Al Alloy/CRFP Friction Stir Welding Processes And Joining Mechanisms Based On Interface Mechanical-chemical Treatments

Automotive lightweight is an important solution to reduce emissions,save energy,and improve vehicle safety and performance,which involves three research directions:the development of light-weight and high-strength materials,the innovation of processing technology,the optimization of component dimension and structure.The hybrid joining of Al alloy and carbon fiber reinforced polymer（CFRP）is considered as a critical issue in the field of automobile lightweight.In this work,the friction stir welding（FSW）process of Al alloy and CFRP was investigated,and the coupling research system of interfacial microstructures-mechanical properties-joining mechanisms was established.The effects of welding tools and joint structures on joint forming were investigated.The technical programme of using welding tool with short stirring pin,the top Al alloy plate/bottom CFRP plate overlapping assisted with interface strengthening process was proposed.The laser ablation process was used to introduce geometric grooves at the joined interface.Combined with the multi-scale finite element（FE）modeling method,the dimensions,morphologies,and formation mechanisms of the laser ablated structures were analyzed.The effects of friction stir lap welding（FSLW）parameters on the joint forming and temperature field were quantified.The dynamic filling process of geometric grooves and the residual stress distribution in the cooling process at the micro-scale were explored.The dynamic evolution process of surface micro-nano structures under hot water treatment（HWT）and silane coupling treatment（SCT）was analyzed,and the coupling of laser ablation and chemical treatments was further proposed,revealing the multi-level interface strengthening mechanism including macro/micro/nano mechanical interlocking and C-O-Al new chemical bond.The welding characteristics of Al alloy and CFRP were analyzed.The main difficulty of welding between Al alloy and CFRP was the huge differences of the material properties,such as thermal expansion coefficient,thermal conductivity,stiffness,and melting temperature.Compared with Al alloy,CFRP possessed poor weldability and strong anisotropy.In the selection and optimization of welding process and interface strengthening process,the thermal damage and thinning of CFRP should be considered emphatically.For Al alloy and CFRP with the thickness of 2 mm,the effectiveness of various welding tools and joint structures was studied.The typical welding defects and formation mechanisms were analyzed.Considering the forming quality,bearing capacity,and interface strengthening potential,the FSLW of Al alloy and CFRP was employed by the welding tool with a short stirring pin with a length of 0.5 mm.The nanosecond laser ablation process on the Al alloy surface was carried out,and a three-dimensional（3D）FE model with a dynamic laser heat source was established.The effects of laser ablation parameters on the surface and cross-section morphologies were studied,and the process characteristics of laser ablation were analyzed.The laser energy density was proportional to the laser power.As the laser power increased from 10 W to 25W,the groove width increased from 4.2μm to 18.2μm,the groove depth increased from28.1μm to 130.9μm,and the increase of depth was more obvious.The dynamic change of the laser absorption coefficient of the Al alloy was influenced by scanning speed,so the effect of scanning speed on the groove depth and width was not obvious.The melting depth threshold in the FE simulation results was defined as the groove depth,and the evaporation width threshold was defined as the groove width.The simulation results were in good agreement with the experimental results,and the visualization of nanosecond laser ablation along with the prediction and precise control of groove geometric characteristics were realized.The FSLW of Al alloy and CFRP assisted by laser ablation process was investigated.The effects of laser ablation parameters on the interfacial microstructures and mechanical properties of Al alloy/CFRP joints were studied.The multi-scale strengthening mechanism of the joints was clarified from the perspective of mechanical interlocking and chemical bonding.Under the pressure of the welding tool,the macro-scale mechanical interlocking was provided by the slightly concave joining interface.The micro-scale mechanical interlocking was provided by the stable anchorage structures formed by the filling process of melted CFRP into the grooves.The nano-scale mechanical interlocking was provided by the micro-nano splash particles generated during laser ablation.The chemical bonding of the interface was enhanced by C-O-Al chemical bond which was formed from the reaction between Al₂O₃ and Polyamide66（PA66）.A multi-scale FE model of Al alloy/CFRP joints was established,including macro FSLW process,micro groove filling process and residual stress analysis.The effects of FSLW parameters on the interface forming were studied.The welding temperature field and joint deformation were analyzed,and the characteristic parameters including peak temperature,high temperature dwelling time,and deformation were quantified.The dynamic filling process of grooves was studied,and the relationship among interface temperature,filling process,and reaction time was quantitatively analyzed.Based on the parameter optimization process,three typical joint optimization indexes were proposed,including interface peak temperature（320-360°C）,average reaction time（3.5-4.5s）and CFRP thinning rate（<6.5%）.For the FSLW of Al alloy and CFRP plates with the thickness of 2mm,the recommended process parameters were welding speed of 200-300 mm/s,rotation speed of about 1000 r/min,and plunge depth of 0.2 mm.The residual stress distributions during the cooling process were studied.The maximum residual stress was located at the edge groove,which was 119.7 MPa.The anchoring effect of the grooves resulted in the non-uniform distribution of the residual stress at the joined interface,and an arc-shaped stress concentration zone was formed at the interface.FSLW of Al alloy and CFRP assisted by chemical and coupling treatments was investigated.The dynamic evolutions of surface morphologies and formation mechanism of micro-nano structures fabricated by HWT and SCT were studied.The coupling process of laser ablation and chemical treatment comprehensively utilized the advantages of interface geometry and chemical bonding,improving the chemical bonding ability of the unablated area while ensuring the mechanical interlocking effect of laser ablated grooves.The results showed that the mechanical properties of Al alloy/CFRP FSLW joints assisted by SCT and laser ablation were optimal,the tensile-shear strength and cross-tension strength were 32.6MPa and 3.2 MPa,respectively.