Fatigue strength of tungsten inert gas-repaired weld joints in airplane critical structures

In this work the effect of Gas Tungsten Arc Welding (GTAW) repairs on the axial fatigue strength of an AISI 4130 steel welded joint used in airframe critical to the flight-safety was investigated. Fatigue tests were performed at room temperature on 0.89 mm thick hot-rolled plates with constant amplitude and load ratio of R = 0.1, at 20 Hz frequency. Monotonic tensile tests, optical metallography and microhardness, residual stress and weld geometric factors measurements were also performed. The fatigue strength decreased with the number of GTAW repairs, and was related to microstructural and microhardness changes, as well as residual stress field and weld profile geometry factors, which gave origin to high stress concentration at the weld toe. (C) 2011 Elsevier B.V. All rights reserved.

Tensile flow behaviour of 2.25Cr-1Mo ferritic steel base metal and simulated heat affected zone structures of 2.25Cr-1Mo weld joint

Tensile tests in the temperature range 298 to 873 K have been performed on 2.25Cr-1Mo base metal and simulated heat affected zone (HAZ) structures of its weld joint, namely coarse grain bainite, fine grain bainite and intercritical structure. Tensile flow behaviour of all the microstructural conditions could be adequately described by the Hollomon equation (sigma = K-1 epsilon(n1)) at higher (> 623 K) temperatures. Deviation from the Hollomon equation was observed at low strains and lower (< 623 K) temperatures. The Ludwigson modification of Hollomon's equation, sigma = K-1 epsilon(n1) + exp (K-2 + n(2) epsilon), was found to describe the flow curve. In general, the flow parameters n(1), K-1, n(2) and K-2 were found to decrease with increase in temperature except in the intermediate temperature range (423 to 623 K). Peaks/plateaus were observed in their variation with temperature in the intermediate temperature range coinciding with the occurrence of serrated flow in the load-elongation curve. The n(1) Value increased and the K-1 value decreased with the type of microstructure in the order: coarse grain bainite, fine grain bainite, base metal and intercritical structure. The variation of nl with microstructure has been rationalized on the basis of mean free path (MFP) of dislocations which is directly related to the inter-particle spacing. Larger MFP of dislocations lead to higher strain hardening exponents n(1).

Orientation preference and fractal character of short fatigue cracks in a weld metal

Short fatigue crack behaviour in a weld metal has been further investigated. The Schmid factor and the fractal dimension of short cracks on iso-stress specimens subjected to reversed bending have been determined and then applied to account for the distribution and orientation characteristics of short fatigue cracks. The result indicates that the orientation preference of short cracks is attributed to the large values of Schmid factor at relevant grains. The Schmid factors of most slip systems, which produced short cracks, are less than or equal to 0.4. Crack length measurements reveal that short crack path, compared to that of long crack, possesses a more stable and relatively larger value of fractal dimension. This is regarded as one of the typical features of short cracks.

Initiation and propagation of short fatigue cracks in a weld metal

Fatigue tests were performed using a purpose designed triangular shaped specimen to investigate the initiation and propagation of short fatigue cracks in a weld metal. It was observed that short fatigue cracks evolved from slip bands and were predominantly within ferrite grains. As the test progressed, the short crack density increased with minor changes in crack length. The growth of short cracks, in the early stage resulted mainly from coalescence with other existing cracks. The mechanism of short crack behaviour is discussed.

Determination of the causes of weld-metal cracking in high-strength steels and the development of heat-treatable low-alloy-steel filler wires for use with the inert-gas-shielded arc-welding process

"Materials Central, Contract no. AF 33(616)-5878, Project no. 7351."

Elemental partitioning and microstructural development in duplex stainless steel weld metal

A thermodynamic analysis which is capable of estimating the austenite/ferrite equilibria in duplex stainless steels has been carried out using the sublattice thermodynamic model. The partitioning of alloying elements between the austenite and ferrite phases has been calculated as a function of temperature. The results showed that chromium partitioning was not influenced significantly by the temperature. The molybdenum, on the other hand, was found to partition preferentially into ferrite phase as the temperature decreases. A strong partitioning of nickel into the austenite was observed to decrease gradually with increasing temperature. Among the alloying elements, average nitrogen concentration was found to have the most profound effect on the phase balance and the partitioning of nitrogen into the austenite. The partitioning coefficient of nitrogen (the ratio of the mole fraction of nitrogen in the austenite to that in the ferrite) was found to be as high as 7.0 around 1300 K. Consequently, the volume fraction of austenite was influenced by relatively small additions of nitrogen. The results are compared with the experimentally observed data in a duplex stainless steel weld metal in conjunction with the solid state δ → δ + γ phase transformation. Particular attention was given to the morphological instability of grain boundary austenite allotriomorphs. A compariso between the experimental results and calculations indicated that the instability associated with irregular austenite perturbations results from the high degree of undercooling. The results suggest that the model can be used successfully to understand the development of the microstructure in duplex stainless steel weld metals.

Laser weld: microstructure and corrosion study of Ag-Pd-Au-Cu alloy of the dental application

The laser Welding process was introduced into dentistry by the end of the 1980s, resulting on a great impulse to that area with the development of cheaper and smaller equipment, using simpler technique. This allowed greater use of that process on the confection of prostheses compared to the brazing process since the heat source for that process is a concentrated light beam of high power, which minimizes distortion problems on the prosthetic pieces. Ag-Pd-Au-Cu alloy used on the confection of dental implant prostheses was observed before and after subjection to the laser welding process. The microstructure was analyzed with the. use of optic microscopy and the corrosion resistance was studied by the traditional electrochemical techniques and by electrochemical impedance, under environmental conditions simulating the aggressiveness found in the mouth cavity. A structural change was detected on the weld area, which presented a refined microstructure deriving from the high-speed cooling. The base metal out of the weld area presented a fusion coarse microstructure. The electrochemical essays showed differences on the potentiodynamic polarization behavior in both weld and metal base areas, indicating superior corrosion resistance in the weld area. The impedance spectra were characterized by capacitive distorted components, presenting linear impedance in the low frequencies area. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Automatic segmentation of the secondary austenite-phase island precipitates in a superduplex stainless steel weld metal

Duplex and superduplex stainless steels are class of materials of a high importance for engineering purposes, since they have good mechanical properties combination and also are very resistant to corrosion. It is known as well that the chemical composition of such steels is very important to maintain some desired properties. In the past years, some works have reported that γ 2 precipitation improves the toughness of such steels, and its quantification may reveals some important information about steel quality. Thus, we propose in this work the automatic segmentation of γ 2 precipitation using two pattern recognition techniques: Optimum-Path Forest (OPF) and a Bayesian classifier. To the best of our knowledge, this if the first time that machine learning techniques are applied into this area. The experimental results showed that both techniques achieved similar and good recognition rates. © 2012 Taylor & Francis Group.

Study of the initial stage of solid electrolyte interphase formation upon chemical reaction of lithium metal and N-Methyl-N-Propyl-Pyrrolidinium-Bis (Fluorosulfonyl) Imide

Chemical reaction studies of N-methyl-N-propyl-pyrrolidinium-bis(fluorosulfonyl)imide-based ionic liquid with the lithium metal surface were performed using ab initio molecular dynamics (aMD) simulations and X-ray Photoelectron Spectroscopy (XPS). The molecular dynamics simulations showed rapid and spontaneous decomposition of the ionic liquid anion, with subsequent formation of long-lived species such as lithium fluoride. The simulations also revealed the cation to retain its structure by generally moving away from the lithium surface. The XPS experiments showed evidence of decomposition of the anion, consistent with the aMD simulations and also of cation decomposition and it is envisaged that this is due to the longer time scale for the XPS experiment compared to the time scale of the aMD simulation. Overall experimental results confirm the majority of species suggested by the simulation. The rapid chemical decomposition of the ionic liquid was shown to form a solid electrolyte interphase composed of the breakdown products of the ionic liquid components in the absence of an applied voltage.

Atmospheric-microplasma-assisted nanofabrication : metal and metal–oxide nanostructures and nanoarchitectures

In this paper, we report on the fabrication of Mo-oxide nanostructures and nanoarchitectures using an atmospheric-microplasma (AMP) system. This AMP system shows a high degree of flexibility and is capable of producing several different nanostructures and nanoarchitectures by varying the process parameters. The low-cost and simplicity of the process are important characteristics for nanomanufacturing, and AMPs offer such advantages. In addition, AMPs have shown the ability of promoting self-organization of nanostructures. © 2009 IEEE.

Satellites in the X-ray photoelectron spectra of transition-metal and rare-earth compounds

An attempt has been made at synthesis and in resolving some of the uncertainties related to the assignments of charge-transfer satellites in the X-ray photoelectron spectra of transition-metal and rare-earth compounds. New satellites are reported in the ligand core-hole spectra as well as in the metal core-level spectra of oxides of second- and third-row transition metals including rare earths. Satellites in the ligand levels and the metal levels tend to be mutually exclusive, a behaviour that can be understood on the basis of metal-ligand overlap. Systematics in the intensities and energy separations of satellites in the first-row transition-metal compounds have been examined in order to gain an insight into the nature of these satellites. A simple model involving the sudden approximation has been employed to explain the observed systematics in intensities of satellites appearing next to metal and ligand core levels on the basis of metal-ligand overlap.

Synthesis of Azo Coupled Crown Ethers Containing Transition Metal and other Chromophoric Environments and Their Ion Complexation Properties

The crown ethers, 2,3-benzo-1,4,7,10,13-pentaoxa-cyclopentadeca-2-ene and 2,3, ll,12-dibenzo-l,4,7,10,13,16-hexaoxscyclooctadeca-2,11-diene are incorporated into H,N'-ethylenebis(acetylacetoneimino) nickel(II) and copper(II), phenol, and β-naphthol by diazo coupling reactions. The selective nature of the coupling reaction has-been demonstrated by the isolation of both asymmetric mono- and symmetric bis(glyoxalarylcrownhydrazoneimino) metal(II) complexes. An interesting binuclear complex containing two intramolecularly rearranged (glyoxal-hydrazonearylimino) metal(II) groups joined by 18-crown-6 result8 when bis(arenediazonium)-18-crown-6 is coupled with the metal(I1) Schiff bases. The substituted ethers form cationic salts with NaClO4, KCNS, NH4CNS, 14g(CNS)2 and Ca(CNS)2. All the synthesised ethers exhibit ion selectivity sequence as K+ > Na+ and Ca2+ > Mg2+.