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Materials Transactions, Vol. 43, No. 3 (2002) pp. 348 to 351 Special Issue on Environmentally Benign Manufacturing and Material Processing Toward Dematerialization c 2002 The Japan Institute of Metals
Environmentally Benign Manufacturing of Automotive Parts via Powder Metallurgy Akira Fujiki1 , Hisayoshi Kojima2 and Tatsuhiko Aizawa3 1
Nissan Motor Co., Ltd. Powertrain Operation Division, Yokohama 230-0053, Japan Nissan Motor Co., Ltd. Tokyo 104-8023, Japan 3 Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan 2
Automotive parts produced using a powder metallurgy (P/M) process are commonly used in automobiles because they can be produced without the use of machining and with a special alloy design. This paper describes how the P/M process can be applied to the production of environmentally friendly automotive parts. The paper begins with a discussion on how the utilization of P/M for net-shape manufacturing makes it possible to conserve both energy and materials due to the fact that machining is unnecessary. Evidence is then presented that shows that it is possible to produce warm-compacted automotive P/M parts with sufficient strength even without the use of special alloy elements and/or heat treatments. As a final example, magnet materials made using spark-plasma sintering are described. This spark-plasma sintering process makes it possible to create high-performance magnet parts that are highly energy efficient. Finally, taking into consideration the above-mentioned example, the ideal P/M process is discussed from an environmentally benign point of view. (Received November 5, 2001; Accepted December 25, 2001) Keywords: powder metallurgy, automotive parts, net-shape manufacturing, plasma sintering
1. Introduction
2. Case Studies
Automotive parts produced using a powder metallurgy (P/M) process are commonly used in automobiles because they can be produced without the use of machining and with a special alloy design. Some P/M automotive parts are environmentally benign. For example, some P/M parts are installed in engine valve timing and/or lift variable system, which are the so-called VTC, VVT, and VTEC systems. Fitting an engine with any of these systems reduces the fuel consumed and gases emitted.1–3) Direct fuel injection systems further reduce fuel consumption. Several P/M parts are used in all of these systems, such as in fuel injector and air intake control systems. High-performance P/M rare earth magnets are also being installed in electric vehicles (EVs) and hybrid electric vehicles (engine and electric motor = HEVs) in order to reduce the emission of harmful exhaust gases and to make the engine energy-efficient. The above examples consider P/M automotive parts from an application point of view. However, considering them from the process point of view is also important. Here we describe several new ways of manufacturing automotive parts using an environmentally friendly P/M process. Three case studies are examined. Case study 1: Net-shape manufacturing (eliminates machining and minimizes material waste) Case study 2: Warm compaction (eliminates the necessity for special alloys and/or heat treatment) Case study 3: Spark-plasma sintering (highly efficient sintering)
2.1 Case study 1: Net-shape manufacturing The accuracy of P/M parts is usually higher than that of wrought materials, and these parts can be produced without the use of machining. Because of this, P/M parts are generally more environmentally sound than other types of parts.4) However, with net-shape manufacturing, the powder pressing movement is unidirectional, making it difficult to create under-cut shapes. But several new developments have recently been made that make it possible to overcome this limitation: sinter-joining, under cut shape powder forming using a CNC (computerized numerical control) press, and green machining. We look at these developments in this section. 2.1.1 Sinter-joining One development is sinter-joining technology. Nissan and Hitachi Powdered Metals have developed sinter-joined sprockets for engines.5) These sprockets have an under-cut shape (Fig. 1). In sinter joining, two parts are independently pressed and assembled before sintering, at which time they are completely joined. New materials and new designs were needed to ensure that the join was strong enough. Sinterjoining technology eliminates the need for machining, which therefore saves energy and materials. Our estimates show that the energy consumed is reduced by approximately 15% and the material saving is 20%, which includes a weight reduction as a result of holes incorporated in the new design of the sprocket. 2.1.2 Under-cut shape forming The second development is under-cut shape forming using a CNC press. Nissan and Yoshizuka Seiki have developed this process.6) This process requires the development of a sliding die that moves perpendicular to the press direction. The punches and dies are controlled by computer in the forming. We therefore developed a CNC powder compacting press first. This hydraulic press can control eight axes by computer.
Environmentally Benign Manufacturing of Automotive Parts via Powder Metallurgy
The compacting movement is shown in Fig. 2 and a sprocket is shown in Fig. 3. Under-cut shape forming also saves energy and reduces material loss. We have not yet worked out just how much energy is saved, but the material saving is estimated to be about 15%. 2.1.3 Green machining A third way of obtaining an under-cut shape has recently been developed. The key to this technology is machining compacted powder before sintering. The compacted powder before sintering is usually called “green parts”, so this technology is called “green machining”. Green parts are neither hard nor strong, so the machining force and the energy of green parts are smaller than those of sintered parts.7) The adoption of green machining in actual production will reduce the machining cost of P/M parts and eliminate the need for special coolant for machining. However, green parts are of
Fig. 1 Sinter-joined idler sprocket for engines.
course weaker than sintered parts, and holding green parts at the machining stage is more difficult than holding sintered parts. Warm compaction of powder provides stronger green parts and overcomes several drawbacks of green machining. Figure 4 shows warm-compacted and green-machined sprockets.8) Warm compaction technology itself is described in the next case study. Green machining is currently under development and there have been very few technical reports on production implementations of this process. 2.2 Case study 2: Warm compaction P/M materials usually have residual porosity and show 90% of their theoretical density as maximum density, so they also have a limitation in terms of strength. Warm compaction technology has been developed to overcome this limitation. With the normal P/M process, powder compaction (powder compacting) is carried out at room temperature. In warm compaction, however, the iron powder, tools and dies are heated to 403 K (130◦ C). Using this process, it is possible for the compacted density to reach a density of 95% of the theoretical density. Nissan Motor Company, Hitachi Powdered Metals, and Jatco Trans Technology have developed warm-compacted and sintered engine sprockets that was changed from conventional P/M alloy steel.9) Figure 5 shows the relative wear of conventional P/M alloy steel sprockets and warm-compacted sprockets. The relative wear for warm-compacted and sintered 7.1 g/cm3 sprockets is greater than that for conventional sprockets. However, the relative wear for warm-compacted and sintered 7.2 g/cm3 sprockets is less than that for conventional sprockets. Special alloys are unnecessary with warm
Fig. 2 Movement of die and tools in under-cut powder forming.
Fig. 4 “Green machined” sprockets (Hoganas AB Sweden).
Fig. 3
Under-cut formed sprocket for engines.
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Fig. 5
Wear test results for motored engine test (Relative wear).
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A. Fujiki, H. Kojima and T. Aizawa
Fig. 7 Warm-compaction apparatus.
Fig. 6 Warm compacted and sintered engine sprocket developed for V6 engines.
compaction, which is an advantage in terms of recycling materials. Figure 6 shows the sprockets we developed, and Fig. 7 shows the apparatus that was used for warm compaction. The above-mentioned P/M alloy steel sprocket was used instead of heat-treated Fe–Cu–C P/M alloy sprockets. Therefore, warmcompacted and sintered high-density (7.2 g/cm3 ) sprockets can be used in place of heat-treated conventional Fe–Cu–C P/M alloy sprockets. Although warm compaction consumes energy when heating both the powder and the dies/tools, the amount of energy that is consumed is much less than when heat treatment is used for P/M steels. Thus using warm compaction, obtaining high-strength P/M parts, and eliminating heat treatment can make the production process more environmentally friendly. 2.3 Case study 3: Spark-plasma sintering There are currently several types of magnet materials that are used in automobiles. It has been predicted that in the future there will be an increase in the number of sensors and actuators made of rare earth magnet materials. Rare earth magnets are usually produced using a P/M process, but the creation of high-density P/M Nd–Fe–B rare earth magnets requires both the consumption of a great deal of electricity and a complicated sintering process. A new spark-plasma sintering (SPS) process is now under development that could make the creation of high-property Nd–Fe–B base exchange-spring magnets10) possible. In this study, the starting ribbon powders, which contain amorphous parts, were prepared using a melt-spinning method. The SPS apparatus is shown in Fig. 8. SPS is a type of solid-compression sintering that is similar to hot pressing. In addition, the sintering promotion factor is the heat that is self-generated by electric discharges between particles when the on-off DC pulse is applied.
Fig. 8 Schematic diagram of spark-plasma sintering.
The production of high-performance P/M rare earth magnets using the SPS method is both simpler and more environmentally friendly than traditional production methods. 3. Discussion 3.1 Focus In Section 2, several case studies were examined. These case studies included raw materials (powders), compaction (powder compacting), and sintering. However, they were each examined independently. In this section, the authors would like to discuss the possibility of combining these technologies and developing an even more environmentally friendly process. Generally speaking, the above-mentioned technologies could be adopted simultaneously; however, sometimes the advantages of using these technologies are not enough to make up for the consumption of energy and materials that they require. Several integral technologies are examined in the following case studies. 3.2 Integral technology case studies 3.2.1 Combining warm compaction and sinter joining Hitachi Powdered Metals has reported on a process in which warm compaction and sinter joining11) are combined. In this process, warm-compacted parts are generally used for
Environmentally Benign Manufacturing of Automotive Parts via Powder Metallurgy
the outer parts of sinter joining because they can easily become cracked at the press fit (assembling) before sintering. With this technology, the machining, heat treatment, and/or special alloy elements could be eliminated. This technology is currently only in use for agricultural parts. The development of larger parts and the acquisition of mass production knowhow will be necessary if this technology is to be adopted for use in the production of automotive parts. 3.2.2 Warm compaction and green machining The combination of warm compaction and green machining was briefly mentioned in Section 2 (case studies). Although the authors have obtained informal information from P/M automotive parts suppliers on this combination, there have been few reports published regarding this field. By adopting the use of this technology, both the consumption of machining energy and sintering energy can be reduced. Since the reduction of machining energy consumption was already discussed in Section 2, in this section we will focus on the reduction of sintering energy consumption. If compacted parts become completely net-shaped before sintering, saving energy is in proportion to green-machined mass. As previously mentioned, there is a possibility that the use of special alloys and/or heat treatments could be eliminated with the utilization of warm compaction. The combination of warm compaction and green machining has a great deal of potential for reducing process energy consumption and eliminating the use of special materials in the production of automotive P/M parts. However, technologies for non-destructive crack detection of green parts should be developed before the use of this process becomes wide spread. Crack checks are usually carried out after sintering, which means that the compaction and green machining conditions can only be examined after sintering has been completed. Therefore, the compaction and green machining conditions must be delayed until after sintering has been checked, which can lead to a loss of time. 3.2.3 Material combination and spark-plasma sintering12) In Section 2, a case study on spark-plasma sintering (SPS) and the efficiency of sintering were explained. In this section, we will be describing several of the advantages of using SPS. First of all, by using SPS, even completely different materials can be sintered at the same time. Due to the fact that each material has its own optimal sintering temperature, which is determined by the melting point, the dissolving temperature, Curie point, etc., it is usually very difficult to sinter mixed materials simultaneously. However, the use of SPS makes it possible to create multiple-phase material. SPS can also be used as a joining technology. In each of these cases, the discharge of electricity plays a very important role. Multiple powder feeding and SPS can be used to create functionally graded materials (FGM). FGM have the characteristics of graded chemical composition, microstructure, and physical properties that make it possible to obtain special properties that monolithic materials do not possess, or increase properties comparing monolithic materials. Recently, ceramic and aluminum FGM materials have been developed that can be used to create pistons for adiabatic internal combustion engines. FGM material is capable of releasing the heat stress
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that results from combustion. Therefore, the heat-shock crack between ceramic and aluminum can be avoided. If adiabatic engines can be used, fuel efficiency would greatly increase and exhaust gas would be reduced considerably. 4. Conclusions (1) Several P/M automobile parts, especially those used in engines, play a great role in reducing fuel consumption and exhaust gas, thus making these P/M parts environmentally friendly. (2) The P/M process is one that is environmentally benign. However, there are several limitations to its actual use in the production of automotive parts. The authors are working to develop a new P/M process that will eliminate these limitations and make it even more environmentally friendly than conventional P/M processes. (3) New developments in this field include materials, the compaction process, and the sintering process. If these technologies can be combined, an even more environmentally sound process could be developed. (4) The ideal process would be one that is both environmentally friendly and can be used to create environmentally friendly automotive parts. Acknowledgements The authors would like to thank Dr. Kohmei Harada (National Institute for Material Science) for his kindness in sending his article to us as a reference, and Mr. Shinichi Takemura (Nissan Motor Co., Ltd.) for his support in giving us information on variable valve timing and the lift control system. REFERENCES 1) S. Takemura, S. Aoyama, T. Sugiyama, T. Nohara, K. Moteki, M. Nakamura and S. Hara: SAE Technical Paper Series 2001–01–0243 (2001). 2) R. Hofman, J. Liebl, M. Kluting and R. Flierl: JSAE (Society of Automotive Engineers of Japan, Inc.) Annual Congress, Spring Convention Proceedings, No. 39–01, (2001) pp. 1–4. 3) P. Kreuter, P. Heuser and J. Reinicke-Murmann: SAE Technical Paper Series 980765 (1998). 4) K. Harada: “Materia Japan” 37 (1998) 42–51 (in Japanese). 5) A. Fujiki, Y. Kishi, M. Ogura, K. Asaka and T. Uemura: SAE Technical Paper Series 950390(1995). 6) T. Hirao, Y. Kishi and T. Katagiri: Journal of the Japan Society of Powder and Powder Metallurgy 48 (2001) 432–439 (in Japanese). 7) J. Asami, N. Hirose, J. Sawa and A. Umeda: Proceedings of the 2001 Spring conference of the Japan Society of Powder and Powder Metallurgy (2001) p. 113 (in Japanese). 8) Catalogues Hoganas Sweden AB. 9) A. Fujiki, T. Hirao, M. Yamaguchi, T. Murata, K. Ueda and Y. Sugaya: Proceedings of the 2000 Powder Metallurgy World Congress (2000) pp. 137–141. 10) H. Ono, N. Waki, M. Shimada, T. Sugiyama, A. Fujiki and H. Yamamoto: Proceedings of the 8th Joint MMM-Intermag Conference (2001). 11) T. Ooba: “Present status and future prospects of warm compaction,” text of the seminar on the “Present status and future prospects of P/M” held at the SOKEIZAI-center in February 1998 (in Japanese). 12) NEDO International Cooperation Study Results, “Report for Basic Industrial Technology Cooperative Study on Functionally Gradient Materials,” 1997, 1998, 1999, 2000 (in Japanese).
