In this study we have assessed the microstructure details of 316L stainless steel produced by the additive-manufacturing selective-laser-melting technique under industrial conditions and correlated ...them with the mechanical properties. The employed micro- and nano-scale imaging electron microscopy techniques revealed the formation of a rigid hierarchical microstructure, which was driven by the rapidly changing solidification rates. The latter also affected the alloying atoms' distribution in the melt-pool boundary area as well as in the dislocation-dense regions. The melt-pool boundaries in themselves did not produce structural irregularities, but were shown to have a slightly different chemical composition. The arrangement of the complex dislocation cells that developed in the whole material volume led to an increase in the yield strength. The calculated twenty-times-higher dislocation density compared to that of the forged material was linked to a very low strength hardening. The similarity of the calculated yield strength, which came from the experimentally determined structure parameters, with the experimental value additionally supported the structure/mechanical properties correlation, derived in this work.
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•Hierarchical growth model development based on structural and compositional analyses•Understanding the melt-pool boundary structure and formation•KAM and ECCI calculations of the dislocation density•The impact of the structure on the yield-strength calculations
Blown powder dynamics is one of the aspects of Directed Energy Deposition (DED) with a major influence on deposition quality and potential for improvement in simulation. Most currently employed ...computational models discard powder grain collisions as negligible, although little explicit evidence for that claim exists. A Discrete Element Method approach is thus employed in the present work to simulate the actual number of grain collisions in a powder stream of a commercial discrete coaxial nozzle and how that number varies with the key processing parameters. While the number of powder grain collisions is found to in fact be negligible at one side of the usable parameter spectrum, the opposite side results in as many as 84% of all the powder grains being involved in inter-granular collisions with an average rebound angle of 14°, challenging the established hypothesis of the negligibility of this phenomena.
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•DED powder stream is shown to feature a large number of inter-granular collisions.•Inter-granular collisions feature non-negligible rebound angles.•Powder stream boundary condition has a key influence on grain collision statistics.
This research investigates the impact of three process parameters of Laser Powder Bed Fusion (LPBF) - laser power, scanning speed, and base plate preheating temperature on the structure and ...mechanical properties of the EOS CoCr SP2 dental alloy. The LPBF process was used to fabricate dental Co–Cr alloy specimens for microstructural analysis and mechanical properties testing. Light and electron microscopy were used to determine microstructural parameters, including porosity, inclusions, and cracks. The material's chemical composition was analysed by EDS, while XRD and EBSD methods were used to determine the presence of microstructural phases and the crystallographic orientation of individual grains. The mechanical properties were evaluated through a static tensile test (Rp0.2, ε), a toughness test (KVa), and a three-point bending test to determine the flexural strength (Rms). In the microstructure, differences were observed that reflected statistically significant differences in mechanical properties (one-way analysis of variance (ANOVA) and Scheffé post hoc test (α = 0.05)) Using the base plate preheating temperature ϑp = 310 °C with a constant scanning speed v = 900 mm/s in combination with increasing laser power P from 160 W to 250 W the proportion of porosity decreased while the mechanical properties of toughness (KVa) and flexural strength (Rms) increase to maximum values.
The key feature of Fe-Mn alloys is gradual degradability and non-magneticity, with laser power bed fusion (LPBF) parameters influencing the microstructure and chemical composition. Our study focuses ...on biodegradable Fe-Mn alloys produced by mechanically mixing pure metal feedstock powders as part of the LPBF process. The Mn content and, consequently, the γ-ε phase formation in LPBF samples are directly correlated with an adapted energy-density (E) equation by combining the five primary LPBF parameters. We varied laser power (P) in a range of 200-350 W and scanning speed at 400 and 800 mm/s, and a comprehensive study was performed on samples with similar E. The study also showed an almost linear correlation between the LPBF's laser power and the material's hardness and porosity. The corrosion resistance was significantly reduced (from 13 to 400 μm/year) for the LPBF samples compared to a conventionally produced sample due to the dual-phase microstructure, increased porosity and other defects. The static immersion test showed that the process parameters greatly influence the quantity of oxides and the distribution of their diameters in the LPBF samples and, therefore, their corrosion stability. The most challenging part of the study was reducing the amount of ε phase relative to γ phase to increase the non-magnetic properties of the LPBF samples.
As biodegradable materials, Fe–Mn alloys have a lot of promise, particularly because they can be employed as metallic implants with excellent mechanical properties. Besides allowing for patient ...customisation, powder-bed fusion of these alloys could help overcome their main drawback, i.e., slow degradation inside the human body, by increasing the component surface with inbuilt structural porosity. The quality of additive-manufactured products depends on their temperature history, making knowledge of the heat-transfer characteristics of the powder-bed fusion process very important. While accurate determinations of temperature gradients and the melt-pool sizes still represent a considerable challenge for all materials, this is particularly true for Fe–Mn alloys, where research is currently limited to a handful of pioneering works, and experimental determinations of the melt-pool contours prove extremely difficult and unreliable. To explore the origins of measurement inconsistency, melt-pool compositions of Fe–Mn specimens were analysed in the scope of this research. Concentric patterns of high- and low-Mn content practically indistinguishable from the melt-pool boundary on the macroscale were revealed within the melt-pool. A microscopic analysis of elemental content distribution was performed and the concentric patterns were attributed to the pronounced segregation of the alloy in conjunction with convective currents. A novel, calibration-free 3D finite-element model of heat transfer during laser powder-bed fusion is proposed to overcome these experimental difficulties and validated against the experimental melt-pool measurements.
We have investigated the impact of the process parameters for the selective laser melting (SLM) of the stainless steel AISI 316L on its microstructure and mechanical properties. Properly selected SLM ...process parameters produce tailored material properties, by varying the laser’s power, scanning speed and beam diameter. We produced and systematically studied a matrix of samples with different porosities, microstructures, textures and mechanical properties. We identified a combination of process parameters that resulted in materials with tensile strengths up to 711 MPa, yield strengths up to 604 MPa and an elongation up to 31%, while the highest achieved hardness was 227 HV10. The correlation between the average single-cell diameter in the hierarchical structure and the laser’s input energy is systematically studied, discussed and explained. The same energy density with different SLM process parameters result in different material properties. The higher energy density of the SLM produces larger cellular structures and crystal grains. A different energy density produces different textures with only one predominant texture component, which was revealed by electron-backscatter diffraction. Furthermore, three possible explanations for the origin of the dislocations are proposed.
Maraging steel grade18Ni300 produced by powder bed fusion (PBF) in its as built condition was plasma nitrided at three different temperatures. The aim of the work was to investigate the impact of the ...nitriding temperature on the microstructural changes as well as on the surface properties such as hardness, wear and corrosion resistance. The microstructural features in the bulk as well as in the nitride layer were investigated using electron-backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-Ray diffraction (XRD) analysis. The bulk microstructure consists of martensite with a small amount of retained austenite, the amount of which increases with a higher nitriding temperature. The nitriding process also causes the formation of precipitates and can therefore also act as an aging treatment. A specific lamellar structure occurs on the surface during the nitriding process, which in the majority of cases consists of the Fe4N phase. The retained austenite also transforms during nitriding to the nitride phase Fe4N. It was found that nitriding at higher temperatures leads to the formation of cracks in the nitride layer. The crack formation is related to nano and micro segregation during the LPBF. These segregations lead to austenite formation, which also takes place along the grain boundaries and transforms during nitriding to Fe4N. Higher nitriding temperatures lead to a thicker nitride compound layer and to better wear resistance. The impact of the cracks on the static mechanical properties is negligible. However, the corrosion resistance is governed by the formation of cracks at higher nitriding temperatures.
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•A higher nitriding temperature increases the amount of austenite and causes cracks.•During nitriding, the austenite on the surface transforms to Fe4N and TiN.•Cracks are related to segregations and transformations of the austenite to nitrides.•A higher wear resistance is obtained at higher nitriding temperatures.•A corrosion enhancement is seen at lower nitriding temperatures.
A single component can often benefit from being built using more than a single processing technique. Here, we investigated the hybrid additive manufacturing (HAM) of Ti6Al4V using a combination of ...powder-bed fusion (PBF) and direct-energy deposition (DED). The aim was to identify critical areas and assess the performance of the hybrid process relative to the individual processes. The PBF sub-parts were built first, and then completed by DED. The builds were in the horizontal and vertical directions, so we could observe the mechanical anisotropy relative to the build direction. X-ray computed tomography, microstructural examinations, and tensile testing coupled with digital image correlation were employed to assess the parts. The as-built PBF surface can be used to build HAM Ti6Al4V samples with DED, thus eliminating steps like machining. The HAM samples built in horizontally had intermediate tensile strengths of about 1050 MPa, and in the vertical direction, about 860 MPa, i.e., lower than the DED samples. Strength-wise, horizontally built parts exceeded the requirements. However, a reduction in deposition size (especially <102 mm2) promoted a different temperature evolution and, in the worst-case scenario, heat accumulation, which led to the formation of an undesirable microstructure and local plastic deformation in the DED part.
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