The formation of stable back beads in joining of the thick materials is important in order to achieve high quality welded metal joints. Since plasma welding employs the high welding current density, it is suitable for the thick materials. Keyhole in the plasma welding depends on the pilot gas and the welding current. The voltage behavior depends on the keyhole situation. If the torch is moved away from the welding line in conventional GMA welding, the welding voltage and the welding current are changed due to variations of the arc length. But, the welding voltage does not change with the arc length in the plasma welding, since the welding voltage depends on the situation of the keyhole. The authors tried to observe the weld pool on top side by using a CCD camera. The timing of the shutter in the CCD camera is investigated to take clear image of the weld pool. As a result, the clear images of the weld pool were taken when the welding current was reduced to 30A and an interference filter of 950nm was attached to the CCD camera. The weld pool shape was changed with the torch position in the groove. The image processing method was developed to detect the top of the weld pool. The torch position was estimated by processing the weld pool images. The digital control was designed to trace the welding line. The performance of the controller was verified by carrying out the tracking experiments.
Microcracking susceptibility in the dissimilar multipass weld metal of alloy 690 and low-alloy steel A533B was evaluated, and the effect of dilution on hot cracking (ductility-dip and liquation cracking) behavior was investigated. In order to simulate the dissimilar multipass weld metal of alloy 690 to A533B steel, the A533B plate was welded under the various dilution ratios using the alloy 690 filler metal with different contents of P and S. The several weld metals, which had different alloy compositions at the fixed (P+S) content, were manufactured, and then ductility-dip and liquation cracking susceptibilities of the reheated weld metals were evaluated by the spot-Varestraint test. The ductility-dip cracking susceptibility heightened as the dilution ratio was increased even when the amounts of P and S were fixed. The increased dilution ratio (contamination of Fe into the weld metal) should reduce the tortuous character of the grain boundary due to inhibiting the constitutional supercooling (the instability of the solidification boundary), as well as enhance the grain boundary embrittlement due to promoting the grain boundary segregation of P and S. Furthermore, the liquation cracking susceptibility slightly heightened with an increase in the dilution ratio at the fixed (P+S) content. The increased liquation cracking susceptibility would be attributed to the enhancement of solidification segregation of P and S with increasing the dilution ratio.
Oxygen plasma arc cutting (PAC) is widely used in various industrial fields. In case of cutting magnetized plates, the magnetic field is concentrated around the cutting front according to the progress of the cutting, and the electromagnetic force induced by leakage magnetic field deflects the plasma jet. The deflected plasma jet causes poor cutting quality and sometimes makes fatal damages on the electrode and the nozzle by double arc as abnormal discharge. This phenomenon is called Magnetic Arc Blow and it is critical issue of applying plasma cutting on magnetized plate. The purpose of this study is to investigate the arc blow behavior and to devise a method to prevent it. We examined the relationship between operating conditions and double arc with external magnetic fields on the plasma jet. We found out criteria of operation conditions that induces the arc blow. In addition, we have succeeded to suppress the double arc generation attributed to the external magnetic field with a mild steel shield cap attachment around the nozzle tip.
Visualization of the microstructure formation process in SUS410 was considered combining the confocal scanning laser microscope (CSLM) and the multi-phase field method (MPFM). According to the in-situ observation using CSLM, γ-phase precipitation temperature falls greatly with an increase in cooling rate. Moreover, solidification and solid-state transformation behavior was analyzed by MPFM. Calculated tensile strength depend on the phase state corresponds to experimental results. It is thought that tensile strength and density at high temperature region are affected by cooling rate due to the delay of γ-phase formation.
K-type thermocouples are fabricated using a chromel wire and an alumel wire of 50&mu:m by non-contact discharge welding method. The welding is carried out at six different butt angles. The size of hot junction tends to decrease with an increase in butt angle, when the butt angle is larger than 1.4rad. The response rate is measured and it is almost proportional to reciprocal of the size of hot junction. It is shown that the size of hot junction is an important factor for response rate. Wire diameter is another important factor for temperature measurement of small objects. When temperature is measured at a same point by a thermocouple under the condition that only hot junction and its vicinity are heated, the output of electromotive force, i.e., indicated temperature will depend on the wire diameter. As the metal wire is good thermal conductor and the rate of heat transport is proportional to the area of wire cross section, deviation from true value increases with an increase in wire diameter. Our experiments showed that the indicated temperature decreased linearly with the wire diameter. Furthermore, we measured the discharge voltage and current at various setting current and discharge gap.
980 MPa grade Cr-Ni weld metal with martensite and retained austenite was produced by single-pass GMAW. Its impact behavior was evaluated using the Charpy impact test. It was found that (1) strain-induced martensite transformation from the retained austenite enhances the impact absorbed energy at various temperatures and contents of oxygen in weld metal; (2) increasing the oxygen content in weld metal reduces the impact absorbed energy; and (3) at 233K, strain-induced martensite transformation can change the fracture mode of the weld metal with extremely low oxygen content from brittle to ductile, but it has no such effect at higher oxygen content levels. Oxide promotes ductile fracture. An increase in the oxygen content lowers the vTrs.
This study was aimed at establishment of a model that can predict tensile shear strength and fracture portion of laser-welded lap joints in the tensile test. To clear the influence of bead length and bead width on them, the joints employed steel sheets with a thickness in the range of 0.8mm to 1.2mm were evaluated. It was found that the tensile shear strength increases with the bead size, and the fracture occurres at base metal (BM), weld metal (WM) or portion between them with a curvature (HAZ), in the tensile test. Also to clarify rotational deformation process around WM during the tensile test, cross-sections of joints were observed, which were applied to several loads in the tensile test. This observation derived relationship between the radius, Ri, at the inner plane of HAZ and the rotational angle, θ, of the center of sheet thickness. Furthermore relationship between Ri and applied load was obtained by linear regression. A plastic analysis for deformation of the joints was carried out based on these functions and some assumptions. These assumtions consider that the joint consists of BM, WM and HAZ, which are under simplified stress mode respectively. Finally estimation of the tensile shear strength and the fracture portion of the joints was achieved. This estimation made a good accordance with the experimental results.