Effects of torch configuration and welding current on weld bead formation in high speed tandem pulsed gas metal arc welding of steel sheets

Abstract

Undercut and humping bead are the common defects that limit the maximum welding speed of tandem pulsed gas metal arc (GMA) welding. In order to increase the maximum welding speed, effects of the inclination angle, interwire distance and welding current ratio between the leading wire and trailing wire on bead formation in high speed welding are investigated. The undercut and humping bead is attributed to the irregular flow of molten metal towards the rear part of the weld pool. This irregular flow can be prevented by the trailing wire with a push angle from 5° to 13° , which provides an appropriate component of arc force in the welding direction. The irregular flow is also related to the distance between the leading wire and the trailing wire, and the flow becomes regular when the distance is in the range 9–12 mm. Moreover, the stabilisation of the bulge of the weld pool between the two wires, the presence of enough molten metal below the trailing arc, and the reduced velocity of molten metal flow towards the rear part of the weld pool, are essential to increase the maximum welding speed. These conditions can be obtained by adjusting the ratio of the leading arc current to the trailing arc current. A maximum welding speed as high as 4–4·5 m min^?1 is achieved by setting the current ratio to a value ranging from 0·31 to 0·5.     

New technological methods and designs of stir head in resistance friction stir welding

Abstract

This paper proposes a new welding method, resistance friction stir welding (RFSW), which provides a novel method to solve some of the challenges of friction stir welding high melting point of ferrous and non-ferrous metals. The RFSW is a new hybrid welding technology, which utilises the advantages of both resistance welding and friction welding. The technological principle of RFSW uses the compound effects of resistance heating and friction heating; the integration methods are discussed and analysed in this paper. It is believed that RFSW is an innovative, practical welding technique. The national patents for invention have already been applied in China.     

Random nature of droplet size and its origins in short circuit CO2 arc welding

Abstract

The nature of the droplet size variation and its effect on the process behaviour were investigated for short circuit CO₂ arc welding using a power supply with constant voltage output. To obtain the droplet images and record the welding current and voltage transient waveforms in phase, an integrated experimental system was established by combining a high speed charge coupled device camera based on laser shadowing and a related optical system with a dual channel analog/digital converter. The droplet size in short circuit transfer mode is defined as the maximum diameter of the molten droplet just before short circuiting. The experimental results reveal that the distribution of droplet size typically has a random nature with a range of between 1 and 2 times the electrode diameter. Excessively large or small droplet size is markedly unfavourable to the process stability. The droplet shape just before short circuiting and the short circuiting experience are also critical factors affecting the process behaviour. The main factors influencing the droplet size and shape are analysed in detail and approaches to control their evolution are then proposed.     

Numerical simulation of heat transfer and fluid flow in GTA/Laser hybrid welding

Abstract

In order to understand the temperature fields, cooling rates and mixing in the weld pool, a comprehensive, three-dimensional heat transfer and fluid flow model is developed and tested by comparing model predictions with two sets of experimental data. The first set of data was taken from the literature. The experiments varied the separation distance between the heat sources for three arc current levels at a constant laser power. The second set of experiments analysed the effect of varying laser power for a constant heat source separation distance. The results demonstrate that the distance between the two heat sources significantly affects the cooling rates. The calculated results showed that the hybrid weld pool was very well mixed with strong convection currents resulting from the interaction between the electromagnetic and Marangoni forces. The calculated and experimental results showed that hybrid welding increases the weld pool width and gap bridgability when compared with laser welding. The weld pool depth in hybrid welding was affected mainly by the characteristics of the laser beam. Hybrid weld pool penetration depth is maximised at an optimal distance between the arc electrode and laser beam. The cooling rate increases significantly when the heat sources are separated beyond a critical distance. At close separation between arc and laser, calculations show that the arc radius must be decreased to achieve the observed weld depths.