Amping Up Manufacturing Efficiency

The applications of tube-shaped workpieces have been growing steadily across many industries, thanks to its advantages like light weight and versatility in assembly. This increase in demand must be met with efficient near net shape manufacturing processes like rotary swaging, which is also gaining industrial prominence in the recent years.

Near Net Shape (NNS) manufacturing processes are crucial for increased productivity and are always aimed at optimizing the number of processes as much as possible. Belonging to this category of processes is Rotary swaging. Formerly known as swaging, it’s a process for precision forming of tubes, bars and other cylindrical workpieces. It is an incremental forming process which has a huge potential in automobile, aviation and aerospace industries.

The process has been pre-dominantly used for reducing cross sections of metal tubes and rods and is categorized under the open die forming processes according to DIN 8583 – Forging. Due to the versatility of its applications and the obvious advantages from a strength to weight ratio perspective, tube-shaped workpieces are finding their way in almost every major industry. And rotary swaging processes are preferred for forming tubes owing to their precision and automation ability.

Learning the Process

In a rotary swaging process, multiple dies are arranged radially around the workpiece. The process mechanism can be understood by referring to Figure 1, where six dies are used to plastically deform the tube to resemble the shape of the mandrel. The dies move up and down with a constant velocity. The mandrel remains stationary and the workpiece is rotated with a velocity synchronous to the motion of dies. By virtue of the high-frequency radial movement of the dies, which in turn leads to the multidirectional forging, the tube undergoes uniform necking. Theoretically, rotary swaging processes can come under two categories namely: infeed method and recess method. The difference between these two methods is based on how the workpiece is fed. As the name suggests, in the infeed method, the workpiece is gradually fed along its axis. In the case of recess method, the workpiece does not have any axial motion. This article is based on the latter method which is often used for reducing the diameter only at a certain position of the workpiece.

FE Simulation of Rotary Swaging

Finite Element (FE) simulations of manufacturing processes have become inevitable these days for reducing the development time as well as for using innovative methods to reduce energy and material consumption. FE simulation of a rotary swaging process with six dies is carried out in order to investigate the process mechanism and establish a generic framework to simulate different variants of the process efficiently.

The commercially available intelligent metal forming simulator AFDEX is used for simulating the rotary swaging process. Some engineering assumptions are made in order to simplify the FE analysis without losing out on the simulation accuracy.

The thermal phenomenon between workpiece and dies is neglected
The effects of acceleration and inertia are neglected
Workpiece material is isotropic
Elastic deformation component of workpiece is neglected
Workpiece obeys von Mises yield criterion and its associated flow rule
A pusher supports the workpiece which initially rests on the mandrel
The mandrel and dies are assumed to be rigid.

Validation of FE methodology

The same simulation program was used to simulate the round-round in-feed swaging process with 4 dies, without a mandrel and with a back-pressing force exerted on the pusher by the axial motion of the workpiece (Figure 2). The predicted tube thickness ranged from 3.66 mm to 3.84 mm where the experimental thickness was 3.75 mm. This shows that the FE methodology for infeed swaging process without mandrel matched well with experiments and the same can be applied for the recess swaging process (current research) with 6 dies.

Current FE Model, material properties and die velocity profile

The rotary swaging process is widely used for producing components like hollow drive shafts, gear shafts as well as the control rod of the steering gear. The process chain consists of cutting the raw material into required length, radially forging the cut blank into desired shape and then trimming off the elongated length by virtue of swaging process. Keeping this in mind, the analysis model is constructed. Figure 3 shows the FE analysis model of the process under investigation. The tube-shaped workpiece is 1.5 mm thick and 143 mm long with an outer diameter of 75 mm.

The material of the tube is AISI_1020 and is characterised by = 300(1 + ⁄0.01194)0.20618 MPa at room temperature. The friction between the dies, mandrel and the workpiece are defined using Coulomb friction model with a frictional coefficient of 0.05. The velocity profiles of the dies and the workpiece is shown in Figure 4. Around 77000 tetrahedral elements are used for discretizing the analysis domain. Remeshing is avoided while simulating this process in order to avoid the undesired but inevitable smoothening of state variables.

Results and Analysis

The rotary swaging process consisted of 8 blows to carry out the incremental forming of the tube. The outer diameter of 75 mm was reduced to 60 mm in 8 blows incrementally as can be seen in Figure 5 which shows the blow-wise deformation history as well as the comparison of initial and final deformed shape. Thinning was observed near the end of the tube as a result of increase in length.

Figure 6 and 7 represent the effective strain and stress distribution respectively. To understand the effective stress distribution predicted, one must investigate the velocity distribution during the forging process (Figure 8) and the internal stress states of the tube (Figure 9). The nodal velocity significantly increases along the axial direction at the sizing zone. This is in line with the theoretical understanding of the process mechanism. Rotary swaging processes of this category experience triaxial stress state. The three stress components (1, 2, 3) in Figure 9 are the axial, radial and circumferential stress components respectively. The axial stress is tensile and the radial, circumferential stresses are compressive in nature. In order to have a minimal variation of wall thickness, the radial stress must be kept minimal compared to the circumferential stress. This is controlled by adjusting the die advancement per blow. Obviously, the stresses increase significantly as the dies contact the workpiece. The axial elongation of the workpiece as well as the necking predicted in the simulation is because of the axial tensile stress by virtue of the deformation happening in the sizing zone. The axial elongation is inevitable in this process owing to the process mechanics and the excess length must be trimmed as per subsequent assembly requirements.

Summing Up

The article focuses on the FE analysis of rotary swaging process for forming tube shaped workpieces using a commercially available metal forming simulation software. The phenomenon predicted in the simulation matches well with the theoretical understanding of the process. Recess swaging processes with mandrel experience triaxial stress states and the axial stress is responsible for elongation of the tubes. With complex material flow phenomenon, usage of FE analysis methodology is very crucial in arriving at creative and optimum process designs quickly.

With complex material flow phenomenon, usage of FE analysis methodology is highly crucial in arriving at creative and optimum process designs quickly.