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When this is molten, the solid zinc is added and is quickly dissolved in the liquid copper before very much zinc has boiled away. 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. By a careful choice of constituents, it is possible to make alloys with unusually low melting points. 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. 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. 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 alloying of tin and lead furnishes an example of one of these special cases. 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.

Legure


One difficulty in making alloys is that metals have different melting points. When cast, the metals would separate into two layers, the heavy lead below and aluminium above. 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. To manufacture this alloy it would be undesirable to melt the few pounds of copper and add nearly twelve times the weight of aluminium. Consequently, in weighing out the metals previous to alloying, an extra quantity of zinc has to be added. 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. 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. But at that high temperature the liquid zinc would also boil away and the vapour would oxidize in the air. 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. When this is molten, the solid zinc is added and is quickly dissolved in the liquid copper before very much zinc has boiled away. Generally the major ingredient is melted first; then the others are added to it and should completely dissolve. 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: For instance, if a plumber makes solder he may melt his lead, add tin, stir, and cast the alloy into stick form. The alloying of tin and lead furnishes an example of one of these special cases. What happens at the other end of the series, when tin is added to lead? Even in this case the behaviour of the alloy on melting is not simple. 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. Once again the freezing point is lowered. Sometimes the making of alloys is complicated because the higher melting point metal is in the smaller proportion. Some pairs of metals do not dissolve in this way. 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. In this, as in many other cases, the alloying is done in two stages. 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. When this is so it is unlikely that a useful alloy will be formed.

Legure

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1 thoughts on “Legure”

Zulkishicage

21.11.2017 at 10:12 pm
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Some pairs of metals do not dissolve in this way. Even in this case the behaviour of the alloy on melting is not simple.

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