| Al-Cu alloys have been widely concerned in aerospace and other industries due to their low density,high strength and good heat resistance.Wire-arc additive manufacturing(WAAM)technology can save raw materials,shorten manufacturing cycles and reduce manufacturing costs while maintaining or improving component performance.However,there are still some problems in WAAM Al-Cu alloy,such as low performance after forming and many defects.The application of interlayer rolling can significantly address these problems.In addition to optimizing the additive manufacturing process,heat treatment of Al-Cu alloys is also a commonly used method to improve the mechanical properties of alloys in industry.However,the microstructure of additive manufacturing alloys is different from that of cast alloys,so the traditional heat treatment process is not necessarily suitable for additive manufacturing Al-Cu alloys.In this paper,the MicroRolling WAAM Al-Cu alloy was used as the research object.The microstructure evolution and precipitation behavior of traditional T6 treatment and two-stage aging treatment WAAM Al-Cu alloy were systematically studied.The effect of mechanical properties,the evolution of precipitated phases during thermal exposure at 300 °C and the fracture failure mechanism of WAAM Al-Cu alloy were also investigated.The main results are as follows :Micro-Rolling can eliminate most of the solidification shrinkage pores in the deposition process of WAAM Al-Cu alloy.After T6 heat treatment,high-density fine pores are formed due to the precipitation of hydrogen.The slight coarsening of the original pores is mainly controlled by the merging of micropores or Ostwald ripening.After T6 treatment,most of the eutectic structures distributed continuously or semicontinuously at the grain boundaries in the as-deposited state are dissolved in the matrix,and the undissolved θ and high-density needle-like θ’’ phases are still retained inside the grains.The ultimate tensile strength and yield strength of the alloy after T6 treatment reached 454 ± 24 MPa and 357 ± 17 MPa,respectively.The strengthening mechanism of WAAM Al-Cu alloy at room temperature is mainly precipitation strengthening.Compared with T6 treatment,two-stage aging treatment can further improve the mechanical properties of the alloy,especially the high temperature properties of the alloy.This is mainly because many grain boundary Fe-rich phases formed during the deposition process are very stable at high temperatures and can effectively prevent grain boundary slip.In addition,the alloy forms more GP zones at the first aging temperature,and the alloy can precipitate high density θ′′ phase.The high temperature tensile strength and yield strength of the two-stage aging WAAM Al-Cu alloy at 300 °C can reach 340 ± 2 MPa and 314 ± 4 MPa.During the thermal exposure process,the properties of the alloy decrease sharply within 0-1 h.The θ′′ phase in the alloy with a coherent relationship with the matrix is rapidly transformed into the θ′ phase with a semi-coherent relationship with the matrix,and the θ′ phase is rapidly coarsened and the number density decreases.The properties of the alloy are basically stable in 2-48 h,and the size and number density of the θ′ phase in the alloy fluctuate but the change is not obvious.The tensile strength and yield strength of the additive manufactured Al-Cu alloy at 300 °C are stable at about 199 ± 3 MPa and 151 ± 2 MPa.In addition,combined with in-situ observation experiments,it is confirmed that continuous or semi-continuous eutectic structure and pore defects on grain boundaries are the main factors leading to the fracture of deposited alloys.Aging heat treatment eliminates most of the eutectic structure,but the further increase of pore defects in heat treatment is still the main position of micro-crack initiation.After thermal exposure,part of the eutectic structure reappears on the grain boundary,which together with the pore defects affect the fracture properties of Al-Cu alloy. |
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