By M. Gagné and C. Labrecque
Rio Tinto Iron & Titanium Inc.
Tracy, Quebec, Canada
Published in the 1999 AFS Transactions

ABSTRACT
     The evaluation of the machinability of a material is usually a lengthy process.  However some indices namely drilling thrust force and torque may be used to rank the machinability of several materials considered for a given application.  This method was used to characterize the machinability of ferritic Ductile Irons.  The results were then compared to those obtained for several competitive materials.  It has been shown that ferritic Ductile Iron is easier to machine than most competing alloys.  The results also indicated that the structural characteristics of a material play a major role in its machining behavior.  

1.  Introduction
     Although the machining of Ductile Iron components is minimized by the casting process itself, secondary operations such as drilling, milling, turning, etc. are often required.  In some cases machining may represent a significant fraction of the total cost of the component.  Therefore understanding the machinability behavior of Ductile Iron is instrumental in maintaining and improving its competitiveness.
     The evaluation of tool life is certainly the most accurate method to assess the machinability of materials.  However, the time required to fully characterize a material is often a major obstacle.  A rapid characterization method might be sufficient for the foundry man to evaluate the machining characteristics of his product or for a casting user to approximate the machining behavior of a material as a function of its mechanical properties and/or structural characteristics.  For instance, the measurement of the drilling thrust force and torque at constant feed rate and rotating speed provides sufficient qualitative information to judge on the quality of the material or to carry out a preselection amongst many candidate materials⁽¹⁾.
     In this paper, a machinability evaluation technique using drilling thrust force and torque as machinability indices is described.  The relationship of these properties to tool wear is discussed for ferritic Ductile Irons and compared to those obtained for other engineering materials, namely wrought steels, powder metal steel and gray iron.

2.  Machinability Evaluation Procedure
     Figure 1 presents a schematic of the experimental set-up designed to measure the drilling thrust force and torque.  It consists of a high power drill press with automatic feed rate control.  The apparatus is equipped with a specially designed specimen holder capable of monitoring the torque applied on the tool and the thrust force transmitted to the test specimen.  The feed rate and the rotating speed are continuously recorded during the operation.  The acquisition system enables the measurement of the four parameters nine times per second.  The data is then transmitted to a computer for processing.  As shown in Figure 2, the rotating speed and feed rate do not significantly vary during a test.
     Typical thrust force and torque curves obtained when drilling a hole with this set up are shown in Figure 3.  As the drill penetrates the work piece, the thrust force and torque quickly increase to reach a steady state.  In most cases, both the thrust force and torque remain at relatively constant levels and then drop when the feed is interrupted.  Occasionally, the torque may increase towards the end of the test probably because of an increased difficulty to expel the chips from the hole.  The average thrust force and torque in the steady state portion of the curves are then calculated for each hole.  The drill is removed at regular intervals to evaluate qualitatively the land wear and the accumulation of the machining debris on the tool.  Drilling chips are also examined.
     When different materials are characterized, one drill is used per material.  In order to minimize the impact of the variation of the drills characteristics on the measurements, a qualification process was established.  The method consists in drilling two holes to a depth of 2.5 cm in a standard material at a feed rate of 0.12 mm/rev and a cutting speed of 2220 rpm.  To be accepted, the thrust force and torque measured on a drill must be within ± 5% of the average values of a lot of twenty drills minimum.  In this study, the cutting tools were high speed steel drills with a helix angle of 118° and a diameter of 6.35mm.  The cutting speed and feed rate were 2220 rpm and 0.12 mm/rev.
     Figure 4 presents an example of curves that can be obtained with such an evaluation method for materials with different machinability levels⁽²⁾.  The thrust force obtained for material B resembles a typical tool wear curve⁽³⁾, with three different stages.  During the first stage, the thrust force increases rapidly which corresponds to the first stage of wear, i.e. the breakdown of the initial cutting edge and the development of a finite land wear on the tool.  This is followed by a region of linear progression of the thrust force (or torque) similar to the steady state of the wear process during which a uniform wear rate is noticed.  Then, the thrust force increases very rapidly and reaches a level at which the drill fails: this is the final accelerated wear stage.  If the machinability of the material is marginal as for material A, the thrust force (or torque) rapidly reaches a high value at which the tool is exhausted without apparent steady-state stage.  On the other hand, however, it may happen that a material exhibits excellent machinability, which makes the determination of tool life a lengthy procedure, such as for material C.  In such a case the thrust force (or torque) measured can be used as a relative indicator of machinability.  In the example of Figure 4, material C, which exhibits the lowest thrust force, is the most machinable alloy followed by materials B and A.

3.  Materials Characterization
     As stated earlier, this study was aimed to the comparison of the machinability of various materials competing with Ductile Irons on the same applications.  As shown in Tables 1 and 2, the test matrix was divided into two series of experiments.  In the first one, the effect of structure and composition on the machinability of materials (two Ductile Irons and one steel) of comparable hardness was investigated.  The objective of the second series was to compare the machinability of the best Ductile Iron to that of competitive materials, namely gray iron, wrought steel and powder metal copper steel.
     The first group of materials (Series A) includes two Ductile Irons with a hardness of 152-156 BHN.  However, as seen in Tables 1 and 2, the composition and structure of these materials are different.  Material D-1 was based on HPI added with FeMn and cast in RIT-Technology pilot plant according to procedures described elsewhere⁽⁴⁾.  The samples consisted in 2.54 cm keel blocks whose structure is described in Table 2.  The second set of Ductile Iron specimens consisted in commercial castings with a section size varying between 1.5 and 3 cm.  The casting was fully ferritic, Table 2, due its high silicon content (2.8% Si) and exhibited a high ferrite micro hardness (Table 2).  For comparison purpose, the machinability of a steel of comparable hardness was also investigated.  This material was a 11L17 steel in which lead and MnS particles act as machinability enhancers.  The composition of the steel is listed in Table 1 and its structure shown in Figure 5.
     The second series of materials, which are also described in Tables 1 and 2, consisted in alloys (P/M steel and gray iron) competing for applications similar to the one for which material D-2 was used.  The gray iron material was a class 40 commercial casting (same type of parts as D-2) whose structure consisted in type A flake graphite embedded in a fully pearlitic matrix.  The castings had a Brinell hardness of 197 BHN.  For comparison purpose, 1045 steel with the same hardness as the gray iron castings was also included in the test matrix with the objective of identifying the role of graphite as machinability enhancer.  Finally, powder metal steel whose structure is shown in Figure 6 was also tested.  Such a FC-0205 material is typical of a P/M alloy competing for parts currently made of cast irons.  The P/M specimens were pressed to a density of 6.8 g/cm³ and sintered at 1120°C for 20 minutes under a controlled atmosphere.  

4.  MACHINABILITY CHARACTERIZATION

4.1  Series A Materials
     Figure 7a presents the change in average drilling thrust force as a function of the drilled depth for the materials of Series A, i.e. for the Ductile Irons and the 11L17 steel with hardnesses of 152-156 BHN.  The three materials display curves of similar shape.  With these materials, the initial increase shown for forged P/M steel in Figure 4 is less pronounced implying that the initial breakdown of the tool probably occurs over a long period of time in these materials due to their relatively good machinability.  However, the torque curves shown in Figure 7b exhibit the typical behavior of the wear curves, i.e. a rapid increase followed by a steady state.  This indicates that the torque applied on the tool is more sensitive to the initial change of the cutting surface (sharpness or build up of debris on the tool) than the thrust force.
     It is a common practice to relate the machinability of a material to its hardness, i.e. the harder the material, the more difficult it is to machine.  However, as shown in Figure 7, although the materials of series A exhibit the same bulk hardness, their machinability as measured by drilling thrust force and torque is different.  The comparison of the curves obtained for the two Ductile Iron materials reveals the following:

i)  the drilling thrust force and torque required to machine the fully ferritic Ductile Iron (D-2) are larger than those needed for the Ductile Iron containing about 15% pearlite (D-1).  After a drilled depth of 80 cm, the difference is about 100 N (20%) for the thrust force and 0.6 N-m (40%) for the torque.

ii) the rates of increase (slope) of the drilling thrust force and of the torque are higher for the fully ferritic iron (D-2) than for the D-1 iron.

     Such differences are related to the following factors.  First the production of the fully ferritic casting D-2 required to alloy the iron with 2.8% Si.  The solid-solution hardening effect of Si resulted in a harder ferrite, which requires higher thrust force and torque.  Second, because ferrite is a ductile phase, it tends to deform rather than break during machining.  The occurrence of a limited amount of brittle pearlite dispersed in the intercellular regions of the D-1 alloy resulted in the rupture of the chips during their formation.  It then limits the adhesion of debris on the tool face and reduces the friction force induced by long chips.  This is illustrated in Figures 8 shows that more long curled chips are formed in the fully ferritic D-2 material than in the D-1 Ductile Iron.  This phenomenon is mainly responsible for the higher torque measured when drilling the fully ferritic material.

iii) A larger nodule count in Ductile Iron should be a positive factor vis-à-vis machinability, the dispersion of graphite particles in the structure ensuring a more continuous lubrication at the tool/chip interface.  However, during these tests, such an effect has probably been dominated by the previously discussed factors.  It nevertheless indicates that a nodule count of 75 is sufficient to ensure a continuous lubrication at the tool/chip interface.

     It is seen in Figure 7a that the 11L17 steel requires the highest thrust force of this series of materials but the lowest torque.  The absence of soft graphite nodules in the steel probably makes more difficult the penetration of the matrix by the drill and results in a higher thrust force.  However, the dispersion of the machinability enhancing compounds in the steel matrix ensures a better lubrication at the tool/chip interface and during the removal of the chips, which results in a lower torque value.  It is nevertheless worth noting that the machinability of Ductile Iron competes with that of a steel specially engineered for applications requiring substantial machining.

4.2 Series B Materials
     The machinability results obtained on the materials of series B are presented and compared to those of Ductile Iron D-1 in Figure 9.  The lowest thrust force is displayed by the D-1 material followed by the gray iron (G-1), the P/M steel (PM-1) and the 1045 steel (S-2).  As in Series A, the characteristics of the phases present in the material control the level of thrust force required to drill the materials:

i) D-1 material, which is significantly harder than the PM-1 material, is found easier to drill.  As seen in Figure 6, a P/M steel with a density of about 6.8 g/cm³ contains 10 to 12% porosity.  Under the stresses applied by the drill, the pores collapse which results in the densification of a layer of material under the tool, Figure 10, and in the strain hardening of the material⁽⁵⁾.  Micro hardness of this densified layer can be as high as 283 VHN⁽⁵⁾ which exceeds the hardness of the 1045 steel.  The occurrence of graphite in Ductile Iron further contributes to ease the cutting of the material by acting as a solid lubricant and as a chip breaker.  Note however that the addition of solid lubricants such as BN⁽⁶⁾ or MnS⁽⁵⁾ to P/M steels improves their machinability.  For example an addition of 0.3% MnS to the PM-1 steel would reduce the thrust force by 40% or to 450 N after 80 cm drilled depth under the drilling conditions used in this study⁽⁵⁾, which is closed to that of the D-1 material.  However, PM-1 steel would have UTS and elongation values significantly lower than those of the D-1 material (450 vs. 550 MPa, 2 vs. 15%).

ii) The pearlitic G-1 gray iron requires a higher thrust force than the Ductile Iron D-1 material.  However as the drilled depth increases, the thrust force for G-1 increases at a significantly larger rate than for D-1(1.1 vs. 0.5 N/cm), resulting in a higher wear rate of the cutting tool.  This is confirmed by the examination of the cutting edge of the tools which reveals a land wear approximately double on the tool used for drilling in G-1, Figure 11.

iii) Although displaying the same hardness as the G-1 material, the 1045 steel (S-2) exhibits a thrust force 40-50% higher than the one observed for G-1.  The rate of increase of the thrust force as a function of the drilled depth is about the same for both materials, implying comparable wear rates.  However, as seen in Figure 12, drilling debris tend to adhere on the cutting face of the tool used in the S-2 material while this phenomenon is minimal when drilling in the gray iron, Figure 11b.  The build-up of debris, which is prevented in gray iron by the occurrence of graphite, is responsible for the higher thrust force measured for the 1045 steel.

     The ranking of Series B materials using the torque curves shown in Figure 10 is slightly different than the one obtained with the thrust force curves.  The gray iron (G-1) required the lowest torque followed the Ductile Iron (D-1), the PM steel (PM-1) and the 1045 steel (S-2).  Figure 13 compares the chips generated when drilling the first and fiftieth (80 cm) holes in the gray iron and the 1045 steel.  As expected, the gray iron chips are small and non-oxydized; no significant difference is seen between the chips from the first and fiftieth holes.  Those generated in the steel are long and their level of oxidation increases significantly as the number of holes increases.  Graphite in G-1 material plays a two fold role: it contributes to chip breaking while being also an efficient lubricant that eases the evacuation of the chips.  This limits the heating of the tool as indicated by the absence of a detectable amount of oxide on the chips.  Nodular graphite in Ductile Iron plays a similar role although being a less efficient chip breaker than flake graphite.

5.  CONCLUSION
     The machinability of Ductile Iron was characterized using the forces generated during drilling under controlled conditions as machinability indices.  The results were compared to those obtained on various other engineering materials competing with Ductile Iron on many applications.  Within the limits of this study, the following conclusions can be drawn:

1. For materials of similar hardness, the micro structural characteristics may significantly influence the machinability.

2.  A fully ferritic Ductile Iron casting containing a high silicon concentration is less machinable than a casting of similar hardness containing about 20% pearlite.  The softer ferrite and the chip breaking effect of pearlite are responsible for such an effect.

3. Drilling in ferritic Ductile Iron requires lower thrust force than in 11L17 machinable steel with a slightly higher torque.  Nodular graphite in Ductile Iron plays a role similar to that of lead and MnS in the 11L17 steel to improve machinability.

4. Ferritic Ductile Iron with pearlite at cell boundaries machines significantly better than a FC-0205 P/M steel containing no machinability enhancer; the addition of such additives to the P/M steel makes the two materials competitive machinability wise.

5. Ferritic Ductile Iron machines better than class 40 gray iron or 1045 steel although the torque measured when drilling in gray iron is slightly lower.

6. Flake graphite is a more efficient machinability enhancer than nodular graphite.  

7. Drilling thrust force and torque are indices that can be used to characterize the machinability of materials.  

References

1. F. Chagnon and M. Gagné, SAE Paper 980634, SAE Conference, Detroit, Feb. 1998.

2. M. Gagné and F. Chagnon, Powder Metallurgy World Congress, Granada, Spain, Oct. 1998.

3. D.F. Moore, Principles and Applications of Tribology, Pergamon International Library, Oxford, 1975.

4. A. Trudel, M. Gagné and F. Lavallée, AFS Transactions, vol. 104, 1996, pp. 123-133

5. M. Gagné and J.A. Danaher, International Conference on Powder Metallurgy and Particulate Materials, Las Vegas, June 1998.

6. M. Gagné, Advances in Powder Metallurgy, MPIF, Princeton. N.J., 1989, pp. 365-375.

List of Figures 

• Figure 1 - Schematic of the Machinability Evaluation Set-up.

• Figure 2 - Recordings of Rotating Speed and Feed Rate during a Drilling Test.

• Figure 3 - Recordings of Thrust Force and Torque during a Drilling Test.

• Figure 4 - Examples of Thrust Force Curves Obtained in Powder-Forged Materials as a Function of Drilled Depth (Reference 2).

• Figure 5 - Unetched Structure of the 11L17 Steel (Material S-1).

• Figure 6 - Typical Etched Structure of the FC-0205 P/M Steel (PM-1).

• Figure 7 - Change in a) Thrust

• Figure 8 - Drilling Chips Obtained when Drilling the First Hole in a) Material D-1 and b) Material D-2. Force and b) Torque as a Function of Drilled Depth for Materials of Series A.

• Figure 9 - Change in a) Thrust Force and b) Torque as a Function of Drilled Depth for Materials of Series B.

• Figure 10 - Densified Layer under the Tool after Drilling in the PM-1 Material.

• Figure 11 - Land Wear Developed after 80 cm Drilled Depth on Tools Used for a) Ductile Iron D-1 and b) Gray Iron G-1.

• Figure 12 - Land Wear and Build-up of Debris on the Cutting Edge of the Tool Used for Drilling in the 1045 Steel (S-2) after 80 cm Drilled Depth.

• Figure 13 - Drilling Chips Obtained when Drilling in Material G-1 ( a) 1^st Hole, b) 50^th Hole) and in Material S-2 ( c) 1^st Hole, d) 50^th Hole).

Table 1 - Description and Chemical Composition of the Materials  
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Table 2 - Structure and Hardness of the Materials
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*  ferrite
**  ferrite-pearlite.