What Is Powder Metallurgy? Its Introduction, Production Process, Advantages, Applications

What Is Powder Metallurgy? Its Introduction, Production Process, Advantages, Applications 

Introduction:-

In Powder metallurgy, parts are produced from metallic powders. This process first was used by the Egyptians in about 3000 B.C. to make iron tools. One of its first modern uses was in the early 1900s to make the tungsten filaments for incandescent light bulbs. In powder metallurgy production, the powders are compressed into the desired shape and then heated to cause bonding of the particles into a hard, rigid mass. Pressing, is accomplished in a press-type machine using tools designed specifically for the part to be manufactured. The tooling, which typically consists of a die and one or more punches, can be expensive, and PM is therefore most appropriate for medium and high production. The heating treatment, called sintering, is performed at a temperature below the melting point of the metal. The availability of a wide range of metal-powder compositions, the ability to produce parts to net dimensions and the overall economics of the operation give this unique process its numerous attractive and expanding applications.

The most commonly used metals in PM are iron, copper, aluminum, tin, nickel, titanium, and the refractory metals. For parts made of brass, bronze, steels, and stainless steels, prealloyed powders are used, where each powder particle itself is an alloy. Metal sources are generally bulk metals and alloys, ores, salts, and other compounds. Powder metallurgy has become competitive with processes such as casting, forging, and machining, particularly for relatively complex parts made of high-strength and hard alloys. Although most parts weigh less than 2.5 kg, they can weigh as much as 50 kg. It has been shown that PM parts can be mass-produced economically in quantities as small as 5000 per year and as much as 100 million per year for vibrator weights for cell phones.

Considerations that make powder metallurgy an important commercial technology include:

1. Parts can be mass produced to net shape, eliminating the need for subsequent processing.
2. The PM process itself involves very little waste of material; about 97% of the starting powders are converted to product. This compares favorably with casting processes in which sprues, runners, and risers are wasted material in the production cycle.
4. Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy.
5. Certain metal alloy combinations can be formed by PM that cannot be produced by other methods.
6. It compares favorably with most casting processes in terms of dimensional control of the product. Tolerances of 0.13 mm are held routinely.
7. PM production methods can be automated for economical production.

There are limitations, these include the following:

1. Tooling and equipment costs are high,
2. Metallic powders are expensive, and 3. There are difficulties with storing and handling metal powders
5. Variations in material density throughout the part may be a problem, especially for complex part geometries.
6. Although parts as large as 22 kg can be produced, most PM components are less than 2.2 kg.

A wide range of parts and components are made by powder-metallurgy techniques:-

1. Advances in this technology now permit structural parts of aircraft, such as landing gear components, engine-mount supports, engine disks, impellers, and engine nacelle frames, to be made by powder metallurgy
2. Automotive components such as piston rings, connecting rods, brake pads, gears, cams, and bushings
3. Tool steels, tungsten carbides, and cermets as tool and die materials
4. graphite brushes impregnated with copper for electric motors, magnetic materials, metal filters and oil-impregnated bearings with controlled porosity
5. Metal foams, surgical implants

Production of Metal Powders :-

The powder-metallurgy process typically consists of the following operations, in sequence
1. Powder production
2. Blending
3. Compaction
4. Sintering
5. Finishing Operations

Powder Production :-

There are three principal methods by which metallic powders are commercially produced.
The methods are
(1) atomization,
(2) chemical and
(3) electrolytic

ATOMIZATION :-

Atomization involves a liquid-metal stream produced by injecting molten metal through a small orifice, that solidify into powders. It is the most versatile and popular method for producing metal powders today. In gas atomization, high velocity gas stream is utilized to atomize the liquid metal. The gas flows through an expansion nozzle, siphoning molten metal from the melt below and spraying it into a container. The droplets solidify into powder form. In some type molten metal flows by gravity through a nozzle and is immediately atomized by air jets. The resulting metal powders, which tend to be spherical, are collected in a chamber below. When high-velocity water stream is used instead of air, this is known as water atomization. The use of water results in a slurry of metal powder and liquid at the bottom of the atomization chamber. Although the powders must be dried before they can be used, the water allows for more rapid cooling of the particles and higher production rates. In centrifugal atomization, the molten-metal stream drops onto a rapidly rotating disk or cup, so that centrifugal forces break up the stream and generate particles. The size and shape of the particles formed depend on the temperature of the molten metal, rate of flow, nozzle size, and jet characteristics.

Chemical Reduction :-

Chemical reduction includes a variety of chemical reactions by which metallic compounds are reduced to elemental metal powders. A common process involves liberation of metals from their oxides by use of reducing agents. The reduction of metal oxides uses gases, such as hydrogen and carbon monoxide, as reducing agents. This approach is used to produce powders of iron, tungsten, nickel, and cobalt and copper. The powders produced are spongy and porous and have uniformly sized spherical or angular shapes.

Electrolytic Deposition :-

Electrolytic deposition utilizes either aqueous solutions or fused salts. The powders produced are among the purest available. In electrolysis, an electrolytic cell is set up in which the source of the desired metal is the anode. The anode is gradually dissolved under an applied voltage, transferred through the electrolyte, and deposited on the cathode. The deposit is extracted, washed, and dried to yield a metallic powder of very high purity. The technique is used for producing powders of beryllium, copper, iron, silver, tantalum, and titanium.

BLENDING AND MIXING OF THE POWDERS:-

To achieve successful results in compaction and sintering, the metallic powders must be thoroughly homogenized beforehand. The terms blending and mixing are both used in this context. Blending refers to when powders of the same chemical composition but possibly different particle sizes are intermingled. Mixing refers to powders of different chemistries being combined.

Blending and mixing are accomplished by following types
1. Rotation in a drum
2. Rotation in a double-cone container
3. Agitation in a screw mixer and
4. Stirring in a blade mixer

Blending of powder is carried out for the following purposes:

1. Powders of different metals and other materials can be mixed in order to impart special physical and mechanical properties and characteristics to the PM product. Proper mixing is essential to ensure the uniformity of mechanical properties throughout the part.
2. Even when a single metal is used, the powders may vary significantly in size and shape; hence, they must be blended to obtain uniformity from part to part.
3. Lubricants can be mixed with the powders to reduce friction between the metal particles, improve flow of the powder into the dies, and die life.
4. Other additives such as binders, are used to develop sufficient green strength

Compaction of Metal Powders:-

In compaction, high pressure is applied to the powders to form them into the required shape. The purposes of compaction are to obtain the required shape, density, and particle-to-particle contact and to make the part sufficiently strong for further processing. The powder is fed into the die by a feed shoe, and the upper punch descends into the die. The presses used are actuated either hydraulically or mechanically, and the process generally is carried out at room temperature, although it can be done at elevated temperatures. The work part after pressing is called a green compact, since it has a low strength. The green parts are very fragile and can crumble or become damaged very easily. The green strength of the part when pressed is adequate for handling but far less than that achieved after sintering.

SINTERING:-

After pressing, the green compact lacks strength and hardness; it is easily crumbled under low stresses. By Sintering its strength increases. Sintering is the process whereby green compacts are heated in a controlled atmosphere furnace to a temperature below the melting point, but sufficiently high to allow bonding of the individual particles. Sintering temperatures are generally within 70 to 90% of the melting point of the metal or alloy. Sintering times range from a minimum of about 10 minutes for iron and copper alloys to as much as 8 hours for tungsten and tantalum.

Finishing Operations:- 

1. Coining and sizing are to impart dimensional accuracy to the sintered art and to improve its strength and surface finish.

2. Preformed and sintered alloy-powder compacts subsequently may be cold or hot forged to the desired final shapes and sometimes by impact forging.

3. Powder-metal parts may be subjected to other finishing operations, such as
• Machining: for producing various geometric features by milling, drilling, and tapping (to produce threaded holes)
• Grinding: for improved dimensional accuracy and surface finish.
• Plating: for improved appearance and resistance to Wear and corrosion.
• Heat treating: for improved hardness and strength.