Papers by Author: Jan Cwajna

Abstract: The paper presents the results of ATD thermal analysis of feed ingots (master heat) and castings made from IN-713C, MAR-247 and MAR-509 superalloys. Metallurgical quality assessment was made based on the analysis of dT/dt curve in rage from T_eut to T_sol. Furthermore, impurities (oxides of Al, Zr, Hf, Cr etc.) have been identified inside the shrinkage cavities of feed ingots and castings. Significant similarities has been observed between impurities found in ingots, ATD test samples and finished castings. It has been demonstrated that ATD analysis of samples taken from different parts of feed ingot can be effective method of metallurgical quality assessment for these ingots.

Abstract: In this paper, an attempt has been made to determine the changes in the structure of UHMWPE under the influence of deformation, which can be the cause of the decrease of functional properties. It has been found that the degree of cross-linking slightly changes due to operational loads (deformation ef=0.15-0.6). The deformation leads, however, to the systematic lowering of the degree of crystallinity and an insignificant decrease of lamella width. Tests of wide- and small-angle X-ray scattering (WAXS, SAXS) prove that this is the result of operational deformation causing a rather weak texture of crystals and the fragmentation of some lamellae. Anisotropy of the lamellar structure also takes place, which changes 2D SAXS images into ellipses with a rapidly growing axial ratio with ef value.

Abstract: Magnesium alloys are widely used in automotive and aerospace industries due to their great connection of low density and good mechanical properties. They are also characterized by good castability and weldability. Their weak high temperature properties and corrosion resistance, led to development of magnesium alloys containing rare earth elements. Casting is the most popular way of manufacturing magnesium elements. However, there is a lack of investigations concerning impact of different factors on fluidity of these alloys. This paper presents results of investigations on influence of pouring temperature on castability and microstructure of QE22 and RZ5 magnesium alloys. In case of QE22 alloy, the filling length of the liquid alloy increased with the increasing pouring temperature. In RZ5 no such dependence was noted. This is probably caused by oxide films in the structure of material. Grain refinement and eutectics volume fraction also didn’t present correlation with pouring temperature.

Abstract: Magnesium alloys, thanks to their low density, are characterized by very high specific strength and specific stiffness. Due to acceptable mechanical properties, these alloys are widely used in automotive and aerospace industries for the elements such as: gearbox and engine housings, steering wheel columns or wheels. The main problem of the most common magnesium alloys – such as AZ91 are their weak high temperature properties. This led to development of alloys containing rare earth elements. These alloys achieve their demanded mechanical properties after grain refinement with the zirconium. Because of a big responsibility of the elements made from Mg-RE alloys, it is important to investigate modification impact on properties of the magnesium alloys. The paper presents results of studies properties of the WE43 and Elektron 21 casting magnesium alloys, modified in three different ways – according to Magnesium-Elektron specification, 50% stronger modification and 100% stronger. For the comparison, unmodified alloys were also investigated. Investigations showed, that alloys modified according to MEL specification presents optimal set of structural and mechanical properties. Further increase of amount of modifiers doesn’t let to significant increase of mechanical properties. Fractographic investigations showed many non-metallic inclusions on the fractures surface, which are result of faulty smelting process.

Abstract: This paper presents the degradation of hollow turbine blades made of ZhS6K nickel-based superalloy after service in DV2 aero engines. The blades were coated with a diffusive aluminide coating (Al-Si) to improve the oxidation and hot corrosion resistance. Turbine blades work under extreme conditions and a complex state of stress. During service, creep and fatigue of various natures take place. The interaction among hot combustion gases causes oxidation of the surface layer and hot corrosion and micro cracking of the coating. Moreover, changes in morphology of the γ’ phase just under the coating and transformations of the primary carbides take place. The factors limiting the lifetime of turbine blades are the quality of the aluminide coating andmicrostructure of the superalloy, depending on the service parameters—the temperature and the duration of service. It was found that exposure to high temperatures above the critical value for several seconds substantially decreased the engine power and its durability. In this paper, analysis of the microstructure, chemical composition and phase composition of the turbine blades after service were carried out. Evaluation of the extent of degradation was performed using the scanning electron microscopy (SEM), energy dispersive X-ray microanalysis (EDS) and electron backscatter diffraction (EBSD) techniques. The EBSD technique was used to analyse the phase composition in micro areas, especially to identify carbides before and after transformations.

Abstract: The paper presents results of tests concerning influence of accuracy in measurements of selected thermal properties of materials of a ceramic mould, insulating mat and an alloy itself on effects of simulation of a casting process provided for elements of an aero-engine. A simulation process was realized by means of the PROCAST program. In initial simulations, thermal parameters were applied and they are accessible in literature and concern materials of a mould and nickel superalloys. Results of simulation revealed presence of numerous defects in a form of gaps, pores and cracks. These results were proved in trial casting. In consequence, it was impossible to select correctly right casting parameters. In order to solve this problem, a value of thermal conductivity was defined experimentally and basing on it, coefficients of heat exchange in metal – mould, mould – insulation mat systems were defined as well. A value of partial coefficients of heat exchange was used to determine values of substitute coefficient of heat exchange for the whole metal – mould, mould – insulation mat systems. By introducing the got data, one managed to define parameters of a casting process, providing improvement in a level of quality of cast elements.

Abstract: A quantitative description of overlaps on fractures in sialon ceramics, is presented in the paper. A conventional analysis, aiming at the determination of the percentage share of overlaps on the basis of quantitative fractography, was preceded by stereometric/fractal analyses. They enabled the selection of representative sections of samples and then, the production of transverse microsections in those places for an analysis of the fractures’ profiles using the light microscopy method and fractographic image analysis. Based on the compared results from both methods, a successful verification was made of the research methodology developed earlier for sintered carbides and proven for a chromium-molybdenum steel.

Abstract: Resistance to brittle cracking is one of the most important functional properties of sialon ceramics. Evaluation of the resistance requires an analysis of decohesion processes occuring in the material. Aiming at a comprehensive description of the morphology of typically brittle fractures, a quantitative characterization was made using different methods and techniques, regarding the surface of fractures formed in a three-point bending test. By using the parameter Sdr, determined via the surface stereometry method, the fracture surface development coefficient, RS, typical for fractography, was calculated. Analysis of the research results indicates a proportional relation between parameter RS and the sialon ceramics’ bending strength.

Abstract: The distance functions such as G(r), K(r), H(r), g(r) and systematic scanning method connected with variance analysis have been used for quantitative characterization of functionally gradient materials structures. The methods have been tested on computer generated 3D model polycrystalline structures with diverse type of gradient function. Usefulness of the presented methods for quantitative description of FGM structures has been evaluated.