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Email Us Today!P/M Process
The P/M ProcessThe P/M process is an economical, environmentally clean, high production method for making parts exactly to or close to final dimensions. With little or no machining operations required. Due to their porosity, they may be impregnated with oil or plastic. The first modern use for powder metallurgy parts came in the 1920’s with the development of the self-lubricating bearings and bronze bearings used in automobiles. With the growth of the automobile industry and more demand, better powders and processes were developed to make better parts at much reduced costs. By the 1960’s, powdered metallurgy processing became more of a proven technique with engineers designing components and assemblies specifically for powder metal, rather than merely using powder metal to replace wrought parts. Improved powder properties allowed for greater design possibilities. By the 1970’s along with the automotive, which has continued to be the greatest utilization, powder metallurgy parts were being used in appliances, farm and garden equipments, hardware, tools, cameras, business machines, sporting goods and military products.
Manufacturing Metal Powders
Metals are first reduced to individual particles through methods including atomization, chemical precipitation, commination and hydrogen reduction. Atomization is the process used commercially to produce the largest tonnage of metal powders. In water and gas atomization the raw material is melted then the liquid metal is broken into individual particles. To accomplish this, the melt stock, in the form of elemental, multi-element metallic alloys, and/or high quality scrap, is melted in an induction, arc, or other type of furnace. After the bath is molten and homogenous, it is transferred to a tundish which is a reservoir used to supply a constant, controlled flow of metal into the atomizing chamber. As the metal stream exits the tundish, it is struck by a high velocity stream of the atomizing medium (water, air, or an inert gas). The molten metal stream is disintegrated into fine droplets which solidify during their fall through the atomizing tank. Particles are collected at the bottom of the tank. Alternatively, centrifugal force can be used to break up the liquid as it is removed from the periphery of a rotating electrode or spinning disk/cup
Mechanical Comminution methods, such as milling, lathe turning, and chipping, comprise the second powder manufacturing group. Milling is the primary method for reducing the size of large particles and particle agglomerates. Ball, hammer, vibratory, attrition, and tumbler mills are some of the commercially available comminuting devices. During milling, forces act on the feed metal to modify the resultant particles. Impact, attrition, shear, and compression all influence powder particle size and shape.
Blending/Mixing
This process is where metal powders or alloys are compressed while restricted in a die, at pressures as low as 10 to 45 tons per square inch.The base metal or alloy, any additional elements, and a powdered lubricant are fed into a blender. These materials are blended into a homogenous mix.
The mixing, or blending, of powder feedstocks for die pressing of Powder Metallurgy parts is carried out for two reasons:
To introduce alloying element additions in a homogeneous form:
Die pressing feedstocks generally consist of elemental mixes in order to maintain as high a level of compressibility as possible. Using this approach means that the compressibility is controlled by that of the soft, annealed base powder (most commonly iron). Use of a fully pre-alloyed powder would mean that the initial particle hardness and work hardening rate would both be increased by the alloying additions and compressibility therefore reduced.To incorporate a pressing lubricant:
Popular lubricants are stearic acid, stearin, metallic stearates or other organic compound of a waxy nature. The purposes of adding the lubricant are to reduce friction (and therefore even out density variations) during compaction, to reduce ejection forces and to minimise the tendency for ejection cracking in the compact.
A homogeneous mix is generally produced from the initial constituents by a tumbling action in an appropriate mixing vessel. Mixing vessels are often of a double-cone geometry, but other vessel shapes are also utilised (V, W or Y-shaped sections).
Compacting
Compaction of components is carried out in specifically designed tools. For the production of P/M components, the metal powder must be compressed so that the individual particles will cold-weld at their contact points to make a part of sufficient “green” strength to be handled and of a density great enough to meet specified properties. The design and quality of the compacting tool must be such that the part will be, after sintering, of the desired strength and dimensions. For achieving uniform density in the part, the respective motions of die and lower punches are calculated and programmed in the press operating cycle. In most cases, tooling is made from high speed steel or carbide, and the life time may range from 10,000 to millions of parts, depending on complexity, materials and tolerances.
The powders are compacted in a die cavity at ambient temperature, after which they are in a near net state, or “green state”.
Sintering
Finally the green parts are sintered in a furnace at temperatures below the melting point of the metal to bond the particles without changing the shape of the part.
The sintering process also increases part strength and controls the porosity of the part. Although the finish parts look solid, they actually consist of small interconnected capillaries, which cause the parts to be approximately 25% porous. During sintering the compacted parts obtain their mechanical strength. Sintering, which takes place below the melting temperature of the major constituent of the material, results in inter particle bonding without appreciably changing the shape of the component. During sintering diffusion and recrystallization occur.
Secondary Operations
The application of finishing processes to the sintered part. In the Powder Metallurgy industry, such processes are often referred to as “secondary operations”.
Sizing - Sintering may produce small dimensional changes in the compacts. Therefore, parts with very close tolerances, are sized in seperate tools.
Oil Impregnation- The parts are impregnated with oil through a vaccum process.
Heat Treating – This is the heating of a sintered metal part in a protective atmosphere and then oil quenching the part. The results are usually improved strength and hardness.
Machining – If required powdered metal parts can be machined to obtain the desired shape before or after heat-treating is completed.
