The　butt　joint　of　A7204P-T4　aluminum　alloy　sheets　with　a　thickness　of　6　mm　was　welded　via　fiber　laser-cold　metal　transition　(CMT)　arc　hybrid　welding,　with　various　sizes　of　niobium　foil　attached　to　the　butt　interface　before　welding　(niobium　mass　fractions　were　0.74%　and　1.36%　in　the　weld).　Well-formed　joints　were　obtained　by　optimizing　the　process　parameters.　The　effects　of　niobium　contents　on　microstructure　and　mechanical　properties　of　hybrid　welded　joints　were　studied,　and　the　tensile　fracture　mechanism　and　fracture　morphology　of　welded　joints　were　also　analyzed.　Experimental　results　show　that　the　microstructure　of　niobium-free　welds　mainly　comprises　fine　crystal　regions,　columnar　crystalline　regions,　and　equiaxed　dendrite　regions.　After　adding　niobium　foil,　grain　refinement　was　observed　in　the　weld　metal　due　to　the　segregation　of　niobium　solute　and　heterogeneous　nucleation　of　niobium　precipitates,　and　the　apparent　disappearance　of　columnar　and　dendritic　structures　were　also　obserbed.　After　mass　fraction　of　0.74%　niobium　was　added,　the　average　grain　sizes　of　fusion　zone　and　weld　center　decreased　by　57.9%　and　55%,　respectively.　The　average　tensile　strength　of　the　joints　without　niobium,　0.74%　and　1.36%　mass　fraction　of　niobium　were　325　MPa,　334.5　MPa　and　328　MPa.　The　post-break　elongation　of　the　welded　joint　with　a　mass　fraction　of　0.74%　niobium　was　6%,　increasing　by　71%　compared　to　joint　without　niobium　addition.　The　fracture　surfaces　of　the　three　tensile　test　samples　were　mainly　dimples　accompanied　by　obvious　tearing　edges,　showing　microporous　aggregate　fracture　characteristics.　©　2020,　Chinese　Lasers　Press.　All　right　reserved.