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.
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.