In the analysis of welding mechanics, it is difficult to analyze large-scale structures because of welding-specific moving local nonlinearity. In this research, the authors proposed a new numerical method for welding mechanics based on the Dynamic Explicit FEM. In the proposed method, the temperature step is divided into hundreds of time steps as implicit FEM and the displacements are computed for each time step based on dynamic explicit FEM until the whole system reaches the static equilibrium state. And, to achieve the static equilibrium state faster, modified mass and damping matrix are introduced. The modified mass and damping matrix are based on the Courant condition and the vibration theory, respectively. The proposed method and static implicit FEM are compared at the final path of multilayer welding of thick bead-on-plate to verify validity and accuracy. The transient and residual deformation and stress distribution of the proposed method show good agreement with those of static implicit FEM. In addition, the computing time and memory consumption of the proposed method are 1/12 and 1/40 times shorter than those of static implicit FEM, respectively, in 243,243 degree of freedom model. It is found that the proposed method has an advantage in large-scale analysis whose nodal points are more than tens of thousands.
An experimental study of a torch for oxygen plasma arc cutting (PAC) was conducted. We made a nozzle equipped with a window and a port for measurement. We observed the hafnium (Hf) electrode during working, using a color high-speed video camera through the window. The color image consists of three images assign red (R), green (G) and blue (B). Each image is taken by different image sensors having different wavelength sensitivity. We estimated the temperature on the surface of the Hf electrode assuming the black body radiation of the surface. And the port of the nozzle is connected to a pressure sensor to measure the pressure in the nozzle. We could estimate the real-time flow rate of plasma gas from the difference between the pressure in the nozzle and the pressure of supply. Using the special nozzle for observation, we investigated the arc phenomena of PAC using an Hf electrode, during arc-start and arc-shut-off.
High power fiber lasers of high beam quality have been developed, and remote welding with a high-power fiber laser has been expected to be one of new high-speed welding procedures in automobile industries. In particular a long-focal-distance focusing optics is beneficial to the welding of a wide area. However, features and effects of a laser-induced plume in such welding process are not fully understood. Therefore, the study was undertaken to investigate weld penetration characteristics with or without removal of a laser-induced plume, to clarify welding phenomena and to develop a stable welding procedure in 4 kW high-power fiber laser welding of a 1.5 mm-thick zinc-coated steel sheet using the focusing optics with a lens of 1,250 mm in focal length. The welds obtained at the focal position without removal of the plume were so unstable that the full penetrations were changed to partial penetrations during laser welding. To fundamentally understand the remote welding phenomena, refractive-index distribution above the laser-irradiated area was visualized by the Michelson interference method. The observation results of fringe patters through a high-speed video camera demonstrated that the incident laser beam was defocused and refracted by a wide formation range and complicated behavior of small refractive-index values derived from the high-temperature metal vapor of the plume, leading to the formation of an unstable penetration weld. It was also confirmed that the removal of the laser-induced plume and the corresponding widespread field of low refractive-index values by using a fan was essential to the stable production of fully-penetrated welds in the high-power fiber laser remote welding with the long-focal-distance focusing optics.
Although several studies on the friction stir welding of high temperature materials have been reported, their practical use has been delayed when compared with soft and relatively low melting point metals such as aluminum, because there have been no tool materials suitable for welding for high temperature materials until recently. The welding tools for these materials are required to have a high performance at elevated temperatures. The purpose of this study is to strengthen iridium (Ir) with an excellent oxidation resistance at elevated temperatures and to develop a welding tool for high temperature materials. As a result, it was clarified that Ir alloys are strengthened by addition of elements with a larger atomic radius than that of Ir. Moreover, the Ir alloy with such added elements showed excellent mechanical properties and oxidation resistance at elevated temperatures. The welding tool made from this alloy was little worn even if SUS304 stainless steel was friction stir welded.
Cast iron with nodular graphite was surface welded on 12mm thick,150mm and 350mm wide,and 600mm long SS400 plates,by self-shielded gas arc welding method. Instead of ordinary preheating,the heat-flows from multiple weld beads were accumulated to raise the temperature of welding zone. For the purpose,the parallel beads were laterally reciprocated at a small bead pitch of 3mm. The effect of welding heat-cycle on the microstructure and mechanical properties of the deposited metal was investigated,changing the reciprocating length, 110mm and 310mm. Main results obtained were as follows;(1) The influence of the heat-flows from preceding and following weld beads on the structure of deposited metals were composed of two major parts. They were the influence of slow cooling,and that of annealing effect on the rapidly-cooled deposited metal. (2) With the short welding heat cycle on the 150mm wide specimen,the molten metal was kept in a temperature range higher than 1500K with a small fluctuation,and cooled slowly resulting bull's-eyes structure with small amount of cementite. (3) With the long heat cycle on the 350mm wide specimen, the deposited metal was rapidly cooled. Following rapid cooling, the temperature of the deposited metal was sharply fluctuated with lower end below 1200K,as the subsequent beads approached and departed,resulting many small graphite nodules in the ferrite matrix. (4) Except the 1st bead, low dilutions resulted from the small bead pitch.
Dissimilar joints between Ni-base superalloy, which is available for turbine impeller for turbocharger, and alloy steel were fabricated by electron beam welding and their microstructure and tensile strength at a room temperature were examined. Macroscopic appearance of the weld part was similar in both joints, however, tensile strength of 247 alloy / SCM435H was smaller than that of 713C alloy / SCM435H joint, and feature of hot-tear fracture was more evident in the fracture surface. SEM-EDX analysis for the weld part found that a 10μm wide solute-segregated region traced the weld interface located in the superalloy side. Small hot-tear cracks were observed only in the segregated region of 247 alloy / SCM435H joint. These hot tear is likely to be distributed over single plane, so have significant effect to joint strength in some case of stress direction.
High-power and high-brightness lasers such as fiber laser or disk laser are promising as desirable heat sources for deeply-penetrated welds, owing to their superior power density. However, their tightly-focused beam spots have low gap tolerance in butt welding. The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in butt welding with several gaps of zero to 0.4 mm for 590 MPa-high-strength steel plates with a 10 kW fiber laser or a 16 kW disk laser. The butt welding was performed on 12 mm-thick plates under several welding conditions including using a cold wire or a hot wire. In the case of 0.4 mm gap, underfilling and burn-through welds were obtained by the conventional 10 kW fiber welding and with using a cold wire. On the other hand, welding defects were greatly reduced with a hot wire. Moreover, a 16 kW high power disk laser with a hot wire could produce sound fully-penetrated welds to suppress not only the welding defects but also oxygen content in the weld fusion zone. The weld penetrations were stable in spite of several gaps from zero to 0.4 mm, owing to removal of excessive melt depending on the gap size from bottom keyhole inlet in the molten pool by a strong laser-induced plume. Consequently, it was revealed that laser power sufficient to produce a full-penetrated keyhole using a hot wire was effective to the improvement of gap tolerance and suppression of oxygen content in the weld fusion zone in butt welding of thick high-strength steel plates with a high-power and high-brightness laser.
The stress relaxation usually appears on the extended tensile specimen during the fixed displacement. This is caused by the room temperature creep straining. In the present work, the creep straining behaviors at room temperature were examined by both of the tensile strain holding and the tensile stress holding. The settled stress after stress relaxation under a fixed strain was evaluated in several plastic straining levels, and the stress-strain curve after stress relaxation was compared with the stress-strain curve obtained under the conventional strain rate. The degradations of the flow stress after stress relaxation were evaluated for the seven kinds of the structural steels with the wide strength range from 197MPa to 826MPa. The effect of the metallurgical strengthening systems on the stress relaxation behaviors was also discussed. The degradation ratio in the flow stress due to the room temperature creep deformation was 30% in the case of interstitial free steel, and 8% in the 780MPa strength class steel. However, the strengthening systems such as a solution hardening, a precipitation hardening and a strain hardening did not give an apparent influence on the stress relaxation resistance.
In recent years, in-process control welding for reducing weld distortion and residual stress is much more required from a view point of improving manufacturing efficiency. In-process control welding by additional cooling has been proposed as one of techniques for reducing weld distortion and residual stress. It has already been partially applied for thin-plate structure in industry. In order to extend the range of application of in-process control welding by additional cooling in the future, further studies are required. Firstly in this study, reduction of angular distortion by heat-sink welding is experimentally discussed. Based on the experimental results, effect of welding conditions on reduction of angular distortion by heat-sink welding is investigated by thermal elastic-plastic analysis. It is concluded that angular distortion is reduced by additional cooling with the appropriate cooling position, and the effect of welding conditions on reduction of angular distortion are clarified by associated with heat input parameter.
Titanium welding processes in MIG welding is not easy to weld due to the arc stability and shielding gas of 100% Ar. The authors tried to take arc welding phenomena by a high speed camera to get the stable metal transfer. The optimum welding conditions were found. Since the thickness of the thin sheet material is 1mm, the heat input must be reduced in conventional welding. But the resultant bead winded along the welding line due to a high speed welding and winding of arc. To improve this situation, it is required that the amounts of the deposited metal are increased. If the amount of the deposited metal depending on the heat input is increased, the burn through takes place. The authors thus employed the switch back welding method for this titanium thin sheet welding. The suitability of the welding conditions was verified by observation of the arc, the weld pool and the external appearance of back beads.
Process parameters are studied for the laser cutting of mild steel plates, whose thicknesses are from 0.8mm to 6mm, by using a small laser machine equipping an 1kW carbon dioxide laser. The ranges of cutting speed and duty are clarified for some plates and the experimental expression is proposed; dc=0.48vt+13, where dc in percentage is the critical value of duty for cutting, v in milli-meter per second is cutting speed and t in milli-meter is the plate thickness. This expression can be applied for the cutting speed over 10 mm/s. Pulse frequency is unrelated factor to the cutting heat input, and then its influence is small on the cutting possibility. The burning near the kerf is increased with increasing cutting heat input1). When the heat input is the same, the burning is increased with the decrease in duty, resulting in the decrease in the cutting speed. It is increased also with the increase in pulse frequency. The kerf width is increased independently of pulse frequency with heat input at the higher duty.