Interface Homogenization And Properties Enhancement In Scanning Laser-arc Hybrid Welding Of Dissimilar Metals

The application of dissimilar metal composite structures is one of the most effective methods for lightweight design,but the high-quality and high-efficiency dissimilar welding is difficult to achieve under existing processes because of the easy formation of brittle intermetallic compounds(IMCs).Generally,fusion welding is of high efficiency,but the severe molten pool reaction easily results in poor controllability of interface growth.In addition,the process window of fusion welding is extremely narrow,which makes it not suitable for industrial application.Solid-state welding is controllable,but has the problems of low efficiency,strict environment requirement,and small application range.In the current study,the Al/Fe butt-joint and Al/Mg lap-joint,with both high demands and considerable difficulties,were used as the objects to carry out the study on scanning laser-arc hybrid welding of dissimilar metals.The effects of beam scanning on the interface growth and the microstructure and properties of dissimilar welds were revealed.The main results are as follows:The weld formation of Al/Fe scanning laser-arc hybrid weld was studied.The results showed that the beam scanning benefited to suppress the poor wetting defects on the root of Fe side such as uneven formation and lack of fusion,and avoid excessive melting of the top and bottom interface on the Fe side.Consequently,the uniformity of the weld and interface were both improved.Under the given parameters,the range of laser power to obtain good weld formation in scanning hybrid welding increased to 800 W,which is 2.7 times higher than that of non-scanning hybrid welding.Based on the Fe element distribution of weld cross-section,the weld overall uniformity was studied.It is found that the beam scanning reduced the percentage difference of average Fe content between the upper and lower parts of fusion zone from 132%to 5%,which means the weld uniformity was increased by approximately 25 times relatively to the none-scanning hybrid welding.The reason is that the molten pool stirring effects driven by the beam scanning widen the lower part of the molten pool and enhance the melt flow and heat transfer,which avoids the segregation of heat and solute,thereby improving the weld formation and uniformity.Based on parameter optimization,the interface of Al/Fe scanning hybrid weld gradually changed from a double-layer of Al₅Fe₂ and Al₁₃Fe₄with uneven thickness to a thin and uniform single-layer of Al₅Fe₂(?1.8?m).Although the obvious non-uniformity in the phase,thickness and morphology along thickness direction existed at the Al/Fe interface,most existing literatures only discussed the interface through a specific area,which is difficult to express its overall characteristics.To this end,this paper proposed a new method to quantitatively characterize the interface uniformity via the thickness distribution variance(?)between the upper/middle/lower parts of IMCs layer:the smaller the?,the better the interfacial uniformity.The results showed that the minimum?reduced to 0.0022 in scanning hybrid welding.Compared to the minimum?(?0.0289)obtained in existing studies on Al/Fe hybrid welding,the interfacial uniformity in scanning hybrid welding was improved by 12 times.Based on the thermodynamic calculation and the analysis of heat source interaction,the effects of beam scanning on the interface growth and uniformity of Al/Fe hybrid weld were discussed.First,the directional beam scanning over a wide range further enhanced the laser-arc interaction,which eliminated the heat accumulation on the Fe side,thereby reducing or inhibiting the melting of the Fe base metal,and decreasing the Fe concentration at the interface and molten pool.Secondly,the stirring of laser keyhole in the molten pool expanded the transition angle between the upper and lower melt flows,promoted the heat/mass transfer from top to bottom interfaces,and formed a local turbulence to reduce the interfacial Fe melt.It reduced the Fe content,and decreased its concentration fluctuates in local areas.Therefore,the reasons for the growth and homogenization of interface by scanning hybrid welding can be described as follows:the Al₅Fe₂ with the smallest Gibbs free energy first reached the thermodynamic conditions and preferentially formed at interface,which prevented the continued dissolution of Fe atoms into molten pool;at the same time,the homogenization induced by the beam scanning further diluted the Fe atoms at interface.As a result,the Fe concentration was difficult to reach its supersaturation in Al melt,which not only delayed the rapid growth of Al₅Fe₂,but also inhibited the precipitation of Al₁₃Fe₄,and finally benefited to obtain a thin Al₅Fe₂ single-layer interface.The quantitative relationship between the weld tensile strength(UTS)and the thicknes uniformity(?),thickness(?)and phase of Al/Fe interface layer was established,which can be divided into two categories according to the?.First,when??0.1156,the interface layer was composed by the uniform single-layer Al₅Fe₂,and the UTS was determined by the?:(1)When 0.9?m???1.8?m,UTS?130 MPa;(2)When 1.8?m<?<3.2?m,UTS?110 MPa.Secondly,when?>0.1156,the interface was composed by the uneven double-layer IMCs of Al₅Fe₂ and Al₁₃Fe₄,making UTS<100 MPa.In addition,because of the improvement of interface uniformity,the UTS and the longitudinal bending angle of the Al/Fe scanning hybrid weld increased to 160 MPa and 90^o,which was 65%and 96%higher than the non-scanning hybrid weld respectively.The enhancement in mechanical properties of Al/Fe scanning hybrid weld can be explained as follows:first,the single-layer IMC interface avoided the uneven mixed fractures between the double-layer IMCs,and the brittle transcrystalline fracture caused by the thickened Al₅Fe₂ layer;secondly,the phase boundary dislocation plugging induced by lattice mismatch between Fe and Al₅Fe₂strengthened the interface;furthermore,the wave interface between Fe and Al₅Fe₂ increased the effective connection length of the interface,and the convex Fe at interface improved the interface toughness,which hindered the crack initiation and propagation.The interface characteristics of Al/Mg scanning laser-arc hybrid lap-weld(based on Ti interlayer)and its interface homogenization mechanism were studied.It is found that under the optimized range of scanning frequency(60?120 Hz),the Ti interlayer can be fully melted at interface,which consumed large amount of Al melt by preferentially forming Al₃Ti with the smallest Gibbs free energy.As a result,the direct contact between liquid Al and Mg was avoided,which effectively suppressed the formation of brittle Al₃Mg₂ and Al₁₂Mg₁₇,and benefited to form a stronger Al₃Ti interface layer.The maximum shear force of the Al/Mg scanning hybrid weld with a width of 15 mm reached up to 3.3 k N,which is about 83%higher than that of the non-scanning laser weld as reported.The enhancement in shear strength of the Al/Mg scanning hybrid lap-weld depends on two factors:first,the beam scanning uniformly distributed the laser energy at interface,which avoided the local unreaction or over-reaction of Ti interlayer due to the uneven heating;secondly,the beam scanning widened the melting range of interface layer and enhanced its effective connection width,which increased the force bearing area of the interface,and thus improved the weld shear strength.