Study on morphology and mechanical properties of stoichiometrically developed Al-Cu-Mg cast alloy

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The present study was carried out to investigate the mechanical properties of newly developed cast aluminum alloy with the (1.0 at. %) of copper and (1.0 at. %) magnesium addition in the mole ratio of 1:1:1 of Al-Cu-Mg in as - cast and thermally aged conditions.

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Nội dung Text: Study on morphology and mechanical properties of stoichiometrically developed Al-Cu-Mg cast alloy

International Journal of Mechanical Engineering and Technology (IJMET) Volume 10, Issue 03, March 2019, pp. 1556–1565, Article ID: IJMET_10_03_156 Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=3 ISSN Print: 0976-6340 and ISSN Online: 0976-6359 © IAEME Publication Scopus Indexed STUDY ON MORPHOLOGY AND MECHANICAL PROPERTIES OF STOICHIOMETRICALLY DEVELOPED AL-CU-MG CAST ALLOY Rakesh Kumar and Varinder Sahni Department of Mechanical Engineering Sant Longowal Institute of Engineering and Technology, Longowal, INDIA ABSTRACT The present study was carried out to investigate the mechanical properties of newly developed cast aluminum alloy with the (1.0 at. %) of copper and (1.0 at. %) magnesium addition in the mole ratio of 1:1:1 of Al-Cu-Mg in as - cast and thermally aged conditions. The atomic weight (at. %) selection of Cu-Mg on the basis of stoichiometric calculations, 1 unit (at. %) of copper and 1 unit (at. %) of magnesium preheated at 200C were mixed in liquid aluminium base material, heated in an electric furnace and melting temperature was kept at 750C for about 10 min alone and this melt was hold at 7300C with alloying additions of copper and magnesium for about 30 min to ensure complete homogenisation, Further, this liquidous aluminium metal matrix was stirred at 800 rpm for five minutes and poured at temperature 7000C 10 0C in permanent mild steel mould preheated at 2000C in order to achieve as- cast aluminium alloy. The solution treatment at temperature 5000C for 2h followed by a quenching in water at 60 °C. Further, it was thermally aged at temperature 1600C for 6 h followed by quenching at room temperature. The effect of solution and aged temperature on mechanical properties by changing metallurgical morphology and further the role of intermetallic compounds on mechanical properties of as-cast alloy have been studied. The optical microscopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) were used to identify the intermetallic phases and formation of different precipitates were studied by using X-ray diffraction (XRD). The improvement in hardness values 35.55% at 1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h and 56.66 % have been recorded and reported at 1% Cu &Mg as-cast alloy with 160 0C,6h. Key words: 6xxx Al alloys, Mechanical properties, Solution treatment Temperature, Thermal Aged temperature Cite this Article: Rakesh Kumar and Varinder Sahni, Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy, International Journal of Mechanical Engineering and Technology 10(3), 2019, pp. 1556–1565. http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=3 http://www.iaeme.com/IJMET/index.asp 1556 editor@iaeme.com
Rakesh Kumar and Varinder Sahni 1. INTRODUCTION Aluminum alloys are next to steels in use as space vehicles, aircrafts as well as many types of surface and water borne vehicles [1]. In automobiles, the components such as engine blocks, head, pistons and wheels etc. are generally aluminum based cast alloys [2]. The low cost and continuous demands for weight reduction and improvements in fuel efficiency have increased pace of research in developing aluminum-based cast alloys [3]. The Al-Cu-Mg alloys offer high hardness and strength. Its components contribute the high degree of damage tolerance [6-8]. The first age-hardening of aluminum alloy as reported in literature was performed by Alfred Wilm in year 1909 who patented duralumin of casting components containing Cu and Mg substances [3]. The steps consist of solution treatment, quenching and artificial aging mechanism is responsible for strengthening is based on the formation of intermetallic compounds during decomposition of a metastable supersaturated solid solution by performing solution treatment and quenching [10-13]. Additionally, hardness and mechanical strength can be increased by aging heat treatment [14-15] The composition of the alloying elements and casting conditions influence the state of intermetallic phases and finally the mechanical properties of the alloy [ 16-19]. To improve the mechanical properties of this aluminium alloys are frequently heat treated by a two-step process i) solution heat treatment and ii) Thermal aging [20]. The copper and magnesium in combination have been used for improving the aging characteristic of the alloys. Some of the investigators have taken compositions of alloying elements in weight fraction or volume fraction but in arbitrary manner. They have used design of experiments for material compositions as input parameters and different mechanical properties as responses, and finally optimum values of alloying elements are suggested for the given objective. In place of taking the fraction of alloying elements in arbitrary manner, a pattern based on stoichiometric weight fraction is explored and presented in the paper. Study has been made on Al-Cu-Mg casting and results in terms of metallurgical and mechanical properties have been presented. Therefore, this study undertakes the effect of the addition of up to1 at.% of Cu-Mg and solution treatment time of 2 h on Al-Cu -Mg alloy to understand the resultant microstructure and its respective hardness. The variation of microstructure, size, morphology and distribution of precipitates, as well as Brinell Hardness are presented and discussed . 2. EXPERIMENTATION 2.1. Methodology Initially to start with stoichiometric ratio of Al-Cu-Mg as 23.496 -55.338 -21.165. Such a high percentage of Cu and Mg cannot be a suitable condition for formation of the Al based alloy, hence 1 (at. %) each of Cu and Mg as per stoichiometric ratio were taken for mixing with the base metal. A graphite crucible of 2-kg capacity was used in electric resistance furnace, and the melting temperature was kept at 750 0C [7] for 5 min. The Cu and Mg as decided above were preheated at 200C for 30 minutes and mixed in liquidous aluminium metal and stirred for five minutes. The melts were hold at 7300C for about 30 min [17] to ensure complete homogenisation and poured after degassing into a permanent mould. It refines the microstructure of metallic material which change the morphology and distribution of intermetallic particulates to enhance the mechanical properties of as-cast aluminium alloy. Finally, metallographic and mechanical properties of the as-cast alloy have been analysed. http://www.iaeme.com/IJMET/index.asp 1557 editor@iaeme.com
Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy 2.2. Metallographic tests The cast specimens were polished as per standard metallographic procedure by using emery paper of progressive finer grades 400,600,800,1000,1200,1500,2200,2500 and 3000 grit size with single disk machine. They were polished with alumina powder to obtain a mirror like surface by solvyt polishing cloth and continue water supply. Kroll’s etchant (Distilled water (92 ml) +Nitric acid (6ml) +Hydrofluoric acid (2ml) was applied for 15 seconds to reveal microstructure. Optical microscopy was used to capture microstructures of as-cast and thermally aged aluminium alloy shown in figure-4. Fractography was done to determine type of fracture for tensile and CVN samples using scanning electron microscopy (SEM) JEOL (JSM-6510LV). 2.3. Testing for mechanical properties Tensile specimen samples were tested on a servo hydraulic based digital controlled tensile testing machine of having capacity 50 kN (make: Tinius Olsen, UK, Model-H50KS) and ultimate tensile strength for as- cast alloy and thermally aged cast base alloys was determined. The tensile specimens were prepared in accordance with ASTM E08/E8M-09 standard as shown in Figure 3. The Charpy V-notch (CVN) test was done to measure the fracture toughness of as-cast and aged as-cast material. The sample for Charpy V-notch test was prepared as per ASTM E23-12c standard and shown in Fig. 2. Micro hardness measurements were taken using Brinell hardness tester (make-: Miraj, Model-B3000(O) with 10 mm diameter indenter at load 500kgf . Figure 1. Test specimen extracted from as-cast plate as per ASTM standard Figure 2. Impact Test Specimen Figure 3. Tensile Test Specimen 3. RESULTS AND DISCUSSION 3.1. Spectro analysis The chemical compositions were tested as per ASTM E 1251-17a using spark emission spectrometer analysis (Model No: LMF-01, Spectromaxx, Make: Germany). The chemical http://www.iaeme.com/IJMET/index.asp 1558 editor@iaeme.com
Rakesh Kumar and Varinder Sahni compositions of as-cast alloy and thermally aged alloy were tested using spectrometer and shown in Table 1. Table 1. Alloying elements composition (wt.%) Si Cu Mg Fe Zn Ni Mn Cr Ti Sn Pb V Al 1% Cu 0.42 0.28 0.5 0.60 0.00 0.00 0.01 0.00 0.1 0.00 0.00 0.00 97.9 &Mg as- 2 07 699 6 56 16 07 27 396 38 41 79 3 cast alloy with solution temperat ure 500 0C,2h 1% Cu 0.47 0.31 0.6 0.64 0.00 0.00 0.01 0.00 0.1 0.00 0.00 0.00 97.7 &Mg as- 3 44 114 2 70 24 09 26 536 51 44 83 5 cast alloy with 160 0C,6h The chemical composition 1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h and 1% Cu &Mg as-cast alloy with 160 0C,6h from spectrometer analysis on addition (1.0 at. %) copper and magnesium in the base material, there is significant development of alloying elements in aluminium metal matrix. 3.2. X-Ray Diffraction (XRD) The X-ray diffraction (XRD) technique was used to show different intermetallic precipitate formations in as-cast and thermally aged alloy which has been observed in XRD spectra as shown in figure 4(a)&(b). In as-cast base aluminium alloy, AlCu compound has less strength and Al12Mg17 compound contributes to hardness of as-cast alloy. However, when aging was done to 160C, formation of Al5Cu6Mg, sigma phase takes place which increases the tensile strength of the material. At the elevated temperature coarse precipitate AlCuMg was also formed. The aging treatment dissolves the AlCu compound and forms complex precipitate compounds which enhance the strength of the matrix. The complex compounds occur and formation of coarse precipitates such as Al2CuMg and Al14Mg13, which enhances the toughness and strength at high temperatures. Sigma phase precipitation was noticed in as cast and aged aluminium alloyed with copper and magnesium which enhances their strength as compared to cast base material. http://www.iaeme.com/IJMET/index.asp 1559 editor@iaeme.com
Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy Figure 4 (a) Figure 4 (b) Figure 4. Intermetallic precipitate formation as per XRD diffraction patterns for (a) 1% Cu &Mg as- cast alloy with solution temperature 500 0C,2h (b) 1% Cu &Mg as-cast alloy with 160 0C,6h. 3.3. Optical Microscopy (OM) studies a b c d http://www.iaeme.com/IJMET/index.asp 1560 editor@iaeme.com
Rakesh Kumar and Varinder Sahni Figure 5. Photomicrographs at 200X for (a) as-cast aluminium, (b) aged as-cast base aluminium, (c) 1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h and (d) 1% Cu &Mg as-cast alloy with 160 0C,6h. This microstructure consists primarily of α-Al with dendritic morphology, eutectic Si, Al- Cu phases. The photographs of microstructure of as-cast aluminium fig.(a), aged as-cast base aluminium fig.(b), as-cast aluminium with 1% Cu, Mg fig.(c) and aged & as-cast aluminium with 1% Cu, Mg fig.(d)are shown in Figure 5. There was negligible precipitate formation along the grain boundaries in as-cast aluminium but after aging, nucleation of precipitates at most of the grain boundaries were noticed which enhances the strength. Alloying with copper and magnesium leads to fine precipitate formation. However, as aging was done, these fine precipitates transform into coarser precipitates at high temperature and nucleation at grain boundaries were enhanced. 3.4. SEM-EDS Analysis The Fe- SEM-EDS samples were analysis for 1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h and 1% Cu &Mg as-cast alloy with 160 0C,6h Mg has been revealed. Element Wt.% At. % Mg K 0.68 0.77 Al K 97.19 98.32 Cu K 2.13 0.91 Totals 100.00 Figure 6 .(a) SEM micrograph, EDS mapping and measurement of chemical composition corresponding to solution treatment at 5000C,2h Element Wt.% At. % Mg K 0.73 0.82 Al K 96.42 97.95 Cu K 2.85 1.23 Totals 100.00 http://www.iaeme.com/IJMET/index.asp 1561 editor@iaeme.com
Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy Figure 6. (b) SEM micrograph, EDS mapping and measurement of chemical composition corresponding to aged temperature at 1600C for 6 h. 3.5. Tensile and microhardness studies Tensile strength & microhardness values of as-cast metal at different stages are shown in Table 2. Their tensile fractography are shown in Fig. 7. dimple morphology was observed in the fractography. In as-cast base metal, small dimples comprising of precipitates were observed but after aging of this as-cast base, the small dimples coalesce together to form large dimples which contain hardened precipitates. These hardened participates contribute to increase in ultimate tensile strength of the aged cast. Copper and magnesium act as inoculants when alloyed with aluminium base metal and induce grain refining in matrix by nucleating more nucleation positions for precipitation. Thus, precipitation hardening was induced in alloyed metal matrix while casting which improves the ultimate tensile strength of cast metal. The resulting improvement of the mechanical properties after heat treatment is correlated to the interdendritic and the precipitation hardening as previously reported. Table 2. Hardness and ultimate tensile strength values S. as-cast alloy stage Area of Ultimate tensile strength Hardness No. tensile test (N/mm2) (BHN) specimen, mm2 1. as-cast base aluminium 36 140 33.5 2. 1% Cu &Mg as-cast alloy 36 157 52.80 with solution temperature 500 0C,2h 3. 1% Cu &Mg as-cast alloy 36 192 77.30 with 160 0C,6h a b Fine dimple formation Coarse dimple formation Figure-(a) Cast Base Metal at 500X Figure-(b) Aged cast base Metal at 500X c d http://www.iaeme.com/IJMET/index.asp 1562 editor@iaeme.com Fine dimple formation Fine dimple formation
Rakesh Kumar and Varinder Sahni Figure 7. Tensile fractography at 500X for (a) as-cast aluminium, (b) aged as-cast base aluminium, (c) 1% Cu &Mg as-cast alloy with solution temperature 500 0C,2h and (d) aged as-cast base Aluminium with 1% Cu, Mg. 3.5. Impact studies The results of impact strength in terms of joules of energy to estimate fracture toughness of cast materials was shown in Tab.3. and their impact fractography are shown in Fig. 8. Table 3. Impact strength values of cast base and Metal matrix alloy in as cast and aged condition S. No. as-cast alloy stage Impact Strength (joules) 1. as-cast base aluminium 15 2. Aged as-cast base aluminium 18 3. 1% Cu &Mg as-cast alloy with 34 solution temperature 500 0C,2h 4. 1% Cu &Mg as-cast alloy with 160 14 0 C,6h b a Coarse dimple formation Figure-(a) as-cast base Metal at 500X Figure-(b) Aging cast base Metal at 500X c d Fine dimple morphology in metal matrix alloy Figure-(c) 1% Cu &Mg as-cast alloy with solution Figure-(d) 1% Cu &Mg as-cast alloy with 160 0C,6h temperature 500 0C,2h http://www.iaeme.com/IJMET/index.asp 1563 editor@iaeme.com
Study on Morphology and Mechanical Properties of Stoichiometrically Developed Al-Cu-Mg Cast Alloy Figure 8. Impact fractography at 500X for (a) as-cast aluminium, (b) aged as-cast base aluminium, (c) as-cast aluminium with 1% Cu, Mg and (d) aged as-cast base Aluminium with 1% Cu, Mg. In cast base metal, dimple fracture was noticed which corresponds to ductile fracture. When alloyed with copper and magnesium, cast aluminium base metal possess very small dimples (Fig. 8.c&d) because of refinement of grain structure of base metal which ultimately improves the fracture toughness of material. 4. CONCLUSIONS  When copper and magnesium were added in as-cast base aluminium change in wt.% were noticed as per spectroscopy analysis. These elements enhance intermetallic precipitate formation at the grain boundary of cast base aluminium alloy.  Significant improvement in tensile strength, impact strength and microhardness were observed in copper and magnesium alloyed as-cast alloy and thermally aged condition. The hardness values improved 35.55% BHN for 1% Cu &Mg as-cast alloy with solution temperature 500 0 C,2h 36.55% and increased 56.66% BHN for 1% Cu &Mg as-cast alloy with 160 0C,6h. This is because of increase in size of dimples and increased nucleation of precipitates in aluminium alloy. ACKNOWLEDGEMENTS This work was carried out with the financial support from Sant Longowal Institute of Engineering and Technology, Longowal under Ministry of Human Resource Development, Government of India. The XRD facilities of IIT Ropar and SEM facilities of IIT Roorkee &SAIL, Thapur University Patiala were used to record metallurgical morphology. REFERENCES [1] Davis, J.R, Aluminium and aluminium alloys, ASM International,2001. [2] John R. Brown, Foseco Non -ferrous foundry’s handbook, eleventh edition,1999. [3] Cui, S., Mishra, R., & Jung, I. Thermodynamic analysis of 6xxx Series AL Alloys: Phase Fraction Diagrams. Journal of mining and metallurgy, Section B: Metallurgy,54(1) B ,2018,119-131. https://doi.org/10.2298/JMMB170512052C [4] Polmear, I. J. Aluminium alloys-A century of Age Hardening, Materials forum volume 28,2004. [5] Medrano-prieto, H. M., Garay-reyes, C. G., & Gómez-esparza, C. D. Evolution of Microstructure in Al-Si-Cu System Modified with a Transition Element Addition and its Effect on Hardness, 19, 2016,59–66. http://dx.doi.org/10.1590/1980-5373-MR-2015-0673 [6] Beroual, S. Boumerzoug, Z. Paillard, P. & Borjon-piron, Y. Effects of heat treatment and addition of small amounts of Cu and Mg on the microstructure and mechanical properties of Al-Si-Cu and Al- Si-Mg cast alloys. Journal of Alloys and Compounds, 784, 2019,1026– 1035. https://doi.org/10.1016/j.jallcom.2018.12.365 [7] Alfonso, I., Maldonado, C., Gonzalez, G., & Bedolla, A.Effect of Mg content and solution treatment on the microstructure of Al-Si-Cu-Mg alloys. Journal of Materials Science, 41(7), 2006,1945–1952. https://doi.org/10.1007/s10853-006-4494-y [8] Hutchinson, C.R., Ringer, S.P.Precipitation processes in Al-Cu-Mg Alloys Micro alloyed with Si, Metallurgical and Materials trans sections A, 31A ,2000,2721- 2733.https://doi.org/10.1007/BF02830331 [9] Lombardi, A., Sediako, D., Ravindran, C., & Barati, M. Analysis of precipitation , dissolution and incipient melting of Al 2 Cu in B206 Al alloy using in-situ neutron http://www.iaeme.com/IJMET/index.asp 1564 editor@iaeme.com
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