Scripta MATERIALIA PERGAMON Scripta Materialia 45(2001)541-546 www.elsevier.com/locate/scriptamat Aging behavior and tensile properties of 6061A1-0.3 um Al2O3p particle composites produced by reciprocating extrusion Hsu-Shen Chu,Kuo-Shung Liu,and Jien-Wei Yeh* Department of Materials Science and Engineering.National Tsing Hua University,Hsinchu 30043,Taiwan,ROC Received 10 March 2000;accepted 5 January 2001 Abstract 6061A1-0.3 um Al2O3p composites are fully densified from powder compact by reciprocating extrusion. Al2O3 particles are dispersed uniformly in the matrix.The composites exhibit excellent ductility caused by the mechanical kneading of reciprocating extrusion.©2O0 I Acta Materialia Inc.Published by Elsevier Science Ltd.All rights reserved. Keywords:Reciprocating extrusion;Composites;Aging response;Work-hardening Introduction Particle reinforced Al composites have become attractive materials for use in aero- space structures,automobile engineering and semiconductor packaging.These com- posites deserve consideration due to their advanced properties,such as low density, high specific stiffness,strength,and wear resistance [1-3]. The composites can be fabricated by casting processes or powder metallurgi- cal methods.Casting processes such as squeeze casting,compocasting and the vortex method are cost effective.However,they involve serious problems of tensile ductility following from segregation,pores and relatively large particle size.The powder metal- lurgical method introduces particle clustering,interparticle oxide layers and con- tamination in the as-pressed state.These defects cause damage cracks and limit the applications of powder metallurgical products [4,5].Improving the fabrication pro- cessing to make fine particles homogeneously disperse and to remove harmful defects in the composites,is therefore important. Corresponding author. E-mail address:jwyeh@mse.nthu.edu.tw(J.-W.Yeh). 1359-6462/01/S-see front matter 2001 Acta Materialia Inc.Published by Elsevier Science Ltd.All rights reserved. PIL:S1359-6462(01)01055-7
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542 H.-S.Chu et al./Seripta Materialia 45 (2001)541-546 This work develops a reciprocating extrusion process to consolidate 6061Al-0.3 um Al2O3p composites by an effective kneading effect from mixing powders,with a view to eliminating the deleterious defects mentioned above.The reinforced particle distribu- tion and age-hardening behavior of the composites are also considered. Experimental Raw materials were commercial 6061Al powder and 0.3 um Al2O3 powder.6061Al powder was dehydrated in a vacuum at 0.1 Torr and 450C for 3 h before mixing with Al2O3 powder.Powder mixtures with different volume fractions (0%,10%and 20%)of Al2O3 were manually shaken in a bottle.After mixing,the powders were hot pressed into round billets,with a diameter of 40 mm and a length of 50 mm,in air at 300C and at 300 MPa. Billets were deformed by 30 passes at 460C through a reciprocating extruder to achieve full consideration.Fig.1 schematically depicts the reciprocating extrusion ap- paratus.The extrusion ratio was 9.5:1.Two billets in each fabrication were stacked in one container and extruded with a back pressure applied on the opposite ram so that the billet shape in the opposite container was recovered.On the final pass,the billet was directly extruded into a long rod with a diameter of 13 mm by moving away the op- posite ram. The distribution of Al2O3 particles in the as-extruded composites was determined using SEM.Samples were solution treated at 530C for 1 h,water quenched,and then aged.Aging hardness was measured with a Vickers hardness tester under an applied load of 5 kg.Tensile tests for specimens with T6 temper were performed on an MTS 810 tensile machine at a strain rate of 10-3s-1.Tensile strain was measured by an ex- tensometer fixed on the specimen.Differential scanning calorimetry (DSC)was used for precipitation kinetics analysis.DSC samples were heated from 25C to 500C at 10C/ min. Die Container B Container A Ram B Ram A Billet Fig.1.The schematic diagram of the reciprocating extrusion apparatus
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H.-S.Chu et al./Scripta Materialia 45 (2001)541-546 543 Results and discussion Microstructures of composites Fig.2 shows the SEM micrographs of the 6061 alloy powder and Al2O3 particles. 6061 alloy powder exhibited primarily a tear-drop shape,typically produced by air atomization.Al2O3 particles present a faceted morphology.Table I shows the density of extruded 6061 alloy and composites,as measured using the Archimedes principle The table shows that 6061 alloy and composites were fully densified.The percentage a little above 100%may be caused by the contribution of Al2O:from the surface of 6061Al particles,since this was ignored in calculating the theoretical density.Fig.3 shows the SEM micrographs of two composites in the as-extruded condition.The fine Al2O3 particles are distributed uniformly in the matrix.This uniform dispersion of such submicron-sized particles could not be achieved by conventional casting or powder methods,since those processes lack the homogenization resulting from kneading. Age hardening behavior Fig.4 plots hardness versus aging time for the composites of different volume fractions,indicating that the hardness of the composites exceeds that of the 6061 alloy in the as-quenched condition.The increased hardness is due not only to the enhanced dislocation density (due to thermal expansion coefficient mismatch)but also to the increased particle density (Orowan hardening)[6,7].As the aging time increased,the 25μm I um (a) (b) Fig.2.SEM micrographs of 6061 alloy and Al2O;powders:(a)6061 powder,(b)0.3 um Al2O3 particles. Table 1 The densification values of 6061 alloy and extruded composites measured by Archimedes principle Material Theoretical density (g/cm3) Experimental density (g/cm3)Densification value(%) 6061 alloy 2.70 2.72 100.7 10%Al2O3 composite 2.81 2.83 100.7 20%Al2O;composite 2.92 2.93 100.3
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544 H.-S.Chu et al./Scripta Materialia 45 (2001)541-546 5μm 5um (a) (b) Fig.3.The distribution of particles in two composites:(a)10%Al2O;composite,(b)20%Al2O3 composite. 140- 30 ■ 2 (AH)SSOUPIEH 70 6061 alloy -一6061-10vol%Al0 60- 6061-20vol%A10 504 0.1 10 as-quenched Aging time (hour) Fig.4.The aging hardness curves of the 6061 alloy and the composites. aging response of the composites became much smaller than that of the 6061 alloy.A higher Al2O3 particle content resulted in a lower response.Similar observations on degradation of precipitation hardening concerning the Al2O3 reinforced age-hardenable Al composites,have already been presented in the literature [5,8,9].These findings have been shown to relate to the reaction between the precipitation hardening element,Mg, with Al2O3 particles to form the spinel,MgAl,O4.The capacity for precipitation hardening of the composite is correspondingly reduced since the Mg in the matrix is consumed during this reaction.Fig.5 presents the DSC thermograms for 6061 alloy and composites.Two exothermic peaks appear in the 6061 alloy at 245C and 288C The structures corresponding to the exothermic peaks were B"(or GP-ID)and B'phases, respectively [10,11].For the composites,the thermograms were quite smooth with al- most no exothermic peak,implying that the formations of the B"zone and B'phase were depressed.The lack of the B"zone would diminish the hardening capability of the
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H.-S.Chu et al./Seripta Materialia 45 (2001)541-546 545 6061 alloy 6061-10%Al203 ---6061-20%A1203 100150200250300350400450500 Temperature(C) Fig.5.DSC analysis of 6061 alloy and composites.Temperature range:100-450C and heating rate:10C/min. composites since B"zone is the only hardening precipitate in the 6061 alloy matrix. Suppression of the B"zone in composites was considered to be a result of the con- sumption of solute Mg by the formation of MgAl,O4. The tensile properties of composites Table 2 lists the tensile properties of the 6061 alloy and composites.Decreased yield strengths of composites are attributable to the consumption of Mg by the formation of MgAl,O4 as stated in the previous section.However,the composites exhibit a large work-hardening capacity,which is indicated by the large difference between the tensile strength and the yield strength.This difference is accredited to the large incompatibility in plastic deformation of the Al2O3 particles and the matrix [9].The composites also exhibit excellent ductility,and are far superior to the 6061-15 vol%(16 um)Al2O3 and 6061-15 vol%(16 um)SiC composites which have an elongation of 3-4%at a tensile strength about 340 MPa [9,12,13].This occurrence is believed to be due to a minimum of pores,particle clustering or poor interface bonding. Conclusions This work has demonstrated that 6061Al-0.3 um Al2O3p composites can be con- solidated and fully densified from powder compact by reciprocating extrusion.These submicron-sized Al2O3 particles can be dispersed uniformly in the matrix.The ag- ing responses of composites are much lower than that of the 6061 alloy.The DSC Table 2 The tensile properties of 6061 alloy and composites in the T6 condition Material Young's modulus(GPa)Yield strength(MPa) Tensile strength (MPa) Elongation(%) 6061 alloy 69 300 318 18.2 6061-10%Al20385 198 290 10.6 6061-20%Al20398 202 363 8.8
9 ! 9 / + +: 0 I + +: / # / 1 # / 5 / 0 5 6A7 0 # - D ; :-" > "= 0 0 0 + ( L ;K3< ( ;+3< 0 ;+3< 4 ;D< A H H D H- AH A D AH HH &'& # &( ) *+ ,- ) ,.) / -:-
546 H.-S.Chu et al./Scripta Materialia 45 (2001)541-546 thermograms reveal that the formations of the B"zone and B'phase of the composites are depressed,because of the loss of the precipitation hardening element,Mg,reacting with the Al2O3 particles to yield MgAl2O4.The large work-hardening capacity of the composites is attributed to the high plastic incompatibility between Al2O3 and the matrix.The composites exhibit excellent ductility,probably due to the full density and the uniform dispersion of Al2O3 particles caused by the mechanical kneading of re- ciprocating extrusion. References [1]Fishman,S.G.(1986).J Metal 38,26. [2]Nair,S.V.,Tien,J.K..Bates,R.C.(1985).Int Mater Rev 30,275. [3]Webster,D.(1982).Metall Trans A 13A,1511. [4]Lewandowski,J.J.,Liu,C.Hunt,W.H.(1989).Mater Sci Eng A 107,241. [5]Lloyd,D.J.(1991).Acta Metall et Mater 39.59. [6]Arsenault,R.J.,Shi,N.(1986).Mater Sci Eng A 81,175. [7]Barlow,C.Y.,Hansen,N.(1991).Acta Metall et Mater 39,1971. [8]House,M.B.,Meinert,K.C.,Bhagat,R.B.(1991).J Metal 43.24. [9]Chu,H.S.,Liu,K.S.,Yeh,J.W.(2000).Metall Trans A 31A,2587. [10]Dulta,I.,Allen,S.M.(1991).J Mater Sci Lett 10.323. [11]Dulta,I.,Allen,S.M.,Halfey,J.L.(1991).Metall Trans A 22A,2553. [12]McKimpson,M.G.,Scott,T.E.(1989).Mater Sci Eng A 107,93. [13]Lloyd,D.J.(1994).Int Mater Rev 39,1
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