Nahed A. El-Mahallawy

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.

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 ...

Effect of some thermal parameters on the directional solidification process

A two-dimensional dynamic heat transfer model was developed to study the effect of some thermal parameters on the directional solidification process. The accuracy of the model was verified by comparing the computed isotherms with experimental solid-liquid interfaces in Al-Al3Ni eutectic grown under different conditions. The results showed that increasing the cross sectional area of a growing composite leads to an increase in solid-liquid interface curvature and a decrease in G/R (G is local temperature gradient and R is local growth rate). Flatter solid-liquid interfaces and higher temperature gradients are obtained for more conducting alloys and more insulating crucible materials.

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.

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 crack susceptibility in the submerged arc welding of medium-carbon steel plates

Abstract The present paper describes cracking behaviour during the submerged arc welding of medium carbon steel plates (0.45% C). It discusses the results of tests made to examine the effect of welding variables (current, welding speed and wire feed rate), plate fabrication conditions (rolling reduction ratio) and plate thickness on cracking susceptibility using trans-varestraint tests. It is found that the cracking susceptibility increases with an increase in the welding current, and decreases with an increase in the welding speed or the electrode wire feed rate. It also increases with increases in the plate rolling reduction ratio and with decreases in the plate thickness. These results have a practical significance for industrial fields, especially where welded machine spare parts are concerned.

Control of the continuous rheocasting process: Part 2Rheological behaviour analysis

The flow rate at the exit port of a continuous rheocaster is dependent on the viscosity of the semi-solid slurry. Slurry viscosity can be determined by the shear rate induced in the stirring chamber and the solid volume fraction attained at the exit port. For this purpose, an analytical study is made on the rheological behaviour and slurry flow rate of Bi-17 wt % Sn alloy in a continuous rheocaster. The rheological equation of state needed for such study is obtained by using measured slurry flow curves.

The relationship between cooling rate and the structure and properties of hypo-eutectic cast iron

The aim of the present work is to make a correlation between the cooling rate of grey cast iron during solidification and the resulting structure, phases and mechanical properties. Liquid cast iron...

Surface roughness and mechanical properties of melt-spun Al-Cu ribbons

Al, Al-5.23, Al-13.46 and Al-33 wt% Cu ribbons with different thicknesses,t, have been prepared by chilled block melt spinning under different processing conditions. Surface roughness, taken as peak to valley values,Rt, measured on both the substrate and air sides of the ribbon, shows great dependence on substrate velocity,v, ejection gas pressure,P, nozzle height above the substrate,H, and a lesser dependence on substrate thermal conductivity,k, and melt superheat, ΔT. The bulk density over all the zones shows an increase over conventionally cast alloys of 7.4% for Al-33 wt% Cu and about 3% for aluminium. Microhardness,Hn, measurements inside each of the three microstructure zones, which were identified across the ribbon section, show variation witht asHn =Hot−m, where the constantHo depends on the microstructure zone andm depends on the alloy composition. The zone at the substrate side has the highestHn values and that at the air side has the lowest ones. The number of bending cycles to fracture, which is taken as a measure of ductility, increased with ribbon thickness and decreased with copper content.