Research On Heat-assisted Incremental Bending Forming Technology Of Large Curvature Metal Plates

Metal forming is one of the most commonly used manufacturing processes.In recent years,with the rapid development of shipbuilding,aircraft,automobile and other industries,the upgrading of products is accelerated,the variety of products is increased,and the workload of sheet bending forming required is also increasing.Different from traditional die stamping for forming thin sheet metal,forming thick sheet metal with 3D free complex curved surface is always a challenge in metal forming field.The traditional processing method is the method of line heating forming,using thermal stress to bend the plate.It is difficult to automate the process as the heating and cooling effects are difficult to predict.This method still relies heavily on the worker's experience and typically requires an apprenticeship of several years.In addition,this method is inefficient and error prone.In the past two decades,many automatic machining methods of 3D free complex surfaces have been proposed.However,many problems remain.For example,bending forming force is too large,bending curvature is too small,forming accuracy is not high.In this paper,a new method for forming thick plate with complex curved surface is presented,which combines heat-assisted and incremental dieless bending technology.First of all,the original dieless incremental plate bending machine was modified,especially the heat-assisted technology was added,which used electromagnetic induction heating to accurately control the local temperature of the thick plate.Secondly,the punching path of the dieless incremental bending process is studied.For simple single direction curvature plate forming,the plate is divided into several parallel slat.It is connected by virtual spring between each slat,however,each slat can be viewed as a simply supported beam,through the bending theory of Euler-Bernoulli beam,the minimum energy point in the processing process is calculated,so as to get the machining trajectory of the single direction curvature plate.However,it is a complicated process to compare the forming of double direction curvature plate with that of single direction plate.By decomposing the plate into rectangular elements,each of which also uses a virtual spring connection,and establishing the objective function of minimum energy.Then,assuming that each element moves to the target shape along the direction of the strain minimum energy,the optimal solution of the strain minimum energy is obtained under the constraint of Gaussian curvature,and the minimum energy loading path of double direction curvature plate forming along Gaussian curvature direction is obtained.The minimum energy loading trajectory can minimize the forming force,effectively reduce the number of stamping points,and increase the forming efficiency and forming accuracy of sheet metal.Thirdly,the springback of single direction curvature plate and double direction curvature plate forming process are studied.Springback behavior is an inevitable phenomenon in metal forming process.According to the law of incremental bending trajectory,the theoretical and simulation models for springback of single direction curvature plate and double curvature plate are established.In order to control and reduce the springback phenomenon and increase the forming limit,the springback of metal sheet forming was studied at room temperature,250? and 500?.The springback law of metal plate bending forming under different heating temperatures was obtained.As the temperature increases,the springback height of the plate decreases.When the temperature is 500?,the springback height of the plate can be almost ignored.which provided a basis for the springback compensation of subsequent sheet processing.Then,combined with the minimum energy incremental bending trajectory and springback law of single direction curvature thick plate,the forming research of large curvature single direction curvature plate was carried out.The single direction curvature plates include constant single direction curvature plates,symmetrical single direction variable curvature plates and asymmetrical single direction variable curvature plates.In view of the interaction between adjacent slats in bending forming process,the method of adding auxiliary supports and heating are adopted to reduce the interaction between adjacent slats.The single direction curvature plate is obtained by simulation and experiment methods.Finally,combined with the minimum energy bending machining trajectory and springback law of double direction curvature plate,the forming research of large curvature double direction curvature thick plate was carried out.The double direction curvature plate includes sail plate and saddle plate.Two different forming strategies are used in the forming process,one is sequential bending forming strategy,the other is alternate bending forming strategy.In order to increase the forming curvature,auxiliary heating and supporting are specially added in the forming process.The double direction curvature plate was obtained by simulation and experiment methods.