Ladislav Kosec

Analysis of casting die failures

Cracks in the surface of a fixed die half resulting in imprints on AlSi9Cu3 alloy castings were analysed. The cracks were revealed and identified by the use of penetrants. Some of them were clearly seen by the use of a magnifying glass or even by the naked eye. Non-destructive metallographic examination by SEM of polymeric replicas was applied.

Laser Cladding of Cold-Work Tool Steel by Pulse Shaping

Repair welding of cold-work tool steels in cold is very risky and almost impossible by conventional processes. The application of pulse shaping in laser cladding with wire to avoid the solidification problems in relevant steel is demonstrated. The results show that sound remelting and/or cladding can be achieved by the right selection of laser parameters and pulse shape, i.e. long pulse duration, moderate pulse peak powers and ramped-down pulse shape. Despite the defects and softening in the cladding due to the formation of retained austenite, the cladding shows better wear resistance at lower loads compared to the heat-treated base material.

Suitability of maraging steel weld cladding for repair of die casting tooling Part II : influence of ageing during aluminium alloy die casting on maraging steel weld microstructure, mechanical properties and crack growth

Abstract This study was done to evaluate precipitation annealing of 18 % Ni maraging steel repair welds during aluminium alloy die casting and to predict the prolonged in-service tool life. The emphasis of this study was the influence of post-weld precipitation annealing heat treatment and aluminium die casting thermal cycling on metallurgical and mechanical properties. A series of specimens of 1.2344 tool steel was prepared to which 1.6356 maraging steel was gas tungsten arc weld clad. Analysis of weld microstructure and hardness was made in order to understand the metallurgical processes during heat treatment at elevated temperatures and at prolonged time. The Response Surface model for prediction of hardness after heat treatment was developed. The edges of immersion test specimens are gas tungsten arc weld clad with 1.6356 maraging steel and machined to the final edge geometry. Two specimens were tested in as-welded condition and two in optimally aged condition. Testing was performed on specially developed immersion test apparatus, which enables the simulation of thermal fatigue during aluminium alloy die casting. After completion of a particular number of thermal fatigue cycles the weld microstructure and hardness were evaluated. A hypothetical model for prediction of precipitation annealing time during aluminium alloy die casting was proposed. The results showed that tool heating lasts 30 % of molten metal injection time, preparing conditions for precipitation annealing at 10 % of injection time, and precipitation annealing at 60 % of injection time. These results, together with a finite element model for prediction of tool temperature and response surface model for prediction of hardness after precipitation annealing, enable accurate prediction of maraging steel tool hardness after any particular number of die casting cycles and consequently in-service tool life. The thermal fatigue resistance of maraging steel welds was compared to AISI H13 tool steel. The results showed superior thermal fatique resistance for AISI H13.

Cutting Properties of PVD and CVD Coated A2lO 3 + TiC Tool Ceramic

The paper presents investigation results of tribological and cutting properties of the coatings deposited with the PVD and CVD techniques on cutting inserts made from the Al2O3 + TiC tool ceramics. Tests were carried out on the inserts made from ceramics, uncoated and PVD or CVD-coated with gradient, mono-, multilayer and multicomponent hard wear resistant coatings composed of TiN, TiCN, TiAlN, TiAlSiN and Al2O3 layers. Substrate hardness tests and micro hardness tests of the deposited coatings were made on the ultra-micro-hardness tester. It was demonstrated, basing on the technological cutting tests of grey cast iron (260 HB), that putting down onto the tool ceramics the thin anti-wear PVD and CVD coatings increases their abrasion wear resistance, which has a direct effect on extending tool life of the cutting edge.

Internal Oxidation of an Ag–1.3 at.% Te Alloy

The internal oxidation of Ag–1.3 at.% Te was studied at 750, 800, and 830°C in pure oxygen (1 atm). The internal oxidation under such high oxygen pressure resulted in formation of two different types of oxide particles and two different fronts of internal oxidation in the internal oxidation zone. The coarser Ag2TeO3 particles were formed through the in situ internal oxidation of Ag2Te particles and the tiny oxide precipitates (most probably also Ag2TeO3) were formed through internal oxidation of tellurium from solid solution. Considering the mechanism of internal oxidation, both diffusionless and diffusive modes were found to be present simultaneously in the oxidation of Ag–1.3 at.% Te alloy. These results were examined with regard to the solubility of tellurium in silver, which was found to be 0.1 at.% Te at 750°C and 0.26 at.% Te at 830°C, as well as the presence and dissolution of Ag2Te particles.

The characterisation of microstructural changes in rapidly solidified Al-Fe alloys through measurement of their electrical resistance

Abstract Microstructural changes of rapidly solidified Al alloy ribbons of various thicknesses and containing different concentrations of iron were analysed. The kinetics and sequence of microstructural changes occurring at a constant heating rate were analysed through the measurement of electrical resistance. This method allowed detection of the transition between metastable and stable phases of rapidly solidified Al – Fe alloy ribbons with various concentrations of iron and various thicknesses. After determining the temperature regions throughout the heating cycle, the quenched microstructure was analysed at transition points using both optical and transmission electron microscopy. Precipitation is the most common reaction for these types of alloys, and the most important alloy formed is Al13Fe4.

Microhardness and microstructure of deciduous enamel with different types of amelogenesis imperfecta

Amelogenesis imperfecta (AI) is an inherited tooth disorder with widely varying phenotypes. The aim of this study was to determine the microhardness and microstructure characteristics of the enamel in AI teeth. The AI phenotypes examined were hypoplastic (pitted and smooth form), hypomaturated, and hypocalcified. Six AI patients were diagnosed according to clinical characteristics. The microhardness of the enamel was measured on axial cuts of AI teeth acquired from the patients. The measurements were done on several sites from the enamel surface towards the dentine-enamel junction using the Vickers scale. Values of microhardness were compared to corresponding control teeth. The microstructure of AI enamel types was evaluated using scanning electron microscopy. The values of microhardness in pitted hypoplastic AI samples were, on average, lower compared to the control enamel and dropped markedly towards the dentine-enamel junction. The smooth hypoplastic enamel was not only extremely thin but also much softer than control enamel. The values for hypomaturated AI fluctuated, but the palatal sites were markedly softer than in the control tooth. Hypocalcified enamel was the softest, with values resembling those of dentin. Microstructural changes varied from altered orientation of enamel prisms in pitted hypoplastic AI to lack of normal prismatic structure and severe porosity in hypocalcified AI. The present results suggest different microhardness profiles and microstructures in each phenotype. Variations among phenotypes are expected with larger case selection in this genetically heterogeneous disease.

Dispersion strengthening of copper by internal oxidation of rapidly solidified Cu-RE alloys Part II: Experimental study of internal oxidation

Combination of rapid solidification and internal oxidation was used for producing a fine dispersion of rare earth oxide particles in the copper matrix. An overall microstructural analysis has shown that the internal oxidation temperature, the rapidly solidified microstructure and its changing ahead of the internal oxidation front strongly influence the mechanism of the internal oxidation process and the resulting microstructure. The internal oxidation in a Cu-Yb alloy took place mainly by direct oxidation of intermetallic particles. Contrary to this, in a Cu-Er alloy two mechanisms of internal oxidation have been clearly observed: (i) Dissolution of intermetallic particles ahead of the internal oxidation front and oxidation of the erbium from the solid solution and (ii) direct oxidation of the Cu-Er intermetallic particles. While a reasonable optimum combination of processing conditions at internal oxidation in the solid state, yielding suitable oxide dispersions, seems to have been identified for the Cu-Er alloy, the same was not true for the Cu-Yb alloy. However, the internal oxidation of the Cu-Yb alloy in the semisolid state, which occurs by alternation of two processes - Yb oxides precipitation and Cu matrix solidification - led to a relatively uniform dispersion of Yb oxide particles.

The mechanics behind formation of secondary ledeburite during tool steel welding

In this article we describe the microstructural changes in the heat affected zone (HAZ) of the tool steel W. Nr. 1.2379, which was surfaced or welded by the submerged arc welding technique (SAW) with different welding parameters. Microstructure of the welds and of the surfacing welds was analysed by optical and scanning electron microscope. In this research, we particularly studied microstructural changes in the area of primary chromium carbides in the HAZ and their effect on the weld crystalization. We came to the conclusion that the temperature in the HAZ is high enough during the welding process that it caused primary carbides to dissolve and concentration of the carbide-forming elements and carbon increased in the surrounding austenite matrix area to eutectic composition, which remelts and solidified as secondary eutectic (ledeburite).

The effect of delayed ettringite formation on fine grained aerated concrete mechanical properties

Abstract Delayed ettringite formation (DEF) is a chemical reaction with proven damaging effects on the mechanical properties of hydrated cementitious composite (concrete). Ettringite crystals can cause cracks and the widening of cracks due to pressure on the crack walls caused by the positive volume difference in the reaction. In this paper, we investigated the potential to utilise the positive volume difference in DEF in order to improve the mechanical properties of hydrated fine grained aerated concrete. Fine dispersed crystallisation nuclei, achieved by adding air-entraining agent (AEA) and by short vibration of specimens, are presented as the main requirement for such improvements. Control tests of expansion and mechanical properties were performed on samples of concrete with and without AEA by inducing DEF. The microstructure of fine grained aerated concrete was examined with an optical microscope and scanning electron microscope. We found that the controlled DEF, which is guaranteed by adding AEA and with the formation of uniformly dispersed air bubbles, which are crystallisation sites for ettringite crystals, improves the mechanical properties. The specimens with induced DEF were measured and found to have a 6.8% increase of compressive strength.