Powder metallurgy was practiced long before ancient artisans learned to melt and cast iron. Egyptians made iron tools using PM techniques back in 3,000 B.C. The ancient Incans made jewelry and artifacts using precious metal powders. The first modern PM product were the tungsten filaments for electric light bulbs developed in the early 1900s. These were followed by tungsten-carbide cutting tools and self-lubricating bearings in the 1930s; automobile parts in the ’60s and ’70s; aircraft turbine engine parts, powder-forged connecting rods, and metal-injection-molding (MIM) in the ’80s; warm compacting in the ‘90s; and nanotechnology and the acceptance of metal 3D printing/additive manufacturing (AM) in the new century.
Benefits and milestones of PM include:
- A cost-effective method of forming precision net-shape metal components that allows for more efficiently designed consumer and industrial products.
- The source of high performance, advanced particulate materials and alloys such as superalloys, tool steels, PM aluminum alloys, dispersion strengthened metals, thermal spray materials, intermetallics, and metal matrix composites.
- New manufacturing processes for better product quality and productivity.
- Increased raw material use through recycling and eliminates costly secondary machining through net and near-net shape design.
- Improves productivity by eliminating manufacturing.
- Delivers precision and special properties such as self-lubrication and controlled filtration.
- Only way to form vital metals such as tungsten carbide, dispersion-strengthened materials, high-speed tool steels, superalloys, and self-lubricating bearings.
- Sustainability: Saves natural resources through recycling and conservation of critical raw materials.
- Strategically important in making automobile engines and transmissions, aircraft turbine engines, riding lawn mowers, surgical instruments, power tools, oil/gas well drilling equipment, and off-road tractors.
The PM parts and products industry in North America has estimated sales of $7 billion. It is comprised of companies that make conventional PM parts and products from iron, copper-base-powders, and stainless as well as companies that make specialty PM products such as superalloys, porous products, friction materials, strips for electronic applications, high strength permanent magnets, magnetic powder cores and ferrites, tungsten carbide cutting tools and wear parts, metal injection molding (MIM) parts, metal additive manufacturing (AM) parts, and tool steels—all from metal powders. PM is international in scope with growing industries in all of the major industrialized countries. Annual worldwide metal powder production exceeds 700,000 tons.
The most common metals available in powder form are iron and steel, tin, nickel, copper, aluminum, and titanium, as well as refractory metals such as tungsten, molybdenum, and tantalum. Alloys such as bronze, brass, stainless steel, and nickel cobalt superalloys are also available in powder form.
Powder particles range in size from 0.1 to 1,000 micrometers. Major methods for making metal powders are atomization of molten metal, reduction of oxides, electrolysis, and chemical reduction.
Trends and New Developments
- Improved manufacturing processes such as cold and hot isostatic pressing (CIP/HIP), powder forging (PF), metal injection molding (MIM), metal additive manufacturing (AM), and direct powder rolling developed and advanced by governments, academics, and industrial R&D programs.
- Fully dense PM products for better strength properties and quality in automobiles, diesel, and turbine engines, aircraft parts, and industrial cutting and forming tools.
- Commercialization of technologies such as metal injection molding, metal additive manufacturing, PM forging, high-temperature vacuum sintering, warm compacting, and both cold and hot isostatic pressing.
- The use of powder forged (PF) connecting rods, PM composite camshafts and main bearing caps, and stainless-steel ABS sensor rings and exhaust system flanges in automobiles.
- New submicron and nanophase powders for cutting tools and other specialized applications.
PM parts are used in a variety of products such as locks and security hardware, garden tractors, snowmobiles, automobile engines and transmissions, auto brake and steering systems, washing machines, power tools and hardware, sporting arms, copiers and postage meters, off-road equipment, hunting knives, hydraulic assemblies, x-ray shielding, oil and gas drilling wellhead components, fishing rods, and wristwatches. Canadian nickels are made from pure nickel powder.
More than an estimated 1.5 billion PM hot-forged connecting rods have been made for light duty vehicles produced in the U.S., Europe, and Japan.
Commercial aircraft engines contain 1,500-4,400 lb of PM superalloy extruded forgings per engine.
Iron powder is used as a carrier for toner in electrostatic copying machines. Americans consume more than two million pounds of iron powder annually in iron-enriched cereals and bread. Iron powder is also used in hand warmers and waterproof cements.
Copper powder is used in anti-fouling paints for boat hulls and in metallic pigmented inks for packaging and printing.
Aluminum powder is used in solid fuels for rockets such as the booster rockets for the space shuttle program.
The basic PM process uses pressure and heat to form precision metal parts and shapes. Powder is compacted (at room temperature) in a rigid precision die at up to 50 tons per square inch into an engineered shape like a gear. Think of 50 compact cars stacked vertically and you have the pressure it takes to press the powder. After parts are compacted and ejected from the die, the part is fed slowly through a special controlled atmosphere furnace to bond particles together. They are metallurgically fused without melting, a phenomenon called “sintering.”
Other processes used to turn powders into finished shapes include cold and hot isostatic pressing (CIP/HIP), powder forging (PF), metal injection molding (MIM), direct powder rolling, gravity sintering, and metal additive manufacturing. Metal AM builds parts layer-by-layer without using a mold or die but by sintering or welding each individual particle of powder.
In contrast to other metal forming techniques, PM parts are shaped directly from powders while castings are formed from metal that must be melted, and wrought parts are shaped by deformation of hot or cold metal, or by machining.