Introduction

Aluminum–silicon alloys are widely used for automotive, aerospace, and electronics industry applications due to their high strength to weight ratio, good castability, low thermal expansion coefficients, good corrosion resistance, and recyclability compared to other materials.1,2,3 The increase in demand of cast aluminum alloys and the surge in aluminum waste scraps has led to the secondary production/recycling of aluminum waste scraps.4,5 The cost and energy usage required to produce primary aluminum from bauxite ores is high compared to the secondary production of aluminum.5 The environmental benefits of secondary aluminum and aluminum alloys has increased the industrial attention on recycling.6 A recent study comparing the secondary aluminum production to primary aluminum production showed that secondary production requires only 6.37% and 4.45% of energy consumption and greenhouse gas emissions, respectively, of primary production.7 In order to achieve global net zero target by 2050, the International Energy Agency has recommended aluminum as a critical metal to increase the recycling and reuse.8 However, the main concern of the secondary aluminum alloys compared to primary aluminum alloys is the presence of impurities which hamper the mechanical properties thereby leading to early failure of these alloys.5

Among various impurity elements present in aluminum scarp, iron is considered to be the harmful impurity element in reducing the mechanical properties of alloys.5,9,10,11 The harmful effects of iron on tensile strength and fracture toughness of recycled Al-Si alloys has been widely reported.7,11,12 Iron has limited solubility in aluminum and forms brittle β-Al5FeSi intermetallic particles during solidification.5,11 It is largely accepted that it is the high brittleness and the morphology of β-phase iron intermetallic particles that allow crack nucleation and crack propagation, thereby reducing the mechanical properties and causing early failure of aluminum alloys.12,13,14,15 The sharp edges of β-phase particles which act as stress raisers aid in crack initiation and pathways for rapid propagation of cracks. Therefore, the ductility of the alloy drastically decreases with the increase in iron content and the size of intermetallic particles associated with aluminum alloys.16,17,18 Superheating temperature has a great effect on the solidification characteristics of Al-Si alloys.19,20,21,22 Although high superheating temperatures in aluminum alloys results in higher energy consumption, furnace wear, and high emissions, an appropriate superheating temperature would be attractive for industrial applications because of its advantages in enhanced alloying and scrap reduction.23 Mondolfo et al. reported that increasing the superheat temperature in aluminum alloys increases the solidification temperature and thereby reduces the interdendritic spacing, primary and eutectic constituent size, and grain size, even though the actual freezing time increases.22 Eskin et al.24,25 and Wang et al.26 studied melt thermal-rate treatment (MTRT) superheating on Fe-containing phases in the Al-15Si-2.7Fe alloy and found that MTRT at 930°C transforms the coarse primary silicon particles and plate-like Fe-containing intermetallic particles into small blocky morphology. They also found an improvement in the hardness of the Al-15Si-2.7Fe alloy with MTRT in a 930°C melt due to the refinement of the primary Si and Fe intermetallic particles.26 Optimum superheat temperature and time are required for a suitable grain refinement which in turn depends on the alloy composition.27,28 When an alloy is superheated, the solubility limit of iron impurities in the alloy melt increases with increasing melt temperature and reaches the optimum value.28,29 Hence, when the alloy is heated to above this temperature range, the iron impurity is found to be eliminated from the surfaces of nucleant particles, resulting in more nucleation and grain refinement.28

Even though superheating of molten metal is an effective method in refining the primary and eutectic constituents, one of the major limitations with it is the increased porosity in the alloy.30 The solubility of hydrogen drastically increases in molten aluminum at higher superheat temperatures than in solid aluminum. The solubility of hydrogen decreases during solidification and this rejected hydrogen results in the formation of porosity in cast aluminum alloys. Recently Wang et al.21 studied the effects of Ce addition and superheat temperatures on the formation of iron intermetallic particles and resulting microporosity in the Al-7.28Si-1.90Cu-0.33 Mg-0.14Fe-0.10Ti-0.10Zr-0.14 V alloy and found that the size of eutectic silicon- and iron-rich intermetallics was the smallest at 730°C compared to the casts at 680°C and 780°C. It is important to study the effect of superheat on the porosity formation and mechanical properties of the alloys. A detailed investigation is required to analyze the effectiveness of the superheating of molten Al-Si alloy melt to progress the research on this topic. In this paper, the morphological modifications and the porosity formation along with the quantified data are evaluated in three dimensions using x-ray computed tomography. This paper studies the effect of superheat temperature on the microstructure and mechanical properties of the Al-7%Si-2%Fe alloy at 700°C, 800°C, and 900°C. In order to better understand the effect of iron on the microstructure and mechanical properties, the Al-7Si alloy was also studied under the same superheat conditions. This detailed investigation of the Al-7Si alloy with higher iron content (2%) shows the benefit of superheat melt treatment in mitigating the harmful effects of iron on the mechanical properties of recycled aluminum alloys.

Experimental

Preparing the Alloys

The alloys were prepared from commercially pure Al, Al-20Si master alloy, and Al-10Fe master alloy supplied from Avon Metals, UK.

The Al-7Si alloys were prepared by melting 99.9% purity aluminum in a clay graphite crucible using a Carbolite high-temperature chamber furnace and then adding Al-20 wt.% Si master alloy to the molten alloy. The mixture was stirred intermittently to ensure proper mixing. After keeping the mixture at 700°C to ensure the master alloy was completely dissolved in the melt, the mixture was then poured into a clay graphite cylindrical mold (55 mm diameter, 97 mm deep). A portion of the Al-7 wt.% Si alloy was then re-melted to prepare the Al-7Si-2Fe alloy. When the Al-7 wt.% Si alloy was in a molten state, Al-20 wt.% Si master alloy and Al-10 wt.% Fe master alloy (ACI Alloys) was added in respective proportions to the melt and stirred well to dissolve them completely in the melt. After keeping the melt at 700°C to ensure all the components in the molten mixture were completely dissolved and homogenous, the casting was performed at the temperature of the melt superheat. The same procedure was repeated at 800°C and 900°C to prepare samples for 800°C (190°C superheat for the Al-7Si alloy and 175°C superheat for the Al-7Si-2Fe alloy) and 900°C (290°C superheat for the Al-7Si alloy and 275°C superheat for the Al-7Si-2Fe alloy) conditions, respectively. The average composition of the prepared alloys was measured by the spark OES method using Hitachi Foundry master pro. The measured composition of the final alloys is given in Table I.

Table I Elemental composition analysis of the prepared alloys (wt.%)
Full size table

Microstructural Characterization

Microstructural studies of the Al-7Si alloy and Al-7Si-2Fe alloys were carried out using scanning electron microscopy (SEM) after polishing the samples using standard metallographic procedures. The samples for microstructural characterization and CT studies were taken from the approximate middle of the cast and the five tensile test samples were taken around the middle of the cast. Elemental distribution maps were captured using a HITACHI TM3030Plus table top microscope coupled with an energy dispersive x-ray spectrometer (EDS).

X-ray Computed Tomography (XCT)

The tensile samples were scanned before and after the test using the Zeiss Versa at CiMat, WMG, University of Warwick, UK. Samples for the XCT measurements were machined as 5-mm-diameter cylinders and scans were performed using the parameters given in Table II. In order to achieve the best resolution possible, a × 0.4 flat panel was used as the detector, which was composed of 2048 × 2048 pixels resulting in a 3.67 µm resolution. The raw data were reconstructed using the Zeiss reconstruction software that uses the Filtered Back Projection algorithm creating a stack of DICOM images. The stack can then be used for analysis with Avizo 9.4.0 (FEI, USA; http://www.fei.com/software/avizo3d).

Table II X-ray tomography scanning parameters
Full size table

Mechanical Property Characterization

Cylindrical tensile samples were prepared using a CNC Lathe according to the dimensions specified in ASTM E−8 M.31 The tensile properties were evaluated using a 100-KN universal tensile testing machine (Instron 5800R) at a constant crosshead speed of 2 mm/min. The measurements were taken with five samples for each condition and the average values used in determining the tensile properties of the alloys. The fracture surface was then studied using a Zeiss Sigma SMT AG instrument.

Results

Figure 1 shows the optical microscopy images of the Al-7%Si alloy for different superheat conditions. Comparing the microstructure of the alloys cast at 900°C (Fig. 1c) and 700°C (Fig. 1a), it is evident that the superheating of the melt refined the dendritic microstructure of the Al-7%Si alloy. As shown in Fig. 1b, the alloy cast at 800°C (Fig. 1b) was found to have more porosity compared to the alloys cast at other superheat temperatures.

Fig. 1
figure 1

Optical microstructure of the Al-7Si alloy cast at melt temperatures of (a) 700°C, (b) 800°C, and (c) 900°C.

Full size image

Figure 2 shows the optical microscopy images of the Al-7%Si-2%Fe alloys for different superheat conditions showing that the intermetallic particles were more prominent at 800°C (Fig. 2b) compared to the other superheat conditions. However, the dendrite microstructure is significantly refined for the Al-7%Si-2%Fe alloys compared to the Al-7Si alloys at high superheats, showing that the iron intermetallic particles restricted the dendrite arm growth.

Fig. 2
figure 2

Optical microstructure of the Al-7Si-2Fe alloy cast at melt temperatures of (a) 700°C, (b) 800°C, and (c) 900°C.

Full size image

Figure 3 shows the elemental mapping of the Al, Si, and Fe phases in the Al-7%Si alloy for different superheat conditions. It is evident from the micrographs that the thickness of the silicon platelets increases with increasing the superheat temperature. This could be due to the increase in the solidification temperature range due to superheating, which increases the time for growth of the silicon flakes during solidification of the alloy.

Fig. 3
figure 3

SEM elemental analysis of the Al-7Si alloy cast with melt temperatures of (a) 700°C, (b) 800°C, and (c) 900°C (red color shows the distribution of aluminum, green color shows the distribution of silicon) (Color figure online).

Full size image

Figure 4 shows the elemental mapping of Al, Si, and Fe in the Al-7Si-2Fe alloy for different superheat conditions. In the presence of iron, the nature of the growth of the silicon phases and iron intermetallic particles is found to be different with different superheating. The iron intermetallic particles are much larger and thicker when the Al-7%Si-2%Fe alloy is heated to 800°C compared to 700°C, whereas superheating to 900°C results in refinement of the iron intermetallic particles (Fig. 4). The silicon particles also show the same behavior as the iron intermetallic particles. The rapid nucleation of the iron intermetallic particles in the alloys cast at 900°C results in the formation of a large number of finer intermetallic particles which act as nucleation sites for the silicon particles.32 These multiple nucleation events of the silicon particles results in the formation of finer silicon particles.

Fig. 4
figure 4

SEM elemental analysis of Al-7Si-2Fe alloy cast with melt temperatures of (a) 700°C, (b) 800°C, and (c) 900°C (red color shows the distribution of aluminum, green color shows the distribution of silicon, blue color shows the distribution of iron) (Color figure online).

Full size image

Figure 5 shows the EBSD maps of the Al-7Si alloy and Al-7Si-2Fe alloy cast at different superheat conditions. In the case of the Al-7Si alloy, grain size decreased on the cast at 900°C compared to that at 700°C. This is due to the melt homogenization effect reported by Eskin et al.33 In the case of the Al-7Si-2Fe alloy, the initial superheating results in dendritic growth followed by iron intermetallic modification, resulting in abnormal grain growth.34

Fig. 5
figure 5

SEM-EBSD analysis of (a) Al-7Si (700°C), (b) Al-7Si (900°C), (c) Al-7Si-2Fe (700°C), and (d) Al-7Si-2Fe (900°C).

Full size image

The mechanism behind this particular behavior in the formation of needle-shaped iron intermetallic particles is explained by Samuel et al.35 γ-aluminum oxide particles form at low superheats and act as nucleation sites for the intermetallic particles during solidification. Studies have reported the nucleation of iron intermetallic particles on aluminum oxide,33 But, at high superheats (above 800°C for 2% iron), the γ-aluminum oxide present in the alloy melt transforms to α-aluminum oxide, which is not a good nucleating surface for iron intermetallic particle formation in the melt, leading to a decrease in the nucleation potential for the iron intermetallic particles. When the melt is superheated to 800°C, the intermetallic particles nucleate and grow on the γ-aluminum oxide particles. Since the temperature change required to start the solidification at 800°C is more compared to at 700°C, the particles will get more time to grow and their size increases. When the melt is superheated at 900°C, the time required for the transformation of the α-aluminum oxide to γ-aluminum oxide is less.33 The nucleation and growth of the intermetallic particles starts on or near the α-aluminum dendrites.34 Since the nucleation sites on the α-aluminum dendrites is more, the rate of nucleation increases, resulting in the formation of a large number of finer intermetallic particles. Dahlborg et al.33 also reported the transformation of γ-aluminum oxide to α-aluminum oxide when the Al-Si melt was superheated to 900°C. Recently, Reza et al.36 reported the formation of α-phase iron intermetallic particles in the A380 alloy instead of the expected β-phase iron intermetallic particles above 890°C. Ahmad et al.37 also reported refinement of the intermetallic plates to globular forms on superheating the Al-Si alloy with 1.12% Fe and 1.94% Fe at 710°C and 1000°C. The mechanism behind the formation of α-phase intermetallic particles was explained on the basis of variation of the β-phase iron intermetallic formation temperature on the superheat temperature.37 The primary β-phase growth time diminishes as the β-phase iron intermetallic formation temperature becomes close to the silicon eutectic temperature.35

Figure 6 shows the 3D reconstructed x-ray CT images of Al-7Si-2Fe alloys cast at 700°C and 900°C. The white-colored structure represents the aluminum–silicon alloy matrix, the red-colored features represent iron intermetallic particles, and the blue-colored features represent the porosity. The iron intermetallic particles were found to have a large platelet shape with sharp edges at 700°C, whereas the intermetallic particles were found to be refined at 900°C, resulting in a large number of smaller structures. The porosity is not clearly visible in these images since the intermetallic particles block the view of porosity. However, from Fig. 6a, it is clear that the porosity formed at 700°C is primarily associated with the intermetallic particles. A detailed analysis of the structure and quantification of porosity is needed to further explain the formation of porosity.

Fig. 6
figure 6

3D reconstructed x-ray CT images of Al-7Si-2Fe alloys cast at (a) 700°C and (b) 900°C (Color figure online).

Full size image

Figure 7 shows the 3D reconstructed x-ray CT images of the porosities in the Al-7Si-2Fe alloys cast at 700°C and 900°C. The blue-colored features represent the porosity. The porosities formed in the Al-7Si-2Fe alloys are not symmetrical and cannot be considered as gas porosities. The equivalent diameter based on the volume of the pores in this figure has been used to calculate the pore size distribution in the alloy, as shown in Fig. 7.

Fig. 7
figure 7

3D reconstructed x-ray CT images of (a) Al-7Si-2Fe alloy cast from melt of 700°C, and (b) Al-7Si-2Fe alloy cast from melt of 900°C (Color figure online).

Full size image

Figure 8 shows the pore size distribution of Al-7Si-2Fe alloys cast at 700°C and 900°C. The pores below the size range of 10 µm were considered as noise present in the imaging or processing. Even though the size of the pores is very small in the Al-7Si-2Fe alloy after superheating, the quantity of pores is greater compared to that at 700°C. The size of pores formed at 700°C is twice that of pores formed on superheating. The largest pores observed at 900°C are in the range of 30–35 µm, whereas at 700°C pores larger than 35 µm are observed and the largest pore is 52.6 µm. This may be due to the larger-sized intermetallic particles formed at 700°C. From Fig. 5a, it is clear that the pores are formed along the surface of the intermetallic particles and in between the intermetallic plates. These pores may be formed during solidification due to the incomplete flow of liquid metal. When the intermetallic plates grow large, due to its higher surface area they restrict/block the flow of liquid metal, resulting in the formation of larger pores between them.38

Fig. 8
figure 8

Porosity distribution of Al-Si-2Fe alloys.

Full size image

Roy et al.39 also reported the poor feeding characteristics and increased shrinkage due to the formation of β-phase iron intermetallic particles in aluminum alloys. They suggested that the β-phase iron intermetallic particles act as nucleation sites for pores. The possible mechanism involved in the formation of porosity was explained as the long needle-shaped morphology of the β-phase iron intermetallic particles which block the interdendritic path, and obstruct the liquid flow leading to micro-shrinkage porosity.40 However, from the 3D observations in this study, it is clear that the platelet-shapes morphology actually blocks the liquid flow, especially in the area between different platelets, the resulting in shrinkage porosity.

In order to understand the effect of superheat on mechanical properties of the alloy, tensile tests were performed, and Fig. 9 shows the tensile stress versus strain curves of the Al-7Si alloys and Al-7Si-2Fe alloys cast at 700°C, 800°C, and 900°C. Table III shows the tensile strength and strain values with standard deviations/errors for all the alloys used in this study. It is evident from the figure and the values in Table III that the ultimate tensile strength and elongation were found to be higher for the alloy cast at 700°C compared to 800°C and 900°C. The grain size refinement should have a direct effect on the mechanical properties of these alloys. However, unfortunately, due to the higher amount of the porosity and iron impurities, the grain size refinement effect cannot be concluded in the mechanical properties observed. Even though the grain size refinement increases the mechanical properties, the porosity and higher iron impurities are major factors affecting the mechanical properties, which could be due to the increase in porosity at higher superheat temperatures of the melt. The addition of 2% iron drastically reduced the tensile properties of the alloys for all superheats. However, it was found that the Al-7Si-2Fe alloy with the superheat of 900°C exhibits tensile strength and elongation lower than the alloy cast at 700°C and higher than the alloy cast at 800°C. This could be due to the particle refinement that occurred in the alloy cast from 900°C although the porosity is much higher at 900°C. Another reason could be due to the fine size of a greater number of particles compared to the large-sized particles observed in the alloy at 700°C. Similar observations were reported on superheating the Al-Si (LM6) alloy where the structural refinement improved its tensile properties.41 The β-phase iron intermetallic particles were found to be refined with increasing the superheat temperature.41

Fig. 9
figure 9

Tensile stress versus strain curve on tensile loading of Al-Si and Al-Si-Fe alloys.

Full size image
Table III Tensile properties of the alloys
Full size table

The present study using the 3D XCT technique clearly shows that superheating Al-7Si-2Fe alloys refines the iron intermetallic platelets and reduces the formation of shrinkage porosity. The tensile test results suggest that the superheating improved the mechanical properties by refining the intermetallic particles, but that the increased porosity on superheating eliminates benefits resulting from the refinement of intermetallic particles. The 3D visualization of alloys not only shows the morphology and size of intermetallic particles but also helps in understanding the effect of increased porosity on early failure of the alloy with higher superheat temperatures. It is evident that, by improving the melt treatments to reduce the porosity formation with superheat could help in improving the mechanical properties of Al-Si-2Fe alloys.

Although microstructural changes were observed with superheating, the increased porosity is probably dominating the outcome of the mechanical test results. Future work is needed in this area to analyze the effect of microstructural changes on the mechanical properties without having a dominating effect of porosity. This will need the analysis of more samples, preparing samples with a more inert atmosphere, and increasing the number of temperature points for the study between 700°C and 900°C.

Conclusion

  • Al-7 Si and Al-7Si- 2Fe alloys were prepared using the casting method at three different temperatures to investigate the effect of superheat on the microstructure and mechanical properties of these alloys.

  • Microstructural studies clearly indicate that the increasing superheat temperatures result in refinement of intermetallic particles in the Al-7Si-2Fe alloy.

  • The Al-7Si alloys have been found to have better tensile strength and elongation compared to the Al-7Si-2Fe alloys for respective superheat temperatures. This indicates the deteriorating property of the iron intermetallic particles.

  • The porosity formed on the melt has been assumed to have considerable negative effects on the tensile strength and elongation, therefore possibly nullifying the property gain expected by refinement of the intermetallic properties on superheating.

  • Controlling the porosity formed may improve the mechanical properties of recycled Al- 7Si alloys along with the microstructure improvement observed on superheating.