Effect of Homogenization on Creep of Additive Manufactured Alloy 718

Creep of additive manufactured (AM) and wrought alloy 718 has been examined in order to understand the difference in nature between the AM and wrought alloy. Three types of homogenization (solution) treatments, 982 degrees C (subsolvus: SUB), 1060 degrees C (supersolvus: SUP) and 1200 degrees C (ultra supersolvus: USS) for 2 h, were employed, prior to the standard aging treatment (760 degrees C/10 h + 650 degrees C/8 h -> AC) known as supersolvus aging in the wrought alloy. Note that, unlike the wrought alloy, fine particles of a few tens of nm in diameter remain with a high number density even after the USS treatment. All the specimens exhibit a similar hardness value of 500 Hv after the aging. Creep test was conducted at 650 degrees C under a constant stress of 630 MPa in air, and the creep rate was monitored through extensometer by linear valuable differential transducer. The creep rate ((epsilon)over dot) in the transient stage becomes slower with increasing homogenization temperature; the minimum creep rate of USS and of SUP at similar to 0.5% creep strain is one order of magnitude slower than that of SUB at 0.8% strain. However, in the accelerating stage, ((epsilon)over dot) at 2.5% strain becomes the same among the three specimens. Eventually the rupture time increases with increasing the homogenization temperature, which is 282 h, 822 h and 1006 h for SUB, SUP and USS specimens, respectively. These values are almost the same as or a little shorter than those of the wrought alloy. The rupture elongation is, however, limited to less than 5%, which is the highest for the USS specimen. The difference in creep deformation among the specimens and between the AM and wrought specimens is discussed in terms of creep rate versus time(strain) curves in conjunction with the nature of the AM and wrought microstructures.