北京化工大学：《材料导论》课程阅读材料（金属材料）Fabrication_of_metal_matrix_composites_by_metal_injection_molding_review.pdf_大学文库

journal of materials processing technology 200 (2008) 12–24 journal homepage: www.elsevier.com/locate/jmatprotec Review Fabrication of metal matrix composites by metal injection molding—A review Hezhou Ye a,∗, Xing Yang Liub, Hanping Hong a a Faculty of Engineering, The University of Western Ontario, London N6A 5B9, Canada b Advanced Materials Design, Industrial Materials Institute, National Research Council of Canada, London N6G 4X8, Canada article info Article history: Received 7 August 2007 Received in revised form 27 September 2007 Accepted 24 October 2007 Keywords: Powder injection molding (PIM) Metal injection molding (MIM) Metal matrix composite (MMC) abstract Metal injection molding (MIM) is a near net-shape manufacturing technology that is capable of mass production of complex parts cost-effectively. The unique features of the process make it an attractive route for the fabrication of metal matrix composite materials. In this paper, the status of the research and development in fabricating metal matrix composites by MIM is reviewed, with a major focus on material systems, fabrication methods, resulting material properties and microstructures. Also, limitations and needs of the technique in composite fabrication are presented in the literature. The full potential of MIM process for fabricating metal matrix composites is yet to be explored. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2. Metal matrix composites (MMCs) by MIM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1. Refractory metal based MMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2. Titanium based MMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3. Intermetallics based MMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.4. Steel based MMCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.5. Bimetal structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3. MMC by microMIM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 ∗ Corresponding author. E-mail address: hye5@uwo.ca (H. Ye). 0924-0136/$ – see front matter. Crown Copyright © 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.10.066

journal of materials processing technology 200 (2008) 12–24 13 1. Introduction Powder injection molding (PIM) is a near net-shape manufacturing technology that combines the shaping efficiency of plastic injection molding with the capability of powder metallurgy for processing metal and ceramic powders (German, 1990). The PIM process normally involves four steps: mixing, injection molding, debinding and sintering, as illustrated in Fig. 1. The evolution of the PIM technology has resulted in many variations, reflecting different combinations of powders, binders, molding techniques, debinding routes, and sintering practices. Metal injection molding, commonly known by its acronym MIM, is by far the most widely used PIM process. The process offers many unique advantages for the mass production of small and complex parts. First, precise and reproducible shapes can be achieved at a notably reduced cost as compared to conventional manufacturing processes. Part quantities varying from 5000 per year (e.g., specialty firearm sights) to over 100 million per year (e.g., cell phone vibrator weights) can be economically produced by MIM (Johnson and German, 2003, 2005). Second, complex shapes can be fabricated relatively easily with MIM, whereas conventional powder metallurgy methods are limited to the fabrication of parts with simple geometries. Third, the use of very fine powders in the feedstock promotes densification during sintering and hence high-performance parts can be produced with material systems that are difficult to sinter using conventional processes. The attractive features of the MIM process can be applied advantageously to the fabrication of metal matrix composite (MMC) or ceramic matrix composite (CMC) parts. Although many MMC or CMC offer unique properties that cannot be normally achieved by monolithic materials, their commercial use are often restricted by the high cost of materials and manufacturing. By applying the MIM process, the cost for commercial use of composite materials can be significantly reduced. In recent years, comprehensive work has been conducted to explore the potential of MIM for the fabrication of metaland ceramic-based composites and components. Companies have even been established to discover the commercial capacity of the MIM technology for the fabrication of composites (Decker, 1989; H.C. Starck Inc., 2003). The most widely studied composites by PIM are metal-based, including stainless steels, refractory metals, intermetallic compounds, and titanium alloys. Although theoretically the reinforcements in a composite can take either continuous (typically long fibres) or discontinuous (particles and short fibres/whiskers) form, the flow and mold filling requirements of the PIM process determine that it is most applicative for processing the feedstock containing particles or short fibres. Consequently, it is not surprising that all of the metallic composites fabricated by MIM to date are strengthened by discontinuous reinforcements. This paper provides a review of the research activities related to composite fabrication through the MIM route. For completeness, bimetal structures and surface-engineered components manufactured by MIM are also included in the metal matrix composites. Furthermore, fabrication of microcomponents by mircometal injection molding will be covered as well in the context even though it is still green at fundamental investigation stage. 2. Metal matrix composites (MMCs) by MIM Based on the matrix materials, the composites fabricated by MIM can be divided into refractory metal based MMC, titanium based, intermetallic based and steel based. Bimetal structures are also discussed in this section. The MIM route has enabled the fabrication of MMCs containing ingredient materials that are not compatible in molten state and difficult to fabricate by conventional routes (Diehl and Detlev, 1990). 2.1. Refractory metal based MMCs Tungsten and molybdenum are two refractory metals that have attracted great interest for high-temperature applicaFig. 1 – Schematic diagram of powder injection molding

14 journal of materials processing technology 200 (2008) 12–24 tions in such industries as consumer electronics, aerospace, telecommunications, medical and defence, due to their excellent heat resistance. The high-melting temperatures of the metals (3422 ◦C for tungsten and over 2623 ◦C for molybdenum), however, make it extremely difficult to process the materials by melting and casting. Conventionally, tungsten and molybdenum-based alloys and composites are fabricated by powder metallurgical method, followed by minor machining or grinding, if necessary. The components manufactured by the conventional powder metallurgy processes are normally limited to simple geometries. The shaping advantage of MIM makes it meritable for producing complex-shaped parts from tungsten or molybdenum-based materials. Therefore, MIM has been widely explored as an alternative manufacturing route for fabricating refractory metal-based materials (Bruhn and Terselius, 1999; Wang et al., 2001; Li et al., 2003; Tarata and Ghita, 2002; German, 1999; Bose, 2003; Yang et al., 2000; Sung et al., 1999). Currently, the fabrication of such refractory components represents one of the most profitable applications of PM technology (German, 1999). Due to the poor sinterability of the refractory system, ultrafine powders, particle size less than 10m or even at submicron scale, are commonly used in the fabrication of refractory parts by MIM in order to achieve high sintering density. A homogenous mixture of the powders should be attained prior to further processes. Mechanical alloying and the reduction of oxidized powders have been employed to get uniform mixtures of the starting materials (Moon et al., 1998a). The binder is a critical element in determining the part quality and the productivity of MIM process. New binder systems have been developed, some of which are listed in Table 1. As shown, wax-based binders are extensively used to formulate the MIM feedstock because waxes have low-melting temperature, good wetting, short molecular chain length, low viscosity and small volume change during thermal process. After injection molding, the green parts are subjected to debinding process, solvently or thermally, or both. In most cases, the debound refractory composite parts are sintered at over 1200 ◦C in pure hydrogen atmosphere. Final density, mechanical, thermal and electrical properties are the key parameters for the product (Moon et al., 1997). An important refractory metal-based composite is the W–Cu system, which has superior thermal management properties and high-microwave absorption capacity. It is predominantly used for heavy-duty electrical contacts and arcing resistant electrodes. W–Cu parts are usually fabricated by infiltrating Cu into W powder compacts or liquid-phase sintering of W/Cu powder mixtures. However, conventional techniques are unsuitable for the mass production of small intricate parts. MIM has been practised as an apposite method for mass production of small and delicately shaped W–Cu parts. In Yoo’s invention (Yoo et al., 1999), MIM is employed to fabricate a tungsten skeleton structure having the porosity in the range of 15–40% after sintering. Copper infiltration is then carried out by putting a copper plate beneath the tungsten skeleton structure and heating in hydrogen atmosphere. Compared to conventional P/M methods, the method can save energy and sidestep a sudden shrinkage during sintering. Furthermore, Yoo adopted the method for fabricating a hermetically sealed W–Cu package container for a microwave device and achieved a strip wire connection having the thermal expansion coefficient similar to that of GaAs without any extra machining process (Yoo et al., 2000). Knuwer (Knuwer et al., 2000) confirmed that using MIM followed by liquid-phase sintering was a promising and economical method for the production of housings for integrated HF-circuits. He obtained satisfactory accuracy as well as the mechanical and physical properties. Besides using ultrafine powders, adding a small amount of a third metal such as Co, Ni or Fe into the system was evidenced to improve the sinterability (Yang and German, 1993a). However, the introduction of the additive element compromises the thermal management properties of the composites, because purity and homogeneity control in theW–Cu system is critical for obtaining high thermal conductivity (Yang and German, 1994b; Yang and German, 1993b). The use of ultrafine powders, in particular nanosized W/Cu powders, provides an effective solution to the sinterability problem of the W–Cu system. W–Cu composites with nearly full density have been achieved (Moon et al., 1994, 1997, 1998a; Kim et al., 1992, 1999, 2000, 2006; Ryu et al., 1998; Yang and German, 1994a, 1997). For example, a W-20%Cu composite made of nanosized particles could be densified to 95% of theoretical density by sintering at 1150 ◦C for 2 h, and the W particles distributed homogenously in Cu (Moon et al., 1998a). To achieve uniform mixing of tungsten and copper powders and to increase the solid loading of nanosized powders, Kim and colleagues (Kim et al., 1999, 2000, 2006; Ryu et al., 1998) adopted low-energy ball milling to modify the shape of commercial Table 1 – Binder systems used in the feedstock of refractory composites for MIM Reference Composition Molding temperature Solid loading (vol.%) Debinding method Powder Moon et al. (1994, 1998a) 45% PW, 15% Bee wax, 30% PE, 10% SA 120–130 NA Thermal W–Cu Kim et al. (2000, 2006) Ryu et al. (1998) Kim et al. (1999) Yang and German (1997) 35% PP, 60% PW, 5% SA NA 52 Solvent + thermal W–Cu Yang and German (1994a) 40% PP, 55% PW, 5% SA NA 54 Solvent + thermal W–Cu Fan et al. (2005) PS, PP, EVA, PW, oil, SA, DOP 140–160 51 Solvent + thermal W–Ni–Fe Suri et al. (2003) PW, PP 170 60 NA 97W–2Ni–1Fe Fan et al. (2002) 74% PW, 20% EVA, 5% HDPE, 1% SA 135 57 NA 90W–7Ni–3Fe Huang et al. (2003) Li et al. (1998) 50% Polystyren, 30% PP, 20% VO 185 47 Solvent + thermal 97W–2Ni–1Fe

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 200(2008)12-24 fine powders.In the course of the low-energy ball milling sinterability is also an inherent problem.While at sintering the tungsten powder exhibited no change in either size temperature, there is essentially no solubility of Cu in M to a three MIM ny1-1.s7 y of M molte of the p owder shape and sulted in impro he fe ing ar s inves : Further high wders and prevent potential contaminations enc e densification.Kirk (Kirk et al 1992ds red tha tere inotherm ing processes. ant all min time to2h Undoubtedly.most of this impurity was ced from the stainless steel vial used for millin diffusivity.Using ultrafne powders seems to be more beneh alloyed W-Cu powder at 600C for 1h in a hydrogen atm thermaldiffusivity of the composite is to amplify thehigh con ents in the vity FCC pha by increasing the v Ryu et al.1998.2002:Moon et al 199b:Lee et al,2000).This ct Is to the rem 2.2 Titanium based MMCs osed Titanium alloys are the most metallic biomate to take advar ntage of different processes,such asg nding and rials for me dical implant appli ions due to their goo com posite powde for MIN used in aerospace applicat ions be ause o ca wide ngth and al.2002:Hu pimanyfocusedonappicationsinthebiomedicalfeld ue to the mbination of high der Thian and his colle agues are the leading rese the materials.The for MI posites for medical implant applications by MIM (Thia fabrication is9w- i-3Fe(Fan et al,2001ab, 021 005;Sur et a 2001.2002ab.c.2003).HAis calcium pr phate com H)2 le W Ni miinand mixed wit tive mat courages bone ingrowthsw dsto 1 natu 2001. the superic degree of mixing of th wders bioactitvity th ent n is of HA (Ha ani et al 1994 erties and negligible distortion were produced by solid- tat make a homoge neous feedstock T6A4 and HA powder I orde hol (PVA as the 03 The PVA y (Fan et al.,2005)propo d a mather natic model of adjustin rem d by hea atine.and the ush ight deviation of cted parts of additives like Mo.Ta a cture HA.A commercial binder syste PAN-250S anical propert syste y MI the experimer set appropriate ding an ndin Bose et al 19.1990).Their results show that vem al solid load quantity of Re ha solid solutio ngthenin ing of the composite powder in the fe fo mol the cations debound thermally and the heating rate and ga ar to the w-Cu composite Mo-C com mportant in the pro ha ely match to semiconductor mater The superiority of a higher rate at the later stage with a high gas flow rate demonstrated as an P

journal of materials processing technology 200 (2008) 12–24 15 fine powders. In the course of the low-energy ball milling, the tungsten powder exhibited no change in either size or shape, while the ductile Cu powder was deformed to a threedimensional equiaxial shape with reduced size. The change of the powder shape and size resulted in improved solid loading and better sinterability. Kim’s investigation indicated the low-energy ball milling could circumvent the agglomerate of nanopowders and prevent potential contaminations encountered in other milling processes. However, iron contamination was detected in the products even when the authors restricted ball milling time to 2 h. Undoubtedly, most of this impurity was introduced from the stainless steel vial used for milling and it could be minimized by using vials made from cemented carbides. Besides mixing and milling, reducing mechanically alloyed W–Cu powder at 600 ◦C for 1 h in a hydrogen atmosphere can also lead to improvements in the green density and shape stability of the powder compact (Kim et al., 2000; Ryu et al., 1998, 2002; Moon et al., 1998b; Lee et al., 2000). This effect is mainly attributed to the removal of internal impurities and oxides. In both Meinhardt’s and Lee’s inventions (Minhardt et al., 2002, 2005; Lee et al., 2005a), they proposed to take advantage of different processes, such as grinding and reducing oxides in hydrogen, mixing powders with a protecting liquid in an inert atmosphere, to make super fine and pure composite powders for MIM. Fabrication of W–Ni–Fe composites by MIM has been widely investigated as well (Fan et al., 2001a,b, 2002, 2004, 2005; Suri et al., 2003; Huang et al., 2003; Li et al., 1998; Lee et al., 2005b; Qu et al., 2000, 2001) due to the unique combination of high density, high strength, good ductility and corrosion resistance of the materials. The primarily studied W–Ni–Fe system for MIM fabrication is 90W–7Ni–3Fe (Fan et al., 2001a,b, 2002, 2005; Suri et al., 2003; Huang et al., 2003; Lee et al., 2005b; Qu et al., 2000, 2001). Nanoscale W–Ni–Fe composite powders are commonly fabricated by mechanical milling and mixed with a multicomponent binder to form homogeneous feedstock (Suri et al., 2003; Huang et al., 2003; Lee et al., 2005b; Fan et al., 2001a,b). Experimental results demonstrated that milling escalated the degree of mixing of the elemental powders, the maximum powder loading, homogeneity of the feedstock, and the final sintering. Fully densified parts with high-mechanical properties and negligible distortion were produced by solid-state sintering in the temperatures range of 1350–1450 ◦C. In order to control the dimensional deviation of the final products, Fan (Fan et al., 2005) proposed a mathematic model of adjusting the weight deviation of the injected green parts, which was verified by experimental investigation. Furthermore, the role of additives like Mo, Ta and Re on the microstructure and mechanical properties of the W–Ni–Fe system by MIM has been scrutinized by German and Bose (German et al., 1989; Bose et al., 1989, 1990). Their results show that even a small quantity of Re has remarkable solid solution strengthening and the grain refining capacity. However, the high cost of Re may restrict the addition of the additive to only special applications. Similar to the W–Cu composite, the Mo–Cu composite has high thermal conductivity and the thermal expansion closely match to semiconductor materials. The superiority of Mo–Cu to Wu–Cu is the lower density at the same weight fraction level. During the fabrication of the composite, poor sinterability is also an inherent problem. While at sintering temperature, there is essentially no solubility of Cu in Mo and only 1–1.5% solubility of Mo in molten Cu. Utilizing the MIM process to fabricate Mo–Cu composite improves sinterability due to the use of ultra fine particles in the feedstock. Further more, additive elements, having a higher solubility for Mo and forming a continuous solid solution with Cu, will promote densification. Kirk (Kirk et al., 1992) discovered that the addition of Ni to the systems could significantly enhance densification during sintering. However, this was achieved at the cost of coarsening Mo grain size, and abating thermal diffusivity. Using ultrafine powders seems to be more benefi- cial because it raises the thermal diffusivity of the composite together with densification. An alternative route to improve thermal diffusivity of the composite is to amplify the high conductivity FCC phase by increasing the volume of Cu (Nan et al., 2004). 2.2. Titanium based MMCs Titanium alloys are the most promising metallic biomaterials for medical implant applications due to their good biocompatibility, high-chemical stability in the physiological environment, and excellent mechanical properties. They are also extensively used in aerospace applications because of their high specific strength and corrosion resistance. The current research on titanium matrix composites through MIM is primarily focused on applications in the biomedical field. Thian and his colleagues are the leading researchers in exploring the fabrication of titanium–hydroxyapatite (HA) composites for medical implant applications by MIM (Thian et al., 2001, 2002a,b,c, 2003). HA is a calcium phosphate compound [Ca10(PO4)6(OH)2] with a composition similar to the mineral phase in natural bones. It is established as a bioactive material that encourages bone ingrowths when used in implants. HA itself, however, is brittle in nature, and cannot be used in load-bearing applications. The purpose of developing the HA/Ti alloy composite is to combine the superior bioactivity of HA and the excellent mechanical properties of titanium alloys, so as to avoid the brittleness and low fracture toughness problems of HA (Halouani et al., 1994). To make a homogeneous feedstock, Ti6Al4V and HA powders were first mixed by a slurry approach, using polyvinyl alcohol (PVA) as the carrier (Thian et al., 2003). The PVA was lately removed by heating, and the powder mixture was crushed into small particles to obtain Ti6Al4V/HA composite powders with an inner core of Ti6Al4V wrapped with an outer layer of HA. A commercial binder system, PAN-250S, was used in the experiment. To set appropriate molding and debinding temperatures, the thermal properties of the multi-component binder was investigated by TGA/DSC. The optimal solid loading of the composite powder in the feedstock for MIM (Thian et al., 2002a) was around 60 vol.%, essentially the same as most other kinds of feedstock for MIM. The molded parts were debound thermally and the heating rate and gas flow rate played an important role in the process (Thian et al., 2001). A slow heating rate at the beginning stage of debinding and a higher rate at the later stage, with a high gas flow rate during the whole process, were demonstrated as an effective approach to remove the binder

16 JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 200 (2008)12-24 tering.a key challe nd noor fabricability of th in fabricating/HA composite is to pr nt HA fron ounds limit their applications.Composite strengthening decomposing.The degree of HA decomposition increases wit On t other hand,to achieve a high echniques have been veloped to produce the Ni,Al-base of the g ten off and A 90 wnat mak omposite with 50wt%of Ti and 50wt%of HA,a sinte Depending on composition,the strength of NigAl increase te hea nin and a c withtemperature up to microhardn ess.while temperature resulted ty of fabricating NiAl-based composite by MIM(Bose and n simulated。 ogical conditions revealed the complete sed in the experim ent.The preferential align ment of th lution of the secondary phase bers in the matrix was attained by a modified form of MIM hate (ICP).tet caophophae tals pre also foun interingofele nental powders under exter chan ted dur identical pre use of the p cipitation of ion thermal rocedure,and pressu as evidenced b MMCs by MIM Many issues.such as the nde ntenng mecha sms ot the stem,the mech Alman and toloff furthe expl the adaptability o al pro d to b Given sintered in controle man 1991;Alman et a han d an r im ents in the mechanical 1p03 an expecte HA-based com ynthesis foll d by hot ng.Results cate 2008 2002 testsre and wang. chanek et al,1997;Hoepfner and Case. tthe Al fbers would trengthen the ostrate,espe Oktar Weng et al.,1 a1g即ey al properties The rep rted/HA composite produc materaduetotshigh-meltnont()x ellent ox 50w de sintering.HA-HA and HA-TisAl4V would be bound togethe ever,hamper the manufacture and application of this reas the Ti6Al4V powde actually restricted der atio Adding vo of chopped for the high porosity of the composite 50%)in the ved fracture sistance and hardness (Stoloff ane tud Therefore the mechanical properti f the Alman,1991:Alm an et al..1992) te are d by the HA POW s an adv ting sh omposition of HA at lower temperature (Wang and Chal orientation of the hbre Research results show that the ng et al..1994) y n the p xpa e the will be able to advance the properties of the MIMed composite oduce a fibre alienment parallel to the flow direction thein Fig.2 9b).B molding,the mechanical pro rties along a c Ithasbeen recognized thatintermetallic compounds basedo an thus be tailored according to performance requirement w ae sity,hig which are hiehl y desirable for hig ittle effect on the degree of alignment (Alma temperature structural applications as rep owever,this met uffers from sev

16 journal of materials processing technology 200 (2008) 12–24 During the high temperature sintering, a key challenge in fabricating Ti6Al4V/HA composite is to prevent HA from decomposing. The degree of HA decomposition increases with sintering temperature, and it becomes significant when the sintering temperature reaches 1100 ◦C (Thian et al., 2002b). On the other hand, to achieve a high degree of densification of the composites requires a relatively high sintering temperature. Thian et al.’s results showed that for a Ti6Al4V/HA composite with 50 wt.% of Ti and 50 wt.% of HA, a sintering temperature of 1100 ◦C, a heating rate of 7.5 ◦C/min and a cooling rate of 5 ◦C/min produced the highest relative density and microhardness, while higher sintering temperature resulted in higher flexural strength and modulus. Furthermore, an in vitro study of the MIMed Ti6Al4V/HA tensile bars performed in simulated physiological conditions revealed the complete dissolution of the secondary phases such as tricalcium phosphate (TCP), tetracalcium phosphate (TTCP), and CaO after 2-week immersion. Following that, calcium phosphate crystals precipitated after 4 weeks of immersion. It was also found that the mechanical properties deteriorated during the initial immersion period and then gradually recovered to almost identical pre-immersion values because of the precipitation of an apatite layer (Thian et al., 2002c). It should be pointed out that the research on Ti/HA-based MMCs by MIM is relatively new. Many issues, such as the sintering mechanisms of the Ti/HA system, the mechanical properties of the composites, and the prevention of the decomposition of HA at high temperatures, need to be elucidated. Given that pure HA could be sintered in controlled conditions to achieve better mechanical properties than that reported for the Ti/HA composite (Halouani et al., 1994), further improvements in the mechanical properties of Ti/HA composites can be reasonably expected. Also, HA-based composites with such reinforcements as silver, silica, titania (Nair et al., 2008; Wang and Chaki, 1993; Gu et al., 2002; Chaki and Wang, 1994; Suchanek et al., 1997; Hoepfner and Case, 2003; Oktar, 2006; Chu et al., 2004; Weng et al., 1994) have also been sintered to get attractive mechanical and biological properties. The reported Ti6Al4V/HA composite produced by MIM consisted of 50 wt.% Ti6Al4V and 50 wt.% HA, with HA coated onto the Ti6Al4V powder in the feedstock. During sintering, HA–HA and HA–Ti6Al4V would be bound together, whereas the Ti6Al4V powder actually restricted densification of the composite (Thian et al., 2002c). That is one of the reasons for the high porosity of the composite (over 50%) in the study. Therefore, the mechanical properties of the composite are mainly contributed by the sintered HA powders and Ti6Al4V/HA interfaces. Also, sintering in vacuum would cause decomposition of HA at lower temperature (Wang and Chaki, 1993; Weng et al., 1994), especially with the presence of Ti. Altering the composition and revising the sintering conditions will be able to advance the properties of the MIMed composite. 2.3. Intermetallics based MMCs It has been recognized that intermetallic compounds based on aluminium have appealing characteristics of low density, high strength at elevated temperatures, along with good corrosion and oxidation resistance, which are highly desirable for high temperature structural applications as replacement for superalloys. However, the brittleness and poor fabricability of the compounds limit their applications. Composite strengthening offers a way to improve the mechanical properties. Among the intermetallic compounds, nickel aluminide Ni3Al is one of the most widely studied materials and many techniques have been developed to produce the Ni3Al-based composites (Stoloff and Alman, 1990). What makes this material unique is its anomalous thermal hardening behaviour. Depending on composition, the strength of Ni3Al increases with temperature up to approximately 600–900 ◦C. Bose and German may be among the first to investigate the feasibility of fabricating Ni3Al-based composite by MIM (Bose and German, 1988a,b; German et al., 1990; German and Bose, 1989a; Bose et al., 1992). Both prealloyed and elemental powders were used in the experiment. The preferential alignment of the fibers in the matrix was attained by a modified form of MIM, where the feedstock was extruded through a special nozzle. After debinding, the consolidation was attained by reactive sintering of elemental powders under an imposed external stress via hot isostatic compaction (HIP). Full densification was possible by appropriate selection of particle sizes, composition, thermal procedure, and pressure. As evidenced by the investigation, it is of general applicability to use relatively inexpensive elemental powder to fabricate hard-forming compounds Alman and Stoloff further explored the adaptability of MIM for production of other intermetallic matrix (NiAl, MoSi2 and TaTiAl2) composites (Alman and Stoloff, 1990, 1991a,b,c,d, 1994, 1995; Stoloff and Alman, 1991; Alman et al., 1991). The Al2O3 fibres were chopped and dispersed into the powders by mixing in alcohol. MIM was used to form the shape and the consolidation of the composites was conducted by reactive synthesis followed by hot isostatic pressing. Results indicated superior alignment was achieved with small spherical powders after injection molding. Microhardness tests revealed that the Al2O3 fibers would strengthen the substrate, especially when aligned by MIM (Alman and Stoloff, 1991d). MoSi2 is another attractive high-temperature structural material due to its high-melting point (2030 ◦C), excellent oxidation resistance and low density. Its extreme brittleness at temperature below 1000 ◦C and the low creep resistance, however, hamper the manufacture and application of this material. Adding 20 vol.% of chopped Al2O3 fibres to the MoSi2 PIM feedstock produced a MoSi2–Al2O3 composite with much improved fracture resistance and hardness (Stoloff and Alman, 1991; Alman et al., 1992). A distinct advantage of PIM in fabricating short-fibre reinforced composites is the capability of controlling the orientation of the fibres. Research results show that the expanding flow of the feedstock tend to line the fibres perpendicular to the flow direction, while the contracting flow will produce a fibre alignment parallel to the flow direction, as schematically shown in Fig. 2 (German and Bose, 1989b). By properly controlling the flow of the feedstock during injection molding, the mechanical properties along a certain direction can thus be tailored according to performance requirements. The key to successful alignment has been confirmed to be the size of the starting powder while the morphology of the powders has little effect on the degree of alignment (Alman and Stoloff, 1991a). However, this method suffers from sev-

journal of materials processing technology 200 (2008) 12–24 17 Fig. 2 – Alignment of the fibres during injection molding. eral obstacles, including the need for fine spherical powders, proper selection of reinforcement and binder, and complete binder removal. Nevertheless, the process has demonstrated the potential for near net-shape processing of intermetallic composites. 2.4. Steel based MMCs Steel based composites seem to be of much less interest to the research society as compared to other metal matrix composites described in the previous sections. This is because steels can be processed relatively easily using the conventional fabrication processes, and the properties of steels can be controlled in a wide range by simply varying alloy compositions and heat treatments. Therefore, few publications could be found on the research of steel matrix composites. The main purpose of fabricating ferrous composites by MIM is to improve the wear-resistance of the steel, coupled with the unique shaping advantage of MIM. Fine TiN and TiC powders, with an average diameter less than 10m, are commonly added as the reinforcement. The amount of the addition is controlled to below 5 wt.%, to avoid compromising the densification and potential damage to the injection molding machine. The steel powders and the additive powders are mixed together to get uniform starting materials before adding the binder. Wax-based binder systems are predominantly employed in the fabrication of steel matrix as well. Table 2 represents some binders successfully applied in labs. The sintering is usually processed in a reduction atmosphere and the temperature is subjected to the steel matrix, usually achieving over 90% theoretical density in the range of 1200–1400 ◦C. Turker and Karatas (Turker and Karatas, 2004) examined the rheological properties of mechanically alloyed composite powder prepared by two different instruments: a high-energy attritor and a three-dimensional turbula mixer. It was found that the composite powders milled by an attritor showed more favourable rheological properties than those by turbula mixing. This was attributed to the fact that mechanical alloying in a high-energy attritor produced nearly spherical or ellipsoidal particles, whereas milling the powder in a ball-containing turbula mixer produced flake-like particles. Spherical particles are more desirable because of their better flowability and elevated packing density, which can result in high solid loading, improved compact strength and little dimensional change during sintering. In addition, powders mechanically alloyed by the high-energy attritor are easy to densify during sintering due to the high strain energy stored in the powders. Although the high-energy attritor mill is an effective instrument for preparing MIM feedstock, special care is needed to minimize contamination during the mixing process. The milling container and milling balls are potential sources of the impurities. Selecting the container and milling balls made from the same materials as the powder is an applicable way to minimize the contamination. MMCs composed of gas atomized 316L stainless steel and TiC powders were fabricated by Loh et al. with MIM (Loh et al., 2001). By optimizing the powder loading, the concentration of TiC in the composites, and the debinding and sintering process parameters, it was shown that MMC parts with about 98% full density could be produced, and the addition of TiC significantly increased the microhardness of the sintered parts. Khakbiz (Khakbiz et al., 2005) investigated the rheological behaviour and stability of the 316L/TiC composite Table 2 – Binder systems used in steel matrix fabrication Reference Composition Molding temperature Solid loading (vol.%) Debinding method Powder Turker and Karatas (2004) 60% PW, 35% PP, 5% SA 175 50 NA Fe–Cr–Al2O3–Ti–Y2O3 Loh et al. (2001) PAN31 (Adeka Fina Chemical) 60–70 53 Themal 316L–3%TiC Khakbiz et al. (2005) 53% PP, 30% EVA, 15% CW, 2% SA 60–80 55–60 Solvent + thermal 316L–3–5%TiC Miura et al. (1996) 69% PW, 20% PP, 10% Carnauba wax, 1% SA NA 68 Thermal SCM440–3%TiN

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 200 (2008)12-24 feedstock.It was identified that the addition of Tic powders had different effects on the viscosity of the stainless stee feedstock at low-she arnreslSoosjandathgh.shearae he2ehanga3ei2eometooaeaiornpd ss with i asing shear rate.For stability,the results ind studies show that TiCis an effective reinforcement to be added By adju ved hardness and wear resistance can be produced by MIM. Step2 Step 1 e in the sinte of steel matrix MMci Copartiagas presure binder and tm Mold rotation o).For com Fig.3-Aschematic diagram of co-injection molding a relative 5t 0h。 2005)Within this density range the wear resistance of the al.,2001:Pischang et al.,1994).Actually,various combinations eased MMCs ntially improved.Forxam of materials can be manufactured into bimetal composite articles showed 10 times lowe are compositions can be achieved.In practice,wax-based binde ematnaiewithoonocsmensum nof the co the sintering be ad 25 Bimetal structures wo single-ba tions and ag ssive environm wo different material layers bond ed together can ofter pe molded part in (Baun elected materials together.In addition to the high cos assoc eta).The second methodis molding with a twin-barre tedwith the athird materaweld-fillers,adhesives,etc)that fre rotation o susetin and th able ocations on the componen aroundor adjacent to e inser apparent advantages in comparison with the first one.The but this approach cn ony be appled to parts the first method involves logistic and quality fhotiheecodmoiahheteslnnthewhedoiea of different chemistries and proper es together and sintering oom temperature.This step may induce the o-mechanica ed bim egarded as transfer of an insert since it remains in the same nget al et al 2002:Pest et al 1996 1997:Heaney et al 2003)This is ng the stress caused by the temperature differe ence Based o bove idea thods derivec nd patente ased on the annlication of the sites different con ent powder materials are each mixed with a binder s binations of materials can be manufactured into bimeta tem and granulated to form feedstocks 11 y or se o0☑desc d compotion which ring were fabricated(Baumgartner and Tan,2001,20 )2:Tan et injecting the sec nd te stock into the cavity whe e the nr

18 journal of materials processing technology 200 (2008) 12–24 feedstock. It was identified that the addition of TiC powders had different effects on the viscosity of the stainless steel feedstock at low-shear rates (500 s−1), owing to the competition between higher packing efficiency and deagglomeration caused by hydrodynamic stress with increasing shear rate. For stability, the results indicated that the addition of a secondary powder degrades the rheological behaviour and elevates the instability indexes. The studies show that TiC is an effective reinforcement to be added into stainless steel. By adjusting the rheological properties and processing parameters, defect-free composite parts with improved hardness and wear resistance can be produced by MIM. A major challenge in the sintering of steel matrix MMC is the precise control of carbon content in the steel matrix. This can be achieved by selecting the binder and controlling the CO partial gas pressure in the debinding and sintering atmosphere, often by a trial and error approach (Miura et al., 1996; Pest et al., 1995; Petzoldt et al., 1995). For steel-based composites, a relative density of 95–99% can be achieved with 3–5 wt.% ceramic addition (Loh et al., 2001; Khakbiz et al., 2005). Within this density range, the wear resistance of the steel-based MMCs can be substantially improved. For example, a MIM composite with a 4140 steel matrix and 3 wt.% TiN particles showed 10 times lower wear mass loss as compared to the matrix material without reinforcements (Miura et al., 1996). 2.5. Bimetal structures Metallic components are often operated under severe conditions and aggressive environments. A bimetal structure with two different material layers bonded together can offer performance advantages for such applications. A traditional way of making the bimetal structure is to weld or bond the two selected materials together. In addition to the high cost associated with the secondary operations, the conventional joining approach also has the disadvantage of requiring the use of a third material (e.g., weld-fillers, adhesives, etc.) that frequently becomes the vulnerable locations on the components and hence impairs the component performance. Press fit or shrink fit can also be used for the fabrication of bimetal structures, but this approach can only be applied to parts with certain specific shapes (Baumgartner and Tan, 2002). MIM provides an alternative solution. Molding two metallic materials of different chemistries and properties together and sintering them to produce a finished bimetal part can overcome these problems. Bimetal structures produced by MIM are sometime regarded as continuously reinforced MMCs (Baumgartner and Tan, 2001, 2002; Tan et al., 2001; Pischang et al., 1994; Alcock et al., 1996; Alcock and Stephenson, 1996; Beard et al., 2002; Arai et al., 2002; Pest et al., 1996, 1997; Heaney et al., 2003). This is true only in the macroscale and is accepted in this paper as well. Based on the application of the composites, different combinations of materials can be manufactured into bimetal composites. Wear resistant pieces with a core of stainless steel and a covering of tool steel or heavy metal, as well as electronic parts of a non-magnetic center surrounded by magnetic ring were fabricated (Baumgartner and Tan, 2001, 2002; Tan et Fig. 3 – A schematic diagram of co-injection molding. al., 2001; Pischang et al., 1994). Actually, various combinations of materials can be manufactured into bimetal composites if similar sintering kinetics and thermal expansion of the compositions can be achieved. In practice, wax-based binder systems are widely used as well. By modifying the solid loading of the component powder, the sintering behaviour of the different materials can be adjusted. A bimetal part can be molded in two ways. The first is molding with two single-barrel injection-molding machines. By this method, one material is injected into the mold with one or several cavities in a single-barrel machine, and then the molded part is transferred to another tooling in another single-barrel machine, where the second material is injected in (Baumgartner and Tan, 2001, 2002; Tan et al., 2001; Pischang et al., 1994). The second method is molding with a twin-barrel injection-molding machine, as illustrated in Fig. 3. The cavity is filled by the first material from a barrel, then the whole mold is modified by rotation or subsection and the second material is injected around or adjacent to the insert (Alcock et al., 1996; Alcock and Stephenson, 1996). The second method has apparent advantages in comparison with the first one. The procedure for the first method involves logistic and quality challenges because the first material must be placed exactly into the second mold, where the first material will be cooled to room temperature. This step may induce thermo-mechanical stresses between the two components after the second molding operation. In contrast, the second method does not require the transfer of an insert since it remains in the same mold. The second material is injected immediately after the first injection, and the two materials cool together without inducing the stress caused by the temperature difference. Based on above ideas, diversified methods were derived and patented (Beard et al., 2002; Arai et al., 2002). In (Beard et al., 2002), different powder materials are each mixed with a binder system and granulated to form feedstocks. The feedstocks are melted and concurrently or sequentially injected into a mold and allowed to solidify as the green part. A patent (Arai et al., 2002) describes the second composition which is molded by injecting the second feedstock into the cavity where the first

JOURNAL OF MATERIALS PROCESSING TECHNOLOGY 200(2008)12-24 9 MIMed compact is arranged to let the second material flow Furthermore.the co-iniection molding method presents parallel to the joining surface.As a result,the second com- a new way for surface engineering.It is particularly advan position can contact the first with neglectable temperature with cor x ge To produce strong bonding betw en the co-injected materi- rials onto an inexpe nsive solid core. high-performanc enng,it is requ etal comp nan engine with chemical compatibility.Cho onent materials etal (Howell and Lawley,1991)has demonstrated that Stellite cle sh 3 cobalt alloy powder be injection molded onto a 109 bonding (Baumgartner and Tan.2001.2002:Tan et Petzoldt et al,1997). ness of the 1090 steel core combined with the magnincen wear tuth。 in anen cabide heavy metal were presented and discussed byis the high sintering temperature involved in the densification of ng (P ang e PIM parts,the bme ncated by mold different shaped composites.layered sample and coated sam annealed condition.which can sometimes compromise the and strength of the core. mater if the not properly selected Therefore,it is important to ensure that the ingre nt pow MMC by microMIM ng the Miniaturization offers the conspicuous advantage of a highe interface would deteriorate the composite mechanical prop. ole produc (Fleis resi has been regarded as one of the leadin technol ies in strate,where the low alloy steel or stainless steel can impar that the M that is fabr ated of 100%tool steel pes ches as mic onics is today (piotter et al 2002 et al (Pest et a 1996,1997 showed mpo In the last decade,microdevices and microco The twin-screw mixer firs ncutineg alf was pia into the upre en et al., t al. debinding,the parts were sintered in hydrog with g phite limited to polymeric and silicon mate rials so fa as Matching offer speci uld no experiment,M4 powder was milled to promote solid-statesin electro-forming and excimer laser ablatior the e and b t the al 1996).Heaney successfully advantages s.micrometal iniection molding (uN IM)has be 316L teel as a bimetal comp as a pre with Liu et al.,2002 esintering metal injection molding (MIM)is the miniaturiza ed These nowders wer e admixed with (MIM and it inherits the features of conventional MIM such wax and presse d into cylindric as low pro ost.g ometric complexity,near net-shape %ar al MIM the of MIM sphere.By comparing the shrinkage of th differen naterial formulation of the feedstock,moldingto achieve design geom M2,it was steel ha nng to displayed sound metallurgical bonding between the ingredi the r ders.binders.facilities are much more stringent for MIMbecause the structure dimensions arereduced to micron

journal of materials processing technology 200 (2008) 12–24 19 MIMed compact is arranged to let the second material flow parallel to the joining surface. As a result, the second composition can contact the first with neglectable temperature gradient. To produce strong bonding between the co-injected materials during sintering, it is required that the two materials have similar thermal expansion and densification behaviour along with chemical compatibility. Choosing component materials with good metallurgical compatibility, suitable particle shape, size distribution and tap density are essential for suitable bonding (Baumgartner and Tan, 2001, 2002; Tan et al., 2001; Petzoldt et al., 1997). Investigations on composite material of iron/iron–silicon, iron/316L, iron–silicon/316L and a low alloy steel/tungsten carbide heavy metal were presented and discussed by Pischang (Pischang et al., 1994). The shrinkage behaviours of the ingredient materials, and the sintering behaviours of two different shaped composites, layered sample and coated sample, were studied. The results confirmed that distortion and cracks could initiate along the phase boundaries of different materials if the ingredient materials are not properly selected. Therefore, it is important to ensure that the ingredient powders will be densified at the same temperature with similar shrinkage. The formation of intermetallic phases along the interface would deteriorate the composite mechanical properties. The co-injection route can realize the production of wear resistant tool steel using a low alloy steel or stainless steel substrate, where the low alloy steel or stainless steel can impart toughness, corrosion resistance and cost preference as compared to a component that is fabricated of 100% tool steel. Pest et al. (Pest et al., 1996, 1997) showed that a bimetal composite of M4 tool steel and Fe–2Ni could be molded and thermally processed. The feedstock of both materials was produced in a twin-screw mixer. Fe2Ni1B green parts were molded first and then cut in the middle. One half was placed into the cavity, and then M4 feedstock was injected into the cavity. After debinding, the parts were sintered in hydrogen with graphite as the carbon source. Matching the densification behaviour of the ingredient powders was realized by altering the particle characteristics and alloy chemistry of the powders. In this experiment, M4 powder was milled to promote solid-state sintering at low temperature and boron was admixed with the Fe–2Ni to form a liquid phase at the sintering stage (Pest et al., 1996). Heaney successfully molded and sintered M2 tool steel with 316L stainless steel as a bimetal composite (Heaney et al., 2003). In his study, two material systems were selected by detecting the shrinkage rate of the component with a dilatometer. The sintering behaviours of M2, Fe–10Cr–0.5B, 4340–0.5B, 316L–0.5B and Fe–2Ni–0.5B (wt.%) were investigated. These powders were admixed with 2 wt.% paraffin wax and pressed into cylindrical compacts. After thermally debinding in a flowing hydrogen atmosphere, the samples were sintered at 1400 ◦C in a 95/5 vol.% argon/hydrogen atmosphere. By comparing the shrinkage of the different materials with M2, it was found 316L–0.5B stainless steel had the best sintering compatibility with M2. The final sintered products displayed sound metallurgical bonding between the ingredient materials, resulting in a defect-free, functionally graded MIMed bimetal composite component. Furthermore, the co-injection molding method presents a new way for surface engineering. It is particularly advantageous when small parts with complex geometries require relatively thick coating. By injecting a layer of desired materials onto an inexpensive solid core, a high-performance bimetal composite with an engineered surface can be formed after sintering (Alcock, 1999; Howell and Lawley, 1991). Howell et al. (Howell and Lawley, 1991) has demonstrated that Stellite 3 cobalt alloy powder can be injection molded onto a 1090- wrought steel core to fabricate a composite cotton picking spindle part. The combination of superior ductility and toughness of the 1090 steel core combined with the magnificent wear resistance of the Stellite material has resulted in an enconomical part with greatly improved wear resistant. Because of the high sintering temperature involved in the densification of PIM parts, the bimetal composites fabricated by molding powder to a solid core will normally produce a core material with annealed condition, which can sometimes compromise the strength of the core. 3. MMC by microMIM Miniaturization offers the conspicuous advantage of a higher functional integration and a wider range of new application fields associated with portable products (Fleischer and Dieckmann, 2006). Therefore, microsystem technology (MST) has been regarded as one of the leading technologies in the engineering world and it is predicted that the MST will become as significant for innovative products in nearly all branches as microelectronics is today (Piotter et al., 2002). In the last decade, microdevices and microcomponents with dimensions ranging from nanometers to millimeters have been applied in various areas, e.g., biomedical and biotechnology, automotive industry, telecommunication, and electronics (Ruprecht et al., 2002; McGlen et al., 2004; Ilic et al., 2006). However, commercialized microsystem products are mainly limited to polymeric and silicon materials so far. Metallic microsystems offer specific properties which could not be attained by polymers or silicon materials. Furthermore, existing microfabrication methods, such as X-ray lithography, electro-forming and excimer laser ablation, have the drawbacks of either low productivity and high production cost or constrained material availability. To overcome these disadvantages, micrometal injection molding (MIM) has been established as a promising route to the mass production of metallic microcomponents (Merz et al., 2004; Tay et al., 2005; Liu et al., 2002). Micrometal injection molding (MIM) is the miniaturization of the conventional metal injection molding process (MIM) and it inherits the features of conventional MIM such as low production cost, geometric complexity, near net-shape, good tolerance and reproducibility (Rota et al., 2002). As in conventional MIM, the four major processing steps of MIM are formulation of the feedstock, molding to achieve design geometry, debinding to remove the binder and sintering to obtain desired properties. As can be expected, the requirements for the powders, binders, facilities are much more stringent for MIM because the structure dimensions are reduced to micron scale.

20 journal of materials processing technology 200 (2008) 12–24 To date, most MIM work was focused on the fabrication of microcomponents using a single material. The exploration of MIM for the fabrication of composites has just started. W–Cu composite fabricated by MIM was reported by Kim (Kim et al., 2005) and WC–Co composite by MIM was mentioned in (Rota et al., 2002). Kim prepared the nanocomposite powder of W-30%Cu by mechanical alloying in a stainless steel container for 50 h and produced irregularly shaped W–Cu agglomerates with a mean particle size less than 100 nm. The binder system for MIM was a wax-based multicomponent system, adapted from their previous study on conventional MIM (Kim et al., 2006). Micromolded PMMA was used as the mold insert. The tiny flow channels used in the micromolds have created significant resistance to the flow of the feedstock. Complete filling of the micromold could be achieved only upon prior evacuation of the cavity by a vacuum pump. The sintering behaviour of the nanocomposite powder was examined by dilatometric analysis to modify the processing parameters. Due to the use of extremely fine powders in the feedstock, W–Cu microparts of full densification without distortion were achieved by solidstate sintering at 1050 ◦C for 5 h with a heating rate of 3 ◦C/min. The sintering temperature was much lower than the one used in conventional sintering. As the conventional MIM, MIM can also be applied to fabricate the bimetal structures. Based on the dilatometric study of the materials during sintering, a considerable number of parts made from different metals and alloys have already been manufactured (Simchi et al., 2006). Depending on the specific application, materials with different mechanical and/or physical properties might be joined with each other. In the process, the feedstocks of different materials are co-injected into the mold in order to obtain a bimetal composite without additional operations. Fig. 4 exhibits an encoder fabricated by IFAM, with the combination of a non-magnetic steel (316L) and ferromagnetic steel (17-4PH stainless steel) by MIM. As encountered in (Kim et al., 2005), incomplete mold filling often occurs in the injection molding of microcomponents with commercially available feedstock or feedstock with convention binder system because of the high viscosity or the insufficient stability in the micron scale (Merz et al., 2004). Three methods are generally applied to avoid the filling problem, i.e., using high injection pressure and holding pressure, elevating the mold temperature and applying vacuum to the mold cavities (Fu et al., 2005; Piotter et al., 1997). Rota (Rota et al., 2002) solved the poor filling of WC–Co due to the high-heat conductivity by heating the mold to the feedstock temperature. Although studies have already demonstrated the feasibility of fabricating composites using MIM, the R&D in this area is still at its embryonic stage and many problems need to be addressed. Even for monolithic materials fabricated by MIM, reproducibility as well as quality assurance, standardized measuring systems are still pending for successful commercialization (Rota, 2005). Compared with conventional MIM, MIM has inherent challenges due to the submicron powders used and the micron scale accuracy. Since the submicron powders tend to be pyrophoric and prone to be contaminated, it is essential to avoid any oxidation and impurity during processing. Handling of such fine powders and formulating the feedstock usually takes place in a glove box under argon (Zauner, 2006). Furthermore, the thermal stresses in the microcomponents induced during debinding and sintering are much higher than conventional components because of the tiny particle size. In addition, uniformity in micron scale in the course of every step is laborious to achieve with conventional facilities, which would result in the deviation of the properties and dimensions of the products. It is expected that the above problems would be more difficult for composites, comprising of two or more different submicron particles. Advances in automation and control, together with enhanced tooling accuracy, will positively minimize the deviation of the final products and problems arising from production, in the future. 4. Summary MIM provides a promising way to fabricate MMC materials and components. It is particularly cost-effective for the fabrication of small, complex parts due to its shaping capabilities. The nature of the MIM process determines that it is mainly suited for the fabrication of composites with discontinuous reinforcements, typically particles or short fibres/whiskers. It can also be used to fabricate Fig. 4 – MicroMIMed encoder composited by a non-magnetic steel (316L) and a ferromagnetic steel (17-4PH). (a) Green and sintered parts and (b) microstructure of the bimetal composite; interface area about 850 m× 850 m (Courtesy of Philipp Imgrund, IFAM, German).

journal of materials processing technology 200 (2008) 12–24 21 bimetal or surface-engineered components, which are considered composites with continuous reinforcements in a wider sense. The composite materials fabricated by MIM so far include the matrixes of steel, refractory metal, intermetallic compounds, and titanium alloys with various types and forms of reinforcements. Bimetal composite structures can be produced by MIM through co-injection of different types of feedstock or molding a powder over a solid core as well. Furthermore the microMIM has shown the capacity of manufacturing composites at micron scale. Limitations and needs of the technique in composite fabrication are also presented in the literature. Previous research has demonstrated the feasibility of fabricating advanced composites and components using the MIM approach. The full potential of MIM process for fabricating metal matrix composites is yet to be explored. references Alcock, J., 1999. 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