| Lightweight dissimilar structure has been receiving much attention, since it has a lot of advantages, such as increasing the load capacity, saving energy, lowering the cost as well as working in the different environment. Automotive manufacturers are striving to realize weight reduction by using lightweight hybrid components. The weight of auto body accounts for30%in the whole weight, with steel as its main materials. The key point here is how to achieve the reliable joinng between Mg and Zn coated steel as well as Mg and steel by advanced laser welding technique.The metallurgical bonding of Mg and Fe could be realized provided intermediate elements during the interfacial reaction since their huge differences in melting points and immscibility. The welding process is difficult to control in welding of Zn coated steel due to the low boiling point of Zn, which makes the conventional welding technique impossible. Laser welding-brazing technique is one of the suitable methods that could avoid the evaporation of Zn during the welding process, but also makes it convenient to add the intermediate elements into the filler.Therefore, in this dissertation, the role of Mg-Zn reaction layer and Fe-Al layer at the interface could be clarified based on the welding characteristics and fracture behavior of laser welded-brazed Mg to Zn coated steel joint. The control of Mg/steel interface by alloying elements is studied, especially during the fast heating and cooling rate induced by laser welding-brazing process. The results could provide theory guidance for other immiscible couples.The influence of key factors such as heating mode, laser offset and welding speed on weld appearance and interfacial microstructure was first investigated. Joints with smooth appearance and without any defects were obtained, with low strength as much as51%of Mg base metal. The failure mechanism of Mg/Zn coated steel joints and the role of Zn and Fe-Al in joining Mg to steel were clarified based on the in-depth analysis of Mg-Zn reaction layer and Fe-Al reaction layer. After that, four different surfaces (Zn+Fe-Al, Zn, Fe-Al, without coating) were fabricated. The laser welding-brazing of Mg to steels with four different surfaces were then conducted. The results further confirmed Fe based compound (Fe-Al phase) is better than Mg based compound (Mg-Zn phase) in terms of adhension at the interface. The critical value of Fe-Al phase in thickness was confirmed as2μm. Zn could easily induce cracks in the seam head deteriorating the joint strength, although it improves wetting-spreading ability.Fe-Al phase was expected to grow adjacent to the steel substrate after selecting Al as the main alloying elements. Three different methods concerning interface control were comparatively investigated, including control with3%~9%Al from filler,100%pure Al interlayer and3%Al from filler assisted by Cr element. The interfacial microstructure and joint strength were then studied. The results indicated that the maximum thickness of newly formed Fe-Al compound using the first method is only1.76μm, which is lower than critical value2μm. The main reason for it is the insufficient diffusion process because of fast heating and cooling rate. While using100%pure Al interlayer, the thickness of Fe-Al layer could reach2-4μm. But the accompanying problem is increasing the brittleness of weld seam due to the formation of a large amount of Mg-Al compound near the interface. The best way to realize the interface control is using Cr to assist the interface control with3%Al from filler. The diffusion problem at the fast thermal cycle was successfully solved and therefore the reliable joining between Mg and steel was achieved. The joint fractured at the fusion side of Mg alloys, with the maximum load of308N/mm reaching75.4%of Mg base metal.Based on thermodynamic analysis, the interfacial reaction involving with multi-components was predicted using Factsage software, to reveal its possible formed phases at different temperatures. The microstructure evolution was clarified when Mg/steel interface was controlled by different micro-alloying elements. The key reason for Cr promoting the growth of Fe-Al phase at the interface was explained that was it increased the range of possible phases formed at the interface, improved the stability of Fe-Al at the precipitation process and inhibited the formation of other phases. |
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