In modern era, industrial technology is changing very rapidly and many advanced materials are being developed to meet industrial requirements. Inconel 625 is a nickel based super alloy which possesses excellent materialistic characteristics like high strength at elevated temperatures, high toughness, better corrosion and wear resistance. Due to its supreme materialistic characteristics, it is extensively used in multifarious applications in industries like aerospace, marine, petro-chemical and solar power stations etc. However, machining and finishing of Inconel 625 is tough work with conventional method [1] [2] [3] [4] . It has been reported that machinability of nickel based alloys is poor with conventional methods. So machining and finishing of Inconel 625 is a challenge for the manufacturing industries [5, 6] . Various researchers have explored different non conventional methods to finish various advanced materials. One of such type of finishing process: Magnetic Abrasive Finishing (MAF) is a newly developed finishing process which is used for many industrial applications to obtain excellent surface finish with minimum surface defects [7] . It has been widely employed for finishing of internal as well as external cylindrical surfaces of stainless steel, brass and aluminium [8] [9] [10] . However, simple MAF is less productive for nickel based advanced materials [11] .
In modern era, industrial technology is changing very rapidly and many advanced materials are being developed to meet industrial requirements. Inconel 625 is a nickel based super alloy which possesses excellent materialistic characteristics like high strength at elevated temperatures, high toughness, better corrosion and wear resistance. Due to its supreme materialistic characteristics, it is extensively used in multifarious applications in industries like aerospace, marine, petro-chemical and solar power stations etc. However, machining and finishing of Inconel 625 is tough work with conventional method [1] [2] [3] [4] . It has been reported that machinability of nickel based alloys is poor with conventional methods. So machining and finishing of Inconel 625 is a challenge for the manufacturing industries [5, 6] . Various researchers have explored different non conventional methods to finish various advanced materials. One of such type of finishing process: Magnetic Abrasive Finishing (MAF) is a newly developed finishing process which is used for many industrial applications to obtain excellent surface finish with minimum surface defects [7] . It has been widely employed for finishing of internal as well as external cylindrical surfaces of stainless steel, brass and aluminium [8] [9] [10] . However, simple MAF is less productive for nickel based advanced materials [11] .
In modern era, industrial technology is changing very rapidly and many advanced materials are being developed to meet industrial requirements. Inconel 625 is a nickel based super alloy which possesses excellent materialistic characteristics like high strength at elevated temperatures, high toughness, better corrosion and wear resistance. Due to its supreme materialistic characteristics, it is extensively used in multifarious applications in industries like aerospace, marine, petro-chemical and solar power stations etc. However, machining and finishing of Inconel 625 is tough work with conventional method [1] [2] [3] [4] . It has been reported that machinability of nickel based alloys is poor with conventional methods. So machining and finishing of Inconel 625 is a challenge for the manufacturing industries [5, 6] . Various researchers have explored different non conventional methods to finish various advanced materials. One of such type of finishing process: Magnetic Abrasive Finishing (MAF) is a newly developed finishing process which is used for many industrial applications to obtain excellent surface finish with minimum surface defects [7] . It has been widely employed for finishing of internal as well as external cylindrical surfaces of stainless steel, brass and aluminium [8] [9] [10] . However, simple MAF is less productive for nickel based advanced materials [11] .
48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to Recently, chemically assisted magnetic abrasive finishing (CMAF) has been developed to make Magnetic Abrasive Finishing (MAF) process more efficient for hard and brittle materials. Chemically assisted magnetic abrasive finishing (CMAF) is a combination of chemical machining and simple Magnetic Abrasive Finishing (MAF) process. In this hybrid process, suitable etchant is applied on work surface and kept at certain elevated temperature for specific period of time. Under suitable chemical conditions, etchant react with work surface which in turn softens the molecular layer of work surface which can be further removed by magnetic abrasive finishing process. Chemically treated workpiece is kept between N-S poles of magnets. A mixture of ferro-magnetic abrasive particles is supplied in the gap between workpiece and magnets. Due to magnetic, ferro-magnetic abrasive particles get attracted towards N-S poles of permanent magnet. A soft magnetic brush is formed which performs the finishing action on work surfaces [12] .
Inconel 625 flat work surfaces have been finished with MAF using multi pole magnetic tool and results revealed that pole rotation speed is most significant factor for Material removal (MR). Moreover, medium finishing time (60 min) and Wt %age of abrasives (35%) also responsible for better MR [13] . Vibration assisted MAF has been successfully employed to finish aluminium work surfaces. It was observed that MR was enhanced with vibration assisted MAF as compared to simple MAF [14] .
Chemically assisted magnetic abrasive finishing process has been successfully implemented for finishing of tungsten flat surfaces. Further Inconel 718 flat surfaces have been successfully finished with Chemically assisted magnetic abrasive finishing process. Experimental results revealed that maximum material removal was obtained with maximum finishing time (90 min) and intermediate Wt %age of abrasives (35%) [15] [16] [17] .
In present research work, an attempt has been made for simultaneous internal and external surface finishing of Inconel 625 tubes using chemically assisted magnetic abrasive finishing process. An indigenous magnetic tool has been designed and developed. The effect of various input process factors on material removal during simultaneous internal and external surface finishing of Inconel 625 alloy tubes has been analyzed and discussed.
The processing principle of simultaneous internal and external cylindrical surface finishing using CMAF is shown in figure1. Initially, workpiece (tube) is chemically treated with suitable etchant. A suitable etchant is applied on work surface and kept at certain elevated temperature for specific period of time. Under suitable chemical conditions, etchant react with work surface which in turn softens the molecular layer of work surface which can be further removed by magnetic abrasive finishing process.
In magnetic abrasive finishing process, two permanent magnets are arranged around the chemically treated workpiece (tube). The upper pole profiles of both magnets (N-S) exactly matches with the outer tube surface. A mixture of ferro-magnetic abrasive particles is supplied inside and outside the tube. A flexible magnetic abrasive brush is formed as ferro-magnetic abrasive particles join the magnetic force lines under the magnetic field. The force (F) is calculated by following formula:
(1) Where V = magnetic particle volume, Xn = magnetic susceptibility, K = magnetic field intensity and grad K =gradient of the magnetic field respectively. Magnetic force Fi is produced inside the tube which pushes the magnetic abrasive particles towards the inner surface of tube. Force Fe is produced outside the tube and magnetic particles are forced towards the outer surface of tube. Due to rotation of tube surface, simultaneous internal and external surface finishing is performed.
An experimental setup for simultaneous internal and external surface finishing of Inconel 625 tubes (Ø25 mm x 150 mm x 2mm) is shown in figure 2. An indigenous magnetic tool has been designed and developed for simultaneous internal and external surface finishing. It consists of two Nd-Fe-B permanent magnets (35 x 35 x 25 mm) which are fixed on aluminium fixture (figure 2). Upper pole profile of both magnets exactly matches with the external surface of tube. Non-woven fabric coated with PTFE (polytetrafluoro-ethylene) tape is used to cover the magnet poles. Magnetic tool is mounted on the tool post of multispeed precision lathe to perform the experimets. 
On the basis of literature survey and precision lathe machine capabilities, five process parameters i.e. processing time, surface rotational speed, weight percentage (Wt %age) of abrasives, chemical concentration and abrasive size have been selected in present research work. Range of process parameters has been decided on the basis of pilot experimentation using one factor at a time (OFAT) technique. Other experimental conditions like electrolytic iron particle size, pole work gap, etching time, etching temperature, quantity of ferro-magnetic abrasive particles are kept constant. Table 1 shows the selected CMAF process parameters and their range. First of all, Inconel 625 tubes have been chemically treated with suitable chemical (Etchant). In this proposed research work, mixture of Ferric Chloride (FeCl 3 ) and Ethanol has been prepared as per selected range of chemical concentration as mentioned in Table 1 . Inconel 625 tubes were dipped into prepared chemical mixture and kept in electric muffle furnace at 65°C for 30 minutes for chemical treatment.
Then chemically treated Inconel 625 tube is fixed in four jaw chuck of precision lathe machine. Two poles of magnetic tool are arranged below the workpiece (Tube). A constatnt gap of 2 mm has maintained during each experiment. Mixtures of silicon carbide (SiC) abrasive particles and electrolytic iron particles have been prepared as per Wt %age of abrasives mentioned in table1. Gaps between the external surface of workpiece (tube) and N-S poles of magnetic tool have been filled with a specific quantity (3 gms on each pole) of ferro-magnetic abrasives particles. Same quantity (3 gms) is also supplied inside the tube. Soluble type barrel finishing compound is also used as a lubricant in this work. Due to high magnetic field, ferro-magnetic abrasive particles get attracted towards N-S magnetic poles. As a result of this, Flexible magnetic abrasive brush is formed inside and outside the tube which performs the finishing action.
All the experiments have been conducted as per experimental conditions as shown in table 2. Initial and final weights of each workpiece have been measured with digital weighing machine. After measuring weight of unfinished and finished workpieces, material removal (MR) is calculated as following:
Material Removal (MR) = Initial Weight of rough workpiece final weight of processed workpiece All values of material removal (MR) after each experiment have been tabulated in Table 2 . 
In this experimental research, response surface methodology has been used to design the experiments. Results of material removal (MR) have been analyzed by analysis of variance (ANNOVA). Table 3 shows the ANNOVA results for material removal. Significant quadratic models and regression equation has been developed using ANNOVA. To analyze impact of selected CMAF process factors on material removal, 3D surface graphs have been generated using significant models with possible interactions. 
It can be observed from figure 3 that material removal (MR) starts increasing with increase in surface rotational speed and processing time. Maximum material removal (MR) has been obtained at processing time (60 min) and higher surface rotational speed (300 RPM). It may be hypothesized that as surface rotational speed is increased, there will be more surface abrasion. On the other hand, material removal is continuously increased with increase in processing time upto 60 minutes. After 60 minutes of processing time, most of the surface asperities have been removed. So material removal starts decreasing. Figure 4 shows combined effect of processing time and chemical concentration on material removal (MR). Initially MR is increased by increasing the chemical concentration and processing time. But after processing for 45 minutes with maximum chemical concentration (700 gms/lt) material removal start decreasing. Further, material removal keeps on increasing with increase in processing time but decrease in chemical concentration of etchant. Maximum material removal is achieved at maximum processing time (75 min) and minimum chemical concentration (500 gm/lt).It may be hypothesized that effect of chemical concentration is limited upto certain processing time. Figure 5 shows the combined effect of surface rotational speed and chemical concentration on material removal (MR). It can be observed that MR is increased by simultaneous increment in surface rotational speed and chemical concentration of etchant. Maximum material removal (MR) is obtained at maximum chemical concentration (700 gm/lt) and surface rotational speed (300 RPM). It may be due to the fact that at maximum chemical concentration (700 gm/lt), surface molecular bonds become very loose. On other hand, as surface rotational speed is increased, more surface abrasion occurs resulting more material removed from workpiece. It is interesting to observe that maximum material is removed with medium Wt % of abrasive particles (35 %) and maximum surface rotational speed (300 rpm). As Wt %age of abrasives is further increased, material removal (MR) decreases drastically. Material removal is minimum at highest value of Wt %age of abrasives (45%) and at minimum surface rotational speed (60 rpm). It proves that higher Wt %age of abrasives becomes less effective at low surface rotational speed (60 rpm).
It can be observed from figure 7 that maximum material removal (MR) is obtained with intermediate values of Wt %age of abrasives (35%) and abrasive size (40 microns). Initially, material removal is enhanced by increasing Wt %age of abrasives till 35% and abrasive size upto 40 microns. but afterwards, sharp decline in material removal is observed by further increasing the Wt %age of abrasives and abrasive size. It can hypothesized that very fine and very coarse abrasive particles with small and higher Wt %age of abrasive are less efficient to remove the material from workpiece surface. 
Surface microstructure of unfinished, chemically treated and finished workpiece has been analyzed using Scanning Electron Microscopy (Fig. 8 ). It proves that processing time is the most influencing parameter during chemically assisted magnetic abrasive finishing process.
From figure 9 (a), it can be observed that at minimum surface rotational speed (60 RPM), number of surface asperities remains on work surfaces. It indicates that less material has been removed from work surfaces (both internal as well as external). As compared to external surface, internal surface seems to be better improved. Figure 9 (b) shows that more material has been removed from work surfaces. Chemically diffused surfaces have been efficiently improved by removing most of surface irregularities at maximum speed (300 RPM). It proves that at higher surface rotational speed, there is corresponding increase in material removal. Again internal surface seems to be better improved than external surface. Figure 9 (c) and 9 (d) represents that surface irregularities and diffused surface are not eliminated from work surface by using minimum (25%) and maximum (45%) range of Wt %age of abrasives. Better results are obtained using 35% Wt %age of abrasives. Still internal surface is better improved than external surface. Figure 10 (a) and 10 (b) show processed work surface microstructure with lower chemical concentration of etchant (500 gm/lt) and higher chemical concentration of etchant (700 gm/lt). It can be concluded that surface craters and scratches are not eliminated efficiently from work surfaces with lower chemical concentration (500 gm/lt). On other side surface craters and scratches are better removed with higher chemical concentration (700 gm/lt). It may be due to the fact that with higher chemical concentration (700 gm/lt), work surface layer become softer which can be easily removed resulting better material removal. Here, again internal surfaces seem to be better improved than external surfaces. Figure 10 
Internal and external surface of Inconel 625 tubes can be simultaneously finished with chemically assisted magnetic abrasive finishing process. On the basis of experimental results and microstructure analysis, following conclusions can be drawn.
1. All five selected input factors: processing time, surface rotational speed, Wt %age of abrasives, abrasive size and chemical concentration significantly affect material removal from work surfaces. 2. Processing time is most significant factor which influences the material removal. Maximum material removal is obtained when work surfaces has been processed for maximum duration (75 minutes). 3. As compared to low surface rotational speed (60 RPM), better material removal can be obtained at higher surface rotational speed (300 RPM). 4. Smaller and larger abrasive sizes are not much effective for efficient material removal. Average sized abrasive particles (40 microns) remove more material from work surfaces. 5. Better material removal can be achieved with intermediate value of Wt %age of abrasive particles (35%).
6. Effect of Chemical concentration of etchant on material removal depends on other process parameters like processing time and surface rotational speed. Still higher chemical concentration of etchant affects material removal from work surfaces. 7. As compared to external surfaces, internal surfaces are better improved.