Alloys are mixtures formed by melting two or more metals together.

They are not, however, mere mechanical mixtures, for they often exhibit properties different from those possessed by the metals in the mixture.

For example, copper and tin are both very malleable metals.

Two parts of copper with one of tin form a white alloy (speculum metal) so hard that it cannot be cut with steel tools, and as brittle as glass.

The tensile strength of this alloy is only 1/5 that of tin and 1/50 that of copper.

Nine parts of copper to one of tin make a tough, rigid gun metal, harder and more fusible than copper, but which cannot be rolled or drawn.

By adding tin (a Bofter metal than copper) to gun metal its hardness is increased !

In preparing alloys the most infusible metal should be melted first, and the others subsequently added.

If the metals are of different specific gravities they must be continually stirred while fluid, or the heavier will sink to the bottom and the alloy will not be homogeneous.

The specific gravity of an alloy is seldom equal to the mean of the specific gravities of the metals in the mixture. It is sometimes more and sometimes less dense.

The tensile strength of an alloy is generally much greater than that of the metals composing it.

Brass is an alloy composed of copper and zinc, the proportions of which vary according to the purpose for which the metal is required.

The zinc is melted first, and the copper added in small quantities. A little old brass in the crucible will facilitate the union of the metals. The crucible must be covered with charcoal powder and a close lid, or the zinc will pass away in vapour.

Colour

The colour depends upon the proportions.

Common yellow brass contains 2 parts of copper to 1 of tin. If the copper be in greater proportion than 4 to 1, the alloy is reddish; if less than 3 to 1, it becomes of somewhat the colour of zinc.

Properties

Brass is tough, as a rule, but is rendered brittle by continued vibration. The presence of iron injures its tensile strength and malleability.

It is more malleable than copper when cold, but cannot be forged at a red heat, because the zinc melts at a low temperature.

The fusibility of brass increases in proportion to the quantity of zinc it contains. The addition of a little phosphorus makes it very liquid and easily run into fine castings.

The proportions of the constituents for the different kinds of brass, and the uses to which these are applied, are shown in the Table on p. 350.

The name Brass is frequently given to all alloys of copper. Those containing tin should properly be called Bronze.

Muntz metal or sheathing is cheaper than common brass and more easily rolled. It is much used for sheathing ships, as it keeps cleaner than copper, and is sometimes employed as a covering for small roofs.

Muntz metal made, as it usually is, of 60 parts copper and 40 zinc, has been found to be attacked by salt water and to lose its zinc. An alloy is therefore used instead of 68 parts copper and 32 zinc.

Delta Metal, sometimes called Dick's metal, is an improved brass, which can be made tough and hard; it can be forged or rolled hot, or worked and drawn into wire when cold. It makes sound fine castings, is of the colour of gold alloyed with silver, and when exposed to the atmosphere tarnishes less than brass.

Kirkaldy's Tests

Stress per square inch in tons.

Contraction of area at fracture per cent.

Extension in 10 inches per cent.

Ultimate.

Elastic limit.

Bar 1 as drawn

33.6

221

15.0

8.8

„ 2 annealed

27.2

8.8

19.9

17.5

Cast in sand

21.6

...

...

...

Bronze is a mixture of copper and tin, the proportions being varied for different purposes, as shown in the Table below.

The different specific gravities of the metals make it difficult to melt them together. The tin is first melted into twice its weight of copper to make hard metal, which ft then added to the proper proportion of copper separately melted.

1 From Patentee's Circular.

Large castings in bronze are often not homogeneous throughout their mass, in consequence of the difference in fusibility of the copper and tin.

Gun Metal also differs in the proportions of its constituents according to the purpose for which it is intended.

At one time it was much employed for casting ordnance, from which it derives its name.

It is harder, more fusible, and stronger than copper, and is used for pump valves and parts of machines.

Bell Metal consists of copper and tin, in the proportion of from 3 to 1 to 5 to 1. Small house bells contain 5 copper to 1 tin. Large bells 4 copper to 1 tin Large church bells 31/2 copper to 1 tin. The metal, after being cast, is heated to redness and quenched, then again heated and allowed to cool slowly.

Aluminium Bronze contains from 90 to 95 per cent copper and 10 to 5 per cent aluminium.

It may be cast or turned in a lathe, also forged cold or hot, but it cannot be welded.

Table Giving The Composition Of Various Alloys

Alloys.

Parts by Weight.

Copper.

Zinc.

Tin.

Lead.

Antimony.

Iron.

Brass, ordinary ...

2

1

„ forlocksanddoorhandles

3

1

„ „ turning and fitting .

3

1

1a/12

„ „ engraving..

3

1

A little

„ „ bushes and sockets ...

18

1

..

1

„ to bear soldering well .

2 2/3

1

„ pot metalb...

2 1/2

. •

..

1

Bronze, hard, for bearings for machinery....

8

...

1

„ for stop cocks and valves

88

10

2

„ „ wheel metal for small toothed wheels..

10

1

„ „ bearings for very. heavy weights..

32

1

5

Manganese.

Manganese bronze..

88

••

10

..

..

• •

2

Gun metal for ordnance..

90 1/2

9

„ of maximum hardness for turning

5

...

1

„ soft ...

16

..

1

Bell metal ....

4

...

1

Less than

Muntz metalc . . .

3

2

1/100

nails for .

87

4

9

Gedge's metal..

60

38.2

1.8

Sterro-metal..

55 to 60

34 to 44

l to 2

2 to 4

Babbit's metal..

4

8

96

White brass..

3

...

90

...

7

Bis.

,, ...

1

...

...

7

7

muth.

Metal to expand in cooling .

...

...

...

9

2

...

1

a The lead prevents the filings from sticking to the tool, but renders the brass unfit for hammering.

b An inferior alloy, used for very common taps, etc., and called also cock metal. c Composition varies between 50 copper and 50 zinc, and 63 copper and 37 zinc.

It is light, very malleable, ductile, and not easily tarnished, but its expense prevents it from being used for anything but instruments.

Phosphor Bronze is any bronze or brass alloy, together with a small proportion of phosphorus. Its qualities may be made to vary by altering the proportions of its constituents.

It wears longer than gun metal in bearings, is very tough, and is useful in positions where it is subject to shocks.

The phosphorus preserves the metal from the effects of the atmosphere.

Manganese Bronze is an alloy (usually white) of pure copper with from 2 to 30 per cent of manganese. It is made in different qualities for casting and for rolling. The latter has a tensile strength of some 30 tons per square inch, with an elongation of from 25 to 45 per cent; it combines the strength and toughness of steel with resistance to oxidation.