6. Say what these statements have to do with. Paraphrase them using the words from the text above. Start each answer in this way, «This statement has to do with...»

1. For the past years there has been noticed a wider use of these processes.

2. For a long time these parameters have been used in the chemical industry.

3. The first factory of this kind was started working at the begin­ning of fifties.

7. Find in the first part of the text, beginning with «In recent years» and ending «in America and Cuba», the key terms. Give their definitions.

8. Make up a summary of the text.

Unit 30

MAKING METAL POWDERS

AND ALLOYS BY POWDER

METALLURGY

Powder metallurgy is the technique by which metals are first produced as powders and subsequently made into metal parts for machinery. The various methods used for making the metal powders arc listed and explained below:

Electrolitic deposition. After electrolysis the metal is usually ob­tained as a soft spongy dendritic structure. Additional crushing or mill­ing is necessary as powder obtained is generally pure, free from oxide, and uniform.

Reduction of the oxides. By treating an oxide of a metal with a reducing gas at an elevated temperature the pure metal is obtained. It is then crushed and milled to form a powder. The method is cheaper, and. by far, the largest amount of metal powder is made by it.

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Condensation of metal vapour. Metals with low boiling points, such as zinc, magnesium, and cadmium, can be boiled and the vapour then condensed in powder form.

Atomization. By spraying a molten metal, such as zinc, lead, or aluminium against a stream of compressed air or inert gas, a metal powder with irregular shapes is obtained. The powder can be collected by a stand- and dust-collector system.

Hydraulic process. Many metals can be combined with hydro­gen to produce hydrides. Hydrides of titanium, zirconium, thorium, co-lumbium, and tantalum are stable at room temperatures, but begin to dissociate into hydrogen and pure metal above 660 °F. These powders can be added directly with other powders and heated to further reduce the oxides present in them. This process is more expensive and limited to special cases.

Shotting. Dropping molten metal from small opening through air or inert gas into water produces fine shot, which is spherical or sometimes pear-shaped. The shot is generally further reduced by other methods, such as crushing or milling.

Milling. By using various kinds of mills, such as stamp mills or ball mills, small pieces of metal can be ground up into powders.

Using chips from the machine shop. These must be further re­duced by crushing and milling.

Precipitation. In this method metal powder is obtained by chemicals. The parts made by powder metallurgy are frequently al­loyed and often even mixed with certain non-metallic constituents. The various kinds of powders must be carefully selected and thoroughly mixed together. The pressing operation is done at room temperature in a die which has a desired shape. The punches, which apply the pressure to the powder, operate the die vertically. Hand dies are removed from the press each time a part is made. They are slow and used generally for experimental work. The dies used in the production are fastened to a press. A lower punch forms the bottom of the die cavity and is oper­ated from below while the upper punch enters the die from above. The die with its upper and lower punches operates in the same manner as the dies for making press forms in plastic molding. In operation the lower punch first moves downward to its lowest position. There the die is filled with loose powder, which is scraped off level with the top of the die. Then, both punches move towards each other, compressing the

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powder under pressure ranging from 10 to 100 tons per square inch. The upper and lower punches instead of having flat end surfaces may be shaped to give a desired form to the parts. A vertical hole may be produced in the parts by having a core rod, either movable or station­ary, projecting from the lower and a corresponding hole above in the upper die. After compressing the compact the upper die moves upward for clearance, and the lower die rises to the table level ejecting the compact. A sweep removes it and the die with loose powder again to start another cycle. For high-production rotary table presses, which have 12 or more die cavities, each provided with top and bottom punches, are frequently used. During production the tabic rotates while the operations of filling, compressing, and ejecting the compacts arc automatically performed at the various stations. All the operator has to do is to keep the hopper filled with metal powder and carry away the finished compacts. One operator can run from three to five machines.

Exercises

1. Answer the following questions

1. What is the definition of powder metallurgy?

2. How many methods of making the metal powders are men­tioned in the text?

3. What powder is obtained after additional crushing or milling?

4. When is the pure metal obtained?

5. What is done to the pure metal after it is obtained?'

6. is this method expensive?

7. What metals can be boiled?

8. How is a metal powder with irregular shapes obtained?

9. What can many metals be combined with?

10. The hydrides of what metals are stable at room temperature?

11. When is fine shot produced? What form does it have?

12. What methods is the shot generally further reduced by?

13. Under what conditions is the pressing operation done?

14. When are hard dies removed from the press?

15. What is the pressure under which both punches compress the powder?

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2. Find in the text «Making Metal Powders and Alloys by powder Metallurgy» synonyms for the following

1. Method

2. Usually

3. To produce

4. To raise, to make higher

5. To grind

6. Quantity

7. Often

1. To work

8. Not fact

10. To attach

3. Give the names to the following definitions

1. The spraying of a molten metal against a stream of com­ pressed air or inert gas.

2. The combining of metals with hydrogen to produce hydrides.

4. Retell the text «Making Alloys by Powder Metallurgy» us­ing the answers to the questions of 1.

4. Read the text

COHERENCE OF METAL POWDERS

Clean metal surfaces will cohere strongly when brought into in­timate contact. The bonding of metal particles in intimate contact will occur at room temperature and is not dependent upon high tempera­tures. If, however, the particle surfaces are hot clean, this coherence will not occur. The properties of the parts so made will depend upon the size and shape of the particles usefl, kind of metals used, pressure used, and the sintering to a great extent of the total area of contact be­tween the particles. After the pressing operation many small voids will remain between the particles resulting in porosity of the metai parts. Often these voids are extremely small and invisible.

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

7. Make up a summary of the text above.

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Unit 31

THE SINTERING PROCESS

Soft metal powders can be compressed quite readily, while hard powders require higher pressures to produce an adequate density. The mechanical presses, used, are rated at from I0 to 500 tons capacity and hydraulic presses have still higher capacities. The dies are generally made of highly polished hardened steel. Here the term «die» is used to include the complete die with its punches. For highly abrasive powders the dies may be lined with a hard-cemented carbide material. The die-cavity depth before filling depends upon the apparent density of the powder being compressed. Since there is al­most no lateral flow in the powder the shapes of the parts which can be pressed are limited. An allowance should be made in the die to retain sufficient allowance on the compacts for sizing operations and also for shrinkage or growth during sintering.

The sintering process improves the bond between the particles and increases the mechanical properties of the finished parts. Al­though, it is true that the metal particles will become bonded together at room temperature, sintering at elevated temperatures facilitates the bonding by increasing surface diffusion and plastic deformation. Usu­ally, the greater strength in the parts is obtained when sintering is done above the critical, or recrystallization temperature. The sintering tem­perature is generally kept below the melting points of the metals in­volved. However, if two or more different metal powders are present, in certain cases, the sintering temperature is raised above the melting point of one of the metals. It will then melt and become a matrix for the other metal particles.

Sintering is usually done in electric-resistance furnaces, although, gas or oil-fired furnaces are sometimes used. The temperatures may-vary from about 1000 to 2500 °F for the copper-base and iron-base compacts. In order to protect the parts from the formation of undesir­able oxide coatings the atmosphere within the furnaces should be kept inert and slightly reducing. Hydrogen, partially burned coke-oven gas. or partially burned natural gas, will provide reducing atmospheres. Ni­trogen will serve well as an inert atmosphere but will not reduce oxides. During sintering there will probably be either a shrinkage or growth of

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the compacts. It has been noted that porosity decreases with higher sin­tering temperatures and this, in turn, generally results in shrinkage.

Considerable effort has been made to combine the compressing and sintering operations. Such a combination would have several ad­vantages, including improved strength and hardness. The chief prob­lem is the lack of a suitable die material which can withstand the high pressure at high temperatures. However, considerable success has been achieved in this direction in the production of cemented carbides.

Exercises

1. Answer the following questions

1. How can metal powders be compressed?

2. What is the capacity of the process used? Is it the same for mechanical presses and hydraulic ones?

3. What are the dies usually made of?

4. What does the die-cavity depth depend upon?

5. Why should an allowance be made in the die?

6. What does the sintering process improve, and what does it in­crease?

7. How does sintering facilitate the bonding at elevated temperature?

8. When is the greater strength in the parts obtained?

9. What can you say about the sintering temperature?

10. Where is sintering usually done?

11. How may the temperature vary?

12. Why should the atmosphere within the furnaces be kept inert and slightly reducing?

13. What gases will provide reducing atmospheres?

14. What is the role of nitrogen?

15. What will happen to the compacts during sintering?