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An experimental and analytical analysis on chip morphology, phase transformation, oxidation, and their relationships in finish hard milling
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 49 (2009) 805–813
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
An experimental and analytical analysis on chip morphology, phase transformation, oxidation, and their relationships in finish hard milling Song Zhang 1, Y.B. Guo Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA
a r t i c l e in fo
abstract
Article history: Received 2 February 2009 Received in revised form 10 June 2009 Accepted 12 June 2009 Available online 18 June 2009
Milling hardened steels has emerged as a key technology in mold and dye manufacturing industry. The effects of cutting parameters on chip morphology, phase transformation and oxidation reaction of the chips during finish milling AISI H13 tool steel (5071 HRc) with coated inserts were investigated in this paper. The chip morphology and phase transformation were examined using an optical microscope and a scanning electron microscope (SEM). The X-ray photoelectron spectroscopy was used to measure the chemical compositions of oxidation layer on chip surfaces. Shear-induced lamella structures characterized by jagged and rough appearance are the basic features of free surfaces. The microstructural analysis indicated that saw-tooth chips and white layers are produced only under certain combinations of cutting parameters of high cutting speed and feed rate. Based on chip color, chip morphology and X-ray photoelectron spectroscopy (XPS) analysis of the chip oxidation layer, the maximal instantaneous temperature at the tool–chip interface is semi-qualitatively estimated using the analytical method developed. In addition, chip color can be predicted based on the oxidation compositions. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Hard milling Chip morphology Oxidation Phase transformation Tool steel
1. Introduction With the advances in cutting tool technologies, hard milling has been recently employed to machine hardened steels (430 HRc) in making dyes and molds for various automotive and electronic components as well as plastic molding parts. Most research in hard milling is focused on the effects of process parameters on tool performances [1], surface quality [2], white layer [3,4], cutting force model [5], cooling/lubrication technology [6,7] and optimal cutting parameters [8]. However, very few studies have dealt specifically with the unique cutting mechanisms induced by hard milling such as the effects of cutting parameters on chip morphology, phase transformation, oxidation reaction and the resultant chip color. An insightful understanding of hard milling mechanism can lead to a better process economics, increased process stability, improved tool life, reduced tooling costs and enhanced surface integrity and component performance. Chip morphology, phase transformation, oxidation and chip color are an important indications to understand the milling mechanism, which is critical in the evaluation of the process performance.
Corresponding author. Tel.: +1 205 348 2615; fax: +1 205 348 6419.
E-mail address: [email protected] (Y.B. Guo). Currently a visiting scholar from Shandong University, PR China.
1
0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.06.006
Due to periodic variation of chip thickness in hard milling, the variation in chip load is the source of cutting force fluctuations, which results in high-frequency vibrations and premature tool failure. Much effort has been placed on addressing the mechanisms of saw-tooth chip formation mechanisms such as adiabatic shearing [9] and surface crack propagation [10–12]. Nakayama et al. [13] suggested that the saw-tooth chips are initiated by periodic crack formation as one of the plausible mechanisms in orthogonal cutting highly cold-worked 40/60 (Zn/Cu) brass. The saw-tooth chip formation starts with the initiation of a crack at the free surface of the workpiece, which further propagates towards the cutting edge of the tool. The crack soon ceases to grow at a point where severe plastic deformation of the material exists under higher level of compressive stresses. Shaw and Vyas [10], Elbestawi et al. [11] and Poulachon and Moisan [14] also employed the periodic crack theory in an attempt to explain the saw-tooth chip formation in hard turning. However, Recht [15] proposed the adiabatic shear theory which suggests that the root cause of saw-tooth chip formation is a catastrophic thermoplastic instability. The decrease in flow stress is due to thermal softening, which offsets strain hardening associated with strain increase. Komanduri et al. [16] adopted the adiabatic shearing theory and explained the saw-tooth chip formation in high-speed machining hardened AISI 4340 steel. Guo and Yen [17] found that the periodic adiabatic shearing crack which involves both crack and adiabatic shearing mechanisms is responsible for the formation of saw-tooth chips in hard machining. An adiabatic shear-induced
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internal crack first initiates and then propagates towards the free surface in front of the tool while the free surface crack develops. Saw-tooth chips are often formed in hard machining and represent an essential feature of the chip morphology. The effects of cutting parameters and material hardness are shown to be interdependent variables in governing saw-tooth chip formation, which affect the transition from continuous to shear-localized chip formation in hard machining [15]. Therefore, the formation of saw-tooth chips is affected by many factors such as material properties and tool geometry. Vyas and Shaw [12] showed that a high hardness of workpiece, large negative rake angle and large undeformed chip thickness promote the crack initiation and the consequent formation of saw-tooth chips in hard machining, while cutting speed has only a modest effect. Poulacho and Moisan [14] has shown that cutting speed and material hardness are interdependent factors in governing the saw-tooth chip formation in turning 100Cr6 steel (AISI 52100). Hard turning of H13 tool steel with a PCBN tool insert indicated that workpiece hardness and cutting speed influence the transition from continuous to saw-tooth chip formation [18]. Wang and Zheng [19] investigated the effect of workpiece hardness on chip formation mechanism in ball end milling of H13 tool steel. At low speeds continuous chips were formed. However, saw-tooth chips were produced at higher speeds. Moreover, the speed at which the transformation from continuous chip to saw-tooth chip occurred depends on the workpiece hardness [16]. As the rake angle becomes more negative or the cutting speed increases, the chip morphology transition from saw-tooth chip to separation of individual segments of the saw-tooth chips may also occur [16,17]. An estimation of cutting temperature in chip formation is crucial to explain the complex phenomena encountered during machining such as tool wear and surface integrity of the machined components. However, it is very difficult to accurately measure the cutting temperatures during the cutting process, especially for milling process, because the milling cutter rotates at high speeds while traveling in the feed direction. Because the majority heat generated during cutting is carried away by the chips, the high-temperature chips would react with oxygen in the air and produce temper colors of the chips. Therefore, chip color can be a characteristic indicator of cutting temperature and may be used as a rough guide to select an optimum cutting speed range. Chip colors also depend on materials composition. Thus, cutting temperature could be qualitatively estimated by examining chip color because cutting temperature affects the extent of chip oxidation and color variation [20,21]. For example, chip color produced in down-milling was golden brown while it was purple and darker in up-milling [22]. Ning et al. [21] claimed that the lighter color on chip surface was mainly due to the intimate contact between the tool and the chip and hence oxidation was suppressed. It was also reported that the darker the chip color, the more the chip oxidation. In general, chip color becomes darker when tool wear increases since oxidation increases due to the increased cutting temperatures. Dhar et al. [23] found that chip color depends on cutting speeds and feed rates in turning AISI 1040 steel. However, a relationship between cutting temperature, chip oxidation and color in hard milling has not been addressed yet.
In this investigation, Taguchi’s method-based design-of-experiment of dry hard milling AISI H13 tool steel (5071 HRc) using (Ti, Al)N–TiN-coated cemented carbide cutters was conducted. The objectives of this study are two-fold. Firstly, the controlling cutting parameters on chip morphology transition and phase transformation are experimentally identified. Secondly, a relationship between cutting temperature, oxidation and chip color is established using a new analytical approach.
2. Experiment procedure and sample preparation 2.1. Work material and cutting tool AISI H13 tool steel combines very good hot-hardness with toughness and covers a wide variety of applications such as pressure die casting, extrusion, hot forging and extrusion mandrels. It can be water cooled in service and gives hightemperature strength and wear resistance. The nominal chemical composition and material properties of H13 steel are listed in Tables 1 and 2, respectively. The workpiece material was hardened and tempered to 5071 HRc. The workpiece samples were prepared as 125 25 20 mm rectangular blocks. Prior to milling, the blocks were face milled on the top and bottom surfaces to remove the heat-treatment-related surface defects and to ensure the flatness by eliminating errors in the experimental results. The cutting tool used in machining tests is a milling cutter of 20 mm diameter, which can be equipped with two inserts. The tool holder and the (Ti, Al)N–TiN-coated carbide inserts were all made by SECO Tool Company. The details of the milling cutter are given in Table 3 and Fig. 1, respectively. In this experiment only one insert was used for each set of experimental conditions, which would eliminate the influence of the tool tip run-out on tool wear associated with a two-insert end mill. Since each surface was milled using a fresh cutting edge so that the tool-wear effect on chip formation would be the same and the variation due to the wear can be minimized. The scanning electron microscope (SEM) images of cutting edge before and after milling in Fig. 1(c) and (d) indicate that tool flank wear is minimal at the most abusive condition (highest speed and feed). The evolution of chip formation with tool wear will be conducted in a future study. 2.2. Experimental design Tables 4 and 5 are the conditions used according to Taguchi’s design-of-experiments. Taguchi’s design-of-experiments with a standard L16 (44) orthogonal array was selected to conduct the
Table 2 Material properties of AISI H13 too steel at room temperature. Density (kg/m3)
Young’s modulus (GPa)
Hardness (HRc)
Yield strength (MPa)
Area reduction (%)
Thermal conductivity (W/m K)
7800
210
5071
1579
23.0
25.6
Table 1 Nominal chemical composition of AISI H13 tool steel (wt%). C
Mn
Si
Cr
Mo
V
Ni
Fe
0.32–0.45
0.20–0.50
0.80–1.20
4.75–5.50
1.10–1.75
0.80–1.20
0–0.30
Balance
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807
finish hard milling experiments with four factors at four levels. The orthogonal array was chosen because of the minimum number of experimental trials required. Each row of the matrix represents one trial. Four levels of each factor were represented by ‘2’, ‘1’ ‘1’ or ‘2’ in the matrix. The collected chips at different cutting conditions are also numbered as shown in Table 5. The dry milling tests were carried out on a CNC vertical machining center (CINCINNATI Arrow-500).
‘‘edge effect’’. The machined chips were examined using optical and scanning electron microscopes ( Philips XL300) to identify the microstructural features of different chip surfaces. Chemical compositions of the oxidation layer on the chip back surface and the bulk H13 steel (free of oxidation layer) were analyzed and compared using X-ray photoelectron spectroscopy (XPS). The XPS
2.3. Sample preparation
Factor
To study the morphology and phase transformation of machined chips, chips were collected from each milling test, cold mounted, polished and etched in a solution of 4% Nital to reveal the microstructure. In order to observe the cross-section of the chips, the top surface shown in Fig. 2 was exposed for polishing so that the free surface and the back surface are both perpendicular to the polished epoxy surface. At the same time, the samples in this study were carefully prepared to eliminate the potential
Table 4 Factors and levels used in finish milling experiments. Level
A—Axial depth of cut ap (mm) B—Radial depth of cut ae (mm) C—Cutting speed v (m/min) D—Feed fz (mm/tooth)
Item
Description
Tool holder Insert designation Coating technique Coating material Insert clamping Tool diameter (mm) Overhang length (mm) Axial rake angle (deg) Radial rake angle (deg) Clearance angle (deg) Number of tooth Corner radius (mm) Insert thickness (mm) Insert length (mm) Insert width (mm)
R217.69–1820.0–12–2A XOMX 120408TR PVD (Ti, Al)N–TiN Collet 20 60 +3 12 14 Single 0.8 4 12 8
1
1
2
1.0 0.3 100 0.05
1.5 0.4 150 0.10
2.0 0.5 200 0.15
2.5 0.6 250 0.20
Table 5 Design-of-experiment based on Taguchi’s method—L16 (44). Experiment #
Table 3 Geometry of milling cutter.
2
Factor
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Chip #
A (ap)
B (ae)
C (v)
D (fz)
2 2 2 2 1 1 1 1 1 1 1 1 2 2 2 2
2 1 1 2 2 1 1 2 2 1 1 2 2 1 1 2
1 2 2 1 1 2 2 1 2 1 1 2 2 1 1 2
2 2 1 1 2 2 1 1 1 1 2 2 1 1 2 2
12mm
80°
14° 8mm 0.8mm
4mm
Geometric dimensions of insert
Tool-holder
Cutting edge before milling
Cutting edge after milling (Exp. #16)
Fig. 1. Milling cutter geometry
Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16
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Feed direction
Back surface
Side cutting edge Top surface A
ae
ap
Free surface End cutting edge
Workpiece
B
Major region
Free surface
Insert
Corner region
Cross section
Fig. 2. 3D schematic of chip formation in end milling (not to scale).
data were collected using monochromatic Al Ka radiation. To eliminate sample charging during the analyses, all the binding energies were referred to as C1s signal at 284.6 eV. Oxidation quantification such as peak locations and widths at half the maximum height was provided after curve fitting. In addition, sensitivity factors for Fe and O have been calculated using the H13 tool steel as the baseline.
3. Experimental results and discussions 3.1. Characteristics of chip morphology Since end milling is a cutting process involving complex tool geometry of the indexable inserts and the interactions between tool/workpiece, a 3-dimensional (3D) view of chip formation is necessary to identify the specific functions by different cutting edges. The 3D view is also vital for understanding the formation of a machined surface and subsequent surface integrity. Fig. 2 illustrates the 3D schematic view of chip formation, specific functions of the cutting edges and the process parameters such as axial and radial depth of cuts. Four surfaces of the chip, i.e., the top surface, the free surface, the back surface and the cross-sections have been defined. The free surface and the back surface are formed by the side cutting edge in sequential cutting, while the machined surface is formed by the tool nose and the end cutting edge. 3.1.1. Lamella-structured free surface The observed lamella structures in Fig. 3 is the basic feature of free surfaces characterized by jagged and rough appearances, which are caused by the shearing mechanism. The lamella structure on the free surface signifies the shearing morphology on the free surface compared to the traditional cross-sectional view. The free surface has two sections of different orientations, i.e., the major section formed by the side cutting edge and the corner section formed by the tool nose edge. The lamella structure in the major section is parallel to the side cutting edge, while the inclined lamella structure in the corner section is approximately parallel to the tool nose edge. Practically, all cutting parameters contribute to the formation and the appearance of the lamella structure. However, cutting speeds and feed rates are the two major process parameters influencing the size and shape of lamella structures of the free surfaces according to the SEM images, while the variations due to
(ap=2.0mm, ae=0.6mm,v=100m/min, fz=0.05mm/tooth) Fig. 3. Lamella structures on the free surface (chip #12).
depth of cut is much smaller. Therefore, this study focuses on the effects of cutting speed and feed rate on chip morphology. When cutting speeds and feed rates are relative low, small but uniform lamella structures are formed as shown in Fig. 4(a) and (b). The distribution of the lamella was very uniform and the average lamella thickness is relatively constant. With the increase in cutting speeds and feed rates, larger lamella structures in terms of width and depth occur as shown in Fig. 4(c) and (d), which indicates the formation of saw-tooth chips. 3.1.2. Smooth back surface During milling as in all cutting operations, the back surface of a machined chip closely contacts with the tool rake face. The plastic deformation of the back surface is constrained by the tool rake face. Thus, the back surface experiences high contact pressure and high frictional force when the chip slides over the tool rake face. The combined actions of high contact pressures, frictional forces and high temperatures make the back surface smooth and shiny (Fig. 5) than the free surface of the chips. It can be noticed that there are some parallel stripes on the back surface induced by irregular cutting edges and/or potential hard particles, but it is generally smooth. Although the distributions of contact stress, friction and temperature at the tool/chip interface vary with cutting process parameters, the back surfaces have very similar characteristics. 3.1.3. The transition from uniform to saw-tooth chip formation Fig. 6 shows the cross-sectional images of the chip top surface. Two different types of chips can be observed: the near uniformly deformed continuous chip and the saw-tooth chip. The continuous chips were produced at low cutting speeds and feeds, while the saw-tooth chips were produced at relatively high cutting speeds and feeds. The chip morphology transition from continuous chip to saw-tooth chip is favored by the combination of increased cutting speed and feeds. The combined effects of cutting speed and feed on chip morphology transition are summarized in Table 6. Based on the observation,
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ap=2.0mm, ae=0.6mm,
ap=2.5mm, ae=0.4mm,
v=100m/min, fz=0.05mm/tooth (chip#12)
v=150m/min, fz=0.10mm/tooth (chip#14)
809
Width of lamella structure
ap=2.0mm, ae=0.4mm,
ap=2.5mm, ae=0.6mm,
v=200m/min, fz=0.15mm/tooth (chip#10)
v=250m/min, fz=0.20mm/tooth (chip#16)
Fig. 4. Combined effects of cutting speed and feed on morphology of chip free surfaces.
parameters defined in Fig. 6(d). Saw-tooth pitch pc (saw-to-saw distance), saw height t1 (saw peak-to-valley distance), chip thickness t2 (distance between saw peak to back surface)a were measured at least three locations for statistical analysis. Fig. 7 shows that at low speeds, chip pitch decreases with increase in speed, while the effect of high speed alone has varied influence. As shown in Fig. 8, the measured inclination angles of the saw-tooth chips range from 561 to 641, which are much larger than those produced in orthogonal cutting [18]. In addition, the inclination angles greater than 451 mean that chip formation mechanism is not pure shear deformation. To incorporate the effect of radial depth of cut on chip morphology, the ratio of the saw-tooth height to chip thickness is defined as r¼ Fig. 5. Back surface topography (chip #12).
feed per tooth is the major contributor of the serration, while cutting speed effect is secondary. 3.1.4. Geometrical characteristics of saw-tooth cross-section of top surface In order to investigate the effects of milling parameters on chip morphology, the chip cross-sections were analyzed with the
t1 t2
(1)
For near uniform continuous chips, the variation in the ratio is insignificant and therefore not applied. As listed in Table 7, the ratio becomes large with increase in cutting speed and feed rate. However, the trend of the ratio variation is consistent for the lamella-structured chips. 3.2. White layer on cross-section of saw-tooth chips The chip back surface experiences high temperatures when the chip slides past the tool rake face, especially at high cutting
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ap=2.0mm, ae=0.6mm,
ap=2.5mm, ae=0.4mm,
v=100m/min, fz=0.05mm/tooth (chip#12)
v=150m/min, fz=0.10mm/tooth (chip#14) pc
View direction for lamella structure in Fig.4 (c)
t2 α
t1
White layer
White layer ap=2.0mm, ae=0.4mm,
ap=2.5mm, ae=0.6mm,
v=200m/min, fz=0.15mm/tooth (chip#10)
v=250m/min, fz=0.20mm/tooth (chip#16)
Fig. 6. Combined effects of cutting speed and feed on cross-section of chip top surfaces.
Table 6 Chip morphology analysis at different cutting speeds and feeds.
0.05 Cutting speed v (m/min) 100 Continuous 150 Continuous 200 Saw-tooth 250 Saw-tooth
0.10
0.15
0.20
Continuous Continuous Saw-tooth Saw-tooth
Continuous Saw-tooth Saw-tooth Saw-tooth
Continuous Saw-tooth Saw-tooth Saw-tooth
Shear angle (°)
Feed fz (mm/tooth)
v=200m/min
70 65 60 55 50 45 40 35 30
v=250m/min
0.05
0.1 0.15 Feed (mm/tooth)
0.2
Chip pitch (µm)
Fig. 8. Variation in inclination angle with feed.
70 65 60 55 50 45 40 35 30
v=200m/min v=250m/min
Table 7 Height-to-thickness ratio of saw-tooth chips. Feed fz (mm/tooth) 0.05
0.05
0.1 0.15 Feed (mm/tooth)
Fig. 7. Variation in chip pitch with feed.
0.2
Cutting speed v (m/min) 100 N/Aa 150 N/A 200 0.6270.019 250 0.5070.050 a
0.10
0.15
0.20
N/A N/A 0.6470.035 0.5570.006
N/A 0.4470.006 0.5370.010 0.5670.015
N/A 0.5570.032 0.6070.020 0.6370.014
N/A denotes uniform chips.
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Table 8 Chemical reactions and equations for Gibbs free energy at high temperatures. Oxidation/reduction reactions
Equations for Gibbs free energy
2Fe(s)+O2(g)-2FeO(s) 4/3Fe(s)+O2(g)-2/3Fe2O3(s) 3/2Fe(s)+O2(g)-1/2Fe3O4(s) 4FeO(s)+O2(g)-2Fe2O3(s) 2H2O(l)-2H2(g)+O2(s) 3Fe2O3(s)+H2(g)-2Fe3O4(s)+H2O(g)
DG ¼ 543,920+138.09T DG ¼ 549,500+183.13T DG ¼ 339,200+172.75T DG ¼ 1648,500+139.35T D ¼ 571,660–326.35T DG ¼ 1654,370–88.75T
90000 Continuous chip (#12) Saw-tooth chip (#16)
4.1. Oxidation reaction of chips It has been often observed and generally agreed that the bulk heat generated in metal cutting is carried away by chips and dissipated into the air. The chip temperature then becomes much higher than the workpiece material. When the chip back surface is exposed to air (an oxidative atmosphere) at high temperatures, the chips are susceptible to oxidation, which results in the formation of an oxide layer on the chip surfaces. The potential gas-metal oxidation reaction between Fe and O are represented as follows: 2FeðsÞ þ O2 ðgÞ ! 2FeOðsÞ
(2)
4=3FeðsÞ þ O2 ðgÞ ! 2=3Fe2 O3 ðsÞ
(3)
3=2FeðsÞ þ O2 ðgÞ ! 1=2Fe3 O4 ðsÞ
(4)
60000
C1s
Intensity (c/s)
4. Relationship between temperature, oxidation and temper colors
Fe2p1 Fe2p3
70000
O1s
80000
50000 40000
Cr2p1 Cr2p3
speeds. The frictional energy was quickly converted to thermal energy, which leads to localized transient high temperatures on the chips. Compared with the friction-induced white layer on the back surface, the white layer in the primary shear zone in Fig. 6(d) was induced by high temperatures resulting from severe plastic deformations at high speeds. When the localized temperatures exceed the austenization temperature 1010 1C (Ac3) of H13 steel, the ferrite phase (a) is transformed to austenite phase (g). Then the austenite phase (g) is quickly quenched by the surrounding air and becomes the ultrafine grained white layer. As shown in Fig. 6(c) and (d), the white layer in the primary shear zone and on the chip back surface is observed. Based on the formation of white layers, the cutting temperature reaches close to 1010 1C during the cutting process.
811
Bulk
30000 20000 10000 0 1000 900
800
700
600 500 400 300 Binding energy (eV)
200
100
0
Fig. 9. XPS survey spectra of chip back surfaces.
4.2. XPS analysis of chip back surface
The degree of oxidation reaction depends on exposure time, surface area, mass transfer, temperature, etc. Gibbs free energy (DG) of a reaction is a measure of the thermodynamic driving force that enables a reaction to take place. A negative value for DG indicates that a reaction can proceed spontaneously without external inputs, while a positive value indicates that it will not. The equation for Gibbs free energy is:
DG ¼ DH T DS
(5)
where DH is the enthalpy change in the reaction, T is absolute temperature and DS is the entropy change in the reaction. The enthalpy change (DH) is a measure of the actual energy that is liberated when the reaction occurs (the ‘‘heat of reaction’’). If it is negative, then the reaction gives off energy, while if it is positive the reaction requires energy. Based on the enthalpy and entropy values of all elements and compounds (H2O, H2 and O2), the standard Gibbs free energy changes (DG) were calculated according to Table 8. Since the changes in Gibbs free energy for the above three reactions (Eqs. (2)–(4)) are all negative when the temperature varies from 273 to 1972 K, the thermodynamics for the formation of FeO, Fe2O3 and Fe3O4 is favorable. Since the colors of FeO, Fe2O3 and Fe3O4 are blue, red and black, respectively, the chip color varies with the presence of specific iron oxide or their combinations, which depend on the degree of oxidation. The temperature at tool–chip interface is a determining factor for the oxidation degree. Colors of the machined chips under milling conditions in Table 4 vary from light purple to dark blue.
In the present paper, to investigate the chemical composition of the oxidation film, XPS spectra of oxidation layer on the back surfaces of the chips and the bulk material (free of oxidation) of the H13 steel were conducted. Peak locations and widths at half the maximum height and peak areas were measured after curve fitting. The XPS survey spectrum of the bulk material (H13 steel), continuous chip (#12) and saw-tooth chip (#16) are shown in Fig. 9. In this figure, binding energy is conceptually, not strictly, equal to the ionization energy of the core electron in each atom. The variation in binding energy results in the shift of the corresponding XPS peak. The peak intensity shows the concentration level of elements, which characterizes the escaped electron counts per second (c/s). Fig. 10 shows that the spectrum is dominated by the presence of Fe2p3 and O1s peaks. The main elements Cr2p3 peak of H13 tool steel is very small. This indicates that the scale mainly contains iron oxides. As shown in Fig. 10, only one O1s peak at about 531 eV can be fitted for the bulk material, which corresponds to the O2 molecule attaching to the bulk surface. For the machined chips, for example, chip #12 and chip #16, there is another fitted peak with bonding energy close to O1s in iron oxide, which indicates the presence of iron oxide on the chip back surfaces. Since the bonding energy of O1s in FeO and Fe2O3 are 529.8 and 529.6 eV, respectively, the bonding energies of O1s in different iron oxides are too close to find the exact chemical compositions of the oxidation layer only by the XPS spectra in Fig. 10. It is necessary to investigate further according to the XPS spectra of Fe2p3/2 of the oxidation layer. Fig. 11 shows the XPS spectra of Fe2p3/2 of oxidation layer on the chip back surfaces. For the continuous chips (chip #12), there are two peaks located at 710.9 and 710.6 eV, which correspond to the bonding energy of Fe2p3/2 in Fe2O3 and Fe3O4, respectively.
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4.3. Correlation of temperature, XPS analysis and chip color
5000 4500
Continouos chip (#12)
Bulk
Intensity (c/s)
4000 Saw-tooth chip (#16)
3500 3000 2500 2000 1500 1000 540.0
537.5
535.0 532.5 530.0 Binding energy (eV)
527.5
525.0
Fig. 10. XPS spectrum of oxygen element on chip back surfaces.
Fe2p3/2 in Fe2O3 5000
Continuous chip (#12) Fe2p3/2 in Fe2O3
4500 Insensity (c/s)
Fe2p3/2 in Fe3O4
Fe2p3/2 in FeO
4000
Fe2p3/2 in Fe3O4 3500 3000
When the temperature exceeds 1478 1C (1751 K), water decomposes into hydrogen and oxygen, which can reduce Fe2O3 to Fe3O4 or even FeO. From the XPS spectra of Fe2p3/2 for the sawtooth chips (chip #16), two peaks located at 710.6 and 709.6 eV confirm the presence of Fe3O4 and FeO, respectively. Therefore, it is reasonable to infer that Fe2O3 oxidation layers on the chip surfaces were reduced to Fe3O4 and FeO when the cutting temperature at the tool–chip interface exceeds 1478 1C (1751 K). The more oxide composition FeO over Fe2O3 makes the saw-tooth chips dark blue. With the decreased amount of Fe2O3, the contents of Fe3O4 and FeO increase gradually and the color of the saw-tooth chip surface becomes darker and darker. The existence of white layer in Fig. 6(c) demonstrates that the cutting temperature is over the austenization temperature 1010 1C (Ac3), so most likely the reduction occurred. The high temperatures could be caused by the exothermic reaction of the chip material after it was formed with the oxygen of the air. The temper colors were not entirely due to different oxidation states of the iron but also due to the oxidation of the alloying elements. Based on the above analysis, it can be inferred that 1478 1C (1751 K) is one critical temperature to determine the occurrence of reduction reaction of Fe2O3 to Fe3O4, while the austenitizing temperature 1010 1C (Ac3) is another critical temperature, which initiates the formation of a white layer. Through the integrated optical, SEM and XPS analysis the continuous chips show light purple and are free of white layers when the chip temperature is less than 1010 1C at low cutting speeds and feeds, while the saw-tooth chips are dark blue and with the presence of a white layer at temperature above 1478 1C when cutting speed and feed are above the critical values.
Saw-tooth chip (#16)
2500
5. Conclusions
2000 718
716
714
712 710 708 Binding energy (eV)
706
704
702
Fig. 11. Trivalent and bivalent oxide peaks of iron on the back surface of the chips.
A Taguchi’s design-of-experiments approach was taken for the finish dry hard milling with coated inserts to investigate the effects of cutting parameters on chip morphology, phase transformation, oxidation reaction, and their relationships. The major conclusions can be summarized as follows:
The main compositions of the oxidation layer of chip #12 consist of more Fe2O3 and slightly less Fe3O4, which make the back surface light purple. The chip free of white layer in Fig. 5a implies that the cutting temperature is below the austenization temperature 1010 1C (Ac3). So oxidation reaction rather than reduction occurred. The chemical reaction is expressed as follows,
Cutting speeds and feed rates are critical process parameters to
4FeOðsÞ þ O2 ðgÞ ! 2Fe2 O3 ðsÞ
(6)
The valences of iron are +2 and +3, i.e., Fe2O3 is the highest valence oxide. The cutting temperature increases with cutting speed, feed rate and depth of cut. When the temperature exceeds a critical value at high cutting speeds, Fe2O3 can be reduced to Fe3O4 or FeO by the hydrogen molecule decomposed from water. The chemical reactions can be expressed as follows:
2H2 O ðlÞ ! 2H2 ðgÞ þ O2 ðsÞ
(7)
3Fe2 O3 ðsÞ þ H2 ðgÞ ! 2Fe3 O4 ðsÞ þ H2 OðgÞ
(8)
The standard free energy changes associated with the above chemical reactions are listed in Table 8.
affect the chip morphology and the transition from continuous chip to saw-tooth chip. Saw-tooth chips and white layers only occur under the specific combinations of higher cutting speeds and feeds. Lamella structures are the basic features for the jagged and rough appearance of the chip-free surfaces. Based on chip colors, chip morphology and XPS analysis of chip oxidation layer the maximal instantaneous temperature at the tool–chip interface can be semi-qualitatively estimated using the developed analytical method. It can be inferred that the dark blue machined chips is induced by a critical cutting temperature, which can decompose water into hydrogen and oxygen.
Acknowledgments This research is based upon work supported by the National Science Foundation under Grant no. CMMI-0447452. The authors would like to thank Dr. Mark Weaver, Mr. Raul Waikar and Mr. Ning Li at the University of Alabama for their assistance in sample preparation and characterization.
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Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
An experimental and analytical analysis on chip morphology, phase transformation, oxidation, and their relationships in finish hard milling Song Zhang 1, Y.B. Guo Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA
a r t i c l e in fo
abstract
Article history: Received 2 February 2009 Received in revised form 10 June 2009 Accepted 12 June 2009 Available online 18 June 2009
Milling hardened steels has emerged as a key technology in mold and dye manufacturing industry. The effects of cutting parameters on chip morphology, phase transformation and oxidation reaction of the chips during finish milling AISI H13 tool steel (5071 HRc) with coated inserts were investigated in this paper. The chip morphology and phase transformation were examined using an optical microscope and a scanning electron microscope (SEM). The X-ray photoelectron spectroscopy was used to measure the chemical compositions of oxidation layer on chip surfaces. Shear-induced lamella structures characterized by jagged and rough appearance are the basic features of free surfaces. The microstructural analysis indicated that saw-tooth chips and white layers are produced only under certain combinations of cutting parameters of high cutting speed and feed rate. Based on chip color, chip morphology and X-ray photoelectron spectroscopy (XPS) analysis of the chip oxidation layer, the maximal instantaneous temperature at the tool–chip interface is semi-qualitatively estimated using the analytical method developed. In addition, chip color can be predicted based on the oxidation compositions. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Hard milling Chip morphology Oxidation Phase transformation Tool steel
1. Introduction With the advances in cutting tool technologies, hard milling has been recently employed to machine hardened steels (430 HRc) in making dyes and molds for various automotive and electronic components as well as plastic molding parts. Most research in hard milling is focused on the effects of process parameters on tool performances [1], surface quality [2], white layer [3,4], cutting force model [5], cooling/lubrication technology [6,7] and optimal cutting parameters [8]. However, very few studies have dealt specifically with the unique cutting mechanisms induced by hard milling such as the effects of cutting parameters on chip morphology, phase transformation, oxidation reaction and the resultant chip color. An insightful understanding of hard milling mechanism can lead to a better process economics, increased process stability, improved tool life, reduced tooling costs and enhanced surface integrity and component performance. Chip morphology, phase transformation, oxidation and chip color are an important indications to understand the milling mechanism, which is critical in the evaluation of the process performance.
Corresponding author. Tel.: +1 205 348 2615; fax: +1 205 348 6419.
E-mail address: [email protected] (Y.B. Guo). Currently a visiting scholar from Shandong University, PR China.
1
0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.06.006
Due to periodic variation of chip thickness in hard milling, the variation in chip load is the source of cutting force fluctuations, which results in high-frequency vibrations and premature tool failure. Much effort has been placed on addressing the mechanisms of saw-tooth chip formation mechanisms such as adiabatic shearing [9] and surface crack propagation [10–12]. Nakayama et al. [13] suggested that the saw-tooth chips are initiated by periodic crack formation as one of the plausible mechanisms in orthogonal cutting highly cold-worked 40/60 (Zn/Cu) brass. The saw-tooth chip formation starts with the initiation of a crack at the free surface of the workpiece, which further propagates towards the cutting edge of the tool. The crack soon ceases to grow at a point where severe plastic deformation of the material exists under higher level of compressive stresses. Shaw and Vyas [10], Elbestawi et al. [11] and Poulachon and Moisan [14] also employed the periodic crack theory in an attempt to explain the saw-tooth chip formation in hard turning. However, Recht [15] proposed the adiabatic shear theory which suggests that the root cause of saw-tooth chip formation is a catastrophic thermoplastic instability. The decrease in flow stress is due to thermal softening, which offsets strain hardening associated with strain increase. Komanduri et al. [16] adopted the adiabatic shearing theory and explained the saw-tooth chip formation in high-speed machining hardened AISI 4340 steel. Guo and Yen [17] found that the periodic adiabatic shearing crack which involves both crack and adiabatic shearing mechanisms is responsible for the formation of saw-tooth chips in hard machining. An adiabatic shear-induced
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internal crack first initiates and then propagates towards the free surface in front of the tool while the free surface crack develops. Saw-tooth chips are often formed in hard machining and represent an essential feature of the chip morphology. The effects of cutting parameters and material hardness are shown to be interdependent variables in governing saw-tooth chip formation, which affect the transition from continuous to shear-localized chip formation in hard machining [15]. Therefore, the formation of saw-tooth chips is affected by many factors such as material properties and tool geometry. Vyas and Shaw [12] showed that a high hardness of workpiece, large negative rake angle and large undeformed chip thickness promote the crack initiation and the consequent formation of saw-tooth chips in hard machining, while cutting speed has only a modest effect. Poulacho and Moisan [14] has shown that cutting speed and material hardness are interdependent factors in governing the saw-tooth chip formation in turning 100Cr6 steel (AISI 52100). Hard turning of H13 tool steel with a PCBN tool insert indicated that workpiece hardness and cutting speed influence the transition from continuous to saw-tooth chip formation [18]. Wang and Zheng [19] investigated the effect of workpiece hardness on chip formation mechanism in ball end milling of H13 tool steel. At low speeds continuous chips were formed. However, saw-tooth chips were produced at higher speeds. Moreover, the speed at which the transformation from continuous chip to saw-tooth chip occurred depends on the workpiece hardness [16]. As the rake angle becomes more negative or the cutting speed increases, the chip morphology transition from saw-tooth chip to separation of individual segments of the saw-tooth chips may also occur [16,17]. An estimation of cutting temperature in chip formation is crucial to explain the complex phenomena encountered during machining such as tool wear and surface integrity of the machined components. However, it is very difficult to accurately measure the cutting temperatures during the cutting process, especially for milling process, because the milling cutter rotates at high speeds while traveling in the feed direction. Because the majority heat generated during cutting is carried away by the chips, the high-temperature chips would react with oxygen in the air and produce temper colors of the chips. Therefore, chip color can be a characteristic indicator of cutting temperature and may be used as a rough guide to select an optimum cutting speed range. Chip colors also depend on materials composition. Thus, cutting temperature could be qualitatively estimated by examining chip color because cutting temperature affects the extent of chip oxidation and color variation [20,21]. For example, chip color produced in down-milling was golden brown while it was purple and darker in up-milling [22]. Ning et al. [21] claimed that the lighter color on chip surface was mainly due to the intimate contact between the tool and the chip and hence oxidation was suppressed. It was also reported that the darker the chip color, the more the chip oxidation. In general, chip color becomes darker when tool wear increases since oxidation increases due to the increased cutting temperatures. Dhar et al. [23] found that chip color depends on cutting speeds and feed rates in turning AISI 1040 steel. However, a relationship between cutting temperature, chip oxidation and color in hard milling has not been addressed yet.
In this investigation, Taguchi’s method-based design-of-experiment of dry hard milling AISI H13 tool steel (5071 HRc) using (Ti, Al)N–TiN-coated cemented carbide cutters was conducted. The objectives of this study are two-fold. Firstly, the controlling cutting parameters on chip morphology transition and phase transformation are experimentally identified. Secondly, a relationship between cutting temperature, oxidation and chip color is established using a new analytical approach.
2. Experiment procedure and sample preparation 2.1. Work material and cutting tool AISI H13 tool steel combines very good hot-hardness with toughness and covers a wide variety of applications such as pressure die casting, extrusion, hot forging and extrusion mandrels. It can be water cooled in service and gives hightemperature strength and wear resistance. The nominal chemical composition and material properties of H13 steel are listed in Tables 1 and 2, respectively. The workpiece material was hardened and tempered to 5071 HRc. The workpiece samples were prepared as 125 25 20 mm rectangular blocks. Prior to milling, the blocks were face milled on the top and bottom surfaces to remove the heat-treatment-related surface defects and to ensure the flatness by eliminating errors in the experimental results. The cutting tool used in machining tests is a milling cutter of 20 mm diameter, which can be equipped with two inserts. The tool holder and the (Ti, Al)N–TiN-coated carbide inserts were all made by SECO Tool Company. The details of the milling cutter are given in Table 3 and Fig. 1, respectively. In this experiment only one insert was used for each set of experimental conditions, which would eliminate the influence of the tool tip run-out on tool wear associated with a two-insert end mill. Since each surface was milled using a fresh cutting edge so that the tool-wear effect on chip formation would be the same and the variation due to the wear can be minimized. The scanning electron microscope (SEM) images of cutting edge before and after milling in Fig. 1(c) and (d) indicate that tool flank wear is minimal at the most abusive condition (highest speed and feed). The evolution of chip formation with tool wear will be conducted in a future study. 2.2. Experimental design Tables 4 and 5 are the conditions used according to Taguchi’s design-of-experiments. Taguchi’s design-of-experiments with a standard L16 (44) orthogonal array was selected to conduct the
Table 2 Material properties of AISI H13 too steel at room temperature. Density (kg/m3)
Young’s modulus (GPa)
Hardness (HRc)
Yield strength (MPa)
Area reduction (%)
Thermal conductivity (W/m K)
7800
210
5071
1579
23.0
25.6
Table 1 Nominal chemical composition of AISI H13 tool steel (wt%). C
Mn
Si
Cr
Mo
V
Ni
Fe
0.32–0.45
0.20–0.50
0.80–1.20
4.75–5.50
1.10–1.75
0.80–1.20
0–0.30
Balance
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807
finish hard milling experiments with four factors at four levels. The orthogonal array was chosen because of the minimum number of experimental trials required. Each row of the matrix represents one trial. Four levels of each factor were represented by ‘2’, ‘1’ ‘1’ or ‘2’ in the matrix. The collected chips at different cutting conditions are also numbered as shown in Table 5. The dry milling tests were carried out on a CNC vertical machining center (CINCINNATI Arrow-500).
‘‘edge effect’’. The machined chips were examined using optical and scanning electron microscopes ( Philips XL300) to identify the microstructural features of different chip surfaces. Chemical compositions of the oxidation layer on the chip back surface and the bulk H13 steel (free of oxidation layer) were analyzed and compared using X-ray photoelectron spectroscopy (XPS). The XPS
2.3. Sample preparation
Factor
To study the morphology and phase transformation of machined chips, chips were collected from each milling test, cold mounted, polished and etched in a solution of 4% Nital to reveal the microstructure. In order to observe the cross-section of the chips, the top surface shown in Fig. 2 was exposed for polishing so that the free surface and the back surface are both perpendicular to the polished epoxy surface. At the same time, the samples in this study were carefully prepared to eliminate the potential
Table 4 Factors and levels used in finish milling experiments. Level
A—Axial depth of cut ap (mm) B—Radial depth of cut ae (mm) C—Cutting speed v (m/min) D—Feed fz (mm/tooth)
Item
Description
Tool holder Insert designation Coating technique Coating material Insert clamping Tool diameter (mm) Overhang length (mm) Axial rake angle (deg) Radial rake angle (deg) Clearance angle (deg) Number of tooth Corner radius (mm) Insert thickness (mm) Insert length (mm) Insert width (mm)
R217.69–1820.0–12–2A XOMX 120408TR PVD (Ti, Al)N–TiN Collet 20 60 +3 12 14 Single 0.8 4 12 8
1
1
2
1.0 0.3 100 0.05
1.5 0.4 150 0.10
2.0 0.5 200 0.15
2.5 0.6 250 0.20
Table 5 Design-of-experiment based on Taguchi’s method—L16 (44). Experiment #
Table 3 Geometry of milling cutter.
2
Factor
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Chip #
A (ap)
B (ae)
C (v)
D (fz)
2 2 2 2 1 1 1 1 1 1 1 1 2 2 2 2
2 1 1 2 2 1 1 2 2 1 1 2 2 1 1 2
1 2 2 1 1 2 2 1 2 1 1 2 2 1 1 2
2 2 1 1 2 2 1 1 1 1 2 2 1 1 2 2
12mm
80°
14° 8mm 0.8mm
4mm
Geometric dimensions of insert
Tool-holder
Cutting edge before milling
Cutting edge after milling (Exp. #16)
Fig. 1. Milling cutter geometry
Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip Chip
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16
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Feed direction
Back surface
Side cutting edge Top surface A
ae
ap
Free surface End cutting edge
Workpiece
B
Major region
Free surface
Insert
Corner region
Cross section
Fig. 2. 3D schematic of chip formation in end milling (not to scale).
data were collected using monochromatic Al Ka radiation. To eliminate sample charging during the analyses, all the binding energies were referred to as C1s signal at 284.6 eV. Oxidation quantification such as peak locations and widths at half the maximum height was provided after curve fitting. In addition, sensitivity factors for Fe and O have been calculated using the H13 tool steel as the baseline.
3. Experimental results and discussions 3.1. Characteristics of chip morphology Since end milling is a cutting process involving complex tool geometry of the indexable inserts and the interactions between tool/workpiece, a 3-dimensional (3D) view of chip formation is necessary to identify the specific functions by different cutting edges. The 3D view is also vital for understanding the formation of a machined surface and subsequent surface integrity. Fig. 2 illustrates the 3D schematic view of chip formation, specific functions of the cutting edges and the process parameters such as axial and radial depth of cuts. Four surfaces of the chip, i.e., the top surface, the free surface, the back surface and the cross-sections have been defined. The free surface and the back surface are formed by the side cutting edge in sequential cutting, while the machined surface is formed by the tool nose and the end cutting edge. 3.1.1. Lamella-structured free surface The observed lamella structures in Fig. 3 is the basic feature of free surfaces characterized by jagged and rough appearances, which are caused by the shearing mechanism. The lamella structure on the free surface signifies the shearing morphology on the free surface compared to the traditional cross-sectional view. The free surface has two sections of different orientations, i.e., the major section formed by the side cutting edge and the corner section formed by the tool nose edge. The lamella structure in the major section is parallel to the side cutting edge, while the inclined lamella structure in the corner section is approximately parallel to the tool nose edge. Practically, all cutting parameters contribute to the formation and the appearance of the lamella structure. However, cutting speeds and feed rates are the two major process parameters influencing the size and shape of lamella structures of the free surfaces according to the SEM images, while the variations due to
(ap=2.0mm, ae=0.6mm,v=100m/min, fz=0.05mm/tooth) Fig. 3. Lamella structures on the free surface (chip #12).
depth of cut is much smaller. Therefore, this study focuses on the effects of cutting speed and feed rate on chip morphology. When cutting speeds and feed rates are relative low, small but uniform lamella structures are formed as shown in Fig. 4(a) and (b). The distribution of the lamella was very uniform and the average lamella thickness is relatively constant. With the increase in cutting speeds and feed rates, larger lamella structures in terms of width and depth occur as shown in Fig. 4(c) and (d), which indicates the formation of saw-tooth chips. 3.1.2. Smooth back surface During milling as in all cutting operations, the back surface of a machined chip closely contacts with the tool rake face. The plastic deformation of the back surface is constrained by the tool rake face. Thus, the back surface experiences high contact pressure and high frictional force when the chip slides over the tool rake face. The combined actions of high contact pressures, frictional forces and high temperatures make the back surface smooth and shiny (Fig. 5) than the free surface of the chips. It can be noticed that there are some parallel stripes on the back surface induced by irregular cutting edges and/or potential hard particles, but it is generally smooth. Although the distributions of contact stress, friction and temperature at the tool/chip interface vary with cutting process parameters, the back surfaces have very similar characteristics. 3.1.3. The transition from uniform to saw-tooth chip formation Fig. 6 shows the cross-sectional images of the chip top surface. Two different types of chips can be observed: the near uniformly deformed continuous chip and the saw-tooth chip. The continuous chips were produced at low cutting speeds and feeds, while the saw-tooth chips were produced at relatively high cutting speeds and feeds. The chip morphology transition from continuous chip to saw-tooth chip is favored by the combination of increased cutting speed and feeds. The combined effects of cutting speed and feed on chip morphology transition are summarized in Table 6. Based on the observation,
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ap=2.0mm, ae=0.6mm,
ap=2.5mm, ae=0.4mm,
v=100m/min, fz=0.05mm/tooth (chip#12)
v=150m/min, fz=0.10mm/tooth (chip#14)
809
Width of lamella structure
ap=2.0mm, ae=0.4mm,
ap=2.5mm, ae=0.6mm,
v=200m/min, fz=0.15mm/tooth (chip#10)
v=250m/min, fz=0.20mm/tooth (chip#16)
Fig. 4. Combined effects of cutting speed and feed on morphology of chip free surfaces.
parameters defined in Fig. 6(d). Saw-tooth pitch pc (saw-to-saw distance), saw height t1 (saw peak-to-valley distance), chip thickness t2 (distance between saw peak to back surface)a were measured at least three locations for statistical analysis. Fig. 7 shows that at low speeds, chip pitch decreases with increase in speed, while the effect of high speed alone has varied influence. As shown in Fig. 8, the measured inclination angles of the saw-tooth chips range from 561 to 641, which are much larger than those produced in orthogonal cutting [18]. In addition, the inclination angles greater than 451 mean that chip formation mechanism is not pure shear deformation. To incorporate the effect of radial depth of cut on chip morphology, the ratio of the saw-tooth height to chip thickness is defined as r¼ Fig. 5. Back surface topography (chip #12).
feed per tooth is the major contributor of the serration, while cutting speed effect is secondary. 3.1.4. Geometrical characteristics of saw-tooth cross-section of top surface In order to investigate the effects of milling parameters on chip morphology, the chip cross-sections were analyzed with the
t1 t2
(1)
For near uniform continuous chips, the variation in the ratio is insignificant and therefore not applied. As listed in Table 7, the ratio becomes large with increase in cutting speed and feed rate. However, the trend of the ratio variation is consistent for the lamella-structured chips. 3.2. White layer on cross-section of saw-tooth chips The chip back surface experiences high temperatures when the chip slides past the tool rake face, especially at high cutting
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ap=2.0mm, ae=0.6mm,
ap=2.5mm, ae=0.4mm,
v=100m/min, fz=0.05mm/tooth (chip#12)
v=150m/min, fz=0.10mm/tooth (chip#14) pc
View direction for lamella structure in Fig.4 (c)
t2 α
t1
White layer
White layer ap=2.0mm, ae=0.4mm,
ap=2.5mm, ae=0.6mm,
v=200m/min, fz=0.15mm/tooth (chip#10)
v=250m/min, fz=0.20mm/tooth (chip#16)
Fig. 6. Combined effects of cutting speed and feed on cross-section of chip top surfaces.
Table 6 Chip morphology analysis at different cutting speeds and feeds.
0.05 Cutting speed v (m/min) 100 Continuous 150 Continuous 200 Saw-tooth 250 Saw-tooth
0.10
0.15
0.20
Continuous Continuous Saw-tooth Saw-tooth
Continuous Saw-tooth Saw-tooth Saw-tooth
Continuous Saw-tooth Saw-tooth Saw-tooth
Shear angle (°)
Feed fz (mm/tooth)
v=200m/min
70 65 60 55 50 45 40 35 30
v=250m/min
0.05
0.1 0.15 Feed (mm/tooth)
0.2
Chip pitch (µm)
Fig. 8. Variation in inclination angle with feed.
70 65 60 55 50 45 40 35 30
v=200m/min v=250m/min
Table 7 Height-to-thickness ratio of saw-tooth chips. Feed fz (mm/tooth) 0.05
0.05
0.1 0.15 Feed (mm/tooth)
Fig. 7. Variation in chip pitch with feed.
0.2
Cutting speed v (m/min) 100 N/Aa 150 N/A 200 0.6270.019 250 0.5070.050 a
0.10
0.15
0.20
N/A N/A 0.6470.035 0.5570.006
N/A 0.4470.006 0.5370.010 0.5670.015
N/A 0.5570.032 0.6070.020 0.6370.014
N/A denotes uniform chips.
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Table 8 Chemical reactions and equations for Gibbs free energy at high temperatures. Oxidation/reduction reactions
Equations for Gibbs free energy
2Fe(s)+O2(g)-2FeO(s) 4/3Fe(s)+O2(g)-2/3Fe2O3(s) 3/2Fe(s)+O2(g)-1/2Fe3O4(s) 4FeO(s)+O2(g)-2Fe2O3(s) 2H2O(l)-2H2(g)+O2(s) 3Fe2O3(s)+H2(g)-2Fe3O4(s)+H2O(g)
DG ¼ 543,920+138.09T DG ¼ 549,500+183.13T DG ¼ 339,200+172.75T DG ¼ 1648,500+139.35T D ¼ 571,660–326.35T DG ¼ 1654,370–88.75T
90000 Continuous chip (#12) Saw-tooth chip (#16)
4.1. Oxidation reaction of chips It has been often observed and generally agreed that the bulk heat generated in metal cutting is carried away by chips and dissipated into the air. The chip temperature then becomes much higher than the workpiece material. When the chip back surface is exposed to air (an oxidative atmosphere) at high temperatures, the chips are susceptible to oxidation, which results in the formation of an oxide layer on the chip surfaces. The potential gas-metal oxidation reaction between Fe and O are represented as follows: 2FeðsÞ þ O2 ðgÞ ! 2FeOðsÞ
(2)
4=3FeðsÞ þ O2 ðgÞ ! 2=3Fe2 O3 ðsÞ
(3)
3=2FeðsÞ þ O2 ðgÞ ! 1=2Fe3 O4 ðsÞ
(4)
60000
C1s
Intensity (c/s)
4. Relationship between temperature, oxidation and temper colors
Fe2p1 Fe2p3
70000
O1s
80000
50000 40000
Cr2p1 Cr2p3
speeds. The frictional energy was quickly converted to thermal energy, which leads to localized transient high temperatures on the chips. Compared with the friction-induced white layer on the back surface, the white layer in the primary shear zone in Fig. 6(d) was induced by high temperatures resulting from severe plastic deformations at high speeds. When the localized temperatures exceed the austenization temperature 1010 1C (Ac3) of H13 steel, the ferrite phase (a) is transformed to austenite phase (g). Then the austenite phase (g) is quickly quenched by the surrounding air and becomes the ultrafine grained white layer. As shown in Fig. 6(c) and (d), the white layer in the primary shear zone and on the chip back surface is observed. Based on the formation of white layers, the cutting temperature reaches close to 1010 1C during the cutting process.
811
Bulk
30000 20000 10000 0 1000 900
800
700
600 500 400 300 Binding energy (eV)
200
100
0
Fig. 9. XPS survey spectra of chip back surfaces.
4.2. XPS analysis of chip back surface
The degree of oxidation reaction depends on exposure time, surface area, mass transfer, temperature, etc. Gibbs free energy (DG) of a reaction is a measure of the thermodynamic driving force that enables a reaction to take place. A negative value for DG indicates that a reaction can proceed spontaneously without external inputs, while a positive value indicates that it will not. The equation for Gibbs free energy is:
DG ¼ DH T DS
(5)
where DH is the enthalpy change in the reaction, T is absolute temperature and DS is the entropy change in the reaction. The enthalpy change (DH) is a measure of the actual energy that is liberated when the reaction occurs (the ‘‘heat of reaction’’). If it is negative, then the reaction gives off energy, while if it is positive the reaction requires energy. Based on the enthalpy and entropy values of all elements and compounds (H2O, H2 and O2), the standard Gibbs free energy changes (DG) were calculated according to Table 8. Since the changes in Gibbs free energy for the above three reactions (Eqs. (2)–(4)) are all negative when the temperature varies from 273 to 1972 K, the thermodynamics for the formation of FeO, Fe2O3 and Fe3O4 is favorable. Since the colors of FeO, Fe2O3 and Fe3O4 are blue, red and black, respectively, the chip color varies with the presence of specific iron oxide or their combinations, which depend on the degree of oxidation. The temperature at tool–chip interface is a determining factor for the oxidation degree. Colors of the machined chips under milling conditions in Table 4 vary from light purple to dark blue.
In the present paper, to investigate the chemical composition of the oxidation film, XPS spectra of oxidation layer on the back surfaces of the chips and the bulk material (free of oxidation) of the H13 steel were conducted. Peak locations and widths at half the maximum height and peak areas were measured after curve fitting. The XPS survey spectrum of the bulk material (H13 steel), continuous chip (#12) and saw-tooth chip (#16) are shown in Fig. 9. In this figure, binding energy is conceptually, not strictly, equal to the ionization energy of the core electron in each atom. The variation in binding energy results in the shift of the corresponding XPS peak. The peak intensity shows the concentration level of elements, which characterizes the escaped electron counts per second (c/s). Fig. 10 shows that the spectrum is dominated by the presence of Fe2p3 and O1s peaks. The main elements Cr2p3 peak of H13 tool steel is very small. This indicates that the scale mainly contains iron oxides. As shown in Fig. 10, only one O1s peak at about 531 eV can be fitted for the bulk material, which corresponds to the O2 molecule attaching to the bulk surface. For the machined chips, for example, chip #12 and chip #16, there is another fitted peak with bonding energy close to O1s in iron oxide, which indicates the presence of iron oxide on the chip back surfaces. Since the bonding energy of O1s in FeO and Fe2O3 are 529.8 and 529.6 eV, respectively, the bonding energies of O1s in different iron oxides are too close to find the exact chemical compositions of the oxidation layer only by the XPS spectra in Fig. 10. It is necessary to investigate further according to the XPS spectra of Fe2p3/2 of the oxidation layer. Fig. 11 shows the XPS spectra of Fe2p3/2 of oxidation layer on the chip back surfaces. For the continuous chips (chip #12), there are two peaks located at 710.9 and 710.6 eV, which correspond to the bonding energy of Fe2p3/2 in Fe2O3 and Fe3O4, respectively.
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4.3. Correlation of temperature, XPS analysis and chip color
5000 4500
Continouos chip (#12)
Bulk
Intensity (c/s)
4000 Saw-tooth chip (#16)
3500 3000 2500 2000 1500 1000 540.0
537.5
535.0 532.5 530.0 Binding energy (eV)
527.5
525.0
Fig. 10. XPS spectrum of oxygen element on chip back surfaces.
Fe2p3/2 in Fe2O3 5000
Continuous chip (#12) Fe2p3/2 in Fe2O3
4500 Insensity (c/s)
Fe2p3/2 in Fe3O4
Fe2p3/2 in FeO
4000
Fe2p3/2 in Fe3O4 3500 3000
When the temperature exceeds 1478 1C (1751 K), water decomposes into hydrogen and oxygen, which can reduce Fe2O3 to Fe3O4 or even FeO. From the XPS spectra of Fe2p3/2 for the sawtooth chips (chip #16), two peaks located at 710.6 and 709.6 eV confirm the presence of Fe3O4 and FeO, respectively. Therefore, it is reasonable to infer that Fe2O3 oxidation layers on the chip surfaces were reduced to Fe3O4 and FeO when the cutting temperature at the tool–chip interface exceeds 1478 1C (1751 K). The more oxide composition FeO over Fe2O3 makes the saw-tooth chips dark blue. With the decreased amount of Fe2O3, the contents of Fe3O4 and FeO increase gradually and the color of the saw-tooth chip surface becomes darker and darker. The existence of white layer in Fig. 6(c) demonstrates that the cutting temperature is over the austenization temperature 1010 1C (Ac3), so most likely the reduction occurred. The high temperatures could be caused by the exothermic reaction of the chip material after it was formed with the oxygen of the air. The temper colors were not entirely due to different oxidation states of the iron but also due to the oxidation of the alloying elements. Based on the above analysis, it can be inferred that 1478 1C (1751 K) is one critical temperature to determine the occurrence of reduction reaction of Fe2O3 to Fe3O4, while the austenitizing temperature 1010 1C (Ac3) is another critical temperature, which initiates the formation of a white layer. Through the integrated optical, SEM and XPS analysis the continuous chips show light purple and are free of white layers when the chip temperature is less than 1010 1C at low cutting speeds and feeds, while the saw-tooth chips are dark blue and with the presence of a white layer at temperature above 1478 1C when cutting speed and feed are above the critical values.
Saw-tooth chip (#16)
2500
5. Conclusions
2000 718
716
714
712 710 708 Binding energy (eV)
706
704
702
Fig. 11. Trivalent and bivalent oxide peaks of iron on the back surface of the chips.
A Taguchi’s design-of-experiments approach was taken for the finish dry hard milling with coated inserts to investigate the effects of cutting parameters on chip morphology, phase transformation, oxidation reaction, and their relationships. The major conclusions can be summarized as follows:
The main compositions of the oxidation layer of chip #12 consist of more Fe2O3 and slightly less Fe3O4, which make the back surface light purple. The chip free of white layer in Fig. 5a implies that the cutting temperature is below the austenization temperature 1010 1C (Ac3). So oxidation reaction rather than reduction occurred. The chemical reaction is expressed as follows,
Cutting speeds and feed rates are critical process parameters to
4FeOðsÞ þ O2 ðgÞ ! 2Fe2 O3 ðsÞ
(6)
The valences of iron are +2 and +3, i.e., Fe2O3 is the highest valence oxide. The cutting temperature increases with cutting speed, feed rate and depth of cut. When the temperature exceeds a critical value at high cutting speeds, Fe2O3 can be reduced to Fe3O4 or FeO by the hydrogen molecule decomposed from water. The chemical reactions can be expressed as follows:
2H2 O ðlÞ ! 2H2 ðgÞ þ O2 ðsÞ
(7)
3Fe2 O3 ðsÞ þ H2 ðgÞ ! 2Fe3 O4 ðsÞ þ H2 OðgÞ
(8)
The standard free energy changes associated with the above chemical reactions are listed in Table 8.
affect the chip morphology and the transition from continuous chip to saw-tooth chip. Saw-tooth chips and white layers only occur under the specific combinations of higher cutting speeds and feeds. Lamella structures are the basic features for the jagged and rough appearance of the chip-free surfaces. Based on chip colors, chip morphology and XPS analysis of chip oxidation layer the maximal instantaneous temperature at the tool–chip interface can be semi-qualitatively estimated using the developed analytical method. It can be inferred that the dark blue machined chips is induced by a critical cutting temperature, which can decompose water into hydrogen and oxygen.
Acknowledgments This research is based upon work supported by the National Science Foundation under Grant no. CMMI-0447452. The authors would like to thank Dr. Mark Weaver, Mr. Raul Waikar and Mr. Ning Li at the University of Alabama for their assistance in sample preparation and characterization.
ARTICLE IN PRESS S. Zhang, Y.B. Guo / International Journal of Machine Tools & Manufacture 49 (2009) 805–813
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