Friction Stir Spot Welding Of 5052 Aluminum Alloy/Carbon Fiber Reinforced Polyether Ether Ketone Composite Structure

The hybrid structure of aluminum alloy and carbon fiber reinforced resin matrix polymer(CFRP)have a promising prospect for the lightweight of automobile.However,due to the different thermal-mechanical properties,it is a challenge to fabricate high-quality hybrid joints by conventional welding technology.As a solid-state welding technology,friction stir spot welding is a technology suitable for manufacturing aluminum alloy/CFRP hybrid joints with high-quality and high-efficiency.Nowadays,there are a few researches on the welding of aluminum alloy/CFRP,the bonding mechanism and fracture mechanism are also unclear.Therefore,the systematic research on the interfacial bonding,failure mechanism and mechanical properties of aluminum alloy/CFRP has an important significance for industrial application.In this thesis,5052 aluminum alloy(AA5052)and carbon fiber reinforced polyether ether ketone(CF-PEEK)were used as base materials,and friction stir spot welding was used to achieve high-quality of AA5052/CF-PEEK hybrid joints.The interfacial interaction and mechanical properties of AA5052/CF-PEEK joints were systematic studied under the effect of welding process(plunge depth and rotational speed)and pretreatment process(anodizing,sandblasting and sandblasting + anodizing).Electron probe microanalysis(EPMA)and field emission scanning electron microscopy(FE-SEM)were used to analyze the cross-sectional morphology,fracture surface and fracture path of joints.The main results could be concluded as follows:When plunge depth was 1.9 mm and rotational speed was 1600 r/min,the tensile shear load of AA5052/CF-PEEK hybrid joints reached the maximum value of 2740 N.Under the same welding parameters,anodizing pretreatment AA5052/CF-PEEK hybrid joints were reduced to 2355 N due to the lower roughness.The joints with sandblasting pretreatment could get a highest strength,and the maximum tensile shear force reached 3187 N.Due to the cracking of the oxide layer,the average mechanical properties of the sandblasting + anodizing pretreatment AA5052/CF-PEEK joints reduced to 2798 N.Meanwhile,the mixed adhesion failure and cohesive failure was the fracture modes of joints.The interface of AA5052/CF-PEEK hybrid joints could be divided into pin-affected zone,shoulder-affected zone,adhesive zone and resin concentrated zone.Pin-affected zone and shoulder-affected zone were strong bonded interfaces with Al-C-O reaction layer.Adhesive zone and resin concentrated zone were weak bonded interfaces under the influence of adhesion force.With the sandblasting and sandblasting + anodizing pretreatment,the defects of interface separation between adhesive zone and resin concentrated zone could be avoided,but the delamination defect in resin concentrated zone cannot be eliminated.AA5052/CF-PEEK hybrid joints could reach a higher mechanical strength,when the crack propagates between the flowing resin and CF-PEEK.However,when the crack expanded between the flowing resin and AA5052,the mechanical properties of hybrid joints were poor.Due to the rotational welding tool,in pin-affected zone and shoulder-affected zone,the fracture surface on aluminum alloy side transformed into a semi-flat and semi-fold structure.The molten resin entered and solidified in the long fold-like structure on the aluminum alloy,which was the essence of micro-mechanical interlocking.Due to the anchoring effect,the area concentrated micro-interlock gradually transformed into anchor point,which was difficult to deform under external load,and the edge of the anchor point transformed into tearing ridge.