Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1999-05-20
2001-05-22
King, Roy V. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S684000, C075S316000, C075S338000
Reexamination Certificate
active
06235078
ABSTRACT:
Iron is generally considered to be an undesired impurity in aluminum. However, small contents of iron (0.15-1.8% by weight) in aluminum influence the mechanical properties of aluminum and make it easier to roll thin aluminum sheets. Aluminum with an increased iron content can also be used in profiles, since the iron improves the extrusion properties.
Aluminium produced by electrolysis contains small amounts of iron originating from the anodes of the electrolytic cell. This iron content, not sufficient for producing aluminium suitable for foils and profiles, and hence iron has to be added.
In the manufacture of iron-containing aluminium the addition of iron can be made in form of iron scrap or lumps of an Al—Fe master alloy containing about 5-30% by weight of iron. Iron powder and iron-powder-based tablets are also used because of the advantages they offer in the form of shorter dissolution time.
The addition of pulverulent materials can be made by injection together with a carrying gas through a lance. The powder is injected either into the ladle, the holding furnace or the casting furnace. The temperature of the aluminium melt is kept in the range of 720-760° C., which is the normal alloying temperature irrespective of the applied alloying method. Higher temperature can be used, but this does not result in a decrease of the dissolution time of the iron powder.
A very important property of the iron powder to be used in the injection process is its particle size. Particles which are small will follow the gas bubbles to the dross on the melt surface and they can also cause dust-forming problems in various stages of the process. Particles being too large will not dissolve fast enough.
It is also important that the surface of the particles is substantially free of an oxide layer which, if present, could deteriorate wetting of the particles by the molten aluminium and thus block or slow down their dissolution. Additionally and as indicated above, the injection process requires special equipment.
When iron powder tablets are used, they are simply thrown into the aluminium melt, through which they sink and dissolve. Some users manufacture the tablets themselves, but there are also commercially available tablets. So-called alloying tablets contain 75-80% of the alloying metal which besides Fe can be Mn, Cr, Cu, Ti, Pb, Ni or Zn. The balance is pure aluminium plus suitable fluxes to accelerate dissolution and to protect the alloying metal as it dissolves. The tablets are made to such an accurate weight and composition that they do not have to be weighed before being used to guarantee the correct dosage.
It has now been found that the previous methods based on the addition of iron-based powders or tablets can be considerably improved, if the iron is added to the metal melt in the form of solid bodies of compacted iron particles consisting of essentially pure iron. In this context the term “non-ferrous metal” includes metals selected from the group consisting of aluminium, copper and copper-based alloys. By using an additive consisting of bodies of compacted iron particles according to the invention, the dissolution rate of iron in the non-ferrous metal melt can be faster. From this follows that the productivity can be increased due to the shorter periods of time at the melting temperature. The use of the compacted iron bodies thus also implies that less energy is consumed. Furthermore, due to the purity of the compacted iron bodies, fewer inclusions are formed and therefore less subsequent purification treatment is needed, which simplifies the manufacture of the alloyed metal.
The advantages obtained by using the compacted bodies according to the present invention are unexpected and quite remarkable in view of the teaching in U.S. Pat. No. 3,935,004 which discloses that compacted bodies of alloying agents, which have been tested for the addition to molten aluminium, were not effective. Specifically this patent discloses that compacted alloying additives for alloying metals to aluminium should contain a fluxing agent as a critical ingredient. This known additive should preferably also contain binding materials. The compacted bodies used according to the present invention are quite the contrary and should not include any fluxing or binding agents.
The new compacted iron bodies can be manufactured from a atomised iron powder or from a sponge iron powder, such as AHC100.29 or M40, M80, M100, M120, W100.25 W40.24 and A40S, all available from Höganäs AB, Sweden. In contrast to the alloying additives disclosed in WO94/17217 no melting step is involved when the compacted bodies according to the present invention are prepared from the solid atomised or sponge iron powders.
The density of the compacted bodies should be sufficiently high so that the bodies do not disintegrate during handling and transportation and so that the bodies do not float on the surface of the metal bath. Thus the densities should be at least 4, preferably at least 5 g/cm
3
. The preferred density interval is between 5.1 and 6.7 g/cm
3
. To this end the powders are compacted in e.g. a conventional mill at a pressure of at least 200 MPa and at most 500 MPa, the preferred interval being between 250 and 400 MPa. The green strength of the compacted body should preferably be at least 5 MPa, most preferably at least 10 MPa. The influence of the compacting pressure on the solubility or recovery rate can be seen in FIG.
1
.
A suitable thickness of the compacted body obtained from the milling operation might vary between 0.5 and 4 mm. The body is subsequently torn to a suitable size. The tearing can be performed in a conventional mill to a size of at least 50 mm
2
, preferably at least 100 mm
2
. It is of course also possible to add the compacted bodies in the form of larger pieces or strips or any other suitable form.
Important factors are also the oxygen and carbon contents of the compacted iron bodies. According to one embodiment of the invention which is especially suitable for use instead of the currently used iron powder tablets, the oxygen content should be between 0.3 and 2%, and preferably the oxygen content varies between 0.5 and 1.5% by weight of the compacted iron bodies. The carbon content should be between 0.02 and 0.75%, and preferably the carbon content should vary between 0.05 and 0.5% by weight of the compacted iron bodies. In this case the iron powder is suitably a non-annealed sponge iron powder.
REFERENCES:
patent: 2986460 (1961-05-01), Babcock et al.
patent: 3935004 (1976-01-01), Faunce
patent: 4416688 (1983-11-01), Greenwalt
patent: 6024777 (2000-02-01), Houser et al.
patent: 6048382 (2000-04-01), Greenwalt
Burns Doane Swecker & Mathis L.L.P.
Coy Nicole
Hoganas AB
King Roy V.
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