Production and characterization of open cell titanium based foams by polymer replication method

Most materials in the nature are in porous forms. These porous materials are also the first materials used by human in history: such as wood, bones and cork. But despite this, production of porous (which also called cellular) materials by humankind is a new process and the idea of producing cellular materials arised when humans realized these natural cellular structures exhibited excellent properties and are superior than dense solid materials; because the unique stracture of cellular materials makes it possible for them to have more strength with less material. The term ''cellular'' is used for materials having more than 90% porosity. These cellular materials consist two phases, one continuous solid phase which also called ''matrix'' and other continuous or discontinuous gaseous phase which is dispersed in this solid matrix. Nowadays, there are lots of studies focused on improving production routes and properties of cellular materials. Production of different types of cellular materials are achieved such as ceramic foams, polymer foams, and rather new type: metallic foams. These materials are being used intensively in the wide range of applications: such as filters, catalyst, silencers, impact absorbers, and honeycomb panels etc. In the literature, there are lots of different classifications for cellular materials. Among them, the most widely used one is dividing them into two: closed cell cellular and open cell cellular materials. Closed cell structures form when the gaseous phase is kept inside the pores so a continuous solid phase and a discontinuous gaseous phase form. On the other hand, open cell structure is characterized by a continuously dispersed gas phase into a solid matrix. Open cell and closed cell cellular materials are used for different applications due to their physical differences. Because of their high strength and load bearing capacity, closed cell foams are used in structural applications whereas open cell foams are used in functional applications where high strength is not required. Among these cellular materials, metallic foams are finding new applications in engineering and technology areas due to their unique combination of properties such as high strength/weight ratio, load bearing capacity, high liquid and gas permeability, ability to preserve its properties at high temperatures, impact and sound absorption capacity. Also, in the wide range of foamable metals, titanium and titanium alloys are getting more and more attention because of their high melting point, superior mechanical properties, lightweightness and biocompatibility. However, their high reactivity with the gases present in the atmosphere and their high melting temperature prevent the usage of liquid and gaseous route foam production methods applied for other materials due to difficult controlling of the reactions. Therefore, during most of the production stages (melting of titanium and powder processing) controlled atmosphere is neccessary. Also, many of the melt infiltration techniques used for foam production are not suitable for titanium alloys due to their high reactivity with most of the mould materials. Thus, for production of titanium and titanium alloy foams, generally powder metallurgy routes are preferred. Methods used to produce titanium and titanium alloy foams are space holder technique, bubble generation (controlled argon expansion in molten titanium), rapid prototayping (RP), freeze casting, conversion of porous ceramic precursor to metallic titanium foam and polymer replication method. Among these methods, polymer replication method catches attention because it is economic, simple and fast compared to others. Also, mechanical properties of the foam can be adjusted easily by changing porosity properties of the model material, because mechanical properties of foams are highly depend on porosity distribution and size. The method is the most widely used industrial technique for ceramic foam production. But despite its advantages, the application of this method to titanium and titanium alloy foams is not well studied. Thus, the aim of this study is to produce open cell titanium foam with polymer replication method and optimize process parameters to achieve superior mechanical properties. Polymer replication method consists preparation of slurry, coating the model material with the slurry, drying and later heat treatment for volatilizing model material and sintering processes. Rheological properties of the slurry solution has a criticical impact on its ability to coat model material properly and uniformly. Despite being a simple method, finding the optimum slurry composition for uniform and proper coating is a difficult process and requires lots of trials. In this work, TiH2 powder with 8,771,61 m particle size is used as the matrix. Besides, to optimize rheological properties of the slurry solution, distilled water as a solvent, for binding properties PVA (polyvinyl alcohol), for homogenous distribution of powders in the slurry Dolapix (polycarboxilic acid) and ammonium are added. The optimum slurry composition is chosen as: 75 gr TiH2 powder, 40 ml distilled water, 6 gr PVA, 1 gr Dolapix and 2 ml ammonium. Prepared slurry ia coated on polyurethane foam, then the green body is dried firstly in room temperature for 24 hours then in an oven for 6 hours at 1100C to get rid of the water in the slurry. Later, to volatilize the polyurethane foam and to sinter the metal coating heat treatment process is applied. For heat treatment, induction sintering is chosen to shorten long sintering time and make the method more industrially applicable. This is the first first known study that investigates induction sintering time and temperature effects on titanium foam production with this method. Firstly, preheating is applied to the samples to prevent fragmentation of the green body due to rapid volatilization of the polyurethane foam. For preheating, holding time is chosen as 6 minutes and applied current is 5 Amper, approximately 4000C. Later, to invastigate temperature and holding time effect on the samples, four different sintering processes are applied. Sample A is sintered for 40 minutes at 13500C ± 500C, Sample B is for 30 minutes at 13500C± 500C, Sample C is for 40 minutes at 12500C ± 500C and Sample D is for 30 minutes at 12500C ± 500C. For characterization of titanium foams produced by polymer impregnation method and induction sintering, macrostructure and microstructure investigations, XRD analysis and Vickers microhardness tests are done. As a result, sintering time and temperature are found to be effective on mechanical and chemical properties of produced foams. Different morphological structures are found on different parts of the foams. This difference might have occurred due to non-homogenous temperature distribution along the body of the metallic foam. The furnace used for the production of foams has an indirect induction heating system. Thus, firstly graphite crucible is heated, and then heat transfer to samples occurred by radiation and convection from the graphite crucible, causing the heat distribution became uneven along the sample body. Uneven temperature distribution might be prevented by suitable furnace design. It is well known that titanium is a very reactive metal and has a great affinity for oxygen, hydrogen, carbon and nitrogen as mentioned. Thus, XRD analysis have done to investigate if these elements are presented in sample bodies. As a result, XRD analysis have given both titanium and titanium carbide peaks on all of the samples, but nitrogen, hydrogen and oxygen is not observed as seen in the other studies in the literature. As the sintering time and temperature is increased, total amount of titanium carbide also increased and titanium peaks became weaker. In the system, firstly polyurethane foam decomposed between 250-4000C and it is followed by the decompositon of TiH2 to titanium and hydrogen at approximately 4950C. Titanium carbide formation reaction from Ti and C starts approximately at 6000C and ends at 11000C. From there, it is understood that higher TiC content is observed as the sintering temperature is increased, also longer sintering time provided more time for the TiC formation reaction to take place. In the system, oxygen contamination is prevented by hydrogen formation during the decomposition reaction of TiH2, if any oxygen was left in the furnace, it firstly reacted with hydrogen because it has a greater affinity for oxygen than titanium, as Ellingham diagram shows. Also, some amount of carbon left in the system by the decompositon of polyurethane foam also might have reacted with the oxygen if any left. Thus, it provided titanium an oxidation protected atmosphere. Maximum solubility of oxygen, nitrogen and carbon in α titanium is 30%, 19% and 2% respectively. For this reason, as the temperature of metallic foams decreased to room temperature, carbon content has crossed the solibility level, leading it to react with titanium. As a result, formation of TiC takes place. Also, in other studies, it is shown that carbon content in produced titanium foams is much higher than oxygen and (ten times more) nitrogen, absence of this small amount of nitrogen in the produced titanium foams might be due to working under vacuum and much shorter sintering time (30-40 min) than in other studies (12-17 hours), which lead to more effective sintering. Moreover, to investigate mechanical properties of the produced samples, micro hardness tests are done. Firstly, metallographic sample preparetion is complated. Samples are mounted and then grinded. For grinding, sandpapers with 240, 400, 600, 800, 1200 and 2500 mesh sizes are used respectively. Then polishing is done with 1 µm and 3 µm diamond solution. After polishing step, Vickers microhardness tests are applied five times and the avarage of these five result is taken. As a result, Vickers hardness values of samples are found to vary between 49846 Hv and 86549 Hv which is a lot higher than titanium Vickers hardness. Hardness value of the samples are increased with longer holding time and higher temperature, due to increasing carbide formation. Working at lower sintering temperatures or much higher vacuumed atmosphere might help to prevent carbide formation.