Dynamic Modulus Anomaly in Metallic Alloys Prepared by Additive Manufacturing

Dynamic modulus vs. temperature was measured in different alloys (stainless steels, Al alloys, Ti alloys, Ni-base superalloys) prepared by additive manufacturing and an anomalous trend was observed in some of them. Dynamic modulus, measured in successive mechanical spectroscopy test runs with heating-cooling cycles, exhibits an anomalous trend in the first test run that is no longer present in the successive runs. The phenomenon consists in the inversion of the decreasing trend of modulus occurring during heating and gives rise to its permanent increase at the end of the complete heating-cooling cycle. The temperature range where the modulus anomaly takes place and the permanent increase observed after cooling depend on the specific alloy. Scanning electron microscopy (SEM) observations and density measurements revealed that the irreversible process causing the anomalous behavior is the closure of pores of nanometric size leading to material densification. This result has been discussed by considering lattice diffusion.