【作者】 严军；

【导师】 曾晓雁；

【作者基本信息】 华中科技大学 ， 物理电子学， 2011， 博士

【摘要】 近年来,光纤激光器以其输出功率高、光束质量好、结构紧凑、能采用光纤传输等优点,在焊接领域具有广阔的应用前景。光纤激光一MIG(熔化极惰性气体保护焊)复合焊接技术集成了两热源的优点,具有更高的焊接效率和更强的适应能力,是目前激光焊接技术的研究热点。但是研究表明,其在焊接高强铝合金厚板时存在焊缝成形不佳,易出现气孔、裂纹等缺陷,接头性能还有待进一步提高等尚未解决的技术难点。为此,本文针对2xxx、6xxx系列高强铝合金光纤激光一MIG复合焊接高强铝合金的工艺特性、焊缝成形特点、气孔产生及抑制机理、接头显微组织演变机制以及接头性能进行了系统研究,旨在为实现复合焊接厚板高强铝合金的工程应用奠定理论基础。具体研究成果如下：对单独光纤激光焊接高强铝合金工艺特点、冶金缺陷、接头性能及其焊接适应性进行了研究。结果发现,冶金缺陷是影响高强铝合金激光焊接接头质量主要因素。其中,咬边缺陷随焊接速度与激光功率密度的增大而增大。而且,针对不同类型的铝合金,母材中低沸点合金元素含量越高,激光束诱发等离子体的能力越强,接头咬边程度越大。通过正交试验发现,激光小孔深度和等离子体波动是影响气孔率的主要原因；保护气种类、激光功率、离焦量显著影响气孔率的置信度为95%。在上述研究基础上,实现了光纤激光焊接高强铝合金的参数优化,获得了质量标准达到C级的光纤激光焊接接头,最大抗拉强度达到母材的72%,比电弧焊接接头高11%。开展了高强铝合金光纤激光-MIG复合焊接研究,得到了最优工艺参数范围。研究认为激光与电弧的最佳热源间距范围随焊接电流的增大而增大,采用氦氩混合气体可以获得更大的焊接熔深和更好的焊缝成形。通过熔池受力分析得到了焊缝成形质量控制方法,绘制了成形质量控制图。对高强铝合金光纤激光-MIG复合焊接接头的气孔产生和抑制机理进行了深入分析,得到了气孔抑制方法。分析认为增大小孔直径、提高工件上方温度、增加电弧电流是实现抑制复合焊接接头气孔的有效手段。增大小孔直径、提高工件上方温度能够降低小孔内等离子体密度、稳定激光小孔,实现对气孔的抑制。增加MIG电流能够提高电弧力,加速熔池流动,消除复合焊接气孔。通过实验验证,首次获得了成形质量好、无气孔缺陷,厚度为8mm的高强铝合金复合焊接接头系统分析了高强铝合金激光-MIG复合焊接接头的微观组织特征,研究复合焊接头内电弧区和激光区在热作用方式、热输入、温度梯度上的差异性及其对焊缝微观组织的影响,得到了焊缝微观组织演变机制。接头下部主要受激光束作用,焊接热输入小、温度梯度大,晶粒尺寸更小,热影响区更窄。接头上部受激光与电弧的共同作用,热输入较大,热影响区更宽,且形成了半熔化区。选用ER5356、ER5087和ER4043三种焊丝研究了复合焊接接头中焊丝成分—显微组织—接头机械性能—断裂机制之间的关系。并在此研究的基础上进一步提高了接头性能。ER4043焊丝焊接的接头晶粒尺寸最大,柱状晶长度最短,接头性能最差,断口观察呈现穿晶准解理断裂特征。ER5087焊丝能细化晶粒,其晶粒尺寸最小,得到的接头强度最高,达到母材的75.6%。此外,其断口的韧窝深度更大,呈韧性正向断裂特征。以上研究取得的结论为实现厚板高强铝合金的激光-电弧复合焊接实际工程应用具有重要意义。更多还原

【Abstract】 The new generation of high power fiber laser presents high power with low beam divergence and flexible beam delivery, which benefits for thick plates welding. By integrating fiber laser beam and arc, the fiber laser-arc hybrid welding has become more and more attractive in recent years due to its obvious advantages including deeper welding penetration, higher melting efficiency, easier addition of welding wires and stronger ability to bridge large gaps etc. Up to now, hybrid welding is the hotspot of laser welding technology. However, the researches had been showed that pores, cracks, bad weld appearance and poor joint properties readily occurred during thick aluminum plates hybrid welding. Therefore, this dissertation investigates the technological process, weld shaping characteristics, the reason and prevention of pore defects, the formation mechanism of microstructure and joint properties in fiber laser-MIG hybrid welding of 2XXX and 6XXX high strength aluminum alloys, which aims to lay the theoretical foundation for hybrid welding thick high strength aluminum plates in engineering application. The following are the main results:In this study, the process characteristics, metallurgical defects and joint properties were studied during fiber laser welding 2A12-T4 aluminum alloy. The experimental results demonstrated that the metallurgical defects are the main impact of poor quality. The undercut defects were increased with the increasing welding speed and laser power density. Furthermore, for different types of aluminum, the higher the content of low boiling point alloying element in base metal, the lower the power density threshold for the plasma formation and the more serious the undercut defects in the weld. A quantitative study on the porosity was carried out by orthogonal experiments. The results showed that the keyhole depth and plasma fluctuation were the main factors influencing porosity. Meanwhile, the shielding gas composition, laser power and defocused length had significant effect on porosity and the degree of confidence was up to 95%. Based on above researches, the optimized process parameters had been realized and the joint quality was accordant with class C by fiber laser welding. The joint tensile strength reached up to 72% of parent materials, which was 11% higher than that of arc joint. The optimized process parameters had been obtained by fiber laser-MIG hybrid welding experiments. The results founded that the alternative optimal heat source distance increased with the increasing of welding current. Meanwhile, deeper weld penetration and better weld shaping can be achieved by using He-Ar mixture shielding gas. Finally, a quality control chart about weld shaping had been drawn.The reason and prevention mechanic of pores were discussed in detail during fiber laser-MIG hybrid welding high strength aluminum. It was thought that enlarging keyhole diameter, improving the temperature above work piece and increasing the arc current were effective methods to restrain pores in hybrid welding joints because the plasma density can be reduced and the stability of keyhole can be enhanced by enlarging the keyhole diameter and improving the temperature above work piece. Furthermore, increasing MIG arc current can increase arc force and accelerate melt flow, which is helpful for eliminating pores. Finally, it was experimentally proved for the first time that 8mm thick full penetration aluminum joint without pore defects was produced by hybrid welding.The microstructure characters of hybrid welding joint were observed in detail. Furthermore, the effect of different thermal effect modes, heat input and temperature gradient on microstructures was studied between laser-zone and arc-zone in hybrid welding joint and the microstructure evolution mechanism was obtained. The under part of joint was heated mainly by laser beam resulting in lower heat input, higher temperature gradient, smaller grain size and narrower heat affect zone (HAZ). However, the upper part of joint was heated by laser beam and MIG arc together, which result in higher heat input and much longer residence time between solidus and liquidus. Accordingly, the partially melted zone was formed.Finally, the 8 mm thick 6061-T651 aluminum alloy was welded with ER5356, ER5087 and ER4043 welding wires by fiber laser-MIG hybrid welding, respectively. Meanwhile, the interrelationship of wire composition, microstructures, joint mechanical properties and fracture mechanism was studied. Furthermore, the joint properties were improved on the basis of above researches. The results showed that the joint welded by ER4043 welding wires present shortest columnar crystals and coarsest grain, which cause to worst joint properties and the fracture surfaces observation showed quasi-cleavage fracture. However, the biggest joint strength can be reached up to 75.6% by ER5087 wires owing to its grain refinement available. The fracture photographs of the joint exhibited deeper dimple and typical ductile dimple fracture pattern was observed. The above results had high engineering significance and practical value for hybrid welding thick aluminum alloy plates.更多还原