Incremental Forming of Titanium Grade 2 Sheet by TPIF-RL Method

ensures a homogeneous wall thickness distribution


Material and Method
In this study, the TiGr2 sheet is formed. This sheet is biomedically compatible. The initial thickness of the sheet is 0.54 mm. From this sheet, axial symmetric cone parts with 40° angle from the horizontal axis are produced. The height of the formed part is 40 mm. Four different factors were examined as pressure, feedrate, increment and forming tool diameter [11][12][13][14]. A total of 18 experiments were performed by the TPIF-RL method.
In the rolling blank holder method, the sheet is not fixed by bolts from the edges. With four pneumatic clamps, the clamping force is applied to the edges of the sheet. A ball bearing is located at the end of each pneumatic clamp. Thus, the flow of the sheet is facilitated. When 2 bar pressure is applied to the pneumatic clamp, a clamping force of 187.28 N is generated. When 9 bar pressure is applied, a clamping force of 1039.02 N is generated. During the forming process, depending on the effect of TPIF-RL method is a new method that gives better wall thickness in incremental forming. In this method, the sheet is not fixed at the edges. Like the deep drawing process, it is compressed with a certain pressure. During the forming process, the sheet flows under the blankholder. Thus, more homogenous wall thickness is obtained.
In this study, Titanium Grade 2 sheet is formed as a cone via TPIF-RL method. Optimum forming parameters were determined by Signal/Noise analysis. In addition, finite element analysis of the process was performed. Using 2 bar clamping pressure, this clamping force, the sheet may flow slightly to the model (Fig. 1). As a result, better wall thickness is obtained. Finite element analysis (FEA) was performed with Abaqus FEM software. Forming tool diameter and model were defined as a rigid body. The sheet was defined as deformable [15][16][17]. The forming tool movement was defined with three different amplitudes. The analysis was divided into three main steps. In the first step, boundary conditions were defined. In the second step, the sheet was formed. In the last step, the forming tool left the sheet.
The sheet was meshed using quadrilateral shell elements with reduced integration (S4R). The formed area of the sheet meshed with 2 mm elements. Other regions meshed with larger elements. The calculation time was reduced by using different mesh sizes [18][19][20].

Experimental Results
The parts obtained from the experiments were marked at fivemillimeter intervals starting from the part center. Wall thicknesses were measured from these marked points. The wall thicknesses on the right side of Table 1 and the experimental set on the left side were given. In experiment 6, a better wall thickness was obtained than the other experiments (minimum thickness of 0.45 mm). In experiment 6, the wall thickness was reducing %13. The smallest wall thickness is 0.42 mm. The maximum thinning amount is 0.12 mm (thickness reduced 22%). According to the table, the wall thicknesses are homogeneously distributed.  The part obtained from experiment 4 is given in Fig. 2. MoS2 lubricator was used in all experiments to reduce friction [21,22]. The thickness distribution obtained from the finite element analysis of experiment 6 is given in Fig. 3.A. As can be seen from the figure, as well as the forming zone, thinning of the wall thickness occurred in the areas where the squeezing pressure was applied. This can also be seen in the test piece. The wall thickness distribution obtained from the finite element analysis of experiment 16 is given in Fig. 3.B.  The main effect of the S/N ratios obtained by S/N analysis is given in Fig. 5. Optimum levels of parameters according to S/N analysis: 2 bar for pressure, 1000 mm/min for feedrate, 0.75 mm for increment, 15 mm for forming tool diameter. When Table 2 is examined, it is seen that the parameters of Experiment 6 are the same as the optimum values. As can be understood from this result, with the small clamping pressure, the sheet metal flows on the model. This results in less thinning of the sheet.

Conclusions
A more homogenous wall thickness distribution can be achieved with the 'rolling blank holder method' (TPIF-RL method), which is the result of the development of the traditional incremental forming method. In this study, TiGr2 sheet is formed in axial symmetrical cone form at 40° angle with the horizontal axis. The part height is 40 mm. Optimum levels of parameters according to S/N analysis: 2 bar for pressure, 1000 mm/min for feedrate, 0.75 mm for increment, 15 mm for forming tool diameter. When optimum values are used, a maximum of 6% thinning in wall thickness is achieved. Within the scope of the study, the FEA model of TPIF-RL method was developed. The experimental results and the FEA results are similar (difference approximately 0.04 mm).

Conflict of Interest Statement
The authors declare that there is no conflict of interest in the study.