Effect of Runner Design and Heat Treatment Parameters on ... · PDF fileEffect of Runner...

IJME • January-June 2013 • Volume 6 • Issue 1

Effect of Runner Design and Heat Treatment Parameters on Microstructure... 1

*corresponding author: Email: [email protected]

Effect of Runner Design and Heat Treatment Parameters onMicrostructure and Mechanical Properties of Cast Modified-

Vibrated Al-7Si-Mg alloy

N. H. Siddalingaswamy a*, M. A. Josepha, N. M. Nagarajana, N. H. Kumarb

aDepartment of Mechanical Engineering, N I T CalicutbMechanical Engineering Department, Wichita State University, Kansas, USA.

ABSTRACT: In the present investigation the effect of low frequency mould vibration and the influence heat-treatmentparameters on microstructure and mechanical properties of modified cast Al-7Si-Mg wt. % alloy in different sand mouldswith two different runner designs/systems are studied. The microstructure and mechanical properties of the alloy arefound to be affected significantly by the combined effect of mould vibration and heat treatment parameters in all thecases. High solution temperatures produce greater refinement, uniform silicon distribution and formation of spheroids.Scanning microscopy of tensile fractured surface reveals the influence of vibration and solution temperature on the modeof fracture. Among the different moulds used for the investigation castings from dextrine no-bake sand mould yieldcastings with better mechanical properties. A comparison of the influence of different moulds with two different runnerdesigns with vibration on the tensile strength of the alloy has shown that filtered rectangular runner (FRR) yield castingswith higher and more reliable tensile strength than the unfiltered vortex flow runner (UVR) system.

Keywords: Cast Al-7Si-Mg alloy; mould vibration; no-bake sand system; solutionising temperature; mechanical properties;microstructure; mode of fracture; gating system.

1. INTRODUCTION

Aluminium alloys with silicon as a major alloyingelement form the most significant part of all shapedcastings manufactured, especially in aerospace andautomotive industries due to its high strength to weightratio, good machinability, corrosion resistance andthermal conductivity. Properties of cast Al-Si alloys arepredominantly governed by the microstructurecharacteristics, such as silicon morphology andintermetallic compounds depending upon the presenceof alloying elements apart from casting process,solidification rate and dendrite arm spacing (DAS).

Sokoloff1 first reported mechanical vibration as atechnique for grain refinement. Several other researchersinvestigated the effect of vibration on microstructure ofcastings2-5

. Numan Abu-Dheir6 found that ductility ismuch influenced by vibration apart from mechanicalproperties and is believed due to structure refinementof the Al-Si alloy.

Apart from the mould vibration the heat treatmentconditions play an important role in controlling themicrostructure and mechanical properties of Al-Sialloys7,8. Recently many attempts have been made byinvestigators to study the influence of heat treatment onmicrostructure and mechanical properties of cast Al-Si-

Mg alloys9,10. The optimum balance of strength andductility attained after the thermal treatments isattributed to the changes in Si particle characteristicsresulting from the solution treatment and to theformation of non equilibrium precipitates of �’(Mg2Si)resulting from the aging treatment.

Recent work at the University of Birminghamspearheaded by Professor John Campbell11 hasdemonstrated that the design of filling systems is crucialto the attainment of sound aluminum castings withreliable mechanical properties. Improper design of thefilling system will usually result in the entrainment ofnot only air, but also the surface oxide films and drossresulting in impaired performance of the castings becauseof unreliable nature of their mechanical properties12.Recent investigations by Sirrell and Campbell13 haveshown that the use of filters considerably improves theflow by offering some resistance so that the sprue can filland also by reducing velocities downstream of the filter.Weibull analyses of tensile strengths13 and fatigueproperties14 of castings obtained from filtered andunfiltered runners confirm the beneficial effects of usingfilters. Experimental findings of Jolly15 et al. seems toimply that the use of either one of the filling systems(filtered or vortex flow) will result in castings with reliablemechanical properties. However, studies have not beenconducted to show which of the two runner systems isbetter when the moulds are subjected to vibration.


2 N. H. Siddalingaswamy, M. A. Joseph, N. M. Nagarajan & N. H. Kumar

The present study is an attempt to investigate thecombined effect of influence of low frequency mouldvibration and heat treatment parameters onmicrostructure, tensile properties, hardness and fracturemode of Al-7Si-Mg alloy castings in different sand mouldwith two runner systems. Also, the present paperpresents results of a comparative experimental studyconducted to establish the effect of a filtered rectangularrunner (FRR) and unfiltered vortex runner (UVR) onreliability of tensile strength of castings from differentmoulds16.

2. EXPERIMENTAL

2.1 Preparation of Castings

The experimental set up for mould vibration is shown inFig. 1. The test castings used for the experiments areprepared by melting commercially available Al-7Si-Mgalloy ingots in a reverboratory tilting type furnace. Thecomposition of the alloy is given in table 1. Addingsodium salt 0.05wt% after degassing the melt with C2Cl6has carried out modification. The molten metal is pouredinto three different sand moulds namely Green sandmould (GS), Sodium silicate carbon dioxide no-bake sandmould (CO2) and Dextrine chromium trioxide no-bakesand mould (DCT) with two different runner systems,namely Unfiltered vortex flow runner (UVR) and Filteredrectangular runner (FRR) systems as shown in Figs. 2and 3. The molten metal is poured into the mould, rigidlyfixed on to the vibration table, the frequency andamplitude is read from the frequency meter fitted in thetable (frequency-0-21 Hz and amplitude 0-2mm). Themould is vibrated according to specified amplitude andfrequency. After solidification the castings are machinedto make the test specimens which in turn given T6 heattreatment having a sequence of solutionising, quenchingand artificial aging. Test specimens from differentmoulds are solutionised at 450°C to 540°C for 3 hoursfollowed by water quenching (30°C) and aging at 180°Cfor 6 hours.

Table 1Chemical Composition of Al-7Si-Mg Alloy, Wt%

Element Si Mg Cu Mn Fe Zn Ti Sn Al

% 7.29 0.35 0.25 0.28 0.5 0.3 0.25 0.05 Bal

2.2 Tensile and Hardness Test

Samples required for various tests are machined fromcastings in different conditions and the values of theproperties are reported. Tensile tests are carried out onspecimens prepared as per ASTM E8 standards usingan INSTRON 5801 Computerized tensile testingmachine. The samples for hardness measurement arepolished with (2/0grade) and Vickers hardness ismeasured at five different locations over the entire crosssection of the test specimen using a load of 5kgf.

2.3 Microscopy

Samples for microstructural studies are cut from thecastings in different conditions. Specimens are polishedusing a standard metallographic procedure and samplesare etched with Keller’s reagent. A ZEISS opticalmicroscope is used for the examination of microstructureof the samples. Scanning electron microscopy studies arecarried out on tensile fractured surfaces.Fig. 1: Schematic Diagram Vibrating Table

Fig. 2: Unfiltered Vortex Flow Runner System (UVR).

Fig. 3: Filtered Rectangular Runner (FRR) IndicatingVertical Placement of Filter



2.4 Statistical Analysis of Tensile Strength

The mechanical properties (e.g. tensile strength) of agroup of nominally similar specimens are usuallyscattered. Green and Campbell17 recently demonstratedthat, compared with several other distributions, the twoparameter Weibull distribution most accuratelydescribed the distribution of tensile strengths obtainedfrom Al-Si-Mg alloy castings. In subsequent studies 12,13

the Weibull statistical technique was used to quantifythe effect of air and film entrainment during mould fillingon the casting quality. The two parameter Weibulldistribution was thus used in the present study toevaluate the effect of the two running systems on thereliability of tensile strengths of the resulting castingsfrom different moulds under low frequency vibrationand is expressed as

�� 1 expu

xF

Where Fu is the cumulative fraction of failures (in a tensiletest), X is the variable being measured, i.e. tensilestrength, � is the characteristic stress at which 1/e (i.e.63.21%) of the specimens survive and � is the parameterquantifying the spread of the distribution, often referredto as the Weibull modulus.

The Weibull cumulative distribution can be plottedas a straight line by plotting ln[ln(1/1–Fu)] versus ln(X)with slope � and intercept ln(ln�). The failure probabilityof the sample is converted to that of a population of thesame specimens using the relation.

��

�0.30.4u

jF

N

Where “j” is the ranked position of the specimen strengthin the population set and N is the total number ofspecimens in the set.

3. RESULTS

3.1 Microstructure Study

Optical micrographs of modified, modified-vibrated andmodified-vibrated and heat-treated alloy are shown inFigs. 4, 5 and 6. It is observed that mould vibrationchanges the morphology of silicon. More over increasein frequency of vibration increases the coarsening of thesilicon particles from acicular state (Fig. 5).

Due to the heat treatment in the next stage, dendritestructure is gradually broken down with an increase insolution temperature along with coarsening of aluminumgrains and spheroidisation of eutectic silicon as shownin Fig. 6. It is observed that coarsening of �-Al grainsand spheroidisation of eutectic silicon particles take placein all the three cases. Further it is noted that there is a

variation in the average size of �–Al phase. This may bedue to difference in thermal conductivity of the mouldsselected for the investigation apart from the heattreatment and vibration effects.

3.2 Mechanical Properties

Influence of mould vibration and solution temperatureson tensile strength, ductility and hardness of the alloyin different moulds are shown in Figs. 7-12. It isobserved that the tensile strength (ultimate), hardnessand ductility of cast Al-7Si-Mg alloy in different mouldsincrease with increase in mould vibration up to 15 Hz.Beyond 15Hz the properties are insignificant as shownin Figs. 7,8and 9. It is primarily attributed to change insilicon morphology and presence of hard siliconparticles. Heat treatment increases tensile strength,hardness and ductility of the alloy as shown in Figs. 10,11 and 12 in all the three types of moulds. The increasein properties of the vibrated alloy after heat treatmentmay be attributed to the spheroidisation of siliconcrystals because stress required for nucleating a voidor crack at a particle matrix interface would increasewith this morphology.

The tensile strengths (Ultimate) of castings fordifferent moulds obtained from two runner systems areshown in table 2 and Weibull plots for these results areshown in Figs. 16, 17 and 18. It is observed from the

Fig. 4: Optical Micrograph of Modified as CastAlloy at X200 (FRR System).

Fig. 5: Optical Micrograph of Modified-Vibrated (15 Hz)Alloy at X200 (FRR System).

Fig. 6: Optical Micrograph of Modified-Vibrated andHeat-Treated Alloy

(Solutionised at 480°C and aged at 210°C) at X100(FRR system).



figures that the FRR system yield castings whose tensilestrength are higher on average and more reliable in allthe three cases as compared to UVR system.

3.3 SEM Study of the Tensile Fractured Surfaces

Tensile fractured surfaces are subjected to SEM studiesto identify the mode of fracture. Figs. 14, 15 and16 showthe SEM images of tensile fractured surfaces of modified,modified-vibrated and heat-treated alloy from differentmoulds with UVR and FRR systems.

These figures reveal well faceted brittle appearanceof silicon particles and black rounded areas, from whichthe hard second phase silicon particles seem to have beenpulled out during the tensile loading. In all the three casesit reveals a mixed mode of fracture consisting of finedimples and intergranular decohesion fracture surfaces.Small dimples indicate that some plastic deformation hastaken place prior to fracture. Cleavage formation maybe attributed to the presence of hard and brittle Mg2Siphase. At high solution and aging temperature (540°Cand 210°C) dimple fracture dominates over the cleavagefracture while at low solution and aging temperatures(450°C and 140°C) the cleavage fracture dominates overdimple fracture.

Fig. 7: Tensile Strength as a Function of VibrationFrequency

Fig. 8: Hardness as a Function of VibrationFrequency

Fig. 9: Ductility as a Function of Vibration Frequency

Fig. 10: Tensile Strength as a Function of SolutionizingTemperature (Aged at 180°C)

Fig. 11: Hardness as a Function of SolutionisingTemperature (Aged at 180°C).

Fig. 12: Ductility as a Function of Solutionising Temperature(Aged at 180°C)



Table 2Tensile Strength (ultimate) of Castings Obtained from

Different Moulds with UVR and FRR Systems

Runner Tensile Strength (MPa) Weibull ModulusType G.S CO2 DCT G.S CO2 DCT

232 233 234 248 249 250 253 254 254

235 236 237 251 252 254 255 256 258

238 240 241 255 256 257 260 260 263

UVR 243 245 246 258 259 260 264 264 266 31.3 44.9 45.3

248 250 251 262 262 264 267 267 270

253 254 255 267 268 269 271 271 273

249 250 251 263 264 265 268 269 269

253 254 256 267 268 269 270 271 272

FRR 257 258 259 270 271 272 274 275 276 50.3 64.2 67.2

261 262 262 273 274 274 277 277 278

263 264 265 275 276 277 279 279 280

266 267 268 277 278 279 281 282 282

4. DISCUSSION

4.1 Influence of Vibration

In general, applying vibration to the solidifying meltresults in two important effects. Firstly, it promotes thewetting of the mold surface by the molten metal, helpingfaster heat extraction from the molten metal through themould wall. Under normal conditions it is difficult toexpect uniform contact between molten metal and mouldwall due to the presence of oxide films on the meltsurface. Application of vibration breaks down thesecovering oxide films, thus making the melt more wettablewith the mould wall and enhancing the contact betweenthem. This aids in promoting nucleation of crystals andgeneration of finer grains at mould surface. Secondly,vibration leads to microstructure changes to both theeutectic composition and dendritic structure ofaluminium. Vibration successfully breaks the dendriticstructure into small islands of aluminium. More over theapplication of external energy works on “crystalmultiplication” phenomenon. Increase in vibration

Fig. 14: SEM Images of Tensile Fractured Surfaces of theModified Alloy with FRR System (X100).

Fig. 15: SEM Images of Tensile Fractured Surfaces of theModified-Vibrated and Heat Treated Alloy(Solutionised at 540°C and aged at 210°C)

with UVR System (X100).

Fig. 16: SEM Images of Tensile Fractured Surfaces of theModified-Vibrated and Heat-Treated Alloy(Solutionised at 540°C and aged at 210°C)

with FRR System X500.

Fig. 17: Weibull Plot of Tensile Strength (Ultimate) ofCastings Obtained from Two Runner

Systems of Green sand Mould.

Fig. 18: Weibull Plot of Tensile Strength (Ultimate) ofCastings Obtained from two Runner

Systems of CO2 Mould.

Fig. 19: Weibull Plot of Tensile Strength (ultimate) ofCastings Obtained from two Runner

Systems of DCT Mould.



frequency increases the tensile strength and hardness ofthe alloy due to the suppression of undesirable dendriticand columnar zones and development of fine-grainedequiaxed structure. This accelerates the spherodisationof grains during heat-treatment because it decreases thediffusion distance.

Fragmentation tendency of primary dendritesobserved with alloy in this investigation is in agreementwith the earlier observations in Al-7Si alloy 18,19 subjectedto rectilinear vibration. It is important to note thatvibration has also shortened the solidification time andsome amount of reduction in gas content.

4.2 Influence of Microstructure

The influence of change in morphology of silicon on thetensile strength and ductility is largely governed by thesize, shape and distribution of silicon particles. Fine andspheroidised phases are known to contribute more toincreased strength and ductility than the acicular andirregular shape particles. In the as cast condition, variousphases act independently against an external load, hencehard silicon crystals act as internal stress raisers in themicrostructure, providing an easy path for the fracture.The size and shape of the second phase silicon particlesgoverns the nucleation and growth of crack in this typeof alloy. Small and homogeneously rounded evenlydistributed eutectic silicon particles results in highductility and strength.

4.3 Influence of Heat Treatment Conditions

The solution temperature establishes the proportion ofsolutes that are in and out of solid solution. Precipitationhardening effect is proportional to the level of supersaturation after quenching. Heat treatment of the alloyrefines and distributes eutectic silicon particles apartfrom spheroidisation. Heat treatment increases thestrength and it may be attributed to the bonding betweenthe hard silicon crystals and soft aluminium matrix andprecipitates Mg2Si besides the spheroidisation 20. Refiningand improving the distribution of eutectic silicon crystalspromotes dimple fracture and reduces the stressconcentration at particle matrix interface. This increasesthe resistance to external load. It is apparent that secondphase particles (dispersoids and precipitates) play acrucial role in the fracture process. They affect the tensilestrength and ductility7-9. Heat treatment promotes dimplefracture compared with cleavage fracture. This is thereason why a heat-treated alloy could with stand highertensile stresses than a cast alloy (with and withoutvibration) in which cleavage fracture dominates. Thefractured surface of an alloy solutionised at 540°C in allthe three cases exhibit finer, uniformly distributed anddeep dimples than at 450°C. This can be attributed toincreased refinement, distribution and spheroidisationof eutectic silicon at higher solution temperatures. Heat

treatment also shows significant increase in hardness inall the three cases, which can be attributed toprecipitation hardening due to formation of Mg2Si andincreased cohesion between silicon crystals and the softaluminium matrix. It resists the flow of the softaluminium matrix under external load. Vibration ofsolidifying castings reduces micro segregation, whichresults in shorter heat treatment time. Greater coherencyand strength achieved by heat treatment coupled withvibration of alloy renders the matrix stronger 7,21,22.

4.4 Influence of Runner Design

The poor performance of the UVR compared with FRRsystem may be attributed to the possible formation of avoid analogous to a vena contracta near the junctionbetween the sprue and vortex flow runner as reportedby Yang et al.23. It is possible that this void has a damagingeffect on the metal entering the cavity, and the voidseemed to be stable at this location and disappeared laterduring the filling operation and was therefore notexpected to cause any serious damage to the metal. It ispossible that the filter did refine the oxide films aspostulated by Sirrell and Campbell13 even though filtersreduce the metal velocity.

The findings of the present study seem to indicatethat combined effect of the runner design and mouldvibration on the microstructure of the alloy plays animportant role to yield castings whose tensile strengthsare higher on average and more reliable. It will beinteresting to compare the performance a filtered runnerwith an unfiltered one that completely eliminates theturbulence at the sprue runner junction, the rolling backwave and also manages to slow down the metal velocityto < 0.5 ms-1 by the time it enters the mould cavity.

5. CONCLUSIONS

1. The combined effect of modification and vibrationof the alloy affects the silicon morphology.

2. Heat treatment of modified vibrated alloy results inimprovement in the strength, ductility, hardness andgrain refining of the alloy besides formation ofspheroids.

3. Moulds with FRR system yield castings with higherand more reliable tensile strength than UVR system.

4. Among the different moulds used Dextrinechromium trioxide no-bake sand mould yieldcastings with higher and more reliable tensilestrengths apart from other mechanical properties.

ACKNOWLEDGEMENT

The authors would like to acknowledge the Department ofMetallurgy and Materials Engineering of National Institute ofTechnology Karnataka (NITK) for its support. Special thanks toDr.K Narayana Prabhu, Dr. A.O Surendranathan and Dr. RajendraUdupa for their support and guidance.



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