Powder Metallurgy(PM) is a metal-processing technique with which solid metal parts are made through the pressing of metal powders. Because this process uses tooling that is shaped specifically for each part to be made, it is mainly used in production of large amounts of parts.
A few
advantages of PM are that:
There are, however, a few limitations with PM:
Parts ranging from just a few grams to 22kg or more can be produced, although most PM parts are less than 2 kg.
The Powder
There are a few factors relating to the powder that change the physical properties of the parts such as porosity. The first is particle size and distribution.
Particle size refers to the physical size of the individual particles. The most common particle size callout is a mesh count. This refers to the number of openings per inch in a mesh used to sift the powder. The higher the mesh count, the smaller the particle size.
Because the mesh is made of small wires the openings are not going to be the same as the callout. If we have a mesh count(MC) of 200, the particles are not 1/200th of an inch in diameter. Instead you take 1 divided by the MC minus the wire thickness.
A second factor relating to the particles is the particle shape. This is a fairly simple concept. The are 6 main shapes of particles; spherical, rounded, cylindrical, flakey, cubic, and aggregated. The best shapes for the particles are spherical and rounded. The other shapes do not compact as well as the spherical ones do, and would create a part that is not as structurally sound.
Interparticle Friction is a third main factor in the structure of the finished PM parts. Friction between the particles will affect the ability for the powder to flow readily, and compact tightly. The most common way of measuring the interparticle friction is measuring the angle that is formed by pouring the powder on a flat surface through a narrow funnel, also known as the angle of repose. Smaller angles result from less friction, while larger angles result from higher friction. Also, larger particles produce less friction while smaller particles produce larger friction, and spherical particles result in the smallest amount of friction. The interparticle friction should be as small as possible, to allow the automated die filling mechanism to completely fill the die rapidly. If the friction is too high, a lubricant can be added to reduce the friction.
Packing density and porosity are based on how much the material will pack down when pressed. This is also known as the packing factor. The packing factor is found by dividing the bulk density(the density of the loose powder) by the true density(the density of the true volume of the material)Typical packing factor values are 0.5 to 0.7, meaning that the material will compact 30-50%.
Production of Powder
Virtually any metal can be made into a powder. There are 3 main ways to produce metal powders; atomization, chemical, and electrolytic.
The most widely used method is atomization. Atomization involves either spraying or smashing molten metal into smaller particles. There are a few ways of doing this. One method is to spray water at a stream of molten metal, causing it to disintegrate into small particles. Water atomization cools the metal very rapidly, and can cause irregular particles. Water can also corrode some metals. A better way of atomization is to spray the molten metal stream with an inert gas. Because this cools the metal at a slower rate, it creates a more circular particle. Another way of atomization is to pour the molten metal onto a rapidly spinning disk, which sprays the metal onto the walls of the container, causing it to create a powder.
Chemical reduction is another process of creating metal powders. It involves a variety of chemical reactions that reduce the metal into elemental powders. A common process involves liberating metals from their oxides by the use of reducing agents, which attach to the oxygen in the oxide, and render metal powders.
Electrolysis is a powderization technique that uses electricity to remove the metal, and render a very fine, and highly pure powder. The system is setup so that the anode is made of the metal that the powder is to be made from. The electricity moves the metal from the anode to the cathode, and covers it in a film that is easily washed and dried to make the powder.
Blending and Mixing
To get good parts from the last two operations, the metal powders must be blended and mixed. Blending is a process in which the powders of the same chemical make-up but different sizes are combined. Mixing is a process where powders of different chemical make-ups are combined to make an alloy.
Blending and mixing is done in 4 ways; rotating drum, rotating double cone, screw mixer, blade mixer. In the drum and double cone methods, the container has baffles inside to prevent the mixture from free falling, which separates the particles by mass. This is the point at which other materials can be added to the mixture. The other materials consist of: lubricants- use to lower friction between the powder and the die, binders- used to hold some metals together between pressing and sintering, and deflocculants- used to allow for better powder flow in the die.
Compaction
In compaction the powder is fed into a die cavity, either automatically or by hand, and is then pressed into the cavity, and the final shape, by a hydraulic press. After pressing, the part is called a green compact. The green compact now has what is called the green density, which is nearly the true density of the material. The part is now in a finished form, but is in no way strong enough to use as a finished part. As the powder is compressed the density increases, and the individual particles are pressed together to plastically deform them, increasing their contact(and friction) and decreasing the pore size.
Two different types of presses are used to form the green compact, one direction, called single-action presses, and two direction, known as opposite ram, double action, and multiple action.
Sintering
Sintering is the final process in PM. It involves heating the part to 70-90% of the metals melting point. During this process the metal remains unmelted. The temperature in the furnace is high enough to get the metal into the recrystalization zone. In this temperature zone the metal particles begin to recrystalize into each other.
During the sintering process, a metal with a lower melting point can be placed on the part so as to be sintered with it. When the second metal melts it will impregnate the part's pores, making it stronger, and filling the pores so they may not absorb other materials. It can also create alloys that cannot be created in any other way. This process is called liquid phase sintering
Another form of sintering called spark sintering uses the press rams as electrodes, to sinter the part in the die.
Secondary Operations
Although PM parts can be used as finished parts, They may also need some further sizing. PM parts may be repressed to improve density, strength, or dimensional accuracy. PM parts can also be coined to add details. Any holes or features that cannot be made in the dies may need to be machined after words.
Because the parts in PM are made from powdered metal, they contain pores. The pore size can be controlled during the pressing to allow for specific porosity. The porosity of the finished part can also be manipulated after pressing by adding materials to the pores through impregnation and infiltration. In impregnation a fluid such as oil is forced into the pores of the part by dropping it in a heated vat of the substance. This process is used on things such as self lubricated bearings and gears. Infiltration, like liquid phase sintering, involves putting the part through a lower temperature furnace to melt a secondary metal into the pores. This does not create an alloy, but does fill the pores, improving the density, and making the part stronger.
PM parts can be heat treated and surface finished in the same ways as cast, forged, or machined parts can, but special care must be taken when heat treating and plating. The pores in the metal can cause problems in both of these operations.
Alternative PM Techniques
Isostatic pressing is an alternative pressing method using a liquid or gas. In cold isostatic pressing(CIP) the powdered metal is placed in a rubber mold, and pressed to the green compact by a fluid. Hot isostatic pressing(HIP) involves using a sheet metal mold that is pressed with a gas, and heated to sinter the part. Dimensional accuracy is not great with parts made through these methods.
Powder injection molding involves using a binder with the powdered metal so that is will flow in a mold. The part is then processed to remove the binder, and then sintered. These operations shrink the part by about 20%. Dimensional accuracy is limited.
PM can also be used to make billets for forging, allowing the billet to be nearly net shape. Forging after pressing bonds the metal to a higher degree than sintering alone, and the forging can be done in fewer hits, reducing cost.
Design Considerations
a main consideration in PM is that the press can only press from 2 opposing directions. This means that the part cannot have holes on the sides, and the part cannot be smaller in the center then it is on the sides.
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/ \ ---Chamfer .*^ ^*. -- Chamfer
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Not Good Good
Chamfers should be large in the punch direction, and a small inside radius is
preferred. There should also be a 1.5mm distance
between holes, or between a hole and the wall.
Fundamentals of Modern Manufacturing - ISBN 0-471-40051-3