Radovan Kovacevic

Finite element modeling of friction stir welding—thermal and thermomechanical analysis

Friction stir welding (FSW) is a relatively new welding process that may have significant advantages compared to the fusion processes as follow: joining of conventionally non-fusion weldable alloys, reduced distortion and improved mechanical properties of weldable alloys joints due to the pure solid-state joining of metals. In this paper, a three-dimensional model based on finite element analysis is used to study the thermal history and thermomechanical process in the butt-welding of aluminum alloy 6061-T6. The model incorporates the mechanical reaction of the tool and thermomechanical process of the welded material. The heat source incorporated in the model involves the friction between the material and the probe and the shoulder. In order to provide a quantitative framework for understanding the dynamics of the FSW thermomechanical process, the thermal history and the evolution of longitudinal, lateral, and through-thickness stress in the friction stirred weld are simulated numerically. The X-ray diffraction (XRD) technique is used to measure the residual stress of the welded plate, and the measured results are used to validate the efficiency of the proposed model. The relationship between the calculated residual stresses of the weld and the process parameters such as tool traverse speed is presented. It is anticipated that the model can be extended to optimize the FSW process in order to minimize the residual stress of the weld.

Thermal modeling of friction stir welding in a moving coordinate system and its validation

A three-dimensional heat transfer model for friction stir welding (FSW) is presented in this paper; a moving coordinate is introduced to reduce the difficulty of modeling the moving tool. Heat input from the tool shoulder and the tool pin are considered in the model. The finite difference method was applied in solving the control equations. A non-uniform grid mesh is generated for the calculation. FSW experiments have been done to validate the calculated results. The calculated results are in good agreement with the experimental results. The calculation result also shows that preheat to the workpiece is beneficial to FSW.

Sensing, modeling and control for laser-based additive manufacturing

Laser-Based Additive Manufacturing (LBAM) is a promising manufacturing technology that can be widely applied to part preparation, surface modification, and Solid Freeform Fabrication (SFF). A large number of parameters govern the LBAM process. These parameters are sensitive to the environmental variations, and they also influence each other. This paper introduces the research work in RCAM on improving the performance of the LBAM process. Metal powder delivery real-time sensing and control is studied to achieve a controllable powder delivery for fabrication of functionally graded material. A closed-loop control system based on infrared image sensing is built for control of the heat input and size of the molten pool in the LBAM process. The closed-loop control results show a great improvement in the geometrical accuracy of the built features. A three-dimensional finite element model is also established to explore the thermal behavior of the molten pool in the closed-loop controlled LBAM process.

A three dimensional model for direct laser metal powder deposition and rapid prototyping

A three-dimensional model for direct laser metal powder deposition process and rapid prototyping is developed. Both numerical and analytical models are addressed. In the case of numerical modeling, the capabilities of ANSYS parametric design language were employed. The model calculates transient temperature profiles, dimensions of the fusion zone and residual stresses. Model simulations are compared with experimental results acquired on line using an ultra-high shutter speed camera which is able to acquire well-contrasted images of the molten pool, and off-line using metallographical and x-ray diffraction analyses. The experiments showed good agreement with the modeling. The results are discussed to provide suggestions for feedback control and reduction of residual stresses.

Sensing and Control of Weld Pool Geometry for Automated GTA Welding

Weld pool geometry is a crucial factor in determining welding quality, especially in the case of sheet welding. Its feedback control should be a fundamental requirement for automated welding. However, the real-time precise measurement of pool geometry is a difficult procedure. It has been shown that vision sensing is a promising approach for monitoring the weld pool geometry. Quality images that can be processed in real-time to detect the pool geometry are acquired by using a high shutter speed camera assisted with nitrogen laser as an illumination source. However, during practical welding, impurities or oxides existing on the pool surface complicate image processing. The image features are analyzed and utilized for effectively processing the image. It is shown that the proposed algorithm can always detect the pool boundary with sufficient accuracy in less than 100 ms. Based on this measuring technique, a robust adaptive system has been developed to control the pool area. Experiments show that the proposed control system can overcome the influence caused by various disturbances.

Real-Time Image Processing for Monitoring of Free Weld Pool Surface

The arc weld pool is always deformed by plasma jet. In a previous study, a novel sensing mechanism was proposed to sense the free weld pool surface. The specular reflection of pulsed laser stripes from the mirror-like pool surface was captured by a CCD camera. The distorted laser stripes clearly depicted the 3D shape of the free pool surface. To monitor and control the welding process, the on-line acquisition of the reflection pattern is required. In this work, the captured image is analyzed to identify the torch and electrode. The weld pool edges are then detected. Because of the interference of the torch and electrode, the acquired pool boundary may be incomplete. To acquire the complete pool boundary, models have been fitted using the edge points. Finally, the stripes reflected from the weld pool are detected. Currently, the reflection pattern and pool boundary are being related to the weld penetration and used to control the weld penetration.

Experimental and numerical modeling of buckling instability of laser sheet forming

New experimental results show that laser bending can be extended to generate a bending angle not only towards but also away from the laser beam, giving more flexibility to the process. In order to explain this buckling instability, a series of experiments have been carried out with real-time measurement of the bending angle for different materials, thicknesses, scanning speeds, laser beam diameters and laser powers, pre-bending conditions, and cooling conditions. Furthermore, a 3-D FEM simulation has been performed that includes a non-linear, transient, indirect coupled, thermal–structural analysis accounting for the nonlinear geometric and material properties. The buckling deformation, bending angle and distribution of stress–strain and temperature, as well as residual stresses, have been obtained from the simulations. The bending angle is affected by the temperature distribution and gradient, the mechanical and thermal properties of the sheet metal material, and the process parameters, such as the laser power, the laser beam diameter, the scanning speed, the material, the sample geometry, and other bending conditions. The buckling mechanism can be illustrated by the simulation results.  2002 Elsevier Science Ltd. All rights reserved.

Computer simulation and experimental investigation of sheet metal bending using laser beam scanning

Abstract Computer simulation and experimental investigation of the sheet metal bending into a V-shape by the laser beam scanning without an external force exerted onto it have been performed. A 3-D FEM simulation has been carried out, which includes a non-linear transient indirect coupled thermal-structural analysis accounting for the temperature dependency of the thermal and mechanical properties of the materials. The bending angle, distribution of stress–strain, temperature and residual stresses have been obtained from the simulations. The sheet metal bending had been performed for different materials, thicknesses, scanning speeds and laser powers. The measurement of real-time temperature and bending angle was carried out. The bending angle is affected by the mechanical and thermal properties of the sheet metal material, the process parameters, and the output of laser energy. The bending angle is increased with the number of laser beam scanning passes and is the function of the laser power and the laser beam scanning speed. The simulation results are in agreement with the experimental results.

Characterization and real-time measurement of geometrical appearance of the weld pool

Abstract The weld pool contains important information about the welding process (Zhang et al., Welding J. 72, 463s–469s, 1993). In this study, a polar coordinate model is proposed to characterize the weld pool geometrically. The identification of its parameters involves complicated non-linear optimization which cannot be done in real time using conventional algorithms. A neural network is therefore proposed to identify the parameters in real time. By using pulsed laser illumination, clear images of the weld pool are captured. The developed image processing algorithm extracts the boundary of the weld pool in real time. Thus, a real-time system is developed to sense and process the image and identify the polar coordinate model. It is shown that the weld penetration can be accurately determined using the model parameters. Thus, a real-time weld penetration monitoring system is also achieved.

Numerical and Experimental Investigations on the Loads Carried by the Tool During Friction Stir Welding

A computational fluid dynamics (CFD) model is presented for simulating the material flow and heat transfer in the friction stir welding (FSW) of 6061-T6 aluminum alloy (AA6061). The goal is to utilize the 3-D, numerical model to analyze the viscous and inertia loads applied to the FSW tool by varying the welding parameters. To extend the FSW process modeling, in this study, the temperature-dependant material properties as well as the stick/slip condition are considered where the material at the proximity of the FSW tool slips on the lower pressure regions. A right-handed one-way thread on a tilted FSW tool pin with a smooth, concaved shoulder is, additionally, considered to increase the accuracy of the numerical model. In addition, the viscous and frictional heating are assumed as the only sources of heat input. In the course of model verification, good agreements are found between the numerical results and the experimental investigations.