柔性变极性等离子弧特性及铝合金横焊穿孔熔池行为

发布时间：2019-06-03 23:03

【摘要】：随着铝合金焊接结构大型化发展,原位焊接制造成为关注的焦点,其中最为典型的应用是重型运载火箭燃料贮箱的焊接,而铝合金横焊技术成为制约其发展的瓶颈。铝合金横焊难度较大、焊接质量和效率较低,主要是重力对熔池作用和铝合金本身物理特性导致。为解决上述问题、提高铝合金横焊效率和质量,开展了变极性等离子弧铝合金穿孔横焊技术的研究。根据常规变极性等离子弧(简称“常规电弧”)穿孔横焊难点和穿孔熔池稳定建立条件,从改变电弧特性角度出发,提出柔性变极性等离子弧(简称“柔性电弧”)。在穿孔焊接前提下,通过改变电弧能量和压力分布,促进穿孔熔池稳定建立,最终实现了8mm厚2A14铝合金板穿孔横焊,焊缝成形良好且组织、力学性能优异。首先,采用常规电弧对6mm、8mm厚铝合金板进行穿孔横焊试验,明确铝合金穿孔横焊的瓶颈问题。板厚6mm时,穿孔横焊的难点是咬边和气孔缺陷的控制;板厚8mm时,穿孔横焊的难点是如何保证穿孔熔池稳定建立。针对穿孔横焊难点,探究咬边缺陷的成因和穿孔熔池稳定建立的条件。正面咬边缺陷主要是由较大热输入和电弧压力导致;背面咬边缺陷主要是由较小热输入或较大电弧压力导致。熔池动态行为观察结果表明,熔池背面上、下侧熔化金属的搭桥连接是穿孔熔池建立的关键,熔化金属在熔池正面的填充能够防止穿孔熔池的崩溃,围绕小孔的流动是稳定穿孔熔池的标志。根据问题产生原因,提出了初步控制措施。“小规范+背面预热”措施可消除6mm板穿孔横焊咬边缺陷;“坡口设计”措施可促进8mm板穿孔熔池的建立,但未能彻底解决穿孔熔池稳定性问题。根据穿孔熔池稳定建立的热、力条件,提出了从改变电弧特性角度控制穿孔熔池稳定性的研究思路。通过使用三孔型水冷喷嘴,改变电弧能量和压力分布,获得柔性变极性等离子弧,改善了横焊缝成形、提高了穿孔过程稳定性。良好作用效果源于其独特的电弧特性。与常规电弧相比,柔性电弧形态扩展,压缩程度降低;静特性下移,阻抗特性降低。通过分裂阳极法测量电弧能量密度分布,利用水冷铜板小孔法检测电弧压力分布。与常规电弧相比,柔性电弧能量集中程度较高,源于三孔型水冷喷嘴的使用和较大的离子气流量;在工件表面柔性电弧能量分布呈椭圆形,改变了穿孔熔池形貌;柔性电弧压力幅值及梯度都大大降低,增加了电弧稳定性。通过穿孔熔池受力计算和流动行为分析,阐释了柔性电弧的作用机理:电弧压力的减小,使焊缝背面最大允许孔径增大,促进了穿孔熔池尾部的填充;其热源特性提高了熔化金属的流动性,促进了熔化金属的搭桥连接和正面填充,提高了穿孔焊接的稳定性。利用流体动力学计算软件Fluent,对穿孔焊接过程进行数值模拟,定量研究电弧形式和电弧压力对熔池行为的影响。为准确描述柔性电弧特性,建立了双椭圆面-锥体复合热源模型和力源模型。通过UDF(User Defined Function)二次开发和VOF(Volume of Fluid)界面追踪技术,实现了三维瞬态穿孔焊接数值模拟并验证了模型的可靠性。在穿孔熔池形成过程模拟中,柔性电弧穿孔速度慢,热传导在工件宽度方向作用显著,熔池温度较高,温度场分布不对称;电弧压力增加使穿孔直径增大、小孔周围金属液膜变薄、向熔池正面的流动速度降低。在穿孔熔池填充过程模拟中,柔性电弧熔池内熔化金属的搭桥连接速度快、填充能力强,熔池凝固速度慢。根据模拟结果,提出了穿孔熔池填充判定条件:表面张力附加力大于电弧力和重力合力;熔化金属搭桥连接和正面填充时间小于熔池凝固时间。判定条件揭示了横焊位置穿孔熔池的热、力平衡机制。最后,对铝合金柔性电弧穿孔横焊工艺及接头组织、力学性能进行研究,并与常规电弧立焊进行对比分析,检验工艺稳定性和接头可靠性。柔性电弧穿孔横焊工艺窗口较宽,具有良好的焊接稳定性;采用大电流、氧化膜刮削和焊前预热的综合措施可有效控制气孔缺陷。柔性电弧横焊接头抗拉强度和断后延伸率略高于立焊接头,断裂模式为塑性断裂,接头中微气孔缺陷和离异共晶是重要的裂纹源。横焊接头上侧热影响区组织晶粒细小、硬度值较高,组织和力学性能具有不对称性。这种不对称性是由横焊缝上、下侧焊接热循环的不对称性引起的,而根源是重力导致的穿孔熔池金属流动的不对称。
[Abstract]:With the development of the large-scale welding structure of the aluminum alloy, the in-situ welding is the focus of attention. The most typical application is the welding of the fuel tank of the heavy-duty vehicle, and the technology of the lateral welding of the aluminum alloy becomes the bottleneck of the development. The transverse welding of the aluminum alloy is difficult, the welding quality and the efficiency are low, mainly due to the effect of gravity on the molten pool and the physical characteristics of the aluminum alloy. In order to solve the above problems, the transverse welding efficiency and quality of the aluminum alloy are improved, and the technology of transverse welding of the variable-polarity plasma arc aluminum alloy is carried out. The flexible variable-polarity plasma arc ("flexible arc") is proposed according to the difficulty of transverse welding of the conventional variable-polarity plasma arc ("conventional arc") and the stable establishment condition of the perforated molten pool. Under the premise of perforation welding, by changing the arc energy and pressure distribution, the stable establishment of the perforated molten pool is promoted, and finally, the through-hole transverse welding of the 8mm-thick 2A14 aluminum alloy plate is realized, and the welding seam is well formed and the mechanical property is excellent. First, by using the conventional electric arc, the 6-mm and 8-mm-thick aluminum alloy plate is used for punching and transverse welding, and the bottleneck problem of the transverse welding of the aluminum alloy is determined. When the plate thickness is 6 mm, the difficulty of the transverse welding of the perforation is the control of the defect of the bite edge and the air hole; when the plate thickness is 8 mm, the difficulty of the transverse welding of the perforation is how to ensure the stable establishment of the perforated weld pool. In view of the difficulty of the cross-welding of the perforation, the cause of the undercut and the conditions for the stable establishment of the molten pool are investigated. The face-edge defects are mainly caused by larger thermal input and arc pressure; the back-edge defects are mainly caused by small thermal input or large arc pressure. The observation of the dynamic behavior of the molten pool shows that the bridging connection of the molten metal on the back of the molten pool is the key to the establishment of the perforated molten pool, and the filling of the molten metal on the front of the molten pool can prevent the collapse of the molten pool, and the flow around the small holes is the mark of the stable perforated pool. According to the cause of the problem, the preliminary control measures are put forward. The "Small Specification + Back Preheating" measures can eliminate the defects of the horizontal welding of the 6-mm plate through the transverse welding, and the "groove design" measures can promote the establishment of the 8-mm plate through-hole molten pool, but the stability of the perforated molten pool can not be thoroughly solved. Based on the heat and force conditions established in the stable molten pool, a new approach to control the stability of the molten pool from the change of the arc characteristic is presented. By using the three-pass water-cooling nozzle, the arc energy and the pressure distribution are changed, the flexible variable-polarity plasma arc is obtained, the transverse welding seam is improved, and the stability of the perforation process is improved. The good effect results from its unique arc characteristics. Compared with the conventional electric arc, the flexible arc shape expands, the compression degree is reduced, the static characteristic is moved down, and the impedance characteristic is reduced. The electric arc energy density distribution is measured by a split-anode method, and the arc pressure distribution is detected by a water-cooled copper plate small hole method. Compared with the conventional electric arc, the flexible arc energy concentration degree is high, the use of the three-pass water-cooling nozzle and the large ion gas flow rate are generated, the flexible arc energy distribution on the surface of the workpiece is elliptical, the shape of the perforated molten pool is changed, and the amplitude and the gradient of the flexible arc pressure are greatly reduced, And the arc stability is increased. The action mechanism of the flexible arc is illustrated by the analysis of the stress calculation and the flow behavior of the perforated molten pool. The arc pressure is reduced, the maximum allowable pore diameter of the back surface of the weld is increased, and the filling of the tail of the molten pool is promoted; and the heat source property of the welding molten pool improves the fluidity of the molten metal, The bridging connection and the front filling of the molten metal are promoted, and the stability of the through-hole welding is improved. The influence of the arc form and the arc pressure on the behavior of the molten pool is quantitatively studied by means of the fluid dynamics calculation software Fluent. In order to accurately describe the characteristics of the flexible arc, a double-ellipsoid-cone composite heat source model and a force source model are established. Through the development of the UDF (User Defined Function) and the Volume of Fluid interface tracking technique, the numerical simulation of three-dimensional transient perforation welding is realized and the reliability of the model is verified. in the process simulation of the punching molten pool, the flexible arc perforation speed is slow, the heat conduction is significant in the width direction of the workpiece, the temperature of the molten pool is high, the distribution of the temperature field is not symmetrical, the increase of the arc pressure causes the diameter of the through hole to increase, and the metal liquid film around the small hole is thin, The flow rate to the front of the bath is reduced. In the process simulation of the filling process of the punching molten pool, the bridge connecting speed of the molten metal in the flexible arc welding pool is high, the filling capacity is strong, and the solidification speed of the molten pool is slow. According to the simulation results, the conditions for determining the filling of the weld pool are put forward: the additional force of surface tension is larger than that of the arc force and the gravity force, and the melting metal bridging connection and the front filling time are less than the setting time of the molten pool. The determination condition reveals the heat and force balance mechanism of the weld pool in the horizontal welding position. In the end, the transverse welding technology and joint structure and mechanical property of the aluminum alloy flexible arc perforation are studied, and compared with the conventional arc welding, the stability of the process and the reliability of the joint are verified. The flexible arc punching transverse welding process window is wide and has good welding stability, and the comprehensive measures of high current, oxidation film scraping and pre-welding pre-welding can effectively control the pore defects. The tensile strength and elongation of the flexible arc transverse welding head are slightly higher than that of the vertical welding head, and the fracture mode is plastic fracture, and the micro-pore defect and the divorced eutectic in the joint are important crack sources. The microstructure and mechanical properties of the lateral heat-affected zone on the lateral welding head are fine, the hardness value is high, and the structure and the mechanical property are asymmetric. This asymmetry is caused by the asymmetry of the heat cycle on the horizontal weld and the lower side, and the root cause is the asymmetry of the metal flow in the molten pool caused by gravity.
【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】：TG456.2

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