Hot Isostatic Pressing of Additive Specimens, Grown from IN718 Alloy Powder Obtained by Gas Atomization

The mechanical properties of IN718 alloy specimens (both horizontal and vertical) made by selective laser melting (SLM) meet the requirements of AMS 5662M and are superior to those of the EP718-VD (ID) alloy. Heat treatment helps to increase and stabilize the strength properties. After the operation of hot isostatic pressing (HIP) and subsequent heat treatment, an increase in the plastic characteristics of the material and Young’s modulus is observed. It was found that vertical specimens (grown in the Z direction) are characterized by a slight deterioration of strength properties (within the requirements of AMS 5662M) and an increase in plasticity compared to the values for horizontal specimens. The heat-resistant properties of heat-treated specimens (grown in both XY and Z directions) meet the requirements of AMS 5662M; the time to failure exceeds ~4.5 times the requirements of AMS 5662M. It should be noted that the values of the long-term strength of horizontal specimens are ~1.6 times lower than those of vertical ones. The macrostructure of specimens made by SLM after HIP and heat treatment is dense and homogeneous, it is characterized by a fine-grained structure with macrograins elongated in the direction of specimen growth. Hot isostatic pressing followed by heat treatment contributes to the almost complete healing of pores and microdiscontinuities concentrated in the bulk of the metal. At the same time, globular and (or) film oxides were revealed in the healing zones. The microstructure of the specimens under study after heat treatment, as well as after HIP and subsequent heat treatment, is characteristic of the IN718 alloy in a normally heat-treated state. Heat treatment contributes to obtaining a more uniform structure due to the equalization of the chemical composition between the zones of layer-by-layer melting. Metallographic studies of IN718 alloy specimens showed that when the temperature of short-term mechanical tests increased to 700°C, the structure coarsened due to the coagulation of intermetallic phases, which clearly led to a decrease mainly in strength characteristics.