Effect of hot isostatic pressing on the microstructure and mechanical properties of Udimet 720Li alloy formed by laser melting deposition.

In this paper, the effect of hot isostatic pressing (HIP) treatment on defects, microstructure, precipitated phases and mechanical properties of Udimet 720Li alloy formed by laser melting deposition (LMD) was investigated. The results showed that the internal microcracks of Udimet 720Li alloy formed by LMD were completely eliminated after HIP treatment, but a few small pores still existed, and the density of the sample were improved from 97.06 % to 99.97 %. Partial recrystallisation occurred in the as-deposited sample. After HIP treatment almost complete recrystallisation occurred, and the grain morphology changed from epitaxially grown columnar grains to equiaxed grains with a decreasement in grain size. After HIP treatment, the MC carbides in the sample were changed from granular to large-sized lumps. For the sample with hot isostatic pressing and heat treatment (HIP + HT), a large number of carbides dissolution, the carbides morphology became granular diffusely distributed in the grain. Meanwhile, the HIP treatment eliminated the γ/γ′ eutectic phases present due to solute segregation, and a large number of secondary γ′ phases were uniformly precipitated within the grain. After the HIP + HT treatment, the average size of the secondary γ′ phase decreased from 283.4 nm to 266.3 nm, and the quantity was significantly enhanced. The HIP treatment reduced the mechanical properties of Udimet 720Li alloy, the microhardness decreased from 480 HV to 404 HV and the tensile strength decreased from 1258 MPa to 1234 MPa, which was mainly related to the decrease in the number of γ′ phases and the increase in the size of carbides. After HIP + HT treatment, the room temperature tensile properties of the samples were enhanced (σ UTS = 1394 MPa, σ YS = 702 MPa, ε f = 18.33 %), which was mainly attributed to the precipitation of more small-sized secondary γ′ phases by heat treatment. [ABSTRACT FROM AUTHOR]