Feedstock powder processing research needs for additive manufacturing development

Additive manufacturing (AM) promises to redesign traditional manufacturing by enabling the ultimate in agility for rapid component design changes in commercial products and for fabricating complex integrated parts. By significantly increasing quality and yield of metallic alloy powders, the pace for design, development, and deployment of the most promising AM approaches can be greatly accelerated, resulting in rapid commercialization of these advanced manufacturing methods. By successful completion of a critical suite of processing research tasks that are intended to greatly enhance gas atomized powder quality and the precision and efficiency of powder production, researchers can help promote continued rapid growth of AM. Other powder-based or spray-based advanced manufacturing methods could also benefit from these research outcomes, promoting the next wave of sustainable manufacturing technologies for conventional and advanced materials.     

Electron beam melting of Inconel 718: effects of processing and post-processing

ABSTRACT

This study reports the effect of process temperature on microstructure evolution of electron beam melted Inconel 718. Samples fabricated at 915°C had fine grain boundary δ (～200?nm) along with coarse intragranular δ spanning through the length of the grains. On the other hand, samples fabricated at 990°C, only had grain boundary δ along with secondary carbides. During hot isostatic pressing, the distribution of carbides governs the grain growth vs. lack of it. The samples fabricated at 990°C having grain boundary carbides had no grain growth owing to the pinning effect of carbides. In contrast, the sample processed at 915°C had significant grain growth owing to dissolution of grain boundary δ phase and absence of grain boundary carbides.     

Application of the fung equation to the constant strain rate behavior of plasticized polyvinyl chloride

A quasilinear viscoelastic equation originally proposed by Fung to characterize the uniaxial viscoelastic behavior of rabbit mesentary is used in this study to characterize the viscoelastic behavior of plasticized polyvinyl chloride for strains up to 24%. An experimentally determined relaxation function is used to predict the constant strain rate behavior of plasticized polyvinyl chloride. The predictive ability of the Fung equation is also compared with the well-known BKZ and Lianis theories. It was found that the Fung equation agrees quite closely with the BKZ and Lianis theories but that all three theories showed only moderate agreement with experiment.     

Nonlinear viscoelastic behavior of plasticized poly(vinyl chloride)

This paper shows how an empirical nonlinear creep relation suggested by Leaderman might be justified on the basis of an approximate nonlinear viscoelastic constitutive equation. In addition, experimental evidence is presented which tends to support the proposed theory.     

Additive Manufacturing of Nickel Superalloys: Opportunities for Innovation and Challenges Related to Qualification

Innovative designs for turbines can be achieved by advances in nickel-based superalloys and manufacturing methods, including the adoption of additive manufacturing. In this regard, selective electron beam melting (SEBM) and selective laser melting (SLM) of nickel-based superalloys do provide distinct advantages. Furthermore, the direct energy deposition (DED) processes can be used for repair and reclamation of nickel alloy components. The current paper explores opportunities for innovation and qualification challenges with respect to deployment of AM as a disruptive manufacturing technology. In the first part of the paper, fundamental correlations of processing parameters to defect tendency and microstructure evolution will be explored using DED process. In the second part of the paper, opportunities for innovation in terms of site-specific control of microstructure during processing will be discussed. In the third part of the paper, challenges in qualification of AM parts for service will be discussed and potential methods to alleviate these issues through in situ process monitoring, and big data analytics are proposed.     

Estimation of nucleation rate and growth rate from time dependence of global microstructural properties during phase transformations

An approach has been developed for calculating nucleation and growth rates from the time variation of the volume fraction, surface area, and integral mean curvature of the product phase during a phase transformation. The local growth rate of the product phase can be estimated without any assumption or knowledge regarding the nucleation behavior. The approach is applicable over the complete range of volume fractions(i.e., from zero to one). Practical feasibility of the approach has been demonstrated by deducing the nucleation and growth rates of austenite during austenitization of pearlite in an Fe-0.83 wt pct C alloy at 730 ‡C, 740 ‡C, and 750 ‡C. It is concluded that the local growth rate and nucleation rate of austenite remain constant during an isothermal austenitization of pearlite. Formerly with the Department of Metallurgical Engineering, Indian Institute of Technology, Kanpur, India     

Molecular cloning and physical mapping of the daptomycin gene cluster from Streptomyces roseosporus

BACKGROUND: Local macrophage proliferation has been described in several animal models of glomerulonephritis (GN), but its significance in human disease is unknown. METHODS: Double immunostaining for CD68 and the proliferating cell nuclear antigen (PCNA) was used to identify macrophage proliferation in 84 biopsies from a variety of glomerulonephridities. RESULTS: A small resident population of glomerular and interstitial CD68+ macrophages was identified in normal human kidney, of which only 1 to 2% showed evidence of proliferation on the basis of PCNA expression. A mild macrophage infiltrate, with only occasional proliferating macrophages, was seen in the less aggressive forms of GN (minimal change disease, non-IgA mesangioproliferative GN and IgA nephropathy). This was in sharp contrast to the more aggressive forms of disease (lupus class IV, vasculitis-associated GN, crescentic GN and mesangiocapillary proliferative GN), in which the prominent macrophage infiltrates contained many proliferating macrophages, accounting for 28 to 47% of the total macrophage population. Macrophage proliferation was largely restricted to areas of severe tissue damage (glomerular segmental proliferative lesions, crescents and foci of tubulointerstitial damage), suggesting that local proliferation is a mechanism for amplifying macrophage-mediated injury. Glomerular and interstitial macrophage proliferation gave a significant correlation with loss of renal function (P < 0.0001) and histologic lesions (P < 0.0001), but not with proteinuria. Interstitial T-cell proliferation also gave a significant correlation with loss of renal function and histologic damage, even though proliferation within the T-cell population was much lower than in the macrophage population. CONCLUSIONS: This study demonstrates that macrophage proliferation is a feature of the more aggressive forms of human GN. Local proliferation may be an important mechanism for amplifying macrophage-mediated renal injury. In addition, the degree of local macrophage proliferation may be a useful diagnostic and prognostic indicator for human GN.     

Chemical potential shifts due to capillarity-unary systems

The effect of curvature on the chemical potential for two-phase unary systems at constant temperature, is usually given in the literature by the Gibbs-Thomson equation [1–6],μβ(H)=μβ(H=0)+2γVβHwhere H is the local mean curvature of a β particle in equilibrium with an α matrix andV^β, the molar volume of the β phase. It can be shown, based on the above equation, that the corresponding shift in chemical potential for the α phase is [1]:μα(H)=μα(H=0)Thus the usual form of the Gibbs-Thomson equation that appears in the literature is not rigorous and hence results in a violation of the condition for chemical equilibrium for a curved interface (μ^α ≠ μ^β(H)) assuming incompressibility and isotropic stresses in the case of the solid phases [7–9]. A rigorous derivation of the Gibbs-Thomson equation for unary two-phase systems at constant temperature is provided, that gives the correct equation for the chemical potential shift:μβ(H)=μα(H)=μβ(H=0)+2γH[VαVβ/(Vα−Vβ)]While the above equation can be approximated (V^{α = G}−V^β≈V^G) in the case of a condensed-gas (G) system to give the usual form of the Gibbs-Thomson equation, a similar approximation for a condensed-condensed system can result in a significant error in both the sign and magnitude of the shift. The corrected expression for the chemical potential shift has an inverse dependence on the difference in the molar volumes (V^α−V^β) between the two condensed phases. For solid (S)-liquid (L) equilibrium in certain systems the shift may be negative (e.g.V^{α = L} < V^{β = S} for Bi, Si, Ge) or even approach infinity (e.g.V^{α = L}≈V^{β = S} for carbon). A similar dependence is obtained for the pressure shifts in each phase. The present formulation is consistent with the Clausius-Clapeyron equation used to represent unary phase diagrams, which has the same dependence on the relative molar volume.