Tomiko Yamaguchi

Modeling of the yoke-magnetization in MFL-testing by finite elements

Abstract The yoke-magnetization is very popular in magnetic particle testing of welds. The detectability of a flaw by using this method largely depends on the magnetic flux density passing through a specimen to be examined or on the intensity of magnetic field acting in/on the specimen. In Japan inspectors have to check and confirm the appropriate magnetizing situation of the specimen by using an A-type standard test specimen specified in the standard JIS G 0565-1992. The development of indications by magnetic particles on the standard specimen is influenced by the air gap between the standard specimen and the specimen surface to be examined. Since the height and breadth of an artificial flaw in the standard specimen also influence the leakage of the magnetic flux density from the flaw, the information about the magnetizing situation is complex. In this paper we first identify influences of some factors on the magnetic leakage flux density from an artificial flaw in the standard specimen by using FEM modeling. Since the check with the standard specimen gives not a unique information to the magnetization state we investigate the technique, in which intensity of magnetic field acting on the specimen surface is used to characterize the magnetization. A finite element approach is applied to model the magnetization situation. The effectiveness of the modeling is confirmed by an experiment.

FEM study on the influence of air gap and specimen thickness on the detectability of flaw in the yoke method

The yoke method is usually used as a magnetic testing method of welds. In this paper, we study the influences of the air gap between the magnetic pole and the specimen surface on the average magnetic flux density passing through the specimen, and the specimen thickness on the leakage magnetic flux density from a flaw using finite element method (FEM). When the air gap increases the average magnetic flux density at the center of the specimen length decreases. We can estimate the intensity of the magnetic field on the specimen surface by extrapolating the magnetic flux density in space to that at lift-off being zero. Moreover, the maximum leakage magnetic flux density from a flaw decreases with increase in the specimen thickness even if the average magnetic flux density passing through the specimen is the same.

Lap welding of titanium and mild steel sheets by seam welding

Abstract Titanium, a metal with superior sea water corrosion resistance, is increasingly used in titanium clad steel applications for ocean structures. Titanium clad steel is normally manufactured by hot rolling or explosive deposition. Joining of titanium and iron leads to the formation of hard and brittle intermetallic compounds that impair the bond strength. It is therefore necessary to use an insert metal to suppress the occurrence of these compounds, particularly during rolling.1–3

Transparent conductive oxide-less dye-sensitized solar cells (TCO-less DSSC) with titanium nitride compact layer on back contact Ti metal mesh

Transparent conductive oxide-less dye-sensitized solar cells with back contact Ti electrodes covered by thin compact titanium nitride (TiN) layer were studied. The TiN compact layer was fabricated on the surface of Ti wire by exposing Ti wires to laser under nitrogen atmosphere. The photovoltaic performance was improved after the compact TiN blocking layer was fabricated on the Ti wire. These results were explained by the suppression of charge recombination between electrons in Ti wires and I3- in electrolytes by the TiN compact layer.Graphical Abstract

Microstructure and Wear Behavior of Cermet/Iron Alloy Cladding Layers on A6061 Alloy Coated by Resistance Seam Welding Method

Cermet/iron alloy cladding layers were coated on the surface of Al–Mg–Si alloy (A6061) plates by resistance seam welding method with tungsten carbide (WC) and high-carbon iron alloy (SHA) powders. The cladding layer consisted of WC reinforcement, SHA binder, A6061 and FeAl3. The effect of WC ratio (30 wt%, 50 wt% and 70 wt%) on the microstructure and wear behavior of the cladding layers was investigated in detail. Abrasive wear test was performed under two kinds of load condition by using a rubber wheel apparatus to evaluate wear resistance. The results showed that the wear resistance of the cladding layer was improved by 3.5–5 times than that of the substrate. At lower load, the wear resistances of the samples 30% and 70% WC were nearly the same, which suggested that FeAl3 played an important role in improvement of the wear resistance instead of WC. While at higher load, the amount of WC determined the wear resistance of the cladding layer. Furthermore, wear behavior of these cladding layers was explained with reference to the observed microstructure of the worn surface.

Interface Microstructure and Weld Strength of Steel/Aluminum Alloy Joints by Resistance Spot Welding

In this study, in order to investigate effects of welding conditions and alloy element Cu in aluminum alloy on growth of intermetallic compounds at weld interfacial zones and weld strength of steel/aluminum alloy joints, the pure aluminum A1050 and the aluminum alloy A2017 to cold-rolling steel SPCC were welded by resistance spot welding. Two intermetallic layers were observed, and the major phases are Fe2Al5 adhering to the steel and FeAl3 adhering to the aluminum. The results of EPMA suggested that diffusion of alloy element Cu in the A2017 happened at the interface, which may be the reason for that the anisotropic growth of Fe2Al5 might be inhibited. The tensile-shear strength of A2017/SPCC joint decreased with an increasing of the thickness of r=0.5, considering the nugget radius as 1.

Evaluation of welds of aluminum alloy AA6022-T4 welded using an electrode force changeable resistance spot welding machine

Resistance spot welding (called spot welding in the following) is used widely in industry, particularly in the automobile industry. In spot welding, welding is typically done with the flow of a fixed current for a fixed period of time, based on a fixed electrode force that is set initially. 1 However, the increase in temperature, expansion, softening, transformation and melting after the application of the electrode force and the hardening, cooling and contraction, after that force is stopped, are accompanied by moment by moment changes in the electrode force at the weld. The authors have developed a resistance welding machine that incorporates a mechanism with which the electrode force can be changed instantaneously, while the current is being applied. When spot welding was carried out on SPCC cold rolled steel sheets using this welding machine, it was possible to form excellent nuggets by lowering the electrode force from the base electrode force, while current was being applied, and it became clear that the maximum tensile shearing force for spot welded joints was obtained with this. Therefore, we took up aluminum alloy AA6022-T4, which is used as a material in passenger cars, in this study and performed spot welding with various conditions and varied the electrode force during welding. We carried out a comparative investigation of the effects these had on the characteristics of the welds and the results of welding using conventional methods. In addition, we carried out tensile shear tests on the welds, and examined the relationship between the welding conditions and the maximum tensile shear load. Furthermore, we carried out a multiple regression analysis and examinations to clarify the relationship between the data obtained from the tensile shear tests and the welding conditions. Furthermore, we took into consideration the macrostructure of the nuggets and observations of the SEM structure of cross-sections.

The Performance of a Wear Resistance Cladding Layer on a Mild Steel Plate by Electric Resistance Welding

This study deals with the performance of a 1 mm-thick wear resistance cladding layer of composite metal powder of 110~160 μm-sized STL-1(Co-Cr alloy) and 100 μm-sized MBF-15 (Ni based brazing alloy) applied by the resistance welding method to clad 1.9 mm-thick mild steel plate. Two metal powders were prepared as the cladding materials, in the ratio of two to one by weight. We studied the microstructures and results of EPMA of cladding layers to obtain the most suitable joint conditions. Also, bending and abrasion tests were performed to evaluate the bond-ability with the substrate and the wear behaviour of the cladding layer. The main results obtained are as follows: The wear resistance cladding layer had sound microstructures when it had been applied under the following welding conditions: welding current higher than 2.1 kA, welding speed of 1.0 m/min, and electrode force less than 392 N. In spite of cracks occurring in the cladding layer after the bending test, the cladding layer, applied under almost all welding conditions, was not separated from the substrate. We obtained the wear behaviour of the mild steel, 16.833 mg/min-wear loss and 0.046 mm/min-abrasion depth. The characteristics of the cladding layer, which had been applied with welding conditions — welding current of 2.5 kA, welding speed of 1.0 m/min, and electrode force of 196 N, had a satisfactory wear resistance behaviour of 15.726 mg/min-wear loss and 0.043 mm/min-abrasion depth.

Metallurgical characteristics of A5083-O aluminium alloy welded by high energy density welding

Summary Since electron beam welding and laser welding have a higher energy density than GTA and GMA welding, both methods allow higher speed welding. The cooling rates estimated in the present research study range between around 500–800°C/sec and around 300–1500°C/sec under the welding conditions respectively adopted in electron beam welding and laser welding. The results of line electron probe microanalysis (EPMA) suggest that magnesium segregates near the dendrite boundaries and that the maximum content tends to decrease from around 14 to 10 mass% as the cooling rate increases. The minimum content at the dendrite cores exhibits the reverse trend. The dissolved amount of magnesium in the weld metal is estimated from the relationship between the lattice parameter and the magnesium content by the X-ray diffraction method. In laser welding, the estimated dissolved amounts of magnesium are around 0.3–0.5 mass% less than in the base metal and tend to increase with an increasing cooling rate. This phenomenon i...

Investigation of feathery crystals developed in weld metal in aluminium alloys by SAM and TEM

Summary The macroscopic solidification structures developed in the weld metal of aluminium alloys are generally columnar crystals and equiaxed dendrites. The development of these macrostructures depends on the welding conditions. Feathery crystals are sometimes developed during low-speed GTA (TIG) welding. This paper describes an investigation of the characteristics of feathery crystals by scanning acoustic microscopy (SAM) and transmission electron microscopy (TEM). SAM enables structures to be observed without etching. The growth features of feathery crystals are clarified by observations made normal and parallel to the welding direction. Measurements of the velocity of the elastic surface wave of feathery crystals by SAM with an anisotropic acoustic lens show the velocity to be around 100 m/sec faster parallel to the welding direction than normal to it, confirming that the feathery crystals are acoustically anisotropic. Feathery crystals are also observed in electron beam welds by SAM, although it is n...