Investigation On The Microstructure And Hot Cracking Behaviors Of The Laser Beam Welds Of The Fine-grain AZ Series Magnesium Alloys Sheets

Magnesium alloys are the lightest green metallic materials for structural application and have very important application values and broad application prospects in many industrial fields,such as aerospace,national defence and military industries,transport vehicles and 3C electronics and etc because of their series of advantages such as low density,high strength/weight ratio,high specific toughness,good thermal conductivity,electromagnetic shielding properties and good damping.Researches show that grain refinement is an effective means to greatly improve the mechanical properties of magnesium alloys,so the investigation on the unltafine grained magnesium alloys is thought to be the main research direction in the field of magnesium alloys,but researches of their weldability are hardly reported.There are many welding processes for magnesium alloys,but only electron beam welding（EBW）and laser beam welding（LBW）are appropriate for welding magnesium alloys.In this thesis,the fine-grained AZ61 and AZ91 magnesium alloys sheets were welded with CO₂ laser beam using filler metal.Effects of the initial microstructures of base metals on liquation reaction and liquation cracking behaviors of the partially melted zone（PMZ）are investigated.Moreover,effects of rare earth Ce alloying on the microstructures and mechanical properties of the fusion zone（FZ）are investigated.Since hot cracking is the main defects which greatly limits the mechanical properties of magnesium alloys welded joints,the results of this thesis can provide referencees for solving hot cracking issues of magnesium alloys.The CO₂ laser beam welding process of fine-grained magnesium alloys with the average grain size of 5μm are studied and the formability of joints,the evolution of the microstructure and mechanical properties of the joints with the heat input are investigated.The results reveal that the welding joints with good formability,fine-grained and densified fusion zone,partially melted zone and unconspicuous heat affected zone with the low liquation degree are obtained under the optimum welding heat input（AZ61: heat input,18J/mm;welding power,1500W;welding speed,5m/min;AZ91: heat input,16.8J/mm;welding power,1400W;welding speed,5m/min）.The microstructures and mechanical properties of the joints of AZ61 and AZ91 magnesium alloys base metals with different initial microstructures are comparatively studied,following conclusions were obtained:（1）the smaller the grain size of base metal is,the lower the liquation degree in the PMZ;（2）the less and the smaller the low melting point phaseβ-Mg17Al12 is,the lower the liquation degree in the PMZ;（3）serious liquation reaction occur in the PMZs of four groups AZ91 magnesium alloy joints since plenty of precipitates（β-Mg17Al12）are inevitably existed in the AZ91 magnesium alloy sheets;（4）four groups AZ61 magnesium alloy autogenous welding joints with densified fusion zone are obtained,and the four groups tensile specimens fail at the PMZ.So the mechanical properties of joints are closely related to the liquation degree of the PMZ.the higher the liquation degree,the lower the mechanical properties are;（5）some microcracks in FZs and serious liquation in PMZs are detected in all joints prepared by the autogenous welding process,resulting in the inferior mechanical properties.Welding process with filler strip can not only avoid notch effect which is inevitable in autogenous welding process,but also the alloying elements within filler strip can be added into the weld,which results in microstructure improvement of the FZ.Grain refinement in the FZ and the improvement mechanical properties can be achieved by the addition of the alloying Ce elements,and even no microcracks are observed in the FZ.In this thesis,several control techniques of hot cracking in AZ series magnesium alloy welding are proposed on the basis of the experimental results.Decreasing Al contents in the base metals,refining the grains of base metals,refining and reducing the low melting point precipitates（β-Mg17Al12）can decrease liquation cracks.Decreasing Al contents and adding rare earth Ce elements to the weld through the filler can decrease solidification cracks.