Optimization of Laser Welding of Galvanized Steel Via Response Surface Methodology

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OPTIMIZATION OF LASER WELDING OF GALVANIZED STEEL VIA RESPONSE SURFACE METHODOLOGY Abstract
The increasing demand of light weight makes ultra thin gage galvanized steels (<0.6mm) attractive for automotive applications such as for body-in-white and closures. Laser welding, well known for its deep penetration, high speed and small heat affected zone, provides a potential solution for welding thin gage galvanized steels in automotive industry. In this study, the effect of the laser welding parameters (i.e. laser power, welding speed, gap and focal position) on the weld bead geometry (i.e. weld depth, weld width, surface concave and aspect ratio) of 0.4mm galvanized SAE1004 steel in a lap joint configuration has been investigated by experiments. The process windows of the concerned process parameters are therefore determined. Then, response surface methodology (RSM) is used to develop models to predict the relationship between the processing parameters and the laser weld bead profile and identify the correct and optimal combination of the laser welding input variables to obtain superior weld joint. Under the optimal welding parameters, defect-free weld are produced, and the average aspect ratio increases about 30%, from 0.60 to 0.83. Introduction

Due to the increasing environmental consciousness and fuel crisis, light-weighting is an inevitable tendency in automobile industry. Moreover, requirements for improving durability in vehicle structure have led to the wide use of galvanized steel sheets as a corrosion resistant material (Ref.1~2). Thus, galvanized thin gage steels (<0.6mm) might be used for automotive applications such as for body-in-white and closures. Currently, resistance spot welding is still the dominant joining method in auto body assembly. However, considerable difficulties confronts in resistance spot welding of ultra thin gage galvanized steels due to the excessive high temperature developed at the electrode surface. The electrodes used in welding thin gage steel need frequently dressing or replacement, which would significantly increase the cycle time and production cost.

Laser welding is well known for its deep penetration, high speed, small heat affected zone, fine welding seam quality, low heat input per unit volume, fiber optic beam delivery and ease of interface with robots (Ref. 3). Furthermore, research has suggested that laser welded automotive components may provide better and more consistent mechanical properties and have superior repeatability compared with those joined by resistance spot welding (Ref. 4). Laser welding therefore provides a potential solution for welding thin gage steel in automotive industry.

However, because the boiling point of zinc (about 906oC) is much lower than the melting point of steel (over 1500oC), highly pressurized zinc vapor is then produced and disturb the molten pool, expel liquid metal, and finally causes a series of weld defections, such as severe spatters and porosity in the welds. These defects would significantly deteriorate the weld quality. Especially, when the sheet thickness decreases, the molten pool becomes shallower, so it is more easily for the pressurized zinc vapor to expel the molten metal from the molten pool and produce even more severe spatters. Many endeavors have been done to mitigate the effect of the highly pressurized zinc vapor over the past decades. A number of proposed solutions include: remove zinc coatings completely by mechanical means prior to welding (Ref. 5), prescribe a small gap between the two sheets (Ref. 6), place metal foils between the galvanized sheets (Ref. 7~8), design specific shielding conditions (Ref.9). Among that, setting up a gap between the sheets is adopted in this study due to its simplicity and economical efficiency.

Though laser welding is extremely advantageous in automotive application, the use of the technique in inappropriate settings can reduce its effectiveness in welding applications...
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