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Machinability evaluation and desirability function optimization of turning parameters for Cr2O3 doped zirconia toughened alumina (Cr-ZTA) cutting insert in high speed machining of steel
Author’s Accepted Manuscript Machinability Evaluation and Desirability Function Optimization of Turning Parameters for Cr 2O3 doped Zirconia Toughened Alumina (Cr-ZTA) Cutting Insert in High Speed Machining of Steel B K Singh, B Mondal, Nilrudra Mandal www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(15)02034-9 http://dx.doi.org/10.1016/j.ceramint.2015.10.128 CERI11575
To appear in: Ceramics International Received date: 7 August 2015 Revised date: 29 September 2015 Accepted date: 16 October 2015 Cite this article as: B K Singh, B Mondal and Nilrudra Mandal, Machinability Evaluation and Desirability Function Optimization of Turning Parameters for Cr2O3 doped Zirconia Toughened Alumina (Cr-ZTA) Cutting Insert in High Speed Machining of Steel, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.10.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Machinability Evaluation and Desirability Function Optimization of Turning Parameters for Cr2O3 doped Zirconia Toughened Alumina (CrZTA) Cutting Insert in High Speed Machining of Steel
B K Singh, B Mondal, Nilrudra Mandal* Centre for Advanced Materials Processing CSIR-Central Mechanical Engineering Research Institute Durgapur-713209, India
*
Corresponding Author Dr. Nilrudra Mandal Scientist Centre for Advanced Materials Processing CSIR-Central Mechanical Engineering Research Institute Durgapur-713209 India Tel.: +91-343-6510218 Fax: +91-343-2546745 E-mail address: [email protected], [email protected]
Abstract In present study, mechanical properties, microstructure and machining parameter optimization of Cr2O3 doped zirconia toughened alumina (ZTA) ceramic insert have been investigated for application in high speed turning of AISI 4340 steel with achieving maximum tool life. The yttria stabilized zirconia (YSZ) in α-Al2O3 matrix with varying percentage of co-doped chromia (Cr2O3) is prepared to study the phase transformation behaviour. The samples are uniaxially pressed in the form of cutting inserts and subsequently sintered at 1600°C to evaluate the mechanical properties. Hardness and fracture toughness reaches the highest value i.e. 17.40 GPa and 7.20 MPa.m1/2 respectively at 0.6 % Cr2O3 doped ZTA due to more
metastable tetragonal ZrO2 phase present in the alumina matrix. After 50 minutes of machining, the flank wear and surface roughness are found well below the tool rejection criteria. The cutting force also does not affect detrimentally on the job-tool interface. Turning experiments have been adopted as per central composite design (CCD) of response surface methodology (RSM) with varying 3 levels of cutting speed (140 m/min , 280 m/min, 420 m/min), feed rate (0.12 mm/rev, 0.18 mm/rev, 0.24 mm/rev) and depth of cut (0.50 mm, 1.00 mm, 1.50 mm). The effect of each input parameter on output responses are investigated using analysis of variance (ANOVA) and modelled using regression analysis. The influence of cutting speed, feed rate and depth of cut is observed maximum for determination of flank wear, cutting force and surface roughness respectively. Cutting speed of 420 m/min with feed rate of 0.12 mm/rev and depth of cut of 0.5 mm has been shown as optimized condition with 83.32% desirability for minimum tool failure and maximum tool life. Keywords: chromia; zirconia toughened alumina; machinability; central composite design; response surface methodology
1. Introduction In metal working industry, as day to day new materials occupies and replaces the old and traditional ones, so, it is very difficult to select an efficient cutting insert for any particular machining application. The performance of cutting tool is dependent on the material to be machined and the type of operation. The supplementary factors that affect the selections of tool are machine tool horsepower, availability of inserts, speed range and rigidity of the machine, tooling budget limitations, productivity demands, machine tool burden rate etc. In order to achieve efficient tool to maintain all the mentioned criteria’s, ceramic inserts have a greater potential due to its high hot hardness, abrasion resistance and chemical stability [1]. From twentieth century, ceramic (alumina) cutting inserts have been used for
machining operation. It was known that only 5% of total tool or inserts used in industries are made up of ceramic (alumina) [2]. To enhance the use of ceramics tool, it is very much imperative to eliminate the hurdles that limit its use. The major problem associated with ceramics inserts has its low toughness and catastrophic failure [3]. Xing Li et al [3] have found a premature or catastrophic failure in cutting inserts by using monolithic Al2O3 due to high stress at high elevated temperature as well as high shock experienced by the cutting edge and face. Hence, to enhance the toughness of the materials, some additives such as yttria stabilised zirconia (YSZ) [4], titanium carbide [5], titanium nitride [6], carbon [7] and ceria [8] have been added to alumina matrix. It is also illustrated by many researchers that the anti wear property of inserts are highly effected by hardness of tool or inserts which can be increased by doping MgO and chromia (Cr2O3) etc to the matrix. As zirconia and alumina is easily available material so ample research has been carried out in the field of zirconia toughened alumina (ZTA) ceramics. ZrO 2 also exists in polymorphism so phase stabilization is essentially required before application of the material. Szutowska et al [9] have revealed that the toughening mechanisms of Al2O3-ZrO2 composite are highly associated with the stress induced phase transformation toughening and microcrack toughening. It is found that the tetragonal phase or second phase of ZrO2 efficiently improve its toughness value. The improvement is also associated with the transformation strengthening method done by phase transformation through mixing with yttria stabilized zirconia (YSZ) [4]. The composite produced by the mixing with YSZ with alumina is known as zirconia toughened alumina (ZTA) and its wear resistance is high due to the transformation toughening mechanism in which the YSZ is surrounded by alumina matrix. It has been also proved that the ZTA has a peculiar phenomena based on polymorphic transformation of ZrO2 (t) tetragonal phase into ZrO2 (m) monoclinic phase during the cooling spam of sintering temperature to room temperature and results in increased value of strength and fracture toughness of ceramics. This improvement may be due to volumetric the t expansion during → m transformation of ZrO2 diffuse in the matrix. The yttria stabilised zirconia (YSZ) in α-Al2O3 increases the fracture toughness of the matrix. There are several other additive materials viz. MgO, NiO, Cr2O3 and TiO2 in α-Al2O3 show increase in fracture toughness of the matrix. Among the additives, chromia (Cr2O3) has a potential to modify the physical properties of alumina. Azhar et al [10] found that chromia doped in alumina forms isovalent solid
solution because both are sesquioxides and have similar corundum crystal structure (approximately hexagonal close-packed oxide ions with the Al3+ and Cr3+ ions capturing two thirds of the available octahedral interstitials sites). While doping of chromia in alumina at high temperature shows complete range of substitution solid solution. It also significantly increases refractoriness and chemical stability [11]. Hardness, tensile strength and thermal shock resistance in chromia doped alumina ceramics is predominantly significant [12]. The increment of grain size and bimodal in size distribution appear when chromia (≈ 2 mol %) is added. It is also found that there is an increase in fracture toughness and flaw tolerance of alumina but decrease in fracture strength. Before machining application, the study of machinability is very much required to be conducted in given work-tool combination. To enhance the productivity and quality of the machined product, it is important to study the different phenomena like surface integrity, tool wear, chip formation and cutting force. In addition to that, development of predictive model for all influencing parameters is also essential to visualise them. Therefore, to manufacture components economically, engineers are trying to find ways for the improvement of machining performance. To achieve high cutting performance, proper selection of optimum parameter is necessary. Generally, the operator’s knowledge or the design data book determines this optimum parameter selection. However, proper experimental data is very limited for machining with the advanced cutting tool. Procedures for statistical design of experiment are quiet extensively used in machinability investigations. Statistical design of experiments refers to the process of planning the experiment, so that the appropriate data can be analyzed by statistical methods, resulting in valid conclusions. Design and methods such as factorial design, response surface methodology (RSM) and taguchi methods are now widely used in place of one factor at a time experimental approach. Benga and Abrao [13] has investigated the surface roughness by using response surface methodology and developed a model by varying cutting speed and feed rate for three different cutting tool i.e. mixed ceramics, whishker – reinforced ceramics and poly crystalline cubic boron nitride (PCBN). It has been found in all three cutting tools, the surface roughness has been affected by feed rate. The similar phenomenon has been observed by Kacal and Yildirim [14] for turning of AISI D6 by ceramics and CBN inserts. Aslan et al [15] has applied analysis of variance (ANOVA) for surface roughness and found that the surface roughness varies with
cutting speed-feed rate and feed rate-depth of cut interactions, for turning of AISI 4140 steel with mixed ceramics inserts. A predictive model has been suggested [16] for utilising neural network and multiple linear regression models to evaluate surface roughness and flank wear in finish turning of AISI D2 steel with wiper ceramic inserts. Meddour et al [17] has investigated and developed a model for cutting force and surface roughness by utilising ANOVA and response surface methodology. In their investigations, inserts made up of mixed ceramics have been used for cutting AISI 52100 steel. The investigation using ANOVA has been performed and it has been concluded that the cutting force is mainly affected by depth of cut followed by feed rate with light contribution. Even though, many research has been carried out for development and machinability investigation of ZTA cutting inserts but little work has been done in respect of chromia addition in ZTA insert. Apart from study the phase purity, density, grain size in terms of doping, machinability investigation has been carried out to visualize the flank wear of the insert, cutting force developed in the time of machining and surface finish of the job. The CCD design of RSM has been employed to model the turning parameters in terms of flank wear, cutting force and surface roughness. ANOVA has been employed to see the effect of each parameter on the responses. Desirability function optimization is utilized to define the optimal machining parameters in high speed machining of AISI 4340 steel using the developed insert.
2. Experimental Details 2.1 Powder Synthesis & Characterization A monolithic Al2O3 (average particle size 60 μm, supplier Merck) is used for preparation Cr2O3 doped ZTA composite material. Yttria Stabilized Zirconia (YSZ) particles (average particle size 1.0 μm, supplier Zirox) is added to Al2O3 matrix with a ratio of 90 wt.% Al2O3 / 10 wt. % YSZ. Samples are synthesized with 0-1 wt % Cr2O3 (average particle size 1 μm, supplier Sigma Aldrich). The powders are wet mixed for 60 min using ultrasonic machine and 30 min using automatic stirrer subsequently. The mixture solution is dried for 24 hr in an oven at 200°C. The powders of Al2O3, YSZ and Cr2O3 are then ball milled for 12 hr using 0.8 wt % of polyethylene glycol
1000 as a plasticizing agent. The morphology of the powders is characterized through particle size analyzer. The powder is calcined at 600°C and sintered at 1600°C for 2 hours. The X-ray diffraction (XRD) of sintered powder at 1600°C is studied for determining the degree of stabilization of phases [18]. In this work, the stabilization of phases is specifically carried out to distinguish between monoclinic and cubic/tetragonal phases at an angle 2θ between 10° and 700.The fraction of tetragonal zirconia is calculated by comparing the peak intensities of Tetragonal phase [101] and Monoclinic phase [111] and [ 111 ] obtained from X-ray diffraction of calcined as well as sintered samples using the following equation 1 & 2 [19]
Vm =
Where, X m =
1.311 X m (1 + 0.311 X m )
(1)
I m (111) + I m (111) X100 I m (111) + I t (101) + I m (111)
(2)
Where, Vm is the volume fraction of monoclinic ZrO2, I = integrated intensity of the respective diffracting plane, Xm is the intensity of m-ZrO2 with respect to total ZrO2, subscript m and t stands for monoclinic and tetragonal ZrO2. The crystallite size of the powder is measured from X ray line broadening analysis using Scherrer’s formula [20]
D=
0.9 l B cos q
(3)
Where, D is the crystallite size, λ is the wavelength of the radiation, θ is the Bragg's angle and B is the full width at half maximum. 2 2 B 2 = Bmeas - Binst
(4)
Where, Bmeas = Observe full width at half maximum from peak values and Binst = Instrumental broadening. 2.2 Fabrication and Sintering of Cutting Insert The tool inserts are prepared after uniaxially compaction of milled powder with a pressure of 5 ton cm-2 in a hydraulic press (Make: Carver, USA). It has been compacted into a designed square shaped (16 mm x 16mm x 6mm) pellets with a die. The drawing of the same is portrayed in Fig 1.
The compacts are sintered at 1600-1625°C with soaking time of 3 hrs in an air atmosphere. The diamond wheel is used to reduce the specimen size in tailor made designed Jig-Fixture and polished slowly to make the inserts very close to the international standard SNUN 120408 (ISO). Finally, the inserts are polished with fine diamond paste (0.5-1.0 µm) in polishing machine. A flat land of angle 20 deg and width of 0.2 mm has been given on each cutting edge to impart edge strength to the inserts. The light honing is also tried on the sharp edges to make rounding off. The final shape of Cr-ZTA insert is depicted in Fig 2. Archimedes Principle is used to calculate apparent porosity and bulk density of the sintered specimens. Sintered samples are weighed in dry state. After that, samples are immersed in kerosene and kept under vacuum of 4 mm of mercury for two hours for filling the pores completely with kerosene. Then, soaked (in kerosene) and suspended samples are weighed. From the measured weight, the apparent porosity and bulk density is calculated as follows:
Dry weight of the samples =Wd Soaked weight of the samples =Ws Suspended weight of the samples =Wa % ApparentPorosity = Bulk Density =
Ws - Wd X100 Ws - Wa
Wd Ws - Wa
(5) (6)
The grain size is determined using a rectangular intercept procedure, following the ASTM E112-88 standard. The average grain size G is calculated by using the following mathematical expression:
G=
4A 3.14 (ni + n0 2)
(7)
where, A is the area of rectangular section, ni and n0 are the grain numbers within the rectangular section and on the boundary respectively. FESEM image of sintered surface of Cr2O3-ZTA insert is taken in Field Emission Scanning Electron Microscope (FESEM) (CARL-ZEISS-SMT-LTD, Germany, Model: SUPRA 40). The hardness of the materials is calculated from the size of the impression produced under load by a pyramid-shaped diamond indenter. The indenter employed
in the Vickers test is a square-based pyramid whose opposite sides met at the apex at an angle of 136°. The diamond is pressed into the surface of the material at loads ranging up to approximately 2 Kgf. The size of the impression (usually no more than 0.5 mm) is measured with the aid of Vickers hardness testing machine Matsuzwa, MXT-70. The Vickers number (HV) is calculated using the following formula [21]
HV =1.854 ( F / D 2 )
(8)
Where, F is the applied load (measured in Kgf) and D 2 is the area of the indentation (measured in mm2). Fracture toughness is also determined by the Vickers indentation technique. KIC is measured by the indentation method, where the length of cracks emerging from the Vickers indentation corners. KIC is calculated using the expression proposed by Evans and Charles in 1976 [22] The fracture toughness KIC is estimated using the equation
K IC = 0.16 (c / a) -1.5 ( Ha 0.5 )
(9)
KIC
= Fracture toughness (MPa•m1/2)
H
= Vickers hardness (MPa)
P
= Test load in Vickers hardener (Newton)
c
= Average length of the cracks obtained in the tips of the Vickers marks
(microns) a
= Half average length of the diagonal (microns)
2.3 Machinability Evaluation The NH-26 lathe (Make: HMT Ltd, India) powered by an 11 KW motor and speed range of 47-1600 RPM is used for turning experiments (Fig 3). The initial diameter of the AISI 4340 steel bar (Hardness 48 HRC) used for machining is 150 mm and the length is 470 mm. The tool holder used is CSBNR2525N43 (Make: NTK) with tool angles -6°, -6°, 6°, 6°, 15°, 15° and 0.8. A piezoelectric dynamometer (Make: Kistler, 9272) fitted in a developed fixture is used for the measurement of cutting forces (Fz). It has been calibrated in the range of 0 to 5000 Newton. A charge amplifier (Make: Kistler, 5015 A) is used to display the
amplified value of force from charge rating using Dynoware software. Cutting force is also measured three times in same speed and average of the same has been taken for machinability investigation. The surface roughness of the AISI 4340 steel has been measured by the portable surface roughness tester SURTRONIC 25. The direction of the roughness measurement is perpendicular to the cutting velocity vector. A total of five measurements of surface roughness are taken at random on each machined surface and the average value is used in the analysis. Flank wear is one kind of abrasive wear caused by abrasive action of the hard work piece material with the cutting inserts. The flank wear is characterized by the abrasive groove and ridges on the flank face of the tool. The flank wear land is measured from the position of the original major cutting edge using a Tool Makers Microscope (Make: Leica, Germany) with X 30 to X 150 magnification and 1 micron resolution. 2.4 Statistical Modelling using Response Surface Methodology Response surface methodology is a modelling technique in which the relationship among different parameters with several responses is determined. It is also helpful for searching out the significance of each parameter and the extent of each on the individual responses. In the present investigation, as the cutting inserts are newly developed and non-isotropic in nature so machinability investigation is very much important before application in high speed turning. In this investigation, cutting speed, feed rate and depth of cut are identified as process parameters which affect the responses such as cutting force produced at the time of turning, surface roughness of job and flank wear of the inserts which are the main criteria for investigation of machinability of material. The effects of the parameters on the responses are tested through a set of planned experiments based on 3 levels 3factor central composite design for mapping it in the quadratic response surfaces. The parameters and their levels are depicted in Table 1. The response function representing the performance can be expressed as
PII: DOI: Reference:
S0272-8842(15)02034-9 http://dx.doi.org/10.1016/j.ceramint.2015.10.128 CERI11575
To appear in: Ceramics International Received date: 7 August 2015 Revised date: 29 September 2015 Accepted date: 16 October 2015 Cite this article as: B K Singh, B Mondal and Nilrudra Mandal, Machinability Evaluation and Desirability Function Optimization of Turning Parameters for Cr2O3 doped Zirconia Toughened Alumina (Cr-ZTA) Cutting Insert in High Speed Machining of Steel, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.10.128 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Machinability Evaluation and Desirability Function Optimization of Turning Parameters for Cr2O3 doped Zirconia Toughened Alumina (CrZTA) Cutting Insert in High Speed Machining of Steel
B K Singh, B Mondal, Nilrudra Mandal* Centre for Advanced Materials Processing CSIR-Central Mechanical Engineering Research Institute Durgapur-713209, India
*
Corresponding Author Dr. Nilrudra Mandal Scientist Centre for Advanced Materials Processing CSIR-Central Mechanical Engineering Research Institute Durgapur-713209 India Tel.: +91-343-6510218 Fax: +91-343-2546745 E-mail address: [email protected], [email protected]
Abstract In present study, mechanical properties, microstructure and machining parameter optimization of Cr2O3 doped zirconia toughened alumina (ZTA) ceramic insert have been investigated for application in high speed turning of AISI 4340 steel with achieving maximum tool life. The yttria stabilized zirconia (YSZ) in α-Al2O3 matrix with varying percentage of co-doped chromia (Cr2O3) is prepared to study the phase transformation behaviour. The samples are uniaxially pressed in the form of cutting inserts and subsequently sintered at 1600°C to evaluate the mechanical properties. Hardness and fracture toughness reaches the highest value i.e. 17.40 GPa and 7.20 MPa.m1/2 respectively at 0.6 % Cr2O3 doped ZTA due to more
metastable tetragonal ZrO2 phase present in the alumina matrix. After 50 minutes of machining, the flank wear and surface roughness are found well below the tool rejection criteria. The cutting force also does not affect detrimentally on the job-tool interface. Turning experiments have been adopted as per central composite design (CCD) of response surface methodology (RSM) with varying 3 levels of cutting speed (140 m/min , 280 m/min, 420 m/min), feed rate (0.12 mm/rev, 0.18 mm/rev, 0.24 mm/rev) and depth of cut (0.50 mm, 1.00 mm, 1.50 mm). The effect of each input parameter on output responses are investigated using analysis of variance (ANOVA) and modelled using regression analysis. The influence of cutting speed, feed rate and depth of cut is observed maximum for determination of flank wear, cutting force and surface roughness respectively. Cutting speed of 420 m/min with feed rate of 0.12 mm/rev and depth of cut of 0.5 mm has been shown as optimized condition with 83.32% desirability for minimum tool failure and maximum tool life. Keywords: chromia; zirconia toughened alumina; machinability; central composite design; response surface methodology
1. Introduction In metal working industry, as day to day new materials occupies and replaces the old and traditional ones, so, it is very difficult to select an efficient cutting insert for any particular machining application. The performance of cutting tool is dependent on the material to be machined and the type of operation. The supplementary factors that affect the selections of tool are machine tool horsepower, availability of inserts, speed range and rigidity of the machine, tooling budget limitations, productivity demands, machine tool burden rate etc. In order to achieve efficient tool to maintain all the mentioned criteria’s, ceramic inserts have a greater potential due to its high hot hardness, abrasion resistance and chemical stability [1]. From twentieth century, ceramic (alumina) cutting inserts have been used for
machining operation. It was known that only 5% of total tool or inserts used in industries are made up of ceramic (alumina) [2]. To enhance the use of ceramics tool, it is very much imperative to eliminate the hurdles that limit its use. The major problem associated with ceramics inserts has its low toughness and catastrophic failure [3]. Xing Li et al [3] have found a premature or catastrophic failure in cutting inserts by using monolithic Al2O3 due to high stress at high elevated temperature as well as high shock experienced by the cutting edge and face. Hence, to enhance the toughness of the materials, some additives such as yttria stabilised zirconia (YSZ) [4], titanium carbide [5], titanium nitride [6], carbon [7] and ceria [8] have been added to alumina matrix. It is also illustrated by many researchers that the anti wear property of inserts are highly effected by hardness of tool or inserts which can be increased by doping MgO and chromia (Cr2O3) etc to the matrix. As zirconia and alumina is easily available material so ample research has been carried out in the field of zirconia toughened alumina (ZTA) ceramics. ZrO 2 also exists in polymorphism so phase stabilization is essentially required before application of the material. Szutowska et al [9] have revealed that the toughening mechanisms of Al2O3-ZrO2 composite are highly associated with the stress induced phase transformation toughening and microcrack toughening. It is found that the tetragonal phase or second phase of ZrO2 efficiently improve its toughness value. The improvement is also associated with the transformation strengthening method done by phase transformation through mixing with yttria stabilized zirconia (YSZ) [4]. The composite produced by the mixing with YSZ with alumina is known as zirconia toughened alumina (ZTA) and its wear resistance is high due to the transformation toughening mechanism in which the YSZ is surrounded by alumina matrix. It has been also proved that the ZTA has a peculiar phenomena based on polymorphic transformation of ZrO2 (t) tetragonal phase into ZrO2 (m) monoclinic phase during the cooling spam of sintering temperature to room temperature and results in increased value of strength and fracture toughness of ceramics. This improvement may be due to volumetric the t expansion during → m transformation of ZrO2 diffuse in the matrix. The yttria stabilised zirconia (YSZ) in α-Al2O3 increases the fracture toughness of the matrix. There are several other additive materials viz. MgO, NiO, Cr2O3 and TiO2 in α-Al2O3 show increase in fracture toughness of the matrix. Among the additives, chromia (Cr2O3) has a potential to modify the physical properties of alumina. Azhar et al [10] found that chromia doped in alumina forms isovalent solid
solution because both are sesquioxides and have similar corundum crystal structure (approximately hexagonal close-packed oxide ions with the Al3+ and Cr3+ ions capturing two thirds of the available octahedral interstitials sites). While doping of chromia in alumina at high temperature shows complete range of substitution solid solution. It also significantly increases refractoriness and chemical stability [11]. Hardness, tensile strength and thermal shock resistance in chromia doped alumina ceramics is predominantly significant [12]. The increment of grain size and bimodal in size distribution appear when chromia (≈ 2 mol %) is added. It is also found that there is an increase in fracture toughness and flaw tolerance of alumina but decrease in fracture strength. Before machining application, the study of machinability is very much required to be conducted in given work-tool combination. To enhance the productivity and quality of the machined product, it is important to study the different phenomena like surface integrity, tool wear, chip formation and cutting force. In addition to that, development of predictive model for all influencing parameters is also essential to visualise them. Therefore, to manufacture components economically, engineers are trying to find ways for the improvement of machining performance. To achieve high cutting performance, proper selection of optimum parameter is necessary. Generally, the operator’s knowledge or the design data book determines this optimum parameter selection. However, proper experimental data is very limited for machining with the advanced cutting tool. Procedures for statistical design of experiment are quiet extensively used in machinability investigations. Statistical design of experiments refers to the process of planning the experiment, so that the appropriate data can be analyzed by statistical methods, resulting in valid conclusions. Design and methods such as factorial design, response surface methodology (RSM) and taguchi methods are now widely used in place of one factor at a time experimental approach. Benga and Abrao [13] has investigated the surface roughness by using response surface methodology and developed a model by varying cutting speed and feed rate for three different cutting tool i.e. mixed ceramics, whishker – reinforced ceramics and poly crystalline cubic boron nitride (PCBN). It has been found in all three cutting tools, the surface roughness has been affected by feed rate. The similar phenomenon has been observed by Kacal and Yildirim [14] for turning of AISI D6 by ceramics and CBN inserts. Aslan et al [15] has applied analysis of variance (ANOVA) for surface roughness and found that the surface roughness varies with
cutting speed-feed rate and feed rate-depth of cut interactions, for turning of AISI 4140 steel with mixed ceramics inserts. A predictive model has been suggested [16] for utilising neural network and multiple linear regression models to evaluate surface roughness and flank wear in finish turning of AISI D2 steel with wiper ceramic inserts. Meddour et al [17] has investigated and developed a model for cutting force and surface roughness by utilising ANOVA and response surface methodology. In their investigations, inserts made up of mixed ceramics have been used for cutting AISI 52100 steel. The investigation using ANOVA has been performed and it has been concluded that the cutting force is mainly affected by depth of cut followed by feed rate with light contribution. Even though, many research has been carried out for development and machinability investigation of ZTA cutting inserts but little work has been done in respect of chromia addition in ZTA insert. Apart from study the phase purity, density, grain size in terms of doping, machinability investigation has been carried out to visualize the flank wear of the insert, cutting force developed in the time of machining and surface finish of the job. The CCD design of RSM has been employed to model the turning parameters in terms of flank wear, cutting force and surface roughness. ANOVA has been employed to see the effect of each parameter on the responses. Desirability function optimization is utilized to define the optimal machining parameters in high speed machining of AISI 4340 steel using the developed insert.
2. Experimental Details 2.1 Powder Synthesis & Characterization A monolithic Al2O3 (average particle size 60 μm, supplier Merck) is used for preparation Cr2O3 doped ZTA composite material. Yttria Stabilized Zirconia (YSZ) particles (average particle size 1.0 μm, supplier Zirox) is added to Al2O3 matrix with a ratio of 90 wt.% Al2O3 / 10 wt. % YSZ. Samples are synthesized with 0-1 wt % Cr2O3 (average particle size 1 μm, supplier Sigma Aldrich). The powders are wet mixed for 60 min using ultrasonic machine and 30 min using automatic stirrer subsequently. The mixture solution is dried for 24 hr in an oven at 200°C. The powders of Al2O3, YSZ and Cr2O3 are then ball milled for 12 hr using 0.8 wt % of polyethylene glycol
1000 as a plasticizing agent. The morphology of the powders is characterized through particle size analyzer. The powder is calcined at 600°C and sintered at 1600°C for 2 hours. The X-ray diffraction (XRD) of sintered powder at 1600°C is studied for determining the degree of stabilization of phases [18]. In this work, the stabilization of phases is specifically carried out to distinguish between monoclinic and cubic/tetragonal phases at an angle 2θ between 10° and 700.The fraction of tetragonal zirconia is calculated by comparing the peak intensities of Tetragonal phase [101] and Monoclinic phase [111] and [ 111 ] obtained from X-ray diffraction of calcined as well as sintered samples using the following equation 1 & 2 [19]
Vm =
Where, X m =
1.311 X m (1 + 0.311 X m )
(1)
I m (111) + I m (111) X100 I m (111) + I t (101) + I m (111)
(2)
Where, Vm is the volume fraction of monoclinic ZrO2, I = integrated intensity of the respective diffracting plane, Xm is the intensity of m-ZrO2 with respect to total ZrO2, subscript m and t stands for monoclinic and tetragonal ZrO2. The crystallite size of the powder is measured from X ray line broadening analysis using Scherrer’s formula [20]
D=
0.9 l B cos q
(3)
Where, D is the crystallite size, λ is the wavelength of the radiation, θ is the Bragg's angle and B is the full width at half maximum. 2 2 B 2 = Bmeas - Binst
(4)
Where, Bmeas = Observe full width at half maximum from peak values and Binst = Instrumental broadening. 2.2 Fabrication and Sintering of Cutting Insert The tool inserts are prepared after uniaxially compaction of milled powder with a pressure of 5 ton cm-2 in a hydraulic press (Make: Carver, USA). It has been compacted into a designed square shaped (16 mm x 16mm x 6mm) pellets with a die. The drawing of the same is portrayed in Fig 1.
The compacts are sintered at 1600-1625°C with soaking time of 3 hrs in an air atmosphere. The diamond wheel is used to reduce the specimen size in tailor made designed Jig-Fixture and polished slowly to make the inserts very close to the international standard SNUN 120408 (ISO). Finally, the inserts are polished with fine diamond paste (0.5-1.0 µm) in polishing machine. A flat land of angle 20 deg and width of 0.2 mm has been given on each cutting edge to impart edge strength to the inserts. The light honing is also tried on the sharp edges to make rounding off. The final shape of Cr-ZTA insert is depicted in Fig 2.
Dry weight of the samples =Wd Soaked weight of the samples =Ws Suspended weight of the samples =Wa % ApparentPorosity = Bulk Density =
Ws - Wd X100 Ws - Wa
Wd Ws - Wa
(5) (6)
The grain size is determined using a rectangular intercept procedure, following the ASTM E112-88 standard. The average grain size G is calculated by using the following mathematical expression:
G=
4A 3.14 (ni + n0 2)
(7)
where, A is the area of rectangular section, ni and n0 are the grain numbers within the rectangular section and on the boundary respectively. FESEM image of sintered surface of Cr2O3-ZTA insert is taken in Field Emission Scanning Electron Microscope (FESEM) (CARL-ZEISS-SMT-LTD, Germany, Model: SUPRA 40). The hardness of the materials is calculated from the size of the impression produced under load by a pyramid-shaped diamond indenter. The indenter employed
in the Vickers test is a square-based pyramid whose opposite sides met at the apex at an angle of 136°. The diamond is pressed into the surface of the material at loads ranging up to approximately 2 Kgf. The size of the impression (usually no more than 0.5 mm) is measured with the aid of Vickers hardness testing machine Matsuzwa, MXT-70. The Vickers number (HV) is calculated using the following formula [21]
HV =1.854 ( F / D 2 )
(8)
Where, F is the applied load (measured in Kgf) and D 2 is the area of the indentation (measured in mm2). Fracture toughness is also determined by the Vickers indentation technique. KIC is measured by the indentation method, where the length of cracks emerging from the Vickers indentation corners. KIC is calculated using the expression proposed by Evans and Charles in 1976 [22] The fracture toughness KIC is estimated using the equation
K IC = 0.16 (c / a) -1.5 ( Ha 0.5 )
(9)
KIC
= Fracture toughness (MPa•m1/2)
H
= Vickers hardness (MPa)
P
= Test load in Vickers hardener (Newton)
c
= Average length of the cracks obtained in the tips of the Vickers marks
(microns) a
= Half average length of the diagonal (microns)
2.3 Machinability Evaluation The NH-26 lathe (Make: HMT Ltd, India) powered by an 11 KW motor and speed range of 47-1600 RPM is used for turning experiments (Fig 3).
amplified value of force from charge rating using Dynoware software. Cutting force is also measured three times in same speed and average of the same has been taken for machinability investigation. The surface roughness of the AISI 4340 steel has been measured by the portable surface roughness tester SURTRONIC 25. The direction of the roughness measurement is perpendicular to the cutting velocity vector. A total of five measurements of surface roughness are taken at random on each machined surface and the average value is used in the analysis. Flank wear is one kind of abrasive wear caused by abrasive action of the hard work piece material with the cutting inserts. The flank wear is characterized by the abrasive groove and ridges on the flank face of the tool. The flank wear land is measured from the position of the original major cutting edge using a Tool Makers Microscope (Make: Leica, Germany) with X 30 to X 150 magnification and 1 micron resolution. 2.4 Statistical Modelling using Response Surface Methodology Response surface methodology is a modelling technique in which the relationship among different parameters with several responses is determined. It is also helpful for searching out the significance of each parameter and the extent of each on the individual responses. In the present investigation, as the cutting inserts are newly developed and non-isotropic in nature so machinability investigation is very much important before application in high speed turning. In this investigation, cutting speed, feed rate and depth of cut are identified as process parameters which affect the responses such as cutting force produced at the time of turning, surface roughness of job and flank wear of the inserts which are the main criteria for investigation of machinability of material. The effects of the parameters on the responses are tested through a set of planned experiments based on 3 levels 3factor central composite design for mapping it in the quadratic response surfaces. The parameters and their levels are depicted in Table 1.