Preliminary Morpho-Mechanical Investigation of X7475 Al-Alloys Produced from Recycled Beverage Can

The need to turn around the beverage can recycle table from conventional can-can to can-automobile (bumper applications) that was seen as a gap to be filled through a novel Experimental 7475 Aluminium Alloy (X7475). In this study, Aluminium, Zinc, Manganese and Copper were sourced from Recycled Beverage Can (RBC), spent DR 20-BG/T 8897.2 2008 Hawk battery and coils of used standing fans respectively. Experimental samples were stir cast. Morphological investigations on over 35 sites revealed elemental composition, nature of defects and possible phases. Annealed (O), naturally aged (T4) and artificial aged (T6) samples were indented for hardness (H). Imperfections and inclusions such as C, O (pores), Na and Fe were observed in the α-Al phase. Hardening precipitates like MgZn2, Mg3Zn3Al2, Mg32 (Zn, Al)49 and Al0.5Fe3Si0.5 were identified. Peak obtainable H of 140.45 Hv (T6), 134.32 Hv (T4) and 89.11 Hv (O) with the least H of 52.18 Hv. Pores and casting defects were observed due to the production route and constituents from secondary sources which affected the mechanical properties. Future research should focus on Optimization of Mechanical Properties. H should be correlated with tensile strength. The use of alumina crucibles may be options.


Introduction
The quest for saving in fuel consumption, weight, customer-driven innovations, environmental and industrial regulations in the automobile industry has spurred an opportunity for the aluminium recycling industry. These are drivers of changing the Can-Can paradigm to Canautomobile applications. This initiative supports the target of magnifying the quantity of aluminium in the automobile industry by 2025 [1]. One of such areas of application is in the beam of the bumper [2][3][4].
The X7475 is a novel alloy cast from RBCs. This alloy has the potentials of being used for production of beam of automobile bumper.
Defects like volumetric shrinkage occurred during solidification within a range from 3.5 to 8.5 %. Pores and shrinkage porosity were formed [3]. These imperfections affect the mechanical, physical, microstructure and morphology of the material in addition to HTs, alloy composition and production route.
Morpho-mechanical characterizations are also affected by the cooling rate during and after solidification. For instance, hydrogen porosity, misruns, cracks, moisture reactions, folds, and tears are inevitable macro and micro structural defects that strongly influence mechanical and fatigue performance [6].
The literature falls short in characterizing the 7xxx aluminium alloy produced from RBCs. The characterizations in George & Knutsen [7] concentrated on a commercial AA7075 and custom-made A713 alloy. The study concluded using only 3 spectra for the investigation. Jiang,et al [8] investigated the coarsening and microstructural evolution of 7005 aluminium alloy in as-received and T6 condition. Shrinkage, pores and precipitates in Al-Cu, Al-Mg-Cu, Al-Mg-Zn and Al-Mg-Si alloys were investigated in the study conducted by Sigmund et al [9], but not the Al-Zn-Mg-Cu alloy produced using 80 % recycled constituents. In the presence of Zn, Mg, Mn, Cu, and Al expected phases including Al 6 CuMg 4 , Al 2 Mg 3 Zn 3 , AlCuMg, Al 2 Cu and MgZn 2 [9] however, defects may affect the phase formations. The aim of this paper was to characterize defects and imperfections in form of pores and volume shrinkage of the novel X7475 stir cast alloy. The morphology was correlated with H after HTs.

Materials and Methods
Ingots of Al, Zn and 70% Cu-30% Al were cast using a gas fired melting furnace and 2 Kg capacity induction furnace of JT0332 model respectively. Al was recovered from RBCs while Zn was obtained from DR 20 standard, BG/T 8897.2 -2008 Hawk battery. Cu was from the coils of used standing fans of 2-ARI 410 standard.
Percentage weight (wt. %) measurement was done on Mettler Toledo Model PL 303 digital weighing machine. Wt. % of Zn varied from 5.0-4.0, Mg was 1.50-1.00 and 0.075-0.025 was for Mn. Improvised stainless steel crucible was used in induction furnace. Al, Zn, 70 % Cu-30 % Al, Mn and Mg were charged in that order. Manual stirring was adopted and pouring into a tailor-made permanent steel mould that was done with a distance of 45 ± 10 cm maintained between the furnace and mould, and pouring temperature was 720-730 o C.
Alloys were taken through heat treatments (HT) using the Carbolite HTF 1800 box furnace in accordance with AMS 2771. Samples were placed on alumina plate to avoid contamination and abrasion. Coupons were sectioned and prepared for investigation using silicon carbide papers of 400, 600 and 800 grits. Mounting on sample holder for EDX was according to the limit of 13 mm height. Morphological characterization (SEM/EDX) was done at different magnifications focusing on over 50 sites of interest, using the made in USA Hitachi VP-SEM, SU1510 technology set at 15.0 kV. The characterization was informed by the peculiarity of the shrinkage pores, gas bubbles, inclusions of foreign matter and cracks [10].

Results and Discussion
The elemental analysis in Figure 1a and b revealed the presence of Al, Zn, Mg, Mn, Fe, Si, Cu and O which supported the growth of precipitates, hardening phases and dendrite growth like MgZn 2 , Al 4 C 3 , Mg 5 Al 8 due to HT and production route. Figure 2, analysis of dislocation sites, is a conglomeration of boundaries supporting the formation of dislocation walls and hardening phases like MgZn 2 that produce plastic deformation. Materials handling route was responsible for the discrepancies observed between calculated and experimental wt. % [11].
Interdendritic separation as in Figure 2 supported the pore formation due to liquid network fragmentation. HT supported the formation of phases and reduced pockets immobilized by surface tension during solidification [3].
Analysis of 3 flash points (figure 3 a-d) revealed that the AC alloy was inhomogeneous as elemental scanning differs from boundaries to matrix. This supported the study of Olawale et al [12] on permeability of the solid system.   The first stage during solidification was characterized by free movement of liquid and solid mass. In figure 3a, point-1 was an interdendritic conglomeration supporting the formation of vacancies for precipitates [9]. The remaining liquid during solidification flowed through the dendritic network [3]. The results of elemental analysis (3b) support the formation of Al-Mg 2 Si, AlMgZn, Mg 32 (Al, Zn) 49 ,MgZn 2 and Mg 7 Zn 3 , MgZn, Mg 2 Zn 3 and Mg 2 Ca [7]. The white phases were AlZnMgCu.
Amidst the AlZnMgCu phase was site C characterized by high Mg-Al-Si and oxide (void) that support the formation of Mg 2 Si, Mg 3 Si 2 and AlMgSi at about 448 o C [12].
For point 2, an AlZnMgSi phase with interdendritic supported the development of strength [3]. Phases like CuMgAl 2 , Al 4 C 3 , Mg 5 Al 3 or Mg 5 Al 8 were also reported in [13] in the presence of AlZnMgCu.
From the 17 points probe in figure 4, spectrum 1 was an MgO rich dislocation with Al 3 Mg 47.90 phase. The highest Mg concentration observed was 47.90 % which was higher than the 20.8 % obtained by Schoenitz & Dreizin [14]. The 49.10 % O was an indication of holes from MgO and material handling process. Phases like Mg 3 Zn 3 Al 2 or Mg 32 (Zn, Al) 49 were reported by Watanabe et al [15] when Mg was higher in the Zn/Mg as in spectrum 1. The point contains a supersaturated solid solution of Mg in the γ-Al phase. The absence of Zn is peculiar to this spectrum. Spectrum 2 may support the formation of Al 67.87 Zn 4.02 Mg 7.31 phase.
The presence of Fe (contamination) as apparently shown in spectrum 3, 10 In all cases, the wt. % of Zn (4.02-5.56) indicates that the X7475 was within the green letter specification and classified as a low Zn 7xxx alloy [17]. Elemental analyses of spectrum 14, 15 and 16 were pores-free sites. The three sites are 5.50, 5.38 and 4.63 wt % Zn respectively. The wt. % of O is lower in comparison to spectra 1 and 2, whereas the duo was at the middle of the pore.
Defects might manifest like extensive piping in contrast to distributed shrinkage porosity [18]. The cylindrical, bright interstitial and vacancy-like defect in Figure 5 is within the α-Al, dominated by C (contaminant) and Zn. From the 8 spectra probe on the pipe, the core composed 64.90 C, 37.04 O, 0.52 Na, 7.52 Al (wt. %). These defects may be typical of the material with the characterization of which the geometry is complex to be linked to the H of the material, but nonmetallic inclusions [18]. In figure 5, spectrum 1 was a site at the core of the pipe with a composition of 52.  Spectrum 6 consists of 54.80 C, 30.55 O, 0.61 Na and 14.04 Al. Na as an impurity might be picked from secondary aluminium. Spectrum 7 has 43.88 C,14.47 O,38.93 Al,2.72 Zn. α-Al phase that was observed within the pores. Spectrum 8 was outside the pore and contains 7.55 O,0.97 Mg,83.40 Al,1.36 Mn,1.76 Fe and 4.97 Zn. However, the presence and any increase in Mn and Fe may increase the Chinese script morphology, intermetallic Al 6 (Fe, Mn). This phase may be favourable to the mechanical property. Rate of cooling is important in the formation and refinement of intermetallic Liu et al [19]. Spots 9 to 12 were C, Na, Mn and Fe free, with wt. % of O, Mg, Al and Zn ranging from 7.65-6.16, 1.33-0.98, 87.19-85.73 and 5.48-5.19 respectively.
Spectrums 13-15 are flicking through both the entire and specific points of the sample and consist of C, O, Mg, Al and Zn. The main phases in 7xxx alloys were α-Al and MgZn 2 . Spectrum 14 was an exception with 1.42 Fe as impurity. Al 0.5 Fe 3 Si 0.5 was phases within this spectrum. The MgZn 2 and α-Al were feasible phases in the presence of 0.86 Mg, 4.06 Zn and 60.62 Al.
Morphological characterizations and H (mechanical property) were products of alloy constituents, solidification conditions and HTs. The result of MH in Figure 6 saw an alloy of 5.0 wt. % Zn and artificially aged (T6)  Defect arrangement, size, intermetallic phase dispersal are morphological characteristics that strongly affect mechanical properties. Other microstructural considerations are the dendrite arm spacing, grain boundary, size and shape. Inclusions in the conditions are the eutectic modification and primary phase refinement which may be altered through HTs [19].
Increase in Mg may result in decrease in H. These categories of alloys had the least H as shown in Figure 6.