US 3627517 A
Description (OCR text may contain errors)
United States Patent Oce 3,627,517 Patented Dec. 14, 1971 Int. Cl. C22c 31/00 US. Cl. 75-134 T 4 Claims ABSTRACT OF THE DISCLOSURE A low temperature melting ternary alloy suitable for thermal fuses in ballasts containing eutectic of indium, tin, and the balance silver or copper.
BACKGROUND OF THE INVENTION This invention relates to fusible or low melting point alloys and, more particularly, to a ternary alloy suitable for use in the manufacture of thermal protective devices for electrical circuitry.
In most industrial countries, authorities such as CSA in Canada, and Underwriters Laboratory in the United States, are set up and maintained for the specific purpose of establishing safety standards in respect of manufactured products, technical installation procedures and the like.
Insofar as electrical circuitry is concerned, regulations have been laid down in the interests of public safety requiring insertion of protective devices which will disrupt associated circuits should any potentially dangerous condition occur.
One type of protective device, widely used, for example, in ballasts or fluorescent lights, is the fusible link comprising a length of a suitable low-melting alloy. Such links are subject to stringent temperature-time standards or, in other words, must be capable of melting rapidly at a specified temperature in order to drop out of and disrupt a circuit when critical conditions arise.
However, while the safety standards set by the authorities must be met, a fusible link, in order to command a competitive position in the market, must be capable of maintaining its associated circuit in operation up to the maximum temperature permitted.
In short, a fusible link, or the alloy from which it is made, to be suitable in every respect, must be characterized by rapid and substantially isothermal phase transition from solid to liquid.
Additionally, the alloy must possess satisfactory mechanical strength and good resistance to the formation of surface films (occurring primarily through the agency of oxidation), such that the desired response to critical temperature changes remains unimpaired.
Put in another way, an alloy, in order to be satisfactory in use as a fusible link, must remain substantially solid with limited expansion or contraction until the temperature in its operative environment reaches a predetermined point. At that time, the alloy must transform rapidly and substantially isothermally to the liquid phase, dropping out of circuit and thus disrupting the latter.
Obviously, the formation of surface films could well alter the melting characteristics of a fusible link. Further, to avoid premature circuit discontinuity arising from permissible temperature fluctuations below the critical point, and to negate excursions slightly beyond that same point, the alloy must change phase in a sharply defined manner so that the link maintains its integrity substantially up to the melting point but not beyond it.
The requirements for a fusible link alloy thus call for the use of a eutectic or near-eutectic composition wherein, as is well known, the phase transition from solid to liquid occurs in an isothermal or near-isothermal manner.
In non-eutectic compositions, repeated thermal cycling through the melt point can cause segregation of elements and gradual variation of the melting and freezing characteristics. However, in the field with which the present invention is concerned, such cycling and its inherent problems are of no consequence since a fusible link is naturally discarded after a single cycle.
The temperature range over which an alloy melts may be termed a function of the variation as between the crystallization rates of the components elements, while the phase transition time may be stated as a function of the various crystallization rates considered together with the latent heat of fusion of the particular composition.
However, the melting point of an alloy bears no apparent relationship to the individual melting points of its components and, similarly, it is extremely difiicult to predict, with any accuracy, the numerical value of latent heat of fusion of an alloy merely by reference to the characteristics of the individual elements involved.
From the foregoing it will be readily appreciated that no logical steps can be followed in establishing a eutectic or near-eutectic composition to meet a very restricted and precise temperature-time target. Even though one alloy may be ideal insofar as melting point is concerned it may not be acceptable from a phase transition time aspect. If melting is too sluggish, a fuse or fusible link will not necessarily fullfil its protective function, failing to drop out of the circuit in the desired manner.
OBJECT OF THE INVENTION It is therefore the object of the present invention to provide fusible alloys possessing all desirable properties and, of most importance, having desired melting-point temperatures together with suitably rapid and complete phase transition characteristics.
SUMMARY OF THE INVENTION The foregoing object is accomplished in accordance with this invention, in its most preferred form, by providing a ternary alloy of indium (In), tin (Sn) and silver The specific percentage content ranges are, of course, governed by the temperature-time limitations imposed for the particular use.
DESCRIPTION OF THE PREFERRED EMBODIMENT Initially, investigation began with a study of the binary system of indium and tin. It was then determined that while the eutectic of this binary system is characterized by a melting point and transition (phase) time suitable for present regulations, it would not be satisfactory to meet the more stringent requirements which are expected to be imposed in the United States in the near future.
As a result, investigations were continued by adding relatively small percentages of Group IB elements to the In-Sn eutectic. I
Copper was found to provide a definite decrease in melting point while silver has an even more desirable effect.
By carrying out appropriate tests it was found that the novel ternary alloy of ln-Sn-Ag melted at ll3.8il C. and completed the phase transition within the time limits set for fusible links by both U.S. and Canadian authorities. This alloy also meets the anticipated U.S. revised regulations.
Further, metallographic examination of a sample of the new alloy revealed a substantially homogenous structure over the entire cross-section, indicative of a eutectic or near-eutectic composition.
Naturally, the true eutectic is the most preferred composition when one considers the desirability of a pure isothermal transition, but observation has established that for the purpose in question, the alloy may be comprised within the approximate composition given and still melt at a temperature in the prescribed area.
A fusible link manufactured from the novel material proved to have satisfactory mechanical strength for its purpose and exhibited good resistance to surface film formation (oxidation).
In all respects therefore, the new ternary alloy of In- Sn-Ag is eminently suitable for use in the manufacture of fusible links while additions of about 0.17% copper to the In-Sn eutectic result in an alloy suitable for use under present regulations, having an M.P. of 1l7.6 C. :1" C.
It is not considered necessary to comment upon production of these new alloys since no peculiar problems exist in this respect. Naturally, however, purity of the components must be afforded consideration.
For the record, initial experimentation was carried out using component elements of 99.9999% purity, but later tests showed that 99.99% is acceptable. Purity considerations are not, of course, essential to the actual point of invention.
In practice, the ternary fusible alloy according to the present invention can be processed to any desired configuration for use as a fusible link. Of course, such use is not intended as being exhaustive of the overall utility of the alloy.
As regards mounting the link, one mode is suggested by reference to Pat. No. 3,354,282 issued to Norbert C. Batsch on Nov. 21, 1967, and assigned to the assignee of the present invention.
Pat. No. 3,354,282 reveals a thermal fuse with capillary action in which the molten fusible link will separate reliably and completely under any practical service conditions, in any position, and after long, useful life. Enclosed within a tubular sleeve is a fusible link and a means attached to each end of the fusible link for providing capillary action on the molten fuse link so that when the fusible link melts upon increase of the environmental temperature beyond its melting point, the molten material is withdrawn into the capillary means thus en suring that the current flow in the electrical lead is entirely interrupted.
While a particular embodiment of this invention has been described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects and, therefore, it is intended that the appended claims cover all such changes and modifications as fall within the true spirit and scope of this invention.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. A ternary fusible alloy comprising a substantially eutectic composition of ln-Sn with the balance made up of silver.
2. A ternary fusible alloy having a composition of about 51.4% In, 47.3% Sn, and 1.3% Ag, characterized by a melting point of 113.811 C.
3. A ternary fusible alloy, as recited in claim 1, wherein the composition is chosen to have the lowest possible melting point, which is approximately 114 C.
4. A ternary fusible alloy, as recited in claim 1, wherein a sufiicient amount of silver is added to the ln-Sn to lower its melting point below the 117 C. minimum melting point of an In-Sn eutectic.
References Cited UNITED STATES PATENTS 11/1950 Zickrick -134 X 3/1955 Kozacka 75134 X U.S. Cl. X.R. 75-134 B