| A new bonded process of joining Al alloy and stainless steel-diffusion weld with two-step temperature is presented basing on their welding characteristic. The systematic study on Al alloy and stainless steel joined by diffusion welding with two-step temperature is conducted by using plating Cu layer, plating Ag layer and Al-Si alloy foil as reactive materials in the case of different joining temperature and holding time. The wetting spreading behavior in the process, grain boundary penetration phenomenon, joints microstructure, welded seam formation are in detail analyzed and discussed by using test mean of Optical Microscope, Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Energy dispersive X-ray spectroscopy (EDS). The mechanical property of joints is examined and the process parameter is optimized. The main research results are as followsUsing diffusion welding with two-step temperature to join Al alloy and stainless steel, the wettability of Al-Cu eutectic liquid on Al alloy matrix is fine, which results from combined action of wetting spreading and reactive spreading. Fe-Al,Al-Cu intermetallic compound are generated in the interface of stainless steel side and the thickness of reactive layer is obviously increased with the process parameter raising. The welded seam center is composed with Al-Cu eutectic structure. The key factor of generating grain boundary penetration is the unequal interdiffusion flux, promoting vacancy diffusing into grain boundary. The growth rate of Al-Cu eutectic liquid penetrating into 6063 Al alloy grain boundaries is 1.3297×10-8m/s The dissolution of plating Cu layer is quick, the maximum width of welded seam liquid zone is 400.583μm. The effect on low diffusion temperature and holding time to joints strength is obvious, when the temperature is too low and the time is too short, the joints strength value shows decreasing trend. The concentration distributions of Fe,Al element in intermetallics of Fe3Al and AlCu4 are analogy calculated, and compared with EPMA actual measurement data, it shows that the theoretical calculation value is basically consistent with the measured value.Using plating Ag layer as intermediate reaction material to bond 6063 Al alloy and 1Cr18Ni9Ti stainless steel, the welded seam microstructure consists of phase rich in Ag,Al(Ag) solid solution,Ag-Al intermetallic compound and Fe-Al intermetallic compound layer. In the persistent heating process, Ag disappears because of volume diffusion with 6063 Al alloy. In welded seam, Ag-Al intermetallic compounds are formed in initial stage, to some extent, the formation of Fe-Al intermetallic compounds is delayed. Contrasted with contact reactive brazing, the growth speed of welded structure is easily controlled. Plating Ni layer can block interdiffusion of Al,Fe atom to form Fe-Al intermetallic compounds. The relation between the thickness of intermetallic compound layer in the interface and square root of holding time is direct proportion; the growth activation energy of intermetallic compound is 162.39KJ/mol.In the process of diffusion welding with two-step temperature of 6063 Al alloy/Al-Si/1Cr18Ni9Ti Stainless Steel, the research shows that the effect on holding time to the wetting spread of Al-12.6%Si eutectic liquid on 6063 Al alloy is little and the influence on 1Cr18Ni9Ti Stainless Steel is very large. The higher the heating temperature is and the longer the holding time is, the less eutectic structure the welded zone has, the primary Si phase and eutectic Si phase get growing. The welded seam formation process in diffusion welding with two-step temperature can be divided into four stages, Filling welded seam; Liquid zone broaden; Isothermal solidification; Removing microdefect through interdiffusion of atom in the low holding temperature. The joints strength is low, resulting from Fe-Al intermetallic compound in the interface and insufficient wetting on the surface of Stainless Steel.In this paper, base on experimental research, the relation between holding time and the growth rate of intermetallic compound in the interface in the case of diffusion welding with two-step temperature is established through Fick′s second law. The mechanism of welded seam formation with diffusion welding with two-step temperature is revealed, the grain boundary penetration phenomenon is qualitatily investigated, Al-Cu eutectic reactive rate is calculated and the critical factors retraining the joints strength is compared. The achievement plays an active role in filling the theory of diffusion welding. |
PDF Full Text Request