Jaroslav Kováčik

Experimental determination of mechanical properties of aluminium foams using Digital Image Correlation

This paper presents an experimental characterization of three different types of closed-cell aluminium alloy foams (AlMg1Si0.6, AlSi12Mg0.6 and AlMg0.6Si0.3) under static compressive loading. This study was carried out on half-cylindrical specimens with skin. The influence of foam density on compressive behaviour was investigated for densities ranging from 430 kg/m3 to 935 kg/m3. The compression tests were performed at room temperature (23°C) with a constant crosshead speed of 0.5 mm/min. Strain distribution, yield stress and compressive modulus values were recorded using Digital Image Correlation. Experimental results show that the mechanical properties (Youngs Modulus, yield stress and plateau stress) increase with density.

Uniaxial Compression Tests of Metallic Foams: A Recipe

In case of metallic foams the stress-strain curve observed during uniaxial compression is often not smooth, expecting plateau is often missing, and the curve instead of slowly increasing stress before final densification takes place often exhibit a lot of peaks with even local stress drops. It is generally accepted that the origin of this behavior is linked to the heterogeneity and/or anisotropy of foams, ductility or brittleness of used matrix alloy and the presence of surface skin. This contribution is designed as a recipe for metal foam investigator how to handle the uniaxial compression test results on metallic foams. Aim of this contribution is to introduce engineers and researchers also to the unusual events that can occur during foam compression test.

Sintering of HDH Ti Powder

Abstract Titanium powders prepared by hydro-dehydration process (HDH powder) were pressure less sintered in vacuum oven at different temperatures, time and green density. The sintering properties of powders of two particle sizes - 30 and 150 microns were investigated. The usual powder metallurgical (PM) results were observed, i.e., decreasing final porosity with increasing sintering temperature and time at constant heating rate. Higher green density leading to higher final density for both powder sizes was also observed. The obtained results will be used as comparative material for future sintering experiments of Ti based composites.

Steels as Materials for Sonotrode Tools

The investigation of suitable steels for sonotrode tool applications was performed. In this case in addition to good sonotrode properties (good transmission of ultrasonic energy, resistance to gigacycle fatigue) the material ought to meet also the tool properties (high hardness and wear resistance). The study was focused on tool steels, which are relatively cheaper than the currently best sonotrode tool material FerroTitanit cermet, and better than usual sonotrode materials AW 7075 aluminum alloy and titanium alloy Ti6Al4V. The suitability of steels for this purpose was evaluated by measuring the ultrasound energy transmission at a frequency of 30 kHz.

Energy Demand Reduction in Nearly Zero-Energy Buildings by Highly Efficient Aluminium Foam Heat Exchangers

The capability periodically to store and release the latent heat of phase transition during melting and solidification of Phase Change Materials (PCMs) has been currently the main subject of interest with regard to cost reduction efforts for cooling, heating of interiors and Domestic Hot Water (DHW) necessary for the operation and maintenance of adequate thermal comfort in new modern as well as old renovated residential buildings. The main principle of PCMs facilities to reduce significantly the energy consumption in the building industry of the future is based on the ability of thermo-active heat exchangers to absorb and later to dissipate into the surroundings excessive heat which can be easily obtained from renewable sources (e.g. solar energy, geothermal heat, etc.) directly in a building or in its immediate vicinity. Smart interior tiling and furnishing systems can provide high energy efficiency by stabilizing the room temperature at a level ensuring sufficient thermal comfort basically governed by the thermal conductivity and heat exchange area between ceiling (respectively also wall and floor if necessary) heat exchangers (radiators) and the heat storage medium in the form of PCMs. Unfortunately, most conventional building materials, e.g. aerated concrete, bricks, gypsum, ceramic tiles, etc. are particularly characterized by very low thermal conductivity, which disadvantages them to be used for these purposes. However, highly porous metallic material such as aluminium foam prepared by powder metallurgy [10, 11] is on the contrary excellently heat conductive, which predisposes it to be used for light-weight design of supporting structure of very energy efficient indoor as well as outdoor thermo-active heat exchangers for building industry of the future. This contribution points to the possibility to apply aluminium foam for both the novel innovative roofing system to cover pitched roofs and the interior ceiling panels, with the minimum energy demands for maintaining the sufficient thermal comfort in future nearly Zero-Energy Buildings (nZEBs).

Poisson’s Ratio of Closed-Cell Aluminium Foams

A nondestructive impulse excitation technique was used to investigate Poisson’s ratio of powder metallurgical pure closed-cell aluminium foams according to ASTM E 1876 within the foam density range of 0.430–1.390 g·cm−3. Instead of a constant value of 0.34, as according to Gibson and Ashby’s assumption for the Poisson’s ratio of metallic foams, the decrease of the Poisson’s ratio with decreasing foam density was observed. Observed Poisson’s ratio data were in the range of 0.21–0.34. To check the validity of the results, the Young’s modulus was calculated using Poisson’s ratio and its dependence on relative density was successfully modelled using the usual power law function with characteristic exponent of 1.72 ± 0.1. This confirms that the obtained experimental results for Poisson’s ratio are valid. Finally, rule of mixture and percolation theory were used to model the observed decrease of Poisson’s ratio with increasing porosity.

Coloring of Pure Zinc Foams

The zinc foams were prepared by powder compaction method with various types (TiH2 or MgH2) and various amount (0.3, 0.6 and 1 wt.%) of foaming agent to investigate the effect of colouring of zinc foams during foaming process. It was observed that due to Archimedes’ principle foaming agent particles with lower density as liquid zinc float over a liquid. As a result, on the foam outer skin they react with oxygen during cooling of the foam. Therefore, the significant colouring (purple, blue and dark blue) of pure zinc foams prepared using TiH2 foaming agent was observed. On the contrary, MgH2 decomposes almost completely during the foaming process of zinc. As a result Mg is either dissolved in zinc foam (Mg2Zn11) or MgO of white colour is created. Therefore no significant colouring of pure zinc foams was observed in this case.

Surface Treatment of Sonotrode Material Aw 7075

Abstract Electro spark deposited TiB2 functional layer and composite functional layers Ni-TiB2 and INC713LC-TiB2 on the aluminium alloy substrate AW 7075 were evaluated with respect to achieved microhardness, roughness, and microstructure. The thicknesses of the layers were observed in the range of 30 to 50 microns. Hardness of the layers (HV) is several times higher than the hardness of the aluminium alloy substrate AW 7075.

The Wear of Particulate Cu-Graphite Composite System

The Wear of Particulate Cu-Graphite Composite System Cu-graphite composites were prepared by HIPing in the range of 0 - 50 vol. % of graphite. The same graphite powder was copper coated and composites with 30 and 50 vol. % of graphite were prepared. With increased graphite content the coefficient of friction and wear rate of composites at first decreased slightly from copper values. When percolation threshold of graphite is exceeded, the friction coefficient starts to decrease significantly. However this decrease stops after a certain critical concentration is reached. Then the friction coefficient becomes almost independent of the composition while the wear rate decreases further. Both percolation thresholds and critical concentration significantly depend on the composite structure, i.e., on the size of used graphite powder. Finally an empirical percolation model was proposed to describe this dependence.