Effect of direct aging on microstructure and properties of 18Ni350 maraging steel fabricated by wire arc additive manufacturing

The 18Ni350 maraging steel（M350） straight-wall component is fabricated using wire arc additive manufacturing（WAAM） process. By employing direct aging heat treatment， the microstructure and mechanical properties of M350 are controlled. The effect of different aging conditions （aging temperature and aging time） on the microstructure and performance of M350 is studied. The results show that the solidification microstructure of M350 fabricated by wire arc additive manufacturing consists of columnar dendrites and cellular dendrites， with segregation of Ni， Mo， and Ti elements observed in the interdendritic regions.During the direct aging process， a reverse transformation occurs in the interdendrite region where Ni， Mo， and Ti elements segregate，leading to the conversion of martensite phase to austenite phase. With the increase of aging temperature and aging time， the size and quantity of reversed austenite increase. The microhardness， yield strength （YS）， and ultimate tensile strength （UTS） first increase and then decrease. The peak microhardness （534HV）， YS （1600 MPa）， and UTS （1658 MPa） are achieved at 530 ℃ aging for 3 h. At the same time， the elongation after fracture remains a value above 13%. In addition， the M350 fabricated by wire arc additive manufacturing demonstrates mechanical anisotropy， with the anisotropy difference peaking under the optimal aging conditions， exhibiting a YS difference of 360 MPa and a UTS difference of 287 MPa.