Study On Simulation Of Friction Stir Assisted Incremental Forming With Synchronous Bonding Of Steel/aluminum Sheet Metals

There are higher requirements to material property with the development of the industrial technology,and the monotonic material is very difficult to meet its requirements.The existing forming methods of heterogeneous sheet metals are to bond them first,and then to form them into the shape by cold or hot stamping,which may exist some shortcomings such as delamination,difficulty in springback control and complicated clamping design.Supported by the National Natural Science Foundation of China(51675332)and the Program of Shanghai Excellent Academic Research Leadership(19XD1401900),this thesis built a coupled thermo-mechanical model in ABAQUS based on the innovative manufacturing process named as friction stir assisted incremental forming with synchronously bonding of steel/aluminum sheet metals.The material flow stress model and the complex contact model were updated and then this simulation model was validated with experimental results in terms of forming force,forming temperature and geometrical accuracy.Furthermore,a criterion of bonding quality was proposed through peeling tests and simulations.Finally,this simulation model was used to study the effects of process parameters on the forming quality and bonding quality.The main achievements of this thesis are shown as follows:(1)The standard Johnson-Cook flow stress model was updated to consider the effect of temperature on the strain hardening and strainrate strengthening.The contact model was updated to considered the effect of the relative sliding velocity of the interface on the frictional heat partition coefficient and the effect of temperature on the friction coefficient.(2)Through analysis of simulation,it can conclude that the temperature of a specified point on the side wall increases with increasing of the ratio between spindle speed and feed rate and with decreasing of the vertical step size.If the wall angle of the designed part increases under the same process parameters,the temperature will also increase.For forming the conical part with a depth of 8.5mm,the process parameters have little effect on the thinning.And the sheet thinning does not follow the Sine law,the thickness of the upper middle region(the depth of 2?3mm)for the upper sheet thickens and then thins over time,while the thickness of upper corner for the lower sheet thickens.For the bulge,it can conclude that higher ratio between spindle speed and feed rate,lower vertical step size and higher feed rate with same ratio may cause higher bulge through simulation.(3)Through peeling tests and simulation,a criterion of bonding quality based on contact stress,equivalent stress and bonding time was proposed which can be used to predict in the following situations:When P_max<92MPa,the bonding quality is poor.When P_max>97MPa and Q<0.476,the bonding quality is poor.When P_max>97MPa and 0.607<Q<7.018,the bonding quality is ordinary.When P_max>97MPa and Q>7.879,the bonding quality was good.(4)Based on the criterion and simulation,it can conclude that the bonding quality varies from different points in interface and the strongest bonding quality exists in the middle part of the side wall.Rise of ratio between spindle speed and feed rate can improve the bonding quality,but the vertical step size has little effect on it..