JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV POWDER METALLURGY Previous Question Paper (February/March - 2022 - R16)

Powder metallurgy has several inherent limitations despite its advantages in net-shape manufacturing. Firstly, tooling costs are extremely high, making it economically unfeasible for small-batch production; it is primarily suited for mass production where costs can be amortized. Secondly, the size and shape complexity of parts are restricted by the press capacity and the ability of the powder to flow into die cavities; for example, parts with very thin walls or intricate re-entrant angles are difficult to produce because powders do not flow like liquids and exhibit limited lateral pressure transmission. Thirdly, material properties can be limited by residual porosity, which acts as stress concentrators, reducing ductility and fatigue strength compared to wrought counterparts. Finally, raw material costs for high-purity, spherical metal powders are often higher than those for bulk cast or rolled materials.

Metal powder particles exist in various morphologies, including spherical, angular, acicular (needle-like), dendritic, and flaky shapes. These shapes are primarily determined by the production method, such as atomization, reduction, or electrolysis.

The shape of the particles significantly influences the flow rate, which is the time required for a specific mass of powder to flow through a standard orifice (e.g., a Hall flowmeter):

• Spherical particles: These exhibit the best flow characteristics due to minimal surface area and point-to-point contact, resulting in low inter-particle friction. They roll over each other easily, leading to high flow rates.
• Angular and Irregular particles: These have higher surface-to-volume ratios and interlocking tendencies. The increased friction between irregular surfaces significantly hinders flow, resulting in lower flow rates.
• Acicular and Flaky particles: These possess high aspect ratios and tend to bridge or arch within the funnel, often resulting in very poor flow or complete blockage of the orifice.

In summary, as the particle shape deviates from a sphere toward more irregular or elongated forms, the inter-particle friction increases and the packing density decreases, leading to a reduction in the flow rate.