Effects of beam shaping on copper-steel interfaces in multi-material laser beam powder bed fusion

Abstract

Multi-material laser beam powder bed fusion (PBF-LB) is a promising approach to meet industrial demands for multifunctional components, due to its enhanced freedom of design. Copper and stainless steel are interesting materials to be combined due to the high electrical and thermal conductivity of copper and the mechanical strength and wear resistance of steel. However, this system suffers from copper contamination cracking (CCC), a phenomenon which leads to intergranular cracking of the steel due to penetration of liquid copper. The more these materials intermix, the greater the amount of copper that can penetrate and damage the steel. Mixing is more intense when steel is below copper during printing, a situation that cannot always be avoided due to design constraints. In this sense, this research tackles the worst scenario of printing CuCrZr onto 316L and analyses the effect of beam shaping on interface quality. For the first time, a ring-mode laser was applied to multi-material PBF-LB with the goal to reduce intermixing. It was observed that using a ring-mode laser source instead of a Gaussian beam decreases mixing of the two alloys and reduces interface defects, albeit being unable to completely suppress cracking. Grain size and texture were also affected by the beam shape, with the Gaussian beam leading to formation of small equiaxed grains along 20 layers of CuCrZr, resulting in random texture. The ring-mode laser source, on the other hand, yields columnar grains and <100> texture along the build direction. Both beam shapes were observed to induce supercooling below the liquid miscibility gap in the Cu–Fe system. Besides the expected γ-Fe and ε-Cu phases, iron was also found to crystallize in a body-centered cubic (BCC) lattice structure, which is believed to happen due to Ni depletion of the steel in Cu-rich compositions, coupled to insufficient copper within the Fe-rich liquid during solidification. BCC fractions are higher for the Gaussian beam. These differences in microstructure and BCC fraction are attributed to the high intermixing of CuCrZr and 316L when applying Gaussian beams, which is around 10 times higher than with the ring-mode laser.