Fabrication of aluminium covetic casts under different voltages and amperages of direct current

2.1 Materials

One commercial type of aluminium used wires (Al) and graphite powder (Gr) were used to produce aluminium-graphite (Al-Gr) Covetic casts. The Gr density was 2.267 g/cm³ with a D50 = 43 μm. Carbon additive particles in micron size are used in the Covetic process of producing 20 metals, including aluminium [1, 10, 11].

2.2 Preparation of Samples

A special design of the Covetic-casting system was made for producing Al-Gr Covetic casts (see Figure 1), including: an electric furnace (ORH5-F102200), an electric blender, a graphite crucible, four Solite batteries (2 × 150, 1 × 100 and 1 × 40 Amp) as DC resources, a CENTURY model battery charger (for recharging batteries after each casting process), and a BT-121 model battery tester (to measure the batteries amperages and voltages).

Figure 1

Covetic-casting system for producing Al-Gr Covetic casts: the graphite crucible (1); molten metal level (2); thermocouple (3); heat saver container (4); electrical blender (5); DC electrodes (6,7); inert gas tube (8) and an electrical furnace body (heater 9, inside body 10, outside cover 11).

The production of an Al-Gr Covetic castwas completed (see Figure 2), as follows: A required amount of Al and Gr was prepared; the graphite powder was dried and rolled up in a thick aluminium foil then, immerged (the Al molten temperature had risen to 850 ± 5°C) in the Al molten when the blender was worked and the DC was applied. Subsequently, the molten was poured into a preheated steel mould.

Figure 2

Production steps of an Al-Gr Covetic cast.

The graphite powder was dried under vacuum conditions in an SRJX-5-13 oven at 250°C for one hour to remove the moisture content and increase wettability. The molten of each cast was held in a high temperature 850 ± 5°C for 3 minutes to decrease the porosity level [13]. The steel mould was lubricated with graphite powder and heated to 450°C to facilitate taking off a cast and to ensure a moderate cooling rate. All casts were left inside the mould to cool in air.

Many trials were conducted before the Casting-Covetic System operated as desired. All of Al-Gr Covetic casts were cast under argon inert gas and they were subjected to the conditions shown in Table 1. The Non-Covetic Al cast (C1) was used for comparison.

2.3 Investigation Process

All of the produced casts were operated mechanically to test their tensile properties according to ASTM E8/E8M - 13A at room temperature by using a universal tensile test machine (WDW-200) with a speed rate of applied load 0.5 mm/ min (gage length = 25 ± 2 mm, working part diameter d₁ = 6 ± 1). The tested samples are presented in Figure 3.

Figure 3

Tensile failure of Al with no current cast (C1); Al-3% Gr Covetic casts at 170 Amps with 12V (C2) and 24V (C3); Al-3% Gr Covetic casts at 290 Amps with 36V (C4) and 400 Amps (C5).

Density was measured according to Archimedes’ principle by using a density measurement device (DA-300M). Each produced cast was prepared with the dimension ϕ12 mm × 20 mm, polished, and sanded sufficiently before the test.

The four-wire digital multi-meter method (AT512 Precision Ohmmeter) was performed to measure the electrical resistance (R, ohm) of all produced casts (see Figure 4). Then, the electrical resistivity (ρ, ohm-meter) was calculated according to Equation 1 [12]. Subsequently, Equation 2 [14], was applied to find the electrical conductivity in a specific conductivity per length (σ, S/ m):

Figure 4

The four-wire digital multi-meter (AT512 Precision Ohmmeter).

Where: I = the distance between clips, mm; A = cross-sectional area of the tested sample, mm².

Then, the results of the electrical conductivity were divided by the conductivity of annealed pure copper, σ = 5.8 × 10⁷ S/ m (100% meeting the International Annealed Copper Standard) (IACS); the electrical conductivity of each Al-Gr Covetic cast was presented as a percentage of IACS [15].

The chemical composition analysis for all produced casts was conducted using an Oxford PMI-Master Pro spark emission spectrometer. Three points were taken for each prepared sample (ϕ12 mm × 10 mm) as shown in Figure 5(a).

Figure 5

Prepared sections of Al-Gr Covetic casts after chemical analysis (a) see the burned surfaces, next for the microstructure analysis (b) the mirror surfaces and casting defects.

In this study it wasn’t preferred to use the X-ray diffraction (XRD) method for the chemical composition analysis because, simply it is hard to reveal any existence of a carbon in aluminium Covetic materials with low carbon content (< 5 wt.% carbon) regarding to [2, 6].

Sections with dimensions of ϕ12 mm × 10 mm were prepared to study the microstructure, see Figure 5(b), of all produced casts by using the grinding papers of silicon carbide with different grit sizes (–400, –800, –1000, –1500, and –2000). They were polished using a diamond paste to obtain a mirror surface. The examination was conducted using a scanning electron microscope (SEM) with Energy-Dispersive Spectroscopy (EDS). In addition, these sections were used for the calculations of their hardness. These calculations were performed using an automatic Brinell Hardness Tester (HBA-3000S) according to ASTM E10-15a with a ball indenter diameter (2.5 mm) and applied load (31.25 kg) for 10 s. The average of the three measurements was recorded.

3.1 Density

Reads of a Measurement Density Device are presented in Table 2. These reads show that the density of Al-Gr Covetic casts decreased from 2.674 g/cm³ to 2.644 g/cm³ with decreasing DC voltage from 24 to 12 V at 170 Amp and from 2.675 g/cm³ to 2.66 g/cm³ with increasing DC amperages from 290 to 400 Amp at 36 V.

The decreasing density of Al-Gr Covetic casts compared with that of the Al cast was due to the slightly low density of graphite powder additives and the increased porosity of Al-Gr Covetic casts, as graphite additives increased the porosity level [16]. The large concentration of carbon atoms in the molten provides lighter weight of aluminum Covetic materials [10].

3.2 Tensile Test

Tensile results in Table 2 confirmed that the ultimate tensile strengths (UTS) of Al-Gr Covetic casts were higher than those of Al cast. However, the increasing of applied DC amperage and the decreasing of the applied DC voltage have not caused a big changing in the UTS of the Al-Gr Covetic casts. The enhancement of their UTS is mostly due to the effect of electrical applied current, which induces the formation of covalent bonds between graphite and Al atoms according to [7]. This enhancement might also be due to the role of impurities in graphite powder, which serve as precipitations in the form of AlFeSi in the aluminium matrix [17].

3.3 Electrical Conductivity

The electrical conductivities of Al-Gr Covetic casts are presented in Table 2 as IACS%. The results show that the conductivity is a function of amperage and voltage of direct current. It increases as the DC voltage decreases (IACS% for C3 = 39.77 at 24V while for C2 = 58.11 at 12V) and as DC amperage increase (IACS% for C4 =38.07 and for C5 = 45.57 at 290 and 400 Amp, respectively).

Applying DC has decreased the grain size of the Al-Gr Covetic casts compared with that of Al cast microstructures, as shown in [7]. Furthermore, the lower voltage and/or higher amperage Al-Gr Covetic casts have more resistance to grain growth and coarsening. The decrease in grain size improves mechanical properties, but increases grain boundaries that can, in turn, increase the electrical resistivity. Nevertheless, decreasing the electrical conductivity of the Al-Gr Covetic casts is necessary, but the nature of the effect of bonds between graphite particles and the host metal (Al), and the relatively high electrical conductivity of graphite (2 to 3 × 10⁵ S/m at 20 °C) [18], have increased the electrical conductivity instead of decreasing it. These results are in accordance with those obtained by [2, 10, 19, 20].

3.4 Chemical Composition and Microstructure Analysis

The chemical composition analysis results are shown in Table 3. The increased content of Fe, Cu, and Ni in the Al-Gr Covetic casts came from known impurities of graphite materials, a commercial instead of a pure graphite type was used in this study because of its availability.

The SEM images of all produced casts are presented in Figure 6. Dendritic structures of aluminium, see Figure 6(a), were formed due to non-equilibrium solidification (equilibrium solidification required a slow cooling rate, and it is not readily available in manufacturing processes) [21]. These images showed the graphite particles in the Al matrix of Al-Gr Covetic casts, which were distributed more uniformly (black arrows in Figure 6 indicate graphite particles). In addition, some inclusions look like dark points were noted in the Al-Gr Covetic casts; however, we observed no such defects in the Al cast. The contents of these points are presented in Figure 7.

Figure 6

SEM images at different magnifications and positions of Al cast (a); Al-3% Gr Covetic casts at 170 Amp with 12V (b), 24V (c); Al-3% Gr Covetic casts with 36V at 290 Amp (d) and 400Amp (e).

Figure 7

EDX- SEM analysis of a dark point (inside the yellow circle) in the microstructure of Al-3% Gr Covetic cast (at 170 Amp with 12 V).

The O₂ content was affected by both the formation of Al₂O₃, as the oxidation of the aluminium base molten through the casting process is difficult to prevent, and the graphite additions. It was clearly detected by EDS-SEM analysis (see Figure 7) that the porosity was increased with increasing graphite additions [16]

Table 2 also shows that the hardness of Al-Gr Covetic castswas higher than that of the Al parent cast. This can be attributed to the strong bonds between graphite particles and the host metal (Al) [11, 19]. This can also be explained by the fact that the graphite particles have higher hardness than the Al alloy, as discussed in [2, 22, 23].

The comparison between this study and other works’ results is presented in Table 4. It shows that the Al-Gr Covetic casts have desirable properties even with no heat treatment and/or extrusion conditions compared to those reported in other studies [7, 8]. The Al-Gr Covetic casts at 170 Amp with 12V had the highest increase in electrical conductivity (33.63%), the highest decrease in density (0.94%), and a notable increase in UTS (18.05%) and HBW (15.18) among other Covetic casts in comparison to the Al parent cast. These improvements of the Al-Gr Covetic casts properties are attributable to the Covetic conversion - carbon infusion.