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M. Brůna

Abstract

This paper focuses on developing an advanced test method and its use to study hot tearing defects in aluminium alloy castings. The paper consists of two parts. The first part introduces the reader to hot tearing in general, and provides theoretical analysis of the hot tearing phenomenon. The second part describes a newly developed method for assessing hot tearing susceptibility, and also gives the results on hot tearing for various aluminium alloys. During the test, the effect of alloy chemical composition on hot tearing susceptibility was analyzed. Three different Al-based alloys with varying Si, Cu and Ti contents were examined. Conclusions deal with the effect of individual elements on hot tearing susceptibility, and confirm that the main objective was achieved and the proposed method proves to be repeatable and reliable.

Open access

M. Brůna, A. Sládek and L. Kucharčík

Formation of Porosity in Al-Si Alloys

Porosity is one of the major defects in aluminum castings and results in a decrease of the mechanical properties of Al-Si alloys. It is induced by two mechanisms: solidification shrinkage and gas segregation. One of the methods for complex evaluation of macro and micro porosity in Al-Si alloys is using the Tatur test technique. This article deals with the evaluation of porosity with the help of Tatur tests for selected Al-Si alloys. These results will be compared with results obtained from the ProCAST simulation software.

Open access

D. Bolibruchová, J. Macko and M. Brůna

Abstract

Submitted article deals with influence of iron based phases segregation by nickel, which is in literature known as iron based phases corrector. Iron is one of the most common impurities that can be found in Al-Si alloys. It is impossible to remove iron from melt by standard operations, but it is possible to eliminate iron negative effects by addition of other elements, that enables segregation of iron in form of intermetallics with less harmful effect. For melt treatment was selected an exact alloy with requested iron content - master alloy AlNi20. Influence of nickel was evaluated quantitatively by chemical analysis (solubility), thermal analysis and microstructure evaluation. Experimental results analysis shows a new view on solubility of iron based phases during melt preparation and treatment with higher iron content and also nickel effect as iron corrector of iron based phases. It can be concluded that nickel did not influenced iron based phases (β-phases), it does not change their type into more favorable form. As an initial impulse for starting this work was insufficient theoretical knowledge of usage secondary alloys Al-Si-Cu with higher iron content and its appropriate elimination in process of castings production for automotive industry. Increased iron content in alloys causes segregation of iron phases in various shapes and types during solidification, which subsequently affects quality, soundness and lifetime of castings. Because of increased demands for casting quality, final mechanical properties and effort to reduce costs, it is necessary to look for compromises in casting production from secondary alloys with occurrence of various impurities.

Open access

M. Brůna, D. Bolibruchová and R. Pastirčák

Abstract

Pouring of liquid aluminium is typically accompanied by disturbance of the free surface. During these disturbances, the free surface oxide films can be entrained in the bulk of liquid, also pockets of air can be accidentally trapped in this oxide films. The resultant scattering of porosity in castings seems nearly always to originate from the pockets of entrained air in oxide films. Latest version of ProCast software allows to identify the amount of oxides formed at the free surface and where they are most likely to end-up in casts. During a filling calculation, ProCast can calculate different indicators which allow to better quantify the filling pattern. The fluid front tracking indicator “Free surface time exposure” has the units [cm2*s]. At each point of the free surface, the free surface area is multiplied by the time. This value is cumulated with the value of the previous timestep. In addition, this value is transported with the free surface and with the fluid flow. Experiments to validate this new functions were executed.

Open access

D. Bolibruchová and M. Brůna

Abstract

Liquid AI-Si alloys are usually given special treatments before they are cast to obtain finer or modified matrix and eutectic structures, leading to improved properties. For many years, sodium additions to hypoeutectic and eutectic AI-Si melts have been recognized as the most effective method of modifying the eutectic morphology, although most of the group IA or IIA elements have significant effects on the eutectic structure. Unfortunately, many of these approaches also have associated several founding difficulties, such as fading, forming dross in presence of certain alloying elements, reduced fluidity, etc. ln recent years, antimony additions to AI-Si castings have attracted considerable attention as an alternative method of refining the eutectic structure. Such additions eliminate many of the difficulties listed above and provide permanent (i.e. non-fading) refining ability. In this paper, the authors summarize work on antimony treatment of Al-Si based alloys.

Open access

L. Kucharčík, M. Brůna and A. Sládek

Abstract

Porosity is one of the major defects in aluminum castings, which results is a decrease of a mechanical properties. Porosity in aluminum alloys is caused by solidification shrinkage and gas segregation. The final amount of porosity in aluminium castings is mostly influenced by several factors, as amount of hydrogen in molten aluminium alloy, cooling rate, melt temperature, mold material, or solidification interval. This article deals with effect of chemical composition on porosity in Al-Si aluminum alloys. For experiment was used Pure aluminum and four alloys: AlSi6Cu4, AlSi7Mg0, 3, AlSi9Cu1, AlSi10MgCu1.

Open access

R. Pastirčák, J. Ščury, M. Brůna and D. Bolibruchová

Abstract

The paper deals with the impact of technological parameters on the heat transfer coefficient and microstructure in AlSi12 alloy using squeeze casting technology. The casting with crystallization under pressure was used, specifically direct squeeze casting method. The goal was to affect crystallization by pressure with a value 100 and 150 MPa. The pressure applied to the melt causes a significant increase of the coefficient of heat transfer between the melt and the mold. There is an increase in heat flow by approximately 50% and the heat transfer coefficient of up to 100-fold, depending on the casting conditions. The change in cooling rate influences the morphology of the silicon particles and intermetallic phases. A change of excluded needles to a rod-shaped geometry with significantly shorter length occurs when used gravity casting method. By using the pressure of 150 MPa during the crystallization process, in the structure can be observed an irregular silica particles, but the size does not exceed 25 microns.

Open access

L. Richtárech, D. Bolibruchová, M. Brůna and J. Caiss

Abstract

This paper deals with influence on segregation of iron based phases on the secondary alloy AlSi7Mg0.3 microstructure by nickel. Iron is the most common and harmful impurity in aluminum casting alloys and has long been associated with an increase of casting defects. In generally, iron is associated with the formation of Fe-rich intermetallic phases. It is impossible to remove iron from melt by standard operations. Some elements eliminates iron by changing iron intermetallic phase morphology, decreasing its extent and by improving alloy properties. Realization of experiments and results of analysis show new view on solubility of iron based phases during melt preparation with higher iron content and influence of nickel as iron corrector of iron based phases.

Open access

Bruna Ibanes, A.M. Sebbenn, V.C.R. Azevedo, M.A. Moreno, F.B. Gandara, E.V. Tambarussi, E.M. Ferraz, K.J. Damasceno-Silva, P. S.C. Lima and M.A. Carvalhaes

Abstract

Genetic studies in tropical tree species have found signs of decreased genetic diversity and increased levels of inbreeding and spatial genetic structure (SGS) in fragmented and exploited populations. The aim of this paper was to investigate genetic diversity, structure, and intrapopulation SGS using eight microsatellite loci for three Orbignya phalerata populations that have undergone different intensities of seed harvesting. From each population, we georeferenced and sampled 30 seedlings, 30 juveniles, and 30 adult trees. The total number of alleles over all loci (k), and observed (Ho) and expected heterozygosity (He) presented lower values for the population experiencing more intense fruit harvesting than less heavily exploited populations, suggesting that fruit harvesting may decrease genetic diversity. Null alleles were detected in practically all loci among seedlings, juveniles, and adults in all populations, indicating that the estimates of Ho, He, and fixation index (F) are biased. When corrected for null alleles (FNull), the fixation index decreased for all samples, resulting in significantly higher than zero results for seedlings of all populations, but not for juveniles and adults of all populations. The comparison of FNull values between cohorts in the most heavily exploited population (ESP) suggests that inbred individuals are eliminated between seedling and adult stages. Significant SGS was detected up to 60 m in all populations, which indicates short distance seed dispersal. Genetic differentiation (G’ST) between pairwise populations was related to spatial distance between populations, with the greatest difference between more distant populations.