By: Mohamed Wahba, Masami Mizutani, Yousuke Kawahito, Seiji Katayama

Central Metallurgical Research & Development Institute, Egypt
Graduate School of Engineering, Osaka University,  Japan
Joining and Welding Research Institute, Osaka University, Japan

Currently, the automotive industry is facing an increasing demand to increase fuel economy and reduce greenhouse gas emissions. Therefore, the trend is moving toward an increase in the percentage of components made of lightweight structural materials such as magnesium alloys. In order to implement these materials, it’s critical to have the ability to produce defect-free welds with reproducible high quality. Laser welding, owing to its versatile advantages, is very promising in this regard. Moreover, the recent development of fiber and disc laser sources with excellent beam quality has increased the capability and enhanced the performance of the laser welding process. Compared to Nd:YAG and CO2 laser sources, the superior beam quality can be utilized to obtain higher welding speeds, and deeper penetration, longer distance to the work piece and lower cost can be realized.

Therefore, a study was performed with the objectives of investigating the process potentials and limitations of welding magnesium alloys with high-brightness disc laser.  A wide range of process parameters were applied to understand the involved welding phenomena. Through high speed video camera observation of a molten pool and a keyhole it was possible to analyze the formation mechanism of different defects encountered in the process of welding magnesium alloys with high-brightness disc laser. The very high power density associated with a focused laser beam induced instabilities to the molten pool and distorted the keyhole inlet. These disturbed melt flows and resulted in various kinds of welding defects. The main welding defect limiting the process was recognized to be incompletely filled grooves. This defect was related to the formation of a large amount of spatters due to the unstable molten pool and keyhole.

Accordingly, the process parameters were optimized in order to stabilize the keyhole and to reduce welding defects. A diagram of the welding process indicating areas of sound and defected welds could be constructed. It was possible to obtain sound welds at high welding speeds up to 7 m/min. Additionally the mechanical properties of sound welded joints were evaluated. The results revealed that sound welded joints with mechanical properties similar to that of the parent material could be produced with controlled energy input.

Based on this study, it was concluded that disc laser could be used to obtain high quality welded joints of magnesium alloys. Such results might be beneficial to expand the utilization of magnesium alloys in the automotive industry.