【作者】 赵东升；

【导师】 杨士勤； 闫久春；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 钛合金和不锈钢的熔点高、线膨胀系数差异大,焊接时会形成较大的焊接应力,而且在高温下容易反应形成硬度高、脆性大的金属间化合物导致接头脆化,所以连接很困难。本文采用真空热轧形变连接的方法成功实现了TC4钛合金与0Cr18Ni10Ti不锈钢的高强度的连接,利用光学显微镜、SEM、XRD、EPMA、纳米压痕等对接头的微观结构进行了观察和测试,对真空热轧形变连接的机理进行了研究,并对焊接接头的应用情况进行了实验研究。钛合金与不锈钢直接真空热轧连接时,焊接温度800℃时连接界面存在未连接的空隙;焊接温度900℃时硬而脆的金属间化合物层尤其是富Cr层在连接界面处形成并导致接头脆化;当焊接温度低于或等于750℃,高于或等于950℃时,钛合金与不锈钢未能形成连接。当焊接温度850℃,压缩率20%时,钛合金与不锈钢直接真空热轧连接接头连接强度达到最大值77.33MPa。用铜作为中间层材料实现了钛合金与不锈钢的连接。铜中间层能够完全阻隔母材之间的扩散,连接界面处没有Fe-Ti金属间化合物形成。不锈钢与铜的连接界面为无规则的镶嵌式结构,没有明显的反应层或扩散层。铜与钛合金的连接界面形成了脆性的Cu-Ti金属间化合物层,随着焊接温度的升高其体积分数增大,接头连接强度随之降低。拉伸试样均断裂于金属间化合物层之间或金属间化合物层与铜之间。连接界面扩散层的厚度主要取决于焊接温度,压缩率对其影响不大。当焊接温度780℃,压缩率20%时接头连接强度达到最大值343MPa。采用镍中间层实现了钛合金与不锈钢的真空热轧连接。镍能够完全阻隔母材之间的扩散,连接界面处没有Fe-Ti金属间化合物形成。不锈钢与镍的连接界面为无规则的镶嵌式结构,没有明显的反应层或扩散层形成。镍与钛合金的连接界面由机械咬合层和扩散层组成。硬而脆的金属间化合物导致接头脆化,随着焊接温度的升高金属间化合物的体积分数增大,接头连接强度随之降低,试样均断裂于脆性的金属间化合物层之间或金属间化合物层和镍之间。连接界面扩散层的厚度主要取决于焊接温度,压缩率对其影响不大。当焊接温度760℃,压缩率20%时接头连接强度达到最大值440.1MPa。采用铌作为中间层材料实现了钛合金与不锈钢的真空热轧连接。铌中间层能够完全阻隔母材之间的扩散,连接界面处没有Fe-Ti金属间化合物形成。不锈钢与铌的连接界面处反应层的厚度随着焊接温度的升高而增大,接头连接强度随之降低。铌与钛合金的连接界面的固溶体层的厚度随着焊接温度的升高而增大。连接界面扩散层的厚度主要取决于焊接温度,压缩率对其影响不大。当焊接温度800℃,压缩率25%时接头连接强度达到最大值430MPa。利用有限元软件MSC.Marc对异种材料热轧连接过程的塑性变形行为进行了计算,计算结果表明:轧制过程中材料连接表面扩展形成新生表面,连接表面之间发生了相对滑移,使得界面之间产生轧制方向上的摩擦力,从而使得连接材料表面氧化膜和其它杂质破除并露出新鲜材料表面,新鲜的材料相互接触形成连接,当采用软质中间层材料时连接表面之间的相对滑移程度增大。对轧制连接的物理过程进行了研究,认为该过程包括连接材料表面的接触、连接材料表面形成新生表面并发生相对滑移从而使得表面的氧化膜和其它杂质破除、新鲜的材料露出并接触实现连接、连接界面处的元素互扩散、连接界面的整体连接的形成这几个阶段。进行了钛合金与不锈钢真空热轧过渡接头的TIG焊接试验。无中间层热轧连接过渡接头在TIG焊接时沿原连接界面开裂;采用铜中间层的过渡接头TIG焊接之后拉伸强度最高约为40.77MPa,Cu-TC4界面沿金属间化合物层开裂;采用镍中间层的过渡接头在TIG焊接之后拉伸强度最高可达431MPa,SS-Ni和Ni-TC4界面结构无明显变化;采用铌中间层的过渡接头TIG焊接之后拉伸强度最高可达421.6MPa,SS-Nb和Nb-TC4界面结构无明显变化。更多还原

【Abstract】 When titanium alloy directly bonded to stainless steel, various intermetallics formed at bonding interface by inter-diffusion between titanium and stainless steel, which makes embrittlement of the bonding joints. In addition, high internal stress results due to the large difference of linear expansion coefficient between these materials. The paper aims at bonding of titanium alloy and stainless steel by vacuum hot roll bonding. The microstructures of the bonding interfaces were analyzed by optical and scanning electron microscopy, X-Ray Diffraction, energy spectrum analysis and nano-indentation. The bonding mechanism of the vacuum hot roll bonding was investigated.When titanium alloy and stainless steel were directly bonded at bonding temperature 800℃, there were unbonded zones across the interface. At bonding temperature 900℃, intermetallic especially Cr rich zone formed across titanium alloy/stainless steel interface make embrittlement of the bonding joints. When bonding temperature equals to or lower than 750℃, titanium alloy and stainless steel cannot bonded directly, and when temperature equals to or higher than 950℃, they cannot bonded, too. The bonded joints achieve the maximum tensile strength of 77.3MPa when processed at bonding temperature of 850℃and reduction of 20%.Vacuum hot roll bonding was carried out between titanium alloy and stainless steel using copper interlayer. Copper interlayer can block the diffusion Fe, Cr and Ni to Ti side and Ti to stainless steel side, and Fe-Ti intermetallics are not formed at the interface. There are no transition layers at SS/Cu interface. The bonding type of Cu-SS is solid solution, and the structure of interface is ruleless inlaid. Brittle Cu-Ti intermetallics are formed at the copper /titanium alloy interface. With the rise in joining temperature, decrease in tensile strength occurs due to incensement of the volume fraction of intermetallics. The tensile specimens fractured at intermetallic layers or between intermetallic layer and Cu interlayer. The thickness of intermetallics at copper/titanium alloy interface depends mainly on bonding temperature, less on reduction. The bonded joints achieve the maximum tensile strength of 343MPa when processed at bonding temperature of 780℃and reduction of 20%.Vacuum hot roll bonding was carried out between titanium alloy and stainless steel using nickel interlayer. Nickel interlayer can completely block the diffusion Fe, Cr and Ni to Ti side and Ti to stainless steel side, and Fe-Ti intermetallics are not formed at interface. There are no transition layers at SS/Ni interface. The bonding type of SS-Ni is solid solution, and the structure of interface is ruleless inlaid. Brittle Ni-Ti intermetallics are formed at the nickel/titanium alloy interface. With the rise in bonding temperature, decrease in tensile strength occurs due to increasing of the volume fraction of Ni-Ti intermetallics. The tensile specimens fractured at brittle intermetallic layers or between intermetallic layer and Ni interlayer. The thickness of intermetallic at nickel/titanium alloy interface depends mainly on bonding temperature, less on reduction. The bonded joints achieve the maximum tensile strength of 440.1MPa when processed at bonding temperature of 760℃and reduction of 20%.Vacuum hot roll bonding was carried out between titanium alloy and stainless steel using Nb interlayer. Nb interlayer can completely block the diffusion Fe, Cr and Ni to Ti side and Ti to stainless steel side, and Fe-Ti intermetallics are not formed at the interface. With the rise in bonding temperature, decrease in tensile strength occurs due to increasing of the volume fraction of Fe-Nb intermetallics. And with the increasing of bonding temperature, the thickness of solid solution layer increases. The thickness of intermetallic at Fe-Nb interface depends mainly on bonding temperature, less on reduction. The bonded joints achieve the maximum tensile strength of 430MPa when processed at bonding temperature of 800℃and reduction of 20%.The behavior of plastic deformation of dissimilar materials experienced during the rolling process was calculated by finite element software MSC.Marc. The calculation results show that the metal plastically deforms and extends, but the oxide is brittle and can respond to stress only by fracturing. During rolling process, the oxide film will therefore be fractured and fragmented, and this allows metal-metal contact and bond. Using soft material as interlayer the extent of mutual slipping across interfaces becomes bigger.The physical process of roll bonding has been investigated. The results showed that bonding of materials is not depend on atomic diffusion but realizes during the process of rolling. The process of the vacuum hot roll bonding was composed of five steps: the bonding surface contact, the fracture of the surface oxide layer and impurity, the extrusion of fresh metal through cracks in rolling and bonding of metals, the inter-diffusion at interface after rolling and the bonding of whole surface.The bonding between titanium alloy and stainless steel transition joint and titanium alloy or stainless steel by TIG welding was carried out. Titanium alloy/stainless steel transition joint bonded directly cracked at their interface while it experiences TIG welding. Transition joint using Cu interlayer after TIG welding, micro-cracks formed on these regions of Cu/TC4 interface between different intermetallic layers and between intermetallic layer and Cu, so the maximum tensile strength of them achieve only 40.77MPa; Transition joint using Ni or Nb interlayer after TIG welding, the microstructure of interfaces does not change significantly, and the maximum tensile strength of them achieve 431MPa and 421.6MPa, respectively.更多还原