US1923592A - Process for improving the mechanical properties of magnesium and its alloys - Google Patents

Description

Patented Aug. 22, 1933 U I UNITED STATES PATENT OFFICE PROCESS FOR IMPROVING THE MECHANI- CAL PROPERTIES OF MAGNESIUM AND ITS ALLOYS No Drawing. Original application March 10, 1930, Serial No. 434,820, and in Germany March '11, 1929. Divided and this application March I, 1932. Serial No. 596,152 9 Clainis. (c1. 148-21.3)

is the case with other metals, extend to all the mechanical properties, but is more or less limited to an improvement of the tensile strength and the properties connected therewith (such as tensile yield limit, elastic limit and elongation under tensile stresses), whereas the improvement of the elastic qualities with respect to stresses havingit'he character of a pressure or a torsion is either considerably smaller 'or'even'practically absent. Thus, the pressure yield limit which in the untreated cast metals closely equals their tensile yield limit,'after plastic deformation rarely exceeds about one half of the latter. 1

A further improvement of the mechanical resistance of magnesium and magnesium alloys when plastically deformed at elevated temperatures can be obtained in a known manner by a subsequent deformation at ordinary temperatures. By this procedure the absolute values of the tensile yield limit as well-as the pressure yield limit are simultaneously'raised but their unfavourable ratio remains substantially unaltered.

Again, the torsional yield limit which, as a rule, in the case of metals amounts to about one half of the tensile yield limit, does not, in the case of magnesium and magnesium alloys, exceed about 25 percent of the yield limit after plastic deformation has taken place under ordinary conditions and at elevated temperatures. Nor is this ratio perceptibly improved by a subsequent deformation and therefore cannot, in this manner, be brought to the level prevailing in other metals.

The numerous technical applications of magnesium and its alloys render an improvement of this unfavourable relation between the tensile yield limitand the above mentioned mechanical properties, particularly the pressure yield limit, desirable. It is an object of the present invention to improve this relation,

In the following general statement, the invention is described with respect to its application 7 alloys of magnesium in which this metal largely predominates, i.e. all alloys of magnesium containing not more than about 25 percent of foreign metals. The termflmagnesium", when employed in the following, therefore includes these alloys as well unless the contrary is especially mentioned.

When subjecting magnesium to plastic deformation at elevated temperatures, preferably between 200 and 500 C., the rate at whichdeformation is effected is, similar to other metals, for reasonsof economy regularlykept so high as to be just compatible with the structural coherence of the material under .deforma''- tion. This rate of deformation, which may just-, 1y be termed the standard"v one, substantially depends onthe temperature at which deformation-is effected andonthe individual plastic properties of the particular metal or alloy conf cerned. The range of the "standard", rate ofv deformation is, accord-ing to the foregoing, characterlzedby a speed of working which is, on the one hand, slightly below the excessive speed of deformation causing afrac'turing of the metal, and on the other hand, .will rarely, if ever, fall below two thirds of that speed.

Some. specific, examples will serve to illustrate the exact meaning of this statement.

The spe d of ,working of a die extrusion press is, once the temperature of working and the material to be extruded are given, governed by the specific pressure brought to bear'upon the material. observed that the application ora' pressure 'exceeding, say, '70 kg/sq. mm results in the production of cracked or disrupted bars, whereas at a pressure of 65 kg/sq. mm the bars are perfect and issue from the di'e'a't a speed of, say, 100 mms length per second, it would seem unreasonable to reduce the pressure to, say, 30 kg/sq mmQthis resultingin bars which are apparently none the more perfect and onlyleave the die at a speed of, say, 30 mms per'se'cond.

wA speedcf the bars of about'8Q to. 100 mms would. in this case, thus correspond to the standard rate of'de'formation. I

In the case of a rolling mill, the speed of deformation may be represented by the decrease .of the thickness of the billet effected by each pass, in. percentages of the "original thickness. If a billet of a'particiflar alloy, when worked a a certain temperature, is found to crack When ina particular instance it is when adjusting the rolls to a decrease of above 20 percent, while remaining uncracked when a decrease of 20 percent and below is selected, it would be devious to apply a decrease which is only a fraction of 20 percent, as long as there is no cogent reason for such an abnormal procedure. In this case, therefore, a decrease rang ing between about 16 and 20 percent would be considered the standard rate of deformation.

Independently, the speed of deformation in a rolling mill may be represented by the speed at which the slabs are passed through the rolls, ie the speed of the rolls themselves. Also in this respect there always exists a standard rate of working which must not be exceeded materially if a breaking up of the metal is to be avoided. and which. on the other hand, will always be chosen as high as possible for reasons of economy.

In the first place, we have found that the numerical relation between the tensile properties on the one hand and the other elastic properties mentioned above on the other hand. in the case of magnesium, is strongly improved by carrying out plastic deformations at elevated temperatures at a rate which falls distinctly below the standard range and amounts to less than half of the latter. We have further found that the same result is obtained when the metal, while being plastically deformed at an arbitrary rate below the upper limit of cohesion but including the standard range, is quenched coincidentally with the ceasing of the effect, upon the metal, of the external forces causing the deformation. The-lowering of the temperature throughout the material by virtue of the chilling must be such as to efilciently prevent any further change in the structure of the constituting crystals with respect to their size and orientation. The quenching of the metal can be performed by means of cold or tepid water, oil, compressed air, or the like. In any case, however, it is essential that the cooling medium should be brought to bear upon the metal in immediate vicinity of the point of issue of the metal from the means effecting deformation, such as dies or rolls.

The application of the present invention will hereinafter be illustrated by a number of various examples. Although in these examples it is supposed that the material is worked up on a die extrusion press of the usual construction, it will be well understood that the invention is not limited to said mode of application but is equally applicable to any kind of plastic deformation, particularly to die stamping, rolling, swaging, or drawing operations.

Example 1 On a die extrusion press a circular'bar of 25 mms diameter is produced from a block of pure magnesium at a pressing temperature of 450 C. The standard speed, i. e. the speed of the bar issuing from the die when working the press at a standard pace as defined above, amounts to 110 mms per second. After the finished bar has been allowed to cool the tensile yield limit of the material is found to be 15 kg/sq.mm and the pressure yield limit 6 kg/ sqmm whereas the torsional yield limit amounts to 2.9 kg/sqmm.

A similar bar is then extruded from another portion of the same material at the same temperature of 450 C. In this case, however, the speed of pressing is, according to the invention,

extremely reduced and only amounts to 5 mms per second. After normal cooling the pressure yield limit is found to have risen to 11 kg/sqnim and the torsional yield limit to 5.1 kg/sqmm whereas the tensile yield limit amounts to 15 kg/sq.mm and thus remains unaltered when compared with the material extruded in the hitherto usual manner.

Example 2 A magnesium alloy consisting of 6.5 percent of aluminium, 1 percent of zinc, 0.3 percent of manganese, remainder magnesium and minor impurities totalling less than 0.2 percent is extruded at a pressing temperature of 300" C. into circular bars of 25 mms diameter. In accordance with the lower plasticity of this alloy, when compared with pure magnesium, the standard speed of the pressing is in this case lower and amounts to about 50 mms per second. When pressed at this rate the tensile y'ield limit of the normally cooled material is found to be 22.5 kg/sqmm, the pressure yield limit is 14 kg/sqmm, and the fatigue limit (under alternating stresses) is 12 kg/sq.mm.

By extruding another portion of the same block at a lower rate, the speed of the bar issuing from the die being only 10 mms per second, the conditions of working otherwise being identical, the pressure yield limit is raised to 22 kg/sqmm and the fatigue limit to 16 kg/sqmm, whereas the tensile yield limit, also in this case, remains unaltered asv compared with that of the same material when pressed at the standard rate.

Example 3 A circular bar having 25 mms diameter and consisting of pure magnesium is extruded at a temperature of 300 C. with a standard pressing speed of 130 mms per second. The shaped bar when subjected to normal cooling conditions and then tested displays a tensile yield limit of 16 kg/sqmm, a pressure yield limit of 6.5 kg/sq.mm and a torsional yield limit of 3.7 kg/sq.mm.

When, however, according to the alternative mode of carrying out the invention, the bar is at its point of issue from the die strongly quenched by means of cold water so as to reduce its temperature throughout to below 200 C. the conditions of working including the speed of pressing otherwise being the same, the material obtained is found to possess a pressure yield limit of 13 kg/sqmm and a torsional yield limit of 5.5 kg/sq.mm whereas the tensile yield limit remains at 16 kg/sq. mm.

Example I,

A block consisting of the magnesium alloy employed in example 2 is extruded into a circular bar of 25 mms diameter at a temperature of 350 C., the bar being quenched by means of cold water at its point of issue from the die. The speed of the bar issuing from the die amounts to 10 mms per second. In this case the tensile yield limit is slightly raised and amounts to 24.8 kg/sqmm, whereas the pressure yield limit is raised to 24.0 kg/sqmm.

It will be well understood that the eflects produced by the two alternative methods as described are practically equivalent. In the case of more massive shapes, however, when it is rather difiicult to effect an instantaneous and thorough quenching, it is preferable to produce the desired effect by lowering the speed sf deformation only. In some cases, however, it is also advantageous to combine both procedures. that is to say, to reduce the speed of deformation as well as to quench the material at its point of issue from the deforming device.

So far as quenching is concerned it will be well understood that this step is entirely distinct from the quenching step employed in known processes for improving mechanical properties of metals. The application of the known process is limited to the'case of alloys which are capable of forming supersaturated solid so lutions and the effect of the quenching consists in forcibly maintaining, in the individual crystals, at ordinary temperatures, a state of distribution of the constituents which is not in accordance with the equilibrium prevailing ator dinary-ztempei a.tures.= ,1 In contradistinction, the

present invention is not limited to alloys forming solid solutions and showing different con-" centrations of saturation at different temperatures-in these solutions. In-orderto obtain the effect according to the present invention. how-' ever, a plastic" deformation is essential and the effect of the quenching consists in maintaining the size and orientation of the individual crystals, whereas the constitution of the latter is entirely irrelevant.

1. A process for impioving the compressive limit, the torsional limit. and the fatigue limit of magnesium and high percentage magnesium alloys, which comprises extruding such metals at temperatures between about 200" and about 500 C. and quenching such metals coincidentally with the ceasing of the effect, on the metal, of the forces causing such extrusion to a temperature at which the structure of the metal remains unaltered with respect to the size and orientation of the individual crystals.

2. A process for improving the compressive limit, the torsional limit, and the fatigue limit of technically pure magnesium, which comprises extruding such magnesium at a temperature of about 450 C. and quenching the extruded magnesium coincidentally with the ceasing of the effect, on the metal, of the forces causing such extrusion to a temperature at which the structure of the metal remains unaltered with respect to the size and orientation of the individual crystals.

3. A process for improving the compressive limit, the torsional limit and the fatigue limit of an alloy consisting of about 6.5 percent of aluminium, about 1 percent of zinc, and traces of manganese, the balance being magnesium, which comprises extruding such alloy at. a temperature of about 350 C. and quenching such extruded alloy coincidentally with the ceasing of the effect, on the metal, of the forces causing such extrusion to a temperature at which the structure of the metal remains unaltered with respect to the size and orientation of the indi vidual crystals.

4. A process for improving the compressive limit, the torsional limit and the fatigue limit of an alloy consisting of about 6.5 percent of aluminium, about 1 percent of zinc, and traces of manganese, the balance being magnesium, which comprises extruding such alloy at a temperature of about 350 C. and quenching the ex truded alloy as it issues from the die to a temperature below about 200 C.

5. A process for improving the compressive limit, the torsional limit, and the fatigue limit of magnesium and high percentage magnesium alloys, which comprises extruding such metals at temperatureabetween about 200 and about 500 C. and at a speed which is substantially lower than the maximum speed at which the metals can be extruded'atthe same temperature and by the same forces, without cracking, and

quenching suchmetals coincidentally with the c'easingof'theeffect, on the-metal, of the forces causing .zsuch z extr'usionj ;;to f a temperature at which the structu 'alumi'riiurn,1about 1 percentpf zinc, and traces of manganese, thebalance being magnesium, which comprises extruding such alloy at a temperature of about 350 and at a speed, measured along the section issuing from the die, of about l0 mms. per second, and quenching such extruded alloy coincidentally with the ceasing of the effect, on the metal, of the forces causing such extrusion to a temperature at which the structure of the metal remains unaltered with respect to the size and orientation of the indiviclual crystals.

'7. A process for improving the compressive limit. the torsional limit, and the fatigue limit of magnesium and high percentage magnesium alloys, which comprises extruding such metals at temperatures between about 200 and about 500 C. and quenching such metals coincidentally with the ceasing of the effect, on the metal, of the forces causing such extrusion to a temperature below about 200" C.

8. A process for improving the compressive limit. the torsional limit, and the fatigue limit of magnesium and high percentage magnesium alloys, which comprises extruding such metals at temperatures between about 200 and about 500 C. and at a speed which is substantially lower than the maximum speed at which the metals can be extruded at the same temperature and by the same forces, without cracking, and quenching such metals coincidentally with the ceasing of the effect, on the metal, of the forces causing such extrusion to a temperature below about 200 C. v

9. A process for improving the compressive limit, the torsional limit, and the fatigue limit of technically pure magnesium, which comprises extruding such magnesium at a temperature between about 300 and about 450 C. and quenching the extruded magnesium as it issues from the die to a temperature below about 200 C. WALTHER SCHMIDT. HANS BOTHMANN. JOSEF RUHRMANN.

CERTIFICATE or CORRECTION.

Patent No. 1,923,592. August 22, 1933.

WALTHER SCHMIDT. ET AL.

lt is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, after line 140, insert the following as claim l0:

10. A process for improving the compressive limit, the torsional limit, and the l-a'tigue limit of technically pure magnesium, which comprises extruding such magnesium at a temperature of about 450C. and quenching the extruding magnesium as it issues from the die to a temperature below about 200C.

And that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 21st day of November, A. D. 1933.

F. M. Hopkins (Seal) Acting Commissioner of Patents.

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