Effects of Rare Earth Er Additions on Microstructure and Mechanical Properties of an Al–Zn–Mg–Cu Alloy
The effects of Er additions on the microstructure and tensile properties of cast Al–15Zn–2.5Mg–2.5Cu aluminum alloy have been investigated. The results show that by adding 1 wt% Er grain refiner in the cast alloy, the grains can be refined to a fine degree. The microstructures and fracture surfaces of cast aluminum alloy samples were examined by SEM. In addition, the Er modified the eutectic structure from a coarse plate-like and acicular structure to a fine branched and fibrous one. The tensile properties were improved by the addition of Er, and good ultimate tensile strength (325 MPa) but poor elongation (6%) were obtained when the Er addition was 1 wt%. Furthermore, fractographic examinations revealed that refined pore and spheroidized a-Al dendrite were responsible for the high ultimate tensile strength. At higher magnifications, unrefined specimens showed cracking along the grains, whereas Er-refined specimens showed cracks in individual intermetallic compounds.
KeywordsRare earth Heat treatments Mechanical properties Microstructure
Constant efforts are being made to design new alloys and improve the properties of existing alloys to meet the demand for aluminum castings with enhanced mechanical properties. In the designing of wrought high strength aluminum alloys, some of the most important factors and attributes to be considered are chemical composition and processing parameters and the resulting effects of the microstructure on the mechanical properties. The cast high strength aluminum alloys must rely on the design of proper chemical composition followed by proper the heat treatment to develop the designed microstructures and properties. Cast aluminum alloys based on the Al–Zn–Mg–Cu system respond very favorably to age hardening and possess a high specific strength . High strength Aluminum alloy have been widely used as structural materials in aircraft structure applications due to their attractive comprehensive properties, such as low density, high strength, ductility, toughness and resistance to the fatigue [2, 3, 4].
Grain refining of aluminum alloys can normally be achieved by melt inoculation with aluminum master alloys containing Ti and/or B. There are many benefits from the use of the master alloys. For example, the mechanical properties can be improved . The use of high concentrations of alloying elements results inhomogeneity in the microstructure and severe segregation of second phases. In casting products, the mechanical properties vary from location to location due to the variation of the grain size, the amount of eutectic phases and the amount of precipitates. Much attention has been made to reduce the segregation of the alloying elements during solidification period of high-alloyed Al alloys [6, 7, 8, 9, 10, 11, 12].
In the last decades, the usage of rare earths, especially La, Ce, Nd, Y, Sc and mischmetall in aluminum alloys has been widely studied [13, 14, 15, 16]. These studies show that the microstructure of these alloys is modified, the mechanical properties and other properties such as electrical conductivity, optical quality and corrosion resistance are also improved. The effects of rare earth and transition elements in aluminum alloy are evident for their special electronic structures, which have received more attention. The effects of rare earth elements in aluminum alloys are determined by their characters. Because of their large atomic radius and tendency to lose two outermost level s-electrons and a 5d or 4f electron to become trivalent ion, rare earth metals are very active in chemical reactions [17, 18].
The main object of this investigation is to study the effect of rare earth Er additions on microstructure and mechanical properties of the Al–15Zn–2.5Mg–2.5Cu alloy.
An Al–15Zn–2.5Mg–2.5Cu aluminum alloy, was used as experimental material. Melting procedure of the alloy was carried out in an electrical resistance furnace using a SiC crucible. Industrially pure elemental Al (99.90%), Mg (99.90%), Zn (99.90) and Cu (99.90%) were used as starting materials to prepare the ingots. The Al–15Zn–2.5Mg–2.5Cu aluminium alloy ingots cut into various small pieces and then placed into a graphite crucible. The graphite crucible was placed in an electrical resistance. Melting of aluminium alloy was done by heating it to a temperature of ~750 °C. Different amounts of Er wt.% (0.5 wt% Er, 1 wt.0% Er, 1.5 wt% Er and 2 wt% Er) were added to the molten alloy at 750 °C.
For microstructural studies, optical microscope equipped with an image analysis system (Clemex Vision Pro. Ver.3.5.025) and SEM (Make: Cam Scan MV2300) equipped with an energy dispersive X-ray analysis (EDX) have been used. The cut alloy sections were polished using SiC based abrasive papers and then etched by Keller’s reagent (2 ml HF, 3 ml HCl, 5 ml HNO3 and 190 ml H2O) to reveal the structure. The average grain size of the specimens were measured in accordance with the ASTM: E112 standard. Phase identification was performed by X-ray diffraction method (Make: Philips PW 1830). Tensile testing on all the samples was performed at room temperature using SANTAM universal testing machine at the strain rate of 1 mm/min. Four test bars were tested for each sample and the average value is reported here.
Results and Discussion
On the other hand, Al–15Zn–2.5Mg–2.5Cu alloy can be strengthened by precipitation of Al3Er particles after addition of Al–30 wt% Er master alloy. The micro or nano size Al3Er particles play a crucial role in the strengthen mechanism. Therefore, the ultimate tensile strength of alloys increased significantly with addition of Er. It’s mainly due to the reinforcement of the precipitated Al3Er particles.
Fractography of Tensile Specimens
Mechanical properties of Al–15Zn–2.5Mg–2.5Cu cast alloys mainly depend on the shape, type and α-Al grain size and distribution of secondary phases.
Al–30Er is an effective master alloy in reducing the grain size, altering dendritic morphology and introducing fine and uniform microstructure.
The increase in the tensile properties by the addition of grain refiner is due to: the breakage of the primary α-Al grains into more uniformly distributed α-Al grains by refinement and fine distribution of the secondary phases.
Grain refinement by the addition of 1 wt% Er improves the strength values.
The ultimate tensile strength of casting alloys increased significantly with addition of Er. It’s mainly due to the refinements of a-Al dendrite and eutectic and reinforcement of the precipitated Al3Er particles.
- 1.J. Hirsch, Aluminium in innovative light-weight car design, Mater. Trans. 52 (2011) 818–824.Google Scholar
- 2.T. Dursun, C. Soutis, Recent developments in advanced aircraft aluminium alloys, Mater. Des. 56 (2014) 862–871.Google Scholar
- 3.A. Haghparast, M. Nourimotlagh, M. Alipour, Effect of the strain-induced melt activation (SIMA) process on the tensile properties of a new developed super high strength aluminum alloy modified by Al-5Ti-1B grain refiner, Mater. Charac. 71 (2012) 6–18.Google Scholar
- 4.M. Alipour, M. Emamy, Effects of Al–5Ti–1B on the structure and hardness of a super high strength aluminum alloy produced by strain-induced melt activation process, Mater. Des. 32 (2011) 4485–4492.Google Scholar
- 5.M. Alipour, M. Emamy, R. E. Farsani, M. H. Siadati, H. Khorsand, Effects of a modified SIMA process on the structure, hardness and mechanical properties of Al-12Zn-3 Mg-2.5Cu alloy, Iranian Journal of Materials Science and Engineering. 12 (2015) 77–88.Google Scholar
- 6.M. Alipour, B. G. Aghdam, H. E. Rahnoma, M. Emamy, Investigation of the effect of Al–5Ti–1B grain refiner on dry sliding wear behavior of an Al–Zn–Mg–Cu alloy formed by strain-induced melt activation process, Mater. Des. 46 (2013) 766–775.Google Scholar
- 7.M. Alipour, M. Emamy, S. H. S. Ebrahimi, M. Azarbarmas, M. Karamouz, J. Rassizadehghani, Effects of pre-deformation and heat treatment conditions in the SIMA process on properties of an Al-Zn-Mg-Cu alloy modified by Al-8B grain refiner, Materials Science and Engineering A. 528 (2011) 4482–4490.Google Scholar
- 8.M. Alipour, M. Emamy, M. Azarbarmas, M. karamouz, Effects of Al-5Ti-1B master alloy on the microstructural evaluation of a highly alloyed aluminum alloy produced by SIMA process, AIP Conference Proceedings 1252 (2010) 1060–1072.Google Scholar
- 9.M. Alipour, M. Emamy, J. Rasizadeh, M. Karamouz, M. Azarbarmas, Effects of Al-8B grain refiner on the structure, hardness and tensile properties of a new developed super high strength aluminum alloy, TMS Annual Meeting, 2 (2011) 309–320.Google Scholar
- 10.G. S. Pradeep Kumar, P. G. Koppad, R. Keshavamurthy, M. Alipour, Microstructure and mechanical behaviour of in situ fabricated AA6061–TiC metal matrix composites, Archives of Civil and Mechanical Engineering, 17 (2017) 535–544.Google Scholar
- 11.M. Alipour, M. Emamy, J. Rasizadeh, M. Karamouz, M. Azarbarmas, Effects of Al-5Ti-1B grain refiner on the structure, hardness and tensile properties of a new developed super high strength aluminum alloy, TMS Annual Meeting, 3 (2011) 833–842.Google Scholar
- 12.Alipour, M, Azarbarmas, M, Heydari, F, Hoghoughi, M, Alidoost, M, Emamy, M.”The effect of Al-8B grain refiner and heat treatment conditions on the microstructure, mechanical properties and dry sliding wear behavior of an Al-12Zn-3 Mg-2.5Cu aluminum alloy” Materials and Design, Volume 38, June 2012, Pages 64–73.Google Scholar
- 13.Mirjavadi, S. S, Alipour, M, Hamouda, A. M. S, Besharati Givi, M. K, Emamy, M.” Investigation of the effect of Al-8B master alloy and strain-induced melt activation process on dry sliding wear behavior of an Al-Zn-Mg-Cu alloy” Materials and Design, Volume 53, January 2014, Pages 308–316.Google Scholar
- 14.AFSHARI, B. M, MIRJAVADI, S. S, DOLATABAD, Y. A, AGHAJANI, M, GIVI, M. K. B, ALIPOUR, M, EMAMY, M.” Effects of pre-deformation on microstructure and tensile properties of Al—Zn—Mg—Cu alloy produced by modified strain induced melt activation” Transactions of Nonferrous Metals Society of China (English Edition), Volume 26, Issue 9, September 2016, Pages 2283–2295.Google Scholar
- 15.Alipour, M, Mirjavadi, S, Besharati Givi, M. K, Razmi, H, Emamy, M, Rassizadehghani, J.” Effects of Al-5Ti-1B master alloy and heat treatment on the microstructure and dry sliding wear behavior of an Al-12Zn-3 Mg-2.5Cu alloy” Iranian Journal of Materials Science and Engineering, Volume 9, Issue 4, 2012, Pages 8–16.Google Scholar
- 16.Alipour, M., Emamy, M., Rasizadeh, J., Azarbarmas, M., Karamouz, M.” Effect of predeformation and heat treatment conditions in the modified SIMA process on microstructural of a new developed super high-strength aluminum alloy modified by A1-8B grain refiner” TMS Annual Meeting, Volume 3, 2011, Pages 843–853.Google Scholar
- 17.Alipour, M., Emamy, M., Rasizadeh, J., Karamouz, M., Azarbarmas, M.” Effects of Al-8B grain refiner on the structure, hardness and tensile properties of a new developed super high strength aluminum alloy, TMS Annual Meeting, Volume 2, 2011, Pages 309–320.Google Scholar
- 18.B. Binesh, M. Aghaie-Khafri, RUE-based semi-solid processing: Microstructure evolution and effective parameters, Materials & Design, Volume 95, 2016, Pages 268–286.Google Scholar
- 19.B. Binesh, M. Aghaie-Khafri, Microstructure and texture characterization of 7075 Al alloy during the SIMA process, Materials Characterization, Volume 106, 2015, Pages 390–403.Google Scholar
- 20.J. Buha, R. N. Lumley, A. G. Crosky, Secondary ageing in an aluminium alloy 7050, Mater. Sci. Eng. A 492 (2008) 1.Google Scholar
- 21.C. Mondal, A. K. Mukhopadhyay, T. Raghu, V. K. Varma, Tensile properties of peak aged 7055 aluminum alloy extrusions, Mater. Sci. Eng. A 454 (2007) 673.Google Scholar