Evolution Behavior of the Surface Oxide Film of Al Alloy Scraps in the Melt

Abstract

The oxide film on the scrap surface is one of the primary sources of oxide inclusions in the aluminum melt. Understanding the evolution of the oxide films in the aluminum melt is an important step for developing efficient recycling technologies and controlling the quality of the product. In the present study, we studied the evolution behavior of the oxide film in the aluminum melt. The oxide films were introduced via aluminum alloy scraps into the melt, and the micro-morphology and composition of the oxide film were analyzed by scanning electron microscope and energy spectrum. Results show that the oxide film on the surface of 1235 alloy foil, A356 alloy turning, and 5083 alloy scalping were broken into small flake oxide film and then transformed into minor granular oxide when the scraps were charged into commercial purity aluminum melt. However, in aluminum alloy melt containing magnesium, the oxide film remained an intact sheet shape up to a certain melt dwelling time.

1. Introduction

Oxides in Al alloys are usually regarded as detrimental defects to casting performance. Oxide inclusions in aluminum alloy affect the fatigue properties, especially the presence of long sheet-shaped oxide film within the aluminum alloy matrix, which leads to cracks and damages the mechanical properties of the alloy. However, oxides are not always harmful to aluminum. The homogeneous dispersion of oxide nanoparticles in metal matrix alloys can play the role of a dispersion-strengthening phase. Oxide dispersion strengthening typically enhances mechanical properties over an extensive temperature range by impeding dislocation motion via Hall-Petch or Orowan mechanisms [1,2,3]. Fan et al. [4] have found that after an intensive shearing, the oxide films entrained within melts can be transformed into fine dispersed uniformly oxide particles in the melt and act as nucleation centers, thereby reducing the grain size. In summary, the effect of oxides on alloy properties depends on the morphological size of the oxide and its distribution in the alloy.

The oxide inclusions in aluminum alloy mainly come from the passive oxide film on the surface of the charge and the oxide film on the surface of the melt formed in the smelting/remelting process. The oxidation behavior of molten aluminum has been studied extensively. Bonner [5] and Impey [6] believed that the surface oxide film of commercial purity aluminum melt would change from γ-Al₂O₃ to α-Al₂O₃ at 750 °C. Aryafar et al. [7] suggested that γ-Al₂O₃ would form on the surface of A356 alloy melt and then be converted to MgAl₂O₄ at high temperatures. Akbarifar et al. [8] believed that the surface oxide film of Al-Mg alloy melts with low Mg content was composed of MgAl₂O₄ and γ-Al₂O₃, while the surface oxide film of Al-Mg alloy melts with high Mg content were MgAl₂O₄ and MgO. The evolution behavior of surface oxide film immersed and suspended in aluminum melt has been reported by researchers. Campbell et al. [9] believed that the oxide film on the surface of aluminum melt would form a double-layer oxide film after being involved in the melt and proposed the concept of “Bifilm”. Bakhtiarani [10], Ahmadpour [11], and Amirinejad [12] have respectively studied the evolution behavior of the surface oxide film in the commercial pure aluminum, A356 alloy, and Al-Mg alloy melt after being immersed in the melt, and they all believed that the oxide film would change to a double-layer structure in the aluminum melt. However, Wang et al. [13] believed that when the gas containing oxygen was introduced into the aluminum melt, the oxide inclusions generated existed in granular form. Li et al. [14] used 1000 series foil to introduce oxides into cp Al melt and observed oxides with different shapes and morphology. Li et al. [15] studied oxide film generated by directly remelting 5182 sheets. Both Mg-Al-O oxide film and cuboid spinel particle were observed to co-exist in the melt.

On the way toward a circular economy, recycling aluminum becomes increasingly important. To reduce the production cost, more and more waste aluminum alloys produced in the production and processing are used for remelting. In order to produce primary quality aluminum products with secondary sources of input alloy, the controlling of non-metallic inclusions (e.g., oxide) plays a critical role. The oxide film on the surface of the return charge during the remelting cycle has become one of the main sources of oxidation inclusion in the aluminum melt. The evolution behavior of the surface oxide film suspended in the aluminum melt has been rarely reported. In this paper, the annealed 1235 alloy foil, 5083 alloy scraps, and A356 alloy scraps were used for remelting studies where oxide films’ evolution behavior in aluminum base melt with different compositions. The microstructure analysis of oxide films was performed to investigate the influence of alloying element magnesium on oxide inclusions evolution behavior. The findings of the present study provide theoretical and practical guidance for the melting and melt treatment process when return scraps are charged into the melt bath.

2. Materials and Methods

A356 alloy and Al-4.5Mg alloy were prepared by using 99.7 wt%Al, 99.7 wt%Mg, and 99.7 wt%Si as raw materials. The aluminum foil used in the experiment was annealed 1235 aluminum foil with a thickness of 0.006 mm produced by a Chinese company. The A356 alloy scraps and 5083 alloy scraps were respectively the first turning scraps and milling scraps provided by a Chinese company.

An amount of 100 g commercial pure aluminum, A356 alloy, and Al-4.5Mg alloy were melted and kept at 750 °C respectively, 10 g 1235 aluminum foil, A356 turning and 5083 scalping were added into the above alloy melt and held for 15 min and 30 min. The conditioned melt was then poured into the steel mold for cooling and solidification.

Samples were sectioned, ground, and polished in compliance with standard metallography techniques. The samples which were used for microstructural characterization were electrochemically polished. The electrolytic polishing solution is made by mixing perchloric acid with anhydrous ethanol in a ratio of 1:9. The FEI QUANTA 200FEG scanning electron microscope (SEM) coupled with EDAX energy dispersive spectrometer (EDS) was used to analyze the micro-morphology and micro-composition of the samples.

3. Results and Discussions

3.1. The Evolution Behavior of the Surface Oxide Film of 1235 Alloy Foils in the Melt

The 1235 alloy foil is a typical aluminum foil. There are many applications of 1235 alloy foil, such as thermal insulation, capacitors, household foils, wraps, bags, etc. [16] Fe and Si are major alloying elements for 1235 alloys. As can be seen from Figure 1, the surface of 1235 alloy foil is smooth, and the second phase in the alloy is visible (as shown by the black arrows), indicating that the surface oxide film is extremely thin. Figure 2 shows the micro-morphology of 1235 alloy foil after holding in the commercial pure aluminum melt at 750 °C for 15 min. It can be seen that the oxide film is broken into many small fragments, which are clustered and distributed, and there are a certain number of small size holes around the pieces (as shown by the black arrows in Figure 2b). Based on the morphology, the piece of oxide observed in Figure 2 appears to be amorphous. When aluminum foils were held in the aluminum melt, due to the different thermal expansion coefficients between the surface oxide film and substrate, the surface oxide film was forced to break and formed small pieces of oxide film during the remelting process in a charging art. The holes may be formed due to the air not being fully consumed in the holding process. When the foils were added to the aluminum melt, some air was wrapped in aluminum foil. On the other hand, the solubility of hydrogen in liquid aluminum and solid aluminum is 19 times different, and hydrogen is segregated from the solid phase to the liquid phase during the solidification of the melt. The pore grows when the hydrogen concentration in the liquid phase reaches saturation. The oxides in the aluminum melt provide a nucleation site for the formation of hydrogen bubbles. The hydrogen bubble may nucleate upon the oxides due to the wettability reason during solidification. After the formation of hydrogen nucleation, the supersaturated gas diffuses to the gas nucleus and makes the bubbles grow. If the bubbles fail to float to the melt surface during the solidification of the melt, they remain in the alloy as pores and shrinkage in close association with oxide films [17,18].

As is shown in Figure 3, the oxide film has changed from a fine flake shape to a crystallized cuboid shape with a uniform size of 1~2 μm. Snijders [19] et al. believed that when the temperature is lower than 300 °C, the oxide film formed on the surface of pure aluminum is amorphous aluminum oxide with a thickness of several nanometers. The amorphous aluminum oxide will start to transform into γ-Al₂O₃ when the temperature is higher than 300 °C. Dignm et al. [20] suggested that the surface oxide film of aluminum foil would convert from amorphous alumina to crystalline γ-Al₂O₃ above 450 °C. Preston et al. [21] indicated that the amorphous oxide film on the surface of aluminum foil would convert to crystalline γ-Al₂O₃ at 680 °C. Wang et al. [13] analyzed the oxide film formed on the surface of aluminum melt at 750 °C and believed that the oxide film was crystalline γ-Al₂O₃. Therefore, it can be inferred that under the experimental conditions in this paper, the surface oxide film of 1235 alloy foil changed from amorphous Al₂O₃ to crystalline Al₂O₃ in the melt with an extension of melt holding time.

Since no physical field with a stirring function was applied to the aluminum melt in this experiment, the aluminum melt was kept in a static state of insulation during the experiment. The oxides formed by the 1235 alloy foil are aggregated in the aluminum melt (as shown in Figure 3). When using the waste 1235 alloy foil as the return charge for the production of 1235 alloy foil, especially the ultra-thin aluminum foil, these aggregated oxide particles formed by waste foil may cause pinholes in the aluminum foil and break the strip during the rolling process, and even lead to roll wear [16]. On the anther side, in the work by Wang et al. [13], oxides formed in pure Al were characterized. Theoretical analysis and the extensive TEM work confirmed that γ-Al₂O₃ and α-Al₂O₃ were potent substrates for heterogeneous nucleation of Al grains. Therefore, if a physical field is applied to disperse agglomerated granular oxides in the aluminum melt uniformly, these small-size oxides can be used as refining agents for the 1235 alloy, refining the alloy grain and thus improving the mechanical properties of the alloy.

3.2. The Evolution Behavior of the Surface Oxide Film of A356 Alloy Scraps in the Melt

A356 alloy is a casting aluminum alloy widely used in the automotive industry. The alloying element magnesium is the principal strengthening element of A356 alloy with a content of 0.25–0.45 wt%. In the machining process of A356 castings, a large number of aluminum chips are generated, and recycling these aluminum chips is a critical way to reduce production costs. It can be seen from Figure 4 that the surface of A356 alloy scraps is rough, and there are cracks. The energy spectrum shows that the surface of A356 alloy scraps had been oxidized. Since a clear magnesium peak can be seen in the energy spectrum, it indicates that the surface oxide film of aluminum chips is not aluminum oxide but aluminum-magnesium oxide. As is shown in Figure 5, the surface oxide film of A356 alloy scraps was broken into minor oxide fragments. The oxygen and magnesium peaks in the energy spectrum of Figure 5 are highlighted, which indicates that the magnesium signal is not emitted from the matrix but from the oxide film. The oxide film is an oxide containing Al and Mg.

As shown in Figure 6, after being held in the commercial pure aluminum melt for 30 min, the surface oxide film of A356 alloy scrap has converted into oxide particles with the size of hundreds of nanometers, and the oxide is composed of Al, Mg, and O. Nano-scale oxide particles can be potent sites for heterogeneous nucleation, resulting in grain refinement. The MgAl₂O₄ particles had a cube-on-cube orientation relationship with α-Al and can be heterogeneous nucleation substrates for α-Al [13]. In addition, these oxide particles can be used as a strengthening phase of A356 alloy to improve the alloy properties under the assumption that they are well dispersed.

It can be seen in Figure 7 that the oxide film, which is thick and in the shape of the film, covers the secondary dendrite of the A356 alloy. The element composition of the oxide film is Al, Mg, and O. During the solidification of the alloy, the oxide film can act as a barrier against the flow of the interdendritic liquid, leading to the formation of a shrinkage pore. In terms of crack initiation, the porosity of castings has an adverse effect on tensile and fatigue properties. The large shrinkage pores are the dominant sources for crack initiation. In the initial stress loading stage, the local pore edge is prone to stress concentration, leading to micro-plastic deformation and reducing fatigue performance [22].

As shown in Figure 8, the relatively thick oxide film covered the secondary dendrite of A356 alloy. With the longer holding time, the oxide film was broken into small fragments. The components of the oxide film are Al, Mg, O, and N. Mahmoud et al. [23] believed that when the surface oxide film of A356 melt was immersed in the melt, the melt would react with the oxygen and nitrogen trapped in the oxide film to generate oxides and nitrides. The energy spectrum shows nitrogen and oxygen elements in the oxide film, which further confirms the previous result.

As mentioned above, the surface oxide film of A356 scrap was broken from a large film shape to a small flake shape and then to a cuboid shape in the commercial pure aluminum melt. Whilst in the A356 alloy melt, the oxide film changed from large to minor. Therefore, it can be indicated that when the A356 alloy scrap is used as the return charge, if the Mg alloying operation precedes the feeding, the oxide film tends to transform into flaky oxide. On the contrary, if Mg alloying was made after a long time of scrap feeding, the oxide film may transform into small-sized particles. In the production of A356 castings using A356 alloy scraps as return charge, scrap is added and held for 30 min before adding alloying elements to the aluminum melt, which can transform the surface oxide film of scraps into small oxide particles. The oxide particles can be used as a heterogeneous nucleation core or strengthening phase to improve the mechanical properties of the alloy when they are well dispersed. The addition of A356 alloy scraps after melt alloying will cause the surface oxide film to remain in the form of the large-size flake oxide film in the aluminum melt. The large size of oxide film remaining in the melt is easy to cause porosity, which harms the mechanical properties of the alloy.

3.3. The Evolution Behavior of the Surface Oxide Film of 5083 Alloy Scraps in Melt

The 5083 alloy is an aluminum alloy with high magnesium content, and its main alloying element is magnesium, with a content of 4.0–4.9 wt%. The 5083 alloy is widely used in the naval, automobile, and petrochemical industries. The machining of the ingot is an essential step in the production process of the 5083 alloy sheets, and a large number of milling chips are produced in this process. Figure 9 shows the morphology of the surface oxide film of 5083 alloy scraps. It can be seen that the surface of the oxide film is undulating and rough. There are many small particles on the surface of the oxide film. The composition of the oxide film is Al, Mg, and O.

Figure 10 shows the microscopic morphology of 5083 milling chips when they were kept in the commercially pure aluminum melt for 15 min. It can be seen that there are many nanoparticles in the oxide film. The energy spectrum analysis result shows that the oxide is composed of Al, Mg, and O.

Figure 11 shows the micro-morphology of 5083 alloy scraps after being held in the commercial pure aluminum melt for 30 min. It can be seen that the oxide film has changed into crystallized granular oxides of hundreds of nanometers in size. The EDX energy spectrum analysis result shows that the oxide is composed of Al, Mg, and O. Aryafar et al. [7] believed that the surface of the Al-Mg alloy was first oxidized to generate Al₂O₃, which further transformed to MgO or MgAl₂O₄. Jorge et al. [24] thought that the oxide film tends to undergo a transition from Al₂O₃ to MgAl₂O₄ in the Al-Mg alloy melt with magnesium content less than 1 wt% at 750 °C. After 10 g 5083 alloy scraps were melted in 100 g commercial pure aluminum melt, the magnesium content in the melt was 0.41 wt%, which was less than 1 wt%. It can be inferred that the oxide in Figure 11 is the MgAl₂O₄ phase. In the Al-Mg alloy, the dispersed discrete MgAl₂O₄ particles can act as potent sites for the heterogeneous nucleation of α-Al grains, which is supported by the well-defined orientation relationship between MgAl₂O₄ and α-Al [13]. Li et al. [25] suggested that the effective dispersion of the MgAl₂O₄ particles can make a significant grain refinement of Al-Mg alloys. In the production of 5083 alloys using 5083 alloy scraps as return charge, scraps are added and held for 30 min before adding alloying elements Mg to the aluminum melt, which can transform the surface oxide film of scraps into small oxide particles. The oxide particles can be used as heterogeneous nucleation substrates to refine the α-Al grains.

Figure 12 shows the micro-morphology of 5083 alloy scraps after being held in Al-4.5Mg melt for 15 min. Unlike the results of 5083 alloy scraps held in the commercial pure aluminum melt, the oxide film was not broken and instead in thick sheets form. The surface of the oxide film is rough, with a large number of small dots. The EDX energy spectrum analysis result shows that the oxide is composed of Al, Mg, and O. As shown in Figure 13, the thickness of the oxide film is about 192 nm after being held in Al-4.5Mg alloy melt for 15 min. The Al-4.5Mg alloy melt contains 4.5% magnesium. The high content of magnesium in the melt has a great effect on the evolution behavior of the surface oxide film of 5083 alloy scraps in the melt, which leads to the surface oxide film of 5083 alloy scraps presence as large size thick flake oxide film instead of the fragment oxide film. In other words, the presence of a high content of magnesium in the aluminum melt makes the surface oxide film of 5083 alloy scraps difficult to fragment in the melt.

As shown in Figure 14 and Figure 15, after being held in Al-4.5Mg alloy melt for 30 min, uniformly sized polyhedral particles grew on the oxide film surface. The EDX energy spectrum analysis result shows that the oxide is composed of Al, Mg, and O. On the side in the direction of contact with the melt, particle oxides with the size of about 850 nm grow out (as shown by the black arrows in Figure 15). The thickness of the oxide film further increased to 310 nm. The thickening of the oxide film and the formation of large amounts of oxides on the surface of the oxide film are the results of the reaction between the surface oxide film of 5083 alloy scrap and the Al-4.5Mg alloy melt. The thickening of the oxide film and the formation of oxides contenting Al and Mg on the surface of the oxide film will inevitably consume the alloying element magnesium in the melt, resulting in the loss of magnesium depending on number of oxide films in the melt.

Field et al. [26] studied the oxidation behavior of solid Al-4.2Mg alloy at 400~575 °C and concluded that MgO and MgAl₂O₄ were formed on the surface of Al-4.2Mg alloy. Bakhtiarani et al. [27] studied the oxidation behavior of Al-4.5Mg alloy held at 750 °C. They believed that the γ-Al₂O₃ on the surface oxide film reacted with the Mg element in the Al-4.5Mg alloy to form MgO, and with a longer holding time, MgAl₂O₄ nucleated and grew at the site between MgO and melt. As a result, the oxide film cracked due to the stress, and the fresh liquid aluminum reacted with the gas sandwiched in the oxide film to form γ-Al₂O₃ and then generated MgO. Since the formation of MgO consumes the Mg element near the oxide film, the Mg content decreases, and MgAl₂O₄ will be formed in the low magnesium environment. It can be inferred that the particle formed on the contacting side of the oxide film is MgAl₂O₄ (as shown in Figure 15).

In summary, when the 5083 alloy scraps were held in the commercial pure aluminum melt, the surface oxide film would transform to cuboid-like oxides, while held in Al-4.5Mg alloy melt, the oxide film became thicker and more difficult to break. Therefore, it can be inferred that the alloying element magnesium played an important role in the evolution of the surface oxide film of 5083 alloy scraps in the aluminum alloy melt. It is the alloying element magnesium in the aluminum melt that causes the oxide film to change to a thicker sheet shape. So, during the production of 5083 alloys, if 5083 alloy scraps are intended to be used as a return charge, it is better to add the scraps and hold for 30 min before adding alloying elements Mg to the aluminum melt.

4. Conclusions

• When 1235 alloy foil was held in the commercial pure aluminum melt, the surface oxide films were broken into fine oxide film in 15 min, and then transformed into cuboid shape oxide in 30 min. When the aluminum alloy melt with 1235 alloy foil was treated, the appropriate holding time was helpful to break the oxide film in the melt.
• The surface oxide film of A356 scrap was broken from a large film shape to a small flake shape, and then into a cuboid shape in the commercial pure aluminum melt. While in the A356 alloy melt, the oxide film changed from large to minor and covered the secondary dendrite of the A356 alloy.
• When the 5083 alloy scraps were held in the commercial pure aluminum melt, the surface oxide film transformed to cuboid-like oxides. When the scrap was charged into and held in Al-4.5Mg alloy melt, the oxide film became thicker and more difficult to break.
• If the alloying element magnesium is added before purification treatment or the oxide film in the melt is not removed thoroughly, the presence of alloying element magnesium will make the oxide film thicker and in the form of a thick sheet shape.