PSI - Issue 13

J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 311–316 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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DCPD for continuously monitoring the crack propagation. The determination of crack growth values was conducted by the system FAM Control, see Sander and Richard (2004). To establish a crack growth curve two different kinds of tests are necessary. For 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 ) an exponential decrease of Δ K at a constant R-ratio and a test frequency of 60 Hz was performed. 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. The optical micrographs of a study by Brüggemann et al. (2017) are displayed in Fig. 3a-b. Figure 3a shows an optical micrograph of an EN AW-7075 specimen which was manufactured with a building platform pre-heated to 473.15 K. The occurring hot cracks could be reduced by using a modified building platform which allows a higher pre-heating temperature of up to 673.15 K. As Fig. 3b shows, the distances between the cracks are increased. The 673.15 K pre-heating therefore has a positive influence on the component quality and is retained for the subsequent modifications described in this paper. 3. Results and discussion

Fig. 3. Optical micrographs of the specimen after SLM processing (a) EN AW-7075 manufactured with a 473.15 K pre-heated building platform, Brüggemann et al. (2017) (b) EN AW-7075 manufactured with a 673.15 K pre-heated building platform, Brüggemann et al. (2017) (c) mixed aluminum alloy manufactured with a 673.15 K pre-heated building platform. To completely avoid hot cracking during the SLM-process 50 Wt.-% EN AW-7075 alloy and 50 Wt.-% AlSi10Mg are blended to create a new mixed aluminum alloy. The silicon is meant to counteract the shrinkage of the alloy during cooling. All manufactured specimen are free of detectable hot cracks. The choice of the process parameters influences the density of the component. To achieve specimens with almost no detectable pores an appropriate parameter set for the new mixed aluminium alloy was found in parameter studies. In order to evaluate the performance of the mixed alloy with regard to the mechanical characteristics, tensile tests are carried out. The load direction of the specimens is parallel to the building direction. Table 1 lists the determined material properties for two conditions (as-built and heat treated) compared to conventionally manufactured EN AW-7075 specimens. The analysis of the experimental data shows an increase in the yield strength by a factor of three because of the heat treatment of the aluminum mixed alloy. Furthermore, the heat treatment can increase the tensile strength, with only a small decrease in the elongation at break. In spite of the subsequent heat treatment, the mixed alloy has a poorer performance in terms of mechanical characteristics compared to the conventional EN AW-7075 alloy. Table 1. Room temperature tensile properties of as-built and heat treated mixed aluminum alloy SLM-specimens compared to conventionally manufactured EN AW-7075 specimens. Condition yield strength [MPa] ultimate tensile strength [MPa] elongation [%] as-built 100 200 10 heat treated 300 350 8 literature (T6), DIN EN 755-2 (2008) 480 540 7