Other Powder metallurgy Process
There are several alternative methods to form parts from powder stock using different pressing and sintering technology .Many of these manufacturing processes have comparable qualities in their technique to conventional powder processing. Similarities in processing will rely on similar reactions and mechanisms within the material. Some factors are the same, alternative powder processes will have differences in their methods. Differences in the manufacturing process will affect the size, geometry and complexity of the part that can be produced. Mechanical properties of the product's material are used dependent upon the type of alternative power metallurgy process. Some special powder processes is well suited for a particular type of part such as:
- Cold Isostatic Pressing (CIP)
- Hot Pressing
- Roll Compaction
- A feeding system, which conveys the powder to the compaction area between the rolls
- A compaction unit, where powder is compacted between two counter rotating rolls to a ribbon by applying a force
- A size reduction unit, for milling the ribbons to the desired particle size.
CIP is produced at room temperature and uses a mold made from an elastomer material such as urethane, rubber, or polyvinyl chloride. The fluid in cold isostatic pressing is usually oil or water. Fluid pressure during the operation is typically from 400 MPa to 1000 MPa). CIP is used to consolidate metal- and ceramic powders to make a "green" part that can be further treated, i.e. rolling, machining or sintering. First the powder is compacted to a very uniform density by cold isostatic pressing. Then the green compact is usually sintered conventionally, to produce the desired part.. The most attractive thing about isostatic pressing is that the powder is compacted with the same pressure in all directions, and, since no lubricant is needed, high and uniform density can be achieved.
Hot pressing involves pressing and sintering a powder part in a single step. The force of the die compact the powder, at the same time heat is being applied to the work. Good mechanical properties and dimensional accuracy can be achieved with this manufacturing. The magnitude of heat and pressure varies and is particularly dependent on powder material. When hot pressing most powders, the mold is made from super alloys. Processing refractory metals will require mold materials with more thermal resistance.
A controlled atmosphere should be maintained for hot pressing. mold materials must be capable of holding integrity under the conditions of extreme temperature and pressure that this process requires Graphite molds may be used for hot pressing such metals. Conventional Hot pressing is a state of the art technology used for the manufacturing of high performance materials. This technology is frequently used for the manufacturing of various materials which are difficult to prepare via liquid phase processing or which are not sufficiently densified by using powder technology such as pressing and sintering. Materials such as pure nitrides, carbides, oxides or borides with densities close to 100 % of the theoretical density are examples.
Roll compaction, or powder rolling, is a process that can manufacture continuous lengths of metal strip, or sheet, from powders. Powder stock is fed into the gap between two rotating rolls. This is usually performed cold. The powder is compacted into a sheet and pushed forward. It then travels into a furnace where it is sintered. After which, the sheet is often rolled again and sintered for a second time. Further processing of the work is dependent on the desired material properties and use.
Roller Compaction generally consists of three major units: