Of Ta15 Titanium Alloy Welded Joint Fatigue And Fracture Behavior

In this paper, hardness, tensile fracture behavior, fatigue fracture behavior, weld defects characteristics on fracture surface and the effect of welded defects on fatigue behavior of TA15 titanium alloy welded joints of tungsten-argon arc welding(TGAW) and electron-beam welding(EBW) were investigated in terms of experimental observations and macro/microscopic characteristics analysis. The mechanism of fatigue crack initiation and propagation were also researched by means of SEM in-situ fatigue tests.The results show that the hardness of fuse zone metal(FZ) and heat effected zone metal(HAZ) of TGAW welded joints is lower than that of base metal(BM). Hardness of FZ and HAZ of EBW welded joints is almost equal to that of BM. Tensile strength of both welded joints is almost equal to that of BM, but the plasticity of the TGAW and EBW welded joints badly decrease.The fatigue loading rates(R) and fatigue loading have effect on fatigue fracture characteristics of TA15 titanium alloy welded joints of TGAW and EBW. When R is 0.06 or 0.5, fatigue striations only can be found in metaphase or anaphase of fatigue crack propagation zone. While R is -1, fatigue striations can be found in fatigue crack propagation zone from prophase to anaphase. And there are a lot of abrasion marks on fatigue fracture surface. The quantity of slick plane and secondary crack of samdwich increase and the fatigue crack propagation zone become rougher with the increasing of fatigue loading.Pore is main welded defect on fatigue fracture surface. The mean of the diameter (d) of welded pores of TGAW is larger than that of EBW. The distance (a) from pore to the surface of sample and the d have a great effect on the fatigue life of TA15 titanium alloy welded joints. The fatigue life decreases when a and/or d decreases. When a>0.3mm, the fatigue crack is tended to initiate from the largest pore. However, when a<0.3mm, the fatigue crack is prone to initiate from the pore that is nearest from the surface.Fatigue cracks initiation mechanism of TA15 titanium alloy welded joints is fatigue cracks initiation along slip lines of sample surface. The process of fatigue crack initiation is as follow: slip lines and slip bands emerge on the surface of FZ near the fusion line on cyclic loading., Slip bands become larger and wider with the cycles increasing. To some degree, the micro-cracks along lamellar structure occure.During the fatigue crack propagation process of TA15 titanium alloy welded joints, slip on the surface of FZ is more obvious than that of HAZ, and the slip lines on the surface of FZ are longer than those of HAZ. There are more secondary cracks on surface of HAZ than those of FZ. The fatigue crack propagation rates(da/dN) of FZ duringâ… andâ…¡s tage of crack propagation is rapider than that of HAZ. The main cause of the difference of fracture crack propagation behavior of FZ and HAZ is the difference of grains and microstructure.