The aim of this study is to observe the effect of process parameters on residual stresses and relative density of Ti6Al4V samples produced by Selective Laser Melting. The investigated parameters were hatch laser power, hatch laser velocity, border laser velocity, high-temperature preheating and time delay. Residual stresses were evaluated by the bridge curvature method and relative density by the optical method. The effect of the observed process parameters was estimated by the design of experiment and surface response methods. It was found that for an effective residual stress reduction, the high preheating temperature was the most significant parameter. High preheating temperature also increased the relative density but caused changes in the chemical composition of Ti6Al4V unmelted powder. Chemical analysis proved that after one build job with high preheating temperature, oxygen and hydrogen content exceeded the ASTM B348 limits for Grade 5 titanium.
Size frequency distributions of the cross‐sections of green glass beads in a thin section of regolith breccia 15427 have been determined. Cross‐section medians of vitrophyric and glassy beads are 0.22 mm and 0.094 mm, respectively. Vitrophyric beads contain exclusively olivine crystals of three crystallographically different morphologies. With a synthetic melt of green glass composition, free flight cooling rates have been determined. Spherules of 0.22 mm and 0.094 mm in diameter cool at rates of 1500°C/s and 4200°C/s, respectively, in the temperature range 1050°C–1000°C. The critical cooling rate for green glass formation, measured under controlled conditions, is about 1°C/s, indicating that lunar green glass beads have not been cooled in free flight but in a hot gaseous medium. By controlled cooling of synthetic green glass melt droplets from above the liquidus temperature, olivine morphologies have been produced that are identical to those in lunar vitrophyric beads. On the other hand, after heating synthetic glass spherules, textures have been observed that do not occur in lunar green glass. It follows that the lunar beads have been continuously cooled without later reheating. It is inferred that green glass melt droplets have been erupted from the lunar interior together with a large mass of gas, the cooling history of which is recorded by the internal textures and the size distributions of the green glass beads.
The current work presents the results of an investigation focused on the influence of process parameters on the melt-track stability and its consequence to the sample density printed out of NdFeB powder. Commercially available powder of Nd7.5Pr0.7Fe75.4Co2.5B8.8Zr2.6Ti2.5 alloy was investigated at the angle of application in selective laser melting of permanent magnets. Using single track printing the stability of the melt pool was investigated under changing process parameters. The influence of changing laser power, scanning speed, and powder layer thickness on density, porosity structure, microstructure, phase composition, and magnetic properties were investigated. The results showed that energy density coupled with powder layer thickness plays a crucial role in melt-track stability. It was possible to manufacture magnets of both high relative density and high magnetic properties. Magnetization tests showed a significant correlation between the shape of the demagnetization curve and the layer height. While small layer heights are beneficial for sufficient magnetic properties, the remaining main parameters tend to affect the magnetic properties less. A quasi-linear correlation between the layer height and the magnetic properties remanence (J r ), coercivity (H cJ ) and maximum energy product ((BH) max ) was found.Materials 2020, 13, 139 2 of 14 specific tooling requirements for each design and limitations in shape flexibility and complexity [15,16]. With the technique of additive manufacturing it is possible to produce parts with more complex design; however, obtaining printing processes for functional materials is still a topic in research and development. Earlier research already showed that techniques such as slurry jetting or laser powder bed fusion (LPBF) proved to be capable of delivering net-shape magnets [17][18][19][20][21][22]. Slurry jetting has been shown to be capable of producing magnets with good magnetic properties (H cJ = 775.3 kA/m, J r = 0.478 T, and (BH) max = 39.78 kJ/m 3 ). These values reach nearly half of the values expected for the bulk material. This manufacturing approach allows the manufacture of the desired magnet in a timeand cost-efficient manner, however, and the resulting magnetic properties will always be lower than the bulk magnets, due to the presence of a significant quantity of bonding resin. Another approach was presented by Kolb et al. [17,18] who utilized laser powder based fusion (LPBF) technique in order to overcome the problem of low relative density. They managed to 3D print cubical magnets of 85% of relative density, which were characterized by a polarization of 0.514 T. The surprisingly lower than expected magnetic properties were assigned to the self-demagnetizing of the samples while measured in an open magnetic circuit inside a Helmholtz coil. Other researchers [19] also applied a laser powder based fusion (LPBF) process to the powdered NdFeB (Nd7.5Pr0.7Zr2.6Ti2.5-Co2.5Fe75B8.8) powder. The 3D printed magnet reached roughly 60% of a bulk counterpart at relative d...
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