Interfacial Behavior And Mechanical Properties Analysis Of Al/steel Joint By Riveting Assisted Friction Stir Welding

Fuel delivery pipelines in rocket engines are dissimilar aluminum/steel composite structure,which require high load capacity and good sealing.The products are produced by brazing currently.But intermetallic compounds(IMCs)at the interface of brazing joints are uncontrolled and the products quality is unstable so that the yield is less than 50%.There is an urgent need to develop new technologies to achieve high quality Al/steel joints.Besides,Al/steel composite structure is an important way to realize the weight saving of cars.The joint of aluminum and steel is also an urgent need for auto industry.In the recent work,riveting asisted friction stir welding between 3-mm-thick 6082-T4 aluminum alloy and 2-mm-thick QSTE340 TM steel was studied ? The material flow behaviour during self-riveting friction stir welding(SRFSW)was presented.The formation mechanism of surface appearance was explained.The joining mechanism of extrinsic-riveting friction stir welding(ERFSW)was clarified.Self-riveting friction stir welding between aluminum and steel was conducted.The microstructural characteristics of different regions in the Al/Fe joints,material flow and mechanical properties were investigated.It was shown that the joint was composed of nugget zone(WNZ),thermo-mechanical affected zone(TMAZ),heat affected zone(HAZ),base material(BM)and self-riveting zone(SRZ).The SRZ was confirmed as the mixture of WNZ and TMAZ in nature in terms of microstruture,material flow and microhardness distribution.The process of plastic aluminum alloys filling into prefabricated holes includes four stages.The process of filling each hole is repeating from the second stage to the fourth stage.The joints fracture at Al/steel interface and aluminum rivets,corresponding to two steep drops on the tensile curve respectively.The strength of aluminum rivets is still weak,and it can be further improved by improving the bearing performance of Al/steel joint further.Extrinsic-riveting friction stir welding between aluminum and steel was conducted.The effect of pre-fabricated holes arrangement on microstructure and mechanical properties was investigated.When the hole is half-hole,intermittentx arranged in the shape of sawtooth,the problem of scarcity of SiC particles above the riveting area is solved,and the bearing area of the steel side is taken into account simultaneously.At rotation speed of 1000 rpm and welding speed of 200mm/min,maximum load of 12.55 kN was achieved.The maximum load per unit weld length was 608N/mm.Fracture occured at the boundary between SiC mixed zone and TMAZ in Al side because elastic modulus of SiC mixed zone near the boundary was 52% higher than aluminum and incompatible deformation occured and made cracks initiate easily at the boundary during tensile tests.Al/steel interface of extrinsic-riveting friction stir welded joint was characterized by laminated composites,Fe4Al13-SiC double-phase dispersed composites and IMCs.Laminated composites were the result of the continuous piling of plastic aluminum and steel shear layers which were driven by pin to fill into the cavity lefted by the traveling pin.The laminated structure can be divided into curved and flat types according to the shape.The curved laminated structure was mainly composed of Al-rich IMCs,while the flat type is mainly composed of Fe-rich IMCs.The Fe4Al13-SiC double-phase dispersion composite structure was formed by the Fe4Al13 layer fragmenting into small particles and mixed with SiC particles under the stirring action of the pin.The formation of IMCs at Al/steel interface was the result of atom diffusion.And the diffusion rate of Fe atoms in solid Al was higher than that of Al atoms in solid Fe under the same conditions because the diffusion distance of Fe atom was higher than that of Al atom.The joining mechanism of aluminum/steel joints by external riveting friction stir welding was analyzed.The strengthening mechanism of SiC particles on rivet structure and the effect of actual plunge depth on IMCs were investigeted.The joining mechanism includes macro-mechanical bonding,micro-mechanical bonding and metallurgical bonding.Macro-mechanical bonding was refered to the SiC particle reinforced rivet structure and the strengthening mechanism of SiC paticles includes dislocation strengthening,dispersion strengthening and fine grain strengthening.Micromechanical bonding structures include hook structure,steel surface wave,micro-pits,and SiC particles embedded in steel.The formation of IMCs is the result of thermal diffusion and deformation induced diffusion.The different actual plunge depths of the pin at different locations in the same weld led to the difference in local temperature and metal deformation,which led to the variation of the shape,type and thickness of IMCs.