【作者】 宋建岭；

【导师】 杨春利；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 博士

【摘要】 铝合金与不锈钢的复合结构具有质轻、高强、耐腐蚀等方面的综合优势,在航空航天、汽车制造及造船等领域中应用越来越广泛,然而两者之间固溶度低,热物理性能差异大,且极易反应生成脆性的金属间化合物,这已成为焊接领域中的难点问题。近来,电弧熔-钎焊方法以其便捷、高效化的焊接特性成为铝/钢异种合金连接中的热门研究方向,促进液态钎料在钢表面的润湿铺展,并控制界面脆性化合物层的生长,是获得优质铝/钢接头的关键问题。本文首次提出了预涂层TIG熔-钎焊方法,针对3mm厚的铝合金与不锈钢板对接结构,全面地研究了其焊接特性,研制出了适用于电弧加热条件下的预涂层,开发出了接头成形控制技术,获得了具有熔焊与钎焊双重特征的复合型接头。在此基础上,研究了接头微观结构特征,深入分析了界面层的组织形态与反应产物,评定了接头力学行为,建立了界面结构与力学性能之间的联系。进一步,从焊接能量控制与合金元素作用两方面展开研究,分析了焊接热输入对界面温度场分布的影响,通过热力学与动力学分析,揭示了TIG熔-钎焊界面层生长机制,深入研究了合金元素对界面结构的作用,分析了其控制界面层生长的作用机理,实现了铝/钢异种合金的可靠连接。通过润湿铺展试验和多组元正交试验,获得了综合性能良好的预涂层,分析了其促进钎料润湿铺展的作用机理,阐述了预涂层TIG熔-钎焊接头成形行为,开发出了单面焊双面成形控制技术,获得了成形良好的铝/钢对接接头。预涂层成分为:改性的Nocolok钎剂(KAlF4 65%+K3AlF6 35%) 55wt%,Zn 20wt%,Sn 20wt%,K2SiF6 5wt%。预涂层的作用主要体现在三个方面:一是熔融氟化物钎剂去除液态钎料表面的残余氧化膜,净化液态钎料表面;二是氟化物钎剂分解并覆在熔池表面,显著降低液-气界面张力σl-g;三是Zn和Sn金属液层沉积在钢表面并溶入液态钎料,降低液-固界面张力σl-s。接头成形过程为:预涂层首先熔化,在钢表面形成液态薄膜;随后,焊丝送到坡口根部,紧贴成形槽底部,并迅速熔化,液态钎料在钢板背面润湿铺展,完成背面成形;最后,液态钎料沿坡口表面进行“上坡”铺展,实现正面成形。研究了铝合金/不锈钢TIG熔-钎焊接头微观结构特征,深入分析了不同填充钎料和焊接热输入条件下的界面结构,测试了接头的力学性能,研究了接头的断裂行为。采用Al-Si12钎料时,界面层厚度分布不均匀,呈锯齿状生长,由τ5-Al8Fe2Si和θ-(Al,Si)13Fe4两层化合物构成,随着热输入量的增加,界面层中θ相生长迅速,整体界面层厚度在3-10μm之间,形态由小锯齿状向粗大锯齿状变化;界面层具有高的硬脆性,在界面层由τ5相组成,厚度在5-7μm之间,呈粗大锯齿状生长时,具有较高的抗裂能力,接头抗拉强度在120-130MPa之间,界面层抗拉强度在80-100MPa之间。采用Al-Cu6钎料时,界面层厚度分布较为均匀,呈短须状生长,由θ-Al13(Fe,Cu)4一层化合物组成,随着热输入量的增加,界面层厚度在2-5μm之间,形态由短须状向粗针状变化;界面层具有较高的抗裂性,当界面层厚度在2-4μm之间,呈短须状生长时,其抗裂能力最强,接头抗拉强度达到170-180MPa,界面层抗拉强度在135-150MPa之间。运用MARC有限元软件实现了对TIG熔-钎焊界面温度场分布的数值模拟,通过热力学和动力学分析,计算了不同化合物的生长自由能,阐述了界面层的生长机制。温度场模拟结果显示,液-固界面温度分布不均匀,坡口顶部位置与底端位置相差近300℃,加热时间相差2.5s,界面峰值温度在700-1200℃之间,加热时间不超过10s。热力学计算结果显示,随着含Al量的升高,化合物的生长自由能逐渐降低,Si元素的加入能够显著降低化合物的生长自由能,界面处形成含Al量最高的Al-Fe-Si三元相。动力学分析结果显示,界面层的生长行为是由溶解动力学控制的,钢基体在铝液中的溶解过程决定着整个界面层的结构特征,而界面峰值温度对溶解过程起到决定性的影响,界面温度控制在1000℃左右时,界面层由τ5相一层组成,厚度在5-8μm之间,呈锯齿状生长,计算分析结果与实际界面结构相符合,对界面层的生长实现最佳能量控制。研究了不同含量Si、Cu元素对界面结构的作用效果,揭示了其控制界面层生长的作用机理。Si元素是通过与Al、Fe反应形成Al-Fe-Si三元相来实现对界面层生长控制的,Si元素极易在界面偏聚,增大了Fe原子向铝液中的溶解速度,Si含量在5wt%左右时,界面由τ5-Al8Fe2Si和θ-(Al,Si)13Fe4两层化合物组成,控制界面层生长的作用效果最佳。Cu元素是通过置换固溶于θ-Al13Fe4相中抑制了其择优生长取向来控制界面层生长的,同时形成少量的Al-Cu结合对提高了界面层的抗裂性,Cu含量在6wt%左右时,界面层由θ-Al13(Fe,Cu)4相组成,接头具有最佳的力学性能。更多还原

【Abstract】 Hybrid structures of aluminum alloy and stainless steel have the comprehensive benefits, such as light weight, high strength and corrosion resistance, etc. Their application has a high technical and economical potential in aerospace vehicle, automobile and shipbuilding. However, joining of aluminum alloy and stainless steel has great difficulty since brittle intermetallic compounds (IMCs) are formed in the joint. Nowadays, arc welding-brazing with the characteristics of convenient operation and high efficiency offers a great potential for material combinations of aluminum alloy and steel. The key issues to obtain high-quality arc welding-brazing joints are to promote the filler metal wetting and spreading on steel surface and to control the brittle IMC layer growth in the Al-steel interface.In this study, the precoating TIG welding-brazing method was first put forward and its welding characteristics of aluminum alloy and stainless steel butt structure was comprehensively studied. The special precoating flux layer applied to arc heating and the butt joint formation control technology were developed and the hybrid joints with typical welding-brazing dual characteristics was obtained. On these basis, the microstructure features of the joint, especially the shapes and reaction products of the interface layer, were analyzed and the mechanical behavior of the joint was evaluated and the effect of interface structure on the mechanical properties of the joint was discussed. Further, the study focused on the welding heat input and the role of alloying elements to control the interface layer growth. The effect of heat input on the interface temperature distribution was analyzed and the growth mechanism of interface layer was revealed through thermodynamic and kinetic analysis. The effect of alloying elements on the interface structure was studied detailedly and the control mechanism of alloying elements on the interface layer growth was analyzed. Finally, the high-quality dissimilar joint of aluminum alloy and stainless steel was obtained successfully.The special precoating flux layer applied in arc heating was obtained through wetting and spreading experiments and orthogonal experiments and its promotion mechanism of wetting and spreading was analyzed. The joint formation behavior was described and its control technology of single-side welding and double-side forming was developed. The composition of precoating layer is that modified Nocolok flux (KAlF4 65wt%+K3AlF6 35wt%) 55wt%, Zn 20wt%, Sn 20wt% and K2SiF6 5wt%. The three major roles of precoating layer are: First, molten fluoride flux removes the residual oxide film to clean the surface of the liquid filler metal; Second, fluoride flux decomposes and floats up in the molten pool to reduce the interface tensionσl-g; Third, Zn and Sn metals deposits on the steel surface and dissolves into the filler metal further to reduce the interface tensionσl-s. The joint formation process is: First, the precoating layer melts to form the liquid film on steel surface; Then, the welding wire is sent to the groove root and melts rapidly to spread on the steel back surface to form the back formation; Finally, the liquid filler metal uphill spreads to form a front formation.The interface microstructures of the joints with different filler metals and different heat input were analyzed and the mechanical properties of the joints were tested and the fracture behavior of the joints was studied. With Al-Si12 filler metal, the interface layer presents a nonuniform and sawtooth shape and consists of two types of IMC phases, which areτ5-Al8Fe2Si phase in the seam side andθ-(Al,Si)13Fe4 phase in the steel side. With the increase of heat input, theθphase layer grows rapidly and the whole layer is in 3-10 thickness and the shape changes from the small sawtooth to the coarse. Both of the IMC phases have high hard and brittle nature, and when the interface layer consists of onlyτ5 phase with the thickness of 5-7μm and the coarse sawtooth shape, it can inhibit the interface cracking effectively and the tensile strength of the joint reaches 120-130MP and the value of the interface layer is 80-100MPa. With Al-Cu6 filler metal, the interface layer presents short whisker shape and consists of onlyθ-Al13(Fe,Cu)4 phase. With the increase of heat input, the whole layer is in 2-5μm thickness and the shape changes from the short whisker to the needle-like. The interface layer has a high crack resistance, and when the interface layer is in 2-4μm thickness and presents short whisker shape, it has the highest crack resistance and the tensile strength of the joint reaches 170-180MP and the value of the interface layer is 135-150MPa.The interface temperature distribution during TIG welding-brazing process was computed by MARC finite element method and the growth free energy of the different intermetallic phases were calculated and the growth mechanism of interface layer was described through thermodynamic and kinetic analysis. The temperature field simulation results show that the liquid-solid interface temperature is uneven with the gap of nearly 300℃and 2.5s from the top to the bottom of the groove and the peak temperature ranges from 700 to 1200℃and the reaction time is less than 10s with different heat input. The thermodynamic analysis results show that the growth free energy of the IMC layer decreases with the increase of Al content and Si element can significantly reduce the growth free energy of IMC layer, so the interface forms the Al-Fe-Si ternary phase with the highest Al content. The kinetic analysis results show that the interface growth behavior is controlled by the dissolution kinetics and the dissolution process of steel matrix into molten pool determine the interface structure and the interface peak temperature plays a decisive impact on the dissolution process. When the interface temperature is at about 1000℃, the interface layer consists of onlyτ5 phase and is in 5-8μm thickness and presents the sawtooth shape. The calculated results are consistent with the actual interface structure, so the interface temperature at about 1000℃is the optimum energy control for the interface layer growth.The effect of different contents of Si and Cu elements on the interface structure was studied detailedly and their control mechanism on the interface layer growth was revealed. The control mechanism of Si elements is that Si enriches at the interface and increases the dissolution rate of Fe atoms into molten pool and then Si reacts with Al and Fe atoms to form Al-Fe-Si ternary phase. With Si content of 5wt%, the interface layer consists ofτ5-Al8Fe2Si andθ-(Al,Si)13Fe4 and this Si content achieves the best control effect on the IMC layer growth. The control mechanism of Cu elements is that Cu can replace some Fe atoms inθ-Al13Fe4 to inhibit its preferred orientation growth. At the same time, Al-Cu bond presents higher binding ability than Al-Fe, soθ-Al13(Fe,Cu)4 has a high crack resistance. With Cu content of 6wt%, the joint reaches the best mechanical properties.更多还原