Brazing process is suitable for joining of thin metal sheets with low heat input and low deformations. Dissimilar metals or complex functional alloys that are hard to weld can be joined with brazing. However, brazing is complex, slow, expensive and needs skilled operators, and these features keep brazing from being easy, simple and versatile joining process. In this study, automation of the brazing system was tried as a solution of the above problems, and a $CO_2$ laser was selected as a fast and reliable heat source of brazing process. To implement an automated laser brazing system, automatic feeding equipments of brazing filler metal wire and flux was devised and attached to the laser system. All of the system components were integrated with a personal computer. With the developed system, basic laser brazing experiments were conducted successfully, and brazed joints were inspected and tested both optically and mechanically. For a complete automation, joints should be specially designed for easy automation and single-side access of the brazing head. To meet these requirements, a plug brazing method of lap joints were devised and developed for general lap joining methods of thin metal sheets. In a lap joint, a small hole was drilled in the upper metal sheet, and with this hole brazing filler metal and flux were fed into the joint gaps while laser beam heats the hole. With this method large amount of lapped area can be easily joined, with easy penetration of molten brazing filler metals.
As a major application of the laser brazing process, rapid laminate tooling was selected and developed. Rapid laminate tooling is a kind of direct rapid prototyping methods, to make metal shapes or metal die tools easily. The process consists of cutting metal sheets, stacking of them, and joining of them to make volumetric shapes. Joining of metal sheets are accomplished by brazing or soldering process. In this study low carbon sheets of 1mm thickness was selected as a base metal, and metal die tools for injection molding of automatic shift gear knobs of automobiles were developed successfully. Recently, there have been some methods developed for improvement of surface accuracy of laminate die tools, such as sloped cutting or laser cladding processes. As a new method, elimination of stair steps of laminate die tools with laser brazing was tried and applied. Wide range of side angles could be completely filled with molten brazing filler metals, and surface hardness was greatly improved with addition of functional powders such as tungsten carbides.
As another application of laser brazing process, repair brazing of laminate die tools was developed. Surface defects on the die cavity can cause severe problems such as mold ejecting problem, decrease of surface accuracy, and decrease of die tool life. In this study, brazing filler metal was molten by the laser and filled the surface defects completely, and the excessive filler metal was removed by mechanical machining process. The repair results were closely related to the laser beam parameters, and their effects were successfully predicted by 3D finite element thermal analysis.
