Flanging is one types of bending process, which is mostly used to manufacture automotive and aerospace components . It is mainly used to apply rigidity and smoothness to the parts and also used to assemble various products [1–5]. Straight, stretch and shrink flanging are three types of flanging process . They are indicated in Fig. 1. In stretch flanging, one edge portion of the blank is fixed and opposite edge of blank is remains free to bend by 900 to form a flange [1, 4]. If the arc length of the final flange is more than the original length then it is called a stretch flanging (SF) process. It is indicated in Fig. 1 (b, c and d). It occurs due to stretching of materials in the circumferential direction . The tension found largest at the edge of die profile radius whereas it is minimum at top and midsection of the flange . Shrink flanging process is opposite to stretch flanging process. In shrink flanging process, material is compressed in the circumferential direction which is illustrated in Fig. 1 (e and f). In SFP, circumferential strain is a main parameter that causes fracture in the flange. In this process, cracks occurs when circumferential strain reaches at the certain limit [3, 5]. Necking, thinning, localized cracks/fracture (Fig. 1 (d)) etc. are the main types of failure in this case . These failures can be forecast and minimize with the support of finite element simulation. The finite element simulation is an important numerical tool that is capable in predicting necking, crack initiation and its location, strain distribution, forming load and thickness variation. It also saves the time in designing and cost of component as compared to experimental procedure due to fast optimization of parameters involved in the process [7–9].
In the past decade, several researchers have attempted to forecast and analyse the failure mechanism in the different sheet metal forming (SMF) process using FE analysis. Feng et al analysed the effects of various sheet geometrical parameters on the formability of flange in the curved SF process through simulation . P. Hu et al generated couple of analytical models for shrink and stretch flanging processes for calculation of blank size . Yogesh Dewang et al carried out the experimental and numerical simulation work for the analysis of binder force effect in stretch flanging of AA5052 sheet . G. Ellen et al created the analytical model for the evaluation of circumferential strain and proposed a semi-empirical failure criteria for analysis of wrinkling in shrink flanging process . Sriram and Chintamani worked on stretch flanging (SF) process to analyse the effect of geometrical parameters on AHSS Steel sheet. It was found that the flange angle is a main parameter for the same process . Cliff Butcher et al predicted the formability in SF process using models of lower bound (Sun-Tvergaard-Needleman) and upper bound (GTN) damage in FEM simulation . Vafaeesefat and Khanahmadlu validated the numerical results with experimental one in stretch z-flange forming process which was based on shell-elements . Several researchers had been studied the effect of different materials on stretch-flangeability [15, 16]. Thinning, fracture, wrinkling etc. were studied in deep drawing process by researchers [17–21]. D. Li et al created the mathematical model for the axisymmetric case and studied the influenc of parameter like sheet and its geometry in V-shape SF process. It was found that the geometrical parameters were more dominated than the materials parameters . Y. Dewang et al investigated stretch flanging (SF) process to see effect of initial flange length, friction coefficient, profile radius of die and punch, gap between punch and die for the prediction of edge crack location and its propagation using FE simulation and it was also justified with experimental result .
In the current era, several researchers have been used different punch profiles and different techniques to enhance the formability of material and also found the effect of these on failures (thinning, necking, localized cracks etc.) in various sheet metal forming processes such as deep drawing, shrink, stretch and hole flanging. Yohei Abe et al improved the formability and minimize the fracture in UHSS steel sheet in SF process using punch gradually that contacts and mitigated the tensile stress . Tong Wen et al used the bar tool with tapered shoulder for analysis of flexible and versatile (stretch and shrink) flanging process in ISMF technology and measured the wall thickness distribution . Surendra K. et al investigated the effect of various punch profiles on aluminium alloy AA5052 blank in SF process. It was found that the minimum crack length can be achieved with in hemispherical/conical punch shape with respect to other shapes . L I Jun-chao et al predicted the mechanical properties and distribution of thickness in blank in the incremental deep drawing process by numerical simulation . Suresh kurra et al performed the experimental and FE simulation for the assessment of formability and thickness distribution in ISMF process using a varying wall angle conical frustum . Yuung-Ming H. used the elasto-plastic FE code for the analysis of the effect of the web width, the flange height and the punch corner radius for prediction of distribution of thickness in SF process . Fracz et al studied the effect of 3 punch profile (i.e. conical, hemispherical and flat) on distribution of thickness in hole flanging process and found that the uniform thickness distribution in the case of flat bottomed punch profile . P. Sarkulvanich et al used three types of punches (conical, flat and spherical punch) to evaluate the influence of blank edge of AHSS steel sheet using FE analysis in SF (hole) process . Zein et al predicted the effect of springback and thinning in deep drawing process . Krawczyk et al performed the hole expansion test, with 3 types of punches (i.e. conical, spherical and cylindrical) and used with 4 types of steels workpiece. Enhancement in the diameter of holes were observed maximum in the case of spherical punch profile is used . Rongjing Zhang et al analysed the effect of spring back and thinning on multilayer sheet metal forming using the hydro-mechanical deep drawing process . Surendra k. et al analysed the influence of geometrical parameters on thinning and crack behaviour of AA5052 sheet in SF process . G. Ingarao et al investigated the influence of thinning and springback of dual-phase steels in U- shape bending operation which was based on multi-objective approach . B. Sarkar et al analysed the influence of materials parameters on thinning distribution in SMF process and determined that the thinning variation is the function of strain path . B.T. Tang et al optimized the design of blank and predicted the distribution of thickness in deep drawing process of square box and clover-shaped cup using enhanced inverse analysis method .
It is observed from the literature that a very less focused has been given, by the past researchers, on the influence of different punch geometry on deformation of sheet in stretch/curved flanging process, which is extensively used in different sectors such as aerospace, automotive industries and household applications. The deformation behaviour/mode in sheet depends on types of contact and area of contact between the surfaces of sheet and punch in SF process.
Non-uniform distribution of thickness and localized cracking are some of the big challenges in SMF process as well as in SF process. To minimize the blank thinning and cracks in SF process, different punch profile tools like cylindrical, 2 step, 3 step, 6 step, conical and hemispherical punch are used in the present research. Influences of these punch profiles have been performed in order to understand the deformation behaviour of blank in the SF process using FEM software ABAQUS/Explicit. Thickness distribution (its location) in blank is predicted along the midsection and the die profile radius and evaluated the contact behaviour/mode of blank with punch at different displacement percentage of punch. Experiments have been conducted on a hydraulic press machine to justify the FEM results.