Synthesis of Especial Al-Si Spare Parts Using Locally Produced Metallurgical Grade Silicon (MG-Si)

Ali HH, Moussa ME and El-Barawy KA

Published on: 2023-12-31

Abstract

Aluminum alloys dominate the world of light metals, especially in the automobile and aerospace industries. Aluminum-Silicon (Al-Si) alloys are preferred for the casting process due to the presence of silicon, which makes them highly castable. In the current study, locally manufactured metallurgical silicon (MG-Silicon) is added to an aluminum melt to produce an alloy based on aluminum that contains 7% silicon by weight (A356). The produced alloy is then used to manufacture a specific aluminum-silicon spare part known as a "star" that is used for the diesel engine's cooling fan, using casting techniques. The physicochemical and morphological properties of the produced alloy will be examined using XRD, spectrophotometer, and optical microscope. Microstructure analysis revealed the existence of primary silicon and aluminum (α-Al) particles surrounded by eutectic silicon and Mg2Si particles. The manufactured spare part "star" is mechanically tested and found to have a Brinell hardness of approximately 75 HB.

Keywords

Al-Si alloys Synthesis of Al-Si Spare part Star A356

Introduction

Aluminum-silicon (Al-Si) alloys are well-known for their low density, high specific strength, ductility, good castability, high thermal conductivity, high corrosion resistance, cosmetic surface quality, resistance to hot tearing, and other properties that make them suitable for large-scale automotive and aerospace applications [1, 2]. Aluminum alloys have been widely employed in the automobile sector among metals due to their outstanding casting qualities and mechanical properties [3]. Aluminum-silicon (Al-Si) alloys favour the casting process because to the presence of Si, which makes them highly castable [4]. Furthermore, it enhances fluidity, decreases shrinkage, and provides exceptional mechanical qualities. Al-Si alloys are produced using various casting techniques, but only a few are economical. The high-pressure die-casting (HPDC) technique controls over 80% of metal castings in today's automotive market because it is more cost-effective and has a high cycle rate [6, 7]. Several considerations go into choosing the casting method, with cost-effectiveness being the main motivator [8]. The easiest and simplest method for casting aluminum is sand casting. Aluminum sand castings contribute to approximately 11% of total weight of aluminum castings produced worldwide [9, 10]. Sand molds are generally simple to make, and mold modifications are simple and use a variety of sands, including silica, zircon sand, chromite sand, and others. These should have tight control over grain size, particle dispersion, permeability, and so on [11, 12]. Different factors affect the Al-Si alloy produced during the solidification of castings, affecting its structure and mechanical properties, such as chemical composition, liquid metal treatment, cooling rate, and temperature gradient. So, it is critical to understand how the alloy solidifies at different cross sections of the cast product and how this affects the mechanical qualities when developing for casting the automobile parts. Among these, the chemical composition is the most important [14]. One method for improving mechanical properties is the addition of chemical modifiers during the casting process, which influence microstructure development during solidification [2]. Some elements such as Sr, Na, Ca, Ba, or Eu could work as chemical modifies because their addition in the hundreds of ppm range changes the eutectic Si morphology from coarse plate-like to fine fibrous, which enhances strength and ductility [15, 12]. Furthermore, iron, calcium, and phosphorus concentrations in silicon are critical for applications. Aluminum alloys containing 7 to 12% aluminum can be age-hardened and have their yield strength doubled by adding a few tenths of percent magnesium so that there is a significant rise in the production of Al-7%Si-Mg (A356) or Al-12%Si alloys. Consequently, to avoid alloy production with large needle-shaped particles, which lead to good corrosion properties, the alloys should be treated with sodium, strontium, or phosphorus [15]. The A356 alloy, which includes

 

around 7 wt. % silicon and 0.3 wt. % Mg is widely used in the power tool, vehicle, and aerospace/military industries due to its good castability, corrosion resistance, and mechanical qualities in heat-treated conditions. The number of impurities determines the grade of this alloy, and the primary alloy A356 contains less iron, providing superior qualities at a higher cost [16].

As a result, the goal of this research is using sand casting technique to produce a spare part "star" that is used for the fixation of the cooling fan of diesel motors in trucks and Lorries made of aluminum-silicon alloy (A356) that is dependent on locally manufactured MG-Silicon (MG-Si) from carbothermic reduction smelting of Egyptian quartz [17].

Experimental Work

Table 1: XRF analysis of produced MG-Si.

Elements

Si

Ti

Al

Fe

Others

Wt%

98

0.02

0.01

1.85

0.12

Figure 1: The photograph of the locally produced MG-Si.

Figure 2: XRD patterns of produced MG-Silicon from Egyptian quartz.

Figure 3: SEM micrograph of produced MG-Silicon.

 

Microstructure Characterization

The optical microscope (OM, model ZEISS) was used to investigate the microstructure of the specimens. All metallographic samples were taken from the same position of 10 mm at the bottom of the castings. The samples were ground on wet silicon carbide papers up to a grit size of 1200 and then polished with alumina paste. After that, the specimens were etched in a 0.5% HF acid solution. The specimens were then rinsed in anhydrous ethyl alcohol and dried using a dry air blast. The samples' phase constituents were analyzed using X-ray diffraction (XRD) with a LabX model and Cu Kα radiation. The scan involved a step scan of 2θ from 20° to 90° with an increment of 0.02° and a scanning speed of 4°/min. Different phases were identified using JCPDS (Joint Committee on Powder Diffraction Standards) cards. The chemical composition of the prepared alloy for producing the star was calculated using optical emission spectroscopy from Oxford Foundry Master in Germany. The morphology of the material was studied using a field emission scanning electron microscope (FESEM, Quanta, FEG 250) at a 20 kV accelerating voltage in the secondary electron (S.E.) mode. A thermal analysis was conducted to investigate the solidification characteristics of the alloy. For this, a sample was taken by pouring the melt at 750°C into a Quik-Cup resin-bonded sand cup with dimensions as shown in Figure (4). A high-sensitivity type K thermocouple (chrome-alum) was placed horizontally at the centre of the cup to record the temperature during solidification. The thermocouple was protected using a silica glass tube. The data for thermal analysis were collected using a data logger and transferred to a personal computer for analysis. A Wilson hardness tester was used to measure the Brinell hardness of the A356 alloy spare part. The test used a 500 kg applied load, a 10 mm diameter steel ball, and 15 s holding time. The average of five readings was taken to obtain accurate values.

Figure 4: Schematic of (a) experimental setup for thermal analysis and (b) resin-bonded sand cup with type K thermocouple.

 

Results and Discussion

Manufacture of Spare Part “Star” Steps

Figure 5 depicts the "Star" spare part, which is made of an aluminum-silicon alloy A356. The spare part "Star” is used to fix the cooling fan of diesel trucks and Lorries.

Figure 5: Selected part “Star” used for fixation of cooling fan of the diesel motor.

Figure 6 shows how used wood patterns and core boxes are used in the casting process. The cast piece is marked in orange, while the core box and supports are represented in black. Wood is commonly used to create sand casting patterns, especially for smaller production runs. Hardwood is the preferred material for these patterns, as it can withstand the heat and humidity of the foundry. Although wood patterns can be used for hundreds of castings, they tend to deform or split over time. Nevertheless, they are still cost-effective and can be used to create very large castings.

Figure 6: Wooden pattern of selected used part “Star”.

Clay, specifically sodium-activated bentonite, is commonly used in the green sand method for casting. Water glass, on the other hand, is used as a binder for making cores. The sand mixture used in this process consists of new sand with 11% sodium-activated bentonite and 4% water. The green sand molding process is used to cast the selected spare part, which is represented in Figure 7. During this process, pressure is applied to the sand to compact it firmly against the face of the pattern and the closed mold is then ready for pouring. Once the aluminum casting is solidified, the sand is removed from the casting through a process called shakeout and is returned to storage bins for reconditioning and later use. What's great about the green sand molding process is that the sand can be reused many times, with only small additions of new sand, clay, and water. This aspect makes it both important and cost-effective.

Figure 7: Sand molding of the selected part “star”.

The melting process was conducted using locally produced metallurgical Si (98 wt. % purity) and Mg (99.96 wt. % purity) mixed with aluminum to prepare A356. Before the melting process, charge calculations were performed, taking into account the estimated 90% efficiency of the melting process and 10% losses during melting based on experience. The process involved melting charges of approximately 20 kg in a graphite crucible using a 200 kW medium frequency induction furnace. The first step was to melt commercial Al ingots to a temperature above 660°C, then add silicon and magnesium to the aluminum melt. Afterward, the melt was heated to a temperature above 750°C and held for roughly 30 minutes for complete homogenization. Finally, the mixture was poured into a sand mold, as shown in Figure 8.

Figure 8: The steps of melting and pouring process of the spare part”star”.

Physicochemical Analysis of the Produced Spare Part “Star”

Chemical Analysis

Table 2 presents the results of the investigation of the chemical compositions of the "star" spare part using optical emission spectroscopy. The alloy composition of the produced spare part "star" is close to that of A356 with 6.87 wt. % of silicon. However, there is a possibility of MG-silicon loss during the casting process, which could explain the slight difference from the 7 wt. % of silicon in A356. The weight percentage of Mg is 0.297 wt. %, which is typical of A356 alloy (Al-7Si-0.3Mg).

Table 2: The chemical composition of the produced alloy of the star (wt. %) and standard A356.

Elements

Al

Si

Mg

Fe

Cu

Mn

Ti

Prepared alloy, wt.%

Bal.

6.87

0.297

0.12

0.01

0.003

0.06

Standard alloy A356,wt.%

Bal.

6.50-7.50

0.25-0.45

0.2

0.2

0.1

0.04-0.2

 

Thermal Analysis

Thermal analysis is conducted to examine the solidification behavior of the manufactured spare part named "star". The data for thermal analysis is gathered by recording the cooling curve during the solidification process, which is depicted in Figure 9. The aluminum alloy mentioned in document number one is a hypo-eutectic aluminum-silicon alloy. In this alloy, primary α (Al) precipitation starts at 624°C, and the eutectic reaction occurs at approximately 569°C.

Figure 9: Cooling curve (yellow) and its derivative (red).

X-Ray Diffraction (XRD)

According to the XRD result, the A356 alloy consists of α-Al matrix, Si particles and Mg2Si intermetallic compounds, as shown in Figure 10. The addition of Mg is crucial for developing strength and hardness in heat-treated Al-Si alloys. It also results in the formation of the Mg2Si phase, which improves the properties of high silicon alloys and changes the nature and amount of primary silicon formed [18, 19].

Figure 10:  XRD pattern of the produced A356 alloy.

Microstructure Investigation

The optical microscope has been used to examine the microstructure of the "star" spare part. The primary aluminum (α-Al) is of light color and it is surrounded by eutectic silicon (dark) and Mg2Si particles (grey) as depicted in Figure (11). The alloy microstructure is mainly composed of two phases: eutectic (α Al + Si) and dendritic α-Al aluminum. The eutectic mixture contains soft Al as a matrix along with Si particles. The presence of Mg and Fe in the alloy results in the formation of various intermetallic compounds in the microstructure of the alloy [20].

Figure 11: Microstructure of the selected spare part “star”.

Microstructure Investigation by (SEM)

Figure 12 displays the scanning electron microscope (SEM) analysis of the produced alloy, known as A356. This analysis is used to examine the components of the alloy and the morphology of the microstructure. An energy dispersion spectrum (EDS) associated with the scanning electron microscope is used to reveal the concentration of alloying elements in specific areas of the microstructure, as displayed in Figure 13. The addition of Mg led to a change in the morphology and size of the eutectic Si particles, resulting in a compact and fine form with a Chinese script-like morphology, as shown in Figure 12. This indicates that Mg concentration can enhance the refinement and modification of silicon at this level [21]. The eutectic network of alloys with a Chinese script morphology contained eutectic Mg2Si particles. This is due to the ternary eutectic reaction during non-equilibrium solidification. The eutectic formation changed from binary (Liquid Al+Si) to ternary (Liquid Al+Si+ Mg2Si), as shown in Figure 13, which lowered the eutectic formation temperature of the matrix. This change affected the size and morphology of the eutectic Si phase in the matrix [21, 22].

The refinement of primary Si and modification of eutectic Si were observed only for alloys with Mg content ≤ 0.3% Mg when compared with the binary Al-Si base alloy. This implies that the presence of Mg can enhance the refinement and modification of Si at this concentration level. The refinement of primary Si could be due to the formation of MgAl2O4. Magnesium-alumina spinel (MgAl2O4) is a common compound in a family of mixed oxide spinels with a cubic structure and a lattice parameter of 8.08 Å [23, 24]. Therefore, the concentration of Mg up to 0.3 wt% might be sufficient to form the MgAl2O4 and then act as a nucleation substrate to refine primary Si [21].

The cast microstructure of Al-Si-Mg alloys is highly dependent on the alloy's chemistry and the casting process. The slower cooling rate in sand casting causes larger dendrites and coarser eutectic phases. A fine microstructure is always preferred in applications requiring high strength. The phases can also be refined through chemical modification without changing the casting process [16]. Adding magnesium can strengthen the material by precipitating fine Mg2Si in the matrix.

Figure 12: SEM micrograph of the produced A356 alloy (star part)

Figure 13:  (a) SEM micrograph with (b) line scan and elemental mapping of (c) Al, (d) Si and   (e) Mg2Si of the produced A356 alloy (star part).

Hardness

The spare part "star" made from A356 alloy has a Brinell hardness of approximately 75 HB. This hardness value is comparable to that of the commercial A356 [25]. Thus, it can be inferred that producing Al-Si spare parts using locally procured MG-Si has properties similar to those produced for export. This work will enhance the local production of spare parts by utilizing local resources and consequently lead to an increase in national income.

 

Conclusion

  • It has been reported that a spare part called "star" has been manufactured using locally produced MG-silicon. The spare part is made of aluminum-silicon alloy and is used to fix the cooling fan of diesel motors. The sand casting technique was used to manufacture it, and the alloy composition consists of approximately 7% Si and 0.3wt. %Mg.
  • XRF and XRD analyses were conducted to determine the microstructure and composition of the alloy. The results showed that the composition of the prepared alloy was within the standard range for this type of alloy (Al-6.87 wt. %Si-0.297 Mg). The microstructure analysis revealed primary aluminum surrounded by eutectic silicon and Mg2Si particles. The addition of magnesium strengthened the material by precipitating fine Mg2Si in the matrix.
  • The Brinell hardness of the investigated spare part was found to be approximately 75 HB.
  • The utilization of locally produced MG-Si for manufacturing Al-Si spare parts has exhibited typical quality compared to export, which could lead to an increase in national income by enhancing local production of spare parts utilizing local resources.

Acknowledgment

The author would like to thank STDF on its support for funding the project entitled to "Production of Special Aluminum -Silicon Spare Parts for Engineering Industries from Primary Resources" through national challenge program. Also, the authors would like to express their gratitude to the late Prof. Dr. Mohamed Waly for his valuable contribution to the present study.

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