US1802265A - Method of improving magnesium alloys - Google Patents

Description

Patented Apr. 21, 1-931 UNl'l'E kl; STATES HEINZ MENKIIN'G, 0F BITTERFELD, GERMANY, ASSIGNOR 1'0 I. G. FABBENINDUSTRIE .AKTIENGESELLSGHAFT, OE FRANKFORT-ON-THE-MAIN, GERMANY METHOD OF IMPROVING MAGNESIUM ALLOYS- No Drawing. Application filed December 23,1929, Serial No. 416,200, and in Germany June 29, 1929.

strength, are considerably improved. Al-

though theseimprovements are particularly conspicuous in the metal as cast,-the invention is not limited to castingsbut may be also applied advantageously when a further mechanical working up of the cast metal is contemplated.

I have observed that one and the same alloy of magnesium, for example a magnesium alloy containing a certain percentage of aluminium, when treated in the molten state with fluxes mainly consisting of chlorides and containing a thickening medium (for example by the processes described in U. S. Patents 1,576,080 and 1,661,526) for purposes of purification, and cast, yields products having different mechanical strength. A closer investigation of this phenomenon has made it appear that the most valuable mechanical properties with respect to tensile strength, elongation, elastic limit, etc., are obtained in the metal when the latter is subjected, during the melting process,

to an overheating above at least 800 0., i. e.

about 200 C. or more above the melting point, for som e time. The duration of overheating which is necessary to produce the desired effect is shorter when a comparatively higher temperature is maintained during the overheating stage, and vice versa. The minimum temperature of overheating, however, varies somewhat aecordingto the particular composition of the alloy to. be treated and must be ascertained by experiment. The effeet is already apparent after a single overheating, but is further enhanced when overheating is repeated. The improvement of.

the mechanical properties thus produced is otherwise practically independent of the casting temperature selected, at least within the limits coming into consideration for all practical purposes. The effect remains the same whether the melt is cooled down to the usual temperature of casting after the overheating stage or whether the overheated melt as such is immediately east.

The improvements obtained in the meehanieal properties of the metal are probablydue to'the formation of a considerably morefinely grained crystalline structure in the solidified metal by virtue of the described heattreatment in the molten state. It is well known that a finely rained structure is generally accompanied y superior mechanical properties.

The following examples will serve to show various preferable modes of carrying out my present invention and the efi'ect produced on the structure and properties of the solidified metal ascompared with those of the untreated alloys of identical composition.-

Example 1 Magnesium-aluminium alloy: 3 percent Al, bzlgnee magnesium (melting point about 630 (a) The allo was heated in an iron erucibleto about 7 0 C.) i. e. 140 C. above the melting point). Thepot. was then immediately removed fromthe furnace and the metal cast at 750 0.: Coarsely grained structure. Mechanical properties of the solidified alloy at about 20 C.

Tensile strength 16.5 kg/mm Elongation 8% Elastic limit 2.7 kg/mm Yield limit 5.0 kg/mm Compressive limit 4.9, kg/mm 6) The same alloy was heated in an iron crucible with the same flux to 900 C. (270 lowed to cool down to 750 C. above the melting point) and maintained at this temperature for minutes, then al- C. and cast: Finely grained structure.

The following mechanical pro rties of the alloy at C. were ascertaine Tensile strength 18.9 kg/mm' Elon tion 10.2%

Elastic limit 3.4 kg/mm Yield limit 6.7 kg/mm Compressive limit 6.7 kg/mm' Example 2 Tensile strength 17.5-kg/mm Elon ation 6% Yiel limit 7.5 kg/nnn (b) The alloy was overheated to 800 C.

'(190 C. above the melting point) and maintained at this temperature for minutes. Casting temperature 780 C., finely grained of foreignmetals, so as sistance to corrosion.

Mechanical properties:

Tensile strength" 19.0 kg/mm Elongatiom", 7%

(a) Heated to 900 C. and maintained at this tem rature during 1 hour. Cast at 740 C.: inely grained structure.

Mechanical properties:

Tensile strength 21.0kg/mm Elongation; 9%

With alloys having comparativel low contents in alloying metals it is genera y advantageous to apply higher overheating temperatures and a onger duration of overheating than with alloys containing larger amounts to obtaln improved ph sical properties.

he process as described may be also applied with the same efl'ect to magnesium alloys in which the usual small percentages of manganese (up to about 0.4 percent) have been incorporated so as to improve their re- I claim:

1. The method of refinin the crystalline structure oflmagnesium a oys which comprises m'e ting the allo s and temporarily subjecting the molten al oys to temperatures of about 200 C. and more above their melting structure. point g alloy i heatefi to (2400 2. process for improving certain physi- C. a ve the melting point), then slowly cal properties of high percentage magnesmm cooled down to 780 C. (casting temperature): Finel grained structure.

Mechanica properties:

alloys which comprises temporarily subjecting the molten alloys to temperatures of about 200 C. and more above their melting h 2 5 Point g ig g g kg/mm 3. A process ior nllliprovmg certain physih cal properties 0 hi percentage magnesium Yleld 11ml kg/mm alloys which compr ses subjecting the molten Example 3 alloys to temperatures of about-200 C. and

Example 4 Ternary alloy magnesium-aluminium-zinc 4 percent Al, 3 percent Zn, balance magneslum.

(a) Heated to 790 C., cast at 740 Coarsely grained structure.

Mechamcal properties:

Tensile strength 18.7 kg/mm Elongation 6% (Ii) Heated to 850 (3., cast at 740 C.:

' Moderately finely grained structure.

' alloys. which comprises repeate more above their melting point for a period of time necessary to cause the formation of a fimfiiy tgrained crystalline structure in the sohd' c alloy.

4. A process for improving certain physical properties of high percentagia ma esium y u jecting the molten alloys to temperatures of about' 200 C. and more above their melting point.

5. A process for improving certain physical properties of high percentage magnesium alloys including aluminium as an alloying metal which comprises subjecting the molten metal to temperatures of about 200 C. and more above t eir melting point for a period of time necessary to cause the formation of a finely grained crystalline structure in the solidified alloy.

6. A process for improving certain physi cal properties of high percentage magnesium alloys, including aluminium as an alloying metal, which comprises subjeetin the molten metal to a temperature between a out 850 and 900 C. for at least about 10 minutes.

--7. A process for improving certain physical properties of hi h percentage magnesium alloys,- mcludinga uniinium as an alloying metal, which. comprisesrepeatedly subjecting the molten metal to tem eratures of about I 200- C. and more above t eir melting point. for a'p'eriod of time necessary to cause the formation of a finely grained crystallinestructure in the solidified alloy. 1 8 The process of producing castings of high percentage magnesium allo s which cmpr1ses-melt1ngthe allo with a ux,heating the molten-metalto a east 800 C.-while protecting the. surface of the metal from oxidation 'by a layer of said flux, maintaining said tempe at'u're for at least about 10 utes, and casting the metal. 9, The. process of producing castings-0f high percentage magnesium allo s "which comprises melting the alloy-with a ux, heat-- ing the moltenmetal to at least-800 (Lwhile protecting the surface of the metal from oxidation 'by a layer of said flux, maintaining said temperature for at least 10 minutes, coolingthe alloy to the desired temperature of casting, and casting said alloy. c ,10. Aprocess for improving'themecham'cal, 1 pro erties of magnesium alloys containing a p percentage of magnesium, which process comprises superheatin the alloysto at'least a 800 C. for. a period 0 time whlch aries inv versel with the content of the alloying ingre 'ent of thealloy and with-the super- 7 heatin temperature employed. 4

w 11. j process for improving'themechanical properties ofinagnesium alloys containing a lngh percentage of magneslum which process: comprises repeatedly superheatmg V the. alloysjto at least 800 C. for a; period of I l time which varies inversely with the. content ofthe alloyinlg'ingredient of the alloy and with'the super eatingtemperature em eyed. f '12. A process of improving the mec 'cal J properties of magnesium alloys having a content of magnesium ranging from 97 to 90 i r 4 'cent' which comprises superheating the oy to temperatures ranging from 900 to 800-Cg; fora period of time necessary to cau'se.'. the

formation of a finely-grained e structure,the temperature decreasmg over.

- this range as the content of magnesium fiecreases, .-time of treating 1 remaining In testimony whereof]; have hereunto set.

i,-m'han'd. e YHEINZ'LIIENKING.,