Wear Mechanism when Machining Compacted Graphite Iron

E. Abele, A. Sahm, H. Schulz (1) Institute of Production Management, Technology and Machine Tools (PTW) -

University of Technology, Darmstadt, Germany

Abstract The industrial application of compacted graphite iron in the automotive industry is taking a rather long time due to its uneconomic machinability, because of a significant decrease in tool life. After six years of holistic research of the PTW in cooperation with foundries, manufactures and material scientists, the wear mechanism was understood and clarified: Sulphur in the microstructure of compacted graphite iron has direct influence on the formation of a manganese-sulphur layer on the cutting edge. For machining gray cast iron this layer protects the cutting edge against abrasive wear. Therefore the design of cutting tools for the machining of CGI must consider the absence of manganese-sulphur layer.

Keywords: Cast iron, Machinability, Tool wear

1 INTRODUCTION Gray cast iron (CI), compacted graphite iron (CGI) and ductile iron (SGI) differ by the shape of the graphite particles. As shown in figure 1 gray iron is characterized by randomly oriented graphite flakes, while the graphite particles in ductile iron are present as individual spheres. In contrast to this, the graphite particles in compacted graphite iron appear as vermicular particles. The particles are elongated and randomly oriented as in gray iron, however they are shorter and thicker and have round edges. Compacted graphite iron appears vermicular when viewed two-dimensional. But deep etched scanning electron micrographs show, that the individual vermicals are connected in a complex coral- like morphology. Together with the rounded edges and the irregular bumpy surfaces of the compacted graphite iron, the coral morphology leads to a strong adhesion between the graphite and the iron matrix. While the smooth-surfaced graphite flakes in gray iron promote crack initiation and growth and thus render the material relatively weak and brittle, the entangled compacted graphite morphology eliminates the natural cleavage paths thus providing strength and stiffness. In comparison with ductile iron (SGI) CGI has better

Figure 1: Cast iron graphite morphology (I: gray cast iron, m: compacted graphite iron, r: ductile iron)

damping properties, better thermal conductivity and good castability [ I ] . Based on this characteristics CGI fulfills the requirements for many light weight- constructions especially automotive engine blocks [2]. The main handicap for introducing CGI into automotive volume production is that it is distinctly more difficult to machine at high cutting speeds than gray cast iron. Tests carried out on a engine block transfer line with parameters for gray cast iron have shown a tremendous decrease in tool life [3]. For high cutting speeds in combination with the use of PCBN as cutting material, tool life is reduced by approximately 50 % in case of interrupted cut operations, like milling. A reduction of the tool life to 10 % of cast iron was recognized for continuous operations like cylinder boring. Due to this decrease in tool life a research program was stated at the PTW in order to find appropriate cutting materials and cutting speeds for economic milling (interrupted cut) and longitudinal turning (continuous cut). Workpieces for milling and turning were casted and designed in order to have the same microstructure as in the engine blocks. During the milling tests the cutter was equipped with only two inserts. The turning test pieces were machined by longitudinal turning with a single insert. Cutting tests were conducted with commercially available insert grades and geometries. The end-of-life criteria for all test conditions was defined as 300 pm flank wear. Benchmark shows results carried out in face milling and longitudinal turning for gray cast iron, low pearlitic CGI (-60 % pearlite content) and high pearlitic CGI (-90 % pearlite content) (Figure 2a, 2b). Low cutting speeds (150-250 rdmin) with conventional coated carbide tools provides approximately 50 % of the tool life of gray cast iron in both milling and turning. Similarly, milling at high cutting speeds (400-800 rdmin) with ceramic or polycrystalline cubic boron nitride also provides approximately 50 % of the gray iron tool life [4, 51. Under these circumstances, the difference in tool life of gray cast iron and CGI is generally depending on the increase of mechanical properties of the workpiece material.

they occur in milling. The diffusion behaviour of compacted graphite iron was investigated in a high temperature oven containing a 99.99 % pure helium atmosphere. Gray cast iron and compacted graphite iron specimens were maintained in contact with PCBN for 6 hours at temperatures of 700 "C and 1050 "C. These temperatures reflect the temperatures occurring on the cutting edge when machining gray iron and compacted graphite iron respectively at high cutting speeds. At both temperatures, elements including boron, tungsten, titanium, were found to diffuse into the specimen material. This result indicating diffusion of elements from the PCBN binder and the CBN crystals themselves. Although the rates of diffusion were found to be higher at increased temperatures, no significant difference regarding the diffusion behaviour could be found

cutting material, cutting speed v, [dmin] between gray iron and compacted graphite iron. This may be no real surprise as there are strong chemical compositions similarities between gray and compacted graphite iron. Thermal gravimetry investigations show oxidation of both binder phase and the CBN grains. A distinct mass gain of the whole insert due to oxidation took place at temperatures exceeding 750°C in a oxygen containing atmosphere. Applying continuous heat conditions up to 950°C by a heating rate of 300 Wh lead to a final mass gain of 0.75 %. Figure 3 shows the oxidation of the binder in form of titanium dioxide (Ti02) needles and the presence of micro cracks at the oxidation sites.

Figure 2a: Tool life for face milling (ap = 2 mm; f, = 0.15 mm; no coolant; i q = 75"; ya = -7"; h, = -8.5")

cutting material, cutting speedv, [mlmin]

Figure 2b: Tool life for longitudinal turning (ap = 0.15 mm - CBN 8 Ceramic 10.2 mm - Carbide;

f = 0.3 mm; no coolant; K = 75"; y = -6"; h = -4")

However, the most significant difference in the machinability of CGI occurs in high cutting speed (>400 mlmin) continuous cutting operations such as turning. These cutting speeds are particularly important as they represent cylinder boring cutting speed in modern transfer lines. Under these conditions, the ceramic and PCBN tool lifes can be 10-20 times lower than those obtained with gray cast iron, which was confirmed under transfer line conditions. For this reason, the recent development work has concentrated on the most critical continuous cutting operation such as turning to simulate cylinder boring.

2 WEAR MECHANISM In conventional machining the abrasive wear is nearly constant over the temperature, adhesive wear is concentrated at lower temperatures [6, 7, 81. At high speed machining diffusion and oxidation effects are increased [9, 10, 11, 121. Pin-on-disc tests were performed in order to determine the abrasive wear behaviour of CGI in comparison to CI. The tests show that, for equal hardness, 33 % less abrasive wear can be expected for CGI compared to gray iron. Adhesive or frictional behaviour with PCBN was measured to be 15 % greater for CGI. This combined effect of increased abrasion resistance and friction helps to explain the 50 % reduction in tool life for CGI at lower temperatures as

Figure 3: Formation of titanium dioxide (Ti02) under oxygen atmosphere and 950°C

All these fundamental investigations provide no answer to the phenomena which can be seen for gray cast iron turning by using of PCBN. Figure 2b shows a decrease in tool life (L, - C125 8 PCBN @ 800 mlmin > L, - C125 8 PCBN @ 400 rnfmin) at reduced cutting speed, which does not fulfill the theory established by Taylor [12]. A potentially very significant difference when machining gray iron (C125) and compacted graphite iron (CGI) at high cutting speeds using PCBN is in the formation of a protective layer on the cutting edge, displayed by Figure 4. To illustrate this, machining investigations at cutting speeds of 10, 100, 400 and 800 rnfmin were performed for both gray cast iron and compacted graphite iron. At low cutting speeds of 10 mlmin and 100 rnfmin no difference on the cutting edges used on both workpiece materials could be observed. No indication of layer formation was visible. However at cutting speeds of 400 rnfmin a layer could be identified on the cutting edge, used for gray iron machining. This layer becomes more dense at 800 mlmin cutting speed. Even at the high cutting speed no layer formation could be found in case of machining compacted graphite iron.

shape and material properties, the most significant difference between gray iron (C125) and compacted graphite iron CGI, respective nodular cast iron SGI is the Sulphur content. Gray cast irons typically contains 0.08- 0.12 per cent sulphur. However, because compacted graphite particles are only stable at low oxygen and sulphur contents, CGI is produced in the range of 0.005- 0.025 per cent Sulphur. In gray cast iron, the Sulphur reacts with the Manganese to form manganese sulphide (MnS) inclusions, which are generally less than 10 pm in size. The soft and pliable MnS inclusions can potentially lubricate or even form protective layers on the cutting edges of the insert when machining. In contrast to gray iron, however, CGI is produced by adding magnesium to the liquid iron to consume both sulphur and oxygen. Because magnesium is a strong sulphide former, magnesium sulphide (MgS) inclusions are formed preferentially to manganese sulphide (MnS). To confirm this theory a desulphurized gray cast iron was machined. Figure 6 illustrates the results for longitudinal turning of desulphurized CI in comparison to high an low pearlitic CGI and regular CI.

Figure 4 top: Layer formation when machining C125, - bottom: No layer formation when machining CGI

for identical tool life (longitudinal turning, PCBN, ap = 0.15 mm; f = 0.3 mm;

no coolant; K = 75"; y = -6"; h = -4")

In a further step the layer was characterized by Electron Scanning Mass Analysis (ESMA) and Electron Dispersive X-Ray. By applying these methods, it was found out, that the observed layer contains the elements Manganese and Sulphur. The presence of an MnS-layer on an insert used for gray iron machining and an abrasively worn flank face on the CGI used insert illustrate this difference. As a result of the formation of a protective layer, the gray iron tool life increases with increasing cutting speed. This counter-initiative behavior is presumably due to the growth and densification of the protective MnS-layer. In contrast to this unique behavior observed in gray cast iron, other materials such as steels, ductile iron and CGI exhibit the expected behavior of decreasing tool life with increasing cutting speeds. This characteristic is displayed by Figure 5. The reason for this behaviour is related to the material composition. Beyond the obvious differences in graphite

cutting speed v, [rnlrnin]

Figure 5: Taylor-diagram of cast iron at longitudinal turning with PCBN (ap = 0.15 mm; f = 0.3 mm; no

coolant; K = 75"; y = -6"; h = -4")

cutting material, cutting speedv, [mlmin]

Figure 6: Influence of sulphur content on tool life for longitudinal turning

(ap = 0.15 mm - CBN 8 Ceramic/ 0.2 mm - Carbide; f = 0.3 mm; no coolant; K = 75"; y = -6"; h = -4")

The results displayed in figure 6 clearly show, that the gray iron tool life is significantly decreased as the sulphur content is reduced from the standard level to CGI-like-levels of 0.008 %. The remaining difference in tool life 0.008 % gray iron and standard CGI can be accounted for by the differences in mechanical properties. It is therefore proposed that the approximately ten times difference in tool life between CGI and gray cast iron at continuous cutting with elevated cutting speeds and the use of PCBN is primarily accounted for by the presence of a protective MnS layer during machining gray cast iron. The remaining differences in tool life may be accounted for by the differences in the mechanical workpiece properties. Investigations have been carried out [3] to produce high- sulphur CGI by adding MnS powders immediately prior the casting. However, the strong thermodynamic affinity of magnesium to prevent MnS from forming in the high sulphur tests and the added MnS powder to MgS.

3 CUTTING TOOLS Due to the absence of MnS in compacted graphite iron an economic machining in continuous cut at elevated cutting speed is not possible with the present available cutting materials. However, the machining investigations carried out with coated carbide inserts and cutting speed in the range of 100 to 120 mlmin, show an acceptable tool life. In order to compensate the productivity when

applying this technology for the most critical cylinder boring operation the number of inserts in the boring tools must be increased by the factor PCBN-cutting speed was decreased. This new tooling concept has been developed together with leading tooling vendors and the automotive industry for both roughing and finishing. In general two different types of multiple insert tools have been successful. Both, the roughing and the finishing tools can be characterized by the staggering of the inserts. By applying a staggered tool some of the inserts cut relatively deep while the remaining axially staggered inserts make a more shallow secondary cut. This concept protects the final inserts and allows the tool maintain geometry and roundness requirements. Tools without staggering have also been tested and have equally shown the ability to satisfy roundness criteria by sharing the load among several inserts. In a long term test the performance of a finishing boring tool was tested on a machining centre with CGI engine blocks over 1300 bores. Over the hole range of 1300 bore the requirements of the automotive manufacturers regarding geometry and roughness could be met. At high cutting speeds, tools with rotating inserts [14, 151 show high productivity. Available tools are equipped with silicon-nitride-ceramic inserts. Due to the rotation of the insert the thermal load acts during a short time. During turning tests tool life was increased by the factor of 300 compared to conventional tools with fixed inserts.

4 CONCLUSION With at least 75 % higher strength, 40 % higher elastic modulus and approximately double of the fatigue strength CGI is the material of the future for diesel engine manufacturers. But compacted graphite iron is more difficult to machine than gray cast iron. When using coated carbide as cutting material for milling and turning the reduction in tool life is commensurate with the increase in mechanical properties. In contrast to this machining at elevated cutting speeds and the use of PCBN operations with continuous cut such as cylinder boring, leads to a drop in tool life up to 10-20 times. By results of PTW it was found out, that the absence of MnS in compacted graphite iron hinders the formation of a protective layer on the cutting edge. The presence of MnS in gray cast iron leads to a protective layer, when machining at cutting speeds higher than 400 rnfmin using PCBN. In contrast to common workpiece material the tool life decreases when increasing cutting speed. Taylor-graph analysis carried out for cast iron illustrate the increase of tool life when increasing the cutting speed for gray cast iron machining at continuous cutting. In an additional investigation evidence was produced by machining desulphurized gray iron with gray iron machining parameters. The results show a decrease in tool life by the factor of ten and confirm the MnS layer theory. In order to realize high productive machining coated carbides perform acceptable tool life at cutting speeds in a range between 100 and 120 rnfmin. To compensate the loss in productivity, special cutting tools for cylinder boring a p p I i cati o n s a re necessary.

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