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 three￾dimensional equiaxial shape with reduced size. The change of the powder shape and size resulted in improved solid load￾ing and better sinterability. Kim’s investigation indicated the low-energy ball milling could circumvent the agglomerate of nanopowders and prevent potential contaminations encoun￾tered 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 atmo￾sphere 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 impu￾rities 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 protect￾ing 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 den￾sity, 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 multi￾component 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 prop￾erties 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 appli￾cations. 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 sinter￾ability 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 con￾ductivity FCC phase by increasing the volume of Cu (Nan et al., 2004). 2.2. Titanium based MMCs Titanium alloys are the most promising metallic biomate￾rials 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 cur￾rent 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 com￾pound [Ca10(PO4)6(OH)2] with a composition similar to the mineral phase in natural bones. It is established as a bioac￾tive 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 devel￾oping 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 frac￾ture 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 alco￾hol (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 load￾ing 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