Repairing And Strengthening Of ZM5 Magnesium Alloy By Laser Surface Welding

With a number of advantages such as low density, high specific strength and good cushioning effect, ZM5 magnesium alloy has expanded its applications in various areas. However, surface casting defects and poor heat resistance have restricted its promoted application in real industries. In order to repair and strengthen thin-walled castings of ZM5 magnesium alloy, the MgAl-x Gd(x = 0.0, 2.5, 5.0 and 7.5 wt.%) coatings were fabricated by fiber laser surface welding with wire feeding and argon protecting. The welding process has been thoroughly researched to achieve the coatings with good quality, and the rare earth element Gd was added to improve the high-temperature performance, which would technologically support the rare earth reinforcement and laser surface modification in magnesium alloy application.Systematic investigation and analysis were carried out on optimizing parameters of laser surface welding on the substrate of ZM5 magnesium alloy, regulating parameters such as defocus, welding speed, laser power and wire feeding speed. In consideration of forming ability, dilution rate, weld-surface angle and microhardness, the optimized processing parameters for laser welded Mg-Al-xGd coatings were determined: defocus +20 mm, welding speed 0.5 m/min, laser power 2300 W, wire feeding speed 3.4 m/min and protecting gas flow 15 L/min. Under the conditions of strict pre-weld cleaning and gas shielding, the weld coatings fabricated in the optimal procedure has achieved good metallurgical bond with the base metal, and no welding porosity or crack defects occurred during mass production.Systematic observation and characterization were carried out on the microstructures and mechanical properties of laser welded Mg-Al-xGd coatings, including the second phase analysis, the ambient-temperature tensile performances and the high-temperature tensile performances. The average grain diameters of laser welded coatings were significantly smaller than that of casting base metal, and the microstructure of laser welded coatings consisted of the matrix α-Mg and the lamellar β-Mg17Al12 in the grain boundaries. After adding the rare earth Gd element, a lot of Al2 Gd particles were precipitated in the grains and their boundaries, the average grain diameters were further refined from 13.5 μm(0.0 wt.% Gd) to 4.8 μm(7.5 wt.% Gd). As revealed in the deformation testing, the tensile strengths and elongations of the laser welded coatings were higher than the base metal. The Gd addition could dramatically improve the high-temperature performance of the laser welded coatings, for example, the yield strength and the tensile strength of the laser welded coating added 7.5 wt.% Gd have reached 53.6 MPa and 88.0 MPa, both of which were higher than the laser welded coating without Gd addition.