*3.3. Experimental Setup*

The experimental work was mainly carried out to validate the simulation results. Figure 9 shows the experimental setup of the incremental sheet forming process. It mainly consists of a CNC forming rig, sheet, forming tool, clamping plate, and fixture plate. Sheet blank was clamped on the fixture plate with the help of a clamping plate through threaded bolts. During the experimentation, a stainless-steel hemispherical tool of 10 mm diameter was used for all the experiments. The tool can move in the X, Y, and Z directions with the help of numerical control at a predetermined feed rate, i.e., 800 mm/min.

**Figure 9.** Experimental setup consisting of (**a**) dieless manufacturing center (**b**) forming setup.

#### **4. Results and Discussions**

This section discusses the comparative results of both the processes in terms of formability, thickness distribution, geometrical accuracy, and surface roughness.

#### *4.1. Results Based on VWATC Test Geometry*

The experiments performed to form the VWATC provides the maximum wall angle for both processes. Further, it compares the formability of both the processes in terms of maximum forming depth while forming the VWATC. The maximum forming depth of the fracture depth reveals the maximum wall angle that can be achieved for respective processes. Figure 10 shows the VWATC formed through ISF and ISH processes. Table 2

presents the maximum forming depth and maximum wall angle observed through VWATC during experimentation for both processes. The maximum forming depths observed for the ISF and ISH processes were 30.71 mm and 32.31 mm, respectively. The wall angles at these depths were observed to be 78.19◦ and 79.29◦, respectively, for the ISF and ISH processes. The experimentation carried out to form the VWATC through both processes reveals that the higher forming depth was achieved with the ISH process compared to the ISF process. This leads to a higher wall angle achieved in the case of the ISH process. Thus, it authenticates that higher formability can be achieved for the ISH process.

**Figure 10.** (**a**) VWATC formed through ISF process and (**b**) VWATC formed through ISH process.

**Table 2.** Observations from VWATC test geometry.


#### *4.2. Results Based on the CWATC Test Geometry*

The maximum wall angle achieved through experimentation of VWATC was 78.19◦ for the ISF process. Therefore, 78◦ CWATC were fabricated through both the processes to further validate that the higher formability associated with the ISH process. The forming time was 16.5 min for components formed through ISF process and it was 18.7 min for components formed through ISH process. Figure 11 shows the CWATC formed through ISF and ISH processes. The maximum forming depth achieved for both the processes is presented in Table 3.

**Figure 11.** (**a**) CWATC formed through ISF process, (**a'**) Scaled fracture section in CWATC formed through ISF process (**b**) CWATC formed through ISH process, (**b'**) Scaled fracture section in CWATC formed through ISH process.

### 4.2.1. Geometrical Parameters

The experimentally formed components were scanned with the help of the ATOS Core 200 (GOM – a ZEISS company, Brunswick, Germany) scanner for the evaluation of geometrical accuracy, thickness distribution, and forming limits. The scanned components were evaluated on the GOM Inspect platform to study the geometric aspects of the component. Figure 12a,b shows the material thickness distribution in the component formed with ISF and ISH processes, respectively, whereas the scanned parts were sliced along the section lines to investigate the material thickness distribution. Figure 13 shows the material thickness distribution on any one side of the sliced section, where the thickness was varying at a similar position for both the formed components. Figure 13 shows that the slope of the thickness distribution is more in the case of components formed through the ISH process as compared to the ISF process. This indicates that the material availability during forming of the component was low in the ISF process as compared to the ISH process. Thus, a more uniform material distribution was observed in the component formed through the ISH process. The minimum value of sheet thickness was achieved in the case of components formed through the ISH process which leads to the delayed fracture of the material as compared to the components formed through the ISF process. It should be noted that the deformation in ISF is a combination of stretching, bending, and shearing [27]. The friction between the tool and the sheet is a major factor that causes the through-the-thickness-shear deformation in the ISF process [28], whereas in the ISH process, the friction between the tool and sheet is minimal because of the intermittent tool-sheet contact. Thus, it could be concluded that the hammering dominates the deformation in bending and stretching of the sheet. The numerical simulation was performed till the fracture depth for both the processes to compare the material thickness distribution for CWATC. Figure 14 shows the material thickness distribution obtained through the numerical simulation. The numerical simulation predicted the thickness distribution with the minimum thickness to be 0.2668 mm and 0.2672 mm for the ISF and ISH processes, respectively.

Table 3 shows the wall angle and depth achieved for the components formed through the ISF and ISH processes. The result obtained for the wall angle achieved was more favorable for the components formed through the ISF process than the ISH process, which indicates that the springback is more in the case of the ISH process than the ISF process. The forming depth obtained supports the formability remarks made in the case of VWATC. The effect of springback is the subject of further research for the ISH process.

**Figure 12.** 3D scanned CWATC components formed with (**a**) ISF process, (**b**) ISH process.

**Figure 13.** Material thickness distribution along the sliced section in the formed CWATC component.

**Figure 14.** Material thickness distribution obtained through numerical simulation for the (**a**) ISF process, (**b**) ISH process.

**Table 3.** Wall angle and depth achieved in ISF and ISH processes for 78◦ wall angle.


#### 4.2.2. Forming Limit Diagram

To evaluate the formability of ISF and ISH processes the forming limits were also evaluated through surface strains developed in the formed components. To measure the strains in the experimentally formed components, circular grids having the size of 1 mm diameter and center-to-center distance of 2 mm, were electrochemically marked on the surface of the sheet blank before experimentation. The experimentally formed components were scanned using an ATOS Core 200 scanner. ARGUS (GOM—a ZEISS company, Brunswick, Germany) digital image correlation (DIC) software was used to measure the surface strains. The distorted circles after forming help to calculate the major (maximum) and minor (minimum) surface strains using the following equations:

$$
\varepsilon\_{major} = \ln \left( \frac{d\_{major}}{d\_o} \right) \tag{3}
$$

$$
\varepsilon\_{\rm minor} = \ln \left( \frac{d\_{\rm minor}}{d\_o} \right) \tag{4}
$$

In the above equations, *<sup>ε</sup>major* and *εminor* are the major and minor strains, while *dmajor* and *dminor* are the major and minor diameters of the ellipse formed. *do* is the diameter of the circle etched on the sheet before forming. Figure 15 shows the comparison of the forming limit diagrams (FLD) for both processes. The higher values of the strain were observed

in the components formed through the ISH process compared to the ISF process. The maximum value of the major strain observed through ISF and ISH processes are 1.54 and 1.78 respectively. This indicates that the higher formability associated with the ISH process compared to the ISF process. Further, as visible from FLD that the strain data are lying in the first quadrant of the FLD indicating the plane strain condition. This implies that the conical objects are subjected to the plane strain condition during forming. The same observations are also reported in the literature [29,30].

**Figure 15.** Comparison of strain values for the components formed through ISF and ISH processes.

### 4.2.3. Surface Roughness

The surface roughness of the formed components was evaluated using the Mitutoyo Surftest SJ-500 (Mitutoyo, Neuss, Germany). The resolution of the device was 0.0001 μm with a measurement accuracy of ±10%. The repeatability of the equipment was <6%. The lateral surface of the formed components was identified as the testing surface. Each test specimen was measured three times to assure the correctness of the results. Figure 16 and Table 4 presents the average roughness values (*Ra*) and maximum average roughness (*Ramax*) values obtained for components formed through ISF and ISH processes. It was observed that the average roughness value for components formed through the ISF process was 0.2860 μm, while it was lower, i.e., 0.2401 μm, for the ISH process. The maximum average roughness value was also lower for the ISH process, i.e., 0.2903 μm, as compared to the ISF process, i.e., 0.4492 μm. The range of mean roughness depth (*Rz*) for components formed through the ISF process is 0.8676–1.3316 μm, whereas it is 1.0839–1.4974 μm for the ISH process. In the ISH process, the tool applies compressive force through the hammering motion, which leaves the punched marks over the formed sheet surface. Thus, the range of mean roughness depth (*Rz*) is higher in the ISH process as compared to the ISF process.

In the ISF process, the deformation behavior is a combination of bending, stretching, and shearing, where the tool stretches the sheet along the predefined forming path. The stretching behavior along the forming path in ISF leads to large scale waviness resulting in large surface strains. Further, it resulted in roughness over the formed area; whereas, in the case of the ISH process, the intermittent hammering restricts stretching over the sheet and generates compressive strain on the punched area. This curtails the stretch marks on the formed area of the sheet. Thus, it can be concluded that the surface quality produced by the ISH process was better than that with the ISF process.

**Figure 16.** Surface roughness profiles of components formed through (**a**) ISF process, (**b**) ISH process.


**Table 4.** Surface roughness in the components formed with ISF and ISH processes.
