Microstructure And Mechanical Properties Of 7B05 Aluminum Alloy Repaired By Laser Directed Energy Deposition

Direct Energy Deposition（DED）has obvious advantages in manufacturing large size parts and repairing damaged parts.7 series aluminum alloy repaired by DED technology has some problems,such as difficult matching of repair materials,many metallurgical defects and low mechanical properties.In this paper,Al-Mg-Sc alloy powder is selected as DED repair material and 7B05 aluminum alloy sheet is taken as the repair object to study the mechanical properties of 7B05 aluminum alloy repaired by DED technology,and the microstructure at the interface between the repair area and the base metal and the repair area.The main conclusions are as follows:（1）The influence of process parameters on phase composition,microstructure and mechanical properties of 7B05 aluminum alloy repaired by DED technology was studied.When the powder feeding amount is too large,the duration of liquid phase decreases,and cracks and spheroidization appear on the interface between the repaired area and the base metal.The amount of powder feeding is too small,the element ablation is serious,and cracks and holes also appear on the interface between the repaired area and the base metal.When the laser energy density is 266 J/mm³,the maximum tensile strength is 315 MPa.Due to the extremely fast solidification rate at the interface between the repaired zone and the base metal,columnar crystals appeared in the molten pool at the interface,and Al₃（Sc,Zr）particles gathered in part of the melting zone,which enhanced the mechanical properties.Considering the mechanical properties and the number of defects,the optimal technological parameters are powder feeding capacity of 0.6r/min,laser power of 1600 W,scanning rate of 600 mm/min,scanning spacing of 1.2 mm and single-layer lifting height of 0.5 mm.（2）In order to reduce defects in repaired samples,the effect of Si content on microstructure and properties of 7B05 aluminum alloy repaired by DED technology was studied.With the increase of Si content,the solidification rate decreases and the number of pores decreases.As the density of Mg₂Si increases,the microhardness increases.The maximum hardness of the deposited samples reaches 88.5HV,and the maximum yield strength and tensile strength increase to 225 MPa and 290 MPa,respectively.Due to the decrease of the number of holes and the increase of the density of Mg₂Si,the elongation first increases and then decreases,with a maximum value of 7%.The optimum Si content powder was used to repair 7B05 aluminum alloy,and the number of holes in repaired samples decreased significantly（3）In order to improve the mechanical properties of samples,the effect of T6 treatment on the microstructure and properties of 7B05aluminum alloy repaired by DED technology was studied.Due to the precipitation of Al₃（Sc,Zr）particles and the refinement of Mg₂Si morphology,the mechanical properties of the repaired samples are improved,the elongation of the repaired samples is 11%,and the maximum tensile strength is 320 MPa.Because Al₃（Sc,Zr）particles pin the grain boundary to inhibit recrystallization,the molten pool shape becomes narrow,and the preferred grain orientation at the junction between the repair zone and the base metal disappears.