J. Barbosa

On assessment of processing variables in vertical centrifugal casting technique

Abstract The aim of the present study is to investigate the influence of the vertical centrifugal casting technique over mechanical and metallurgical properties of a hypereutectic Al–18Si alloy. Due to the inherent vibration of the centrifugal casting technique, and in order to study and understand the individual effects of the equipment vibration and the centrifugal force itself (pressure or fluid dynamics), as well as the combined effect of both, three different tests were performed: gravity casting, gravity casting with vibration and centrifugal casting. It was concluded that the metallurgical and mechanical properties of castings obtained by the centrifugal casting process depend on the combined effect of the centrifugal pressure and/or fluid dynamics and on the inherent vibration of the technique itself. Correlations between the different casting techniques and obtained mechanical and metallurgical properties are presented.

Characterisation of metal/mould interface on investment casting of γ-TiAl

Abstract This paper describes the evaluation of different refractory compounds – SiO2, ZrO2 and Y2O3 – as face coats of investment casting shells for γ-TiAl. The effect of the different refractories on the metal–mould interaction is studied in different aspects. Experimental results include characterisation of the constituents present at the metal/mould interface, the segregation profiles of residual elements, namely oxygen, from the interface to the inner part of the samples, the extension and microhardness of the samples external hard case and the samples surface finishing.

Grain refinement of Al-Mg-Sc alloy by ultrasonic treatment

In foundry practice, ultrasonic treatment has been used as an efficient technique to achieve grain refinement in aluminium and magnesium alloys. This article shows the strong effect of pouring temperature and ultrasonic treatment at various temperatures on the grain refinement of Al-1 wt% Mg-0.3 wt% Sc alloy. Without ultrasonic treatment, a fine grain structure was obtained at the pouring temperature of 700 °C. The average grain size sharply decreases from 487 ± 20 to 103 ± 2 μm when the pouring temperature decreases from 800 to 700 °C. Ultrasonic vibration proved to be a potential grain refinement technique with a wide range of pouring temperature. A microstructure with very fine and homogeneous grains was obtained by applying ultrasonic treatment to the melt at the temperature range between 700 and 740 °C, before pouring. Cavitation-enhanced heterogeneous nucleation is the mechanism proposed to explain grain refinement by ultrasound in this alloy. Moreover, ultrasonic treatment of the melt was found to lead to cast samples with hardness values similar to those obtained in samples submitted to precipitation hardening, suggesting that ultrasonic treatment can avoid carrying out heat treatment of cast parts.

Effect of ultrasonic degassing on performance of Al-based components

The effect of high intensity ultrasound based on the novel multi-frequency multimode modulated technology on the final density, porosity, mechanical and fatigue properties of an AlSi9Cu3(Fe) alloy after different processing time was studied. Reduced pressure test was used to evaluate the density of alloys. The tensile and fatigue tests were used to evaluate the static and dynamic properties for the different time of ultrasonic degassing, respectively. It is found that ultrasonic degassing is effective in reduction porosity as well as to improve the final density of castings. Furthermore, the experimental results suggest that the porosity level does not have a substantial influence on the static properties contrary to what is observed on fatigue properties.

Influence of crucible material on the level of contamination in TiAl using induction melting

This paper describes the processing results of TiAl alloys melted in a controlled atmosphere induction furnace, using crucibles made of graphite, calcia and zirconia stabilized with yttria, calcia and magnesia. Samples were poured into graphite permanent moulds or solidified and cooled inside the crucible, in order to simulate the worst case scenario in a metal/refractory reaction using this process. This paper presents segregation profiles of residual elements and their influence on the micro hardness of different phases and the extent of the “alpha-case”.

Effect of grain and secondary phase morphologies in the mechanical and damping behavior of Al7075 alloys

The present study evaluates the role of the microstructure in the static and dynamic mechanical behavior of as-cast Al7075 alloy promoted by ultrasonic treatment (US) during solidification. The characterization of samples revealed that US treatment promoted grain and intermetallics refinement, changed the shape of the intermetallic phases (equilibrium phases of soluble M and/or T (Al, Cu, Mg, Zn) and their insoluble Al-Cu-Fe compounds) and lead to their uniform distribution along the grain boundaries. Consequently, the mechanical properties and damping capacity above critical strain values were enhanced by comparison with values obtained for castings produced without US vibration. This results suggest that the grain and secondary phases refinement by US can be a promising solution to process materials to obtain high damping and high strength characteristics.

Effect of the Bi content on the mechanical properties of a Sn-Zn-Al-Bi solder alloy

Because of environmental and health concerns, some alternative solder alloys, named lead-free ones, are being developed. Among them, the Sn-Zn-Al system has been studied and reveals promising properties. Selection of the solder alloys, for the electronic industry applications, is conditioned by their mechanical properties due to the stress produced in service. The studied alloys were produced by melting the pure elements in a resistance furnace, under inert atmosphere, and pouring in a steel mould. The samples were heat treated for homogenization of the microstructure. The produced alloys were analyzed by XRF spectrometry and Scanning Electronic Microscopy (SEM/EDS) for chemical and microstructural characterization. In this work the presence of bismuth, in the range of 0-7 weight %, was evaluated in what concerns to the as-cast and homogenized microstructures and mechanical properties. The mechanical properties of the produced alloys, in the as-cast and homogenized conditions, have been determined by tensile strength and hardness tests. The results obtained showed that bismuth has an important effect on the mechanical behavior of the alloys, namely, in the transition from ductile to brittle behavior. This behavior was correlated with the samples microstructures.

Qualifound – a modular tool developed for quality improvement in foundries

Purpose – Castings defects are usually easy to characterize, but to eradicate them can be a difficult task. In many cases, defects are caused by the combined effect of different factors, whose identification is often difficult. Besides, the real non‐quality costs are usually unknown, and even neglected. This paper aims to describe the development of a modular tool for quality improvement in foundries, and its main objective is to present the application potential and the foundry process areas that are covered and taken into account.Design/methodology/approach – The integrated model was conceived as an expert system, designated Qualifound, which performs both qualitative and quantitative analyses. For the qualitative analyses mode, the nomenclature and the description of defects are based on the classification suggested by the International Committee of the Foundry Technical Association. Thus, a database of defects was established, enabling one to associate the defects with the relevant process operations ...

EFFECT OF TRACE ELEMENTS ON THE INTERFACE REACTIONS BETWEEN TWO LEAD-FREE SOLDERS AND COPPER OR NICKEL SUBSTRATES

Traditional Sn-Pb solder alloys are being replaced, because of environmental and health concerns about lead toxicity. Among some alternative alloy systems, the Sn-Zn and Sn-Cu base alloy systems have been studied and reveal promising properties. The reliability of a solder joint is affected by the solder/substrate interaction and the nature of the layers formed at the interface. The solder/substrate reactions, for Sn-Zn and Sn-Cu base solder alloys, were evaluated in what concerns the morphology and chemical composition of the interface layers. The effect of the addition of P, at low levels, on the chemical composition of the layers present at the interface was studied. The phases formed at the interface between the Cu or Ni substrate and a molten lead-free solder at 250o C, were studied for different stage times and alloy compositions. The melting temperatures, of the studied alloys, were determined by Differential Scanning Calorimetry (DSC). Identification of equilibrium phases formed at the interface layer, and the evaluation of their chemical composition were performed by Scanning Electron Microscopy (SEM/EDS). Different interface characteristics were obtained, namely for the alloys containing Zn. The obtained IML layer thickness was compared, for both types of alloy systems.

Leaching of brasses in long-term direct contact with water

Copper-zinc alloys (alfa-beta brasses) are currently used to produce house water piping and further parts, that might be in direct contact with drinking water. Lead is added to these alloys in order to improve their machinability. It is well known that lead is unhealthy for humans and, according to EU Council Directive 98/83/EC on the quality of water for human consumption, the admissible Pb content in drinking water has to decrease during the next years because of its high toxicity. The interest in improving public health evocated the need of developing new ecological materials for application on water piping and systems because extensive research indicates that the occurrence of increased Pb content in drinking water can be due to the release (leaching) of Pb from leaded copper alloys. As a consequence of such findings, the dissolution /erosion behaviour of a commercial sanitary brass tap, submitted to a dynamic test simulating the service conditions, has been studied. For that purpose an equipment that enables a long ‐ term direct contact with water has been designed and developed. The chemical composition of the tap and of the final residue produced during the test have been determined by XRF spectrometry and the solutions analysed by atomic absorption spectrometry. Results concerning metal values dissolution have been compared with the limit values established by the legislation. For the experimental conditions, results show that Pb level in leaching solution is lower than the admissible value indicated for the quality of water for human consumption.