| 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.
|
PDF Full Text Request