MATERIALS ENGINEERING, Vol. 16, 2009, No. 3
published on June 2009

CONTENTS and ABSTRACTS

Štefánik, P., Beronská, N., Iždinský, K. (pp. 1-6)
INFLUENCE OF THERMAL CYCLING ON MICROSTRUCTURE AND THERMAL EXPANSION OF CARBON FIBRES/COPPER COMPOSITES Full text (PDF 995 kB)
The preparation of copper matrix reinforced by high modulus carbon fibres (Thornel K1100) as well as the microstructure and dilatation changes during thermocycling is presented.Unidirectional composites with two types of matrix - pure copper and/or copper alloy with 0.2 wt. % of chromium - were thermally cycled between 30-600 °C three times.The composite with pure Cu exhibited larger voids and weak interfacial bonding. Due to the chemical reaction with K1100 fibres a reactive interfacial bonding has been formed. During thermocycling the hysteresis, but no large disintegration was observed. The coefficients of thermal expansion (CTEs) strongly depend on fibre orientation. In direction parallel to the fibre orientation in the temperature range of 220-500°C CTEs were very low (0.7-1.0x10-6/K), but in perpendicular direction the CTEs were higher than that of pure copper.

Mišičko1, R., Kvačkaj, T., Vlado, M., Gulová, L., Lupták, M., Bidulská, J. (pp. 7-12)
DEFECTS SIMULATION OF ROLLING STRIP Full text (PDF 5816 kB)
The defects in the continuous casting slabs can be developed or kept down in principle by rolling technology, especially depend to sort, size and distribution of primary defects, as well as used of rolling parameters. Scope of the article is on observation behavior artificial surface and undersurface defects (scores) without filler (surface defects) and filling by oxides and casting powder (subsurface defects). First phase of hot rolling process have been done by software simulation DEFORM 3D setting to the limited condition for samples with surface defects. Samples of material with low-carbon steel of sizes h x b x l have been chosen and the surface defects shape „U” and „V” of scores have been injected artificially by software. The process of rolling have been simulated on the deformation temperatures 1200°C and 900°C, whereas on the both of this deformation temperatures have been applied amount of deformation 10 and 50 %. With respect to the process of computer simulation, it is not possible to truthful real oxidation condition (physical – chemical process during heat of metal), in the second phase of our investigation have been observed influence of oxides and casting powders inside the scores for a defect behavior in plastic deformation process (hot and cold rolling process) in laboratory condition. The basic material was STN steel class 11 375, cladding material was steel on the bases C-Mn-Nb-V. Scores have been filled by scales to get from the heating temperatures (1100°C a 1250°C), varied types of casting powders, if you like mixture of scale and casting powders in the rate 1:4. The joint of the basic and cladding material have been done by peripheral welded joint. Experiment results from both phases are pointed on the evolution of original typology defects in rolling process.

Bubenko, L., Konečná, R., Nicoletto, G. (pp. 13-18)
OBSERVATION OF FATIGUE CRACK PATHS IN NODULAR CAST IRON AND ADI MICROSTRUCTURES Full text (PDF 1579 kB)
When speaking about quality of construction materials, fatigue crack propagation resistance is one of the most important considered properties. That is essentially influenced by character of matrix. Here presented contribution deals with the fatigue crack propagation mode through the matrix of as-cast nodular cast iron (NCI) and austempered ductile iron (ADI), whereas influence of microstructure has been considered and discussed. Experimental materials used in presented contribution were pearlitc-ferritic NCI and heat treated ADI 800. Pearlitic-ferritic NCI was used as the base for ADI production. Experiments were performed on mini round compact tension (RCT) specimens using an Amsler vibrophore. Fatigue crack paths in both materials were investigated and compared. Light microscopy was used to analyze the microstructure, crack initiation and propagation within broken specimens. In both tested materials fatigue cracks always initiated at graphite-matrix interface, while graphite nodules remained generally unbroken, eventually only surface of nodules was damaged. Though, comparing two materials with different microstructures, the diversity of fatigue crack propagation modes at high deltaK and low deltaK was observed.

Viňáš, J., Ábel, M., Kaščák, Ľ. (pp. 19-23)
DEFORMATION INFLUENCE ON A LIFETIME OF WELDING ELECTRODE TIPS Full text (PDF 243 kB)
The contribution deals with the influence of welding electrode tips deformation on their lifetime. The influence of material properties, production technology and the intensity of welding electrodes load on their lifetime are presented. The electrode tips of the most used type of CuCr1Zr alloy of three basic standard shapes before and after the process of welding are evaluated. The process of welding is realized with low, middle and maximum welding parameters on programmable pneumatic spot welding machine VTS BPK 20. The influence of welding parameters on chosen material characteristics of welding tips is observed. Through the use of upsetting test, dependency of forming strength and deformation of material on used technology of welding tip production is observed.

Fintová, S., Konečná, R., Nicoletto, G. (pp. 24-28)
STATISTICAL DESCRIPTION OF LARGEST PORE SIZE IN MODIFIED AL-SI ALLOYS Full text (PDF 615 kB)
Casting defects in AlSi7Mg alloys are owing to the processing technology inseparable features of final products. The porosity consisting of gas pores or microshrinkages is the most decisive factor influencing the fatigue life of castings. Extreme value statistic method has been shown a suitable procedure for prediction of a size of the largest defects, which can occur in real castings, which are substantially larger than the area of a control specimen prepared for defect distribution analysis. In this contribution application of Murakami’s statistical method based on the description of the largest extreme value distribution to the evaluation of porosity in AlSi7Mg alloy prepared by casting technologies with two types of modifiers, namely Sr and Na is presented. The results of statistical analysis are compared with the fatigue performance of specimens manufactured from particular materials.

Oravec, K., Vojtko, M. (pp. 29-32)
MECHANICAL PROPERTIES OF A PRESSED PIECE EN AW 2618A Al–ALLOY AFTER HEAT TREATMENT Full text (PDF 610 kB)
The aim of this contribution is to find out the influence of thermal treatment and small cold work on the microstructure and mechanical properties in the cross section of the EN AW 2618A Al – alloy. Four types of secondary particles were identified in the experimental alloy. Major portion in secondary particles is occupied by Al3NiFe particles, which are up to 5 µm large. Secondary particles Al3NiFe and AlCuNi are not soluble enough during the thermal treatment. The strength properties of the experimental Al - alloy are influenced most of all by precipitation hardening - ageing. The thermal treatment containing solution annealing and next artificial ageing increased the tensile strength 52% above the prime state value. The combined treatment containing solution annealing, cold work and artificial ageing resulted the increase of strength 59.4 %.


© University of Žilina, Faculty of Mechanical Engineering, ISSN 1335-0803