| The dissimilar metal structure of Ti/Al shows wide application perspective inaeronautical, aerospace、high speed railway and other fields. But it is hard to joiningtitanium to aluminum for its weak weldability. In this paper,3mm thick of TC4titanium and2A14aluminum dissimilar metals were joined by friction stir weldingmethod. The influence of process parameters on weld formation and tensile strengthare investigated, moreover, micro-structure、the forming of the brittle phase and itsimpaction on the joints properties are studied by the scanning electronmicroscope(SEM)、the transmission electron microscope(TEM) and the micro-areaX-ray diffraction. The results show that:The distance between the pin axis biasing aluminum alloy base materials andTi/Al contacted surface (offset, δ) has a significant impact on weld formation and jointtensile strength. As the offset rose the joints conducive to get performance better. It iseasy to form longitude cracks when the offset δ equals to1mm. when δ=2mm、δ=2.5mm, the tool rotation (n) changed from400r/min to700r/min and the weldingspeed (V) between20mm/min and80mm/min, better weld forming can be obtained.When δ=2.5mm, the joint tensile strength increased first and then decreased withincreasing rotation speed and welding speed. And when n=700r/min、V=60mm/min，the joints tensile strength is346.7MPa which is82.5%of the aluminum alloy basematerial.There are three kinds of fracture positions of the joints, fractured at theheat-affected zone (HAZ), the Ti/Al interface and the Ti particles concentrations zone.The fracture mode at the HAZ is micro-void accumulation fracture; the interfacefracture mode is brittle fracture; the fracture mode in the Ti particles concentrationszone is attributed to the holes defects formed by interaction between titanium particles.The layer phenomenon showed in the nugget zone (NZ) within the joint of Ti/Al:the upper is stripy grain-refining area, aluminum alloy dynamic recrystallization regioncomes to the middle, and the lower is titanium particles plasticizing zone. The smallsize "streamlined" liked titanium particles are distributed in advancing side; Inretreating side, the large size "strip" liked titanium particles are distributed along thethickness direction, where holes defects formed. As the offset increasing, the structures in the interface changed from dynamic recrystallization to interphase distribution of theTi/Al; the number of the holes in the NZ decreases, and the structure has shown onionring appearance; Increasing the rotation speed and welding speed, the area ofaluminum alloy recrystallization region narrows, onion ring area increases, titaniumparticles is distributed uniformly in the nugget, and the mechanical properties of jointis excellent; Increasing the rotation speed and welding speed further, the flow area ofplastic titanium alloy decreases, the structure is messy and has no rules, the mechanicalproperties of joint becomes poor.There are two mainly kinds of brittle phases in the joints: TiAl3and TiAl whichare formed in the areas where the plasticity titanium alloy flowed. TiAl phase formedby diffusion reaction between Ti、Al atoms exist in the NZ. TiAl phase which is innano level size、present a smooth ellipsoid, less in number, and is not connected into asheet, so as to have the dispersion strengthening effect on the joint. TiAl3phase existsin the junction of titanium particles and aluminum matrix in the form of theTi/TiAl3/Al interphase distribution. layered Ti/TiAl3/Al hybrid structure and TiAlshould be generated in the interface of titanium alloy base metal and weld nugget andthe lamellar thickness is very thin (about a few microns). The number of the brittlephase reduces with the offset δ increases; increasing the rotation speed and weldingspeed, the species of brittle phase change from a single TiAl3to TiAl3and TiAl, andTiAl brittle phase quantity increases first and then reduces. |
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