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

Keyhole plasma arc welding achieves much deeper penetration than do all other existing arc welding processes. Because of its ability to penetrate thicker material, the control of the keyhole in plasma arc welding becomes critical. From an analysis of the physical process, a sensor to detect the state of the keyhole for keyhole process control has been proposed and developed. This sensor measures the electrical effect of the plasma cloud generated during keyhole plasma arc welding. It is found that the plasma cloud, which rapidly decreases to zero upon establishment of the fully penetrated keyhole, can be used to detect the state of the keyhole reliably. The effectiveness of the proposed sensor for detecting the keyhole state has been verified during pulse keyhole plasma arc welding.

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A plasma cloud charge sensor for pulse keyhole process control

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

Three-dimensional simulation of the plasma arc welding process

A first order study of the interfacial (keyhole) shape between a penetrating argon plasma arc jet and a stationary liquid metal weld pool is presented The interface is determined using the Young-Laplace equation by assuming that the plasma jet behaves as a one-dimensional ideal gas flow and by neglecting flow within the weld pool The solution for the keyhole shape allows an approximate determination of the liquid-solid metal phase boundary location based on the assumption that the liquid melt is a stagnant thermal boundary layer Parametric studies examine the effect of plasma mass flow rate, initial plasma enthalpy, liquid metal surface tension, and jet shear on weldment shape under both normal and zero gravity Among the more important findings of this study is that keyhole and weld geometries are minimally affected by gravity, suggesting that data gathered under gravity can be used in planning in-space welding

B. Rubinsky

Papers

Keyhole and weld shapes for plasma arc welding under normal and zero gravity