Prepared By: Eng.Ahmed Rabeea Under the supervision of: Dr.Ali H. Haleem 2014-2015 INTRODUCTION In cold-pressed products, mechanical properties are very weak and in some cases can turn back to the powder under the influence of minimal force and to raise the resistance and durability of these pressed and give it the necessary physical and chemical properties, products need to sintering process. Sintering is the thermal treatment of a powder or compact at temperature below the melting temperature point of mean constituent for the purpose of increasing the strength by bonding together of the particles. sintering is carried out at temp. in the range of 0.7_ 0.9 TM of the base metal in multi component system. EFFECT OF SINTERING ON PROPERTIES Sintering will increase: 1- strength. 2- density. 3- ductility. 4- thermal conductivity. 5- electrical conductivity. 6- change in composition is expected due to the formation of solid solution. 7- grain size. PHENOMENA ASSOCIATED WITH THE SINTERING These phenomena can be divided into basic and sub: 1- basic phenomena: A. diffusion and cohesion powder particle. B. rounded pores. C. shrinkage or expansion of product. 2- Secondary phenomena: A. release Residual stresses. B. formation of solid solution or chemical component. C. release of gases. SINTERING PROCESS Primary variables defining a powder sintering operation are time, temperature and furnace atmosphere. Sintering temperature is typically .7 to .9 of the powder's melting point. Sintering time is dependent on manufacturing process factors and material. Tungsten, for example, is sintered for a relatively long time. Standard industrial powder sintering times for different processes and materials vary from 10 minutes to 8 hours. ATMOSPHERE OF SINTERING A controlled atmosphere is critical during powder sintering. The purpose of the atmosphere in sintering is to control carburization and decarburization, prevent oxidation and remove existing oxides, prevent unwanted chemical reactions ,assist in the burning off of additives, aiding the removal of lubricants and composition control and adjusting the impurity levels. Common atmospheres used for industrial powder processes are carbon monoxide, disassociated ammonia, hydrogen, partially combusted natural gas and inert gases such as argon or helium. Sometimes parts are also sintered in a vacuum. Vacuum sintering is mainly applicable to refractory metals and stainless steel. STAGES OF SINTERING Sintering of a green compact occurs in three stages. First, the powder compact is subject to preheating. Preheating will raise the part to a relatively low temperature, providing the burning off of additives. Preheating will also start to strengthen bonds within the part, increasing its integrity for the next stage. In the second stage the temperature is raised to the sintering temperature and maintained for a specific duration necessary for the desired amount of bonding to occur. Temperature is lowered as the part is allowed to cool during the third stage. Keeping the work in the controlled furnace atmosphere during cool down is critical in preventing unwanted chemical reactions between the part and the environment. TYPES OF FURNACES In industrial powder manufacture their are two types of furnaces, batch and continuous. In a batch furnace low quantities of parts are placed in the furnace, undergo the entire sintering process and are removed. Continuous furnaces provide flow through production and have three zones for the three stages of the manufacturing process, (preheat, sinter, and cool down). A moving belt carries a continuous supply of parts through the chambers. Heat doors can rapidly open and close to allow parts through, while keeping heat in. The belt travels at the exact speed to give parts the correct amount of time in each chamber. Consistent products and high productivity rates make continuous furnaces the most common choice for powder sintering. While batch operated furnaces have a lower productivity rate and are less often used, they do provide more control of the atmosphere and hence part purity. Vacuum atmospheres can generally only be provided by batch furnaces. SINTER HARDENING Sintering furnaces are available that can apply accelerated cooling rates in the cooling zone and material grades have been developed that can transform to martensitic microstructures at these cooling rates. This process, together with a subsequent tempering treatment, is known as sintering hardening, a process that has emerged, in recent years, has a leading means of enhancing sintered strength. DRIVING FORCE FOR SINTERING The main driving force that enacts this particle bonding is considered to be a reduction of energy due to a reduced surface area. Powders with a greater surface area will have a higher driving force towards bonding and a lowering of this potential energy. SINTERING MECHANISM In general sintering can be divided in to three stages: Stage 1: necks are formed at the contact points between the particles then neck growth proceeds rapidly but powder particles remain discrete. Stage 2: most densification occurs, the structure recrystallizes and particles diffuse into each other. Stage 3: isolated pores tend to become spheroidal and densification continues at a much lower rate and small pores remain even after long sintering times. TYPES OF SINTERING 1- solid state sintering. 2- liquid state sintering. 3- Transient liquid phase sintering. 4- Viscous flow sintering SOLID STATE SINTERING Solid state sintering occurs when the powder compact is densified wholly in a solid state at the sintering temperature Sintering occurs by diffusion of atoms through the microstructure. This diffusion is caused by a gradient of chemical potential – atoms move from an area of higher chemical potential to an area of lower chemical potential. The different paths the atoms take to get from one spot to another are the sintering mechanisms.The six common mechanisms are: 1. Surface diffusion – Diffusion of atoms along the surface of a particle 2. Vapor transport – Evaporation of atoms which condense on a different surface 3. Lattice diffusion from surface – atoms from surface diffuse through lattice SOLID STATE SINTERING 4. Lattice diffusion from grain boundary – atom from grain boundary diffuses through lattice 5. Grain boundary diffusion – atoms diffuse along grain boundary 6. Plastic deformation – dislocation motion causes flow of matter Also one must distinguish between densifying and non-densifying mechanisms. 1–3 above are non-densifying – they take atoms from the surface and rearrange them onto another surface or part of the same surface. These mechanisms simply rearrange matter inside of porosity and do not cause pores to shrink. Mechanisms 4–6 are densifying mechanisms – atoms are moved from the bulk to the surface of pores thereby eliminating porosity and increasing the density of the sample. LIQUID PHASE SINTERING liquid phase sintering occurs when a liquid phase is present in the powder compact during sintering. For certain materials, such as cemented carbides (like WC,SiC and others) or hard metals, a sintering mechanism involving the generation of a permanent liquid phase is applied. type of liquid phase sintering involves the use of an additive to the powder, which will melt before the matrix phase and which will often create a so-called binder phase.The process has three stages: 1- Rearrangement As the liquid melts, capillary action will pull the liquid into pores and also cause grains to rearrange into a more favorable packing arrangement. LIQUID PHASE SINTERING 2- Solution-precipitation In areas where capillary pressures are high, atoms will preferentially go into solution and then precipitate in areas of lower chemical potential where particles are not close or in contact. This is called contact flattening and densifies the system in a way similar to grain boundary diffusion in solid state sintering.. 3- Final densification Densification of the solid skeletal network, liquid movement from efficiently packed regions into pores. For permanent liquid phase sintering to be practical, the major phase should be at least slightly soluble in the liquid phase and the “binder” additive should melt before any major sintering of the solid particulate network occurs, otherwise rearrangement of grains will not occur. mechanism for sintering metal powders: (a) solid-state material transport; (b) liquid-phase material transport .R= particle radius, r=neck radius, and p=neck profile radius LIQUID PHASE SINTERING In general, compared with solid state sintering, liquid phase sintering allows easy control of microstructure and reduction in processing cost, but degrades some important properties, for example, mechanical properties TRANSIENT LIQUID PHASE SINTERING Transient liquid phase sintering is a combination of liquid phase sintering and solid state sintering. In this sintering technique a liquid phase forms in the compact at an early stage of sintering, but the liquid disappears as sintering proceeds and densification is completed in the solid state .An example of transient liquid phase sintering may also be found in the schematic phase diagram in Figure above when an A–B powder compact with composition X1 is sintered above the eutectic temperature but below a solidus line, for example at temperature T2. Since the sintering temperature is above the A–B eutectic temperature, a liquid phase is formed through a reaction between the A and B powders during heating of the compact. During sintering, however, the liquid phase disappears and only a solid phase remains because the equilibrium phase under the given sintering condition is a solid phase. TRANSIENT LIQUID PHASE SINTERING In a compact that contains only iron powder particles, the solid state sintering process would generate some shrinkage of the compact as the sintering necks grow. However, a common practice with ferrous PM materials is to make an addition of fine copper powder to create a transient liquid phase during sintering. At sintering temperature, the copper melts and then diffuses into the iron powder particles creating swelling. By careful selection of copper content, it is possible to balance this swelling against the natural shrinkage of the iron powder skeleton and provide a material that does not change in dimensions at all during sintering. The copper addition also provides a useful solid solution strengthening effect. VISCOUS FLOW SINTERING Viscous flow sintering occurs when the volume fraction of liquid is sufficiently high, so that the full densification of the compact can be achieved by a viscous flow of grain – liquid mixture without having any grain shape change during densification ELECTRIC CURRENT ASSISTED SINTERING These techniques employ electric currents to drive or enhance sintering. English engineer A. G. Bloxam registered in 1906 the first patent on sintering powders using direct current in vacuum. The primary purpose of his inventions was the industrial scale production of filaments for incandescent lamps by compacting tungsten or molybdenum particles. The applied current was particularly effective in reducing surface oxides that increased the emissivity of the filaments. In 1913, Weintraub and Rush patented a modified sintering method which combined electric current with pressure. The benefits of this method were proved for the sintering of refractory metals as well as conductive carbide or nitride powders. The starting boron–carbon or silicon–carbon powders were placed in an electrically insulating tube and compressed by two rods which also served as electrodes for the current.The estimated sintering temperature was 2000 °C. ACTIVATED SINTERING IN this, an alloying element called ‘doping’ is added in small amount improves the densification by as much as 100 times than un doped compact samples. Example is the doping of nickel in tungsten compacts. REACTION SINTERING IN this process, high temperature materials resulting from chemical reaction between the individual constituents, giving very good bonding. Reaction sintering occurs when two or more components reacts chemically during sintering to create final part. A typical example is the reaction between alumina and titania to form aluminum titanate at 1553 K which then sinters to form a densified product. VARIABLES AFFECTING SINTERABILITY Variables affecting sinterability and microstructure Variables related to Powder: shape, size, size distribution, raw materials agglomeration, etc. (material variables) Chemistry: composition, impurity, homogeneity, etc. Variables related to Temperature, time, pressure, sintering condition atmosphere, heating and cooling (process variables) rate, etc. SUMMARY 1- Sintering is the thermal treatment of a powder or compact at temperature below the melting temperature point of mean constituent for the purpose of increasing the strength by bonding together of the particles. 2- Primary variables defining a powder sintering operation are time, temperature and furnace atmosphere. 3- Sintering of a green compact occurs in three stages preheating, sintering and cooling. SUMMARY 4- sintering types are: solid state sintering, liquid state sintering, Transient liquid phase sintering and Viscous flow sintering. 5- Variables affecting sinterability are material variables (powder shape,size,size distribution and others) and process variables (Temperature, Time, atmosphere and others). REFERENCES 1- The use of powder metallurgy products in the industry by Primakov Fenikov. 2- http://en.wikipedia.org/wiki/Sintering 3-http://www.ipmd.net/Introduction_to_powder_metallurgy/Sintering 4- riad.pk.edu.pl/~mnykiel/iim/KTM/.../pdf/CHAPT06.PDF 5- http://thelibraryofmanufacturing.com/pressing_sintering.html 6- http://en.wikipedia.org/wiki/Powder_metallurgy 7- Previous seminars and other research.
