23-04-2011, 04:15 PM
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INTRODUCTION
Powder metallurgy is concerned with the production of metal powders and converting them to useful shapes. It is a material processing technique in which particulate material are consolidate to semi finished and finished products. Generally the emphasis is on the metallic material but the principal of the process apply with little modification to ceramic, polymers and a variety of composite materials composed of metallic and non metallic phases. Nowadays powder metallic techniques are increasingly used to provide exceptional properties that are required in highly sophisticated aerospace electronic and nuclear energy industries. However an automobiles industry is the major consumer of powder metallurgy product today. There are two important reasons to use powder metallurgy by industries. Products like tungsten filament, tungsten carbide, porous self lubricating bearings etc. are either difficult or impossible to make by other methods. The other reason is that powder metallurgy process of manufacturing structural components competes with other manufacturing products such as casting machining and forging. Powder metallurgy process minimizes or eliminates the machining, and scrap losses at the same time is suited to high volume production of components. The process offers economy, savings in energy and raw materials along with mass production of quality precision components.
HISTORY OF POWDER METALLURGY
Powder metallurgy principle of shaping metallic objects without melting from powdered materials can be traced back to the early civilizations. These include the ancient Egyptian iron implants which date from at least 3000 B.C. In Greece the manufacture of iron components were widespread in 800-600 B.C. The manufacture of large objects were known to Indians as early as 300A.D. and the famous Delhi iron pillar weighing more then six tons is a typical master piece indeed . These are processed by direct reduction of iron oxide without melting, since the technology to obtain temperature high enough to melt pure iron was not available until about 1800. The significant development in the use of the powder metallurgy principle took place during the early part of nineteenth century for processing platinum and the credit to this is to be given to Wollaston in England and sobolevskiy in Russia. These developments ultimately led to the modern renaissance of powder metallurgy in the beginning of twentieth century with the manufacture of tungsten filaments for the incandescent lamp industry. The invention of electric lamp by Thomas Edison and Swan a century ago has contributed substantially to the rapid progress of this field. Powder metallurgy emerged as a new dimension in materials technology in twentieth century particularly during the world war period and subsequent years. Today the technology is used advantageously to process advanced material for the nuclear, electronics and aerospace industries. But in modern India the progress made in this field is mainly during the past two decades.
The Powder Metallurgy Process.
METAL POWDER TREATMENT
Annealing
It is customary that the powder producer delivers the powder to the fabricator ready for mixing. The aims of annealing are:
1) To soften the powder
2) To reduce the residual amount of oxygen, carbon and/or nitrogen from
the powder.
The annealing operation may be done in an atmosphere furnace or a vacuum furnace. The former may be of batch or continuous type. Annealing temperatures are kept as low as possible to minimize sintering.
Powder Mixing
The term ‘blending’ is strictly applied to a one component operation, whereas mixing involves more than one type of powder, e.g. mixing of solid lubricant with a metal powder or powders of several other metals. Sometimes the additive acts as lubricant as well as alloying addition, e.g. graphite in iron powder.
Various variables in the powder mixing process have been highlighted
by Hausner.1 They are:
1. Type of mixer
2. Volume of the mixer
3. Geometry of the mixer
4. Inner surface area of the mixer
5. Constructional material and surface finish of the mixer
6. Volume of the powder in the mixer before mixing
7. Volume of the powder in the mixer after mixing
8. Volume ratio of component powders
9. Volume ratio of mixer to powder
10. Characteristics of component powders
11. Type, location and number of loading and emptying devices
12. Rotational speed of mixer
13. Mixing time
14. Mixing temperature
15. Mixing medium (gaseous or liquid)
16. Humidity, when mixing in air.
Mixing efficiency is best when the powder volume is about 50% to 60% of the mixer volume. Optimum mixing time may be from between 5 to 30 minutes but this can be determined only by experience with a given mixture in a particular mixer. The aim is to mix the powders only as long as necessary to achieve a thorough mix and to fix a uniform apparent density of the mix from batch to batch. The apparent density of the mix tends to increase with mixing time.
Types of Mixers
Among various types of mixers available, the following are most common for metal powders (Fig.1):
Double Cone Mixer:
This consists of vertical cylinders with conical ends, which rotate about a horizontal axis. This rotation imparts a continuous rolling motion which spreads and folds the powders as they move in and out of the conical area. This action thoroughly mixes the powders with little or no change in the size and shape of the individual particles.
V-Mixer:
This is constructed by joining two cylinders of equal length into a ‘V’. As the ‘V’ rotates about its horizontal axis, the powder charge splits and refolds.