PSI - Issue 76

Xabat Orue et al. / Procedia Structural Integrity 76 (2026) 3–10

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The DED-LB/CW systems used consist on a 6+2-axis robotic cell, and a 3-axis machine called TITAN which was designed and manufactured by Tekniker for research and development purposes. Both systems are equipped with a continuous wave 4 kW Yb fiber laser (1070 nm) from IPG Photonics and the coaxial wire laser head CoaxPrinter from Precitec GmbH & Co. The deposition was accomplished by a bidirectional strategy alternating 90º in each layer, and an additional contouring to ensure the integrity in height growth. To generate an inert atmosphere, plastic welding chamber of 700 x 700 x 500 mm 3 size filled with commercially pure argon was employed. The oxygen concentration was monitored and maintained below 100 ppm to avoid the formation of any oxides in the deposited Ti-6Al-4V. The samples were extracted in horizontal direction from rectangular solid blocks manufactured in as-built conditions with no heat treatment by Wire-based Electro Discharge Machining (EDM-w). This was executed deep inside the solid blocks to obtain homogeneous material. A safety margin of 10 mm from the surface was considered enough for this purpose. This also ensures to avoid the possible residual alpha-case layer that might have been generated in the surface despite being manufactured in an inert atmosphere. Afterwards, all samples were machined. For microstructural characterization, 12 mm wide and 2 mm thickness specimens were extracted along the height of the blocks. These specimens were metallurgically prepared by grinding with 2500-grit and polishing with colloidal silica suspension 50 nm alkaline, and afterwards Weck's reagent was used for etching. The microstructure, as well as the fractographical analysis of post-mortem specimens, was undergone by the Olympus GX71 Optical Microscope (OM) and ZEISS Ultra Plus Scanning Electron Microscope (SEM). Additionally, the software MountainsMap® was used to postprocess the acquired images and to obtain microstructural characteristics such as the length, the width and the aspect ratio of α -laths. Related to the mechanical characterization, Vickers hardness, tensile, High Cycle Fatigue (HCF) and Fatigue Crack Growth (FCG) tests were executed. Vickers hardness tests were undergone along the height of microstructural samples by FUTURE-TECH FM-700 with 0,5 kg indentation-load. For tensile tests, rectangular specimens of 4 x 6 mm 2 cross-section were tested according to ASTM E8 [14] using the INSTRON 3369 testing system (50 kN). For HCF and FCG tests, INSTRON 8852 servohydraulic biaxial machine (100 kN and 1000 Nm) was used instead. HCF assessment was carried out at R = 0,1 with 16 Hz of frequency ( f ), and a run-out of 10 million cycles was set. Axial force controlled constant amplitude procedure according to ASTM E466 [15] was applied using cylindrical specimens of 6 mm diameter and 12 mm calibrated length. To determine the fatigue limit, the up-and-down method was applied according to Dixon ’s evaluation procedure [16]. Regarding FCG tests, compliance method was applied with a Crack Opening Displacement (COD) gauge of 10 mm length and 4 mm travel. Here, Compact Tension (CT) specimens of 12,5 mm thickness ( B ) and 50 mm width ( W ) extracted in L/T-L/T (H-H) and L/T-S (H-V) directions [17] were used. FCG tests were executed at R = 0,1 and R =0,7with f = 16 Hz applying K-Decreasing procedure according to ASTM E647 [18]. The construction of the K-T diagram was executed considering the experimental results obtained from the above mentioned tests. The prospective fatigue limits were calculated considering the specimens that failed before 10 million cycles (fractures). According to the fatigue design guide of non-ferrous materials for helicopters [19], all data point were moved to 50 million cycles keeping the same Gaussian normalized residual, i.e., extrapolating with the slope of the S-N curve. The intrinsic fatigue limit of the material free of defects was calculated by the relationship (4) for R = -1 [6], and to consider the mean stresses of the tests at R = 0,1 Walker’s model was applied with (5,6) [7]. Based on Linear Elastic Fracture Mechanics (LEFM), the long crack region of the K-T diagram was determined by ΔK th,lc obtained from FCG tests in each direction. Related to the consideration of equivalent defect size, Murakami’ s √ was used and Y was determined accordingly based on the results obtained from fractographic analysis (see section 3). 3. Results and Discussion The microstructural analysis by OM and SEM revealed a columnar prior-beta grains with martensite (Fig. 2). This is in line with other authors due to the fast cooling rates towards the base plate that are common in DED processes [1]. The undergone post-processing by MountainsMap® revealed a length of alpha-lath of 11 µm.