Issue34

A. Riemer et alii, Frattura ed Integrità Strutturale, 34 (2015) 437-446; DOI: 10.3221/IGF-ESIS.34.49 440 pores and residual stresses on the crack growth performance has been analysed. Therefore the CT specimens were subjected to different heat treatments aimed at stress relieving and pore reduction respectively. Titanium alloy Ti-6-4 The fracture-mechanical performance of parts produced by SLM depends on the material used. For Ti-6-4 in its untreated condition low and insufficient crack growth data was found. For this material heat treatment is necessary. Fig. 4a and b show the results for threshold values for various heat treatments in both examined orientations. The threshold values are plotted vs. Yield strength and Elongation at break respectively (for more details cf. [7]). Fig. 4c illustrates the crack path in as-built (that means untreated condition) and 800° (that means following heat treatment at 800°C) condition. For as-built condition high residual stress in range of the Yield strength were found by X-Ray diffraction measurements. These high residual stresses have huge influence on the threshold values, Fig. 4a and b, as well as on the crack path, Fig. 4c. For the material condition resulting from the heat treatment at 800°C hardly any residual stresses were found. This is the reason for crack growth direction in line with notch direction following treatment at 800°C. Additionally, Fig. 4a and b include the results for the treatment at the higher temperature of 1050°C where modifications in microstructure were achieved. In this case, the highest level of anisotropy (despite heat treatment) was found. Within the HIP treatment (hot isostatic pressing) the samples were applied to high temperature and pressure. This ensures that residual stresses are removed and micro-pores partly compressed. Figure 4 : The dependence of threshold values on the Yield strength a) and Elongation at break b) . The effect of the residual stresses on the crack path is shown in c) . In order to classify the fracture-mechanical data obtained for selective laser melted Ti-6-4 tests for conventionally processed material were carried out. In these tests a threshold value of 3.3 MPa·m 1/2 was found. Furthermore, in literature a value of 4.0 MPa·m 1/2 for Ti-6-4 processed by conventional techniques is published. The findings from tests on laser melted Ti-6-4 show that residual stresses have the major influence on the fracture-mechanical performance of Ti-6-4. Heat treatment is absolutely necessary in order to reduce residual stresses and consequently to achieve or even to exceed