US3790365A - Method of making metal foams by sequential expansion - Google Patents

Abstract

Metal foams are produced by decomposing a blowing agent in a molten metal such that there is an initial and a subsequent expansion due to blowing gas. The sequential expansion can be conducted by more than one addition of blowing agent to the molten metal to be expanded, or by allowing the molten mass to expand, applying agitation to collapse the foam, and allowing it to expand again, or by forming an intermediate product by expanding and cooling, remelting the intermediate, stirring, and foaming again. Zinc-based foams of fine quality are produced. In a preferred embodiment, substantially pure zinc, titanium hydride (0.1 to 1.5 weight per cent based on weight of metal, and a reaction temperature of from about 475* to about 500*C. are employed.

Description

United States Patent 1 Niebylski et al.

[ Feb. 5, 1974 METHOD OF MAKING METAL FOAMS BY SEQUENTIAL EXPANSION [75] Inventors: Leonard M. Niebylski, Birmingham; Chester P. Jarema, Detroit, both of Mich.

[73] Assignee: Ethyl Corporation, Richmond, Va.

[22] Filed: June 21, 1971 [21] Appl. No.: 155,210

Related US. Application Data [63] Continuation-impart of Ser. No. 879,515, Nov. 24,

1969, abandoned.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. R. Satterfield Attorney, Agent, or Firm-Donald L. Johnson; Robert A. Linn [5 7] ABSTRACT Metal foams are produced by decomposing a blowing agent in a molten metal such that there is an initial and a subsequent expansion due to blowing gas. The sequential expansion can be conducted by more than one addition of blowing agent to the molten metal to be expanded, or by allowing the molten mass to expand, applying agitation to collapse the foam, and allowing it to expand again, or by forming an intermediate product by expanding and cooling, remelting the intermediate, stirring, and foaming again. Zinc-based foams of fine quality are produced. In a preferred embodiment, substantially pure zinc, titanium hydride (0.1 to 1.5 weightper cent based on weight of metal, and a reaction temperature of from about 475 to about 500C. are employed.

4 Claims, No Drawings METHOD OF MAKING METAL FOAMS BY SEQUENTIAL EXPANSION This is a continuation-in-part of application Ser. No. 879,515, filed Nov. 24, 1969 now abandoned.

BACKGROUND OF THE INVENTION Foamed metals have been described in the prior art; see for example, U.S Pat. Nos. 2,895,819, 3,300,296, and 3,297,431. Such foams are produced by adding a gas-evolving compound to a molten metal and thermally decomposing the compound to evolve blowing gas. The gas forming compound can be a metal hydride such as ZrH or TiH US. Pat. No. 2,983,597.

It is known that zinc foams can be produced; confer the aforementioned U.S. patents. In those patents, alloys having an appreciable amount of metal other than zinc are taught to form foams. This invention provides means to foam substantially pure zinc as well as other metals and alloys.

SUMMARY OF THE INVENTION In essence, this invention provides metal foams and processes for their formation. In an important aspect, this invention provides a process for preparation of a metal foam, said process comprising decomposing a blowing agent in a molten metal to produce blowing gas such that there is an initial and a subsequent evolution of blowing gas from said blowing agent; and subsequently cooling the expanded mass produced after the final evolution of blowing gas, thereby forming a set cellular product.

Methods for the sequential expansion include more than one addition of blowing agent to the molten metal to be foamed,

allowing the molten mass to expand by decomposition of blowing gas, causing the foam to collapse and expand again, and

forming a set intermediate product by expanding and cooling, remelting the intermediate, and foaming again.

By such techniques, zinc-based metals including substantially pure zinc or alloys having 85 per cent zinc or more, can be efficaciously expanded.

DESCRIPTION OF PREFERRED EMBODIMENTS In a preferred embodiment, substantially pure zinc is employed as the metal to be foamed. Zinc alloys having up to about weight per cent of alloying metal(s) such as aluminum-zinc and magnesium-zinc alloys can be used. Typical alloys which can be employed are ZDC No. 3 (AG4OA), ZDC No. 5 (AC4lA) and Alloy ZDC No. 7. Compositions of such alloys are set forth on pages 28-9 of ASARCO, Zinc Die Casting Alloys, American Smelting and Refining Company Bulletin VI-l. The compositions of those alloys recited on the cited pages are incorporated by reference herein as if fully set forth. Another typical alloy is ILZRO 12; it has the composition aluminum 1 1-13 percent copper 0.5-1.25 percent magnesium 0.01-0.03 percent zinc balance Also lead-zinc alloys can befoamed according to the process of this invention, so long as they are molten at applicable temperatures, such as the ranges discussed below. Moreover, this invention can be extended to lead and lead alloys. The lead may be alloyed with magnesium or aluminum. Other alloys can be employed as well as other substantially pure metals.

The temperature employed is not a truly independent variable but is dependent at least in part on the nature of the metal and the nature of the blowing agent employed. The temperature employed is an elevated temperature sufficient to maintain the metal in a molten state while expansion takes place and also sufficient to thermally decompose the blowing agent to form blowing gas at a rate which yields foaming at the desired speed. The speed of foaming should not be so long as to unduly delay the process; likewise, it should not be too fast for blow-hole formation or other untoward event may occur. Usually it is desirable to employ a temperature which allows the sum of the plurality of expansion periods to total from about 1 to about 45 minutes, but shorter and faster times can be employed. Given a desired metal-blowing agent system, a skilled practitioner can determine a desired temperature and suitable expansion time sum with a minimum of experimentation. Preferred sums are 2-30 minutes and most preferred from about 3 to about 15 minutes. Thus, temperatures of 395655C. can be employed and it is preferred to use temperatures of from about 425 to about 625C. More preferable, temperatures of from about 425 to about 525C. and most preferably from about 475 to about 500C, are employed.

Such temperatures are conveniently used when conducting the process by utilizing a metal hydride such as magnesium or titanium hydride as the blowing agent. In general, for this invention, hydride blowing agents are of choice. The titanium hydride of commerce usually has a little less hydrogen than the stoichiometric amount. Such commercial material is very satisfactory, as are materials wherein more hydrogen has been removed (by treatment at about 400C. for from about 1 to 24 hours). When foaming a zinc-based metal, that is, a metal which has weight per cent zinc or more, titanium hydride affords a finer pore structure than Mgll ZrI-I prepares foams usually of intermediate pore size.

Thus, where coarser zinc-based foams are desired,

tion. However, greater and lesser amounts can be emv ployed if desired. As the ,amount of blowing agent increases (when other parameters are allowed to remain constant) preparation of less dense foams is favored.

When using titanium hydride, a preferred amount is I from about 3 to about 10 grams per each 1,000 gram portion of zinc-based metal to be foamed.

As taught above, the process of this invention entails more than one expansion of a molten mass utilized to form a set cellular product. In a preferred embodiment, two expansions are employed, that is, an initial expansion is followed by a final expansion. If desired, more than two expansions can be carried out. Thus, three, four or more expansions may be used.

When the process is conducted, all the blowing agent to be employed can be introduced by one addition. When this expedient is employed, one method of achieving a plurality of expansions is to allow the material to foam, then cause the foam to collapse, and then allow the molten mass (thereby produced) to foam again. Good results are obtained when the initial expansion increases the mass to be foamed from about two to about three times the original volume, and the foam is made to collapse to the original volume or thereabouts, say, up to about 1.1 or 1.2 times the original volume.

The foam can be made to. collapse by stirring as described above, however, it is not necessary to do so.

One can add all the blowing agent in one addition, allow the molten mass to foam, cool to form an intermediate set cellular product, remelt and stir, and allow the mass to foam again, and then cool to form a final set cellular product. This embodiment can be carried out in a number of ways. For example, a comparatively large amount of blowing agent can be added, say up to about 5 to about weight per cent based on weight of metal to be formed, allow the molten mass thus formed to foam to an extent greater than desired for the final product, cool to form an intermediate, and then remelt and stir, and finally, allow the molten mass to foam to the extent desired. Alternatively, after the original addition of blowing agent, the foaming can be stopped with less or about the same amount of foaming as desired in the final product. These products can be remelted and refoamed as pointed out above. Moreover, one can make one addition of blowing agent, allow the first expansion to take place to the extent desired, cool and add some or all the intermediate thereby produced to a further amount of molten metal, allow foaming to occur to the desired extent, and then allow the final product to cool and set. Routine experimentation can determine the extent of initial expansion and the amount of non-blown molten metal which will yield a desired product for any given amount of blowing agent and initial amount of molten metal used to form the intermediate product.

Alternatively, the blowing agent employed may not all be added in one addition. In other words, a plurality of additions of blowing agent can be utilized. Good results are achieved using two blowing agent additions, but more can be employed if desired. Thus, one can make three, four or five or even more separate additions of blowing agent to the molten mass. When using two additions, generally at least 5 weight per cent of the total amount of blowing agent to be employed is added in the first addition. In some instances, it is desirable to add at least 10 percent of the blowing agent. In many instances it is desirable to add about 50 per cent of the blowing agent in the first addition. Greater or lesser amounts than those discussed above can be added in the first addition, as desired.

The blowing agent need not be added to the molten metal at the same temperature. In fact, in a preferred embodiment of this invention using zinc-based foams, a portion of the blowing agent is added at a temperature of from about 425 to about 450C. and subsequently the remainder of the blowing agent is added at a temperature of from about 475 to about 525C. When this operation sequence is employed, foaming occurs over a longer period of time and more uniform mixing of the blowing agent in the molten metal can be achieved. These beneficial results are enhanced by the temperatures of 425 450C., since that usually affords very slow foaming.

The above embodiments may be combined. For example, one can add blowing agent to a molten mass, allow it to foam to form an intermediate set cellular product, remelt, add a second quantity of blowing agent, allow the mass to foam to the desired extent, and then cool to form the final set cellular product.

For the process of this invention to achieve its best results, it is desirable that the initial expansion occur to a significant amount. The exact amount of initial expansion required per given embodiment and per given amount of blowing agent in molten metal can be determined for each agent and metal via routine experimentation. Generally, good results are achieved if upon initial expansion the molten mass expands to form about two to three times the initial volume. Greater or lesser amounts of expansion can be employed. For example, good results can be achieved when the initial expansion occurs to about 1 10 per cent of the initial volume. Similarly, good results have been achieved when the initial expansion was about 15 times the original volume and three times the final volume. Thus, the final expansion can be to a greater or lesser extent than a previous expansion. When an intermediate set cellular product is to be formed and this is to be added to additional molten metal (and no more blowing agent is to be added to the final melt) the amount of initial expansion, generally, is equivalent to about 1.1 to about three times the volume of the final amount of molten metal.

The ability to form satisfactory foams utilizing an initial expansion followed by a cooling step to form an intermediate set cellular product has two advantages. First, it allows scrap foam to be utilized. Second, it allows the formation of set cellular product at one location and shipment to another for final expansion.

In an important aspect, this invention involvesthe discovery that better results are achieved if the molten mass to be foamed had been subjected to an initial prior treatment with blowing gas. The reason for the beneficial result obtained is not known with certitude. Not wishing to be bound by any theory, the beneficial result may occur through increased wetting of the blowing agent employed. It has been noted that in certain metals such as zinc, upon initial addition of blowing agent such as titanium hydride, that even after rapid, efficient stirring, some metal hydride particles will come to the surface of the molten metal. If the molten metal is subjected to an initial addition of titanium hydride followed by sufficient stirring and some expansion as described above, it has been noted that there is generally much less-blowing agent on the surface after a second addition of blowing agent. These results tend to hypothesize that pre-gassed metal may have an enhanced ability to wet blowing agent particles such as metal hydrides.

In a preferred embodiment, the temperature of the massto be foamed with Til-I, is maintained by an induction field. For reasons unknown to us, it appears that induction heating affords higher quality foams than resistance heating methods. In addition, induction heating afi'ords closer process control.

As appreciated by a skilled practitioner, better foams are produced if the blowing agent is substantially'uniformly mixed throughout the molten metal to be foamed. Thus, we prefer to use mixing techniques which tend to give efficient mixing. High speed stirring is a method of choice.

Our process proceeds well at ambient pressure, but greater and lower pressures can be used if desired.

The following exemplary material serves to illustrate the process of this invention.

EXAMPLE I In a cylindrical reaction vessel, a 1,200 gram charge of substantially pure zinc was heated to 440C. To this was added four grams of Til-l enclosed within lead foil. The addition of titanium hydride was carried out while dispersing, using high speed mixing, for approximately 30 seconds. Very slight foaming occurred. An additional 5 gram portion of titanium hydride was added and stirred in for approximately 30 seconds while the molten metal was at a temperature of 470C., utilizing an induction field. While this temperature was maintained, foaming continued for 4-5 minutes. Upon cooling, a good quality foam of l2l 3 per cent density with very small (1/64-1/32 inch) average pore size cells was produced.

Similar foams are made when the amount of titanium hydride is from 1 to grams per 1,000 gram portion of zinc. Similar foams are also produced when the same amounts of magnesium hydride are employed but in general they have larger pores than analogous TiH foamed materials. Likewise, analogous foams are produced from zinc alloys having up to about 15 weight per cent of alloying material selected from magnesium, aluminum, zinc, and combinations thereof. Similar foams are produced when the process of the above invention is continued at temperatures up to about 625C. However, the foaming is more rapid and in many instances, greater amounts of blowing agent are preferably employed.

Our work has indicated that zinc foams are made in fine pore qualities only if they'have been subjected to at least two foam operations. If only one expansion is allowed to take place, the resultant foam will be large celled, non-uniform, andwill usually have a heavy skin bottom. Such inferior foams can be produced for example, using pure zinc and 0.75 1.25 grams of titanium hydride per each 100 gram portion of zinc employed.

EXAMPLE II A 1,120 gram portion of substantially pure zinc was melted and 60 grams of substantially pure aluminum were added to the zinc at 450C. The temperature was raised to 600C.

Thereafter, the temperature was brought to 460C. and 10 grams of Til-l were mixed with the Al-Zn alloy. The temperature dropped to 420C., and in spite of admixing with efficient stirring, some of the hydride remained on top of the alloy. The resultant mixture was reheated such that it reached 480C. In approximately 5 minutes. During this reheating, it foamed slowly.

The foam was restirred such that the melt approached the initial volume and reheated to 480C. After 4 minutes at this temperature foaming stopped.

The crucible containing the molten mass was removed from the induction furnace. It was noted that some foam collapse had occurred. Sectioning revealed collapsed cells and a k inch thick bottom skin.

EXAMPLE, 111

A 953 gram portion of substantially pure zinc was melted and 43 grams of substantially pure aluminum were added thereto at 450C. The temperature of the resultant mass was raised to 600C. to insure alloying.

The temperature was subsequently brought to 500C. and solid carbon dioxide snow was added to the molten mass until the temperature decreased to 400C. At that temperature the metal was too cold to add any more carbon dioxide snow. A total of l 19 grams of CO had been added. The resultant melt was heated to 600C. and an additional 132 grams of CO were added. This brought the temperature to 485C. and the molten metal was very thick.

At 470C, 10 grams of titanium hydride were added. The metal appeared cold as many solids had formed. The melt was reheated for 2 minutes and at a temperature of 490C. Foaming occurred until the volume was about twice the original volume. The mixture was restirred until the volume approached the original; the metal was very heavy and as a result mixing was difficult. The induction field was applied again, foaming reoccurred and when the foam reached about 2 inches from the top of the crucible containing it, the crucible was removed from the induction furnace.

Upon standing the foam shrunk somewhat. Sectioning revealed a better foam quality than that obtained in Example ll. Cell collapse .was minimal and the bottom was one-eighth inch as compared to the /2 inch skin in Example ll. Thus, carbon dioxide improves the quality of aluminum-zinc foams. Similar improvements in quality are achieved with water, nitrogen, and argon. Air or oxygen can also be used to increase the viscosity of the melt.

EXAMPLE IV A 1,200 gram portion of zinc balls was melted and the temperature raised to 450C. At that temperature, two lead foil packets of 5 grams titanium hydride each were added with stirring. The stirrer was removed and the resultant mass was heated for 5 minutes. Very little foaming occurred during that period in which the final temperature was 430C. The molten mass was reheated to 450C. and foaming began. The mixture was restirred causing the volume to decrease to about the original volume. Thereafter the stirrer was removed and the resultant mass reheated for 5 minutes at about 480500C. During that period a heavy lid had been placed on the crucible containing the foam. During the resultant second foaming operation, the zinc foamed to fill the crucible and push the lid up. After cooling to obtain a set cellular product, sectioning revealed a very fine pore foam with no appreciable skin. Utilizing this technique such that before the final expansion the mixture is allowed to foam to 2-3 times the original volume affords good results.

EXAMPLE v After minutes in an oven, it was noted the foam had a purplish color; after 8 minutes, the foam was found to have collapsed somewhat. Sectioning after cooling revealed collapsed cells.

A 960 gram portion of the intermediate foam pro- Compressive Strengths of Various 1" X l" X 1" Zinc Foarn s Strength to Weight Ratio in psi/pcf Foam Description Wt. (pcf) Comp. Strength (psi) Pure Zn w"/TiH 4.2g I60 56 3.50 Do. 4.5g 17.1 56 3.27 Do. 4.2g l6.0 55.8 3.49 Pure Zn w/CO and TiH 3.7g [4.] 43.8 3.l2 Do. 5.2g 19.8 60.0 3.03 5% Al 95% Zn w/CO and TiH 9.9g 37.7 328 8.70 Some crumbling Do. 10.6g 40.4 433 10.7 Do. Alloy No. 5 WITH: ll.0g 41.9 317 7.56 Do. Do. 13.4g 5].[ 698 13.7 Do. 371 Mg-97'7c Zn w/TiH 15.2g 57.9 345 5.96 More severe crumbling and breaking Do. 169 64.4 372 I35 Do. 371 M g-97% Zn w/ZrH. 2|.7g 82.7 2000 24.- Do.

w=with duced above was melted and began to foam around 450C. Stirring was conducted, the stirrer was removed and the induction field applied to give a temperature of about 480C. At that temperature foaming occurred again and the induction field was shut down. The resultant mass. was air cooled and sectioning revealed a very fine pore uniform foam. Density was 48.3 pcf, 1] per cent metal density.

dride will'ordinarily liberate enough gas to foam a metal mass at 375400C. and higher, titanium hydride will do so at 435-450C. and higher, and zirconium hydride at 465485C. and higher. In general, best results are obtained in this invention with the above blowing agents by utilizing a molten metal at a.

temperature within from about to about 70C. of the lower temperature cited above for the selected blowing agent. More preferably, the molten metal should be generally about 50C. above the lower temperature. Utilizing this temperature-blowing agent relationship, good results are obtained utilizing magnesium hydride, zirconium hydride or titanium hydride in processes similar to the above examples employed with many diverse metals and alloys. Suitable alloys may be binary, ternary or quaternary. Such combinations of lead, zinc, magnesium, tin, silicon, copper and aluminum are useful in this invention. We can use the low melting eutectic of 32 per cent Al and 68 per cent magn esium.

Ternary systems applicable for-this invention include combinations of the above named metals exemplified by aluminum-magnesium and silicon. Thus, we can use an alloy with about 3 to about 15 per cent silicon and 04-10 per cent magnesium, the balance being aluminum. Other ternary systems which we can use are Al- Cu-Mg, Al-Zn-Mg and the like. Quaternary mixtures include combinations of the above named metals. Typical alloys of this type'are Al-Mg-Cu-Zn such as A1 78.5 per cent, Mg 5 per cent, Cu 5 per cent, and 11.5 weight due to the small sample size. Nevertheless, trends can be ascertained. These results indicated that although zinc alloys of aluminum and magnesium are more difficult to foam than pure zinc, such alloys have a much better strength to weight ratio than substantially pure zinc foams. The results also indicate that carbon dioxide thickening of pure zinc produces a foam having a strength-weight ratio about the same as a nonthickened pure zinc foam. Also, the data indicate that magnesium or aluminum additions to zinc tend to cause foam embrittlement.

The foamed products of this invention can be utilized in many instances wherein the non-foamed base metals are employed. Foamed zinc can be utilized in structural materials where zinc is now used and less weight is desirable. The zinc foams of this invention can be used in injection molding and diecasting complex shaped hardware for automobiles, aircraft, and other vehicles. For structural use, slabs of foamcan be made by pouring the mixture to be foamed into a mold having a flat surface. lf the mold surface has a temperature of, say, 50 to C. below the temperature of the expanded mass, then a skin of non-cellular metal can be formed on the foamed mass. Similar conditions hold for lead and lead alloy products of this invention. The lead and lead alloy foams of this invention lend themselves well to use as acoustical materials.

We claim:

1. Process for preparing a metal foam, said method comprising a. decomposing a blowing agent in a molten metal.

thereby producing gas which expands said molten metal to about two up to about threetimes the initial volume of said molten metal,

b. collapsing the foamed mass to about the initial volume of the molten metal by agitation with sitrring means,

c. heating to re-expand said molten metal, and

d. cooling the re-expanded mass to form a set cellular product.

2. A process of claim 1 wherein said metal is a zincbased metal having up to about 15 weight percent of one or more alloying metals.

3. A process of claim 1 wherein substantially pure zinc is employed.

4. Process of claim 1 wherein said blowing agent is selected from the class consisting of magnesium hydride, zirconium hydride, and titanium hydride.

UNITED STATESPA'EENT UFFICE" (/-1) m CERTIFICATE F @QREQTI N P acent NO. 3,790 I Dated February 5:

Renard M. Niebylski et a1 a in tha ahove-idantif gmd pment i It 15 certifiiefi mm: 12mm appaw am?" that mid Lettara mama mm hamby awmcm a3 shown balm Invntofls) r Goluiam l, 7 line 21, imam am the follmwing sentence:

-- This is a cant-mmtiondmgar-t m applicatian Serial No. 879,515, filw fiavember 2 1969,

now abandonefi.

Signed and sealed this 15th day of October 1974.

-(SEAL) Attest:

MCCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents

Claims (3)

1. 2. A process of claim 1 wherein said metal is a zinc-based metal having up to about 15 weight percent of one or more alloying metals.
2. 3. A process of claim 1 wherein substantially pure zinc is employed.
3. 4. Process of claim 1 wherein said blowing agent is selected from the class consisting of magnesium hydride, zirconium hydride, and titanium hydride.