18O/SIMS characterization of the growth mechanism of doped and undoped α-Al2O3

Sequential oxidation experiments at 1200°C and 1500°C using¹⁶O and >95%¹⁸O-enriched environments were conducted on undoped and Y- and Zr-doped -NiAl and FeCrAl alloys. After oxidation, samples were analyzed by SIMS sputter depth profiling. At 1200°C, a clear pattern was established where the undoped -Al₂O₃ was found to grow by the simultaneous transport of both Al and O. Zr-doped -Al₂O₃ was found to grow mainly by the inward transport of oxygen. The profiles in all cases indicate O diffusion primarily by shortcircuit pathways. Results at 1500°C (only on -NiAl) indicated a similar behavior but were less conclusive. Y and Zr were found to segregate to the oxide grain boundaries at 1200°C and 1500°C. The segregation of these dopants is believed to impede the cation transport in the -Al₂O₃ scale and thereby change the oxidation mechanism.     

Preparation and electrochemical corrosion behaviour of 3D large-sized nanocrystalline copper bulk in solutions with chloride ions

ABSTRACT

A 3D large-sized nanocrystalline (NC) copper bulk was prepared by inert gas condensation and in situ warm compress, the density of which was 99.51% of theoretical value. Electrochemical methods and XRD、EDS、SEM technique were used to investigate the anodic polarisation behaviour of NC copper in solutions with chloride ions. NC copper exhibited a different anodic polarisation behaviour from its coarse-grain (CG) counterpart in 0.47?wt-% NaCl solution. No second passivity was observed for NC copper, while there was a notable second passivity for CG copper. Compared with CG copper, the corrosion resistance of NC copper decreased. The anodic polarisation behaviour of NC copper changed significantly when 10^4?ppm Cl^? was added in 0.3% Na₂SO₄ solution. Attributed to the CuCl protective film on NC copper, the critical current intensity decreased and the decreasing rate of anodic current intensity lowed notably in activity-passivity zone.     

Effect of the θ–α-Al2O3 Transformation in Scales on the Oxidation Behavior of a Nickel-Base Superalloy with an Aluminide Diffusion Coating

The high-temperature oxidation behavior of a nickel-base superalloy andaluminide-diffusion coating has been investigated over the temperaturerange from 800–1100°C and analyzed by TGA, XRD, EDAX, andSEM. The -NiAl coating was formed by low-pressure gas-phasecementation at 950°C for 3 hr. It was found that the formation of-Al₂O₃ from -Al₂O₃ on the -NiAl coating resulted in asharp decrease in the parabolic rate constant kp by one order ofmagnitude at 1050°C during transient oxidation. The transformationcaused the anomalous behavior of the oxidation kinetics curves of thisdiffusion coating in the temperature range 800–1100°C withinthe first 100 hr. A change in the morphology of scales also occurredwith the transformation. A growth stress was characterized by theformation of convoluted scales, which were observed on the surfaceafter oxidation. The oxidation mechanism of this -NiAl diffusioncoating is described.     

Optimization of Ti6Al4V titanium alloy laser–arc hybrid weld parameters

This article reports the results of a study aimed at using statistical methods to optimize the parameters for laser–arc hybrid butt welding of Ti6Al4V titanium alloy sheets with a thickness of 3.0 mm. The study has examined the effects of the hybrid welding process parameters, such as laser beam power, arc pulse frequency, arc length, arc current, wire speed, laser and arc relative positions, and weld speed. Microstructure has been studied using light microscopy and morphological analysis of weld bead cross sections. This article reports the results of energy and morphological tests.     

High-temperature oxidation behavior of pure Ni3Al

The high-temperature oxidation behaviour of pure Ni₃Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni₃Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni₃Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni₃Al was oxidized at 1000°C, Al₂O₃ was present as -Al₂O₃ in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable -Al₂O₃ to stable -Al₂O₃ was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of -Al₂O₃ was observed beneath a thin and fine-grain outer layer of -Al₃O₃. The oxidation mechanism of pure Ni₃Al is described.     

Measurement of groove and weld bead shape by spot laser sensor

Abrasion is one of the most significant forms of wear affecting mechanical parts. It is mainly reduced by hardening the surfaces involved and one of the systems used involves the application of brazing to deposit tungsten carbides as antagonists to abrasion.

This paper analyses the types and the characteristics of the carbides commonly used, evaluates the oxy-acetylene flame depositing system, and describes the major forms present on the market and their developments. The paper also describes the latest improvement trends for obtaining increased performance and compares laboratory test results.     

Effect of homogenisation heat treatment on heat affected zone cracking susceptibility.Study of weldability of Inconel 718 cast alloy (3rd Report)

ALW has developed a new tungsten inert gas (TIG) robotic welding process which combines the quality of the TIG process with the productivity of the MIG process. The keystone is the particular configuration of the torch: the weld wire is fed directly into the arc zone, where the temperatures are higher resulting in ‘continuous liquid-flow’ transfer. This configuration also offers the advantage of reduced overall dimensions and greater accessibility of the torch for robotic welding of complex geometries. There is no longer any need to position and direct the weld wire with respect to the torch and the joint to be welded. Hence, the sixth axis of the robot is free.

The applications to date relate to the welding of stainless steel containers, butt and lap welding, carbon steel mechanical parts with intermittent beads in the thin or medium thickness range. Applications on thin, galvanised sheeting, using CuSi₃ wire with no spatter, have been successfully tested in the weld-brazing field, in partnership with the automobile industry. Other applications may include the welding of stainless steels and light alloys for the food and furniture sectors, among others.     

Theoretical analysis of grain boundary liquation in heat affected zone of Inconel 718 alloy. Study of laser weldability of Ni‐base superalloys (3rd Report)

In this work, with the aid of the implicit gradient method, the fatigue behaviour of different types of arc-welded joints was analysed. The structural parts, made of steel, were modelled as three-dimensional solids without applying any geometric exemplification, directly calculating the fatigue life numerically. The advantage offered by this approach is that of being able to maintain the unvaried calculation procedure regardless of the type of joint in question. Moreover, adopting the gradient method, there is no obligation to impose particular rules for the creation of the calculation mesh on the finished elements.