This　study　investigates　a　novel　Al-Mg-Mn-Er-Zr　alloy　with　high　strength　and　elongation.　The　uneven　distribution　of　L12-Al3Er　and　Al3(Er,　Zr)　phases　in　the　molten　pool　structure　results　in　a　trilevel　equiaxed　heterogeneous　structure　in　the　alloy.　The　as-printed　alloy　exhibits　various　forms　of　Al3(Er,　Zr)　phases　and　Er-containing　primary　phases　(such　as　Al3(Zr,　Er)5),　demonstrating　an　excellent　combination　of　strength　(ultimate　tensile　strength　of　531　MPa,　yield　strength　of　455　MPa)　and　elongation　(fracture　elongation　of　27%).　The　strengthening　mechanism　and　strain　distribution　behaviour　of　the　heterogeneous　structure　are　explored　to　understand　the　alloy＇s　elongation　and　strength　mechanism.　The　large　equiaxed　grains　concentrated　in　the　centre　of　the　molten　pool　contribute　to　high　elongation,　while　heterogeneous　deformation-induced　(HDI)　strengthening　caused　by　heterogeneous　grains　increases　the　strength.　The　ultrafine　grains　at　the　molten　pool　boundaries　ensure　that　strength　remains　at　a　high　level,　achieving　a　good　balance　of　strength　and　elongation.　©　2024　The　Author(s).　Published　by　Informa　UK　Limited,　trading　as　Taylor　&　Francis　Group.