Research On Laser-arc Hybrid Additive Manufacturing Of High-strength Aluminum Alloy Grid Structure

High-strength aluminum alloys represented by Al-Cu alloy have been widely used in lightweight components of the aeronautic and aerospace fields due to their high strength,good fracture toughness,excellent weldability,strong heat resistance and corrosion resistance.As the typical representative of lightweight structures,grid structure has the advantages of high strength-weight ratio,large spatial stiffness and damage tolerance,and low manufacturing cost,thus has great potential for application.Additive manufacturing technology realizes structural manufacturing by accumulating materials layer by layer,which is one of the effective methods to manufacture high-strength aluminum alloy grid structures.However,there are intersection features in the grid structure.Laser additive manufacturing specimens are easy to crack,while arc additive manufacturing may lead to coarse microstructure,which seriously affects the service performance of manufactured components.In order to solve the problems,a new method for laser-arc hybrid additive manufacturing grid structure of Al-Cu alloy was proposed in this paper.The effects of laser-arc hybrid heat source and the scanning path on the microstructure evolution were explored,and a numerical model of molten pool at the intersection of grid structure was established.The relationship between microstructure and mechanical properties was analyzed,and the law of microstructure evolution and performance changes during hybrid additive manufacturing was revealed.The main research contents and related conclusions are as follows:(1)The hybrid additive manufacturing experiment was carried out,and the numerical model of the intersection forming process was established.The grid structure additive manufacturing was realized.Appropriate process parameters were selected for preparing grid structure specimens of Al-Cu alloy,such as laser power,arc current,scanning and wire feeding speeds,overlap degree and scanning path.Based on the computational fluid dynamics method,the three-dimensional numerical models including geometric model,heat source model,droplet model and molten pool force model were constructed to simulate the additive manufacturing process of the intersection in grid structure.The deviation between the simulated results with the actual was only 1.1%,and obvious arc-shaped cross features were observed at the intersections.(2)The microstructure distribution and evolution of the cross and longitudinal sections at the intersections of grid structure specimens were revealed.According to the differences in grain morphology,the two sections were divided into the top layer and the remaining layers.The microstructures inside single layer of the remaining layers were divided into the arc zone(AZ)and the laser zone(LZ).The AZ was mainly composed of coarse columnar,while the smaller G/R,larger G·R and recoil pressure promoted the growth of fine equiaxed in LZ.The distribution of Cu was relatively uniform,and the acicular θ’ phases were observed.The decrease in G/R from the boundary to the center in the remelting zone at the intersection(RZI)of the top layer promoted the transformation of equiaxed dendrites.The Cu was distributed in the dendritic shape,and the precipitates were mainly the large-size θ phases.(3)The microhardness and tensile properties of the deposited specimens were evaluated,and the fracture mechanism was clarified.It was found that the average microhardness in the RZI were 98.6±4.5 HV0.1 and 97.1±5.9 HV0.1,respectively.Compared with the unremelting zones,the maximum increase was 12.1%,but it was 7.9% lower than that of the remaining layers.The yield strength,tensile strength and elongation were 146.5±2.2 MPa,290.3±5.9 MPa and 13.6±0.8%,respectively.The precipitation strengthening and refinement of eutectics were the main factors for the strength improvement.The fracture mode in AZ was mainly intergranular fracture,while it was dominated by transgranular fracture in LZ.Compared with the little pores,the eutectics distributed along the grain boundaries were the primary causes of the fracture.