PSI - Issue 13

J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 317–321 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 2. (a) Particle size distributions of the two powder lots (b) spherical particle shape of Ti-6Al-4V powder.

To evaluate the influence of the changed particle size distribution on the material properties tensile specimens based on the standard DIN 50125 (2008) and CT specimens with a thickness of 3.0 mm, a width of 40 mm and a V-shaped notch with a length of 8 mm according to the ASTM 647-08 (2008) standard were manufactured with the standard process parameters for Ti-6Al-4V. These specimens were manufactured on a SLM 280 HL (SLM Solutions Group AG) system with a build chamber of 280 mm x 280 mm x 350 mm, a 400 W Yttrium laser and in inert gas atmosphere. The platform was heated up to 473.15 K. Following SLM processing the chamber was flooded with nitrogen and the oxygen content is reduced below 0.2 %. The mechanical and fracture mechanical properties were investigated for two material conditions: Some specimens were heat-treated at 1073.15 K after the building process for two hours to reduce residual stresses, the other half was set to hot isostatic pressing which leads to the reduction of residual stresses and pores. Therefore, the specimens made of Ti-6Al-4V are pressurized in a pressure chamber under inert gas atmosphere with an isostatic pressure of 1000 bar and a temperature of 1193.15 K for two hours. For the characterization of the quasi-static properties a universal testing machine, I NSTRON 5569, was utilized. The tensile tests with a minimum of three specimens for each condition were conducted displacement controlled with a crosshead speed of 5 mm/min according to the DIN EN ISO 6892-1 (2009) standard. The elongation at room temperature (293.15 K) was measured using an optical extensometer. The analysis of the fracture mechanical properties was executed using an I NSTRON electro-mechanical tensile testing machine E LECTRO P ULS E10000. The analyzed characteristics are determined at ambient conditions and under mode-I-loading. Each test series contains a minimum of three specimens. A periodic sinusoidal loading at a stress ratio of R = 0.1 was applied to the sample. The current potential drop method was used for the continuous determination of the crack length during the experiment. This process is carried out with the help of the crack length measuring system DCM-2 of the company M ATELECT LDT. In this case, the measuring device has two channels for potential drop measurement and for energizing the samples a constant current source is available. In the current potential drop method, a constant current I is given into the test sample isolated from the testing machine, and the potential drop U at a defined location is measured. If now the crack propagation and the concomitant reduction of the cross section in the test sample results, according to Ohm's law a change, in this case an increase, of the electrical resistance R Ω and thus also of the potential drop U . The determination of crack growth values was conducted by the system FAM Control, see Sander and Richard (2004). A complete crack propagation curve requires two different tests. The investigation of the near threshold behavior and the threshold value (Δ K th ) of the stress intensity factor range at low stress intensity factor ranges (Δ K ) is performed by an exponential decrease of Δ K at a constant R-ratio and a test frequency of 20 Hz. The crack growth behavior at elevated stress intensity factor ranges (P ARIS regime and above) was investigated using a constant force ratio with a test frequency of 10 Hz.

3. Results and discussion

To determine the static material properties, tensile tests are performed. The results of these investigations comparing the two PSDs for the 1073.15 K heat-treated and the HIP condition are listed in Table 1. Both the yield strength and the tensile strength have only small deviations. These minor differences in the mechanical characteristics can be affected by environmental parameters such as air temperature, tolerances of the experimental system and the measurement system.