the method of powder metallurgy

WHAT IS P/M?
Powder Metallurgy or P/M is a highly developed method of manufacturing precision
metal parts. Over the last seventy years, the technology has matured from making selflubricating bearings for autos to complex carrier gear sets in automobile transmissions and high strength powder –forged connecting rods in engines. When we use the term P/M within the industry, it also refers to related technologies such as metal injection molding, isostatic pressing and powder forging.
P/M manufacturing technology consists of three steps; mixing elemental or alloy
powders, compacting those powders in a die at room temperature and then sintering or heating the shape in a controlled atmosphere furnace to bond the particles together metallurgically. Generally, scrap rates for the process are less than 3 per cent. Because the process has so little waste and the part, is often finished when taken from the furnace, the process is very cost effective when compared to manufacturing processes that must contend with flash, machining chips, and sprues and gates. The speed of the presses is such that simple or complex parts can be made to close tolerances, often eliminating machining. Production runs range in number from a few hundred to thousands of parts per hour. Conventional P/M parts are limited to parts which can be formed uniaxially. Frequently, the technology is used for material systems that are hard to machine such as tungsten or molybdenum or hard to cast due to detrimental solidification behavior of the material systems chosen. The utility of the process can also be found by combining previously separate shapes into a one-piece P/M part, avoiding additional assembly or joining steps in the component’s design. In some cases, P/M parts can be joined by bonding or other joining techniques to give greater utility to the designed component. Today’s pressesenable parts to range between simple, single level shapes to complex, eight level shapes.
The basic versatility of P/M is demonstrated by the use of components in the
automotive, aerospace, business machine, electronics and appliance markets.
Thousands of different, reliable cost-saving designs now serve these industries. The
average U. S. full-size passenger car contains more than 16.4 kilograms (36 pounds) of P/M parts.
P/M’s flexibility is becoming more pronounced with the advent of its use as a forming
technique to manufacture components with very novel material combinations. The
designer can adjust the chemistry and other P/M characteristics, such as density, to
provide a custom result, uniquely suited for the application. Materials can include
ceramics, nanophase materials, semi-metals and functionally gradient materials.
 

THE ADVANTAGES OF THE P/M PROCESS
As we get more detailed in this discussion, it will become more apparent how the P/M
process delivers its many advantages. Perhaps P/M’s strongest benefit is its ability to eliminate or minimize machining. Tolerances, which I will compare later, are the key issue here. Like many component design processes, the earlier a competent P/M parts designer is brought into the design process, the more likely a design can be optimized to meet the needs of the component’s function, which should be followed by form.
Following on the minimization of machining, and since the component is shaped in a
closed die, there is little or no trim or other process related scrap. Strict process control and the reproducibility inherent in the process result in consistent quality and little rejection due to off specification components.
These days, close dimensional tolerances, are essential to the reliability and
performance of an assembly. P/M’s ability to control tolerances is one of the secrets to its successful adoption in the conversion of many components from competing forming technologies.
There is essentially no limit to the variety of alloy systems that can be used to produce a shaped component. This gives the designer considerable latitude in matching function and application requirements with a material system to meet those needs such as strength, corrosion resistance or other particular metallurgical or mechanical properties.
P/M parts have a surface finish of between 32 –48 microinches (Ra). Produced from a die with surfaces of 20 –32 microinches.
P/M parts can be enhanced using heat-treating, coining, burnishing or other secondary operations that can result in property improvement.As we mentioned above, one of the first modern uses of P/M components was in selflubricating bearings. By controlling the density and particle size, very specific pore characteristics can be designed into an application. If the porosity is interconnected, the component can be used in very fine, high temperature filter applications.
Complex shapes can be made easily and efficiently. The ingenuity of the designer and the ability of the part to be ejected from the compaction die are the only limits to the choices.
Most P/M dies are made of high strength, durable alloys such as tungsten carbide or
high-speed steel, which permits high volumes of parts to be produced before wear of
the tooling becomes a consideration. Once a part is being produced in a “steady-state” fashion on a set of tools, the results are consistent from part -to -part.
Since the dies and tooling are so durable, high volumes of parts can be economically produced at high speeds, subject to the limits of the press capacity.
The largest market for PM components is the automotive sector. Nearly 70 % of all P/M parts are used in this industry. The applications are frequently found in the drive train and transmission where the demands for performance and reliability are great. One major application is connecting rods. Millions of rods have been produced over the last twelve years, without a failure.
Following on that comment, it goes without saying that the most cost conscious of
buyers can be found in the auto industry. If P/M weren’t economical, the auto industry would use a different technology.
Most powder is produced from scrap metals and as such helps limit environmental
impact through recycling. The net shape characteristic of the component and the
limited use of energy for heating imply that a minimum amount of energy is used to
meet the needs of the application. Minimizing machining lowers the likelihood of water pollution due to cutting fluids and oils.

THE BASIC P/M PROCESS STEPS
As mentioned above, the three basic steps for P/M manufacturing are mixing,
compacting and sintering.
P/M powders can be produced by taking elemental, partially alloyed or prealloyed metal powders and mixing them with lubricants such as graphite or waxes to produce a homogeneous mixture of ingredients. Since the density and consistency of fill of the die is important to maintain uniform part production, the importance of this step of theprocess can not be minimized. The additives can also be introduced to aid
machinability, wear resistance or lubricity of the base alloy composition.
A controlled amount of mixed powder is automatically gravity-fed into a precision die
and is compacted. Usually this is done at room temperature at compaction pressures as low as 138 MPA (10 tons per square inch) to as high as 827 MPa (60 tons per square inch) depending on the density requirements of the part and powder being pressed. Normally the compaction pressures are 345 to 690 MPa (25 – 50 tons per square inch). The compacted or “green” part has the size and shape of the finished part when ejected and has sufficient green strength to be handled and transported to the sintering furnace.
In the sintering step, the green compact is placed on a mesh belt, and slowly moves
through a controlled atmosphere furnace where the parts are heated to a temperature below the melting point of the base metal, held at the sintering temperature and then cooled. The sintering step transforms compacted mechanical bonds between the powder particles into metallurgical bonds by a solid state transformation process.

SINTERING
The traditional furnace found in the P/M industry is a mesh belt furnace with three
operating zones; a pre-heat or de-lube zone, a hot zone and a cooling zone.
The furnaces usually operate between 1120 to 1150 degrees centigrade (2050 and
2100 degree F) for ferrous parts and 790 to 845 degrees centigrade (1450- 1550
degrees F.) for bronze materials. The trip through the furnace for a single part takes
about 2 to 3 hours, depending upon the size of the part. Some components aretemperature sintering, an effort made to enhance mechanical properties. Another
variation on the process is called sinter-hardening, accomplished by using a controlled cooling rate in the cooling section of the belt furnace, transforming the steel matrix of a ferrous part to martensite, thereby eliminating the need for a secondary hardening step.
 

SECONDARY OPERATIONS
With the exception of repressing, impregnation and infiltration, a sintered P/M
component can be finished or treated, if necessary, just like any other metal component.
P/M components can be plated or coated for corrosion resistance and can also accept a black oxide coating to act as a paint base. Deburring can be performed to relieve sharp edges and ferrous parts can be burnished to control size and surface finish. Ferrous P/M components can be successfully welded using just about any welding technique, given allowances for density, alloy composition, proper joint design and elimination of contaminants. Furnace brazing is often used to join other materials.
P/M components can be heat treated to improve strength and hardness and can be
steam treated to make the surface hard and more wear resistant and well as
contributing to improved corrosion resistance and the sealing of porosity.
Repressing or coining is performed on P/M components to densify or modify the surface shape and provide stricter dimensional control. This can also be achieved by sizing a component.
Oil impregnation, used on P/M self-lubricating bearing components since the late
1920’s, involves either soaking or vacuum processing. The components can absorb
between 12 and 30 % oil by volume. Resin impregnation is performed on P/M
components to improve machinability or to prepare the surface for plating by making the surface liquid or gas tight and the surface “pore free”.
Infiltration is a secondary process step used to either improve strength or seal parts and make them gas or liquid tight. Optional, like resin impregnation, it can also be used to enhance machinability and improve ductility and prepare parts for plating. It is not used to prevent P/M defects.