PSI - Issue 17

Thomas Simson et al. / Procedia Structural Integrity 17 (2019) 843–849

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 4. Oxygen content of particles from different size fractions for the three powder qualities “new”, “used” and “waste” (surface/v olume oxide).

A further difference is that, on the used powders, occasionally massive surface oxides appear. Such massive surface oxides are increasingly present on the “waste” powder (see arrows in F ig. 2b and 2c). Those powders stem from hot areas of the laser process (e.g. directly from the melt pool) and are much larger in size than the original powder. Therefore, these particles are prone to be oxidized due to a longer retention time at elevated temperatures. Via energy dispersive X-ray analysis (EDX) these oxides are identified to be Ti-based. Local EDX measurements on single particles away from such visible oxides (2x2 µm² area measurements on the surface of at least 10 different particles) also clearly demonstrate the oxygen pick-up after the multiple use of the steel powders in the process (Fig. 4). The mean surface/volume oxide content increases remarkably. The influence of building direction and heat treatment on mechanical properties such as hardness, tensile strength and notched impact strength is investigated. Therefore samples were produced in two different building directions (horizontal and vertical) and the values are compared to those for wrought maraging steel. All samples are reworked by grinding processes before examination in order to achieve the same initial condition. The results of all mechanical properties are summarized in Tab. 3. The values show average values from several samples. For hardness values, 10 tests per sample were carried out, 5 samples for the tensile test and 3 samples for the notched impact. The hardness of the solution annealed samples is ranging from 333~341 HV10, while the hardness of samples raises to 640~656 HV10 after heat treated at 490 ºC and 6 hour duration for age-hardening. There are no significant differences between the building directions of the AM samples and the wrought material. The intermetallic Ni, Co, and Mo rich precipitates formed during aging serve to strengthen the martensitic matrix. Kempen, K. et al. (2011). Aging-treatment leads also to an improvement of tensile performances. The ultimate tensile strength ( Rm ) increases from 1056~1096 MPa to 1964~2102 MPa, while the break elongation ( ε tot ) was reduced from 11.3~16.0% to 2.0~4.5%. This indicated that the strength was well enhanced while the ductility was seriously reduced. The notch impact tests are done according to ASTM E23 standard ASTM E23 (1996). After the preparation of the LPBF samples, they were grinded to reduce the surface effects. Some of the samples were then heat treated, while others were kept in their solution annealed state for comparison. Notch impact tests show a significant decrease in toughness for the aged samples. The aging leads to higher hardness and results in lower toughness. The intermetallic precipitates promote brittle fracture so that less energy is required to achieve complete fracture at higher hardness. This is also shown by the impact tests at low temperatures (-20 °C and -50 °C). At low temperatures, body-centered cubic (bcc) materials break more brittle, which causes the energy absorbed to drop. The aged samples are not affected by the low temperatures. Again, there is no difference between the building directions and wrought material. 3.2. Mechanical properties