| Automotive manufacturers are striving to realize weight reduction using lightweight hybrid components. The key point here is how to achieve the reliable joining between Al and steel by advanced welding techniques. There have been some problems on the joining of Al to steel. Al and steel have much difference in physical properties, such as melting point, density and elastic thermal coefficient (CTE), etc. As a result, the joint was prone to fracture. Besides, metallurgic reactions occurring between Al and Fe would result in the formation of hard and brittle Fe-Al intermetallic compounds, which have very negative effects on the joint mechanical properties. Hence, it is of importance to control the formation of Fe-Al intermetallic compounds.By virtue of low heat input, high cooling rate and capability of adding filler metals, the formation of Fe-Al intermetallic compounds can be controlled to some extent. Therefore, pulsed Nd:YAG key hole laser welding and continuous diode laser welding/brazing of Al to steel were carried out in this paper.Pulsed Nd:YAG laser key hole laser welding of Al to steel was investigated Due to the low heat input in this welding technology, the formation and growth of intermetallic compounds would be controlled. Thus, the change mechanism of intermetallic compounds with heat input was investigated. Two typical joints were obtained by adjusting the process parameters, which were high-depth and low-depth joints. The interfacial intermetallic compounds changed from Al-rich intermetallic compounds in high-depth joint to Fe-rich intermetallic compounds in low-depth joint, which led to a sharp increase in joint strength. The mechanism of interfacial reactions was revealed based on SEM and EDS analysis. With the increase of heat input, the inter-diffusion of Fe and Al at the interface between Al and fusion boundary would be exacerbated, leading the change of interfacial reactions. It in turn caused the change of intermetallic compounds. It was noted that the constantly appeared weld defects in the joint made unfavorable joint strength, even though the process parameter was optimized by Taguchi method. The formation amount of Fe-Al intermetallic compounds increased rapidly due to the melting of Al and steel in key hole mode. It led to the low joint strength. Besides, the key hole was unstable during the key hole welding process, which made an unstable welding. It then led lots of weld defects.Further, by considering the problems that encountered in Nd:YAG welding process, a more stable laser welding mode was applied, i.e., conduction mode. A continuous diode laser welding/brazing method was used in joining of Al to steel with various filler metal. The addition of alloy elements was benefit to control the formation type and distribution of intermetallic compounds. Thus, the change mechanism of intermetallic compounds with alloy elements was analyzed, except for the investigation in the change mechanism of intermetallic compounds with laser heat input. It showed that the type of intermetallic compounds change with the laser heat input, it in turn leading to the change of joint strength and fracture mode. When low and medium power were applied, the brazed interface was mainly composed of θ-Fe(Al,Si)3 and τ5-Al7.2Fe1.sSi. However, when high power was applied, the interfacial intermetallic compounds changed into θ-Fe(Al,Si)3, τ5-Al7.2Fe1.8Si and η-Fe2(Al,Si)5 within some microcracks. It was confirmed that the formation and growth of interfacial intermetallic compounds were mainly depend on the diffusion of Fe towards from steel to liquid fusion zone by EDS line scanning. In the Initial stage, the diffusion process would only alter the morphology of the intermetallic compounds, and then it might eventually alter the type of the intermetallic compounds. With the addition of Zn into the filler metal, a vital change happened in the type of interfacial intermetallic compounds. The interfacial intermetallic compounds were changed from layered Fe2Al5 and needle-like FeAl3 to layered Fe2A5-xZnx and dispersed FeZn10 with minor Al-rich amorphous phase. It caused the increase of joint strength. To reveal the correlation between FeZn10 and joint mechanical properties, the hardness testing was performed. It showed that FeZn10 was relatively soft and tough which was able to absorb the fracture energy by plastic deformation. It in turn prevented the initiation and propagation of crack in the joint and improved the joint strength. According to the thermo-dynamic theory calculation, the formation sequence of intermetallic compounds was confirmed, and the evolution of intermetallic compounds with the change of laser heat input and alloy elements were analyzed.At last, the effect mechanism of the thickness, type and distribution of intermetallic compounds was analyzed. Base on theory of the correlation between interplanar mismatch and interfacial energy as well as interfacial strength, edge-to-edge crystallographic model was applied to calculate the interplanar mismatch at the interface, to evaluate and compare the interfacial energy at different interfaces, to predict the fracture mechanism of the interfacial fracture. It was found that a higher interplanar mismatch meant a higher interfacial energy, a lower interfacial strength and higher tendency to fracture at interface. |
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