COMPARATIVE ANALYSIS OF CHARACTERISTICS OF STAINLESS STEEL CELLULAR MATERIAL PREPARED THROUGH POWDER METALLURGY USING ACCICULAR AND CRUSHED UREA AS SPACEHOLDER SHAILENDRA JOSHI* *Department of Mechanical Engineering, NPSEI, Pithoragarh, Uttarakhand, India. ABSTRACT: Stainless steel foams are widely used in structural applications where light weight is required along with high strength especially used in impact energy absorbtion applications where damping capability is required such as in vehicles and buildings. The properties of stainless steel foam, to a large extent depends on the pores arrangement which is decided by the space holder utilized during manufacturing SS foam through powder metallurgy. In the present paper we will discuss the pores distribution and true stress-strain curve of stainless steel foam obtained using accicular and crushed urea as space holder. Depending upon their structure and above characteristics, resulting applications are studied. Keywords: foams, damping, arrangement, accicular, crushed INTRODUCTION Metal foams are porous structures in solid metal matrix where pores occupy a large volume. They have various excellent properties like greater compression strength,low density, better stiffness, etc. which make it desirable for structural & functional applications. The commonly used metal foams are made up of aluminium, titanium, tantalum and steel. Foams microstructure comprises mainly solid and voids. As a result of distinctness in various voids structure in terms of dimension and shape, the behavior of metal foams is more. As such analysis of this uncertainty in the microstructure of metal foams is of much importance. Various methods had been developed for manufacturing stainless steel foams e.g. melt-gas injection, powder metallurgy, investment casting [5], etc. Since powder metallurgy is cost effective and flexible it has been considered a suitable method for stainless steel foam fabrication [5-7]. Also it results in better uniform distribution of pores in foams, more precise control of process variables and pore size[8] & high relative density. Steel foam is a material that can create materials with a variety of different morphologies that have desirable mechanical properties. Metal foams have an extended stress plateau in their compressive stress-strain curves, which is effective for absorbing energy at a constant stress level. Metal foams are affective in automotive, railway, aerospace and chemical applications, where low weight, reduced chemical pollution and high degree of comfort and safety is desired [1] EXPERIMENTAL WORK Firstly, Stainless steel powder of appropriate size as well as acccicular urea are weighed as per to obtain porosity as 40, 50 & 60% respectively .Afterwards they both are kept in a crucible apparatus in a definite amount for obtaining proper mixture . In same manner, the samples of crushed urea particles are prepared maintaining porosity 40, 50 & 60 %. These mixtures are than pressed in a hydraulic machine by putting inside a cylindrical die of 10 mm diameter at a pressure of 260 MPa for half minute producing green compacts(cylindrical shape) producing definite number of samples. Then, preheating of green compacts 0 is carried inside the tubular furnace near 340 C, for nearly two and half hour. During sintering process the samples are heated up to 11100C for a time interval of 115 minute at a heating rate of 120C /min. Then the samples are kept in the furnace for a time period of 65 min. so that the samples volume attains a constant temperature of 1100 °C. Then the samples are kept in the furnace for cooling. The pore morphology of both types of distinct samples obtained through accicular and crushed carbamide is studied on SEM. Then, various properties of the samples are characterized after performing compression test on the specimen on universal testing machine. RESULTS AND DISCUSSION Green compact samples compacted surface are analyzed on SEM. The micro structural image depicts the distribution of urea particles in the matrix as shown in figure. In the image the black area shows SS particles & white regions represent particles of urea. We can observe from the SEM images that larger size pores results in case of green stainless steel compacts containing accicular urea whereas small size uniformly distributed pores in case of that containing crushed urea particles. This occurs due to variation in the size of urea particles and contributing mixing in crucible. Due to the evaporation of urea particles, whether accicular or crushed, during preheating of green compact samples the pores gets produced. The pores obtained are larger in case of samples resulted from accicular urea than that of crushed urea particles due to deformation and distortions resulted by increase in temperature and evaporation. Pores dimension are more as compared to f the dimensions of urea particles because on getting sufficiently heated, the urea particles evaporate after the mechanical bonds broken in the foam structure which produced larger sized voids. Both types of samples were than sintered to give strength and produce metallurgical bonds in the stainless steel foam samples. The sintering is done at a suitable temperature between preheating temperature & melting point. The samples with higher percentage of urea are irregular in shape because after preheating of green compacts, the samples with higher percentage of urea have higher porosity making the pre-heated compact much weaker. Due to less bonding strength the samples having higher percentage of urea did not sustain it’s shape after preheating. The irregularity in shape is more in case of samples obtained through accicular urea particles than that obtained from crushed ones but the strength is increased after sintering. The voids obtained are less in dimensions than that of the pores obtained after preheating due to the large driving force producing more pore shrinkage during sintering. SEM Micrograph of green compact ss matrix containing accicular urea SEM Micrograph of preheated sample containing acicular urea particles SEM Micrograph of preheated sample containing crushed urea particles SEM Micrograph of the green compact ss matrix comprising of crushed urea particles Sintered stainless steel foam samples with varying porosity processed using acicular crystals of urea accicular while in foam samples containing crushed urea are approximately circular. This is due to the shape of space holder particles and metallurgical bonding in stainless steel matrix during sintering. Sintered Stainless steel foam samples with varying porosity processed using fine urea particles During the compression test of stainless steel foam samples of 40, 50 & 60% porosity, when we use accicular urea as space holder ,the plateau region results when the porosity reaches above 50 vol % while this regions is not distinct in case of samples obtained from crushed urea as space holder due to large size accicular pores obtained in previous case. They have high energy absorbtion capacity. Stainless steel foam’s yield strength samples lowers due to increasing porosity in later case. Also the material is ductile and can be used in lightweight structural applications. SEM Micrograph of sintered foam sample (using acicular urea space holder) SEM image showing sintered foam sample (using crushed urea space holder) The voids resulted in sintered foams while utilizing accicular urea as space holder are Stress versus Strain diagram of ss foam (acicular urea space holder) Stress versus Strain diagram of ss foam (crushed urea space holder) CONCLUSION Thus stainless steel foams can be successfully processed using PM method. Also the degree of porosity can be varied with the quantity and nature of space holder. The pores dimensions depend on the space holder utilized, sintering temperature and preheating &sintering period. The foams having more dimensions pores showed plateau regions at higher porosity while those possessing circular pores doesn’t possess distinct plateau region. The accicular foams therefore have high impact energy absorbtion characteristics & the later crushed particles foams can be utilized in high strength structural applications. 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