Powder Injection Molding Processes
Sintering operations are the most important step in the powder metallurgy techniques. You will find detailed information about the sintering operations and the operations that applied before and after the sintering operations.
First of all, the difference between the blending and the mixing of the metal powders. Blending is the blending of the same metal powders with the help of a blending chamber. Blending is very important for the mixing of the different-sized metal powder particles homogeneously.
Mixing is the mixing of the different types of metal powders in one mixing chamber. Mixing also very important for the productioın of the metal alloys, which can not be alloyed in conventional methods. If the different types of metal powders are mixed homogenously, successful sintering of these metals can be achieved.
There are different types of mixing and blending mechanisms are available.
In the first mixing mechanism, it is a very basic rotating drum that includes the metal powder to be mixed. Rotating double-cone is another type of mechanism that has the physically same characteristics as the rotating drum.
Screw-mixer is a different mechanism that includes a big screw that rotates inside a cylinder to mix the metal powder. Blade-mixer is another type of mixing mechanism that includes a blade, instead of a screw.
To make the sintering operation easier, there are different additives are added in the mixing or blending step to metal powders.
Compaction is the pressing of the mixed and blended metal powders inside the mold cavity, with the help of mechanical or hydraulic presses. Compaction is very important before the sintering operations because the contact between metal particles must be increased.
The compacted part can be handled inside the manufacturing site to send them to sintering operations. The bulk density is increased in the compaction process, which is called green compact density. The produced part is also called a green compact.
Different kinds of mechanisms are used in the compaction of the metal powders. Also, hydraulic and mechanical hybrid systems are used.
Volume is also reduced with the compaction of the metal powder.
The sintering operation is the most important step to produce parts with powder metallurgy techniques. Sintering is heat treatment of the produced compacted part, which created solid-state bonding between compacted pores. With these solid-state bonds, more rigid and strong structures were obtained.
The sintering temperatures range between 0.7-0.9 times the melting temperature of the metal.
The physical phenomenon behind the sintering heat treatment is the surface energy of the separate metal particles. When the surface energy of the metal particles with the application of heat, the bonds are created. Necking occurs between metal particles and these necks turn into grain boundaries.
Shrinkage occurs because of the compacting and sintering operations. The decrement of the volume of the total part must be calculated before the operation. In these calculations, decrement between particle spaces with compacting and sintering operations must be considered.
The sintering application usually has three steps. In the first step of the sintering operation, pre-heat is applied to eliminate the additives such as lubricants and others. These additives are burned off. And the total heat is applied for the main sintering operation for a while. Cooling of the material is done after the sintering operation.
Furnaces must be specially designed for the sintering operation. The internal environment of the furnaces must be special for sintering operations.
The sintering furnace atmosphere must be provided to prevent internal and external oxidations of the part. Also, the atmosphere must eliminate the burned additives inside the sintered part. Vacuum atmospheres are used for stainless steel production and ammonia, hydrogen, natural gas environments in the sintering furnaces are available.
To improve the characteristics of the produced sintered part, there can be secondary operations are applied.
Sizing and densification of the sintered parts are done by squeezing the part between presses. The densification process increases the density of the sintered part to obtain a more solid and rigid structure. Sizing is applied to obtain better dimensional accuracies.
After the sintered parts are produced, oil or other liquids are impregnated to pores. With this method, self-lubricating bearings or other mechanical parts are produced.
The pores between the particles of the sintered parts are infiltered with another molten metal. Molten metal goes into the pores of the sintered part with the capillary action. And the melting temperature of the molten metal must be lower than the metal of the sintered part. The obtained structure is more strong and rigid.
Heat treatments and finishing operations are applied to sintered parts like other metal parts that are produced with the conventional metal shaping processes.
Coating, electroplating, and painting operations are applied for the appearance of the sintered part. Heat treatment is also applied. The most used plating materials are; copper, nickel, cadmium, and chromium.
This is the general information about the sintering operations in the powder metallurgy techniques.
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