| China is one of the countries which have abundant magnesium resource in the world. The magnesium storage in our country reaches to 22.5% throughout the world. Magnesium and magnesium alloys are the lightest structural metals with excellent properties such as high specific strength, low density, good heat and electric conductivity, good damp absorption of vibration, good electromagnetism shielding properties and good process-ability. As a consequence, they are widely used in the automotive and aeronautic industries, as well as in the communications and household applications industries. Magnesium and its alloys can be joined with many appropriate welding technologies. Soldering is particularly promising for the applications where precisely bonded joints are required. The closed-packed hexagonal structure of magnesium has poor deformability, and oxide and inclusion can be easily produced during soldering. Thus the mechanical properties of magnesium soldering joint are not satisfied in many applications. Therefore, it is necessary to develop new filler metals which have fine crystal and few inclusions. By using appropriate soldering process, the fine grain structure can also be obtained in the soldering joint, which can enhance its mechanical propertie. This is significative in the application of magnesium and magnesium alloys.In this work, an Er doped Mg-Zn-Al magnesium alloy solder by ultrasonic treatment was developed. The effect of ultrasonic vibration on the solder melt was investigated. According to the actual heating process of ultrasonic assisting soldering, the oxidation behavior of the solder in atmosphere at different temperature was discussed. The mechanism of oxide broken and the liquid solder filling the clearance were revealed. The strengthening effect of the ultrasonic treated solder and the microsturcute of soldered joint were studied. The detainls were as following:Based on the ternary Mg-Zn-Al phase diagram, an Er doped Mg-Zn-Al filler metal were developed. By measuring the melting range and the properties of the Mg-Zn-Al solder, 53% Mg and 0.3% Er were added in the developed solder. Accroding to the Mg-Zn and Al-Er binary diagram, the microstructure of Mg-Zn-Al solder and the effect of Er in the solder were analyzed. The microstructure of Mg-Zn-Al filler metal was mainly composed of α-Mg solid solution, equilibrium α-Mg+Mg Zn eutectic phase, eutectoid α-Mg+MgZn phase, and quasieutectic α-Mg+MgZn phase. There were three exiting form of Er in the solder, that is, the supersaturated solid solution containing Er, the primary Al3 Er compound and Al+Al3Er alienation compound. The primary Al3 Er compound and Al+Al3Er compound distributed in the grain and grain boundary.The Er doped Mg-Zn-Al solder was ultrasonic treated in order to get rid of inclusions and refine microstructure. Accroding to the microstructure and the statistic result of inclusions, it was found that the content of MgO inclusions in the middle layer of the solder was the lowest and their size was in the range of 0.3-0.6μm. After ultrasonic treating the solder melt, those compound inclusions with big size sank to the bottom of the crucible, which had effect of purifying the melt. By calculating the attenuation the ultrasonic intensity and analyzing the applied force of particle in the liquid solder, the mechanism of getting rid of inclusions are discussed. When the solder melt was treated with high power, the cavitation produced by the ultrasonic vibration could break up the dendritic structure of α-Mg and Al3 Er compound into pieces. The local temperature in the melt reached to 104 K due to the heat effect of ultrasonic vibration, which could melt the irregular Al3 Er compound and make the Al3 Er smoother. The grain α-Mg and Al3 Er was distributed throughout the solder under the action of acoustic streaming. The grain size of α-Mg was 23.26μm, which was only 73% that of the α-Mg which was not ultrasonic treated. Based on the ultrasonic basic theory, the effect of ultrasonic parameters on the microstructure of the solder was analyzed, and the mechanism of microstructure refinement was revealed.In order to understand the oxidation behavior of Mg-Zn-Al solder in atmosphere, the influence of temperature on oxidation behavior of the solder was studied. According to the varity of oxygen content in the cross section of the solder, the surface structure was divided into the outer layer and the transition layer. Based on the kinetics of oxidizing, the P/B ratio was calculated. The oxidation production, the properties of oxide and the effect of temperature on the properties of oxide were analyzed. An oxide model of the Mg-Zn-Al solder in atmosphere at different temperature was established. The results showed that when the heating temperature was lower than 130℃, the oxide film formed on the surface of the solder was a protective film. When the heatin temperature was higher than 230℃, the surface of the solder was oxidized seriously, and the oxide film on the surface of the solder was completely magnesium oxide, which could not prevent the surface of the solder from being oxidized.The spreading and wetting behavior of Mg-Zn-Al solder under action of ultrasonic vibration was conducted in atmosphere for the purpose of understanding the mechanism of oxide removal. By comparing with the spreading and wetting behavior of the solder under the action of flux, the effect of ultrasonic vibration on the spreading and wetting behavior was studied. The results showed that the effect of acoustic streaming of ultrasonic vibration forced the Mg-Zn-Al solder to spread along the base metal. The impact wave induced by ultrasonic cavitation broken the oxide film on the surface of base metal and the solder, which could promote the solder to wet the base metal. It was found that the Mg-Zn-Al solder spread firstly and then wetted the base metal under the action of ultrasonic vibration.Based on ultrasonic theory, the sound pressure and ultrasonic intensity were calculated. It was found that the driving forces of Mg-Zn-Al solder filling the clearance of lap joint were come from the sound pressure produced by the ultrasonic vibration. The value of sound pressure was as high as 106 Pa, while that of capillary action was 960 Pa. As a consequence, the force of capillary effect could be ignored during ultrasonic assisting soldering. The sound intensity propagated along the clearance of lap joint was far greater than that along the contrary side of the solder. Therefore, the liquid filler metal would prefer to fill the clearance of the lap joint. The microstructure of α-Mg and the exiting form and distribution of Al3 Er compound were analyzed. By comparing with the microstructure of the solder and the joint obtained from the furnace soldering, the strengthing mechanism of the joint soldered with the ultrasonic treated solder was revealed. It was found that, on the one hand, compared with the original structure, the grain size of some α-Mg phase in the lap joint grew coarse after ultrasonic vibration soldering. On the other hand, the dispersed α-Mg solid solution could act as nuclei due to the fact that the α-Mg solid solution solidified before the Mg-Zn eutectic phase, which could refine the microstructure of the soldered joint. In addition, the grain dispersed Al3 Er compound distributed in the grain and grain boundary, which had the effect of microstructure refinement and the second phase strengthening. |
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