Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
1999-11-10
2001-05-01
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
Reexamination Certificate
active
06225591
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to welding electrodes, and to a process for forming same.
Resistance welding has long been used as a quick and effective method of joining metal members. The workpieces to be welded are placed in abutting relationship and a large electrical current is caused to flow through the workpieces by a pair of opposed electrodes that contact the workpieces on opposite sides of the weld point. The current causes the abutting surfaces of the workpieces to be heated sufficiently to effect the formation of a weld nugget. Typically, the electrodes apply significant pressure to the workpieces during welding. This facilitates the welding process by urging the material together and, also, reducing electrical resistance between each electrode tip and the adjacent workpiece material that it contacts.
Since welding is accomplished by resistance heating of the material being welded, it will be appreciated that the electrodes will also be heated substantially. It is important to have electrodes of high electrical conductivity in order to minimize the power loss in the electrode, and the resulting heating of the electrode.
Over time, the repeated heating and pressing operations involved in resistance welding cause breakdown, softening, mushrooming and other deformation of the electrodes. The current density of the current going through the workpieces drops. As this occurs, electrical current requirements for welding increase with the enlarged welding tip face contacting the workpiece material until ultimately, redressing or replacement of the electrode is required. Accordingly, it is also important to have an electrode which is capable of withstanding significant distorting force at the elevated temperatures which result from the welding process so as to minimize the number of times it becomes necessary to redress or replace the electrode within a given period of time.
It is known in the art to form resistance welding electrodes by combining a copper electrode body with an anneal resistant, high hardness insert. Typically, the insert performs much better than the copper material from which the electrode body is formed. However, the insert material is much more expensive than the copper used to form the electrode body.
The insert may be brazed onto the shank of the electrode. The brazing step is disadvantageous, however, as it adds an additional step to the electrode manufacturing process and, hence, increases the cost of the electrode. Furthermore, the brazing operation may anneal and soften the electrode body.
It is also known to force the insert into an electrode body via a press-fit operation. The steel being welded today is often galvanized or coated with a zinc or other, softer metal coating. As a result, resistance welding electrode may tend to stick to the coated metal. An electrode tip joined to an electrode body only by means of a press-fit may tend to pull out of the body as the electrode is retracted following resistance welding of coated materials, thus requiring replacement of the electrode.
Accordingly, there is a need for an improved resistance welding electrode which can be manufactured via an efficient and more cost effective process and, yet, is capable of performing in an acceptable manner.
SUMMARY OF THE INVENTION
This need is met by the present invention, whereby an improved resistance welding electrode, and a process for forming the same are provided. The process involves providing a billet having an inner cavity, inserting a dispersion strengthened copper insert into the billet and deforming the insert-containing billet via cold-working operations so as to lock the insert in place mechanically in the billet. The forming operations may be performed in a single step such that the electrode can be manufactured in an efficient and cost effective manner. Furthermore, because the insert is mechanically locked in place within the billet, it is unlikely that the normal amount of sticking that occurs during resistance welding of coated steel will pull the insert out of the billet. It is also noted that the billet is preferably formed from a CDA C10700 silver bearing copper which is a high conductivity material. Previously, it was generally thought that silver bearing copper should not be used in forming welding electrodes as it was believed that such material would anneal at the temperatures involved in resistance welding. However, by virtue of cooling water located in an inner cavity of the silver bearing copper main body portion and because the main body portion makes only limited, if any, contact with a workpiece, annealing of the main body portion is prevented.
According to a first aspect of the present invention, a process for forming a resistance welding electrode is presented. A billet formed from a high conductivity metal is provided. The billet includes a first portion having a first inner cavity being defined by a first wall and a first stop face. An insert is inserted into the first inner cavity of the billet with the insert having a first portion positioned substantially adjacent the first stop face. The insert is deformed such that an outer diameter of the first portion of the insert is increased, thereby mechanically locking the insert in place in the billet.
The step of deforming the insert may comprise containing a first section of the first portion of the billet by a forming element and then applying pressure to the billet so as to displace at least the first portion of the billet not contained by the forming element and the first portion of the insert thereby increasing the outer diameter of the first portion of the insert and mechanically locking it in the billet. The step of providing a billet may comprise providing a generally cylindrical cut-off portion of high conductivity metal, upsetting and forward extruding the cut-off portion so as to form the billet having the first portion and a second portion, and forming the first inner cavity in the first portion of the billet. The process may further comprise the step of forming a second inner cavity in the second portion of the billet. The step of forming the first inner cavity in the first portion of the billet and the step of forming a second inner cavity in the second portion of the billet may be performed substantially simultaneously. Preferably, the insert is formed from an internally oxidized copper-aluminum alloy or dispersion strengthened copper. The billet may be formed from a high conductivity copper or a silver bearing copper.
According to another aspect of the present invention, a process for forming a resistance welding electrode comprises providing a generally cylindrical cut-off portion of high conductivity metal. A billet is formed from the generally cylindrical cut-off portion having a first portion with a first inner cavity therein and a second portion with a second inner cavity therein. The first inner cavity is defined by a first wall and a first stop face. An insert is inserted into the first inner cavity of the billet. The insert includes a first portion positioned substantially adjacent the first stop face. A first section of the first portion of the billet and a second portion of the insert are contained via a forming element. Pressure is applied to the billet thereby increasing an outer diameter of a second section of the first portion of the billet and an outer diameter of the first portion of the insert so as to lock the insert in place mechanically in the billet, thereby forming the resistance welding electrode.
The step of containing a first section of the first portion of the billet and a second portion of the insert via a forming element may comprise the steps of positioning the first section of the first portion of the billet and the second portion of the insert in an inner cavity of the forming element, the forming element being part of a punch assembly. The inner cavity of the forming element includes an inner diameter substantially equal to an outer diameter of the first portion of the billet. The second section of the first
Nippert Russell Alan
Swank Brian Eugene
Killworth, Gottman Hagan & Schaeff LLP
Shaw Clifford C.
The Nippert Company
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