Microstructure and mechanical properties of directed energy deposited U75V/15–5PH structurally graded material

In this study, 15–5 precipitation hardening (PH) martensitic steel powder was successfully deposited on U75V eutectoid pearlitic steel to form structurally graded material (SGM) using directed energy deposition (DED) technology. Microstructure evolution mechanism and mechanical properties of U75V/15–5PH SGM were investigated systematically. Results show that the microstructural transition zone (MTZ) is formed above the fusion line due to the dilution effect of eutectoid pearlitic steel. The phase composition of MTZ consists of martensite, retained austenite, and carbides, which is distinct from that of directed energy deposited 15–5PH steel. MTZ possesses the highest microhardness owing to the solid solution strengthening and second phases strengthening of carbon elements provided by eutectoid pearlitic substrate. Special intrinsic thermal cycles of DED lead to the formation of heat affected zone (HAZ) below the fusion line, in which the spheroidization phenomenon of lamellar cementite occurs. The spheroidization degree of lamellar cementite in HAZ is exacerbated with increasing distance from the fusion line. Compared with wrought U75V substrate, the lower tensile strength of SGM is mainly attributed to the weak phase boundaries strengthening of granular pearlite at the bottom of HAZ.