Compaction sintering and mechanical properties


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Compaction sintering and mechanical properties

  1. 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME AND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online) IJMETVolume 3, Issue 3, September - December (2012), pp.565-573© IAEME: Impact Factor (2012): 3.8071 (Calculated by GISI) © COMPACTION, SINTERING AND MECHANICAL PROPERTIES OF Al-SiCp COMPOSITES Jeevan.V1, C.S.P Rao2 and N.Selvaraj3 1,2,3 Department of Mechanical Engineering, National Institute of Technology Warangal, Warangal, Andhrapradesh, India. Email: ABSTRACT A trend has been observed in the field of aluminum based composite materials to employ silicon carbide as reinforcement material in developing composites of unique properties. In the present study, an attempt has been made to fabricate the unreinforced Al and its composites were synthesized using the Powder Metallurgy (P/M) manufacturing route with blending, pressing and sintering allows the near net shape fabrication of precision parts. The composites are further solution heat treated at 5290C for two hours and artificially aged at 1750C for 18 hours. Optical Microscopy, Scanning Electron Microscopy has been carried out to analyze powder morphology and composite structure. An increasing trend towards micro-hardness and compressive strength with increase in weight percentage of silicon carbide has been observed. KEYWORDS: Al-SiCp, Mechanical Properties, Microstructure, Powder Metallurgy. 1. INTRODUCTION Particulate Reinforced Aluminum Matrix Composites (PR AMCs) have evoked a vehement interest in recent times for potential applications in aerospace, defence and automotive industries. PR AMCs exhibit improved physical, mechanical and wear resistant properties such as higher stiffness, superior strength-to-weight ratio, improved wear resistance, increased creep resistance , low coefficient of thermal expansion, improved high- temperature properties, and high workability of the composites over those of the monolithic metals oralloys [1-5]. Earlier studies on Metal Matrix Composites (MMCs) addressed the behaviour of continuous fiber reinforcement composite based on aluminum, zinc and titanium alloys matrices. The wide usage of these composites is restricted because of high production cost of composite and composite fiber. MMCs that include both particulate and whiskers have attracted considerable attention than fiber reinforced MMCs, because of their low cost and considerable ease of manufacturing. A wide range of PR AMCs manufacturing processes has 565
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEMEbeen developed. These are generally manufactured either by solid state (Powder Metallurgyprocessing) or by liquid state (stir casting) processes effectively [6]. To fabricate PR AMCs, among the various manufacturing technologies powder metallurgy isone the most advantageous techniques to fabricate isotropic distribution of particles in matrix, gooddimensional accuracy, complex, net shape lightweight components can be produced cost effectively.Powder metallurgy is especially suitable for producing PR AMCs as it prevents some wettabilityproblems of silicon SiCp and deleterious reactions that may appear during casting routes. Blendedfine powder mixtures in the solid state with particulates, whiskers or platelets along with bindersproduce materials of uniform microstructure. The conventional powder metallurgy process can easilyformulate different composition by mixing elemental or premixed powders along with reinforcement,and pressing the powder mixture to form green compact by applying hydraulic pressure and sinteringthe green compact in inert gas atmosphere. Few microstructural parameters control and contribute tothe advancement in the properties of PR AMCs. These involve the matrix alloy, the morphology, size,and weight fraction of the reinforcement particulate; the material processing technique; and the heattreatment adapted [1-7]. PR AMCs powder is highly compressible. Mostly, green densities of more than 90 % oftheoretical can achieve utilizing low compacting pressures around 200MPa, allowing the use ofpresses with smaller capacity. Sintering of PR AMCs parts is more economical than for most otherPM materials due to the relatively low sintering temperatures. Due to the low density of PR AMCs,more than twice number of parts can be produced from unit weight of powder as compared to ferrous,copper and tungsten based powders. During last decade, several researchers have reported the fabrication of Al-SiCp compositesand testing of their properties such as tensile strength, hardness, wear resistance and microstructuralcharacterization. Most of the researchers have observed that an increase in tensile strength, hardnessand wear resistance while decrease in ductility with increase in reinforcement content and aluminumalloy powders are difficult to sinter because of the stable aluminum oxide film covering the powderparticles and thus reducing sinter-ability. In addition, the presence of hard ceramic particles inaluminum ductile matrix increases the processing difficulty. Related work carried out on aluminumalloy by reinforcing ceramic particles such as SiC, Al2O3, ZrO2, TiO2 etc., with varyingreinforcement sizes, volume/weight fractions, lubricants, compaction pressures, sinteringtemperatures, sintering time, and sintering atmospheres. By varying these parameters will resultoptimal set of parameters lead to resultant microstructure and properties [7-22]. The 6xxx series aluminum alloys have a widespread application, especially in the building,aircraft and automotive industry due to their properties. Increasing demand for these materials haveresulted in increasing research and development for high strength and high-formabilityaluminum alloys. Among 6xxx series aluminum alloys AA6082 one of the most common engineeredaluminum alloy. It offers a combination of better corrosion resistance and weldability due to its lowerstrength values in the welding zone. In numerous applications, AA6061 can be replaced with AA6082due to its higher strength [11-12]. The objective of the present investigation is to fabricate the unreinforced Al and itscomposites. Hence, the present studies are aimed at fabrication of Al and Al-5 wt% of SiCpcomposite that is fabricated by powder metallurgy route followed by solution heat treatment andartificially aged. Microstructure, micro-hardness and compressive strength of the developedunreinforced Al and its composites are studied. alloys [1-5]. 566
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME2. EXPERIMENTAL PROCEDURE2.1 Materials It is necessary to select pure metal powder and optimal processing parameters for thepreparation of specimens. Commercial pure aluminum is obtained from M/S Metal PowderCompany Ltd, Tamil Nadu, India. Silicon, Magnesium, and Manganese are supplied bypremier industrial corporation limited Maharashtra, India. Silicon carbide is obtained fromoutside vendor at Tamil Nadu, India. The morphology of raw powders (Al, SiCp) was madewith Scanning Electron Microscopy (SEM), JSM-6390 Model (JOEL) shown in figure 1(A),1(B), 1(C), 1(D) and 1(E). The EDAX analysis has shown in figure 2(A) and 2(B).Particlesize and purity details for raw materials are given in table 1. Fig. 1(A) SEM of Aluminum Powder Fig. 1(B) SEM of Silicon Powder Fig. 1(C)SEM of Magnesium Powder Fig. 1(D)SEM of Manganese Powder 567
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME Fig. 1(E) SEM of Silicon Carbide Powder Fig. 2(A)EDAX of Aluminum Powder Fig. 2(B)EDAX of Silicon Carbide Powder Table 1: Details of Raw Material Sl.No Raw Material Particle Size Purity 1 Aluminum -200/+325 mesh 99.50% 2 Silicon -325 mesh 99.57% 3 Magnesium -150 mesh 99.67% 4 Manganese -325 mesh 99.78% 5 Silicon Carbide -1200 mesh 98.0%2.2Mixing The chemical composition of the AA6082 prepared by elemental mixing is as follows: Al–1.0Si–0.9Mg–0.7Mn/5.0 SiCp (all concentrations by weight). Contech Precision Balance (Type: CA223) is used for weighing elemental powder. Metal and ceramic powders were blended in a Turbulamixer with Jar container. Blending is one of the crucial processes in powder metallurgy where themetallic powders have mixed with the ceramic reinforced particles.Good blending produces noagglomeration of both the metallic and ceramic powders. 1.5% of acrawax by weight was added to thebase Aluminum powder and mixed separately for 15 minutes. In general lubricant was added andhomogeny blended to reduce friction between the powder mass and the surface of the die and obtain agood compaction. Addition of 1.0 Si, 0.9 Mg, 0.7Mn as elemental were made to the lubricated basepowder and mixed for 15 min each, after which a composition similar to that of wrought 6082 Alalloy was gained. Finally by addition of 5% of SiC particulates by weight to the 6082 Al alloy powderand mixed for 20minutes. The obtained powder mixtures with ceramics were homogeny atmacroscopic level. 568
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME2.3 The Specimens Compacting For pressing, a hydraulic press (Model: plus one machine fabric) was used to obtaingreen compacts. Die wall is brushed with zinc stearate powder for easy ejection of pallet andto reduce the friction between them. Blended Powders were compacted at 200 ± 5 Mpa in ahardened steel die. In order to avoid damage of the samples during ejection, the compactionpressure was decreased to 5Mpa after maximum pressure was obtained. The dimensions ofgreen compacts are 13.3 x 13.3 x 13.3 mm3. The theoretical density assuming zero porositywas calculated by Rule of Mixture (ROM). The green density of the compacts wasdetermined from weight and volume measurements. The AA6082 and AA6082-SiCp powdermixtures exhibit uniform die filling and provides good reproduction of part configuration.Theoretical density and green density are shown in Table 2. Table 2: Theoretical Density and Green Density Material Theoretical Green Density Density (%) (g/cm3) AA6082 2.62 94.32 AA6082-SiCp 2.64 91.362.4Sintering and Heat Treatment The mild steel boat with dimensions 30x15x5 cm3 and 0.4cm thickness is filled withfine sand and the green compactswith achieved dimensions are placedin the boat. The boat ismoved slowly inside pre heating zone with hydraulic arm. The temperature within the furnacerises slowly in the preheat zone till it reaches the actual sintering temperature. The greencompacts are de-lubricated in the preheat zone at 3500C for 30 minutes. After de-lubricationof pallets the boat enters into hot zone or sinter zone where the temperature raised slowly to6200C it remains essentially constant for 45 minutes in a protective atmosphere crackedammonia. The sintering temperature is kept below the melting point of the base metal. Theboat is pushed into the cooling zone where the drop in part temperature is controlled preciselyand cooled to room temperature. As the parts travel through the furnace, the temperaturecycle results change in composition, microstructure and properties. In the preheat zone, thelubricant volatilizes, leaves the part as a vapor, and is carried out by the dynamic atmosphereflow. In the hot zone, metallurgical bonds develop between particles and solid state alloyingtakes place. The part then moves through the cooling zone. The microstructure developedduring sintering determines the properties of the part. Dimensional changes encountered aftersintering. The premixed elemental AA6082 specimens are subjected to volumetric expansion.Sintered densities of specimens were measured by the Archimedes principle (waterdisplacement technique). The porosity is increased during the sintering process compared tothe green one. The large porosities reduced the sintering densities due to wide polymer burnoff range leaving residual porosity. Proper bonding between metallic matrix and ceramicparticles at interface and the morphology and distribution of pores and carbides in the matrixare achieved. The composites are further solution heat treated at 5290C for two hours andartificially aged at 1750C for 18 hours in a muffle furnace. 569
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME3. RESULT AND DISCUSSION3.1 Microstructure The purpose of microstructure examination was to investigate grain size and shapemorphology and distribution of the silicon carbide particles. The microstructures of theunreinforced Al and Al-SiCp composites were studied using optical microscope. For thispurpose small samples were cut from the cube fabricated by powder metallurgy process.Theflat samples were polished using silicon carbide paper (320, 400, 800, 1000, and 1500 grit)and finally using a short-nap cloth with fine alumina powder as slurry. The samples were thenetched using the Keller’s reagent. Figure 4(A) and 4(B) shows the optical microscopephotographs for the the unreinforced Al and Al-SiCp. Micrograph indicates the nearlyuniform distribution of the SiCp particles in the aluminum matrix and some clustering ofsilicon carbide arise reinforcement in the matrix. Fig. 4(A) Microstructure of AA6082 Fig. 4(B) Microstructure of AA6082-5SiCp3.2 Micro-hardness Test Vickers Microhardness measurements were performed on polished flat specimensaccording to ASTM E384-08 with indenting load of 200gf and dwell time 15 seconds. Theaverage microhardness data given in this paper resulted from five measurements. Theposition of indentation on the sample was chosen randomly. The microhardness test gives agood indication on the strength of the material. As the SiCp increases from 0 to 5 percentagehardness also increased. The results were shown in Figure 5. 50 48 Micro-Hardness 46 44 42 40 38 36 AA6082 AA6082-5SiCp Fig. 5 Microhardness of P/M AA6082 and AA6082-5%SiCp 570
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME3.3 Compression Test The compression test was chosen as it requires small size specimen. The sampleswhich are a problem in case of powder metallurgy produced aluminum silicon carbidecomposites. For each combination, four compression Specimens were tested. Figure 6,illustrates the effect of Silicon carbide particulate reinforcement content on the compressionstrength of the composite. It is observed that the compressive strength of the compositeincreases as the reinforcement content increases from 0 to 5 weight percent. This increase inthe compression strength is attributed to the presence of hard particles, which imparts highstrength to the composite. This may be due to very small amounts of particulates at differentorientations, which can make significant difference in stress-strain behavior. The rigidity andcrushing strength of particles is much higher than that of matrix material hence the strengthincreases. 600 580 Compression Test 560 540 520 500 480 460 AA6082 AA6082-5SiCp Fig. 6Ultimate Compressive Strength of P/M AA6082 and AA6082-5%SiCp4. CONCLUSION During compaction of powders, the shape and quality of final component dependsupon the quality of initial manual compact. Therefore, the manual compact should beprepared carefully and proper allowances should be in dimensions to get the desired finalcomponent. Compaction at 200 MPa followed by sintering at 6200C has been successfullyused to produce Al alloy and Al-SiCp composites. During thepecipitation hardeningthe alloyis transformed to a homogeneous, one phase solution. Micro-hardness, compressive strengthof powder metal Al alloy and Al-SiCp composites increases with increase in reinforcementcontent from 0 to 5% weight of SiCp.5. Acknowledgements The authors wish to thank Mr.VinayChoudary (C.E.O) and Mr. Saibaba (GM),Innomet Powders, Hyderabad, for their support and encouragement during the researchstudies. 571
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