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Fracture toughness of AA2024 aluminum fly ash metal matrix composites

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https://www.eduzhai.net International Journal of Composite Materials 2014, 4(2): 108-124 DOI: 10.5923/j.cmaterials.20140402.10 Fracture Toughness of AA2024 Aluminum Fly Ash Metal Matrix Composites Ajit Bhandakkar1,*, R C Prasad1, Shankar M L Sastry2 1Department of Metallurgical Engineering and Materials Science, IIT Bombay 2Mechanical, Aerospace and Structural Engineering, Washington University in St. Louis, USA Abstract The aluminum fly ash metal matrix composites (MMCs) find important applications in automobile and aerospace where high strength and modulus is important. The fly ash by product of coal burning is drawing lot of attention as reinforcement for MMCs due to its low cost and reduction in environmental pollution. The ash particles, generally being hollow in nature, display lower densities while oxides present as constituents make them possess high modulus and strength thereby enhancing specific strength and stiffness along with lower densities compared to many metal based systems. The uses of MMCs are impeded in critical applications due to its low fracture toughness as compare to metals. LEFM (Linear Elastic Fracture Mechanics) has been used by researchers to characterize the plane strain fracture toughness using various specimen geometries and notches. However there were very few studies using EPFM (Elastic Plastic Fracture Toughness) are reported in open literature. In the present paper the influences of weight fraction of fly ash reinforcement on hardness, tensile strength and fracture toughness have been evaluated. Hardness of aluminium fly ash metal matrix composites increases with the addition of fly ash particulate reinforcement. However the tensile strength and fracture toughness KIC and JIC of the aluminum fly ash composite decreases that of base alloy. The fracture toughness of AA2024 varied between 17-18 MPa√???????? as compared to 21 MPa√???????? for remelted base alloy AA2024, which is consistent with the reported data. The JIC fracture toughness of AA2024 fly ash composites varied between 6-15 KJ/m² as compared to 25 KJ/m² for the re melted base alloy AA2024. The load and COD plot shows hysteresis loop in loading and unloading compliance curve. This hysteresis loop is indicative of crack closure due to fly ash particles. The reason for crack closure may be surface roughness resulting from reinforcement particles in the composites. The fracture behavior and micro-mechanism of failure in base alloy and composites have been observed under SEM and optical microscopy. Keywords Fracture Toughness, Aluminum Fly Ash Composites, MMCs, Damage Mechanics 1. Introduction The commercial applications of MMCs have been limited due to their higher cost and low fracture toughness as compared to metal alloys. However in spite of higher cost MMCs has emerged as an important class of materials due to high specific strength and stiffness as well as other desirable properties. The Aluminium alloy matrix composites are being used extensively for high performance applications in automobile and aerospace. Because of several advantages, continued efforts are being made to process Aluminum ceramic reinforced metal matrix composites with low cost reinforcement. Fly ash which is by product of coal burning is drawing lot of attention as reinforcement for MMCs due to its low cost and reduction in environmental pollution. The fly ash particles, generally being hollow in nature, display lower densities while oxides present as constituents make them * Corresponding author: ajitbb@iitb.ac.in (Ajit Bhandakkar) Published online at https://www.eduzhai.net Copyright © 2014 Scientific & Academic Publishing. All Rights Reserved possess high modulus and strength thereby enhancing specific strength and stiffness along with lower densities compared to many metal based systems. The measurement of valid plane strain fracture toughness, (KIC) and Elastic plastic fracture toughness (JIC) for particulate reinforced metal matrix composites is an important step in the process of developing useful products from these materials and increasing confidence in their properties and performance. However limited work has been reported in open literature on influence of fly ash on fracture toughness of aluminum metal matrix composites. The value of the KIC and JIC characterizes the fracture resistance of a material in the presence of a sharp crack under tensile loading, where the state of stress near the crack front is a triaxial plane strain, and the crack-tip plastic region is small compared with the crack size and specimen dimensions [3-6]. In 1977 Barker [7] proposed the short-rod specimen for determining plane strain fracture toughness. Waszczak [8] investigated the applicability of LEFM (KIC) to Boron /Aluminium composites and reported the non linear behavior of load vs COD (crack mouth Opening displacement) curve due to large scale plasticity of AA6061 International Journal of Composite Materials 2014, 4(2): 108-124 109 aluminium alloy. The LEFM method of fracture toughness of Al/SiC metal matrix composites were studied by few researches [9-11] and reported that the fracture toughness of the composites depends on the volume fraction and aspect ratio of the particles. Hong et al. [11] showed that the fracture toughness of SiC/2024 Al alloy composite decreases from 20.16 MPam1/2 to 14.67 MPam1/2 when the volume fraction of the SiC particles increases from 3% to 10%. Hasson and Crowe [12] showed that the KIC value is 15.8 MPam1/2 for a 25 vol.% SiCp reinforced 6061 Al alloy composite in T6 condition, while the value is only 7.1 MPam1/2 for a 20 vol.% SiCw reinforced 6061 Al alloy composite in T6 condition. The uses of MMCs are thus impeded in critical applications. LEFM (Linear Elastic Fracture Mechanics) has been used by researchers to characterize the plane strain fracture toughness using various specimen geometries and notches. [13–23]. However there were very few studies using EPFM (Elastic Plastic Fracture Toughness) are reported in open literature. In this paper, the aluminum fly ash metal matrix composites AA2024 were processed by low cost liquid metallurgy route. The composites were secondary processed by hot extrusion and evaluate the mechanical properties, fracture toughness and micro-mechanisms of failure. The composites in the present investigation showed a stable crack growth therefore JIc test was conducted using three point bend specimens. Typical loading unloading curves for base alloy and composite are plotted and the fracture toughness KQ for AA2024 base alloy and fly ash composites were calculated from J-R curve at maximum load. (800℃) fly ash particulates (25-45µm) were and 900℃ for silicon carbide (1500 grit) added through a preheated pipe by manual tapping into the slurry, while it was being stirred. Table 3 gives the stir casting process details. A post-addition stirring time of 30 min was allowed to enhance the wetting of particulates by the metal. The temperature of the slurry was sufficiently raised above the melting range of the matrix alloy before pouring the composite melt into preheated permanent mould. Figure 1. Experimental set up for processing of AA2024/fly ash composites 2. Experimental 2.1. Material Aluminum alloy AA 2024 are used as base matrix with composition (weight percent) listed in Table.1 The reinforcement used are silicon carbide and fly ash having particles of sizes 25-45 in 5%, and 10% by weight and the chemical composition of fly ash reinforcement is as per Table.2. Figure 2. Experimental set up for fabrication of aluminum metal matrix composite 2.2. Processing of Aluminum Fly Ash Composites Fig.1 and Fig.2 shows the experimental setup for fabrication of aluminum metal matrix composite through liquid metallurgy route. About 1 kilograms of the AA 2024 alloy is cleaned and loaded in the silicon carbide crucible and heated to above its liquidus temperature. The temperature was recorded using chromel-alumel thermocouple. To maintain the solid fraction of about 0.4, the temperature of the melt was lowered before stirring. The specially designed mechanical graphite stirrer is introduced into the melt and stirred at ~ 400 rpm as shown in Fig.3. The depth to which the impeller was immersed is approx 1/3rd the heights of the molten melt from the bottom of the crucible. The preheated Figure 3. Graphite stirrer for uniform distribution of Aluminum metal matrix composite 110 Ajit Bhandakkar et al.: Fracture Toughness of AA2024 Aluminum Fly Ash Metal Matrix Composites Grade AA2024 Al Base Table 1. Chemical composition of matrix alloy AA2024 % Elements Cu Mn Mg Zn 4.17 0.68 1.3 0.11 Fe Cr Si 0.63 0.090 0.35 Table 2. Chemical composition of fly ash reinforcement Grade % Elements Indian Fly ash Al2O3+ SiO2+ Fe2O3 CaO MgO Na2O K2O SO3 92.49 - 2.13 0.73 - 1.06 Table 3. Stir casting process details for fabrication of aluminum fly ash composites Sr. No Composite System Reinforcement Size (µm) Preheat Temp of reinforcement Total Stirring time Pouring Temp. (℃) 1 2024+5% Fly ash (wt%) 25-45 800℃ 30min 750 2 2024+10% Fly ash (wt%) 25-45 800℃ 30min 800 Table 4. Extusion ratio used for secondary processing of AA2024/fly ash composites Material Initial Diameter(mm) Final Diameter % Reduction AA 2024 base alloy 49.5 AA2024 +5%P60 49.5 AA2024 + 10%P60 49.5 17.74 17.74 17.74 64.16 64.16 64.16 2.3. Secondary Processing The as-cast composite billets were extruded/hot rolled at 450 oC (Soaking for 4 hrs) in order to get rid of the porosities induced during primary processing. It also improves the distribution of the reinforcement in the aluminum matrix. Secondary processing improves distribution of fly ash reinforcement in the matrix, imparts directional properties, whereby mechanical properties are improved. The hot extrusion/rolling details of Metal Matrix Composite (AA2024 + fly ash) are shown in Table 4. 2.4. Specimen Preparation The tensile specimens were fabricated from the extruded rods of the base metal and composite extrusions. as shown in Fig.4 as per ASTM E8 were used for tensile testing. The SENB specimens for KIC are prepared in LT direction with notch and intended direction perpendicular to the rolling direction as per ASTM E-1820 and ASTM E-647 standards as shown in Fig 5 and Fig.6. Three Point bend test specimen with a 4.5 mm thickness were machined from round bars of 12.5 mm in diameter Fig.5 show the specimen dimensions. Fatigue precracks were grown by keeping the BISS servo hydraulic machine under displacement control, with frequencies between 10 to 15 Hz by maintaining a/w ratio between 0.55-0.70. Straight notches were used in the specimen in order to enhance the initiation of the fatigue crack. The tests were made in the BISS machine using displacement control with a load point displacement rate of 0.1 mm/min. Load vs. Load point displacement (P vs. d) and amplified Load vs. mouth opening displacement (P vs. V) plots were obtained. The values of JIC and J-R were obtained following the ASTM E-1820 standards. From the obtained JIC, the equivalent K, and KIC, were calculated by equation-1. International Journal of Composite Materials 2014, 4(2): 108-124 111 Figure 4. Tensile test specimen as per ASTM E-8 Figure 5. Fracture toughness test specimen SENB for JIc and FCGR testing test sample as per ASTM E-1820 and as shown in Fig.7. The conditional fracture toughness was calculated using following Eqn.1 and the values of fracture toughness of base alloy and composites are listed in Table.5 and Table.6. 3.2. Equation 1 Fracture Toughness of Composite KQ Figure 6. Fatigue crack starter notch configuration Figure 7. Test set up for Fracture toughness testing 3. Results and Discussion 3.1. Elastic Plastic Fracture Toughness Testing Elastic plastic fracture toughness JIC tests and fatigue crack growth rate (FCGR) were conducted on BiSS 50 KN servo hydraulic Universal Testing Machine by using SENB KQ = PQ B.W 3 xsxf  a  ,  w 2 KQ = Conditional Fracture Toughness PQ = Load value obtained by 95% secant line. S = Span length A = Crack length W = Width of the specimen The Elastic plastic fracture toughness JQ for the base alloy AA2024 is 25.81 KJ/m2 and for AA2024-5% FA is 15.70 KJ/m2 and AA2024-10% FA 6.69 KJ/m2 as listed in Table.5.This decrease in the fracture toughness of the composites is due to weak interface between the fly ash reinforcement and aluminum alloy matrix which acts as small micro cracks as shown SEM microstructure in Fig.18. Also during stir casting lot of casting defects such as void, porosity generates during stirring of fly ash reinforcement. The similar results were reported by Ashby et.al for the Aluminum Silicon carbide composites as shown in Fig.8, the fracture toughness of the composites is in the range of 6-10 and that of base alloy is in the range of 10-30 KJ/m2. 112 Ajit Bhandakkar et al.: Fracture Toughness of AA2024 Aluminum Fly Ash Metal Matrix Composites Composite grade AA2024 BASE AA2024-5%FA AA2024-10%FA Table 5. Fracture toughness of AA2024/fly ash composites S (mm) 41.18 41.18 41.18 B(mm) 4.20 4.50 4.24 W(mm) 8.80 9.12 8.80 a(mm) 4.70 5.00 4.92 a/W f(a/W) 0.53 2.97 0.54 3.13 0.56 3.25 Pmax (N) 597 529 445 K(MPa√???????? 21.11 17.41 17.03 Grade AA2024 AA2024 AA2024 FLY ASH % 0 5 10 Table 6. Elastic Plastic fracture toughness of AA2024/fly ash composites Fracture Toughness testing of AA2024 Fly ash composites W (mm) B (mm) L (mm) Initial (a/w) After Pre cracking (a/w) Load (KN) 8.9 4.27 41.18 0.40 0.52 0.541 Jmax 60.54 9.1 4.5 41.18 0.40 0.52 0.554 34.522 8.6 4.15 41.18 0.40 0.53 0.483 16.06 JQ 25.81 15.70 6.69 ΔaQ 0.244 0.20 0.224 Specimens AA2024+0% FA AA2024+5% FA AA2024+10% FA Table 7. Crack tip opening displacement (CTOD) test result Pmax ( KN) 0.668 0.591 0.401 VP (mm) 1.01 0.863 1.515 K (Mpa. m1/2) 27.829 23.141 17.886 CTOD (mm) 0.244 0.200 0.224 Figure 8. Elastic Plastic fracture toughness of Composites - Al-SiC International Journal of Composite Materials 2014, 4(2): 108-124 113 JR TEST REPORT Width : 8.8 mm Thickness : 4.2 mm Net Thickness : 4.2 mm Disp. Inc. : 0.1 mm Loading Rate : 0.01 mm/sec Unloading Rate : 0.005 mm/sec Span : 41.18 mm Specimen ID : 70.00 Date : 15-12-2013 Time : 11:51:08 Modulus : 70 GPa Modulus Correction : 1 Yield Strength : 238 MPa Tensile Strength : 353 MPa Possions Ratio : 0.33 Pre-cracked a/W : 0.534 J (kJ/sq-m) v/s Crack Increment (mm) 60.00 50.00 40.00 30.00 20.00 10.00 0.00 0.000 0.200 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800 JQ = 25.819 kJ/sq-m Jmax = 60.548 kJ/sq-m delta aQ = 0.244 mm JQ qualifies as J1c! Validation Conditions 1) Thickness, B > 25*JQ/Yield Stress 2) Initial ligament, b0 > 25*JQ/Yield Stress 3) Slope of power law regression line, dJ/da, evaluated at delta aQ is less then Yield Stress C:\MTL32\Data\J1C TRAIL 2024-base-1.jrm Figure 9. J- Δa curve of base alloy AA2024 base alloy 114 Ajit Bhandakkar et al.: Fracture Toughness of AA2024 Aluminum Fly Ash Metal Matrix Composites JR TEST REPORT Width : 9.16 mm Thickness : 4.5 mm Net Thickness : 4.5 mm Disp. Inc. : 0.1 mm Loading Rate : 0.01 mm/sec Unloading Rate : 0.005 mm/sec Span : 41.18 mm Specimen ID : Date : 15-12-2013 Time : 12:42:29 Modulus : 70 GPa Modulus Correction : 1 Yield Strength : 141 MPa Tensile Strength : 181 MPa Possions Ratio : 0.33 Pre-cracked a/W : 0.549 J Vs Delta a Power (J max kJ/sq-m) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 JQ = 15.7 kJ/sq-m Jmax = 33.271 kJ/sq-m delta aQ = 0.200 mm JQ is not calculated! Validation Conditions 1) Thickness, B > 25*JQ/Yield Stress 2) Initial ligament, b0 > 25*JQ/Yield Stress 3) Slope of power law regression line, dJ/da, evaluated at delta aQ is less then Yield Stress C:\MTL32\Data\J1C 2024-5-1.jrm TRAIL Figure 10. J- Δa curve of base alloy 2024 + 5% FA Composite International Journal of Composite Materials 2014, 4(2): 108-124 115 JR TEST REPORT Width : 9 mm Thickness : 4.5 mm Net Thickness : 4.5 mm Disp. Inc. : 0.1 mm Loading Rate : 0.01 mm/sec Unloading Rate : 0.005 mm/sec Span : 40 mm Specimen ID : 30.00 Date : 29-12-2013 Time : 14:37:56 Modulus : 70 GPa Modulus Correction : 1 Yield Strength : 120 MPa Tensile Strength : 155 MPa Possions Ratio : 0.33 Pre-cracked a/W : 0.593 J (kJ/sq-m) v/s Crack Increment (mm) 25.00 20.00 15.00 10.00 5.00 0.00 0.000 0.500 1.000 1.500 2.000 JQ = 6.692 kJ/sq-m Jmax = 25.204 kJ/sq-m delta aQ = 0.224 mm JQ qualifies as J1c! Validation Conditions 1) Thickness, B > 25*JQ/Yield Stress 2) Initial ligament, b0 > 25*JQ/Yield Stress 3) Slope of power law regression line, dJ/da, evaluated at delta aQ is less then Yield Stress C:\MTL32\Data\J1C TRAIL 2024-10-2.jrm Figure 11. J- Δa curve of base alloy 2024 + 10% FA Composite 2.500 116 Ajit Bhandakkar et al.: Fracture Toughness of AA2024 Aluminum Fly Ash Metal Matrix Composites The typical load and COD curve of base alloy and its composites are shown in Fig. 12-14. Serires1 Figure 12. load V/s COD of AA 2024 base alloy Series1 Figure 13. load V/s COD of AA2024-5%FA composites International Journal of Composite Materials 2014, 4(2): 108-124 117 Series1 Figure 14. load V/s COD of AA2024-10%FA Composites Figure 15. S-N plot of AA2024 base alloy and composites It may be noted from the Figure.9-11 J V/s ∆a curve for the composite and the base alloy, the AA2024 fly ash composites offers considerable stable crack growth. The load and COD plot shows a typical observation i.e hysteresis loop in loading and unloading compliance curve. This is indicative of crack closure. The reason for crack closure may be surface roughness resulting from fly ash particles in the composites. It increases with the addition of reinforcement however results in observation of JQ from 15.7 KJ/m2 and 6.692 KJ/m2 for the AA2024 fly ash composites 5 and 10wt% respectively. The stable crack growth may be attributed to crack blunting at pores dendrite lobes and surface roughness. These results in reduction in stress intensity at the crack tip, more such sites easing will be growth to fracture and thus reduction in fracture toughness. 3.3. Influence of Fly ash Reinforcement on Fatigue Fatigue is the phenomenon of mechanical property

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