Mechanism Analysis And Study On Numerical Simulation Of Single-point Incremental Forming

Single-point incremental forming(SPIF)is driven directly by the 3 D model of the product according to the numeric information of the product.It achieves incremental forming of the three-dimensional surface by the forming tool moving along horizontal trajectory and vertical feeding.Relative to the traditional stamping technique,the SPIF has more advantages such as not requiring the special die, combining with other forming techniques such as multi-point forming.The SPIF can make the whole forming replaced by local incremental forming,economize the forming force,reduce the demand of equipment tonnage.It can achieve dieless forming,economize the expenses of the die designing and manufacturing.It can process some kinds of different shape sheet metal in the same equipment.It can achieve the variational path forming.It can shorten the exploiting cycle of new products and reduce the product cost.It can easily achieve incorporate CAD/CAM/CAE and forming automatically.At present,the forming mechanism has been understood a certain extent,but the essential reason hasn't been found.That is the hot issue and imperative solving all over the word.Only obtaining breakthrough in the study of forming mechanism,then can be resolved the problem of forming technics.It hasn't been lucubrated on the sheet metal SPIF and hasn't the systematic theories system supported too.In order to master the forming technology as soon as possible,reduce the times and expenses of the experiment,it is the good idea that the whole forming process can be appeared and the keys can be acquired by numerical simulation method.The SPIF process concerns nonlinear on geometry, material and state,as well as complex discontinuous boundary prolem.Nowadays,the finite element method(FEM) is a cogent means to deal with the nonlinear prolems.In this paper, dynamic explicit algorithm FEM is successfully used to analyze the SPIF process of sheet metal.Study on how the defects can produce and predict the possible defects by numerical simulation method,choose the proper technical parameters to restrain or eliminate forming defects, and the forming quality of the parts and efficiency of manufacture can be improved consequently.It provides an effective means for optimizing the technique design and judging forming ability and quality of the parts. Also,it promotes the development of the SPIF technology. The main contents and results are as follows:1) Research on the forming mechanism of the SPIFIt is very important to research the forming mechanism for choosing technics parameter.In this paper,the SPIF forming mechanism is analysed.The forming area stress state is the uniaxial compressive stress and biaxial tensile stress.That makes the contact area beween the forming tool and sheet metal have more plastic.The strain in direction of thickness is compressive strain,however,shear strain and circumferential strain are tensile strain.Analysing the five stages of the forming process:the elastoplastic pre-tensile,the plastic bending along the forming tool surface,the reversely plastic bending to former bending,the forming process finished and unloading,producing spring-back.The design of forming trajectory must follow the principle of forming uniformity,the principle of moderate vertical reduction and horizontal offset.It will appear compressive stress during forming process,then there are two forming states including over-SPIF and lack-SPIF.2) Buliding the finite element model for the two methods of SPIF processIt is very important to build the reasonable finite element model for numerical simulation.The reasonable finite element model is constructed for SPIF process with two forming method according the mechanics model.Studying some questions such as spatial discretization for SPIF process,the choice of finite element unit,the choice of material pattern,boundary condition etc.,they have a great influence on the numerical simulation. It provides the principles and basis for the corret finite element model of SPIF process.3) Numerical simulation on the technical parameters of the SPIFIn this paper,take the square pyramids as an example,different technical parameters are considered to simulate the SPIF process by finite element method. The influence of the different parameters to forming quality is researched,and the difference is analysed. The excessively small forming tool ball radius can have the local stress phenomenon,creat the similar cutting condition, cause the workpiece surface fineness to change to be very big, even appears the crack.On the contrary,with the increase of forming ball radius,it reduces the local stress centralism phenomenon on the contact area between forming tool ball and sheet metal, and the forming ball size couldn't be oversized, otherwise it could have the situation which partial position processing did not arrive.The smaller the vertical reducation is, the better sheet metal forming ability is.The bigger the vertical reduction is, the bigger the contacted area between the forming tool and the sheet metal is.The friction force is increaseing.It results in the forming tool deformed, affecting the forming precision.In the forming efficiency aspect, vertical reduction smaller forming efficiency to be lower. Horizontal offset smaller,formed inclination angle smaller,sheet metal forming limit is smaller.Incre- asing the offset,sheet metal forming ability better,sheet metal forming limit is bigger.So,multi-steps forming technics can be used when the forming inclination angle is smaller.This paper proposed that the connection between the forming ball and the main body is altered to the rolling condition. That can reduce friction between ball and sheet metal.The scratch of sheet metal effected by the forming tool can be avoided.Thus it can enhance smoothing and forming ability and reduce sheet thickness reduction.The gap width between die and forming tool affect forming also.With the reduces of gap width, the crack trends are reduced during forming sheet metal of the same depth.4) Numerical simulation on the forming quality of the SPIFThe sheet metal forming quality is the important guideline in proving the formimg method whether good or bad.The changes of sheet thickness play an important role in the crack and wrinkling of sheet metal forming.The thinner the sheet metal is, the crack trends become more evident.That is because the thinner sheet has weaker anti-pulls ability.The too thin sheet easily appear wrinkling, with the increase of sheet metal thickness,the wrinkling gradually reduces,until vanishing.Simultaneously,the deforming degree surpasses the marginal value,with the increase of deforming degree.And the spots achieving destabilization increase,the wrinkling trends increase during forming process.There are some difference on thickness reduction and forming precision between the two forming methods.The thickeness reduction is very serious in the corner of the parts for dieless single-point incremental forming,but in the main forming area for die single-point incremental forming.In the forming precision,there are some certain error in the forming bottom both the two forming methods.And the bottom contacted between the sheet metal and die appears bulge.The divided "level" forming is the most major characteristic for single-point incremental forming.The strain history of the sheet metal section are characterised by a typical step trend in the whole.With the forming tool moving continuously,the strain have the gradual increasing trend.In the change of sheet metal thickness,thickness reduction changes subtly in the initial and ending stage of sheet metal forming,but it is very serious in the middle stage namely the main deforming area.