镁合金超声辅助激光焊接特性研究

发布时间：2018-05-05 08:48

本文选题：超声辅助激光焊接 + 熔池流动　；参考：《哈尔滨工业大学》2017年硕士论文

【摘要】：镁合金是近年来在工程领域应用广泛也是最轻的金属材料,广泛地应用于各个领域。因为镁合金自身的一系列特殊的物理性质,导致在激光焊接过程中极易出现气孔、晶粒粗大等问题。超声辅助加工技术在金属凝固、表面改进以及焊接领域得到了广泛的研究,实现了细化晶粒,消除气孔和裂纹,从而改善材料综合性能的效果。因此,本课题提出超声辅助激光焊接工艺,利用超声波在焊接熔池中产生的空化效应、声流效应以及机械与热效应,消除镁合金在焊接时出现的一系列问题,并研究其机理,为镁合金的高质量焊接打下理论基础。与常规的镁合金激光焊相比,施加超声后,焊缝表面平整美观,但超声辅助无法解决焊缝的下塌缺陷;增大激光功率或降低焊接速度,焊缝熔宽和下塌量均有所增加;在其他工艺参数相同的情况下,只有负离焦能够得到稳定的焊接过程,正离焦和零离焦均会出现匙孔塌陷而导致焊接过程不连续;偏移量和变幅杆压力主要影响传播到镁合金熔池的超声能量。随着偏移量的增大,损失的超声能量越多,超声对焊缝成形的影响越小;变幅杆压力大小主要影响超声从变幅杆端部传到工件表面处的能量损失,当压力很小或很大时,超声能量损失较大,引起焊缝成形不良。通过扫描电镜以及EDS能谱分析表明镁合金焊接时主要产生氢气孔以及匙孔不稳定产生的“小孔型”气孔。在焊接速度较低的情况下,向熔池中施加超声波可以减小焊缝中气孔缺陷的产生;采用示踪元素法对熔池流动进行分析研究,发现施加超声后,焊接熔池的流动性增强。但随着焊接速度的提高,这种现象变得越来越不明显;结合超声对熔池流动特性以及超声波产生的效应,阐述了超声抑制焊接气孔机理:施加超声波辅助后,液态熔池中产生空化效应、声流效应以及机械搅拌效应,促进气泡的形核、长大及逸出过程,从而抑制气孔缺陷的产生。对焊缝微观组织及力学性能进行分析,AZ31B镁合金激光焊焊缝相组成主要由α-Mg基体和第二相β-Mg17Al12构成,β相弥散分布于晶界和晶粒内部,焊缝熔合线处的柱状晶区较大。施加超声辅助后,熔池中产生空化效应、声流效应以及机械搅拌作用,能够改善液态金属的润湿性,有利于熔池金属的形核,同时超声波效应也会促进熔池的流动,使已结晶的晶粒更容易被流动的金属卷到熔池中,促进等轴晶的形成;且由于超声波的加入,焊接接头的力学性能有所提高。
[Abstract]:Magnesium alloy is widely used in engineering field and is the lightest metal material in recent years. It is widely used in various fields. Due to a series of special physical properties of magnesium alloy, it is easy to appear porosity and coarse grain in laser welding process. Ultrasonic assisted machining technology has been widely studied in the fields of metal solidification, surface improvement and welding. It can refine grains, eliminate pores and cracks, and improve the comprehensive properties of materials. Therefore, the ultrasonic assisted laser welding process is proposed in this paper. The cavitation effect, acoustic flow effect and mechanical and thermal effect of ultrasonic wave in welding pool are used to eliminate a series of problems in welding of magnesium alloy, and its mechanism is studied. It lays a theoretical foundation for high quality welding of magnesium alloy. Compared with the conventional laser welding of magnesium alloy, the weld surface is flat and beautiful after ultrasonic application, but ultrasonic assistance can not solve the collapse defect of weld, and the weld width and collapse amount increase with increasing laser power or decreasing welding speed. When other process parameters are the same, only negative defocusing can obtain a stable welding process, both positive defocusing and zero defocusing will cause keyhole collapse, which leads to discontinuity of welding process. The ultrasonic energy propagating to the molten pool of magnesium alloy is mainly affected by the offset and the pressure of the variable amplitude bar. With the increase of deviation, the more ultrasonic energy is lost, the smaller the influence of ultrasonic on weld formation, and the pressure of the horn affects the energy loss from the end of the horn to the surface of the workpiece, when the pressure is very small or large, The loss of ultrasonic energy is large, which leads to bad weld formation. Scanning electron microscopy and EDS energy dispersive spectrum analysis showed that hydrogen pores and keyhole instability were mainly produced in magnesium alloy welding. In the case of low welding speed, ultrasonic wave applied to the weld pool can reduce the formation of porosity, and the fluidity of the weld pool is enhanced by the tracer element method. However, with the increase of welding speed, this phenomenon becomes less and less obvious. Combined with the ultrasonic effect on the flow characteristics of molten pool and ultrasonic wave, the mechanism of ultrasonic suppression of welding porosity is expounded: after the application of ultrasonic aid, Cavitation effect, acoustic flow effect and mechanical stirring effect are produced in liquid molten pool, which can promote the nucleation, growth and escape of bubbles, thus inhibiting the formation of pore defects. The microstructure and mechanical properties of the weld are analyzed. The weld phase composition of the laser welding of AZ31B magnesium alloy is mainly composed of 伪 -Mg matrix and the second phase 尾 -Mg17Al12. The 尾 phase is dispersed in grain boundary and inside the grain, and the columnar crystal zone is larger at the weld fusion line. With the aid of ultrasonic, cavitation, acoustic flow and mechanical stirring can improve the wettability of liquid metal, which is beneficial to the nucleation of molten pool metal, and the ultrasonic effect can also promote the flow of molten pool. The crystallized grains are more easily rolled into the molten pool by the flowing metal to promote the formation of equiaxed crystals, and the mechanical properties of welded joints are improved due to the addition of ultrasonic wave.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG457.19

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