Selective laser melting of micron-sized niobium functionalized Al7075 alloy: Element evaporation and grain refinement

The broad industrial applications of selective laser melting (SLM) are hindered by the limited printable alloy systems. Among the unprintable alloys, the high-strength Al7075 alloy remains challenging to be processed with SLM due to the formation of hot cracks. In the present work, the inoculation treatment of micron-sized niobium (Nb) powders into Al7075 powders could refine the grains due to the formation of Al3Nb particles, healing cracks by reducing the crack length density. Furthermore, a processing compromise existed between the element evaporation of Mg/Zn and processability during SLM of Al7075 and micron-sized Nb functionalized Al7075 powders. The lower scanning speed, leading to a more severe evaporation of Mg and Zn, could result in a better processability with a lower crack area and length density. The improved printability of SLMed Al7075 with less Mg and Zn content could be explained by the decrease of hot cracking susceptibility. Coupled with the low scanning speed and inoculation treatment of Nb, the as-built Al7075 + 1 vol% Nb alloy could exhibit a low crack length density of 1.29 × 10−5/μm and exhibited a yield strength of 258 ± 13 MPa with an elongation to fracture of 11.1 ± 3.7%. The yield strength and elongation to fracture increased to 283 ± 2 MPa and 16.0 ± 2.8% after the T6 heat treatment. The marginal enhancement of strength is due to the evaporation of strengthening elements of Mg and Zn, lacking precipitation strengthening phase. The compromise among the element evaporation of Mg and Zn, processability, and mechanical properties provided the strategy to balance the printability and mechanical properties during SLM of Al7000 or micron (nano)-functionalized Al7000 series alloys.