Effects Of CeO₂ Addition On The Microstructure And Properties Of 6061-T6 Aluminum Alloy Weld

Laser welding technology has the advantages of low heat input, small thermal deformation, high depth-width ratio and high welding speed. So the laser welding of aluminum alloy in automotive lightweight production has a broad application prospect. In this dissertation, the 6061-T6 aluminum alloy was welded by filling CeO₂ and Al mixed powder used YLS-6000 6 k W fiber laser system. The microstructure, porosity and mechanical properties of the 1 mm and 2 mm thick joints with filling different content of CeO₂ powder were studied. Part of the 2 mm thick joints were treated by single and double side laser shock peening（LSP）. The microstructure, porosity, residual stress, micro-hardness and tensile strength of the weld after LSP were also studied, in this dissertation. The main research contents and conclusions are as follows:（1） All the joints had no cracks but the porosity existed. The microstructure of the weld showed the change characteristics from columnar grains near fusion boundary to dendritic and equixed grains in the center of fusion zone, and the second dendrite arm spacing in the center of fusion zone were finer.（2） Effects of no powder and filling Al+0.5 wt-%CeO₂、Al+1.0 wt-%CeO₂ powder on 2 mm thick 6061-T6 aluminum alloy weld were studied. The weld of filling Al+1.0 wt-%CeO₂ powder had finer grain than filling Al+0.5 wt-%CeO₂ powder, and the weld without filling powder had coarsest grain. Filling Al+0.5 wt-%CeO₂ powder can get the lowest porosity ratio. Right amount of rare earth element Ce can reduce the generation of porosity. But instead, too much element Ce increases the generation of porosity. The weld with filling Al+0.5 wt-%CeO₂ powder had the highest value of micro-hardness and tensile strength which were 81.5 HV, about 67.9% of the base material, and 221.61 MPa, about 67.3% of the base material, respectively.（3） The 1 mm thick 6061-T6 aluminum alloy was welded with filling 0.5, 1.0 and 1.5 wt% content of CeO₂ powder. Results show that with the increase of CeO₂ content, the porosity increased and the micro-hardness and tensile strength decreased. Porosity content and heat input were two main factors that affect the weld tensile strength under different laser power. Porosity can increase the crack sensitivity, and make the joints fractured more easily. Under the same heat input, the tensile strength was mainly influenced by porosity ratio in the weld.（4） Laser shock peening can effectively reduce the surface and sub-surface layer porosity and refine the grain of the 2 mm thick 6061-T6 aluminum alloy weld. Tensile strength of the weld after LSP also improved. The smallest grain size was in the range of 0.3-0.5 μm after LSP, which was finer than that of that without LSP（grain size was more than 3 μm）. The porosity ratio was reduced from 2.42% to 0.82% after double-side LSP. The change of porosity morphology changed the residual stress after LSP. Tensile strength of the weld after double-side LSP was improved from 203.98 MPa to 237.9 MPa. But the effect of single-side LSP was limited.In this dissertation, the influence of laser power, rare earth element and LSP on the microstructure, porosity and mechanical properties of 6061-T6 aluminum alloy laser weld were studied. Laser shock peening was used to reduce the surface porosity so as to improve the mechanical properties of the weld, which was an innovative of this research work. This study provides a theoretical basis in the design of powder composition, selection of process parameters and properties improvement after LSP for automotive aluminum alloy laser welding.