Drilling of high quality features in green powder metallurgy components

Materials Science and Engineering A 458 (2007) 195–201

Drilling of high quality features in green powder metallurgy components Etienne Robert-Perron a,∗ , Carl Blais a , Sylvain Pelletier b , Yannig Thomas b a

b

Department of Mining, Metallurgical and Materials Engineering, Universit´e Laval, Quebec City, Que., Canada Powder Forming Research Group, Industrial Materials Institute, National Research Council of Canada, Boucherville, Que., Canada Received 10 October 2006; received in revised form 10 December 2006; accepted 12 December 2006

Abstract With the increasing demand for sinter-hardened PM components, there is a growing need to solve the poor machining behaviour that characterizes them. Approaches based on green machining appear promising to reduce machining costs and compete favourably with other shaping processes. Advancements in binder/lubricant technologies have led to the development of high green strength systems that enable green machining of high quality features. This study deals with the drilling of through holes in high green strength PM components. Design of experiments has been used to optimize the drilling parameters as well as tool selection. The usage of optimum cutting conditions led to the generation of holes having excellent geometrical conformance. © 2007 Elsevier B.V. All rights reserved. Keywords: Powder metallurgy; Green machining; Drilling; Green strength; Binder/lubricant

1. Introduction Powder metallurgy (PM) is a process that allows the fabrication of components with complex geometries. At first, a discussion about the machining of PM components may appear as a paradox since powder metallurgy is said to be a “near net shape” process, which minimizes the need to use secondary shaping operations such as machining [1]. Nevertheless, due in part to the limitations of the process (for example, the difficulty to generate undercuts in axial pressing) as well as strict dimensional conformance, machining operations are necessary for approximately one third of the ferrous PM parts produced in North America [2]. Furthermore, one of the major challenges facing PM is the need to produce parts having improved mechanical properties. Thus, harder and/or tougher microstructures (martensite and/or bainite) have to be used to reach the required performances. Such microstructures are obtained nowadays by using low alloyed steel powders better known as sinter-hardenable powders. Such base material allows manufacturers to harden parts during the last stage of the sintering cycle by adequately controlling the cooling rate in the sintering furnace. By using the latter approach, no secondary heat treating operation is necessary to obtain the harder and/or tougher microstructures required. This proves as

∗

Corresponding author. Fax: +1 418 656 5343. E-mail address: [email protected] (E. Robert-Perron).

0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.050

a tremendous advantage since it significantly reduces production costs while eliminating the usage of quenching media that are detrimental to the environment. On the other hand, such microstructures make the components harder to machine leading to accelerated cutting tool wear and increased production costs, which tend to counterbalance the gains obtained with sinterhardenability. PM components generally show lower machining behaviour than those produced from wrought materials [3]. The poor machinability of PM components is usually explained by three postulates: interrupted cutting, lower thermal conductivity and the presence of hard particles/phases [2–6]. These characteristics of PM parts lead to increased temperatures at the tool/chip interface and promote wear mechanisms on the cutting tool such as erosion, diffusion and deformation [7]. A prospective avenue to circumvent the poor machinability of sinter-hardened PM parts is green machining. This process involves the machining of components while they are in their “green state”, i.e. before sintering. The main advantage of this process is its ability to triple the production rate of the machining operations [8]. Furthermore, green machining extends tool life since the strength of green compacts comes from the binder, the mechanical interlocking and, to some extent, cold welding, instead of metallurgical bonds as it is the case for sintered parts. Moreover at that stage, the harder phases have not yet been formed since such transformations only occur during the cooling stage of the sintering cycle. Thus, the cutting forces

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Table 1 Drill type specifications ISO no.

Material

Coating

Helix angle

Point geometry

Point angle (◦ )

KHSS10316 KWCD00350 KWCD00461 K105A02500

High speed steel Carbide K600 Carbide K600 Carbide K10

Uncoated Uncoated Uncoated Uncoated

Standard (28◦ ) Low (15◦ ) High (35◦ ) Standard (28◦ )

Split point Conventional Conventional Three flutes

135 118 118 130

applied on the tool as well as the heat generated are kept to a minimum. Green machining is not a straightforward procedure. Steel parts pressed to a green density of 6.8 g/cm3 have typical green strength values of 12–17 MPa (∼1750–2500 psi). These values are insufficient to allow proper holding of the parts in the chuck of a lathe or a machining center and would lead to catastrophic failure during machining [9]. However, recent developments, such as warm compaction and binder/lubricant technologies, allow us to obtain improved green strength values. Warm compaction consists in pressing a preheated powder at a temperature typically ranging from 90 ◦ C to 150 ◦ C to obtain green strengths about four times higher as those provided by room temperature compaction [10]. The binder/lubricant technologies consist in substituting the conventional lubricant (e.g. ethylenebisstearamide (EBS)) for polymeric based compounds that promote mechanical interlocking and cold welding between particles, which in turn strengthens the part. Moreover, these polymeric lubricants have the ability to form a strong network that strengthens the green specimens during compaction and/or by performing a curing treatment at approximately 190 ◦ C in air for 1 h. The green strength values of such components are increased by a factor of two to five compared to those obtained from conventional powder blends [11,12]. Several authors investigated the possibility of machining such high green strength PM components especially in the case of drilling operations. The machining performances of green PM components are usually based on the surface finish and on the average width of breakouts formed near the edges of machined surfaces [13–16]. These breakouts are formed by particle pull out during machining and are typically found in the vicinity of outlet edges; defined as the last edge seen by the cutting tool as it leaves the component. Surface finish and edge integrity usually improve as the feed rate decreases; a value of 0.0254 mm/r is suggested for drilling through holes while the surface speed does not seem to affect the quality of the outcome [13–15]. According to those studies, holes drilled following the optimum feed rate will present a surface finish (Ra ) of 1–2 ␮m and a width of breakouts near the outlet edge of 250 ␮m. Other studies investigated the machining behaviour of high green strength PM parts for a turning operation in interrupted cutting [16,17]. Again, a low feed rate is suggested (0.0127 mm/r or 0.0254 mm/r) while the surface speed does not seem to affect the results. The objective of this study was to improve the machining behaviour of green PM components while drilling through holes. This was achieved by using a high green strength powder system

and optimizing the cutting parameters. Design of experiments (DOE) and analysis of variance (ANOVA) were used to optimize surface speed, feed rate and drill type. Moreover, dimensional change after sintering of holes drilled using the optimum condition was characterized using a coordinate measuring machine (CMM). 2. Experimental procedure 2.1. Material investigated A powder system was produced based on Quebec Metal Powders 4601 sinter-hardenable powder (Fe–1.8 wt.% Ni–0.5 wt.% Mo–0.2 wt.% Mn) to which was added 2.0 wt.% Cu and 0.6 wt.% graphite. The latter premix follows the denomination FLC4608 based on MPIF Standard 35 [18]. Lubrication was done using 0.65 wt.% of a proprietary binder/lubricant (FLOMET HGSTM ) specifically adapted for high green strength and green machining [12]. This mix was pressed into rectangular plates (10.8 cm × 10.8 cm × 1.6 cm) to a green density of 7.00 g/cm3 . These samples were submitted to a curing treatment in air at 190 ◦ C for 1 h to increase their green strength and their machinability in terms of surface finish and edge integrity. 2.2. Green drilling experiments The machining operation performed on green plates was the drilling of through holes. The drilling operation was done using a computer numerically controlled (CNC) machining center (Republic Lagun, model VCM 4824). The drilling process was performed dry, i.e. no coolant was used. Thrust force measurement was performed during drilling using a dynamometer placed underneath the sample (Kistler, type 9443B). Four drill types (diameter 6.35 mm), manufactured by Kennametal, were investigated; their specifications are presented in Table 1. There are two cutting parameters in drilling: surface speed and feed rate. Table 2 summarizes the range of the latter parameters considered in this study. Since the matrix of experiments involves 64 tests, design of experiment (DOE) using orthogonal arrays and signal to noise (S/N) ratios was used [19,20]. The selected orthogonal array is a L16 , which allows us to reduce the number of tests to 16, as shown in Table 3. Such orthogonal array allows the determination of the relative influence of each parameter studied. Moreover, the usage of S/N ratios instead of averages permits to optimize the cutting condition while minimizing the variance of the latter. The equation used for calculating S/N ratio is the following smallerthe-better type, where n is the number of measurements and y

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Table 2 Cutting parameters and levels investigated for the drilling experiments Parameters

Levels

Drill type Surface speed (m/min (rpm)) Feed rate (mm/r)

1

2

3

4

KHSS10316 50 (2500) 0.0254

KWCD00350 80 (4000) 0.0762

KWCD00461 110 (5500) 0.1270

K105A02500 140 (7000) 0.1778

is the measurement values:  n  S 1 2 = −10 log10 yi N n

(1)

i=1

Series of 16 holes were drilled in the same specimen and the test was repeated on three different plates for reproducibility. The usage of DOE permitted to minimize the number of experiments while allowing the determination of the optimum level for each parameter as well as the relative influence of the latter on the variation of the results. This is achieved by performing an analysis of variance (ANOVA) [19]. Nevertheless, the selected array does not permit to identify potential interactions between the different parameters studied. Therefore, the assumption was made that there is no interaction between the parameters studied. The validity of the latter assumption comes from the fact that the parameters studied are operation based and could be judged mutually independent [19,21]. 2.3. Characterization of machining performances Three machinability criteria were used to optimize the machining performances of drilling through holes in green PM steel components: thrust force, average width of breakouts and surface finish. The average width of breakouts is the mean width of the pulled out particles measured on the face of the component surrounding the outlet edge. The average width of breakouts was measured only on the outlet edge since the width of breakouts

surrounding the inlet edge was independent of the machining parameters (approximately 70 ␮m for each hole characterized). Fifty measurements were performed on the outlet edge using an image analysis routine (Clemex Vision) in optical microscopy. Four micrographs were acquired for each outlet hole to construct a mosaic, as shown in Fig. 1. Such an approach allowed us to significantly increase the resolution of our measurements. Fig. 1A shows a typical micrograph used for generating the mosaic while Fig. 1B presents the result of the image analysis routine developed to characterize the average width of breakouts. The surface finish of the freshly machined surface inside the hole was measured using a profilometer (Mitutoyo, SJ-201P). Two different measures were performed: Ra and Rz [22]. 2.4. Dimensional change Plates where holes were drilled following the optimum cutting conditions were sintered at 1120 ◦ C in 90% N2 –10% H2 atmosphere during 25 min. Fast cooling rate (1.5 ◦ C/s from 650 ◦ C to 400 ◦ C) was used to obtain a hardened microstructure and improved mechanical properties. Such cooling rate is typical of what is currently used in the industry to sinter sinter-hardenable powders. Tempering (175 ◦ C/1 h in air) was performed to increase the toughness of the specimen, as suggested by MPIF Standard 35 [18]. Dimensional change was measured for the length of the plates and for the diameter of holes using a CMM (Mitutoyo, Machine model CMM BRT-A504).

Table 3 Orthogonal array L16 selected for the experiments

3. Results and discussion

Experiment no.

3.1. Green strength

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Parameters and levels Drill type

Surface speed

Feed rate

1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

1 4 3 2 4 3 2 1 3 2 1 4 2 1 4 3

Table 4 presents the green strength of the systems investigated measured according to MPIF standard 15 [23]. The green strength was measured on transverse rupture strength (TRS) bars (length 3.18 cm, width 1.27 cm and thickness 0.64 cm) both for the as pressed (FLOMET HGS as pressed) and the cured (FLOMET HGS cured) condition. TRS bars were compacted at room temperature to a green density of 7.00 g/cm3 (same conTable 4 Green strength of the materials investigated Systems investigated

Green strength (MPa)

EBS FLOMET HGS as pressed FLOMET HGS cured

16.9 20.5 52.2

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Fig. 1. (A) One of the four micrographs used for generating the mosaic and (B) results of the image analysis routine developed for characterizing the average length of breakouts (unpolished-unetched). In the case presented above, the average width of breakouts is 180 ␮m. Table 5 Results obtained after drilling through holes in green PM plates Experiment no.

Thrust force (N)

Average width of breakouts (␮m)

Surface finish Ra (␮m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

69 562 381 181 900 455 242 93 520 274 103 577 306 146 447 337

74 540 412 167 845 502 240 110 489 293 98 550 307 138 461 330

71 589 382 178 876 436 253 103 499 278 116 597 301 144 476 329

209 197 209 141 >500 >500 227 126 124 96 156 195 401 120 >500 >500

158 182 155 150 >500 >500 138 139 131 115 156 240 389 133 >500 >500

ditions as plates). The green strength of a conventional system using 0.75 wt.% of EBS was also measured for comparison. As shown in Table 4, there is a strong increase of the green strength by using the FLOMET HGS system, especially in the cured condition. The green strength of the latter system is more than three times larger than the conventional system. 3.2. Optimization of cutting parameters for drilling through holes Table 5 presents the four sets of results (thrust force, average width of breakouts and surface finish Ra and Rz ) characterized in each hole of the three plates. Note that the results for the average width of breakouts for experiments 5, 6, 15 and 16 are over 500 ␮m. In order to conserve the maximum information from these experiments, and to ensure a significant ANOVA, they were used as is in the data analysis. Table 6 presents the S/N ratios calculated from the values in Table 5 for each experiment. These S/N ratios were averaged by parameter level to determine the optimum cutting condition for drilling through holes in a green PM component, as shown in Table 7 and in Fig. 2. The optimum cutting condition for each parameter is the closest value

166 171 149 154 >500 >500 145 135 124 124 137 275 264 126 >500 >500

3.1 4.8 4.9 4.3 4.5 3.4 3.2 3.6 5.9 5.2 2.6 5.6 5.5 2.9 5.9 5.9

Rz (␮m) 3.6 4.9 5.3 4.2 4.5 3.5 4.3 4.7 5.1 5.8 2.1 6.4 5.8 3.1 7.1 5.4

3.3 5.1 4.8 4.5 5.0 4.9 4.6 4.6 5.6 3.8 3.6 6.1 5.9 2.9 6.7 6.1

23.0 36.8 34.9 35.3 37.4 32.5 22.8 26.4 34.2 24.5 22.7 43.5 47.8 26.7 44.7 38.3

24.1 35.3 37.3 34.5 35.3 33.2 36.3 35.9 36.3 42.3 21.1 49.2 42.2 27.3 44.6 42.4

23.6 46.3 35.2 36.6 37.0 37.8 37.7 36.3 39.0 36.0 34.0 39.2 46.5 25.2 41.6 44.7

Table 6 S/N ratios Experiment no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Thrust force

−37.1 −55.0 −51.9 −44.9 −58.8 −53.4 −47.8 −40.2 −54.0 −49.0 −40.5 −55.2 −49.7 −43.1 −53.3 −50.4

Average width of breakouts

Surface finish Ra

Rz

−45.1 −45.3 −44.8 −43.4 −54.0 −54.0 −44.8 −42.5 −42.0 −41.0 −43.5 −47.6 −51.0 −42.0 −54.0 −54.0

−10.4 −13.8 −14.0 −12.7 −13.4 −12.0 −12.2 −12.7 −14.9 −14.0 −9.0 −15.6 −15.1 −9.4 −16.4 −15.3

−27.4 −32.0 −31.1 −31.0 −31.3 −30.8 −30.4 −30.4 −31.3 −30.9 −28.5 −32.9 −33.2 −28.4 −32.8 −32.4

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Table 7 ANOVA as a function of machinability criteria Criteria

Parameters

Levels

Sum of square

Relative influence (%)

Thrust force

Drill type Surface speed Feed rate

−47.2 −49.9 −40.2

−50.0 −50.1 −47.8

−49.7 −48.4 −52.4

−49.1 −47.7 −55.6

19 17 534

3 3 94

Average width of breakouts

Drill type Surface speed Feed rate

−44.6 −48.0 −43.3

−48.8 −45.6 −45.1

−43.5 −46.8 −48.7

−50.3 −46.9 −50.2

126 12 122

48 5 47

Ra

Drill type Surface speed Feed rate

−12.7 −13.5 −10.4

−12.6 −12.3 −13.5

−13.4 −12.9 −14.1

−14.1 −14.1 −14.8

5 7 46

9 12 78

Rz

Drill type Surface speed Feed rate

−30.8 −30.8 −28.7

−30.7 −30.5 −31.4

−30.9 −30.7 −31.4

−31.7 −31.7 −32.2

4 3 28

11 9 80

Surface finish

to zero since the desired results for each criterion is zero, i.e. minimum thrust force, minimum width of breakouts and lowest number of Ra and Rz . Considering the results obtained from the ANOVA in Table 7, the feed rate is responsible for 94% of the variation of the thrust force. Therefore, a low feed rate of 0.025 mm/r is suggested for reducing the thrust force while the variation of the latter criterion is unaffected by the drill type and the surface speed. The variation of the average length of breakouts is mainly caused by the drill type and the feed rate, which contribute for 48% and 47%, respectively. Considering the average width of breakouts, the optimum cutting conditions for the sets of parameters studied are: KWCD00461 drill type, feed rate of 0.0254 mm/r. The last criterion studied was the surface finish, which is mostly affected by the feed rate. The latter parameter is responsible for almost 80% of the variation of both the Ra and Rz criteria. Thus, a value of 0.0254 mm/r is suggested for improving the surface finish of the machined surfaces. The drill type and the surface speed

showed much lower contributions on the variation of the results, i.e. approximately 10% each. The results presented above indicate that among the tools studied, the drill KWCD00461 is the best suited for generating high quality through holes in green PM steel components since it minimizes particle pull out near the outlet edge. The good results obtained with this drill are explained by the high helix angle, which is adapted for the machining of soft ferrous material [24]. The second parameter studied was the surface speed. Within the range studied, the latter shows a very small contribution on the final outcome of the drilling process regardless of the machinability criterion. Therefore, a high surface speed of 140 m/min is suggested for increasing productivity. The last parameter investigated was the feed rate. The control of this parameter is essential for producing high quality features in green drilling and a value of 0.0254 mm/r is suggested. The influence of this parameter on the quality of the final outcome is explained by the increased thrust force when increasing the feed rate. At high feed rate,

Fig. 2. Graph of parameter effects. (A) Thrust force, (B) average width of breakouts, (C) surface finish (Ra ), and (D) surface finish (Rz ).

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Table 8 Comparison of the predicted and the experimentally measured results Machinability criteria

Results predicted by the ANOVA

Results measured experimentally

Relative error of the ANOVA (%)

Thrust force (N) Average length of breakouts (␮m)

92 101

107 115

16 14

Surface finish Ra (␮m) Rz (␮m)

3.7 27.6

3.7 25.3

0 8

Fig. 3. Micrographs of an outlet edge of a hole machined under the optimum cutting conditions: (A) typical mosaic used to characterize the average width of breakouts (unpolished-unetched) and (B) typical edge showing the breakouts (SEM).

particle pull out in the area near the outlet edge is promoted since the bonds between the particles and the binder/lubricant are broken due to insufficient strength to sustain the thrust force. The optimum cutting conditions have not been tested simultaneously; therefore a confirmation test was needed to validate the approach taken. Using the ANOVA presented in Table 7, it is possible to predict the results using the optimum cutting conditions presented above and comparing them to the empiric results obtained with the confirmation test [19,20]. The predicted values, as well as the empiric results, are shown in Table 8. As expected, the empiric results are the best in terms of the machinability criteria considered in our study (ref. Table 5). Moreover, the average with of breakouts obtained is more than half of the minimum value obtained in previous studies [13–15], which was 250 ␮m. This result is explained by the optimization of the machining conditions as well as the type of the binder/lubricant used. The differences between the predicted and the empiric values for the thrust force and the average length of breakouts are 16% and 14%, respectively. The relatively large differences, when compared to those obtained for the surface finish, are explained by the large range of results used for modeling the process. Indeed, from Table 5, standard deviations of 214 N and 156 ␮m are measured for the thrust force and the average length of breakouts, respectively, which are 60% of the respective mean value. The empiric measurements of the surface finish are closer to the predicted values due to the lower standard deviation of the results: 1.2 ␮m and 7.4 ␮m for Ra and Rz , respectively, which are 25% and 20% of the respective mean value. Moreover, it is important to note that the thrust force measured during drilling through holes in green PM components is approximately 10 times lower than that measured during drilling sintered steel with

similar composition [24,25]. The lower thrust force associated with the green drilling ensures longer tool life when compared to the machining of sintered components [24]. Thus, the cutting edges of the drill stay sharp for a longer time which minimizes particle pull out due to tool wear (dull cutting edge) and yields higher quality through holes. Fig. 3 shows a typical outlet edge of a hole machined under the optimum cutting conditions. Fig. 3A presents the mosaic used for the characterization of the average width of breakouts while Fig. 3B shows a typical micrograph in scanning electron microscope (SEM) near the outlet edge. Fig. 4 presents the machined surface inside the hole using a SEM. It is seen that most of the particles were cut during drilling. However, some were pulled out which explains the rougher surface finish measured on surfaces machined in green state when compared to

Fig. 4. Typical surface finish after drilling a hole in a green component.

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that of surfaces machined post sintering. Indeed, surface finish of 1 ␮m (Ra ) is often measured after drilling sintered PM steel [26].

201

Leander F. Pease III for compaction and sintering and Maude Larouche from Universit´e Laval for the image analysis. References

3.3. Dimensional control The dimensional change measured on the length of the plates showed that sintering induced swelling of 0.28% from green size, which is typical for this type of powder [27]. The diameter of the holes drilled in green state using the optimum cutting conditions exhibited a swelling of 0.07%. This dimensional change is comparable to that of holes incorporated into the components during compaction, which is 0.06% (as measured on rings: i.d. 12.7 mm). As shown in Fig. 4, there is no densification near the machined surface after drilling, which explains the similar dimensional change as the one measured on inside diameter of rings. 4. Conclusions • It has been shown that high quality through holes can be drilled in green PM steel components (FLC-4608 powder type). To increase edge integrity and surface finish, the cutting conditions suggested are the following (based on the range of parameters investigated): o Drill type: KWCD00461 (diameter 6.35 mm), produced by Kennametal or equivalent. o Surface speed: 140 m/min (7000 rpm) o Feed rate: 0.0254 mm/r • Following these conditions, the thrust force measured during drilling is 107 N, the average width of breakouts near the outlet edge is 115 ␮m and the surface finish inside the hole is 3.7 ␮m and 25.3 ␮m for criterion Ra and Rz , respectively. • The dimensional change of the diameter of holes drilled in green PM components is identical to that measured for holes incorporated in the components during compaction. Therefore, the green machining process does not affect the dimensional change of features during sintering. Acknowledgements The authors wish to thank Dr. Sylvain St-Laurent from Quebec Metal Powders for the fruitful discussions as well as Dr.

[1] R.M. German, Powder Metallurgy and Particulate Processing, Metal Powder Industries Federation, Princeton, 2005. [2] D.S. Madan, Adv. Powder Metall. Part. Mater. 8 (1995) 55–67. [3] J.S. Agapiou, M.F. DeVeries, Int. J. Powder. Metall. 24 (1) (1988) 47–57. [4] R.J. Causton, Adv. Powder Metall. Part. Mater. 8 (1995) 149–170. [5] R.J. Causton, C. Shaede, Adv. Powder Metall. Part. Mater. 7 (2003) 154–170. [6] J.L. Seefelt, D.W. Smith, P. Machmeier, Adv. Powder Metall. Part. Mater. 1 (1990) 323–340. [7] E.M. Trent, P.K. Wright, Metal Cutting, 4th ed., Butterworth-Heinemann, Boston, 2000. [8] E. Robert-Perron, C. Blais, S. Pelletier, Y. Thomas, APMI/MPIF Powder Metallurgy Machinability Seminar, 2005. [9] F. Chagnon, L. Tremblay, S. St-Laurent, M. Gagn´e, SAE Technical Paper no. 1999-01-0337, 1999, pp. 71–76. [10] S. St-Laurent, F. Chagnon, Adv. Powder Metall. Part. Mater. 3 (1997) 3–18. [11] L. Tremblay, Y. Thomas, Adv. Powder Metall. Part. Mater. 2 (1999) 141–156. [12] L. Tremblay, J.E. Danaher, F. Chagnon, Y. Thomas, Adv. Powder Metall. Part. Mater. 12 (2002) 123–137. [13] A. Benner, P. Beiss, Euro PM2000 Conference on Material and Processing Trends for PM Components in Transportation Proceedings, 2000, pp. 101–109. [14] O. Andersson, A. Benner, Adv. Powder Metall. Part. Mater. 6 (2001) 16–28. [15] M. Ramstedt, O. Andersson, H. Vidarsson, B. Hu, Adv. Powder Metall. Part. Mater. 12 (2001) 151–162. [16] A. Benner, P. Beiss, Adv. Powder Metall. Part. Mater. 6 (2001) 1–15. [17] E. Robert-Perron, C. Blais, Y. Thomas, S. Pelletier, M. Dionne, Mater. Sci. Eng. A: Struct. 402 (2005) 325–334. [18] Metal Powder Industries Federation, Standards 35, Princeton, 1998. [19] M.S. Phadke, Quality Engineering Using Robust Design, Prentice-Hall, Englewood Cliffs, 1989. [20] G. Taguchi, Quality Engineering in Production Systems, McGraw-Hill, New York, 1989. [21] P.J. Ross, Taguchi Techniques for Quality Engineering, McGraw-Hill, New York, 1988. [22] Mitutoyo, Surface Roughness Tester SJ-201P User’s Manual no. 99MBB079A series no. 178. [23] Metal Powder Industries Federation, Standard 15, Princeton, 1998. [24] A. Salak, M. Selecka, H. Danniger, Machinability of Powder Metallurgy Steels, Cambridge International Science Publishing, Cambridge, 2005. [25] C. Blais, G. L’Esp´erance, Powder Metall. 45 (1) (2002) 39–45. [26] G. L’Esp´erance, APMI/MPIF Powder Metallurgy Machinability Seminar, Cleveland, 2005. [27] http://www.qmp-powders.com.