PSI - Issue 7

2

Fedor Fomin and Nikolai Kashaev/ Structural Integrity Procedia 00 (2017) 000–000

F. Fomin et al. / Procedia Structural Integrity 7 (2017) 415–422

416

2016). Nevertheless, the results of fatigue testing are not so encouraging, and failure is always located in the fusion zone (FZ) regardless of the surface quality of the welding seam. The underlying reason for this behaviour is attributed to the inherent flaws in the laser beam welded titanium structure. The detrimental effect of welding-induced defects on fatigue performance is even further exacerbated by the formation of hard and notch-sensitive martensitic structure within the FZ. It was shown by authors in previous research (Fomin et al., 2017) that to maximize the fatigue strength of the joints, all geometry-specific stress raisers such as weld toes and underfills must be removed. Under these circumstances, fatigue strength is still lowered by the presence of internal porosity, especially near the surface. A suitable type of post-weld heat treatment, resulting in more ductile microstructure in the FZ, can slightly increase the fatigue strength of the machined joints. This effect and the microstructural transformations occurring upon post-weld annealing have already been reported (Fomin et al., 2017); however, the mechanism of fatigue failure and the crack growth from internal defects, especially at the early stage, has not been fully elucidated yet. The aim of this study is to investigate and quantify the effect of subsurface porosity on fatigue cracking in the high cycle fatigue (HCF) regime. The detailed mechanism of the initiation and growth of a crack from a pore is necessary for the sufficiently reliable design with respect to fatigue. In this paper, we propose a fatigue life assessment model based on a fracture mechanics approach for laser beam welded Ti-6Al-4V machined butt joints. This predictive scheme combines a number of approaches adopted for the specific material and crack geometry. It was shown that to achieve satisfactory agreement with experimental data, the model should take into account the size and position of the pore, the notch effect introduced by the defect and short crack behaviour. 2. Material and experimental procedures The material investigated was a Ti-6Al-4V (Grade 5) titanium alloy in the form of hot-rolled and mill-annealed sheets with the thickness of 2.6 mm. The basic mechanical properties of the material were as follows: yield strength 995 MPa, ultimate tensile strength 1039 MPa, and modulus of elasticity 110.9 GPa. LBW was performed in argon atmosphere by an 8-kW continuous-wave ytterbium fibre laser YLS-8000. The process parameters employed for welding of the coupons were as follows: laser power was 5.5 kW, welding speed was 4.0 m/min, and filler wire (Ti 6Al-4V, Ø 1.0 mm) feed rate was 3.0 m/min. Fig. 1(a) shows the martensitic microstructure of the FZ in the as-welded condition. Mechanical properties of the laser beam welded joints were characterized in two conditions: machined and heat-treated machined. Post-weld heat treatment (PWHT) was carried out in a vacuum furnace at the temperature of 920 ˚C for 45 min, followed by cooling in argon atmosphere. Fig. 1(b) shows the coarse lamellar microstructure of the FZ after PWHT. Load-controlled uniaxial fatigue tests were conducted at room temperature using a Testronic 100 kN RUMUL resonant testing machine. The experiments were performed in accordance with ASTM E466-07 at the frequency of 80 Hz and stress ratio R = 0.1. The specimens with a uniform test section of 8-mm width and 20-mm length were used. The welding seam was located in the centre of the gage length, and fatigue loading was applied transverse to the weld direction. The fatigue tests were carried out up to 10 7 cycles. 3. Experimental results The results of fatigue testing, expressed by the S-N curves, are shown in Fig. 1(c). For reference, base material data are also provided. The failures of the welded joints always occur in the welding seam regardless of the heat treatment unless the applied stress was close to the yield stress of the material. As was shown by Fomin et al. (2017), as-welded Ti-6Al-4V butt joints exhibit poor fatigue behaviour due to the deleterious effect of surface defects. Milling the surface flush with the sheet surface can considerably extend the fatigue life. The fatigue limit of the laser beam welded milled Ti-6Al-4V butt joints is approximately 500 MPa (see Fig. 1(c)) and corresponds to approximately 70% of the BM fatigue limit. The mentioned reduction of 30 % implies the existence of internal defects within the FZ that reduce the fatigue strength of the joint. The effect of these internal flaws on the overall fatigue behaviour of the machined joints is of major interest in the current study. PWHT increases the fatigue limit by approximately 10% up to 550 MPa and