US2550879A

US2550879A - Bimetallic piston

Info

Publication number: US2550879A
Application number: US126627A
Authority: US
Inventors: Jr Charles E Stevens
Original assignee: Fairchild Engine and Airplane Corp
Current assignee: Fairchild Engine and Airplane Corp
Priority date: 1949-11-10
Filing date: 1949-11-10
Publication date: 1951-05-01
Anticipated expiration: 1968-05-01

Description

Patented May 1, 1951 BIMETALLIC PISTQN N o I Charles E. Stevens, Jr., Garden City, 31 8-.

Signor to 'Fairchild En po'r'ation, Farmingdale,

Maryland girls an A rpl 0911 N. Y., a corporation of Application November 10, Serial (o1; soc- 14) Claims. 1

, 'iiii sf mention relates to means nines anclhas ticular reference to theiinsertion of a' piston ring band into an aluminum piston, V

a It has been proposed heretofore to improve the wearing characteristics of aluminum pistons by inserting' bands near thepiston head in which the piston ring grooves are formed. Steel and cast iron have been suggested for such bands because of the improved wearing Capacity of these metals over aluminum and aluminum base alloys. However, becauseof the widelydifierent teem cients of thermal expansion between, cast iron and steel on'th'e' one, hand, and aluminum and aluminum base alloyson the other, it'has be en further proposed to incorporate in the iron I and steel an alloying metal such as nickel, which imparts thereto a coefiicient of ,thermalexpans' ion more nearly matching that of the aluminum or the aluminum basefalloyconstituting the body of the piston. However; even in 7 that case," the disparity in thermal expansion between "the in: serted band and the aluminum base body of the piston causes the aluminum to grow against the restraining iron or steel band during the heating cycle, which results in minute upsetting of the aluminum so that When it cools, the band will not fit as tightly as" it did originally. Subsequent he'ating cycles exaggerate this condition until the ring band finally'becom'e's loose in the aluminum base piston and failure occurs. Therefore, while helpful and desirable, high nick-e1 cast iron bands of high'coefiicient' of expansion inserted in aluminum pistons do not entirely solve xp bl mt v i c In accordance with the present invention, aluminumbaseand aluminum base alloy pistons having wear reducing ring bands inserted therein are provided in which the ring band is molecularly bonded to the aluminum under such conditions that a difference in coefficient of thermal expansion between the aluminum andsthe ferrous metal is insufiicient to produce a strain which will exceed thebond strengthduring cool? 1 ,0? h o 'af ruthe casting Operation; Accordingly th-e subsequent subjection of, the

piston to wide temperature fluctuations in operation will not result'in rupture. of .the bond, and the ringrband remains firmly seated in the piss ton, as has been demonstrated by rigorous testin actual use.

In m k nsth bim ta icp stonof thisinvention,;- a ring band of highv nickel cast iron of high oe ient erm e pansion or a ray i on ring b and of low modulus, of elasticity f rst coated with a thin film of molten aluminum overlying a ferro aluminurn alloy layperziat the interface and while the aluminum-coated ring ba d. i at a temp ra ur -a least as hi h: ast e. melting temperature at which the aluminum or a uminum a e a oy to bei as; the ody: o the piston is cast, and while the aluminum film Q .P1 ?P%n 1i mc e vorv a t c. t u num b Limei c st. a ound the. r n a i. p. :e r blyma ma m acc rdi o known casting techniques; When the piston is cooled and removed irom the mold, the band will be eql ul rlx b n to he aluminum thr u h th inte venin eh e qv-a uin m loy l y r. which is preferably held; to a thickness on the order of athousandth-cf an inch because of its r n l brittlemt a: a ,4 ciltiwi b seeni at. th me i ston .01 t i n nti n a i iqn zandtwearta t compo ite product having the properties of lightnessof lumin m str n th o thetca t i onor ste l ati mint Wh hev greatest wea u s and am equ ni ytr su i iter sh rt u a e nd whichis avoided by use of the piston of this invention. .c

a more complete un er a din of. .the, in.- uentionmeference may be had to the accompanying dra ing inWhichL i ,7 v

,l igure 1 isa, across-septicn through a bimetallic p t n f this i venti n; and t 1 .v Fig, 2iis an enlarged cross-sectionthrough the piston insert of Fig. 1 to illustrate the bimetallic arrangement in greater detail. a.

Referring ,to the drawing, numeral l0 designates arin aband of cast iron or cast steelinserted in a piston H ofcast aluminum or aluminum base alloy. The ring band 10 preferably is formed as acoinplete ring, without the piston ring groove I? which is machinedtherein' after the composite iron-aluminum piston is formed. Preferably also, the ring band II] is larger in outside diameter than the-piston, so that it may be properly located in a recess in the permanent mold into which the aluminum or aluminum base alloy is cast.

Prior to insertion in the mold, the cast iron or steel ring band I is heated to the temperature at which aluminum alloys with the iron or steel, preferably by immersing the band II] in a bath of molten aluminum or aluminum base alloy for a sufficient period of time to reach the said temperature, and then allowing the alloying action to take place between the surface metal of band and the molten aluminum or aluminum base alloy, until the desirably thin layer of thealloy and the overlying aluminum film are formed thereon. Although the ferro-aluminum alloy is necessary to a molecular bond between the aluminum and ferrous metals constituting the composite piston, the brittleness of the alloy requires that the layer be kept down to a thickness on the order of a thousand of an inch.

The coated ring band l0, while heated to a temperature such that the surface film of aluminum is still molten or plastic, is then placed in the permanent mold of the'desired configuration, and the molten aluminum or aluminum base alloy body metal ll cast around the ring band [0. Owing to the fusion union between the molten or plastic film of aluminum on the ring band l0 and the molten casting metal, the aluminum is homogeneous outwardly from the surface of the ferroaluminum alloy layer 13, which constitutes the molecular bond between the aluminum or aluminum alloy body I I of the piston and the ferrous metal ring band 10. Further details of the general process for forming such bimetallic structures may be had upon reference to the Whitfield et al. Patent No. 2,396,730, issued March 19, 1946.

As is well known, the thermal and mechanical or impact and wear stresses to which the piston is subjected in use are of considerable magnitude, and therefore, minimization thereof is desirable in the first instance. Hence, the ring band I0 is desirably formed of a high nickel cast iron containing about 12.5% nickel, or more or less, in order to reduce the disparity in'coeflicients of thermal expansion between the material of which the ring band I0 is formed and the material: ofwhich the piston body II is formed. One suitable commercial form is marketed'under the trade name Ni-resist. 'This thermal expansion disparity may be further reduced by utilizing as material for the piston body ll, an aluminum base alloy containing alloying ingredients reducing the thermal expansion of aluminum. One such alloy is aluminum containing 12% silicon, 2.5% nickel, 1.2% magnesium and 0.8% copper, and is known commercially as Alcoa, A132 alloy. Assumin that the aforementioned high nickel cast iron ring [0 and low expanding aluminum alloy body H are employed, an analysis of the stresses involved will show the conditions which enable the piston of this invention to withstand most rigorous usage without impairment beyond normal wear.

Assuming that insert [0 has an inside diameter at room temperature of 4.375 inches, its radial contraction (u) from the solidification temperature of the A132 alloy will be The solidification temperature, AT, as reflected by an identical shrinkage in A132 alloy is Adding a room temperature of 70 F. brings the temperature up to 962 F. This is an indication that the restraint offered by the cores in the mold give a casting having a final size which is the equivalent of a casting obtained with unrestrained cooling from a hypothetical solidification point of 962 F. This indication is reasonable since it has been found that the restraint of a core is often more than this. To carry the analysis further, consider Alcoa 142 alloy containing 4% copper, 2.0% nickel and 1.5% magnesium, as substituted for the A132 alloy, and using the AT of 892 F. as applying to'that aluminum alloy and 1070 F. as applying to the high nickel cast iron of band 10, then an 142= 2.187X14.9 1O- 892 (1) 029' um: 2.187 X 10.7 10- X (1140 -70) (2) w .025" um 142'I.LNi= .004" (3) solidification temperature of #142 Alloy.

This shows that if the composite structure is to remain intact, then a force must be imposed on the insert 10 at the bond I 3 which is great enough to cause a radial distortion inward of .004". In order to obtain this factor, the principles of hydrostatic pressure are employed as applicable here because the conditions are the same, considering that the freezing aluminum alloy is essentially a liquid. It follows that Considering the inner surface of an inserted ring band ID to be 1" long axially, the strength contribution of the bond at I3 is at least 9,000 pounds, and the two lateral edges of the band are stressed in shear. With an end area for a 1" band of .563 1.000 2=1.13 sq. in., and assuming a reasonable shear value of 6,000 pounds per square inch, the unit force will be 6,000 l.13=6800 pounds, or a total of all forces of 15,800 pounds contributed solely by the bond at [3 and neglecting the shrink pressure on the sides of the insert which is negligible. This analysis accordingly shows that the bond at l3 between the insert 10 and the aluminum body of a piston is adequate to withstand the stresses of use, and many hours of use of the pistons of this invention demonstrate that the computation s is conservative, as there have been no failures of such pistons. I I

Although it has been suggested heretofore "that high expanding ring band's be used, although without use of the "essential bond heretofore described, applicant also has found that "gray cast iron/oi low modulus of elasticity can be used as ring band material with beneficialresuits providing that gray iron of a modulusbelow 16x19 is selected, preferably between 11x10 and 15 x Assuming the same conditions as before with Alcoa A132 alloy as the piston body material and a low modulus gray cast iron as the band material, then adapting Formulae -l, 2 and 3 as 1', 2' and 3, it follows that Substituting in Formula 4 and changing values as indicated:

=8200 lbs/in.

or a stress condition only 30% more than the case of high nickel cast iron and Alcoa 142 alloy which has withstood many hours of rigorous engine testing.

When the use of low modulus gray iron with Alcoa 142 alloy is considered, an artificiallyproduced distortion of .029.01'7 or .012 is produced in the aluminum alloy. substituting this value for .007 in Formula 4' gives an indication of stresses of the order of 14,000 pounds per square inch unit force, which considering the conservatism of the computation, serves amply to explain why such pistons showed no sign of failure in an engine after over 500 hours of rigorous usage.

The foregoing computations are based on the assumption that for relatively short axial lengths of insert Hi, axial stresses can be neglected. Howover, where the axial length of an insert like iii is materially greater than the 1 inch length assumed above, the axial stress is a very considerable part of the whole stress and, therefore, cannot be neglected. However, the strength of the bond at I3 of the piston of this invention provides a large factor of safety and hence, the band In may be safely widened axially to accommodate the next adjacent groove 12, if desired, although the upper one or two rings usually sulr'er the greatest wear and hence, need the iron band support more than lower rings.

Although a preferred embodiment of the bimetallic piston of this invention has been illustrated and described herein, it is to be understood that the invention is not limited thereby, but is susceptible of changes in form and detail within the scope of the appended claims.

I claim:

1. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength, a band of a high strength ferrous metal encircling said body portion and embedded therein, and a film of an alloy of the body portion metal and the ferrous metal of the band interposed between the body and the abutting band surfaces for bonding them together.

2. A piston for an internal combustion engine, comprising a cylindrical body portion formed Q' -eases 6 or a light than .of relatively 10w stren th or the class consisting of aluminum and aluminum base alloys, a band of a high strength ferrous metal encircling said body portion and embedded therein, and a film of ferro-aluminum alloy interposecl between the body and the abutting band surfaces for bonding them together.

3. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength, a band of a high strength ferrous metal having a mod ulus of elasticity below 16 10 and encircling said body portion and embedded therein, and a film of an alloy of thebody portion metal and. the ferrous metal of the band interposed between the body and the abutting band surfaces for bonding them together.

4. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength, a band of a high strength ferrous metal having a modulus of elasticity of between about 11x10 and about 15 10 and encircling said body portion and embedded therein, and a film of an alloy of the body portion metal and the ferrous metal of the band interposed between the body and the abutting band surfaces for bonding them together.

5. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength, a band of low modulus cast gray iron encircling said body portion and embedded therein, and a film of an alloy of the body portion metal and the ferrous metal of the band interposed between the body and the abutting band surfaces for bonding them together.

6. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength, a band of cast ironhaving a modulus of elasticity below 16x10 and encircling said body portion and embedded therein, and a film of an alloy of the body portion metal and the ferrous metal of the band interposed between the body and the abutting band surfaces for bonding them together.

'7. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength, a band of cast iron having a modulus of elasticity between about 11 10 and 15 10 and encircling said body portion and embedded therein, and a film of an alloy of the body portion metal and the ferrous metal of the band interposed between the body and the abutting band surfaces for bonding them together.

8. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength of the class consisting of aluminum and aluminum base alloys, a band of a high strength ferrous metal having a modulus of elasticity of between about 11 10 and about 15x10 and encircling said body portion and embedded therein, and a film of ferro-aluminum alloy interposed between the body and the abutting band surfaces for bonding them together.

9. A piston for an internal combustion engine, comprising a cylindrical body portion formed of a light metal of relatively low strength of the class consisting of aluminum and aluminum base allows, a band of low modulus cast gray iron encircling said body portion and embedded therein, and a film of ferro-aluminum alloy intere posed between the body and the abutting band surfaces for bonding them together. 10. A piston for an internal combustion engin comprising a cylindrical body portionformed of a light metal of relatively low strength of the class consisting of aluminum and aluminum base alloys, a band of cast iron having a modulus of elasticity between about 11x10 and 15x10 and encircling said body portion and embedded therein, and a film of ferro-aluminum alloy interposed between the body and the abutting band surfaces for bonding them together.

CHARLES E. STEVENS, JR.

REFERENCES CITED 7 The following references are of record in the file of this patent:

8 UNITED STATES PATENTS Number Number 15 581,527 566,312

Name Date Deputy June 2, 1931 Koch Feb. 6, 1934 Mahle Nov. 6, 1934 Deputy July 12, 1938 Graham Sept. 2, 1941 McCullough et al. Dec. 9, 1941 White et al Apr. 7, 1942 Gay June 9, 1942 Deputy Sept, '7, 1943 FOREIGN PATENTS Country Date Great Britain Oct. 16, 1946 France Nov. 20, 1923