In　order　to　realize　lightweight　manufacturing　of　gear　parts,　the　cold　extrusion　forming　process　of　steel-aluminum　composite　metal　gears　was　studied.　Using　numerical　simulation　software　Deform,　the　effects　of　five　steel　rings　with　different　thicknesses　on　forming　pressure　and　shapes　of　formed　forgings　were　compared.　The　simulation　results　showed　that　final　forming　force　of　the　gear　was　smaller　and　the　radial　deformation　degree　of　steel　ring　was　larger　when　using　thinner　steel　ring.　This　radial　deformation　degree　was　characterized　by　height　difference　between　the　convex　and　the　concave　in　steel-aluminum　bonding　position,　and　great　deformation　presented　as　obvious　bimetal　occlusion.　The　physical　experiment　verified　numerical　simulation　results,　and　meanwhile　found　that　the　thickness　of　steel　sleeve　had　an　important　effect　on　bonding　tightness　of　steel-aluminum　contact　surface　of　formed　part　and　the　composite　gear　formed　by　thin　steel　ring　had　a　gap　at　tooth　tip　between　steel　and　aluminum.　Comprehensive　analysis　suggested　the　ratio　of　outer　diameter　to　inner　diameter　(R0/Ri)　of　steel　sleeve　reflected　different　deformation　regularity.　Consequently,　the　thick-walled　axial　compression　model　was　used　to　analyze　forming　process　of　steel　sleeve　with　high　R0/Ri　ratio,　and　the　thin-walled　cylindrical　shell　model　was　used　to　analyze　forming　process　of　steel　sleeve　with　low　R0/Ri　ratio.　The　axial　compression　of　thin　cylindrical　shell　produced　local　buckling　deformation,　which　led　to　the　appearance　of　gaps　in　steel-aluminum　bonding　position.　In　order　to　avoid　gaps　in　bimetal　contact　surface,　the　influence　of　steel　ring　thickness　on　deformation　coordination　of　two　metals　should　be　fully　considered　in　cold　extrusion　of　steel-aluminum　composite　metal　gear.　©　Published　under　licence　by　IOP　Publishing　Ltd.