Mitsuaki Katoh

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.

Effect of Heat Treatment on Bond Characteristics of Aluminum Clad Steel. Development of Clad Materials by Vacuum Roll Bonding and its Characteristics. (Report 2).

An aluminum clad steel was produced using the vacuum roll bonding. The clad steel was heated at 560 to 650°C for 0 to 20 s to study formation of intermetallic phases and bond shear strength in the bond interface between the aluminum and the steel of the aluminum clad steel given weld thermal cycles. Main results obtained are as follows ; (1) Although the clad steel was heated at 650°C without holding, an intermetallic phase was not formed in the bond interface. (2) However, when the clad steel was heated for a certain time, intermetallic phases of FeAl3 and Fe2Al5 were formed in the bond interface and the activation energy for formation of the phases was 41 kcal/mot. (3) When the thickness of the phases was more than around 1.5μm, the bond shear strength of the clad metal was abruptly decreased in comparison with that of the clad steel in as-produced condition.

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.

Effect of Laser Fusing on Hot Corrosion Resistance of Nickel Based Self Fluxing Alloy Coating

The thermal sprayed coatings are widely used in waste incineration boilers and fossil fuel-fired boilers. However, the defects, such as porosity, cracks and unmelted particles, in these coatings are detrimental to corrosion performance. In this study, the nickel based self fluxing alloy coating was fused by YAG laser to improve hot corrosion resistance of the coating. Under appropriate laser parameters, the nonporous, crack-free coating was produced. Hot corrosion test, conducted in the presence of a mixed salt of Na2SO4/NaCl/KCl at 550Гshowed that the modified coating exhibited excellent corrosion resistance compared with the as sprayed and gas fused coatings.

Surface Modification of Titanium Using Laser Beam

Pure titanium has an excellent biocompatibility in comparison with stainless steels and Ti-Al-V alloys. We would expect pure titanium to have application for artificial joints and artificial bones if the wear resistance of the pure titanium were to be improved. So the surface modification of the pure titanium was performed using YAG laser beam. The laser power was 1.5 kW and Ar was used as the shielding gas. The shielding gas flow rate was changed from 5 to 40 L/min with a constant laser torch traveling speed of 500 mm/min. First, we investigated effects of the shielding gas flow rate on the Vickers hardness of the laser melted zone. When the shielding gas flow rate decreases, the average hardness increases and the oxygen and nitrogen concentrations of the laser melted zone also increase. We made clear the relationship between the average hardness and the nitrogen equivalent in the laser melted zone as follows. When the square root of the nitrogen equivalent (Neq=N+O/2) was less than 0.1, a plot of the average hardness for the square root of the nitrogen equivalent reveals a linear relationship. However, the average hardness of the laser melted zone increased more than the value indicated by the linear relationship when the square root of the nitrogen equivalent was above 0.1. Next, metallurgical analyses of the laser melted zone were performed using an electron probe micro analyzer (EPMA), an X-ray diffraction method (XRD) and a transmission electron microscope (TEM), and effects of the behavior of oxygen and nitrogen on the hardness of the laser melted zone were studied. A uniform dislocation structures in the laser melted zone is observed over a wide area where there is the linear relationship between the hardness and the square root of the nitrogen equivalent. Lamellar structure, which alternated between two phases of αTi and TiN in the laser melted zone, was formed where the hardness is greater than those indicated by the linear relationship. One phase of TiN contained a large quantity of nitrogen, and the other phase of αTi contained little nitrogen. It is found that the lamellar structure composes of αTi and Ti-nitrides (TiN and TiN0.26). It is also observed that a wide area of αTi possesses a twin structure with a high dislocation density.

Study on demagnetizing field in coil method using FEM

The central demagnetizing factor was calculated when the B-H curve of the specimen is given by a straight line using three-dimensional axial symmetrical finite element method (FEM). It is shown that the results obtained with FEM are almost similar to those reported by Bozorth, The central demagnetizing factor was calculated and the distribution of lines of magnetic flux was examined when (1) the coil length is modified for a constant specimen length and (2) when the specimen length is changed for a constant coil length. Moreover, we examined the influence of the relative permeability on the central demagnetizing field and calculated the central demagnetizing factor for the material whose B-H curve is non-linear.

Mechanical properties of aluminium alloy welds by laser beam

Since aluminium and aluminium alloy have good thermal conductivity and good specific strength, they have found a wide range of applications in aircraft, construction, electrical machines, etc. They have also been used in recent years in response to the demand for reductions in the weight of various transportation vehicles, particularly in efforts to reduce the energy consumption of motor vehicles. Since the 5000 series of alloys has a higher strength than other aluminium alloys and also has other excellent characteristics required of a structural material, such as good weldability, corrosion resistance, formability and low temperature characteristics, it occupies a central position among materials used for welded structures. The uses of these alloys covers a wide range including, ships, railways, land vehicles, car wheels, cranes, shipping containers and vessels. MIG welding or TIG welding is generally used for the fusion welding of aluminium. In recent years, there has been a demand for a welding technique for aluminium alloys that is of high quality, high precision and high speed, and thus laser welding has been considered. Here, we describe the microsegregation of Mg and the quantity of dissolved Mg in Section 2, microsegregation and hardness in Section 3 and microsegregation and dissolved Mg quantity and impact characteristics in Section 4, in laser welds of aluminium alloy.

Wear characteristic of titanium using laser surface modification method

Owing to its high specific strength and superior corrosion resistance, titanium has been used as material for steam and nuclear power stations, heat exchangers of seawater desalination devices, construction and aerospace, and space machinery and instruments. It has also been used as biomaterial owing to its biocompatibility. However, it is known to be inferior in wear resistance, and therefore, if its wear resistance can be improved, its use can be expected to expand further. As for the wear property of titanium alloys, there have been reports on the Ti–6Al–4V alloy by forming TiN with laser irradiation and by forming TiN and TiC with electron beam irradiation, but there has been hardly any investigation for the improvement in the wear property of pure titanium. With respect to the laser fusion layer of pure titanium with the YAG laser, the authors have conducted an analysis to investigate the effect of oxygen and nitrogen on the hardness of the laser fusion zone, and found that, by varying the argon shielding gas flow rate, it is possible to obtain the laser fusion layer with a hardness three times higher than that of the base material. In this study, to improve hardness as well as wear resistance further, the YAG laser modification of pure titanium was performed using nitrogen as a shielding gas so that a material analysis, a hardness test and a wear test could be carried out with the modified surface obtained. Also, a further investigation was made on the relationship between wear resistance and hardness.