METHOD OF DIRECT RESISTANCE WELDING - SELF BRAZING OF ALUMINUM TO MOLYBDENUM PIN
A direct welding process for joining a current collector to a terminal pin in the construction of electrochemical cells is described. The resistance welding process utilizes increased current combined with an applied force to bond dissimilar metals with a melting temperature differential of preferably more than 500° C. Preferably, the method is used to bond the terminal pin to the cathode current collector. This method of attachment is suitable for either primary or secondary cells, particularly those powering implantable biomedical devices.
1 . A method of creating a direct resistance bond between two dissimilar melting temperature metals, comprising the steps of:
a) providing a first metal;
b) providing a second metal, having a higher melting temperature than that of the first metal;
c) placing the first and second metals between a negative terminal welding electrode and a positive terminal welding electrode, the negative terminal welding electrode connected to a negative terminal of an electrical power source, the positive terminal welding electrode connected to a positive terminal of the electrical power source; and
d) applying an electrical current and a mechanical force to the first and second metals between the negative and positive terminal electrodes to create a bond between the first and second metals.
2 . The method of claim 1 including providing the positive terminal welding electrode having a planar surface so that the second metal contacts the planar surface of the positive terminal welding electrode.
3 . The method of claim 2 including providing the planar surface of the positive terminal electrode a rectangular surface area having a width and a length that is at least twice that of that of the width or a diameter of the second metal
4 . The method of claim 1 including providing the second melting temperature of the second metal at least about 125° C. greater than a first melting temperature of the first metal.
5 . The method of claim 1 including applying at least about 800 amperes of electrical current to the first and second metals positioned between the negative and positive terminal welding electrodes.
6 . The method of claim 1 including applying at least about a 10 Newton force to the first and second metals positioned between the negative and positive welding electrodes.
7 . The method claim of 1 including selecting the first metal from the group consisting of aluminum, titanium, nickel, steel, stainless steel, niobium, copper, gold, silver, palladium and combinations thereof.
8 . The method of claim 1 including selecting the second metal from the group consisting of molybdenum, tantalum, tungsten, and combinations thereof.
9 . The method of claim 1 including forming the bond having a fusion zone, wherein the fusion zone is comprised of columnar shaped grains of the first metal.
10 . The method of claim 9 including forming the fusion zone so that heat generated from the welding electrodes melts the first metal so that the first metal is wetted to a surface of the second metal at a fusion zone interface.
11 . The method of claim 1 including forming the bond having a heat affected zone, wherein the heat affected zone is comprised of heat affected grains of the first metal having a size that is larger than first metal grains that are substantially unaffected by heat created during formation of the bond.
12 . The method of claim 1 including forming the bond having a heat affected zone, wherein the heat affected zone is comprised of heat affected grains of the first metal being at least two times the size of grains of the first metal that are substantially unaffected by heat created during formation of the bond.
13 . The method of claim 1 including forming the bond in which an intermetallic alloy composed of the first metal and the second metal is not formed.
14 . The method of claim 1 including forming the bond in which the first metal substantially comprises columnar shaped grains.
15 . The method of claim 1 including providing the second melting temperature of the second metal at least about 500° C. greater than a first melting temperature of the first metal.
16 . The method of claim 1 including forming the bond in which the second metal comprises a grain size up to 0.1 micrometers.
17 . The method of claim 1 including providing the first or second metal a component of a medical device.
18 . The method of claim 1 including providing the first or second metal a component of an electrochemical cell.
19 . The method of claim 1 including forming the bond comprising a fusion zone wherein the first and second metals join together and a heat affected zone spaced distally from the fusion zone, wherein the grains of the first metal are not columnar in shape.