US2363394A - Heat treatment of magnesium alloys - Google Patents

Description

Patented Nov.'.21,

I D S TE-s "PATENT OFFICE I by HEAT rassn m r rrrmaunsmm I Robert 8. Bush, Midland,l\lich., assignor to-The Dow Chemical Company, 'llflicllaml. Mich., a corporation-oi Michigan i No Ihawing. AppllcationDecember 23, 1943,

7 Serial N0. 515,419

7 Claims. (Cl. 148-413) I This invention relates to the heat-treatment of magnesium-base alloys.- It particularlyconcerns ,a preliminary sequence for minimizing germination during solution-heat-treatment oi magnesium-base alloy. castings.

During the heat-treatment '1 metals, there is j sometimes observed. the phenomenon of germination, or sporadic uncontrolled grain growth.

(See, for example, the discussion in" The Science.

- of Metals, p. 133 fl., Jeflries and Archer, Mc- Graw-Hill, New York, 1924.) Thus,after prolonged solution-heat-treatment 01' certain masnesium-base alloy castings; local areas of the metal mayexhibit, insteadof the usual large number of small grains, one or a few relatively enormous grains having adiameter of a tenth 1 of an inch or more; single grains more than half anv inch in size are occasionally seen. Inasmuchas the occurrence of such macro-crystalline areas may so weaken the metal as to render. it unfit ior service, the avoidance of germination during heat-treatment of magnesium alloys is of great practical importance. I

, According to the present invention, uncontrolled crystal growth during heat-treatment oi heat-tr'eatable magnesium-base alloys characterized bya tendency towardgermination may be largely prevented by a pre-treatment which comprises heating the metal-at a solution-heattreating temperature for a time sufllcient to'efiect partial solution of any major second phase present but insumcientto cause appreciable 4 germination, and then coolingthe metal and subjecting it to an aging temperature for a time sumcient to eifect substantial precipitation 01 a major. second phase; This pre-treatment, in-

-volving partial solution and reprecipitationof' a major second phase present-in the alloy, appears to alter'the ultimate. structure of the metal. pos- ,sibly by reorienting crystals-or by creating new centers of nucleation,- so that, excessive grain growth does not occur during subsequent'solution-heat-treatment. In so far as known, the process or the in'vem tion may be successfully applied to articles formed of, any heat-treatable magnesium-base" alloy =which tends to undergo germination on prolonged heating. In this connection, however, it

' may be noted thatthe tendency toward unconk trolled; crystal growth'is most common with cast metal, especially metal which has been locally stained by drastic'chilling in the mold. The

initiate, germination during subsequent heattreatment of the casting. Commercial magnesium alloys containing. from 8 to 14 percent of aluminum as the major alloying element, especially those in which the usual 0.1 to 0.2 per centmanganese is absent, and to a lesser extent those I alloys containin'gup to 4 per cent of zinc, as

' wellas aluminum and the usual amount of manganese,- appear'to be particularly susceptible to lo sporadic grain growth. For this reason; the pretreatment according to the invention is most commonly used as a preliminary step in the solution he'at-treatmeut of castings poured from superheated magnesium-aluminum-zino alloys of the type mentioned.

Among thecommercial magnesium base alloys Ito-whichthe invention is most frequently applied arernow'metal 0., containing from 8.3 to

9.7- per 'cent aluminum, 1.! to 2.3 per cent zinc,

and about 0.1 per cent manganese; Dowmetal G,

containing from 9 to'l1 per cent of aluminum, up to 0.3 'per cent zinc, about 0.1 per cent manganese; and Dowmetal R, containing from 8.3 to 9.! per cent aluminum, about 0.2 per cent 26 manganese, and up to 1.0 per-cent zinc, the balance of each alloy being magnesium.

In practicing the invention, the casting or other article to-be treated is first heated at a solution-heat-treat ing temperature, 1. e. at a temperature well above the temperature limit of solid solubility oi the subsisting concentration of the major alloying element in magnesium, In the case of magnesium-base alloys containing 8' 'to'l4 per cent of aluminum, the preferred temperature is in' the range of 715 to 800- F., be-

ing usually 750 to 'l'm F. The-duration of this treatmentshould be suflicient to cause substantial solution of a any second major phase present but not so long as to cause germination, with 40 the magnesium-aluminum alloys f mentioned,

' are usually satisfactory, the longer times being' required at the lower temperatures; 21 131 770? F. is a common proce ure. 3 1

The preliminary heating "in a standard furnace provided'with a protective atmosphere, or in'a molten salt bath. The metal should be raised to therequired temperature as rapidly as possible, but, as in other heat-treatments of magnesium alloys, heating should not be so fast that the eutectic phase melts and" burns before itgoes into solid solution. For

example, a heating time of about 2 hours may be necessary to raise a magnesium-aluminum alloy is best accomplished casting from about 640 F. to a treatment temperature of 770 F.

After the brief partial solution-heat-tre'atment just described, the magnesium alloy article is.

cooled rapidly to an aging temperature or below and is then given an aging treatment for a time sufficient to effect substantial precipitation of a second phase. -Cooling is ordinarily done in still air, although forced-draft or other means may 'be used. Aging may be carried out in convenhours are effective, the higher temperatures requiring the shorter times.

Following the pre-heating and the aging steps, the castings or other articles may be given a conventional solution-heat-treatment, e. g. at 715 to 800 F. for to hours for magnesiumaluminum alloys, without undergoing substantial germination, a result usually not otherwise possible with alloys having germinative tendencies. The pretreatment according to the invention in general decreases somewhat the time required to eflect the conventional solution-heat-treatm'ent.

In particularly diilicult cases of germination, as with castings subjected to extreme local chilling, it may occasionally be necesary to repeat the partial solution-heat-treating and aging steps of the invention one or more times in order entirely to prevent germination during prolonged solution-heat-treatment.

Regarding the invention from another viewpoint, it may be seen that the prevention of germination is accomplished essentially by interrupting an ordinary solution-heat-treatment prior to the occurrence of germination and subiecting the article to an aging temperature for a time sumcient to effect substantial precipita tion-hardenlng before resuming the solutionheat-treatment.

The following example will further illustrate the invention but 'is not to be construed as limiting its scope:

Example A considerable number of identical aircraft castings of intricate designwere pouredfrom a tive, and that the invention is co-extensive in scope with the following claims.

What is claimed is:

1. An improved procedure for producing in an ungerminated condition solution-heat-treated articles formed of a heat-treatable magnesiumbase alloy characterized by a tendency to undergo germination on prolonged heating, which comprises pre-heating the article at a solution-heattreating temperature for a period sufllcient to effect partial solution of a major second phase in the alloy but insufllcient to produce appreciable germination, then subjecting it to an aging temperature for a time sufficient to eflect substantial precipitation, and thereafter subjecting the article to solution-heat-treatment.

2. An improved procedure for producing in an ungerminated condition after solution-heattreatment castings poured from a superheated heat-treatable magnesium-base alloy containing from 8 to 14 per centaluminum and up to 4 per cent zinc as the major alloying elements, which comprises pre-heating the casting'at a solutionheat-treating temperature of at least 715 F. for a period sufficient to eifect partial solution of any second phase in the alloy but not exceeding about 8 hours, then cooling the casting rapidly and subjecting it to an aging temperature in the range 325 to 700 F. for a time suflicient. to effect substantial precipitation, and thereafter subjecting the casting to solution-heat-treatment at a temperature of at least 715 F.

3. 'A. process according to claim 2 in which the magnesium-base alloy contains from 8.3 to 9.7

per cent aluminum and from 1.7 to 2.3 per cent .ings formed of a heat-treatable magne ium-base alloy characterizedby a tendency "undergo germination on prolonged heating, a pre-treatment for minimizing germination during subsequent heat-treatment which comprises heating the casting at a solution-heat-treatlng tempera- .ture for a time sumcient to effect partial solution of any second phase in the alloy but insuflicient to cause appreciable germination, and then cooling the casting and subjecting it to an aging temperature for a time sufllcient to effect substantial precipitation.

- 7. An improvement'in'the solution-heat-treat- In contrast, with an otherwise identical series of castings which were pre-heated at'770' F. for

2 hours and aged at 650 F. for 4 hours in accordance with the invention, and were then solution-heat-treated at 770 F.for 18 hours, less than 5-.per cent of the castings showed even traces of germination.

It is to be understood that the foregoing description is illustrative rather than strictly limitament of articles formed of a heat-treatable magnesium-base alloy characterized by a tendency to undergo germination on prolonged heating which comprises interrupting the solution-heat-treatment prior to the occurrence. of appreciable germination and subjecting the article to an aging temperature for a time sumcient to effect substantial precipitation before resuming the solution-heat treatment.

ROBERT S. BUSK.