13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- amplitude of 0.4%, (a) an overall view, (b) crack initiation site and (c) crack propagation zone Figure 9. SEM images of fracture surfaces of a post-welded joint in the STA condition, fatigued at a strain amplitude of 0.4%, (a) an overall view, (b) crack initiation site and (c) crack propagation zone 4. Conclusions 1. The microstructure across the dissimilar welded joint exhibited a considerable change, mainly consisting of acicular and fine martensite α′ both in the FZ and HAZ of Ti-6Al-4V side, and acicular martensite α′, primary α and metastable β in BT9 side, while both BMs of Ti-6Al-4V and BT9 had a microstructure of equiaxed α grains and inter-granular α + β lamellae. There was no significant difference in the microstructure of both FZ and HAZ between the as-welded and the aging conditions. However, after STA the microstructure of the FZ and HAZ at Ti-6Al-4V side mainly consisted of coarse α platelets with interlamellar β, while α platelets, primary α and interlamellar β were observed in the HAZ at BT9 side. 2. A characteristic asymmetrical microhardness profile across the dissimilar welded joint was observed with a higher value on the BT9 side than on the Ti-6Al-4V side. The hardness in the FZ was higher than that of both BMs due to the formation of α′ martensite arising from the rapid cooling during EBW. A narrow soft zone in the outer-HAZ of BT9 side was observed due to the presence of β phase, while the hardness in the HAZ of Ti-6Al-4V side was higher than that of the Ti-6Al-4V BM. After aging treatment at 550C the high hardness in FZ remained nearly the same or slightly higher and the narrow soft zone disappeared due to decomposition of β. However, the hardness value of the post-welded joint after STA in both FZ and HAZ was slightly higher than or nearly the same as that of BMs due to the presence of coarsened α. 3. The post-welded joints in both aging and STA conditions displayed cyclic saturation or stabilization in the entire cyclic deformation process at low strain amplitudes up to 0.6%, while cyclic softening occurred after initial cyclic stabilization at higher strain amplitudes. With increasing strain amplitude, the stage of initial cyclic stabilization became gradually shorter. 4. The fatigue life of the aging condition was slightly longer than that of the STA condition at the lower strain amplitudes. In general the strain-controlled fatigue resistance of the post-welded joints in both aging and STA conditions was roughly equivalent. 5. The fatigue fracture of the post-welded joints after aging and STA treatment occurred mostly in Ti-6Al-4V BM. Fatigue crack initiation occurred from the specimen surface or near-surface defect, and crack propagation was mainly characterized by the characteristic fatigue striations coupled with secondary cracks in both aging and STA conditions. Acknowledgements The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and AUTO21 Network of Centres of Excellence for the financial support, and (a) (b) (c)
RkJQdWJsaXNoZXIy MjM0NDE=