| This thesis investigated chiefly three open and essential issues, which were the guarantee of stable weld shaping, the control of intermetallic compound (IMC) and the mechanism of interfacial reaction of the solid, and the liquid in non-equilibrium state respectively, during the process of TIG welding-brazing of aluminum-stainless steel. To begin with, the joints of excellent weld shaping were achieved by modifying the flux, adjusting the wire feeding system and backing plate, which contributed to the further application of the technology, i. e., hot wire of aluminum by high frequency induction heating, expanded the process parameters ranges, and increased the strength and stability of the joints. Further, in light of the two methods of controlling the intermetallic compound, which are declining the heat input and adjusting both the growth and decomposition of intermetallic compound, the control of microstructure and the properties of joints corresponding to ER2319filler and ER1100filler was achieved. As a result, the dependable bonding of dissimilar metals, which were aluminum and stainless steel respectively, eventually came true. Moreover, the microstructure characteristics of the joints have been further investigated and analyzed including both the interfacial intermetallic compound and precipitates in the weld. After identifying the fracture behavior of the joints, the relationship among process, microstructure and properties was finally established. Besides, with the help of the simulation of interfacial temperature field and the theory of thermodynamics and kinetics, the mechanism and law of the formation of interfacial IMC layers was revealed corresponding to different situations.The modified flux, which exerted many positive effects on the joining process in the situation of alternating welding current, came into being by adding optimized amount of pure powder of aluminum into the Nocolok flux. With the existence of this homemade flux on the surface of stainless steel, the stability of arc was greatly improved, and the wetting angle of the filler metal declined, which led to a high-level spread of filler metal on the stainless steel. As for the composition of the flux, the concentration of the aluminum powder had a significant influence on the microstructure and the properties of the joints. Specifically, with the aluminum mass fraction from0to40wt.%, the wetting angle decreased and the spreading area increased obviously, the thickness of IMC nearly kept constant; while with the aluminum mass fraction over40wt.%, the wetting angle and spreading area kept stable, the thickness of IMC increased significantly. It can be concluded that the mechanism of flux was changed by adding the aluminum. In other words, the aluminum in the flux got molten first, and then followed the wetting on the steel, which means that the spread process of filler metal turned out to be the process of mixing the aluminum in the weld pool and molten metal in the flux. What’s more, the problem that thin plates can easily get burn-through and weld shaping is sensitive to the position of wire feeding would be exactly solved by employing couple wires feeding system. By different wire combination and appropriate welding current, the stable weld shaping of joints was achieved when the thinness of base metal were1.5mm,3mm, and4mm respectively. Besides, the welding penetration has improved by employing stainless back plate instead of copper plate which usually leads to uneven back weld shaping due to high thermal conductivity. Eventually, uniform and satisfactory weld shaping was realized.In light of the idea of lowing heat input, the control of microstructure and the properties of joints corresponding to ER2319filer could be realized. First, hot wire system by high frequency induction heating was established according to the high electric and thermal conductivity of the aluminum. The system is able to feed single or double wires with different diameters and temperature ranges of20oC-400oC continuously. With this technique of hot wire, the heat input was reduced and the whole temperature field was affected, and the maximum temperature in the field came to be changed from810oC to698oC with the reaction time3.5s and the thickness of interfacial IMC at bottom3.3μm approximately. In this situation, the tensile strength of the joints reached280MPa, and the interfacial IMC consisted of θ-(Fe,Cu)4Al13chiefly and minor Cr0.7Fe0.3Al6, the precipitates in the weld was identified as Al2Cu. However, with the increase of heat input, the maximum temperature and the reaction time of the solid and the liquid during the joining process both increased, as well as the thickness of the IMC, which finally results in the deterioration of joints’ properties. What’s more, two fracture modes were obtained. Specifically, for the joints with high tensile strength, the fracture originated from the weld close to fusion zone; on the other hand, for the joints with low joints tensile strength, the fractures evolved from the interfacial IMC at bottom and extended to the weld.Based on the idea of the dynamic control of the formation and decomposition of the interfacial IMC, the microstructure and the properties of the joints corresponding to ER1100filler could be controlled. At first, the process parameters were optimized via responding surface methodology (RSM) combined with central composite design (CCD), where peak current, duty ratio, base current and frequency were included, and the responses were the tensile strength of the whole joints and the ones without reinforcement. Then a regression model of two order was established and employed to optimize the process parameters and the prediction of joints properties. It was observed that the maximum tensile strength reached238Mpa corresponding to the optimized process. Moreover, the single factor and the interactions between factors could be achieved by this obtained model. Besides, by analyzing the microstructure of the joints, the thickness of interfacial IMC, which consisted of η-Fe2Al5and θ-Fe4Al13, was about200nm, and the strip-like or the net-like precipitates were identified as FeAl6for ER1100. With the increase of the welding current, the thickness of the IMC increased first, reached the maximum and then declined, and the precipitates of FeAl6increased. Consequently, the properties of the joints also increased to the maximum and then declined. As for the fracture in this situation, there were two separately fracture modes. For the joints with lower strength, the fractures evolved from the interfacial IMC at bottom and extended to the weld, while for the joints with higher strength, the fractures occurred at interfacial IMC exactly.According to the different filler metals and ways to control the IMC as well as thermodynamics and kinetics, the mechanism of interfacial reaction on different situations were analyzed in depth. Firstly, in the situation of ER2319, based on the dissolution and diffusion process controlled by dissolution kinetics, the growth model of interfacial IMC was established, which made possible the prediction of growth rate and thickness of IMC for some fixed temperature. While in the situation of ER1100, based on the idea of the dynamic control of the formation and decomposition of the interfacial IMC, the trend of IMC growth and the rate of decomposition were analyzed from two intervals, which revealed the internal relationship among thickness of IMC, amount of precipitates which is FeAl6, and welding current. In conclusion, different ways to control the IMC could be selected on these two filler metals in order to obtain joints with high quality. As observed, though the tensile strength of joints corresponding to ER1100filler was relatively lower, the ductility and impact toughness, however, were far superior to the joints corresponding to ER2319filler, which means that the former showed better comprehensive properties. |
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