In　this　study,　2219-T87　aluminum　alloys　were　butt　welded　by　the　double-pass　tungsten　inert　gas　arc　welding　process.　And　the　softening　behavior　of　fusion　zone　(FZ)　and　heat-affected　zone　(HAZ)　was　evaluated　with　the　analysis　of　welding　temperature　field,　grain　size,　alloying　element　distribution　and　precipitates　evolution.　Results　show　that　the　two　FZs　are　almost　the　weakest　regions　in　the　joint,　where　the　microhardness　value　is　76　and　78　HV,　respectively.　Microhardness　of　the　HAZ　generally　grows　along　with　increasing　distance　from　fusion　line　except　a　valley　value　at　the　distance　of　about　4.5　mm.　The　mean　grain　size　of　two　FZs　is　about　74.4　and　79.2　mu　m,　whereas　41.5,　44.9　and　43.4　mu　m　for　the　two　measured　HAZs　and　base　metal　(BM),　respectively.　There　is　about　60.4%　and　54.2%　Cu　consumed　in　the　coarse　whitish　particles　of　FZs　that　have　little　strengthening　effect,　while　the　percentage　is　about　24.6%　of　BM　that　is　almost　the　same　as　HAZ.　A　large　number　of　strengthening　phases　theta＇　distribute　dispersively　in　BM,　whereas　hardly　any　precipitates　exist　in　FZ　and　HAZ　adjacent　to　FZ.　So　the　coarsening　of　grain　size,　reduction　and　segregation　of　alloying　element　content,　and　the　precipitate　evolution　are　regarded　as　the　main　causes　of　softening　in　FZ,　while　the　precipitate　evolution　is　the　main　factor　of　softening　in　HAZ.