Paul N. Hasluck - Metalworking

Aluminium bronze consists of aluminium and copper in varying proportions. A special hard alloy contains 11 per cent. aluminium, and others contain from 10 to 1 per cent. aluminium. The 10 per cent. alloy has a tenacity of about 100,000 lb. per square inch, but shrinks upwards of twice as much as brass. As the metal solidifies rapidly, it is necessary to pour with very large gates, and every precaution must be taken to prevent the metal setting in the gates before the casting has been completely poured. The great difficulty with large cored work is to prevent the metal cracking. This must be done by using baked cores consisting of sand and cinders ground together and just held with a little resin. Aluminium bronze of the above quality oxidises rapidly when poured from the crucible to the mould. If the film or skin of oxide passes into the mould, it will appear on the surface as dirt, and will probably spoil the casting. The scum may generally be kept back by the ordinary skimmer, if the pourer takes special care. As aluminium and copper volatilise only at very great heats, there is no appreciable loss in continued casting. Metal made simply by mixing aluminium and copper will most probably be brittle, and does not acquire its best properties till it has been remelted several times. If the casting is uniformly thin, it will generally come sharp, but, if there are thin and thick parts, it is necessary to put runners or offshoots from the thin parts to enable the metal to fill and soak back into the casting as it cools. This will overcome the difficulty often met of thumbing, or the formation of shallow places at the thick parts of the castings. A good bearing brass is an aluminium bronze containing 95 parts of copper and 5 of aluminium, to which is added 8 per cent. of tin. A good method is as follows :In using a 140-lb. crucible, melt 124 lb. of copper under a good layer of charcoal; then add cautiously, bit by bit, 6 lb. of ingot aluminium, or fairly heavy aluminium scrap. When the metal is quiet, add 10 lb. of ordinary cake tin, thoroughly well stir in, an-d pour into ingots. When making brasses, take about half the weight of the casting required in scrap metal, and make up with the new ingot metal. This, when melted, must be well stirred and then poured. It is necessary to have large gates to allow proper shrinkage, as there is considerable liability to draw. The mould must be rather loosely rammed, and when poured the metal must not be touched for some time, as it remains liquid in the mould longer than the usual alloys.