Metal fusion bonding – Process – Diffusion type
Utility Patent
1998-10-13
2001-01-02
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Diffusion type
C228S194000, C228S195000, C148S430000
Utility Patent
active
06168071
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a method for joining materials together by a diffusion process using silver/germanium alloys and a silver/germanium alloy for use in the method. More particularly, the present invention concerns such a method comprising using a copper/silver/germanium alloy which does not require the introduction of an additional filler material.
BACKGROUND OF THE INVENTION
Conventionally, metals are joined by three principle methods: welding, brazing and soldering. When welding using a filler, a filler material is chosen which has a composition which is similar to the two parts being joined and the actual contact regions of the adjacent metal parts are melted to effect the join. When resistance welding or laser welding, e.g. spot welding, the adjacent metal parts to be joined are raised to a high temperature and melted, no filler metal being required to effect the join. When brazing or soldering, a filler metal is used which has an appreciably lower melting point than that of the two parts being joined such that the contact regions of the metal parts being joined are not melted. Typically, solders melt below about 450° C. Brazing uses filler metals of greater strength, which have a melting temperature of greater than 450° C. These methods all require either the use of a filler metal, which forms a liquid phase upon heating to facilitate the joining of the parts, or the melting of the adjacent metal parts to be joined. The filler metal or the melted parts of the metals to be joined then cools and solidifies, thereby creating a bond at the molecular level.
Silver and silver alloys have conventionally been joined using a brazing process. However, so-called “fire staining” occurs at the high temperatures required to effect a bond using brazing techniques.
Because of its high thermal and electrical conductivity silver and silver based alloys are well known for their use in electrical and electronic contacts. Silver is also a precious metal and is used in the manufacture of jewelry and silverware. Accordingly, it becomes necessary to create bonds between both silver and silver alloys and other metals, whether they are ferrous or non-ferrous. It also is desirable to bond ceramics or plastics to metals. This largely has been done in the past by tack welding, soldering or brazing the joints together, which introduces impurities from the fluxing agent or solder into the silver/silver alloy.
A fire stain or fire spot is caused by the oxidation of the copper at high temperatures. Additionally, it is well known that silver tarnishes easily. This is because the silver, which does not readily oxidize at ambient temperatures, reacts with sulphur or hydrogen sulphide to cause the familiar tarnishing effect. It is known that the addition of a small quantity of germanium to a silver/copper alloy slows down the rate at which the silver reacts with sulphur and hydrogen sulphide, thus reducing the tarnishing effect as well as reducing fire stain when the silver/copper/germanium alloy is heated to high temperatures at which the copper would normally oxidize.
It is possible to braze or flame anneal in air a silver/copper/germanium ternary alloy without causing the production of fire stain and to provide a finished product which is less susceptible to tarnishing. However, a filler metal is required to perform the brazing to create a bond or joint and it is also necessary to raise the temperature of the silver alloy being worked to a high temperature, thus producing large temperature gradients in a bond area.
SUMMARY OF THE INVENTION
The present invention provides a method for joining two elements using a silver based alloy having a germanium content, and silver based alloy having a germanium content, that addresses the problems noted above. One embodiment of the method comprises first providing two elements to be joined together, at least one of the elements comprising a silver/copper/germanium alloy having a silver content of at least 77% by weight, and germanium content of between 0.4 and 7% by weight, more typically from about 0.5% to about 3%. The remainder of the alloy, apart from any impurities, principally is copper and boron, particularly elemental boron, as a grain refiner at a concentration of greater than 0 parts per million and less than 20 parts per million, typically greater than about 0 parts per million and less than about 10 parts per million, with working embodiments including from about 1 to about 2 parts per million boron. The two elements are placed adjacent one another such that a portion of a free surface of the silver/copper/germanium alloy contacts a portion of a free surface of the other element without interposing a filler material between the two free surfaces. The surfaces adjacent one another are then heated to a temperature below that of the solidus temperature of any one of the constituent materials of the elements, thereby joining the two elements by a diffusion process. The heating step can comprise heating the two adjacent free surfaces of the elements to a temperature as low as about 500° C. The method also can comprise joining a plurality of superimposed elements together to create a multilayered product.
The present invention also provides an alloy useful for practicing the method. The silver/germanium alloy typically has a silver content of at least 77% by weight and a germanium content of between 0.4 and 7% by weight, typically from about 0.5 to about 3%, which alloy also contains boron as a grain refiner at a concentration of greater than 0 parts per million and less than 20 parts per million, typically greater than about 0 parts per million and less than about 10 parts per million, with working embodiments including from about 1 to about 2 parts per million boron.
Accordingly, one object of the present invention is to provide a method for joining metals without needing to provide an additional filler metal.
Another object of the present invention is to provide a method for joining metals at temperatures below the solidus temperature of the constituent materials of the parts being joined.
Still a further object of the invention is to provide a silver/germanium alloy which is particularly suited to joining by the method of the invention or by non-filler welding techniques.
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Johns Peter Gamon
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Pittman Zidia D.
Ryan Patrick
LandOfFree
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