1. Introduction
A wide range of applications is provided and its alloys, with a unique combined advantage which makes it the material to choose for many applications such as aerospace, the automotive, military, etc. due to their low density, coefficient of thermal expansion, high strength, wear-resistance, and improved damping properties [
1,
2]. Aluminum–metal matrix composites have received comprehensive attention for functional and fundamental reasons among the materials of tribological significance. Due to theirhigh compressibility, less density, specific strength, and economical processing, aluminum alloy composites can be quickly produced by the powder metallurgy technique compared with the other available fabrication techniques [
3]. Aluminum alloys and aluminum-based metal matrix composites have found applications in the manufacture of various automotive engine components. The metal matrix composites’ main advantage is mechanical properties such as hardness and yield strength to be adequately regulated by strengthening the matrix and substantial mass control, which is necessary, mainly when used mutually [
4]. Aluminum metal matrix composites are also used in the transportation sector due to their lower density, lower airborne emission, and less noise, which helps maintain the environmental regulations and provides good fuel economy [
5]. The high strength to weight ratio of Aluminum matrix composites has also successfully cemented their place in military applications. The Young’s modulus of pure Aluminum can be enhanced by 300% (70 GPa to 240 GPa) by reinforcing the aluminum fibers [
5]. Aluminum-based powder metallurgy alloys were used to produce near-net-shape products with high material utilization with less cost, lower processing temperature, and refined homogeneous microstructure with a lesser amount of porosity [
6,
7].
Further research into powered metal’s ability to absorb and dissipate microwave radiation has opened new powder metallurgy opportunities. Microwave sintering is an efficient, economic, and valuable approach to processing some P/M materials [
8]. Microwave sintering has advantages like enhanced diffusion processes, reduced energy consumption, and rapid heating rates. Microwave sintering considerably reduces processing times, decreases sintering temperatures, improves physical and mechanical properties, and has lower environmental hazards. These are some features that have not been observed in conventional sintering [
9,
10].
Much research has been carried out for aluminum metal powder mixed with varied compositions of the alloying elements. Aluminum–manganese alloys and aluminum–coppers are two of the most-used combinations. Al-Mn alloys have elevated formability and corrosion resistance with high heat transfer coefficients, making them feasible for radiators, packaging, and roofing applications [
11]. Such alloys have a very high strength-to-weight ratio and a density slightly higher than that of various plastics. Inter metallic-phase formation of Al-Mn elements by the adsorption of Mn within the liquid phase of Al acts as the driving force for solidification [
12,
13]. Al-Cu alloys are heat-treatable, and hence they possess high strength, especially at high homologous temperatures (200–300 °C), with higher toughness, resulting in a wide range of aircraft and transportation industry applications [
14]. The compact density shows improvement after sintering due to precipitation due to the copper swaging nature of copper [
15,
16].
The present research attempts to study the effect of heating mode on the physical and mechanical properties of aluminum alloy composites produced through the powder metallurgy route. The effects of copper and manganese addition on the sinterability of the same were also studied.
2. Materials and Methods
For the present investigation, gas atomized pure elemental form aluminum, copper, and manganese powders were purchased from Krish Met Tech Pvt. Ltd.(Annamalai Colony St, Annamalai Colony, Virugambakkam, Chennai, Tamil Nadu, India) Chennai. By varying the Al, Cu, and Mn contents, three distinct alloys are designed, as shown in
Table 1. The characteristics of the as-received powders are listed in
Table 2. According to the designed composition, elemental powders are separately blended using a mortar for 60 min to obtaina uniform composition.
The powders were subjected to uniaxial compaction at 400 MPa and were made as cylindrical pellets (~25 mm diameter) using a Universal Testing Machine (model: Instron 8801, Norwood, MA, USA). Zinc stearate was used as a die wall lubricant [
13]. All compacted alloys are sintered at 550 °C for 60 min using (i) conventional sintering in a tubular furnace (model: VBCC, TUBULAR FURNACE, Chennai, Tamilnadu, India) at a heating rate of 5 °C/min, and (ii) microwave sintering furnace (model: VBCC HYTERM FURNACE Chennai, Tamilnadu, India, multimode cavity 2.45 GHz, 6 kW) at a heating rate of 30 °C/min. Alumina boats were used for placing the samples inside the furnaces. Age-hardening of sintered samples was performed at 150 °C for 60 min in a box furnace (model: R257 INDFURN SUPERHEAT FURNACES, Chennai, Tamil Nadu, India)at a heating rate of 5 °C/min, and was water quenched to room temperature. By calculating the volume and weight of the pellets, the sintered densities of the pellets were determined. The densification parameter of the samples was calculated as:
The sintered samples were initially polished with 220, 400, 800, 1000, 1200, 1500, 1800, and 2000 grit SiCemery papers, progressively, and mirror polishing was done on the disc polishing machine with diluted alumina solutions on a velvet cloth. After polishing, the samples were etched using Keller’s reagent (3 mL-HNO
3, 2 mL-HCl,1 mL-HF, and 100 mL-H
2O) [
17]. The polished sample microstructures were evaluated with optical microscopy (model: ZEISS-AXIO vert A1, Jena, Germany). Microhardness of all the compositions was tested using the Vickers hardness testing machine (model: MMT-X7B, no: MM5562X, Matsuzawa Co.,Ltd, Akita, Japan)with a load of 0.5 kgf, with 10 s dwell time. The indent’s diagonal lengths were measured, and the experiment was repeated three times to obtain a precise value.
The Vickers hardness of the sintered alloys is found out using the following equation:
where p = Load applied, a = Average length of the diagonal =
, d
1 = length of diagonal 1, d
2 = length of diagonal 2.
A scanning electron microscope [model: ZEISS-EVO18, Jena, Germany] was used in backscattered electron imaging mode for microstructural examination. The sample’s electrochemical activity was tested using the electrochemical method in a freely aerated 0.1N HCl solution. Before polarization, the polished samples were allowed to stabilize for 1 h to obtain stable open circuit potential (OCP). Electrochemical tests were carried out in a flat corrosion cell using a standard three-electrode configuration of the sample as the working electrode, platinum mesh as the counter electrode, and Ag/AgCl reference electrodes. Potentiodynamic polarization tests were carried out from −250 mV versus OCP to +1600 mV versus the reference electrode at a CT scan rate of 0.1667 mV. From the corresponding anodic and cathodic curves, the Tafel curves were created. The corrosion potential (E
corr), corrosion current (I
corr), and corrosion rate were determined from the polarization curves. The corrosion rate was determined by using the 1st-Stern method [
18] and is expressed as follows:
where e is the equivalent weight (g)
is the density within the material (mg/m
3), and I
corr is the corrosion current (mA/m
2).
4. Conclusions
Using microwave sintering and conventional sintering, three alloys (77.5Al-22.5Mn, 57.28Al-18.6Cu-24.12Mn, and 65Al-20Cu-15Mn) were sintered at 550 °C for 60 min. After sintering, they were aged at 150 °C, and corrosion tests were also under taken with the aged alloys. The following conclusion was made from this study.
Microwave sintered compacts exhibited higher densification factors and hardness when compared to conventional sintering. Microwave sintered alloys have finer microstructures and narrower pore sizes than conventional alloys, as evidenced by SEM microstructures. The results of EDS proved that the alloy formation of Mn with Al was more likely and was more prominent in the case of microwave sintering. Al-20%Cu-15%Mn showed less hardness and smaller grain size in the microwave sintering than Al-18.6%Cu-24.12%Mn. Higher corrosion resistance was observed in the case of microwave sintering due to the uniform dissolution of elements. Microwave sintering can be considered the best alternative to the conventional sintering technique for Al alloys processed via powder metallurgy.