**1. Introduction**

The incremental sheet forming (ISF) process is a flexible dieless manufacturing process [1]. The ISF process starts with the three-dimensional (3D) computer-aided design (CAD) model of the geometry of the component to be formed. The desired CAD model can be generated with any modelling software (e.g., SOLIDWORKS, CATIA, Siemens NX, PTC Creo, etc.). The required toolpath to incrementally form the desired geometry is evolved on the basis of the sliced 3D model. A forming tool is given the controlled motion along the generated toolpath and deforms the sheet into the desired shape [2]. The sheet is clamped at its outer periphery. Stresses are generated on the sheet due to the continuous motion of the forming tool, which leads to local plastic deformation on the sheet [3,4]. As

**Citation:** Shahare, H.Y.; Dubey, A.K.; Kumar, P.; Yu, H.; Pesin, A.; Pustovoytov, D.; Tandon, P. A Comparative Investigation of Conventional and Hammering-Assisted Incremental Sheet Forming Processes for AA1050 H14 Sheets. *Metals* **2021**, *11*, 1862. https:// doi.org/10.3390/met11111862

Academic Editors: José Valdemar Fernandes and Marta Oliveira

Received: 22 September 2021 Accepted: 17 November 2021 Published: 19 November 2021

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the process does not require dies to manufacture the components, thus, it can be used in forming customized products at a lower cost in comparison to the conventional process. With an increase in the demand for customized products, the ISF process is drastically gaining popularity [5]. The ISF process finds its applications mainly in the automobile, aerospace, and medical implant industries. In the automobile industry, the headlight reflector, heat or vibration shield, solar oven, and silencer of the trucks are manufactured using the ISF process. Apart from this, the ISF process also finds its application in the medical implant industry, where it mainly helps to form facial and cranial implants. Even though the ISF process has numerous advantages over the conventional forming process, it has some limitations while forming materials with poor forming characteristics such as geometric inaccuracy, non-uniform thinning, poor surface finish, higher processing time than conventional forming processes, and difficulties in forming geometries with 90◦ (or more than 90◦) wall angles. Formability of the material is the degree of deformation during the forming operation without any unwanted flaws like tearing, buckling, necking, or any other defect [6]. The different forms of the ISF process are studied to overcome the flaws that arise during the ISF process.

The ISF process is broadly classified according to the number of contact points during the forming process, i.e., single point incremental forming (SPIF) process and two-point incremental forming (TPIF) process. In the SPIF process, there is only one forming tool, which is controllable, whereas in the TPIF process, there is an additional supporting tool with the master tool to control the flow of the deforming material [7,8]. The TPIF process is further extended with the help of a partial die which limits the neck formation during the process [9]. Researchers had worked on heat-assisted incremental forming techniques to improve formability and geometrical accuracy [10]. Further, an ultrasonic-assisted incremental forming setup had been devised to investigate the formability and the forming forces arising due to ultrasonic energy infused during the process [11]. Vihtonen et al. [12] compared the robot-assisted incremental forming by pressing (RAIFP) and robot-assisted incremental forming by hammering (RAIFH) based on the strain measurement and found that the formability of the material can be enhanced with the help of hammering. Luo et al. [13,14] designed and developed a new sheet metal forming system based on incremental punching. They have developed a mechanics model, which helps to predict the final shape of the component using the minimum energy principle. One-step FEM is developed to predict the stress and strain distribution of the component using inverse finite element modelling. Further, they have proposed that the incremental sheet metal punching (ISMP) is effective for the rapid prototyping of sheet metal components, whereas it is effective in developing free form sheet geometries without bottom support. Further, Wang et al. [15] performed incremental sheet punching based on the sinusoidal toolpath and the results indicated that the wavelength and amplitude of the sinusoidal toolpath affect the material formability, surface quality, and fatigue life. The dimensional accuracy was investigated in the incremental sheet metal hammering process and the results revealed that the springback and surface roughness can be minimized by increasing the punch diameter [16]. Though limited research articles are available on the effect of hammering in the incremental sheet forming, the scope of the work is not limited, as shot peening, one of the types of hammering process, had revealed that the material characteristics can be improved with the help of the controlled impact on the material surface [17].

The present study aims to integrate the hammering operation with the incremental sheet forming process to enhance the material properties of the sheet during the forming. The process uses a working principle similar to the ISF process, i.e., the sheet is clamped at its ends and is deformed into desired geometry with the help of forming tool. The forming tool is provided with a hammering induced toolpath. As the process is applied for the sheet materials, thus, it is also called the incremental sheet hammering (ISH) process. Figure 1 shows the schematics of the ISH process. The present work comprises experimental and numerical investigations of the ISF and ISH processes on the AA1050 sheet and a comparative conclusion has been drawn based on the analysis.

**Figure 1.** Schematics of incremental sheet hammering (ISH) process.

#### **2. Deformation Methods and Methodology**
