PSI - Issue 24

Jacopo De Nisi et al. / Procedia Structural Integrity 24 (2019) 541–558 Paolo Folgarait et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Also in this case, it is confirmed that the heat treatment conditions recommended by EOS for PH1 (B1 group) allowed reaching the highest HRC values for the metal alloy. From the other side, HV values assumed by the C group (highest ageing temperature and holding time) showed a drop in strength and hardness behavior, due to the formation of precipitates having semi-coherency or incoherent with ma trix or crystal’s lattice and coarser in size, conditions allowing mobile dislocations to bypass obstacles easier.

3.3. Tensile properties characterization

Samples were designed according to Standard ISO 6892-1:2016 in order to obtain proportional cylindrical tensile specimens ( Fig. 5 ), featuring: - Nominal diameter: 5 mm - Gauge length: 25 mm

Samples were oriented with their major axis both horizontally (h) and vertically (v).

Fig. 5 . Tensile specimen geometry, dimensions and characteristics (according to Standard ISO 6892-1:2016).

Specimens were fabricated on an EOS M290 equipment, leaving an overlayer of 1 mm, removed by machining tool operations to final dimensions and tolerances, after heat treatment. Surface roughness were measured on samples before machining, resulting in average values for Ra=7,3±1,1 µ m and Rz=38,2±6,5 µ m in the horizontal direction and Ra=11,2±1,0 µ m and Rz=58,6±6,52 µ m in the vertical direction. Tensile tests have been performed according to ASTM E8/E8M-16a using a tensile machine INSTRON 5985 with load cell 250 kN (range: 0.25÷250 kN) and automatic extensimeter Instron AUTOX750 (class 0,5 UNI EN ISO 9513:2013 and class B-1 ASTM E83-16). Load cell and tensile machine calibrated before testing (class 0,5 UNI 7500 1, compliant ASTM E4). Elongation (El%) and Reduction of Area (ROA%) have been assessed using calibrated centesimal mechanical calibers. Results do not show any significant dependency of the measured mechanical parameter on the sample location on the building platform. The larger variability in tensile properties than HRC tests is due to single measure points, i.e. experimental points are not representing an average (as for hardness measurements) and output variability in tensile is typically wider than Rockwell C indentation pattern. 0 group ( as-built condition) Average measured UTS for vertical specimens (UTS v =1276±3 MPa) and horizontal (UTS h =1256±16 MPa) as well as average measured YS for vertical specimens (YS v =956±18 MPa) and horizontal (YS h =952±27 MPa) are equivalent within the error bar (full error bar=2σ ( - σ; + σ; symmetric), σ being the standard deviation, calculated for each group of samples). Except for YS measured in horizontally fabricated samples (see Fig. 6 to 9 ), all mechanical quantities are greater than the reference value supplied by EOS in the alloy datasheet (see Table 8 ). The case for YS h , position A to F, shows values equal or lower to the minimum value of the uncertainty interval declared by EOS in Table 5. This case has been fatherly investigated with the Technology Provider. A possible cause may be found in the presence of residual stresses in the horizontally fabricated samples. This effect may induce a mechanical hysteresis in the material during further machining operations and a consequent measured YS lower than actual (in fact, this seems to be the case for