Powder sintering involves raising the temperature of the green compact, (pressed powder part), to a certain level and keeping it at that temperature for a certain amount of time. The sintering temperature is usually between 70% and 90% of the melting point of the powder metal. This will cause bonding mechanisms to occur between powder particles pressed together in the compact. Bonding within the green compact is weak and this pressed unsintered part usually has just enough structural integrity to be handled. Bonding that occurs during sintering greatly strengthens the part.
During sintering, the individual particle structures disappear and the material forms as a mass. Conventional sintering will not eliminate all porosity in the part, however it does reduce the porosity further. In addition to being reduced in volume, sintering may also isolate areas of the interconnected open porosity in the green compact. These isolated areas become closed porosity, since they are cut off from the outside environment. Sintering is characteristic in the burning off of additives in the powder that were necessary for earlier processing stages. Elimination of constituents no longer needed such as lubricants, binders and deflocculates, is essential to maintain the purity of the material of the final product. In addition to increasing the strength and density of the part’s material, sintering also increases ductility, thermal conductivity and electrical conductivity. Shrinkage will occur during sintering, but will be calculated for when manufacturing process factors are controlled.
Mechanisms that cause bonding during sintering are varied and complex. The main mechanism by which bonding occurs is considered to be diffusion, mechanisms will vary based on manufacturing process factors and powder characteristics. Some other mechanisms that may occur along with diffusion are plastic flow, recrystallization, grain growth, liquid phase material transport and vapor phase material transport. Physical characteristics of different types of bonding may be different. Particle bonding by two different mechanisms is illustrated below. The diffusion bonding shrinks the distance between particles, reducing space. The phase material transport adds material, while keeping the particles the same distance apart.
Bonding mechanisms during sintering are complicated and different, however 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.
Alloying of different metal powders also occurs during sintering. The sintering temperature must always be lower than the melting temperature of at least one of the powder constituents. In some cases, the sintering temperature is above the melting point of one of the materials but below the melting point of the other. This is called liquid phase sintering. Liquid phase sintering can eliminate porosity and produce parts with excellent material properties.
Post time: Mar-04-2017