Mohamed A. Taha

On the influence of process variables on the thermal conditions and properties of high pressure die-cast magnesium alloys

Abstract The influence of pressure and velocity in high-pressure magnesium die casting on the thermal conditions and on the casting properties is studied. Specimens with the shape of a tensile test plate with a thickness of 12 mm and a length of 295 mm were cast using the alloys AM20HP, AM50HP, AS41, AE42, AZ91HP. Two gate velocities of the liquid metal 40 and 80 m s −1 , were used for die filling and two pressures, 30 and 70 MPa, were applied to the metal during the solidification phase. Other processing conditions were kept constant. Temperature measurements at different positions in the die and at the metal/die interface were made during the operating cycle. Temperature distributions obtained from a simple two-dimensional numerical heat-flow model were found to agree generally with the measured values. The temperature distribution did not change significantly when varying the pressure and/or velocity. Calculated cooling curves for the alloy AZ91 indicate that the specimens solidified completely within 4.5 s. The measured bulk density of the casting was found to increase with velocity and/or pressure: Consequently, the tensile strength also increased. The density distribution along the specimen was examined, the density being found to decrease as the distance from the gate increases. The surface hardness for each alloy was generally similar at all positions in the test specimen and did not vary much with velocity or pressure. However, the surface hardness was always higher than the hardness inside the specimen, being due to the fine structure at the surface and the coarse structure inside the specimen resulting from different solidification times.

Practicalization of cast metal matrix composites (MMCCs)

Abstract This article is an overview which briefly defines the main research points which have been considered towards the industrialization of cast metal matrix composites ‘MMCCs’. Major issues in liquid processing are defined. This includes the identification of the major phases and the selection criteria and requirements for MMCCs, namely material design requirements, compatibility requirements, processing requirements and cost requirements. Also post-processing such as heat treatment, forming and machining, which can be operated on the MMCCs, are discussed. The modifications which can lead to commercial foundry technologies are also considered. Such modifications include those concerning melt preparation, holding and handling, degassing, gating system, molding techniques, and equipment modification and foundry layout. The efforts made by the international associations, for the standardization of MMCCs are overviewed. Finally, examples are given for the available practical MMCCs, either in the form of raw composite ingots or shape casting, and are evaluated.

On the reaction between aluminium, K2TiF6 and KBF4

The reaction between molten Al and KBF4 and K2TiF6 was analyzed. Additions of the two salts separately, consecutively and simultaneously were made at 800 and 1000 °C. The phases formed were identi ...

Metal–matrix composites fabricated by pressure-assisted infiltration of loose ceramic powder

Abstract A composite fabrication method has been developed in which pressure is applied to infiltrate loose ceramic particles by liquid metal. The pressure applied has been exerted by two different techniques: centrifugal and squeeze casting. Both techniques are found to be successful in preparing Al–Al 2 O 3 metal–matrix composites. In this method the alumina powder, followed by a rod of aluminium, is inserted into a tube which is heated to a temperature above the melting point of the alloy. In centrifugal casting the tube is rotated around an axis perpendicular to that of the tube and a centrifugal force is induced which acts on the liquid metal. In squeeze casting a squeezing force is applied to the liquid metal using a similarly heated plunger. The composite is formed by the infiltration of the liquid metal through the powder interstices under the action of the force applied. The infiltration mechanisms in both techniques are found to be different. Whilst the formation of the composite was in the direction of the squeezing force, it was in the direction opposite to the centrifugal force. However, four cases of infiltration are obtained in both techniques: no infiltration because the pressure applied is lower than the back pressure due to surface tension; partial infiltration with remaining metal above the alumina powder due to low pressing time; full infiltration due to there being sufficient metal charge and sufficient time; and partial infiltration due to an insufficient metal charge. The infiltration mechanisms in both cases are suggested in view of the application of different processing conditions, different Al 2 O 3 particle sizes and the use of a commercial aluminium and an Al–Si alloy. The structural features of the composite, namely the particle distribution, the metal/ceramic interface and the soundness, were studied. Comparison between the structures obtained by both techniques is made. In both cases, a high volume fraction of Al 2 O 3 ranging between 50 and 65% was obtained.

Effect of melt superheat and chill material on interfacial heat-transfer coefficient in end-chill Al and Al-Cu alloy castings

Solidification of metal castings inside moulds is mainly dependent on the rate of heat removal from the metal to the mould. During casting solidification, an air gap usually develops at the interface between the solidfying metal and the surrounding mould or chill. This condition occurs in most casting geometries, except in some cases such as the cast metal solidifying around a central core. An overall heat-transfer coefficient, which includes all resistances to heat flow from the metal to its surroundings can be determined. The objective of this work was to determine the overall heat-transfer coefficient,h, using experimental and computersimulation results on commercial purity aluminium and Al-4.5 wt% Cu alloy solidifying in a vertical end-chill apparatus. The cast ingots had a cylindrical shape with 12.5 mm diameter and different lengths of 95 and 230 mm. It solidified at different superheats (ranging from 50–110 °C) against two different chill materials: copper, and dry moulding sand. A computer program solving the heat-conduction equation and taking into consideration the convection in the melt, was used to compute the temperature history at numerous points along the ingot length. Differenth values were assumed as a function of time, until agreement between experimental and computed cooling curves was obtained. The variation ofh as a function of time, surface temperature, specimen length for each melt superheat and chill material was found. The thickness of the air gap was also evaluated. The results indicate that the variation of heat-transfer coefficient with time followed a pattern of sudden increase for the first few seconds, followed by a steady state, after whichh decreased and reached another lower constant value. Theh values were also found to decrease rapidly when the liquidus temperature was reached in the melt. For longer specimen and higher melt superheat, the heat-transfer coefficient increased. It was also higher for a copper than for a sand chill.

Dendrite morphology of several steady state unidirectionally solidified iron base alloys

The experimental data are presented of the dendrite morphology of steady state unidirectionally solidified steels. The dendrite arm spacings were correlated with the equation A =KRmGmwhereR is the growth rate and G the temperature gradient. The exponentsm andn for the primary arms are fairly close to the theoretical values (m = -0.25,n = -0.5). For the secondary arms they are about the same (-0.4). The primary arm spacings do not depend much on composition. The secondary arm spacings, however, decrease at fixed carbon content with increasing content of substitutional elements, and they were found to be smaller in a steel freezing as ferrite compared to steels freezing as austenite.

Influence of solidification parameters on dendrite arm spacings in low carbon steels

Influence de la vitesse de croissance et du gradient de temperature sur la morphologie dendritique

Relationship between formability and cast structures in end-chill directionally solidified Al–Cu alloys

Abstract End-chill unidirectional solidification experiments have been conducted on pure Al and Al–Cu alloys with 1, 3 and 4.5 wt.% Cu, using melt superheat varying between 50 and 200 K. The specimens produced were 50 mm in diameter and 150–200 mm long. Studying the solidification structure indicated that most of the specimens exhibited a columnar structure growing perpendicular to the chill surface, followed by equiaxed grains. The volume fraction of the columnar structure and the grain size of the equiaxed one are related to the Cu content, superheat and the cooling rate. The volume fraction of the eutectic phase varied, depending on the casting parameters. Compression testing has been performed, in order to study the formability of the columnar and equiaxed grain structures. Constant testing conditions of a specimen slenderness ratio (8: 8) and strain rate of 1 mm/s were used, as well as a lubricant. In the relationship σ= k e n , describing the true stress/true strain relationship for the region between the limits of initial yielding and necking in ductile materials, the strength coefficient ( k ) and strain hardening coefficient ( n ) are used as indices for formability. Hence, the difference in formability behaviour between the columnar and equiaxed grains is presented.

On the microstructure and mechanical properties of squeeze-cast Al-7 wt% Si alloy

Abstract Specimens of commercial Al-7 wt% Si alloy have been solidified under different pressures, varying from atmospheric to 70 MN/m 2 , into rods 10 mm in diameter and 50 mm in length. Two types of porosity have been examined in these specimens, macro-porosity (observed at magnifications x and micro-porosity (observedf at magnifications > 100 x ), and the pressure was found to have reduced the volume fraction and size of both. Critical pressures have been found for each type of porosity, above which the pores disappear. Different features of the microstructure have been examined. Whilst the type of structure does not change with variation of pressure, the volume fraction of primary dendrites increases and their secondary arm spacing decreases. A more homogeneous structure with more perfectly arrayed dendrites is obtained in squeeze-cast specimens. The improvements in soundness and microstructure in squeeze castings are reflected in their mechanical properties, which are found to be superior to those of gravity-cast castings. By increasing the pressure, the tensile fracture strain and strength and the impact toughness are increased. Correlation between the fracture tensile strength and the microstructure is established.

INDUSTRIALIZATION OF CAST ALUMINUM MATRIX COMPOSITES (AMCCs)

This paper briefly describes the main research points and activities which have been considered toward industrialization of cast aluminum matrix composites (AMCCs). First, the issues in liquid processing of metal matrix composites are reviewed. The different requirements for successful AMCCs are then described. Post-processingsuch as heat treatment, forming, and machining, which can beperformed on AMCCs, are discussed. The up-to-date, specially modified commercial foundry technologies that are needed for the practical production of AMCCs are briefly described. The main efforts made bysome associations and industrial firms focusing on AMCCs are also summarized. Finally, examples are given for the available practical AMCCs, either in the form of raw composite ingots or shape casting.