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Machinability study of Zr-Cu-Ti metallic glass by micro hole drilling using micro-USM
Journal of Materials Processing Technology 240 (2017) 42–51
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Machinability study of Zr-Cu-Ti metallic glass by micro hole drilling using micro-USM Sandeep Kuriakose a,∗ , Promod Kumar Patowari a , Jatin Bhatt b a b
Department of Mechanical Engineering, National Institute of Technology Silchar, Assam 788010, India Department of Metallurgical and Material Engineering, Visvesvaraya National Institute of Technology Nagpur, Maharashtra 440010, India
a r t i c l e
i n f o
Article history: Received 1 February 2016 Received in revised form 7 August 2016 Accepted 25 August 2016 Keywords: Metallic glass Amorphous alloy Micro hole drilling Micro-ultrasonic machining Machinability study
a b s t r a c t Metallic glasses are the new class of materials researched and developed extensively for their applications in fabrication of micro components because of their exceptional mechanical properties. Micro machining of metallic glass to the required tolerance without any structural damage is a challenging task. Micro-ultrasonic machining (micro-USM) is a nontraditional machining process which can machine micro features in hard and brittle materials without heat generation. In present research work the machinability of Zr60 Cu30 Ti10 metallic glass is investigated by micro hole drilling using micro-USM. Machinability study is carried out by analyzing the performance measures like overcut, edge deviation, taper angle, material removal rate (MRR) and tool wear rate (TWR) with respect to the input machining parameters such as feed, abrasive grit size and concentration of abrasive slurry. The edges of the drilled holes are analyzed using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) to study the micro structural changes which are taken place in metallic glass due to micro machining. SEM and EDS analysis revealed that the amorphous structure of metallic glass is not affected during machining by micro-USM. A range of feed rate, abrasive grit size and slurry concentration are recognized for microUSM drilling in metallic glass using multi-objective optimization on the basis of ratio analysis (MOORA) method. The micro-USM drilling in metallic glass provides a new horizon for the precision machining of glassy alloys otherwise which are very difficult to machine conventionally without any change in its amorphous structure. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metallic glasses are the metallic alloys having random distribution of atoms resulting in amorphous structure. Review study by Wang et al. (2004) on bulk metallic glasses (BMGs) have indicated combination of unique properties like high strength, high hardness, superior wear resistance, corrosion resistance and having ability to obtain good surface finish. BMGs are widely researched recently due to their applications in consumer electronics frames and casings, small functional metal parts, orthopedic screws, cardiovascular stents, precision surgical instruments as testified by Johnson (2015) and for micro-electro-mechanical systems (MEMS) as reported by Chen et al. (2008). The research of Wang et al. (2011) in Ca-Mg-Zn based metallic glass with elastic modulus similar to that of human bone made the most revolutionary application of
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Kuriakose). http://dx.doi.org/10.1016/j.jmatprotec.2016.08.026 0924-0136/© 2016 Elsevier B.V. All rights reserved.
metallic glass as biodegradable implants. The study of Chu et al. (2014) and Sun et al. (2014) on Zr-based BMGs for bio-implants revealed its antibacterial properties and better human bone marrow mesenchymal stem cell adhesion morphology, respectively. Many of these applications are pointing towards the need of microfabrication methods for BMG. The main method used for the design and fabrication of metallic glass parts is thermoplastic forming (TPF). Schroers et al. (2007) and Schroers (2010) proved that TPF can be effectively used for the fabrication of metallic glass micro parts too. The works of Liu et al. (2015) showed the capability of TPF to replicates even features in the nanometer scale with high strength, homogeneity, free of internal stresses and porosity. However, the limited mold-tool life time and annealing-induced embrittlement of processed parts due to exposure of high temperatures result in property variations as reported by Johnson (2015). Manufacturing of molds for the complex micro geometries for TPF is also a tough job and limits the use of TPF for micro fabrication of BMG. Mechanical subtractive processes such as drilling, milling, turning etc. can be used for the machining of complex geometries in
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BMG. Bakkal et al. (2004c), studied the turning of Zr-based BMG and testified that cutting force and specific cutting energy of BMG were analogous with those of Al6061 and SS304. Research reported by Bakkal et al. (2004b) on chip formation revealed the poor thermal conductivity of metallic glasses resulted in high heat generation in the 2400–2700 K range. This can result in light emission and oxidation at high cutting speed with low thermal conductivity tools as confirmed by Bakkal et al. (2005a) while drilling BMG. The optical microscopy of the polished and chemically etched layer of the chips formed at high cutting speed by Bakkal et al. (2004a) revealed the oxidation and crystallization and this deteriorates the properties of metallic glass. Also, the investigation of Bakkal et al. (2005b) about thrust force, torque, and tool wear in drilling of the bulk metallic glass discovered that high heat generation adversely affects the tool life. Yet the conventional micro hole drilling in Zr41.25 Ti13.75 Cu12.5 Ni10 Be22.5 BMG of thickness 1.2 mm by Zhu et al. (2013) could machine micro holes without crystallization. Furthermore, thermal softening due to high heat generation and burrs produced deteriorates the hole’s edges and geometry. Therefore, the conventional micro machining methods are incapable of machining geometries to the required tolerances in metallic glass and further secondary finishing operations may entail removal of the burrs produced to achieve the required surface finish. This suggests for the nontraditional micro machining methods as pointed out by Masuzawa (2000), for more accurate and burr free machining of metallic glass. Nontraditional micro machining methods can be used for the micro machining of very accurate, smooth surfaces and features for the MEMS, optical and photonic applications. The research works in this field for micro machining of metallic glass are rare. To the best of authors’ knowledge, the work reported by Chen et al. (2008) about machining of micro-holes and three-dimensional microstructures on the La62 Al14 Ni12 Cu12 , Zr55 Al10 Ni5 Cu30 and Cu46 Zr44 Al7 Y3 BMGs by micro-electrical discharge machining (micro-EDM) is the prominent one. The micro holes are machined without any crystallization but defects like recast layers, burrs and heat affected zones are found with decrease in drilling diameter. For micro-EDM at higher energy input, Yeo et al. (2009) found that melting point of the electrode material is very significant. The investigations in traditional or nontraditional drilling neither studied the important hole features like overcut, edge deviation and taperness nor the feasibility of machining with material removal rate (MRR) and tool wear rate (TWR) till date for metallic glass. Sun et al. (1996) and Masuzawa (2000), studied about micro machining using micro-ultrasonic machining (micro-USM) and reported that micro-USM is capable of making almost any three-dimensional microstructure with high aspect ratio on most materials, particularly on hard and brittle materials. It can be used where the most well-known precision micromachining methods such as microEDM, LIGA and excimer laser drilling are very restricted either due to machinable shapes or in machinable materials. Review studies of Kumar (2013) and Thoe et al. (1998) established that ultrasonic machining is nearly a stress less machining without any heat accumulation. The micro-USM can be used as an alternative to conventional micro machining or micro-EDM for high aspect ratio structures in metallic glass without heat generation, light emission, crystallization and shear band formation. The research in the field of micro-USM of metallic glass is found to be very limited. In this research work the machinability study of metallic glass using micro-USM is carried out by micro hole drilling operation. Shaped micro tools are fabricated for micro-USM drilling with respect to the specifications for the optimum machining performance. Proper input and output parameters are selected and the appropriate ranges of the parameters are established. Experimentation is carried out by changing the input parameters with respect to the full factorial design. A new systematic approach is applied
43
Fig. 1. XRD pattern of metallic glass Zr60 Cu30 Ti10.
for the calculation of important hole features such as overcut at entry and exit, edge deviation at entry and exit, taperness and for the feasibility of machining such as MRR and TWR. Micro hole features and feasibility of machining are studied with respect to the micro-USM input parameters like feed rate, abrasive size and abrasive slurry concentrations. The multi-objective optimization on the basis of ratio analysis (MOORA) method is carried out effectively to identify the optimum input parameters by analyzing the output parameters of micro machining.
2. Experimental design and procedure 2.1. Micro drilling tool fabrication and drilling test setup The micro hole drilling experiments are performed using Sinker Model AP-1000 table top USM of Sonic-Mill. The metallic glass used for this study is Zr60 Cu30 Ti10 of thickness 20 m, synthesized by melt spinning route. XRD analysis is done in Panalytical X’Pert Pro MPD X-ray Diffractometer using Cu target confirming the amorphous structure of metallic glass and the XRD pattern is shown in Fig. 1. The metallic glass work piece is attached on a mildly heated glass slide using a polymeric wax. This glass slide is attached on the USM work base which is fixed to the USM machine base magnetically. After machining, the wax is removed completely using acetone as solvent. The tools for the micro-USM are fabricated in a half inch tool head of the machine. In USM the tool profile should be of the profile to be machined. So for the micro hole drilling, hollow micro cylindrical tool is chosen. Hollow tool is selected because of its higher MRR capability compared to the solid tool as observed by Kumar (2013). The tool wear characteristic study of Adithan (1981) in USM showed that the stainless steel tool exhibit low tool wear as compared to tungsten carbide and mild steel tool. In addition to that stainless steel tool is cheap and easily available. Therefore, stainless steel is chosen as tool material. The surgical quality stainless steel micro cylindrical tubes are cut using wire electrical discharge machine (WEDM) to the required length and weight for resonance according to the tool design criteria suggested by Thoe et al. (1998) and Kumar (2013). The micro tubes cut are attached to the tool tips by brazing or silver soldering to withstand the ultrasonic vibration during machining. Fig. 2(a) shows the hollow cylindrical micro tool prepared of outer diameter 326 m and inner diameter 209 m Fig. 2(b) shows the SEM image of the tip of the tool surface used for micro-USM. The EDS is done to ascertain the chemical element present in the tool. The EDS plot is shown in Fig. 3 and it confirms that the stainless steel alloy composed with alloying elements C, Cr, Mo, Zn, Ni, Mn, Al etc. The prepared tool is attached to the horn of the micro-USM and tuned for resonance.
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Fig. 2. (a) The micro tool prepared of outer diameter 326 m b) SEM image of micro tool of outer diameter 326 m and tool surface selected for EDS. Table 1 Process parameters with levels for micro-USM drilling in metallic glass. Parameters
Levels
Values
Feed (m/s) Abrasive size (grit no) Slurry concentration (%)
4 2 3
10, 30, 50, 70 400, 600 40, 50, 60
Fig. 3. EDS result of the tool surface.
2.2. Experimentation and calculation of output parameters The schematics of the experimental procedure is shown in Fig. 4. The full factorial design of experiments is considered to conduct the experiments in metallic glass samples by varying one parameter at a time to study the machining characteristic in detail. The input machining parameters with their levels are as shown in Table 1. The total numbers of experiments conducted are 24. Boron carbide (B4 C) mixed in distilled water is taken as abrasive slurry. After every eight experiments, the tool is removed from the machine and the tip is grinded and flattened to compensate change in geometry and for higher accuracy. Other USM parameters such as amplitude of vibration and feed force are kept constant.
Leica DM 2500 M trinocular metallurgical microscope is used to obtain optical micrograph of the drilled micro holes and for the measurement of the output parameters. The optical microscope images are taken for the hole entry side and exit side and the calculations are done for the overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle, MRR and TWR. Methodology adopted for calculation is shown in Fig. 5 and in Eqs. (1)–(8). The overcut is the measure of increase in the radius of a drilled hole and it is measured as the difference in radius of drilled hole and tool as given in Eq. (1) Overcut =
(Da − Dt ) 2
(1)
Where Dt = Outer diameter of tool, Da = D1 for calculation of overcut at entry and Da = D2 for calculation of overcut at exit. Where
Fig. 4. Schematic of the experimental procedure.
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
45
Fig. 5. (a) Measurement of overcut (b) Measurement of edge deviation.
D1 = average diameter of hole at entry and D2 = average diameter of hole at exit are calculated as the average of the diameters of the circles inscribed through the maximum irregularities (Dmax) and minimum irregularities (Dmin ) of drilled micro hole at entry and exit respectively. Edge deviation of the hole indicates finishing of the drilled hole at the edges by calculating difference between the maximum and minimum height of the irregularities in the edge of the micro hole. Edge deviation at entry and exit of the micro hole are calculated as shown in Eq. (2) Edge deviation =
(Dmax − Dmin ) 2
(2)
The calculation of taper angle provides information on change in diameter of drilled hole with respect to increase in depth of drilling. It is measured using Eq. (3) Taper angle, = tan−1 [
(D1 − D2 ) ] 2h
(3)
Where h = Thickness of the workpiece MRR in micro hole drilling is calculated as the volume of material removed per second as per Eq. (4) MRR =
D2 h 4t
(4)
Where t = Time for machining and D = Average hole diameter and is calculated as average of D1 and D2. TWR is calculated as the decrease in length of the tool with respect to machining time. It is measured using Eq. (5) TWR =
T t
x11
x12 . . .
x
x
X=[ . . . xm1
yij =
xij
m
(7)
x i=1 ij
Parameters to be maximized are added and parameters to be minimized are subtracted as per Eq. (8) to calculate the assessment value.
Ri =
g j=1
yij −
n
yij
(8)
j=g+1
Where ‘g’ is the number of parameters to be maximized, (n–g) is the number of parameters to be minimized and Ri is the assessment value of ith experiment with respect to all the parameters. When arranged in descending order, the best experiment is the one which has the highest assessment value. The hole quality is designated as minimum of overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle, TWR and maximum MRR. The best quality, good quality and worst quality holes are classified based on MOORA method and the structural changes are investigated using SEM and EDS analysis. SEM and EDS analysis are conducted with Hitachi S-3400N SEM. The optimum combination of input parameter levels for drilling the best quality micro hole is identified.
(5)
Where T = Tool wear The hole quality and optimum machining conditions are investigated by analyzing the output parameters using MOORA method. Karande and Chakraborty (2012) have proved MOORA method as an easy to implement multi objective decision making tool which provide precise rankings. Analysis is done using Eqs. (6)–(8). A decision matrix is made of the output values as shown in Eq. (6),
21
Where xij is the performance measure of ith alternative on jth criterion, ‘m’ is the number of experiments and ‘n’ is the number of output parameters. The matrix is then normalized using Eq. (7),
22
...
x1n x
2n
.
.
.
. .
. xm2 . . .
xmn
]
(6)
3. Results and discussion The machinability characteristics of metallic glass using micro tool of 326 m is investigated for two different abrasive grit sizes i.e. for grit 400 and 600 with varying abrasive concentrations and feed rate. Total 24 micro hole drilling experiments are conducted. Micro holes are drilled successfully on metallic glass using the micro-USM drilling process without any heat generation. The temperature of the workpiece was same as that of the slurry since slurry was circulating over that. The slurry temperature at starting of experiment had a little variation depending on room temperature but in all the experiments were below 30 ◦ C. The slurry and workpiece temperature remained same without any change by micro-USM drilling. The reason is attributed to the large volume of slurry flowing over a very small machining zone. Metallic glass showed a very good machinability to USM machining.
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SC=60% 70 60 50 40 30 20 10 0
SC=50%
SC=40%
b) Overcut exit (µm)
a)
Overcut exit (µm)
Fig. 6. Overcut at entry vs. feed for abrasive of (a) grit 400, (b) grit 600.
0
20
40
60
80
Feed (µm/s)
70 60 50 40 30 20 10 0
SC=60%
0
SC=50%
20
40
Feed (µm/s)
SC=40%
60
80
Fig. 7. Overcut at exit vs. feed for abrasive of (a) grit 400, (b) grit 600.
3.1. Analysis of machinability responses The main machining responses of micro hole drilling such as overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle, MRR and TWR are analyzed. The variations between output variables and feed for different concentrations of abrasive slurry for each grit size are plotted separately. The variation of overcut is shown in Figs. 6 and 7.
3.1.1. Overcut An average value of 40.92 m and 31.58 m were obtained for the overcut at entry for the grit 400 and 600, respectively. Overcut at exit value also followed the same trend with 39.58 m and 28.48 m corresponding to 400 grit and 600 grit. A least overcut of 10.75 m at entry and 10.25 m at exit of the hole was achieved by the micro-USM drilling using slurry of 600 grit. The Figs. 6 and 7 show that overcut decreases with increase in the feed rate and is minimum in the higher feed rate of 50–70 m/s. Feed rate is limited to 70 m/s because of the possibility of crushing of abrasive at very high feed rate as reported by Thoe et al. (1998). Fig. 7(a) and (b) shows overcut at exit follows same trend as that of overcut at entry with a minor reduction in overcut values. Abrasive slurry concentration (SC) of 50–60% is observed to be giving better results. Overcut is mainly caused by the unwanted interaction of abrasives around circumference of the tool. The overcut at entry and exit are found to be less for the slurry of 600 grit abrasives than that of 400 grit. The average diameter of 600 grit abrasives is lesser compared to 400 grit and which reduces the impact area of abrasives around the tool and thereby reducing the overcut. Overcut decreases with respect to the decrease in abrasive size. Higher feed is also found to be significant in reducing overcut in microUSM drilling of metallic glass. Yan et al. (2002) drilled precision micro-holes in borosilicate glass using micro EDM combined with micro-USM, and suggested that fast material removal reduce the friction between the micro-tool and the micro hole wall in the ultrasonic machining process. The increase in feed causes an increase in MRR and reduces the time for drilling which in turn reduces abrasive interaction time and thus the overcut.
3.1.2. Edge deviation The average edge deviation values for 400 grit and 600 grit are found to be 6.83 m and 12.58 m at entry and 7.67 m and 14.50 m at exit, respectively. Edge deviation is higher at exit than entry. The decrease of slurry reach at the exit side and the burrs left causes this increase. The best value of edge deviation is obtained as 4.50 m at entry and as 3.50 m at exit using slurry of 400 grit. The edge deviation is found to be better for higher abrasive size, 400 grit for the same feed. A trend for the variation of edge deviation could not be observed at entry and exit of the micro hole for the micro-USM drilling and is shown in Figs. 8 and 9. The medium feed rate in the range of 50 m/s, 400 grit abrasive and medium slurry concentrations are observed to be suitable for minimum edge deviation. Edge deviation on the other hand found to be lesser for slurry of higher sized abrasives, 400 grit than that of 600 grit. Study of Benedict (1987) about USM established that higher abrasive size increases the MRR. Slurry of higher sized abrasives could machine the hole edges more uniformly because of the higher MRR. This reduces the edge deviation of drilled micro hole, but at the expense of higher overcut. A higher concentration of slurry clogs and prevents the slurry flow to the micro machining zone and affects the edge deviation. Lower slurry concentrations even though reduces the number of abrasive particle for machining, but slightly improves MRR because of increase in flow ability of abrasive slurry to reach the micro machining zone as observed by Kumar (2013). So a lower slurry concentration of 40% or medium slurry concentration of 50% favors higher MRR and the lesser edge deviation.
3.1.3. Material removal rate The MRR as shown in Fig. 10 shows almost linear relationship with respect to the feed rate. At the higher feed rate tool moves faster towards the work piece and a good machining rate of metallic glass at the listed feed rate is observed with micro-USM. The 400 grit abrasive showed better machining rate with an average MRR of 0.0039 mm3 /min than 0.0032 mm3 /min with 600 grit abrasive. A grand average value of MRR 0.00355 mm3 /min is obtained for the micro-USM drilling using tool of 326 m with time just less than one minute to machine a micro hole. The abrasive concentration, within the range of 30–60%, has not seemed to make much
SC=60%
SC=50%
SC=40%
b)
35 30 25 20 15 10 5 0 0
20
40
60
Edge dev. entry (µm)
a)
Edge dev. entry (µm)
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
SC=60%
47
SC=50%
SC=40%
35 30 25 20 15 10 5 0 0
80
20
Feed (µm/s)
40
60
80
Feed (µm/s)
Fig. 8. Edge deviation at entry vs. feed for abrasive of (a) grit 400, (b) grit 600.
a)
SC=50%
SC=40%
b)
SC=60%
Edge dev. exit (µm)
Edge dev. exit (µm)
SC=60% 35 30 25 20 15 10 5 0
0
20
40
Feed (µm/s)
60
80
35 30 25 20 15 10 5 0
0
SC=50%
20
40
Feed (µm/s)
SC=40%
60
80
Fig. 9. Edge deviation at exit vs. feed for abrasive of (a) grit 400, (b) grit 600.
Fig. 10. MRR vs. feed for abrasive of (a) grit 400, (b) grit 600.
Fig. 11. SEM of tip of micro tool showing the wear profile.
difference in MRR. A slight higher material removal rate is observed for 40% slurry concentration because of the increase in flow ability of the slurry to reach the micro machining zone. 3.1.4. Tool wear rate The stainless steel tool showed a good wear resistance. TWR was found to be negligible with an average value of 1.76 m/s. The SEM of tip of micro tool after completion of experiments is shown in Fig. 11. The change in the geometry of the tool by micro-USM was very minimal. A minor reduction in outer diameter of the tool was
observed but the changes observed to the diameter in the linear length of the tool was very negligible when compared to the depth of cut applied. The use of hollow tool for drilling decreased the tool wear and increased the MRR by the principle of trepanning. Stainless steel hollow cylindrical tool proved as the promising tool for the micro drilling because of its wear resistance, ease of availability and cost effectiveness. A specific trend could not be observed in case of tool wear in micro-USM drilling of metallic glass.
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Fig. 12. SEM of micro hole in the best category (a) Entry side of the micro hole (b) Edge of the entry side the micro hole (c) Exit side of the micro hole (d) Edge of the exit side of the micro hole.
Fig. 13. EDS result of the burr on edge of the micro hole in best category.
3.1.5. Taper angle Taperness of the micro holes drilled in metallic glass using micro USM were with an average value of 9.29◦ and 3.81◦ respectively for 600 grit and 400 grit. Taper angle is found to be higher for the holes drilled using 600 grit than that with 400 grit. Machining with 400 grit drilled micro holes of near zero taper angle. Machining with 400 grit slurry machined micro holes with an average low taper angle value of 3.81◦ . Higher sized abrasive slurry was found to be drilling micro holes with lower taper angle in metallic glass. This is because of more uniform machining of the exit edges of the micro hole because of higher MRR. Finer abrasives reduce the overcut but at the completion of depth of cut, incapable of machining the edges of the micro holes at the exit side perfectly. The burrs left, thereby increases the taper angle of the drilled micro hole. The levels of parameter for optimum conditions are contradictory
for different response variables necessitating the use of scientific decision making tool.
3.2. MOORA analysis for optimum conditions The selection of levels of machining parameters for a combination of desired output variables is a multi-objective decision making problem. A decision matrix of 24 × 7 order is formulated with output variables using Eq. (6) for MOORA analysis. There are 24 numbers of experiments (drilled micro holes) and 7 number of output parameters. The decision matrix is normalized and the assessment values are calculated by adding the criteria to be maximized and subtracting the criteria to be minimized as given in Eqs. (7) and (8). MOORA analysis is given in Appendix A. Here in this case overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle and TWR are the criteria to be minimized
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49
Fig. 14. SEM of micro hole in the worst category (a) Entry side of the micro hole (b) Edge of the entry side the micro hole (c) Exit side of the micro hole (d) Edge of the exit side of the micro hole.
Fig. 15. EDS result of the burr on edge of the micro hole in worst category.
and MRR is the criterion to be maximized. The experimental run with highest assessment value is checked for the optimum drilling condition. Input parameters corresponding to the experiments are identified. Optimum condition is obtained for the input machining parameters of (feed 50 m/s, slurry concentration 40% and grit 600). Optimum condition is obtained near high feed rates of 50–70 m/s. Feed rate is found to be the most significant parameter in quality of micro hole in micro-USM drilling of metallic glass. High feed rate reduced the machining time and there by reduced the overcut considerably. Slurry of 400 grit abrasive was found to be suitable for less edge deviation, lower taper angle and for higher MRR. But abrasive slurry of 600 grit was observed to be outweighing the slurry of 400 grit when considering the whole response parameters for micro-USM drilling of metallic glass because of significant reduction in overcut. The best quality, good quality and
worst quality holes are classified based on assessment values. Two micro holes from each category are selected and the edges of the micro holes at the entry and exit are inspected for shear bands, stress affected zones, heat affected zones or change in material properties through SEM and EDS analyses. The SEM images of the micro holes in the best category and worst category are shown in Figs. 12 and 14 respectively. Burr height is found to be less in case of 600 grit than that in case of 400 grit because the smaller sized abrasives reduces the unit material removal and thereby increases the accuracy. The EDS analysis is done on the burrs on edge of the micro hole and also on a point which is radially a little outside of the edges. The EDS result of the burr on edge of the hole in best category and worst category are shown in Figs. 13 and 15 respectively. The EDS results showed that the chemical composition of the material remained same during machining. The EDS on the burrs on edge of
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the micro hole in worst category showed a little higher percentage of carbon content than the holes in the best category. Since temperature change during machining was negligible, the reason for this is attributed to the higher numbers of B4 C slurry particles entrapped in the burr. The mechanical properties of metallic glasses are different from crystalline alloys. Zhao et al. (2015) and Li et al. (2015) demonstrated that metallic glasses do not strain harden and they strain inhomogeneously by the formation of localized shear bands, propagation of shear bands and finally fails catastrophically. Atroshenko et al. (2010) found that even though metallic glass exhibit similar elastic modulus as that of the crystalline alloys of same composition, the strength of metallic glasses is much higher. Because of high strength as well as low thermal conductivity, the conventional machining of metallic glass results in high heat generation accompanied with light emission. The temperature of slurry and workpiece during micro-USM drilling were below 30 ◦ C in every experiments. For structural changes of the metallic glass material, especially for any crystallization, temperature must rise to its crystallization temperature. For that, it has to rise several hundreds of Celsius. In this particular case, workpiece temperature during machining was insignificant for any structural changes to the metallic glass material. The SEM and EDS analysis of the micro hole edges confirms that no shear bands, stress affected zones or heat affected zones are formed by the micro-USM drilling. The SEM images of edges of the best quality hole at entry and exit, as shown in Fig. 12, showed very good edge finish with minimum burrs. The worst quality hole, as shown in Fig. 14 only differed with more burrs. The reason for this stress free machining without any crystallization and property change is that because of the mechanism of material removal of USM. The sharp point of the abrasives in the slurry, which is at the interface of the shaped tool and the metallic glass workpiece, hit on the surface of workpiece with several thousands of its weight and produces an inelastic deformation zone by the ultrasonic vibration of the tools. At some threshold force, this deformed zone develops into a crack called as median crack. During unloading this median crack begins to close and leading to the formation of lateral cracks (Ichida et al., 2005; Kumar, 2013; Zhao et al., 2015). The lateral cracks outspreads to the work surface and lead to the micro-chip formation or spallation during complete unloading,
as reported by Atroshenko et al. (2010). Material removal by micro chipping and slurry flow through the machining zone prevents the heat generation and accumulation. The machining without heat generation helps to retain the amorphous structure of the metallic glass and its exceptional properties. The best feature of micro-USM is that it can machine any shape that can be made on the tool. The geometry of the tool is machined on the surface of the workpiece. So the study pertains for the micro channels, square holes or other complex micro- shape that can be machined by micro-USM in metallic glass for the applications of micro fluidics, MEMS, bio-implants etc. 4. Conclusions In this research work, machinability study of metallic glass for the machining of micro features is investigated by micro hole drilling operation using micro-USM. The applicability of USM for the machining of metallic glass and the effects of the variation of the input machining parameters to the drilled hole quality are studied. The following conclusions are perceived from this research. 1. Micro-USM can be used to machine micro holes in metallic glass in room temperature without any heat generation and crystallization. The amorphous structure and original properties of metallic glass are retained without any changes. 2. The metallic glass has a very good machinability for micro-USM with high MRR and with very low TWR. Micro holes with very less overcut, minimum edge deviation and near zero taper angle are machined in metallic glass by micro-USM. The burrs produced also were very minimal. 3. Micro hole quality was observed to be very good in the higher feed rate and by the use of abrasive slurry with abrasives of higher grit number. The micro-USM provided a new way for the precision machining of metallic glasses which are conventionally difficult to machine because of higher heat generation with light emission and crystallization. Appendix A.
Table A1 Decision matrix of the output parameters for MOORA analysis. Exp No.
Overcut entry (m)
Overcut exit (m)
Edge dev. entry (m)
Edge dev. exit (m)
Taper angle (◦ )
Tool wear rate (m/s)
MRR (mm3 /min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
36.50 41.75 37.25 29.00 40.50 39.00 36.00 23.75 62.00 54.25 46.50 44.50 42.50 33.00 22.75 23.75 21.75 29.25 31.75 35.00 49.00 48.75 30.75 10.75
36.50 40.75 35.75 26.50 40.25 38.00 35.50 22.50 61.00 52.25 44.00 42.00 39.75 29.00 21.00 22.50 20.25 22.25 23.25 28.25 48.00 47.75 29.50 10.25
8.00 4.50 5.50 6.00 8.00 9.00 6.00 7.50 7.00 7.50 6.00 7.00 34.00 12.00 8.50 12.50 19.50 9.50 4.50 5.00 12.00 11.50 6.50 15.50
8.00 3.50 6.50 6.00 8.50 8.00 6.00 8.00 10.00 11.50 11.00 5.00 30.50 15.00 10.00 15.00 18.50 15.50 16.50 12.50 9.50 10.50 6.00 14.50
0.00 2.86 4.29 7.13 0.72 2.86 1.43 3.58 2.86 5.71 7.13 7.13 7.83 11.32 5.00 3.58 4.29 19.30 23.04 18.66 10.63 2.86 3.58 1.43
1.66 1.41 2.85 1.21 0.33 1.57 2.17 2.60 1.07 1.15 0.51 3.36 3.12 1.40 1.63 2.22 0.45 0.61 1.24 2.60 2.39 1.59 1.50 3.63
0.0016 0.0042 0.0045 0.0049 0.0010 0.0041 0.0043 0.0044 0.0028 0.0044 0.0047 0.0064 0.0018 0.0027 0.0028 0.0048 0.0008 0.0024 0.0040 0.0048 0.0022 0.0038 0.0044 0.0042
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
51
Table A2 Normalized decision matrix and assessment values of MOORA analysis. Exp No. Overcut entry (m) Overcut exit (m) Edge dev. entry (m) Edge dev. exit (m) Taper angle (◦ ) Tool wear rate (m/s) MRR (mm3 /min) Assessment value, Ri 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0.042 0.048 0.043 0.033 0.047 0.045 0.041 0.027 0.071 0.062 0.053 0.051 0.049 0.038 0.026 0.027 0.025 0.034 0.036 0.040 0.056 0.056 0.035 0.012
0.045 0.050 0.044 0.032 0.049 0.047 0.043 0.028 0.075 0.064 0.054 0.051 0.049 0.036 0.026 0.028 0.025 0.027 0.028 0.035 0.059 0.058 0.036 0.013
0.034 0.019 0.024 0.026 0.034 0.039 0.026 0.032 0.030 0.032 0.026 0.030 0.146 0.052 0.036 0.054 0.084 0.041 0.019 0.021 0.052 0.049 0.028 0.067
0.030 0.013 0.024 0.023 0.032 0.030 0.023 0.030 0.038 0.043 0.041 0.019 0.115 0.056 0.038 0.056 0.070 0.058 0.062 0.047 0.036 0.039 0.023 0.055
References Adithan, M., 1981. Tool wear characteristics in ultrasonic drilling. Tribol. Int. 14 (6), 351–356. Atroshenko, S.A., Morozov, N.F., Zheng, W., Huang, Y.J., Sudenkov, Y.V., Naumova, N.S., Shen, J., 2010. Deformation behaviors of a TiZrNiCuBe bulk metallic glass under shock loading. J. Alloys Compd. 505, 501–504. Bakkal, M., Liu, C.T., Watkins, T.R., Scattergood, R.O., Shih, A.J., 2004a. Oxidation and crystallization of Zr-based bulk metallic glass due to machining. Intermetallics 12, 195–204. Bakkal, M., Shih, A.J., Scattergood, R.O., Liu, C.T., 2004b. Machining of a Zr–Ti–Al–Cu–Ni metallic glass. Scr. Mater. 50, 583–588. Bakkal, M., Shih, A.J., Scattergood, R.O., 2004c. Chip formation, cutting forces, and tool wear in turning of Zr-based bulk metallic glass. Int. J. Mach. Tools Manuf. 44, 915–925. Bakkal, M., Shih, A.J., McSpadden, S.B., Liu, C.T., Scattergood, R.O., 2005a. Light emission, chip morphology, and burr formation in drilling the bulk metallic glass. Int. J. Mach. Tools Manuf. 45, 741–752. Bakkal, M., Shih, A.J., McSpadden, S.B., Scattergood, R.O., 2005b. Thrust force, torque, and tool wear in drilling of the bulk metallic glass. Int. J. Mach. Tools Manuf. 45, 863–872. Benedict, G.F., 1987. Nontraditional Manufacturing Processes. Marcel Dekker Inc., New York, pp. 67–86. Chen, X.H., Zhang, X.C., Zhang, Y., Chen, G.L., 2008. Fabrication and characterization of metallic glasses with a specific microstructure for micro-electro-mechanical system applications. J. Non-Cryst. Solids 354, 3308–3316. Chu, J.P., Liu, T.Y., Li, C.L., Wang, C.H., Jang, J.S.C., Chen, M.J., Chang, S.H., Huang, W.C., 2014. Fabrication and characterizations of thin film metallic glasses: antibacterial property and durability study for medical application. Thin Solid Films 561, 102–107. Ichida, Y., Sato, R., Morimoto, Y., Kobayashi, K., 2005. Material removal mechanisms in non-contact ultrasonic abrasive machining. Wear 258, 107–114. Johnson, W.L., 2015. Is metallic glass poised to come of age? Nat. Mater. www. nature. com/naturematerials. Karande, P., Chakraborty, S., 2012. Application of multi-objective optimization on the basis of ratio analysis (MOORA) method for materials selection. Mater. Des. 37, 317–324.
0.000 0.018 0.027 0.045 0.005 0.018 0.009 0.023 0.018 0.036 0.045 0.045 0.050 0.072 0.032 0.023 0.027 0.123 0.147 0.119 0.068 0.018 0.023 0.009
0.039 0.033 0.067 0.029 0.008 0.037 0.051 0.062 0.025 0.027 0.012 0.080 0.074 0.033 0.039 0.053 0.011 0.014 0.029 0.062 0.056 0.038 0.035 0.086
0.019 0.049 0.053 0.057 0.011 0.048 0.049 0.051 0.033 0.051 0.054 0.075 0.021 0.031 0.033 0.056 0.009 0.028 0.047 0.055 0.025 0.044 0.051 0.049
−0.171 −0.132 −0.176 −0.131 −0.163 −0.167 −0.144 −0.151 −0.224 −0.214 −0.178 −0.202 −0.460 −0.255 −0.164 −0.184 −0.232 −0.269 −0.275 −0.268 −0.301 −0.215 −0.129 −0.192
Kumar, J., 2013. Ultrasonic machining-a comprehensive review. Mach. Sci. Technol. 17 (3), 325–379. Li, G., Jiang, M.Q., Jiang, F., He, L., Sun, J., 2015. The ductile to brittle transition behavior in a Zr-based bulk metallic glass. Mater. Sci. Eng. A 625, 393–402. Liu, Y., Liu, J., Sohn, S., Li, Y., Cha, J.J., Schroers, J., 2015. Metallic glass nanostructures of tunable shape and composition. Nat. Commun. 6, 7043, 10.038/ncomms8043. Masuzawa, T., 2000. State of the art of micro machining. CIRP Ann. Manuf. Technol. 49 (2), 473–488. Schroers, J., Nguyen, T., Keeffe, S.O., Desai, A., 2007. Thermoplastic forming of bulk metallic glass-applications for MEMS and microstructure fabrication. Mater. Sci. Eng. A 449–451, 898–902. Schroers, J., 2010. Processing of bulk metallic glass. Adv. Mater. 22, 1566–1597. Sun, X., Masuzawa, T., Fujino, M., 1996. Micro ultrasonic machining and its applications in MEMS. Sens. Actuators A: Phys. 57, 159–164. Sun, Y., Zhang, W., Kai, W., Liaw, P.K., Huang, H., 2014. Evaluation of Ni-free Zr–Cu–Fe–Al bulk metallic glass for biomedical implant applications. J. Alloys Compd. 586, 539–543. Thoe, T.B., Aspinwall, D.K., Wise, M.L.H., 1998. Review on ultrasonic machining. Int. J. Mach. Tools Manuf. 38 (4), 239–255. Wang, W.H., Dong, C., Shek, C.H., 2004. Bulk metallic glasses. Mater. Sci. Eng. R Reports 44, 45–89. Wang, Y.B., Xie, X.H., Li, H.F., Wang, X.L., Zhao, M.Z., Zhang, E.W., Bai, Y.J., Zheng, Y.F., Qin, L., 2011. Biodegradable Ca–Mg–Zn bulk metallic glass for potential skeletal application. Acta Biomater. 7, 3196–3208. Yan, B.H., Wang, A.C., Huang, C.Y., Huang, F.Y., 2002. Study of precision micro-holes in borosilicate glass using micro EDM combined with micro ultrasonic vibration machining. Int. J. Mach. Tools Manuf. 42, 1105–1112. Yeo, S.H., Tan, P.C., Aligiri, E., Tor, S.B., Loh, N.H., 2009. Processing of zirconium-based bulk metallic glass (BMG) using micro electrical discharge machining (Micro-EDM). Mater. Manuf. Process. 24, 1242–1248. Zhao, Y., Zhang, G., Estévez, D., Chang, C., Wang, X., Li, R., 2015. Evolution of shear bands into cracks in metallic glasses. J. Alloys Compd. 621, 238–243. Zhu, J., Kim, H.J., Kapoor, S.G., 2013. Microscale drilling of bulk metallic glass. J. Micro Nano-Manuf. 1, 041004.
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Machinability study of Zr-Cu-Ti metallic glass by micro hole drilling using micro-USM Sandeep Kuriakose a,∗ , Promod Kumar Patowari a , Jatin Bhatt b a b
Department of Mechanical Engineering, National Institute of Technology Silchar, Assam 788010, India Department of Metallurgical and Material Engineering, Visvesvaraya National Institute of Technology Nagpur, Maharashtra 440010, India
a r t i c l e
i n f o
Article history: Received 1 February 2016 Received in revised form 7 August 2016 Accepted 25 August 2016 Keywords: Metallic glass Amorphous alloy Micro hole drilling Micro-ultrasonic machining Machinability study
a b s t r a c t Metallic glasses are the new class of materials researched and developed extensively for their applications in fabrication of micro components because of their exceptional mechanical properties. Micro machining of metallic glass to the required tolerance without any structural damage is a challenging task. Micro-ultrasonic machining (micro-USM) is a nontraditional machining process which can machine micro features in hard and brittle materials without heat generation. In present research work the machinability of Zr60 Cu30 Ti10 metallic glass is investigated by micro hole drilling using micro-USM. Machinability study is carried out by analyzing the performance measures like overcut, edge deviation, taper angle, material removal rate (MRR) and tool wear rate (TWR) with respect to the input machining parameters such as feed, abrasive grit size and concentration of abrasive slurry. The edges of the drilled holes are analyzed using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) to study the micro structural changes which are taken place in metallic glass due to micro machining. SEM and EDS analysis revealed that the amorphous structure of metallic glass is not affected during machining by micro-USM. A range of feed rate, abrasive grit size and slurry concentration are recognized for microUSM drilling in metallic glass using multi-objective optimization on the basis of ratio analysis (MOORA) method. The micro-USM drilling in metallic glass provides a new horizon for the precision machining of glassy alloys otherwise which are very difficult to machine conventionally without any change in its amorphous structure. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Metallic glasses are the metallic alloys having random distribution of atoms resulting in amorphous structure. Review study by Wang et al. (2004) on bulk metallic glasses (BMGs) have indicated combination of unique properties like high strength, high hardness, superior wear resistance, corrosion resistance and having ability to obtain good surface finish. BMGs are widely researched recently due to their applications in consumer electronics frames and casings, small functional metal parts, orthopedic screws, cardiovascular stents, precision surgical instruments as testified by Johnson (2015) and for micro-electro-mechanical systems (MEMS) as reported by Chen et al. (2008). The research of Wang et al. (2011) in Ca-Mg-Zn based metallic glass with elastic modulus similar to that of human bone made the most revolutionary application of
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Kuriakose). http://dx.doi.org/10.1016/j.jmatprotec.2016.08.026 0924-0136/© 2016 Elsevier B.V. All rights reserved.
metallic glass as biodegradable implants. The study of Chu et al. (2014) and Sun et al. (2014) on Zr-based BMGs for bio-implants revealed its antibacterial properties and better human bone marrow mesenchymal stem cell adhesion morphology, respectively. Many of these applications are pointing towards the need of microfabrication methods for BMG. The main method used for the design and fabrication of metallic glass parts is thermoplastic forming (TPF). Schroers et al. (2007) and Schroers (2010) proved that TPF can be effectively used for the fabrication of metallic glass micro parts too. The works of Liu et al. (2015) showed the capability of TPF to replicates even features in the nanometer scale with high strength, homogeneity, free of internal stresses and porosity. However, the limited mold-tool life time and annealing-induced embrittlement of processed parts due to exposure of high temperatures result in property variations as reported by Johnson (2015). Manufacturing of molds for the complex micro geometries for TPF is also a tough job and limits the use of TPF for micro fabrication of BMG. Mechanical subtractive processes such as drilling, milling, turning etc. can be used for the machining of complex geometries in
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
BMG. Bakkal et al. (2004c), studied the turning of Zr-based BMG and testified that cutting force and specific cutting energy of BMG were analogous with those of Al6061 and SS304. Research reported by Bakkal et al. (2004b) on chip formation revealed the poor thermal conductivity of metallic glasses resulted in high heat generation in the 2400–2700 K range. This can result in light emission and oxidation at high cutting speed with low thermal conductivity tools as confirmed by Bakkal et al. (2005a) while drilling BMG. The optical microscopy of the polished and chemically etched layer of the chips formed at high cutting speed by Bakkal et al. (2004a) revealed the oxidation and crystallization and this deteriorates the properties of metallic glass. Also, the investigation of Bakkal et al. (2005b) about thrust force, torque, and tool wear in drilling of the bulk metallic glass discovered that high heat generation adversely affects the tool life. Yet the conventional micro hole drilling in Zr41.25 Ti13.75 Cu12.5 Ni10 Be22.5 BMG of thickness 1.2 mm by Zhu et al. (2013) could machine micro holes without crystallization. Furthermore, thermal softening due to high heat generation and burrs produced deteriorates the hole’s edges and geometry. Therefore, the conventional micro machining methods are incapable of machining geometries to the required tolerances in metallic glass and further secondary finishing operations may entail removal of the burrs produced to achieve the required surface finish. This suggests for the nontraditional micro machining methods as pointed out by Masuzawa (2000), for more accurate and burr free machining of metallic glass. Nontraditional micro machining methods can be used for the micro machining of very accurate, smooth surfaces and features for the MEMS, optical and photonic applications. The research works in this field for micro machining of metallic glass are rare. To the best of authors’ knowledge, the work reported by Chen et al. (2008) about machining of micro-holes and three-dimensional microstructures on the La62 Al14 Ni12 Cu12 , Zr55 Al10 Ni5 Cu30 and Cu46 Zr44 Al7 Y3 BMGs by micro-electrical discharge machining (micro-EDM) is the prominent one. The micro holes are machined without any crystallization but defects like recast layers, burrs and heat affected zones are found with decrease in drilling diameter. For micro-EDM at higher energy input, Yeo et al. (2009) found that melting point of the electrode material is very significant. The investigations in traditional or nontraditional drilling neither studied the important hole features like overcut, edge deviation and taperness nor the feasibility of machining with material removal rate (MRR) and tool wear rate (TWR) till date for metallic glass. Sun et al. (1996) and Masuzawa (2000), studied about micro machining using micro-ultrasonic machining (micro-USM) and reported that micro-USM is capable of making almost any three-dimensional microstructure with high aspect ratio on most materials, particularly on hard and brittle materials. It can be used where the most well-known precision micromachining methods such as microEDM, LIGA and excimer laser drilling are very restricted either due to machinable shapes or in machinable materials. Review studies of Kumar (2013) and Thoe et al. (1998) established that ultrasonic machining is nearly a stress less machining without any heat accumulation. The micro-USM can be used as an alternative to conventional micro machining or micro-EDM for high aspect ratio structures in metallic glass without heat generation, light emission, crystallization and shear band formation. The research in the field of micro-USM of metallic glass is found to be very limited. In this research work the machinability study of metallic glass using micro-USM is carried out by micro hole drilling operation. Shaped micro tools are fabricated for micro-USM drilling with respect to the specifications for the optimum machining performance. Proper input and output parameters are selected and the appropriate ranges of the parameters are established. Experimentation is carried out by changing the input parameters with respect to the full factorial design. A new systematic approach is applied
43
Fig. 1. XRD pattern of metallic glass Zr60 Cu30 Ti10.
for the calculation of important hole features such as overcut at entry and exit, edge deviation at entry and exit, taperness and for the feasibility of machining such as MRR and TWR. Micro hole features and feasibility of machining are studied with respect to the micro-USM input parameters like feed rate, abrasive size and abrasive slurry concentrations. The multi-objective optimization on the basis of ratio analysis (MOORA) method is carried out effectively to identify the optimum input parameters by analyzing the output parameters of micro machining.
2. Experimental design and procedure 2.1. Micro drilling tool fabrication and drilling test setup The micro hole drilling experiments are performed using Sinker Model AP-1000 table top USM of Sonic-Mill. The metallic glass used for this study is Zr60 Cu30 Ti10 of thickness 20 m, synthesized by melt spinning route. XRD analysis is done in Panalytical X’Pert Pro MPD X-ray Diffractometer using Cu target confirming the amorphous structure of metallic glass and the XRD pattern is shown in Fig. 1. The metallic glass work piece is attached on a mildly heated glass slide using a polymeric wax. This glass slide is attached on the USM work base which is fixed to the USM machine base magnetically. After machining, the wax is removed completely using acetone as solvent. The tools for the micro-USM are fabricated in a half inch tool head of the machine. In USM the tool profile should be of the profile to be machined. So for the micro hole drilling, hollow micro cylindrical tool is chosen. Hollow tool is selected because of its higher MRR capability compared to the solid tool as observed by Kumar (2013). The tool wear characteristic study of Adithan (1981) in USM showed that the stainless steel tool exhibit low tool wear as compared to tungsten carbide and mild steel tool. In addition to that stainless steel tool is cheap and easily available. Therefore, stainless steel is chosen as tool material. The surgical quality stainless steel micro cylindrical tubes are cut using wire electrical discharge machine (WEDM) to the required length and weight for resonance according to the tool design criteria suggested by Thoe et al. (1998) and Kumar (2013). The micro tubes cut are attached to the tool tips by brazing or silver soldering to withstand the ultrasonic vibration during machining. Fig. 2(a) shows the hollow cylindrical micro tool prepared of outer diameter 326 m and inner diameter 209 m Fig. 2(b) shows the SEM image of the tip of the tool surface used for micro-USM. The EDS is done to ascertain the chemical element present in the tool. The EDS plot is shown in Fig. 3 and it confirms that the stainless steel alloy composed with alloying elements C, Cr, Mo, Zn, Ni, Mn, Al etc. The prepared tool is attached to the horn of the micro-USM and tuned for resonance.
44
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
Fig. 2. (a) The micro tool prepared of outer diameter 326 m b) SEM image of micro tool of outer diameter 326 m and tool surface selected for EDS. Table 1 Process parameters with levels for micro-USM drilling in metallic glass. Parameters
Levels
Values
Feed (m/s) Abrasive size (grit no) Slurry concentration (%)
4 2 3
10, 30, 50, 70 400, 600 40, 50, 60
Fig. 3. EDS result of the tool surface.
2.2. Experimentation and calculation of output parameters The schematics of the experimental procedure is shown in Fig. 4. The full factorial design of experiments is considered to conduct the experiments in metallic glass samples by varying one parameter at a time to study the machining characteristic in detail. The input machining parameters with their levels are as shown in Table 1. The total numbers of experiments conducted are 24. Boron carbide (B4 C) mixed in distilled water is taken as abrasive slurry. After every eight experiments, the tool is removed from the machine and the tip is grinded and flattened to compensate change in geometry and for higher accuracy. Other USM parameters such as amplitude of vibration and feed force are kept constant.
Leica DM 2500 M trinocular metallurgical microscope is used to obtain optical micrograph of the drilled micro holes and for the measurement of the output parameters. The optical microscope images are taken for the hole entry side and exit side and the calculations are done for the overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle, MRR and TWR. Methodology adopted for calculation is shown in Fig. 5 and in Eqs. (1)–(8). The overcut is the measure of increase in the radius of a drilled hole and it is measured as the difference in radius of drilled hole and tool as given in Eq. (1) Overcut =
(Da − Dt ) 2
(1)
Where Dt = Outer diameter of tool, Da = D1 for calculation of overcut at entry and Da = D2 for calculation of overcut at exit. Where
Fig. 4. Schematic of the experimental procedure.
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
45
Fig. 5. (a) Measurement of overcut (b) Measurement of edge deviation.
D1 = average diameter of hole at entry and D2 = average diameter of hole at exit are calculated as the average of the diameters of the circles inscribed through the maximum irregularities (Dmax) and minimum irregularities (Dmin ) of drilled micro hole at entry and exit respectively. Edge deviation of the hole indicates finishing of the drilled hole at the edges by calculating difference between the maximum and minimum height of the irregularities in the edge of the micro hole. Edge deviation at entry and exit of the micro hole are calculated as shown in Eq. (2) Edge deviation =
(Dmax − Dmin ) 2
(2)
The calculation of taper angle provides information on change in diameter of drilled hole with respect to increase in depth of drilling. It is measured using Eq. (3) Taper angle, = tan−1 [
(D1 − D2 ) ] 2h
(3)
Where h = Thickness of the workpiece MRR in micro hole drilling is calculated as the volume of material removed per second as per Eq. (4) MRR =
D2 h 4t
(4)
Where t = Time for machining and D = Average hole diameter and is calculated as average of D1 and D2. TWR is calculated as the decrease in length of the tool with respect to machining time. It is measured using Eq. (5) TWR =
T t
x11
x12 . . .
x
x
X=[ . . . xm1
yij =
xij
m
(7)
x i=1 ij
Parameters to be maximized are added and parameters to be minimized are subtracted as per Eq. (8) to calculate the assessment value.
Ri =
g j=1
yij −
n
yij
(8)
j=g+1
Where ‘g’ is the number of parameters to be maximized, (n–g) is the number of parameters to be minimized and Ri is the assessment value of ith experiment with respect to all the parameters. When arranged in descending order, the best experiment is the one which has the highest assessment value. The hole quality is designated as minimum of overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle, TWR and maximum MRR. The best quality, good quality and worst quality holes are classified based on MOORA method and the structural changes are investigated using SEM and EDS analysis. SEM and EDS analysis are conducted with Hitachi S-3400N SEM. The optimum combination of input parameter levels for drilling the best quality micro hole is identified.
(5)
Where T = Tool wear The hole quality and optimum machining conditions are investigated by analyzing the output parameters using MOORA method. Karande and Chakraborty (2012) have proved MOORA method as an easy to implement multi objective decision making tool which provide precise rankings. Analysis is done using Eqs. (6)–(8). A decision matrix is made of the output values as shown in Eq. (6),
21
Where xij is the performance measure of ith alternative on jth criterion, ‘m’ is the number of experiments and ‘n’ is the number of output parameters. The matrix is then normalized using Eq. (7),
22
...
x1n x
2n
.
.
.
. .
. xm2 . . .
xmn
]
(6)
3. Results and discussion The machinability characteristics of metallic glass using micro tool of 326 m is investigated for two different abrasive grit sizes i.e. for grit 400 and 600 with varying abrasive concentrations and feed rate. Total 24 micro hole drilling experiments are conducted. Micro holes are drilled successfully on metallic glass using the micro-USM drilling process without any heat generation. The temperature of the workpiece was same as that of the slurry since slurry was circulating over that. The slurry temperature at starting of experiment had a little variation depending on room temperature but in all the experiments were below 30 ◦ C. The slurry and workpiece temperature remained same without any change by micro-USM drilling. The reason is attributed to the large volume of slurry flowing over a very small machining zone. Metallic glass showed a very good machinability to USM machining.
46
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
SC=60% 70 60 50 40 30 20 10 0
SC=50%
SC=40%
b) Overcut exit (µm)
a)
Overcut exit (µm)
Fig. 6. Overcut at entry vs. feed for abrasive of (a) grit 400, (b) grit 600.
0
20
40
60
80
Feed (µm/s)
70 60 50 40 30 20 10 0
SC=60%
0
SC=50%
20
40
Feed (µm/s)
SC=40%
60
80
Fig. 7. Overcut at exit vs. feed for abrasive of (a) grit 400, (b) grit 600.
3.1. Analysis of machinability responses The main machining responses of micro hole drilling such as overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle, MRR and TWR are analyzed. The variations between output variables and feed for different concentrations of abrasive slurry for each grit size are plotted separately. The variation of overcut is shown in Figs. 6 and 7.
3.1.1. Overcut An average value of 40.92 m and 31.58 m were obtained for the overcut at entry for the grit 400 and 600, respectively. Overcut at exit value also followed the same trend with 39.58 m and 28.48 m corresponding to 400 grit and 600 grit. A least overcut of 10.75 m at entry and 10.25 m at exit of the hole was achieved by the micro-USM drilling using slurry of 600 grit. The Figs. 6 and 7 show that overcut decreases with increase in the feed rate and is minimum in the higher feed rate of 50–70 m/s. Feed rate is limited to 70 m/s because of the possibility of crushing of abrasive at very high feed rate as reported by Thoe et al. (1998). Fig. 7(a) and (b) shows overcut at exit follows same trend as that of overcut at entry with a minor reduction in overcut values. Abrasive slurry concentration (SC) of 50–60% is observed to be giving better results. Overcut is mainly caused by the unwanted interaction of abrasives around circumference of the tool. The overcut at entry and exit are found to be less for the slurry of 600 grit abrasives than that of 400 grit. The average diameter of 600 grit abrasives is lesser compared to 400 grit and which reduces the impact area of abrasives around the tool and thereby reducing the overcut. Overcut decreases with respect to the decrease in abrasive size. Higher feed is also found to be significant in reducing overcut in microUSM drilling of metallic glass. Yan et al. (2002) drilled precision micro-holes in borosilicate glass using micro EDM combined with micro-USM, and suggested that fast material removal reduce the friction between the micro-tool and the micro hole wall in the ultrasonic machining process. The increase in feed causes an increase in MRR and reduces the time for drilling which in turn reduces abrasive interaction time and thus the overcut.
3.1.2. Edge deviation The average edge deviation values for 400 grit and 600 grit are found to be 6.83 m and 12.58 m at entry and 7.67 m and 14.50 m at exit, respectively. Edge deviation is higher at exit than entry. The decrease of slurry reach at the exit side and the burrs left causes this increase. The best value of edge deviation is obtained as 4.50 m at entry and as 3.50 m at exit using slurry of 400 grit. The edge deviation is found to be better for higher abrasive size, 400 grit for the same feed. A trend for the variation of edge deviation could not be observed at entry and exit of the micro hole for the micro-USM drilling and is shown in Figs. 8 and 9. The medium feed rate in the range of 50 m/s, 400 grit abrasive and medium slurry concentrations are observed to be suitable for minimum edge deviation. Edge deviation on the other hand found to be lesser for slurry of higher sized abrasives, 400 grit than that of 600 grit. Study of Benedict (1987) about USM established that higher abrasive size increases the MRR. Slurry of higher sized abrasives could machine the hole edges more uniformly because of the higher MRR. This reduces the edge deviation of drilled micro hole, but at the expense of higher overcut. A higher concentration of slurry clogs and prevents the slurry flow to the micro machining zone and affects the edge deviation. Lower slurry concentrations even though reduces the number of abrasive particle for machining, but slightly improves MRR because of increase in flow ability of abrasive slurry to reach the micro machining zone as observed by Kumar (2013). So a lower slurry concentration of 40% or medium slurry concentration of 50% favors higher MRR and the lesser edge deviation.
3.1.3. Material removal rate The MRR as shown in Fig. 10 shows almost linear relationship with respect to the feed rate. At the higher feed rate tool moves faster towards the work piece and a good machining rate of metallic glass at the listed feed rate is observed with micro-USM. The 400 grit abrasive showed better machining rate with an average MRR of 0.0039 mm3 /min than 0.0032 mm3 /min with 600 grit abrasive. A grand average value of MRR 0.00355 mm3 /min is obtained for the micro-USM drilling using tool of 326 m with time just less than one minute to machine a micro hole. The abrasive concentration, within the range of 30–60%, has not seemed to make much
SC=60%
SC=50%
SC=40%
b)
35 30 25 20 15 10 5 0 0
20
40
60
Edge dev. entry (µm)
a)
Edge dev. entry (µm)
S. Kuriakose et al. / Journal of Materials Processing Technology 240 (2017) 42–51
SC=60%
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SC=50%
SC=40%
35 30 25 20 15 10 5 0 0
80
20
Feed (µm/s)
40
60
80
Feed (µm/s)
Fig. 8. Edge deviation at entry vs. feed for abrasive of (a) grit 400, (b) grit 600.
a)
SC=50%
SC=40%
b)
SC=60%
Edge dev. exit (µm)
Edge dev. exit (µm)
SC=60% 35 30 25 20 15 10 5 0
0
20
40
Feed (µm/s)
60
80
35 30 25 20 15 10 5 0
0
SC=50%
20
40
Feed (µm/s)
SC=40%
60
80
Fig. 9. Edge deviation at exit vs. feed for abrasive of (a) grit 400, (b) grit 600.
Fig. 10. MRR vs. feed for abrasive of (a) grit 400, (b) grit 600.
Fig. 11. SEM of tip of micro tool showing the wear profile.
difference in MRR. A slight higher material removal rate is observed for 40% slurry concentration because of the increase in flow ability of the slurry to reach the micro machining zone. 3.1.4. Tool wear rate The stainless steel tool showed a good wear resistance. TWR was found to be negligible with an average value of 1.76 m/s. The SEM of tip of micro tool after completion of experiments is shown in Fig. 11. The change in the geometry of the tool by micro-USM was very minimal. A minor reduction in outer diameter of the tool was
observed but the changes observed to the diameter in the linear length of the tool was very negligible when compared to the depth of cut applied. The use of hollow tool for drilling decreased the tool wear and increased the MRR by the principle of trepanning. Stainless steel hollow cylindrical tool proved as the promising tool for the micro drilling because of its wear resistance, ease of availability and cost effectiveness. A specific trend could not be observed in case of tool wear in micro-USM drilling of metallic glass.
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Fig. 12. SEM of micro hole in the best category (a) Entry side of the micro hole (b) Edge of the entry side the micro hole (c) Exit side of the micro hole (d) Edge of the exit side of the micro hole.
Fig. 13. EDS result of the burr on edge of the micro hole in best category.
3.1.5. Taper angle Taperness of the micro holes drilled in metallic glass using micro USM were with an average value of 9.29◦ and 3.81◦ respectively for 600 grit and 400 grit. Taper angle is found to be higher for the holes drilled using 600 grit than that with 400 grit. Machining with 400 grit drilled micro holes of near zero taper angle. Machining with 400 grit slurry machined micro holes with an average low taper angle value of 3.81◦ . Higher sized abrasive slurry was found to be drilling micro holes with lower taper angle in metallic glass. This is because of more uniform machining of the exit edges of the micro hole because of higher MRR. Finer abrasives reduce the overcut but at the completion of depth of cut, incapable of machining the edges of the micro holes at the exit side perfectly. The burrs left, thereby increases the taper angle of the drilled micro hole. The levels of parameter for optimum conditions are contradictory
for different response variables necessitating the use of scientific decision making tool.
3.2. MOORA analysis for optimum conditions The selection of levels of machining parameters for a combination of desired output variables is a multi-objective decision making problem. A decision matrix of 24 × 7 order is formulated with output variables using Eq. (6) for MOORA analysis. There are 24 numbers of experiments (drilled micro holes) and 7 number of output parameters. The decision matrix is normalized and the assessment values are calculated by adding the criteria to be maximized and subtracting the criteria to be minimized as given in Eqs. (7) and (8). MOORA analysis is given in Appendix A. Here in this case overcut at entry, overcut at exit, edge deviation at entry, edge deviation at exit, taper angle and TWR are the criteria to be minimized
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Fig. 14. SEM of micro hole in the worst category (a) Entry side of the micro hole (b) Edge of the entry side the micro hole (c) Exit side of the micro hole (d) Edge of the exit side of the micro hole.
Fig. 15. EDS result of the burr on edge of the micro hole in worst category.
and MRR is the criterion to be maximized. The experimental run with highest assessment value is checked for the optimum drilling condition. Input parameters corresponding to the experiments are identified. Optimum condition is obtained for the input machining parameters of (feed 50 m/s, slurry concentration 40% and grit 600). Optimum condition is obtained near high feed rates of 50–70 m/s. Feed rate is found to be the most significant parameter in quality of micro hole in micro-USM drilling of metallic glass. High feed rate reduced the machining time and there by reduced the overcut considerably. Slurry of 400 grit abrasive was found to be suitable for less edge deviation, lower taper angle and for higher MRR. But abrasive slurry of 600 grit was observed to be outweighing the slurry of 400 grit when considering the whole response parameters for micro-USM drilling of metallic glass because of significant reduction in overcut. The best quality, good quality and
worst quality holes are classified based on assessment values. Two micro holes from each category are selected and the edges of the micro holes at the entry and exit are inspected for shear bands, stress affected zones, heat affected zones or change in material properties through SEM and EDS analyses. The SEM images of the micro holes in the best category and worst category are shown in Figs. 12 and 14 respectively. Burr height is found to be less in case of 600 grit than that in case of 400 grit because the smaller sized abrasives reduces the unit material removal and thereby increases the accuracy. The EDS analysis is done on the burrs on edge of the micro hole and also on a point which is radially a little outside of the edges. The EDS result of the burr on edge of the hole in best category and worst category are shown in Figs. 13 and 15 respectively. The EDS results showed that the chemical composition of the material remained same during machining. The EDS on the burrs on edge of
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the micro hole in worst category showed a little higher percentage of carbon content than the holes in the best category. Since temperature change during machining was negligible, the reason for this is attributed to the higher numbers of B4 C slurry particles entrapped in the burr. The mechanical properties of metallic glasses are different from crystalline alloys. Zhao et al. (2015) and Li et al. (2015) demonstrated that metallic glasses do not strain harden and they strain inhomogeneously by the formation of localized shear bands, propagation of shear bands and finally fails catastrophically. Atroshenko et al. (2010) found that even though metallic glass exhibit similar elastic modulus as that of the crystalline alloys of same composition, the strength of metallic glasses is much higher. Because of high strength as well as low thermal conductivity, the conventional machining of metallic glass results in high heat generation accompanied with light emission. The temperature of slurry and workpiece during micro-USM drilling were below 30 ◦ C in every experiments. For structural changes of the metallic glass material, especially for any crystallization, temperature must rise to its crystallization temperature. For that, it has to rise several hundreds of Celsius. In this particular case, workpiece temperature during machining was insignificant for any structural changes to the metallic glass material. The SEM and EDS analysis of the micro hole edges confirms that no shear bands, stress affected zones or heat affected zones are formed by the micro-USM drilling. The SEM images of edges of the best quality hole at entry and exit, as shown in Fig. 12, showed very good edge finish with minimum burrs. The worst quality hole, as shown in Fig. 14 only differed with more burrs. The reason for this stress free machining without any crystallization and property change is that because of the mechanism of material removal of USM. The sharp point of the abrasives in the slurry, which is at the interface of the shaped tool and the metallic glass workpiece, hit on the surface of workpiece with several thousands of its weight and produces an inelastic deformation zone by the ultrasonic vibration of the tools. At some threshold force, this deformed zone develops into a crack called as median crack. During unloading this median crack begins to close and leading to the formation of lateral cracks (Ichida et al., 2005; Kumar, 2013; Zhao et al., 2015). The lateral cracks outspreads to the work surface and lead to the micro-chip formation or spallation during complete unloading,
as reported by Atroshenko et al. (2010). Material removal by micro chipping and slurry flow through the machining zone prevents the heat generation and accumulation. The machining without heat generation helps to retain the amorphous structure of the metallic glass and its exceptional properties. The best feature of micro-USM is that it can machine any shape that can be made on the tool. The geometry of the tool is machined on the surface of the workpiece. So the study pertains for the micro channels, square holes or other complex micro- shape that can be machined by micro-USM in metallic glass for the applications of micro fluidics, MEMS, bio-implants etc. 4. Conclusions In this research work, machinability study of metallic glass for the machining of micro features is investigated by micro hole drilling operation using micro-USM. The applicability of USM for the machining of metallic glass and the effects of the variation of the input machining parameters to the drilled hole quality are studied. The following conclusions are perceived from this research. 1. Micro-USM can be used to machine micro holes in metallic glass in room temperature without any heat generation and crystallization. The amorphous structure and original properties of metallic glass are retained without any changes. 2. The metallic glass has a very good machinability for micro-USM with high MRR and with very low TWR. Micro holes with very less overcut, minimum edge deviation and near zero taper angle are machined in metallic glass by micro-USM. The burrs produced also were very minimal. 3. Micro hole quality was observed to be very good in the higher feed rate and by the use of abrasive slurry with abrasives of higher grit number. The micro-USM provided a new way for the precision machining of metallic glasses which are conventionally difficult to machine because of higher heat generation with light emission and crystallization. Appendix A.
Table A1 Decision matrix of the output parameters for MOORA analysis. Exp No.
Overcut entry (m)
Overcut exit (m)
Edge dev. entry (m)
Edge dev. exit (m)
Taper angle (◦ )
Tool wear rate (m/s)
MRR (mm3 /min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
36.50 41.75 37.25 29.00 40.50 39.00 36.00 23.75 62.00 54.25 46.50 44.50 42.50 33.00 22.75 23.75 21.75 29.25 31.75 35.00 49.00 48.75 30.75 10.75
36.50 40.75 35.75 26.50 40.25 38.00 35.50 22.50 61.00 52.25 44.00 42.00 39.75 29.00 21.00 22.50 20.25 22.25 23.25 28.25 48.00 47.75 29.50 10.25
8.00 4.50 5.50 6.00 8.00 9.00 6.00 7.50 7.00 7.50 6.00 7.00 34.00 12.00 8.50 12.50 19.50 9.50 4.50 5.00 12.00 11.50 6.50 15.50
8.00 3.50 6.50 6.00 8.50 8.00 6.00 8.00 10.00 11.50 11.00 5.00 30.50 15.00 10.00 15.00 18.50 15.50 16.50 12.50 9.50 10.50 6.00 14.50
0.00 2.86 4.29 7.13 0.72 2.86 1.43 3.58 2.86 5.71 7.13 7.13 7.83 11.32 5.00 3.58 4.29 19.30 23.04 18.66 10.63 2.86 3.58 1.43
1.66 1.41 2.85 1.21 0.33 1.57 2.17 2.60 1.07 1.15 0.51 3.36 3.12 1.40 1.63 2.22 0.45 0.61 1.24 2.60 2.39 1.59 1.50 3.63
0.0016 0.0042 0.0045 0.0049 0.0010 0.0041 0.0043 0.0044 0.0028 0.0044 0.0047 0.0064 0.0018 0.0027 0.0028 0.0048 0.0008 0.0024 0.0040 0.0048 0.0022 0.0038 0.0044 0.0042
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Table A2 Normalized decision matrix and assessment values of MOORA analysis. Exp No. Overcut entry (m) Overcut exit (m) Edge dev. entry (m) Edge dev. exit (m) Taper angle (◦ ) Tool wear rate (m/s) MRR (mm3 /min) Assessment value, Ri 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0.042 0.048 0.043 0.033 0.047 0.045 0.041 0.027 0.071 0.062 0.053 0.051 0.049 0.038 0.026 0.027 0.025 0.034 0.036 0.040 0.056 0.056 0.035 0.012
0.045 0.050 0.044 0.032 0.049 0.047 0.043 0.028 0.075 0.064 0.054 0.051 0.049 0.036 0.026 0.028 0.025 0.027 0.028 0.035 0.059 0.058 0.036 0.013
0.034 0.019 0.024 0.026 0.034 0.039 0.026 0.032 0.030 0.032 0.026 0.030 0.146 0.052 0.036 0.054 0.084 0.041 0.019 0.021 0.052 0.049 0.028 0.067
0.030 0.013 0.024 0.023 0.032 0.030 0.023 0.030 0.038 0.043 0.041 0.019 0.115 0.056 0.038 0.056 0.070 0.058 0.062 0.047 0.036 0.039 0.023 0.055
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0.000 0.018 0.027 0.045 0.005 0.018 0.009 0.023 0.018 0.036 0.045 0.045 0.050 0.072 0.032 0.023 0.027 0.123 0.147 0.119 0.068 0.018 0.023 0.009
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0.019 0.049 0.053 0.057 0.011 0.048 0.049 0.051 0.033 0.051 0.054 0.075 0.021 0.031 0.033 0.056 0.009 0.028 0.047 0.055 0.025 0.044 0.051 0.049
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