Additive manufacturing and mechanical properties of the dense and crack free Zr-modified aluminum alloy 6061 fabricated by the laser-powder bed fusion

For additive manufacturing such as laser powder bed fusion (LPBF), commercial aluminum alloy (AA) 6061 is typically considered unsuitable due to formation of solidification cracking and/or excessive porosity. In this study, to improve buildability/printability of AA6061, 1 wt% of Zr was alloyed to produce Zr-modified AA6061 by LPBF. Powders of unmodified and Zr-modified AA6061 were produced by gas atomization, and utilized as a feed-stock for the LPBF to fabricate specimens for microstructural examination and mechanical testing. The as-built unmodified AA6061 exhibited poor printability due to formation of cracks and porosity in the microstructure regardless of LPBF parameters. However, the Zr-modified AA6061 exhibited near full density, with substantial reduction in porosities without any solidification crack for a certain LPBF processing window. The improved printability of Zr-modified AA6061 was attributed to a significant grain refinement, which would reduce the solidification cracking susceptibility by hampering the epitaxial growth of long columnar cracks, as observed in unmodified AA6061. Yield strength, tensile strength and strain-at-failure for the as-built Zr-modified AA6061 were determined to be 210 MPa, 268 MPa, and 26.5%, respectively. These are superior to the tensile properties of AA6061 in O-annealed condition or in as-cast condition. After T6 heat treatment, yield strength, tensile strength and strain-at-failure of Zr-modified AA6061 were determined to be 300 MPa, 327 MPa, and 14%, which were again superior to the tensile properties of wrought AA6061 in T6 heat treated condition. Effects of Zr addition on the buildability/printability improvement and mechanical properties of AA6061 were corroborated by a variety of electron microscopic characterization.