Papers by Author: Janez Grum

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Authors: Sebastjan Žagar, Janez Grum
Abstract: In the paper two aluminium alloys, i.e. 6082 and 7075, which were cold hardened by shot peening under different conditions, are treated. Surface hardening was carried out with S170 steel shot of the same diameter, particle hardness being 56 HRC. Other conditions were the operating pressure, mass flow, which provide different Almen intensities. The hardened layer was described by surface integrity. Macroscopic and microscopic analyses consisted in analyses of hardened profiles of hardness, and residual stresses in the thin surface layer. Research results indicated that there were significant differences among the characteristics chosen to describe surface integrity and that they had an important influence on the final condition of the surface layer. With too severe settings of the peening parameters, the surface properties got worse because of damages, which resulted in crack initiation and growth of the existing cracks.
Authors: Janez Grum, Tomaž Kek
Abstract: The paper describes an analysis of the AE signals captured with PZT sensor during and after laser cutting. In the course of cutting the continuous AE signals are treated in a defined time interval. After cutting, bursts of acoustic emission will occur. They are due to solidification of the molten material and cooling of the laser-cut surface. The results shown refer to cutting of steel plate DC04 and of austenitic stainless steel plate X5CrNi18-10. Both plate types are frequently used in automobile industry, where, because of mass production, optimisation of the cutting process is desired. Numerous laser-cutting conditions assuring different quality levels of the cut made were chosen. It was proved that the laser cutting conditions had a significant influence on the amplitude value and intensity of the continuous signal and AE activity after the termination of laser cutting. Separately it was also confirmed that there was a relation between the measured signal and laser-cut quality, which is shown, to a large extent, by the presence of dross at the lower edge of the cut.
Authors: Janez Grum, Tomaž Kek
Abstract: Residual stresses are an inevitable result of mechanical and heat treatments of steel. The stress field generated at the surface and through the cross section of a product is usually non-uniform, and there can be important stress gradients. In general residual stresses can be defined as self-balanced internal stresses existing in a free body on which no external forces are acting. In the present study the focus was on the residual stresses occurring in a steel deep-drawn product of a complex form. The residual stresses occur mainly because of different degrees of plastic deformation at different locations. The residual stresses occurring in the product as a result of the manufacturing process of deep drawing can be comparatively strong. In slitting or cutting a useful product out of a deepdrawn part by laser cutting, a portion of the residual stresses will get relieved and produce distortion of the deep-drawn product. The studies made showed that the distortion of the deep-drawn product depended on the mode of cutting-out. Also the state of the residual stresses after slitting or cuttingout of the deep-drawn product will depend on the mode of cutting-out, i.e., on the sequence of individual cuts chosen. The product used in the analysis made was a deep-drawn steel mud-guard, which makes a component part of a working machine. The various modes of cutting-out a useful product of 1260x800x370 mm in size were performed with laser cutting using cutting oxygen. The residual stresses in the mud-guard were determined with a semi-destructive hole drilling strain gage method.
Authors: Zoran Bergant, Janez Grum
Abstract: In this paper, the residual stresses on flat specimens, made from 12-nickel precipitation hardening maraging hot-working tool steel coated with Fe-Ni-Co-Mo and subsuquent laser remelted using Nd:YAG laser, were determined using the hole drilling method. In this newly proposed multi-step process for hot working tool steel reparation, the residual stresses are generated during each step by different physical mechanisms. The flame spraying is a highly productive technique to replace worn out material but with limited bonding to the substrate material. Subsequent laser remelting improves coating's bonding with substrate, coating's density and deoxidation to improve wear resistance, erosion and corrosion resistance of the flame sprayed coatings. After laser remelting, the tensile stresses in coatings slightly reduces or shifts into compressive which in turns improves the crack initiation resistance of the repaired surface of the thermomechanicaly loaded die casting tool.
Authors: Uros Trdan, Janez Grum, Michael R. Hill
Abstract: The influence of different parameters of laser shock processing applied to a precipitation-hardened aluminium alloy 6082-T651, on residual stress, surface tophraphy and microhardness was investigated. Processing was performed with an innovative Nd:YLF laser with the power densities of 2 and 4 GW/cm2, with a uniform pulse duration of 18 ns. Laser shock processing experiments were performed with the closed ablation method to ensure a higher shock-wave pressure. In the first phase, the study was focused on an evaluation of surface topography, with the record of the surface roughness profile and with the surface evaluation at a scanning electron microscope JEOL JXA-8600M. Then followed measurement of microhardness HV0.2 in the cross section region. In the second phase comparison of residual stresses which were measured using the X-ray diffraction, was performed. Laser shock processing turned out to be a very efficient technique to improve surface properties. On the basis of the micro plastic deformation induced by shock waves, an increased dislocation density in the specimen surface was obtained. The gradient of dislocation piling through the specimen depth improved the variation of microhardness and residual stresses, which, in turn, improves fatigue strength of the material under dynamic loading.
Authors: Janez Grum
Abstract: Induction surface hardening creates very desirable residual stresses in the hardened surface layer. Residual stresses are always of a compressive nature and are usually present to the depth of the induction-hardened layer. By the appropriate selection of grinding wheel and grinding conditions and taking into account the physical and mechanical properties of the workpiece material very favourable compressive residual stresses in the hardened surface layer can be retained. How is it possible to assure a desirable surface and surface layer quality after induction hardening and fine grinding? Finding an answer to this question requires a very good knowledge of the process of grinding on the micro-level as well as knowledge of mechanical and heat effects acting on the layer of the workpiece including the type and condition of the grinding wheel. An allinclusive consideration of the numerous influences of the kind and condition of the tool on the changes on the surface and in the surface layer of the workpiece in the given machining conditions is described by the term “surface integrity”.
Authors: Roman Šturm, Janez Grum
Abstract: The present paper deals with residual stress and strain changes in thin flat specimens of nodular cast iron during laser remelting process. In laser remelting process of a workpiece we have used several laser beam passages across the workpiece surface. The state of internal stress in the workpiece changes because of different ways of guiding the laser beam across the workpiece surface, and consequently strain of the workpiece may occur. In the process of gradual remelting of the specimen surface, i.e. during its heating and cooling, volume changes in the specimen surface layer occur due to thermal and microstructural changes. To find out more about volume changes in the thin surface layer of specimens, the latter were monitored by measuring the strain at the lower side of the specimens during and after the remelting process, and also by residual stress measurement in the thin surface layer. The results obtained confirm that the laser remelting process could be successfully optimised on the basis of knowing the residual stresses and the strain generated during the remelting process.
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