Research On Laser Welded Mg-rare Earth Alloy NZ30K And Improvement Of Joint Properties

The welding technique of Mg and its alloy can’t meet the demands of manufacturingat present and it has been the main problem which limits the application of magnesiumalloy structural components. The mechanism of interaction between CO₂laser and NZ30Kalloy is studied. Spectral analysis equipment and high speed camera are used to investigatethe plasma. The microstructure and mechanical properties of laser welded joints underdifferent heat treatments had been tested and analyzed. The method of combiningtheoretical calculations and experimental results was used to study the strengthening roleof different morphologies precipitateds in the laser welded NZ30K alloy. The corrosionmechanism of the rare earth element Nd in improving the corrosion resistance of laserwelded NZ30K joints is revealed. Therefore, research on the weldability of Mg-Rare alloyis of great significance in theoretical study and industrial application.The mechanism of interaction between laser and materials is studied. The rate ofabsorption of the magnesium alloy surface for the laser is calculated. Spectral analysisequipment and high speed camera are used to investigate the plasma. In order to calculatethe temperature of the plasma, the method of relative intensity of two spectral lines withthe same ion is applied. The influencing factors of the welding forming were investigatedsuch as the laser power、 welding speed、 laser focal position and so on. Thecorrespondence between laser welding parameters, weld joint microstructure andmechanical properties of Mg-Rare alloy is studied. Using the high power CO₂laser theMg-Rare earth alloy NZ30K can be welded with good joint appearance under the properwelding parameters, where joint is full penetrated and free of macro defects in the fusionzone. If the heat input is too large during the laser welding, the solidification cracking willappear in the welding joint of Mg-Rare alloy. The cracks are mainly caused by thecombined effect of tensile stress and low melting point eutectic Mg12Nd.The microstructure and the mechanical properties of laser welding joints of Mg-Rare earth alloy are observed and tested. Due to the high rate of cooling in laser welding, thefusion zone of joint is made up of fine equiaxed grain. There is no obvious soft zone in thejoint of NZ30K. The FZ mainly consists of α-Mg as base and Mg12Nd which distributesalong the grain boundary. The results of mechanical property experiments show that thetensile-strength of NZ30K is about80%of that of base metal when the heat input is84J/mm.The microstructure and mechanical properties of laser welded joints under differentheat treatments had been tested and analyzed. The precipitation sequence in laser weldedNZ30K alloy during aging treatment has been investigated using transmission electronmicroscopy （TEM）. The results indicated that the heat treatment plays an important role inthe mechanical strength of laser welded joint of NZ30K. The microstructure of samplesafter the solution treatment as well as aging treatment is different from that of theas-received welded joint. For solution treatment, although the microstructure is muchdifferent from that of as-received welded joint, the solution strengthening effect is notobvious. The precipitation in laser welded NZ30K alloy follows the sequence of Mg（SSSS）�'G.P.�'β″（DO19）�'β′（fcc）.The ratio of different strengthening is△τ棱：△τ杆：△τ球：△τ片=3.57：3.14：1.86：1. The prismatic shape of precipitates perpendicular to the basal planeis the most important factor that enhances aging treatment strengthening. The flack shapeof precipitates parallel to the basal plane is the worst important factor. There are lots ofprecipitates in the fusion zone after the aging treatment, which will significantly enhancethe tensile strength of the welding joint. After aging treatment at200℃×8h, the UTS oflaser welded NZ30K joint is the highest.The corrosion behavior of laser welded joints in different regions and under differentheat treatment had been investigated. The corrosion mechanism of the rare earth elementNd in improving the corrosion resistance of laser welded NZ30K joints is revealed. It isfound that the corrosion resistance of fusion zone is better than that of the base metal andthe corrosion resistance of NZ30K is better than that of AZ31. After appropriate heat treatment the corrosion resistance of laser welded NZ30K can be significantly improved.The electrochemical activity of Mg-Rare earth compounds Mg12Nd phase is low, soMg12Nd solid solution containing rare earth can improve the corrosion resistance. Mg12Ndphases are basically continuous distribution along the grain boundaries in the smallequiaxed grains of fusion zone. Therefore, barriers produced by Mg12Nd phases can bequickly formed on the surface and the corrosion rate of the alloy will be reduced.