Search

The stirring effect of a rotating laser on a molten pool can expand the range of the laser heat source and effectively inhibit defects such as pores and improve the quality of the weld joint. Due to laser rotation, the physical characteristics of the heat source and the dynamic behavior of the keyhole and molten pool are more complicated than those of conventional laser welding. This paper adopts a numerical simulation method. A three-dimensional model is developed, which takes into account the coupling of the keyhole, recoil pressure, and molten pool. The model can describe the dynamic behavior characteristics of keyholes and fluid flow and the formation process and mechanism of keyhole-induced pores in welds during rotating laser welding. It can be concluded that in conventional laser welding, the keyhole is deep, narrow, and unstable, which usually results in the formation of bubbles. If the bubbles in the molten pool fail to overflow in time, pores are formed. With an increase in laser rotation frequency, the keyhole becomes shallow and wide, and the dynamic behavior of the keyhole tends to be stable, which can effectively inhibit the formation of pores. When the rotating frequency is increased up to 150 Hz, the formation of pores can be completely suppressed. [ABSTRACT FROM AUTHOR]

A three-dimensional transient model is proposed to investigate the weld pool dynamic behavior in laser + metal inert gas (MIG) hybrid fillet welding of aluminum alloy in the horizontal position, which allows for the joint configuration and coupling of the keyhole, droplet and weld pool as well as the heat and mass exchange between gas and liquid phases and is able to simulate the temperature distribution, fluid flow and formation process of a keyhole-induced pore in hybrid welding with a horizontal fillet joint. The weld porosity is also measured using x-ray nondestructive testing technology. Keyhole behavior and the formation mechanism of keyhole-induced porosity were analyzed. The calculated results are in generally good agreement with the experimental ones. A clockwise vortex always exists at the middle part of the weld pool. The formation and growth of the molten metal bulge on the keyhole wall are responsible for the occurrence of a gas bubble, which has a variation in size and shape and can be split during welding. The keyhole collapses easily at its middle or upper part in horizontal fillet welding, and the capture of the bubble by the upper molten pool boundary enhances the possibility of porosity formation to some degree. The keyhole-induced pore is mainly formed at the regions near the keyhole bottom and the upper fusion line of the weld pool. [ABSTRACT FROM AUTHOR]

Repair welding is a popular method to repair the cracked zones of pipes in power plants. However, residual stresses induced by repair welding are inevitably generated and have a significant effect on the performance of repair-welded components during service. The purpose of this research is to investigate the effects of solid-state phase transformation (SSPT) on the repair welding residual stress distribution of multi-layer and multi-pass welding in P91 steel. Based on thermal elastic-plastic theory, a three-dimensional thermal–metallurgical–mechanical finite element model considering the SSPT is established, and shielded metal arc welding (SMAW) heat input is modelled as one double elliptical heat source. The effect of phase transformation on residual stress is investigated by considering effects with and without phase transformation in the database of materials. The results show that transverse compressive stress and a low longitudinal stress area are generated in the final pass due to martensite transformation. Compared with different zones along the thickness direction, the effect of SSPT is greater around the outer surface than around the internal fusion zone. The rationality of the developed finite element model is verified by the experimental method, and good agreement is obtained between those results from the numerical analysis and experimental methods. [ABSTRACT FROM AUTHOR]

A three-dimensional transient model is proposed to investigate the weld formation during laser + metal inert gas (MIG) hybrid welding of aluminum alloy for horizontal fillet joint, which considers the coupling of keyhole, droplet, and liquid metal pool as well as the influence of joint geometry feature and welding position. An asymmetrical double-elliptic heat source is utilized to model the arc heat input and a conic heat source with increased peak heat density is used to describe the laser energy. The coordinate rotation is used to take into account the inclination of heat source and the related forces. According to the experimental and simulated results, weld formation mechanism of horizontal fillet joint is studied. At a lower laser power, the fluid flow pattern in hybrid welding is similar to that in MIG welding and a strong downward flow caused by both gravity and arc pressure occurs at the liquid metal zone of the vertical plate, leading to the accumulation of molten metal at the horizontal plate and the resultant weld asymmetry. At high laser power, a clockwise vortex appears above the keyhole. In this case, the sagging of liquid metal near weld pool surface due to effect of gravity is an important factor responsible for the asymmetry of fillet weld. Besides, with increasing laser power, the tendency of weld pool free surface sagging is enhanced to some extent and weld bead surface becomes concave. [ABSTRACT FROM AUTHOR]

A cable-type welding wire (CWW) gas metal arc welding (GMAW) method was proposed as a novel approach, using CWW for the consumable electrode. Droplet transfer influences the welding process, and the forces on the droplet were analyzed to elucidate the metal transfer phenomenon observed during the welding process. The effects of the arc pressure, rotating force, and welding parameters were analyzed to understand the metal transfer. The special structure of the CWW affected the arc characteristics and forces during metal transfer as part of the welding process. The droplet formed by droplets from each thin wire, the arc, and electromagnetic forces on droplet formation and the coupling process were analyzed. The arc pressure and rotating forces are beneficial to metal transfer and increase the droplet transfer frequency. The droplet size decreases with increasing welding parameters. [ABSTRACT FROM AUTHOR]

Cold cracking susceptibility of the newly developed 9Cr2WVTa steel plate welded with the corresponding gas tungsten arc welding wire has been evaluated using the implant test and the Y-groove cracking test. Measurement of diffusible hydrogen content shows that the welding wire is ultra-low welding consumables (0.40 mL/100 g). Results of the implant test show that the fracture critical stress without pre-heating is only 300 MPa and increases up to the yield strength with pre-heating to 150 °C. The fracture critical stress under high heat input decreases to 250 MPa owing to the formation of quenched microstructure. Fractographies show ductile dimple fracture under higher loads and quasi-cleavage fracture under lower loads. Results of Y-groove cracking tests show that the minimum pre-heating temperature is 150 °C and the maximum transverse residual stress is located at the root of the weld metal. Therefore, the heat input should be chosen in a proper range and the pre-heating temperature is no less than 150 °C. [ABSTRACT FROM AUTHOR]