【作者】 董鹏；

【导师】 肖荣诗；

【作者基本信息】 北京工业大学 ， 光学工程， 2011， 博士

【摘要】 异种合金复合结构可以综合利用两种合金的优势性能,在现代工业中有着广泛的应用前景。但是异种合金之间的物理、化学性质差异以及不同元素在界面相互反应生成的新相,使异种合金的焊接存在很大困难。因此,对异种合金激光焊接工艺技术及机理的研究,实现异种合金的可靠连接,对于异种合金复合结构的应用具有重要的理论和实际意义。本文针对熔点差异较大的异种合金组合,提出了一种激光深熔钎焊的方法,即在焊接过程中,采用聚焦的激光作用在低熔点母材一侧进行深熔焊接,低熔点母材熔化后在接头界面铺展浸润高熔点母材从而形成钎焊接头。激光深熔钎焊在低熔点母材一侧为激光深熔焊特征,在高熔点母材一侧为钎焊特征。本文以黄铜-低碳钢、铝合金-紫铜以及铝合金-钛合金三类不同异种合金为代表,系统研究了三种组合激光深熔钎焊接头成形特点及影响因素,接头显微组织和力学性能,并采用有限单元法对激光深熔钎焊传热过程进行了模拟和分析。在黄铜-低碳钢组合中,液态黄铜在低碳钢界面可以良好地浸润。另外铁和铜两种元素在固相形成固溶体,不会形成金属间化合物,因此黄铜-低碳钢激光深熔钎焊接头成形较好。在铝合金-紫铜组合中,铝和铜两种元素能够形成共晶,有助于液态铝润湿铜界面形成良好深熔焊接头。但是铜与铝较高的导热率,在激光能量密度较小时,冷却速度较快,易出现未熔合缺陷。在铝和铜的焊接过程中需要较多的能量输入。对于铝合金-钛合金组合,由于液态铝在钛界面的浸润性较差及铝和钛易形成金属间化合物,在铝合金-钛合金激光深熔钎焊过程中界面易出现未熔合缺陷。通过优化工艺参数可以促进液态铝在钛合金界面的铺展浸润并控制金属间化合物的生成,得到成形较好的焊接接头。利用光学显微镜、扫描电子显微镜及透射电镜等手段研究了三种异种合金接头组合的接头显微组织,结果表明对于黄铜-低碳钢等形成固溶体的异种合金组合,在焊缝界面没有明显的过渡层,而对于能形成共晶或金属间化合物的异种合金组合,界面会形成由金属间化合物或共晶及其混合物构成的过渡层。如铝合金-铜接头界面存在由金属间化合物层以及α(Al)+Al2Cu共晶构成的过渡层,熔化的铜母材在焊缝中以α(Al)+Al2Cu共晶的形式存在,而在铝合金-钛合金焊缝界面位置,只存在由金属间化合物构成的过渡层。研究了在铝合金-紫铜激光焊接中,不同铜母材的熔化量对于接头的力学性能的影响。铝合金-紫铜激光熔焊接头的抗拉强度为79MPa左右,断裂在焊缝。铝合金-紫铜激光深熔钎焊-熔焊部分接头断裂发生在铝母材位置,部分断裂发生在界面处,其抗拉强度为大于94.5MPa。铝合金-紫铜激光深熔钎焊的抗拉强度较低,抗拉强度为52MPa,断裂发生在界面金属间化合物层与共晶层结合的位置,此时焊缝的显微硬度要高于铝母材但是低于铜母材,而铜母材存在一定熔化的焊缝的显微硬度都高于两种母材。铝合金-钛合金激光深熔钎焊接头的最大抗拉强度为217MPa,接头的断裂为韧性断裂,接头的抗拉强度随着界面未熔合率的下降而增加。但是在1m/min情况下,靠近界面的钛合金母材晶界位置析出的氧化物会导致出现微裂纹,以及当激光入射角大于9°时,激光直接作用在钛合金界面,在焊缝中产生大量缺陷影响接头强度。采用移动离散热源模型,在有限元分析软件Ansys中编写APDL程序对异种合金激光深熔钎焊温度场分布以及界面热循环进行了模拟计算。分析了不同材料以及工艺参数对焊接过程温度场以及界面热循环的影响。更多还原

【Abstract】 Dissimilar alloys composite structures have wide applications in many industrial fields, because they can provide each alloy’s advantage. However, the great differences in physical, chemical properties of dissimilar alloys and elements interaction in molten pool make joining them become a challenging task. It is important for investigations and comprehensions on the technique, mechanism and elements interaction in molten pool during laser welding of dissimilar alloys for the application of dissimilar alloys composite structuresIn this dissertation, for welding dissimilar alloys which differ considerably in melting point and metallurgical properties, an approach, namely laser penetration brazing (LPB), has been applied and investigated. A focused laser beam acts on the lower melting point base metal, which results in the low melting point base metal melted by means of the key-hole effect. The higher melting point base metal, however, still maintains the solid state. A penetration brazing joint is formed through the interaction of the higher melting point base metal and molten lower melting point baser metal.As a representative of variety dissimilar alloys couples, brass-steel, aluminum-copper and aluminum -titanium were used in laser penetration brazing experiments. The formation characteristic, microstructure and mechanical properties of laser penetration brazing joints of variety dissimilar joints were investigated. The heat-transfer process was analyzed by finite element analysis.The formation of the laser penetration joint is mainly influenced by the type of dissimilar alloys couple and the processing parameters. The formation of brass-steel joint is formed well, because the wettability of copper on steel is well and the interaction between Cu and Fe can form only solid solution. In the laser penetration brazing of copper and aluminum, more laser energy is needed for wetting of aluminum on copper, due to the high thermal conductivity of copper. The melting of part copper doesn’t bring about the negative effect on the formation of joints due to the eutectic reaction between Cu and Al. The poor wettability of aluminum on titanium may lead to incompleted fusion part in the interface of aluminum-titanium joint. It can be improved by optimizing the processing parameters. The formation of Al-Ti intermetallic compounds has negtative effect on the joints, the melting rate of titanium, therefore, must be controlled. For dissimilar alloys couple which can form only solid solution, such as brass-steel, there is no obvious transition layer along the interface in the joint. However, for dissimilar alloys couples which form eutectic or intermetallic compounds, there is a transition layer along the interface in the joint. In the aluminum-copper joint, a transition layer composed of Al-Cu intermetallic compounds and a (Al)+Al2Cu eutectic. The melted copper exists as the form of Al-Cu intermeallic compounds and a (Al)+Al2Cu eutectic in the weld metal. The microstructure of aluminum-copper joints is different with the amount of the melted copper. The transition layer along the interface in the aluminum-titanium joint is only composed of Al-Ti intermetallic compounds.The amount of molted copper results in the different microstructure of aluminum-copper joint, which influence the mechanical properties. For the joints with excessive amount of copper melted, the microhardness is higher than that of base metal. The maximum tensile strength of joints is 79MPa, and the failure occurred at the weld metal. For the joint with part melting of copper, the microhardness is higher than that of base metal. The maximum tensile strength for tensile test is 100.6 MPa. The failure occurred at aluminum side. The maximum tensile strength of failure in interface samples is 94.5 MPa. The microhardness in the weld metal of the joint with small amount of copper melting is higher than that of aluminum base metal, however, lower than that of copper base metal. The maximum tensile strength of joints is 52MPa. The failure occurred at the copper-weld interface. The maximum tensile strength of aluminum-titanium joints is 210Mpa. The failure occurs at the interface. The incompleted fusion rate in titanium is the main factor of the strength of aluminum-titanium joints. The tensile strength increased with decreasing the incompleted fusion rate. However, the defects in weld metal have negative effect on the tensile strength, such welding velocity lm/min joints and deflect angle more than 9°joints.A moving integration heat source was used in the simulation of temperature field. The temperature field and heat cycling were analyzed via Ansys software. The method can be used as a reference in the simulation of laser penetration brazing.更多还原