US2789050A - Aluminum-magnesium alloys and method of producing same - Google Patents

Description

United States Patent ALUMINUM-MAGNESIUM ALLOYS METHOD OF PRODUCING SAME Charles Eric Ransley, C heshain Boi's, England, assignor to The British Aluminium'Company Limited, London, England, a British company No Drawing. Application December 7, 1955, Serial No; 551,512

7 Claims priority, applicationGreat Britain December. 9, 1954 Claims. C1. 75-147 This invention relates to the production of aluminiumbase alloys containing magnesium as the major alloying element.

As is known, sodium, is commonly present in very small percentages (as a contaminating element) in aluminium alloys and in the pure aluminium ingots normallyused for the production of alloys. Such alloys may be in the form of castings produced by normal sand and permanent mould processes, or in the form of billets or blocks intended for subsequent extrusion or rolling. The residual sodium contamination in such alloys is rarely higher than 0.005% and more usually lies in the range 0.0005% to 0.004%. In certain types of aluminium alloys sodium contamination of this order has no definable eifects, and is certainly not regarded as detrimental, but it has been conjectured in recent years that the marked embrittlement and hot-shortness to which aluminium-base alloys containing magnesium as the major alloying element are subject must be due, in fact, to the presence of small amounts of sodium in the alloy.

Ihave now establishe'dby research that this conjecture is correct and that the deleterious eifects of sodium in these particular alloys are noticeable in practice in alloys containing about 2% of magnesium and become more pronounced as the percentage of magnesium is increased above this value. For example, in alloys containing about 5% magnesium (e. g. alloys to 'B'SEGE Specification N6, normally used for extruded or rolled products) sodium contents in excess of about 0.002% can lead to very pronounced crackin'g'during rolling, and even smaller sodium contents are harmful though to a smaller degree. The defects may take the form of edge-cracking, surface crazing or fcrocodiling (i. e. partial rupturing of the block alon'g'tlie central plane, parallel to rolling surface). At sodium contents of the order of 0.003% the cracking is likely to be sufiiciently severe to cause complete rejection of th'eniaterial at a very early stage in processing.

Thesodium contamination has a marked efiect on the mechanical properties of the alloys as measured by a standard tensile test. This is particularly apparent in tests at elevated temperatures (e. g. 450 C.) when the percentage elongation may be reduced by 0.002-0.003% sodium to as little as one-tenth of that obtainable on the almost sodium-free metal.

Similarly in the casting alloys, for example, those. to BSzGE Specification LM containing 911% magnesium, traces of sodium have a very striking effect, 0.002% being sufiicient to reduce the elongation on fully heat-treated cast test bars from over 20% to about 3%.

It has not been clear until now why traces of sodium should be innocuous in many aluminium alloys, and yet have the pronounced deleterious effects described above in aluminium-base alloys containing magnesium as the major alloying element. I have found that in the alloys in which sodium contamination is apparently harmless,

the sodium is present in a combined form, normallyas a ternary aluminium-silicon-sodium compound. This is the form in which sodium is present in commercial purity aluminium, which contains an appreciable amount of silicon (about 0.1% or more). In the alloys which are subject to embrittleme'nt, however, the-sodium appears to be present in a free or uncombined state, and in this at elevated temperatures. H n Y The susceptibility of the mague sium containing. alloys can be ascribed to the as: th atinagnesiumhas amarked' afiinity for silicon and tends to :fo ri n the stable compound MgSi. Under suitable conditions it appears to react with the ternary aluminium-silicon-sodium compound (which we may refer to for convenience as [AlSiNa]) and thus release free sodium after the fol lowing scheme:

My work has established the conditions under which the reaction goes to the right, so that the sodium is in the embrittling state. It appears that in the solidalloys a magnesium content of greater than 1% is sutficient to produce this result, although it is not marked in practice until about 2% magnesium is reached, and becomes more pronounced as the alloying content rises above this figure.

It would be extremely dilficult to follow the constitutional changes in these alloys at very low sodium levels by normal metallurgical means, and we have made extensive use of a hydrogen-absorption technique for this purpose. This entails heating a sample of alloy in pure hydrogen at an elevated temperature, the pressure of hydrogen being chosen to be greater than the dissociation pressure of sodium hydride NaI-I at this temperature: a convenient temperature is usually 450 C., and a hydrogen pressure of 4 atmospheres is then required. Under these conditions, if the sodium in the alloy is present in the alloy as an intermetallic compound (e. g. the aluminium-silicon-sodium ternary compound) then the amount of hydrogen absorbed, as measured by a sub sequent vacuum hot-extraction of the metal is small and usually less than 0.1 cc. S. T. P./ g. If, however, some of the sodium is present in the uncombined state, there is a marked absorption of hydrogen corresponding to the formation of sodium hydride within the metal. Since the conversion of only 0.001% sodium to hydride involves the absorption of hydrogen equivalent to 0.49 cc. S. T. P./ 100 g. it will be seen that this measurement provides a very sensitive test for free sodium, and has enabled the general effects of sodium in various alloys to be elucidated. In carrying out these tests, however, it is important to reduce porosity in the alloy samples to a low level before absorption otherwise spurious results may be obtained; a satisfactory precaution against this is to hot press the alloy in adie for fifteen minutes under a pressure of about 10 tons/sq. in., at a temperature of about 450 C. prior to exposing it to hydrogen.

It is important to 'define at this stage the alloys which are subject to embrittlement with sodium by the mechanism outlined above, and are thus considered within the scope of the present invention. The most important aluminium-base alloys containing magnesium as the major alloying element are the normal aluminium-magnesium wrought and cast alloys which contain more than 1% magnesium, e. g. alloys toBSzGE Specification N4, N5, N6, N7 and LMS, LMlO. These contain magnesium contents from 2% up to aboutl1% and apart from small amounts of iron and silicon, usually (except inthe case of LMlO) have alloying additions of manganese or chromium only. These do not affect the sodium equilibrium.

In aluminium-base alloys containing magnesium in which the magnesium is not the major alloying element,

state can lead to intercrystalline weakness, particularly Patented Apr. '16,- 1957- it may be possible to have magnesium contents in appreciable excess of the 1% stipulated above, and still not observe any embrittlement by sodium. Alloys containing zinc as the major alloying element appear to behave in this way, presumably because the zinc greatly reduces the chemical activity of the magnesium and so prevents the decompositon of the ternary aluminiumsilicon-sodium compound. Thus an alloy with a composition approximating to the DTD.687 Specification range as follows:

Percent Magnesium 3.4 Zinc 5.6 Copper 1.35 Manganese 0.40 Chromium 0.12 Iron 0.44 Silicon 0.19

and with a sodium content of 0.006% showed a hydrogen absorption of only 0.2 cc. S. T. P./100 g. at 450 C. under 4 atmospheres pressure. 'This alloy hot-rolled well without appreciable cracking. Alloys of this type are thus not susceptible to sodium embrittlement and as such are not considered within the scope of the present invention. 7 a

In the production of aluminium-base alloys containing magnesium as the major alloying element the proportion of free sodium in the melt may be reduced to the order of 0.004% by selective oxidation of the surface of the molten metal and may be further reduced to the order of 0.002% by bubbling nitrogen through the molten metal. It is customary to seek further to reduce the sodium contamination to a minimum by flushing the molten alloy with gaseous chlorine or with chlorinated compounds (such as hexachlorethane or carbon tetrachloride) either alone or in combination with a vehicle gas such as nitrogen. Heavy fluxing with magnesium chloride-based fluxes is also used. These measures are not always fully efiective, particularly if the initial sodium contentof the melt happens to be high (e. g. greater than 0.004%); considerable quantities of the reagents must be applied; the treatment is costly and time-consuming and gives rise to unpleasant fumes; and a loss of magnesium from the melt is also caused.

It is an object of the present invention to provide an improved and highly efiective yet simple procedure whereby the deleterious efiects of sodium in aluminiumbase alloys containing magnesium as the major alloying element may be greatly reduced or eliminated. Another object is the provision of aluminium-base alloys containing magnesium as the alloying elements in excess of 1% which may contain sodium in amounts up to at least 0.01 and still remain free from objectionable cracking during hot-working while having improved properties in the as-cast, extruded or partially worked conditions.

According to one feature of this invention, the deleterious efifects of the sodium contaminant present in an aluminium-base alloy containing magnesium as the major alloying element are reduced by adding a small proportion of bismuth to the alloy.

Another feature of the present invention consists in a new aluminium-base alloy containing magnesium as the major alloying element in the proportion of at least 1% by weight and having incorporated therein a small proportion of bismuth.

Bismuth is' not normally a' detectable impurity in aluminium alloys and the amount present is usually considerably less than 0.001%. This is insufiicient to produce any advantageous effects and the procedure according to the present invention entails a deliberate addition of bismuth at some stage in the manufacture 4 as the major alloying element in the proportion of a least 1% by weight, and contaminated by sodium in amounts of not more than 0.01% by weight, having incorporated therein at least 0.001% by weight of bismuth.

Preferably the proportion of bismuth incorporated in the alloy is in the range of from 0.001% to 0.1% by weight. Advantageously the proportion of bismuth incorporated in the new alloys is in the range of from 0.002% to 0.02% by weight.

Some examples of the behaviour of alloys prepared according to the invention compared with similar alloys not so prepared will now be given.

Example I A melt of alloy was prepared to BSzGE Specification N.6 (i. c. with a magnesium content of about 5%) and containing 0.0033% sodium and 0.02% bismuth. This was cast into a suitable block for rolling by the usual semi-continuous casting process and was hot-rolled down to the normal finishing thickness (e. g. reduction) prior to cold rolling, without any visible signs of'cracking of an objectionable nature. A control alloy, however, containing 0.0038% sodium, but no deliberate addition of bismuth (bismuth not detected chemically), cracked badly after only 15% reduction in thickness in hot-rolling under identical conditions and was rejected as quite unsuitable for further rolling.

A hydrogen-absorption test carried out on these two materials at 450 C. under 4 atmospheres pressure, after first hot-pressingv them to ensure that no appreciable porosity was present, gave a result of 0.21 cc./ g. on the bismuth-containing alloy and 1.64 cc./ 100 g. on the normal alloy without any bismuth addition. A similar test carried out under 9 atmospheres gave corresponding figures of 0.27 cc./ 100 g. and 1.80 cc./ 100 g. respectively. 7

- Example 2 A large production melt of alloy to BS:GE Specification N.6 was treated with a moderate quantity of chlorine gas and then cast by the semi-continuous process into blocks of 8 ins. thickness for rolling. The metal was found to have a sodium content of about 0.002%. During hot-rolling down to A in. thick considerable internal and surface cracking occurred. Approximately 25% of the metal had to be scrapped at an early stage because of this difliculty and the remainder had to be trimmed uneconomically at the end of hot-rolling.

A second similar melt was heavily treated'with hexachlorethane, with the result that the sodium content was reduced to below 0.001%. This metal hot-rolled down to A in. thickness satisfactorily without any rejections and at the end of this process required relatively little trimming. V I

A third similar melt was given no treatment apart from an addition of 0.01% bismuth by adding the appropriate proportion of 2% bismuth hardener alloy. This metal had a sodium content of about 0.002% but still rolled satisfactorily down to A in. thickness and was comparable in quality with the second batch.

The material from these three trial batches was even-, tually cold-rolled down to 0.036 in. thick sheet. The normal cold mechanical properties of the sheets in both the as-rolled and annealed conditions were all similar but the results of hot tensile tests at 450 C. were as follows:

A hydrogen absorption test carried out on these materials at 450 C., under 4 atmospheres pressure, gave results of 0.48, 0.04 and 0.01 cc./ 100 g. on the chlorine, hexachlorethane and bismuth-treated alloys respectively.

Example 3 A melt to BSzGE Specification LM and having the following nominal composition:

and made up on a 99.7% pure aluminium base, was very thoroughly fluxed with a pure anhydrous carnallite flux and then treated with hexachlorethane. D. T. D. type sand test bars were cast from this metal. A small addition of sodium was then made to the melt, and further bars were cast. Finally an addition of 0.01% bismuth was made to the melt in the form of a 2% aluminium hardener alloy and a third set of test bars was cast.

These bars, after solution treatment for 16 hours at 430 C. and quenching in oil, had the following (average) properties:

Alloys according to BSzGE Specification LM10 are known to be capable of developing extremely good properties with both a high ultimate tensile strength and a high elongation. Their use has been somewhat restricted, however, because they tend to show erratic behaviour in practice. However, if an addition of bismuth is made to the alloy (of the order, for example, of 0.005% to 0.02% by weight) it is found that improved properties and much more consistent behaviour can be obtained and it follows that this invention will have an important eflfect on the general use of these alloys.

I have found that in order to neutralise the range of sodium contents which normally occurs in production, i. e. 0.0005% to 0.005%, bismuth contents of the order of 0.002% to 0.2% are desirable. A bismuth content of the order of 0.002% by weight has been found to give beneficial results. It may be noted that it is difficult in the higher magnesium alloys to retain bismuth contents much greater than 0.02%, and in alloys containing 5% or more of magnesium, 0.02% bismuth may be regarded as a convenient, but not a restrictive, maximum content to achieve.

Bismuth contents of the order required may be obtained by suitable additions to the molten alloy of metallic bismuth, or of a suitable bismuth hardener (e. g. commercial purity aluminium with 2% of bismuth). Treatment of the melt with certain bismuth compounds is also feasible and such methods of introducing bismuth into the alloy are to be regarded as yielding material within the scope of the present invention.

What I claim is:

1. An aluminium-base alloy characterized by good rolling properties and consisting of magnesium as the major alloying element in the proportion of at least 1% by weight and bismuth in the proportion of from 0.002% to 0.02% by weight.

2. An aluminium-base alloy characterized by good rolling properties and consisting of magnesium as the major alloying element in the proportion of from 1% to 11% by weight and bismuth in the proportion of from 0.002% to 0.02% by weight.

3. An aluminium-base alloy characterized by good rolling properties and consisting of magnesium in excess of 1% by weight as the major alloying element and sodium as a contaminant in the proportion of from 0.002% to 0.005% by weight and bismuth in the proportion of from 0.002% to 0.02% by weight.

4. In the manufacture of an aluminium-base alloy consisting of magnesium as the major alloying element in the proportion of from 2% to 11% by weight and free sodium as a contaminant, the incorporation of bismuth in the alloy in the proportion of from 0.002% to 0.02% by weight.

5. In the manufacture of an aluminium-base alloy consisting of magnesium as the major alloying element in the proportion of from 2% to 11% by weight and free sodium as a contaminant in the proportion of from 0.002% to 0.005 by weight, the incorporation of bismuth in the alloy in the proportion of from 0.002% to 0.02% by weight.

References Cited in the tile of this patent UNITED STATES PATENTS 1,932,848 Dean et al Oct. 31, 1933 1,932,858 Wood Oct. 31, 1933 1,966,481 Berthelemy et a1. July 17, 1934 2,288,513 Canac et a1. June 30, 1942

Claims (1)

1. 3. AN ALUMINIUM-BASE ALLOY CHARACTERIZED BY GOOD ROLLING PROPERTIES AND CONSISTING OF MAGNESIUM IN EXCES OF 1% BY WEIGHT AS THE MAJOR ALLOYING ELEMENT AND SODIUM AS A CONTAMINANT IN THE PROPORTION OF FROM 0.002% TO 0.005% BY WEIGHT AND BISMUTH IN THE PROPORTION OF FROM 0.002% BY WEIGHT.