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A New Advanced Ceramic for Dry Machining
A New Advanced Ceramic for Dry Machining N. Narutaki (Z),Y. Yamane, S. Tashima, H. Kuroki Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, 739, Japan Received on January 6,1997
ABSTRACT Concerns toward safety of the work environment as well as the environment in general are compelling industry to adopt 'green' or 'dry' machining, i. e. without the use of any cutting fluid. Since. some of the benefits of the cutting fluids are not going to be available in dry machining, need exists for the development of a more refractory, tougher, chemically wear resistant, and hard cutting tool materials. In this paper, the synthesis and evaluation of a new alumina ceramic tool material using very pure (99.99%) and submicron grain size (0. 22 vm) alumina powder with practically no binder sintered at low temperature (1230OC) (i.e. without the need for HIP'ing) are presented. This material is found to be stronger and harder than conventional HIP'ed alumina ceramic. The new alumina ceramic was found to be more wear resistant and fracture resistant (against both mechanical and thermal shock) than conventional alumina in dry turning and milling of gray cast iron and S45C carbon steel. Key words: Ceramic, Tool, Machining
1. INTRODUCTION It is evident that soon environmentally clean or 'green' machining will be considered no longer as a desirable option but a necessary one for sustainable manufacturing enterprise. Concern for the environment, health and safety of the operators, as well as the requirements to enforce the environmental protection laws and occupational safety and health regulations are compelling industry to consider 'dry' machining as a viable alternative instead of using cutting fluids. Byrne and Scholta [4] and others [ 1 2 ] have proposed some strategies to address this important problem. The machining costs (labor and overhead) in the U. S. alone is estimated at $300 billion/year (7).The costs associated with the use of cutting fluids is estimated to be about 16 % of the manufacturing costs (41 which is many more times than the labor and overhead figures quoted above. Consequently, elimination on the use of cutting fluids, if possible; can be a significant incentive. The key benefits of dry machining is more than mere saving of costs, namely, the provision of a healthy manufacturing environment, worker safety, protection from adverse effects of the chemicals, and overall protection of our precious environment. Of course, the lubricants or cutting fluids are used in the first place for several important reasons. Chief amongst them are: 1. to remove the heat generated in cutting, 2. to reduce friction (lubricating action), 3. to remove chips from the cutting region, 4. to protect the workpiece against rust and other undesirable reactions, 5. to prevent built-up in cutting or the metal chips adhering to the tool surface as in milling where the chips that are hot are bonded to the cutting edge which are subsequently cooled as the tool leaves the workpiece. Of course, if we were to practice dry machining or the use of lubricants is banned for one reason or the other, we have to address all the above issues satisfactorily. In reality, it may be possible to address only some of the benefits of cutting fluids in dry machining. For example, the use of new coated tools or new advanced ceramics may provide additional heat resistivity but may not provide all the other benefits of the cutting fluid. Of course, the cutting fluid industry is very
Annals of the ClRP Vol. 46/7/7997
sensitive to this issue and are reformulating new compositions that are more environmentally friendly, for example without Pb, S, or CI containing compounds which were used extensively in the past to improve the lubricating action [1,2]. It is clear that this huge industry will make significant improvements to make the cutting fluid more acceptable at least for applications where use of cutting fluid is an imperative. It is entirely possible that cutting fluids in the future may be used more selectively, or use sparingly, or not use at all.
For a long time, because of the limitations on the tool materials available, the use of cutting fluids was considered an essential integral part of the machine tool system. All the ill effects associated with the use of cutting fluids were considered as a necessary evil. Methods were developed to minimize its effects although progress was far less than desired. The detrimental effects of the cutting fluids include the cost of the cutting fluid system, i.e. the fluid itself, pumping systems, collection and filtering system, storage and disposal, and sometimes a recirculating system etc; the physiological effects on the operator, namely, toxic vapors, unpleasant odors, smoke fumes, skin irritations (dermatitis), or effects from bacteria cultures from the cutting fluid; and its overall effect on the environment. Consumption of the cutting fluids has been reduced drastically by using mist lubrication. However, mist in the industrial environment can have a serious respiratory effect on the operator. Consequently, standards are being set to minimize this effect. Use of cutting fluids will become more expensive as stricter enforcements of new regulations and standards are imposed leaving no alternative but to consider 'dry' machining. 'Dry' machining connotes that the above mentioned desirable features are no longer available. The options available under these conditions include 1. the use of very high positive rake angles (ca +30°) on submicron cemented carbide tools which would reduce the overall cutting energy significantly [5], 2. the use of tools that are more refractory and therefore can withstand the hightemperatures generated in cutting, 3. development of
43
coatings on the cutting tools that can withstand the high temperatures andlor provide a lubricating layer for reduced friction, O r 4. reduce the cutting speed SigfliflCantly t0 obtain the desired to01 life and eCOnOmiCS of machining. The high temperatures (ca 1oooOc) generated in machining especially at high cutting speeds, necessitate the use of a refractory cutting tool that can withstand the high temperatures and to provide long tool life [3.8.9.12.13]. In this paper, the synthesis and performance evaluation of a new submicron sized alumina based advanced ceramic towards the goal of dry' machining are presented [6.10.11]. Though alumina ceramic tool has the lowest toughness among the ceramics, it has the advantages of high hot hardness, high heat resistance, and low reactivity with steel. The lower toughness is no longer a serious limitation as techniques such as whisker reinforcement, presence of second phase particles are available for toughening brittle materials. In fact alumina has the highest stability amongst ceramics in relation to Fe. It is well known that grain size and impurities segregating at the grain boundaries have great influence on strength and performance of a ceramic tool. The new alumina ceramic tool used in this study is made from a very fine and pure alumina powder (0.22 um of particle size and 99.99% purity) without any additives. Use of finer grain size enabled lower sintering temperatures. Also, conventional sintering was sufficient to produce a dense body thereby reducing the cost of manufacturing. As will be shown, this material exhibited superior cutting performance in terms of wear resistance and fracture resistance compared to commercial, conventional HIP'ed (hot isostatic pressing) alumina. 2. DEVELOPMENT OF A NEW ALUMINA CERAMIC TOOL To produce high.strength ceramics, grain size must be as small as possible and grain boundary as pure as possible. With the advancements in the synthesis of materials, such as sol-gel technique, it is possible to produce very small grain size (nanometer size) and very pure material (99.9996) economically without the need for comminution. Grain growth during sintering takes place especially at higher sintering temperatures. To inhibit grain growth, sintering aids, such a MgO are added but they tend to form low temperature glassy phases which segregate at the grain boundaries. While MgO is a classic grain growth inhibitor, it also reduces the grain boundary strength and promotes intercrystalline fracture. This results in reduced strength in proportion to the amount of additives. Therefore, decreasing the amount of additives is a very important consideration for strengthening ceramics. However, if alumina powder can be sintered at fairly low temperature where grain growth hardly takes place, it will not be necessary to use these additives. In this investigation, very fine (0.22 pm of particle size) and pure (99.99%) alumina powders are used [6,10,11]. This size of powder has very large specific surface area (ca 15.1 m2/g), so that the compact from these particles has very large surface energy. Due to the assist of this large energy, the required sintering temperature is significantly reduced compared to sintering of conventional material with large grain size (several pm). In this study, it was found sufficient to sinter the alumina powder at 123OOC. Use of such a low sintering temperature means that additives are not necessary, and consequently grain boundary is kept free from impurities. In order to produce a ceramic compact, a slip from alumina powders was poured into a metal mould and compacted using a high-speed centrifugal compaction
44
(HS-CC) process in which the slip was segregated into a compact and water by applying large centrifugal force corresponding to 10-20 x 103 G's as shown in Fig.1, Details of the slip are given in Table 1, The compacts were dried at 4OoC for 4 hours and at 100°C for 4 hours, then sintered at 123OoC for 1.5 hours under atmospheric conditions. Table 2 gives the mechanical propenies of ceramic tools. To compare the performance of the new ceramic tools, it is compared with a conventional HIP'ed ceramic. HU, HUHIP, and AW (W 80) in Table 2 are the new alumina ceramic tool developed here, HIP'ed new alumina ceramic tool, and HIP'ed commercially available alumina ceramic tool. respectively. Figures 2 (a) to (c) are typical SEM micrographs of the fracture surfaces of a commercial HIP'ed alumina (AW), new submicron, pure alumina (HU), amd HIP'ed new alumina (HU-HIP) ceramics, respectively. The average grain size, measured from the fracture surface, was 0.7 pm for HU, 2 p m for AW, and 3 pm for HU-HIP. The HU ceramic material being very pure is found to be void free except for some very fine microporosity (<0.1 um). Since no grain growth inhibitors were used in this material, the HIP'ed material showed considerable grain growth due to higher operating temperatures and longer processing times. Also, the fracture of the AW tool was found to be predominantly intercrystalline while that of the HU tool to be transcrystalline. Table 1 Details of Slip
Slip concentration, mass06 Dispersion agent, mass% (Poly-carboxylic ammonium) Binder (Acrylic polymer), mass% Dispersion time, hours Dispersion temperature, OC Viscosity, mPa s
75 0.6 0.1 16-18 12-15 100-300
Table 2 Mechanical Properties of Ceramic Tools Property 3 point bend strength (TRS), MPa Vickers hardness, kg/mm2 Fracture toughness. MPalm l2 Density, gkm3
AW
HU
HU-HIP
780
1330
825
1650
2100
1450
3.5
3.3
3.4
3.94
3.95
3.97
3. MACHINING TEST RESULTS AND DISCUSSION
Dry turning and milling tests were conducted on a gray cast iron (continuously cast) (FC250-CC) and a S45C (or AlSl 1045) carbon steel workmaterials. Figures 3 (a) and (b) show the progress wear (average VB and maximum VB"' flank wear) of the three ceramic tools [commercial HIP'ed alumina (AW), new submicron, pure alumina (HU), and HIP'ed new alumina (HU-HIP) ceramics], tested in turning of gray cast iron. For convenience, the cutting conditions used are also included in the figures. It can be seen from Figures 3 (a) and (b) that wear of HU (and HUHIP) tool is significantly lower (half or more) than the conventional alumina ceramic (W80). A similar trend was observed in the feed range of 0.21 -0.50 mm/rev. Figure 4 shows SEM micrographs of the nose, rake face, and
clearance face of the ceramic tools after dry turning (cutting time: 5 min.) of cast iron. Examination of the micrographs show very little wear on the new ceramic (HU) fool material compared to the commercial ceramic (AW) tool or the large grain sized HIP'ed tool material. It was also found that the worn surface of the commercial alumina ceramic (AW) to be very rugged compared to the new ceramic (HU) tool. lntercryalline fracture and consequent dislodgement of grains appears be responsible for the rugged surface of commercial AW tool with large grain size. In contrast, with the new alumina ceramic (HU) tool, wear seems to be due to transcrystalline fracture. The higher wear rate of commercial alumina ceramic compared to the new alumina ceramic shown in Figure 3 lends further support. The differences in the wear modes as well as the wear rates of these tool materials suggest that the superior wear resistance of HU tool is not only due to the higher hardness but also due to higher bond strength between the grains due to pure grain boundary with no additives in the case of new alumina ceramic. Figures 5 and 6 show progressive average flank wear (VB) of the three ceramic tools tested (AW, HU, and HUHIP) ir! turning of S45C steel at two different depths of cut (d=l mm and 1.5 mm). The superior wear resistance of the new alumina ceramic (HU) compared to commercial alumina (AW) is very clear at the lower depth of cut (Figure 5). It can also be seen that the commercial alumina fractured after 1 min. cutting due to the higher depth of cut used (d=1.5 mm) while the new alumina ceramic continued to perform well even after 5 min. of cutting (Figure 6). This shows clearly the improved wear resistance as well as the fracture resistance of the new alumina ceramic over conventional HIP'ed alumina. Figure 7 shows SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry turning (cutting time : 5min for HU and HU-HIP and 1 min for AW) of S45C steel. The depth of cut for AW and HU was 0.21 mmlrev. It can be seen that the wear on the commercial alumina tool was considerable resulting in fracture after 1 min of cutting. The wear on the new alumina ceramic was gradual with no fracture even after cutting for 5 min. Table 3 shows the results of the face milling tests on an AlSl 1045 steel. Each test was carried out three times under the same cutting conditions (as indicated by the repetition of the letters P andor F in Table 3), and if a tool can cut the work material for a.definite length (480 mm) with no fracture, then higher feed rate was applied. It is clear from Table 3 that new alumina ceramic (HU) tool has exhibited larger resistance against fracture or chipping than the conventional alumina ceramic tool (AW).
(cutting time : 4 mins) of S45C steel. Examination of the micrographs show very little wear on the new alumina ceramic (HU) tool or new alumina HIP'ed tool (HU-HIP) material while severe wear on the commercial ceramic (AW) tool. In the case of commercial alumina (AW) tool, chipping was found on the cutting edge after 2 rnin. cutting. while no such chipping is found in the case of HUHIP tool. The worn surfaces are found to be very similar to those of turning, namely, rugged surface on the AW tocl and smooth surface on HU-HIP tool, indicating larger intergranular fracture and dislodgement of grains in the case of commercial alumina and smaller transgranular fracture in the case of new alumina ceramic developed in this investigation. The superior performance in milling of the new alumina ceramic over the commercial alumina ceramic (Figure 8) also indicates that the new ceramic material is more wear resistant and fracture resistant against thermal and mechanical shocks than the commercial alumina. As for the fracture resistance of cutting tool in intermittent cutting, such as face milling, it is said that fracture toughness and heat cracking resistance are the most important characteristics. Considering the fact that AW tool and HU-HIP tool have almost the same fracture toughness as shown on Table 2, heat cracking resistance of these tools seems to be different, since heat cracking resistance is influenced by bonding strength between particles of the tool.
4. CONCLUSIONS 1.
Very pure (99.9996) fine grain alumina powder with an average particle size of 0.22 pm, with no additives was compacted under high speed centrifugal forces and sintered at low temperatures (123OOC).
2.
Cutting performance of the newly developed alumina ceramic tool material and commercial HIP'ed alumina tool material were evaluated in turning and in face milling tests using gray cast iron and S45C carbon steel workmaterials. The new ceramic tool material showed not only superior wear resistance but also superior fracture resistance compared to commerciai white alumina ceramic tool material.
3.
The lower sintering temperature and the ability to compact the pure, submicron grain alumina powder by sintering instead of HIP'ing or hot pressing reduces the cost of producing these tool materials significantly.
4.
The superior performance of the newly developed fine grain, pure alumina ceramic over the conventional alumina ceramic in dry turning and milling of gray cast iron and S45C carbon steel was demonstrated, thus paving the way towards dry machining.
Table 3 Results of Fracture Resistance Tests in Face Milling Feed rate, mmltooth 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 P: no fracture
REFERENCES AW
HU
PPP PPP PPP FFF
PPP PPP PPP PPP PPP PPP PPF PFF FFF
F: fracture
Figure 8 shows SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry milling
1.
- -- - ---- ,
1994, "Dry Machining's Double Benefit," Machinery and Production Engineering, 14-20 2. Aronson, R. B., 1995, "Why Dry Machining," Manufacturing Engineering, 114:33-36 3. Brinksmeier, E. and S. Bartsch, 1988, "Ceramic Tools - Material Characteristics and Load Types Determine Wear Mechanisms,' Annals of CIRP, 37/1:97-100 4. Byrne, G. and E. Scholta,l993, "Environmentally Clean Machining Process A Strategic Approach," Annals of CIRP, 42/1:471-474 5. Crawford, J.H. and M. E. Merchant, 1953, "The Influence of Higher Rake Angles on Performance in Milling," Trans ASME, 75561-566 6. Koike, J.., Tashima, S., Wakiya, S., Maruyama, K. and H.Oikawa, 1996, "Mechanical Properties and Micro-structure of Centrifugally Compacted Alumina
-
45
and HlPed Alumina," Materials Science and Engineering, 220A:26-34 7. Komanduri, R., and J. Desai. 1983, " Tool Materials." Encyclopedia of Chemical Technology, 23:273-309 8. Narutaki, N., Yamane, Y., and K. Hayashi,l991, "Cutting Performance and Wear Characteristics of an Alumina-Zirconia Ceramic Tool in High-speed Face Milling," Annals of CIRP, m : 4 9 - 4 2 9. Narutaki, N.. Yamane, Y.. Hayashi, K., and T. Kitagawa. 1993. " High-speed Machining of lnconel 718 with Ceramic Tools," Annals of CIRP, 42/1:103106 10 Tashima, S.. Sumida. N., and H. Kuroki,1992. "Centrifugal Compaction of Submicron High Purity Alumina Power." J of the Japan Society of Powder and Power Metallurgy, 39:39-43 11 Tashima, S.. Hashimoto.S.. and H. Kuroki.1994. "High Speed Centrifugal Compaction and Low Temperature Sintering of Submicron Alumina Powders," J of the Japan Society of Powder and Power Metallurgy. 41 :180-183 12. Vigneau. J., and-J. J. Boulanger, 1982, " Behavior of Ceramic Tools during the Machining of Nickel Base Alloys," Annals of CIRP, 31/1:35-39 13. Vigneau, J., Bordel. P.. and A. Leonard,"l987, Influence of the Microstructure of the Composite Ceramic Tools in Their Performance when Machining Nickel Alloys," Annals of CIRP, 36/1:13-16 ACKNOWLEDGEMENTS The authors would like to acknowledge the significant contribution made by Professor R. Komanduri,Oklahoma State University in U S A . , towards the discussion of the results obtained. This research was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education Science and Culture in Japan.
b J
Motor
Figure 1. Schematic showing the high-speed centrifugal compaction (HSCC) apparatus used for the compaction of alumina ceramic compacts
Figures 2 (a) to (c) Typical SEM micrographs of the fracture surfaces of commercial HIPed alumina (AW), new submicron, pure alumina (HU), and HIP'ed new alumina (HU-HIP ) ceramics, respectively
46
E
d=l . O m
v
f3 > o
t
r
so 1
L
3
L
3
c wso o m
In’-HIP
Cutting time (min)
Cutting time (min)
Figures 3 (a) and (b) Progressive wear (average VB and maximum VB”’ flank wear) of the three ceramic tools tested (AW, HU, and HU-HIP) in turning of gray cast iron.
Figure 4 SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry turning (cutting time : 5 mins) of cast iron
0.5,
I
\Vork S4jC V=?OOm’min d= 1.Omm
f-0.20mm’tooth
V=2OOm/m in d=l j m m
Dn. 0.1 L
c;
0
0 0
1
7
-c) 3
I
J
1
Cutting time (min) Figure 5 Progressive average flank wear (VB) of the three ceramic tools tested (AW, HU, and HU-HIP) in turning of S45C steel at a depth of cut, d of 1 mm
0
I
I
l
I
I
t
I
I
l
3
Cutting Time (min) Figure 6 Progressive average flank wear (VB) of the three ceramic tools tested (AW. HU,and HU-HIP)in turning of S45C steel a t a depthsof cut, d of 1.5 mm
47
Figure 7
Figure 8
48
SEM micrographs of the nose, rake face, and clearance face Of the ceramic tools after dry turning of S45C steel
SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry milling (cutting time : 4 rnins) of S45C steel
ABSTRACT Concerns toward safety of the work environment as well as the environment in general are compelling industry to adopt 'green' or 'dry' machining, i. e. without the use of any cutting fluid. Since. some of the benefits of the cutting fluids are not going to be available in dry machining, need exists for the development of a more refractory, tougher, chemically wear resistant, and hard cutting tool materials. In this paper, the synthesis and evaluation of a new alumina ceramic tool material using very pure (99.99%) and submicron grain size (0. 22 vm) alumina powder with practically no binder sintered at low temperature (1230OC) (i.e. without the need for HIP'ing) are presented. This material is found to be stronger and harder than conventional HIP'ed alumina ceramic. The new alumina ceramic was found to be more wear resistant and fracture resistant (against both mechanical and thermal shock) than conventional alumina in dry turning and milling of gray cast iron and S45C carbon steel. Key words: Ceramic, Tool, Machining
1. INTRODUCTION It is evident that soon environmentally clean or 'green' machining will be considered no longer as a desirable option but a necessary one for sustainable manufacturing enterprise. Concern for the environment, health and safety of the operators, as well as the requirements to enforce the environmental protection laws and occupational safety and health regulations are compelling industry to consider 'dry' machining as a viable alternative instead of using cutting fluids. Byrne and Scholta [4] and others [ 1 2 ] have proposed some strategies to address this important problem. The machining costs (labor and overhead) in the U. S. alone is estimated at $300 billion/year (7).The costs associated with the use of cutting fluids is estimated to be about 16 % of the manufacturing costs (41 which is many more times than the labor and overhead figures quoted above. Consequently, elimination on the use of cutting fluids, if possible; can be a significant incentive. The key benefits of dry machining is more than mere saving of costs, namely, the provision of a healthy manufacturing environment, worker safety, protection from adverse effects of the chemicals, and overall protection of our precious environment. Of course, the lubricants or cutting fluids are used in the first place for several important reasons. Chief amongst them are: 1. to remove the heat generated in cutting, 2. to reduce friction (lubricating action), 3. to remove chips from the cutting region, 4. to protect the workpiece against rust and other undesirable reactions, 5. to prevent built-up in cutting or the metal chips adhering to the tool surface as in milling where the chips that are hot are bonded to the cutting edge which are subsequently cooled as the tool leaves the workpiece. Of course, if we were to practice dry machining or the use of lubricants is banned for one reason or the other, we have to address all the above issues satisfactorily. In reality, it may be possible to address only some of the benefits of cutting fluids in dry machining. For example, the use of new coated tools or new advanced ceramics may provide additional heat resistivity but may not provide all the other benefits of the cutting fluid. Of course, the cutting fluid industry is very
Annals of the ClRP Vol. 46/7/7997
sensitive to this issue and are reformulating new compositions that are more environmentally friendly, for example without Pb, S, or CI containing compounds which were used extensively in the past to improve the lubricating action [1,2]. It is clear that this huge industry will make significant improvements to make the cutting fluid more acceptable at least for applications where use of cutting fluid is an imperative. It is entirely possible that cutting fluids in the future may be used more selectively, or use sparingly, or not use at all.
For a long time, because of the limitations on the tool materials available, the use of cutting fluids was considered an essential integral part of the machine tool system. All the ill effects associated with the use of cutting fluids were considered as a necessary evil. Methods were developed to minimize its effects although progress was far less than desired. The detrimental effects of the cutting fluids include the cost of the cutting fluid system, i.e. the fluid itself, pumping systems, collection and filtering system, storage and disposal, and sometimes a recirculating system etc; the physiological effects on the operator, namely, toxic vapors, unpleasant odors, smoke fumes, skin irritations (dermatitis), or effects from bacteria cultures from the cutting fluid; and its overall effect on the environment. Consumption of the cutting fluids has been reduced drastically by using mist lubrication. However, mist in the industrial environment can have a serious respiratory effect on the operator. Consequently, standards are being set to minimize this effect. Use of cutting fluids will become more expensive as stricter enforcements of new regulations and standards are imposed leaving no alternative but to consider 'dry' machining. 'Dry' machining connotes that the above mentioned desirable features are no longer available. The options available under these conditions include 1. the use of very high positive rake angles (ca +30°) on submicron cemented carbide tools which would reduce the overall cutting energy significantly [5], 2. the use of tools that are more refractory and therefore can withstand the hightemperatures generated in cutting, 3. development of
43
coatings on the cutting tools that can withstand the high temperatures andlor provide a lubricating layer for reduced friction, O r 4. reduce the cutting speed SigfliflCantly t0 obtain the desired to01 life and eCOnOmiCS of machining. The high temperatures (ca 1oooOc) generated in machining especially at high cutting speeds, necessitate the use of a refractory cutting tool that can withstand the high temperatures and to provide long tool life [3.8.9.12.13]. In this paper, the synthesis and performance evaluation of a new submicron sized alumina based advanced ceramic towards the goal of dry' machining are presented [6.10.11]. Though alumina ceramic tool has the lowest toughness among the ceramics, it has the advantages of high hot hardness, high heat resistance, and low reactivity with steel. The lower toughness is no longer a serious limitation as techniques such as whisker reinforcement, presence of second phase particles are available for toughening brittle materials. In fact alumina has the highest stability amongst ceramics in relation to Fe. It is well known that grain size and impurities segregating at the grain boundaries have great influence on strength and performance of a ceramic tool. The new alumina ceramic tool used in this study is made from a very fine and pure alumina powder (0.22 um of particle size and 99.99% purity) without any additives. Use of finer grain size enabled lower sintering temperatures. Also, conventional sintering was sufficient to produce a dense body thereby reducing the cost of manufacturing. As will be shown, this material exhibited superior cutting performance in terms of wear resistance and fracture resistance compared to commercial, conventional HIP'ed (hot isostatic pressing) alumina. 2. DEVELOPMENT OF A NEW ALUMINA CERAMIC TOOL To produce high.strength ceramics, grain size must be as small as possible and grain boundary as pure as possible. With the advancements in the synthesis of materials, such as sol-gel technique, it is possible to produce very small grain size (nanometer size) and very pure material (99.9996) economically without the need for comminution. Grain growth during sintering takes place especially at higher sintering temperatures. To inhibit grain growth, sintering aids, such a MgO are added but they tend to form low temperature glassy phases which segregate at the grain boundaries. While MgO is a classic grain growth inhibitor, it also reduces the grain boundary strength and promotes intercrystalline fracture. This results in reduced strength in proportion to the amount of additives. Therefore, decreasing the amount of additives is a very important consideration for strengthening ceramics. However, if alumina powder can be sintered at fairly low temperature where grain growth hardly takes place, it will not be necessary to use these additives. In this investigation, very fine (0.22 pm of particle size) and pure (99.99%) alumina powders are used [6,10,11]. This size of powder has very large specific surface area (ca 15.1 m2/g), so that the compact from these particles has very large surface energy. Due to the assist of this large energy, the required sintering temperature is significantly reduced compared to sintering of conventional material with large grain size (several pm). In this study, it was found sufficient to sinter the alumina powder at 123OOC. Use of such a low sintering temperature means that additives are not necessary, and consequently grain boundary is kept free from impurities. In order to produce a ceramic compact, a slip from alumina powders was poured into a metal mould and compacted using a high-speed centrifugal compaction
44
(HS-CC) process in which the slip was segregated into a compact and water by applying large centrifugal force corresponding to 10-20 x 103 G's as shown in Fig.1, Details of the slip are given in Table 1, The compacts were dried at 4OoC for 4 hours and at 100°C for 4 hours, then sintered at 123OoC for 1.5 hours under atmospheric conditions. Table 2 gives the mechanical propenies of ceramic tools. To compare the performance of the new ceramic tools, it is compared with a conventional HIP'ed ceramic. HU, HUHIP, and AW (W 80) in Table 2 are the new alumina ceramic tool developed here, HIP'ed new alumina ceramic tool, and HIP'ed commercially available alumina ceramic tool. respectively. Figures 2 (a) to (c) are typical SEM micrographs of the fracture surfaces of a commercial HIP'ed alumina (AW), new submicron, pure alumina (HU), amd HIP'ed new alumina (HU-HIP) ceramics, respectively. The average grain size, measured from the fracture surface, was 0.7 pm for HU, 2 p m for AW, and 3 pm for HU-HIP. The HU ceramic material being very pure is found to be void free except for some very fine microporosity (<0.1 um). Since no grain growth inhibitors were used in this material, the HIP'ed material showed considerable grain growth due to higher operating temperatures and longer processing times. Also, the fracture of the AW tool was found to be predominantly intercrystalline while that of the HU tool to be transcrystalline. Table 1 Details of Slip
Slip concentration, mass06 Dispersion agent, mass% (Poly-carboxylic ammonium) Binder (Acrylic polymer), mass% Dispersion time, hours Dispersion temperature, OC Viscosity, mPa s
75 0.6 0.1 16-18 12-15 100-300
Table 2 Mechanical Properties of Ceramic Tools Property 3 point bend strength (TRS), MPa Vickers hardness, kg/mm2 Fracture toughness. MPalm l2 Density, gkm3
AW
HU
HU-HIP
780
1330
825
1650
2100
1450
3.5
3.3
3.4
3.94
3.95
3.97
3. MACHINING TEST RESULTS AND DISCUSSION
Dry turning and milling tests were conducted on a gray cast iron (continuously cast) (FC250-CC) and a S45C (or AlSl 1045) carbon steel workmaterials. Figures 3 (a) and (b) show the progress wear (average VB and maximum VB"' flank wear) of the three ceramic tools [commercial HIP'ed alumina (AW), new submicron, pure alumina (HU), and HIP'ed new alumina (HU-HIP) ceramics], tested in turning of gray cast iron. For convenience, the cutting conditions used are also included in the figures. It can be seen from Figures 3 (a) and (b) that wear of HU (and HUHIP) tool is significantly lower (half or more) than the conventional alumina ceramic (W80). A similar trend was observed in the feed range of 0.21 -0.50 mm/rev. Figure 4 shows SEM micrographs of the nose, rake face, and
clearance face of the ceramic tools after dry turning (cutting time: 5 min.) of cast iron. Examination of the micrographs show very little wear on the new ceramic (HU) fool material compared to the commercial ceramic (AW) tool or the large grain sized HIP'ed tool material. It was also found that the worn surface of the commercial alumina ceramic (AW) to be very rugged compared to the new ceramic (HU) tool. lntercryalline fracture and consequent dislodgement of grains appears be responsible for the rugged surface of commercial AW tool with large grain size. In contrast, with the new alumina ceramic (HU) tool, wear seems to be due to transcrystalline fracture. The higher wear rate of commercial alumina ceramic compared to the new alumina ceramic shown in Figure 3 lends further support. The differences in the wear modes as well as the wear rates of these tool materials suggest that the superior wear resistance of HU tool is not only due to the higher hardness but also due to higher bond strength between the grains due to pure grain boundary with no additives in the case of new alumina ceramic. Figures 5 and 6 show progressive average flank wear (VB) of the three ceramic tools tested (AW, HU, and HUHIP) ir! turning of S45C steel at two different depths of cut (d=l mm and 1.5 mm). The superior wear resistance of the new alumina ceramic (HU) compared to commercial alumina (AW) is very clear at the lower depth of cut (Figure 5). It can also be seen that the commercial alumina fractured after 1 min. cutting due to the higher depth of cut used (d=1.5 mm) while the new alumina ceramic continued to perform well even after 5 min. of cutting (Figure 6). This shows clearly the improved wear resistance as well as the fracture resistance of the new alumina ceramic over conventional HIP'ed alumina. Figure 7 shows SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry turning (cutting time : 5min for HU and HU-HIP and 1 min for AW) of S45C steel. The depth of cut for AW and HU was 0.21 mmlrev. It can be seen that the wear on the commercial alumina tool was considerable resulting in fracture after 1 min of cutting. The wear on the new alumina ceramic was gradual with no fracture even after cutting for 5 min. Table 3 shows the results of the face milling tests on an AlSl 1045 steel. Each test was carried out three times under the same cutting conditions (as indicated by the repetition of the letters P andor F in Table 3), and if a tool can cut the work material for a.definite length (480 mm) with no fracture, then higher feed rate was applied. It is clear from Table 3 that new alumina ceramic (HU) tool has exhibited larger resistance against fracture or chipping than the conventional alumina ceramic tool (AW).
(cutting time : 4 mins) of S45C steel. Examination of the micrographs show very little wear on the new alumina ceramic (HU) tool or new alumina HIP'ed tool (HU-HIP) material while severe wear on the commercial ceramic (AW) tool. In the case of commercial alumina (AW) tool, chipping was found on the cutting edge after 2 rnin. cutting. while no such chipping is found in the case of HUHIP tool. The worn surfaces are found to be very similar to those of turning, namely, rugged surface on the AW tocl and smooth surface on HU-HIP tool, indicating larger intergranular fracture and dislodgement of grains in the case of commercial alumina and smaller transgranular fracture in the case of new alumina ceramic developed in this investigation. The superior performance in milling of the new alumina ceramic over the commercial alumina ceramic (Figure 8) also indicates that the new ceramic material is more wear resistant and fracture resistant against thermal and mechanical shocks than the commercial alumina. As for the fracture resistance of cutting tool in intermittent cutting, such as face milling, it is said that fracture toughness and heat cracking resistance are the most important characteristics. Considering the fact that AW tool and HU-HIP tool have almost the same fracture toughness as shown on Table 2, heat cracking resistance of these tools seems to be different, since heat cracking resistance is influenced by bonding strength between particles of the tool.
4. CONCLUSIONS 1.
Very pure (99.9996) fine grain alumina powder with an average particle size of 0.22 pm, with no additives was compacted under high speed centrifugal forces and sintered at low temperatures (123OOC).
2.
Cutting performance of the newly developed alumina ceramic tool material and commercial HIP'ed alumina tool material were evaluated in turning and in face milling tests using gray cast iron and S45C carbon steel workmaterials. The new ceramic tool material showed not only superior wear resistance but also superior fracture resistance compared to commerciai white alumina ceramic tool material.
3.
The lower sintering temperature and the ability to compact the pure, submicron grain alumina powder by sintering instead of HIP'ing or hot pressing reduces the cost of producing these tool materials significantly.
4.
The superior performance of the newly developed fine grain, pure alumina ceramic over the conventional alumina ceramic in dry turning and milling of gray cast iron and S45C carbon steel was demonstrated, thus paving the way towards dry machining.
Table 3 Results of Fracture Resistance Tests in Face Milling Feed rate, mmltooth 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 P: no fracture
REFERENCES AW
HU
PPP PPP PPP FFF
PPP PPP PPP PPP PPP PPP PPF PFF FFF
F: fracture
Figure 8 shows SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry milling
1.
- -- - ---- ,
1994, "Dry Machining's Double Benefit," Machinery and Production Engineering, 14-20 2. Aronson, R. B., 1995, "Why Dry Machining," Manufacturing Engineering, 114:33-36 3. Brinksmeier, E. and S. Bartsch, 1988, "Ceramic Tools - Material Characteristics and Load Types Determine Wear Mechanisms,' Annals of CIRP, 37/1:97-100 4. Byrne, G. and E. Scholta,l993, "Environmentally Clean Machining Process A Strategic Approach," Annals of CIRP, 42/1:471-474 5. Crawford, J.H. and M. E. Merchant, 1953, "The Influence of Higher Rake Angles on Performance in Milling," Trans ASME, 75561-566 6. Koike, J.., Tashima, S., Wakiya, S., Maruyama, K. and H.Oikawa, 1996, "Mechanical Properties and Micro-structure of Centrifugally Compacted Alumina
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45
and HlPed Alumina," Materials Science and Engineering, 220A:26-34 7. Komanduri, R., and J. Desai. 1983, " Tool Materials." Encyclopedia of Chemical Technology, 23:273-309 8. Narutaki, N., Yamane, Y., and K. Hayashi,l991, "Cutting Performance and Wear Characteristics of an Alumina-Zirconia Ceramic Tool in High-speed Face Milling," Annals of CIRP, m : 4 9 - 4 2 9. Narutaki, N.. Yamane, Y.. Hayashi, K., and T. Kitagawa. 1993. " High-speed Machining of lnconel 718 with Ceramic Tools," Annals of CIRP, 42/1:103106 10 Tashima, S.. Sumida. N., and H. Kuroki,1992. "Centrifugal Compaction of Submicron High Purity Alumina Power." J of the Japan Society of Powder and Power Metallurgy, 39:39-43 11 Tashima, S.. Hashimoto.S.. and H. Kuroki.1994. "High Speed Centrifugal Compaction and Low Temperature Sintering of Submicron Alumina Powders," J of the Japan Society of Powder and Power Metallurgy. 41 :180-183 12. Vigneau. J., and-J. J. Boulanger, 1982, " Behavior of Ceramic Tools during the Machining of Nickel Base Alloys," Annals of CIRP, 31/1:35-39 13. Vigneau, J., Bordel. P.. and A. Leonard,"l987, Influence of the Microstructure of the Composite Ceramic Tools in Their Performance when Machining Nickel Alloys," Annals of CIRP, 36/1:13-16 ACKNOWLEDGEMENTS The authors would like to acknowledge the significant contribution made by Professor R. Komanduri,Oklahoma State University in U S A . , towards the discussion of the results obtained. This research was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education Science and Culture in Japan.
b J
Motor
Figure 1. Schematic showing the high-speed centrifugal compaction (HSCC) apparatus used for the compaction of alumina ceramic compacts
Figures 2 (a) to (c) Typical SEM micrographs of the fracture surfaces of commercial HIPed alumina (AW), new submicron, pure alumina (HU), and HIP'ed new alumina (HU-HIP ) ceramics, respectively
46
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r
so 1
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3
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3
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In’-HIP
Cutting time (min)
Cutting time (min)
Figures 3 (a) and (b) Progressive wear (average VB and maximum VB”’ flank wear) of the three ceramic tools tested (AW, HU, and HU-HIP) in turning of gray cast iron.
Figure 4 SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry turning (cutting time : 5 mins) of cast iron
0.5,
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\Vork S4jC V=?OOm’min d= 1.Omm
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V=2OOm/m in d=l j m m
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Cutting time (min) Figure 5 Progressive average flank wear (VB) of the three ceramic tools tested (AW, HU, and HU-HIP) in turning of S45C steel at a depth of cut, d of 1 mm
0
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I
l
I
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Cutting Time (min) Figure 6 Progressive average flank wear (VB) of the three ceramic tools tested (AW. HU,and HU-HIP)in turning of S45C steel a t a depthsof cut, d of 1.5 mm
47
Figure 7
Figure 8
48
SEM micrographs of the nose, rake face, and clearance face Of the ceramic tools after dry turning of S45C steel
SEM micrographs of the nose, rake face, and clearance face of the ceramic tools after dry milling (cutting time : 4 rnins) of S45C steel