Investigation of machinability by electrical discharge machining method of ZA27MWCNT composites prod

Abstract

Keywords: EDM; Machinability; Metal Matrix Nano Composite; MWCNT; ZA27

Introduction

In recent years, research on metal matrix composites (MMC), which are preferred in many sectors such as medicine, defense, automotive, aerospace, energy, infrastructure, maritime, transportation, food, and sports are increasing [1,2]. In these researches, mainly aluminium and magnesium alloys constitute the majority. However, recently, the production and examination of the properties of zinc matrix ceramic reinforced composites has been a field of interest

for many researchers. ZA27 coded zinc-aluminum alloy (Zamac) is widely used in the automotive industry, sports equipment, toys, hardware, decoration, white goods, etc., due to its suitability for mass production, high corrosion resistance, low melting temperature and superior workability. In addition, zinc-aluminium (ZA) alloys are known to be important bearing materials especially suitable for highload and low speed applications. They are reported to be environmentally friendly materials with good processing

*Corresponding author. *E-mail address: msubasi@gazi.edu.tr This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkilic 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/).

Sigma J Eng Nat Sci, Vol. 42, No. 6, pp. 1965−1972, December, 2024

properties and low initial cost, highlighting their excellent castability [3]. ZA27 alloy is used as the base material in the development of MMC materials and the reinforcement element is as effective as the base material in the development of the properties of composite materials. The reinforcement element carries a large part of the load on the composite. The use of nanomaterials as reinforcement elements in the production of such composites is becoming increasingly common [4]. Among these materials, materials such as nano-sized B4C, SiC, SiO2, Al2O3, MgO, TiB2, TiC, carbon nanotubes (CNT) and nanographene (GNP) stand out in the technology. In addition to these reinforcement elements, some researchers have produced “pumice reinforced AA7075 syntactic foams as composites” and investigated their machinability by face turning [5]. In recent years, materials containing one-dimensional CNT and two-dimensional graphene have emerged as an important new class for structural engineering and functional device applications due to their exceptionally high elastic modulus and mechanical strength. Along with their aspect ratio (i.e. length-to-thickness or length-to-diameter ratio) properties, graphene and CNT are considered the most promising reinforcing fillers for the fabrication of composite materials [6]. There are many studies in the literature in which the very superior properties of carbon nanomaterials are expressed [7-9]. CNTs are the most important of all nanomaterials, and they are tubular structures with nanoscale diameter and micron size in the simplest sense. In other words, from a structural standpoint, CNTs can be imagined as hollow cylinders made of one or

more sheets of graphene (a single layer of graphite) (Figure 1). The cylinder diameter may range from 0.4 nm to 100 nm or higher. The sizes of CNTs, which can vary depending on the production method and conditions, can range from hundreds of μm to cm [10]. It has been observed that CNTs have stiffness up to 1000 GPa, tensile strength of 100 GPa and thermal conductivity values up to 6000 W/mK. These studies show that CNTs are the strongest fibres known to mankind with extraordinary properties [11]. In studies on metal matrix composites, a large number of aluminium-based nano-composite studies have been carried out. However, it has been seen that there are not enough domestic and foreign studies on ZA27-based nanocomposites, which is a zinc-based alloy, in the literature. Metal matrix composite materials are subjected to some shaping processes depending on where they will be used. These shaping processes can be traditional chipless or machining methods, as well as some non-traditional machining methods can be applied because metal matrix composites are difficult to shape with traditional methods due to the high hardness that occurs in their structures due to the additives reinforced into them [12]. Therefore, recently, non-traditional processing methods such as EDM have been the subject of research to overcome this problem. In the literature, there are some studies on the mechanical properties and machinability of nanomaterial doped, ZA27 and different metal matrix composites [13-18]. At this point, the importance of tool conductivity was emphasised in some similar studies. In comparing the performance of CuBe and E-Cu electrodes, it was expressed that the E-Cu electrode performed better than the CuBe electrode in processing TiN-coated Ti6Al4V because of its higher conductivity [19]. However, no study has been encountered to examine the machinability of ZA27-based nano-doped composites with EDM. In this experimental study, the machinability of ZA27 matrix MWCNT added composites produced by powder metallurgy technique, unlike the literature, was investigated by EDM method. The MMR and TWR ratios were compared with the results in machining the MWCNT-free ZA27 alloy.

Materials And Methods

Figure 1. Formation of (a) single and (b) multi-walled carbon nanotubes from graphene nanoplate.

Some technical properties of the ZA27 matrix material and MWCNT reinforcement element in powder form obtained from the Nanograph Nano Technology Turkey Company are given in Table 1. In this study, composite materials previously produced by the powder metallurgy method within the scope of Scientific Research Project numbered 2018/043 were used. Composite materials were produced by doping 0.5% by weight of MWCNT in ZA27 matrix. The powder mixture, prepared by adding 0.5% MWCNT by weight into the ZA27 matrix material, was mixed in a ball mill at 300 rpm for 2 hours and then compressed in a hydraulic press under

Sigma J Eng Nat Sci, Vol. 42, No. 6, pp. 1965−1972, December, 2024

Table 1. ZA27 matrix and MWCNT reinforcement material specifications [20-22] ZA 27 Al %

Mwcnt

500 MPa pressure in a steel mold. Then, the compressed composite samples were placed in the heat treatment furnace. The temperature of the heat treatment furnace was gradually increased to 450°C in 45 minutes and the production process was completed by sintering at these temperatures for 90 minutes. Images of the composite samples whose production process was completed were taken with a scanning electron microscope (SEM) to examine their microstructures. Then, hardness measurements of composite materials were made according to the Brinell method.

In hardness measurements, a load of 15,625 kg was applied with a 2.5 mm diameter steel ball [20-22]. Machinability experiments were carried out in two stages as composite materials and pure ZA27 blank sample. EDM machinability tests were carried out using Furkan 50A ZNC-1 brand EDM machine. Experiments using the full combination experimental design presented in Table 2, like as three different currents (6, 15, 25 A), three different pulse on times (ton; 48, 99, 150 µs) and two different pulse off times (toff; 99, 150 µs) were used. The power level values given in Table 2 are used to set the processing current intensity. Increasing the power level without changing the arc interval/ arc duration (ton/toff ) ratio increases the MRR (mm3/min) due to the increase in current intensity.

SEM images at different magnifications taken from 0.5% MWCNT added composite materials produced by powder metallurgy method are given in Figure 2. The production parameters such as compression pressure, sintering temperature and time applied in the production of composites are other factors affecting the homogeneity of the composite structure [20]. When the SEM images (Figure 2a) are examined, it is understood that the sintering process generally provides the neck and bond formation between the ZA27 particles. However, it is also seen that partially porous (porous) regions occur within the composite structure. This structural defect, which is frequently encountered in particle-reinforced metallic composites, has been reported in the literature in different studies. Dalmış et al. their study, it was reported that the addition of nano-graphite decreased the density values of the composites and increased the porosity ratio. It has also been

Results And Discussion

Sigma J Eng Nat Sci, Vol. 42, No. 6, pp. 1965−1972, December, 2024

stated that porosity is known as a common and undesirable feature of metal matrix composites produced by powder metallurgy method and that it affects the properties of the composites tremendously [23]. El-Sayed et al. their a similar study, it was stated that the mixing process of raw materials is an important first step in controlling the distribution and porosity of particles that affect the mechanical and tribological behavior of composites [24]. Saheb et al. their study, it was stated that porosity was mainly caused by CNT agglomeration [25]. Zhou et al. some other factors such as the distribution, texture, interfacial bonding and porosity of CNTs in the composites produced by the powder metallurgy method will significantly affect the strength of the composites. They also pointed out other factors in their studies. It has been stated that the porosity amounts in all nanocomposites are much higher than those in the matrix, and as the weight percentage of CNTs increases, the porosity level increases. It has been stated that this situation is due to the agglomeration of CNTs. It has also been reported that CNT clusters may prevent effective bonding between the matrix and CNTs, leading to very small cracks and increasing porosity [26]. As can be seen from the SEM images, the not very well dispersed MWCNT particles tended to aggregate and agglomerate and covered the ZA27 particles (Figure 2b). In the SEM image at 20000x magnification in Figure 2c, densely agglomerated MWCNT particles completely surrounding the ZA27 fragment are seen. In this case, the contact between the ZA27 particles is reduced. The aggregated MWCNT particles acted as a thermal barrier and somewhat prevented the thermal transfer between the ZA27 matrix material. In this case, the sintering temperature applied is insufficient, and it is considered to prevent the bond formation between the ZA27 particles at the desired level. When evaluated in terms of thermal properties, there are very important differences between the matrix material ZA27 alloy (123 W/mK) and the reinforcement element MWCNT (3000 W/mK). Another important issue is the difficulties in producing such composite structures homogeneously. It is difficult to ensure the homogeneous distribution of very small nano-sized reinforcement elements such as graphene

or CNT in the metal matrix. However, the powder metallurgy technique is the most advantageous among the composite material production methods. Therefore, the powder metallurgy method was preferred in this study. The importance of the mixing process in the preparation process of composite structures emerges here. The mixing method should be chosen according to the matrix material of the composite structure to be produced and the characteristics of the reinforcement element to be used. In this study, the mechanical mixing method was applied. According to the obtained microstructure images, it can be said that it would be more appropriate to increase the applied 120 min mixing time and 300 rpm mixing speed values. Hardness The measured hardness values of the 0.5% MWCNT added ZA27 composite produced by powder metallurgy and the pure ZA27 sample without MWCNT reinforcement according to the Brinell method are given in the graphic in Figure 3.

Sigma J Eng Nat Sci, Vol. 42, No. 6, pp. 1965−1972, December, 2024

When the graphics in Figure 3 are examined, it is understood that the hardness of the composite material has decreased significantly with the effect of MWCNT reinforced into the ZA27 matrix material. While the hardness of the pure ZA27 test sample was 102.3 HBW, the hardness value of 0.5% MWCNT reinforced ZA27 composite was

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