The authors have developed an apparatus for varying an electrode force of a spot welding machine. This is equipped with a servomotor by which we can change rotation instantaneously from normal to reverse and vice versa, and also has a display by which we can monitor the change in the electrode force. Lap resistance spot welding was performed on cold rolled steel (SPCC) and temporal changes of the electrode force were compared with those by a conventional process. In the conventional process the electrode force changed complexly with the elapsed time and the changes were divided into three kinds depending on welding current. When we decreased the initial electrode force than that in the conventional process good welds could be obtained even in lower welding current and the decrease in the thickness of the welds could be lowered. In this case the temporal changes of the electrode force could be explained during the expansion and shrinkage of the welds, though the electrode force changed complexly depending on the initial electrode force and welding current.
Lap resistance spot welding was performed on cold rolled steel (SPCC) with an electrode force changeable lap resistance welding machine the authors had developed. We evaluated the welds by ultrasonic testing (immersion method, C-scope representation) and a tensile shear test, and compared the results for this method with those by the conventional method. It was useful to apply the ultrasonic testing to evaluate the welds. Usually plug-type fracture was observed in the tensile shear test and the maximum tensile shear load was larger than those estimated using the value of the minimum strength (290 MPa) of SPCC specified by the Japanese Industrial Standard, though some specimens fractured along the weld interface. Regression analysis was performed by setting the maximum tensile shear load as a dependent variable, and setting the initial electrode force, the welding current, the initial electrode force timing and the latter electrode force as independent variables. The result shows that only the initial electrode force and the welding current were significant. The maximum tensile shear strength increased by increasing the welding current and decreasing the initial electrode force. It was made clear that we could obtain better spot welds by using the process we developed than the conventional spot welds.
Process parameters of the Laser-Arc hybrid welding of HSLA-590 steel have been studied using 2.4 kW CO2 laser machine and a MAG welding machine. The effects of power ratio, laser power, arc power and leading process have been investigated in this study. The results shows that the power ratio between laser power and arc power is higher, the bead shape is better. And the effects of root gap and bevel angle in butt joint were investigated. From the experiments, the maximum gap tolerance and suitable bevel angle are found, that are 1 mm and 45°, respectively. The effects of shielding gases, helium or argon, are also investigated for finding the effects on penetration, bead width and bead appearance. The microstructures of welds are studied by laser scanning microscope and Scanning Electron Microscope (SEM). The Dendrite Arm Spacing is measured in laser, MAG and hybrid welds. The maximum hardness is not appeared in the HAZ, but in the weld metal. And there is a soften zone on the fusion line.
The space GHTA (Gas Hollow Tungsten Arc) welding with a high-voltage high-frequency device for arc initiation may cause electro-magnetic noise problems for the computer equipment placed on the ISS (International Space Station). It was also clear from the experiments using a high-voltage DC device for arc start at the ISS orbital pressure, that there is a shifting phenomenon in which a spark discharge shifts to either a glow discharge or an arc discharge when starting the arc. Welding projects around the ISS in space need an arc discharge and a device which does not produce electro-magnetic noise for arc start. Therefore, in this report, arc starting experiments of the space GHTA welding using a touch start technique for arc start were carried at the ISS orbital pressure, 10-5 Pa. As a result, the space GHTA welding using a touch start technique can be used for arc initiation when welding at the ISS orbital pressure. It was also clarified that the application of the touch start technique is optimum in order to use the space GHTA welding process in space.
Hybrid welding was carried out on Type 304 stainless steel plate under various conditions using a YAG laser combined with a TIG arc, and the effects of various welding conditions on the penetration and porosity formation were investigated for respective arrangements of a YAG laser beam or TIG torch ahead in hybrid welding, and the welding results were compared with the ones obtained with a TIG arc only or a YAG laser alone. In most cases the hybrid weld beads were deeper and wider in comparison with the YAG laser ones; however, this fact did not always take place. It was consequently found that a laser beam should be shot inside the molten pool made readily with a TIG arc alone to produce deeper weld beads effectively. It was also confirmed that, under the focused condition, the hybrid weld beads at the arc current of 200 A had a smaller amount of porosity than the YAG laser ones. The behavior of arc plasma and laser-induced plume as well as the relationship between a laser-shot spot and the molten pool made with TIG arc only was observed through a CCD camera and a high-speed camera during arc and hybrid welding to understand the hybrid welding phenomena and factors affecting penetration.
YAG laser only or YAG laser-TIG arc hybrid welding was carried out on Type 304 stainless steel plate with a low S content in air or in the chamber with Ar gas including 0% to 15% oxygen gas. It was confirmed in both laser and hybrid welding that the penetration and geometry of welds depended upon the volume of the oxygen in the ambient atmosphere, and that a pronounced "nail head" part disappeared in a weld metal containing a higher content of oxygen in both the laser and the hybrid welding at 100 A. In addition, laser and hybrid welding were performed on polished Type304 plate with high S content and surface-oxidized Type304 plate with low S content in pure Ar atmosphere to discuss the influence of the minor element on the surface-tension driven melt flow. Then the geometry of their weld beads was similar to that made in the chamber with Ar and oxygen gas mixed. These experimental results suggest that convective flow induced by surface-tension driven force produces the upper part of the weld bead and consequently the weld displays a wine glass shape in both laser welding and hybrid welding at 100A in Ar atmosphere.
To know whether an arc plasma burns down into a keyhole during hybrid welding is very important in terms of the interpretation of welding phenomena and the production of deeper weld beads. In this paper, therefore, with the objective of understanding the existence level of the arc plasma in the keyhole during YAG laser-TIG arc hybrid welding, electric currents were measured by utilizing normal specimens and two different experimental samples of Type 304 stainless steel plates with simplified keyholes. In the lap- and butt-joint weld-type experiments, special samples consisting of the upper and lower plates of 2 mm in thickness were set with some gaps, and the currents flowing through respective plates were measured while the keyhole was formed in the lower plate during hybrid welding. It was confirmed in the lap-joint type weld experiment that the current flowing through the lower plate was not so high as it increased the penetration. And in the butt-joint type weld experiment, the current forced to pass through the lower plate decreased by the formation of laser-induced plume. It was consequently concluded that the arc was pinched around the keyhole inlet but did not burn down into the keyhole during hybrid welding.
In pulsed arc welding, there are a lot of factor that have influence on weld bead shape, and the relationship between the welding parameters and the weld phenomena is complicated. So, it is often selected the welding condition by experiment and experience. In this research, we conduct the heat conduction analysis, and clarify the influence of the pulse condition of the heat source on the welding bead formation. We clarify that melting efficiency in pulsed arc welding can be improved greatly when the peak ratio is high and the frequency is low even if the average heat input was the same to no-pulsed arc welding.
In order to understand the microstructure change during welding process, aiming to design high intensity and toughness weld structure, kinetic information about the phase transformation is essential. In our research group, in-situ observation system consisting of undulator beam and imaging plate have recently been used. The welding torch is driven by stepping-motor in the system. Those make possible that phase transformation can be identified in real-time under the condition of directional-oriented solidification and the spatial resolution of 100 μm width. The time-resolution is 0.3125 seconds. In the present work, combination of analyzing method: the in-situ observatrion system by X-ray diffraction technique using intense synchrotron radiation, morphological observation by high-temperature laser scanning confocal microscopy and observation of post-weld material by OM, SEM and micro diffraction-system, is suggested to analyze the phase transformation during welding process. Using the all results acquired by those analysis methods, Phase evolution of hypereutectoid carbon steel, during fusion welding was analyzed. The primary phase was directly identified as an austenite phase, during welding. Precipitation of pearlite phase was observed at 560 deg., during cooling followed by the martensitic transformation.
The authors considered the fracture toughness of plasma sprayed YSZ coating from the view of the effect of spray distance by using TDCB method. It was recognized that the cracking propagated through the interface of plattened particles in YSZ coating. The fracture toughness of YSZ coating was uniform on short spray distance. But the longer a spray distance became, the smaller a fracture toughness became on the spray distance over some value. This phenomenon of YSZ coating is similar to that of Al2O3 coating. The dependence of fracture toughness of YSZ coating on spray distance was same as that of bonging rate at the interface of plattened particles on spray distance. It was recognized that the fracture toughness of YSZ coating was controlled by bonding rate.
The fixed conditions of butt welds between straight pipe and valve or pump in the actual piping system are different from those of straight pipes. However, the effect of fixed condition on the residual stress and the stress intensity factor for evaluation of structural integrity of cracked piping was not clear. In this study, the finite element analyses were conducted by considering the differences in the distance from the center of weld to the fixed end L to clarify the effect of fixed condition on the residual stress and the stress intensity factor. For the 600A piping, the axial residual stress distribution was not affected by the distance L. Furthermore, the stress intensity factor of circumferential crack under the residual stress field with fixed condition could be estimated by using the existing simplified solution for piping.
Stress intensity factor estimated by using an appropriate modeling of components is essential for evaluation of crack growth behavior in stress corrosion cracking. For the appropriate modeling of welded components with a surface crack, it is important to understand the effects of residual stress distribution and the geometry of component on the stress intensity factor of the crack. In this study, stress intensity factors of surface crack under two assumed residual stress fields were calculated. As the residual stress fields, bending type stress field (tension-compression) and self-equilibrating stress field (tension-compression-tension) through the thickness were assumed. The geometries of welded components were set to plate and piping. The assumed surface cracks for evaluations were long crack in the surface direction and semi-elliptical surface crack. Furthermore, the crack growth evaluations were conducted to understand the effects of residual stress distribution and the geometry of component. From the comparison of stress intensity factors and crack growth evaluation for surface crack under residual stress fields, the effects of residual stress distribution and the geometry of component on the stress intensity factor of surface crack and on the appropriate modeling of cracked components were discussed.
The stoichiometric NiAl exhibits attractive properties in terms of high melting point (1911K), high thermal conductivity, oxidation resistance and low density (5.9kg/m3). However, it has been limited to apply the material for industrial fields because of its extreme brittleness at ambient temperature range. It is considered that the refinement of grain size to nano meter level seems to be a promising way to overcome the above mentioned problem because the enhancement of hardness and strength can be expected by nano-structure, according to Hall-Petch law. In the present study, synthesis of nano-structured NiAl coating was carried out by using MA (mechanical alloying) process and HVOF (high velocity Oxy-fuel) spraying. In addition, effect of nano-structure to the mechanical properties of NiAl coating was also investigated. It was observed that the coating obtained by the process used was composed of both nano-structuered NiAl and Ni phase. However, mechanical properties of the coating were not affected so severely by the grain size of sprayed powders prepared, with the difference between approximately 20nm and 200nm.
The computer-aided optimisation system ("Bayesian Expert System") for transient liquid phase bonding process which integrated the alloy design of insert metal and optimisation of processing parameters has been systematised utilising the radial basis function neural network with Bayesian inference. Bayesian Expert System consisted of three main systems, i.e. the supporting system for planning the experimentation, the supporting system for alloy design of insert metal and the supporting system for optimisation of processing parameters. The latter two systems involved the formulation and its verification module with Bayesian RBF network, the search module of extremum by the genetic algorithm, the multi-objective optimisation module by the satisficing trade-off method and visualisation tool for outputting the obtained results. The Bayesian Expert System realised the more accurate formulation of evaluation factors based on the measured data with scatters and resulted in the feasibleness of becoming the advanced optimisation of TLP-bonding process.
The optimisation of TLP-bonding process for Ni-base superalloy, IN738LC has been carried out using the Bayesian Expert System for TLP-bonding. The evaluation factors introduced as an index of bonding performance of an insert metal involved the melting point of insert metal, hardness and formability of brittle phases in the bond layer. The chemical composition of Ni-3.0Cr-8.1Si-1.0B (mass%) was determined as the optimal composition of an insert metal. The processing parameters have been optimised with two evaluation factors; the joint strength and toughness. The optimal bonding condition was determined as 1433K×5.6ks. Brittle phases were not formed in the bond layer using a newly developed insert metal, and the joint strength and toughness was superior to the average of the base metal properties.
Authors tried to butt-weld a mild steel plate to a magnesium alloy plate by the solid state welding using a rotating pin. This study investigated the effects of pin rotation speed, the position for the pin axis to be plunged on the strength and the microstructure of the joint. The main results obtained are as follows. Butt-welding of a steel plate to a magnesium alloy plate was easily and successfully achieved. The maximum tensile strength of a joint reached about 70% of the magnesium base metal tensile strength and the fracture path was along the joint interface. When pin rotation was slow, some defects appeared in the magnesium matrix of a joint due to insufficient plasticization of the magnesium. The joint strength increased with the pin rotation speed. This seems to be because the plasticization of the magnesium was increased and the pressure for pushing the plasticized magnesium onto an activated faying surface of the steel was increased. However, the excessive increase of the pin rotation speed caused the ignition of the magnesium, resulting in the decrease of a joint strength. At the pin offset of 0.1mm toward steel, steel fragments scattered in the magnesium matrix in the form of small piece which had no influence on the joint strength. However, larger offsets over 0.2mm made the steel fragment scattering in the magnesium matrix continuous and parallel to the weld interface. Since fracture path tended to occur along the continuous steel fragments perpendicular to the tensile direction, the joint strength decreased.