Influence of Graphite Morphology, Matrix Structure on Gray Iron Machining.
The machinability of gray iron is influenced by a complex interaction between the matrix structure and graphite morphology. Past studies have examined aspects of gray iron machinability, however, the influence of microstructure on the mechanisms of chip formation during machining remains elusive.
Gray iron's soft graphite phase provides the metal's free-machining characteristics. However, the shape, amount and distribution of flake graphite in gray iron has a major impact on its free-machining attributes. The graphite is soft with negligible hardness. It provides discontinuances that are thought to facilitate chip breaking. In addition, graphite flakes could result in areas of high stress concentration and paths of easy matrix fracture during machining.
Increasing the amount of graphite has been reported to reduce tool forces during machining. However, an excess of coarse graphite adversely affects mechanical properties and leads to poor surface finish. High carbon equivalent (CE) values and low cooling rates favor the formation of graphite and, consequently, can influence machining characteristics. Hypoeutectic irons and irons subjected to rapid solidification rates exhibit small, short flakes that disrupt the matrix to a lesser extent and are more desirable when maximum tensile properties and a fine smooth surface finish are required.
In gray iron, five basic classifications of graphite flakes ranging from Type A to E have been established jointly by ASTM and AFS. If solidification takes place at slow cooling rates in the presence of a number of potent nuclei, little undercooling occurs and Type A graphite--thick flakes with limited branching randomly distributed and oriented throughout the iron matrix--is formed. Under conditions of greater undercooling, faster solidification rates occur with less effective substrates and finer Type D graphite is formed. Type D graphite is found to reside in the interdendritic regions and is randomly distributed.
The randomly oriented Type A graphite promotes good mechanical properties and has been reported to improve machinability. Medium-size Type A graphite provides higher tensile strength and elongation than does Type D graphite, which has flakes that are fine-tipped and "more stress concentrating." In addition, Type A graphite irons also exhibit the greatest ductility. The effect of Type A graphite on mechanical properties becomes more pronounced as the CE increases. The small Type D flakes have been reported to promote a fine machined surface finish by minimizing surface pitting.
This article studies the influence of graphite morphology and matrix structure on the micromechanism of chip formation during the machining of gray iron. The role of graphite size and shape is studied at a microscopic level to determine its impact during machining. In addition, the interaction of graphite with the matrix structure and how this affects machining of gray iron is explored. Lastly, the question of graphite lubrication when machining gray irons also is examined. Through this study, foundries will better understand the effects of machining on gray iron and be able to deliver castings that machine better and are more cost-effective to the customer.
Two experimental methods were used to accomplish the objectives of the study--slow speed and quick-stop device (QSD) machining.
Slow-Speed Machining--These tests were conducted on polished and etched specimens using an orthogonal cutting machine. The machining conditions for these slow-speed tests, as well as subsequent QSD tests, are shown in Table 1.
For all tests, the cutting speed, depth of cut and rake angle were held constant. The only independent variable was the material under study. The surface deformation characteristics of the graphite and the matrix during slow machining of polished and etched specimens were examined from video footage. Quantitative analysis of the deformation and fracture of the microstructures ahead of and beneath the tool were performed directly from the video images. The chip thickness and chip length also were estimated, as well as the machining "shear angle."
QSD Machining--The second set of experiments involved dry machining tests at a normal production cutting speed on a lathe fitted with a QSD. The QSD simulated semi-orthogonal cutting and allowed the cutting tool to be rapidly removed during high-speed machining. The process leaves a representative chip still in place with the chip root undisturbed. Simultaneously, cutting force data signals were collected with the help of a dynamometer. The cutting conditions for the experiments also are shown in Table 1.
At this point, the specimens underwent the same analysis steps.
Specimen Analysis--The side face of the specimen was prepared by polishing and etching prior to machining. After QSD machining, the specimens were analyzed using a scanning electron microscopy (SEM). The photomicrographs obtained from the SEM then were used to examine chip formation, the root of the chip, the newly cut surface and the material condition ahead of and underneath the cutting tool. The fracture events around the tool also were characterized. The extent of fracture and matrix deformation ahead of and beneath the cutting tool were used to estimate the size of the machining-affected region. Cutting forces typically measured during conventional machinability studies also were obtained. The forces gave an indication of the magnitude of the stress to fracture in the primary shear plane and the resultant compressive stresses exerted on the tool.
Specimen Characteristics--The microstructures and chemical composition of the five gray irons with different matrix structures are detailed in Tables 2 and 3.
The gray iron specimens had both Type A and D graphite flakes with either pearlite or ferrite matrix structures (Figs. la-e). Specimen 1 was Type A graphite in a fine pearlitic matrix (ASTM A48 Class 40). Specimen 2 consisted of Type A graphite flakes in a coarse pearlitic matrix (ASTM A48 Class 30). Specimen 3 was similar to 2 but had longer graphite flakes and a coarser pearlitic matrix. Specimen 4 consisted of Type D graphite in a predominantly pearlitic matrix. Specimen 5 also was Type D graphite in a ferrite matrix.
Slow-Speed Machining Results
The slow orthogonal machining experiments showed that fracture events ahead of the cutting tool dominate chip formation in gray irons. The mechanisms of fracture ahead of and beneath the tool when cutting the Type A graphite specimens 1, 2 and 3 were similar. During slow-speed machining, video pictures indicated that widespread fracture events occurred along graphite flakes ahead of and beneath the cutting tool. These deformation and fracture events took place at large distances ahead of and on the underside of the tool. Separation of fracture chips from the specimen occur at frequent intervals ahead of the tool, resulting in discontinuous chips.
The deformation and fracture ahead of and beneath the tool in specimen 2 were more extensive compared to those in specimen 1. This observation may be attributed to the nature of the graphite flakes as well as the coarseness of the matrix structure. In addition, the pearlitic matrix structure for specimen 2 also was coarser than in specimen 1 and had a higher hardness.
The coarsest graphite structure was found in specimen 3. The video pictures showed that the fracture in specimen 3 occurred along the graphite flakes, and the material literally "collapsed in all directions" ahead of and underneath the cutting tool. Consequently, the deformation and fracture around the tool for specimen 3 was more extensive when compared to both specimens 1 and 2. Graphite flake length clearly influenced the scale of the fracture events occurring ahead of and underneath the cutting tool.
The slow orthogonal machining of Type D graphite specimens 4 and 5 showed similar deformation mechanisms ahead of and below the tool. During machining of specimen 4, it was observed that deformation and fracture events also occurred throughout the region ahead of the tool, resulting in discontinuous chips. Fracture occurred preferentially in the interdendritic regions where the graphite flakes were aligned. Chips came off as big chunks ahead of the tool. These flake-rich interdendritic regions were easy fracture paths that caused large chunks of material to readily separate from the base material. Deformation on the underside of the tool was present but was more limited for specimens with Type D graphite. This contributed to the improved surface finish for Type D (compared to Type A) graphite specimens.
When machining specimen 5, the video pictures showed that the matrix deformation and fracture was more extensive ahead of and beneath the tool as compared to specimen 4. The type and size of flake graphite in both specimens was the same, but specimen 4 had a pearlitic matrix while that of specimen 5 was predominantly ferritic. Since the ferritic matrix was softer and more ductile, the matrix deformation was more extensive for specimen 5 than for 4.
The general observations from the video pictures described in the previous section were investigated further by quantitative analysis. Figures 2a-d describe a model of chip formation processes in gray irons. Figure 2a shows the start of the process where the tool compressed the work material ahead of itself, thus creating an irregular fracture front that traves ahead. Microcracking is observed throughout the deformed region ahead of the tool. As the tool moves forward, chip fragments completely fracture and separate (Fig. 2b). Microcracks connect with each other, creating a shattered region where separation occurs at all graphite matrix interfaces. At the same time, a similarly damaged region with compacted fragments of the material is created below the tool.
Irregular fracture occurred at some intervals, creating craters beneath the cut surface (Fig. 2c). In such cases, the tool travels freely without cutting any material until the next chip formation and fragmentation cycle begins. A subsurface damaged region, referred to in this study as the fracture depth, is clearly noticeable. This region contains shattered and compacted fragments of the cut material that is grazed over by the tool tip as it passes. The uneven surface that is caused by irregular fracture contributes to machined surface roughness.
Figure 3 shows the various measurements taken around the tool and presented in Table 4. The fracture distances ahead of and beneath the tool are described in the diagram and table as fracture front (y) and fracture depth (d), respectively.
The chip thickness showed significant differences among the specimens. The chips obtained from Type A graphite specimens were generally thicker than those from Type D graphite. The Type A graphite specimens were deformed and fractured much more readily ahead of and below the tool during machining. Consequently, the formed chip spread out parallel to the tool face, resulting in larger chip thickness.
The behavior of Type D graphite was similar but the spread of the chips occurred to a lesser extent. This chip variation observed in gray irons violates an important assumption used in the Merchant theory of orthogonal cutting. This explains why commonly used analysis for characterizing the machining behavior of other materials cannot be blindly used to characterize the machining behavior of cast gray iron.
QSD Machining Results
Examination of the SEM photomicrographs of gray iron specimens after QSD machining revealed a complex zone ahead of and underneath the cutting tool where the material was affected. This zone is named the machining-affected zone (MAZ). The three separate regions observed in the MAZ are shown in Fig. 4--the decohesion zone, the fracture zone and the shattered zone.
In the decohesion zone (at low strain far ahead of the tool), decohesion occurs between the matrix structure and many of the graphite flakes. This microcracking eventually creates paths of easy fracture.
The fracture zone is characterized by larger cracks that are formed by the link-up of selected microcracks that also join up with cracks propagating from the tool interface. The cracks follow the graphite flake paths of least resistance and are randomly oriented and distributed.
The shattered zone has two parts--the region ahead of the tool and the region below the tool. The region ahead of the tool consists of material that is shattered into small matrix fragments from microfracturing at almost all graphite-matrix interfaces. The discontinuous chips that form are fragments, loosely held together with fractured graphite/matrix interfaces. The material below the tool also is shattered, but compressive forces exerted by the relative motion of the tool and the work cause compaction of the chip fragments, resulting in a matted structure.
Fracture Events in the MAZ Model
When machining Type A and D graphite irons, the compressive forces cause decohesion between graphite flakes and the matrix to occur in the region ahead of and below the cutting tool. On further straining, plastic deformation and fracture of the matrix occur, resulting in a network of microcracks. As machining progresses, these microcracks coalesce and form larger microcracks.
The link-up of selected small and large microcracks form the primary crack front in the fracture zone. Finally, when several crack fronts link up ahead of the tool, the material shatters into many fragments that form the discontinuous chip.
The graphite flakes for Type A specimen 1 (Fig. 5a) were randomly oriented and distributed. The fracture occurred along the flakes and the damage ahead of and beneath the cutting tool is extensive. The fracture mode for the other Type A graphite irons, specimens 2 and 3, were similar to that for specimen 1. Figure 5b shows a photomicrograph of the shattered zone for specimen 2. The three regions of the MAZ may be seen. In comparison with specimen 1, the damage ahead of and on the underside of the tool for 2 was much more extensive.
The photomicrograph showing the shattered zone for specimen 3 is presented in Fig. 5c. The fracture also occurred along the graphite flakes ahead of and on the underside of the tool, and the MAZs may be seen. Since the graphite flakes were much longer in specimen 3 than in either 1 or 2, deformation and fracture in this material occurred in all directions and were much more extensive.
A similar MAZ view for Type D graphite specimen 4 is shown in Fig. 5d. Deformation and fracture preferentially occurred in the intercellularregions containing the graphite flakes. Again, the damage ahead of and beneath the cutting tool is defined by the three zones.
The photomicrograph for specimen 5 Type D graphite in a ferrite matrix is shown in Fig. 5e. Similar fracture events occurred along the flakes with the three regions of the MAZ visible. The deformation ahead of and on the underside of the tool for Type D graphite specimens was more extensive for the ferritic specimen, than for the pearlitic specimen. The flake size of these materials was similar, therefore, the difference in fracture characteristics can be attributed to the difference in matrix structure.
MAZ Quantitative Analysis
To convert the proposed MAZ characteristics into a quantifiable and reproducible model, a method has been developed to determine zone distances from the cutting tool. The distances are obtained by taking measurements on four different planes from the MAZ photomicrographs.
The data shows that for Type A graphite, specimen 3 (with long Type A flakes and coarse pearlite matrix) had the largest deformation distances. On the other hand, specimen 1 (with short Type A flakes and fine pearlite matrix) had the shortest distances. The deformation distances for specimen 2 were in between but closer to 3, due to their similar microstructure of long flakes and coarse matrix.
Specimen 3 exhibited higher MAZ distances in the various planes just as it did in the slow-speed experiments. The longer graphite flakes and lower hardness of specimen 3 caused more deformation and easy fracture to occur than in both specimens 1 and 2. Similarly, the MAZ distances for specimen 2 were closer to those of 3 because of longer graphite flakes and lower hardness. In addition, both 2 and 3 had coarser pearlite matrix, while specimen 1 had a fine pearlite matrix structure with higher hardness.
In the case of Type D graphite irons, specimen 4 with a pearlite matrix had much shorter fracture distances than did specimen 5 with a ferrite matrix for all the planes. Moreover, the fracture distances for specimen 5 compared closely with those of the Type A graphite irons. The main influence on the fracture distances is the matrix structure. This result indicated that the higher the amount of ferrite in the matrix, the higher the ductility and the easier it is to machine the material.
If free graphite exists at the tool work interface during the machining of gray irons, it can be expected to influence the machining characteristics of these materials. To determine conclusively if free graphite was present at the cutting interface during machining, photomicrographs of the chip root for QSD specimens were evaluated. Figures 6a-c are photomicrographs that show the root of the cut chip for Type A specimens 1, 2 and 3. They all reveal the presence of free graphite at the root of the cut chip.
The photomicrographs also show evidence of graphite smearing. Figures 6d and 6e are photomicrographs of QSD specimens for Type D specimens 4 and 5. The photomicrographs show that free surface graphite is present at the root and that graphite smearing occurs during machining.
These observations confirm that free graphite exists at the tool-chip interface during the machining of all gray irons. As a result, it can be concluded that the free graphite serves an important lubrication role between the tool and the work material during machining.
The chip formation process of gray irons is fundamentally influenced by the graphite morphology and the interaction of the graphite with the matrix structure. The fracture of these materials during machining occurs along the graphite flakes. The extent of deformation and fracture in the MAZ ahead of and beneath the cutting tool depends on the size and shape of the graphite. The longer the graphite flakes, the larger the MAZ.
The machining characteristics of gray irons also are influenced by the type of matrix structure. The results in this study show that a coarse pearlitic matrix structure is more favorable to large MAZs than is a fine pearlite matrix. Similarly, larger MAZs are obtained with a ferrite matrix structure than with a pearlite matrix.
The results from the study show evidence of the existence of free graphite at the tool-chip interface during machining of gray cast irons. In then can be argued that fine, free graphite is present at the cutting boundary of the tool and the work material where it affects the local frictional conditions and, therefore, tool life. The fine graphite at the boundary can form a thin solid film that separates the tool from the work. As a result, chip shear strain is lowered and friction at the tool-chip interface is reduced. Consequently, the tool-chip interface temperatures are lowered and machinability is enhanced.
In conclusion, the detailed study of the MAZ in gray irons has revealed the fundamental mechanics of chip formation in gray irons. Following are specific conclusions drawn from this study:
1. Fracture of Type A and D flake graphite irons during machining occurs along the graphite flakes, forming discontinuous chips. The longer the graphite flakes, the longer the fracture distances ahead of and below the tool.
2. Three regions that describe the MAZ for flake graphite irons are identified--the decohesion zone, the fracture zone and the shattered zone.
3. The sizes of the MAZs were influenced by both the matrix structure and the graphite morphology of the gray iron.
4.Free-surface graphite is present at the tool-chip interface of flake graphite irons, and it is concluded that graphite plays a major role in influencing frictional and chip formation conditions during the machining.
This article was adapted from a paper (9980) presented at the 1999 AFS Casting Congress and is available from AFS Publications at 800/537-4237.
Cutting Conditions for Slow-Speed and QSD Machining Slow-speed Parameter machining QSD machining Cutting speed: 2 in./min 120 feed/min Depth of cut: 0.015 in. 0.015 in. Width of cut: 0.125 in. 0.125 in. Tool material: High-speed steel High-speed steel Rake angle: 0[degrees] 0[degrees] Magnification: X160 -- Microstructure Characteristics for the Five Gray Iron Specimens Specimen Class Description 1 ASTM A48 Class 40 Type A graphite size 5-6 in. pearlite 2 ASTM A48 Class 30 Type A graphite size 4-5 in. coarse pearlite 3 -- Type A graphite size 2-3 in. coarse pearlite 4 -- Type D graphite in mainly pearlitic matrix 5 -- Type D graphite in ferrite with some pearlite Chemical Composition of the Five Gray Iron Specimens Element 1 2 3 4 5 Carbon 3.390 3.360 2.680 3.260 3.070 Silicon 2.570 2.190 1.950 2.720 2.620 Manganese 0.804 0.437 0.705 0.282 0.157 Sulfur 0.044 0.051 0.044 0.010 0.012 Nickel 0.031 0.020 0.041 0.015 0.039 Chromium 0.039 0.031 0.057 0.023 0.032 Copper 0.073 0.040 0.084 0.636 0.040 Phosphorus 0.023 0.020 0.017 0.013 0.020 Molybdenum 0.006 0.006 0.010 0.006 0.006 Magnesium NR NR NR NR NR Tin (Sn) NR NR NR NR NR NR: Not Reported Fracture Measurements Taken from the Video for the Five Gray Iron Specimens Fracture Fracture Contact Chip Front, y Depth, d Length Thickness Spec. (mean [pm]3[sigma] in.) (mean [pm]3[sigma] in.) x (in.) [t.sub.c] (in.) 1 0.033 [pm]0.0003 0.013 [pm]0.0004 0.011 0.024 2 0.034 [pm]0.0005 0.015 [pm]0.0009 0.014 0.023 3 0.051 [pm]0.0010 0.021 [pm]0.0006 0.020 0.030 4 0.030 [pm]0.0005 0.009 [pm]0.0003 0.014 0.020 5 0.031 [pm]0.0003 0.013 [pm]0.0005 0.009 0.021 Shear Angle Spec. (deg.) 1 25 2 24 3 22 4 28 5 26
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|Author:||Cohen, Paul H.|
|Date:||May 1, 2000|
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