PSI - Issue 7

Gianni Nicoletto et al. / Procedia Structural Integrity 7 (2017) 133–140 Gianni Nicoletto/ Structural Integrity Procedia 00 (2017) 000–000

135

3

Post-fabrication heat treatments have been investigated to improve the ductility of Ti6Al4V products manufactured by SLM and to achieve a variety of desired mechanical properties. Temperature of β -transus transformation is critical in the heat treatment: heating below or above it influences the relative quantity of α and β phases , which results in a lamellar morphology or in the development of columnar grains, Rafi et al (2013). Four different post fabrication heat treatments were investigated in this study. Details are as follows: HT 1 - 740°C for >1 hr, vacuum cooling to 530° C in >1 hr and cooling to room temperature in Ar. HT 2 - 740°C for >1 hr and cooling to room temperature in Ar. HT 3 - 1200°C for >1 hr, vacuum cooling to 530°C in >1hr and cooling to room temperature in Ar. HT 4 - 900°C for > 1 hr, vacuum cooling to 530°C in >1 hr and cooling to room temperature in Ar All these heat treatments are classified as Annealing HT. In particular, HT1 and HT2 are annealing treatments below the β - transus temperature while HT3 and HT4 are above this temperature. 2.3. Metallographic preparation Metallographic samples were cut from specimens with a K “Remet” SiC blade and cleaned with acetone in an ultrasonic bath and dried with compressed air. Lapping operation with abrasive SiC papers from the coarsest grain size to the finest one (i.e. 400, 800, 1200, 2000 mesh) was carried out before a final OPS polishing for 40 min. Kroll’s etchant (6 ml HNO 3 ,2 ml HF, 92ml H 2 O) was used to reveal the Ti6Al4V microstructure. The optical microscopy investigation and recording was performed on Zweiss Axio Observer system. 2.4. Mechanical tests\ Mechanical properties of the material such as ultimate strength, yield stress and elongation were characterized by uni axial tensile tests. Tensile specimens were machined from cylindrical bars of SLM Ti6Al4V according to the ASTM F2924-E8 (M) standard geometry, see Fig. 1, where the length of reduced section A=30mm, reference diameter D=6mm, gage length G=25mm, and fillet radius R= 6 mm.

Fig. 1 Tensile specimen geometry according to ASTM F2924-E8 (M) Fig. 2 shows fatigue specimens with the two conditions of surface finish examined: as-built and machined. Key geometrical parameters were: reference diameter D = 6 mm, total length L = 80mm, gripping diameter G = 12mm and fillet radius R= 20 mm. The stress concentration factor was K t = 1.1 Fatigue tests were performed in fully reversed, rotating bending mode at room temperature and with a frequency of 50 Hz on a multi-station fatigue machine. Samples of each heat treated material were tested with stress amplitudes ranging from just below the yield strength down to levels that produced lifetimes up to 10 7 cycles. Unbroken specimens after 10 7 cycles were removed and termed “run out”.

Fig. 2 Rotating bending specimens with the as-built and machined surfaces.