Study Of Microstructure And Mechanical Properties Of CO₂Laser Welded Joint On ZK60Magnesium Alloy

In recent years, the demand of lightweight has made magnesium alloyincreasingly and widely used in the field of aerospace, motor vehicle andtelecommunication. The welding technology of magnesium alloy is the decisive keyto its applied development and the hot spot to study. In this thesis，the processingof CO₂laser beam welding（LBW）of magnesium alloy ZK60plates with2.0mmthickness were studied. The microstructure and mechanical properties of joints wereanalyzed by optical microscope （OM）, energy dispersive spectrometer （EDS）,scanning electron microscope （SEM）, tensile machine and hardness machine.Meanwhile，the characteristics of magnesium alloy ZK60welding have been furtheranalyzed through CO₂laser beam welding.The experimental results show that a large heat input is needed to weld ZK60alloy successfully. If the output power of laser beam is in coordination with weldingspeed appropriately, the heat input is appropriate, and the geometry of weld jointslooks well. There are little defects, such as crack, porosity in the weld metal. Thewelding zone is composed of tiny equiaxed grain, and the heat affected zone is verynarrow with coarser crystal. The microstructure in room temperature consists ofα-Mg, MgZn and MgZn₂phase. The MgZn and MgZn₂phase are distributed on thegrain boundary, and play the role of strengthening the butt joint tensile performance.Moreover, the precipitated particles also have tremendous influence on the microhardness distribution. The micro hardness of weld zone is slightly lower than that ofthe parent metal, but higher than that of the heat affected zone. When laser power isP=1200W, welding speed is V=3.5m/min, the number of precipitated phase is larger,which are distributed homogeneously, and also the micro hardness of the weldedjoint is increased. In the tensile test, the ultimate tensile strength of the successfullywelded joint can reach to81.1%of the parent metal. The fracture position appears inthe heat affected zone, and the fracture surface possesses the mixed-rupturecharacteristics.The microanalysis shows that the main defects in laser welded joint of ZK60magnesium alloy are porosity and hot crack. The main reason of pore formation isthat the solubility of hydrogen suddenly drops in magnesium alloy during the weldmetal solidification. In the ZK60magnesium alloy welded joint, the formation of hot crack mainly caused by the low melting point eutectic. The above problem can besolved by the methods of mechanical processing and chemical processing on theparent metal surface before welding, as well as appropriate welding parameters andpreheating process.