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The metal containing pores have been developed in last few decades. The interest of metals containing pores is rising up in development for application of material. The research is not only by utilizing the inherent low density but also by utilizing the pores themselves and the large specific surface area (L. J. Gibson et. al, 2000).This type of metal is accepted as new engineering materials and can be classified into two. The first one is 'porous metal'. It is normally sintered by metallic powder and the range of porosity from 15% to 75%. The second one is 'metal foams' with porosity range from 80% to 98%. It is also can be known as high porosity metal. Porous metal provide specialized product for application such as self-lubricating bearing, battery electrodes, fluid flow control, filtration, and etc. While, metal foam provides a different application over solid metal such as for sound absorption, energy absorption, thermal management, vibration suppression, and etc (S. K. Hyun and H. Nakajima, 2002).
There are various fabrication methods to develop porous metal such as foaming method with gas bubbling, electroplating method for metal deposit in polyurethane form, powder sintering metallurgy method, and etc. However, there are poor mechanical performances of porous copper by these methods. The reasons of this problem are the pore shape plays an important role on the porous metal behaviour under critical load. Typically, the pores formed in sintered metal are irregular. They have a great number of acute angles acting as stress promoters. Generally, the pores shape are inhomogeneous and disorder. These are resulting in stress concentration and rupture at the weakest point (H. Du et. al, 2010).
Copper is the oldest metal used by human. This metal has been mined for more than 10,000 years. This is proved with the finding of copper pendant in Iraq being dated to 8700BC. The use of copper was increased dramatically during Industrial Revolution in late 19th and early 20th centuries with the progress of the electrical industry (Davis J. R., 2001).
This metal is a mostly suitable material for thermal management application. It is due to its high thermal conductivity and pretty high sintering activity. Other than that, copper is also excellent metal used to study several metallurgical activity in certain high temperature mechanism such as dynamic recovery and dynamic recrystallization [9 -10]. According to Johnson and Lye-King Tan, the development of copper MIM parts using a variety of copper powder feedstocks made by different processes. In their researched, they focused on the importance of porosity and impurities in affecting the conductivity of finished parts (German and Bose, 1997).
Other than that, copper is suitable for a wide range of application because of its special combination of physical properties such as strength, conductivity, corrosion resistance, machinability and ductility. The variations in composition and manufacturing methods can make these properties can be further improved (Vin Calcutt, 2001).
Table 2.1: Attributes and Applications of Copper and Copper Alloys (Vin Calcutt, 2001)
Type of Application
Architecture, sculpture, jewelry, clocks, cutlery.
Door hardware, marine internal combustion engines, crop treatments.
General, hydraulic and marine engineering, metalworking, aerospace, power generation, shipbuilding, offshore oil and gas platforms.
Plumbing tubes and fittings, roofing, general and marine engineering, shipbuilding; chemical engineering, industrial processes including pickling, etching and distilling; domestic plumbing, architecture, desalination, textiles, papermaking.
Ease of fabrication
All the above plus printing.
Electrical power generation, transmission and distribution, communications, resistance welding, electronics.
Essential for health of humans, animals and crops
Agriculture, preservation of food and wood.
Cryogenics, liquid gas handling, superconductors.
General engineering, marine engineering, defense, aerospace.
Instrumentation, geological survey equipment, minesweepers, offshore drilling.
Mining and other safety tools, oxygen distribution.
Electrical springs and contacts, safety pins, instrument bellows, electronic packaging.
Heat exchangers and air-conditioning/refrigeration equipment, automotive radiators, internal combustion engines, mining.
Table 2.2: Thermal and Electrical Conductivities of different elements (Davis J. R., 2001).
Thermal Conductivity W/m.K
Electrical Conductivity % IACS*
*International Annealed Copper Standard (IACS) and 100% IACS represents a conductivity of 58 MegaSiemens per meter (MS/m); this is equivalent to a resistivity of 1/58 Ohm per meter for a wire one square millimetre in cross section.
2.3 Powder injection moulding (PIM) or metal injection moulding (MIM)
Nowadays, powder injection molding (PIM) or metal injection molding (MIM) has become important technology for processing metal or ceramic into parts of desired shape. PIM/MIM is a cost effective method in producing simple or complex part from almost all type of material such as metals, ceramics, intermetallic compound, and composites close to final dimension (near net shape) at production rate which range from few hundred to several thousand parts per hour (R. Supati et. al, 2000). Other than that, this manufacturing method is combining traditional powder metallurgy process and plastic injection molding. Lately, PIM/MIM has been studied not only for hard metal but also for material like titanium, copper, and aluminium (Kazuaki Nishiyabu et. al, 2005).
The PIM/MIM process usually starts with mixing selected powders and binders in the correct proportions. It is also known as feedstock preparation. This is a critical step because lack quality of the feedstock cannot be corrected by subsequent processing adjustments. Therefore, it is important to ensure that the feedstock is homogeneous and free powder binder separation or segregation of particles. Failure to break up or disperse the powder uniformly or unsuitable rheological behaviour of the feedstock will cause molding defects such as distortion, cracks, or voids that will lead to non-uniform shrinkage or warping in the sintered parts (K. S. Roetenberg et. al, 1992).
After that, the selected powders and binders are mixing together and then granulated into homogeneous feedstock. The feedstock is molded using the same equipment and tooling that are used in plastic injection molding.Â Â The molded "green" part is ejected and then cleaned to remove all flash. After molding, the binder is removed by debinding process. The "green" part is heated in a low temperature oven. This process is allowing the polymer binder to be removed via evaporation. As a result, the "green" part becomes "brown" metal part. Lastly, the "brown" metal part sintered at an elevated temperature. The purpose of this process is to remove pores from the material. This process will cause the part to shrink from its molded size. The resulting part is still maintain the original molded shape with high tolerances, but the density is much greater (R. Supati et. al, 2000).
Figure 2.1: Powder Injection Molding Process (http://www.custompartnet.com/wu/metal-injection-molding)
Preparation of feedstocks is one of the critical parts in PIM. The feedstock is prepared through mixing a suitable ratio of powder and binder. The used of too little binder may create voids in the mixture, high viscosity feedstock and difficult to mold. Otherwise, excessive binder will gives a low molded strength and may cause separation of powder and binder during molding (N. H, Loh et. al, 2000). Excessive binder will slow down the debinding process and may cause part slump when the particles settle or migrate during debinding. The dimensional changes during sintering will also be larger (R. M. German, 1989).
Other than that, the type of mixer and mixer blade, mixing speed and temperature, mixing time and the sequence also will affect the homogeneity of the feedstock. The mixer blades are lighter in construction and have a relative smaller surface area as opposed to a horizontally mounted blade (C. Johnson, 1990). Even though higher mixing speed is efficient in breaking agglomerates, it could induce micro-bubble formation in the feedstock, creating inhomogeneity. A lower speed helps to prevent air trap but is not effective in breaking the agglomerates. A balance needs to be achieved and a medium speed may be the most suitable (N. H. Loh et. al, 1998). If the mixing time is too long, it can cause degradation of the binder components. The mixing temperature has also an effect on dispersion since of its effect on feedstock rheology. If the temperature is very close to the melting of the polymer, it will cause extensive of particle interactions and poor dispersion. This also attributed to poor wettability of the polymer (M. D. Sacks et. al, 1978).
Debinding process consist of two stages which are solvent and thermal debinding. The debinding schedule comprises three main elements: heating rate, debinding temperature and debinding time. A suitable combination of the three parameters will produce a defect-free part which is one of the main objectives of this study.
To reduce the possibility of defects with safe and fast binder removal, the multi component binder chosen includes the lower stability components of paraffin wax and stearic acid, which are removed in early stage of debinding, and generate pore channels (Ali S. Muhsan et. al). It is considered that after removing some percent of the binder, there exists some interconnected capillary porosity inside of the samples which makes leaving of gaseous products in subsequent thermal debinding easy in short time. Since nearly most of binder was removed in solvent debinding step, subsequent thermal debinding can be performed with higher speed in comparison with usual thermal debinding process (Ahmad F., 2010).
Sintering gives strong antiparticle bonds and removes or reduces the void space by densification. Hence, sintering lead to substantial shrinkage in the PIM parts. Several parameters influence the sintering process: these include initial density, material, particle size, sintering atmosphere and temperature, sintering time and heating rate (R. M. German, 1990). A rapid heating rate can trap surface contaminant within the part and cause degradation properties. On the other hand, too slow a heating rate will give poor densification and inferior properties. Too high a temperature or too long a sintering time will cause distortion, slumping, excessive grain growth and even bloating (H. E. Amaya, 1991).
Figure 2.2: 2D reconstructions (virtual slices) perpendicular to the cylindrical axis showing Cu particles at different stages of the sintering process: (a) before sintering, (b) after sintering at 1000Â°C, and (c) after sintering at 1050Â°C (O. Lame et. al, 2002).
2.4 Hot compaction or hot die compaction process
Hot compaction process similar as PIM/MIM process. In this process, the pressure and temperature is combined simultaneously. It is also known as pressure assisted sintering. There are four type of hot consolidation such as hot pressing, hot isostatic pressing (HIP), forging, and hot extrusion. Figure 2.3 shows the basic schematic diagram of these four processes.
Figure 2.3: Schematic of four conventional hot consolidation processes (R. M. German, 1996).
Hot die compaction is the simplest form of hot consolidation process to compress or materials at high temperature. This process consists of loading the loose powders material to be consolidated into a die. Normally, the cross-section of the die is simple and made from graphite. To reduce reaction between the powder material and the die, the graphite die is generally filled with non-reactive carbonaceous. Other materials that can be use as die are refractory metals and alloys, and some ceramics (Bose A., 1996).
To compact the powder materials, the pressure applied in vertical direction to the punches through a ram. The temperature is dependent on the type of powder material and the maximum pressure is applied (Chen P. Et. al, 2007). The schematic of the process is shown in Figure 2.3(a).
2.5 Space holder method
There are many methods that can use to produce or to fabricate porous metal. One of them is by solid-state-based powder metallurgy. By using this method, porous metal can be produce at low temperature. One of the powder metallurgical metal foam or porous metal production techniques is the space holder sintering method. By using appropriate space holder material, this method can yield a highly porous part with desirable pore size, shape, and volume. There are two ways to remove the space holder from "green" part either by using water leaching or thermally. According to Nuray Bekoz et al, the famous way is by using water leaching because thermal removal takes longer time and need low heating rate to avoid crack to the part. Another reason to use water leaching is, when using thermal method, the decomposition of space holder material may release harmful gas that may interact with the metal (Nuray Bekoz and Enver Oktay, 2012).
The concept of production of porous metal is shown in Figure 2.4. The feedstock materials are mixture of metal powder and binders. The high densification after debinding and sintering process is very important for high quality product. The space holder method is applied to produce pores in the product. The fraction of space holder particle and processing parameters set the ratio between closed and open porous structure (Kazuaki Nishiyabu et. al, 2005).
Figure 2.4: concept of production porous metal by space holder method (Kazuaki Nishiyabu et. al, 2005).
There are many type of material that can be use as a space holder such as sodium chloride (NaCl), potassium carbonate, carbamide, and etc. Zhang et al. was produce micro-alloyed steel foam with porosities between 50% and 85% by using potassium carbonate as space holder material (Zhang et. al, 2008). Bakan et al. was using carbamide as a space holder material and applying water leaching technique to produce highly porous 316L stainless steel with 70% porosities. From the researched, they conclude that the pore shape, size and distribution can be control by applying water leaching and sintering process (Bakan et. al, 2006). Other than that, Mutlu et. al. also used carbamide particles as a space holder material. They was used this material to fabricate highly porous 17-4PH stainless steel specimens with porosities between 39% and 82%. From the researched, they state that the porosity was directly related to added fraction of carbamide (Mutlu I. and Oktay E., 2011).
Halil I. Bakan was done experiment on a novel water leaching and sintering process for manufacturing highly porous stainless steel. He was used water leaching technique to remove the space holder on the 'green' part. The "green" part was immersed in distilled water for 30 to 120 minutes. Then, the specimen was removed from the bath and dried in air at 40oC for 6 hours. The specimen was going on thermal pyrolys at a ramp rate of 300K/h up to 350oC to remove remaining small amount of bonding agent, Polymethylmethacrylate (PMMA). For sintering process, the sample without boron was heated at temperature of 1370oC for 120 minutes, and temperature of 1245oC for 45 minutes for sample with boron. This process was carried in vacuum at a base pressure of 6.5 Pascal (Halil I. Bakan, 2006).
Figure 2.5: Schematic presentation of the water leaching and sintering process for preparation of highly porous 316L stainless steel components (Halil I. Bakan, 2006)
The experiment and research on effects of carbamide shape and content on processing and properties of steel foams was done by Bekoz et. al. In this experiment, the different shape of carbamide was used as a space holder. They used water leaching technique to remove the space holder. From the experiment, water leaching technique can cause volumetric expansion of the foam structure. This expansion becomes more obvious when less space holder material was added. Other than that, sintering process can cause volumetric shrinkage on the foam structure. The volumetric shrinkage increased gradually with the increased of porosity. The shape and fraction added of space holder material was directly affecting the percentage of porosity and the pore shape. Foams with uneven shape of pores have higher compressive yield strength and lower Young's Modules than foams with spherically shape pores. The foams strength increased with increasing relative density (Nuray Bekoz and Enver Oktay, 2012).
Figure 2.6: Typical morphologies of raw materials: (a) steel powder, (b) irregular, (c) spherical carbamide particles, and (d) coated carbamide particles with the steel powder (Nuray Bekoz and Enver Oktay, 2012).
Figure 2.7: Steel foam specimens having 71.0% porosity fabricated by using (a) irregular and (b) spherical carbamide particles (Nuray Bekoz and Enver Oktay, 2012).
2.6 Characteristic of porous metal
Yasser M.Z. et al have done investigation on the mechanical properties of sintered porous copper compacts. The effect of porosity, applied stress and sliding velocity on wear rate, and stress-strain relationship of porous compacts was studied (Yasser M. Z. Et. al, 2007). The result has shown in the Figure 2.8 and Figure 2.9.
2.6.1 Effect of porosity, applied stress and sliding velocity on wear rate
Figure 2.8: Effect of porosity on wear rate of porous copper (Yasser M. Z. Et. al, 2007).
The effect of porosity on wear rate of porous copper compacts at various sliding velocity and applied stress levels is shown in Figure 2.8. It is showing that the wear rate increases linearly with porosity. According to Rapoport et. al, this may be caused by the reliance of wear resistance on the strength of the rubbed surface. It is also depends in turn on the geometrical parameters of porous compacts including the size of the pores as well as the porosity of the sample (Rapoport et. al, 2002).
For the effect of applied stress, it is clearly shows that increasing applied stress at any constant porosity and velocity leads to an increase in the wear rate. Zhang et. al state that, this happen because the real contact area between the pin and the disc increases with increasing the applied stress. The compaction of the wear particles in the pores lead to the formation of a smooth surface. In producing smooth surface, the applied stress needs to be increased so that the pores completely closed (Zhang et. al, 2005).
From the Figure 2.8 also, it is shown that the increasing sliding velocity of the wear disc, at any constant porosity and applied stress, leads to considerable increase in wear rate. Lim et. al stated that, this is due to the fact that the effect of sliding velocity of the wear disc is sufficiently large to affect the applied stress. Increasing sliding velocity gives rise to frictional heating to cause increased wear rate (Lim et. al, 2003).
2.6.2 Stress-strain relationship of porous compacts
Figure 2.9: Stress-strain relationship for porous copper compact (Yasser M. Z. Et. al, 2007).
According to the experiment that have done by Yasser M.Z et. al, the stress strain curves result of porous copper compacts with high (51.5%) and low (20.63%) porosities is shown in Figure 2.9 (Yasser M. Z. Et. al, 2007). There is a distinct ultimate strength point followed by complete fracture for the high porosity specimen and no such clear ultimate strength point can be found for low porosity specimen. Zhu et. al stated that, the ultimate strength point of the high-porosity specimen that due to the strength of porous metal not only affected by the number of pores and their size but also by their shape (Zhu et. al, 2005). At the sharp edge of the pore, the stress concentration can be identified easily. It is implying that irregular pore shape in high porosity specimen reduce the mechanical properties, especially the plasticity.
Figure 2.10: SEM of porous copper specimens having high and low porosities (Yasser M. Z. Et. al, 2007).
Otherwise, in the low porosity specimen, most of the pore shapes are rounded as shown in Figure 2.10. According to Wang et. al this behaviour happened due to two competing modes of deformation that exist during compression. The first one leads to closure of the voids due to the axial compression and the other one yields void nucleation and concentration (Wang et. al, 2003). As a result, for high porosity specimen, the later mode prevails and being the cause of the formation of fracture and eventually disintegration of the sample.