Microstructure evolution and mechanical properties of reduced activation steel manufactured through laser directed energy deposition

Abstract

A new laser metal desposition process based on an inside-laser coaxial powder feeding system was successfully applied to manufacture reduced activation steel, which is featured with fine microstructure and excellent mechanical properties. In addition, infrared thermal imaging experiments and Abaqus numerical simulation were conducted to characterize the complex thermal history during the laser metal deposition process. The microstructure and mechanical properties of reduced activation steel were systematically investigated in the as-fabricated and heat-treated samples. The results indicat that the peak temperature increased and the cooling rate decreased in the melt pool when the additional layers were deposited as a result of a cumulative effect of heat in the fabricated thin wall samples. The reduced cooling rate directly contributed to the decreased heterogeneous nucleation rate and the coarsening of austenite grains in the top domain. The differences in terms of microstructure and hardness of the as-fabricated samples along the building direction were also in a good agreement with the evolution of temperature field. The thermal cycling experimental and cyclic heat treatment results confirmed that in-situ thermal cycles were unable to trigger recrystallization because the stored strain energy was insufficient to induce nucleation of new austenite grains during laser directed energy deposition.