R Kovacevic

A unified model of transport phenomena in gas metal arc welding including electrode, arc plasma and molten pool

This paper presents a theoretical model for describing globular transfer in gas metal arc welding. The heat and mass transfer in the electrode, arc plasma and molten pool are considered in one unified model. Using the volume of fluid method, the transport phenomena are dynamically studied in the following processes: droplet formation and detachment, droplet flight in arc plasma, impingement of droplets on the molten pool and solidification after the arc extinguishes. The simulation of heat and mass transfer in the arc plasma considers the developing surface profile of the electrode and molten pool and also the effect of the flying droplet inside the arc plasma. Furthermore, the heat inputs to the electrode and the molten pool result from the simulation of the arc plasma. In addition, a He–Ne laser in conjunction with the shadow-graphing technique is used to observe the metal-transfer process. The theoretical predictions and experimental results are shown to be in good agreement.

Keyhole formation and collapse in plasma arc welding

A mathematical model is developed to describe the heat transfer and fluid flow in stationary keyhole plasma arc welding. Using the volume of fluid method, the processes of heating, melting, collapsing and subsequent solidifying in the molten pool are dynamically studied. The predicted results have been compared with the experimental weld and keyhole shape. Using the LaserStrobe video system, the captured images of the upper surface of the weld pool are used to verify the predicted transient development of the weld pool. It is indicated that theoretical predictions and the experimental results are in close agreement.

Automated torch path planning using polygon subdivision for solid freeform fabrication based on welding

A continuous path is one of the most important requirements for solid freeform fabrication (SFF) based on welding. This paper proposes a method for torch path planning applicable to SFF based on welding with an emphasis on minimum human intervention. The suggested approach describes a method based on the subdivision of a two-dimensional (2-D) polygonal section into a set of monotone polygons to generate a continuous path for material deposition. A two-dimensional contour is subdivided into smaller polygons (subpolygons). The path for each individual subpolygon is generated. The final torch path is obtained by connecting individual paths for all the subpolygons and trimming along the points of intersection of the paths for individual subpolygons. The final path is a closed loop; therefore, any point can be selected as the starting point for material deposition. The proposed method can be used to develop the toolpath for CNC milling.

Dynamic analysis of globular metal transfer in gas metal arc welding—a comparison of numerical and experimental results

A mathematical model to describe the globular transfer in gas metal arc welding is developed. This work is both theoretical and experimental. Using the volume-of-fluid (VOF) method, the fluid-flow and heat-transfer phenomena during the impingement of a droplet on a solid substrate, arc striking, the impingement of multiple droplets on the molten pool and finally the solidification after the arc extinguishes are dynamically studied. A He-Ne laser in conjunction with the shadow-graphing technique is used to observe the metal-transfer processes. Theoretical predictions and experimental results are shown to be in good agreement, suggesting that the theoretical treatment of the model is sound.

Dynamic Analysis and Identification of Gas Tungsten Arc Welding Process for Weld Penetration Control

In this study, gas tungsten arc welding is analyzed and modeled as a 2-input (welding current and arc length) 2-output (weld depression and width) multivariable process. Experiments under a number of typical welding conditions are performed to excite and identify the process characteristics and variations. It is observed that the model parameters vary in a large range with the experimental conditions. A real-time model frame with only a few parameters to be identified on-line is proposed. Based on the obtained models, the process characteristics in terms of inertia, delay, nonminimum phase, and coupling are given. These characteristics suggest an adaptive predictive decoupling control algorithm. By designing and implementing the suggested control algorithm with the real-time model, excellent results have been achieved for both simulation and practical control. This shows that the dynamic analysis and identification provide sufficient process information for design of the control system.

Mechanically assisted droplet transfer process in gas metal arc welding

Abstract Gas metal arc welding has been generally accepted as the preferred joining technique due to its advantages in high production and automated welding applications. Separate control of arc energy and arc force is an essential way to improve the welding quality and to obtain the projected metal transfer mode. One of the most effective methods for obtaining separate control is to exert an additional force on the metal transfer process. In this paper, the droplet transfer process with additional mechanical force is studied. The welding system is composed of an oscillating wire feeder. The images of molten metal droplets are captured by a high-speed digital camera, and both the macroscopic appearance and the cross-sectional profiles of the weld beads are analysed. It is shown that the droplet transfer process can be significantly improved by wire electrode oscillation, and a projected spray transfer mode can be established at much lower currents. By increasing the oscillation frequency, the droplet transfer rate increases while the droplet size decreases. In addition, the improvement in the droplet transfer process with wire oscillation leads to an enhancement of the surface quality and a modification of the geometry of the weld beads that could be of importance for overlay cladding and rapid prototyping based on deposition by welding.

Three-dimensional model for gas tungsten arc welding with filler metal

Abstract This paper presents a three-dimensional model to describe the transient heat and fluid flow in gas tungsten arc welding (GTAW) with a filler metal. Using the volume of fluid (VOF) method, the melt depth and the surface profile of the weld are predicted. The GTAW experiments show that the predictions of the weld pool shape based on the model are in good agreement with the measured values.

Sensitivity of Front-Face Weld Geometry in Representing the Full Penetration:

The measurement of weld penetration using a front-face sensor has been an important issue in welding automation. The fundamental problem is to find a measurable front-face parameter that can adequately represent the weld penetration. In this paper, both the front-face average weld depression depth (a novel weld geometrical parameter) and the weld width are selected as possible representations of full penetration in GTA welding. Two types of sensitivities of front-face weld geometry, with respect to variations in welding conditions and with respect to control variables, are proposed as criteria for determination of promising parameters. Sensitivity calculation of experimental data shows that the front-face weld width is not promising while the average front-face weld depression depth is. This conclusion is also confirmed by a specific experiment and statistic models.