Selection and Fabrication of HMMC (AL6063-SIC-B4C-MG)

Abstract

This unit deals with the selection and fabrication of HMMC (Al6063-10SiC-5B4C-Mg) constituents by extensive biography review and satisfactory fabrication design. Researchers have promoted an extensive collection of Al6063 composites employing organic and inorganic reinforcements. The fundamental purpose of the broken-up stages is to constrain the metal matrix in a relevant capacity to strengthen the properties of the base materials. In the case of Al6063, the reinforcement weighty subject matter in the composite varies from 5 wt.% to 30 wt.%. Diverse classes of reinforcements had sought to integrate and operate in the composite formulation as hybrid reinforcements. This chapter further discusses the comprehensive development stages of 84% wt of Al 6063, 10% wt of SiC, 5% wt of B4C with 1% wt of Mg hybrid metal matrix composite (HMMC) through the stir casting approach. During the stir casting process, the melting action of the material emanates numerous gases and residuals apart from the expected composite. The residuals have numerous environmental concerns, which require discussion since some of the vapors and substantial waste can lead to detrimental effects on the environment in terms of air and soil pollution.

1. Introduction

Figure 1 displays the flowchart of the HMMC development. During stir casting, a non-homogeneous mixture pattern has been an apprehension. The inclination is due to inappropriate segregation of reinforcement because of incorrect process parameters (rotation of stirrer, angle of stirring application, condition of wetting, and density). The material properties likewise have been reported to modify the characteristics of the homogeneous mix. The main metal matrix melted to obtain a molten state by melting it above its liquid temperature. The preheated reinforcement material is combined gradually so that a semi-solid-state is achieved. Repeatedly, the entire mix is needed to get heated to produce a molten state, and in between, stirring is done to attain the entirely conceivable consistency. The capability of the stir casting method predominantly rests on stirring speed, stirring duration, and stirring temperature [1, 2].

Figure 1.

Flow-chart of HMMC development.

Here a crucible composed of ceramic or graphite is being utilized to melt the parent metal in a furnace. A mechanized stirrer with a graphite impeller with a rotation speed of around 150–800rpm is engaged to agitate the melt (see Figure 2) periodically. The reinforcement materials are preheated to eliminate the humidity substances, facilitating wettability during stirring. Sakthivelu [4] had recommended a maximum limit of 30% of reinforcement for stable composites.

Figure 2.

Stir casting technique for the MMCs fabrication [3].

1.1 Stirring speed

The uniform dissemination of the reinforcement materials in the parent metal is essential for the advance in the coveted properties such as stiffness, toughness, tensile strength, etc. The stirrer with inadequate rpm provides an ineffectual activating force on the central metal matrix, contributing to an inadequate association [5]. Cluster arrangement and agglomeration inclination were recorded at slow rpm of mixing. The stirrer operated at high rpm provides considerable benefits in the creation of the expected composite since, at high rpm, the shear force supports the reinforced material to get the transfer inside the metal matrix dispersed phase and better bonding action with the metal matrix deep inside it, thereby setting up a coherent mix [5]. It has also been reported that porosity inclination can be stepped up at enhanced stirrer speed since gas particles induce inside the matrix.

1.2 Stirring duration

Stirring time likewise influences the distribution of dispersion into the metal matrix. Clustering of the material is observed at the lower stirring time, and further non-uniform mixture with fewer inclusions of reinforcement materials [6].

1.3 Stirring temperature

With a rise in temperature of the matrix metal, the viscosity was established to reduce, generating an effect in the reinforcement materials distribution. In extension, the chemical reaction was further revealed to develop with a rise in the temperature of the molten material [7].

2. Preparation of HMMC materials

The development of HMMC through stir casting typically uses the following phases. Figure 3 illustrates the phases of melting of metal matrix composite to its melting point. The stirring of molten metal is managed to utilize an electric motor.

Figure 3.

Phases of the stir casting process.

Table 1 displays the stir casting process parameters retained during the fabrication of HMMC.

S. No.	Parameters	Values
1	Preheating chamber temperature	850°C
2	Furnace temperature	900°C
3	Core temperature	750-800°C
4	Voltage	440 V
5	Frequency	50 Hz
6	Stirrer speed	300–400 rpm
7	Die pre-heating temperature	200°C

Table 1.

Stir casting process parameters.

The reinforcement material is delivered with continuous stirring movement through a stirrer to associate the reinforcements in the matrix of the parent metal. The mixture is eventually poured into the mold and solidifies naturally. The pertinent equipment employed for HMMC development is summarized.

2.1 Furnace

Figure 4 presents the original furnace adopted for the development of the HMMC. It has a temperature gauge with a regulator switch to regulate the temperature. The maximum temperature obtainable is around 1400°C. A convenient mechanical stir system generates a vortex in the melt, facilitating an exquisite melt blending, composing the metal matrix and related reinforcements. In order to evade the chances of solid particles settlement at the base of the crucible, a bottom pouring furnace is likewise suggested.

Figure 4.

Electric furnace used for HMMC development.

2.2 Mechanical stirrer rotor

The mechanical stirrer plays an essential role in forming an acceptable vortex in the melt to bring about the best possible coherence. Distinctive impeller stirrers can be used, i.e., single, double, and multiblade impeller. The double blade impeller (Figure 5) is employed mainly to develop AMCs. The single and multiblade impeller is handled primarily in chemical industries.

Figure 5.

Actual photo of mechanical stirrer used.

Figure 6 displays impeller stirrers accepted for HMMC development. The blade was applied with a coating of zirconia onto a stainless-steel stirrer. The zirconia layer helps in averting probable reactions between the molted aluminum material and stainless steel of the stirrer. The impellers have been investigated for developing a sufficient vortex during the mixing process.

Figure 6.

Impeller stirrer and types.

2.3 Crucible

Crucible is a container in which the metal matrix is melted to its molten temperature and the desired refracting materials are being added. Nowadays, diverse materials consisting of Alumina, Tungsten, Graphite, etc. are being adopted as a crucible. For HMMC development, the reinforcement materials (SiC and B₄C) were pre-heated in the Alumina crucible (Figure 7), whereas the parent metal (Al6063) is melted in a graphite crucible shown in Figure 8.

Figure 7.

Alumina crucible.

Figure 8.

Graphite crucible.

The Graphite crucible experiences the following advantages.

1. High melting temperature (2500°C).

2. It is easily accessible.

3. The cost is less in comparison to tungsten.

4. Graphite has good electrical conductivity.

2.4 Power supply

The induction resistance furnace with a temperature regulator is linked with a three-phase electricity supply. To control the current and voltage supply, an ammeter and voltmeter were associated with the circuit. Figure 9 illustrates the ammeter and voltmeter. The ammeter indicates the instantaneous current flowing in the circuit. The induction resistance furnace is engaged with moving iron type (M-Tech industries) with range 0-10A. The Voltmeter is likewise utilized for measuring the instantaneous voltage value across the circuit with a range 0–300 volt.

Figure 9.

Ammeter and voltmeter.

The current drawn by the electrical inductor furnace, depends on the furnace size, shape, and capabilities. The furnace shown withdraws around 55A to 75A. The efficiency spectrum of an electrical furnace is surprisingly modest; all modern electrical furnaces have an AFUE of 100%. That means that entire electric furnaces convert electricity into heat energy without any losses. Due to energy losses in ducts and the energy required to run a blower, the electric furnace is slightly expensive for operation [8]. The energy requirement is AC 380/7kw/50 Hz. The induction furnace also comprises a temperature regulator and digital display unit of temperature. Figure 10 shows the Digital display unit with a regulator switch.

Figure 10.

Digital display unit with regulator switch.

2.5 Die

A graphite material die was utilized to shape and solidify molten material obtained after the rigorous stirring of Al 6063, SiC, and B₄C. The size of the die is 100 × 50 × 30 with a tapered shape. Figure 11 displays the die adopted for fabrication.

Figure 11.

Graphite die with HMMC brick.

3. Steps accepted for HMMC fabrication

STEP 1: The stir casting setup employed for fabricating 84% Al-10% SiC-5% B₄C-1% Mg is shown in Figure 12. It consists of a furnace with a temperature range up to 14,000°C for heating the materials utilizing electrical resistance heating, which is the generally used technique of heat development. The mechanical stirrer fixed to the motor was supported inside the graphite crucible.

Figure 12.

Electric furnace.

STEP 2: A correct weight measurement of the constituents is determined utilizing an electronic balancer (see Figure 13a-c). The silicon carbide and boron carbide particles were preheated at 850-900°C to eliminate any traces of moisture and oxidize their surface, forming a silicon oxide (SiO2) layer (see Figure 14). This layer enhances the wettability of the composite [9].

Figure 13.

Wt. measurement of (a) Al 6063 material, (b) SiC (c) B₄C.

Figure 14.

Preheating of SiC and B₄C.

STEP 3: The Al 6063 billets were later melted in a new graphite crucible (see Figure 15). The temperature of the furnace was controlled between 850 and 950°C. A less slag was observed on the edges and was cut out by a graphite spoon. A uniform temperature of 700-750°C was secured, and flux was included in the melt to restrain oxidation [10].

Figure 15.

Aluminum billets melting in furnace.

STEP 4: The molten metal was later cooled naturally to a semi-solid state at around 550-600°C, and gradually preheated SiC, and B₄C was included in the melt in fragments, and mechanical stirring was performed at 300–400 rpm (Figure 16).

Figure 16.

Motorized stirring of HMMC.

STEP 5: Less than 1% wt of magnesium (Mg) was also added to develop the wettability of the mixture [7]. The mix was continuously stirred for 5 minutes, and the consistent mixture was poured into the die. Figure 17 exhibits the graphite die along with the developed HMMC brick.

Figure 17.

Graphite die with HMMC brick.

STEP 6: The solidified HMMC brick (Figure 18) is taken out from the die and is processed and cut into suitable identical pieces (30 × 20× 5 mm) further for experimentation. The specimens were then packed in polythene pouches with unique identification marks. Few specimens were then forwarded to authorized labs for SEM analysis and mechanical testing.

Figure 18.

Solidified HMMC bricks.

4. Samples of HMMC

Figure 19(a) and (b) show the weight measurement of brick one and brick two. The brick was cut into a smaller size of 30 mm × 20mm × 5 mm for experimentation on Die Sinking EDM. The wire EDM (Annexure 4) was used for preciously cutting the bricks so that the internal grain structures were not disturbed. Figure 20 shows the line diagram of the HMMC sample, and Figure 21 shows the actual sample.

Figure 19.

(a). Wt. measurement of brick 1. (b). Wt. measurement of brick 2.

Figure 20.

Line diagram of the specimen.

Figure 21.

View of the sample.

5. HMMC properties and test analysis

5.1 Properties of the individual constituents

Aluminum 6063 is broadly employed as a general-purpose alloy in many engineering applications such as the extrusion process, owing to its fair strength [11]. Table 2 exhibits the constituents of Al6063.

Al6063	Wt. %	Al6063	Wt. %
Al	Max 97.5	Mn	Max 0.2–0.7
Cr	Max 0.11	Si	0.2–0.9
Cu	Max 0.09	Ti	Max 0.11
Fe	Max 0.36	Zn	Max 0.92
Mg	0.44–0.89

Table 2.

Composition of Al6063 [2].

Table 3 (a-c) presents the physical and thermal properties of B₄C [12], Al6063 [2], and SiC [13], respectively.

(a) B4C		(b) SiC		(c) Al 6063
Properties	Value	Properties	Value	Properties	Value
Specific Heat (°C)	700	Coefficient of Thermal Expansion (°C)	4	Coefficient of Thermal Expansion (per ^o C)	0.000022
Melting Point (°C)	2783	Specific Heat (°C)	750	Thermal Conductivity (cal/cm^2/ cm/ Celsius at 25°C)	0.285
Density (g/cm^3)	2.55	Melting Point (°C)	2730	Electrical Conductivity (% copper standard at 20 °C)	33.5
Thermal Conductivity (W/mk)	17–42	Density (g/cm^3)	3.21	Density (g/cm^3)	2.64
Hardness (Kg/mm^2)	2900–3580	Thermal Conductivity (W/mk)	120	Freezing Range (°C) approx.	625–525

Table 3.

(a, b) physical properties of B₄C and SiC (c) thermal properties of Al6063.

Boron carbide (B₄C) is one of the hardest materials available. Above 1250°C, it has been harder than cubic boron nitride and diamond. B₄C is an alluring reinforcement substance owing to its unique balance between thermal and chemical properties. Moreover, it possesses a smaller density and greater hardness value of order 30 GPa. Thus, B₄C-reinforced HMMCs fabricated through the moderate-cost stir casting structure have gained higher attractiveness among researchers [14, 15]. B₄C has good mechanical strength with desired properties of neuron absorption [16].

Silicon carbide (SiC) is constituted of tetrahedra of silicon and carbon atoms with influential bonds in the crystal lattice. The SiC material has less thermal enlargement, immense strength, and thermal conductivity of greater order and has been recorded to be resistant against thermal shock [17, 18]. The SiC can tolerate severe temperatures and has got high hardness coupled with low density.

Magnesium (Mg) is acknowledged for promoting grain refinement, wettability, and reinforcing the solid solution [19].

5.2 Properties of the HMMC

The spark atomic emission spectrometry (SAES) was conducted with ASTM E1251–11 standards (test procedure for Al and Al alloys) to determine the elements present in the HMMC samples. Table 4(a) illustrates the composition and %wt of elements. Table 4 (b) indicates the HMMC significant properties. The density of HMMC (2637 kg/m³) as obtained through the test report has been used to calculate the MRR and EWR [20, 21].

(a)				(b)
HMMC	% Wt	HMMC	% Wt	Properties	Value
Al	91.43	Mg	0.087
Si	4.151	Cr	0.037	Melting point	750–800
Cu	1.516	Ti	0.027	U.T.S	110Mps
Zn	1.498	Sn	0.022	Tensile Strength	118 Mpa
Fe	0.806	V	0.006	Break Load	9.44KN
Mn	0.149	Cd	0.0019	Yield Stress	82 Mps
Ni	0.13	Co	<0.001	Hardness	72HB
Pb	0.089			Density	2637 kg/m^3

Table 4.

(a) Composition of HMMC (b) significant properties of HMMC.

5.3 SEM analysis of the HMMC

For establishing the homogeneity of the HMMC, the sample was tested by employing scanning electron microscopy (SEM). Figure 22 displays the uniform dispersion of SiC and B₄C in the specimen. No segregation of SiC grains along with B₄C particles was stationed along the grain edges. Dissemination of grains is acknowledged to be intra-granular, in which the maximum particles locate inside the grains. The uniform distribution is commensurate with the efficient and timely stirring action during the stir casting process [22]. The crater’s size is less with B₄C particles this could be because of the creation of a boron oxide (B₂O₃) layer on the B₄C ceramic, because of liquid-to-liquid reaction leading to an expansion in the wettability, which is observable at a specialized high temperature [23]. Many researchers proposed that reinforcement in the particulate form up to wt. 30% may be included in a molten metal matrix to perform a more substantial reinforcement distribution [24]. Reinforcement is added emphatically into the molten stage of aluminum. The stirring speed, time of stir, stirrer blade angle, pouring temperature, solidification rate, reinforcement size, and elements percentage influence the fabricated composite consistency.

Figure 22.

SEM image of the stir casted HMMC.

6. Environmental concerns

The stir casting process involves melting the metal at around 800-1000°C. The metal matrix used is Aluminum 6063 with Boron Carbide (B₄C) and Silicon Carbide (SiC) as reinforced materials. The melting operation produces specific unwanted gases and residual waste, which must be discussed. Table 5 illustrates the relevant unwanted gases/residual waste with apprehensions on the environment and human beings [27].

Sr. No	Unwanted Gases/ Residual Waste	Environmental concerns and human health
1	Aluminum hydroxide	Exposure to Aluminum hydroxide may cause repulsion, vomiting, hyperacidity, pungency, Low blood phosphates (hypophosphatemia), distaste, causticity leading to bowel obstruction, Fecal impaction [25].
2	Aluminum oxides	Indicative toxicity has been reported, followed by chronic inhalation of the aluminum oxides. Long-term aluminum oxide inhalation may cause pneumoconiosis with cold and exertion and a restrictive pattern of rib cage function. In severe cases, death has been reported due to respiratory failure.
3	Aluminum sulfates	Eating or gulping aluminum sulfate produces serious disturbance to the digestive organs and stomach. An influenced individual may encounter retching, queasiness, and runs, adding water to aluminum sulfate can make a sulfuric acid structure. The sulfuric acid may cause soil damage by reducing its constituents.
4	Boron Oxides	Acute effects: The boron oxides contacts can aggravate the skin and eyes. Breathing in Boron Oxide can bother the nose and throat, causing hacking and wheezing. Introduction to Boron Oxide may cause heaving wooziness, cerebral pain, sickness, and so forth.
		Chronic Effects: The accompanying long-haul wellbeing influences may happen after some time getting an introduction to Boron Oxide and can keep going for months to years [26]. Boron oxide may make permanent damage to kidney and livers.
5	Silicon Dioxide	Silicon Dioxide exists naturally on earth and our bodies. No evidence has been reported to advocate its implication on human health, but more research is required to ascertain its role on the body. Inhalation of silica dust may cause diseases related to breathing.
6	Fly Ash particles	It can get placed in the deepest part of the lungs, where it may cause an asthmatic attack, inflammation, and immunological reactions. They contribute to Particulate Matter 2.5 and 10.
7	Magnesium Oxide	Exposure to Magnesium Oxide can cause “metal fume fever” which is a symptom in which the patent gets a metallic taste in the throat with headache, sneezing symptoms, cold symptoms.

Table 5.

List of unwanted gases/residual waste/effects on the environment.

7. Summary

This chapter focuses on the comprehensive development stages of the HMMC (84%wt of Al 6063–10%wt of SiC-5%wt of B₄C with 1%wt. of Mg) through the stir casting method. A comprehensive description of the essential ingredients (electric furnace, stirrer, the crucible, die) required for HMMC fabrication and the procedures has been covered. Reinforcement is added emphatically into the molten stage of aluminum. The stirring speed, time of stir, stirrer blade angle, pouring temperature, solidification rate, reinforcement size, and elements percentage influence the fabricated composite consistency. The developed HMMC was further analyzed for composition and specific mechanical and thermal tests. The HMMC density of 2700 kg/m³ was noted for MRR calculations. For confirming the homogeneity of the HMMC, the sample was analyzed using an SEM test. Dissemination of grains was noticed to be intra-granular, in which the maximum particles reside inside the grains. The uniform distribution is proportional due to the efficient and timely stirring action during the stir casting process. The crater’s size is observed to be less with B₄C particles because of the creation of a boron oxide (B₂O₃) layer on the B₄C ceramic because of liquid-to-liquid reaction leading to an expansion in the wettability, which is observable at a specialized high temperature. During the stir casting process, the melting action of material emits out certain gases and residuals apart from the required composite. The residuals have specific environmental concerns. The severe effects caused by aluminum hydroxide, aluminum oxide, aluminum sulfate, boron oxide, silicon dioxide, magnesium oxide, and fly ash on the environment have also been covered.