Experimental Investigation of SPIF Process for Conical Frustum

In sheet metal industries, single point incremental forming (SPIF) is a popular technology for quick prototyping and small series manufacturing techniques. In producing specific components with complicated geometries, the technique has become known for its flexibility, effectiveness, and simplicity of usage. The objective of this research work is to carry out the numerical investigation of the SPIF process to study the effect of different parameters on the formability for truncated cone. Al 1060 grade of aluminium is used for this study which is commonly used for kitchenware, reflectors, electronic components etc; where strength is not a major concern. Three dimensional models were developed for different set of parameters using commercial finite element software ABAQUS 6.13 with Dynamic/Explicit solver using solid mechanics model. Taguchi analysis has been carried out to find the effect of parameters (Tool rotational speed, Feed, Step size, Tool diameter and Wall angle) on thickness reduction. Each parameter was varied for three levels and L9 orthogonal array was used. The ANOVA tool has been used to summarize the contribution of each variable. Results have been compared with reported experimental results for validation. The process modelling is concluded with a brief overview of approaches for improving part quality.


Introduction:
The creation of flexible manufacturing methods that can easily maintain the difficult product proficiency in the market is currently in growing market.A relatively new technique of moulding is called incremental sheet forming (ISF).Its flexibility is the key for increasing use of the process in the industries that can conform to the needs of current scenario.ISF holds a competitive advantage for fabricating low volume functional sheet metal products in an economic and efficient way.Complex shapes can be manufactured using this method and the with less set up cost, less manufacturing time, without use of dies and higher degree of flexibility.Mason evaluated the actual estimation of the popular present ISF in 1978 for use with small batches of customized parts.The idea suggests using a singular spherical tool with three axes of numerical control.The development of technology, more particularly the appearance of numerically controlled machines, made it possible to use this method.Later, Mason's work was carried in Japan, leading to additional inventions [1].This has been proved to be an efficient process for small batch production and customized products in sheet metal forming.1291 (2023) 012035 IOP Publishing doi:10.1088/1757-899X/1291/1/012035 2 Tomaž Pepelnjak et al compared finite element (FE) analysis to experimental validation of a progressively built truncated pyramid.The emphasis was on the potential simplicity of the FE process modelling and its impact on the dependability of the obtained findings, particularly on the geometric precision of the component and the bottom pillowing effect.The FE modelling of the SPIF process was carried out using the programme ABAQUS [2].Slim Bouzidi et al, used the SPIF technique on AZ31B Magnesium alloy.A comparison of the gathered responses from portions acquired without holes and those obtained from perforated plates revealed inconsistencies [3].M. Sbayti et al. developed hot single point incremental forming (HSPIF) procedures to increase the shape complexity and formability of the Ti6Al4V alloy.They used four contemporary meta-heuristic algorithms for met modeling: Multiverse (MVO), Moth-flame (MFO), Harris Hawk (HHO), and Marine Predictor (MPA).The computational findings show that the strategies under consideration are quite competitive in geometry optimization.Among these four optimizers, MPA is the best in estimating the pillow defect in HSPIF rapidly [4].Sherwan Mohammed Najm et al. used Single Point Incremental Forming (SPIF) to perform several forming experiments under various forming circumstances.The arithmetical mean roughness (Ra) and ten-point mean roughness (Rz) of an AlMn1Mg1 sheet were predicted using an Artificial Neural Network (ANN).The findings demonstrated that ANN with a single output provides an optimal R-Square [5].Adham E. Ragab et al. investigated the impact of four process factors (tool diameter, feed rate, step size, and sheet thickness) on the final product properties.They use principle component analysis (PCA) to find the correlations between answers [6].Weining Li et al. investigated heating-assisted SPIF systems for high-strength alloy sheets in order to overcome current constraints.They investigated heating system methods, tools, tool path design, lubricants, and macro-and micro-numerical assessments [7].
Single Point Incremental Forming (SPIF) is a kind of AISF and a novel technique for highly localized deformation by the gradual feed of a generic tool following a predefined and programmed path with the help of a CNC machine leading to the final geometry of the parts shown in Figure 1.Unlike its close forming processes, shear forming and spinning, ISF is able to form complex asymmetric geometries due to computer assisted forming [8].ISF is a sheet material formation method built on additive manufacturing process concepts.Slices cut horizontally deform the sheet portion locally.The NC technique is used to move the locus of the forming tool (also known as the tool path) in these slices that are built to the work piece.

Figure 2 Schematic diagram of ISF
Using CAPP techniques, the tool path is directly generated from the CAD model of the finished object.The forming instrument has an end that is hemispherical.The edges of the metal sheet are rigidly fixed on a straightforward frame as it travels along the tool path [9].The ISF process's entire main steps are shown in Figure 2.
i.The steel plate is securely clamped to the structure.ii.The forming tool descends and makes touch with the sheet.iii.The tool moves in accordance with the initial tool path, which has the end product's geometric shape.iv.The tool keeps performing these actions until it reaches the conclusion of its path.
In the incremental sheet metal forming, generally two types of tool are used to form the sheet: Water Jet (WJ) and solid tools such as punches.Solid tool generally hemispherical and flat headed.Compared to hemispherical end tools, flat end tools can offer greater contour precision and formability.Additionally, especially in comparison to hemispherical end tools, flat end tools needed comparatively less forming force [10].The Forming Limit Curve in SPIF is roughly 2.7 times higher than standard Forming Limit Curves (FLC) than traditional processes, such as deep drawing and stamping [11].The large through thickness shear or the irregular strain path caused by cyclic plastic deformation are the two factors responsible for this rise in formability [12].In spiral tool paths give better surface finish in comparison to stepped contours and no entry and exit marks are observed.However, spiral paths are typically not allowed by CAM programmes.The easier approach to defining spiral path is via set of coordinates.When moving at faster velocities, surface flaws like sheet waviness get worse [13].Surface roughness also increases with speed.For the FEM modelling of the SPIF procedure, use ABAQUS/Standard (implicit solver).The authors enhanced the aluminium of the plastic deformation zone in the SPIF process using a FE sub-modelling method.The outcome demonstrated that there are differences in the contact pressure distribution between the sheet and the shaping instrument.The contact could typically be divided into two sections, according to a comparison of the distribution of the contact pressure under various operating circumstances.To reduce processing time in their research, a cone model with a 40° pie is used for modelling [14].

Experimental Work
To raise the calibre of products and procedures, the Taguchi technique is applied.When a better degree of performance is consistently attained, improved quality follows.By choosing the ideal mix of design elements, the greatest efficiency is achieved.Making the product/process resistant to the impact of uncontrollable factors allows for uniformity in performance.In the Taguchi method, the best design is chosen using design-of-experiment principles, and performance constancy is attained by running the trial condition while taking noise factors into consideration.To explore their influence on the final result, certain factors, including tool design, methods, and substance parameters, have been chosen.The selection of parameters in accordance with formability, are selected.Four parameters (Tool rotational speed, Feed, Step size and Wall angle) have been selected with an objective to observe the effect of these process parameters on product geometry and percentage thinning.Four levels of factors were selected for DOE.The factors and there levels used in present study are listed in Table 1.
Constant parameters for the experiment design are listed in Table 2.The trials can now be chosen and their methodology created using the variables and levels identified during the ideation session.The Taguchi technique is founded on conducting evaluations or experimentation to evaluate the sensitivity of response variables to a set of control parameters by taking into consideration trials in "Orthogonal Array (OA)" with a goal to achieve the ideal setup of process parameter.An optimal collection of well-balanced (minimum) trials is provided by orthogonal arrays.In the present study orthogonal array was designed using five factors with four levels.Here we choose L9 orthogonal array was selected.Table 3 shows the L9 orthogonal array for defined factors and their levels.Percentage thickness reduction and profile error in the simulated results was compared with practical output in order to validate the model.Further effect of process parameters over thickness reduction are analysed using "Analysis of Variance (ANNOVA)" tool.

Modelling and Simulation
Modelling is representation of graphical data into the model of actual part with the help of aluminium software.Modelling is at the heart of modern engineering practices.In this study efforts are made to perform FEM analysis of SPIF of AA1060 for conical frustum.The commercial FEA software ABAQUS 6.13 is used with explicit aluminium.Aluminium alloys have a wide range of application from general purpose to industry specific works.It possesses good mechanical strength, excellent weight to volume ratio, machinability and weldability.Aluminium / Aluminium 1060 alloy is a low strength and pure Aluminium / Aluminium alloy with low strength and good corrosion resistance characteristic, basically used for manufacturing of general purpose items where strength is not a major concern like utensils and electrical appliances.Economical cost and ease of machining favour it for the selection.
'Hemispherical Punch' is created as 3D Analytical rigid revolved shell object, 3D model of which is shown in Figure 3 (a).Different radius is taken for the geometry of Hemispherical Punch for different simulation runs as per DOE.Sheet metal blank is considered to be '3D deformable shell planar' object with a dimension of 90 mm × 90 mm.(Figure 3 b). 6 No material has been required to define for the deforming tool as it is considered to be an analytical rigid object.Sheet metal is assigned with the material property of AA1060 as per Table 4. Part geometries are assembled according to the actual configuration in single coordinate system (Figure 4).Tool radius compensation is considered while positioning the tool over the sheet.Tool Radius Compensation is given as per the equation 1.After creating assembly of tool and blank, the surface to surface type interaction between the outer surface of the hemispherical tool, taken as master surface, and the top surface of the sheet metal blank, taken as slave surface is assigned (Figure 4).The contact property taken is tangential behaviour type, directionality taken is isotropic and friction coefficient between the two surfaces is taken as 0.09 [16].The Figure 5 shows the view of the assembly with contact definition.3D model of final assembly is shown in Figure 6.
Where;  = tool radius compensation r = radius of the tool α = wall angle The Sheet blank was fixed at its four edges, to hold the sheet at its place as it is done by blank holder in experimental tests.The mid surface area of the sheet is not supported and is kept free to deform freely as shown in Figure 7.A radial path is created by selecting the nodes (Figure 10).Field output data is plotted against this path for analysis Product geometry of simulated cone is compared with ideal geometrical profile (Table 5) and deviations are observed (Figure 11).Deviation from the ideal geometry is observed in the cone profile.Material behaviour and tool path have significant influence over the geometrical profile.However backing plates help to lower this deviation considerably.During simulations element size functions as an important factor for profile accuracy but simultaneously it costs the simulation time.From the visualization of plastic equivalent strain (Figure 12), it is observed that strain distribution is not same in the deformation zone.Maximum strain is observed along in middle zone of the cone wall.

Conclusion
One of the most researched shapes in the area of incremental sheet formation is the conical frustum.Basic modelling setup (element type, meshing, steps, boundary conditions, and schemes (dynamic/explicit)) are reviewed and explored through practice on a FE simulation of a cone.Additionally, emphasis has been placed particularly on geometrical precision, thickness strain, and thickness reduction in the study of modelling results.Solver time for different models varied with feed and number of contour path.FE modelling for conical frustum was successfully done with ABAQUS 6.13.Deviation of simulated profile from ideal geometry was observed.Spring back, Tool path, absence of back plate and element size are speculated reasons for the cause.Thickness strain is found changing drastically in deforming zone.Maximum strain is observed in the middle zone of the deformed wall.Change in strain rate is also observed with element position.Percent thickness reduction data of simulation runs for designed orthogonal array were compared with experimental results and error was found in range of 6.42% -22.74%.Reduction in element size and mass scaling factor can improve the output with significant cost over solver time.The ability to minimize the number of measurements necessary to preserve the quality of the final SPIF output was the key contribution of the research given in this study.The findings reported in this research provide valuable insight into the link between the various properties of SPIF products.By applying the same approach to other materials, product shape, and process parameters, expertise in this field may be enhanced.

Future Scope
It is critical to understand the process limiting variables in industrial micro forming implementations.
The limiting issues might be related to the micro forming's unique characteristics.Many studies are being conducted to enhance the micro forming process, such as

Figure 6 .
Figure 6.Assembly showing interaction between tool and blank

Figure 7 .Figure 9 .
Figure 7. 3D Model of assembly with Encastred edgeThe load is defined in terms of velocity assigned to it.First tool movers downward following the wall angle and then planar movement takes place to form the circle.For the circular movement of tool amplitudes with smooth steps are incorporated for planar velocities in X and Y in terms of time and velocity as described by the equation 2 and 3 for each of the circular contour. =  (For amplitude along X axis)(2) =  (for amplitude along Y axis)(3)

Figure 10 .
Figure 10.Path selection for output data

Table 1 .
Values of Factors

Table 3 .
L9 Orthogonal Arrays for defined factors and levels