Metal treatment – Stock – Aluminum base
Reexamination Certificate
1999-02-22
2001-08-14
Wyszomierski, George (Department: 1742)
Metal treatment
Stock
Aluminum base
C420S548000, C420S554000
Reexamination Certificate
active
06273970
ABSTRACT:
The present invention relates to an aluminum alloy having therein a homogenous distribution of bismuth, and to methods for its continuous casting.
A typical slide bearing alloy consists of three main materials: a relatively hard matrix material (aluminum or copper), a combination of soft components for providing self-lubricating properties to bearings, and small quantities of various additives which modify the structure and properties of the matrix metal.
The most common Al-based engine bearing alloys comprise 6-20% of tin as a soft component. Al—Sn alloys may be produced by conventional casting methods; however, the lubricating properties of Sn are low, as compared to materials such as Pb and Bi.
An additional problem with aluminum-tin alloys is that tin forms continuous net surrounding matrix grains in Al—Sn alloys, causing these alloys to have a relatively low fatigue resistance.
Aluminum base alloy bearings containing lead as a soft component, are of a higher quality than Al—Sn bearings. Higher seizure resistance is achieved in Al—Pb bearings at 2-3 times that of lower soft phase contents. In addition, lead is dispersed throughout the aluminum matrix in the form of separate spherical particles. These properties enhance the fatigue resistance of engine bearings containing lead.
Despite the evident advantages of aluminum-lead bearings, this alloy is not widely used because of two main manufacturing problems: (1) the low miscibility of lead in liquid aluminum; and (2) the large difference between the densities of the two metals. These problems result in the gravitational segregation of heavy lead droplets during the cooling down and solidification of the Al—Pb alloy. Therefore, conventional casting methods do not enable the manufacture of a homogeneous aluminum matrix containing lead particles.
Metallurgical problems of the Al—Pb alloy also relate to another aluminum base system, Al—Bi. Bi also has limited miscibility in liquid aluminum. Furthermore, the density of liquid Bi is four times higher than that of liquid aluminum. Therefore, conventional casting of Al—Bi alloy causes gravitational segregation of the heavier phase Bi in the bottom region of the casting.
In contrast to lead, bismuth is an environmentally friendly metal. Because bismuth possesses most of the properties of lead, including self-lubrication, several attempts to make aluminum-bismuth engine bearing alloys have been made.
An aluminum base alloy, containing bismuth in a quantity of 4.25-7 wt. %, was proposed in U.S. Pat. 4,590,133. The aluminum-bismuth alloys of said Patent were demonstrated to have excellent anti-seizure properties. In order to impart wear to the alloy, 2-2.5% Si was added, and 1.25-2.3% lead was added to enhance surface property. Small additions of about 1% Cu were made to increase the strength of the Al—Bi material. The addition of a number of other additives was proposed, such as nickel, manganese, chromium, tin, antimony and zinc.
It is noted in said Patent that there are practical limitations to the amount of bismuth which can be accommodated in an aluminum alloy produced by a casting process, because of the liquid immiscibility of aluminum and bismuth. This was probably the reason why a relatively low, maximum Bi content of 7% was described therein.
Test results presented in said Patent demonstrate the superiority of Al—Bi alloys to aluminum-tin alloy. However, some of the results are contradictory; this contradiction may be attributed to non-uniform distribution of Bi particles in the aluminum matrix. A typical example of the inconsistent results reported in said Patent is found with regard to the description therein of an alloy containing only 3% of Bi and 4.3% of Si. The tensile strength of this alloy, presented in said Patent, is 16,419 psi. Such a low value of tensile strength of Al—Si alloy (lower than that of Al-20% Sn) must be assumed to be caused by very bad bismuth distribution (large Bi particles and gravitational segregation of bismuth). This example shows the importance of both a proper metallurgical structure of Al—Bi alloy for engine slide bearings and a method of casting which enables producing such a structure.
U.S. Pat. No. 5,286,445 teaches an Al—Bi alloy having different additives, including Zr, which precipitates during thermal treatment after rolling operations and causes the division of stretched-out Bi particles. This method achieved fine bismuth inclusions, but is not able to prevent the gravitational segregation of Bi. In addition, the method does not control the size of Bi particles formed during solidification. This may cause the formation of a coarse Bi cast structure, resulting in long Bi ribbons which divide into fine grains during subsequent annealing operations. However, these fine inclusions form long chains, which considerably decrease the fatigue resistance of the bearing.
Bi has also been proposed as an auxiliary supplement for improving the seizure resistance quality of alloys, but the quantity of Bi is either relatively low (~2%), e.g., as described in U.S. Pat. No. 5,122,208, or difficulties are declared in the preparation of the alloy, because of a non-uniform distribution of Bi, e.g., as described in U.S. Pat. No. 4,471,032.
Thus, it is seen that no method has been disclosed for producing aluminum-bismuth alloys with fine, homogeneous Bi dispersion.
Because the two systems Al—Pb and Al—Bi are very similar to each other, the methods used for producing aluminum-lead may also be used for producing aluminum-bismuth.
One method which has been proposed for the continuous casting of a homogeneous alloy consisting of immiscible metals is described in U.S. Pat. No. 5,333,672. The method of said Patent comprises cooling and solidification of the alloy under crossed electric and magnetic fields and modifying the gravitation force acting on the alloy components. The method determines the values of intensity of the electric and magnetic fields which provide indifferent equilibrium of the alloy components. The method also takes into account the fact that sizes of dispersed inclusions depend upon the cooling rate of the melt and upon deviations in the intensities of the electric and magnetic fields. In order to obtain a mean particle size of the dispersed phase having the predetermined value, cooling was described as being carried out according to the formula:
V
>
Tcm
-
Tkp
nd
(
1
)
wherein:
d is average particle size, &mgr;m;
n is an empirical coefficient, equaling 3≦n≦30 sec/&mgr;m;
v is the cooling rate of the melt, degrees/sec;
Tcm is the temperature of the melt at which the components are in a state of molecular solution, ° C.; and
Tkp is the crystallization temperature of the melt, ° C.
Unfortunately, the range of empirical coefficient n determined in U.S. Pat. No. 5,333,672 is too wide, and therefore it does not enable a person skilled in the art to use the relationship in calculation for real systems. In addition, said patent does not take into consideration the fact that sizes of metal microstructure elements depend not only on the cooling rate, but also on the concentration of nucleating particles. The addition of nucleants into aluminum alloy melt for grain refining is a widely used method of microstructure control.
U.S. Pat. No. 5,053,286 discloses a method of dissolving lead in molten aluminum and horizontal continuous casting of the melt in a twin-roll caster at a cooling rate of more than 200° C./sec. The microstructure obtained when the alloy is cast with such a high rate of cooling, is very fine. The cast strip produced by said method contained 5% lead and demonstrated very little lead segregation towards the bottom of the cast. The maximum lead particle size was 25 microns. However, even at such a high cooling rate and low lead content, the spheres in the bottom half of the casting were 2-2.5 times larger than those in the top half.
Metallurgical structure is claimed in the patent as containing uniformly distributed lead particles no more than 25 microns in diameter. Lead content claimed in the patent is between 4% and 10% by we
Kopeliovich Dmitri
Shagal Vladimir
Shapiro Alexander
Elecmatec Electro-Magnetic Technologies, Ltd.
Stroock & Stroock & Lavan LLP
Wyszomierski George
LandOfFree
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