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Impact of T6 Heat Treatment on the Hardness of Boron Carbide Reinforced Aluminum Composites

Gaurav1 , Dr. V. K. Singh2 , Dr. Sakshi Chauhan3

1PhD scholar, Dept. of Mechanical Engineering, GBPUAT Pantnagar, Uttarakhand, India

2Professor, Dept. of Mechanical Engineering, GBPUAT Pantnagar, Uttarakhand, India

3Asst. Professor, Dept. of Mechanical Engineering, GBPUAT Pantnagar, Uttarakhand, India

Abstract - The study examines the effects of T6 heat treatment on the hardness of aluminumcomposites reinforced with boron carbide (B4C). Aluminum alloy 2024 (AA2024) served as the matrix material, with boron carbide particles incorporated to enhance its mechanical properties. A standard T6 heat treatment process comprising solutionizing, quenching, and aging was applied to the composite samples. Microstructural analysis revealed significant precipitation hardening and uniform dispersion of B4C particles as a result of the heat treatment. Comparedto as-cast composites, the T6treated samples exhibited a marked increase in hardness, attributed to the combinedeffects ofmatrixstrengtheningand reinforcement. The findings underscore the synergistic impact of heat treatment and B4C reinforcement on enhancing the mechanical performance of aluminum composites, pavingthe way for high-performance applications.

Key Words: Aluminium Matrix Material, Aluminium alloy, Rockwell Hardness, Stir casting, SEM

1.INTRODUCTION

Overthepasttwodecades,aluminiummatrixcomposites (AMCs)havemadesignificantadvancements,demonstrating improved performance characteristics [1]. Aluminium matrixcompositespossessadvancedpropertiessuchaslow density, excellent electrical and thermal conductivity, exceptionalresistancetooxidationandwear,highspecific strength and stiffness, and durability at elevated temperatures[2].Thesecompositesareidealforautomotive andaerospaceapplications,astheyaredesignedtoreduce weight while enhancing mechanical properties such as strength and stiffness [3]. Non-metallic ceramic particles suchassiliconcarbide,boroncarbide,andtitaniumcarbide are commonly used as reinforcements in aluminium matricestoproducealuminiummatrixcomposites(AMCs) [4].

Ceramic reinforcement composites have garnered significantattentionacrossvariousindustrialsectorsdueto their outstanding properties and versatility. These composites typically consist of a metal or ceramic matrix reinforcedwithceramicfibers,particles,orwhiskers,which significantly enhance their mechanical, thermal, and electrical performance. The addition of ceramic reinforcements improves the composite’s hardness, wear

resistance, thermal stability, and electrical conductivity, makingthemidealfordemandingapplicationsinaerospace, automotive, electronics, and energy sectors [5], [6], [7]. Titaniummatrixcomposites(TMCs)reinforcedwithboron carbide (B4C) exhibit excellent mechanical properties, including high strain rates and impressive compressive strengths [5]. Ceramic reinforcement composites offer a widerangeofpossibilitiesfortailoringmaterialproperties to meet specific application needs. The fabrication techniques, such as additive manufacturing, powder metallurgy, and stir casting, play a crucial role in determining the composite's microstructure and performancecharacteristics[8],[9],[10].

Stircastingisawidelyusedandcost-effectivemethodfor fabricating metal matrix composites (MMCs), particularly AMCs. This process involves mechanically stirring reinforcing particles into molten metal to ensure their uniformdistributionthroughoutthematrix[11],[12].The stir casting method is preferred for its simplicity, proven effectiveness, and suitability for large-scale production, making it an ideal choice for manufacturing aluminium matrixcomposites[11].Thequalityandpropertiesofstircast composites are influenced by several key factors, includingtheselectionofmatrixandreinforcingmaterials, alongwithparameterssuchasstirringtemperature,speed, andduration.Thesevariablesplayacrucialroleinensuring ahomogeneousdistributionofreinforcementandachieving thedesiredmechanicalandtribologicalproperties[12],[13]. Remarkably, when compared to traditional methods, the application of ultrasonic energy during stir casting has shownsignificantimprovementsinparticledispersionand theresultingmechanicalproperties.Theuseofultrasound helpsbreakdownparticleagglomerationsandenhancesthe uniformity of the reinforcement distribution within the matrix, leading to better overall performance of the composite[14].Nonetheless,challengessuchasachieving consistentparticledispersionandensuringproperwetting ofthereinforcementparticlesremainprevalent[15].

The composition and manufacturing methods of aluminiumcompositematerialssignificantlyinfluencetheir Rockwell hardness values. Typically, the addition of reinforcementssuchasceramicparticles,likeboroncarbide or silicon carbide, enhances the hardness of aluminium compositeswhencompared topurealuminium[16],[17],

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[18].Aluminiumalloysreinforcedwithnano-TiO2 particles through friction stir processing (FSP) exhibit enhanced toughness[19].Similarly,theadditionofceramicparticles like titanium diboride (TiB2) to aluminium matrices significantlyenhancestheirhardnessandothermechanical properties.TiB2,beingahardceramicmaterial,contributes toincreasedwearresistanceandhardness[20].Incertain instances, it was discovered that the peak hardness of reinforced composites was either slightly higher than or equivalenttothatofunreinforcedalloys[21].

2. MATERIAL & METHODOLOGY

2.1 Acquiring of Materials

The AA2024 alloy was procured in block form from a supplierbasedinMumbai.Boroncarbidewasprocuredin powderformfromasupplierbasedinMumbai.Theaverage particlesizeofboroncarbidewas40-50microns.

2.2 Casting & Sample Preparation

The AA2024 block was carefully cleaned to remove impuritiesanddirtbeforemelting.Thisstepwascrucialto ensure the quality of the final alloy and minimize contaminants that could negatively affect the material's properties.ThecleanedAA2024blockwasplacedinacoalfired furnace. Coal furnaces were chosen for their accessibility and common usage, despite electric or gas furnaces being more prevalent due to their superior temperature control for melting. During the smelting process,slag,anon-metallicwaste,andotherimpuritiesthat hadaccumulatedatthesurfaceofthealuminiummeltwere removed.Thisprocesshelpedmaintainthealloy'spurityby eliminating oxides and other contaminants that could negativelyimpactitsmechanicalproperties

Boroncarbidewasaddedinweightpercentagesof8,10, 12, 14, and 16, and the molten mixture was vigorously stirred to ensure thorough mixing. Stirring is crucial in castingoperationsasitpreventstheformationoflocalized regions where alloying components could concentrate. Uniform mixing is essential to maintain a consistent microstructure throughout the material, which in turn ensures the uniformity of its mechanical properties. The moltenliquidwaspouredintopreheatedmoldstominimize thermalshockandensureasteadyflowofthemoltenmetal. Thisapproachreducestheformationofinternalstressesin the alloy, which could otherwise lead to cracking or deformation due to rapid cooling. The molds were specifically designed to produce circular samples, which were then further processed to create hardness test specimens.

Thecastingswereallowedtocoolnaturallyinambientair, rather than using forced cooling methods such as water quenching. This approach was chosen to prevent thermal stresses that could lead to cracking or warping of the

samples.Theslowcooling rateintheambientairenabled more controlled solidification of the aluminium alloy, ensuring a more uniform and stable microstructure. Once thecastingshadfullysolidified,theywerecarefullyremoved fromthemolds.

Aftercasting,thecircularsampleswerecutforhardness testing. The surfaces of these specimens were smoothed usingemerypaperandthencleanedwithacetonetoremove anycontaminants.

2.2 Heat Treatment

AspecificprocessknownasT6heattreatmentisapplied toaluminiumalloystoimprovetheirmechanicalproperties, such as strength and hardness. The composite samples underwent a standard T6 heat treatment process, which involvedsolutionizing,quenching,andaging.

Duringthesolutionizingprocess,thealuminiumalloyis heated to about 500°C, allowing the soluble alloying elements, such as copper in AA2024, to dissolve into the aluminiummatrixandformasolidsolution.

Afterthesolutionheattreatment,thealloyingelements aretrappedinthesolutionbyrapidlycoolingthematerial withwater.Thisquenchingprocesspreventstheformation ofprecipitatesatroomtemperature,keepingthematerialin asupersaturatedsolidsolution.

Followingthequenching,thematerialundergoesartificial aging at a lower temperature, typically around 180°C, for approximately24hours.Thisprocessallowstheprecipitates toformandenhancethematerial'sstrengthandhardness.

3. EXPERIMENT

3.1

Rockwell Hardness Testing

TheRockwellhardnesstestisacommonlyusedmethod toassessthehardnessofmaterials,includingalloys.Inthe Rockwellhardnesstest,ahardenedsteelballindenterisused to apply an initial force of 10 kgf to the specimen. The diameterofthehardenedsteelballindenterusedinthetest was1/16inch.Thisinitialforce,referredtoasthe"preload," ensuresthattheindentermakescontactwiththesurfaceof thespecimen.Subsequently,alargertestforce(majorload) of100kgfisapplied,causingtheindentertopenetratethe material.Afterapredeterminedduration,themajorloadis removed, and only the minor load remains applied to the specimen. The depth of the indentation created under the minorloadisthenmeasured.TheRockwellhardnessvalueis determinedbymeasuringthedepthoftheindentationand applyingEquation(1)correspondingtothescaleinuse.

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Where HR is the Rockwell hardness number, N is a constantbasedonthescaleused(e.g.,N=130forRockwellB scale),histhetotaldepthoftheindentationafterapplying themajorload,anddisthedepthcausedbytheminorload

Inthepresentstudy,theBscaleoftheRockwellhardness test was used to evaluate the hardness of the prepared aluminiumcomposite.TheDigitalRockwelltestingmachine, asshownin Fig - 1,isusedtocalculatethehardnessofthe specimen.Inthissetup,thedepthcalculationsareperformed automatically by the machine itself. For each composite sample, around five hardness readings were taken on the surfacetoensureaccurateandconsistentresults.

3.2 Microstructure Analysis

Tofurtheranalysethedistributionofthereinforcement and matrix material in the composite, Scanning Electron Microscopy(SEM)wasutilizedtoobservethedispersionand distribution.Thisanalysishelpsusgaindeeperinsightsinto thephenomenadrivingtheobservedtrendsintheproperties of the composite. This serves as a valuable indicator for understanding the hardness pattern of the composite. Microstructure analysis also helps us establish the relationshipbetweenpropertiesandmaterialdistribution.By observingthedistributionofboroncarbideparticleswithin the aluminum matrix, we can gain insights into how uniformity or clustering of the reinforcement affects the mechanicalproperties,suchashardnessandoverallstrength ofthecomposite.

4. RESULT AND DISCUSSION

4.1 Hardness

First,compositesS1,S2,S3,S4,andS5,reinforcedwith 8%, 10%, 12%, 14%, and 16% weight of boron carbide, respectively, were tested for hardness using the Rockwell hardnessmachine.Thehardnessvaluesofthesecomposites wererecordedandarepresentedin Table 1

Table -1: HardnessofB4Creinforcedcomposite

ThehardnessofT6heat-treatedcomposites,designated asT1,T2,T3,T4,andT5,with8%,10%,12%,14%,and16% weight reinforcement of boron carbide, respectively, was evaluated using the Rockwell hardness machine. The recordedhardnessvaluesarepresentedin Table 2.

Table -2: HardnessofB4Creinforcedcompositetreated withT6Heattreatment

To clearly visualize the hardness pattern of the composites, a graph of these values was plotted and compared,asillustratedin Chart 1.

The chart clearly indicates that the hardness value increaseswithanincreaseintheweightpercentageofboron carbide. However, after reaching 14% weight of boron carbide,thereisaslightdecreaseinhardness.Thiscouldbe attributedtothenon-uniformdistributionofboroncarbide particlesathigherconcentrations.Thisdecreaseinhardness can be mitigated by further optimizing the stir casting parameters,suchasincreasingthestirringspeed,duration,

Fig -1:RockwellHardnessSetup

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or temperature, to ensure a more uniform distribution of boroncarbideparticleswithinthematrix.

4.2 Scanning Electron Microscopy (SEM)

ToassessthedistributionofboroncarbideintheAA2024 composite,SEManalysiswasconducted.Forthispurpose,the specimensurfaceswerepreparedusingfineemerypaperand alumina particles In aluminum composites, boron carbide playsacrucialroleinenhancingthecomposite'sload-bearing capacity.Ifthedistributionofboroncarbideishomogeneous, this strengthening phenomenon will be more effectively realized. For this analysis, the composite with the highest boron carbide content was selected to examine the distributionasshownin Fig -2

TheSEMimageclearlyrevealsapredominantlyuniform distribution of boron carbide particles, with only minor clusteringobservedincertainareas.

5. CONCLUSIONS

According to this study, the T6 heat treatment substantiallyenhancesthemechanicalandmicrostructural propertiesofaluminumalloys.TheT6heattreatmentresults inamaximumincreaseof7.32%inhardness. Aconsistent improvementinhardnessisobservedforthesameweight percentofreinforcementaftertheT6heattreatment.These improvementsinhardnessenhancethecomposite'slifespan, making it suitable for long-term applications while also providingexcellentwearresistance.

Microstructuralanalysisfurtherconfirmsthesuccessful casting of aluminum composites with boron carbide reinforcement. The uniform distribution of boron carbide particlesobservedinthemicrostructuralanalysisvalidates the successful enhancement in hardness and indicates potentialimprovementsinothermechanicalpropertiesofthe composite.

The SEM image clearly reveals a predominantly uniform distribution of boron carbide particles, with only minor clusteringobservedincertainareas.

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Chart -1:Hardnesscomparisonofcomposite

Fig -2:SEMofcompositehaving16%B4C

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