13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- alloys, with the chemical compositions listed in Tables 1 and 2, respectively. Both alloys were machined into the plates of 140 mm × 80 mm × 10 mm, and then mechanically and chemically cleaned before welding. EBW was performed using HDZ-15B EBW machine with an accelerating voltage (V) of 60 kV, an electron beam current (Ib) of 68 mA, an focus current (If) of 2230 mA, and a welding speed (v) of 500 mm/min. Table 1. Chemical composition of Ti-6Al-4V titanium alloy Element Al V Fe C N H O Ti Content (wt.%) 6 4 0.3 0.1 0.05 0.015 0.2 balance Table 2. Chemical composition of BT-9 titanium alloy Element Al Mo Zr Si Fe C N H O Ti Content (wt.%) 6.5 3.5 1.5 0.3 0.25 0.1 0.05 0.012 0.15 balance PWHT was carried out in a vacuum furnace at a vacuum degree of 10-2 Pa. One of the welded joints was subjected to solution treatment at a temperature of 950C for 1 h followed by furnace cooling and then aging at 550C for 4 h followed by air cooling, and the other joint was directly subjected to aging at a temperature of 550C for 4 h followed by air cooling. Microstructures were examined (after etching using Keller’s reagent) via optical microscopy. Microhardness was determined across the welded joint of mid-thickness (i.e., at a distance of 5 mm from the bottom surface) using a computerized Buehler hardness tester with a load of 500g and a dwell time of 15s at an interval of 0.1 mm. Fatigue specimens with a gauge length of 12 mm and a width of 3 mm were machined perpendicularly to the welding direction using electro-discharge machining (EDM). The gauge area was ground up to #600 SiC papers to remove the EDM cutting marks and to achieve a smooth surface. Total strain-controlled, pull-push type fatigue tests were conducted in air at room temperature using a computerized Instron 8801 fatigue testing system at different strain amplitudes up to 1.2%. A triangular waveform with a strain ratio of Rs=-1 was applied at a constant strain rate of 1×10-2 s-1, where Rs is the ratio of the minimum strain to the maximum strain. The strain-controlled test at low strain amplitudes was continued until 10,000 cycles, after which it was changed to load control at 50 Hz. At least two specimens were tested at each strain amplitude. Fatigue crack initiation site and crack propagation mechanisms were examined on the fracture surfaces of failed samples using SEM. 3. Results and Discussion 3.1 Microstructure Fig.1(a) and (b) show the microstructure of the two BMs, respectively. It is seen that both Ti-6Al-4V and BT9 had a typical bimodal microstructure, consisting of a combination of equiaxed α grains and inter-granular α + β lamellae. However, Ti-6Al-4V contained more equiaxed α grains than BT9. Fig.1(c), (d) and (e) show a significant microstructural changes in the FZ and HAZ after EBW between Ti-6Al-4V and BT9 alloys. The FZ was mainly composed of acicular and fine martensite α′ (Fig.1(d) and (e)) due to the rapid cooling during EBW. The HAZ at Ti-6Al-4V side also consisted of acicular martensite α′ (Fig.1(e)) but larger in size. The inner-HAZ at BT9 side consisted of a mixture of acicular martensite α′, primary α and metastable β (Fig.1(d)), while the outer-HAZ mainly consisted of primary α and metastable β (Fig.1(c)). Fig.2 shows the microstructure of the joints in the aging condition. From these optical images, no significant
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