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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, if we just put pieces of copper and zinc in a crucible and heated them above 1,083C, both the 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, the solid 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 the weight 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 (melting point 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 more of the alloy becomes solid until finally at 1,248C it has completely solidified. Except in 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, the freezing 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 lead is cooled, the mixture reaches a temperature of 217C before it begins to solidify. Then, as the 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, it can be seen that with 80 per cent tin, the alloy starts solidifying at 203C, and finishes only when 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 to freeze 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 cent composition 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 the tin-lead alloys) is called the eutectic temperature.

By a careful choice of constituents, it is possible to make alloys with unusually low melting points. 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. Practical jokers 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 temperature rises sufficiently high, the alloy melts and the water is released through the jets of the sprinkler.

(From Metals in the Service of Man by W. Alexander & A. Street.)