| Magnesium alloys have high specific strength, stiffness and excellent shock absorption, thermal conductivity, electromagnetic shielding, recyclability and show their superior value and broad application prospects in the automotive transportation, aerospace and other fields, In the increasing application of magnesium alloys, whether to get magnesium alloy structural parts which are unable to obtain by one-time forming or fixation with other component, connection is the first thing to consider. Soldering has a low heating temperature, little effect on the microstructure and properties of the base material and the small stress and deformation after welding and the simple process. Magnesium alloy soldering can avoid oxidation and melting defect pores, cracks which often occur in the fusion welding. So soldering is becoming one of the preferred connection method of magnesium alloy.Removing the oxide film is a key step during soldering and the main methods are flux gaseous medium, mechanical and physical stripping. Flux as the most widely used and most effective method to remove oxide film plays a major role in the wetting process of liquid solder and base metal. However, studies of magnesium alloy fluxes, especially on soldering fluxes, removal mechanism of oxide film are at the beginning. The lack of studies on flux restricts the promotion and application of soldering technology in magnesium alloy connections. This paper focus on flux of magnesium alloy with low-melting point. By means of researching oxide film composition on surface of magnesium alloy, selecting component of soldering flux, designing and optimizing soldering flux formulation, researching microstructure and properties of soldered joints and flux performance evaluation, we try to find a suitable flux formulation for magnesium alloy soldering.Thesis firstly analyzed the composition of AZ31B magnesium alloy surface oxide film. XRD and XPS data show that AZ31B magnesium alloy surface oxide film contains Al2O3 and MgO. Results of Sn-9Zn solder spreading experiment showed that when organic acids A+ (5～10wt.%)NH4Cl or (1～10wt.%)ZnCl2, spreading area is larger; the flux can use organic acids A as the matrix component, NH4Cl and ZnCl2 as an active agent or remover added in small amounts.By optimizing the design of the flux formulation using limited uniform design method, obtained the regression equation expression of xi(NH4Cl), x2(ZnCl2) between S of solder spreading area which significantly credible is S=84.643-734.588x1+819.382x2-1408.509x1x2+3092.476x12-5144.404x22. After the optimal solution of the equation, we obtained that mass ratio between ZnCl2 with organic acids A is 8:92 is the best flux formulation, and solder spreading is largest which reaching 117.52mm2.By researching the shape, structure and properties of microstructure, we found that region closing soldering seam has a white interface layer, which maybe is Mg2Sn IMC and β-Sn solid solution phase containing a small number of Al and Zn. Central region of soldering seam is mainly rich-Zn phase and β-Sn phase, in which a portion of the elongated needle rich-Zn due to the poor corrosion resistance happens selective corrosion and comes off looking as black grooves. By testing the micro hardness of every part of the soldered joints, we found that the average micro hardness of interface region is 130 HV which is harder than soldered seam due to it contains IMC which is the hard phase.By classification and performance evaluation of flux, we found that NO of the newly developed flux is FS222B, non-volatile content is up to 86.4%, the corrosion rate is 0.0133g/cm2, indicating that the flux is very corrosive and after welding requires a thorough cleaning. |
PDF Full Text Request