Powder Metallurgy Process Quiz

Chapter 7: Powder Processing

• Powder metallurgy (PM) is a process that involves metal powders, compacted into a desired shape, and then heated (without melting) to form a solid piece.
• Common metals used in P/M include iron, copper, aluminum, tin, nickel, titanium, brass, bronze, and various steels. Pre-alloyed powders are used for parts requiring specific alloys.
• The PM process consists of powder production, blending, compaction, sintering, and finishing operations.

Introduction

• The PM process involves metal powders compacted into desired shapes using heat without melting.
• Commonly used metals in the process include iron, copper, aluminum, tin, nickel, and titanium, as well as refractory metals. Other alloys such as brass and bronze, are also used making components from pre-alloyed powders.

Production of Metal Powders

• Powder metallurgy involves several steps: powder production, blending, compaction, sintering, and finishing operations.
• Methods to produce metal powders include: Atomization, Reduction, Electrolytic deposition, Carbonyls, Comminution, and Mechanical Alloying.

Methods of Powder Production

• Atomization: Involves directing a liquid-metal stream through a small orifice to break it up using jets of inert gas, air or water (gas/water atomization). This method depends on the temperature of the molten metal, rate of flow, nozzle size, and jet characteristics and results in varied particle sizes and shapes. Centrifugal atomization involves a molten metal stream dropping onto a rapidly rotating disk or cup, using centrifugal force to generate particles.
• Reduction: Involves using gases like hydrogen and carbon monoxide as reducing agents to reduce metal oxides to their metallic state. This production results in spongy and porous powders with uniform spherical or angular shapes.
• Electrolytic Deposition: Uses aqueous or fused salts to produce highly pure powders.
• Carbonyls: Forms metal carbonyls by reacting iron or nickel with carbon monoxide. Reaction products decompose into iron or nickel, creating small, dense, uniformly spherical particles of high purity.
• Comminution: Involves crushing or grinding brittle or less ductile metals into smaller particles, done using ball mills.
• Mechanical Alloying: Involves mixing powders of two or more pure metals in a ball mill. The impact of the hard balls creates fracture and bonding through diffusion, forming alloy powders.

Step 2: Blending Metal Powders

• Different metal powders and additives (lubricants, binders) are blended to achieve specific properties for the final product.
• Binders and other additives help create desired green strength, and facilitate sintering.
• Uniformity in size and shape of the powders is important. This is commonly done by mixing powders using different bowl geometries (e.g. rollers, mixers).

Step 3: Compaction of Metal Powders

• Blended powders are pressed into dies to create desired shapes
• Compaction aims to achieve the required shape, density, and particle-to-particle contact necessary to form a workable and strong part.
• Methods include:
  • Isostatic pressing: uses hydrostatic pressure to create uniform compaction and density
  • Injection molding: similar to die casting, involves blending the powder with a binder, injecting the mixture into a mold at a certain temperature, creating a green compact. These are then put in a low-temperature oven to remove the binder.
  • Rolling: compacts the powder into a continuous strip by feeding the powder into a two-high rolling mill.
  • Spray deposition: creates preforms using an atomizer, spray chamber, and mold.
• Hot isostatic pressing (HIP): uses high temperature inert gas or a liquid medium.

Step 4: Sintering

• Heated green compacts in a controlled atmosphere furnace to allow bonding (fusion) of individual powder particles.
• Sintering temperature and time vary depending on the metal.
• Processes use continuous furnaces with multiple chambers to improve bond strength, prevent cracking, and avoid contamination.
• Sintering mechanisms include diffusion, vapor-phase transport, and liquid-phase sintering, with liquid phase sintering allowing for alloying between different metals.

Step 5: Secondary and Finishing Operations

• Operations to further improve the properties of the sintered P/M product, often including:
  • Coining and sizing: Compacting for dimensional accuracy, improving strength, and surface finish.
  • Machining: Producing various geometric features using milling, drilling, and tapping.
  • Grinding: Improving dimensional accuracy and surface finish.
  • Plating: Improving appearance and resistance to wear and corrosion.
  • **Heat treating:**Improving hardness and strength.
  • Infiltration: reducing porosity by introducing a lower-melting-point metal that melts in the porous material and fills the pores, resulting in a higher density and strength product.

Test your knowledge about the powder metallurgy process with this quiz. Covering topics from production methods to material properties, challenge yourself on the key concepts and techniques used in this advanced materials science field.