Influence of Welded Pores on Very Long-Life Fatigue Failure of the Electron Beam Welding Joint of TC17 Titanium Alloy

Abstract

The electron beam welding process is widely used in the connection among titanium alloy material parts of aero-engines. Its mechanical properties need to meet the requirements of long life and high reliability. In this paper, the static strength and the fatigue failure behavior of the electron beam weldments of TC17 titanium alloy were investigated experimentally under low amplitude high frequency (20 kHz), and the mechanical response and failure mechanism under different external loading conditions were analyzed. In summary, the samples were found to have anisotropic microstructure. The tensile strength of the PWHT of TC17 EBW joint was ~4.5% lower than that of the base metal. Meanwhile, compared with the base metal, the fatigue strength was reduced by 45.5% at 109 cycles of fatigue life. The fracture analysis showed that the fatigue failure of the welded joint of TC17 alloy was caused by the welded pores and the fatigue cracks initiated from the welded pores. A fine granular area (FGA) was observed around the crack initiation region. The existence of pores caused the stress intensity factor of the fine granular area (KFGA) to be inversely proportional to the fatigue life. The KFGA calculation formula was modified and the fatigue crack propagation threshold of the welded joint of TC17 alloy was calculated (3.62 MPa·m1/2). Moreover, the influences of the effective size and the relative depth of the pores on the very long fatigue life of the electron beam welded joint of TC17 titanium alloy were discussed.

The microstructure, mechanical, and fatigue behaviours of MAG welded G20Mn5 cast steel

“…The shrinkage surface or subsurface defects introduced by the casting become the primary failure origins. Therefore, the fatigue failure is prone to initiating from the near‐surface shrinkage introduced by the casting process rather than those welded defects . Furthermore, in each case, a single dominant crack originating from a critical defect is evidently observed on the fracture surface.…”

Microstructure evolution and mechanical properties of two kinds of weathering steel welded joints

“…In contrast to previous investigations, welded joints typically fracture in the stress range below the yield point, 36,37 or just above it. 11 According to the Hall-Petch relationship, 22 the grain size is inversely proportional to the strength of the material, which means that the finer the grains are, the stronger the material is. In the FZ and HAZ of welded joints, coarse and elongated grains are present, but the failure site still occurs in the BM where the grains are much finer, which means that the strength of the welded seam is higher than the strength of the BM.…”

“…Although the crack initiates from the specimen interior, its fatigue life is only 7.838 Â 10 5 cycles, so it does not form a fine grain zone around the defect, which is a typical VHCF characteristic. 11,39,40 Cracks initiate from larger welded defects, and the stress intensity factor around the defect is much greater than the threshold value of the crack expansion of the material itself, so that the crack directly skips the initiation stage, directly into the crack expansion stage, resulting in short fatigue life. 11 After fatigue failure, the fracture morphology of Q450AWR5 welded joints was observed, and the results were shown in Figure 14C,D.…”

“…7 These welded structures are always undesirable, such as welded defects, inhomogeneous microstructure, and residual stresses, which can become the weakest part of the whole structure, leading to premature failure of the welded joints, 9 especially under cyclic loading. [9][10][11] Therefore, as a large number of welded structures are used inside trains, the mechanical properties of the welded joints directly affect the safe operation of trains, in particular, the fatigue properties. 9,11 The service life of trains is usually more than 20 years, 12 and their internal welded structures often face very high cycle fatigue (VHCF) problems, thus constantly threatening the operation of trains.…”

“…[9][10][11] Therefore, as a large number of welded structures are used inside trains, the mechanical properties of the welded joints directly affect the safe operation of trains, in particular, the fatigue properties. 9,11 The service life of trains is usually more than 20 years, 12 and their internal welded structures often face very high cycle fatigue (VHCF) problems, thus constantly threatening the operation of trains. Tsutsumi et al 13 investigated the linear stir welded joints of SMA490AW and SPA-H weathering steels.…”