Microstructure And Mechanical Properties Of Friction Stir Welded/processed Advanced Ultra-high Strength Steels

Advanced high strength steels are widely used in automobile manufacturing and other fields due to their high strength,considerable toughness and good plasticity.However,advanced ultrahigh strength steels(AUHSSs)with strengths above 1 GPa usually contain relatively high contents of metastable microstructures which can be easily degenerated under heat effect.This leads to great challenges for the high-quality welding of AUHSSs.Severe coarsening and softening often occur in AUHSSs during fusion welding processes,which dramatically reduces the mechanical properties of welded joints and impedes the industrial application of AUHSSs.Friction stir welding(FSW),a solid-phase welding technique based on severe plastic deformation,can improve the microstructure and properties of joints by controlling the plastic deformation,peak temperature and cooling rate during welding.Hence,it is expected to achieve high-quality joints of AUHSSs by FSW.However,there have been limited research on the FSW of AUHSS as of now,resulting in lack of in-depth understanding of the weldability of AUHSS.In this study,two typical AUHSSs—ferrite/martensite dual phase(DP)steel(DP 1180)and quenching and partitioning(Q&P)steel(Q&P 1180)were selected as the target materials,whose evolution of microstructure and mechanical properties during FSW were systematically investigated and the joint’s performance was improved by additional cooling.On this basis,low-carbon steels with ultrafine grains,ultrahigh strength and excellent plasticity were successfully fabricated using friction stir processing(FSP)technique by innovatively adopting a co-design strategy of both macro-and microstructures.The tensile deformation behaviors of the FSP steels were studied in depth.The main conclusions are as follows:Firstly,the weldability of the two steels of DP 1180 and Q&P 1180 was systematically investigated via FSW.Friction stir lap welding of DP 1180 steel was successfully carried out by using three welding tools with different pin lengths.The joint consisted of stir zone(SZ),heat affected zone(HAZ)and parent metal(PM).The HAZ could be subdivided into fine grain(FG)HAZ,inter-critical(IC)HAZ and subcritical(SC)HAZ.A typical self-tempering structure of single-phase martensite with a large number of fine carbide particles was produced in the SZ.Due to the self-tempering effect,no obvious hardening was found in the SZ compared to the PM.The mechanical properties of the lap joints were determined by welding mode,pin length,and joint defects at the lap interface.The lap joint reached a maximum tensile shear strength when the welding form of retreating side loaded in the upper sheet was adopted and the stir pin was inserted into the lower sheet with a depth of 0.4 mm.DP 1180 steel was successfully friction stir spot welded by adopting a concave shoulder tool with a zerolength pin.No hook defect or obvious keyhole was observed in the spot joints.By using additional water cooling,the joint’s morphology was improved,the softening of the HAZ was suppressed,and the tensile shear strength of the joint was enhanced to a maximum value of 17.9 kN.Equal strength joints to the PM were successfully fabricated during the butt FSW of ultrahigh strength Q&P 1180 steel.The simulation results of the temperature field showed that the peak temperature of the SZ was over Ac3.The cooling rates of the SZ and HAZ were far above the minimum cooling rate for fully martensitic transformation,which gave rise to the formation of fresh martensite in the SZ,FG-HAZ and IC-HAZ.Metastable phases such as retained austenite and martensite decomposed in the SCHAZ accompanied by precipitation of nano-carbide particles.No obvious difference in the microstructural characteristic and minimum hardness was found in the HAZs of joints under different tool rotation rates.This phenomenon was in accord with the heat source zone-isothermal phase transition layer model.Compared to that of the PM,the strength of the joint was not decreased palpably and impregnable with the increase of rotation rate,which was due to the improved work hardening ability of the softened zone profiting from the generation of ultrafine DP structure,nano-carbide particles and ultrafine ferrite.Although the weldability of Q&P 1180 steel was better than that of DP 1180 steel,a significant drop in the strength of the Q&P 1180 joint was observed when the welding method with high linear energy was used.However,it was easy to achieve equal strength joint to the PM by using welding approaches with relatively low linear energies such as FSW and laser welding.Secondly,delaminated ultrafine DP steel as well as ultrafine multiphase steel with low alloy content were successfully prepared by using FSP.In the delaminated DP steel,the lamellar structure exhibited three-dimensional distribution characteristics.A difference of martensite fraction of～30%was found in the adjacent two lamel las.Thus alternately distributed soft zone with a lower fraction of martensite and hard zone with a higher fraction of martensite were generated.Dislocations were the main substructures of martensite in the two regions,while lath block or bundle was hard to be observed in martensite.Ultrafine equiaxial grains with a grain size of～0.7 μm were formed in both the soft zone and hard zone.The work hardening rate of the delaminated DP steel obtained by FSP was significantly higher than that of the PM of low carbon steel and DP 1180 steel,which enabled an ultrahigh ultimate strength as high as 1.7 GPa and a uniform elongation of 8%.The enhancement of strength and plasticity of the delaminated DP steel was correlated with the back stress strengthening and work hardening caused by the inhomogeneous deformation of the DP structure and the delaminated structure.After heat treatment at 720℃ for 10 min,the delaminated ultrafine DP structure was transformed into an ultrafine multiphase structure composed of ferrite,martensite,retained austenite and nano-carbides.Due to the transformationinduced plasticity effect and a high proportion of ferrite,substantial plasticity improvement was achieved in the ultrafine multiphase steel whose elongation was 39.7%and strength was maintained at an ultrahigh level of～1 GPa.Finally,in view of the apparent distinctions in the tensile deformation and fracture behaviors of ultrafine-grained steel materials with various phase compositions,the comparative study in this paper shows that plastic strain was hard to develop in conventional ultrafine-grained pure iron due to the lack of dislocation source.However,the plastic deformation ability of ultrafine ferrite could be enhanced by introducing dislocation sources via the design strategy of DP or multiphase structure based on FSP.The ultrafine DP and multiphase steels prepared by FSP owned considerable toughness and fracture caused by the aggregation of microvoids occurred during their failure processes.However,the ultrafine-grained pure iron showed inferior toughness and brittle fracture morphology was observed on the fracture surface.