Fatigue properties of parts fabricated by laser powder bed fusion, a kind of additive manufacturing, are inferior to those of conventional ways. Defects and inclusions are the causes of low fatigue properties, but the effect is not clarified. Inconel 718 specimens were fabricated by laser powder bed fusion. Cross-sectional observation and fatigue tests were performed. Gas porosity, cracks, and inclusions were observed in cross-section. TiN inclusions were observed in fatigue fracture origins for specimens with hot isostatic pressing. To enhance the fatigue properties of Inconel 718 fabricated by laser powder bed fusion, it is necessary to remove TiN inclusions within the powder.
The powder characteristics are one of the most important factors to affect the processability in additive manufacturing technology, especially in laser powder bed fusion. In this research, the influence of highly spherical gas-atomized Ti-6Al-4V alloy powder on the processability in laser powder bed fusion was investigated. The cubic specimens of highly spherical gas-atomized Ti-6Al-4V alloy powder were fabricated at various laser power and scan-speed using a laser powder bed fusion type machine with 1 kW fiber laser, and examined the density and surface roughness of the cubic specimens. As a result, the optimum fabrication condition was obtained using the process map evaluated by the density and surface roughness of the cubic specimen. The alloy fabricated at a laser power of 150 W and a scan speed of 700 mm/s showed the maximum relative density of 99.94% and the surface roughness, Sa, of about 5 µm. The tensile strength and elongation of the as-built alloy were more than 1300 MPa and 8%, respectively. It was thus found that the use of highly spherical gas-atomized powder greatly improves the density, surface roughness and tensile properties of the as-built alloy.
Fused deposition modeling (FDM) with a 3D printer, one of so-called “Additive Manufacturing (AM)”, is a promising advanced industrial production method. However, there are much difficulty in the fabrication of dense ceramics using a FDM-type 3D printer; i) preparation of “filament” used for supplying raw materials to the printer, ii) thermal management during AM to adjust both the viscosity and yield stress of melting filament appropriately, and iii) sintering densification of powder compacts. To meet these requirements, ceramics powders consisting of bimodal fine/coarse particles in zirconia-alumina system and unimodal hydroxyapatite powders were adopted for the starting raw materials. Homogenous filament with the compositions of (ZrO2-Al2O3)/binder = 65/35vol% were prepared by reducing the thickness of resin layer to 110~140 nm and extruded into the strand filaments with the diameter of 1.8-2.0 mm around 423 K. After AM under the viscosity γ and yields stress σy, less than 1.60·102 Pa·s and 2.0 MPa, respectively, of kneaded body, and defatting at 773 K in air, the powder compacts were densified by microwave sintering at 1723 K for 6.0·102 s in N2. As-obtained material showed a high relative density more than 95.0%. In addition, capsule-free HIP treatment brought much densification by removing the closed pores inside materials.
This research evaluated the effect of nitrogen absorption into austenitic stainless steel in selective laser melting (SLM) process. Microstructures and mechanical properties of SUS316 specimens fabricated by SLM in different atmosphere, nitrogen and argon, were investigated. The results of microscopy observation of as-built specimen fabricated in nitrogen showed a fine cellular microstructure. In contrast, coarse columnar grains grew up from melt pool boundaries were observed in the microstructure of as-built specimen fabricated in argon. The ultimate tensile strength, yield strength and hardness of as-built specimen in nitrogen were considerably higher than those of specimen in argon and conventionally processed material. These were considered to be the cause of the difference in microstructures and nitrogen absorption. Post-SLM solution heat treatment (SHT) reduced the tensile strength and hardness of both specimens and improved their ductility. However, the tensile strength and hardness of the specimen in nitrogen were still higher than those in argon.