A novel seal-flow multi-vortex friction stir lap welding of metal to polymer matrix composites

The friction stir lap welding(FSLW)of metal to polymer is a challenging work due to the unavoidable polymer overflowing.Facing this problem,a novel seal-flow multi-vortex friction stir lap welding(SM-FSLW)technology based on the subversively-designed multi-step pin was put forward.Choosing 7075 aluminum alloy and short glass fiber-reinforced polyether ether ketone(PEEK)as research subjects,the welding temperature,material flow,formation and tensile shear strength of dissimilar materials lap joint under the SM-FSLW were studied and compared with those under traditional FSLW based on the conical pin.The multi-step pin rather than the conical pin effectively hindered the polymer overflowing due to the formation of vortexes by the step,thereby attaining a joint with a smooth surface.Compared with traditional FSLW,the SMFSLW obtained the higher welding temperature,the more violent material flow and the larger area with high flow velocity,thereby producing the macro-mechanical and micro-mechanical interlockings and then heightening the joint loading capacity.The tensile shear strength of lap joint under SM-FSLW was 27.8% higher than that under traditional FSLW.The SM-FSLW technology using the multi-step pin provides an effective way on obtaining a heterogeneous lap joint of metal to polymer with the excellent formation and high strength.

Analysis and Control of Modular Multi-terminal DC Power Flow Controller with Fault Current Limiting Function

In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current(MTDC)grids,this paper proposes a modular multi-terminal DC power flow controller(MM-DCPFC)with fault current limiting function.The topology structure,operation principle,and equivalent circuit of MM-DCPFC are introduced,and such a structure has the advantages of modularity and scalability.The power balance mechanism is studied and a hierarchical power balance control strategy is proposed.The results show that MM-DCPFC can achieve internal power exchange,which avoids the use of external power supply.The fault characteristics of MM-DCPFC are analyzed,fault current limiting and self-protection methods are proposed,and the factors affecting the current limiting capability are studied.The simulation models are established in PLECS,and the simulation results verify the effectiveness of MM-DCPFC in power flow control,fault current limiting,and scalability.In addition,a prototype is developed to validate the function and control method of MM-DCPFC.

A Novel Friction Stir Spot Riveting of Al/Cu Dissimilar Materials

In order to eliminate the disadvantages of the keyhole in conventional friction stir spot welding joint and attain the highstrength lap joint of Al/Cu dissimilar metals,a novel welding technique,named as friction stir spot riveting(FSSR),was proposed.A pinless tool and an extra filling stud were employed.The Al/Cu spot joints without keyhole defect were achieved by the FSSR.A Cu anchor-like structure was formed,which greatly increased the mechanical interlocking between the upper Al sheet and lower Cu sheet.The thin intermetallic compounds containing CuAl2 and CuAl at the Al/Cu interface strengthened the joining interface between the Al sheet and the Cu stud.Increasing rotating velocity increased frictional heat and plastic deformation and then eliminated the interfacial joining defects.The FSSR joint with the maximum tensile shear load of 3.50 kN was achieved at a rotating velocity of 1800 rpm and a dwell time of 20 s,whose fracture path passed through the softened region of upper Al sheet.In summary,the novel FSSR technique has the advantages of strong mechanical interlocking and metallurgical bonding between dissimilar materials,thereby attaining the high-strength spot joint.

Silver particle interlayer with high dislocation density for improving the joining of BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3-δ) electrolyte and AISI 441 interconnect

One of the critical challenges for the protonic ceramic fuel cell stack is sealing electrolytes and interconnects.However,the traditional AgeCuO sealant will aggravate the oxidation along the interconnect surface and result in brittle compound layers at the BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3-δ)(BZCYYb)electrolyte interface.The present work demonstrates that a silver particle interlayer with high dislocation density can be adopted to join BZCYYb electrolyte to the interconnect(AISI 441 stainless)in air instead of traditional AgeCuO sealant.Elevating temperatures result in a sufficient bonding at the Ag/BZCYYb interface,and a defect-free joint is obtained at 950℃.Atomic bonding at Ag/BZCYYb interface is confirmed by TEM.Also,a dense and thin oxide layer(2-3 μm)is formed along the AISI 441 interface.Ag particles in the interlayer provide the main driving force for the sintering joining.The massive dislocations promote the recovery and recrystallization of the Ag interlayer,as well as the interdiffusion of BZCYYb/Ag.After aging in the wet oxidizing atmosphere at 600℃ for 300 h,joints remain intact and dense,indicating superior oxidation resistance and aging performance.Besides,the joint shear strength(25.3 MPa)is 59%higher than that of the joint brazed by traditional AgeCuO.