Metal additive manufacturing has emerged as a new manufacturing option for aerospace and biomedical applications. The many challenges that surround this new manufacturing technology fall into several ...different categories. The paper addresses one of these categories, the physical mechanisms that control the additive manufacturing process. Physical mechanisms control the effects of processing parameters on microstructures and properties of additively manufactured parts. Some mechanisms might not have been recognized, yet, and for those that are currently known, detailed quantitative predictions have to be established. The physical mechanisms of metal additive manufacturing are firmly grounded in metallurgy, branching into laser physics and the physics of granular materials. Powder bed additive manufacturing is described from the powder storage to post-processing and elements of metallurgy are highlighted that are relevant for the different aspects of the additive manufacturing process. These elements include the surface reactions on powder particles, the heating and melting behavior of the powder bed, solidification, and post-processing. This overview of the different metallurgical aspects to additive manufacturing is intended to help guide research efforts and it will also serve as a snapshot of the current understanding of powder bed additive manufacturing.
Laser powder bed fusion (LPBF) additive manufacturing (AM) has been adopted by various industries as a novel manufacturing technology. Powder spreading is a crucial part of the LPBF AM process that ...defines the quality of the fabricated objects. In this study, the impacts of various input parameters on the spread of powder density and particle distribution during the powder spreading process are investigated using the DEM (discrete element method) simulation tool. The DEM simulations extend over several powder layers and are used to analyze the powder particle packing density variation in different layers and at different points along the longitudinal spreading direction. Additionally, this research covers experimental measurements of the density of the powder packing and the powder particle size distribution on the construction plate.
The thermal diffusivity of powder bed plays a crucial role in laser powder bed fusion (LPBF) additive manufacturing. The mechanical properties of the parts built by LPBF are immensely influenced by ...the thermal properties of the powder bed. This study aims to measure the thermal diffusivity of metallic powder, nickel-based super alloy Inconel718 (IN718), in LPBF using laser flash three-layered analysis in a DLF1600 instrument, which incorporates a special powder cell to encapsulate the powdered sample. Measurements were performed at different temperatures. The thermal diffusivity of several reference samples was measured for the purpose of validating the test results, and it was compared to published data for identical measures. It was observed that experimental results for powder samples were smaller than the actual thermal diffusivity of the sample. R software analysis was used to analyze test data in order to obtain powder thermal diffusivity values that were close to the actual values.
•Energetically stable geometries are determined for various concentrations of oxygen, nitrogen and hydrogen on hexagonal close packed Ti(0001) surface.•The surface phase diagrams are determined to ...investigate the stability of various oxide, nitride and hydride phases as a function of finite temperature and gas partial pressure.•The spin polarization evolving as a consequence of structural changes for various gas concentrations of all adsorbed cases are described.
Density functional theory and ab initio thermodynamics studies are performed to investigate the interaction of oxygen, nitrogen and hydrogen molecules with hexagonal close packed Ti (0001) surfaces. The system is modeled as a slab where the concentration of the adsorbed atoms is systematically increased and the most favorable configuration is determined based on total energy comparisons. Varying tendencies are observed for the ground state structures of oxygen, nitrogen and hydrogen adsorption. Oxygen and hydrogen atoms are found to be adsorbed on the surface with increasing concentration and tend to migrate to interior layer only after surface saturation. However, nitrogen prefers to migrate into the interior layers. For all elements, the favorable position in the interior layers is the octahedral interstitial site. Taking into account the partial pressure of oxygen, nitrogen and hydrogen and the effect of temperature, we develop surface phase diagrams that display stable surface configurations. Our studies provide an atomic insight into the structure of the surface scale, passivation effects, and surface tension of Ti for advanced powder manufacturing processes and applications in catalysis.
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Additively manufactured (AM) components usually have nonequilibrium microstructures. Post-built heat treatments are recommended for AM components to achieve homogenous microstructures. In this study, ...the effects were investigated of conventional solutionizing and precipitation hardening (H-900) heat treatments on the microstructure evolution of 17-4PH AM and wrought components. Microstructural characterization techniques including SEM, TEM and EBSD analysis were used on 17-4PH AM and wrought components to obtain quantitative information about the microstructure and phase evolution during these heat treatments. These microstructural studies demonstrate that 17-4PH AM components can achieve microstructures and hardnesses similar to those of wrought samples by post-built heat treatments.
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•As-built 17-4 PH has steel an inhomogeneous columnar microstructure composed of ferrite, martensite, and some austenite.•Solution heat treatments homogenized the grain structure and eliminated the {100} texture observed in as-built samples.•Oxide inclusions pinned grain boundaries during heat treatment thereby refining the grain structure.•Precipitation heat treatment yielded Cu-rich nano-precipitates without further changes in the grain structure.•Heat treatments can achieve comparable microstructures and hardness in additively manufactured and wrought 17-4 PH samples.
Ti-6Al-4V powders from six different vendors were compared with respect to their microstructures, size-distributions, chemistries, surface appearances, flow behavior, and packing densities. The ...analysis approaches followed closely ASTM F3049, the standard guide for characterization of additive manufacturing metal powders. Chemistries, including impurity content, agreed well with the standard requirements. Powder particle microstructures revealed acicular alpha prime for all vendors. Measurable differences were observed primarily in the size-distributions and the flow behavior.
In this study, our goal is to design solid solution strengthened aluminum alloys for manufacturing technologies that involve high cooling rates. This investigation starts with an analysis of solid ...solution strengthening using first principles calculations to determine elastic property changes and local lattice distortions from the introduction of different elements into a host aluminum lattice. These results, coupled with both equilibrium and non-equilibrium solubility data, leads to the selection of cerium and cobalt as the primary candidate alloying elements. Alloys of AlCe and AlCo at concentrations of 0.5, 1.0, and 3.0 at. % are then synthesized and subjected to laser glazing to produce non-equilibrium microstructures. The microstructure and solid solution characteristics are determined using a combination of scanning electron microscopy and transmission electron microscopy. Furthermore, nanoindentation is used to measure the hardness showing that both candidate systems harden significantly after glazing. In addition, Al-1.0Co at. % achieves a hardness comparable to Al6061-T6. These results conclusively show that cerium and cobalt are promising elements in the next generation aluminum alloys which make use of non-equilibrium processing conditions such as additive manufacturing.
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•Design of new aluminum alloys using a combination of computational and experimental tools•First-principles calculations reveal lattice strains for Al-X systems (X: elements), leading to selection of Co and Ce•Laser surface glazing reveals phase formation and solubility extension for AlCo and AlCe systems•Nanoclusters develop in rapidly quenched AlCo alloys that vary from fcc to Al9Co2 phase depending on alloy composition•Hardness of Al-3at%Co laser glazed phase approximately twice that of Al6061-T6.
► Novel deformation mechanism in FeCo alloys via severe plastic deformation. ► Observation of the strip formation (“Noodles”) at an initial stage of milling. ► Self-nanoscaling that leads to grains ...of about 8nm. ► Correlation between the morphology and magnetic properties.
Nanocrystalline Fe100−xCox (x=20, 35, 50, 60) alloys have been prepared by mechanical alloying of Fe and Co powders via high energy ball milling. The alloy formation process and microstructure evolution of the samples have been investigated. Energy filtered transmission microscopy (EFTEM) observations revealed the strip formation of the Fe and Co phases at an initial stage of milling. The final grain size of the equiaxed grains in the obtained alloys reached 8nm upon milling for 20h. The saturation magnetization of the mixtures of Fe and Co increases with milling time, indicating an increasing homogeneity in composition and the phase formation. It is found that the saturation magnetization is also dependent on the Co content, which reaches the highest value of 240emu/g at Fe65Co35. The phase transformation of the as-prepared FeCo alloys was also studied using differential scanning calorimetry.
Fabrication of bulk nanocomposite materials, which contain a magnetically hard phase and a magnetically soft phase with desired nanoscale morphology and composition distribution has proven to be ...challenging. Here we demonstrate that SmCo/Fe(Co) hard/soft nanocomposite materials can be produced by distributing the soft magnetic α-Fe(Co) phase particles homogenously in a hard magnetic SmCo phase matrix through a combination of high-energy ball milling and a warm compaction. Severe plastic deformation during the ball milling results in nanoscaling of the soft phase with size reduction from micrometers to ~15 nm. Up to 35% of the soft phase can be incorporated into the composites without coarsening. This process produces fully dense bulk isotropic nanocomposite materials with remarkable energy-product enhancement (up to 300%) owing to effective inter-phase exchange coupling.
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