Chapter 7: Powder Processing

Chapter 7: Powder Processing

• Powder metallurgy (PM) is a process that involves metal powders, compacted and then sintered (heated without melting) to form a solid piece.

• Common metals used in PM include iron, copper, aluminum, tin, nickel, titanium, and refractory metals. Brass, bronze, and steels, often use pre-alloyed powders.

• PM processing consists of powder production, blending, compaction, sintering, and finishing operations.

Introduction

• The choice of metal powder production method depends on the desired properties of the final product, including microstructure, bulk and surface properties, chemical purity, porosity, shape, and particle size distribution.

• Particle sizes produced range from 0.1 to 1000 μm.

• Different methods yield different particle shapes (acicular, irregular rodlike, flake, dendritic, spherical, irregular, rounded, porous, angular).

Methods of Powder Production

• Atomization: involves a liquid-metal stream created by injecting molten metal through a small orifice (gas or water atomization). Size and shape depend on molten metal temperature, flow rate, nozzle size, and jet characteristics. Centrifugal atomization uses a rapidly rotating disk or cup to break up the metal stream. Atomization with a rotation consumable electrode involves rapidly rotating the electrode within a helium-filled chamber.

• Reduction: Uses gases like hydrogen and carbon monoxide as reducing agents to reduce metal oxides to their metallic state. The resulting powders are porous and spongy.

• Electrolytic Deposition: Uses aqueous or fused salts to produce highly pure metal powders.

• Carbonyls: Forming carbonyls from iron or nickel reacting with carbon monoxide, then decomposing into pure, spherical metal particles.

• Comminution: Crushing or grinding brittle or less ductile metals in a ball mill to produce small particles. A ball mill is a machine with a rotating hollow cylinder partially filled with steel or white cast-iron balls.

• Mechanical Alloying: Mixing powders of two or more pure metals in a ball mill. Impact of balls causes powder fracture and diffusion, forming alloy powders.

Step 2: Blending Metal Powders

• Blending (mixing) of metal powders is essential for uniform physical and mechanical properties.

• Additives like lubricants and binders (used in sand molds) improve flow characteristics and facilitate sintering.

• Powders must be blended under controlled conditions to prevent contamination.

Step 3: Compaction of Metal Powders

• Blended metal powders are pressed into desired shapes within dies.

• Compaction aims to achieve the required shape, density, and particle-to-particle contact.

• Methods: Isostatic pressing (CIP) uses hydrostatic pressure in a flexible rubber mold. Hot Isostatic Pressing (HIP) uses high-temperature inert gases or a vitreous (glasslike) fluid within a high-melting-point metal container. Powder injection molding (PIM) blends fine metal powders with a polymer or wax-based binder. Rolling compacts metal powders into a continuous strip using shaping rolls. Spray Deposition is a shape-generation process that uses atomization and a spray chamber with an inert atmosphere, and a mold to for example produce seamless tubing and pipe. Different compaction methods yield varying properties, like different mechanical and physical properties of the compacted metals.

• The pressed powder is called a green compact. Higher pressure leads to higher density in the green compact. The size distribution of the particles also impacts the density.

Step 4: Sintering

• Sintering is the heat treatment of green compacts in a controlled atmosphere furnace to bond individual particles together at a temperature below their melting points.

• Sintering temperature and time vary with different materials and can be determined by a sintering process chart.

• Continuous-sintering furnaces usually have three chambers: a burn-off chamber, a high-temperature chamber, and a cooling chamber.

• Sintering mechanisms are diffusion, vapor-phase transport, and liquid-phase sintering. Liquid-phase sintering occurs when the particles of different metals have varied melting points.

Step 5: Secondary and Finishing Operations

• operations performed on sintered products to improve properties, dimensional accuracy, and surface finish such as coining and sizing, cold and hot forging or impact forging, and other machining processes (e.g., milling, drilling, tapping).

• Additional finishing operations such as machining, grinding, plating, and heat treating can further enhance hardness, strength, dimensional accuracy, surface finish, and corrosion resistance in finished P/M products.

• Impregnation and infiltration reduce porosity in the sintered parts.