Role of microstructural phases in enhanced mechanical properties of additively manufactured IN718 alloy

The effect of different microstructural phases such as laves, metal carbides, 6, y & PRIME;, and y & DPRIME;on mechanical and fatigue properties were extensively examined for additively manufactured Inconel 718 alloy at three different material conditions: as-built, as-built-machined, as-built-heat-treated. The as-built samples microstructure revealed the melt pools having the continuous structure of columnar and cellular dendrites of laves phases which form due to microsegregation of alloying elements. The machining of the as-built specimens reduced the surface roughness to 0.82 pm and removed the micro-sized pores existing on the surfaces, thereby increasing the tensile and fatigue strength by 5% and 50%, respectively. However, the heat-treatment of as-built specimens resulted in dissolution of melt pool boundaries along with the breakdown of continuous dendritic structure of laves phase, and evolution of the strengthening phases. As a result, the mechanical and fatigue properties improved appreciably as compare to the as-built ones: the microhardness by 27%, the tensile strength by 23.20%, and the fatigue strength by 150% (at R = -1). The examination of damaged surfaces revealed the existence of four types of fatigue damage mechanisms: lack of fusion, surface roughness, facets, and gas porosities. The observed changes in mechanical and fatigue properties could be very well correlated with the changes in microstructural level and are discussed in this article.