Investigation On Laser Welding With Filler Metal Process Of AZ31 Magnesium Alloy And The Microstructures And Properties Of The Welds

Magnesium alloys become very important structural materials and have find increasing application prospects in many industrial fields,such as aerospace, automotive,electronics and etc.The increase in magnesium applications highlights the need to develop a reliable welding technology. Nowdays,many welding processes can be used to joining magnesium alloys,but laser beam welding (LBW) with high energy, low and precise heat input and the high welding'speed has attracted great attention to weld magnesium alloys.In this thesis,the AZ31 magnesium alloy sheets were welded with laser beam using filler metal in the shape of strip.The results reveal that the high quality welded joints can be achieved by CO2 laser beam welding with filler strip process.Compared with the autogenous welding process,welding with filler strip process can bring about a decrease in the fusion zone (FZ) area and a reduction in grain growth in the heat-affected zone (HAZ).In addition, a final completely flat surface of the weld can be obtained after the removal of the residual filler strip,which results in the tensile properties of the joints espceically the fracture strain to be enhanced, due to the notch effect avoided.The effects of welding parameters on the microstructures and mechanical properties of the joints were studied.The results reveal that the tensile properties of the welded joints were decreased with the heat input increasing. But it can promote mix in molten pool by increasing heat input.The alloying elements of Ca, Mn, and Al were selected to add into the weld respectively. And the effects of the alloying elements metioned above on microstructures and properties of the AZ31 alloy welds were investigated.Compared with the joints welded with AZ31 filler strip,grains in the FZ were refined and the mechanical properties of the joints were heightened due to the addition of the alloying elements of Ca, Mn, and Al.The alloying elements of Ca on microstructures and properties of the AZ31 alloy welds were studies by welding with the AZC02,AZC04, and AZC1 filler strip.In FZ,the grains were firstly refined then coarsen with Ca increasing. Simultaneously, the precipitates in the FZ were obviously refined with appropriate content of Ca addition.The average UTS of the welds with filler strip of AZC04 (the content of Ca in the weld being 0.19wt%) can be up 308MPa, which is equal to the base metal,and the fracture strain can reach 76.7% to the base metal.A significant grain refinement in the FZ can be observed with the addition of the filler metal containing Mn.The equiaxed grains with the average size of about 6.0μm,only about 33% of that in the weld welded with filler strip of AZ31,were obtained with the filler metal of AZM1.5 (the content of Mn in the weld being 0.58wt%),and the average UTS of the joints can reach about 306MPa, which is 99.2% to the base metal, and the fracture strain can reach 81.6% to the base metal.The grains got finer and the distribution of precipitates changes from a relatively small number of scattered particles to a dense population of dendrites as the Al content increasing in fusion zone. And the tensil properties of the welded joints were found to be greatest with AZ61 filler strip.