JNTU B.Tech III POWDER METALLURGY Previous Question Paper (August - 2022 - R18)

Powder metallurgy is a critical manufacturing process for producing near-net-shape components from metal powders. Its primary importance lies in its ability to produce complex parts with high precision, minimal material wastage, and the capability to create unique alloys or composite materials that are impossible to achieve through conventional melting and casting. It is particularly valuable for processing refractory metals, porous materials like self-lubricating bearings, and materials with high melting points.

Milling is a mechanical process used to reduce particle size and modify the shape or characteristics of metal powders. Key methods include:

• Ball Milling: Metal powders are placed in a rotating drum with grinding media (balls). The impact and attrition forces break down the particles. This is effective for brittle materials and for achieving fine or nanocrystalline powders.
• Attritor Milling: An intensified version of ball milling where a central shaft with arms agitates the balls at high speeds, significantly reducing processing time and achieving finer particle sizes.
• Hammer Milling: Uses high-speed rotating hammers to impact and shatter particles, primarily used for coarser reduction.
• Vibratory Milling: The container is vibrated at high frequencies, causing the grinding media to collide with the powder, suitable for producing very fine, uniform powders with minimal contamination.

The carbonyl process is a chemical method used to produce high-purity iron powder. The process relies on the formation and subsequent thermal decomposition of iron pentacarbonyl, Fe(CO)₅.

1. Carbonylation: Impure iron is reacted with carbon monoxide (CO) gas under elevated pressure (approx. 50-200 atm) and moderate temperature (approx. 150-200°C). This reaction forms iron pentacarbonyl, which is a liquid at room temperature.
Fe + 5CO → Fe(CO)₅
2. Purification: The liquid iron pentacarbonyl is distilled to remove impurities, as it has a relatively low boiling point, ensuring the final metal powder is of extremely high purity.
3. Thermal Decomposition: The purified vapor is introduced into a decomposition chamber heated to approximately 250-300°C. At this temperature, the iron pentacarbonyl decomposes, releasing pure iron and carbon monoxide gas.
Fe(CO)₅ → Fe + 5CO
4. Recovery: The resulting iron powder is spherical and very fine. The carbon monoxide gas is recycled back into the process to react with more iron, making it an efficient closed-loop system.