| With the global consensus of low-carbon transition,the concept of dual carbon economy has been born.Lightweighting,as one of the important means to realize the dual carbon economy,has become the current trend of global manufacturing industry.By introducing aluminum alloys into stainless steel structures,a heterogeneous composite structure can be obtained that combines the advantages of both.However,the core problems of chemical incompatibility and physical mismatch between aluminum and steel make the high-quality aluminum/steel joining a great challenge.To solve the problems of poor wettability and insufficient mechanical properties in aluminum/steel welding,a novel laser-chemical composite processing of surface microtexture regulation technique was proposed in this study.Through this technology,a reliable laser welding-brazing joint in the form of 3 mm thick 6A01aluminum alloy and 301L stainless steel lap joint with 4043 aluminum alloy as filler material was successfully achieved.The dynamic wetting mechanism of aluminum/steel under laser irradiation was elucidated,the directional enhancement mechanism of microtexture on spreading was analyzed,the optimization path of microtexture on joint connection and failure behavior was clarified,and the regulatory role of surface microtexturing on laser welding-brazing process was revealed.A novel laser-chemical composite processing process for surface microtexturing had been developed based on laser processing technology and chemical etching technology.The process can effectively remove the remelted defects generated by laser and formed a multi-level microtextured surface with large-size groove（hundred-micron level）and small-size ridge（micron level）on the substrate surface.The process could significantly improve the surface microtexture quality and was suitable for the regulation of wetting and welding process.Isothermal wetting tests on different substrates showed that microtexturing can enhance the interfacial reactivity and capillary effect.Wherein,small-sized ridge can promote the spreading of droplets in any direction by a small amount,while large-sized groove caused a decrease in spreading activation energy in parallel direction and an increase in spreading activation energy in vertical direction,which finally exhibit anisotropic wetting behavior promoted in the parallel direction and inhibited in the vertical direction.The laser-induced dynamic wetting and spreading behavior of 4043 Al/301L SS system was investigated.Under laser irradiation,a strong inhomogeneous temperature distribution would appear on the substrate surface.Driven by the thermodynamic gradient,Si atoms in the melt adsorbed onto the surface of Fe substrate faster than Al atoms,and gradually thickenedτ5-Fe2Al8Si+θ-Fe（Al,Si）3+η-Fe2（Al,Si）5 phase was generated at the interface of the central region with increasing irradiation time,and a thin IMC layer consisting ofτ5-Fe2Al8Si phase was formed near the triple line.The contact angle gradually decreased and the spreading radius gradually increased.In the absence of microtexture introduction,the wetting behavior was controlled by the reaction behavior of breaking the oxide film in the early stage of spreading and by the atomic adsorption behavior in the later stage.The introduction of microtexture increased the specific surface area and the dissolution amount of the material,intensifing the interfacial reaction,thus changing the controlling factor of the late stable spreading from atomic adsorption behavior to diffusion behavior.The spreading behavior under laser irradiation was determined by the synergistic effects of interfacial reactions,surface energy and thermodynamic driving forces.Microtexture provided capillary driving forces in the parallel trench direction to promote spreading and generated energy barriers in the vertical direction to inhibit spreading,resulting in anisotropic spreading on microtextured substrate.The anisotropy became more pronounced as the groove spacing decreased.The aluminum/steel laser welding process was optimized by response surface methodology with laser power of 2836 W,welding speed of 0.46 m/min and wire feeding speed of 4.75 m/min.With the optimized parameters,the introduction of microtexture promoted the wetting behavior of the filler material on the microtextured surface.The dense groove arrangement（microtexture spacing of 0.3 mm）and the angled groove design toward the rear of the weld（microtexture angle of 135°）facilitated further spreading performance.The typical joint morphology of welded-brazed joints consists ofτ5-Fe2Al8Si andθ-Fe（Al,Si）3.The microtexture can induce additional nano-sizedη-Fe2Al5 phase generation at the interface betweenθ-Fe（Al,Si）3and the steel side.In addition,it can improve the local plastic deformation capacity to a certain extent,homogenize the interface difference,and change the fracture behavior of the joint from predominantly brittle to tough-brittle composite fracture.The surface microtexture increased the effective bonding area,enhanced the local plastic deformation capacity of the joint,and transformed the joint fracture mode of the joint from predominantly brittle to tough-brittle composite fracture,thus realizing the improvement of the joint performance.After microtexturing,the maximum linear load of joints can reach 495.9 N/mm,which was nearly 61%higher than that before microtexture introduction. |
