13C. These faceted dendrites form due to kinetic difficulties in forming new planes of atoms (Ref. 21). In this type of dendrite, the growing direction of dendrite arms are ones that are capped by relatively slow growing planes (usually low-index planes) (Ref. 21). The slowest growing plane would be expected to be the closestpacked planes. Weinberg and Chalmers (Ref. 22) reported that the axis of a pyramid, whose sides are the most closely packed planes, is generally the major dendrite direction. As a result, for AlNi faceted dendrites with BCC structure, this direction is <100>. Therefore, the process of solidification at the middle part of the joint starts with the nucleation and growth of the AlNi-faceted dendrites along the <100> growth direction. Fourthly, if the Ni content of the remaining liquid between the steel side and formed AlNi precipitates is high enough, Mg2Ni with a melting point of 762°C nucleates (see Fig. 13C). Formation of Mg2Ni depends on sufficient Ni concentration in the remaining liquid near the steel-FZ interface after formation of the AlNi IMC. The Ni content of the remaining liquid after precipitation of AlNi increases from 2.4 at.-% at the top side of the interface to 10.6 at.-% at the bottom portion because formation of the AlNi IMC layer consumed the Ni atoms near the interface and the volume fraction of this phase increased from the bottom to the top portion of the joint. Based on the above analysis, high enough concentration of Ni in the remaining liquid close to the bottom side of the joint after formation of AlNi IMC resulted in formation of the Mg2Ni + α-Mg lamellar eutectic in the form of a gray phase between the AlNi IMC and steel. In order for this lamellar eutectic to grow, the local composition of the fusion zone should be close to the eutectic composition (10 at.-% Ni, according to the Mg-Ni binary phase diagram) (Ref. 21). Reactions between Mg in the fusion zone and Ni along the interface caused formation of the Mg-Ni eutectic phase. This reaction can be represented by the following balanced chemical reaction: L(10 at.-% Ni)←508°C→Mg2Ni (33 at.-% Ni)+Mg(0 at.-% Ni) (1) Therefore, at the bottom of the interface, two reactions occurred; the first one was precipitation of AlNi from the liquid and the second was the eutectic reaction between Mg and Ni in the FZ (reaction 1). In the case of reaction sequences, first AlNi forms near the interface and then the remaining liquid with a low Al content between the AlNi IMC and steel-FZ interface, which is still rich in Ni, undergoes a eutectic reaction with Mg and results in the formation of the lamellar α-Mg + Mg2Ni eutectic. With the formation of the AlNi IMC layer, diffusion of Ni atoms from the steel side to the FZ is blocked. Therefore, the concentration of Ni in the remaining liquid between the interface and preformed AlNi phase is expected to be higher than the remaining liquid on the other side of the AlNi phase. The result is the formation of Mg2Ni just between the AlNi phase and steel (see Fig. 8A, B). In the top portion of the interface, with the nucleation and growth of the AlNi particles, most of the Ni atoms are consumed. Therefore, the Ni content of the remaining liquid would not be enough for formation of the Mg2Ni phase. Conclusions 1. With the addition of an electrodeposited Ni interlayer on steel sheet, single flare bevel lap joints of AZ31B-H24 Mg alloy to steel sheet were rendered possible by the laser brazing process, and a uniform brazed area with good wetting and bonding of both base metals was achieved. 2. Dissolution of the Ni coating layer during the laser brazing process led to the formation of new AlNi IMC phases and also a Mg-Ni eutectic zone along the interface. The AlNi intermetallic layers at the steel-FZ interface formed in the sequence of diamond-shaped, dendritic, and nodules from the bottom to the top portion of the joint. 3. The formation of a nano-scale Fe(Ni) transition layer on the steel by solidstate interdiffusion between Fe and Ni during laser brazing was found to be responsible for the formation of a metallurgical bond between the steel and the Mg-Al brazing alloy. 4. The average shear strength of the joints reached 96.8 MPa, 60% that of the base metal of AZ31B Mg alloy. Fracture surface analysis showed that fracture occurred in the FZ close to the steel-FZ interface. Acknowledgments The authors wish to acknowledge support of the American Welding Society (AWS) Graduate Fellowship program, the Natural Sciences and Engineering Research Council of Canada (NSERC), and WELDING JOURNAL 9-s WELDING RESEARCH Fig. 13 — Formation of transitional layer and intermetallic compounds during laser brazing of Ni-plated steel-AZ31B with Mg-Al filler metal: A — Wetting of the Ni-plated steel by molten filler metal and dissolution and diffusion of Ni into the FZ and steel substrate; B — formation of the transitional layer and aggregation of Ni along the interface; C — nucleation and growth of AlNi IMC, and epitaxial growth of the remaining liquid in the form of α-Mg + Mg2Ni eutectic onto the thin Fe(Ni) interlayer.
Welding Journal | January 2013
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