Research On Process Of Refill Friction Stir Spot Welding For Aluminum Alloy And SCF-PPS Composite

As the requirements of lightweight manufacturing in aerospace,rail transit,automobile manufacturing and other industries increasing,a composite structure of lightweight material CFRP and aluminum alloy is proposed.However,due to the large physical and chemical differences between CFRP and aluminum alloy,especially with different thermal expansion rates,the connection and assembly of this composite structure are very difficult.And there are many limitations in conventional process for joining CFRP to metals.Refill friction stir spot welding is considered to be a promising alternative technology for joining aluminum alloy and CFRP due to its simple connection process,low temperature and short production cycle.In this paper,3mm thick short carbon fiber polyphenylene sulfide(SCF-PPS)and 2 mm thick AA6061-T6 aluminum alloy were used in this work,and the surface of the aluminum alloy was pretreated with phosphoric acid anodizing as the way to improve the joint strength.The connection process of AA6061-T6 / SCF-PPS refill friction stir spot welding was revealed.And the influence of process parameters on the joint appearance,the macro mechanical interlock of the interface,the microstructure,and the mechanical properties was analyzed.The interface connection mechanism was provided for theoretically guidance on obtaining high-quality metal-CFRP composite structural joints.During the connection process,due to thermal melting characteristics of SCF-PPS,especially the low thermal conductivity,the heat is concentrated near the interface.In order to prevent defects such as penetration of PPS to the joint surface and serious warpage of the joint,the welding tool is inserted into the AA6061-T6 aluminum alloy plate inside without touching the interface.The maximum depression is no more than0.8 mm.The single variable control was used to study the evolution of the joint macro-forming and interfacial structure of the rotating speed(600 rpm-1800 rpm),the plunge depth(0.2-0.8 mm)and the welding speed(40-100 mm / min).The study found that under all process parameters,no obvious defects were found in the joint surface.With the increase of the rotation speed,the plunge depth and the reduction of the welding speed,the backfilling of the joint surface droped,and the amount of resin extrusion of the joint increased.At 600 rpm,the joint surface was rough.AA6061-T6 aluminum alloy has different thermal effects on different parts.It can be divided into stirring zone,heat mechanical affected zone,heat affected zone and base metal according to the grain status.The joint interface can be divided into anchor zone,transition zone and fringe zone.With the increase of the rotation speed,the plunge depth and the decrease of the welding speed,due to the enhancement of heat input and stirring effect,the plastic deformation ability of the aluminum alloy is enhanced,and the macroscopic interlocking efficiency of the interface can reach more than 10%.PPS is melted under heat input,and its fluidity is enhanced.It can fill the gaps on the surface of AA6061-T6 aluminum alloy to form a micro-mechanical interlock.The melted layer is sensitive to heat input.Under the process conditions of 1000 rpm,the melted layer can be observed at the interface.The melted layer is mainly concentrate on the edge or transition zone of the joint.The hardness distribution of AA6061-T6 aluminum alloy plate is "W" type.The hardness value is affected by the dynamic recrystallization of crystal grains,the dissolution of the precipitation phase,and the competition mechanism of grain recovery.The hardness of the SCF-PPS crystallized near the interface is higher than that of the base material and can reach more than 20 HV.The hardness of the SCF-PPS below the anchoring zone is reduced by melting,and the welding parameters have no obvious influence on the hardness of the interface SCF-PPS.The mechanical properties of the joint are optimal at the speed of 1000 rpm,the plunge depth of 0.4 mm,and the welding speed of 80 mm / min.The hardness of the aluminum anchor is 78.92 HV,and the tensile load of the joint can reach 2405 N.Observing the fracture morphology of the joint,it is found that the PPS of the joint adhesion area is neatly arranged in a nest shape,which is a brittle fracture.The carbon fiber lurking under the surface of the PPS can be seen in the plastic deformation area and the PPS undergoes some plastic deformation.The pores caused by thermal shrinkage in the joint transition zone expand to the plastic deformation zone.AA6061-T6 aluminum alloy surface after 10% phosphoric acid electrolyte was energized at 30 V for 10 min to generate a dense oxide film,fine and uniform pores can be observed under the electron microscope,elemental analysis found that the oxygen content of the aluminum alloy surface increased 11.23 wt%.After the surface treatment,the mechanical properties of the joints have been significantly improved,and the tensile load can reach more than 2700 N.Moreover,under the optimal welding process parameters,the joint fracture mode changed from interface failure to fracture on the SCF-PPS side.Through the observation of fracture morphology,it was found that a large number of fibers participated in bearing the load,and fiber pulling and breaking behaviors occurred.The connection mechanism of the interface is the combined effect of mechanical interlocking-adhesion-chemical combination.When the volume of the deformed aluminum alloy is near 4.7 mm3,the macro mechanical interlocking effect of the joint is good,which helps to improve the tensile strength of the joint.The adhesion force is mainly due to the wetting of the aluminum alloy by molten PPS and the interaction between the electron cloud of the arc pair and the free electrons of the metal.The chemical bonding at the interface is weak,and the diffusion of Al,C,Mg,O elements may occur.