Microstructure And Mechanical Properties Of Friction Stir Lap Joints Of Metal And Plastic-based Materials

Polyether ether ketone(PEEK)and carbon fiber reinforced polyether ether ketone(CF-PEEK)are widely used in aerospace,transportation,automobile and other fields due to its excellent corrosion resistance,abrasion resistance,excellent flame retardance,and high specific strength.Aluminum alloys are also widely applied in modern industry because of the advantages such as high specific strength,good corrosion resistance,and good workability.In the process manufacturing,there is an increasing demand for the joining of metals and CFRPs because the dissimilar joint will combine the advantages of both metal and carbon fiber reinforced resin matrix polymer(CFRP)and achieve lightweight components.However,due to the huge differences in physical and chemical properties,it is difficult to achieve high quality joining,which limits its industrial application.In this paper,5052 aluminum alloy,PEEK and CF-PEEK were taken as the as-received materials.The friction stir lap welding(FSLW)and its variants were employed to join aluminum alloy to PEEK-based resin materials,and the effect of different welding processes and pretreatment methods on microstructures and mechanical properties of the dissimilar joints was investigated,and the welding mechanism and joint failure mechanism were also revealed.Based on the study above,it will provide a theoretical reference to extend the industrial application of metal/resin-based dissimilar joints.In order to meet the actual engineering requirements of improving the performance of dissimilar welded joints without completely destroying the metal structure,a form of lap joint of resin-based material on top and metal on bottom was designed to join 5052 aluminum alloy with PEEK by friction stir welding(FSW).Due to the poor thermal conductivity of the upper PEEK,incompatibility between PEEK and metal,and intense material mixing of metal and resin caused by high rotation rate of the tool shoulder and stir pin,a well-formation joint could not be achieved via FSW.Moreover,there were large size holes in the joint,and the tensile shear force(TSF)of the joint was only 0.86 k N,which reduced the bearing capacity of joint.In order to obtain well-formation joints with a high strength,a new welding tool with separate shoulders and stirring pins was designed,where the shoulders and pins had different rotation velocity.The welding method with this new designed tool was called dual-rotation friction stir welding(DRFSW).A well-formation,high-strength joint with a TSF of 1.13 k N was obtained via DRFSW.The high velocity rotating stirring pin provided the heat required for interfacial joining and the low velocity rotating shoulder facilitated the good formation for the joint,resulting in controlled forming and controlled mechanical properties.Since the joint strength obtained from the lapping form of the resin-based material on the top still needs to be improved,friction lap joining(FLJ)was applied to join 5052 aluminum alloy to PEEK with the lapping form of metal on top and resin-based material on the bottom.High-strength dissimilar joints were achieved with an average maximum TSF of 1.76 k N,95% of the as-received PEEK strength,at a rotation rate of 1000 rpm,a welding speed of 800 mm/min,and a plunge depth of 1.0 mm.The main reason for the formation of bubbles was that the interface temperature exceeded the PEEK thermal decomposition temperature,resulting in the thermal decomposition of the resin.When the rotation rate increased,the heat input increased while the axial pressure decreased,resulting in an increase in the interface bubble area.In addition,the interface joining was achieved mainly by chemical bonding(C-O-Al)assisting by mechanical interlocking.Friction stir spot welding(FSSW)of 5052 aluminum alloy and CF-PEEK was utilized with a lap with the metal on top and the resin-based material on the bottom.The maximum tensile shear force(TSF)of 2.68 k N was obtained at a rotation rate of 1400 rpm,a plunge depth of 1.6 mm,and a dwell time of 8 s.By pre-fabricated hole and presetting the interlayer near 5052 aluminum alloy lapped zone,the bubbles defects in the shoulder-affected zone(SAZ)during the directly welding were transferred to the pre-fabricated hole.The melted solidified PEEK was also deposited in the pre-fabricated hole,forming a macroscopic mechanical interlocking,which increased the TSF to 2.92 k N,with an increase rate of 9%.By the surface treatment of silane coupling on the 5052 aluminum alloy,the joint TSF increased to 3.6 k N,with a growth rate of about 34%,which was attributed to the interfacial chemical bonding introduced by the silane coupling treatment.In the welding process,the melting of CF-PEEK with high fiber content resulted in the reduction of resin matrix,which resulted in insufficient molten resin.As a result,voids and bubble defects formed at the joint interface,which is not beneficial for dissimilar joining.By presetting 0.2 mm PEEK interlayer to supple the resin matrix,combined with the silane coupling treatment,the TSF can be improved to5.15 k N by increasing the resin content at the interface and improving the chemical bonding,with a growth rate of about 92% compared to the direct welding.