Mg-Al filler metal. Detailed microstructural analysis of the fusion zone and AZ31B Mg alloy after the laser brazing process has been reported in our previous investigation (Ref. 15). This paper focuses on microstructural analysis of the steel-fusion zone interface. Microstructural Evolution along the Steel-FZ Interface Figure 4 shows the microstructure at different locations of the steel-FZ interface. The Ni coating was not detected as a separate layer along the interface after the LBP, which would suggest that it had entirely melted and gone into solution in the liquid immediately adjacent to the interface. It was observed that the microstructure of the FZ-steel interface changed significantly across the FZ-steel interface from the bottom (Position A, Fig. 4B) to top (Position F, Fig. 4E) side of the joint. In order to explain this change of microstructure during the laser brazing process, temperature distribution across the interface vs. time was measured during laser brazing using two thermocouples, one attached to the top side and the other to the bottom side of the steel sheet (see Fig. 2A). According to the measured temperature profiles shown in Fig. 5, the steel sheet experienced maximum temperatures of 1151.1° and 652.7°C on the top and the bottom side, respectively. Therefore, a 500°C temperature gradient was measured between the top and bottom side of the steel sheet during the laser brazing process, since the laser beam was focused on the top of the filler metal, as shown in Fig. 2A (Ref. 15). This temperature difference and gradient across the joint interface during the laser brazing process is believed to be the main reason for the prominent change of microstructure across the FZ-steel interface. As shown in Fig. 4B, at the bottom of the interface a few diamond-shaped bright phases were formed near the steel-FZ interface. In order to identify these phases, a TEM foil was prepared from position B of Fig. 4A. Figure 6 shows the TEM images, EDS plot, and selected area diffraction pattern (SADP) of these submicron particles. The diffraction pattern shows a standard diffraction pattern of AlNi (with BCC structure) with 011 zone axis of the particle. According to an EDS analysis of the diamond-shaped bright phases shown in Fig. 4B, the composition of the particles was 49.6 ± 1.3 at.-% Ni, 45.4 ± 4.7 at.-% Al, and 5.0 ± 2.5 at.-% Mg, thus confirming that the diamond-shaped particles were mainly composed of AlNi intermetallic compound (IMC). Representative concentration profiles of Ni, Al, and Mg across one AlNi particle are shown in Fig. 6D, which indicates that a trace A B C D amount of magnesium was found in this particle. It has been reported that each of the Al-Ni binary intermetallics has some solubility for substitutional magnesium atoms (Ref. 19). Figure 7 shows the XRD spectra obtained from the middle of the steel-FZ interface. The area covered by the X-ray beam was a 300-μm-diameter circle. This XRD result confirmed the existence of AlNi IMC, Fe, β-Mg17Al12, and α-Mg. The AlNi IMC compound was not found at the middle of the FZ area, whereas the XRD pattern in Fig. 7 showed some weak peaks suggesting that AlNi IMC had formed mainly at the steel-FZ interface. It was observed that upon moving from the bottom to the middle of the interface, which was associated with increasing temperature, the morphology of the IMC phase along the interface changed from the diamond shaped AlNi to a faceted dendriticshaped phase (see Fig. 4C, D). Energy-dispersive X-ray spectrometer analysis results indicated this dendritic phase contained 43.0 ± 1.6 at.-% Ni, 52.1 ± 2.0 at.-% Al, and 4.9 ± 0.5 at.-% Mg. This composition again corresponded with the AlNi IMC phase. In this area, the first precipitated phase from the liquid was AlNi IMC, the same as at the bottom of the joint. This phase grew steadily in a faceted dendritic shape. As the interface temperature increased with moving from position A to position E in Fig. 4A, the growth morphology of the AlNi phase changed from diamond shaped to a faceted dendritic shape, as demonstrated in Fig. 4D. Continuous growth of the AlNi was observed in this area with some dendrites having long secondary dendrite arms (see Fig. 4D). At the top of the joint (position F in WELDING JOURNAL 5-s WELDING RESEARCH Fig. 6 — AlNi particle characterization at position B shown in Fig. 4A: A, B — TEM images; C — SADP in the 011 zone axis of this particle; D — EDS composition line scans across an AlNi particle indicating line scans of Ni, Al, and Mg. Fig. 7 — X-ray diffraction pattern of the steel-FZ interface.
Welding Journal | January 2013
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