2. Find in the text «The Sintering Process» synonyms for the following

1. To demand

1. Easily, without difficulty

2. A word serving as a specific name of something

3. Plain, obvious

4. Link

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6. To make easier or less difficult

7. To include as a necessary condition

8. Nevertheless

9. Layer

10. Benefit; profit

3. Retell the text «The Sintering Process» using the answers to the questions of 1 as a plan.

4. Read the text

SIZING OR COINING

After the parts have been sintered, additional operations may be necessary. Parts requiring close tolerances may necessitate a final siz­ing or coining operation. This is a cold-working operation in which the parts are again compressed in accurate dies to obtain close dimensional tolerances and increased strength and density. Sometimes, it may be desired to impregnate parts which are usually porous filled with oil.

5. Give the name to the following: «The compressing of the parts in accurate dies»,

6. Find in the text the key terms. Give their definitions.

7. Make up a summary of the text «Sizing or coining».

Unit 32

ADVANTAGES

AND DISADVANTAGES.

APPLICATIONS

Advantages. The powder metallurgy process for making parts has several advantages which can be given as follows.

1. Alloys and laminated metallic structures, which are impossi­ble to obtain by any other process, can be produced.

2. The chemical analysis of the parts can be closely controlled. Metal powders, which are above 99 per cent pure, can be obtained commercially.

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3. A high production rate can be obtained on automatic presses.

4. Close dimensional tolerances can be maintained, especially, if a sizing operation is used. Tolerances on small and medium-size parts can be held to ±0.001 in.

5. The yield of almost 100 per cent is obtained, since powder losses generally run below about 0.5 per cent.

6. A wide range of properties such as density, porosity, and par­ticle size can be obtained for particular applications.

7. Highly skilled labour is not required. Disadvantages.

1. The strength and other mechanical properties of the parts pro­duced are generally lower than those of similar parts made by forging or casting. This is due to the porosity which may be present in the parts.

2. The size of the parts produced is limited because of the large presses needed to obtain the compressing pressures resulting in in­creased tool and press costs.

3. Powdered metal is an expensive raw material when compared with materials used in other processes.

4. At present it is not commercially feasible to press several al­loy powders. For instance, aluminium powders tend to cold-weld to the wall of the die causing the die walls to become scored.

5. There is almost no flow with metal powders as with liquids or plastics during compressing because of friction between the particles. As a result, intricate shapes cannot be produced, and complicated dies may be required for some parts which appear at first to be suitable.

6. The dies and the presses used for the pressing operations have high initial costs. These costs must generally be offset by the use of mass production methods in order to secure low part costs.

7. There is a tendency for the powders to adhere to the sides of the die and, thereby, to induce unequal pressures throughout the com­pact during pressing. As a result, parts compressed from the top tend to have a low density at the bottom, and those compressed from both top and bottom tend to be less dense in the middle.

8. Some powders in a finely divided state present explosion and fire hazards, and precautions must be taken to keep these dusts out of the air.

Parts made by powder metallurgy have important characteristics and are found in numerous interesting applications. Probably, the first

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use was the production of tungsten wire which is used for filaments in electric-light bulbs.

Since tungsten is a refractory metal, it cannot be commercially melted and cast, and so, the production of the wire presents a problem. However, fine tungsten wire is now produced by this process by first making a tungsten compact from the powder followed by suitable working and heat treating operations. Similarly, other high melting point metals such as molybdenum, tantalum, and columbium can be fabricated by using this process. When using two or more different powders, it is possible to retain the individual characteristics of each constituent. For instance, electric contacts should be wear resistant, re­fractory, and have a good electric conductivity. By combining the proper powders these properties can be obtained. Excellent small mag­nets can be produced from iron, aluminium, nickel and cobalt powders. Welding electrodes made by this process combine high melting resis­tance and high electrical conductivity.

Exercises

1. Answer the following questions

1. Which metal powders can be obtained commercially?

2. What tolerances on medium-size parts can be maintained?

3. Why is the yield of almost 100 per cent obtained?

4. What proportions can be obtained for particular applications?

5. What is said about strength and other mechanical properties of the parts produced?

6. What is the reason for the strength and other mechanical properties to be lower?

7. Why is the size of the produced parts limited?

8. What do aluminium powders tend to and what does this cause?

9. Can intricate shapes be produced with metal powders? Why?

10. How can the problem of high initial costs for the dies and the presses be solved?

11. What tendency is there for the powders?

12. What can you say about the production of tungsten wire?

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13. When is il possible to retain the individual characteristics of each constituent? Give some examples to illustrate this.

2. Find in the text above synonyms for the following

1. Benefit, profit

2. Productivity

3. Plain, obvious

4. Owing to, because of

5. Costly, high-priced

6. As compared to, in comparison with

7. Capable of being done, or accomplished

8. For example

9. In consequence

10. Complex

11. Trend

12. To bring about, to cause

13. To be inclined

14. Component, element

3. Speak about advantages and disadvantages of the powder metallurgy process. Use the answers to the questions of 1.

4. Read the text

POWDER METALLURGY LOOKS TOWARDS THE FUTURE

Developments and advances in powder metallurgy, a technology created some 50 years ago can save manufacturing industry great amounts of valuable materials. Powder metallurgy is a cheap alterna­tive to many conventional manufacturing processes.

When components, simple or complex, require precision and high quality at a comparatively low cost, powder metallurgy can pro­vide the solution of the problem. An important feature of powder met­allurgy is that it can provide the industry with such material composi­tions which are not achievable by any other means.

15!

Components produced by the powder metallurgy process can go straight into the manufacturing cycle or, if required, undergo further processing including heat treatment. Powder metallurgy is finding new applications in various industries, i.e. in electronics, aviation, machine-building, etc.

The unique physical properties of powder metallurgy parts en­able oil to be retained in minute porous cavities within the part. This self-lubricating characteristic is long-lasting and can eliminate other lubrication systems.

Future processes to be introduced by the association will pro­duce self-lubricating bearings, metal and non-metal alloys and other materials with pre-set properties based on combinations of various powders. Metal powder with its unusual characteristic features and properties is listed in the category of new materials.

6. Ask questions to the text above.

7. Retell the text.

Unit 33

COMBINING WITH NON-METALLIC POWDERS

It is possible to combine metal powders with non-metallic powders to produce useful materials. For example, Car-boloy is made by mixing the carbide powder, such as tungsten carbide, with cobalt powder which will act as the bond. This is used for cutting tools and other parts which have the hardness of tungsten carbide with the toughness of cobalt. Electric motor brushes consist of varying mix­tures of copper and graphite depending on the degree of electric con­ductivity and wear resistance desired. Friction surfaces for clutches, brakes, or other devices may be made by combining powdered materi­als, such as graphite, silicon carbide, alumina, and silica with iron and other metals. Since most friction plates are thin a backing plate of low-

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carbon steel is used with a thin facing of friction material bonded to it. The powder metallurgy process can be used to obtain a direct bond be­tween the thin facing of friction material and the backing plate. Clutch and brake disks made with this material have operated excellently on heavy-duty machinery and large vehicles. They perform satisfactorily at higher temperatures and with greater loads as compared with other friction materials. They wear longer, do not deteriorate in the presence of oil or water, withstand shock loads, and are not affected by arctic cold or tropical heat.

Obtaining Desired Properties. This process makes it possible to obtain varying degrees of porosity. Porous metal bearings can be im­pregnated with oil to make them self-lubricating. Used for fairly light loads, these bearings will operate satisfactorily for the useful lifetime of a machine without additional lubrication. Sometimes, the combina­tion of copper, graphite, and tin is used for porous bronze bearings. Iron powder for a combination of iron and copper powder may also be used. Porous metal filters of bronze or nickel can be produced. They have greater strength and shock resistance than ceramic filters and are made with porosities up to 80 per cent.

Exercises

1. Answer the following questions

1. Why may metal powders be combined with non-metallic powders?

2. How is Carboloy made?

3. Where is Carboloy used?

4. What do electric motor brushes consist of?

5. How may friction surfaces for clutches, brakes, or other de­vices be made?

6. Why is a backing plate of low-carbon steel used?

7. What else can the powder metallurgy process be used for?

8. What are the advantages of using clutch and brake disks made with this mate rial?

9. What does this process make it possible to obtain?

10. What makes porous metal bearings self-lubricating?

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2. Find in the text above synonyms for the following

1. Extent

2. The property of conducting electricity

3. To function

4. To become worse

5. To stand, to resist

6. To influence

7. To soak, to saturate

8. Rather

9. Extra

3. Retell the text «Combining with Non-Metallic Powders» using the answers to the questions of 1 as a plan.

4. Make up an abstract of the text above in writing.

5. Render the following