Process Optimisation for a Squeeze Cast Magnesium Alloy Metal

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Journal of Materials Processing Technology 168 (2005) 262269

Process optimisation for a squeeze cast magnesium alloy metalmatrix composite

M.S. Yong a , , A.J. Clegg ba Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore

b Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK

Received 5 January 2004; received in revised form 5 January 2004; accepted 27 January 2005

Abstract

The paper reports the inuence of process variables on a zirconium-free (RZ5DF) magnesium alloy metal matrix composite (MMC)containing 14 vol.% Safl bres. The squeeze casting process was used to produce the composites and the process variables evaluated wereapplied pressure, from 0.1 MPa to 120 MPa, and preform temperature from 250 C to 750 C. The principal ndings from this research werethat a minimum applied pressure of 60MPa is necessary to eliminate porosity and that applied pressures greater than 100 MPa cause breclustering and breakage. Theoptimum applied pressure was established to be 80 MPa. It was also established that to ensure successful preforminltration a preform temperature of 600 C or above was necessary. For the optimum combination of a preform preheat temperature of 600 Cand an applied pressure of 80 MPa, an UTS of 259 MPa was obtained for the composite. This represented an increase of 30% compared tothe UTS for the squeeze cast base alloy. 2005 Elsevier B.V. All rights reserved.

Keywords: Magnesium alloys; Squeeze casting; Metal matrix composites; Mechanical properties

1. Introduction

Metal matrix composite (MMC) components can be man-ufacturedby severalmethods. Themetal casting route is espe-ciallyattractive in terms of itsability to produce complex nearnet shapes.However,castingsproducedby conventionalcast-ingprocessesmay contain gasand/or shrinkage porosity. Thetendency for porosity formation will be exacerbated when -bres are introduced because they tend to restrict the ow of molten metal and cause even greater gas entrapment withinthe casting. It is pointless to use bres to reinforce a casting if

defects are present, since the addition of bres will not com-pensate for poor metallurgical integrity. In order to full thepotential of bre reinforcement and produce pore free cast-ings the squeeze casting process can be selected. The uniquefeature of this process is that metal is pressurised throughoutsolidication. This prevents the formation of gas and shrink-age porosity and produces a metallurgically sound casting.

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

Selection of this process is also based on its suitability formass production, ease of fabrication and its consistency inproducing high quality composite parts.

With the development of MMCs, magnesium alloys canbetter meet the various demands of diverse applications. Theaddition of reinforcement to magnesium alloy produces su-perior mechanicalproperties [13] and good thermal stability[4,5] . Of the various composite types, the discontinuous andrandomly oriented bre-reinforced composites provide thebest value to strength ratio.

Despite the potential advantage of using magnesium

MMC for lightweight and high strength applications, littleis known about the inuence of squeeze inltration parame-ters. Key parameters, such as applied pressure and preformtemperature must be optimised, especially for the squeezeinltration of a magnesiumzinc MMC. These process pa-rameters were researched and the results are presented in thispaper. However, it was rst necessary to select appropriatebres and binders since their selection is fundamental to thesuccess of the MMC. The main criterion determining the se-lection of bre type is compatibility with the matrix. Two

0924-0136/$ see front matter 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2005.01.012


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bre types that are known to be compatible with magnesiumare Safl and carbon [6]. Silica and alumina-based bindersare widely used in preform production, mainly due to theirhigh temperature properties [7]. However, there are concernsabout chemical reactions between magnesium alloys and sil-ica [8].

To ensure full inltration of liquid metal into the bre pre-form, researchers [911] have emphasised the importanceof preheating the preforms. However, there has been lit-tle research to determine optimum preform temperature formagnesium alloys and that reported has focused on AZ91(magnesiumaluminium) alloy. The wetting capability of these alloys is different, for instance the wetting and the in-terfacial reaction between Al 2O3 reinforcement and cerium,lanthanum (both rare earth elements) or magnesium is farbetter in comparison to aluminium.

Most work on applied pressure has focused on aluminiumalloys and their composites. However,Ha [12] and Chadwick [13] investigatedthe inuenceof applied pressure on theshortfreezing range MgAl family of alloys. The effect on solid-ication will inevitably be different for long freezing rangealloys, such as the MgZn family alloys that are the focusof this research. The difference in solidication morphologywill be signicant when inltrating the melt into a porous -bre preform. Long freezing range alloys retain a liquid phaseover a longer period during inltration and this may promotebetter inltration, reduce voids and consequently improve thesoundness of the composite.

2. Experimental methodology

A zirconium-free magnesium4.2% zinc1%-rare earthsalloy, designated RZ5DF, was used for this research. Severalbrepreform materials, proportions and binder systems,wereevaluated to determine their compatibility with the magne-sium alloysand the mechanical properties that they deliveredto the composite [14]. This preliminary research establishedthat a compopsite based on a silica-bonded, 14 vol.% Saflbre preform delivered the best characteristics in terms of ease of production and maximum value to strength ratio.

The effect of applied pressure, between 0.1MPa and120 MPa, on the RZ5DF-14vol.% Safl bre composite wasrst evaluated. The maximum permissible applied pressurewas limited by both thecapability of thesqueeze casting pressand die design. The metal pouring temperature was main-tained at 750 C, the die temperature at 250 C, the durationof applied pressure at 25 s, and delay before application of pressure at 4 s. These conditions replicated those employedfor the base alloy that was reported previously [15].

Following this, the inuence of preform temperature wasevaluated for a restricted range of applied pressures. Fourpreform temperatures were selected: 250 C (similar to thedie temperature), 400 C (intermediate temperature), 600 C(at which temperature the RZ5DF alloy is a mixture of liquidand solid), and 750 C (at which temperature the RZ5DF

alloy is in the fully molten state). These experiments wereconducted at three applied pressures: 60 MPa, 80 MPa and100MPa.

The mechanical properties were evaluated using tensileandhardnesstests. These testswere complementedby opticalmicroscopy and, for the tensile fracture surfaces, SEM.

2.1. Test casting

The test casting was a rectangular plate of 126mm inlength, 75 mm in width and 16mm in depth.

2.2. Melt processing

The alloy was melted in an electric resistance furnace us-ing a steel crucible, the uxless method and an argon gascover. The die was coated with boron nitride suspended inwater to protect it from excessive wear.

2.3. Tensile testing

Tensiletestswere conductedon a 50 kN Mayestesting ma-chine using position control. Modied test specimens weremachined according to BS18 (1987) and magnesium Elek-tron Ltd RB4 specications [16].

2.4. Hardness testing

Hardness was measured to determine and study the inu-ence of reinforcement on the magnesium and the isotropy of bre distribution. The locations of hardness measurements

are shown in Fig. 1 . Hardness measurements were conductedusing the Rockwell B scale for both the alloys and com-posites. The preference for the Rockwell rather than Vick-ers hardness measurement was due to the larger indentationneeded to ensure a more consistent measurement on the com-posite. The area of the Vickers hardness indentation is sosmall that, in some cases, the measurement could be takenfrom the hard bre causing large variations in hardness val-ues.

Fig.1. Locationsof hardnessmeasurements (eachdot represents thepositionof a hardness measurement) taken in both Longitudinal and Transversedirections.


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2.5. Metallography

An optical microscope and stereoscan 360 electrom mi-croscope (SEM) were used to examine the microstructure of the MMC specimens. Metallographic samples were preparedusing standard techniques andwere etchedusing an acetic pi-

cral solution. The electron microscope was equipped with aback-scatter detector and was used to characterise fracturesurfaces from the tensile test specimens.

2.6. Cell size

The cell size was established using the intersectionmethod. Five areas were selected at random and 21 mea-surements of cell size were taken for each area. The averagevalue for the 105 readings was determined.

3. Results and observations

The results are reported in the sequence in which the ex-periments were conducted. In the rst series of experiments,the effect of applied pressure wasevaluated. In the secondse-ries, thecombinedinuencesof applied pressureandpreformpreheat temperature were evaluated.

3.1. Series 1 experiments: the inuence of applied pressure

3.1.1. Tensile propertiesThe effect of applied pressure on UTS and ductility of

squeeze cast, RZ5DF-14 vol.%, Safl bre composites isshown in Fig. 2. It can be seen that the highest UTS valuewas obtained with an applied pressure of 80 MPa. It wouldappear from the gure that a pressure in excess of 40 MPais essential to develop a signicant improvement in UTS butthat levels above 80 MPa have a detrimental effect.

3.1.2. HardnessThe hardness values along the longitudinal and transverse

directions of the composite castings produced at different

Fig. 2. The effects of squeeze inltration applied pressure on the tensileproperties of the RZ5DF matrix with 14 vol.% fraction Safl bres.

Fig. 3. The average material hardness along the longitudinal and transversedirectionof the squeeze inltrated RZ5DF alloy with14 vol.% fractionSaflbres, cast withconstantpouring temperature of 750 C anddie temperatureof 250 C.

applied pressures are shown graphically in Fig. 3. Whilst thedominating inuence on hardness is provided by thepresenceof the Safl bres, the results show that the hardness at thetwo lowest levels of applied pressure (0.1 MPa and 20 MPa)is distinctly lower than that associated with applied pressurelevels of 40MPa and above.

3.1.3. MetallographyMetallography was conducted to examine the inuence

of applied pressure on the cast structure. Selected opti-cal microstructures are presented in Fig. 4. The metal-lographic examination identied the presence of microp-orosity in those samples produced with applied pressuresbelow 60 MPa. The microporosity, as expected, occurredmainly at cell boundaries and was most easily conrmedby adjusting the depth of eld. It also identied the ten-dency for bre clustering and fracture at applied pressuresgreater than 80 MPa. The presence of fractured bres isdemonstrated more clearly in the SEM micrographs shownin Fig. 5. These micrographs show fractured bres in theplane transverse to that of load application during the tensiletest.

3.2. Series 2 experiments: the inuence of preformtemperature

The preliminary experiments showed that the optimumapplied pressure was 80 MPa. However, to ensure robustnessin theexperimentation, theeffectsof preform preheat temper-ature were evaluated for the optimum applied pressure andpressures of 60 MPa and 100 MPa.

3.2.1. Tensile testsThe effects of preform temperature and applied pressure

on UTS are summarised in Fig. 6. The results show that apreform preheat temperature of 750 C produced the mostconsistent UTS values across the range of applied pressures


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Fig. 4. Optical microstructure of squeeze inltrated RZ5DF-14 vol.% fraction Safl bres produced under (i) atmospheric pressure, 0.1 MPa, applied pressureof (ii) 20MPa, (iii) 40MPa, (iv) 60 MPa, (v) 80 MPa, (vi) 100MPa and (vii) 120 MPa.

and that the maximum UTS of 259 MPa was obtained witha preform temperature of 600 C and an applied pressure of 80 MPa. These results conrm the status of 80 MPa as theoptimum value of applied pressure.

3.2.2. HardnessThe results of the hardness tests are shown in Fig. 7. The

greatest variation in hardness was demonstrated by the testcasting produced with the lowest value of applied pressure

Fig. 5. SEM micrograph of the fracture face of a squeeze inltrated RZ5DF-14vol.% fraction Safl bres produced under applied pressure of (i) 100 MPa and(ii) 120MPa.


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Fig. 6. The plot of UTS for RZ5DF-14vol.% Safl MMC produced fromvarious combinations of applied pressure and preform temperature.

(60 MPa) and preform temperature of 400 C. The range of variation was 8 HRB compared to 6 HRB observed forthe other combinations of preform temperature and appliedpressure.

3.2.3. MetallographyMetallographic examination of the composite structures

showed that more densely packed bres occurred at the pre-form surface at the lowest preform temperature. This effectis illustrated in Fig. 8. The sequence of microstructures showthat preform deformation and bre clustering were less evi-

dent at higher preform temperatures. The SEM micrographsof tensile fracture surfaces, Fig. 9, conrm the clustering of bres and provide evidence of bre to bre contact, for thepreheat temperature of 400 C. This effect was not evidentfor the preheat temperature of 600 C.

4. Discussion

To achieve the successful inltration of a bre preform theliquidmetalmustpenetrate the preform completely. Potentialbarriers to this are presented by: the density of the preform,which can be represented by the preform permeability [14];

Fig. 7. The average material hardness along the longitudinal and transverse direction of the squeeze inltrated RZ5DF alloy with 14 vol.% fraction Safl bresproduced under different combinations of preform temperatures and applied pressures.

Fig. 8. A micrograph taken at the preform inltration region of a squeeze inltrated specimen produced with a preform temperature of (i) 750 C, (ii) 600 C,(iii) 400 C and (iv) 250 C.


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an insufcient pressure head, necessary to displace the airand overcome resistances to metal ow; and/or a low pre-form temperature that promotes premature solidication of the solid before complete inltration.

Increasing either the applied pressure or the preform pre-heat temperature, independently or in combination, may im-

prove inltration. However, there may be adverse conse-quences. Too high a level of applied pressure may physi-cally damage the preform through compression. This leadsto compactedpreforms that resist inltration together with -bre clustering andbre breakage that reduce the bres effec-tiveness for strengthening the matrix. Although researchers[11,17,18] have resorted to high preform temperatures toachieve inltration, this too can have adverse effects. Forexample, an increased heat content in the system may retardsolidication.This in turn extends thetimeduringwhich thereis the opportunity for adverse interfacial reactions to occurbetween the alloy and bres. Furthermore, an extended pe-riod of solidication can promote the formation of larger cellsizes that in turn impair the mechanical properties.

The inuence of applied pressure is quite clearly demon-strated in Fig. 2. The gure can be divided into three distinctregions: 91MPa. The rst of theseregions is associated with the presence of porosity and voidsin the castings and this porosity is associated with low UTSvalues. As the applied pressure is increased the porosity iseliminated and the composite develops its optimum UTS of 259MPa at an applied pressure of 80 MPa. Thereafter, anincrease in applied pressure produces bre clustering andbreakage leading to more initiation points for fracture and sothe UTS declines. The tensile evidence is supported by evi-

dencefrom hardnesstestsand metallography. Thepresenceof porosity, revealed in Fig. 4, adversely affects the hardness of the castings. Quite simply, low levels of applied pressure arenot sufcient to either suppress porosity formation or com-pletely inltrate the bre preform. It is interesting to note thattheoptimumapplied pressure level of 80 MPa forthe compos-ite is20 MPa higherthan that necessary to developthehighestlevel of strength in the bre-free base alloy [15]. Metallo-graphic examination revealed that preform deformation andbre clustering was less evident and bres were less denselypacked at the surfaces of the preforms preheated to 600 C or750 C (see Fig. 8) when compared with 400 C and 250 C.

It wasfoundthat thehighestpreformtemperature(750 C)produced the most consistent UTS values over the range of applied pressures considered. This preform temperature isabove the liquidus temperature of the alloy. It would, there-fore, be expected that inltration of the preform would notbe impeded by the early onset of solidication of the alloy on

the bre preforms. The preform temperature of 600

C pro-duced a higher variation in UTS than was observed for the750 C preform preheat temperature. However, the highestvalue of UTS of all the experiments was produced with thispreheat temperature in combination with an applied pressureof 80 MPa.

A preform temperature of 750 C supported a wider rangeof applied pressure because, even at the lowest level of 60 MPa, there was a minimal resistance to inltration. Itwas also noted that there was less variation in bre distri-bution. An even distribution of bres was also evident inthe specimens produced at a preheat temperature of 600 C,see Fig. 9. This temperature is 33 C below the alloys liq-uidus temperature. Although inltration was not a problem,it can be postulated that solidication would occur quitequickly under these conditions. This postulation is supportedby microstructural evidence and cell size measurements, seeFig. 8, that show a smaller cell size, associated with betterUTS, in the samples produced with a preform temperature of 600 C.

With preheat temperatures of 400 C and, especially,250 C the UTS values are generally poor and there is clearevidence of ineffective inltration. The microstructural evi-dence clearly shows that preform deformation, bre cluster-ingandbrebreakage is evidentto varyingdegrees.However,

such effects were not uniform and produced inconsistent ef-fects.For example, forthe combination of 400 Cand60MPaapplied pressure, microstructural evaluation, see Fig. 10, re-vealed a high concentration of bres in the centre of the inl-trated preform. This effect wascaused by twofactors. Firstly,the low preform preheat temperature promoted rapid solid-ication of the alloy prior to the application of pressure atboth the preform surface and at locations near to the die wall.Secondly, the low applied pressure resulted in irregular andcurtailed inltration. In consequence, the applied pressurecompactsrather thaninltratesthepreform. Thisproducesin-ltrated regions that have a higher concentration of bres and

Fig. 9. SEMmicrograph of thefractureface of a squeeze inltrated RZ5DF-14vol.% Safl MMC produced with (i)400 C and (ii)600 C preform temperature.


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Fig. 10. Microstructure showing different parts of the squeeze inltrated RZ5DF-14 vol.% fraction Safl specimen produced with a preform temperature of 400 C and applied pressure of 60 MPa. The sequence is (i) top, (ii) centre and (iii) bottom portion of the fabricated composite.

this can produce higher values of UTS, see Fig. 10. However,the effect is inconsistent and therefore undesirable. Hardness

measurements also conrmed the inconsistency. For exam-ple, the specimen produced with 60 MPa and 400 C preheatdemonstrated the greatest variation in hardness, see Fig. 7,andthis was attributable to thecentral clustering of bres.Ex-amination of the fracture surface of the specimen producedwith a preform temperature of 400 C and an applied pres-sure of 80 MPa clearly shows the bres in contact with oneanother, see Fig. 9.

4.1. The inuence of zinc

Alloyingmagnesiumwith 4.2% of the lowermelting point

metal zinc produces a binary alloy that has a long freezingrange. Experimentation [16] determined the values of theliquidus and solidus of the RZ5DF alloy to be 633 C and474 C, respectively, a freezing range of 159 C. Althoughlong freezing range alloys are the most prone to shrinkageporosity, this problem is overcome by squeeze casting. Thelong freezing range may in fact be benecial in the produc-tion of a composite since the extended period during whicha liquid phase is present may promote inltration. The pres-ence of zinc may also be signicant for the preform preheattemperature.

The results show that the optimum UTS of 259 MPa wasobtained with a preform temperature of 600 C, a tempera-ture just 33 C below the alloys liquidus temperature. Cellsize measurements revealed that specimens produced at thispreheat temperature had a smaller average cell size, typically30 m, see Fig. 8 . For specimensproduced with preheat tem-peratures of 750 C, 400 C and 250 C the average cell sizewas >50 m. This variation can be explained by consider-ation of the nucleation and growth sequence in the variousspecimens. The high preform preheat temperature retards therate of solidication because time is necessary for the heatof the preform to be transferred through the alloy to the die.Nucleating cells have time to grow. Conversely, at low pre-heat temperatures of 400 C and 250 C, the alloy solidies

quickly in contact with the relatively cold bres. The rstsolid formed is rich in the primary phase and the remaining

liquid becomes richer in the low melting point eutectic. Al-though primary phase still forms by nucleation and growth inthe inter-bre regions, the number of cells formed is reducedand their size is larger.

5. Conclusions

1. The optimum applied pressure for the squeeze casting of RZ5DF-14 vol.% Safl bre composites was determinedto be 80 MPa. At applied pressures below 60 MPa, micro-porosity was not suppressed. Conversely, a high appliedpressure of 100 MPa or above causes bre clustering andbreakage and a concomitant reduction in UTS.

2. The optimum preform preheat temperature was estab-lished tobe 600 C.At this temperature consistent bre in-ltrationwas achieved andtheoptimum cell size of 30 mwas obtained in the matrix.

3. The optimum combination of applied pressure and pre-form preheat temperature was determined to be 80 MPaand 600 C, respectively. For this combination, a UTSvalue of 259 MPa was obtained. The composite delivereda 30% increase in UTS compared with that developed inthe squeeze cast base alloy.

Acknowledgements

Dr. Yong gratefully acknowledges the receipt of an Over-seas Research Students Award and a Loughborough Univer-sity Research Studentship.

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Process Optimisation for a Squeeze Cast Magnesium Alloy Metal