On the Cooling Rate-Microstructure Relationship in Molten Metal Gas Atomization

Abstract Gas atomization is the most used powder production technique since it provides good control on particles shape, surface oxidation and dimension. It is a rapid solidification technique involving fast cooling rates, which are strictly correlated to particle size. This relationship is typically described with a power law function that can be determined experimentally by measuring the microstructural length-scale or through the application of a heat transfer model. Both paths were exploited in the present work focusing on a gas-atomized Al-4.5Cu alloy. Atomized powders were characterized by means of X-ray diffraction, differential scanning calorimetry, light and scanning electron microscopy to investigate the relationship between cooling rate and microstructure length-scale. A recently proposed semi-empirical model was validated and discussed in terms of a physically based heat transfer approach. The change in gas-to-melt mass flow ratio (GMR) was also investigated showing that it does not affect appreciably the relationship between solidification rate and particle size, but does increase the Cu supersaturation in the powders of finer size

A time-saving and cost-effective method to process alloys by Laser Powder Bed Fusion

Definition of the main process parameters, laser power (P), scanning speed (v), hatching distance (hd) and scanning strategy useful for producing dense samples, is fundamental to develop novel alloy compositions for Laser Powder Bed Fusion (LPBF). The present work has two aims: on one side, to verify the processability of a new AlSi10Mg + 4Cu alloy of mixed powders by LBPF; on the other side, on the basis of the experimental analysis, to define a method for processing new alloys. Producing Single Scan Tracks (SSTs), samples with unidirectional scanning strategy and samples with 67° rotated scanning strategy, the proper P-v-hd combinations were identified reaching a final porosity lower than 1.5%. A scenario of hardness and build-up rate vs. energy density is given, to adopt the main process parameters suitable to maximize mechanical properties or productivity. According to the novel method, P-v-hd combinations can be defined through the production and characterization of SSTs and samples with 67° rotated scanning strategy. Through two production steps dense samples can be then obtained, allowing the development of new compositions saving time and reducing costs related to the powder usage. Keywords: Laser Powder Bed Fusion (LPBF), Single scan tracks (SSTs), Hatching distance (hd), Overlap, Process parameters, Hardness, Build-up rat

A possible role for nitric oxide in modulating the functional cyclosporine toxicity by arginine

A possible role for nitric oxide in modulating the functional cyclosporine toxicity by arginine. The renal damage consequent to cyclosporine A (CsA) administration ranges from hemodynamic alterations to irreversible chronic lesions. The initial vasoconstriction depends upon the imbalance between the various modulators of the renal vascular tone, among which the most powerful are endothelins and nitric oxide (NO). CsA could play a crucial role by inhibiting the Ca++/calmodulin-mediated activation of the constitutive NO synthase (NOS) isoform, which converts L-arginine (L-Arg) into NO and citrulline, with a 1:1 stoichiometry. To investigate the possibility of modulating CsA nephrotoxicity with L-Arg we studied six groups (G) of Lewis rats treated with daily gavage up to eight weeks: G1, CsA 40 mg/kg; G2, G1 plus L-Arg 300 mg/kg; G3, G2 plus the competitive inhibitor of NOS, NG-nitro-L-Arg (L-NNA); G4, L-Arg alone; G5, L-NNA alone; and G6, controls receiving vehicle alone. After eight weeks L-Arg treated rats were protected against the toxic effects of CsA [creatinine (Cr) values, G2, 0.62 ± 0.05 mg/dl vs. G1, 0.99 ± 0.16 mg/dl, P < 0.001; proteinuria (P), G2, 7.2 ± 1.02 mg/day vs. G1, 15.1 ± 1.9 mg/day, P < 0.01]. The administration of L-NNA abolished the protective effect of L-Arg (G3, Cr 1.23 ± 0.16 mg/dl; P 16.9 = 2.3; P < 0.02 and P < 0.005, respectively vs. G2). The levels of Cr in G2 rats were superimposable to control groups. The NOS activity, evaluated by measuring [3H]citrulline formation from [3H]L-Arg in kidney homogenates, was blocked by L-NNA in G5 (0.019 ± 0.009 pmol/min/mg proteins vs. G6 0.047 ± 0.002, P < 0.01). NOS activity was significantly increased versus controls in G1 (0.110 ± 0.032, P < 0.01) and G2 (0.088 ± 0.009, P < 0.02), while L-NNA reversed this phenomenon (G3, 0.052 ± 0.03). The expression of mRNA encoding for cNOS and iNOS was only slightly increased in CsA-treated rats. We suggest that CsA treatment increases NOS activity in the kidney by a mechanism which does not require a de novo synthesis of the enzyme. Such an increase, that may be devoted to counterbalance the vasoconstrictive effects of the drug, is unable to reduce the toxic effect of CsA in the absence of exogenous L-Arg. Administration of L-Arg is likely to reduce CsA nephrotoxicity by accomplishing the higher request of activated NOS for its substrate, thus potentiating NO synthesis in the kidney.