layer (as shown in Fig. 10B) confirmed good coherency between this layer and steel as well as the fusion zone. According to EDS point analysis, the Ni content of the transition layer varied between 17 and 40 at.-%. Figure 10C shows the selected area diffraction pattern (SADP) on the transition layer that identified it as Fe(Ni) solid solution with face-centered cubic (FCC) structure. Therefore, this layer proved to be the key factor for realizing a metallurgical bond between the steel and fusion zone. Representative concentration profiles of Fe, Ni, and Mg across the interface between the fusion zone and the steel are shown in Fig. 10D. It is evident from these line scans that Fe, Ni, and Mg diffused into each other as a result of the high temperature experienced during the laser brazing process. As a result, two diffusion or transition layers formed between the steel and fusion zone. According to the element distributions of Fe, Ni, and Mg (see Fig. 10D), in transition layer I with a thickness of almost 70 nm from the steel side, the Fe content decreased gradually while the Ni content increased. In this layer, solid-state diffusion of Ni and Fe into each other is believed to control the overall thickness of this layer. Another diffusion layer (transition layer II) was observed in Fig. 10D between the transition layer I and the fusion zone. The thickness of this layer was ≈ 60 nm. A slight diffusion of magnesium from fusion zone into transition layer II was detected. It would appear that there was sufficient solubility of the Mg in this Fe(Ni) interlayer for diffusion of the Mg to occur and that wetting and bonding of the α-Mg + Mg2Ni eutectic phases had in fact occurred with the thin Fe(Ni) interlayer that had formed during laser brazing, and not directly with the steel. Mechanical Properties Due to the nonsymmetric configuration of the 5-mm-wide tensile-shear test specimens (see Fig. 2B), a combination of shear and tensile forces existed at the interface. Consequently, the joint strengths are reported here as fracture load, since it is not possible to separate tensile and shear stresses. The average tensile shear strength of the laser brazed steel-Ni- AZ31B joints using the Mg-Al filler metal was found to be 153.7 ± 2.7 kgf (or 1506.3 ± 24.5 N). This is 153% higher than tensile shear strength of the laser brazed Alcoated steel-AZ31B Mg alloy specimens obtained in our previous study (Ref. 15). The low standard deviation of the tensile shear strength of the laser brazed steel-Ni- AZ31B joints (±2.7 kgf) compared with the laser brazed steel-Al-AZ31B joints (±11 kgf) indicated that the laser brazing process was inherently stable and reproducible. If only the shear plane is considered, the average shear strength of the joints was 96.8 MPa, or 60% of that of AZ31B-H24 Mg alloy base metal. All tensile-shear specimens fractured in the FZ very close to the steel-FZ interface. Typical fracture surfaces of both the fusion zone side and steel side after tensile shear testing are shown in Fig. 11. Figure 11A, C are low-magnification SEM micrographs of the fracture surfaces of the fusion zone side and steel side, respectively, and dimples are shown at high magnification in Fig. 11B, D. This uniform distribution of the dimples is characteristic of ductile fracture surfaces. These fracture surfaces indicated that the specimens failed under conditions similar to tensile test with a strong shear stress component (tensile-shear test). The effect of shear stress on the morphology of the dimples is very evident in these micrographs. The vertical direction in each of the micrographs is parallel to the direction of the shear, and the elongation of the dimples under the action of shear stress is evident in Fig. 11B, D. The AlNi IMC compound was not found at the fracture surfaces. The EDS analysis results of the fracture surfaces of both the steel and FZ side also indicated that crack propagation during the tensile shear tests had occurred entirely in the FZ. Based on the EDS results, the composition of the fracture surface for both steel side and FZ side were similar to the FZ, meaning fracture passed through the FZ near the steel-FZ interface. Figure 12 shows the XRD pattern from the fractured surface of the joint on the steel side. Fe, α-Mg, and AlNi peaks were seen in this X-ray diffraction result. These findings were consistent with the SEM and EDS analysis results. Discussion From the above results, the interaction between the filler metal and surface of the Ni-plated steel can be explained as follows (see Fig. 13): Firstly, the solid-state Ni-plated steel is in contact with the liquid filler metal (Mg- Al alloy) at the laser brazing temperature and, subsequently, the liquid Mg-Al alloy flows over the Ni surface — Fig. 13A. Secondly, dissolution and diffusion of Ni atoms into the liquid occur, as shown in Fig. 13B. At the same time, some solidstate diffusion of Ni atoms into the steel also occurs. A slight diffusion of Fe atoms WELDING JOURNAL 7-s WELDING RESEARCH A C B D Fig. 10 — A— TEM image of the steel-fusion zone interface; B— higher magnification of the selected square area in A; C — SADP in the 011 zone axis of the interfacial phase; D — EDS line scan analysis of Fe, Ni, and Mg at the steel-fusion zone interface.
Welding Journal | January 2013
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