Powder Metallurgy

Powder  Metallurgy

In powder metallurgy process (PM), fine powders of metals and alloys are compacted together by pressing the powder in a mould or die to control the shape of the finished product. The pressure used for compaction are high so that the metal or alloy particles get mechanically interlocked. The part also develops enough strength, so that it may be taken out of the die or mould cavity without damage or crumbling back to powder form. The product of this compaction process is known as ‘‘green compact’’. Its strength is low, and its density is below that of corresponding solid metal or alloy. It is also somewhat porous. To produce useful strength level, the green compact is sintered at a high temperature but below the melting point of the powder-metal in a neutral or reducing atmosphere. An exchange of atoms between individual particles welds them together into a slightly porous piece of metal of the approx. shape and size of the die or mould cavity. The sintered component may be used as such or may undergo some secondary operations before being used.

Now we shall discuss, in some detail, the various steps involved in making PM components:

Metal  powder  production:

Metal powders can be produced in several ways of which atomisation is the most important. In this process, metal or alloy is heated until it melts. The molten metal is then gravity fed through a nozzle where it is impinged with a high velocity stream of water, air or nitrogen atomising it. Upon solidification, the atomised particles of metal or alloys are of various shapes and sizes. These may have to be pulverised to a fine powder of size below 100 microns.

 Blending:

Blending means agitation of powder for homogenizing the particle sizes. While blending some lubricants are also added so that during the next operation of compacting, the die wear and the friction between the metal particles may be reduced. Usual lubricants used are powdered graphite or lithium stearate. Blending is usually done dry—no water is added.

 Compacting:

After blending, the powders are placed in a die and compacted by pushing a punch in under pressure. The dies are usually made of tungsten carbide to reduce wear of the die. The use of lubricants is necessary to reduce wear of dies and to reduce compacting force and in order that the density of ‘‘green compact’’ is almost as high as the density of solid metal. Ejection of green compact from the die also becomes easier with the use of lubricants. Compacting requires high pressures of the order of 700 MPa to cause mechanical interlocking in particles.

Before sintering, however, the lubricant must be driven out by a low temperature heating cycle.

Sintering:

This is the next step in PM process. The green compacts are heated in a muffle type furnace in a controlled atmosphere. For ferrous metals, a dissociated ammonia atmosphere is used to control carburization or decarburization of the powder compact. Temperatures are maintained between 60–80% of the melting point of metal or alloy concerned. Sintering time may range from 20 minutes to 60 minutes. Sintering raises the ultimate strength of the product. It results in diffusion bonding of particles.

Advantages  of  powder  metallurgy  process:

The main advantage of PM process is that accurate control over powder can be exercised permitting variation in physical and mechanical properties. If so desired a part can be made with different densities in different portions of the same part. Parts can be made in different shapes accurately so that no subsequent machining is required. Small gears, parts with spline or irregular shapes can be produced cheaply and accurately. PM process is metal and energy efficient. PM parts are also relatively free from defects. Main disadvantage is that initial tooling costs are high and it cannot produce parts with weak thin sections.

Applications:

Powder metallurgy is used in the manufacture of parts for: Automobile industry: motors, gear assemblies, brake pads Abrasives: polishing and grinding wheels

Manufacturing:  cutting  and  drilling tools (using  hard  metals)

Electric and magnetic devices: magnets, soft magnetic cores, batteries Medical and dental: prostheses, amalgams

Aerospace: motors, heat shields, structural parts Welding: solder, electrodes

Energy:  electrodes,  fuel cells

Other:  porous  filters,  bearings,  sporting  goods  etc.

Moulding of Plastics and Power Metallurgy

 PLASTIC:  Introduction:

Plastic or Polymers are classified into two categories:

1. Thermoplastics

2.  Thermosets

Thermoplastics:

Thermoplastic polymers soften when heated and can be reshaped, the new shape being retained on cooling. The process can be repeated many times by alternate heating and cooling with minimal degradation of the polymer structure.

Thermosets:

Thermosetting polymers (or thermosets) cannot be softened and reshaped by heating. They are plastic and moldable at some state of processing, but finally set to a rigid solid and cannot be resoftened. Thermosets are generally stronger and stiffer than thermoplastic.

Plastic  Processing:

Though there are a wide variety of plastic product manufacturing processes in use, the discussion of all of them is beyond the scope of this book. We shall describe three common methods. These are

(i) Injection moulding, 

(ii) Extrusion, and

(iii) Blow moulding.

 Injection  moulding:

This is the method used for large scale production of thermoplastics components. The plastic-powder is filled into a hopper connected to a cylinder-piston mechanism. As the piston withdraws, some plastic powder is inducted into the cylinder and the piston then moves it forward by exerting pressure on it. The cylinder is heated so that the plastic powder gets heated to a temperature between 175–275°C. Under the action of heat and pressure, the plastic softens and is forced through a nozzle into a water cooled die.

After the plastic part has cooled and solidified, it is ejected out of the die and the cycle starts all over again.

Extrusion:

This process is also called extrusion moulding. This method is also suitable for thermoplastics, thermosetting plastics are generally not suitable for extrusion. By extrusion, solid rods, pipes, tubing and different sections can be made. A hopper feeds polymer material into a chamber, which is kept heated. A screw rotates in the centre of this chamber feeding the polymer material forward. Under the action of heat and pressure, it starts flowing. In the front portion of the chamber, a (heated) die is fitted, which provides the only exit for the material. As more and more material is screw fed, a continuous stream of material is squeezed out from the die, its cross-section acquiring the shape of the die. The material coming out of the die is cooled and carried off by a suitable belt conveyer. The scheme of extrusion process is shown in Fig.4.2

Blow  moulding:

This process has been used for making plastic bottles, toys, hollow dolls and similar other items. The blow moulding begins with a heated tubular piece of plastic, which is called PARISON. The parison is positioned in between a two piece mould as shown in Fig. 4.3.

The bottom opening of the parison gets pinch closed and sealed as the two piece mould closes. Air is then injected into the parison under pressure (0.7–10 kg/cm2) thereby forcing the plastic to acquire the shape of the mould. The mould is opened and the part formed inside the mould is removed after it has cooled sufficiently. The above process is similar to making articles of glass by blowing air into a mass of molten glass.