High Temperature Mechanical Properties of Al­Si­Mg­(Cu) Alloysfor Automotive Cylinder Heads

Chang-Yeol Jeong+

Department of Nuclear and Energy System Engineering, Dongguk University, 707 Seokjang-Dong, Gyeongju 780-714, Korea

To improve the fuel efficiency and reduce automobile emissions, there has been growing demand of more durable alloys for enginecomponents with the improved thermal and fatigue resistance. This study examined the effect of alloying elements on the high mechanicalbehavior of Al­Si­Mg­(Cu) casting alloys for cylinder heads. Depending on the alloying elements affecting the strength of the matrix, thethermal expansion coefficient decreased with increasing Mn and Cu content at high temperatures with a concomitant increase in the elasticmodulus, hardness and tensile strength. Quantatative analysis showed that the mechanical properties of the Al2Cu precipitate hardened alloywere maintained at temperatures over 250°C, whereas the degradation of mechanical properties of the Mg containing alloy occurred at 170°Cdue to coarsening of the Mg2Si precipitation phase. The LCF (low cycle fatigue) lives decreased with increasing alloy content according to theCoffin-Manson relation due to the smaller elongation. On the other hand, an analysis of the fatigue lives with the hysteresis loop energy, whichconsists of both strength and elongation, showed that the fatigue lives were normalized with an alloy of the same strengthening mechanismsregardless of the test temperature. [doi:10.2320/matertrans.M2012285]

(Received August 22, 2012; Accepted January 7, 2013; Published February 22, 2013)

Keywords: aluminum­silicon­magnesium­(copper) alloy, automobile, cylinder head, low cycle fatigue

1. Introduction

The recent legislative and environmental demands on theautomotive industry have prompted automobile companiesto produce lighter-weight fuel-efficient vehicles with loweremissions. The most effective way of producing a light-weight car is to apply light-weight materials, such asaluminum alloys. Al­Si­Mg­(Cu) casting alloys, as impor-tant light metals, are used widely in automotive componentsowing to their excellent mechanical properties and cast-ability.1­3) In recent years, diesel engines have becomeprevalent because of their low total emissions, and thecombustion pressures of diesel engines have increased upto 20MPa. Because many components used in automotiveengines are subjected to complex loading cycles at hightemperatures,4­7) it is important to understand the loadingmechanisms and damage accumulation, as well as increasethe fatigue lives under given conditions. In addition, themicrostructural parameters, i.e., the DAS (dendrite armspacing), porosity, morphology of silicon and second phaseparticles, are very important parameters in the thermal fatigueand high temperature tensile properties. The aim of this studywas to improve the high temperature mechanical propertiesof cylinder head materials with modified chemical compo-sitions. This study evaluated the mechanical properties ofaluminum casting alloys prepared using a permanent-moldgravity cast, and examined the effect of alloying elements,such as Cu, Si, Mg and Mn, on the tensile and fatiguebehavior. A series of mechanical tests were conducted toevaluate the deformation resistance of Al­Si­Mg­(Cu) castalloys, in which the chemical compositions were varied.Based on the experimental results, the durability ofautomotive engine parts can be increased by reinforcingthe microstructure with minor changes in the process andalloying elements.

2. Experimental

Three types of Al­Si­Mg­(Cu) alloys for the cylinder headwere cast using the permanent-mold gravity method. Table 1lists the chemical composition and heat treatment steps ofthe alloys. All the alloy compositions and heat treatmentschedules in this study were based on the mass productionconditions of the cylinder heads in a motor company ofKorea. Alloy A is similar to A356 (AC4CH) and alloy Bis similar to A365 (AC4A). The T6 heat treatment wasperformed on both alloys to obtain the maximum strengthbecause of the higher stress due to the increasing combustionpressures. On the other hand, alloy C is similar to A319(AC2B) except a lower Fe content. The T7 heat treatmentwas used for alloy C due to the stability of the measurementsof the final products. The high temperature physical andmechanical properties, such as the thermal expansioncoefficient (Netzsch DIL 402PC), dynamic mechanicalanalysis (DMA, TA instruments Q800) and micro hardnessmeasurements (Shimadzu DUH-W201S) were evaluated as afunction of temperature. Cylindrical type specimens for thetensile and LCF tests were machined and polished longitudi-nally up to 2400 grit crocus cloth to remove the machiningnotch effect. Strain-controlled uniaxial LCF tests were carriedout at a strain rate of 1 © 10­3 s­1 at 100 and 250°C using adynamic Instron model 8861 machine. The tensile tests werecarried out at a range of temperatures using an Instronmachine. All the test results were recorded using a computerdata acquisition system and analyzed under the conditionswhere cyclic softening and/or hardening were completed toachieve stable strain and stress.

3. Results and Discussion

3.1 MicrostructuresFigure 1 shows optical microscopy images of the cast

Al­Si­Mg­(Cu) matrix with different Cu, Si, Mg and Mn+Corresponding author, E-mail: jcy@dongguk.ac.kr

Materials Transactions, Vol. 54, No. 4 (2013) pp. 588 to 594©2013 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

contents after T6 (alloy A and B) and T7 (alloy C) heattreatments. The basic microstructure of the alloys consistedof primary ¡-Al dendrites with eutectic Si and intermetallicparticles distributed between the ¡-Al dendrites to form aperiodic cell pattern repeated across the metallographicsurface. The DAS was reported to decrease with increasingsolidification rate for a given alloy composition.8­10) In

this study, the lowest DAS value for gravity casting wasapproximately 23 µm in an Al­7.0mass% Si alloy, asshown in Fig. 1(a). With the same solidification rate with2.76mass% Cu, the mean DAS was approximately 22 µm,as shown in Fig. 1(c). On the other hand, with increasing Sicontent up to 9.4mass% and increasing Mn and Ti content,the mean DAS decreased to approximately 17 µm at the same

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Fig. 1 Optical microscopy images showing the microstructure after heat treatment, (a) alloy A, (b) alloy B and (c) alloy C.

Table 1 Chemical compositions and heat treatment steps of the Al­Si­Mg­(Cu) alloys (mass%).

Cu Si Mg Zn Fe Mn Ni Ti Sr Al

Alloy A 0.09 6.94 0.37 0.06 0.15 0.06 0.01 0.03 0.13 Rem

Alloy B 0.03 9.35 0.38 0.02 0.16 0.43 0.02 0.07 0.12 Rem

Alloy C 2.76 5.97 0.22 0.01 0.14 0.01 0.01 0.02 ® Rem

Heat treatmentSolution treatment: 535°C/6 h¼ water quenchingAging: 180°C/5 h¼ air cooling (alloy A and B)Aging: 250°C/5 h¼ air cooling (alloy C)

High Temperature Mechanical Properties of Al­Si­Mg­(Cu) Alloys for Automotive Cylinder Heads 589

cooling rate of the 7.0mass% Si alloy, as shown in Fig. 1(b).This was attributed to the undercooling effect8­10) withincreasing Mn and the decreasing freezing range of thealloy with increasing Si content. The addition of Ti providedfiner ¡-Al formation, which helps restrict the growth ofprimary Si and intermetallic phases.11,12) Therefore, a finerDAS was achieved. In addition, by Sr modification, the shapeof eutectic Si was spheroidized, and their sizes decreasedconsiderably, as shown in Figs. 1(a) and 1(b), whereas aneedlelike Si morphology was observed in the unmodifiedalloy, as shown in Fig. 1(c).

Figure 2 shows SEM (scanning electron microscope)images of the precipitates and X-ray diffraction patterns ofthe three alloys. As shown in Fig. 2(a), Mg2Si precipitates,which are the main strengthening particles of the Mg addedalloy, were observed in alloy A. Al15(Mn,Fe)3Si2 interme-tallic particles and Al2Cu precipitates were observed after

addition of Mn and Cu, respectively, as shown in Figs. 2(b)and 2(c).

3.2 Physical propertiesFigure 3 shows the high temperature physical properties of

an Al­Si­Mg­(Cu) casting alloy with the change in alloyingelements. In particular, Figs. 3(a) and 3(b) present thethermal expansion coefficient and elastic modulus, respec-tively. The thermal expansion coefficient increased withincreasing temperature to 350°C and then decreased withfurther increases in temperature. This can be explained bysoftening of the solid with increasing temperature > 0.7Tm(melting temperature), in which the general properties of thesolid cannot be maintained.13) Therefore, the reliability ofthe thermal expansion coefficient over 350°C is limited dueto collapse of the solid. The thermal expansion coefficientdecreased with increasing proportion of Si, Mn and Cu. In

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Fig. 2 SEM (scanning electron microscope) images of the precipitates and X-ray difraction patterns after heat treatment, (a) alloy A,(b) alloy B and (c) alloy C.

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particular, the addition of Cu reduced the thermal expansioncoefficient significantly at the same temperature and a stablestate was obtained up to 350°C for alloy C, whereas this statewas achieved at 250°C for alloys A and B. On the other hand,the elastic modulus decreased almost linearly with increasingtemperature, and the elastic modulus increased as much as3GPa with increasing Mn and Cu content. Mn and Cu, whichhave relatively high temperature stability compared to Mg,restrict thermal expansion and increase the stiffness. Cuincreases the age hardening precipitation of Al2Cu as wellas that of Mg2Si for Mg, and Mn is an element inAl15(Mn,Fe)3Si2 intermetallic particle formation.14,15) There-fore, these elements result in an increase in the elasticmodulus at a given temperature. The next chapter discussesthe high temperature stability of particles with elements.

3.3 Mechanical properties3.3.1 Hardness

Figure 4 shows the high temperature hardness of the threealloy sets using a micro indenter. The initial hardness ofalloys A and B was higher than that of alloy C because theT6 heat treatment was conducted for alloys A and B, whereasthe T7 heat treatment was applied to alloy C. As thetemperature was increased, the hardness tended to decreaseand was reduced rapidly over 150°C for alloys A and B.On the other hand, with the addition of Cu to alloy C,the tendency of a hardness reduction decreased significantlyand was maintained over 200°C.

This phenomenon was attributed to the stability of theprecipitate with chemical compounds. Exposure to temper-atures > 150°C for an extended period results in severecoarsening of the precipitates, particularly for Mg hardenedalloys, which has deleterious effects on strength. On the otherhand, the stability of Al2Cu is believed to be retained over250°C because the aging treatment was already performedat 250°C after a solution treatment, as shown in Table 1.Figure 5 shows the DMA results as a function of temper-ature. DMA provides the general DTUL (deflection temper-ature under load), which is a standard method for evaluatingthe softening temperature of materials16) but also providesdynamic mechanical structure information unattainable byDTUL.17) DMA discerns the elastic and viscous componentsof deformation and provides a very sensitive profile of the

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Fig. 3 High temperature physical properties of Al­Si­Mg­(Cu) alloys,(a) thermal expansion coefficient, (b) elastic modulus.

Fig. 4 High temperature hardness results of the Al­Si­Mg­(Cu) alloys.

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Fig. 5 DMA results of Al­Si­Mg­(Cu) alloys, (a) alloy A and (b) alloy C.

High Temperature Mechanical Properties of Al­Si­Mg­(Cu) Alloys for Automotive Cylinder Heads 591

viscoelastic properties, including storage modulus, lossmodulus and tangent ¤, as they change with temperature,where tan ¤ is the ratio of the loss modulus to the storagemodulus. Regarding the tangent ¤, an oscillatory straindeformation is applied to a sample, then the stress response ofmaterial is measured. As a result, the phase angle ¤, or phaseshift, between the deformation and response was measuredas a viscoelastic material response. The stress in a dynamicexperiment is referred to as the complex stress ·*(= ·A + i·″), which can be separated into two components.One is an elastic stress in phase with the strain, ·A = ·*cos ¤.·A is the degree to which material behaves like an elasticsolid. The other is a viscous stress out of phase with thestrain, ·″ = ·*sin ¤. ·″ is the degree to which materialbehaves like an ideal liquid. Therefore, tangent ¤ representsthe damping ability of the material caused by a change in themicrostructure and precipitates.17) For alloy A, as shown inFig. 5(a), the tangent ¤ value changed suddenly at 168°C,which suggests that the coarsening of Mg2Si particles affectsthe stiffness of the matrix, which is coincident with artificialaging being generally conducted at 160­180°C for Mghardened aluminum alloys. On the other hand, for alloy C, achange in tangent ¤ was observed near 277°C, which is theprecipitation and growth temperature of Al2Cu. This can bein line with the results shown in Figs. 3 and 4 in that theaddition of Cu contributes to the high temperature stability.Therefore, an increase in Cu content is effective because thekey requirement of a cylinder head is to maintain the hightemperature strength and low thermal expansion character-istics, which reduce the thermal fatigue damage.3.3.2 Tensile property

Tensile tests were carried out to measure the mechanicalproperties of Al­Si­Mg­(Cu) with different chemical com-positions and testing temperatures, and the results arepresented in Fig. 6. The yield strength and ultimate tensilestrength of alloy A were 263 and 317MPa, respectively,at room temperature, and 116 and 121MPa at 250°C. Boththe yield and tensile strength were not affected significantlyby the Mn and Cu content in alloys at room temperature.On the other hand, the tensile properties at high temperatures(>200°C) showed a different tendency to that at roomtemperature, resulting in improved strength, particularly theyield strength. In alloy C, with increasing Cu content, theyield strength was improved up to 167MPa and the tensilestrength was 186MPa at 250°C, as shown in Fig. 6. Thisconfirms the previous physical and mechanical propertiesdue to coarsening of the precipitation phase of Mg2Si andthe high temperature stability element effect of Cu. Theelongation decreased with increasing proportion of the addedcontent. This is believed to be in line with the requirementsof a cylinder head alloy, i.e., low thermal expansion anddeformation characteristics.3.3.3 Fatigue property

LCF tests of the three alloys were conducted to evaluatethe high temperature deformation resistance, which is the keyfactor of the automotive cylinder head. Figures 7 and 8 showthe change in the hysteresis loop and peak stress according tothe cyclic deformation at 250°C and a total strain range of«0.3% (¦¾t = «0.3%). The change in the loop was analyzedat the start and half of the fatigue life (Nf ), and the change in

the tensile and compressive peak stress was measured duringcycling. In the case of alloy A, the obvious cyclic softeningbehavior was observed, where the tension and compressionpeak stress decreased drastically at the initial cycling, anddecreased gradually with the progress of cyclic deformation.For alloys B and C, the degree of cyclic softening decreasedsignificantly and the peak stress was maintained by repeateddeformation at high temperatures. The tensile peak stress andplastic strain range ("¾p) at the half of the fatigue life wererespectively, 118MPa and 0.251% for alloy C, 63MPa and0.426% for alloy A, and 99MPa and 0.339% for alloy B.In particular, in alloy C, the peak stress was much higherthan the other alloys and the plastic strain range decreasedsignificantly under the given conditions. These results werein line with the tensile property, where the strength wasmaintained over 250°C because of the stability of the Al2Cuparticle.

Figure 9 shows the low cycle fatigue results of the threealloys over multiple temperature and strain ranges. Thefatigue life was evaluated as a function of the plastic strainrange using the Coffin-Manson relation, which revealed ahigher temperature, larger elongation and longer fatigue life,as shown in Fig. 9(a). If the fatigue life only is evaluated asthe plastic strain range, the fatigue life might be distortedas a result of considering the increase in elongation due toan increase in temperature neglecting the strength of thematerial.13) Therefore, when the fatigue life was evaluated as

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Fig. 6 Tensile test results of the Al­Si­Mg­(Cu) alloys, (a) YS (yieldstrength) and UTS (ultimate tensile strength), (b) EL (elongation).

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the hysteresis loop energy, which considers the strength andplastic strain of the materials under the given conditions,a similar low cycle fatigue life was observed without anincrease in fatigue life due to the increase in temperature,as shown in Fig. 9(b). In addition, the low cycle fatigueof alloy C revealed a similar life with increasing alloy

content. This shows that other high temperature mechanicalcharacteristics can be improved without deteriorating thefatigue features.

4. Conclusions

(1) A finer microstructure and uniform precipitation wasobserved with increasing Si, Mn and Cu concentrationsin the Al­Si­Mg­(Cu) cast alloy.

(2) With increasing Cu content at high temperatures, thethermal expansion coefficient decreased with a con-comitant increase in the elastic modulus, hardness andtensile strength. The Al2Cu precipitate was stable over250°C, whereas coarsening of the Mg2Si precipitationphase occurred at 170°C.

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Fig. 7 Cyclic stress­strain behavior of the Al­Si­Mg­(Cu) alloys,(a) alloy A, (b) alloy B and (c) alloy C.

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Fig. 8 Cyclic softening behavior of the Al­Si­Mg­(Cu) alloys, (a) alloy A,(b) alloy B and (c) alloy C.

High Temperature Mechanical Properties of Al­Si­Mg­(Cu) Alloys for Automotive Cylinder Heads 593

(3) The high temperature fatigue results showed that thecyclic softening behavior decreases with increasing Mnand Cu content. The low cycle fatigue lives expressedas the hysteresis loop energy, which is the amount ofenergy consumed over a cycle, provide a reasonablefatigue life evaluation.

Acknowledgments

This work was supported by the Industrial StrategicTechnology Development Program [10042593, A develop-ment on the aluminum diesel cylinder block and head forestablishing 0.9 ps/kg (power to weight ratio) of a passengercar] funded by the Ministry of Knowledge Economy (MKE,Korea). The authors express their gratitude for their financialsupport.

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Fig. 9 Low cycle fatigue results of the Al­Si­Mg­(Cu) alloys, (a) Coffin-Manson relation, (b) hysteresis loop energy.

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