By: Shanglu Yang, Rouzbeh Sarrafi, and Radovan Kovacevic
Research Center for Advanced Manufacturing
Southern Methodist University
Dallas, TX USA 75205
With respect to excellent mechanical properties and corrosion-resistant capability, galvanized steels have been used intensively in different industries such as in the automotive industries. However, the successful welding of galvanized steels in a gap-free lap joint configuration is still a great challenge. The highly-pressurized zinc vapor is readily developed at the interface of the two metal sheets due to the lower boiling point of zinc (around 906 ˚C) than the melting point of steel (over 1500˚C). The failure of mitigating the highly pressurized zinc vapor at the interface of the two metal sheets leads to the formation of different weld defects such as the spatters and blowholes, which not only damage the weld surface quality but also deteriorate the mechanical properties of welds. In order to mitigate the effect of the highly pressurized zinc vapor, many welding methods have been developed which includes the removal of zinc coating at the interface of two metal sheets, redesigning the lap joint configuration, setting of alloy element at the interface, and so on. All of these methods require the pre-processing and post-processing actions and are costly to be used in the practice.
An innovative hybrid welding procedure, which combines the laser welding with the gas tungsten arc welding (GTAW) used as a preheating source, has been successfully developed to weld the galvanized high strength dual phase steels in a gap-free lap joint configuration. In this new welding procedure, GTAW that leads the laser beam at the specific distance is used to preheat the workpieces. Under the controlled heat input from the GTAW, zinc coating at the top surface is burned and the metal oxides are generated at the top surface of workpience. At the same time, the zinc coating along the weld zone at the interface of two metal sheets is transformed into the zinc oxides, which has the high melting point than that of steel. Furthermore, the thin layer of the generated metal oxides at the top surface of workpiece dramatically increases the coupling of laser beam energy into the welded material. Under these welding conditions, the stable keyhole is produced, which provides the channel for the highly pressurized zinc vapor to be vented out. This new welding procedure offers an efficient and robust way for lap joint of galvanized steels in a gap-free configuration and can be practically used in the industry. The completely defect-free lap joints are achieved. In comparison with the other methods for welding of galvanized steels in a gap-free lap joint configuration, the productivity efficiency can be dramatically increased by the use of this new welding procedure. At the same time, it avoids the high time-consuming and cost of pre-processing and post-processing associated with the traditional ways of welding galvanized steels in a gap-free lap joint configuration.
A machine vision system is also developed to real-time record the images of molten pool during the welding process, which is based on a high speed CCD camera with the frame rate of 4000 fps and one green laser as the illumination source. By the analysis of the recorded images of molten pool, the instability mechanism of laser welding of galvanized steels in a gap-free lap joint configuration is better understood. Furthermore, the developed machine vision system can be used to on-line monitor the various defect formations and provides the calculated geometry of molten pool for controlling the weld quality, thus accomplishing the automation of welding process.
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here