Summary
Highlights
Density directly correlates with tensile strength; higher density generally means greater strength. Further strength and wear resistance can be achieved through quench and temper processes, which alter the microstructure. Different microstructures like ferrite, pearlite, and martensite, achieved through carbon addition and heat treatment, significantly increase yield strength.
Conventional press-and-sinter technology involves molding metal powder through compaction in a closed die, followed by sintering and finishing. These parts are used in various industries, including automotive, lawn and garden, recreation, hydraulics, and agriculture.
Size limitations in powder metal components are primarily dictated by equipment, but aspect ratio is a crucial concept. A higher aspect ratio (e.g., a tall, thin component) can lead to a significant density variation, known as a neutral axis, which results in varying mechanical properties throughout the part. Understanding these implications is vital for design.
The basic PM process begins with metal powders, which are blended with lubricants and alloying ingredients. This 'press-ready mix' undergoes compaction in a closed die to achieve a prescribed density. Subsequent sintering forms strong metallurgical bonds between particles, and parts are often ready for use or further finishing.
Most metal powder for conventional PM is produced by water atomization. High-quality metals are melted, poured through a tundish, and hit with high-pressure water, converting the molten metal into powder. After dewatering, the black powder is annealed in a protective atmosphere to remove oxidation, then pulverized, screened, and packed.
Powders are prepared by blending and mixing. Blending involves intermingling powders of the same composition (e.g., two types of iron powder), while mixing combines powders of different materials (e.g., iron and copper) with lubricants and other alloying elements like nickel and graphite.
After mixing, the powder goes to the compacting press. The powder fills a die cavity, and punches compress it, forming the component in a closed die. Compaction establishes the part's density, which is critical for mechanical properties. Increasing density during compaction reduces porosity and increases the contact area between powder particles.
Following compaction, parts are sintered in a continuous mesh belt furnace. Lubricants are removed, and then parts enter a high-heat zone (2050-2100°F) where strong metallurgical bonds, or 'sinter necks,' are formed between powder particles. The process includes de-lubing, high-heat sintering, transition, and cooling zones.
Powder metallurgy is used in various applications, including notch and pocket plates for automotive clutches (made from sinter-hardened steel), automotive planetary carrier systems (pressed and sintered components, with gears made from powder metal forged material), and stator/rotor cores for electric motors (made from soft magnetic composite or sintered soft magnetic material).
Powder metallurgy is a global and well-studied science. Conventional PM offers cost-effective solutions for high-tolerance applications. More information about powder metallurgy can be found at PMPM.com.