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 nr18 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) and at high-shear rates (>500 s−1), owing to the competition between higher pack￾ing efficiency and deagglomeration caused by hydrodynamic stress with increasing shear rate. For stability, the results indi￾cated 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 atmo￾sphere, often by a trial and error approach (Miura et al., 1996; Pest et al., 1995; Petzoldt et al., 1995). For steel-based com￾posites, 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 exam￾ple, 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 condi￾tions and aggressive environments. A bimetal structure with two different material layers bonded together can offer per￾formance 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 associ￾ated 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 fre￾quently 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 struc￾tures, but this approach can only be applied to parts with certain specific shapes (Baumgartner and Tan, 2002). MIM pro￾vides 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 com￾binations 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 elec￾tronic 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 load￾ing 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 mate￾rial 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 mold￾ing 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 induc￾ing 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 sys￾tem 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