We have developed a scalable approach to directly incorporate grain-refining nanoparticles into conventional hot-tear-susceptible pure aluminum or aluminum alloy powders. These aluminum alloy powders may be additively manufactured into high-strength, crack-free aluminum alloys with fine equiaxed microstructures by incorporating nanoparticle nucleants to control solidification during additive manufacturing. Some variations provide an additively manufactured aluminum alloy comprising aluminum and at least one grain-refining element, wherein the additively manufactured aluminum alloy has a microstructure with equiaxed grains. Pure aluminum or aluminum alloys, combined with grain refiners, are useful in many processes beyond additive manufacturing. Some variations provide an aluminum alloy comprising aluminum and grain-refining nanoparticles selected from zirconium, tantalum, niobium, or titanium, wherein the aluminum alloy has a microstructure that is substantially crack-free with equiaxed grains.