A relationship between the build and texture orientation in tensile loading of the additively manufactured maraging steels

Abstract

This research reveals the effect of building direction on the microstructure, mechanical properties and the deformation behaviour of additively manufactured maraging steels. 18Ni-300 maraging steel samples were manufactured throughout the laser powder bed fusion (LPBF) process in horizontal and vertical directions. Uniaxial tensile tests were then conducted for as-built and heat-treated specimens to study the mechanical properties before and after the heat treatment process. The horizontal samples showed higher strength and ductility than the vertical ones, while the heat-treatment procedure reduced the building direction effect on the mechanical strength. Ductility was slightly different amongst the four sets of specimens. The heat-treatment increased the mechanical strength significantly at the expense of decreasing the ductility. Fractography through scanning electron microscopy showed that the aged fracture surfaces contain cleavage along with dimples. Transmission electron microscopy revealed the austenite phase enclosed by the martensite laths and segregated alloying elements and precipitations, resulting in a hierarchical microstructure. Further studies on the deformation behaviour in the four cases of as-built/aged, horizontal/vertical combinations were conducted. X-ray diffraction technique was adopted to study the phase changes through the deformation process. The as-built specimens showed a fully transformed austenite phase, while the aged samples reduced the austenite fraction to half at the fracture. Finally, the EBSD technique was conducted to show the microstructure evolution through the manufacturing process. The material showed an elongated grain structure towards the building direction. Heat-treatment did not eliminate the texture along the building direction, while the deformation process led to a texture evolution from < 100 > and < 111 > into < 110 > directions.