US3475168A - Method of making metallic articles containing mineral fibers - Google Patents

Description

United States Patent 3,475,168 METHOD OF MAKING METALLIC ARTICLES CONTAINING MINERAL FIBERS Orlando A. Batlista, Yardley, Pa., and Frank J. Karasinski, Trenton, N.J., assignors to FMC Corporation, Philadelphia, Pa., a corporation of Delaware No Drawing. Application Aug. 7, 1967, Ser. No. 658,657, which is a continuation-in-part of application Ser. No. 578,118, Sept. 9, 1966. Divided and this application Oct. 15, 1968, Ser. No. 767,836

Int. Cl. B22f 3/10 U.S. Cl. 75-206 7 Claims ABSTRACT OF THE DISCLOSURE A shaped article of improved physical properties consisting of a major proportion of a low melting point (below about 1575 C.) metal and a minor proportion of submicron sized particles of a chrysotile which has been chemically modified to produce a ratio of SiO to MgO of between about 1.05 and 1.30. Also disclosed is a method of producing the article by making a dry mixture of metal powder and the submicron sized chemically modified chrysotile, pressing the mixture to produce billet and then sintering, extruding under pressure or otherwise working the billet.

This application is a division of application Ser. No. 658,657 and a continuation-in-part of application Ser. No. 578,118, filed Sept. 9, 1966 and now abandoned.

This invention relates to shaped articles formed of a relatively low melting point metal or metal alloy reinforced by having incorporated therein a minor percentage of microcrystalline mineral silicate fibers of a chemically modified chrysotile and to methods of making such articles.

As used herein, the term relatively low melting point means below about 1575 C. and, as will appear later, the invention is believed to have its greatest utility in connection with metals and alloys having melting points below about 675 C. Some examples of metals having melting points below 675 C. are aluminum, magnesium, tin, zinc, and lead. Aluminum is a particularly suitable metal for the application of this invention and, accordingly, it will be more specifically described in connection with that metal. However, it is to be understood that the techniques hereinafter described are applicable to other relatively low melting point metals and alloys including copper, which has a melting point above 675 C. but below 1575 C. Many low melting point metals such as alumi num, magnesium, tin, zinc, etc., have a rather low extensibility or ductility as well as low impact strength. These properties may be improved in some instances when the metals are alloyed with other metals, but even so, they are not as high as may be desired for certain uses. It has now been found that the impact strength and ductility as well as the tensile strength of articles formed of some low melting point metals and alloys may be substantially improved by incorporating with the metal or alloy a minor amount of a recently developed chemically modified mineral silicate, the nature of which will presently be explained.

It is an object of this invention to provide a shaped article of a low melting point metal or alloy in which is incorporated a minor amount of a chemically modified chrysotile whereby the article has greater impact strength, ductility, and/or tensile strength than a similar article formed of the same metal or alloy without the addition of the modified chrysotile.

It is a further object of the invention to provide a method of making a shaped article having a composition 3,475,168 Patented Oct. 28, 1969 comprising a major proportion of a relatively low melting point metal or alloy and a minor proportion of a chemically modified chrysotile.

Other and further objects, features, and advantages of the invention as well as the means of attaining the same, will become apparent as the description of preferred embodiments thereof proceeds.

The chemically modified chrysotile is a unique microcrystalline colloidal mineral silicate material and is the subject of copending aplication Ser. No. 479,620, filed Aug. 13, 1965 in the name of Orlando A. Battista, one of the joint inventors of the present invention. This application teaches that relatively long fibered asbestos is advantageously chemically modified and then mechanically disintegrated to obtain the desired product. In the chemical treatment, the materials which produce the desirable change in chemical composition consist essentially of any acid or acid salt. The chemical modification step is controllable by varying combinations of H ion, temperature, time and/or pressure, as well as by the severity of mechanical disintegration during the digestion step. However, best results are obtained by treat ment at elevated temperatures, preferably the reflux temperat-ure of the particular treating agent involved, in aqueous suspension at rather low solid content. For example, 0.2 N hydrochloric acid can be used to treat chrysotile at about 5% solids for /2 to 4 hours at reflux. This treatment will produce an optimum change in the SiO /MgO ratio to raise the same to about 1.21. The use of a pressure digester to permit digestion under pressure permits a reduction in the time of treatment to affect a change to the optimum SiO /MgO ratio.

In the initial treatment, satisfactory results have been obtained with hydrothloric acid, sulfuric acid, nitric acid, on acetylating mixture consisting of acetic acid, acetic anhydride, and trace amounts of sulfuric acid, and phosphoric acid. It is important that the acid concentration be adequately controlled. For example, 0.4 normal sul furic acid at reflux of a 5% mixture will remove far too much magnesium oxide in five minutes, causing an excessive change in the SiO /MgO ratio, together With a rapid loss of yield. On the other hand, an acetylating mixture of 600 ml. acetic acid plus ml. acetic anhydride plus 3 ml. of concentrated sulfuric acid can be used safely, providing good control to prevent the re action from going beyond the desired yield and the SiO /MgO ratio, even after an hour.

Consequently, dilute acid is preferred and particularly at about 5% solids, a 0.2. normal strength hydrochloric acid solution at atmospheric pressure is about optimum from the point of view of getting the reaction completed in a short time without the danger of too great a loss of product, and without severely detracting from the capability of producing stable aqueous dispersions of it. At higher solids contents the concentration of hydrochloric acid should be adjusted accordingly upwards in order to obtain the desired SiO /MgO ratio in the final product. For purposes of the present invention, the ratio of SiO /MgO brought about by the chemical treatment should be between 1.05 and 1.30 by weight.

Following the chemical pretreatment, the modified chrysotile is filtered and mechanically disintegrated directly in the presence of the retained mother liquor from the digestion treatment using shearing action as, for example, in a Waring Blendor or Osterizer, a Cowles Dissolver, a Rietz Extructor and similar types of mechanical shearing equipment. A preferred procedure is to perform the chemical pretreatment simultaneously with the me chanical disintegration. Normally, the product is washed to a chloride content of about 0.10% or less when HCl is used, prior to drying and packaging.

It is of the utmost importance to combine an optimum chemical pretreatment step with the appropriate mechani cal disintegration to procedure the desired basic microcrystalline colloidal particles of submicron size with the preferred dimensions of these submicron particles being of the order of 200-300 angstroms in diameter and 2000- 5000 or more angstroms in length. Mechanical disintegration of the chemically modified chrysotile is required until such time that at least about 10% of the mechanically disintegrated product comprises particles under one micron in all dimensions and preferably comprises or even much more particles having a maximum dimension in all directions of less than a micron. The resultant products will form stable dispersions and gels with water and other polar liquids in concentrations of the order of a few percent. On the other hand, stable dispersions and gels cannot be formed with natural chrysotile.

Prior to the above chemical modification treatment, the chrysotile may be first opened by subjecting it to mechanical action with or without the presence of dimethyl sulfoxide. Or, as still another route, the acid modified chrysotile is mechanically attrited in the presence of the dimethyl sulfoxide. It has been found that when the asbestos fibers described above are soaked in dimethyl sulfoxide (D'MSO) and slurried to permit uniform wetting out of the fibers by the dimethyl sulfoxide, a much more efficient mechanical breakdown of the chrysotile to submicron particles occurs. Dispersion of the slurry with the attendant release of individual fibrils of fibril fragments, respectively, may be aided by treatment of the DMSO slurry in a simple pulping device such as a heater or a Bauer Refiner to provide the appropriate mechanical action desired. After the mechanical treatment, the excess DMSO is separated from the opened-up asbestos as by centrifuge, filter, or squeeze rolls, and asbestos washed free of DMSO. If desired, it is satisfactory to perform the chemical modification step in the presence of DMSO and simultaneously with mechanical disintegration step, followed by washing and drying.

Although the preferred treatment for soaking or slurrying is to use the DiMSO undiluted, dilution with water or any miscible solvent is operable. The DSMO, either undiluted or diluted may be also reused for the steeping and/or slurrying treatments inasmuch as there is no evidence that it is significantly changed chemically during the soaking or slurrying steps.

By using elevated temperatures, the rate of dispersing the fibrils may be enhanced, although operating temperatures of the order of 100 degrees centigrade or less are preferred. Soak or slurry times of about 60 minutes are usually adequate, although much longer times may be desirable in some instances, depending on the natural compactness of the fibril aggregations. Inasmuch as time and temperature are not critical, they can be varied to suit the convenience of the operator.

It has been pointed out that the acid chemical treatment removes some of the magnesium oxide from the chrysotile and increases the SiO /MgO ratio. This removal of magnesium oxide changes the surface character of the chrysotile from relatively smooth to one which is pitted, cavitated and rough and it is thought that this rough surface contributes to a good mechanical bond between the chemically modified chrysotile and the metal in which it is incorporated in accordance with the present invention. Using the nitrogen adsorption method described by S. N. Nelsen and S. T. Eggertsen in Analytical Chemistry, volume 30, pages 1387-1390 (1958) natural chrysotile having an SiO /MgO ratio of 0.99 has been found to have a surface area of between 20 and square meters per gram (m. /gm.). This is after fiberization of the material by grinding and after removal of silt and non-asbestos components by air classification. When this same chrysotile was treated as described above to increase the SiO /MgO ratio to 1.15 and mechanically disintegrated, it was found to have a surface area of between 65 and 75 m. /gm., when measured by the nitrogen adsorption method. When the chemical treatment was such as to increase the SiO /MgO ratio to 1.20, the surface area was found to be between and mP/gm. This great increase in surface area is taken to indicate the presence of cavities and other irregularities. Shabani chrysotile from South Africa has in its natural state a ratio of SiO /MgO of slightly more than 1.05 (actually about 1.057) and this is within the range set forth above but this chrysotile, in the absence of an acid treatment such as described herein, has a relatively smooth surface and absent the chemical treatment is not satisfactory for carrying out the present invention.

The microcrystalline colloidal mineral silicate or chemically modified chrysotile wherein at least about 10% of the particles are of submicron size described above is the material which according to the present invention is added to aluminum, magnesium, zinc, tin, lead, copper, or other relatively low melting point metal or metal alloy to improve the impact strength, the ductility or extensibility and/ or the tensile strength of shaped articles made nominally of such metals. While it is essential to the present invention that at least about 10% of the particles of the modified chrysotile be of submicron size, it is desirable to have larger percentage of the particles below one micron and in fact it would be most desirable to have all of the particles below one micron. The reason for the use of a product with an abundance of small particles under one micron is that homogeneous distribution of the modified chrysotile within the metal is essential.

It has been previously mentioned that the invention is most useful in connection with metals and alloys having a melting point below about 675 C. and this is because most methods of fabricating metal articles involve raising the temperature of the metal to close to its melting point. At about 700 C., the chemically modified chrysotile loses its Water of hydration and consequently may lose some strength. However, the chemically modified chrysotile does not melt below about 1550 C. and may be effectively used with metals having melting points ranging up to approximately 1550 C. As a matter of fact, in some instances, it is desirable to heat the modified chrysotile to about 700 C. in order to deliberately drive off the water of hydration and thereby enable the production of larger percentages of submicron sized particles. These particles may then be incorporated into metals having melting points below about 675 C. as well as metals such as copper having melting points between 675 C. and 1550 C.

It has been found that an extruded aluminum rod containing by weight 1.0% of the chemically modified chrysotile wherein at least 10% of the particles were whiskers less than one micron in length had an elongation at break of about 25-35% or more, as compared with an elongation at break about 5-10% for a control rod of commercially pure aluminum. The tensile strength of both the sample and the control was about 22,000 p.s.i. By drawing the sample to reduce its extensibility down to that of the control, the tensile strength of the sample may be greatly increased. When larger percentages of the particles of the modified mineral silicate are of submicron size, it is possible to use as little as 0.1% of this material and attain improved tensile strengths and elongations. Such an aluminum hybrid has widespread utility, one such use being as wire. Considerably higher tensile strength and/or elongation can be attained by using the modified chrysotile having larger percentages of submicron size particles.

The aluminum containing the modified chrysotile may be fabricated by conventional methods such as by extrusion under heat and pressure or by powder metallurgy techniques. It is also possible with the use of the modified chrysotile to produce a foamed aluminum, in which case a relatively large amount, for example, up to about 40%, of the modified chrysotile should be added. By means of induction heating, the article, which may have been formed for example by extrusion, is rapidly interiorly heated to a temperature above which the chrysotile loses its water of hydration; this is released in the form of steam providing vapor to create small pockets within the structure. By quickly quenching the article, the gas is trapped within the pockets thus providing a foamed product having an exceptionally low apparent density.

The manner of incorporating the chemically modified ehrysotile with the aluminum or other metal is a most important aspect of this invention. When metals are alloyed with other metals, it is normal to use temperatures such that both, or all, metals are in a fluid state whereby they may be homogeneously mixed together. However, in the case of the present invention wherein it is preferred that the modified ehrysotile be used in the fibrous or whisker form, the melting of the colloidal mineral silicate particles at temperatures of 1575 C. or higher normally is avoided. In order to incorporate the modified chrysotile in whisker form into the metal, attempts were made to disperse the ehrysotile in dry form throughout the metal by stirring it into the molten metal, the metal having a lower melting point than the ehrysotile as noted above. However, this approach proved unfruitful because the surface tension of the metal precluded the wetting of the ehrysotile whiskers with the result that the latter always remained on the surface of the melt, no matter how vigorous the physical mixing.

As mentioned, uniform or homogeneous distribution of the modified ehrysotile throughout the metal is essential. This may be accomplished to a reasonable degree by mixing the ehrysotile, either with or without its water of hydration, in dry form with a dry powder of the metal. This powdery mixture is then subjected to a high pressure to form a shaped article such as a slug or billet which may be extruded through a die under heat and pressure to form an elongated article having a cross section corresponding to the shape of the die opening. Instead of extruding the pressed powder article, it may be sintered to harden and give it greater cohesion. If desired, a lubricant may be incorporated with the powder and in this manner a sintered, selflubricating hearing may be formed.

It has been found that better homogeneity and a greater increase in strength may be obtained if the dry powder mixture is provided in the manner now to be described. As previously mentioned, when at least about ten percent of the particles of the chemically modified ehrysotile are of submicron size in all dimensions, a colloidal dispersion may be formed. According to the preferred method of carrying out the present invention, such a colloidal dispersion is formed and to this is added the metal or alloy in powdered form. The mixture is then stirred to provide a homogeneous blend or the stirring may be carried out as the metal powder is gradually added. The blend is then dried to provide the dry powdery mixture which is processed in the manner previously described.

To improve the bond between the modified ehrysotile whiskers and the metal, the whiskers may be provided with a metallic coating by well-known methods such as vacuum deposition or a precipitation process. Alternatively, the modified ehrysotile particles may be provided with a coating of a metal salt such as for example aluminum acid phosphate or aluminum nitrate where the whiskers are to be incorporated in aluminum or aluminum alloys. Such techniques will yield a coating of almost atomic thinness and thus the coating does not appreciably increase the size of the whiskers or interfere With the homogeneous distribution of the ehrysotile within the metal. If the colloidal mineral silicate is to be added to aluminum, a further example of a satisfactory coating material is colloidal which has a positive surface charge whereas the chemically modified mineral silicate carries a negative charge. Thus, the alumina becomes electrically bonded to the ehrysotile. The compatibility and wettability of the colloidal microcrystalline silicate particles with metals are greatly improved by deposition of surface metal coatings, the preferred metal coating varying with each hybrid composition.

Having thus described the invention, what is claimed is:

1. The method of making a shaped article comprising making a dry mixture consisting of metallic particles selected from the group consisting of metals and metal alloys having a melting point below about 1575" C. and a ehrysotile which has been chemically modified to obtain a ratio of SiO to MgO of between about 1.05 and 1.30 and further characterized. in that the surface area as determined by the nitrogen adsorption method is at least 65 m. gm. and wherein at least about 10% by weight of the modified ehrysotile is of submicron size, and cold pressing the dry mixture to form a shaped article.

2. The method set forth in claim 1 including the step of coating the modified chrysotile particles with a metal salt film prior to making the dry mixture.

3. The method set forth in claim 1 comprising the additional step of extruding the shaped article under heat and pressure to further form and shape the article.

4. The method set forth in claim 1 comprising the additional step of sintering the shaped article.

5. The method set forth in claim 1 wherein the dry mixture is obtained by the steps of adding the metallic particles to a colloidal dispersion of the chemically medified ehrysotile, stirring the dispersion to provide a homogeneous blend, and drying the blend.

6. The method set forth in claim 1 wherein the metallic particles are aluminum.

7. The method set forth in claim 1 wherein the chemically modified ehrysotile constitutes between about 0.1% and about 40% of the weight of the dry mixture.

References Cited UNITED STATES PATENTS 2,253,608 8/1941 Bauer 106-36 2,369,502 2/1945 Walker 10636 3,019,103 1/1962 Alexander 29-185 XR 3,181,231 5/1965 Breck 29182.5

FOREIGN PATENTS 1,130,324 10/ 1968 Great Britain.

CARL D. QUARFORTH, Primary Examiner ARTHUR J. STEINER, Assistant Examiner US. Cl. X.R.

my UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 475 168 Dated Qctaher 28 1 96 9 Inventor(s) Orlando A. Battista and Frank I1, Karasinski It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shoym below:

Column 2 line 34, "hydrothloric" should be hydrochloric; line 35 "on" should be -an-. Column 3 line 2, "procedure" should be -produce-; line 28, "of" (second occurrence) shoul be -or. Column 6, line 9, insert alumina-- after "colloidal" line 40, (Claim 5) change "medi" to -modi in the references cited, the sub-class for Patent No. 3 ,Ol9,l0 should be -l82.5.

SIGNED AND SEALED Atteat:

7 WILLIAM E. sum. Edward M. Fletcher. 16 Commissioner of Patents Attesting Officer