Diffusion Behaviors Induced by Severe Deformation in Friction Stir Welding on Al/Steel and Joint Mechanical Properties

Author:Wan Long

Supervisor:huang yong xian


Degree Year:2019





The Al/steel hybrid structures have huge demand in the automotive,ship-building and aerospace industries.The present paper is focused on friction stir welding(FSW)of dissimilar 6082-T6 aluminum alloys and QSTE340TM steel,an area that is getting great concern recently.However,the diverse physical properties and thermal expansion make them difficult to join together.To reduce the tool wear and realize the control of intermetallic compounds(IMCs)is the key point.The novel severe deformation driven FSW technique was developed to solve these problems and the new FSW tool was designed.The fundamental research was carried out by the combination of experimental study,theoretical analysis and mathematical model establishment.The present study focused on the explanation of element fast diffusion mechanism,the control of the IMCs,the establishment of the relationship between joint microstruture and properties,and joint reliability evaluation.The novel FSW tool contained a rotating shoulder made of W6 high-speed steel and a rotating pin fabricated of WC-Co alloy steel.This kind of design method improved the machinability and adaptability of welding tools.The enlarged pin head with circumferential notches was beneficial to accelerating flow behavior of the softening materials,which further increased the effective interfacial width.The influence of novel tool on temperature distributions,effective strain rates and the velocity of material flow was revealed by means of numerical simulation experiment.The width of high temperature region above 723 K was 4.8 mm and 4.7 mm at the advancing side and retreating side,respectively.The width of high temperature region of the novel Al/steel FSW joint was wider than those of the conventional cone shape pin and cylinder shape pin tool.The higher effective strain rate of greater than 1000s-1 on the circumferential notches shape pin,compared to that of about 180 s-1 on the conventional pin,produced more strain energy which provided potential activation energy for the interface diffusion and reaction.The area of the turbulent flow region on the circumferential notches shape pin increases by 185%,which avoided the occurrence of welding defects and improved mechanical properties of the lap joints.The typical hook defects existed at the advancing side and cold lap defects existed at the retreating side in conventional FSW lap joints due to material displacement were reduced or eliminated in the lap joints by the novel designed tool.The mechanism of material flow and elimination of hook structure at interface was revealed.The maximum shear strength of the 6082-T6 and 2A12-T4 Al alloys lap joints reached 7566 N,which was 85%of base 6082-T6 alloy and the facture location lied at the HAZ instead of the joining interface.The effective interfacial width was increased from about 6.0 mm by the conventional pin to 7.5 mm by the enlarged pin head with circumferential notches.The shear strength of the Al/steel joint by the enlarged pin head with circumferential notches reached 442 N/mm,which had 27%improvement compared with the results in published references.The concave surface on the top of the circumferential notches shape pin could accumulate plasticized materials between the upper Al and lower steel sheets.This further prevented the pin contacting with steel directly,and then avoided the pin wear for the soft/hard configuration.The circumferential notches shape pin was beneficial to eliminating hook and cold lap defects,and increasing bonding area as well as avoiding pin abrasion.Influences of welding parameters on interfacial macrostructure and defects of Al/steel lap joints were studied,and the process optimization window was established.With the increase of welding speed,laminated structure consisted of IMCs and steel fine grain layer disappeared,and interface shape was transformed from the serrated shape to flat shape gradually with the rotation speeds of 700 and 900 r/min.Under the combined action of severe deformation and high temperature thermal cycle produced by novel tools,grain deformation,dislocation rearrangement and recrystallization occurred in steel surface structure.A model of microstructure evolution of steel under plastic deformation was established.The microhardness value changed abruptly at interface due to the existence of IMCs and deformation hardening effect.Nano-hardness reached 9.4GPa in the upper surface layer 10μm away from the interface.The relationship between interfacial structure and shear strength of Al/steel lap FSW joint was studied.When the rotation speed was 700 r/min,the optimum range of welding speed was 50-250 mm/min.With the increase of rotation speed to 900r/min,the optimum range of welding speed was decreased to 50-200 mm/min.When the rotational speed was further increased to 1200 r/min,it was found that the optimum range of welding speed was further narrowed,and the optimum shear strength appeared near the welding speed of 50 mm/min.The average shear loads of the testing samples with various welding speeds and rotation speeds showed an increase tendency and then decreased with increasing welding speed from 30 to 400mm/min.The thickness of IMCs layer decreased from 3.3 to 0.46μm with welding speed increasing from 30 to 300 mm/min.When the welding speed exceeded 300mm/min,the IMCs layer disappeared,and the effective metallurgical bonding was not formed at the interface,which resulted in the decrease of interfacial load-bearing performance.There were four different paths of interface failure modes,namely,direct cracking from IMCs layer,cracking along interface,cracking from aluminium alloy welding nugget zone and alternating intermittent cracking along aluminium alloy welding nugget zone and interface.The strain concentration phenomena of successive occurrence,development and transfer of interface failure showed obvious spacing characteristics in time and space.Response surface methodology was used to optimize the FSW process of Al/steel lap joints,and the mathematical model of welding parameters and failure load was established.The interfacial fast diffusion behavior was characterized.The interface region was mainly composed of particle composite structure,laminated structure and IMCs layer,and some regions had amorphous atomic configurations.When rotation speed and welding speed was 1200 r/min and 30 mm/min,the thickness of IMCs layer was3.3 and 1.1μm under the function of novel designed tool and conventional cylinder tool.The corresponding diffusion coefficient was 0.39 and 0.04μm2/s.The diffusion coefficient was two orders of magnitude greater than Al/steel diffusion welding under873 K.The effective heat of formation model was used to predict first phase forming in Fe-Al system,which predicted Fe4Al133 or Fe2Al5 formed firstly.However,there were still some limitations.The evolution models of particle composite structure and interfacial layer structure were established.The local silicon enrichment suppressed the further growth of the IMCs,which attributed to the ultrathin interficial layer containing a Fe4Al13 layer,a single Fe2Al5 layer,a bright Fe-rich layer and a thin chaotic layer.The Si element is mainly concentrated in the Fe4Al13 and Fe2Al5 IMCs layers,mainly in the Fe4Al13 IMC layer.The content was 8 at.%and the diffusion range was about 100-140 nm.The enrichment of Si effectively inhibited the thickness of IMCs layer by occupying vacancies and blocking diffusion channels.The thickness of IMCs layer at the interface was explained by diffusion dynamics model.The diffusion distance decreased with the increase of welding speed.The local stress and strain induced by FSW process could refine the grain size and produce high concentration dislocations,which provided more paths for the diffusion of Fe elements and nucleation points for the nucleation of Fe4Al13 grains,thus revealing the essence of fast diffusion phenomenon.