- Email: [email protected]
Effect of Al-15Zr Master Alloy and Extrusion Process on Microstructure and Mechanical Properties of Al-6%Mg Alloy
Available online at www.sciencedirect.com
ScienceDirect Procedia Materials Science 11 (2015) 438 – 443
5th International Biennial Conference on Ultrafine Grained and Nanostructured Materials, UFGNSM15
Effect of Al-15Zr Master Alloy and Extrusion Process on Microstructure and Mechanical Properties of Al-6%Mg Alloy H. Hosseinya ,M. Emamya, G. Ashuria,* a
School of Metallurgy and Materials, University of Tehran, Tehran, Iran
Abstract In current research different amounts of Al-15Zr master alloy were added to Al-6%Mg alloy to obtain various Zr concentrations (0.01, 0.03, 0.05, 0.1, 0.2, 0.3 % weight percent) . The microstructural study was carried out on Al-6%Mg extruded alloy. It was found that with the increase of Al-15Zr master alloy, Al (α) matrix morphology was changed from dendritic to equiaxed grains due to the suppression of grain growth by the formation of homogeneous Zr containing disperoids at grain boundary. The grain size of as-cast alloys was significantly decreased by adding Al-15Zr master alloy to same extent. An increase in the amount of Zr content resulted in improvement of ultimate tensile strength (UTS) and elongation (%El) values. Fractographic examinations of the alloy showed a ductile mode of fracture via introducing more homogenous fine dimples after grain refining process. © 2015Published The Authors. Published Ltd. © 2015 by Elsevier Ltd. by ThisElsevier is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of UFGNSM15. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of UFGNSM15 Keywords: Al-15Zr grain refiner; microstructure; ultimate tensile strength; fracture surface.
1. Introduction Al-Mg base alloys are one of the main class of light-weight alloys, with high strength and good corrosion resistant properties that are used in welded applications, and transportation for making dump truck bodies, large tanks for carrying petrol, and pressure vessels, particularly where cryogenic storage is involved, Polmear (1995) Refining of the grains by grain refiners, precipitation and solid-solution-hardening are the most important techniques for increment of the strength of Al alloys and also creating a microstructure with uniform distributed of fine equiaxed or even ultrafine grains, Toropova et al. (1998), Polmear (1995). An equiaxed fine grain performs a serious
* Corresponding author. Tel.: 09376486506 ; fax: 09376486506. E-mail address: [email protected]
2211-8128 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of UFGNSM15 doi:10.1016/j.mspro.2015.11.034
H. Hosseiny et al. / Procedia Materials Science 11 (2015) 438 – 443
duty in enhancing mechanical properties, ameliorates feeding process to remove shrinkage porosity and distribution of intermetallic particle throughout the casting and amended surface finish, Gao et al. (2013), Ghadimi et al. (2013).The binary Al-Mg phase diagram is shown in Fig. 1a. Al-6% Mg is known as a high strength alloy in Al-Mg series. Al–6%Mg alloy is a wrought commercial alloy and has attracted great attention of many researchers and scientists. However, Al-6%Mg alloys are commonly suffered from softening effect during its usage and these series of aluminum alloys are used in a work-hardened condition. Extruded of Al–6%Mg–Zr alloy is expected to lead to the precipitation of Al3Zr and as a result, the microstructure will contain fine dispersions of Al3Zr precipitates within the recrystallized grains. Zr exploits peritectic reaction with aluminum to form metastable with LI2 structure aluminides phases, Lee et al. (2002). Al3Zr particles are usually coherent with the matrix and thermally stable because of their high melting points. Because of a few data regarding to the grain refining of Al–Mg alloys. It merits a further investigation on these alloys. Therefore, in this research, the grain refining behavior of extruded Al–6%Mg alloy will be studied by using Al-15Zr master alloy. Nomenclature UTS DRX DRV
Ultimate tensile strength Dynamic recrystallization Dynamic recovery
2. Experimental procedure Industrially materials were used as starting materials for preparing primary ingots and extruded rod of Al-6Mg alloy. For this purpose pure Al (99.8%) and Mg (99.9) were melted in a 10 kg SiC crucible via an electrical resistance furnac, In order to prepare alloys with different content of Zr during preparation of small billets the weight losses of Mg were selected to be 15%. The Al–15Zr master alloy was appended to the molten metal at 760°C to provide various amounts of Zr (0.01, 0.03, 0.05, 0.1, 0.2, and 0.3 wt. %) in the cast specimens. Finally, after stirring process of the molten metal and cleaning off the dross, for all tests, after a period of 5 min the melt was poured into a cylindrical permanent iron mold as shown in Fig. 1b. Before extrusion process, cast billets were homogenized at 500°C for 10. Cylindrical castings were cut into billets with 30 mm in diameter and 30 mm in heights in order to fit into the extrusion container. These billets were preheated to 500 °C for 1 h and then extruded using a hydraulic press at a ram speed of 1 mm/s with the extrusion ratio of 6:1. Extrusion process was carried out applying graphite based oil between metal, die and container. Round tensile samples were machined along the extrusion direction according to ASTM: E8/E8 M-11. The sketch of a tensile test specimen is seen in Fig. 1c. Tensile tests were carried out using a computerized testing machine (SANTAM STM-20) at a strain rate of 0.1mm/min at room temperature. In order to reveal the microstructure, the cut sections were polished and then etched by Keller’s reagent (2 mL HF, 3 mL HCl, 5 mL HNO3 and 190 mL H2O).
Fig. 1. (a) Al-Mg phase diagram; (b) schematic of mold iron; and (c) tensile test specimen.
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3. Results and discussion 3.1. Macro and microstructural evolution Figure 2 illustrates the macrostructures of Al–6%Mg cast alloy after grain refining by different concentrations of Zr. It can be seen that the grains obtained from Al–6%Mg alloy specimens after grain refining process are much finer in comparison with nonrefined alloy. Figure 3 also shows the microstructures of Al–6%Mg alloy after grain refining, homogenising and hot extruding process by different contents of Zr. The microstructures of Al–6%Mg alloy are shown in Fig. 3a. According to Al– Mg binary phase diagram, the Al–6%Mg alloy contains primary a-Al phase and eutectic structure in other words ߚAl3Mg2 intermetallic and a-solid solution phase are formed. Also, the presence of iron and silicon impurities may result in the formation of such phases as Al 15 (Fe, Mn)2 Si3 phases or insoluble Mg2Si particle, which possesses more favourable skeletal morphology, Zolotorevsky et al. (2007). It is clear from Fig. 3a, 3d and 3g that the grain refining process elevated the number of grain boundaries. By adding Al-15%Zr master alloy not only the grain size decreases, but also the columnar dendrite morphology changes to rosette-like, Fakhraei and Emamy (2014). The mechanism of grain refinement by Al-15%Zr master alloy is related to the formation of Al3Zr particles in the microstructure. Primary Al3Zr particle precipitate from the melt during solidification and refine the grain size of the alloy, Kashyap and Chandrashekarbull (2001) , Yin et al. (2000). Figure 3b, 3e and 3h illustrates the homogenized Al-6%Mg alloy. During solidification process of Al cast parts, much of alloying elements segregate from liquid metal and result in an inhomogeneous distribution of large particles and also formation of constitutive particles on grain boundary regions or inside the grains, Robson and Prangell (2001). Coarse residual particles have deteriorated effect on the extrudability of Al alloys, Rokhlin et al. (2004) High mechanical and extrudability properties are achieved, if these particles dissolve in the structure. In other hand applying homogenization process, results in the removal of the inhomogeneous distribution and dissolving the second-phase particles of alloying elements on a micro-scale causes the disposed that are able to inhibit recrystallization zone generate (like Al3Zr). Considering the effect of such intermetallics on grain boundary pinning which results in finer grain size and consequently higher mechanical properties, Robson and Prangell (2001). Figure 3c, 3f and 3i illustrates the microstrutures of extruded alloy that consists of fine and equiaxed grains with a fully recrystallized microstructure and the relatively no non-recrystallized grains elongates in extrusion direction. The variation of average grain sizes of Al-6%Mg alloy with different concentration of Zr, before and after extruding process are shown in Fig. 4. The microstructures of Al–6%Mg extruded alloy illustrate refined precipitates distributed at dynamically recrystallized grain a boundary which clarifies the pinning effect of precipitates for the grain boundaries (Fig. 3c). During the extrusion of some metals such as aluminum alloys, a fibrous microstructure (consisting of original as-cast grains elongated in the extrusion direction) is commonly developed due to the operation of dynamic recovery (DRV). In Al alloys, however, dynamic recrystallization (DRX) occures during hot deformation and a fine-grained microstructure is progressively developed. There are more precipitates in the Al6%Mg extruded alloy after refining by Al-15Zr master alloy as expected Al3Zr intermetallic particle prevents dynamically recrystallized grain growth; however they are coarser than that of primary extruded alloy (Fig. 5f-i). Thus, a finer dynamically recrystallized grain structure is obtained. In other words, the amounts of recrystallized grains are increased with increasing the Al-15%Zr grain refiner. Recrystallization behavior of aluminum alloys show that the size and distribution of particles has important effects on the structure of recrystallized grains. Basically, the particles can act in two ways. Coarse particles can act as nucleation sites for crystallized grains. The presence of these particles can be create strain in the deformation process and therefore will be a good place for the formation of new nucleation for dynamic recrystallization during hot deformation. On the other hand fine particles can be pin the grain boundary by Zener phenomenon, and reduce the growth rate of recrystallized grains during hot deformation process. These fine particles can prevent recovery procces, Gandhi and Raj (1991), Mayo et al. (1990).
H. Hosseiny et al. / Procedia Materials Science 11 (2015) 438 – 443
Fig .2. Macrostructures of Al–6%Mg alloy with different concentration of Zr (a) 0%; (b) 0.01%; (c) 0.05% ;(d) 0.1%; and (e) 0.3% Zr.
Fig . 3. Metallographic images of (a) 0 % Zr and (g) 0/05 % Zr and (d) 0/3 % Zr as cast, (b) 0 % Zr and (e) 0/05 % Zr and (h) 0/3 % Zr homogenized and (c) 0 % Zr , (f) 0/05 % Zr and (i) 0/3 %Zr extruded Al-6Mg alloys.
Fig. 4. Average grain-size of the Al–6%Mg alloy with different amount Zr, before and after extruding process.
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H. Hosseiny et al. / Procedia Materials Science 11 (2015) 438 – 443
3.2. Mechanical properties Figure 5 illustrates UTS and elongation results it is seen that UTS increases with increasing Zr concentration. Al15%Zr master alloy enhances the UTS values of the Al-6%Mg extruded alloy from 335 MPa to maximum 381 MPa, and the elongation values of the alloy from 26 % to maximum 31%. As mentioned above, Zr has a refining effect on the microstructure of as-cast alloy and it forms Al3Zr intermetallics particle .According to the results achieved from structural studies, also, by adding Al-15%Zr to the Al–6%Mg alloy a structure with finer grains and Al3Mg2 intermetallics is obtained. Dynamic recrystallization during hot deformation cause extremely fine grains due to more grain boundaries of initial structure which perform as nucleation sites for dynamically recrystallized grains , Taku et al. (2014). Thus improving mechanical properties mainly attributed to the refinement effect of microstructure. Reduction interdendritic segregation between dendritic structure by homogenizing process before extrusion, the loss of porosity in the structure of the extrusion process and distribution of fine secondary particle phase in the structure, Mulazimoglu et al. (1996).
Fig. 5. (a) UTS; (b) Elongation valuse of Al-6%Mg extruded alloy.
3.3. Fractography Figure 6 reveales the fracture surfaces of the extruded alloy in Zr free and 0.2% Zr added condition. In both conditions fracture surfaces revele several fine dimples. In normal ductile fracture three stages are seen: (i) void initiation, which usually occurs at second-phase particles or inclusions, (ii) void growth and (iii) hole coalescence through the separation of the ligaments, which links the growing voids ,ASM Handbook (1992) However in Fig. 6 two classes of dimples are observed, namely the large-sized dimples caused by the coarse intermetallic particles or the grain boundary precipitates, and the high density small-sized dimples nucleated in the grain interiors.
Fig. 6. SEM micrographs of fractured surface of (a) Al–6%Mg; (b) Al-6%Mg-0.2 % Zr extruded alloy.
H. Hosseiny et al. / Procedia Materials Science 11 (2015) 438 – 443
4. Conclusions In current research the effects of different concentrations of Al-15%Zr master alloy on the structure evolution and mechanical behavior of Al-6%Mg extruded alloy were studied and the following conclusions can be drawn: 1. The grains are refined by Zr addition and extrusion procces Zr addition reduces the average grain size of Al6%Mg cast alloy from 782 μm to 351μm. And extrusion procces reduces from 208 μm to 174 μm by increasing Zr content. 2. UTS and elongation values of the Al-6Mg alloy afterZr addition and extrusion improved from 335 MPa and 26% to maximum 381 MPa and 31%, respectively. 3- Fractography examination of the Al-6%Mg refined extruded alloy shows more ductile fracture by Al-15Zr addition.
Acknowledgements The authors would like to thank the University of Tehran for their moral and financial support. References ASM Handbook, 10th ed., 1992.vol. 14, Metal Forming, Asm international, Metals Park, OH,. Fakhraei. O, Emamy. M, 2014. Effects of Zr and B on the structure and tensile properties of Al–20%Mg alloy, Materials and Design 56 557–564. Gandhi C., Raj R., 1991. A model for subgrain superplastic flow in aluminum alloys, Acta metall. Mater, 39(4), pp. 679-688. Gao Z., Li H., Lai Y., OuandY., Li D., 2013. Effects of minor Zr and Er on microstructure and mechanical propertiesof pure aluminum Mater. Sci. Eng., A 580 92–98. Ghadimi H., Nedjhad S. H. and Eghbali B., 2013.Enhanced grain refinement of cast aluminum alloy by thermal and mechanical treatment of Al5TiB master alloy Trans. Nonferrous Met. Soc. China 231563-1569. Kashyap KT, Chandrashekarbull T., 2001, Effects and mechanisms of grain refinement in aluminum alloys. J Mater Sci; 24: 345–53. Lee S., Utsunomiya A., Akamatsu H., Neishi K., Furukawa M., Horita. Z and Langdon. T. G, 2002. Influence of Scandium and Zirconium on Grain Stability and Superplastic Ductilities in Ultrafine-Grained Al-Mg Alloys, Acta Mater, Vol. 50, pp. 553-564. Mayo M. J., Kobayashi M., Wadsworth. J, 1990. Superplasticity in Metals, Ceramics and Intermetallics, Pittsburgh, PA. Mulazimoglu M. H., Zaluska A., Gruzleski J. E., Paray F, 1996. Electron microscope study of Al-Fe-Si intermetallics in 6201 aluminum alloy”, Metal. Mater. Trans. A 27 929-936. Polmear I.J., 1995. Light alloys: metallurgy of the light metals, 3rd edition, Arnold, London, pp131-235. Robson J.D. ,Prangell , 2001. Dispersoid precipitation and process modeling in Zirconium containing commercial aluminium alloys, Acta mater. 49 599–613. Rokhlin L.L., Dobatkina T.V., Bochvar, Lysova N.R., and E.V, 2004. Investigation of Phase Equilibria in Alloys of the Al-Zn-Mg-Cu-Zr-Sc System, J. Alloys Compd, , 367, p 10-16. Taku S., Andrey B. , Rustam K., Hiromi M. ,J. Jonas J., 2014.Dynamic and post-dynamic recrystallization under hot, cold and severe plastic deformation conditions, Progress in Materials Science 60 130–207. Toropova L.S., Eskin D.G., Kharakterova M.L. & Dobatkina T.V., 1998. Advanced Aluminum Alloys Containing Scandium, Structure and Properties Baikov Institute of Metallurgy, Moscow, Russia. Yin Z., Pan Q., Zhang Y., Jiang F., 2000. Effect of minor Sc and Zr on the microstructure and mechanical properties of Al–Mg based alloys. J Mater Sci Eng A; 280:151–5. Zolotorevsky V. S., Belov N. A., Glazoff. M. V, 2007. Casting Aluminum Alloys, vol. I, UK, Pp; 386-396.
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ScienceDirect Procedia Materials Science 11 (2015) 438 – 443
5th International Biennial Conference on Ultrafine Grained and Nanostructured Materials, UFGNSM15
Effect of Al-15Zr Master Alloy and Extrusion Process on Microstructure and Mechanical Properties of Al-6%Mg Alloy H. Hosseinya ,M. Emamya, G. Ashuria,* a
School of Metallurgy and Materials, University of Tehran, Tehran, Iran
Abstract In current research different amounts of Al-15Zr master alloy were added to Al-6%Mg alloy to obtain various Zr concentrations (0.01, 0.03, 0.05, 0.1, 0.2, 0.3 % weight percent) . The microstructural study was carried out on Al-6%Mg extruded alloy. It was found that with the increase of Al-15Zr master alloy, Al (α) matrix morphology was changed from dendritic to equiaxed grains due to the suppression of grain growth by the formation of homogeneous Zr containing disperoids at grain boundary. The grain size of as-cast alloys was significantly decreased by adding Al-15Zr master alloy to same extent. An increase in the amount of Zr content resulted in improvement of ultimate tensile strength (UTS) and elongation (%El) values. Fractographic examinations of the alloy showed a ductile mode of fracture via introducing more homogenous fine dimples after grain refining process. © 2015Published The Authors. Published Ltd. © 2015 by Elsevier Ltd. by ThisElsevier is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of UFGNSM15. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of UFGNSM15 Keywords: Al-15Zr grain refiner; microstructure; ultimate tensile strength; fracture surface.
1. Introduction Al-Mg base alloys are one of the main class of light-weight alloys, with high strength and good corrosion resistant properties that are used in welded applications, and transportation for making dump truck bodies, large tanks for carrying petrol, and pressure vessels, particularly where cryogenic storage is involved, Polmear (1995) Refining of the grains by grain refiners, precipitation and solid-solution-hardening are the most important techniques for increment of the strength of Al alloys and also creating a microstructure with uniform distributed of fine equiaxed or even ultrafine grains, Toropova et al. (1998), Polmear (1995). An equiaxed fine grain performs a serious
* Corresponding author. Tel.: 09376486506 ; fax: 09376486506. E-mail address: [email protected]
2211-8128 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of UFGNSM15 doi:10.1016/j.mspro.2015.11.034
H. Hosseiny et al. / Procedia Materials Science 11 (2015) 438 – 443
duty in enhancing mechanical properties, ameliorates feeding process to remove shrinkage porosity and distribution of intermetallic particle throughout the casting and amended surface finish, Gao et al. (2013), Ghadimi et al. (2013).The binary Al-Mg phase diagram is shown in Fig. 1a. Al-6% Mg is known as a high strength alloy in Al-Mg series. Al–6%Mg alloy is a wrought commercial alloy and has attracted great attention of many researchers and scientists. However, Al-6%Mg alloys are commonly suffered from softening effect during its usage and these series of aluminum alloys are used in a work-hardened condition. Extruded of Al–6%Mg–Zr alloy is expected to lead to the precipitation of Al3Zr and as a result, the microstructure will contain fine dispersions of Al3Zr precipitates within the recrystallized grains. Zr exploits peritectic reaction with aluminum to form metastable with LI2 structure aluminides phases, Lee et al. (2002). Al3Zr particles are usually coherent with the matrix and thermally stable because of their high melting points. Because of a few data regarding to the grain refining of Al–Mg alloys. It merits a further investigation on these alloys. Therefore, in this research, the grain refining behavior of extruded Al–6%Mg alloy will be studied by using Al-15Zr master alloy. Nomenclature UTS DRX DRV
Ultimate tensile strength Dynamic recrystallization Dynamic recovery
2. Experimental procedure Industrially materials were used as starting materials for preparing primary ingots and extruded rod of Al-6Mg alloy. For this purpose pure Al (99.8%) and Mg (99.9) were melted in a 10 kg SiC crucible via an electrical resistance furnac, In order to prepare alloys with different content of Zr during preparation of small billets the weight losses of Mg were selected to be 15%. The Al–15Zr master alloy was appended to the molten metal at 760°C to provide various amounts of Zr (0.01, 0.03, 0.05, 0.1, 0.2, and 0.3 wt. %) in the cast specimens. Finally, after stirring process of the molten metal and cleaning off the dross, for all tests, after a period of 5 min the melt was poured into a cylindrical permanent iron mold as shown in Fig. 1b. Before extrusion process, cast billets were homogenized at 500°C for 10. Cylindrical castings were cut into billets with 30 mm in diameter and 30 mm in heights in order to fit into the extrusion container. These billets were preheated to 500 °C for 1 h and then extruded using a hydraulic press at a ram speed of 1 mm/s with the extrusion ratio of 6:1. Extrusion process was carried out applying graphite based oil between metal, die and container. Round tensile samples were machined along the extrusion direction according to ASTM: E8/E8 M-11. The sketch of a tensile test specimen is seen in Fig. 1c. Tensile tests were carried out using a computerized testing machine (SANTAM STM-20) at a strain rate of 0.1mm/min at room temperature. In order to reveal the microstructure, the cut sections were polished and then etched by Keller’s reagent (2 mL HF, 3 mL HCl, 5 mL HNO3 and 190 mL H2O).
Fig. 1. (a) Al-Mg phase diagram; (b) schematic of mold iron; and (c) tensile test specimen.
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H. Hosseiny et al. / Procedia Materials Science 11 (2015) 438 – 443
3. Results and discussion 3.1. Macro and microstructural evolution Figure 2 illustrates the macrostructures of Al–6%Mg cast alloy after grain refining by different concentrations of Zr. It can be seen that the grains obtained from Al–6%Mg alloy specimens after grain refining process are much finer in comparison with nonrefined alloy. Figure 3 also shows the microstructures of Al–6%Mg alloy after grain refining, homogenising and hot extruding process by different contents of Zr. The microstructures of Al–6%Mg alloy are shown in Fig. 3a. According to Al– Mg binary phase diagram, the Al–6%Mg alloy contains primary a-Al phase and eutectic structure in other words ߚAl3Mg2 intermetallic and a-solid solution phase are formed. Also, the presence of iron and silicon impurities may result in the formation of such phases as Al 15 (Fe, Mn)2 Si3 phases or insoluble Mg2Si particle, which possesses more favourable skeletal morphology, Zolotorevsky et al. (2007). It is clear from Fig. 3a, 3d and 3g that the grain refining process elevated the number of grain boundaries. By adding Al-15%Zr master alloy not only the grain size decreases, but also the columnar dendrite morphology changes to rosette-like, Fakhraei and Emamy (2014). The mechanism of grain refinement by Al-15%Zr master alloy is related to the formation of Al3Zr particles in the microstructure. Primary Al3Zr particle precipitate from the melt during solidification and refine the grain size of the alloy, Kashyap and Chandrashekarbull (2001) , Yin et al. (2000). Figure 3b, 3e and 3h illustrates the homogenized Al-6%Mg alloy. During solidification process of Al cast parts, much of alloying elements segregate from liquid metal and result in an inhomogeneous distribution of large particles and also formation of constitutive particles on grain boundary regions or inside the grains, Robson and Prangell (2001). Coarse residual particles have deteriorated effect on the extrudability of Al alloys, Rokhlin et al. (2004) High mechanical and extrudability properties are achieved, if these particles dissolve in the structure. In other hand applying homogenization process, results in the removal of the inhomogeneous distribution and dissolving the second-phase particles of alloying elements on a micro-scale causes the disposed that are able to inhibit recrystallization zone generate (like Al3Zr). Considering the effect of such intermetallics on grain boundary pinning which results in finer grain size and consequently higher mechanical properties, Robson and Prangell (2001). Figure 3c, 3f and 3i illustrates the microstrutures of extruded alloy that consists of fine and equiaxed grains with a fully recrystallized microstructure and the relatively no non-recrystallized grains elongates in extrusion direction. The variation of average grain sizes of Al-6%Mg alloy with different concentration of Zr, before and after extruding process are shown in Fig. 4. The microstructures of Al–6%Mg extruded alloy illustrate refined precipitates distributed at dynamically recrystallized grain a boundary which clarifies the pinning effect of precipitates for the grain boundaries (Fig. 3c). During the extrusion of some metals such as aluminum alloys, a fibrous microstructure (consisting of original as-cast grains elongated in the extrusion direction) is commonly developed due to the operation of dynamic recovery (DRV). In Al alloys, however, dynamic recrystallization (DRX) occures during hot deformation and a fine-grained microstructure is progressively developed. There are more precipitates in the Al6%Mg extruded alloy after refining by Al-15Zr master alloy as expected Al3Zr intermetallic particle prevents dynamically recrystallized grain growth; however they are coarser than that of primary extruded alloy (Fig. 5f-i). Thus, a finer dynamically recrystallized grain structure is obtained. In other words, the amounts of recrystallized grains are increased with increasing the Al-15%Zr grain refiner. Recrystallization behavior of aluminum alloys show that the size and distribution of particles has important effects on the structure of recrystallized grains. Basically, the particles can act in two ways. Coarse particles can act as nucleation sites for crystallized grains. The presence of these particles can be create strain in the deformation process and therefore will be a good place for the formation of new nucleation for dynamic recrystallization during hot deformation. On the other hand fine particles can be pin the grain boundary by Zener phenomenon, and reduce the growth rate of recrystallized grains during hot deformation process. These fine particles can prevent recovery procces, Gandhi and Raj (1991), Mayo et al. (1990).
H. Hosseiny et al. / Procedia Materials Science 11 (2015) 438 – 443
Fig .2. Macrostructures of Al–6%Mg alloy with different concentration of Zr (a) 0%; (b) 0.01%; (c) 0.05% ;(d) 0.1%; and (e) 0.3% Zr.
Fig . 3. Metallographic images of (a) 0 % Zr and (g) 0/05 % Zr and (d) 0/3 % Zr as cast, (b) 0 % Zr and (e) 0/05 % Zr and (h) 0/3 % Zr homogenized and (c) 0 % Zr , (f) 0/05 % Zr and (i) 0/3 %Zr extruded Al-6Mg alloys.
Fig. 4. Average grain-size of the Al–6%Mg alloy with different amount Zr, before and after extruding process.
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3.2. Mechanical properties Figure 5 illustrates UTS and elongation results it is seen that UTS increases with increasing Zr concentration. Al15%Zr master alloy enhances the UTS values of the Al-6%Mg extruded alloy from 335 MPa to maximum 381 MPa, and the elongation values of the alloy from 26 % to maximum 31%. As mentioned above, Zr has a refining effect on the microstructure of as-cast alloy and it forms Al3Zr intermetallics particle .According to the results achieved from structural studies, also, by adding Al-15%Zr to the Al–6%Mg alloy a structure with finer grains and Al3Mg2 intermetallics is obtained. Dynamic recrystallization during hot deformation cause extremely fine grains due to more grain boundaries of initial structure which perform as nucleation sites for dynamically recrystallized grains , Taku et al. (2014). Thus improving mechanical properties mainly attributed to the refinement effect of microstructure. Reduction interdendritic segregation between dendritic structure by homogenizing process before extrusion, the loss of porosity in the structure of the extrusion process and distribution of fine secondary particle phase in the structure, Mulazimoglu et al. (1996).
Fig. 5. (a) UTS; (b) Elongation valuse of Al-6%Mg extruded alloy.
3.3. Fractography Figure 6 reveales the fracture surfaces of the extruded alloy in Zr free and 0.2% Zr added condition. In both conditions fracture surfaces revele several fine dimples. In normal ductile fracture three stages are seen: (i) void initiation, which usually occurs at second-phase particles or inclusions, (ii) void growth and (iii) hole coalescence through the separation of the ligaments, which links the growing voids ,ASM Handbook (1992) However in Fig. 6 two classes of dimples are observed, namely the large-sized dimples caused by the coarse intermetallic particles or the grain boundary precipitates, and the high density small-sized dimples nucleated in the grain interiors.
Fig. 6. SEM micrographs of fractured surface of (a) Al–6%Mg; (b) Al-6%Mg-0.2 % Zr extruded alloy.
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4. Conclusions In current research the effects of different concentrations of Al-15%Zr master alloy on the structure evolution and mechanical behavior of Al-6%Mg extruded alloy were studied and the following conclusions can be drawn: 1. The grains are refined by Zr addition and extrusion procces Zr addition reduces the average grain size of Al6%Mg cast alloy from 782 μm to 351μm. And extrusion procces reduces from 208 μm to 174 μm by increasing Zr content. 2. UTS and elongation values of the Al-6Mg alloy afterZr addition and extrusion improved from 335 MPa and 26% to maximum 381 MPa and 31%, respectively. 3- Fractography examination of the Al-6%Mg refined extruded alloy shows more ductile fracture by Al-15Zr addition.
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