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Metallurgical Engineering

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Powder Metallurgy

Powder metallurgy (PM) is a metal working process for forming precision metal components from metal powders. The metal powder is first pressed into product shape at room temperature. This is followed by heating (sintering) that causes the powder particles to fuse together without melting.

The parts produced by PM have adequate physical and mechanical properties while completely meeting the functional performance characteristics. The cost of producing a component of given shape and the required dimensional tolerances by PM is generally lower than the cost of casting or making it as a wrought product, because of extremely low scrap and the fewer processing steps. The cost advantage is the main reason for selecting PM as a process of production for high – volume component which needs to be produced exactly to, or close to, final dimensions. Parts can be produced which are impregnated with oil or plastic, or infiltrated with lower melting point metal. They can be electroplated, heat treated, and machined if necessary.

The rate of production of parts is quite high, a few hundreds to several thousands per hour.

Industrial applications of PM parts are several. These include self – lubricating bearings, porous metal filters and a wide range of engineered shapes, such as gears, cams, brackets, sprockets, etc.

Process Details: In the PM process the following three steps are followed in sequence: mixing (or blending), compacting, and sintering.

Mixing: A homogeneous mixture of elemental metal powders or alloy powders is prepared. Depending upon the need, powders of other alloys or lubricants may be added.

Compacting: A controlled amount of the mixed powder is introduced into a precision die and then it is pressed or compacted at a pressure in the range 100 MPa to 1000 MPa. The compacting pressure required depends on the characteristics and shape of the particles, the method of mixing, and on the lubricant used. This is generally done at room temperature. In doing so, the loose powder is consolidated and densified into a shaped model. The model is generally called “green compact.” As is comes out of the die, the compact has the size and shape of the finished product. The strength of the compact is just sufficient for in – process handling and transportation to the sintering furnace.

Fig 1 Typical set of powder metallurgy tools

To illustrate the process, let us take a straight cylindrical part such as a sleeve bearing. Fig 1 shows a typical set of tools used for producing this part. The compacting cycle for this part (Fig 2) follows the following steps.

Fig 2 Powder metallurgy compacting cycle

1. With the upper punch in the withdrawn position, the empty die cavity is filled with mixed powder.

2. The metal powder in the die is pressed by simultaneous movement of upper and lower punches.

3. The upper punch is withdrawn, and the green compact is ejected from the die by the lower punch.

4. The green compact is pushed out of the pressing area so that the next operating cycle can start.

This compacting cycle is almost the same for all parts.

Sintering: During this step, the green compact is heated in a protective atmosphere furnace to a suitable temperature, which is below the melting point of the metal. Typical sintering atmospheres are endothermic gas, exothermic gas, dissociated ammonia, hydrogen, and nitrogen. Sintering temperature varies from metal to metal; typically these are within 70 to 90% of the melting point of the metal or alloy. Table gives the sintering temperatures used for various metals. Sintering time varies with size and metal of part. Table 1 also gives typical range of sintering time needed for various metals.

Table 1 Sintering temperature and time for various metal powders


Sintering is a solid state process which is responsible for producing physical and mechanical properties in the PM part by developing metallurgical bond among the powder particles. It also serves to remove the lubricant from the powder, prevents oxidation, and controls carbon content in the part. The structure and porosity obtained in a sintered compact depend on the temperature, time, and processing details. It is not possible to completely eliminate the porosity because voids cannot be completely closed by compaction and because gases evolve during sintering. Porosity is an important characteristic for making PM bearings and filters.

Secondary and finishing operations

Sometimes additional operations are carried out on sintered PM parts in order to further improve their properties or to impart special characteristics. Some important operations are as under.

1. Coining and sizing: These are high pressure compacting operations. Their main function is to impart (a) greater dimensional accuracy to the sintered part, and (b) greater strength and better surface finish by further densification.

2. Forging: The sintered PM parts may be hot or cold forged to obtain exact shape, good surface finish, good dimensional tolerances, and a uniform and fine grain size. Forged PM parts are being increasingly used for such applications as highly stressed automotive, jet – engine and turbine components.

3. Impregnation: The inherent porosity of PM parts is utilized by impregnating them with a fluid like oil or grease. A typical application of this operation is for sintered bearings and bushings that are internally lubricated with upto 30% oil by volume by simply immersing them in heated oil. Such components have a continuous supply of lubricant by capillary action, during their use. Universal joint is a typical grease – impregnated PM part.

4. Infiltration: The pores of sintered part are filled with some low melting point metal with the result that part's hardness and tensile strength are improved. A slug of metal to be impregnated is kept in close contact with the sintered component and together they are heated to the melting point of the slug. The molten metal infiltrates the pores by capillary action. When the process is complete, the component has greater density, hardness, and strength. Copper is often used for the infiltration of iron – base PM components. Lead has also been used for infiltration of components like bushes for which lower frictional characteristics are needed.

5. Heat Treatment: Sintered PM components may be heat treated for obtaining greater hardness or strength in them.

6. Machining: The sintered component may be machined by turning, milling, drilling, threading, grinding, etc. to obtain various geometric features.

7. Finishing: Almost all the commonly used finishing method is applicable to PM parts. Some of such methods are plating, burnishing, coating, and colouring.

Plating: For improved appearance and resistance to wear and corrosion, the sintered compacts may be plated by electroplating or other plating processes. To avoid penetration and entrapment of plating solution in the pores of the part, an impregnation or infiltration treatment is often necessary before plating. Copper, zinc, nickel, chromium, and cadmium plating can be applied.

Burnishing: To work harden the surface or to improve the surface finish and dimensional accuracy, burnishing may be done on PM parts. It is relatively easy to displace metal on PM parts than on wrought parts because of surface porosity in PM parts.

Coating: PM sintered parts are more susceptible to environmental degradation than cast and machined parts. This is because of inter – connected porosity in PM parts. Coatings fill in the pores and seal the entire reactive surface.

Colouring: Ferrous PM parts can be applied colour for protection against corrosion. Several methods are in use for colouring. One common method to blacken ferrous PM parts is to do it chemically, using a salt bath.

8. Joining: PM parts can be welded by several conventional methods. Electric resistance welding is better suited than oxy- acetylene welding and arc welding because of oxidation of the interior porosity. Argon arc welding is suitable for stainless steel PM parts.