The International Hobby Metal Casting ezine.
Brought To You By:
Col Croucher, administrator of: www.myhomefoundry.com
Australia.
Volume: 05. Number: 55. May 2008.
Welcome to all our new & current subscribers alike.
Each month when I sit down at the computer to produce this ezine, I wonder what is happening out there in metal casting land. the products and things that people want to cast in metal vary enormously. If you have not jumped onto the metal casting bandwagon yet, then there is no time like the present to begin learning how to cast and bring your ideas and dreams into metal reality.
Below is a short article on extraction of aluminium from the ore of bauxite, if you are not already familiar with the smelting process hopefully this will help you to understand more about the process, not that you will get to smelt ore into metal, but it will help you to understand more about where aluminium metal is derived from.
We have also put together an explanation about metal solidification, to some of you who are new to metal casting, this may be a bit on the technical side, but it is interesting to read none the less, and if you have been melting metal for awhile and you have struck a few problems, you may even have an "Ah ha" moment while you read about what happens with molten metal as it cools from liquid to a solid.
Just recently while on a holiday break on the mid north coast of New South Wales (Australia), I was able to catch up with one of our many Aussie hobby foundry ebook customers, (David), in a future edition we will have an article about David’s metal casting exploits.
It was quite a coincidence to call in on David, as he had only just completed the construction of a gas fired tilting furnace, so after a short yarn over a cup of coffee and biscuits we adjourned to the foundry down in the backyard where he unveiled a very impressive professionally built tilting furnace. The furnace is naturally aspirated and gas fired, but it is also compressed air and oxygen injection capable for very high temperature work. David has an engineering background and his skills certainly showed through in the furnace construction, but he is a novice at metal casting, while I was there he invited me along to witness the commissioning of his new furnace on a Saturday morning, so how could I refuse a moment like this, it is not often that I get the chance to see first hand what some of our customers are doing, it was a pleasure to watch and provide help, guidance and encouragement, I'm sure that within a few months that David will be producing many good quality metal castings.
If I ever get the chance to go abroad in the next year or so, it would be great to be able to catch up with some (maybe you) of our overseas hobby foundry people, there are just so many who are scattered across the globe now, and it would not be possible to visit everyone.
Metal casting is just like any other new pursuit, you have to learn the basics first before you progress to the more advanced stuff, make simple patterns and then mould & cast them, the experience & success gained with each little project will progress you further along the road to success.
Begin Here Now.
Aluminium Smelting
Furnace Cell.
Smelting of
aluminium is quite different to that of melting ingots or scrap
aluminium metal in a gas or oil fired furnace. Read the article below
to learn more.
Metal Solidification.
From Small Nuclei To
Dendrites.
Minute nuclei form, then dendrites begine to grow,
secondary arms form and grow in the opposite direction.
Early Growth
Stages Of Metallic Dendrite.
Secondary & tertiary arms develop from the main
backbone of the dendrite, like branches & twigs on a tree.
Dendrites grow
until outer the arms touch, when metal is completely solid, only the
grain boundaries are visible.
Minerals containing aluminium are difficult to decompose, and, for this reason, samples of the metal were not produced until1825. The Danish scientist H. C. Oersted used metallic potassium to chemically reduce aluminium from one of its compounds. Consequently, in those days aluminium cost about £250 per kg to produce, and was far more expensive than gold.
The modern electrolytic process for extracting aluminium was introduced simultaneously and independently in 1886 by Hall (in the USA) and W Roult (in France). Nevertheless, the metal remained little more than an expensive curiosity until the beginning of the present century. Since then, the demands by both air- and land-transport vehicles for a light, strong material have led to the development of aluminium technology and an increase in the production of the metal, until now it is second only to iron in terms of annual world production.
The only important ore of aluminium is bauxite, which contains aluminium oxide (A1203). Unfortunately, this cannot be reduced to the metal by heating it with coke (as in the case of iron ore), because aluminium atoms are too firmly combined with oxygen atoms to be detached by carbon. For this reason, an expensive electrolytic process must be used to decompose the bauxite and release aluminium. Since each kg of aluminium requires about 91 mega joules of electrical energy, smelting plant must be located near to sources of cheap hydro electric power, but often at great distances from the ore supply
Crude pig iron can be purified (turned into steel) by blowing oxygen through it, to burn out the impurities, but this would not be possible in the case of aluminium, since the metal would burn away first, and leave us with the impurities. Instead, the crude bauxite ore is first purified by means of a chemical process, and the pure aluminium oxide is then decomposed by electrolysis, since aluminium oxide has a very high melting point, it is mixed with another aluminium mineral called cryolite, to form an electrolyte which will melt at a low temperature.
The furnace 'cell' (smelter) usually measures about 2.5 m x 1. 5 m x 0.6 m in size, and employs a current of 8000 to 30 000 A, at 7 V. The sacrificial anodes, which gradually burn away, are made from a mixture of petroleum-coke and tar-pitch. (Several smelter cells are employed in the porocess)
When the electric current passes, the aluminium particles, being positively charged, are attracted to the lining of the furnace, which constitutes the negative electrode (or cathode). Hence, molten aluminium collects at the bottom of the furnace, and is tapped off when necessary. In the meantime, oxygen is given off at the anodes, which burn as a result, and need to be replaced at frequent intervals.
Although aluminium has a high affinity for oxygen, and might therefore be expected to oxidise (or 'rust') very easily, in practice it has an excellent resistance to corrosion. This is due largely to the thin but very dense film of oxide, which forms on the surface of the metal and effectively protects it from further atmospheric attack. You will be familiar with the comparatively dull appearance of the surface of polished aluminium; this is due to the oxide film, which immediately forms. The protective oxide skin can be artificially thickened by a process known as anodising. Since aluminium oxide is extremely hard, anodising also makes the surface more wear- resistant.
Dendritic Metal Solidification.
When the temperature of a molten pure metal falls to its freezing point, crystallisation will begin. The nucleus of each crystal will be a single unit of the appropriate crystal lattice. For example, in the case of a metal with a body-centred cubic lattice, atoms will come together to form a single unit, and this will grow as further atoms join the lattice structure. These atoms will join the 'seed crystal' so that it grows most quickly in those directions in which heat is flowing away most rapidly. Soon the tiny crystal will reach a visible size, and form what is called a 'dendrite'. Secondary and tertiary arms develop from the main 'backbone' of the dendrite-rather like the branches and twigs, which develop from the trunk of a tree, except that the branches in a dendrite follow a regular geometrical pattern. The term 'dendrite' is derived from the Greek word dendron – “a tree”.
The arms of the dendrite continue to grow until they make contact with the outer arms of other dendrites growing in a similar manner near by. When the outward growth is thus restricted, the existing arms thicken until the spaces between them are filled, or, alternatively, until all the remaining liquid is used up. As mentioned earlier in this chapter, shrinkage usually accompanies solidification, and so liquid metal will be drawn in from elsewhere to fill the space formed as a dendrite grows. If this is not possible, then small shrinkage cavities are likely to form between the dendrite arms.
Since each dendrite forms independently, it follows that outer arms of neighbouring dendrites are likely to make contact with each other at irregular angles, and this leads to the irregular overall shape of crystals, as indicated in a similar manner, the trees of a forest push and jostle each other as they reach towards the light, so that some forest trees are sometimes not of pleasing, regular shapes.
The rate at which a molten metal solidifies affects the size of the crystals which form. A gradual fall in temperature results in the formation of only a few nuclei, and so the crystals can grow unimpeded to a large size. Rapid cooling, however, leads to the formation of a sudden 'shower' of nuclei. Since the resultant crystals are large in number, they will be small in size. As the foundry man says, 'Chilling causes fine-grain castings'. Because of the difference in the rates of cooling, the resultant grain size of a die-casting is small as compared with that of a sand-casting. This is an advantage, since fine-grained castings are generally tougher and stronger than those with a coarse grain size.
If the metal we have been considering is pure, then there will be no hint of the dendritic process of crystallisation once solidification is complete, because all atoms in a pure metal are identical. If impurities were dissolved in the molten metal, however, these would tend to remain in solution until solidification was almost complete. They would therefore remain concentrated in that metal which solidified last; that is, between the dendrite arms. In this manner they would reveal the dendritic pattern when a suitably prepared specimen was viewed under the microscope. This concentration or segregation of impurities at crystal boundaries
A small amount of impurity in the metal can have such a devastating effect on mechanical properties, making the cast metal brittle and likely to fail along the crystal boundaries. In addition to this local segregation at all crystal boundaries, there is a general accumulation of impurities in the central 'pipe' of a cast ingot. This is where metal solidifies last of all, and has become most charged with impurities; relatively pure metal having crystallised during the early stages of solidification.
Of course there is a lot more to this story, but I think that you will have enough to ponder right now after reading this far. The next time that you are melting some metal you may want to take a few moments to try to understaned what is actually going on down there in the crucible, while the process of melting is pretty simple in itself, there is also a complex science while the metal is altered from a piece of scrap to a molten state then poured and allowed to cool as a new casting.
If you have just glanced over the words above in bold font and maybe not taken too much notice, stop now and read through this article again, there are little secrets here that will make the penny drop for a lot of people, you will begin to understand the high importance of preparing, degassing & fluxing molten metal before it is poured into the mould. Whatever you do throughout your melting - casting process will have a direct end result with your finished casting, if you are carefull and have good practises in place then your casting reject rate should be low indeed.
Col.
Links: Learn More>>
Comalco.
http://www.comalco.com/31_weipa_bauxite_mine.asp
Alcoa.
http://www.alcoa.com/ingot/en/info_page/smelting.asp