When low transformation-temperature welding consumables are applied to welding of high strength steels, there is chemical mismatch between high strength base metal and the Cr-Ni system low transformation-temperature weld metal. The chemical mismatch may affect galvanic corrosion of welds in outdoor steel structures such as a bridge. In this study, atmospheric corrosion behavior was investigated for chemically mismatching weld joints formed by low transformation-temperature welding consumables. Corrosion depth profile at weld interface was measured by using laser microscope after outdoor exposure test of a year and the measured profiles were analyzed by plotting on Gumbel probability paper. Results of measurement and analysis showed that corrosion depth of base metal was independent on Cr content of weld metals and distance from weld interface.
The thermal elastic-plastic analysis for the welding transient and residual stresses and deformations needs the huge computation time to obtain the accurate results. In this study, a new method, a step iterative total strain method, was developed to shorten the computation time largely, keeping high accuracy. The theory can take account of not only the temperature-dependencies of the mechanical properties, as usual thermal elastic-plastic theory, but also the change from elastic state to elastic-plastic state in one large temperature increment. Based on the theory (the thermal elastic-plastic constitutive equation), the equilibrium equation (the stiffness equation) was developed in the finite element method (FEM). New FEM program for thermal elastic-plastic analysis was developed, based on the above theory. A simple important problem of thermal elastic-plastic behavior was analyzed to verify the capability (accuracy) of the new method. In the next report, complex typical welding problems will be analyzed. As a result, it will be confirmed that the computation time is very largely shortened, compared with the usual incremental method, keeping the high accuracy.
The thermal elastic-plastic analysis for the welding transient and residual stresses and deformations needs the huge computation time to obtain the accurate results. In this study, a new method, a step iterative total strain method, was developed to shorten the computation time largely, keeping high accuracy. The theory can take account of not only the temperature-dependencies of the mechanical properties, as usual thermal elastic-plastic theory, but also the change from elastic state to elastic-plastic state in one large temperature increment. Based on the theory (the thermal elastic-plastic constitutive equation), the equilibrium equation (the stiffness equation) was developed in the finite element method (FEM). New FEM program for thermal elastic-plastic analysis was developed, based on the above theory. A simple important problem of thermal elastic-plastic behavior was analyzed to verify the capability (accuracy) of the new method. In this report, some complex typical welding problems were analyzed. The residual stresses and deformations were compared with the accurate results obtained by the usual incremental method with very small temperature increments. As a result, it was confirmed that the computation time is very largely shortened, compared with the usual incremental method, keeping the high accuracy.
A newly developed low-transformation-temperature welding wire, of which transformation start temperature is lower than that of conventional welding wires for 490-MPa class structural steels, was applied to fabrication of fillet welded T-joints. The welding angular distortion and the temperature profile of the weld metal were continuously measured during the welding process. The angular distortion of the fabricated T-joint was reduced when the weld metal reached the martensitic transformation start temperature. The residual angular distortion was less with the low-transformation-temperature welding wire than that with the conventional wire. It was also determined that the transformation start temperature of the weld metal rose from 205°C to 380°C resulting from dilution of chemical composition from the base metal. The welding distortion of T-joints was calculated by a numerical simulation with consideration of the effect of phase transformation under weld thermal cycles. Material properties used in the numerical simulation such as continuous cooling transformation diagrams and dilatometric curves were determined by actual measurement with specimens extracted from the weld metals of the fabricated joints. The welding distortion was reproduced with high accuracy in the numerical simulation. Results of numerical simulation also determined that there was a direct correspondence between the transformation expansion of the weld metal and the angular distortion.
It is well known that angular distortion in conventional welding has been predicted by heat input parameter proposed by Satoh and Terasaki, however, it is also well known that angular distortion in new welding process such as Tandem Arc welding has not always been predicted by conventional heat input parameter. Thus, welding distortion characteristics in multiple-heat-source welding have been investigated thermal conductivity theory and numerical simulation. First, an influence factor concerning angular distortion in multiple-heat-source welding has been extracted by thermal conductivity theory. As the result, the following factors are given as an influence factor for angular distortion in multiple-heat-source welding; heat input parameter, distance between two heat sources, heat input ratio, and welding speed. Based on them, the effect on angular distortion in multiple-heat-source welding has been evaluated by numerical simulation. In multiple-heat-source welding, angular distortion is possible to differ greatly by influence of distance between two heat sources or heat input ratio, even though heat input given to weld joint is equal. That is, multiple heat sources are appropriately arranged, then, angular distortion is expected to be able to be reduced effectively.
Scrap is raw materials of the recycled steels refined in electric arc furnaces, and this recycle process is good for keeping our living environment. The reports on microstructures and mechanical properties of heat affected zones (HAZs) of these recycled steels are not so many. This report contains mechanical properties and microstructures of HAZs in thermal cycled mother steels, and these HAZs were estimated a heat input of 42-62 kJ/cm. The following experimental results are obtained. TiN particles act as the nucleation sites of acicular ferrite, and also fasten on γ grain boundaries, and improve the toughness of HAZs.
High power laser is a promising tool to weld heavy section plate members. One of the problems, in this case, is formation of some weld defects such as porosity and hot cracking. In the present paper, formation mechanism of the porosity has been investigated in the deep penetration laser welding. 20 kW CO2 laser was used to attain the deep penetration in excess of 20 mm. Dynamic keyhole behaviour was observed during welding using a micro-focused x-ray transmission imaging system. Significant fluctuation in the keyhole depth of about 40% was observed even in the CW welding. The bubble was formed at the keyhole tip during abrupt decrease in the keyhole depth. The keyhole was closed with surrounding molten metal just above the tip, resulting in bubble formation. This is caused by capillary instability of the cylindrical keyhole. Most of the bubbles were combined with each other in the molten pool and remain after solidification. It causes the porosity. The bubble formation is promoted by fluctuation of the keyhole in the radial direction. Increasing the power density in the keyhole is effective in suppressing the porosity by stabilizing the keyhole. However, optimization of the welding variables was not enough to prevent the porosity.
The microstructure and mechanical properties of 600 MPa grade high strength steel welds made by CO2 laser welding and CO2 laser-MAG hybrid welding have been investigated. The microstructures of weld metals and heat-affected zone (HAZ) were observed by optical microscope and scanning electron microscope (SEM). The distribution of alloying element in weld metal was identified by electron probe microanalysis (EPMA). Microstructure and chemical composition were different between the locations of the hybrid weld metal. There was more ferrite and bainite in the nail head part than nail part (bottom part) in the hybrid weld metal. Tensile tests of the laser welded joints and the hybrid welded joints were performed according to JIS-Z-2201 and JIS-Z-2202. The notch toughness of the weld metals were also measured using half-size Charpy V-notch impact test specinen. The hybrid weld metals showed higher toughness than the laser weld metal at the testing temperature range of -60°C to +15°C.
This paper described an experimental study of the friction stir welding of dissimilar metals between commercially pure titanium (CPTi) and AZ31B magnesium alloy. Butt joints were produced by changing the joining parameters such as tool rotating speeds, offset distances of a probe and probe diameter. Evaluation of the joints was performed by the observation of the weld surface appearance, X-ray radiographic test, tensile test and SEM and EDX analysis. The main results obtained are as follows. Butt-joint welding of the CPTi plate to the Mg plate was easily and successfully achieved. The ignition of Mg occurred during welding at the tool rotation speeds over 1200rpm. The fragments of CPTi existed in a continuous form in Mg matrix. The tool rotation speed of 1200rpm and the offset distance of 0.2mm attained the maximum tensile strength of a joint, which was about 75% of that of Mg base metal. Fracture occurred at the weld interface in most joint. EDX analysis revealed that Al in the Mg diffused into CPTi through the weld interface. It was found that the decrease in Al concentration in the Mg around the weld interface caused the low tensile strength of the joints. Since the joints welded using the probe of 6mm diameter tended to cause the defects such as cracks and voids compared with the probe of 3mm diameter, the probe with 6mm diameter made the tensile strength of the joints decreased.
TiAl intermatallic compound coatings for diamond polish were fabricated by mechanical alloying (MA) and thermal spraying in this study. Titanium powder and aluminum powder were mixed by MA method. Then the MA powder was used as the feedstock powder for low pressure plasma spraying (LPPS). It was possible to fabricate thick coatings with dense microstructure by controlling of MA time, spraying distance and chamber pressure. Furthermore, almost completely TiAl constituent coating was fabricated by controlling of Ti and Al powder mixing ratio upon the raw material of MA. The TiAl coating had good diamond polishing property. Therefore, thermal sprayed TiAl coating is suitable for the diamond polisher.
In order to clarify such brazing conditions for making sound joints of Al-alloys to stainless steel as to wet the Al-Si brazing filler on the stainless steel before rapid isothermal solidification of the filler, the solidification time is estimated by the numerical analysis and is also measured experimentally. The numerical analysis is conducted for the brazing process of the A1050 pure aluminum base metal and BA4045 aluminum-silicon brazing filler. The analysis contains both the initial dissolution of the base metal by the molten brazing filler and the follow-ing isothermal solidification behavior. The movement of the solid-liquid interface is calculated and the total solidification time is estimated. The brazing experiment between the same combination of metals shows the total solidification time is much less than the numerically esti-mated time. The element analysis by EPMA indicates that the zinc in the flux plays the dominant role in the brazing process. According to the results, the time to solidify the brazing filler layer is 30×10-3H2 s, where H is the initial brazing filler layer width in μm. To obtain the brazed joint between the Al alloys and stainless steel, the initial brazing filler layer of at least 30 μm and the holding time of at least 30 s at the braz-ing temperature are considered to be suitable for the brazing.
There is need for speeding up of MAG welding to improve productivity but it is known that irregular bead is formed more easily as welding speed increases. A simulation model for the welding is very useful to predict the forming of welding defects. In the present work, a three dimensional (3-D), non-stationary thermal conduction model for thin plate is developed. The temperature distribution in the base metal is calculated to estimate the molten pool size using a finite difference model based on the 3-D heat conduction equation, and the surface profile of the molten pool is calculated taking account of the balance of gravity, surface tension and arc pressure. As a result of comparison between experiment and calculation, it is shown that the proposed model is useful as an engineering tool to predict the high speed MAG welding. Additionally, it is shown that the proposed model is effective to clarify the mechanism of the bead formation in the high speed MAG welding.