F. H. 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.

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 interpass time on austenite grain refinement by means of dynamic recrystallization of austenite

A 0.06 pct C-0.3 pct Mn and a 0.07 pct C-0.6 pct Mn-0.028 pct Nb steel were deformed in torsion at a constant strain rate of 2/s. Two schedules were used. In schedule A, seven roughing passes executed between 1260°C and 1130°C were followed by a single large finishing pass with a strain of 3.5 at constant temperatures between 1010°C and 840°C. The time between roughing and finishing was 200 seconds. In schedule B, the seven roughing passes were followed by 10 finishing passes, again applied isothermally, with strains of 0.3 and interpass times of 0.6, 2, and 10 seconds. The results indicate that for the Nb steel, low rolling temperatures (870°C) and strains above 2 are required for complete dynamic recrystallization, which results in austenite grain sizes under 6μm. Cooled at a rate of 10°C/s, the dynamically recrystallized austenite grain structures transform into ferrite with grain sizes under 4 μ. Extrapolations from the present data suggest that at industrial strain rates and cooling rates, ferrite grain sizes under 2 μm should be achieved.

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.

Mechanism of heterogeneous

The present article describes a novel theoretical approach to the mechanism of heterogeneous nucleation of pores in metallic systems. The proposed mechanism is based on the behavior of foreign particles at the advancing solid/liquid (S/L) interface. Foreign substrates act as a barrier to the fluid flow as well as to the diffusion field at the S/L interface, giving rise to enhanced gas segregation and viscous pressure drop. Mathematical analyses have been employed to pre- dict the gas segregation and pressure drop in the gap between the particle and the S/L interface. The equations which arise are solved using available experimental data in the literature. An order of magnitude analysis is done, and it is shown that pressures in the range of the activation barrier (fracture pressure) can be obtained in normal castings. The effect of particle properties and solidification parameters, such as wettability, density, thermal conductivity, solidification rate and morphology of S/L interface, are discussed. A complete assessment of all possible kinds of particles is not possible, since the material values and experimental parameters are not known in most of the cases. To consolidate the mechanism, therefore, further quantitative measurements of material values, interfacial energies in particular, are required on systems of interest.

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.

Effect of melt cleanliness on the properties

An experimental study was conducted to examine the effect of melt cleanliness with respect to the presence of inclusions on the properties of an Al-10 wt pct Si metal matrix composite (MMC) reinforced with 10 vol pct SiC particles. The occurrence of inclusions was controlled by filtra- tion, using ceramic foam filters of 10, 20, and 30 ppi sizes, under gravity and pressure. Test bars obtained from filtered and unfiltered melt castings, prepared from fresh (as-received) and recycled composite materials, were T6-tempered and tensile tested at room temperature. The casting quality was examined using X-ray radiography. The results indicate that various factors influence the casting quality and mechanical properties of the cast composite. The A12O3 films and spinel MgAl2O4 — the main inclusions observed in the present composite — are chiefly responsible for the degradation in the mechanical properties. In addition, SiC sedimentation, Al4C3 formation, the hydrogen level of the melt, and the starting material used can also influence these properties. Fracture studies reveal that the inclusions and associated microvoids act as the crack initiation sites during composite fracture. Simple filtration using 10 ppi ceramic foam filters under gravity serves adequately in removing these inclusions and producing the desired mechanical properties.

Porosity formation in Al-9 wt% Si-3 wt% Cu-X alloy systems: measurements of porosity

A set of 72 experiments was carried out to study the effects of solidification conditions, hydrogen content and additives on the formation of porosity in Al-9 wt% Si-3 wt% Cu-X alloy systems. It was found that not all alloying elements contribute to porosity formation in the Al-Si-Cu base system. Some of these elements, e.g. magnesium, titanium and phosphorus, tend to reduce both pore size and density. Hydrogen is the strongest element that induces porosity formation, its effect being reinforced either by the addition of strontium or by increasing the solidification time, or both. Grain refining is found to reduce pore density and pore size, and results in a fine dispersion of the pores throughout the alloy matrix. The necessary precautions to be taken in measuring the porosity in these alloys are reviewed in this paper. Accurate measurements of porosity using image analysis need careful adjustment of optical parameters, namely focus, illumination and grey level, as well as a careful selection of the number of field measurements required to represent correctly the sample surface.