Rotary　friction　welding　dissimilar　metals　of　AA1100　aluminum　to　H59　brass　was　conducted　with　a　20-mm　cylinder　sample.　The　friction　pressure　is　90 MPa　and　the　welding　time　is　15 s.　Preheating　to　600 K　and　873 K　were　conducted　to　soften　H59　brass　(i.e.,　the　hard　side),　so　that　the　both　metal　with　great　differences　in　physical　properties　could　deform　simultaneously　at　the　friction　interface.　The　deformation　rate　was　calculated　by　solving　constitution　equation,　and　the　necessary　parameter　interfacial　temperature　was　obtained　by　infrared　thermal　imaging　analysis.　SEM　observation　and　EDS　analysis　were　jointly　conducted　to　clarify　microstructure　and　elements　distribution　across　the　interface.　The　results　show　that　the　joint　produced　without　preheating　brass　contain　some　unwelded　areas　and　therefore　presents　lower　tensile　strength　(30 MPa)　just　33%　of　the　aluminum　base　metal.　Preheating　results　in　large　temperature　difference　(166 K)　on　both　side　of　interface.　Ratio　of　deformation　rate　Ψ　is　defined　to　characterize　deformations’　difference.　Ψ　drops　two　orders　of　magnitude　(from　600　decrease　to　5)　when　brass　was　preheated　to　600 K,　but　raising　preheating　temperature　to　873 K,　its　value　has　minor　change　(from　5　decrease　to　1).　Smaller　Ψ　value　implies　a　comparative　shear　deformation　and　viscoplastic　flow　on　both　sides’　surface.　The　welding　interface　presents　layered　structure,　which　is　composed　of　CuAl2　+　CuAl　+　Cu9Al4　laminate.　With　a　thin　IMCs　layer　(3 μm),　the　joint　has　highest　tensile　strength　63 MPa.　©　2022,　International　Institute　of　Welding.