A. M. Samuel

Effect of solidification rate and metal feedability on porosity and SiCAl2O3 particle distribution in an Al-Si-Mg (359) alloy

Abstract In the production of particle-reinforced metal-matrix composite castings, particle sedimentation during the melting process and particle redistribution during solidification can lead to particle segregation in the as-cast structure, the effects of which, in addition to those of porosity, can be highly detrimental to the properties and quality of the casting. Solidification rate and metal feedability are considered mainly responsible for the two problems. The present work reports on the influence of these factors on the particle distribution and porosity in 359 alloy composites reinforced with SiC and Al 2 O 3 particles. The results show that the microporosity observed in 359/SiC (p) composites is a consequence of pore nucleation at the SiC particle sites and hindered liquid metal flow due to particle clustering; the former is responsible for the skewed porosity distribution profiles typically observed in these composites, similar to the type I distributions observed in A356 alloy. In the 359/Al 2 O 3(p) composite, limited feedability and the wider range or larger particle sizes of the alumina particles result in the bell-shaped porosity profile observed, as well as the larger maximum pore size range (100–180 μm 2 against 0–40 μm 2 for the SiC (p) composites). The interparticle distance distributions for the SiC (p) composites show that finer dendrite arm spacings (DASs) produce a more uniform distribution of the SiC particles, while higher spacings lead to particle clustering, usually at separations of about 5 μm, the probability increasing with increase in SiC (p) content. In the 359/Al 2 O 3(p) composite, the distribution profile changes from a normal, random distribution to an exponential type as the DAS is increased. Together with the microstructural observations, the distributions indicate that particle pushing is the dominant phenomenon in the SiC (p) composites during solidification, whereas in the Al 2 O 3(p) composite, mechanical trapping of the particles takes place at smaller DASs, that changes to particle pushing at larger spacings.

Various aspects involved in the production of low-hydrogen aluminium castings

The various areas involved in the casting process of aluminium alloys are all interrelated with each other and revolve around one primary concern: the hydrogen content present in the molten alloy prior to and during casting and its consequential effect on the porosity and quality of the cast product. Focusing on this concern, the present article reviews the problems associated with the production of aluminium alloy castings, in particular those areas on which the hydrogen content has a direct bearing. Current procedures in each area are discussed.

Effect of magnesium content on the ageing behaviour of water-chilled Al-Si-Cu-Mg-Fe-Mn (380) alloy castings

A study of the effect of magnesium concentration on the ageing behaviour as measured by the hardness of 380 alloy was conducted for three levels of magnesium, namely 0.06 (base alloy), 0.33 and 0.5 wt%, for water-chilled castings (dendrite arm spacing ∼ 10–15 μm). Differential scanning calorimetry analysis of as-cast samples was carried out to determine the changes in the reactions of the phases obtained during alloy solidification, employing heating rates of 0.1 and 1.0°Cs−1, up to approximately 700°C. Two heat treatments were applied to the as-cast alloys: T5 comprising ageing at 25 (room temperature), 155, 180, 200 and 220°C, for times up to 200 h, and T6 comprising solution heat treatment at 480 °C or 515°C for 8 h, followed by quenching in warm water at 60°C, followed by immediate artificial ageing at 155 or 180°C for varying times up to 100h. The results show that the higher hardness values obtained with T6 treatment can be explained by the excess precipitation of magnesium-containing phases in the as-solidified alloys. This precipitation could be eliminated under the high cooling-rate conditions prevalent in die-casting operations so that T5 treatment may be used to replace T6 treatment to produce the same hardness values. In addition, solution heat treatment in the low-temperature range (480–515°C) is adequate to produce the required changes in silicon morphology and dissolution of magnesium in the matrix. No significant difference in hardness behaviour was observed when the magnesium content was increased beyond 0.3 wt%.

Effect of alloying elements and dendrite arm spacing on the microstructure and hardness of an Al-Si-Cu-Mg-Fe-Mn (380) aluminium die-casting alloy

The microstructure of aluminium alloys based on 380 die-casting alloy was studied in detail, as a function of the alloying elements iron, magnesium, copper and manganese, and the solidification rate. Three methods of solidification were employed to simulate cooling rates obtained from investment, permanent, and die-casting processes, corresponding to ∼ 0.4, ∼ 12 and ∼ 260 °C s−1, respectively, with emphasis on the highest cooling rate. Hardness measurements were carried out on samples obtained from the latter, in the as-cast and T5 tempered conditions (4 h at 25, 155, 180, 200 and 220 °C). The results have been discussed and the correlation between the hardness and microstructure as a function of alloying elements is presented. The effect of solution heat treatment on the variations in the microstructure and hardness has also been discussed.

Effect of melt treatment, solidification conditions and porosity level on the tensile properties of 319.2 endchill aluminium castings

An experimental investigation of the tensile properties of endchill castings of 319.2 commercial aluminium alloy was carried out to determine the effect of Sr modifier, TiB2 grain refiner and hydrogen content, and the resulting porosity on these properties. It was found that with respect to solidification time, the interaction effect of other parameters on the porosity followed the order H2 > Sr > TiB2. Pore nucleation and pore morphology were solidification time-dependent, with Sr addition enhancing the sphericity of the pores. Both ultimate tensile strength (UTS) and ductility were sensitive to variations in porosity and solidification conditions, while the yield strength remained practically unaffected. Increase in the porosity volume fraction above 0.5% reduced the ductility to negligible levels in the unmodified, non-grain refined base alloy. It was also observed that Sr modification and grain refining allow for an increase in the porosity level before the same level of degradation in ductility is reached.

On the castability of Al-Si/SiC particle-reinforced metal-matrix composites: Factors affecting fluidity and soundness

Abstract This paper presents results of studies carried out to determine the effect of melt and mold temperatures on the castability of four Al-Si/SiC (p) reinforced metal-matrix composites containing two levels of silicon (7 and 10%wt) and two levels of SiC (10 and 20%vol.), in terms of the fluidity and soundness. These were assessed by monitoring the Al 4 C 3 formation, SiC distribution, porosity volume fraction and melt cleanliness (in terms of the oxide content) in specimens prepared under different melting and casting conditions. The results show that a low silicon content coupled with a high SiC level accelerates the formation of Al 4 C 3 which is detrimental to the fluidity and hence castability of the composite alloy. Increasing the silicon level from 7 to 10%wt improves the castability through a significant decrease in Al 4 C 3 content. Increasing the SiC content from 10 to 20%vol. results in a relatively homogeneous distribution of the particles within the matrix, even at low cooling rates of about 10°C s −1 . The presence of oxides in an otherwise fluid composite melt considerably reduces the castability.

Effect of melt, solidification and heat-treatment processing parameters on the properties of Al-Si-Mg/SiC(p) composites

With the recent renovations in casting technology and foundry procedures, Al-Si-Mg alloys reinforced with SiC particulates are being increasingly employed in automotive and aerospace applications. The SiC reinforcement particles influence the solidification process in various ways, affecting the fluidity and castability of these composites.This articles reviews the results of an extensive study carried out on different aspects of Al-Si-Mg/SiC(p)-type metal-matrix composites containing 7 or 10 wt% Si, reinforced with either 10 or 20 vol% SiC particulates. Aspects investigated include the castability and soundness in terms of melt fluidity, the effect of the solidification rate and of inclusions on the mechanical properties, and optimization of the heat-treatment parameters with regard to these properties. The influence of the processing parameters on the mechanical properties was determined by monitoring the microstructural changes taking place during the various stages of processing, by measurements of the dendrite arm spacing, porosity and SiC-particle content and distribution in the castings obtained, as well as the amount of oxide inclusions and other harmful reaction products such as Al4C3 present therein. The effect of employing the fluxing procedure commonly used in Al-Si-Mg alloys on the mechanical properties of one of the four composites studied is also reported.

Influence of casting and heat treatment parameters in controlling the properties of an Al-10 wt% Si-0.6 wt% Mg/SiC/20p composite

The influence of melting, casting and heat treatment parameters in determining the quality and tensile properties of an Al-10wt% Si-0.6wt% Mg/SiC/20p composite in comparison to its base alloy (359) has been studied. For the composite, melt-temperature, hydrogen level, and the cleanliness and stirring procedure, control, respectively, the harmful melt reactions of the SiC reinforcement with the alloy matrix, gas porosity, inclusion and oxide-film contamination, whereas casting conditions are mainly controlled through the use of a proper mold temperature and appropriate mold coating materials that enhance the feedability and reduce or eliminate the effects of shrinkage. The beneficial effect of the SiC reinforcement particles is two-fold: 1. they act as preferential sites for the nucleation of the eutectic silicon particles, leading to an overall refinement of the latter and lowering the amount of strontium modifier required from 150 to 90 ppm to achieve the same level of refinement in the as-cast microstructures of both composite and base alloy; 2. their presence also results in a more uniform redistribution of the silicon particles in the as-cast structure (cf. the large, irregular interdendritic regions of eutectic silicon observed in the base alloy). Both composite and base alloy exhibit a similar heat treatment response with respect to tensile properties for the various heat treatments applied. Addition of 20 vol% SiC to the base alloy (359) is seen to increase the Youngs modulus and yield strength by 30–40%, marginally affect the ultimate tensile strength, but reduce the ductility by almost 80%.

Heat-treatment parameters for a 359/Al2O3/10p composite modified with 0.07 wt% strontium

Abstract In melt-produced composites, the properties of particle-reinforced aluminum metal-matrix composites (MMCs) can be compromised by the interfacial reactions that may occur between the molten matrix and the reinforcement depending on the melt conditions. The reinforcement can also considerably influence the response of the matrix alloy to solution heat treatment and age-hardening. A study of the melting and heat-treatment parameters and their effect on the tensile properties and fracture behaviour of 359/Al 2 O 3 /10p composite modified with 0.07 wt% (700 ppm) strontium (Sr) was conducted, with the aim of assessing the properties of particulate alumina (Al 2 O 3 ) as reinforcement compared to silicon carbide (SiC), as well as the effect of a high Sr content. It was found that the latter significantly improved the fluidity/castability of the composite alloy. Although overmodification occurred, no associated Sr-containing intermetallics were observed, owing to the high silicon content of the matrix. During solidification, mechanical trapping of the alumina particles was observed for particle sizes much larger than that of the α-aluminum dendrites, as well as nucleation of eutectic silicon on their surfaces. Spinel particles were also observed to form at the alumina-particle/matrix interface, their formation progressing during solution heat treatment. Prolonged solution treatment at high temperatures resulted in a significant lowering of the strength upon subsequent ageing. Isothermal/isochronal ageing of the composite showed that the alumina particles only accelerate the kinetics but do not alter the precipitation behaviour of the matrix. Composite fracture can occur within the aluminum matrix, along the matrix/reinforcement interface, as well as by shearing of the alumina particles.

Relevance of the use of the quality index concept in cast SiCP-reinforced Al-Si-Mg composites

The quality index, Q, defined as Q=UTS + klogE1 was introduced as a means to better interpret tensile test data. However, its use in the case of composite materials is often questioned. The difficulty arises from the fact that the elongations obtained are usually close to, or less than, unity. Based on a heat-treatment study of cast Al-Si-Mg/SiCp composites (359/SiCp) and an analysis of the tensile properties obtained, it is shown that the concept of quality index, as it is commonly applied, is inappropriate in describing the combined effects of UTS and E1, and it is much better, instead, to use the probable yield strength (PYS) for such materials. Respective expressions for Q and PYS have been obtained for the 359, 359/SiC/10p and 359/SiC/20p alloys studied.