Directed energy deposition of Al-Mg-Si alloys with fully-equiaxed microstructure and isotropic high strength/ductility

Al-Mg-Si alloys fabricated by additive manufacturing (AM) are known to suffer from the coarse columnar microstructure, which usually causes severe hot cracking and anisotropy in mechanical properties. Introducing potent nucleants to promote columnar to equiaxed transition (CET) is an efficient method to solve this problem. However, nanoscale nucleant particles are prone to agglomerate and form clusters during high-temperature AM, which are detrimental to the ductility. In this work, in the AA6061 fabricated by directed energy deposition (DED), we designed a novel approach to alleviate the agglomeration of TiB₂ particles by adopting interlayer pause (IP) strategy on the basis of TiB₂ nucleant addition. Resultantly, due to the discontinuous heat input interrupted by IP and thus a reduced heat accumulation, dispersed fine TiB₂ particles can be achieved, which reduce the stress concentration and allow for a good metallurgical binding. Moreover, these dispersed fine TiB₂ particles, (a) effectively promote the CET by providing numerous heterogeneous nucleation sites, resulting in an ultra-fine fully-equiaxed grain microstructure (∼ 21 µm) with a reduced texture; (b) significantly inhibit the formation of pores, reducing both their density and size. Consequently, crack-free high-performance DEDed AA6061 without anisotropy in mechanical properties are successfully fabricated, achieving high strength and high ductility at T6-treated state along both the vertical direction (YS: 266 ± 2 MPa, EL: 8 ± 1%) and the horizontal direction (YS: 262 ± 2 MPa, EL: 7 ± 1%), simultaneously. This combination of nucleant addition and deliberately-controlled IP strategy is expected to be applicable to a wide range of wrought aluminum alloys (e.g. 2xxx and 7xxx) fabricated by AM.