Microstructures and mechanical properties of laser solid formed nickle base superalloy Inconel 718 prepared in different atmospheres

Due to the rapid melting and solidification in laser molten pool, the laser solid forming (LSF) samples have generally much finer microstructure than that of the conventional cast, and are also free of macrosegregation. To date, more and more researchers have reported the application of LSF technology in the forming of superalloy, which contains some easily segregated elements such as Nb, Ti and Al. To prevent the molten pool from oxidation, the LSF process usually should be performed within a protective atmosphere. But in practice, the forming of oxidation-resistant metals are always performed in the air atmosphere and only the molten pool is protected by noble gas. Therefore, it is important to investigate the microstructures and properties of LSF materials prepared in the air and noble gas. In the present study, the microstructures, tensile properties and high cycle fatigue properties of LSF Inconel 718 alloy samples prepared in the air and argon atmospheres were investigated. It is indicated that the microstructures in as-deposited and heat-treated LSF Inconel 718 alloy samples prepared in both atmospheres are similar. The as-deposited microstructure consists of columnar dendrites grew epitaxially along the deposition direction from the substrate, and changes to the equiaxed grains after solution heat treatment due to the recrystallization. The tensile properties of the two kinds of the samples after heat treatment reached the standard for wrought Inconel 718 alloy. The tensile strength of the sample prepared in air is slightly higher than that of the sample prepared in the argon atmosphere, and the high cycle fatigue lifetime of the latter is 30% higher than that of the former, which is related to the fewer defects such as micro pores and small size oxide inclusions existed in the former. The lower high cycle fatigue property of LSF Inconel 718 alloy than that of wrought Inconel 718 alloy is attributed to the metallurgical defects and the coarsening and uneven distribution of the recrystallized grains.