Stable　powder　injecting　into　a　weld　pool　in　gas　metal　arc　welding　(GMAW)　process　cannot　be　guaranteed　by　a　regular　by-pass-feed　method　due　to　the　strong　repelling　from　the　shielding　gas.　A　novel　trial　of　co-axial　through-the-arc　powder　injecting　is　thus　proposed　to　study　its　effect　on　the　bead　formation,　microstructure,　and　surface　hardness　of　aluminium-alloy　welds.　The　weld　depth　and　bead　width　could　be　both　increased.　Cellular　grains　grow　on　most　of　the　surface　of　TiC　particles,　some　of　which　even　grow　into　columnar　grains　and　dendrites.　Effective　grain　refinement　of　the　melted　zone　(MZ)　is　observed.　The　surface　hardness　of　the　cross　section　is　significantly　increased.　These　positive　results　would　be　beneficial　for　arc　welding　and　additive　manufacturing　processes.