Study On Protection Layer For Precision Improvement Of Hot Forging Of Bevel Gear

Gears are the most important transmission parts used to transmit motion and power between two space shafts,which have some advantages of stable transmission,wide application range etc.With the rapid development of modern manufacturing,high precision gears are required.At present,the small modulus gear is usually fabricated through cold precision plastic forming.However,there still exists a challenge in achieving gears with modulus higher than 10 processed by the cold precision plastic forming.The deformation resistance of the cold precision plastic forming is large and the large tonnage cold precision plastic forming equipment cannot be purchased at home and abroad.Although the deformation resistance of the gear can be reduced by hot forging,it is inevitable that the accuracy and surface quality of gears are poor,which is caused by high-temperature oxidation.In this work,the coating protection method is proposed to solve the problem of dimension precision and surface quality decline caused by high temperature oxidation of gear forgings.The main contents and conclusions are as follow:1.Simulation and analysis of hot-forging forming process of gear with Deform-3D software.The hot forging process of the gear was analyzed,and the finite element model was established.Through numerical simulation analysis,it was found that when the width of flash was 4.6 mm and the thickness of flash is 0.72 mm,the billet was completely filled with the cavity,and the forming load of billet would not be too large.In forging process,the temperature distribution,strain distribution,stress distribution,metal fluidity of the billets met the requirements of the hot forging of gear.It was propitious to form hot-forging gear with complete filling,clear outline,safety and quality requirement.At the same time,other requirements for high temperature oxidation resistant coating were put forward.The load,movement and collision of the billet required the coating's good bonging property with the substrate to avoid shedding failure.The strain and metal flow of the billet required that the coating had good plasticity and could be uniformly covered on the surface of the billet.2.Design and performance test of high temperature oxidation resistant coating.A high temperature protective coating was prepared based on the properties of 20CrMnTi steel.The bonging property between coating and substrate was tested.It was found that the coating both had a good bonding property with the substrate before and after heating.After the sample was heated at 1100 ? for 30 min,the high temperature oxidation behavior of the steel was investigated by SEM,EDX and XRD.After heating,an oxide layer of 153 ?m formed on the surface of the uncoated sample,however no oxide layer formed on the surface of the coated sample.The coating could reduce the oxygen partial pressure around the sample by reacting with oxygen in the furnace.The formation of a dense MgFe22O4,MgAl2O4,Fe2.45Si0.55O4 and Fe(Cr,Al)2O4 spinels layer could decrease the diffusion velocity of oxygen.3.Application of high temperature antioxidant coating on hot forging of bevel gears.The protective coating was applied to the billet surface,and appropriate equipment was selected for gear hot forging.Then the influence and reasons of the coating on precision of gear forgings were analyzed by SEM,EDX,XRD and numerical simulation.The results show that,after hot forging,the thickness of scale on the surface of uncoated gear was 89 ?m and the thickness of coating on surface of coated gear was 39 ?m.The smooth gear surface was obtained after removing the coating.When applied on the steel,such a coating material not only enhanced the dimensional accuracy but also improved the surface quality of the gear.The element and phase analysis of the cross section of the gear forgings revealed that the composition of MgFe2O4,MgA12O4 and Fe(Cr,Al)2O4 spinels and Fe2.45Sio.55O4 fayalite formed in the coating,which effectively prevents the metal from high temperature oxidation.The numerical simulation of the gears forging process by DEFROM-3D showed that there was the friction on the billet surface and the surface area of the billet will increase.The oxide scale would crack and led to the further oxidation of the metal for the un-coated bullet.But for the coating bullet,the coating thickness was reduced and the coating slurry could be uniformly adhered on the surface of the billet to prevent the further oxidation of the metal.So the coating could effectively improve the dimensional accuracy and surface quality of the gear forgings.