接头进行试验研究,观察其显微组织,对其力学性能以及腐蚀性能进行测试。研究结果表明:AZ31接头焊缝组织主要为树枝晶,接头常温抗拉强度可达224MPa,高温抗拉强度仅为105MPa,高温下接头性能下降明显。焊缝区域会出现软化现象。分别对NZ30K和AZ31两种接头进行浸泡腐蚀试验以及电化学腐蚀试验,所得结果显示NZ30K的耐腐蚀性能比AZ31好。稀土镁合金焊接接头中含有稀土元素Nd,可以提高焊缝组织中α-Mg相的耐腐蚀能力。同时在稀土镁合金接头焊缝区域组织主要为大量细小的等轴晶,在晶界上连续分布着Mg12Nd相,可以起到阻碍腐蚀的作用,从而降低整个焊缝接头的腐蚀速度,提高焊缝耐腐蚀性能。
[Abstract]:Magnesium alloys are widely used in aerospace, mechanical manufacturing and electronic products, but their low strength and poor corrosion resistance seriously restrict the development of magnesium alloys.In order to improve the properties of magnesium alloys and promote the application of magnesium alloys, new magnesium alloys containing rare earth elements and their bonding technology have been widely concerned in recent years.Welding magnesium alloy by traditional welding method will result in large deformation and great change of heat affected zone properties.As one of the high energy beam welding methods, laser welding is one of the ways to solve the above problems.In this paper, laser self-fusion welding was used to weld NZ30K rare earth magnesium alloy of 4mm. The influence of different welding parameters on weld formation, joint structure and properties was studied.The evolution behavior and characteristics of microstructure in different parts of the joint were observed and analyzed by means of OMSAM EDSX XRD, and the types of precipitated phases in the weld were determined, and the mechanical properties of the joints were analyzed by tensile tests.The relationship between the welding process parameters and the microstructure and properties of the joints was clarified, and the tensile properties of AZ31 magnesium alloys and NZ30K rare earth magnesium alloys at high temperature were compared and analyzed. At the same time, the difference of corrosion resistance of different joints at room temperature was studied.The mechanical properties of the joints obtained from the NZ30K magnesium alloy test were tested, and the mechanical properties of the joints at high temperature (200 掳C) were tested.The microstructure of weld and the distribution of alloying elements were observed by means of SEM EDS- XRD.The results show that when the laser power is 3 kW, the welding speed is 3 m / min and the defocus is -5 mm, the tensile strength of the weld joint reaches 228 MPA, which is about 90% of the tensile strength of the base metal.The high temperature tensile strength of the joint reaches 162 MPA, which is about 70% of the normal temperature tensile strength of the joint.After laser welding, the mechanical properties of the joints are good.The microstructure of the weld is mainly composed of a large number of equiaxed crystals. The ND element is mainly concentrated at the grain boundary, and the main composition at the grain boundary is mg _ (12) N _ (d). There is no obvious softening phenomenon in the welded joint after laser welding.The microstructure, mechanical properties and corrosion properties of AZ31 laser welded joints were investigated.The results show that the microstructure of the weld is mainly dendritic, the tensile strength of the joint can reach 224MPa at room temperature and the tensile strength at high temperature is only 105MPa, and the performance of the joint decreases obviously at high temperature.The weld zone will soften.Immersion corrosion tests and electrochemical corrosion tests were carried out on NZ30K and AZ31 joints respectively. The results showed that the corrosion resistance of NZ30K was better than that of AZ31.The rare earth element Ndin the welded joints of rare earth magnesium alloys can improve the corrosion resistance of 伪 -Mg phase in weld microstructure.At the same time, the microstructure of rare-earth magnesium alloy joints is mainly composed of a large number of fine equiaxed crystals, and the Mg12Nd phase is distributed continuously on the grain boundaries, which can hinder the corrosion and reduce the corrosion rate of the whole weld joints.Improve the corrosion resistance of weld.
【学位授予单位】：江苏科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG456.7

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