Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2001-10-26
2004-08-24
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S085140, C219S085190
Reexamination Certificate
active
06781084
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus and a method for joining work pieces together and, more particularly, to an apparatus and method for performing resistance brazing utilizing high current and low voltage electric flow with precise temperature control to rapidly heat the interface between the work pieces to create a molten zone of metal which forms a solid weld when cooled.
In the manufacture and construction of many types of structures, brazing has been used to reduce or replace the use of various fastening methods, such as bolting, riveting and the like. Resistance brazing is a well-known process whereby work pieces are joined together by use of electric current that creates heat along the high resistance interface between the work pieces. During the brazing process, a fusible material is inserted along the high resistance interface in order to wet the surfaces of the work pieces and to ensure dissolution of any oxides that may be formed during the brazing operation. A low voltage, high current electric flow is used to rapidly heat the interface causing the fusible material to melt then consolidate and cool to form a brazed joint. In order to properly form the joint, the melted fusible material must wet the entire surface along the interface between the work pieces while maintaining the melting temperature of the fusible material below the melting temperature of the work pieces being joined In order to avoid the undesirable formation of voids, microcracks, and brittle intermetallic compounds, resulting in joints that are weak and exhibit poor resistance to thermal shock, it is important that diffusion and alloying between the fusible material and the work pieces being joined should be kept to a minimum. One problem, which is often associated with subjecting work pieces to high temperature, is the formation of tensile residual stresses created in the work pieces during the brazing process by the expansion and then contraction of the region adjacent to the interface between the work pieces. Such tensile residual stresses are well known to reduce both fatigue life and increase sensitivity to corrosion-fatigue and stress corrosion cracking in a wide variety of materials. Further, where two different work pieces having different sizes are brazed together, any residual stress within the work pieces are amplified due to the difference in their heat capacity.
In order to minimize the formation of such tensile residual stress, it is desirable for brazing to be carried out by heating the interface and the fusible material just above the melting point of the fusible material and cooling immediately after adequate flow and wetting has taken place. It has been found that by careful monitoring and control of the temperature being generated during the brazing process along the interface between the work pieces being joined, it is possible to minimize such detrimental thermal effects.
Accordingly, a need exist for an apparatus for brazing whereby a low voltage, high current electric flow be used to braze work pieces together formed from various materials while monitoring and controlling the temperature along the interface between the work pieces to form a joint having physical properties that are superior to joints formed using conventional brazing apparatus.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and a method for brazing work pieces together and which eliminates the disadvantages associated with conventional apparatus currently in use for brazing parts together. In a preferred embodiment of the invention, the brazing apparatus comprises an electric connector for connecting to an electrical power source; at least two electrodes adapted for attaching to and providing electrical contact with the work pieces to be brazed together; means for converting the electric power being directed from the electrical power source into a low voltage, high current electric flow directed to the work pieces; and a control system for controlling the brazing operation. The brazing apparatus further comprises means for automatically controlling the time duration of the brazing operation and means for monitoring and automatically controlling the brazing temperature being generated along the interface between the work pieces.
In another preferred embodiment of the invention, the brazing apparatus further comprises means for permitting the user to manually control the time duration of the brazing operation.
In another preferred embodiment of the invention, the brazing apparatus further comprises means for permitting the user to manually control the brazing temperature being generated along the interface between the work pieces.
In another preferred embodiment of the invention, the control system comprises a programmable controller or microprocessor for controlling the brazing operation.
In another preferred embodiment of the invention, the brazing apparatus comprises means for cooling the electrodes.
In another preferred embodiment of the invention, the brazing apparatus comprises water-cooled electrodes.
In another preferred embodiment of the invention, the brazing apparatus comprises means for allowing work pieces having various heat capacities to be joined together by brazing.
In another preferred embodiment of the invention, the brazing apparatus comprises a plurality of electrodes spaced along the brazing line to provide a uniformed brazing temperature along the interface between the work pieces.
Another preferred embodiment of the invention is a method for brazing at least two work pieces together comprising the steps of inserting a fusible material along the high resistance interface between the work pieces; defining a temperature level to be maintained along the interface during the brazing operation, applying a low voltage, high electric current flow to rapidly heat the interface causing the fusible material to melt, then consolidate and cool to form a brazed joint; wherein the temperature along the interface is monitored and regulated by increasing or decreasing the current flow to maintain the temperature at the predefined level.
In another preferred embodiment of the invention, the method for brazing further comprises the step of terminate the brazing operation after a predefined time.
A primary object of this invention, therefore, is to provide an apparatus for brazing together work pieces such that the brazed joint has improved physical properties over brazed joints formed using other conventional brazing apparatus.
Another primary object of the invention is to provide and apparatus for brazing which can be used to braze together temperature sensitive parts.
Another primary object of the invention is to provide an apparatus for brazing together work pieces that reduces or eliminates the formation of tensile residual stresses typically produced during conventional brazing processes.
Another primary object of the invention is to provide an apparatus for brazing together work pieces that reduces or eliminates the formation of voids, microcracks, and brittle intermetallic compounds.
Another primary object of the invention is to provide an apparatus for brazing together work pieces that eliminates or reduces the formation of joints that are weak and exhibit poor resistance to thermal shock.
Another primary object of the invention is to provide an apparatus for brazing together work pieces having different heat capacities.
Another primary object of the invention is to provide an apparatus for brazing together work pieces having variable dimensions.
Another primary object of the invention is to provide an apparatus for brazing together a plurality of work pieces together.
Another primary object of this invention is to provide an apparatus for brazing that is relatively inexpensive to manufacture.
Another primary object of this invention is to provide an apparatus for brazing that is relatively inexpensive to operate and maintain.
Another primary object of this invention is to provide an apparatus which is relatively lightweight and portable.
Britton Christopher
Brockman Howard
Fuztech, Ltd.
Shaw Clifford C.
Smith Mark F.
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