Determination of local strain in cutting steel chips by direct measurement
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https://www.eduzhai.net International Journal of M aterials Engineering 2013, 3(5): 97-102 DOI: 10.5923/j.ijme.20130305.02 Determination of Local Strains in Cut Steel Chips through Direct Measurement Andy Simoneau Department M echanical Engineering, University of New Brunswick, Fredericton, E3B 5A3, Canada Abstract Metal cutting is a severe plastic deformation process involving large strains, high strain rates, and high temperatures. To determine the strains during the chip formation process the conventional analysis is based on bulk materia l deformation which disregards the inhomogeneous nature of the materia l mic rostructure and tends to under-predict the strains in metal cutting. This paper uses the workpiece materia l microstructure before and after deformation to determine the strains encountered during the metal cutting process. A series orthogonal cutting tests of AISI 1045 and 1144 steel we re conducted yielding similar process characteristics and chip format ions. With similar shear angles and cut chip thicknesses, average shear strains in the primary and secondary shear deformat ion that would normally be predicted for a ho mogeneous material are under-predicted. Using AISI 1144 cut chips, it is shown experimentally that local strains in the primary shear deformation zone can reach an average value above 6.0, and in some cases can reach a value of 10 in the secondary shear deformat ion zone which agrees with prev iously predicted local strains for AIS 1045 steel based on finite element modeling. The distinct shape of manganese-sulfide (MnS) precip itates in 1144 steel is found to be an effective means for determin ing local strains and strain variability across a cut steel chip. The findings and their imp lications are presented and discussed. Keywords Orthogonal Machining, Metal Cutting, Chip Fo rmation, M icrostructure, Local Strain 1. Introduction The metal cutting process involves the removal of material in the form of a chip fro m a workpiece. The actual removal process is a shearing process where the workp iece material is sheared by a tool with a geo metrically defined cutting edge. From Fig. 1, the two p rimary faces of the cutting tool are the rake and flank faces, which together meet to form the cutting edge. In its simplest form, the cutting process resembles concentrated shear along a distinct plane commonly referred to in metal cutting as the shear plane. The direction of shearing occurs at an angle relative to the cutting plane (referred to as the shear angle - φ) and the tool or workp iece moves with some p redetermined cutting velocity. Machining is a severe plastic deformation process involving large p lastic strains (>1), very h igh strain rates (>10-4s-1), and elevated temperatures in excess of half the melt temperature of the metal being mach ined. While mach ining of metals is not a new process and widely utilized in shaping raw materials, the conditions under which metal cutting occurs are unique. As the workpiece material approaches and passes through the shear plane it undergoes a substantial a mount of plastic strain. The bulk of the materia l * Corresponding author: firstname.lastname@example.org (Andy Simoneau) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved then flows as a chip, continuing to strain extensively along the contact length between the chip and the tool rake face in the secondary shear deformation zone. While the cutting process can take place in many different forms, one of the most widely recognized, modeled, and understood forms of metal cutting is the orthogonal metal cutting process in Figure 1, wh ich is a two-dimensional plane strain process. Figure 1. General configuration for orthogonal machining highlighting the primary and secondary shear deformation zones Depending on workpiece material and cutting parameters such as cutting speeds and feedrates, different chip formation types are possible fro m the classical continuous, continuous with a built up edge (BUE), and discontinuous chips, to cyclical, continuous chips[3-6], and segmental chip formations[7-12]. Continuous chip format ion is not a desirable result fro m a p roduction standpoint as these severely work hardened chips can damage the machined surface, and will require constant supervision and manual 98 Andy Simoneau: Determination of Local Strains in Cut Steel Chips through Direct M easurement removal fro m the work or cutting area. Ho wever, continuous chips are ideal for analy zing the machining p rocess and in particular, the plastic deformation process in the different deformation zones during the chip format ion process. Depending on workpiece material and cutting parameters such as cutting speeds and feedrates, different chip formation types are possible fro m the classical continuous, continuous with a built up edge (BUE), and discontinuous chips, to cyclical, continuous chips[3-6], and segmental chip formations[7-12]. Generation of a continuous chip during orthogonal metal cutting has been a staple for researchers analyzing the metal cutting process given the two-dimensional nature of the p rocess and plane strain condition of continuous chip format ion in orthogonal cutting. By analyzing the cut chip microstructures using optical and scanning and transmission electron microscopy (SEM and TEM, respectively), geo metric features of the cutting process such as identification of the shear deformat ion zones and the shear plane and shear angle, can be readily observed. From this, plastic strains, strain rates, and even temperatures in the shear deformation zones can be determined analytically. Merchants theory[13,14], Lee and Shaffer’s triangular slip line field, and Oxley ’s parallel-sided shear zone theory have all been based on orthogonal cutting and the formation of a continuous chip[16-19]. The plastic strains of a cut chip are determined by comparing the workpiece material before and after the cutting process (via the chip). Treating the materia l structure as a homogeneous one, the resulting deformation in the primary and secondary shear deformation zones can be determined through analysis of the mechanics of the cutting process and knowledge of the constitutive law that governs the workpiece materials mechanica l properties under cutting conditions. It is believed that this approach underestimates the actual plastic strains that occur during metal cutting throughout the cut chip cross-section. Adding to this speculation, is has been shown that as the scale of cutting decreases to the microscale, the heterogeneous structure of a metals microstructure has a significant effect on the chip formation process. At a scale corresponding to the grain size of 1045 steel, Simoneau, Ng, and Elbestawi showed that local strains could be 2-3 t imes larger than the strains predicted by the bulk measurement approach and that not only did this have an impact through the cut chip but also along the machined surface[21,22]. This same effect was shown for conventional cutting across multip le scales of cutting and for d ifferent grain structures and sizes. While it is hypothesized, and nu merically shown that the strains in metal cutting at a conventional macroscale are underestimates of the actual shear strains which result from the materials heterogeneous structure, it has not been shown experimentally and is thus the aim of the current work. 2. Experimental 2.1. Cutting Tests A series of orthogonal cutting tests were performed on a Haas CNC Lathe. At cutting speeds of 100, 200, and 300 m/ min, feedrates of 0.05, 0.1, 0.2, and 0.3 mm/rev were used to generate a series of continuous chips. These tests were performed for both AISI 1045 and AISI 1144 steel. In all cases the rake angle was 0o the clearance angle was 11o, and all cutting was done dry without the use of any coolant. After each cutting test the cutting insert was indexed to ensure that a new cutting edge was used for each series of cuts. While not a focus in this body of work, cutting forces were measured and recorded using a 3-co mpenent piezo-electric force dynamo meter in conjunction with the Lab View data acquisition system. After each cutting test the resulting chip was collected for analysis. 2.2. Work piece Materials (a) (b) Figure 2. Microstructure of (a) AISI 1045 steel and (b) AISI 1144 steel. Both have a pearlite-ferrite structure (dark-light grains respectively), but the 1144 st eel has small MnS precipitat es in the ferrit e grains AISI 1045 and 1144 steel were the two materials orthogonally machined for the study. The 1045 steel was used because of the already extensive research that has been done with this steel[20-24], and the 1144 steel was used to highlight how strain localizat ions could be determined using the material microstructure. These two steels have a similar microstructure as shown in Figure 2. As well, the two are similar in terms of carbon content and overall mechanical properties. Both material microstructures are predominantly comprised of pearlite (dark grains in Figure 2) and ferrite (light grains in Figure 2). The difference between the two is that 1144 is a resulfurized steel, it has the same carbon content, but the sulfur and manganese content have both International Journal of M aterials Engineering 2013, 3(5): 97-102 99 increased as indicated in the chemical co mpositions in Table 1. The increase in sulfur and manganese is enough to cause a manganese sulfide (MnS) precip itate to form in the microstructure. These precipitates are easily identified as small circular precipitates in the ferrite grains as shown in Figure 2b. Table 1. AISI 1045 and 1144 steel chemical compositions AISI Chemical Composition (%) St eel C S Mn P Fe 1045 0.45 0.045 0.7 0.04 98.76 1144 0.45 0.28 1.5 0.04 97.73 2.3. Sample Preparation and Microscopy Following the cutting tests, the resulting cut chips were cold mounted in an epoxy and polished to a 3 micro meter fin ish. A 2% Nital etch was used in all cases to bring out the microstructure of the deformed ch ips for observation using both optical and scanning electron mic roscopy (SEM). 3. Results and Discussion generated. This was an important and necessary requirement to ensure that a proper comparison of AISI 1045 and 1144 steel cut chips could be done. Similarly it was also a necessary requirement to ensure that a proper analysis of the plastic strains through the chip cross-sections could be determined. The resulting chip cross-sections in Figure 3 highlight that there is a clear and d iscernible d irection of shearing that results from the workpiece material passing through the primary shear deformation zone during the cutting and chip formation process. As well, there is also a thin shear deformation zone (secondary shear deformation zone) that is observed along the back surface of the chip which relates to the tool-chip interface and the sliding contact between the cut chip and the cutting tool rake face. To ensure plane strain conditions the depth of cut should be an order of magnitude larger than the uncut chip thickness. As a result the depth of cut was set to 3 mm for all o f the mach ining tests. Based on the cut chip cross-sections the shear angle φ is determined based on the ratio of the uncut to cut chip thickness (chip ratio r) and the rake angle α, of the cutting tool fro m (1). φ = r cosα 1 − r sin α (1) With a continuous chip in a ll cases, the shear angles were found to be very similar for both the 1045 and 1144 steels, as highlighted in Figure 4. (a) Figure 4. Shear angle comparison for both AISI 1045 and 1144 steels based on the cut chip analysis (b) Figure 3. Cut chip cross-sections of (a) AISI 1045 steel and (b) AISI 1144 st eel. Cutt ing speed V = 200 m/min and feedrat e is 0.3 mm for both chips In all of the machining trials a continuous chip was Based on the shear angles and known rake angle of the cutting tool, the shear strain, γ, can then be determined fro m equation (2) which relies on continuous chip format ion. The equation of course assumes that the materia l is homogeneous, and ignores localization effects that may be present throughout the chip cross-section. γ = cos α sin φ cos(φ −α) (2) The shear strain value obtained fro m equation (2) is the strain across the chip cross-section and is only a measure of the plastic strain as a result of material passing through the primary shear deformation zone. Based on the cut chip samples, shear strains of 2.1 up to 3.0 were found for both the 1045 and 1144 chips which falls into the same range observed by other researchers[20, 21]. Determination of the shear strains using this approach represents a classical 100 Andy Simoneau: Determination of Local Strains in Cut Steel Chips through Direct M easurement approach to shear strain estimat ion and simply treats the material as a homogeneous structure before and after the chip format ion process, ignoring any affects that the heterogeneous structure of the material might have. In particular, any shear strain localizat ions that might be prevalent are not taken into account and the result is a potentially under-estimated shear strain value which has been argued by researchers[22, 23], but only shown through modeling. conditions of orthogonal cutting and the near circular shape of the MnS p recip itate in the microstructure, the deformed and undeformed MnS precip itates could be used to determine the strains encountered as the material passes through the primary shear deformation zone and the secondary shear deformat ion zone. 3.1. Determini ng Local Strain Values Previous modelling research using finite element methods to model the cutting of a heterogeneous workp iece material has shown that strains through the chip cross section can exceed 4.0 throughout the chip cross-section and reach values as high as 10 in the secondary shear deformation zone. Th is has never been shown experimentally however. By e xa mining the MnS inclusions in the 1144 steel using optical microscopy and SEM tools, large, local shear strains can be quantified. Using optical and scanning electron microscopy, Figure 5 h ighlights the MnS precipitates that have deformed into long oblong or elliptical shapes as a result of passing through the primary shear deformation zone. Analyzing these precipitates it is apparent that the conventional approach to treating the deformed chip as a single homogeneous structure to determine the shear strains resulting from the chip format ion process do indeed underestimate the actual shear strains that occur throughout the microstructure. Attempts to identify local p lastic strains throughout the chip cross-section are extremely difficult. Th is is due to the fact that in order to properly measure plastic deformation some in itial geo metry or frame of reference across the uncut chip thickness is required. At the same time that same point or frame of reference must be discernible after the cutting process. There are two manners in which this can be achieved. The first requires placing a grid or geometry on the material and observing that deformed geometry before and after cutting. Unfortunately this cannot be used because it means that instead of examining the middle of a chip cross-section, the edge of the chip must be used as a frame of reference. Hence plane strain conditions cannot be maintained and the analysis is no longer valid. The other approach is to of course use the material microstructure as the geometrical frame of reference. Again this poses difficult ies simply because of the irregular and random nature of a materials microstructure such as 1045 steel. As well, it is not possible to v iew the microstructure ahead of the cutting tool and keep track of it without sectioning the workp iece material prior to cutting and rendering the plane strain condition cannot unmaintainable. To overcome the previously stated obstacles, the MnS precipitates are treated as circles prior to the deformation process. This simplification is not far fro m the actual microstructure as shown in Figure 2b. Given the plane strain (a) (b) Fi gure 5. Deformed MnS precipit ates in the cut chip cross-sect ions using (a) optical microscopy, and (b) SEM. Inclusions are indicated by white arrows There are two potential methods for relating the deformed to undeformed MnS precipitate in the 1144 steel during the chip format ion process. The first is to determine the original diameters of the MnS p recipitates and use a statistical average for the overall undeformed MnS precipitates. Using this same approach for the deformed precipitates the two averages could be compared to obtain a crude estimate of the shear strain of the MnS precipitates. This approach is difficult, tedious, and would be h ighly inaccurate, representing a very rough estimate at best. A second method is to analyze and measure only the deformed MnS precipitates. By treating the deformed MnS precip itates as a deformed circle – an ellipse, the major and minor axis of the ellipse can be used to find the cross-sectional area of the deformed MnS precipitate which can then be translated back to an undeformed MnS precipitate as illustrated in Figure 6. With the undeformed and deformed MnS precipitate dimensions known, the shear strains can then be determined. International Journal of M aterials Engineering 2013, 3(5): 97-102 101 Figure 6. Relating a deformed MnS inclusion back too its undeformed shape to estimate strains Precip itates were examined at the mid-line along the length of the chip cross-section to estimate the strains through the primary shear deformat ion zone. Precipitates were also considered along the back face of the cut chip (along the tool-chip interface) to determine the strains that would be encountered in the secondary shear deformation zone. The location of the precipitates used to determine the strains was kept constant in order to ensure that the e xa mined precipitates were all passing at or near the same point in the two shear deformat ion zones. Precipitates near the chip free surface were avoided as it is expected that their strains would decrease as we approach the chips free surface. While it has not been experimentally shown, plastic strains are most likely not consistent across the cross section of the cut chip thickness even when ignoring the effect of the secondary shear deformation zone. The transformation fro m a circular shape to an ellipse was assumed to involve deformat ion along a straight line. Curved MnS precipitates were excluded fro m the examination (this only occurred occasionally in the secondary shear deformation zone). While the deformation process is not physically an instantaneous process, it is treated as such since the thickness of the primary shear deformation zone is on the same order in size as the MnS precip itates. At the interface between the MnS precipitates and the surrounding ferrite matrix it is assumed that there is a 1:1 transfer of strain informat ion. With no evidence of de-cohesion between the MnS precipitate and the surrounding ferrite matrix, a 1:1 ratio between the MnS particle and the ferrite matrix is considered acceptable, but only in the material at the ferrite-MnS interface. This rat io does not exist as we move into the ferrite matrix, away fro m the MnS particle, thus highlighting that even inside a single ferrite grain there can be strain variations. As a result, the variations in the material microstructure; MnS precipitates, ferrite grains, and pearlite grains, and their interactions with each other will lead to strain variations and localization effects . Using the approach of relat ing the deformed to undeformed MnS precip itate cross-sectional areas the resulting shear strains were found to range fro m 5.97 up to 6.8 for cutting speeds up to 300 m/ min in the primary shear deformation zone. In the secondary shear deformat ion zones, shear strains from 7.0 up to a maximu m of 10 were observed. These values are extremely large, but they are also in-line with p reviously published findings based on finite element modeling and the impact of microstructures during mu ltiscale cutting. The treatment of the cut metal chip as a ho mogeneous structure underestimates the shear strains by 2 to 3 times. This is significant when considering the size of the shear strains and their potential impact on the dynamics behavior of the workpiece materia l as it plastically deforms. The shear strain determination impacts shear strain rates and temperature predict ions during the metal cutting process. The bulk of e xperimental and numerica l research in strain localization and strain measurements during orthogonal cutting of the type examined in this research has focused on AISI 1045 steel. Given the similarity in mechanical properties to AISI 1144 steel, the results fro m the current study can be extended to AISI 1045 steel. The MnS precipitates examined were only found in the ferrite grains. In studies working with AISI 1045 steel, it is the ferrite grains that lead to the strain localizat ion effect. Therefore, these results can be used to support previous claims that current estimations of plastic strains during orthogonal cutting are typically underestimated because they do not account for localization effects that occur as a result of material microstructure. 4. Conclusions Determining the strains in metal cutting by treating the material as a ho mogeneous structure during the chip formation process will lead to an underestimation of the actual plastic strains encountered during the chip formation process. This fact has been demonstrated using numerical simu lation but until this point had not been shown experimentally in metal cutting. When considering the localization affects that are prevalent as a result of a workp iece materials microstructure, shear strains can in actuality be 2 to 3 times larger than previously thought. In the analysis, elastic properties of MnS and the surrounding ferrite grains were not considered. This may lead to slight discrepancies if elastic recovery is considered, however, to what extent is unknown, and it has not been considered to date when focusing on machined chips. The strain localization is present though, and it is significant enough to warrant further investigation. Of course the impact of this is that in simulat ion work, the constitutive laws used to model the dynamic behavior during large deformations may need to be reworked in order to better replicate and illicit information fro m processes such as metal cutting. These localizat ion effects will not be limited to the midsection of the cut chip thickness, and will of course be prevalent throughout the cut chip cross-section. The amount of localization and subsequent magnitude of the shear strains will likely vary across the cut chip. At or near the chip free surface the shear strains may be similar to the values obtained when treating the material as a homogeneous one. As shown in the current work, in the middle of the cut chip shear strains may increase by a factor of 2 to 3. Ho wever, as 102 Andy Simoneau: Determination of Local Strains in Cut Steel Chips through Direct M easurement we approach the secondary shear deformation zone this  H.O. Gekonde, G. Zhu, X. Zhang, S.V. 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