John Goldak

Bio: John Goldak is an academic researcher from Carleton University. The author has contributed to research in topics: Welding & Finite element method. The author has an hindex of 19, co-authored 66 publications receiving 3386 citations.

A new finite element model for welding heat sources

Abstract: A mathematical model for weld heat sources based on a Gaussian distribution of power density in space is presented. In particular a double ellipsoidal geometry is proposed so that the size and shape of the heat source can be easily changed to model both the shallow penetration arc welding processes and the deeper penetration laser and electron beam processes. In addition, it has the versatility and flexibility to handle non-axisymmetric cases such as strip electrodes or dissimilar metal joining. Previous models assumed circular or spherical symmetry. The computations are performed with ASGARD, a nonlinear transient finite element (FEM) heat flow program developed for the thermal stress analysis of welds.* Computed temperature distributions for submerged arc welds in thick workpieces are compared to the measured values reported by Christensen1 and the FEM calculated values (surface heat source model) of Krutz and Segerlind.2 In addition the computed thermal history of deep penetration electron beam welds are compared to measured values reported by Chong.3 The agreement between the computed and measured values is shown to be excellent.

Computer modeling of heat flow in welds

Abstract: This paper summarizes progress in the development of methods, models, and software for analyzing or simulating the flow of heat in welds as realistically and accurately as possible. First the fundamental equations for heat transfer are presented and then a formulation for a nonlinear transient finite element analysis (FEA) to solve them is described. Next the magnetohydrodynamics of the arc and the fluid mechanics of the weld pool are approximated by a flux or power density distribution selected to predict the temperature field as accurately as possible. To assess the accuracy of a model, the computed and experimentally determined fusion zone boundaries are compared. For arc welds, accurate results are obtained with a power density distribution in which surfaces of constant power density are ellipsoids and on radial lines the power density obeys a Gaussian distribution. Three dimensional, in-plane and cross-sectional kinematic models for heat flow are defined. Guidelines for spatial and time discretization are discussed. The FEA computed and experimentally measured temperature field,T(x, y, z, t), for several welding situations is used to demonstrate the effect of temperature dependent thermal properties, radiation, convection, and the distribution of energy in the arc.

Three-dimensional numerical analysis of heat and mass transfer in heat pipes

Abstract: A three-dimensional finite-element numerical model is presented for simulation of the steady-state performance characteristics of heat pipes. The mass, momentum and energy conservation equations are solved for the liquid and vapor flow in the entire heat pipe domain. The calculated outer wall temperature profiles are in good agreement with the experimental data. The estimations of the liquid and vapor pressure distributions and velocity profiles are also presented and discussed. It is shown that the vapor flow field remains nearly symmetrical about the heat pipe centerline, even under a non-uniform heat load. The analytical method used to predict the heat pipe capillary limit is found to be conservative.

A new method for the model-independent assessment of thickness in three-dimensional images

Abstract: Three-dimensional (3-D) structural parameters derived from lower-dimensional measurements using indirect morphometric methods may be strongly biased if the measured objects deviate from the assumed structure model. With the introduction of 3-D microscopic measuring techniques it is possible to obtain a complete depiction of complex spatial structures. As a consequence, new 3-D methods have recently been developed for the estimation of morphometric parameters such as volume, surface area and connectivity by direct processing of the 3-D images. Structure thickness is an important morphometric parameter which is usually defined for specific structure models only. In this paper we propose a general thickness definition for arbitrary structures allowing us to calculate the mean structure thickness and the thickness distribution of 3-D objects in a direct way and independently of an assumed structure model. Additionally, an efficient implementation for the practical usage of the method is described using distance transformation. The new method is applied to trabecular bone structures measured with a 3-D micro-computed tomography system.

An overview of Direct Laser Deposition for additive manufacturing; Part I: Transport phenomena, modeling and diagnostics

Abstract: Laser-based additive manufacturing (LBAM) processes can be utilized to generate functional parts (or prototypes) from the ground-up via layer-wise cladding – providing an opportunity to generate complex-shaped, functionally graded or custom-tailored parts that can be utilized for a variety of engineering applications. Directed Energy Deposition (DED), utilizes a concentrated heat source, which may be a laser or electron beam, with in situ delivery of powder- or wire-shaped material for subsequent melting to accomplish layer-by-layer part fabrication or single-to-multi layer cladding/repair. Direct Laser Deposition (DLD), a form of DED, has been investigated heavily in the last several years as it provides the potential to (i) rapidly prototype metallic parts, (ii) produce complex and customized parts, (iii) clad/repair precious metallic components and (iv) manufacture/repair in remote or logistically weak locations. DLD and Powder Bed Fusion-Laser (PBF-L) are two common LBAM processes for additive metal part fabrication and are currently demonstrating their ability to revolutionize the manufacturing industry; breaking barriers imposed via traditional, ‘subtractive’ metalworking processes. This article provides an overview of the major advancements, challenges and physical attributes related to DLD, and is one of two Parts focused specifically on DLD. Part I (this article) focuses on describing the thermal/fluidic phenomena during the powder-fed DLD process, while Part II focuses on the mechanical properties and microstructure of parts manufactured via DLD. In this current article, a selection of recent research efforts – including methodology, models and experimental results – will be provided in order to educate the reader of the thermal/fluidic processes that occur during DLD, as well as providing important background information relevant to DLD as a whole. The thermal/fluid phenomena inherent to DLD directly influence the solidification heat transfer which thus impacts the part's microstructure and associated thermo-mechanical properties. A thorough understanding of the thermal/fluid aspects inherent to DLD is vital for optimizing the DLD process and ensuring consistent, high-quality parts.

Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies

Abstract: A concentrating solar power (CSP) system converts sunlight into a heat source which can be used to drive a conventional power plant. Thermal energy storage (TES) improves the dispatchability of a CSP plant. Heat can be stored in either sensible, latent or thermochemical storage. Commercial deployment of CSP systems have been achieved in recent years with the two-tank sensible storage system using molten salt as the storage medium. Considerable research effort has been conducted to improve the efficiency of the CSP system and make the cost of electricity comparable to that of the conventional fossil-fuel power plant. This paper provides a comprehensive summary of CSP plants both in operation and under construction. It covers the available technologies for the receiver, thermal storage, power block and heat transfer fluid. This paper also reviews developments in high temperature TES over the past decade with a focus on sensible and latent heat storage. High temperature corrosion and economic aspects of these systems are also discussed.