1sqezze cast3

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Journal of Materials Processing Technology 145 (2004) 134141

Process optimisation for a squeeze cast magnesium alloy

M.S. Yong a,, A.J. Clegg b

a Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singaporeb Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University,

Loughborough, Leicestershire LE11 3TU, UK

Accepted 28 July 2003

Abstract

The paper reports the influence of key process variables on zirconium-free (RZ5DF) and zirconium-containing (RZ5) magnesiumzinc

rare earths alloys by examination of the microstructure and mechanical properties of specimens produced by squeeze casting. Appliedpressures from 0.1 to 120 MPa were considered and it was established that an applied pressure greater than 40MPa was required to suppress

the formation of microporosity. Increasing the applied pressure from 0.1 to 60 MPa, produced a reduction in cell size from 127 to 21m.

The metal pouring and die temperatures considered in the investigation were within the range of 720780 and 225275 C, respectively. It

was established that the intermediate die temperature of 250 C produced the highest tensile properties and that the presence of zirconium

did not improve the as-cast properties of the squeeze cast alloy. The highest UTS value obtained for the zirconium-free RZ5DF alloy was

198 MPa compared to 195 MPa for the ZR5 alloy. These UTS value were approximately 50% higher than those for material cast under

atmospheric pressure.

2003 Elsevier B.V. All rights reserved.

Keywords:Magnesium alloys; Squeeze casting; Mechanical properties; Grain refinement

1. Introduction

Magnesium alloys have properties that make them attrac-

tive for certain applications. However, even complex alloys

have limitations in respect of strength, stiffness and abra-

sion resistance. It is possible that these limitations can be

overcome by using metal matrix composites (MMC) with

a magnesium-based alloy. Although several manufacturing

processes can be used to produce such composites, the cast-

ing route is especially attractive given its ability to produce

complex shapes. However, in order to obtain the benefits

of reinforcement, the casting process must deliver castings

that are free of defects such as gas or shrinkage porosity.

Squeeze casting is capable of delivering such castings and

consequently has been used to produce cast MMC. Before

advocating squeeze casting for MMC production, however

it is necessary to understand the influence of process vari-

ables on the base alloy and that was the purpose of the work

described in this paper.

Corresponding author. Present address: Singapore Institute of Manu-

facturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore.

E-mail address: [email protected] (M.S. Yong).

The majority of investigations to evaluate the effect of

squeeze casting parameters have considered aluminium al-

loys and their composites. The most important parameters

in squeeze casting have been identified as melt temperature,

melt quality (i.e. the absence of oxide films and inclusions)

and quantity, die temperature, applied pressure, and duration

of applied pressure[1,2].In the case of composites, infiltra-

tion velocity and preform temperature can be added to the

list [35]. These variables are equally relevant to magne-

sium alloys and their composites was confirmed by several

researchers[613].A significant difference between magne-

sium and aluminium is the formers lower volumetric heat

of fusion, which means that solidification should occur at a

faster rate[14].

An understanding of the effects of process variables is es-

sential because the structure and properties of alloys can be

optimised without recourse to expensive alloying elements

or nucleating agents[6]. This paper reports an investigation

of the influence of zirconium grain refinement, applied pres-

sure, and pouring and die temperatures on the as-cast prop-

erties of specified magnesium-based alloys. This work was

a prerequisite to that on composites that will be reported in

a future paper.

0924-0136/$ see front matter 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmatprotec.2003.07.006


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M.S. Yong, A.J. Clegg/ Journal of Materials Processing Technology 145 (2004) 134141 135

2. Experimental methodology

Two magnesium alloys were used: commercially avail-

able RZ5, which contains zirconium, and a zirconium-free

version (hereafter referred to as RZ5DF). Their composi-

tions are presented inTable 1.

The effect of applied pressure on the RZ5DF alloy wasfirst evaluated whilst maintaining metal pouring temperature

at 750 C, die temperature at 250 C, duration of applied

pressure at 25 s, and delay before application of pressure at

4 s. Following establishment of the optimum applied pres-

sure (at 60 MPa), the influence of metal pouring and die

temperatures was investigated whilst the applied pressure,

delay and duration were kept constant. The pouring tem-

peratures investigated were 720, 750 and 780 C and the

die temperatures were 225, 250 and 275 C. The selection

of the experimental ranges for pouring and die tempera-

tures was guided by the literature. Pure magnesium melts

at 640 C and the recommended pouring temperatures for

the alloys is 720800 C [15]. A die temperature rangeof 200300 C is advocated for commercial die-casting

[1,16].

Test casting: The test casting was a rectangular plate with

a length of 126 mm, a width of 75 mm and a depth of 16 mm.

Melt processing: The alloys were melted in an electric

resistance furnace using a steel crucible, the fluxless method

and an argon gas cover. The die was coated with boron ni-

tride suspended in water to protect it from excessive wear.

The alloys were cast using the direct squeeze casting pro-

cess. The squeeze casting process was described in previous

papers[17,18].

Tensile testing: Tensile tests were conducted on a 50 kNMayes testing machine using position control and the modu-

lus was determined using a strain gauge attached to the gauge

section parallel to the direction of tensile loading. Modified

test specimens were machined according to BS18 (1987) and

Magnesium Elektron Ltd. RB4 specifications[19]. The ten-

sile properties were determined from as-cast material tested

at ambient temperature.

Hardness testing: Hardness measurements were con-

ducted using the Rockwell B scale in preference to Vickers

hardness testing as the former provided improved consis-

tency. The locations of hardness measurements conducted

on the castings are shown inFig. 1.

Metallography: Metallographic samples were prepared

using standard techniques. Specimens were finally ground

using 1000 grit silicon carbide paper, prior to polishing

with six micron and finally 1 m diamond paste. A 5%

Table 1

Specification of magnesium alloys used in the investigation

Zinc

(%)

Rare

earths (%)

Zirconium

(%)

Magnesium

RZ5 alloy 4.2 1 0.7 Balance

RZ5DF alloy 4.2 1 Balance

Fig. 1. Locations of hardness measurements (each dot represents the posi-

tion of a hardness measurement) taken in both longitudinal and transverse

directions.

nital etchant was used for the RZ5 alloy but an aceticg-

lycol etchant was found to be necessary for the RZ5DF

alloy.

Cell size: The cell size was established using the inter-

section method. Five areas were selected at random and

twenty-one measurements of cell size were taken for eacharea. The average value for the 105 readings was determined.

The results and observations from this experimental pro-

gramme are presented in the following section, which also

includes a comparison of tensile strength, material hardness

and metallographic structures.

3. Results and observations

The results are reported in the sequence that the ex-

periments were conducted. As the first objective of the

investigation was to establish an optimum applied pressurelevel, these experiments were the first to be conducted us-

ing the zirconium-free RZ5DF alloy. Once this level had

been established, a second series of experiments was con-

ducted using the RZ5DF alloy to evaluate the influences

of metal pouring and die temperature. A third series of

experiments was conducted using the zirconium-containing

RZ5 alloy. By comparing the results of the second and third

series of experiments, the influence of zirconium could be

established.

3.1. Series 1 experiments: the influence of applied

pressure

3.1.1. Tensile properties

The effects of applied pressure on the tensile proper-

ties of squeeze cast RZ5DF alloy are presented in Fig. 2.

It can be seen that the highest tensile properties were ob-

tained with an applied pressure of 100MPa and that the

lowest were produced at atmospheric pressure (0.1 MPa),

i.e. by gravity die-casting. The graph shows that there is

a significant increase in tensile properties as the applied

pressure is increased to 60 MPa but that after this point, a

further increase in applied pressure produces little further

improvement.


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136 M.S. Yong, A.J. Clegg/ Journal of Materials Processing Technology 145 (2004) 134141

Fig. 2. Effects of applied pressure on the tensile properties of squeeze

cast RZ5DF alloy.

3.1.2. Hardness

The hardness values along the longitudinal and trans-

verse directions of the RZ5DF alloy castings produced

with different applied pressures are shown graphically in

Fig. 3. The results show that applied pressure appears tohave little effect on hardness, since the majority of values

fall within the range 1420 HRB. However, close inspec-

tion of the detailed hardness values of castings produced

using low applied pressures (0.1, 20 and 40 MPa) revealed

that these had the lowest hardness values. Low hardness

was expected in these castings because of the presence of

porosity.

3.1.3. Metallography

Metallography was conducted to examine the influence of

applied pressure on the cast structure. Examination of the

RZ5DF alloy specimens produced at different applied pres-sures revealed that a critical pressure greater than 40 MPa

was required to suppress microporosity. Selected microstruc-

tures that show the effect of applied pressure on struc-

ture are shown in Fig. 4. These are complemented by the

graphical representation of the relationship between applied

pressure and cell size presented in Fig. 5. These figures

show a pronounced reduction in cell size for squeeze cast

material.

-

--

-

-

- - - - - - -

25

20

15

10

5

00.1 20 40 60 80 100 120

Applied Pressure (MPa)

RockwellHardness(

HRB)

Fig. 3. The average material hardness along the longitudinal and transverse

directions of the squeeze cast RZ5DF alloy, cast with constant pouring

temperature of 750 C and die temperature of 250 C.

3.2. Series 2 experiments: the evaluation of squeeze cast

RZ5DF alloy


The relationships between pouring temperature, die tem-

perature and tensile properties are shown in Figs. 6 and 7.


Metallographic specimens were examined to evaluate the

effects of pouring and die temperature on the RZ5DF mi-

crostructure. The examinations were conducted on speci-

mens selected from those that had the highest, intermediate

and lowest UTS values. The structure associated with the

highest UTS value is shown inFig. 8.

3.3. Series 3 experiments: the evaluation of squeeze cast

RZ5 alloy


The relationships between pouring temperature, die tem-perature and tensile properties are shown inFigs. 9 and 10.


Metallographic examinations were conducted to evaluate

the effects of pouring and die temperature on the squeeze cast

RZ5 microstructure. These examinations were conducted on

specimens selected from those that produced the highest, in-

termediate and lowest UTS values.Fig. 11shows the struc-

ture associated with the highest UTS value.

4. Discussion

As in all casting processes, the rate of solidification in

squeeze casting is determined primarily by the rate at which

heat is transferred by the metal to the die. In most casting

processes, heat flow is controlled to a significant extent by

resistance at the metalmould interface. The thickness of the

solid metal that forms is typically a parabolic function of

time, being initially very rapid and then decreasing as the

mould is heated[14]. However, in squeeze casting, the pres-

sure applied through the punch promotes an intimate con-

tact between the metal and die and this largely overcomes

the resistance to heat flow. According to Campbell [20],the

transfer of heat across the interface can be enhanced signif-

icantly in squeeze casting. For squeeze cast aluminium, the

figure may be up to 60,000 W/m2 K compared to the more

normal values of 1001000 W/m2 K. The heat to be trans-

ferred consists of the volumetric heat of fusion (the major

component) and the superheat. Higher pouring temperatures

increase the superheat contribution. It should be noted that

whilst the volumetric heat of fusion for magnesium is ap-

proximately 15% lower than that for aluminium, the specific

heat for the liquid metal is approximately 15% higher for

magnesium than aluminium. The temperature of the die in-

fluences its capacity to absorb heat. However, it is the dies


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Fig.

4.

OpticalmicrostructuresforsqueezecastRZ5DFproducedwithvario

uspressure.


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---

-

---

- - - - - -

140

120

100

80

60

40

20

0

0.1 20 40 60 80 100 120

Applied Pressure (MPa)

GrainSize(m)

Fig. 5. Influence of applied pressure on the cell size of the squeeze cast

RZ5DF alloy.

- - --

---

-----205

200

195

190

185

180

175

170

165

160

2250

C 2500

C 2750

C

Die Temperature (degree C)

UltimateTensileStrength(MPa)

Pouring Temp (0C)

7200C

7500C

7800C

Fig. 6. The effects of pouring and die temperatures on ambient temperature

UTS (RZ5DF alloy).

Fig. 8. Optical microstructure of the squeeze cast RZX5DF alloy with the highest UTS of 198 MPa at ambient temperature, providing an average cell

size of 18m.

-

-------

-

-

- - -

7200C

7500C

7800C

Pouring Temp (0C)

14

12

10

89

8

7

6

5

4

32250C 2500C 2750C

%AreaReduction

%Elongation


Fig. 7. The effects of pouring and die temperature on percentage elongation

and percentage area reduction of RZ5DF alloy specimens tested at ambienttemperature.

thermal diffusivity that exerts the major influence on the rate

of solidification[14].

From the results, it would appear that the optimum ap-

plied pressure range is from 50 to 100 MPa. The low UTS

values produced by applied pressures below 40 MPa are pri-

marily the consequence of porosity present in the castings

because the pressure acting on the molten metal was insuffi-

cient for its elimination (Fig. 2). As the applied pressure was

increased to 60 MPa, not only was porosity reduced, the rate

of cooling was increased and the cell size reduced with a

concomitant improvement in tensile properties. This is em-

phasised when the microstructures of castings produced by

different applied pressures, shown inFig. 4,are compared.

An average cell size of 127 m was produced by gravity


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- -

---------

205

200

195

190

185

180

175170

165

160

2250C 2500C 2750C


UltimateT

ensileStrength(MPa)

Fig. 9. The effects of pouring and die temperature on UTS at ambient

temperature (RZ5 alloy).

---

-

-

---

-

- - -

12

11

10

9

8

8

7

6

5

4

2250C 2500C 2750C


%AreaReduction

%Elongation

7200C

7500C

7800C

Pouring Temp (0C)

Fig. 10. The effects of pouring and die temperature on percentage elongation and percentage area reduction of RZ5 alloy specimens tested at ambient

temperature.

Fig. 11. Optical microstructure of the squeeze cast RZ5 alloy with the highest UTS of 195MPa at ambient temperature, providing an average cell size

of 21m.

die-casting at atmospheric pressure (0.1 MPa) but the cell

size was reduced to 21m at an applied pressure of 60MPa,

a significant six-fold reduction in cell size. The reduction

in cell size was attributed to the intimate contact between

the melt and die wall that promoted rapid heat transfer, as

applied pressure was increased.

Solidification is a process of nucleation and growth andthis process is influenced by the rate at which heat is trans-

ferred which in turn influences the structure and properties

of the casting. In squeeze casting, we might expect solidifi-

cation to commence as soon as the metal contacts the die,

i.e. before pressure is applied. Once pressure is applied, heat

transfer is promoted and concurrently the temperature of

the metal increases, as predicted by the ClausiusClapeyron

equation and this might, in combination with the long


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freezing range of the alloys, be expected to promote con-

stitutional undercooling. Such conditions are normally con-

ducive to the formation of a dendritic structure but the co-

pious nucleation promoted by the massive chilling effect of

squeeze casting[7]produces an extremely fine casting struc-

ture.

The results for the RZ5DF alloy suggest that the high-est UTS was associated with the higher pouring temperature

(780 C) and the intermediate die temperature of 225 C. A

steep temperature gradient, a consequence of combining a

high pouring temperature with a low die temperature, should

yield a fine microstructure and produce higher mechanical

properties [2124]. Conversely, castings produced with a

shallow temperature gradient are likely to have a large uni-

form cell structure, which will generally lead to lower me-

chanical properties. However, the results for the RZ5 alloy

suggest that, although the intermediate die temperature was

still as important, the highest UTS was obtained when this

was combined with the intermediate pouring temperature

(750 C). This suggests that a severe temperature gradient isless important because of the refining effect of the zirconium.

4.1. The role of zirconium

By comparing the results generated by the series 2 and 3

experiments, it is possible to provide a view on the need for

zirconium in a magnesiumzincrare earths alloy intended

for processing by squeeze casting. Zirconium, in excess of

its solubility limit, is used to produce a grain refining effect

when the alloy is sand cast and consequently subjected to

a relatively slow cooling rate. Zirconium in magnesium is

the most effective grain refiner in commercial use [14].However, its presence adds to the cost of the alloy and re-

quires process controls and procedures that counteract the

tendency for gravity segregation of the zirconium. It is pos-

sible that more than one mechanism is at work. However,

the most likely is a peritectic reaction in which separating

zirconium particles react with the liquid to acquire a layer of

zirconium-enriched solid solution that serves as nuclei[24].

When the number of nuclei is large, crystallisation proceeds

from a large number of points. The presence of zirconium

in magnesium produces a fine equiaxed cell structure with

typical cell sizes of 3050m in sand castings and this

generally leads to higher mechanical properties [25].

However, the values of the cell size obtained in this in-

vestigation were in the range of 1832 m, which is al-

most half that reported above. This may be due to the speed

of solidification in a squeeze casting that is faster in com-

parison to that for sand castings. Differences in the cell

shapes were observed between castings produced with and

without the addition of zirconium. These differences can be

seen by inspectingFig. 11(with zirconium, i.e. RZ5 alloy)

and Fig. 8 (without zirconium, i.e. RZ5DF alloy). It can

be seen from the figures that the zirconium addition caused

the individual cells to assume a more regular and rounded

form.

Contrary to the anticipated effects of a grain refinement

addition, reported in the literature [2528], the metallo-

graphic examinations did not show a significant difference in

cell size. Castings produced with the addition of zirconium

(RZ5 alloy) contained cell sizes which ranged from 21 to

26m whereas those without (RZ5DF alloy) contained cell

sizes ranging from 18 to 32m. The addition of zirconium tothe RZ5 alloy had little effect because the process of squeeze

casting refined the cell structure. It can be concluded that the

use of grain refinement in squeeze casting is unnecessary.

This research has shown that the cell structure can be manip-

ulated by such process variables as applied pressure, pouring

and die temperature. This confirms Chadwicks view that the

microstructure can be controlled by controlling casting vari-

ables alone and without recourse to a nucleating agent[6].

5. Conclusions

1. The pressure applied in squeeze casting promotes rapid

solidification and a refined cell structure. Increasing

the applied pressure beyond 60 MPa provided little

improvement in the tensile properties of squeeze cast

RZ5DF alloy. An applied pressure of 60 MPa was suffi-

cient to eliminate all traces of shrinkage and gas porosity

within the casting. Metallographic examination of the

castings revealed that the cell size reduced from 127 to

21m when the applied pressure was increased from

0.1 to 60 MPa.

2. It is possible to achieve comparable tensile properties

in the zirconium-free RZ5DF alloy to those in RZ5

alloy grain refined with a zirconium addition by se-lecting appropriate processing parameters. The highest

UTS value obtained in the zirconium-free RZ5DF alloy

was 198 MPa compared to 195 MPa for the RZ5 alloy.

These values are significantly higher (approximately

50%) than those obtained when the alloys were cast

at 0.1 MPa.

Acknowledgements

Dr. Yong gratefully acknowledges the receipt of an

Overseas Research Students Award and a Loughborough

University Research Studentship.

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1sqezze cast3