Experimental investigation of metal removal and micro-hardness during MAF process

This work aims to provide an experimental investigation into metal removal and micro-hardness through the Magnetic Abrasive Finishing process (MAF) and study the impact of some process parameters (feed rate, coil current, and spindle speed) on these responses with (1008-AISI) workpieces and spherical electromagnetic tool based on Taguchi design of the experiment using Minitab 17 software. The results show that metal removal was strongly affected by the feed rate, while the strongly influential variable on micro-hardness was the coil current. The highest value of metal removal rate is achievable at conditions 10 mm/min, 2.5 A, and 700 RPM for feed rate, coil current, and spindle speed respectively.


Introduction
Magnetic abrasive finishing (MAF) is an electro-magnetic field-assisted process. The forces in this process are controlled by the magnetic field. The movement made by the pole brush and fixed plate work surface will cause a little amount to be removed from the workpiece and produce a mirror finishing surface [1,2]. The machining gap between the magnetic pole and workpiece is filled by abrasive powder which is consist of abrasive and iron powder plus a binder material because of the relative motion that occurs the abrasive friction force will cause the work surface machining [3]. Due to the flexibility of the magnetic brush, the machined surface will be free of the micro-cracks that appeared by traditional finishing processes such as grinding which uses a rigid tool [4]. Joshi et al. (2014) investigated the cylindrical MAF process of brass and cast iron workpieces using an electromagnetic pole. The results prove that the increase of coil turns to increase the metal removal due to the increase that occurs in magnetic flux. The initial surface roughness and hardness were taken into account to make a comparison between the initial and final values of both hardness and roughness [5]. Yuewu et al. (2020) studied the finishing process of (SS304) plate by MAF process utilizing three types of abrasive powder (diamond particles embedded in a spherical iron matrix, CBN, and alumina). The study included four machining parameters which are the size of magnetic abrasive powder, rotational speed, gap, and feed rate on the responses to metal removal and surface roughness. Finite element analysis was used to estimate the normal pressure of magnetic abrasive powder. The results illustrated that the diamond powder was the best one when compared with the other powder which gives higher material removal and better finish at the same machining conditions [6]. Nazar Kais M. Naif (2012) investigated the effect of some machining parameters during the MAF process. The abrasive powder is made from quartz and iron powder with percentages of 60%, and 40 % respectively. The experiments were performed on the brass plate (CuZn33) and the output responses are surface roughness and microhardness [7]. O. Kadhum (2018) predicted the value of the working time which obtains the required value for metal removal and surface roughness through designing and implementing an automated machine. The values were predicated by controlling the machining factors (velocity of the magnetic pole, electromagnetic voltage, machining gap, and machining time) during the MAF process. As a result of this work, about 97% for metal removal and 95% for surface roughness were the matchings between predicted values and experimental results [8]. S. Shather and M. Abd (2019) studied the metal removal and surface roughness during the MAF process utilizing different concentrations and mesh sizes of silicon carbide as the abrasive powder. As a result of this study, the higher metal removal was0.004 g gained using 25% Si and 75% Fe with1.5 mm gap and 100 μm mesh size. The optimum change in surface roughness was 0.007 μm utilizing 33% Si and 67% Fe, gap 2 mm and, 200 μm mesh size. [9]. The main aim of this work is to study the impact of machining parameters on the metal removal and microhardness of machined surfaces during the MAF process. .

Experimental procedure
MAF is performed on an NC milling machine, which is used to control the tool movement. In this work, the tool is a spherical electromagnetic tool with a (20 mm) diameter which is shown in Fig.1, whereas Fig.2 illustrated the machined sample it Consists of an iron core covered with copper wire. The specimens of (1008-AISI) plate with dimensions (100 X 50 X 0.9) mm, were used to implement the 9 experiments. The workpiece's chemical composition is shown in Table1. The experiments were carried out at rotation speed 700 RPM and the density of magnetic field 1113.5, 3047.6, 4415.9 µ Tesla for 1.5, 2, 2.5 A respectively.  A50 percent is mixed from tungsten carbide and pure iron powders were performed to get the magnetic abrasive powder with 350°C sintering temperature after the sintering process the high ball mill and sieving machines were utilized to have a 300 µm mesh size of the magnetic abrasive powder. The process parameter levels are shown in Table 2. Table 3 illustrates the Taguchi L9 Orthogonal array of the parameters. The hardness was measured, before and after the MAF process, the differences in HV values at three different locations, and then the value was averaged by utilizing a Micro-hardness tester device.   Table 4 and Table 5 illustrate the experimental results of metal removal and hardness respectively.

Table 5. Experimental microhardness and S/N results
The main effect plot of metal removal and microhardness are illustrated in Fig. 2 and Fig. 3 respectively.  Fig. 3, it is clear to see lower metal removal when higher feed rate, because more magnetic abrasive passes into the machining gap with lower feed rate values, which causes an enhancement of metal removal. On the other side, the abrasive particle passes quickly with increasing the feed rate and produces a small amount of metal to be removed from the machining zone. Increasing the magnetic flux density means more material removal. The increase in tool rotational speed will cause an increase in the value of metal removal. The reason behind this action is belonging to a higher frictional force occurs at a higher rotational speed because of the irregular jumbling of the abrasive that happened under these conditions, which produces over-removal of the material by the aggressive strikes of the jumbling abrasives.  4 demonstrated when increasing the rotational speed, the normal magnetic force will decrease that meaning a decrease in the improvement of surface hardness. At high speed the magnetic abrasives will Splash from the machining gap when normal magnetic force decreases, Fig.4 also illustrated it seen that the increase in coil current causes the change in Micro-Vickers hardness to increase. A higher number of indentations in the workpiece surface occur when increasing the coil current because of an increase in the strength of the magnetic brush [9]. Some influence of feed rate on change in micro-hardness is illustrated in Figure 4, the increases in feed rate values from (10 to 30) mm/min an improvement occurs in the change in micro-hardness.  Fig.6 illustrated the signal-to-noise ratio for metal removal and microhardness respectively, from Fig.5 can be noticed that the maximum values of the S/N ratio were obtained at the lower level of feed rate and the higher level of coil current and spindle speed. Fig.6 demonstrated that the maximum value of the S/N ratio at higher levels of feed rate and coil current whereas the lower level of spindle speed results in the maximum value of the S/N ratio. Fig7. and Fig.8 represent the effect of input parameters on metal removal, from these figures can be observed that the highest value of metal removal achievable at the lower right corner of the contour plot where the maximum value of coil current and spindle speed while the feed rate at the minimum level. This is because the abrasive particle passes quickly with increasing the feed rate and produces minimizes the metal removal to be removed from the machining zone. From Fig.9 can be observed that the highest value of metal removal was obtained at the upper right corner at the maximum levels of both coil current and spindle speed. This attribute can be explained due to higher frictional force occurring at a higher rotational speed.

Conclusions
In this work, metal removal and microhardness have been investigated experimentally using three machining factors on (1008-AISI) workpieces based on the Taguchi design of the experiment. The following conclusions are listed: 1-The investigation was conducted by considering the varying parameter of (feed rate, coil current, and spindle speed) with the Taguchi method.
2-The (feed rate) is the most influential variable for the metal removal flowed by speed and then coil current.
3-The (Coil current) is the most influential variable for the microhardness flowed by speed then feed rate. .