US2797460A - Casting light metal against iron and article formed thereby - Google Patents

Description

July 2, 1957 CASTING M. G. WHITFIELD 2,797,460

LIGHT METAL AGAINST IRON AND ARTICLE FORMED THEREBY Filed Sept. 16, 1952 2 Sheets-Sheet l INVENTOR. Mm sfldu G #411 #7540,

QMI QIZZ ATTORNEYS.

M. G. WHITFIELD July 2, :1957

' CASTING LIGHT METAL AGAINST IRON AND ARTICLE FORMED THEREBY Filed Sept. 16, 1952- 2 Sheets-Sheet 2 INVENTOR. Mars/m GI IVIIIrF/na,

alluw A1 TOR N EYS.

CASTING LIGHT METAL AGAINST IRON AND ARTICLE FORMED THEREBY Marshall G. Whitfield, Garden City, N. Y., assignor m Whitfield & Sheshunolf, Inc., Garden (Iity, N. K, a corporation of New York Application September 16, 1952., Serial No. 309,773

. 7 Claims. (c1. 22-404 My invention relates to the problem of producing composite articles of heavy and light metals, of which the formation of aluminum forgings or castings with steel or cast iron inserts will serve as an illustration. For example, ithas been proposed to make aluminum pistons with grooved cast iron inserts of annular formation to receive the rings with which the piston will be used in service. Great difficulty has hitherto been encountered in forming a satisfactory and satisfactorily permanent bond between the aluminum and the iron.

Eflortshave been made tosecure bond by mechanically roughening, knurling or grooving the iron inserts, but without eifectingsatisfactory results. The procedure of first coating'the iron insert by hot-dipping with aluminum has been proposed, and in this connection the employment of ,variouspreliminary coatings'on the iron of metals morerea'dily wet'by. the molten aluminum has been ,suggested; Typical patents in this field are the between a light metal and a ferrous base without the necessity of a hot-dipping procedure.

These and other objects of the invention which will be set forth hereinafter or will be clear to the skilled worker in the art upon reading these specifications, I accomplish by that procedure and in that article of which I shall now describe exemplary embodiments. Reference is made to the accompanying drawings in which Figures 1 to. 7 inclusive (hereinafter more fully described) are photomicrographs of structures in which aluminum is bonded to iron.

; formed.

Whitfieldef al. Patent 2,396,730 and Deputy 1,807,689.

The teachings of this application relateto light metals 1 which. are to. be bonded to ferrous bodies. The light metals are. inclusive of-magnesium, aluminum, alloys of magnesium and aluminum, and alloys of aluminum, by which lmean allo'ys consisting preponderantlyof aluminumbut including other metallic ingredients such as silicon, copper, nickel, iron, vanadium and others or combinations of them. Hereinafter I shall describe my invention in connection with aluminum or aluminum alloys in certain specific embodiments, its application to other light metals being obvious in the light of the foregoing. By-ferrous bodies I mean to include irons, mild steels, and castirons, with or without alloying ingredients frequently-found in such materials. I

A principal object of my invention is the provision of a method which is simple, straightforward and economical :for producing a proper bond between a ferrous body and a light metal cast in'place. While ordinarily the ferrous body will be located in a mold and a'light metal body formed about it by casting, my, procedure is inclusive of one Where both 'a ferrous body and a light metal forging or the like are located in a mold and are bonded together by casting molten light metal between the two. 7

/ which it is located in a mold and additional light metal It isan object of my invention to provide a method of forming an excellent bond between a light metal and fa ferrous body when the light metal contains sub stantial quantities. of alloying ingredients ordinarily interfering with bond formation. g 1

Itis an object of the invention to form a proper bond Briefly, in the practice of my invention, the ferrous body is thinly coated with a metal easily wetted by molten aluminum or other light metal, the body is then located in a mold wherein it-is at an elevated temperature but below the melting temperature of the metal being cast against it. The molten light metal is then caused to flow into the mold and over the surfaces of the ferrous body to which it is to be bonded in such fashion that there is a substantial flow of the molten metal past these surfaces with a flushing action as will hereinafter be explained. These factors will now individually be discussed.

It has been known that in hot-dipping processes aluminum or'other light metal will not adequately wet the surface of iron if that surface is covered with oxide.

It is necessary in such processes to maintain contact between the molten metal and the iron for a suflicient time to permit the solution or. reduction of the oxide. Thereupon wetting occurs and an interface alloy is It has been suggested that this condition could be ameliorated by'firstcoating the iron with a metal having a lower melting'point than the light metal and more readily wet by it than iron. Such metals are in general 1 shorter immersion time is effected. The result of immersion, however, is the formation of a coating of the lightmetal on the ferrous base accompanied by a-sharply' defined interface alloy layer. In the case of a'tin coating on iron which is'then hot-dipped in aluminum, the

interface alloy layer isprobably'a tin-iron-aluminum alloy. The alloy layer is somewhat rough on the light metal side of the composite, but usually appears relatively smooth andsharply demarked on the iron side of the composite.

In the procedures of the patents noted above a ferrous body is first hot-dipped in the molten light metal, after cast against it; The casting of additional light metal in molten condition against a previously formed hot-dip coating does not diminish the interface alloy layer, but

on the contrary has a tendency to increase its thickness, if anything; While in general in casting procedures better results are secured by casting the light metal against a previously formed hot-dip coating, it has been found that a source of weakness lies in the interface alloy layer. For example, aluminum pistons having cast iron inserts exhibit a fracturing of the bond, either immediately after formation, or after a very short service life. In exaggerated instances, the ferrous body may be entirely loosened from the surrounding light metal casting.

In most instances the bond fractures in part only initially, the fracture tending to spread during the service life of the article.

In the practice of my process I employ on the ferrous body a coating of metal such as tin, zinc or cadmium which will promote wetting, but the step of hot-dipping is; omitted by control of the other factors mentioned above;

when the molten light metal is cast against the ferrous Patented July 2, 1957 body ready wetting occurs; but the characteristic hot-dip alloycoating-'doesmob appear tmbe formedr In-some-- instances there is a visual absence of alloy coating when the composite structure issectioned, etched, and viewed under the microscope: Inother instances-the interface alloy layer appears not only to be diminished in deptlr but-also to have-a ditfercnt fo'rm or configuration; The

nature of the bond is greatly; improved.

The degree-of preheat of the ferrous body-in the mold will vary= wit-hits thickness and" the -proporti0n-of casting" metal-to the siz'eof the-'insert or'body. But the body will be heated to atemperature -below-the-melting point of the molten coating metah One-result: ofthis appears to be: the formation, in place of=or adjacentto theinterface alloy layer, of a zone in which alloying constituents-of the-molten flight metalare precipitated in very-finely divided conditiont- Without wishing to be bound bytheory, I believe-'that'thiseffect results=from -a chilling of the'molten-unetal adjaeentthe inSert-or body, and that the formation 0f= a zone'of-fine precipitation substantial-lywimproves the nature-' of the bond; By this-I mean toindicatesthat, whereas-with a highly alloyed light-metal (say. an: aluminum containing -large quantities of silicon and/or other metals) more=diflicultyis ordinarily encountered in forming an-adequate bond with iron, asubstantially betterbond appears toresultwhen thereis formed the fine precpitation Zone aforesaid.

Another' important characteristic of my process is the carrying on ofthe castingoperation in such fashion thatthemoltenmetal being'cast flows over and aroundand flushes: the: surfaces of the metal body or insert for an appreciable interval of time. Again without wishing to be-boundby theory, I believe that this flushing action has the effect ofbreaking up oxide layers which always exist on the surfaces of molten aluminum and magnesium and their alloys even when a non-oxidizing atmosphere is maintained. over theplated surfaces of the ferrous bodies. I believe that by the flushingaction the surface of the ferrousbody are cleansed of occluded gases while any oxides on the surface of themolten light metal areparted, allowinga clean metal-to-metall contact and inter-solution.

Whether the flushingxaction has any effect on the formation of the interfacealloy, or to whatextent this is affected by the-formationlof the fine rprecipitatiomzone, or whether the interface alloy is minimized or obviated merely by casting the molten light metal against the relatively cooler-surfaces ofthe ferrous body, is not.known.

However; afterpracti-ce of my invention, examination of theinterfacebetweenthecast metal and the ferrous body indicatesuthat the formation of iron-aluminum alloy isv tosecure the efiect or flushing by deliberate agitation of the molten metal bythe time it has solidified, so as to cause it to move'across the surfaces of the ferrous body. Inmostinstances an adequate effect of fiushing can most readily be secured by continuingthe, pouring after themold has filled, so that excessmetal exits from the. risers.v

Asv the castingmetalgoes .past, the plated-insert of ferrousbody, the plated metal, having a-lowerimelting point than the molten castinganetal, is caused to flow and initiate a bonding action with the surface'metal of", the ferrous body. The molten castingmetalthat finally .comes to rest ,againstthe ferrous body is, under. rthespreferred conditions of my process, the hottest metal in the-casting.

This helps in the absorption of the; plated metal xintothe- 4 casting metal and in this way presumably removes a potential source-of weakness at the bonding zone.

The above teachings will now be contrasted with prior art procedures, and illustrated by reference to certain examples. Figures 6 and 7 are typical photomicrographs at a magnification of 250 of different composite structures. In each of the figures the cast iron is the lowermost layer, and thealuminum theuppermo'st layer. Between the-two there will be seen a sharply demarked alloy layer, very thick in the case ,of Figure 7. In each case the cast iron body was electro-tinned, then dip-coated in a molten, high silicon-aluminum bath, followed by castinga high siliconaluminum alloy against the hot coated surface in a mold. The aluminum alloy. was. a material-produced in accordance with the teachings of the Schwarz Patent 2,131,076, and contained from 21 to 23 percent of silicon together with small amounts of nickel, copper, vanadium and magnesium, the balance being aluminum. The alloy is known as vanasil, and is an advantageous alloy for the formation of pistons for internal combustion engines by reason of its heat expansion characteristics. It may be noted that alloying constituents of the molten coating metal precipitate'd in largemasses adjacent to and sometimes actually insidethednterface' alloy layer.

Thebonds of the articles illustrated in Figuresfi and 7',

were foundto be functionallydeficient;

Example. 1.

A' cylindrical tube with, a variable diameter ranging fromiapproximately 1.8 inch to approximately. 3 inches,

made of low carbon steel and having a thickness of .015 inch was bright annealed in a hydrogen furnace, electro plated with tin in'a conventional alkaline tin plating bath toa thickness of .0002 over the outer. surface. sandcores were fitted into the ends .ofthe tube to fill'it and-support him an outer mold which was also made of baked sand. The tube was placed in the sand mold, which was so gated that'the'metal'to be cast would flow past and up around the tin plated tube and out the top of the mold; An aluminum alloy (Alcoa 355) at a temperature of 1380 was poured into the gate so as to flow past the surface of the tube for approximately 10 to 20 seconds. When the casting had-so1idified an excellent'bond was seen to have developed;

A sectionof the composite" article, etched with nital, is shown in'Figure 4 of the drawings. Again the-lower layer represents the steel, the upper layerthe aluminumalloy andtheintermediatelayer an alloy bond; The thickness ofthe alloy bond is'substantially diminished as can be seen by comparing Figure 4 with'Figure 7, and there is no irregular penetration of'the basewhich 'commonly occurs with low carbon steel. In this particular article there is no visual indication of a fine precipitation A grey iron ring 5% inch in outside diameter and 4 inch in inside diameter, having a thickness of inch was plated with tin inan alkaline (sodium stannate) .tin bath to a thickness of .00015' inch. It wasplaced in an iron mold which had been heated to atemperature of about 600 .to.700 E; and when the ring itself had risen in temperature of approximately 400 F., molten: high silicon-aluminum; alloyat a temperature of about 1525' F. was poured over the ring surfaces on whichbbndingwas desired vin-such a way that thevmoltenz-metalifiowed over the surfacesof the ringforS to 10 seconds This pistonringinsert casting was then placed in an: electric heat treating, oven for one hour at a temperatureof .960? 'F.,

cooledin an air blast, placed in anelectric ,oven for three,

hours at 310 F., andslowlycooled to room temperature.

Baked It will besample shows no visual'evidence of the typical iron'-. Instead there lies above aluminum interface alloy layer. the iron a layer of aluminum containing its alloying constituents precipitated in a very fine state of subdivision.

Example 3 Figure 2 is a photomicrograph at 250 X magnification of a section of a similar structure similarly treated excepting that the conditions were not so carefully controlled. Here again vanasil was cast against an electro-tined cast iron insert ring. In the case of the particular section, a thin layer of recognizable interface alloy is shown overlying the iron lower layer; but there is clearly apparent above it a line precipitation zone in which the alloying constituents appear in finely divided form. The bond produced in the article sectioned in Figure 2 was tested and found to be highly satisfactory.

Figure 2 is an illustration of the fact that relatively slight variations in temperature or in the volume and duration of the fiow of molten metal past a surface to which it is to be bonded, will produce visual variations in the bond itself. Moreover in different parts of a given mold, or in different areas of the surface of a given insert there may occur variations in temperature and in volume flow of the molten metal which will produce visual differences in the bond. The best possible results appear to be attained when the ferrous insert or body is heated so that the coating of tin, zinc or cadmium on its surface is rendered fluid in the presence of an inert or reducing gas such as argon, helium or hydrogen, and when the greatest possible volume of flow of the molten light metal or alloy over the surfaces of the insert or body is attained. Even with the variations which have been noted, however, the teachings herein set forth will produce a bond which is mechanically adequate, and far superior to any bond which has hitherto been produced by other procedures.

Example 4 A composite article was produced by the method outlined in connection with Examples 2 and 3 excepting that the cast iron body was electroplated with cadmium instead of tin and the aluminum alloy which was cast against it contained only 12 percent of silicon instead of the 21 to 23 percent characteristic of the vanasil mentioned above. Figure 3 is a nital etched section at a magnification of 250 x. The cast iron is the lower layer and the aluminum alloy the top layer. Between the two there will be seen some slight evidence of an alloy inter face. In spite of the lower content of silicon a fine precipitation zone is clearly apparent. The bond was good.

Example 5 Two circular steel sheets approximately .10 inch thick were electroplated with tin to a thickness of about .0001 inch and were placed in a mold adjacent to an alloy aluminum forging which had been preheated to a temperature approximately 100 F. below the melting point of the aluminum alloy, under an atmosphere of argon or helium and molten aluminum alloy of the same analysis as the forging and at a temperature of approximately 1400 F., was poured into the mold so as to flow over the tin-plated surfaces of the iron bodies, filling the space between the sheets and the preheated forging, bonding to both simultaneously. The aluminum alloy was one known as 7-5 and contained 5 percent silicon, 7 percent copper and .5 percent nickel. The bond was good.

Example 6 An iron insert was electroplated with zinc, and was The sectioned I then heated in a protective atmosphere (to minimize oxidation of the platedlayer) to a temperature a littleabove the melting point of zinc. An aluminum alloy containing 5 percent silicon, 7 percent copper and 0.5 percent nickel at a temperature of 1375 F. was poured over the insert.

art, although the bond was not as good as that, for

example, of Figure 1.

The conclusions which I have reached in the development herein outlined are that superior bonds can be attained between a ferrous body and a cast light metal structure if instead of hot-dipping the ferrous body the light metal is so cast against it as to flush the surface of the ferrous body for an appreciable interval of time. The use of a plated coating of tin, zinc, cadmium or the like greatly promotes wetting but is not suflicient to assure a good bond with cast light metal in the absence of the flushing action which has been described. Preheating the ferrous body in a non-oxidizing or reducing atmosphere appears to help in preventing lack of wetting by reason of oxides interposed :between the molten light metal and the ferrous body, and to this extent is a partial substitute for the flushing action. But inasmuch as the flushing action will produce a superior bond to that produced when the molten light metal is simply cast against the plated surface of the ferrous body even though that surface was treated in and protected by a reducing atmosphere, the flushing action is more properly regarded as a primary inexpensive and extremely effective way of eliminating oxide films which prevent wetting. The particular preferred conditions outlined result in a new type of bond characterized by a diminution and in some instances elimination of the alloy interface. A chilling of the molten metal resulting from a lower temperature of the ferrous body is of advantage in promoting bond by causing a finely divided precipitation of alloying constituents in the light metal, and this is of espectial value when the quantity of alloying constituents such as silicon, copper, nickel, vanadium and the like is large. The temperature of the iron body, considering its mass should not be so low that a plated coating of tin, cadmium or zinc cannot be raised 7 above its melting point by heat from the molten light metal. With this lower limitation, however, the advantage appears to lie with lower temperatures of preheat of the ferrous body rather than temperatures approaching that of the molten coating metal. The visual appearance of a fine precipitation zone is apparent in my etched samples only when the quantity of alloying constituents present is fairly high; but I believe that the value of the fine precipitation of alloying constituents extends to fairly low quantities, and that the chilling effect is worth while at least down to contents of 5 percent silicon or less.

Modifications may be made in my invention without departing from the spirit of it. Having thus described my invention in certain exemplary embodiments, What I claim as new and desire to secure by Letters Patent is:

1. A process of casting a light metal against a ferrous body which comprises electroplating the said ferrous body with a wetting metal chosen from a class consisting of tin, zinc and cadmium, locating the ferrous body in a mold, the said body being heated to a temperature approaching but below the melting point of the light metal which is to be cast thereagainst, said temperature being chosen in the light of the effective mass of the said ferrous body, the temperature of the light metal and the length of time of the casting operation, so that during the casting operation the said wetting metal will be 7, liquefied while atthe same time thetsaid ferroumbody will, produce a chilling action on the light metal, cast thereagainst, and casting, against the said treated ferrous body alight metal chosen froma class consisting: of aluminum and alloys of aluminum and other metalsin which alloys, aluminum is preponderant, andcausingthe.

moltenlightmetalto flow against-andpast, and flush the exposed surfaces of the;ferrousubody fora .suflicient length:of:time to remove oxidefilms.between;the..light metal and the the surfaces of the ferrous .bodyand, carry the saidioxidefilms awayirom'theferrous body.

2. The process claimedzainclaim'l in which the flush ing action is accomplished. by providing entrance and exit means in connectionwith the mold, so. locatingtthese thatmetal issuing from .the. exitmeans must. have-flowed. over the surfaces of the ferrous body, and continuing the pouring for a substantial interval after the mold has been filled.

3. The process claimed in claim.2 in which the said wetting metal is tin. V

4. The process claimedsin claim 3 wherein said light metal is an aluminum alloy rich in silicon, said chilling action serving to causefinelydivided precipitation of the. said silicon adjacentthe interface between. thesferrous body and the light metal castthereagainst.

5; The process claimed in .claim 4wh'erein. the article 8.1 7. ,A',comppsite article,produced in accordance with claim 4,

References :Citedinthe file of this patent UNITED STATES PATENTS 286,459 McCarthy Oct. 9, 1883 534,979 Stark Feb. 26, 1895 1,043,577 Eldred Nov. 5, 1912 1,528,947 Preston Mar. 10, 1925 1,669,071 Thomas May 8, 1928 1,732,515 Hunter Oct. 22, 1929 1,738,056 Hunter Dec. 3, 1929 1,938,707 Mann Dec. 12, 1933 2,265,243 McCullough et al. Dec. 9, 1941 2,435,991 Whitfield Feb. 17, 1948 2,544,670 Grange et a1 Mar. 13, .1951 2,544,671 Grange et a1 Mar. 13, 1951 2,634,469 Pershing et a1 Apr. 14, 1953 2,672,666 Enfer et a1 Mar. 23, 1954 FOREIGN PATENTS 351,977 Great Britain June 25, 1931 428,586 Great Britain May 15, 1935 634,599 1 Great Britain Mar. 22, 1950 OTHER REFERENCES

Claims (1)

1. A PROCESS OF CASTING A LIGHT METAL AGAINST A FERROUS BODY WHICH COMPRISES ELECTROPLATING THE SAID FERROUS BODY WITH A WETTING METAL CHOSEN FROM A CLASS CONSISTING OF TIN, ZINC AND CADMIUM, LOCATING THE FERROUS BODY IN A MOLD, THE SAID BODY BEING HEATED TO A TEMPERATURE APPROACHING BUT BELOW THE MELTING POINT OF THE LIGHT METAL WHICH IS TO BE CAST THEREAGAINST, SAID TEMPERATURE BEING CHOSEN IN THE LIGHT OF THE EFFECTIVE MASS OF THE SAID FERROUS BODY, THE TEMPERATURE OF THE LIGHT METAL AND THE LENGTH OF TIME OF THE CASTING OPERATION, SO THAT DURING THE CASTING OPERATION THE SAID WETTING METAL WILL BE LIQUEFIED WHILE AT THE SAME TIME THE SAID FERROUS BODY WILL PRODUCE A CHILLING ACTION ON THE LIGHT METAL CAST THEREAGAINST AND CASTING AGAINST THE SAID TREATED FERROUS BODY A LIGHT METAL CHOSEN FROM A CLASS CONSISTING OF ALUMINUM AND ALLOYS OF ALUMINUM AND OTHER METALS IN WHICH ALLOYS, ALUMINUM IS PREPONDERANT, AND CAUSING THE MOLTEN LIGHT METAL TO FLOW AGAINST AND PAST, AND FLUSH THE EXPOSED SURFACES OF THE FERROUS BODY FOR A SUFFICIENT LENGTH OF TIME TO REMOVE OXIDE FILMS BETWEEN THE LIGHT METAL AND THE SURFACES OF THE FERROUS BODY AND CARRY THE SAID OXIDE FILMS AWAY FROM THE FERROUS BODY.

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