Caiwang Tan

Influence of Zn Coating on Interfacial Reactions and Mechanical Properties During Laser Welding-Brazing of Mg to Steel

To investigate the influence of Zn coating on the joining of magnesium alloy AZ31xa0to Zn-coated steel, dissimilar metal joining both with and without Zn coating was performed by the laser welding-brazing (LWB) process. Welding characteristics including joint appearance, identification of interfacial reaction layers, and mechanical properties were comparatively studied. The results indicated that the presence of Zn coating promoted the wetting of liquid filler wire on the steel substrate. Heterogeneous interfacial reaction layers formed along the interface between the Mg alloy and Zn-coated steel, whereas no distinct reaction layer and increased concentration of Al were identified at the interface between the Mg alloy and noncoated steel. The maximum tensile-shear strength of Mg/steel lap joint with Zn coating reached 180xa0N/mm, which was slightly higher than that achieved without Zn coating (160xa0N/mm). Failure of joint in both cases occurred at the interface; however, the fracture mode was found to differ. For Zn-coated steel, the crack propagated along the Mg-Zn reaction layer and Fe-Al phase, with little Mg-Zn reaction phases remaining on the steel side. As for noncoated steel, some remnants of the seam adhered to the steel substrate.

Laser Welding-Brazing of Immiscible AZ31B Mg and Ti-6Al-4V Alloys Using an Electrodeposited Cu Interlayer

nMetallurgical bonding between immiscible system AZ31B magnesium (Mg) and Ti-6Al-4V titanium (Ti) was achieved by adding Cu interlayer using laser welding-brazing process. Effect of the laser power on microstructure evolution and mechanical properties of Mg/Cu-coated Ti joints was studied. Visually acceptable joints were obtained at the range of 1300 to 1500xa0W. The brazed interface was divided into three parts due to temperature gradient: direct irradiation zone, intermediate zone and seam head zone. Ti3Al phase was produced along the interface at the direct irradiation zone. Ti-Al reaction layer grew slightly with the increase in laser power. A small amount of Ti2(Cu,Al) interfacial compounds formed at the intermediate zone and the (α-Mgxa0+xa0Mg2Cu) eutectic structure dispersed in the fusion zone instead of gathering when increasing the laser power at this zone. At the seam head zone, Mg-Cu eutectic structure was produced in large quantities under all cases. Joint strength first increased and then decreased with the variation of the laser power. The maximum fracture load of Mg/Cu-coated Ti joint reached 2314xa0N at the laser power of 1300xa0W, representing 85.7% joint efficiency when compared with Mg base metal. All specimens fractured at the interface. The feature of fracture surface at the laser power of 1100xa0W was characterized by overall smooth surface. Obvious tear ridge and Ti3Al particles were observed at the fracture surface with increase in laser power. It suggested atomic diffusion was accelerated with more heat input giving rise to the enhanced interfacial reaction and metallurgical bonding in direct irradiation zone, which determined the mechanical properties of the joint.

Influence of Laser Power on the Microstructure and Mechanical Properties of a Laser Welded-Brazed Mg Alloy/Ni-Coated Steel Dissimilar Joint

In this work, we describe a method to improve the bonding of an immiscible Mg/steel system using Ni as an interlayer by coating it on the steel surface. Laser welding-brazing of AZ31B Mg alloy to Ni-coated Q235 steel using Mg-based filler was performed in a lap configuration. The influence of laser power on the weld characteristics, including joint appearance, formation of interfacial reaction layers and mechanical properties was investigated. The results indicated that the presence of the Ni-coating promoted the wetting of the liquid filler metal on the steel surface. A thermal gradient along the interface led to the formation of heterogeneous interfacial reaction layers. When using a low laser power of 1600xa0W, the reaction products were an FeAl phase in the direct laser irradiation zone, an AlNi phase close to the intermediate zone and mixtures of AlNi phase and an (α-Mgxa0+xa0Mg2Ni) eutectic structure near the interface at the seam head zone. For high powers of more than 2000xa0W, the FeAl phase grew thicker in the direct laser irradiation zone and a new Fe(Ni) transition layer formed at the interface of the intermediate zone and the seam head zone. However, the AlNi phase and (α-Mgxa0+xa0Mg2Ni) eutectic structure were scattered at the Mg seam. All the joints fractured at the fusion zone, indicating that the improved interface was not the weakest joint region. The maximum tensile-shear strength of the Mg/Ni-coated steel joint reached 190xa0N/mm, and the joint efficiency was 70% with respect to the Mg alloy base metal.

In Situ SEM Observations of Fracture Behavior of Laser Welded–Brazed Al/Steel Dissimilar Joint

Laser welding–brazing of 6061-T6 aluminum alloy to DP590 dual-phase steel with Al-Si12 flux-cored filler wire was performed. The microstructure at the brazing interface was characterized. Fracture behavior was observed and analyzed by in situ scanning electron microscope. The microstructure of the brazing interface showed that inhomogeneous intermetallic compounds formed along the thickness direction, which had a great influence on the crack initiation and propagation. In the top region, the reaction layer at the interface consisted of scattered needle-like Fe(Al,Si)3 and serration-shaped Fe1.8Al7.2Si. In the middle region, the compound at the interface was only serration-shaped Fe1.8Al7.2Si. In the bottom region, the interface was composed of lamellar-shaped Fe1.8Al7.2Si. The cracks were first detected in the bottom region and propagated from bottom to top along the interface. At the bottom region, the crack initiated and propagated along the Fe1.8Al7.2Si/weld seam interface during the in situ tensile test. When the crack propagated into the middle region, a deflection of crack propagation appeared. The crack first propagated along the steel/Fe1.8Al7.2Si interface and then moved along the weld seam until the failure of the joint. The tensile strength of the joint was 146.5xa0MPa. Some micro-cracks were detected at Fe(Al,Si)3 and the interface between the steel substrate and Fe(Al,Si)3 in the top region while the interface was still connected.

Dissimilar metal joining of magnesium alloy to zinc-coated steel by laser welding-brazing process

In automobile industry, there is a strong driving force for reducing the weight of car body by using hybrid structural body parts for improving fuel efficiency and decreasing air pollution. As one of solutions, the dissimilar combination of magnesium alloy and steel tends to be effective for the car body to save the weight while maintaining the sufficient strength for the automobiles. However, it offers great challenges and potentials to joining the dissimilar metals of magnesium alloy and steel, owing to the large differences in physical and chemical properties between them. In this work, a promising approach, laser welding-brazing process with filler wire was proposed. The weldability of dissimilar alloys of magnesium alloy and zinc-coated steel was studied. The influence of process parameters on the wetting and spreading ability of liquid filler were investigated. Furthermore, the brittle intermetallic compound phases at the interface were characterized and the mechanical properties were evaluated. From the results, failure of shear test specimen occurred at the interface from the toe of weld, which had a detrimental effect on mechanical properties of the joint.In automobile industry, there is a strong driving force for reducing the weight of car body by using hybrid structural body parts for improving fuel efficiency and decreasing air pollution. As one of solutions, the dissimilar combination of magnesium alloy and steel tends to be effective for the car body to save the weight while maintaining the sufficient strength for the automobiles. However, it offers great challenges and potentials to joining the dissimilar metals of magnesium alloy and steel, owing to the large differences in physical and chemical properties between them. In this work, a promising approach, laser welding-brazing process with filler wire was proposed. The weldability of dissimilar alloys of magnesium alloy and zinc-coated steel was studied. The influence of process parameters on the wetting and spreading ability of liquid filler were investigated. Furthermore, the brittle intermetallic compound phases at the interface were characterized and the mechanical properties were evaluated. Fro...