making alloys

7/30/2019 Making Alloys

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Metallurgy: Making Alloys

The majority of alloys are prepared by mixing metals in the molten state; then the

mixture is poured into metal or sand moulds and allowed to solidify. Generally the major

ingredient is melted first; then the others are added to it and should completely dissolve. For

instance, if a plumber makes solder he may melt his lead, add tin, stir, and cast the alloy into

stick form. Some pairs of metals do not dissolve in this way. When this is so it is unlikely

that a useful alloy will be formed. Thus if the plumber were to add aluminium, instead of tin

to the lead, the two metals would not dissolve - they would behave like oil and water. When

cast, the metals would separate into two layers, the heavy lead below and aluminium above.

One difficulty in making alloys is that metals have different melting points. Thus copper

melts at 1,083C, while zinc melts at 419C and boils at 907C So, in making brass, ifwe just put pieces of copper and zinc in a crucible and heated them above 1,083C, boththe metals would certainly melt. But at that high temperature the liquid zinc would also boil

away and the vapour would oxidize in the air. The method adopted in this case is to heat

first the metal having the higher melting point, namely the copper. When this is molten, thesolid zinc is added and is quickly dissolved in the liquid copper before very much zinc has

boiled away. Even so, in the making of brass, allowance has to be made for unavoidable

zinc loss which amounts to about one part in twenty of the zinc. Consequently, in weighing

out the metals previous to alloying, an extra quantity of zinc has to be added.

Sometimes the making of alloys is complicated because the higher melting point metal is in

the smaller proportion. For example, one light alloy contains 92 per cent aluminium (melting

point 660C) with 8 per cent copper (melting point 1,083C). To manufacture this alloy it

would be undesirable to melt the few pounds of copper and add nearly twelve times theweight of aluminium. The metal would have to be heated so much to persuade the large bulk

of aluminium to dissolve that gases would be absorbed, leading to unsoundness. In this, as in

many other cases, the alloying is done in two stages. First an intermediate 'hardener alloy' is

made, containing 50 per cent copper and 50 per cent aluminium, which alloy has a melting

point considerably lower than that of copper and, in fact, below that of aluminium. Then the

aluminium is melted and the correct amount of the hardener alloy added; thus, to make l00lb

of the aluminium-copper alloy we should require 84lb. of aluminium to be melted first and

16lb of hardener alloy to be added to it.

In a few cases, the melting point of the alloy can be worked out approximately by

arithmetic. For instance, if copper (melting point 1,083C) is alloyed with nickel (meltingpoint 1,454C) a fifty-fifty alloy will melt at about halfway between the two temperatures.Even in this case the behaviour of the alloy on melting is not simple. A copper-nickel alloy

does not melt or freeze at one fixed and definite temperature, but progressively solidifies

over a range of temperature. Thus, if a fifty-fifty copper-nickel alloy is liquefied and then

gradually cooled, it starts freezing at 1,312C, and as the temperature falls, more and moreof the alloy becomes solid until finally at 1,248C it has completely solidified. Except in


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certain special cases this 'freezing range' occurs in all alloys, but it is not found in pure

metals, metallic, or chemical compounds, and in some special alloy compositions, referred to

below, all of which melt and freeze at one definite temperature.

The alloying of tin and lead furnishes an example of one of these special cases. Lead melts

at 327C and tin at 232C. If lead is added to molten tin and the alloy is then cooled, thefreezing point of the alloy is found to be lower than the freezing points of both lead and tin

(see figure 1). For instance, if a molten alloy containing 90 per cent tin and 10 per cent leadis cooled, the mixture reaches a temperature of 217C before it begins to solidify. Then, asthe alloy cools further, it gradually changes from a completely fluid condition, through a

stage when it is like gruel, until it becomes as thick as porridge, and finally, at a temperature

as low as 183C, the whole alloy has become completely solid. By referring to figure 1, itcan be seen that with 80 per cent tin, the alloy starts solidifying at 203C, and finishes onlywhen the temperature has fallen to 183C (note the recurrence of the 183C).

What happens at the other end of the series, when tin is added to lead? Once again the

freezing point is lowered. An alloy with only 20 per cent tin and the remainder lead starts tofreeze at 279C and completes solidification at the now familiar temperature of 183C.One particular alloy, containing 62 per cent tin and 38 per cent lead, melts and solidifies

entirely at 183C. Obviously this temperature of 183C and the 62/38 per centcomposition are important in the tin-lead alloy system. Similar effects occur in many other

alloy systems and the special composition which has the lowest freezing point of the series

and which entirely freezes at that temperature has been given a special name. The particular

alloy is known as the 'eutectic' alloy and the freezing temperature (183C in the case of thetin-lead alloys) is called the eutectic temperature.


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By a careful choice of constituents, it is possible to make alloys with unusually low meltingpoints. Such a fusible alloy is a complex eutectic of four or five metals, mixed so that the

melting point is depressed until the lowest melting point possible from any mixture of the

selected metals is obtained. A familiar fusible alloy, known as Wood's metal, has a

composition:

Bismuth 4 parts

Lead 2 parts

Tin 1 part

Cadmium 1 part

and its melting point is about 70C; that is, less than the boiling point of water. Practicaljokers have frequently amused themselves by casting this fusible alloy into the shape of a

teaspoon, which will melt when used to stir a cup of hot tea.

These low melting point alloys are regularly in use for more serious purposes, as for

example, in automatic anti-fire sprinklers installed in the ceilings of buildings. Each jet of the

water sprinkler system contains a piece of fusible alloy, so that if a fire occurs and the


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temperature rises sufficiently high, the alloy melts and the water is released through the jets

of the sprinkler.

(FromMetals in the Service of Manby W. Alexander & A. Street.)

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