US3400752A - Treatment of readily oxidisable metals - Google Patents

Description

p 0, 1968 w. UNSWORTH 3,400,752

TREATMENT OF READILY OXIDISABLE METALS Filed Nov. 28, 1967 INVEIVTOR.

(Jim- 9m UN5WORTH- B'TTORAJETS United States Patent 3,400,752 TREATMENT OF READILY OXIDISABLE METALS William Unsworth, Swinton, England, assignor to Magnesium Elektron Limited, Swinton, England Filed Nov. 28, 1967, Ser. No. 686,085

Claims priority, application Great Britain, Dec. 2, 1966,

54,112/ 66 3 Claims. (Cl. 16466) ABSTRACT OF THE DISCLOSURE Readily oxidisable metal is protected from oxidation by applying sulphur hexofluoride to the molten metal.

This application relates to the protection of readily oxidisable metals during casting especially magnesium and magnesium base alloys.

It is well known that small oxide skin inclusions arise in the running systems of magnesium alloy sand castings during the actual pouring operation. With good foundry practice these inclusions are not, however, large or numerous enough to materially affect the mechanical properties of commercial alloy castings but in the case of the high strength zirconium alloys castings they can have a detrimental effect. Inclusions of this type are too small to be detected by normal radiographic techniques but are visible on fractured surfaces, particularly when examined under a low power microscope.

It is well known in the magnesium foundry art that flushing the mould with sulphur dioxide before pouring assists in minimising oxidation but the degree of protection is not sufficient to avoid the formation of these very small oxide skins.

It has now been discovered that by flushing the mould with sulphur hexafiuoride the incidence of these small oxide skins can be greatly suppressed. Sulphur hexafluoride has the advantages of being a colourless, non-toxic gas and, unlike sulphur dioxide, is also odourless. The beneficial effect of using sulphur hexafluoride is illustrated by the following table which compares the incidence of oxide skins found on fracture testing of an experimental sand casting poured under various conditions. The elfect is shown in terms of both the number of oxide skins per unit area and also the fractional area occupied by the skins.

Number of Percentage oxide skin inarea covered Conditions clusions by oxide Per sq. cm. Skins 3,400,752 Patented Sept. 10, 1968 The alloy was that known as ZE63 containing about six percent zinc, 3 per-cent rare earth metals and 0.7% zirconium (the subject of patent application No. 45,142/ 63).

This invention is also applicable to the production of gravity die castings. It is particularly suitable for use in precision and instrument castings of all types, and may be used in the production of cast stock for working.

The invention may also be applied to the production of castings in mangesium lithium alloys or in other metals which are specially liable to undergo oxidation during filling of the mould, for example aluminium alloys containing lithium.

The invention is illustrated in the accompanying diagrammatic drawing which shows a sectional view of a mould.

A sand mould 10 may be loosely covered with a metal plate 11 and a pipe 12 for introducing sulphur hex-afluoride may be entered into the sprue 12 into which the molten metal is subsequently poured. The sulphur hex-afluoride enters the mould and replaces substantially all the air in the mould cavities 14.

The invention is not only useful for flushing moulds but sulphur hexafiuoride may also be used for protecting molten oxidisable metal in a crucible or pressure vessel or for protecting a stream of the metal.

Iclaim:

1. A method of preventing oxidation of readily oxidisable metal wherein sulphur hexafluoride is applied to the molten metal.

2. A method of casting readily oxidisable metal wherein the mould cavity of a mould is flushed with sulphur hexafiuoride whereupon the molten metal is poured into the cavity.

3. A method as claimed in claim 1 wherein the metal is selected from the group consisting of magnesium and magnesium base alloys.

References Cited UNITED STATES PATENTS 1,720,286 7/1929 Michel -96 3,119,745 1/1964 Caillat et al 75-96 X FOREIGN PATENTS 404,518 1/1934 Great Britain.

I. SPENCER OVERHOLSER, Primary Examiner.

V. RISING, Assistant Examiner.

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