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Effects of Rare Earth Er Additions on Microstructure and Mechanical Properties of an Al–Zn–Mg–Cu Alloy

  • S. Kord
  • Mohammad Alipour
  • M. H. Siadati
  • Masumeh Kord
  • Praveennath G. Koppad
Conference paper
Part of the The Minerals, Metals & Materials Series book series (MMMS)

Abstract

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.

Keywords

Rare earth Heat treatments Mechanical properties Microstructure 

Introduction

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 [1]. 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 [5]. 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.

Experimental Procedure

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.

After successful addition of Er, ultrasonic treatment and uniform mixing throughout, the melt was poured into a permanent mold designed and fabricated according to ASTM B557M-10 standard (see Fig. 1). The advantage of using permanent mold is the provision of appropriate uphill filling system and feeding design. This design will have a low turbulence manner of fluid flow resulting in reduced air entrapment and porosity in the fabricated composite specimens.
Fig. 1

Tensile specimen geometry and dimensions a cast iron mold and b tensile sample dimensions

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

Structural Characterization

Figure 2 shows SEM micrograph and map analysis of unrefined Al–15Zn–2.5Mg–2.5Cu as cast alloy. The segregation of solute occurred during solidification of the alloy led to the high concentration of Cu, Mg and Zn in the inter-dendritic eutectic regions. The microstructures of Al–15Zn–2.5Mg–2.5Cu alloy contains highly primary α-phase (solid solution of aluminum) and second phases based on η-MgZn2 (in eutectic region), S–Al2–CuMg and T–Al2Mg3Zn3 phases (Fig. 3) [9]. The lamellar eutectic structures like α-Al and η-MgZn2 phases were clearly seen from the micrographs. It is important to note that the solubility of η-MgZn2 phases for Cu is relatively high, which is observed in the eutectic structure. In addition to this, the EDS carried out on this alloy showed that the presence of MgZn2 intermetallic which indicates that Mg and Zn percentages are higher than the average level of chemical composition in the Al–15Zn–2.5Mg–2.5Cu alloy. It is obvious that the η-MgZn2 phases dissolved the Cu and Al elements, and formed the Mg(Zn,Cu,Al)2 phase, which is consistent with the present literatures [8]. The segregation of solute that occurred during casting led to the high concentration of Cu, Mg and Zn in the interdendritic eutectic regions. It is well known that the diffusion velocity of Cu is lower than Zn and Mg, which results in the higher concentration of Cu in the regions of eutectic structures during solidification. The driving force for the phase transformation to Al2CuMg from Mg(Zn,Cu,Al)2 phase must be super saturation of Cu in the areas of eutectic structures, which makes Al2CuMg a stable phase.
Fig. 2

Results from map analysis in sample unrefined

Fig. 3

a BSE image of the eutectic structure (η-MgZn2) and S phase presenting in sample unrefined, b X-ray energy spectrums of the η phase, c X-ray energy spectrums of the S phase

Figure 4 shows the influence of various amounts of Al–30Er on the grain refinement of Al–15Zn–2.5Mg–2.5Cu alloy. The average grain size of unreinforced Al–15Zn–2.5Mg–2.5Cu alloy was found to be in range of 550 μm. The addition of Er to Al–15Zn–2.5Mg–2.5Cu alloy has significantly refined the coarse columnar primary α-Al grains to fine equiaxed α-Al grains of average size of 62 μm. The main reason is that Al3Er particles act as nucleating agents during the solidification of α-Al grains. It can be clearly seen that the microstructure of Al–30 wt% Er master alloy consists of a-Al matrix and intermetallics Al3Er phase. The Al3Er phase is uniformly distributed in the a-Al matrix. Several mechanisms have been proposed for the grain refining process. In some mechanisms the presence of some particles like Al3Er are known to be effective for grain refinement procedure. The presence of some alloying elements, particularly Mg and Cu are known to improve the efficiency of some grain refiners such as Al–30Er master alloys. With increasing content of Er wt%, these particles pin the grain boundaries and increase the grain refinement, crack pinning and cause increase of strength. When wt% of Er is more than 1 wt%, the grain size will be constant and effect of grain refinement diminished.
Fig. 4

Grain size variations with Er contents

Figure 5 shows the micrographs of as cast microstructures of unreinforced and varying weight percentage of Er reinforced Al–15Zn–2.5Mg–2.5Cu alloy. The micrographs clearly show there is significant change in dendrite morphology of the Al–15Zn–2.5Mg–2.5Cu alloy after adding Er. The microstructures of alloy revealed a rosette-like microstructure of primary α-Al grains solid solution surrounded by interdendritic secondary phases. In comparison with Er added specimens, unrefined specimens showed coarser morphology. From Fig. 5, it is noticeable that adding Er increases the number of grain boundaries and therefore promotes a more homogeneous distribution of intermetallic precipitates. It is obvious that the addition of Er changes the shape and size of the eutectic phase.
Fig. 5

SEM microstructures of refined specimens, with a 0.0 wt% Er, b 0.5 wt% Er, c 1 wt% Er and d 2 wt% Er

Figure 6 shows SEM micrograph and map analysis of 1 wt% Er refined Al–15Zn–2.5Mg–2.5Cu as cast alloy. The various phases were clearly seen from the micrographs. In addition to this, the EDS carried out on this alloy showed that the presence of Al3Er intermetallic which indicates that Al and Er contents are higher than the average level of chemical composition in the alloy.
Fig. 6

Energy dispersive X-ray and map analysis of the Al–15Zn–2.5Mg–2.5Cu/1% Er and distribution of the major elements

Tensile Properties

Figure 7 shows the mechanical properties of wrought Al–15Zn–2.5Mg–2.5Cu alloy as standard conditions for comparing the different steps of processes in this study. In Fig. 7, UTS values of the specimens in the different conditions are shown. As it is shown in Fig. 7, the average ultimate tensile strength (UTS) of the specimens after adding Er increases from 225 ± 8 MPa to about 310 ± 7 MPa. Mechanical properties (tensile) of the Al–15Zn–2.5Mg–2.5Cu–xEr alloy mainly depend on the shape, size and size distribution of the α-Al grains, eutectic structure and the distribution of the intermetallics in the interdendritic or grains [19]. The main reason for this improvement is high probably due to the reduced grain size of the castings, leading to a finer distribution of second phases (i.e. intermetallics) in Al–15Zn–2.5Mg–2.5Cu–xEr alloy. It is well known that according to Hall–Petch theory the finer the grains, the higher the strength [20]. But, due to the several mechanisms engaging in strengthening of Al 7xxx alloys, especially precipitation hardening, the dependence of the strength to grain size was unclear. Intermetallic compounds are brittle and considered as important crack initiating sites during loading. Several reports indicate that metastable coherent or semicoherent η precipitates are formed during aging treatment [21]. Ultimate strength of as-cast microstructure specimens has a low value which is due to the presence of shrinkage porosities inside the grains and boundaries.
Fig. 7

UTS and %El. of unrefined and Er refined samples at the different conditions

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

Figure 8 presents the fracture surfaces of the cast alloys. It can be clearly seen that extensive irregular cleavage planes and some tearing ridges are apparent on the entire fracture surface of the unmodified alloy as shown in Fig. 8a. It indicates that the fracture characteristics exhibit quasi-cleavage fracture, resulting in low strength value of the specimen (Fig. 7). Due to the modification of Er on the eutectic, the area of the cleavage planes decreases and the number of dimples increases, as shown in Fig. 8b. Moreover, the modification efficiency enhances with the addition of 1 wt% Er in the alloy, as a result, the 1 wt% Er modified alloy exhibits more dimples and higher strength, comparing with that of other wt% Er modified alloy.
Fig. 8

Fractographs of the tensile samples from alloys: a unmodified-as cast, b 1 wt% Er

Conclusion

The following conclusions can be drawn from this study.
  1. 1.

    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.

     
  2. 2.

    Al–30Er is an effective master alloy in reducing the grain size, altering dendritic morphology and introducing fine and uniform microstructure.

     
  3. 3.

    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.

     
  4. 4.

    Grain refinement by the addition of 1 wt% Er improves the strength values.

     
  5. 5.

    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.

     

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Copyright information

© The Minerals, Metals & Materials Society 2018

Authors and Affiliations

  • S. Kord
    • 1
    • 2
  • Mohammad Alipour
    • 1
    • 3
  • M. H. Siadati
    • 1
  • Masumeh Kord
    • 2
  • Praveennath G. Koppad
    • 4
  1. 1.Faculty of Materials Science and EngineeringK. N. Toosi University of TechnologyTehranIran
  2. 2.Department of BiomaterialPasteur Institute of IranTehranIran
  3. 3.Department of Materials Science and EngineeringUniversity of TabrizTabrizIran
  4. 4.Department of Mechanical EngineeringNagarjuna College of Engineering and TechnologyBangaloreIndia

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