Effect of MGZN2 addition on the sintering density microstructure and hardness of aluminum alloys pre

Abstract

Keywords: Aluminum; Powder Metallurgy; Porosity; Intermetallic Compounds; Sintering Density

Introduction

Due to the high corrosion resistance, electrical drive capability, machinability, and relatively low cost, Aluminum (Al) and its alloys are significantly used in the aerospace industry [1-3]. Recently, Al alloys have been employed mainly in the automotive, maritime, aviation, and defense industries with improved resistance and impact properties. The Organization for Economic Cooperation (OECD) and Development International Energy Association (IEA) reported [4] that aluminum alloys are classified as good

candidates that led to increased usage in transportation systems, including light vehicles, railcars, and aircraft as efforts to reduce fuel consumption. However, these alloys have drawbacks to notably used [5], due to their low strength. Thus, adding different alloying elements, such as Zn, Mg, and Cu [6, 7], is essential to improve hardness and strength [6, 7]. Furthermore, the higher solid solubility of Mg and Zn into the Al matrix may be considered to produce the strengthened hardening and thus improve the mechanical properties [8, 9]. Numerous experimental and theoretical

*Corresponding author. *E-mail address: cardakli@atauni.edu.tr This paper was recommended for publication in revised form by Regional Editor Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

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studies have been performed with precipitates effects on the mechanical property’s enhancement and the alloying elements’ additions [10, 11]. It was reported that there are several intermetallic phases, like MgZn2 and Al2Mg3Zn3 are formed in the commercial alloy of Al-Zn-Mg-Cu and might exist below the solidus temperature [12, 13]. Moreover, aluminum alloys are typically fabricated using casting, forging, extrusion, and semisolids [14]. The mechanical characteristics of aluminum alloys developed by the powder metallurgy process have been increasingly stated recently. Powder metallurgy is a method of production used to manufacture the final shape or near to shape components [15]. Moreover, it is easier and cheaper to manufacture complex engineering components with this method. Combined with various powder particle sintering, which is used successfully in small component production for different industrial applications [16]. Light metals such as aluminum are used in the automobile industry to minimize the weight of vehicles in order to minimize fuel consumption. Since the mid-1990s, industrial applications for automotive components, like camshafts and bearing caps, have been made of aluminum via powder metallurgy methods. Because of its high strength, 7XXX series aluminum alloys are increasingly used in powder metallurgy nowadays [17]. The literature review revealed that the second phase’s formation could increase the strength of aluminum alloys. Still, the relationship between various percentages and microstructures has not been studied yet. This study thus attempts to explain the effects of the second phase (intermetallic compounds), which formed in the structure according to the additive percentage of powder metallurgy Al alloys’ behavior in terms of density and porosity microstructure and microhardness.

Archimedes’ principles [18]. The average value (3 trials) of porosity density was calculated using the following equation [19]:

Experimental Procedure

A 99.9% purity of Mg and Zn rods were purchased from Sigma Aldrich used in this study. A 5 g of the Zn and 0.93 g of Mg were melted in a Zirconia crucible using an electric induction furnace under a vacuum atmosphere. The casted MgZn alloy was then crushed using a hammer and followed by a ball milling at 300 rpm for 3 hrs to obtain MgZn2 powder with fine particle size. The aluminum powder with a purity of 99.9% was also purchased from Sigma Aldrich, mixed with the produced MgZn2 powder at different volume fractions of 2.5%, 5%, 10%, and 20%, shown in Table 1. Then, the mechanical ball milling was carried out in an argon atmosphere at a rotation speed of 300rpm for 18h. The mixed powder was compacted into green samples using a hydraulic press with an applied 5 tons of load. The produced samples were in the shape of a cylinder with a 20 mm diameter and 3 mm thickness dimension. The green samples were then sintered in a vacuum furnace at 575 oC for 3 hrs, followed by polishing. The density of the porosity and sintered samples were calculated according to

Where ρ is the experimental density (based on Archimedes’ principles) and ρa is the theoretical density Al.

The microstructure and phase composition due to the different volume fractions of MgZn2 addition were characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. Fully recrystallized grains with average sizes through above alloying (the average grain size was measured by the “line intercept method”, according to the microstructures observed by Scanning electron microscopy (SEM); all the high-angle grain boundaries, including twin boundaries were counted). For the phase analysis, X-ray diffraction (XRD-600, Japan) with Cu Ka radiation at 40 kV and 100 mA was used. The microhardness measurements were calculated using a Shimadzu microhardness tester with an applied load of 0.2 Newton for 30 sec. The calculated values are the average of five measurements at different locations.

Results And Discussion

Effect of MgZn2 content on sintering density and porosity of Al alloys Figure 1 shows the sintering relative density of Al samples with different MgZn2 contents. The relative density of Al increased from 78.9% to 95.1% as the percentage of MgZn2 risen from 2.5% to 5%. However, adding MgZn2 with 10% and 20% reduced the relative density by 86% and 82.3%, respectively. The enhancement of the relative density is attributed to the optimum percentage of the Zn (which smaller particle than Mg) filled the interval space (porosity) during the compaction process, resulting in a higher relative density. Furthermore, as the particles’ spacing becomes closer, the diffusion process takes a shorter time to produce a completed bonding between the particles and makes a homogenized microstructure. Nevertheless, when

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the MgZn2 addition was increased to 10% and 20%, the relative density was reduced due to the partial replacement of Mg by Zn atoms, which lead to a decrease in the mismatching of the lattice of aluminum [20, 21]. These results were also confirmed by measuring the porosity (Figure 2), which shows the same variation trend as the percentage of MgZn2 added.

This result is also consistent with the density and volume fraction of the porosity discussed earlier. The grain refinement process changes the planar or cellular interface to an equiaxed structure, which minimizes porosity [22], diminishes hot tearing, improves feeding, and reduces volumetric shrinkage [23]. This phenomenon raises the fracture toughness significantly [22] with the decrease in the grain size of aluminium alloy [24]. Figure 3a shows the characteristic microporous microstructure of the sintered Al alloy. The addition of 2.5 wt.% MgZn2 (Figure 3b) shows micro-segregation and coarse grains of the MgZn2 phase, which is non-uniformly distributed in the aluminium matrix. Furthermore, the Al alloys’ grain size slightly decreased as the addition of the MgZn2 increased from 2.5% to 5% along with uniform distribution in the formed particles (see in Figure 3c) than other further additions (i.e., 10% and 20%), as shown in Figure 3d and e. These reductions in the grain size may be attributed to the structure of the Mg and Zn, as both of them belong to a close-packed hexagonal structure. The Zn atoms diffusion rate is considered faster than Mg atoms due to the difference in the atom size, whereby it diffuses in the aluminum matrix and produces a solid solution or intermetallic compounds [25]. Despite this, the particle size distribution mainly affects the powder properties, affecting the sinterability of the prepared alloys. Liu et al. [26] found that the optimum sintering density was achieved within a particle size for the Al powder nearly ∼100 𝜇m. Figure 4(a-e) shows the SEM micrographs of the Al alloys with and without MgZn2 addition; it was found that the MgZn2 was mainly distributed at the Al particles’ boundaries along with few particles were formed at the intragranular of Al particles. The SEM also shows that the sintering precipitates have formed at both center and boundaries of grains, as shown in Figure 4d. The elemental analysis was conducted by EDX to determine the composition of the area scanning and found that the composition of Mg and Zn varied according to the added contents. Besides that, the presence of these precipitates (marked with magnified scanned area in Figure 4f) could be seen clearly as the content of MgZn2 increased. The second phase on the grain boundary impedes the grain boundary movement and prevents grain growth [27, 28]. In consequence, the Al grains refined with the increased MgZn2 content. Figure 5 shows the XRD patterns of the Al samples with and without MgZn2 addition. It can be seen that the bare sample is composed of the α-Al phase, while with the addition of MgZn2, there are three phases were obtained; α-Al phase, Al5Mg11Zn4, and AlMg4Zn11. The intermetallic compounds Al5Mg11Zn4 and AlMg4Zn11 were formed with different volume fractions, and their related XRD peaks were increases as the MgZn2 increased. The main reason for the intermetallic formation is that with increasing the amount of MgZn2 addition, the Mg and Zn atoms will be diffused at a higher rate. However, due to the Zn atoms

Figure 1. The density of the sintered Al alloy with different MgZn2 contents.

Figure 2. Porosity volume fraction of the sintered Al alloy with different MgZn2 contents.

Effect of MgZn2 content on microstructure and phase structure of Al alloys Figure 3(a-e) shows the optical micrographs of Al samples with the different MgZn2 addition contents. It can be seen that with increasing the MgZn2 contents from 2.5% to 5%, the pore size of the sintered Al sample is reduced.

Sigma J Eng Nat Sci, Vol. 41, No. 3, pp. 538−544, June, 2023

Figure 3. Optical micrographs of (a) Al, (b) Al +2.5% MgZn2, (c) Al +5% MgZn2, (d) Al +10% MgZn2, (e) Al +20% MgZn2, and (f) grain size measurement.

diffusion ability limitation [28], the Zn atoms particulate near the sintered necks and distort the Al matrix. Moreover, during the transformation of the sintered necks to form grain boundaries, the MgZn2 particles are situated near the grain boundaries, which results in double-confirming the microstructure presence in Figure 4.

Effect of MgZn2 content on microhardness of Al alloys Figure 6 shows the variation of the microhardness values corresponding to the amount of MgZn2 addition. It can be seen that the hardness of the Al samples was increased drastically as the MgZn2 was added. The highest hardness was obtained from the Al sample with 5% of MgZn2, achieving

Sigma J Eng Nat Sci, Vol. 41, No. 3, pp. 538−544, June, 2023

Figure 4. SEM surface morphology and EDS analysis of (a) Al, (b) Al +2.5% MgZn2, (c) Al +5% MgZn2, (d) Al +10% MgZn2, (e) Al +20% MgZn2 and (f) magnified precipitates formation.

a hardness of 112.4 HV. The hardness improvement may be attributed to the strength hardening as well as the presence of the precipitates, which it hinders the movement of dislocations and thus increases the hardness [29]. As a result of porosity and grain size reduction associated with

the existence of intermetallic compounds, the mechanical properties in terms of hardness improved. However, the hardness decrement for the 10% and 20% samples may result as a consequence of the increment in the size of precipitates formed in the structure (oversaturation).

Sigma J Eng Nat Sci, Vol. 41, No. 3, pp. 538−544, June, 2023

Figure 6. Microhardness of Al with different MgZn2 contents. Figure 5. XRD diffraction patterns of Al with different MgZn2 contents.

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