Reaction sintering of Fe-Al-Si alloys

The present study concerns the production of multiphase alloys of the Fe-Al-Si system using a powder metallurgical approach. Several compositions have been considered based on α-Fe, α′-FeAl; and α″-Fe₃Al intermetallic phases. Elemental powder mixtures were compacted and sintered in a dilatometer. In this way, the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperature. The effect of heating rate has been considered. The characterization of the final products was based mainly on x-ray diffraction (XRD) analyses, particularly as concerns the quantification of the crystalline phases present in the final products. Grain and porosity morphologies were characterized by scanning electron microscopy (SEM). In this way, clear indications of the reaction and densification processes occurring during the sintering treatments were obtained.     

Influence of Atomizing Gases on the Oxide-Film Morphology and Thickness of Aluminum Powders

Fine powders of aluminum were produced in a pilot-plant, inert-gas atomizerwith a confined-design nozzle, which operated vertically upward. Argonand helium at 1.85 MPa and nitrogen at 1.56 MPa were used as the atomizingagent. The morphology of the powder particles was examined by SEM. Powderswere sieved dry and wet. The Sauter mean diameter of the powders varied from20.70 to 10.25 m depending on the atomizing gas. The distribution ofsizes was bimodal. The mean thickness of oxide on the surface of the powderwas calculated from the total oxygen contents of powder samples (determinedby a Leco analyzer). In addition, ESCA measurements and BET tests werecarried out for surface-oxide thickness and area measurements,respectively. The finest powder produced under helium incorporated thinnersurface-oxide layers than the coarser ones produced under argon andnitrogen. This was due to differences in physical properties (such asdensity, thermal conductivity) and flow properties (such as gasvelocity and relative velocity) of the atomizing gases used, i.e., helium,argon, and nitrogen. The oxide was very irregular in thickness in thecoarse-size range of the Al powders produced under argon and nitrogen. Thiswas presumably because of the high- and low-temperature oxidation ofaluminum droplets during the atomization and subsequent solidification andcooling periods leading to the rough surfaces observed with SEMinvestigation in the present work.     

Size and pressure effects on solid transition temperatures of ZrO2

Transition temperatures between tetragonal and monoclinic polymorphs of ZrO₂ nanoparticles, thin films and nanostructured materials are calculated by considering energetic contributions of surface (interface) energy and surface stress on total Gibbs free energy. The transition temperatures drop as the size of the nanocrystals decreases, which is consistent with available results.     

Sensing tool breakage in face milling with a neural network

A new approach using a neural network to process the features of the cutting force signal for the recognition of tool breakage in face milling is proposed. The cutting force signal is first compressed by averaging the cutting force signal per tooth. Then, the average cutting force signal is passed through a median filter to extract the features of the cutting force signal due to tool breakage. With the back propagation training process, the neural network memorizes the feature difference of the cutting force signal between with and without tool breakage. As a result, the neural network can be used to classify the cutting force signal with or without tool breakage. Experiments show this new approach can sense tool breakage in a wide range of face milling operations.     

Internal Stresses, Corrosion and Electrochemical Behavior in Soils, and Stress Corrosion of Pipe Steels

The results of field tests carried out at main gas pipelines (MP) are shown. The distribution of internal stresses along the MP perimeter is obtained. Polarization currents at differently strained pipeline fragments are determined in soils taken at different distances from the failure epicenter. The mechanoelectrochemical effect is shown to be twice greater in failure soils. Its value can serve as a criterion of the stress corrosion probability.     

Surface finish generated in hard turning of quenched alloy steel parts using conventional and wiper ceramic inserts

Significant progress has already been achieved in green manufacturing including dry and hard, often high-speed, machining technologies. For instance, the demand for higher productivity has resulted in the wider application of ceramic and PCBN tools with special multi-radii (wiper) geometry. This paper reports some important characteristics of the surface roughness produced in the turning of a hardened low-chromium alloy steel using mixed alumina–titanium carbon (TiC) ceramic cutting tools equipped with both conventional and wiper inserts. The characteristic geometrical features of surfaces obtained in both these turning operations have been assessed by means of representative two-dimensional (2D) surface roughness parameters, and some 3D visualizations, which allowed more complete characterization of the surface topography and prediction of its service properties. Results show that keeping equivalent feed rates, i.e. 0.1 mm/rev for conventional and 0.2 mm/rev for wiper tools, the surfaces obtained have similar 3D height roughness parameters, and comparable values of skew and kurtosis. At defined cutting parameters, surfaces produced by wiper tools contain blunt peaks with distinctly smaller slopes resulting in better bearing properties. Only marginal changes of Ra parameter were recorded during 15 min machining trials.     

Role of platinum in the Na2SO4-Induced hot corrosion resistance of aluminum duffusion coatings

Electrochemical corrosion measurements have been carried out with Pt-containing and Pt-free Al-diffusion coatings on IN 738 LC in a 90Na₂SO₄+ 10K₂SO₄ (mol%) melt at 1173 K. Pt improves the resistance to basic fluxing while there are no significant differences between both coating types in their resistance to acidic fluxing. The corrosion resistance of the Pt-containing coating is also higher in the passive potential region where protective scales rich in Al₂O₃ are formed. The reason for the different behavior of both coating types appears to be related to the high corrosion resistance of the Pt-rich surface layer of the coating and an increased Al₂O₃ content in the scale of the Pt-containing type.     

Suppression of carbon monoxide formation in oxide-coated TZM molybdenum X-ray rotating anodes

Higher performance designs for rotating anode X-ray tubes have increased the average rotating anode temperature from below 1100 °C to well above 1300 °C. This temperature increase has accelerated the formation of carbon monoxide by reaction of carbon from the alloy substrate with oxygen from the emissive coating. The dominant carbon source is thought to be Mo₂C grain boundary precipitates in the TZM molybdenum alloy substrate. The dominant oxygen source is thought to be TiO in the emissive coating. Placement of a monocarbide-forming reactive layer between the alloy substrate and the emissive coating has been demonstrated to lower the thermodynamic activity of the carbon source and dramatically reduce the rate of formation of carbon monoxide.