Comparative study of microstructure and mechanical properties of laser welded–brazed Mg/steel joints with four different coating surfaces

Abstract

This paper presents a comparative study of laser welding-brazing Mg to steel with four different coating surfaces, including (Znzpre-existing Fe-Al phase), pure Zn coating, pre-existing Fe-Al phase and fresh steel without any coating. The presence of Zn coating was found to significantly improve the wettability of liquid filler on steel. However, Mg-Zn products enriching at the seam head tended to cause cracking. The weak bonding of Mg-Zn products and Fe-Al layer was mainly responsible for the decreased tensile strength and interfacial failure that occurred in joints with the first two coatings. For joints with the latter two coatings, the thickness of newly formed Fe-Al layer determined the mechanical properties. The reaction layer formed at the Mg/fresh steel was thin, inducing interfacial failure, whereas the joint with pre-existing Fe-Al phase fractured at the seam, indicating that the pre-existing Fe-Al phase was beneficial to formation and growth of the Fe-Al phase.

Joining of AZ31 Magnesium Alloy and Steel Sheet under Four Different Coating Conditions Based on Gas Metal Arc Weld-Brazing

“…In the case of the AZ31-GI steel joint, the interface of the brazed joint is composed of an α-Mg and Mg-Zn eutectic layer, which has a high Zn content; the 25-μm-thick transition layer of this joint is composed of an Fe-Al intermetallic compound which is estimated the FeAl or Fe 3 Al phase, which has a high Al content, as determined via EDS analysis. Tan 25,26) identi ed the FeAl and Fe 3 Al phases of Fe-Al intermetallic compound and Mg-Zn eutectic structure between Mg-Zn coated steel dissimilar joint by transmission electron microscopy (TEM) and selected area electron diffraction pattern (SADP). Conversely, a weakly developed eutectic layer and a small island, which consists of the eutectic microstructure and transition layer, occur at the interface of the AZ31-GA steel joint.…”

“…This fracture propagates along a direction perpendicular to the interface, until the joint fractures. Tan 25,26) considered the fracture mechanism of the Mg/steel brazed joint: the decrease in tensile strength and consequent interfacial failure of the Mg alloy/steel brazed joint with Zn coating, was attributed mainly to the weak bonding between the Mg-Zn phase and the Fe-Al layer. Similarly, the strength of the Mg alloy/steel brazed joint, without Zn coating, varied with the thickness of the Fe-Al phase.…”

“…Fusion welding processes have been used in order to overcome the disadvantage of solid-state welding between magnesium alloys and steel; these processes include a laser hybrid [9][10][11][12][13][14][15][16][17][18][19][20][21] , laser braze [22][23][24][25][26][27][28] , and resistance spot welding [29][30][31] on the dissimilar joint. Liu [9][10][11][12][13][14][15] and Qi [16][17][18][19] have applied a laser-arc hybrid process to various dissimilar joints of magnesium alloy and steel.…”

“…Laser braze welding also was applied to the joint of magnesium alloy and steel [22][23][24][25][26][27][28] . Wahba and Katayama 22) characterized the lap joint formed between an AZ31B magnesium alloy and a zinc-coated steel.…”

“…Wahba and Katayama 22) characterized the lap joint formed between an AZ31B magnesium alloy and a zinc-coated steel. Li et al and Tan et al [23][24][25][26][27][28] examined the microstructure and evaluated the mechanical properties of laser welded-brazed joints formed between magnesium and Zn-coated/non-Zn-coated steel. These studies revealed that, in the case of the galvanized steel, the crack propagated along the interface between the Mg-Zn reaction layer and the pre-existing Fe-Al layer, owing to weak bonding between these layers.…”