Introduction

Powder metallurgy {PM) technology can be used to manufacture a diverse range of components with properties that may be difficult or uneconomic to achieve using alternative processes. Examples of these components include self-lubricating bearing, elec­trical and magnetic parts, and hard materials such as tungsten carbide. PM can also be used to produce parts at or near their required final di­mensions, thus, minimising or eliminating the requirement for secon­dary machining. This can result in cost savings in terms of reduced

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time needed for machining and improved material utilization. The ap­plication of knowledge based techniques to PM materials selection of­fers considerable potential for improving the accessibility to engineers, of the benefits of PM technology. Knowledge based techniques consti­tute a consistent and reliable means of capturing and preserving knowledge relating to a particular domain, e.g., materials, processes etc. These techniques, which may include rule bases and mathematical models, can be used to develop powerful decision support tools. PM materials Advisor-I, which is being developed as such a decision sup­port tool, contains empirical models relating processing conditions and composition to tensile properties. The mechanical properties of a com­ponent, which are critical for ensuring satisfactory performance, de­pend upon the material selected and the subsequent processing opera­tions. Although, applications usually require a mechanical specifica­tion which includes tensile as well as fatigue and impact properties, at present PM materials Advisor-I use only the tensile properties as a ba­sis for power selection. The system provides an output of suitable ma­terial compositions with the required sintering conditions using the mathematical models and the database files resident in the system. These models are derived from data on PM materials sourced from companies, standards, including manufacturing handbooks such as the ASM Handbook. Statistical analyses of this data enable relationships to be established between tensile properties and certain material/process parameters. Incorporation of this information within the PM materials advisor provides the type of process related information needed to evaluate PM as a production method for a given component.

System Architecture

The two elements of PM materials Advisor-I consist of an expert system, employing a materials advisor knowledge base, and database files, containing coefficients used in expressions stored in the expert system to relate the tensile strength of a PM material to its density and composition.

An IBM PC-AT compatible machine (a90MHz Pentium PC), was used for development of PM materials Advisor-I. The software packages used are commercially available, consisting of the XiPlus ex-

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pert system shell and dBase database. The data was collected for sev­eral hundred PM materials and values of tensile strength, density, and chemical composition were entered into a file before being processed by a proprietary statistical analysis package. Linear and multiple re­gression was employed to fit curved to the data. This regression analy­sis resulted in derivation of coefficients for tensile strength of the fol­lowing materials: plain iron, carbon steel, nickel steel, copper steel, and low alloy steel. These coefficients were stored in dBase files, so that during an expert system consultation they were automatically read into expressions within XiPlus, and used to inform the design engineer of the type of material, density range, and range of carbon content needed to produce a component with the required tensile strength.

The Expert System

To use PM materials Advisor-I, it is necessary to consult the XiPlus expert system. This is achieved by loading the XiPlus knowl­edge base, which contains rules relating to materials selection, and which is used by the inference engine to answer a query specified by the user. In XiPlus expressions that are used in rules and which are ef­fectively variables are known as identifiers. For example, consider the following rule: if the temperature is > 1120, then, the sintering tem­perature is within the range. The words «temperature», «sintering tem­perature», and «within the range» are identifiers («if», «then» and «is» are recognized by XiPlus as key words).

Another example of the rule is that if the sintering temperature is within the range and the required tensile strength is < 287, then, the plain iron is a candidate material.

The rules of this type are incorporated in the PM advisor knowl­edge base.

When it is necessary to determine whether iron is a suitable ma­terial, the inference engine generates a question asking the user for a value for the furnace temperature. XiPlus has inferred that in order to find out wtheter plain iron is a candidate material it has to determine whether the temperature is within the range. If the user enters a value of more than 1120 °C the sintering temperature is within the range. It is, then, necessary to ask the user if the required tensile strength is less than 287 MPa, and if it is, the answer to the query is that iron is a can-

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didate material. This is an example of backward chaining, which is one of the main methods of inference employed within XiPlus.

The Database

From a dBase file the PM material advisor obtains coefficients and constants for the use of the material modelling equations. Also, ob­tained from the database, are the range of densities and compositions from which the equations were derived. By using an interfacing pro­gram supplied with XiPlus, it is possible to read a field of the dBase record and assign its value to an identifier. Such identifiers are, then, available for inclusion in the rules to be used in the knowledge base for the inference. The nature of the inferences made determines the answer to the original queries and the final result of the consultation.

The Knowledge Base

The rules within the PM materials Advisor-1 knowledge base represent models used to calculate densities and compositions needed to obtain the required tensile strength. To enable development of these models a collation of process/material data and standardization of units and terms employed is required followed by statistical analysis to iden­tify relevant relationships. Materials data suitable for this type of analysis is available in standards such as MPIF Standard 35, as well as, research publications, Danninger, Jangg, and Stickler. Modelling of the relationship between the tensile strength and variation in material and process parameters identifies the various ways in which the specifica­tion can be attained. Inclusion of these models in an expert system knowledge base and specification of the operating conditions imposed upon the component allows determination of the optimal material specification for the application.

Factors Affecting Tensile Strength

Density is generally considered to be of primary importance in determining the strength of PM materials, and various models have been developed to establish an empirical relationship density and strength. However, in addition to density and composition there are a number of factors which have an influence on tensile strength. These include pore shape and orientation, mean pore size, pore size distribu­tion, grain size, sintering parameters, such as atmosphere, temperature

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and duration, and post-sintering treatments. When investigating rela­tionship between density and mechanical properties, it is important that the effects of variations in the above factors are kept to a minimum. This is achieved by analysing data for powders, which for each type of material have similar physical properties, and where the material is produced by employing conventional pressing and sintering. The ex­pressions for tensile strength quoted here relate to cold compaction and sintering between 1120 and 1150 °C for 20-30 minutes. It is usually found that, within limits, sintering at a higher temperature for a longer time results in higher tensile strength. For example, Danninger et.al. report that for iron powder compressed at 400 MPa sintering at 1120 °C for 30 minutes produces iron with tensile strength of 140 MPa, while 1250 °C for 4 hours gives 186 MPa.

Appendix 3 SOME INTERESTING FACTS ABOUT METALS

It is supposed that gold was the first of all the metals ever used by man. And, probably, this was the case everywhere in the ancient world. We may certainly believe this, for beautiful gold rings, bracelets, collar ornaments, etc. have been found in ancient tombs and buried towns of Egypt, Peru and other places.

From time immemorial men have hunted gold as we hunt it to­day. They travelled very long distances and often risked their lives to get it. It does seem strange why they wanted gold. They did not use it as money, for among both ancient folk and peoples of much later peri­ods other things had served for money long before gold was so used. Some of the things that served for money in those times were, for in­stance, shells and beads in Europe, pearls and copper among the

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American Indians, iron treated with vinegar in Spartan Greece, copper rings in many places.

Most likely, people wanted gold only for ornaments. People have certainly liked to wear ornaments from the earliest times - animal teeth and ivory, pearls and jade, even coloured shells. But, though, men had travelled great distances to get these bits of jewellery, they did not face such dangers as they faced when they hunted gold.

Gold is a wonderful metal in many ways.

It is a yellow metal, bright and shining when it is polished - the colour of the sun. And that, in itself, is, perhaps, the answer why gold seemed to these ancient folk to be a piece of the sun found on the earth. The early gold-seeking peoples were the Incas of America, the ancient farming folk of China, India, Mesopotamia, Egypt, Italy, France, Spain and Britain which were worshippers of the Sun.

Whatever the reason, this early search for gold (gold hunting) became in later years more important to us than even gold itself has ever been, because it led to the use of the other metals.

Copper came after gold. Then came tin, lead, silver and iron. Before the beginning of history almost each of the base metals had been discovered and used. But some metals, that are common today, were still rare in the early days.

Perhaps, it will not surprise you to learn that copper is a beauty. But few people have really seen it. What most people have seen is a mask, which copper puts on when it is shown to the world.

If you want to see the metal as it really is you must take off its covering. You can do it if you heat it to redness in a glass tube. Then, copper will show its beauty. You will see a shining silver metal, deli­cately tinted with pink like the petals of a rose.

But the moment you draw the copper from the closed tube and a breath of air hits its face it will grow red. Then, it will darken to a dull bronze. And, finally, more contact with the air will give it a greenish and bluish tint.

«Not worth a copper» goes the well-known saying. Yet, copper is worth much to the world in this Age of Electricity and Light. It is an excellent conductor of heat. It is almost as good as silver and much bet­ter than iron. A pure copper wire will convey three times as much cur­rent as an iron wire of the same diameter. But the copper must be pure.

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There are many traces of the early use of copper in Europe, Asia, America and Northern Africa. Here, archaeologists discovered many relics that show how prehistoric peoples lived.

Probably, it was in Egypt, after all, that man first found copper.

In the word copper we have, in fact, a living history of the fa­mous mine in Cyprus. This mine gave the early world a large part of its copper supply. Rome got most of the metal from the island of Cyprus.

We know from the earliest records that this island started mak­ing copper in 4,000 B.C. And because it had great mineral riches, dif­ferent people tried to conquer it.

First, it was dominated by the Egyptians. Later on, the Assyrians took hold of it. Then, came the Greeks. After the Greeks it fell into the hands of the Persians. And, finally, the Romans took it over. All these peoples wanted to possess the rich copper mine of Cyprus.

In the early days copper was too sacred, too beautiful and too hard to get from the earth, to allow its use in the houses of the ordinary people.

In those days, only temples and public buildings were orna­mented with it, for instance, China's famous Temple of Heaven in Pe­king, as well as, many Japanese temples were roofed with copper.

At Kamakura in Japan there is a shrine, which is built in the form of a colossal bronze Buddha. It is 36 feet high. For 670 years this Figure, Japan's greatest work of art, has withstood tidal waves that have swept away many temples.

Copper has been used for roofing in America. Most of the build­ings have been protected by copper. For example, next to the Nation Capitol in Washington you can see one of America's most beautiful buildings, the Library of Congress.

The most striking feature about it is i& dome of gold - «22 carat fine». But the golden dome is really of black copper. It supports a layer of very fine real gold. And in the building itself large quantities of copper are used in wires and pipes.

As a matter of fact, throughout the history of the world copper and bronze have always been used for making works of art.

Many bronze figures have been found in excavations of Assyria and Babylonia. The Assyrians and Babylonians made excellent relief work in metal.

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Their rivals, the Egyptians, developed metal work to a greater extent. Among them, the cult of the dead was prominent, So, they elaborate tombs for the important members of the society.

Tin is a heavy metal, silver white in colour. It was scarce in the ancient world. Sometimes, it was found in stream beds as nuggets just as gold was found. And sometimes, it was mined. Copper and tin were also melted together to make the alloy, bronze.

Silver, like gold, as you know, is one of the precious metals. It was first used for ornaments and later for coins. In the early days, sil­ver was very valuable. It was priced almost as much as gold itself.

Silver is a white and shining metal which can be beaten into sheets or drawn into threads. It is harder than gold and will, in time, tarnish and lose its lustre.

Somehow, the Romans priced silver more than gold. When, for instance, the armies of Carthage had been defeated and forced to pay tribute to Rome, the Romans demanded the tribute in silver, not in gold. Similarly, the Romans knew that the Carthagians had long worked some of the great silver mines of the ancient world.

The Romans in those days used silver in different ways. They made cloth woven of silver thread. And they even covered their war machines with silver. They used to cover their war machines with sil­ver because it made these machines so bright that they could be seen from far away. And their desire was to frighten their enemies with flashing war machines.

Silver came rather early into use for ornaments, money, mirrors and table ware. The Egyptians made silver beads, necklaces and brace­lets almost as early as they made these things of gold.

The use of silver for mirrors was even more interesting. The first metal mirrors were made of bronze, cast and polished. Later on, a mix­ture of lead, silver and copper was used. But polished bronze, as we know now, does not really reflect very well, and silver tarnishes easily. The Egyptians discovered that very soon and so, they stained their mir­rors black by rubbing them with an egg. A black surface, of course, cannot give a good reflection. At any rate, it cannot give as good a re­flection as a silver one, but it does not tarnish either. We learn from old records that there were no glass mirrors even in Roman times, though, the Romans knew how to make glass and thin sheets of silver or tin.

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But they did not know how to apply these to glass. Polished metal mir­rors of silver, bronze and, sometimes, steel came into common use only in the sixteenth century.

Mercury was not used for mirrors in the ancient world, still it was well-known then. It was used to purify gold and silver ores; for mercury has a curious property it can pick up gold silver dust. From old records we learn how the fine clothing of the rich Romans was burnt when it became too old to wear, and the ashes were mixed with mercury. This was done because the Romans used to decorate their clothes with gold embroidery. And when the ashes were mixed with mercury, then, mercury would pick up the gold from the ashes. To do this, the mixture was put into a cloth bag and squeezed. And thus, all the mercury came out through the cloth leaving behind pure gold.

Though, mercury had been known all through the ancient his tory, no one, in fact, knew what it really was until the days of the al­chemists of the Middle Ages. It was one of these alchemists Albert le Grand who discovered pure, free mercury. These mediaeval folk thought there was something magic about mercury, so they were a little afraid of it. It is not difficult to see why they were afraid of it. Mercury is a curious metal, almost like a thing alive. Its common name is quick­silver. We use mercury today in medicine, mirrors, colours, electric lights, thermometers, etc.

Lead, probably, followed silver as a useful metal. Sometimes, it was mixed with bronze. The more common use of lead was for roofs, pipes, paints. In Babylon the roofs of the terraces that formed the Handing gardens were made of lead. In Rome it was very common for roofs, gutters and water pipes. The Romans got some of their lead from Britain, but, very likely, most of it came from Spain. The ancient lead smelters cast lead in lumps of convenient size for the plumbers. Each lump was marked with its weight, and, sometimes, with the name of the founder. Many such lumps were found in England and Spain.

In Rome itself water was piped to the houses just as we pipe it today. Lead, in fact, is so much the plumber's metal that the name of his craft comes from the Latin word for lead (plumbum).

Lead was also used as a weapon in different ways and at differ­ent times. It was the ancient practice to heat great kettles of lead until it was molten and pour it over castle walls upon the head of the attackers.

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We use lead in some of these ways today. But, perhaps, it is more important for us to use it in making glass and, especially, paints.

Cobalt, a reddish-grey metal, was known, too, in the ancient world, and it was used for making colours. But people were afraid of it, as they were afraid of mercury. Even as early as the Middle Ages, co­balt was looked upon as an evil metal and its name meant «black devil».

When King Solomon wished to repay Hiram for his help, he of­fered him some gold and silver mines. But Hiram refused to take the gift after inspecting the mines, because he found that they also con­tained cobalt, and he was afraid of the danger from the miners.

Nickel, a hard, silver-white metal, was known in Greece and In­dia, but it was, probably, quite rare. It really did not come into general use until the 16^th century in Germany, where it got its name of nickel which meant «Old Nick» or «demon». It was so called because it was difficult to work.

Zinc, a white metal, which we use today in a hundred ways, was not known in the ancient world. It was almost at modern times that pure zinc was first made in Germany. But, though, the ancient people had no pure zinc, they still used the ore of zinc, even, when they could not separate the metal from its ore. Zinc and copper ores, when smelted together, form the alloy of brass. Brass was used by the Greeks and Romans. But it is doubtful, if it was common in use. Bronze, indeed, was fairly common in the ancient world, but brass was rare.

Generally speaking, zinc, lead, copper, tin, mercury, iron, nickel, cobalt, antimony, gold and silver were known and used in the ancient world. At least, three of them - copper, tin and iron - were important for the toolrnaker, while gold represented the wealth of the world. But, in spite of the usefulness and need of these metals, the ancient people, actually, knew very little about any of them. This sounds very strange, indeed since the Egyptians and the Greeks were good at metal-working.

In ancient Greece and Rome metals seemed mysterious to peo­ple, who didn't study them properly and knew very little about them.

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Appendix 4

ENGLISH FOR SCIENTIFIC

DISCUSSIONS

Talking of.

Syn. Speaking about...

What I'm going to say

(to point out, to pay attention to...) is that... In my opinion...

Syn In my view..,

I think. .

As far as I'm concerned...

To tell the truth...

As the case stands...

I'm going to be talking about.

At this point...

Well...

By the way...

They say...

Firstly...

First of all ...

Syn.To begin with...

Secondly...

Finally...

Togo back to...

Syn. To come back to.

Anyway...

To be honest...

SPEECH PATTERNS FOR SCIENTISTS

Итак..., ну...

Между прочим, кстати

Говорят...

Сначала...

Прежде всего...

Затем, после этого...

В конце..., в заключение...

Возвращаясь к...

Во всяком случае..., как бы то

ни было...

Честно говоря..., откровенно

говоря...

Говоря о...

Я хочу сказать (отметить, обра­тить внимание на то), что...

По моему мнению, на мой взгляд...

По правде говоря... При данном положении дел... Я собираюсь рассказать о... Здесь, сейчас, в этом месте...

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As a matter of fact...

Syn. In fact...

Actually...

I should say before we start...

Let's get down to... It goes without saying... The real question is that.

In other words... Summing it up... Syn. To summarize... Let's look at...

I'd like to make some brief comments about... My interest in this problem is... It's safe to say...

This brings me to...

Now to digress for just

a moment...

In the long run...

To make a long story short...

Syn. In short

To put it in a nutshell

I must emphasize that...

In spite of...

That's about all...

there is to it...

Syn. That's all I have for

the moment

I'd like to draw your attention

to...

Then I'll move on to...

В действительности, на самом деле...

Прежде чем мы начнем, я дол­жен сказать, что... Приступим к... Само собой разумеется... Вопрос действительно в том, что...

Другими словами... Подводя итог... Подытоживая... Давайте взглянем на.,., Давайте рассмотрим... Я бы хотел сделать краткие за­мечания по поводу... Меня интересует этот вопрос... Можно с уверенностью ска­зать...

Это меня наводит на мысль о... Сейчас, если отвлечься на секунду... В конечном счете... Короче говоря..., в двух сло­вах...

Я хочу подчеркнуть, что... Несмотря на..., невзирая на... Пожалуй, это все...

Я бы хотел обратить ваше вни­мание на... Затем я перейду к...

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I was just wondering Я хочу (знать, спросить, выяс-

нить) (to know, to ask, to find out)...

The main idea of the article is Главная мысль статьи - это то, that... что...

Интересно отметить, что.., Извините, что прерываю вас...

Syn. The points of the article... The keynote of the article... It's interesting to note that... Sorry to interrupt you... Syn. Excuse my interrupting you...

В тексте говорится о...

The text deals with... Syn. The text has to do with...