Pressing And Sintering Of Powder Parts

Porosity, Impregnation, And Infiltration

Porosity is a characteristic trait of powder processed materials. In some cases the goal is to mitigate or eliminate porosity. In other cases a certain level of porosity is desired. As discussed, porosity exists within the green compact. Amount of porosity in the green compact can be controlled to some extent by the level of pressure used to press the compact. If the compact is not fully pressed, more porosity will occur than with complete compaction. In fact, in loose sintering the powder is not pressed at all, achieving very high porosity for special components such as metal filters.

Measurement of porosity in a metal is usually quantified as the percentage of empty space in the material. Less than 10% is low porosity, 10%-20% is medium porosity and over 25% is considered a highly porous material. Porosity within the pressed green compact is mostly interconnected porosity, where the networks of pores are open to the outside environment. During sintering the volume of the porosity is reduced and many of the voids are isolated from the main pore networks and the outside environment. When pores become isolated within the material they are no longer considered open porosity, but closed porosity.

Impregnation is the filling of the pores in a metal with a fluid. A common application of this is in the production of self lubricating components such as bearings and gears. In these cases, the powder processed part is usually soaked in hot oil. Parts are typically 10%-30% oil impregnated by volume. Sometimes a part will be impregnated with polymer resin to prevent other substances from entering the pores or to assist with further processing.

Infiltration is the filling of a metal's pores with another metal of lower melting point than the base material. The infiltration metal is heated to a temperature above its melting point but below that of the porous metal part. Liquid metal is allowed to enter into the porous network and solidifies, filling the pores with solid metal. Infiltration can produce parts with special mechanical properties. Iron infiltrated with copper is a common example of this process in manufacturing industry.

As mentioned, pores may become isolated during powder processing. The amount of interconnected, open, (not isolated), pores is a critical factor for impregnation and infiltration. Material can not enter pores cut off from the outside environment. Amount of open porosity can be measured by the amount of fluid necessary to saturate the part.

Further Processing Of Powder Manufactured Parts

With powder metallurgy, as with most other major manufacturing processes, there is often a need for further processing of the product. Porosity of parts manufactured by powder methods is a special factor in secondary, or finishing, operations performed on such parts.

The combination of powder metallurgy and forging is able to produce extremely high quality parts. A part is powder manufactured with about 15%-20% porosity, and then hot forged. Typically the forging is flashless. Hot forging eliminates porosity, increasing the density of the part. Hot forging of the powder processed part also creates a uniform wrought grain structure. Secondary processing of a powder manufactured part by forging greatly enhances the mechanical properties of the part. Since the powder forming produces the part to near net shape, the forging is often performed in one step. Other metal forming processes such as rolling and extrusion can also be employed as secondary operations on already pressed and sintered parts. These processes may also be performed cold or hot.

Machining of parts produced by powder metallurgy is common in industrial manufacturing practice. Machining is not often used for bulk removal, but for fine detail. Certain particular part features, such as side holes, cannot be produced by powder processing. Features not created during the pressing and sintering of the part can be produced latter by machining. A common problem with the machining of powder processed parts is the unwanted impregnation of the part's porous structure by coolant and lubricating fluids used during machining. Techniques such as machining dry and infiltration, (or impregnation), of the work with other materials are employed to solve this problem.

Sizing and coining are common finishing operations. Sizing is sometimes used on parts manufactured by powder processes in order to form the part to its final dimensions. Sizing involves only a small, but accurate, geometric change in the work. While sizing can be used to increase geometric accuracy, coining can be employed to improve surface finish and add details to parts created by powder processing.

Heat treatment of powder processed products can be performed, provided the porosity of the work is calculated as a factor in the manufacturing process. Increased porosity will decrease the thermal conductivity of the metal part, causing it to heat and cool slower than otherwise.

Surface processes such as painting or electroplated are also used in the finishing of powder processed products. As with other secondary and finishing processes, the porosity of the work must be considered. Materials for surface coatings should not be absorbed into the porous structure of the work. Infiltration and impregnation of porous parts is commonly employed to allow for surface treatments.