Plasma arc welding: Process, sensing, control and modeling

It is a key issue to establish an appropriate model of the heat source in the simulation of keyhole plasma arc welding (PAW). It requires that the model account for the keyhole effect and have the characteristic of volumetric distribution along the direction of the plate thickness. For available heat source models, neither Gaussian nor double ellipsoidal modes of heat source is applicable to the keyhole PAW process. Considering the force of the high-speed plasma jet and the associated strong momentum, a modified three-dimensional conical heat source model is proposed as the basis for the numerical analysis of temperature fields in the keyhole PAW process. Further, a new heat source model for quasi-steady state temperature field in keyhole PAW is developed to consider the "bugle-like" configuration of the keyhole and the decay of heat intensity distribution of the plasma arc along the direction of the workpiece thickness. Based on this heat source model, finite-element analysis of the temperature profile in keyhole PAW was conducted and the weld geometry was determined. The results showed that the predicted location and locus of the melt-line in the PAW weld cross section are in good agreement with experimental measurements.

A new heat source model for keyhole plasma arc welding in FEM analysis of the temperature profile

Once the keyhole was established, the width of the keyhole did not change with an increasing welding current and a decreasing welding speed.

Observation of the keyhole during plasma arc welding

Plasma arc welding: Process variants and its recent developments of sensing, controlling and modeling

Keyhole plasma arc welding (PAW) has the potential to achieve deep, narrow penetration with low cost and high tolerance of joint preparation. However, the narrow parameter window associated with the keyhole condition limits its wide application, especially for welding thicker plates. To reach its full potential, a control system for keyhole PAW is developed. The efflux plasma voltage signal is detected in real-time to characterize the keyhole size and dimension. The welding current waveform with two slow dropping substages and variable slopes is designed to improve the controllability of keyhole dynamics via reducing both heat input and arc force when a keyhole transforms from its opening status to the closure status. The keyhole PAW experiments are conducted to examine the control effectiveness of the system. The developed system operates in the mode of "one keyhole per pulse" reliably. Closed-loop control tests of varied-thickness plates show that even if the plate thickness changes as much as 50% (from 8 to 4 mm), the developed system adjusts the peak current and its dropping slopes automatically to keep both weld width and penetration consistent.

A Control System for Keyhole Plasma Arc Welding of Stainless Steel Plates with Medium Thickness

In the present investigation, the individual and interactive effects of the main welding parameters on weld quality of plasma arc keyhole welding of conventional structural steel, high strength microalloyed steel and strong formable microalloyed steel have been examined using welding of butt joints with a square groove in various welding positions, and welding of joint roots with a single-V-groove and the root face in the flat position. The most important welding parameters are welding current, welding speed and welding gases, especially plasma gas flow rate. Welding parameter combinations producing the best quality welds are presented. It is shown that it is possible to achieve defect-free high-quality welds with good strength and toughness properties, but the allowable range of variation of welding parameters, especially for the highest weld quality, is narrow. An argonhydrogen mixture for the plasma gas together with argon as shielding and backing gases give the best results with respect to weld quality.

J.K. Martikainen

Papers

Investigation of the effect of welding parameters on weld quality of plasma arc keyhole welding of structural steels