The　flow　and　slip　rate　of　material　in　friction　stir　welding　mainly　determined　the　weld　quality,　flow　rate　depended　not　only　on　the　shape　of　pin　and　welding　parameters　but　also　on　slip,　and　an　instantaneous　centre　of　speed　was　generated　by　rotating　speed　and　welding　speed.　The　relative　speed　of　contact　interface　is　generally　an　important　factor　in　material　flow.　The　material　experienced　different　viscous　slips　when　rotating　around　the　pin,　which　disengaged　pin　at　a　larger　slip　and　adhered　to　pin　at　a　smaller　slip,　resulting　in　large　differences　in　two　sides.　The　first　derivative　of　the　dividing　line　at　the　boundary　of　SZ　was　extracted　to　investigate　material　flow.