US3508599A - Lightweight structural articles - Google Patents

Description

Apri28,1970 M. BVORDAHL 3,508,599

y LIGHTWEIGHT STRUCTURAL ARTICLES Original Filed June 5. 1961 2 Sheets-Sheet 1 Milton B.Vordohl ATTORNEY April 28, 1970 M. B. voRDAHL 3,508,599

LIGHTWEIGHT STRUCTURAL ARTICLES Original Filed June 5. 1961 2 Sheets-Sheet 2 INVENToR Milton B. Vordohl ATTORNEY Figs United States Patent Office -3,508,599 Patented Apr. 28, 1970 3,508,599 LIGHTWEIGHT STRUCTURAL ARTICLES Milton B. Vordahl, Beaver, Pa., assigner to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New `lersey Original application June 5, 1961, Ser. No. 114,903.

Divided and this application Sept. 23, 1964, Ser.

Int. Cl. B22d 23/00 U.S. Cl. 164-81 3 Claims ABSTRACT OF THE DISCLOSURE This invention pertains to lightweight, high strength elements for constructional applications and, more particularly, to composite structural articles comprising spacedapart face sheets or panels and an improved lightweight, high strength filler material in the form of preformed hollow spherical bodies or bubbles, The invention is especially directed to methods for the manufacture of such hollow spherical bodies. The method comprises the steps of forming a spherical bubble in a bath of molten material from which the sphere is to be made and passing the formed bubble into a cooling medium overlying the bath to solidify the bubble.

This application is a divisional application of co-pending patent application Ser. No. 114,903 entitled Lightweight Structural Articles in the name of Milton B. Vordahl, filed June 5, 1961, and now abandoned.

Many facets of those currently rapidly expanding, technological areas relating to lightweight, high strength constructional applications, Such as, for example, high speed aircraft, missiles, etc., have resulted in the need for constructional materials exhibiting minimum weight together with high strengths, over a wide range of temperatures. Many such new applications have resulted in the development of metals and alloys, such as iron, nickeland cobaltbase superalloys, the precipitation hardenable stainless steels and allied refractory metals and alloys, such as, titanium, columbium, molybdenum, beryllium, etc., having many properties suitable for use under rigorous environmental conditions, such as, high temperatures, extreme temperature changes and corrosive atmospheres. Many of these metals, such as, titanium, beryllium and the like, have highly desirable strength-to-weight ratios and, accordingly, have been subjected to intensive development for those applications wherein high strengths and low densities are prerequisites. However, many of the newly available materials of construction, as well as others of the more remote prior art, while possessing one or more desirable attributes, eg., high strength at elevated temperatures, corrosion resistance, etc., are relatively dense materials. Moreover, even the lighter weight materials are frequently too heavy to use, in full sectional form, to the extent necessary to obtain the desired strengths.

Consequently, the foregoing and other materials of construction having been utilized, to a limited extent, in the form of laminates comprising two or more relatively thin panels or sheets of the primary material of construction, together with relatively lightweight cores. One example of such composite structure is the so-called sandwich panel upon which extensive developmental work is currently being expended, particularly in those areas, such as aircraft and missile skin construction, wherein the aforementioned properties of light weight together with high strength are especially necessary. Such lightweight sandwich structures, as for aircraft use, are difficult to make in even the simplest embodiments, such as flat, spaced apart panels or skins having corrugated or honeycomb interior fill. Complex, curved shapes of such construction are almost impossible to manufacture. In those instances wherein such shapes are absolutely essential, the cost of the completed panels ordinarily is one hundred times or more times the cost of the constructional materials themselves. The ditliculty encountered in the construction of laminates of such complex shapes is occasioned, in part, by the necessity for the provision Ofcomplicated brazing fixtures required to maintain the close dimensional tolerances needed for such structures. Moreover, intimate contact of the spaced apart panels with the interior fill is diicult to achieve throughout a large structure of complex shape, requiring extensive and complicated machining operations to tit the lill to the contour of the panels. Failure to achieve substantially complete contact between fill and face panels results in a defective product which is apt to fail under applied stress.

Accordingly, it is an object of the present invention to provide a lightweight, high strength sandwich structure susceptible of ready manufacture in either simple or complex shape, and of consequent low cost.

It is a further object of the invention to provide a method of making new and improved high strength, low density, low cost sandwich structures.

It is another object to provide a novel, lightweight, high strength fill for laminated structural applications.

It is still another object to provide a method for making thin walled, hollow metal articles suitable for core fill for structural laminates.

In accordance with the above objectives, the invention provides, in a preferred embodiment thereof, a sandwich structure comprising relatively thin gauge metal sheets or panels held together and supported in a predetermined, spaced-apart relationship by a plurality of preformed, hollow metal spheres or bubbles. The spheres, constituting the interior fill of the sandwich structure, are preferably provided with a film of suitable material such as a brazing alloy, which, upon heating of the filled structure, melts and, upon subsequent cooling, solidifies to effect adherence of the spheres one to another and to the confining sheets or panels.

The foregoing and other objectives of the invention will be readily apparent from an inspection of the following specification and accompanying drawings herein.

FIG. l is an enlarged, cross sectional view of one form of a suitable hollow iiller sphere for use in manufacture of the sandwich structures of the invention, the sphere being provided with a brazing coating;

FIG. 2 is an enlarged, cross sectional view of an alternative form of a filler sphere;

FIG. 3 is an enlarged plan View of the filler spheres, in a two-layer ideal packing arrangement upon a bottom panel;

FIG. 4 is an enlarged, elevational View of a sandwich structure in accordance with the invention, wherein the filler comprises a three-layer, ideal packing arrangement of ller spheres between two spaced-apart panels;

FIG. 5 is an isometric view of a structural article in accordance with the invention comprising flat, spaced panels enclosing, and held in spaced-apart relationship by, a quantity of hollow filler spheres;

FIG. 6 is an enlarged, cross sectional, elevational view of a portion of a sandwich structure in accordance with the invention, illustrative of the method of aflixing the filler spheres to each other and to the face panels;

FIG. 7 is an enlarged, cross sectional View of a punch and die means for forming a first hemispheric portion of the hollow filler sphere illustrated in FIG. 2;

FIG. 8 is an enlarged, cross sectional, elevational view of a punch and die means for forming a second hemispheric portion of the filler sphere of FIG. 2; and

FIG. 9 is an elevational schematic view, partly in cross section, of a suitable apparatus for producing hollow filler spheres in accordance with the invention, i.e., forming bubbles of a suitable metal and solidifying the same to form the spheres.

Referring now to the drawings, and more particularly to FIGS. 3, 4 and 5, those drawings illustrate a simple at sandwich structure made in accordance with the invention. Thus, the invention contemplates the provision of relatively thin gauge side panels 11 of suitable material. Side panels 11 may comprise various metals, the nature of which is dependent upon the application intended for the finished article. Thus, for the especially important high strength, low density applications required in the aerospace industries, panels 11 may be fashioned from a high strength, high temperature metal or alloy, such as a precipitation hardenable stainless steel, titanium or alloys thereof, columbium, molybdenum, tungsten, zirconium, beryllium, etc. or alloys thereof.

As illustrated, the side panels 11 are spaced apart by a plurality of hollow filler spheres 12. In order to realize the extraordinary constructional advantages of the product of the invention, the spheres 12 are constructed of a suitable metal, the nature of which, as in the case of the panels 11, depends upon the particular application for which it is intended. Thus, the spheres 12 may themselves be made of the same high strength alloy as the panels 11. Alternatively, the spheres 12 may be constructed of a metal or alloy diiferent from that of the panels 11. In the case of panels 11 made of a high strength, high temperature alloy, the advantages of the invention may be realized even if the spheres 12 are made of less strong material, since the spherical shape of the individual filler members is adapted to receive and to bear applied high stresses, such as compressional stresses, with a minimum of deformation. Over-all strength and rigidity of the inventive composite structures may be further enhanced by internally pressurizing the spheres 12 as with a pressurizing fluid such as air or an inert gas or other uid as will become apparent hereinbelow. Desirably, the pressurizing uid may be a lightweight gas, as helium or hydrogen, to further increase the strength-to-weight ratio of the pressurized spheres.

The spheres 12 may be of any reasonable dimension, depending 4upon the configuration of and the properties desired for the composite article, so long as the diameter of the filler spheres is small relative to the distance between the confining, spaced-apart panels; that is, the spheres are preferably small enough to accommodate a minimum of two, and preferably three or more, complete layers of close packed configuration between the confining face sheets. This size relationship is important in that so long as the spheres are small, they may be poured between panels of preformed, complex shape and/or curvature and still eiectively lill the volume between panels. Moreover, the stated minimum number of layers promotes strength uniformity in the sandwich when subjected to applied stresses of variable magnitude and direction. Thus, the invention contemplates spheres ranging in diameter from a few tens of microns Iup to several millimeters and larger depending on the minimum distance between panels. The thickness of the walls of the individual spheres 12 may also vary widely in accordance with the properties desired in the nished article and with the method utilized in the manufacture of the spheres.

FIGS. 3 and 4 show the spheres 12 deposited in an ideal packing arrangement. Such packing is conducive to minimization of excessive voids and to maximization of strength and rigidity. It is to be understood, of course, that complete achievement of the illustrated ideal packing arrangement is not necessary nor does it usually occur in normal practice. However, such packing arrangement of the spheres 12 is generally closely approximated in the practical construction of the articles of the invention which, consequently exhibit, substantially completely, the desirable attributes of that packing arrangement.

As illustrated in FIG. 5, the articles of the invention may additionally comprise end or edge members or panels 13.

It is to be further understood, of course, that the invention is not limited to but, indeed, explicitly contemplates shapes other than the simple flat shapes illustrated in FIGS. 3-5. Thus, an enormous advantage olfered by the present invention is the provision of methods and means for producing complex structural shapes without the attendant disadvantages and consequent high costs of the prior art. Thus, large constructional members of simple or compound curvature can be easily and quickly produced by simply lixing, in the desired spaced apart relationship, the preformed sheets or panels, of any shape whatsoever, and the preformed spheres 12 then simply poured therebetween. There is, therefore, no necessity for extensive, time-consuming and costly operation upon an integral filler or spacer material in order to conform the same precisely to the contours of the conning sheets or panels.

The hollow ller spheres 12 may be produced in any suitable manner, depending upon the size and the material of construction involved. Thus, for example, FIGS. l and 2 illustrate a hollow ller sphere suitable for use in the invention. As will be noted from those drawings, the spheres 12 may be formed in two, hemispheric sections, which sections may subsequently be joined together to form the completed sphere. Thus, in the case of the sphere shown in FIG. l, there is provided a first hemispheric section 14 having an inwardly directed offset portion 16 dem'ng a shoulder 17 and rim 18 adapted, respectively, Ifor the abutment and resilient holding of a second hemispheric section 19. Similarly, in FIG. 2, a first hemispheric section, designated generally by the numeral 21, is provided with an outwardly directed offset portion 22 defining a shoulder 23 and a flange 24 adapted, respectively, to abut and to resiliently hold a second hemispheric section, designated generally by the numeral 26.

A hollow iller sphere, as illustrated in FIG. 2, may be made, for example, by punch and die means as illustrated in FIGS. 7 and 8. Thus, in FIG. 7 there is provided a first die block 27 provided with a plurality of die cavities 28 for reception of lirst punches 29. By use of such apparatus, lirst hemispheres 21 may be easily and quickly made in large numbers simply by placing a flat sheet structure 31 upon the surface of die block 27 and actuating the punches 29. Similarly, second hemispheres 26 may be rapidly made in quantity by means of a die block 32 having a plurality of die cavities 33 for cooperation with a commensurate number of second punch members 34.

The spheres 12, of course, serve not only to space the panel members apart but serve also to fixedly hold the same in a predetermined position. This is accomplished, for example, by providing each of the spheres 12 with an overlying layer 36 (FIG. l) of a suitable bonding material such as a brazing alloy. The bonding medium may fbe applied to the spheres 12 by any suitable means, as by dipping the spheres in molten bonding medium, spraying, etc. Thereby, after the preformed hollow spheres are poured into place lbetween the confining panels, the entire assembly is heated to a temperature at which the bonding medium becomes sufliciently Huid to How about the spheres and to collect and be held by surface tension forces, in large part, at the points of contact be tween the individual spheres and between the spheres and the confining panel sheets. Upon subsequent cooling and solidiication of the bonding medium, the component parts of the assembly are converted into an integrally bonded, unied whole. The bonding of the composite parts of the assembly iby the bonding medium is illus trated in FIG. 6y wherein the bonds between spheres 12 are illustrated at 37 and those lbetween spheres and panels 11 are illustarted at 38.

As an alternative to the mechanical formation of the hollow filler spheres 12 illustrated iri FIGS. 7 and 8, the spheres may be formed in the manner illustrated in FIG. 9. Thus, in that drawing there :is illustrated an apparatus comprising a relatively elongated vertical columnar vessel 39 which may be provided with suitable heating means in a lower portion thereof, as heating coils 41. Heat loss from the vessel 39 may be minimized by providing an insulating cover 42 thereabout. A lower portion of the vessel 39 contains a first uid medium 43 comprising a metal from which it is desired to form the spheres 12. Floating on top of the first fluid medium 43 is a lighter, second uid medium 44. Substantially constant amounts of the two fluid media may be maintained within the vessel 39 by providing reservoirs 46' and 47, connected to the appropriate portions of the vessel 39 by inlets 48 and 49 respectively, the inlet lines being provided, respectively, with control valves 45 and 50. Pumps 51 and 52 are pro vided to supply fluid from reservoirs 46 and 47, respective ly, to the vessel 39. There is also provided adjacent the bottom of the vessel 39 an inlet line 53 which is connected to a vertically extending nozzle 54. A controller valve 56 is provided in the inlet line 53 and is connected to a pressurizing pump 57 which, in turn is connected, by supply line 58, to a source of pressurizing uid (not shown). A discharge chute 59 is provided adjacent the top of the vessel 39 and may have an exit extremity there of adjacent a suitable transferral means, such as conveyor 61.

In operation of the apparatus illustrated in PIG. 9, a pressurizing gas, the composition of which is depend ent upon the composition of medium 43, is pumped into the vessel 39 through nozzle 54 whereupon a bubble 62 is formed within the medium 43 and rises therethrough and passes into the lighter, immiscible medium 44, the upper portions of which are maintained at a temperature below the freezing point of medium 43 whereby the film of medium 43, comprising the skin of the bubble 62, freezes to form a solidified, hollow sphere 12. The spheres 12 oat and collect upon the surface of medium 44 and are removed therefrom, through the chute 59, to the conveyor 61 by means of which they may be transferred to the location of any subsequent processing to which it may be desired to subject them.

The particular process features utilized in connection with the operation of the apparatus .in FIG. 9 Will depend upon many factors, among the foremost of which are, of course, the nature of the bubble-forming medium 43. As an example of spheres which may be produced in the aforesaid manner, molten aluminum is provided as the medium 43 and over the aluminum is floated a molten mixture of halide or other substantially unreactive salts having a melting point below that of aluminum as, for example, a low melting mixture, such as a near eutectic, of certain salts, e.g. chlorides of sodium, potassium, lithium, calcium, etc. A suitable temperature gradient is set up in the salt layer as, for example, by the provision of suitable cooling means (now shown) adjacent the top of the vessel 39. A small jet of air is then blown into the molten aluminum through nozzle 54 thereby forming bubbles 62 having an inner, continuous aluminum oxide film wet exteriorly thereof with molten aluminum. As the bubble rises and passes across the aluminum-salt interface into the salt layer 44 the aluminum film solidifies, providing an air filled aluminum bubble.

By way of further example, the lower layer 43 may comprise molten zinc containing titanium to the limit of its solubility and which additionally contains iinely divided, suspended particles of titanium or titanium alloy. The upper layer 44 again comprises a molten mixture of halide salts having a melting point below that of the metallic layer 43. Bubbles are then formed in the metal slurry by means of a jet of inert gas, argon or helium, containing a small partial pressure of a gas such as air, carbon monoxide or a suitable carbonyl which will react with the slurry to form a stabilizing skin. After solidification by rising through the salt layer 44 and removal from the vessel 39, the metal bubbles are heated in a partial vacuum to remove the zinc and to Sinter the titanium particles thereby producing a continuous titanium sphere.

In a still further example, the lower layer 43- may comprise tin, aluminum or zinc containing dissolved sodium and the upper layer 44 may comprise a molten halide mixture containing a lower chloride of titanium or other metal of which it is desired to form the bubble and which is reducible by the sodium. In such case, as the sodiumcontaining bubble rises through the halide layer, there is formed on the surface thereof, by reduction of the metal chloride, a thin film of the corresponding metal.

The advantages of the invention are not restricted to the specified apparatus mentioned, but may lbe utilized in a wide variety of other, less rigorous applications. Thus, the low cost and inherently advantageous properties, such as great strength, low density, substantial absence of directionality, vis-a-vis applied stress, admirably suit these novel sandwich structures to architectural construction and the like. Thus, in addition to possessing high load bearing capacity, the inventive structures may inherently possess, depending upon the materials of construction, a high degree of soundproofng and/or insulating properties.

As noted, the articles of the invention exhibit substantially uniform resistance to deformation of various applied stresses. Thus, the resistance to shear stress applied at various angles to the plane of the suface panels is substantially as great as the resistance to compressive stress applied normally to the panels. The inventive articles have high resistance to tensile stress also. These prop erties distinguish the inventive articles from prior art sandwich structures with their variable directionality. Thus, corrugated sandwich filler is Weak in bending with the bend axis parallel to the corrugations. Honeycomb filler is weak in resistance to shearing movements of the face sheets. The sandwich structure of this invention, on the other hand, exhibits a high degree of uniformity of strength properties. Thus, although the inventive structures are not as strong as corrugated structures in the best direction of the latter, they are much stronger than a corrugated structure in its worst direction. My new structures are also exceptionally strong in resistance to applied shear stress-in contradistinction to honeycomb structures. My novel structures are also to be distinguished from sandwich structures comprising a foamed metal filler. In the latter type of articles, the interstices between the foam bubbles are filled with metal, thereby contributing significantly to the weight of the sandwich and, consequently, making such structures unsuitable for those applications requiring highest strength-to-weight ratios. Moreover, bubble wall thickness is uncontrollable in the foamed-type material, with a concomitant decrease in uniformity of properties. Additionally, use of the foamed metal procedure severely limits the choice of metals which can be utilized. The herein described inventive structures avoid those last-mentioned disadvantages.

By way of further example of specific structural articles in accordance with the invention, I contemplate sandwich structures comprising face sheets of a high strength, high temperature, low density metal, as titanium or its alloys and a filler of titanium spheres, the whole joined by a bonding medium comprising magnesiumin accordance with the teachings of my co-pending United States patent application Ser. No. 114,903, now abat-1- doned. Further examples comprise face sheets of titanium, aluminum or stainless steel face sheets and a filler of aluminum spheres, the whole joined by an epoxy resin bonding medium. I also specifically contemplate face sheets of titanium, titanium filler spheres and a low melting point titanium base brazing alloy, the latter as described and claimed (3% beryllium, balance titanium) in co-pending application Ser. No. 770,608 inthe name of Howard B. Bomberger, Jr. A further example of particular, useful embodiment of the invention comprises face sheets of titanium, titanium alloy or stainless steel and ller spheres of titanium, stainless steel or aluminum, wherein bonding is elected by thermosetting metal alloy pastes, as, for example, those described in my United States Patent No. 2,837,425.

What is claimed is:

1. A method of making a thin-walled hollow metal sphere, comprising forming a gas spherical bubble in a bath of molten, sphere-forming metal, said bubble being enclosed by a partially solidied layer of said sphereforming metal, passing the bubble into a layer of a liquid cooling medium overlying the bath, and cooling the bubble within the liquid cooling medium to form a solid, Wallorming skin of sphere-forming metal thereabout.

2. A method of making a thin-walled, hollow metal sphere, comprising forming a molten metal bath, injecting gas into said bath below the surface thereof to form a bubble therein, passing Vthe bubble into a' cooling zone comprising a molten salt, oflesser density and lower melting point than the metal, overlying the metal bath, cooling an upperextremity of the cooling zone below the freezing point of the metal, and passing the bubble into said cooling zone `whereby a metal wall thereof freezes to form the desired hollow sphere.

3. A method of making a thin-walled, hollow aluminum sphere, comprising forming a moltenf aluminum bath,

` form a self-supporting, sphere wall of aluminum.

References Cited UNITED STATES PATENTS 852,396 4/1907 Pease. 3,013,311 12/1961 Meissner 18-57 3,230,056 1/1966 Arant et al. 164-267 X 2,136,096 ll/l938 Benner et al 65-'-21 X 1,236,606 8/1917 Sanford 65--229 1,871,792 8/1932 Horseld 65-21 X '2,583,452 l/l952 Watts et al. 65-21 X 2,624,069 1/1953 Fisher 18-5 X J. SPENCER ovERHoLsER, Primary Examiner V. K. RISING,'Assistant Examiner

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