The Study Of Behavior And Mechanism Of Tensile And Fatigue Of TC4-DT Titanium Alloy Welded Joints

In this paper, TC4-DT titanium alloy base metal and electron beam welding (EBW) joints were took as the research object, the behavior of tensile and fatigue of base metal and welded joint were analyzed by the in-stiu observation test, the tensile and fatigue fractography were observed through scanning electron microscope,the mechanism of fatigue crack initiation and propagation was investigated.The tensile behavior and fractography of titanium alloy base metal and welded joints shows that:the parallel cross-sectional sample of the welded joint was fractured in base metal, its behavior was similar to the base metal with no obvious slip characteristics, the crack was initiated and propagated at the phase boundary or grain boundary, much dimple was obviously observed in the fractography, which belongs to ductile fracture. The circular cross-section sample of welded joints fracture in the weld area, much slip characteristics were observed during tensile process, the crack was initiated along slip bands, much crystallographic plane and grain shape was observed in the fractography, which belongs to brittle fracture. Much slip bands were observed in the notch tip of the notched specimens of the fusion zone and the heat affected zone, the crack was initiated and propagated along slip bands. According to the fractography, the notched specimen of the fusion zone belongs to brittle fracture, but the notched specimen of the heat affected zone belongs to ductile fracture.The fatigue behavior and fractography of titanium alloy base metal and welded joints shows that:the parallel cross-sectional sample of the welded joint was also fractured in base metal; the micro crack was formatted by cracking slip bands and propagated along or through the primary a phase. The micro crack of the notched specimens of base metal and welded joints was formatted by phase boundary cracking; the crack of base metal was propagated directly through the primary a phase, the crack of the notched specimen of fusion zone was propagated either along acicular martensite a’phase or directly through acicular martensite α’ phase, the crack of the notched specimen of the heat affected zone was propagated along sheet β transformation organization, when the crack was grown to the primary a phase, the specimen of process1was propagated through primary a phase, but the specimen of process2was propagated along primary a phase.The behavior of tensile and fatigue of welded joint was affected by changing the organizational structure of welding process. As circular waveform scanning was used in process2, the sheet β phase was very small in fusion zone, slip bands generated difficultly, So slip bands in process2is littler than process1for circular cross-section sample and notched specimen of the fusion zone. The fatigue behavior of base metal and the parallel cross-sectional sample of the welded joint were affected very little by the process as they were fractured in base metal. The fatigue behavior of notched specimens of the fusion zone and the heat affected zone were affected also not very obviously by the process, but the fatigue crack growth rate of them was affected by the process. The fatigue crack growth rate was relatively close before150um; the crack growth rate of the specimens of process-1was significantly greater than the process2after150um, it is same to notched specimens of the heat affected zone. Considered from the fatigue life and safety factor of titanium alloy structural parts, process2was better than process1, we should give priority to the second welding process.