In this study, Al-8Fe-8Si-1.4V/SiCp composites fabricated by squeeze casting process were age-hardened to Study the influence of heat treatment on mechanical properties, such as hardness, bending strength and modulus of elasticity. The cast samples were solid sоluted at 540 °C for one hour, then quenched in water to room temperature, and finally aged at 190 °C for 2, 4, 6, 8 and 10 hours for hardness test and at 195 °C for 2, 6 and 10 hours for bending strength determination

Essari, A.

Petrović, Z.

Samardžić, I.

Sariuglo, F.

Sedmak, Aleksandar

English

machinery

189METALURGIJA 55 (2016) 2, 189-192A. ESSARI, F. SARIUGLO, Z.PETROVIC, A.SEDMAK, I. SAMARDŽIĆEFFECT OF AGING ON MECHANICAL PROPERTIES OF Al-8Si-8Fe-1.4V/SiCp COMPOSITESReceived – Primljeno: 2015-04-25Accepted – Prihvaćeno: 2015-10-20Preliminary Note – Prethodno priopćenjeA. Essari, Elmergb University, Libya, F. Sariuglo, Middle East Tech-nical University (METU), Ankara, Turkey, Z. Petrovic, A. Sedmak, Faculty of Mechanical Engineering, University of Belgrade, Serbia, I. Samardžić, Faculty of Mechanical Engineering, University of Osijek, CroatiaIn this study, Al-8Fe-8Si-1.4V/SiCp composites fabricated by squeeze casting process were age-hardened to study the influence of heat treatment on mechanical properties, such as hardness, bending strength and modulus of elas-ticity. The cast samples were solid sоluted at 540 °C for one hour, then quenched in water to room temperature, and finally aged at 190 °C for 2, 4, 6, 8 and 10 hours for hardness test and at 195 °C for 2, 6 and 10 hours for bending strength determination. Key words:  Al alloy, die custing, heat treatment, mechanical properties, harness test, bending testINTRODUCTIONCast aluminum alloys are used extensively for mak-ing various engineering products due to their light weight and high strength to weight ratio. The principle alloying elements in Al based cast alloys are Si, Cu, Zn, Mn, Cr and Ti. These alloys are used up to the tempera-ture 453 K. Beyond this temperature the mechanical properties deteriorate due to coarsening of precipitates. An Al transition metal system has the potential to with-stand relatively high temperatures and is likely to be suitable for use in the aerospace and automobile sectors. In particular, Al-Fe-Si-V alloys have attracted consider-able interest since they maintain strength at tempera-tures higher than other Al-Fe-X system alloys.  Excel-lent high temperature mechanical properties of Al-Fe-Si-V alloys are attributed to the high volume fractions of ultrafine cubic silicon intermetallic phase, Al12(Fe,V)3Si, and its slow coarsening rate [1]. As the percentage of ceramic increases, composite displays more and more properties of ceramic, like improved stiffness and decreased ductility. As the volume of ceramic increases, the amount of matrix between particles decreases. The ability of ma-trix to deform is the basis of ductility of composite. Ductility of the unreinforced matrix in the case of alu-minum can be over 10 %, as measured by strain to fail-ure of test samples, [2]. As the percentage of reinforce-ment increases to 40 %, the elongation decreases to less than 2 %, [3]. The objective of this study is to evaluate the influence of heat treatment on some mechanical properties of SiC particulate reinforced aluminum alloy. ISSN 0543-5846METABK 55(2) 189-192 (2016)UDC – UDK 669.715:621.747:621.78:620.17:620.178.1=111Hardness and three-point bending tests were conducted on metal matrix composites with SiC particles of differ-ent amounts. Results were compared with the non-rein-forced aluminum matrix alloy.EXPERIMENTAL PROCEDUREMaterials and production processesThe nominal chemical composition of the matrix was approximately Al-8Fe-8Si-1.4V by wt %. The composites produced contained 4 % and 7 % SiCp. The volume fraction of SiCp in the composites was deter-mined experimentally by the image analyzer. Particles of SiC having diameter 2 9 ± 1,5 m, were produced by KLA Exalon, Norway. The alloys were modified by adding nearly 1,5 % Mg, and melted in an induction furnace using master al-loys of Al–Si and Fe-V. After melting, sufficient time was given for homogenization of the melt. The molten alloy was degassed, and then SiC particles were added into the alloy at its mushy state. Molten Al alloys were poured into a mold specially designed for vertical filling squeeze-casting machine (Figure 1).Bending and tensile test specimens were produced simultaneously in this mold. During squeeze casting the applied pressure was 80 MPa. The alloys were heat-treated to T6 condition that is solid saluted at 540 °C for Figure 1 Squeeze/Pressure casting die190A. ESSARI et al.: EFFECT OF AGING ON MECHANICAL PROPERTIES OF AL-8SI-8FE-1.4V/SICP COMPOSITES METALURGIJA 55 (2016) 2, 189-1921 h / water quench + aging up to 10 h. Aging tempera-ture was 190 °C ± 5 °C.Bending TestBending strength of cast composite specimens, pro-duced by squeeze/pressure die casting machine, was evaluated by the three point bending test. The number of tested specimens was three samples for each case. Dimensions and shape of three point bending test speci-men were: Span length = 50 mm, width = 10 mm, height= 5 mm. The three point bending fixture of the tensometer device was used for these tests.Brinell Hardness TestHardness measurements were carried out according to ASTM standard E10-84, standard test method for Brinell hardness. All tests were performed with 2,5 mm diameter ball using31,25 kg loads. On each sample av-erage of eight measurements were taken.The number of tested specimens was 4-6 samples for each case (different aging times and different SiCp content).Hardness levels for materials obtained by HB tests are given in Table 1 and Figure 2.Table 1 Average Brinell hardness values / HBAging time / hMatrix Matrix + 1,5 % Mg + 0 % SiCMatrix + 1,5 % Mg + 4% SiCMatrix + 1,5 % Mg + 7% SiC0 66 83 99 972 62 87 126 1074 58 100 129 1276 70 119 131 1258 67 122 128 13010 59 119 128 128For instance, the 6 hr aging matrix without Mg showed an average hardness value of 70 HB. With Mg addition, hardness improved to 119 HB which is 70 % increase. However, with 4 % SiC addition, for the same aging time, hardness increased to 131 HB correspond-ing to approximately 10 % increase compared to the matrix with Mg. It seems that Mg addition to the matrix is more effective factor than SiC. One should also no-tice a small decrease in hardness (125 HB) as SiC con-tent is increased to 7 %.Figure 2 shows that effect of SiC content and aging time on hardness values. For unreinforced and unmodi-fied matrix there was no significant change in the hard-ness level with increasing aging time because the amount of hard phases and precipitates present were not significant to improve the hardness. The hardness of Mg modified matrix, significantly increased with in-creasing of aging time because of the nucleation of new hard phases like Mg2Si and increase in dislocation den-sity. Hardness increased for 2, 4, 6 and 8 h of aging time because of the increase in dislocation density, and then decreased at over aging (OA). This reduction in hard-ness is attributed to the increased size (coarsening) of precipitates which led to the decreased dislocation den-sity, and increase in incoherency between matrix and particles. To investigate the effective hardness of the composite by means of the rule of mixtures (ROM), fol-lowing relations are used: H̄up = Hh  fh + Hs  fs (1) H̄ low = (fh / Hh  + fs / Hs  )-1 (2)where Hh and Hsare the hardness values of hard and soft phases, and fh and fs are the volume fraction of hard and soft phases, respectively. The subscripts “up” and “low” represent the upper bound and lower bound of hardness.In Figure 3, the calculated results are compared with the measured results, indicating reasonable values since most of the measured points lie between the upper and lower bounds. Figure 2  Effect of aging time and SiC content on the hardness behaviour (1) Matrix, (2) Matrix with 1,5% Mg, (3) Matrix with 1,5 % Mg and 4 % SiC, (4) Matrix with 1,5 % Mg and 7 % SiCFigure 3  Comparison of measured and calculated hardness values191A. ESSARI et al.: EFFECT OF AGING ON MECHANICAL PROPERTIES OF AL-8SI-8FE-1.4V/SICP COMPOSITESMETALURGIJA 55 (2016) 2, 189-192Bending strengthThe average of bending strength values, measured by three point bending tests, are given in Figure 4, indi-cating that there is no significant change, as SiC content increases from 0 to 7 %. Peak strength is observed at two hours aging (also shown in Figure 5), since the strength increased sharply within first two hours, and then decreased gradually as a result of over aging phe-nomenon. The early peak strength due to aging may be attributed to the relatively high aging temperature which accelerates the dislocations, and nucleation of hard phases.Modulus of elasticity:The modulus of elasticity, E, was determined using the force deflection equations formulated for three point bending conditions as follows:  (3)where P is load / N;L span / mm; y mid-point deflection / mm; b width / mm; and t height / mm. Measured values are shown in Table 2, for different content of SiC (in Volume percent – Vp) and different aging times.Table 2   Modulus of elasticity / GPa – measured values for matrix with 1,5 % MgAging time / hrSiC Vp / % 0 2 6 100 48,2 52,8 51,7 49,34  58,0 60,2 59,1 50,37 41,7 44,8 37,9 43.6The modulus of elasticity of Al/SiCp composites in-creased as the volume fraction of SiC increased. The Hashin - Shtrikman model (H-S model) was used to predict the modulus of Al/SiCp composites, [3]. In this model, the modulus is expressed as:  (4) where index “m” stands for matrix, “p” for particle, and “c” for composite.Another equation was proposed by Halpain and Tsai (H-T), [3]:        (5)        (6)where s is the aspect ratio of SiC particles. The average value, s = 1,72, was measured from Figure 6, s = c/a, where c is longer axis and a is shorter axis.Applying the isostress condition of ROM for modu-lus of elasticity:     (7)Figure 4  Relationship between strength and SiC content with increasing in aging timeFigur e 5  Flexure strength related to aging time with different SiC contentsFigure 6  SEM photograph of SiC particles (Magnification 1 000 X)192A. ESSARI et al.: EFFECT OF AGING ON MECHANICAL PROPERTIES OF AL-8SI-8FE-1.4V/SICP COMPOSITES METALURGIJA 55 (2016) 2, 189-192All three models are used to calculate the modulus of elasticity, as shown in Table 3, together with the measured results. Table 3 indicates that ROM is the nearest model to the measured values. For 7 % SiC con-tent the modulus decreased in an unexpected way to reach 59,5 % of its value by ROM. This trend may be attributed to data scatter or increased porosity of speci-mens with higher SiC content Figure 7. The H-S and H-T models overestimate the modulus of elasticity of composites compared to ROM. Table 3  Modulus of elasticity values with respect to SiCp content (Vp) and aging timeVp/ %H-S /GPaH&T /GPaROM /GPaMeasured/GPaMeasured/ ROM0 69 69 69 51,2 74,30 %4 73,2 82,7 71,4 56,6 79,20 %7 76,5 93,7 73,4 43,6 59,50 %Figure 7 shows one of the tested specimens with high porosity. Existence of some cavities is also ob-served. These cavities may have formed as a result of entrapped gases during casting process.CONCLUSIONSAppreciable improvement in hardness levels was ac-hieved by age hardening in modified and reinforced mat-rix. This improvement increased with addition of Mg and also with increased SiC content in the matrix. By compa-rison of measured values with calculated data by ROM, the experimental results were found reasonable. Bending strength improved significantly by appl-ying heat treatment to the modified and reinforced mat-rix, but no obvious trend between bending strengthand SiC content was observed.For modulus of elasticity, the comparison of the mea-sured results and the calculated data, using three different models, indicates that the closest model for this case is ROM. The obtained results were satisfied for matrix with 0 % and 4 % SiCp, but for matrix with 7 % SiCp the mo-dulus decreased, as a result of high porosity. The effect of aging obviously appeared on the aver-age of modulus at peak aging and decreased at over ag-ing. In other words, it follows the same trend of bending strength.REFERENCES[1]  K. L. Sahoo, C. S. Sivaramakrishnan, A. K. Chakrabarti. (2000). Some studies on Al-8.3Fe-0.8V-0.9Si Alloy. Journal of Material Processing Technology 135 (2003) 253-257.[2]  M. Burzić, S. Sedmak, Z. Burzić, D. Jaković, D. Mom-čilović, The Effect of Reinforcement Content on Impact Toughness of Cast Al-SiCp Metal Matrix Composite(in Serbian), Structural Integrity and Life 2 (2002) 11-14.[3]  M. Schwarts.Composite materials, I (1996) 44-48.[4]  C. Barile, C. Casavola, C. Pappalettere, F. Tursi, RFI Com-posite Materials Behaviour,Structural Integrity and Life 10 (2010) 209-213.[5] M. Burzić, Z. Radaković, M. Manjgo, Z. Burzić, The Ef-fect of Fiber Orientation on Fracture Properties of Glass Fiber- Epoxy Composite, Structural Integrity and Life 12 (2012) 35-38.Note:  Responsible person for English translation is prof. Martina Šuto (J.J. Strossmayer University of Osijek) and prof. Marina KaršićFigure 7  Fracture surface of tested specimen with high porosity and cavities

Effect of aging on mechanical properties of Al-8Si-8Fe-1.4v/SiCp composites

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Effect of aging on mechanical properties of Al-8Si-8Fe-1.4v/SiCp composites

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