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hardness and tensile properties of cast Al6Si0.3Mg alloys. The influence of squeeze pressure on macro- and microstructures of Al6Si0.3Mg

applications has increased during the last decade, partly

properties; the addition of Mg makes the alloys heat

treatable [15].

Generally, alloy composition effects are independent of

improved thermal contact between the casting and die results

in the formation of fine grained structures [1012]. Con-

sequently, these also improve casting soundness and

ontribution, aspects

Materials Letters 59 (200as a result of the increased amount of light metals

being used for transportation purposes. One advantage is

that light-metal components lead to an overall reduced

weight and, thus, to reduced energy consumption.

Another advantage, which may be just as important

from an environmental point of view, is the fact that

aluminium components may be recycled with relative

low energy demands. For such purposes, alloys of the

type AlSiMg are found to give good results since they

show excellent casting characteristics and mechanical

casting techniques where as the rate of solidification of

cast structure, and heat treatment procedures are process

dependent [68].

Also, the mechanical properties of the castings are

affected significantly by the morphology of the eutectic Si

and porosity. The porosity is known to affect adversely the

mechanical properties of Al alloys [1,4,9,10].

In the squeeze casting process, the high pressure applied

during solidification can eliminate porosity caused by both

gas and shrinkage, and the increased cooling rate caused byalloys has been investigated. Some of castings were solution treated at 540 8C for various times and others were subjected to aging at 170 8Cafter solution treatment. The results indicated that precipitation occurred within about 30 min for both cast and squeeze cast alloys. The

hardness began to increase and maximum values were observed after about 10 h for as-cast alloy. Increasing of squeeze pressure (70160

MPa) accelerated strength of the alloys from 8 to 4 h, respectively. Squeeze pressures decreased the percentage of porosity and increased the

density, also it decreased the grain size of a-Al and modified the Si eutectic. Hardness and tensile properties increased with both heattreatment and increasing of squeeze pressure.

D 2004 Elsevier B.V. All rights reserved.

Keywords: AlSiMg; Squeeze casting; Soundness; Macro-/microstructure; Aging; Tensile properties

1. Introduction

The use of light-metal components in various

The mechanical properties of cast AlSiMg alloys are

determined by chemical composition, rate of solidification,

cast structure and its integrity and heat treatment.Microstructure characteriza

squeeze-cast

M.T. Abo

Non Ferrous Laboratory, Central Metallurgical Research and

Received 24 March 2004; received in revised fo

Available online

Abstract

A research program was conducted to study the effects of squeez0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2004.11.041

* Tel.: +20 25010642; fax: +20 25010639.

E-mail address: rucmrdi@rusys.eg.net.n and tensile properties of

SiMg alloys

l-khair*

lopment Institute (CMRDI), P.O.B. 87 Helwan, Cairo, Egypt

November 2004; accepted 27 November 2004

ecember 2004

sure (70, 100 and 160 MPa) and heat treatment T6 on the structure,

5) 894900

www.elsevier.com/locate/matletmechanical properties. In the present crelated to the casting technique and heat treatment of the

casting are considered.

pore sizer device micro merities model 9310 to quantify the

porosity percent.

3. Soundness

The density measured by Archimedes principle and also

the densities measured by the pore size device are tabulated

in Table 1.

From the results both measurements show that with

increasing the squeeze pressure, the density increases.

rials Letters 59 (2005) 894900 895The squeeze cast specimens were cut into two parts in the

longitudinal direction, and one part was prepared for

observation of macrostructure by polishing and etching

with Turkers reagent. Then the casting was formed as

tensile specimens of A-370 standard.This paper presents results on the effect of optimum

process conditions for obtaining sound castings and also to

explore the squeeze pressure effect on macro-and micro-

structure, soundness, aging curves, eutectic morphology and

tensile properties.

2. Experimental procedure

An AlSi alloy of composition Si 5.9, Fe 0.52, Cu 0.385,

Mg 0.333 and Al balance was used as casting material. After

melting in an electric resistance crucible furnace, degassing

with previously purified liquid nitrogen was carried out.

Liquid metal with a certain superheat (50 8C) was pouredinto the preheated tool steel die with a cylindrical cavity of

an internal diameter of 50 mm, a height of 100 mm and a

wall thickness of 20 mm.

After pouring the melt into the die cavity, pressurization

was achieved using a 60 T hydraulic press. The delay time,

which is necessary for the pressurization of the melt after

pouring was 120 s. The die temperature was 250 8C and thepouring temperature was 750 8C and the squeeze pressureswere 70, 100 and 160 MPa.

The densities of as-cast and squeeze specimens were

determined using Archimedes principle and also by using

Table 1

The Density measurements and porosity percent

Squeeze pressure

As cast 70 MPa 100 MPa 160 MPa

Total pore area sq-m/g 2.783 0.698 0.659 0.490

Bulk Density g/ml 2.5846 2.6735 2.6753 2.6976

2.6527* 2.6894* 2.6902* 2.6917*

Porosity % 2.27 0.71 0.63 0.48

* Archimedes measurements.

M.T. Abou El-khair / MateSome of the specimens were solution treated at 540

8C for 32 h, the other for hardness and tensile tests weresubjected to T6 condition (solution treated at 540 8C for8 h, quenched in water at 25 8C and aged at 170 8C fordifferent times till 36 h).

The as-cast and solution-treated specimens were pol-

ished and etched, and the microstructures were examined

using optical microscopy. The tensile properties were

evaluated with the test specimens fabricated by ScHE-

MATZUE DCS testing machine under the cross-head

speed of 0.3 mm/min and fracture surfaces of the test

specimens were examined with JEOL scanning electron

microscope.

Fig. 1. Macrostructure of squeeze cast alloys (a) 70 (b) 100 and (c) 160

MPa. 1X.

Density increases markedly from as-cast to 70 MPa, and the

increase is gradual thereafter from 100 to 160 MPa. Also,

the pore area and porosity percent decrease. Increasing of

pressure from 0 to 70 MPa decreases the porosity about 69%

while it is 32% from 70 to 160 MPa.

As the squeeze casting process does not make use of feeder

material, the cavity resulting from metal shrinkage must be

compensated by the application of pressure. Hashemi et al.

[11] stated that the density measurements indicated that there

is a critical pressure that can be achieved. All squeezed cast

specimens at a pressure above 50 MPa were fully dense.

4. Macrostructure

Fig. 1 shows the macrostructural characterization con-

ducted on the squeeze cast alloys at 70, 100 and 160 MPa,

respectively. It revealed no evidence of blowholes or

macropores. The macrostructure is an equiaxised structure

and the grain sizes decrease with increase of squeeze

pressure from 70 to 160 MPa.

5. Microstructure

The major purpose of this work is to clarify, through

microstructural studies, the influence of squeeze pressure

and heat treatment on the mechanical properties of

Al6Si0.3Mg alloy.

The microstructure of as-cast alloy is shown in Fig. 2a

which shows the existence of acicular eutectic Si surrounded

by a-Al dendrites . The relatively coarse a-Al dendrite ofabout 150 Am size together with the coarse Si particles wereobserved in the gravity cast material. This was attributed to

the slow solidification rate during casting. On the other hand,

in Fig. 2b, the application of pressure 70 MPa results in a

structure of a-Al of about 50 Am size with fine eutectic Si.Increasing the applied pressure from 100 to 160 MPa reduces

the grain size of the primary phase from 30 to 20 Am (Fig. 2cand d). Also, increasing the applied pressure leads to the

formation of a very fine eutectic Si. However, application of

pressure does not affect the eutectic Si morphology.

On the solidification under pressure, the following

changes in microstructure were observed Fig. 2:

1. increasing in the volume fraction of the Al-rich a-phasewith increase in pressure

2. decrease in the size of the primary Al-rich dendrites

3. decrease in the volume fraction of the eutectic

4. considerable refinement of the Si of the eutectic

The microstructures of as-cast and squeeze cast solution

treatment alloys are shown in Fig. 3. The microstructure in

the solution treatment condition consists of a-Al cells

M.T. Abou El-khair / Materials Letters 59 (2005) 894900896Fig. 2. Microstructure of (a) as-cast and squeeze cast alloys at (b) 70, (c) 100 and (d) 160 MPa.

rials LM.T. Abou El-khair / Matebounded by irregularly shaped Si particles. Eutectic is

acicular and randomly distributed.

The typical distribution of Si particles after solution

treatment is shown in Fig. 3. For squeeze cast alloy the Si

particles have spheroidized and coarsened to some extent, as

compared with as-cast alloy. Most of them are more or less

spherical. Some particles still have a longitudinal shape.

Initially, Si particles are broken down into smaller fragments

and are gradually spheroidized. Prolonged solution treat-

ment leads to coarsening of the particles. Both spheroidiza-

tion and coarsening are surface energy-driven, i.e., the

system tries to reduce excess surface area to the minimum

possible [13,14].

Fig. 3. Microstructure of solution-treated alloyetters 59 (2005) 894900 8976. Aging behavior

The aging curves at room temperature hardness of as-cast

and squeezed cast alloys are shown in Fig. 4.

The curves behave in a similar trend. They reveal

that the precipitation-strengthening effect become pro-

nounced after 30 min of aging and reaches a peak

value at 10, 8, 6 and 4 h for as-cast and squeeze cast

(70160 MPa) alloys, respectively. Little difference in

time to peak hardness was observed for different

conditions.

The alloys are strengthened by the precipitation of an

intermetallic compound Mg2Si during the aging treatment.

s (a) as-cast (b) squeeze cast at 70 MPa.

The enhancement of strength properties obtained during

aging treatment is primarily owing to the metastable phase

from the supersaturated solution [8]. When both the solution

and aging treatment effects on the tensile properties are

considered, the properties would show an increase in

strength and decrease in ductility. The present results

therefore agree quite well with the results reported in

literature.

The increase of strength with increasing squeeze pressure

is due to the increased solubility of Si and the virtual

elimination of shrinkage and/or gas voids could all

contribute towards the observed improvements in strength

of the alloys [6,10,11].

The elongation percent increases with increasing squeeze

pressure, the values of solution-treated alloys are the largest

and lower mechanical properties appreciably. The Si particle

characteristics can be altered by subjecting the casting to a

squeeze pressures at 170 8C.

rials Letters 59 (2005) 894900This enhancement of strength was obtained also with

increasing the squeeze pressure from 70 to 160 MPa.

Increasing the pressure increases the Si solubility so the

strength increased with increasing the pressure [10].

Because of the presence of excess Si in the solid solution,

precipitation of Mg2Si occurs much faster in squeezed

alloys [6].

The solution treatment stage of the T6 heat treatment

performs several important functions: dissolution of Mg2Si

phase; homogenization of the solid solution; and fragmen-

tation, spherodization and coarsening of the eutectic silicon.

Dissolution of Mg2Si and homogenization of the matrix

occurs within 15 min in A356 alloy at 540 8C [1].The changes to eutectic silicon morphology are

generally slower, taking up to several hours, and depend

on parameters such as solution temperature and original

particle size/shape which in turn are determined by

solidification conditions, grain size and eutectic modifi-Fig. 4. The aging curves of as-cast and squeeze cast alloys at different

M.T. Abou El-khair / Mate898cation [9].

7. Tensile properties

Table 2 shows the values of ultimate tensile strength

(UTS), yield strength (YS) and elongation percent (El%) of

the non-heat-treated, solution-treated and aged alloys at

different squeeze pressures. From the table UTS and YS

show an increasing trend with increasing of squeeze

pressure from 70 to 160 MPa for all alloys.

Aged alloys have the largest value of UTS and YS, the

values of solution-treated alloys are less while the values of

nontreated alloys are the lowest.

The eutectic Si in as-cast alloy is present as coarse,

acicular needles which act as stress raisers, and conse-

quently, the material is easy fractured. The heat treatment

modifies the morphology of the Si from acicular to

spherodized shape, thereby improving the mechanical

properties [5,9,14].high temperature heat treatment for long periods. Therefore,

for prolonged solution treatment, the observed change in

tensile properties are attributed to change in Si particle

characteristic [4,9,10].

8. Tensile fracture surface

Fig. 5 reveals the SEM micrographs of the typical

fracture surfaces of nontreated and aged tensile specimens.

A mixed mode of brittle cleavage and ductile fracture with

dimples was observed at both heat-treated and nontreated

alloys. Application of squeeze pressures improve the

fracture surfaces. It indicates a more ductile failure mode.

Table 2

Mechanical properties (UTS, YS and El.%) of investigated alloys

Pressure MPa Effect of heat treatment

Non treated Solution treatment Aging

UTS MPa As cast 103 147 150

70 128 173 198

100 131 185 200

160 132 190 208

YS MPa As cast 64 114 82

70 87 136 136

100 103 139 165

160 114 140 190

EI.% As cast 2.5 4 3.5

70 4 13 5

100 5.5 14 6ones. The improvements in elongation values are most

likely to be due to the increased volume fraction of the

primary a phase and improved soundness of the alloys.The eutectic Si morphology plays a vital role in

determining the mechanical properties. Particle size, shape

and spacing are factors that characterize Si morphology.

Under normal cooling conditions, Si particles are present as

coarse acicular needles. The needles act as crack initiators160 6.5 15 8

rials LM.T. Abou El-khair / MateThe fracture behavior of the alloys is affected by the size

of a-particles and Si morphology [15].

9. Conclusions

1. Both heat treatment and squeeze casting increase the

strength of the investigated alloys.

2. Increasing of squeeze pressure decreases the a-Al grainsize and modified the eutectic Si.

3. Increasing of squeeze pressure (70160) MPa decreases

the porosity and improves the tensile properties.

4. Squeeze pressure accelerates hardness peak from 8 to

4 h.

Fig. 5. SEM micrographs of tensile fracture of (a) as-cast andetters 59 (2005) 894900 8995. Aging enhances UTS and YS and reduces the ductility.

6. The Si particles start to fragmentize and spheroidize

almost immediately with solution treatment. This leads

to pronounce improvement in mechanical properties of

treated alloys.

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M.T. Abou El-khair / Materials Letters 59 (2005) 894900900

Microstructure characterization and tensile properties of squeeze-cast AlSiMg alloysIntroductionExperimental procedureSoundnessMacrostructureMicrostructureAging behaviorTensile propertiesTensile fracture surfaceConclusionsReferences