Papers by Author: Róbert Bidulský

Abstract: The main aim of the present paper is evaluated the mechanical properties, microstructures and porosity of Ti6Al4V and CoCrW alloys produced by Laser Powder Bed Fusion (L-PBF) as an additive manufacturing (AM) technology. The mechanical properties were follows: For Ti6Al4V alloy the UTS was 1180 MPa; the YS was in the range <600; 745 MPa>. For CoCrW alloys, the UTS were in range <750; 950 MPa> and YS was in range <400; 500>. Evaluation of porosity was realized on non-etched samples using by quantitative image analysis in order to describe the dimensional and morphological porosity characteristics. The pores in the Ti6Al4V alloy showed homogeneous distribution without significant large pores.

Abstract: The paper deals with the influence of severe plastic deformation on the typical powder metallurgy (PM) microstructural characteristics, such as porosity, of a PM aluminum alloy. A commercial ready-to-press aluminum based powder was used as material to be investigated. After applying different compacting pressures (400, 500, 600 and 700 MPa), specimens were debinded in a ventilated furnace at 400 °C for 60 min. Sintering was carried out in a vacuum furnace at 610 °C for 30 min. The specimens were ECAPed for 1 pass. The dimensional and morphological porosity of investigated materials were measured individually for each pore. Results show that ECAP generates shearing stress breaking down the oxide film; this, coupled to particles deformation under local constraints, enables strong bonding and stability. Therefore, ECAP supports next progressive decreasing of pore size as well as strongly influences both dimensional and morphological porosity characteristics, considering that small pores evolve easily to a circular form. Moreover, ECAP cause strong bonding between adjacent particles, which results in a significant increase of mechanical properties.

Abstract: The main aim of the present contribution is to show how different heat treatment conditions influence the microstructure of a Fe - [1.5Cr - 0.2Mo] - 0.6C powder system. In vacuum furnaces, the cooling rate is generally determined by the pressure of the gas (basically N2) introduced into the chamber. Different gas pressures have been applied, from 0 to 6 bars. The average cooling rates were calculated in the range of 1180 °C to 400 °C and were varying from 0.1 to 6 °C/s, according to the gas pressure. Considering the cooling rates, increasing the nitrogen pressure resulted in an increased amount of bainite/martensite microstructure. The microstructure constituents ranged from 97% pearlite + 3% ferrite in the system treated at 0 bar to 82 % martensite + 18 % bainite (with small amount of tempered martensite) in the system cooled applying N2 at 6 bars. Mechanical properties have been evaluated in terms of toughness, TRS and hardness for all processing conditions; the analysis of the properties allowed to plot graphs correlating the different properties as function of the characteristic microstructures.

Abstract: The aim of the presented paper is to describe the sintered duplex stainless steels manufactured in sinter-hardening process and their structural and mechanical properties. Duplex stainless steels were obtained through powder metallurgy starting from austenitic 316L or ferritic 410L prealloyed base powders by controlled addition of alloying elements powder. Prepared mixes were compacted at 700MPa and sintered in a vacuum furnace with argon backfilling at temperature of 1240°C for 1h. After sintering different cooling cycles were applied: rapid cooling (6°C/s) using nitrogen under pressure and slow cooling (0.1°C/s) with furnace in argon atmosphere. Produced sintered duplex stainless steels were studied by scanning and optical microscopy and EDS chemical analysis of microstructure components as well as X-ray analysis. Mechanical properties were studied through tensile and three-point bending tests and Charpy impact test. It was demonstrated that austenitic-ferritic microstructures with regular arrangement of both phases and absence of precipitates can be obtained with properly designed powder mix composition as well as sintering cycle with rapid cooling rate. Produced sintered duplex steels show good mechanical properties which depend on austenite/ferrite ratio in the microstructure and elements partitioning (Cr/Ni) between phases. The optimal mechanical properties were obtained for compositions based on ferritic 410L powder where the balanced distribution of α and γ is present and the tensile strength can reach value about 500MPa with 16% of elongation and impact energy about 120J. The precipitations of hard intermetallic σ-FeCr phase take place when sintering with slow cooling cycle what cause substantial decrease of plastic properties, including reduce of elongation to 7% and in particular decrease of impact energy to 68 J.

Abstract: The effect of high temperature sintering and high cooling rate on shifting the microstructural composition to the favourably of martensite-bainite structures and thus effective improvement of mechanical properties of sintered steels based on Astaloy CrL powder with an addition of 1 and 2% Cu or 50% Distaloy AB powder and 0.65% C was investigated. All the systems were processed by both sinter-hardening and conventional sintering. The vacuum sintering at high-temperature of 1240 0C and at common temperature of 1180 0C were integrated with high (6 0C/s), medium (3 0C/s) and slow (0.1 0C/s) cooling rates; conventional sintering at 1180 0C with cooling rate of ~0.17 0C/s was carried out in a N2+10%H2 atmosphere. In dependence on chemical composition, the yield and tensile strengths of 890-1150 MPa and 913-1230 MPa respectively and impact energy of 10-15 J were achieved by sinter-hardening. The yield and tensile strengths are approximately double than those resulting from conventional sintering.

Abstract: The present paper is focused on the wear characteristic of vacuum sintered Cr-Mo-[Mn]-[Cu] steels. The effect of chemical composition and the processing conditions in a vacuum furnace were evaluated. In such furnaces the cooling rate is generally determined by the pressure of the gas (N2) introduced into the chamber, the average cooling rates were calculated in the range of 1240°C to 400°C. The wear characteristics were analyzed as function of the processing and microstructures of the tested alloys through pin on disk test. Sintering of specimens in vacuum together with rapid cooling resulted in the formation of dominant martensitic microstructures with some small bainitic areas. The effect of both surface hardness and microstructure on the wear behaviour of the investigated steels shows the relation between the hardness and the wear rate. The influence of processing condition on the amount of martensite is also presented.

Abstract: The main aim of this paper is to show how ECAP influences the porosity distribution of PM aluminium alloys. The dimensional and morphological characteristics of investigated materials were measured individually for each pore. A commercial ready-to-press aluminium based powder (ECKA Alumix) was used as material to be investigated. After applying different compacting pressures (400 - 700 MPa), specimens were debinded in a ventilated furnace (Nabertherm) at 400 °C for 60 min. Sintering was carried out in a vacuum furnace at 610 °C for 30 min. The specimens were ECAPed for 1 pass. When severe plastic deformation is applied, the stress distribution in deformed specimens causes the powder particles to squeeze together to such an extent that the initially interconnected pores transform to small isolated pores, determining a given value of the dimensional characteristics. Consequently, ECAP influences the porosity distribution in terms of the involved severe shear deformation and therefore influences the pore morphology.

Abstract: The wear behaviour on prealloyed sintered steel parts TiCN PVD coating was analysed. Cylindrical specimens ( 40x30 mm3) were compacted to the green density of ~7.0 g.cm-3. The specimens were sintered in pusher furnace at the temperature of 1180°C for 40 minutes in an atmosphere of 25H2+75N2. Part of sintered specimens was then coated by Physical Vapour Deposition. Wear tests were carried out by means of a pin-on-disk tribometer. The results of profilograms (the values of surface roughness) and the wear curves (mass loss versus sliding distance) showed positive effect of coating on the wear resistance of the tested specimens.

Abstract: The present work deals with the compressibility behaviour of a new generation of coated metal/ceramic composite powders. A commercial 316L stainless steel powder was coated with the alumina layer by an aqueous sol-gel colloidal process and then metal/ceramic composite materials were produced with core/shell structure. The powders were uniaxially cold pressed in a cylindrical die in the range from 200 to 1000 MPa. In order to reach the best green density, composite powders have been added with 1% in weight of lubricant. The compressibility behaviour was analysed using the equation P = P0.exp (-K.pn). P0 is the relative apparent density and p is applied pressure; K and n are the parameters with defined physic-metallurgical substance. The values of the parameters K and n were determined by regression analyses. The correlation coefficients r of experimental compaction curves were up to 0.98. The results showed that difference between metal/alumina weight ratios affected the compressibility parameters as well as compressibility curves.