EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

This paper compares the fatigue properties of Inconel 625 elaborated by Laser Powder Bed Fusion (L-PBF) and Direct Energy Deposition (DED), at room and service temperatures. The two processes are based on metallic powder fused by laser, and the microstructures obtained are similar. A high chemical inhomogeneity is revealed by dendritic structures surrounded by the micro-segregation of heavy elements, and a grain size and shape that depend on the process parameters chosen. Similarly, porosities and lack of fusion are defects commonly encountered in both. The major difference is that DED has coarser grains and dendrites due to lower solidification and cooling rates, as well as bigger defects.
First, stress controlled fatigue tests at room temperature were performed using a loading ratio of R=0,1. For the DED, it was shown that the high cycle fatigue limit was higher than its yield tensile strength (YTS), while as-built L-PBF barely withstands 40% of its YTS. Fractographic investigation highlighted that the initiation takes place on surface or sub-surface lack of fusions in both cases, but the propagation mechanisms are different. In spite of the bigger size of defects and its lower mechanical properties, the DED microstructure accommodates the defects better than the L-PBF.
Then, the oligocyclic fatigue behaviours at the service temperature (650°C) of the microstructures obtained by the two additive manufacturing processes were compared using the strain controlled fatigue tests with a strain ratio of R=-1. The difference in their tensile properties makes it difficult to compare their performance in terms of total strain. It is suggested to compare the Manson-Coffin behaviour using the inelastic strain, and thus at similar mechanisms plastic phenomenon.
Finally, the effects of different annealing treatments on the metallurgical and mechanical properties of these two microstructures were explored.