Experimental and numerical investigation of the forming limit in the hydroforming process of metal pipes

Dissertation for receiving a master's degree in the field

Mechanical engineering, construction and production orientation

Abstract

Production of complex industrial products from metal pipes with the aim of increasing the strength to weight ratio of structures and especially reducing the fuel consumption of vehicles In recent years, it has attracted the attention of artisans. In this regard, one of the new processes used is hydroforming. Rupture in the forming processes of sheets and tubes is evaluated as one of the main limiting factors using the forming limit curves (FLD). Using the forming limit curve derived from traditional forming methods is not valid in the hydroforming process. The use of the forming limit curve resulting from the sheet forming methods is also error-prone in the pipe hydroforming processes.

In this research, with the aim of experimentally predicting the forming limit of Zangenzen 304 steel pipe, firstly, in the process of axially symmetric hydrobulging, the effect of loading method and mold geometry on the path of plastic straining and instability created in the pipe was investigated numerically. In order to investigate the effect of loading method, three types of free loading, loading with pipe axial feeding and loading with a fixed end of the pipe were studied. The effect of mold geometry was studied by changing the corner radius (R) and the length of the deformation area (W). In this study, the effect of axial feeding value of the tube in axial feeding loading mode on the strain ratio (?) was investigated. Numerical results showed that the path of pipe straining in two states of free loading and loading with a fixed end of the pipe is almost identically located on the right side of the yield limit curve. In this condition, as the length of the deformation zone (W) increases, the strain ratio (?) tends to zero, which is independent of the boundary conditions. By increasing the mold corner radius (R) in the free loading state, with the decrease of the strain ratio (?), the strain path approaches the plane strain state; While in loading with tube axial feeding, increasing the mold radius does not have a significant effect on the strain path of the part and the strain ratio. In the case of loading with axial feeding, the strain ratio (?) decreases significantly with the increase of tube feeding amount. In the experimental part of this research, among the various simulated tests, 10 tests with appropriate dispersion of the loading path on the strain plane were selected. After the design and construction of the mold, the graded pipes were loaded under controlled conditions until they ruptured, and by measuring the strain in the areas adjacent to the rupture, the curve of the steel pipe's forming limit was drawn. The FLD curve obtained for the 304 stainless steel pipe was compared with the FLD curve of the same type of sheet from other researchers' studies.

In the final stage, in order to check the efficiency and accuracy of the extracted ductility criterion, it was used to predict rupture in the process of making the cam from the tube. The results of numerical and experimental analysis in this section showed that the FLD curve obtained with acceptable accuracy can predict the ductility in the cam hydroforming process. In addition, the examination of the ductility of the industrial part confirmed that the yield limit curve obtained in this research can be used to predict the ductility of industrial parts.

Keywords: Experimental strain analysis, finite element simulation, forming limit diagram, pipe hydroforming Higher quality as well as flexible production system have been adopted. For this reason, it is necessary to use new materials and develop advanced production processes. Therefore, researchers and industrialists have turned to advanced manufacturing processes with high flexibility. One of these processes that has attracted the attention of producers today is hydroforming[1]. Hydroforming is a process that, due to the need for relatively high technology, its use has been limited to special cases for a long time.With the advancement of technology, production machinery, sealing systems and computer control processes in the last decade, fluid pressure forming has been introduced as a method that can be used in the industry [1].

In this chapter, metal forming processes are first introduced and categorized, and the place of hydroforming among them is mentioned. After introducing the hydroforming process and its types, a brief explanation about the forming limit curves, its applications and the method of obtaining it will be provided. Then, an overview of the research done by other researchers is presented in connection with this thesis. Finally, the goals and features of this thesis are described. 1-2-Introduction of metal forming processes In general, metal forming processes can be classified into two main groups [1]: A- volume forming [2] B- Sheet forming [3]

Volume forming has the following two distinct characteristics [1]:

1- The shape or cross-section of the workpiece is permanently and highly deformed.

2- The amount of deformation of the deformation [4] in this process compared to the deformation of the elastic [5] is usually so high that it cannot be returned. The spring [6] of the part is omitted after the deformation.

Forging processes[7], forging[8], rolling[9] and stretching[10] are examples of volume forming processes of metals.

The characteristics of sheet forming processes are as follows [1]:

2- This forming process usually creates a significant change in the geometry of the part, but the cross-sectional area of ??the object does not change much.

3- Sometimes wax and elastic deformations are of the same order. Therefore, spring return cannot be ignored. Deep drawing [11], bending [12] and rotary forming [13] are examples of sheet forming processes. Metal forming is a process in which a pressurized fluid is used to cause plastic deformation in a sheet or tube-shaped primary piece. In each of the hydroforming processes, a press, mold and a pressure boosting system are always needed [2, 3]. In general, in the hydroforming process, a product with favorable mechanical properties is obtained due to the uniform distribution of fluid pressure on the part surface. Other advantages of hydroforming include the ability to produce complex parts, better dimensional accuracy, and improving the forming of materials that have little forming ability. On the other hand, this process has disadvantages, including the slow production cycle and expensive equipment [2]. In addition to its many applications in the aerospace industry, hydroforming has been widely used in the automotive industry since the 1990s. One of the major applications of this process in the automotive industry is the manufacture of car exhaust system parts, chassis parts, engine components and car body parts. 1-3-1- The history of hydroforming process The history of using fluid to shape metals goes back more than 100 years. The primary applications of this process were in making steam boilers and musical instruments. However, the foundations of hydroforming were established in the 1940s. The first recorded application of the hydroforming process was by Milton Garvin of Scheibel America in the 1950s, who used this process in the manufacture of kitchen utensils. Making copper T-shaped fittings in the plumbing industry was the most common application of this process until the 1990s. Since the 1990s, due to industrial advances in computer control, hydraulic systems and new strategies of design and manufacturing processes, this process has found a special application in the industry and has replaced many forging and stamping processes, especially in vehicle parts [4].