Structural Design And Microstructures/Properties Of High-speed Friction Stir Welded Joints Of Aluminum Hollow Extrusions

Aluminum hollow extrusions have been extensively used in railway vehicle manufacturing industry,due to their high rigidity,favorable strength/weigh ratio,good flatness and excellent corrosion resistance.Friction stir welding(FSW),as a solid-state joining technique,can eliminate the welding defects caused by the conventional fusion methods,and can be used to join aluminum hollow extrusions.Researches on friction stir welding of aluminum hollow extrusions are carried out in recent years,and there are two main problems: the structural design of FSW joint lacks theoretical basis,and it’s urgent to improve the welding efficiency.To solve the problems,a sequentially coupled thermo-mechanical model was established in the present thesis,and the structure of high-speed FSW(HFSW)joint was designed considering the bearing capacity and weight of aluminum hollow extrusions.A large-shoulder FSW tool with the Archimedes spiral groove owning varying depth was developed,and HFSW of aluminum hollow extrusions was successfully realized.Finally,the evolutions of microstructures and mechanical properties of HFSW joints were revealed.A modified analytical model for heat generation in FSW was established,and the effect of thermo-physical parameters on thermal simulation was studied.Compared with the conventional analytical model,the volume for heat generation was divided into several layers in the modified analytical model,and different yield strength was used in each layer to calculate the non-uniform volume heat flux.Meanwhile,the relationship between the heat-producing area of shoulder and the plunge rate of tool was given to remove the sudden change of temperature.By comparing the simulated thermal cycles in two models with the measured ones,it can be found that the modified thermal model owned a higher simulating accuracy.Additionally,each parameter was taken as room temperature(20℃)value,middle-temperature(300℃)value,high-temperature(600℃)value,and the the value as functions of temperature in the thermal simulation,and the thermal cycles obtained using contant value were compared with that using the function.The results showed that the yield strength had significant influence on thermal simulation,and the other parameters including the specific heat,thermal conductivity and mass density had little influence on thermal simulation.The optimized welding structure and joint type for friction stir welded aluminum hollow extrusions were determined.A sequentially coupled thermo-mechanical model was built to investigate the stress distribution,and the results showed that the “most dangerous” location was located at the arc transition region on the AS of supporting rib at the plunge stage.Increasing the width of supporting rib SW and the arc radius AR could decrease the welding stress significantly,and the structure was chosen as the optimized one when SW =6mm and AR =5mm.Then three different-pin-length tools were used to weld aluminum plates in order to design three kinds of joints,namely,lack-of-penetration butt(LPB)joint,complete-penetration butt(CPB)joint and butt-lap(BL)joint.Root flaw defect was formed at the faying surface of the LPB joint,and Hook defects can be observed both on the retreating side(RS)and advancing side(AS)in the BL joint.The welding defects had negative influence on the tensile properties of joints.The CPB joint had no welding defects and exhibited the highest tensile strength,which made it the best joint type.HFSW of aluminum hollow extrusions was successfully realized,and the microstructure evolutions in different zones of HFSW joints for aluminum hollow extrusions were clarified.The shoulder diameter of the tool was five times the thickness of workpiece,and an Archimedes scrolled groove owning varying depth was made on the flat shoulder.Two right-hand tapered threads with different pitches were machined on the pin side.The aluminum hollow extrusions could be welded at high welding speed using the self-designed large-shoulder tool.Based on the microstructure observation and thermal simulation,the grain structure in the heat-affected zone(HAZ)of HFSW joint was similar to that in the base material(BM),but some grain coarsening occurred.The high welding speed decreased the dwelling time at high temperature,and inhibited the coarsening and dissolution of β’’ precipitates.The predominant precipitates in the HAZ were high-density β′′ precipitates and low-density β′ precipitates.TMAZ was characterized by elongated grains with the highest density of dislocations,β′′ precipitates completely dissolved into the matrix,and only low-density β′ precipitates was seen in grains.Continuous dynamic recrystallization(CDRX)occurred in the NZ under the interaction of intense plastic deformation and high welding heat input,and the zone was composed of fine and equiaxed grains,in which high density of dislocations were found.All the β′′ precipitates and most of β′ precipitates dissolved into the matrix,and very few β′ precipitates are transformed into equilibrium phase β.With increasing the welding heat input,the dissolution degree of β′′ precipitates in the HAZ increased obviously,and the amount of β precipitates in the NZ increased slightly.The internal relationship between HFSW joint properties and microstructures was revealed,and the effect of welding defects on HFSW joint properties was studied.Moreover,the optimization of welding parameters used in high-speed FSW was conducted.The tensile properties of joints without defects were dependent on the microhardness.The microhardness evolution in the NZ was governed by the fine-grain and dislocation strengthening,the microhardness evolution in the TMAZ depended on the dislocation and precipitation strengthening,and the microhardness evolution in the HAZ was controlled by the precipitation and solution strengthening.With the increase of rotation speed,axial force and welding speed,the changing trend of one strengthening mechanism was contrary to the other in each zone,and the microhardness in different zones increased monotonously,which resulted in the increase of joint properties.When adopting the improper welding parameters,the welding defects including root flaws and voids were likely to occur in the lower part of the weld.The root flaws can be divided into kissing bond and no bond.The kissing bond had no influence on tensile and fatigue properties of joints.The no bond with the size smaller than 0.1mm had no adverse impact on the tensile properties,but it decreased the fatigue properties of joints significantly.The voids with the size smaller than 0.3mm had no influence on the tensile properties,while they had significant adverse impact on the fatigue properties.The mathematical relationship between the tensile strength and the welding parameters was built by using the response surface methodology,and the tensile strength of joint reached 219MPa(75% of the BM)using the optimized values of rotation speed(2200r/min),welding speed(2000 mm/min)and axial force(19kN).Meanwhile,the joint showed excellent fatigue properties,and the fatigue characteristic values at a 95% survival probability was 125.5MPa.The optimized structure of FSW joint had been used as floors of railway vehicle,and the self-designed tool and the optimized welding parameters had been applied to join the floors.