Study On Dieless Incremental Forming Process For Producing Tubular Parts

Incremental forming technology, a highly flexible digital sheet metal forming technology, has attracted much attention in recent years for its great potential in rapid and economic sheet metal forming field. However, the technology is rarely applied in tube forming area so far, which keeps us from taking the advantages of this flexible technology fully.This paper firstly concluded and classified the tube components manufactured by incremental forming technology according to the typical tube geometry and forming characteristics. Then, the typical incremental tube forming processes including tube end expansion(reduction), tube end inversion, hole-flanging on tube wall and tube wall grooving were selected to be investigated both theoretically and experimentally. Dedicated forming tools and fixtures were designed and manufactured for different incremental tube forming processes. The forming characteristics of the tubes with different materials and sizes were compared. The finite element method were also used to simulate those incremental tube forming processes. The influences of different processing parameters on various forming processes were analyzed. The processing optimization were implemented for each forming process. Main conclusions are as follows:① The end expanded tube parts with different expanding angles and heights can be acquired just by changing processing tool path when applying the dieless incremental forming technology into tube end expansion process, which shows the flexibility of this method. In the meantime, the forming ability of the tube can be improved. While, when compared with tube end expansion by pressing, the quality of the tube surface which expanded in incremental forming way is relatively poor, on the other hand, the forming efficiency of the incremental forming technology is lower, which makes it mainly suitable for trial-manufacturing small batched products.② Tube with large relative wall thickness and good plasticity can be reduced smoothly in the end area by dieless incremental forming technology. And the thickness of the tube wall increases almost linearly form the bottom to the end of the reduced area. On the contrary, tube with small relative wall thickness is prone to wrinkle because of the big springback when reducing the tube end by dieless incremental forming technology, which may lead to the failure of the process.③ The processing parameters of tube end inversion by dieless incremental forming technology should be strictly controlled, otherwise many defects, including the distortion of the forming area, cracking in the tube end and buckling of the tube, will occur during the process. The cone angle q of the tool in the forming area and the transition fillet radius R have vital influence on tube end inversion process.④ Branched tube parts with different shape and size can be manufactured when applying dieless incremental forming technology into tube wall flanging process. Good quality in transition area between branching tube and mother tube can be acquired when cone angle q of the tool is selected appropriately. Typical forming defects including wall thinning, local wrinkling, shape distortion, cracking, and unevenness of the tube end will occur when the processing parameters are not well controlled. The cone angle q of the tool in the forming area has great influence on whether the wrinkle occurs or not in the transition area between the branching tube and mother tube. Critical cone angle differs from different wall thickness and friction condition. The formable size of the branching tube is related to outer diameter D of the mother tube, size of the preformed hole and tube material. The recommended formable size of the branching tube are as follows, height range of the branching tube: h <0.28 D, diameter range of the branching tube: d <0.9D.⑤ The thickness of the tube wall under pressure always increases firstly and decreases afterwards when applying the dieless incremental forming technology into the tube wall grooving process. The tube material has great influence on the shape of the tube after grooving process. An phenomenon called "End effect" occurs when pressure slot position is close to the tube end.⑥ The springback of the tube wall has a greate influence on the size of tubular parts during the dieless incremental forming processes mentioned above. Adjustments should be made when planning the processing path of the forming tool to compensate the springback of the tube wall and the elastic deformation of the forming tool.⑦ Tube parts that manufactured hardly by traditional stamping technology can be formed easily with the application of the dieless incremental forming technology into tube forming area, which not only greatly expands the application range of the incremental forming technology and meet the demand of single piece or small batched thin-walled parts, but also realizes the lightweight structure for products. The application prospect of this method thus will be vast.