Papers by Author: János Dobránszky

Abstract: In this article the complex research and development project of the laser cutting micromachining of nitinol alloys are shown. The laser cutting parameters of the 1.04 mm inner diameter and 0.1 mm wall thickness nitinol tubes are also shown. The laser cutting parameters of micromachining and the cut surface of nitinol tubes are summarized when 3 mJ pulse energy, 0.02 ms pulse duration, 6 bar Ar gas pressure, 3000 Hz frequency, 10 mm/s rotation speed and 5 mm/s² speed-up were used. The effect of the laser cutting to the raw material is characterized by microstructural and micromechanical examinations. A detailed description is given of the energy input by laser beam machining. The pulse and the impact of the applied pressure parameters of the gas to the raw material are also shown.

Abstract: This paper examines pulsed Nd:YAG laser cutting edge quality of 0,4 mm thin AISI 304L austenitic stainless steel sheets. We analyze the effect of laser beam focal plane distance from the upper plane of the sheet and the effect of cutting speed on the striation of the cutting edge. We obtain the acceptable focus position range from the analysis of percussion drilling. At the evaluation we determine the optimal focus position for the maximal cutting speed.

Abstract: Nowadays the development of composite materials has increasing importance. In this study Nextel 440 type alumina fibre reinforced aluminium matrix composite wires were investi-gated. Composite wires produced using the continuous process are suitable to simplify the introduc-tion of fibre reinforcements into aluminium castings as well as the production of double composite, sandwich and preferentially reinforced structures. This paper focuses on the porosity of composite wires because minimizing porosity is the primary condition of good mechanical properties. Compo-site wires were produced with different infiltration pressures (0.83 MPa, 1.03 MPa, 1.24 MPa, 1.52 MPa, 1.65 MPa 1.86 MPa and 2.07 MPa) and different diameters (1 mm and 2 mm) to determine the correlation between infiltration pressure and the porosity of wires. 10 grinded cross-sectional samples were made from each type of composite wires. Based on the micrographs of these samples the volume fraction of aluminium was determined by image analysis, which also yielded informa-tion on the porosity of wires. The results show that there is direct linear correlation between the infiltration pressure and the change in porosity. These findings, however, are valid only for the in-vestigated range of pressure.

Abstract: The high-carbon steel strips are one of the most widely used base materials of bandsaw blade manufacturing. These materials have sufficient strength and ductility to cope with the high fatigue load of the bandsaw blades. These endless strips are produced by welding, and therefore the weld and the heat affected zone have different mechanical properties, like tensile strength and fatigue resistance, than the base material. These properties of the weld can be influenced by preheat and post weld heat treatment. Regarding to the latest industrial requirements, the application of laserbeam welding was examined to produce higher standard bandsaw blade. The laserbeam welded joints has lower heat input and narrower heat affected zone compared to metal inert gas (MIG) welding, which is currently used in bandsaw blade manufacturing. To assure the proper mechanical properties and sufficient resistance to fatigue, an examination was carried out to determine the effect of preheat temperature and post weld heat treatment time on the mechanical properties and fatigue behaviour of the laserbeam welded joint.

Abstract: This paper gives an overview of the laser cutting equipment developed for stent cutting: Nd:YAG, fiber and water-jet guided lasers; work piece positioning systems and different materials for stent manufacturing were presented. The side effects of laser cutting like oxide layer, heat affected zone and dross deposition were examined. Pulsed Nd:YAG laser cutting of AISI 304L type austenitic stainless steel high precision tube with 1,800 mm diameter and 0,117 mm wall thickness was performed. The relationship between the average power and kerf was characterised.

Abstract: Produce of coronary stents demands advanced precision. In the present study, electropolishing was performed on stainless steel slotted tube coronary stents made by laser cutting. The surface quality of stents has a significant influence on biocompatibility, therefore the optimal method for electropolishing were explored. Additionally, acid pickling as the pretreatment of electropolishing was also conducted. Pickling was necessary prior to electropolishing for decreasing roughness of the cutting zone and for removing the oxide films covering the stent surface. An optimal condition for electropolishing could also be established and it caused a smooth stent surface. Material removal (weight loss and strut width change) in the process of both pickling and electropolishing was investigated. Furthermore, material characterization of the stents was determined by means of composition analysis, metallographic characterization and microstructural analysis.

Abstract: The clinical results of drug eluting stents [DESs] depend on the type of the coating, the coating technology and the dosage and efficiency of the drug. The current study shows our examinations on drug eluting stents. Different types of commercially available coronary stents were investigated. Various methods were used for these experiments, for example optical microscopy, confocal fluorescent microscopy, scanning electron microscopy and EDS analysis. The properties characterizing applicability of drug eluting stents and the changing of the coatings during expansion are shown.

Abstract: This paper presents the possibility of composite block production by using pressure infiltration technology. This method uses the pressure of an inert gas (usually argon or nitrogen) to force the melted matrix material to infiltrate the reinforcing elements. Three types of materials were considered: open cell metallic foam, metal matrix syntactic foam and carbon fiber reinforced metal matrix composite. Physical and mechanical investigations – such as SEM and compression tests – were performed. The results of measurements were summarized briefly.

Abstract: In this study Nextel 440 type alumina fibre reinforced aluminium matrix composite wires were investigated. Composite wires produced using the continuous process are suitable to simplify the introduction of fibre reinforcements into aluminium castings as well as the production of double composite, sandwich and preferentially reinforced structures. This paper focuses on the porosity of composite wires because minimizing porosity is the primary condition of good mechanical properties. Composite wires were produced with different infiltration pressure (0.83 MPa, 1.03 MPa, 1.24 MPa, 1.52 MPa, and 1.65 MPa) to determine the correlation between infiltration pressure and the porosity of wires. 10 grinded cross-sectional samples were made from each type of composite wires. Based on the micrographs of these samples the volume fraction of aluminium was determined by image analysis, which also yielded information on the porosity of wires. The results show that there is direct (linear) correlation between the infiltration pressure and the decrease in porosity. These findings, however, are valid only for the investigated range of pressure. The measured values were in good agreement with the theoretical model used for comparison.

Abstract: Investigations of microstructural differences of tempered eutectoid steel strips are presented. The constituent phases – fine mixture of ferrite and cementite and a small amount of retained austenite – considerably affect the mechanical properties, especially the resistance to high cycle fatigue. The amount of retained austenite was determined by XRD and EBSD analysis. Thermoelectric power (TEP) measurements were performed for 270 different batches of C75S type eutectoid steel strips. TEP measurement shows characteristically the fine microstructural differences. The TEP of the investigated samples varied between 5650 and 7030 nV/°C. The amount of retained austenite can be significantly higher at the surface (20 %) than at the internal part (0-5 %). The sensitivity of XRD analysis was reduced because of the presence of coarse cementite particles. Using Cu anode, the measurements were more successful than in case of using Co anode. The EBSD analysis showed that also samples immeasurable with XRD contain retained austenite, but its detectability with XRD analysis declines with the coarsening of cementite.