Powder Metallurgy Research Paper

Centrifugal Atomization: in this method the raw material is made as an electrode and plasma arc is created by bringing a non-consumable electrode to close proximity. The raw material electrode is rotated at high speeds and a splash of liquid metal is ejected from the electrode and is then cooled by the inert gas and metal powders are collected.

Special Characteristics of Powder Metallurgy
Various characteristics of powders are depending upon both strength and density of green compact and sintered product.
1. Surface Area: this is the area per unit mass of metal powders. It indicates the area for bonding.
2. Density: this indicates degree of porosity in green compact or sintered product. There are three types of density (porosity) is defined.

Mixing and Blending: Mixing and blending is generally done in a ball mill process. To obtain uniform distribution of particle sizes and making of homogeneous alloyed powders mixing and blending are required. To ensure filling up of interstices between large particles and additives to aid the process of sintering although lubricants are added to reduce die wall friction.
COMPACTING: The main purpose of compacting is to obtain sufficient green strength and also to increase apparent density. Compacting pressure may vary between 80 to 800 MPa. for every powder there is an optimum pressure. During the compacting process green strength is obtained due to following reasons.
(a) Sliding action with pressure promotes adhesion (b) mechanical interlocking takes place (c) bonding agents occupy the interstices and increases the adhesion.

Figure 3.5 Schematic Diagram of Compacting Process
1. With the upper punch in the withdrawn position, the empty die cavity is filled with mixed powder.
2. The metal powder in the die is pressed by simultaneous movement of upper and lower punches.
3. The upper punch is withdrawn, and the green compact is ejected from the die by the lower punch.
4. The green compact is pushed out of the pressing area so that the next operating cycle can start.

Cold Isostatic and Hot Isostatic Process Figure 3.6 Schematic Diagram of Cold isostatic Pressing

The temperature used for sintering is below the melting point of the major constituent of the Powder Metallurgy material.

Preheating Cooling

Conveyor
Figure 3.7 Sintering Operation
The stages of sintering are i) Preheating ii) Heating lower than melting point iii) cooling
Characteristics obtained during the sintering operation are
1. Voids tend to be closed
2. Residual stress on the powders caused during compaction is reduced.
3. Impurities are forced to be out during process.

Applications of Powder Metallurgy Parts
Some parts can only made by powder metallurgy route.
1. light bulb filaments made with powder technology
2. linings for friction brakes
3. metal glasses for high-strength films and ribbons
4. heat shields for spacecraft reentry into Earth's atmosphere
5. electrical contacts for handling large current flows
6. magnets; microwave ferrites
7. Production of self lubricating bearings
8. Production of welding electrodes
Advantages of Powder