MIG welding process contains arc plasma, metal transfer and weld pool phenomena. In this process, electrode wire melts by heat energy from the arc plasma, and molten metal at the wire tip deforms by various driving forces such as electromagnetic force, surface tension and arc pressure. Subsequently, the molten droplet detaches from the tip of wire and transfers to the base metal. The arc plasma shape changes together with the metal transfer behavior. In the present work, we constructed the unified model of metal transfer phenomena in which the interaction between the arc plasma and the metal drop was considered. In the numerical model developed, wire melting rate experimentally derived was provided as a function of arc current. But heat conduction and heat convection were not taken account in the metal. The numerical result by the developed model could demonstrate transition of metal transfer mode from globular transfer to spray transfer with increase of current. It is found that spray transfer mode in higher current range is formed mainly by electromagnetic force directly acting on the molten metal. In case of low arc current, surface tension plays dominant role in globular transfer mode. Furthermore, the properties of droplet are strongly affected to determine the transfer mode.
This research was performed with the objective of clarifying the effect of welding speed on melt flows during melt-run welding of SUS304 stainless steel plates with a 6-kW power laser beam on the basis of three-dimensional X-ray transmission in-situ observation. As welding speed increased from 25 mm/s to 250 mm/s, three kinds of welds characterized by porosity formation, no defects or underfilling due to spatters were produced. The average and the maximum values of measured melt flow velocity were three and ten times higher than the welding speed, respectively. Two kinds of circulation flows at the inlet or the tip of a keyhole were confirmed to control heat transfer in a molten pool. It was found that the circulation flows were so sensitive to the welding speed that bubbles resulting in porosity or spatters were often formed. Accoriding to the X-ray observation of the spatters formation with tungsten carbide (WC) tracers, as the melt flow rose along the keyhole wall, the velocity was accelerated from 0.24 m/s to 0.54 m/s near the keyhole inlet. Consequently, the melt flows made the convex surface behind the keyhole grow higher, resulting in spattering.
Friction Stir Welding (FSW) can weld dissimilar metal joints without a thick and brittle intermetallic compound (IMC) layer at the weld interface. In this study, the dissimilar lap joint of A3003 aluminum alloy and SUS304 stainless steel was successfully welded by FSW, and the joint obtained was tested to examine the properties of fracture toughness and fatigue crack growth rate. Its fracture toughness was different by the directions of crack propagation. The fracture toughness of advancing side (AS) to retreating side (RS) was stronger than that of RS to AS, and that of cryogenic temperature was stronger than that of room temperature (RT). Its fatigue crack growth rate also showed the same tendency as its fracture toughness. These data were compared with the past data and discussed.
The microstructural refinement of cobalt based alloy (Stellite NO. 6) by laser cladding and friction stir processing (FSP) was studied. A nanometer-sized microstructure consisting of fine carbide (particle size: 100∼200 nm) and a grain (grain size: 150∼250 nm) was successfully fabricated by the FSP on the laser clad cobalt based alloy. The nanostructured cobalt based alloy (Stellite NO. 6) had an extremely high hardness of about 750 HV.
In order to improve the strength of FSSWed PVC joint, SiC particle was compounded into the welded region at the same time with welding process. The joining process and its effect on strength of FSSWed PVC joint were investigated in joining experiment with a PVC plate having different guide hole diameter for filling the SiC particle. In case of the specimen compounded SiC particle with guide hole diameter of 8mm, SiC particle could be dispersed uniformly in the welded area. In the specimens with guide hole diameter of 5mm and 2mm, dispersion of SiC particle was not observed in side of welded area. Degradation and increase in local mechanical property was significantly observed at edge of the keyhole and inside the welded area respectively in specimen with guide hole diameter of 8mm. Width of welded area increased with increasing guide hole diameter. The joining strength was improved by fabricating composite material in welded region. The strength increased with increasing guide hole diameter. It is considered that joining strength was significantly affected by width of welded area. According to temperature distribution and observation of material flow using a high speed camera, contact between welding tool and SiC particle before plunging into a lower sheet induced increasing the heat input during the process and resulted in promoting the friction stirring. Therefore, it is considered that change in guide hole diameter leads to change in contact area between welding tool and SiC particle and also change in width of welding area then resulted in change in the joining strength.
GTA (Gas Tungsten Arc) welding has been used in manufacturing field and studied as a higher level welding process. To understand the arc phenomena that are complicated, it is significant to measure the temperature distribution of arc. Although Fowler-Milne method is one of the popular measurement methods, it is applied only under the assumption of LTE (Local Thermodynamic Equilibrium). In this study, the reliability of Fowler-Milne method is investigated by comparison of results between Fowler-Milne method and numerical simulation for six shielding gas conditions (Ar, He, N2, Ar+50%He, Ar+20%N2 and Ar+5%H2). The area in which the agreement between both ways is more than 95% is considered as that in which the LTE assumption is valid. The validity of this evaluation is discussed from the viewpoint of the collision frequency of species in the arc. The present study shows that LTE is valid for the area with the collision frequency of 2.0×1034 m-3s-1 for pure Ar arc, and the area with the collision frequency of more than 1.0×1035 m-3s-1 for other four conditions (N2, Ar+50%He, Ar+20%N2 and Ar+5%H2). However, for pure He arc, the collision frequency is much lower than the other conditions. Therefore, He arc is not in LTE for the whole area, which suggests that another method is required to measure the accurate temperature distribution for pure He arc.
In order to realize practical application of micro-plasma arc welding, the generation method of sub-millimeter sized plasma arc was developed through controlling the DC electrical discharge in the range from glow to arc discharge with use of the experimental torch device and the newly designed high-voltage power source. The electrical discharge was established between a cathode of the tungsten electrode and an anode of stainless steel sheet through plasma constriction nozzle made from heat-resistant material of Boron Nitride. It is shown that the micro-plasma arc with use of sub-millimeter diameter nozzle is stabilized as the plasma beam of which size corresponds to the nozzle diameter. And decrease of nozzle diameter increases the power density of micro-plasma arc at the anode surface. Sub-millimeter sized melt spot can be obtained with the current of 1 A and discharge time of 5 ms.
The precipitation and growth of sigma phase in duplex stainless steel was investigated to clarify the quantitative effects of temperature and chemistry on growth rate in isothermal heating process. Intermetallic phase such as sigma phase is harmful for mechanical properties of duplex stainless steels. The increasing of alloying elements of Cr and Mo useful for improving of corrosion resistance promotes the sigma phase precipitation but that phase is not always formed rapidly. So it is meaningful to clarify the kinetics of precipitation to give the solution the trade off relation of high corrosion resistance and prevention of harmful phase precipitation. A physical model of growth rate as function of temperature including the parameter regarding chemistries was suggested on the basis of traditional nucleation theory. The measurement of sigma phase conducted experimentally after heated at various temperature, employing 25%Cr duplex stainless steel containing various level of molybdenum, tungsten and nickel contents. Referring those data obtained, it was confirmed that the effects of temperature and molybdenum, tungsten and nickel contents on the growth rate is explained by the physical model suggested in this work.
The time-temperature-precipitation (TTP) diagram called as C-curve of sigma phase in duplex stainless steel was investigated to clarify the effect of chemistry and its mechanism. Intermetallic phase such as sigma phase is harmful for mechanical properties of duplex stainless steels. The increasing of alloying elements of chromium and molybdenum effective for improving of corrosion resistance promotes the sigma phase precipitation. It is meaningful to clarify the effect of the alloying elements on the nose temperature and time of TTP curve which indicate the critical heating condition to be free from harmful phase precipitation. Employing 25%Cr duplex stainless steel containing various level of molybdenum, tungsten and nickel contents, The time-temperature-precipitation (TTP) curve was obtained by microstructure observation test with optical microscope after heated at various temperature. The nose time of C-curve was shifted to shorter and nose temperature was to higher by the increase of molybdenum or nickel. The nose temperature was also shifted to higher by 2% tungsten addition but the nose time was almost same. The TTP curve of each steel tested was also calculated from the physical model of growth rate considering the effect of alloying elements. Those calculated TTP curves almost fitted the experimental results. The influence mechanism of chemistry on the TTP curve was analyzed by this model.
In this research, Idealized Explicit FEM (IEFEM), which was developed to achieve analysis of welding residual stress of actual structures, was applied to V groove and X groove multi-pass pipe welding problem. Both problems have more than 3 million degrees of freedom. In the analysis of V groove problem, the influence of work hardening rule was discussed through the comparison of residual stress obtained by using isotropic, kinematic and composite hardening rule. As a result, it was found that residual stress is evaluated excessively in the case of isotropic hardening. In the analysis of X groove problem, the influence of welding pass grouping technique on residual stress distribution was investigated. By comparing the residual stress distributions among the analysis considering the actual welding pass sequence, using welding pass grouping technique and experimental measurement, it was found that the residual stress might be different if the welding pass grouping technique is used. In addition, the influence of welding pass sequence on residual stress was also investigated in the analysis of X groove problem. By comparing the residual stress among the cases considering actual welding pass sequence, reversed in-layer sequence, reversed build-up sequence, it was found that the influence of welding sequence on residual stress is small. In the analysis of V groove problem and X groove problem, the analyses were finished in 70 hours and 250 hours, respectively. These computing times were short enough for the practical use.
Conventional selection of welding conditions has a high dependence over the experience of the welder. Since the effect of process parameter on the penetration has not been grasped quantitatively when the target groove configuration and a welding position change, selection of welding conditions has to be performed each time. Moreover, there is the necessity of cutting and observing a cross-section in the penetration shape check to select optimal conditions, and it requires much time. Therefore, the demand to predicting and controlling penetration shape is increasing. In this research, the relation of an output waveform formation parameter, arc phenomena, and penetration shape is estimated for the controlling of the penetration shape change by a current pulse waveform. As a result, it becomes clear that a large factor in the penetration shape change is a Droplet Transfer Mode. Then, the possibility of penetration shape control is found. Moreover, the above-mentioned relation is incorporated into the conventional penetration shape simulation model and the validity of a simple heat source model which imitates change of the Droplet Transfer Mode as change of heat source configuration is evaluated.
In this study, new butt welding technique was proposed to join the polymeric materials, in which the polymeric material is softened by the heated tool due to the Joule's effect heating of the electric current flow through the tool, and the coalescence of material is done by the stirring action due to the tool rotation. 3 mm thick Polycarbonate (PC) sheets were joined in various joining conditions, from which joining mechanism, mechanical properties of joints and process parameters affecting joint performance were investigated. In the experiments, in-situ observation with CCD camera and material temperature measurement during process, as well as the observation of surface appearance and cross-section of the joint and tensile test were performed for these purposes. It was shown from the in-situ observation and material temperature measurement that the molten and softened region is formed around the weld tool. It was also shown that sufficient heat input was required to form the sound joints with acceptable performance, which depended upon the joining speed and amount of electric current flow through the tool. The observation of joint appearance and cross-section revealed that the joint with comparable thickness to base material was obtained under the condition of revolution pitch below 0.08 mm, defined by the ratio of joining speed to tool rotation. It is noticed that the joint obtained from the proper conditions have same mechanical properties as the base material, and that the process parameters of this method were tool rotation speed, welding speed and amount of electric current. These results suggest proposed method is useful for joining the polymeric materials.
This paper describes the effect of the inclination of the weld faying surface on joint strength of friction welded joint and its allowable limit for austenitic stainless steel (SUS304) solid bar similar diameter combination. In this case, the specimen was prepared with the inclination of the weld faying surface pursuant to the JIS Z 3607, and the joint was made with that diameter of 12mm, a friction speed of 27.5s-1, and a friction pressure of 30MPa. The initial peak torque decreased with increasing inclination of the weld faying surface, and then the elapsed time for the initial peak increased with increasing that inclination. However, the steady torque was kept constant in spite of the inclination of the weld faying surface increasing. The joints without the inclination of the weld faying surface, which were made with friction times of 1.5 and 2.0s with a forge pressure of 270MPa, had achieved 100% joint efficiency with the base metal fracture. Those joints had 90 degrees bend ductility with no crack at the weld interface. The joints with the inclination of the weld faying surface of 0.3mm (gap length of 0.6mm), which were allowable distance, was also obtained the same result with this condition. Furthermore, those joints with a friction time of 2.5s were obtained the same result. On the other hand, the joints with the inclination of the weld faying surface of 0.6mm (gap length of 1.2mm), which was twice inclination of the allowable distance, were also obtained the same result in a friction time of 2.5s. However, the joints without the inclination of the weld faying surface at this friction time were not obtained the base metal fracture, although those achieved 100% joint efficiency. In conclusion, to obtain 100% joint efficiency and the base metal fracture with no cracking at the weld interface, the joint must be made with the inclination of the weld faying surface, which was allowable distance pursuant to the JIS Z 3607.