Roman Hamar

Combined heat treatment of metal materials

Purpose – The purpose of this paper is to propose and analyze a combined heat treatment of metal materials, consisting in classic induction pre-heating and/or post-heating and full heating by laser beam. This technology is prospective for some kinds of surface hardening and welding because its application leads to lowering of temperature gradients at the heated spots, which substantially reduces local residual mechanical strains and stresses. Design/methodology/approach – The task was solved like the 3D hard-coupled problem for electromagnetic field, temperature field and field of displacements. It was solved numerically using the techniques based on the FEM. For solution was used commercial software COMSOL Multiphysics, some parts were solved using own scripts in the software Agros. Findings – In the paper are shown results of the numerical solution and experimental measured data. Due the work the authors found that the influence of the pre-heating and post-heating really leads to limit the temperature g...

Numerical modeling of hybrid laser welding taking into account phase change of material

Purpose This paper aims to present a three-dimensional (3D) model of hybrid laser welding of a steel plate. Before welding, the plate is pre- and/or post-heated by induction to avoid mechanical stresses in material due to high gradients of temperature. Welding itself is realized by laser beam without welding rod. The model takes into account existence of both solid and liquid phases in the weld. Design/methodology/approach Presented is the complete mathematical model of the above heat treatment process, taking into account all relevant nonlinearities (saturation curve of the processed steel material and temperature dependences of its physical parameters). Its numerical solution is realized by the finite element method. Some important results are compared with experimental data. Findings In comparison with the former model developed by the authors that did not take into account the phase change, the results are more realistic and exhibit a better accordance with measurements. On the other hand, they strongly depend on sufficiently accurate knowledge of material parameters in both solid and liquid levels (that represent the input data). Research limitations/implications The quality of calculated results strongly depends on the material properties and their temperature dependencies. In case of alloys (whose chemical composition may vary in some range), such data are often unavailable and must be estimated on the basis of experiments. Another quantity that has to be calibrated is the time dependence of power delivered by the laser beam, which is due to the production of a plasma cloud above the exposed spot. Practical implications The presented model and methodology of its solution may represent a basis for design of the complete technology of laser welding with induction pre-heating and/or post-heating. Originality/value Fully 3D model of hybrid laser welding (supplemented with pre- and/or post-heating by magnetic induction) taking into account both solid and liquid phases of welded metal and influence of the plasma cloud is presented.

Model of laser heating with induction pre- and post-heating and its experimental verification

Mathematical model of laser heating with induction pre- and/or post-heating is presented and solved. The purpose of this combined way of heat treatment is to reduce residual mechanical stresses in the surface layers of the processed material produced by high temperature gradients at the spots heated by the laser beam. The 3D model takes into account all important nonlinearities including the temperature dependencies of physical parameters of involved materials (such as the magnetic permeability, electric or thermal conductivity, specific heat capacity etc.). Its numerical solution is carried out by the finite element method. The methodology is illustrated with a practical example whose selected results are verified by measurements.

Higher-order finite element modeling and optimization of actuator with non-linear materials

An electromagnetic actuator with nonlinear structural parts is modelled and optimized. The aim of the optimization is to obtain a flat static characteristic and reach the highest possible force acting on its plunger (manufactured of permanent magnet) at the smallest possible dimensions. The solution of the problem is carried out numerically by the professional software package COMSOL Multiphysics fully controlled by a number of special in-house scripts and procedures. First, two deterministic optimization algorithms are applied to evaluate several different actuator configurations and determine the optimal actuator design from the viewpoint of the maximum force acting on its movable part. Then, a genetic algorithm is used for a multiobjective optimization providing the flattest possible static characteristic. The methodology is illustrated by a typical example whose results are discussed.

Novel algorithm for modeling combined laser and induction welding respecting keyhole effect

Numerical model of combined laser and induction welding is presented and solved. From the physical viewpoint, the process represents a coupled problem of nonlinear and nonstationary interaction of the magnetic and temperature fields respecting the phase change and evaporation of heated molten metal (keyhole effect). A specific algorithm was developed for manipulation with the space and time variation of interface between solid and molten metal. Selected results are compared with the realized experiment.

Aspects of PD measurement in hybrid gaseous insulation at DC voltage

The increased application of HVDC components in electrical energy systems has led to new challenges in diagnostic testing and measurement of electrical insulation. In addition to other diagnostic methods Partial Discharge (PD) diagnosis at DC voltage is currently subject to much discussion worldwide. This is because the established and approved procedures, methods and evaluation techniques at AC voltages cannot be so easily applied under DC conditions. This paper deals with some problems associated with PD measurement, which can be carried out in hybrid gaseous insulation using DC testing. In principle, such insulation types are mostly characterized by the presence of insulating (solid) materials. These interfaces can become electrically charged, should an electric field be present within the insulation, e.g. by the application of an external electric field during PD testing. For the study of such phenomena, a simple needle-plane electrode set with an insulating sheet was investigated at DC voltage with different polarities and electrical stress durations. It could be shown, that the charging process on the insulating sheet already takes place at test voltages below the measured PD inception voltage level. The charge intensity on interface, and as a consequence, its impact on the electrical field conditions e.g. the inception field strength, depends both on polarity and applied voltage duration. It is assumed, that any charging process below the (self-sustaining) ionisation level, characterized by the electrical inception field strength, should be caused by natural ionisation processes which take place in the test object. This assumption was supported by different results obtained if the test set was under open- and closed- volume condition. In the same manner, the results obtained were confirmed by electrical field calculations for different charging conditions on the insulating sheet and applied voltage polarity. The results described in this paper lead to the main conclusion that the charging ability of interfaces in hybrid gaseous insulation must be considered if PD measurement at DC voltage should be performed. For a deeper understanding of these processes further investigation, e.g. in a more compact insulation type appears to be necessary.

Modeling of selected 3D electroheat coupled problems with time-varying geometries

Novel approach to modeling of 3D coupled electroheat problems with time-varying geometries is presented. Instead of time-expensive remeshing of the whole system in every time step it is used the initial mesh that takes into account the expected geometric changes. In the course of computations, its relevant elements are able to adaptively change their shapes and material parameters. The methodology is illustrated with an example of combined cladding.

Optimization of inductor for pre- and post-heating metal parts during their edge laser welding

The shape of an inductor for pre-heating and/or post-heating metal parts is optimized with the aim to reach the prescribed pattern of temperature along the place to be consequently welded by laser beam. The task represents a nonlinear inverse multi-parametric problem that is solved for a given range of field current frequencies. While the direct part of the problem is solved by a fully adaptive higher-order finite element method using own code, the inverse part is solved by selected genetic algorithms. Tested is also the influence of frequency of the field current. The methodology is illustrated with a typical example.

Complex Permittivity Models of Composite Dielectrics

This paper deals with analytical simulations of behaviour of loss frequency-dependent composite dielectric. Several equations of dielectric mixture theory were verified and the results were compared with measured values of these materials, which were prepared in good accord with the theoretical model. The studied dielectric spectroscopy frequency range was from 100 Hz to 1 MHz and we present one stochastic and one properly defined particular composite material composed of constituents with significantly different properties and volume factors. A definition of a suitable dielectric mixture and a volume factor of each component specification is necessary for qualified results evaluation of dielectric properties simulations. The materials used as insulation and/or an insulation part were chosen for the described experiment. First, epoxy resin was used as a host phase (matrix). The choice of sort of the inclusion (filler) phase is the problem of highest importance as far as analytical simulations are concerned. The results of the final composite properties are largely dependent on the inclusions properties, matrix and inclusion permittivity differences, inclusion distribution in the matrix, volume factor and especially the size and shape of the inclusion.

Numerical Solution of Electroheat Problems with Time-varying Geometries

Novel approach to modeling of 3D coupled electroheat problems with time-varying geometries is presented. Instead of time-expensive remeshing of the whole system in every time step it is used the initial mesh that takes into account the expected geometric changes. In the course of computations, its relevant elements are able to adaptively change their shapes and material parameters. The methodology is illustrated with an example of induction-assisted laser cladding.