Formability Evaluation of AA1100 Aluminium Sheet in Incremental Sheet Forming Process

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1 Formability Evaluation of AA1100 Aluminium Sheet in Incremental Sheet Forming Process Mr. Aniket K Patel Student, Mechanical Engineering Department, Institute of Technology, Nirma University Ahmedabad. 15MMCM06@nirmauni.ac.in Dr. B A Modi Professor, Mechanical Engineering Department Institute of Technology, Nirma University Ahmedabad. bharat.modi@nirmauni.ac.in Prof. A M Gohil Assistant Professor, Mechanical Engineering Department, Institute of Technology, Nirma University, Ahmedabad. ashish.gohil@nirmauni.ac.in Abstract Single Point Incremental Forming (SPIF) gains its application in building prototypes out of sheet metals. It has cutting edge over the conventional sheet metal forming from the tooling cost, forces involved, development time and flexibility view point besides very high formability. Even formability of the materials with low formability (AA1100-F Aluminium alloy) can be enhanced 7 to 20 times using SPIF for a groove geometry. A force measurement system has been developed to study the effect of step-depth on the forming force. Moreover, FE analysis of the SPIF process for the groove geometry has been performed to predict the force involved in SPIF. It has been observed that during the SPIF process the force involved is in the order of few hundred Newton. Keywords Groove component, Single Point Incremental Metal Forming, Vertical Machining Center, Design of experiment. I. INTRODUCTION Single Point Incremental Forming (SPIF), also called as Incremental Sheet Forming (ISF) is an important technique in which forming of sheet metal is carried out with a hemispherical forming tool using VMC machine. In ISF process, sheet is clamped by blank holder and hemispherical headed tool is moved along the defined contour path using VMC machine. The process has specific advantage of manufacturing products in batch size over conventional forming process, where costly dies and punches are essential. ISF is highly flexible as it doesn t require dies and punches as in conventional forming. Detailed literature review and status of the Incremental Sheet Forming is well documented [1,2]. Yanle Li et al. [3] have carried out FE simulation of ISF process to form cone shaped component and presented deformation mechanism which involves stretching, bending and shearing. Szekeres et al. [4] developed spindle mounted force measuring set-up to measure the forces involved in ISF process. Hussain et al. [5] proposed an experimental method to find thinning limit in forming variable angle cone and verified it with cosine law, Centeno et al. [6] concluded that the enhancement of formability above the FormingL imit Curve (FLC) in SPIF is much higher. Duflou et al. [7] presented the effect of wall angle of the component on the force required for forming. Duflou et al. [8] carried out parametric study and inferred that with increase in vertical step-depth, tool diameter, wall angle and sheet thickness, the force required for forming increases. Ghamdi et al. [9] proposed force-based strategy to control defects in SPIF. Petek et al. [10] developed autonomous online system for fracture identification based on the forming force curve and using statistical analysis. Santiago et al. [11] studied the forming force for variable angle cone by FE simulation. Ambrogio et al.[12]developed the strategy to prevent failure by using force as spy variable. Fratini et al.[13] concluded through the statistical analysis that strain hardening exponent(n) has the most significant effect on the maximum safe strain in plain strain deformation mode (FLD 0). Hagan and Jeswiet [14] studied the effect of step-depth and spindle speed on surface roughness of the formed component in ISF. Surface roughness found to deteriorate exponentially with step-depth. Ingarao et al. [15] compared Single Point Incremental Forming and stamping process on the basis of energy consumption and material utilization. Looking to the literature, it is attempted to study the forces involved in forming a groove using ISF. In this paper, it is proposed to study formability of an aluminium alloy AA1100 in SPIF and the forces involved in it. Agroove has been

2 formed using SPIF. An experimental set-up has also been developed for force measurement. FE analysis of SPIF for groove forming has also been carried out. II. DEVLOPMENT OF EXPERIMENTAL SET-UP A. Development of Load cell fixture and Groove test fixture An experimental set-up has been developed as shown in (Fig.1). A blank as shown in Fig. 2a is rigidly held on the fixture (Fig.1). A tool (Fig.2b) mounted on a VMC, tool head is traversed along the groove trajectory to form a groove (Fig.2c). A part program has been used to give feed and stepdepth required to achieve the required groove forming. 4. Laptop 6. Tool 3. DAQ card 2. DC amplifier 5. Fixture 1. S-Beam load cell ` Fig. 1. Schematic diagram of Experimental setup for forming force measurement Fig. 2. (a) Blank Fig.2(b). Hemispherical head tool Fig.2 (c).componant with groove forming S-beam load cell (Fig.3) has been mounted under the fixture for measurement of the force involved in SPIF of the groove in Z direction. The force signal have been captured and recorded using the system consists of amplifier, DAQ card and Lab VIEW signal Express software as shown in Fig.1. Actual set-up is shown in Fig. 4. Z Fig. 4. Experimental set-up for forming force measurement Fig. 3. Conceptualized 3-D CAD model of fixture for forming force measurement

3 B. Calibration of S-beam load cell S-beam type Load Cell of 5000N capacity have been used in the set-up.it can measure force with an accuracy of ± 0.01%.The load cell output signal is 0 to 30 mv, hence DC amplifier has been used to amplify the signal to 0-10 V. National Instruments Data Acquisition card USB-6008 is used to capture analog signals. The load cell is calibrated on compression testing machine as in the experiments it experiences compressive forming load. Standard calibration procedure is followed. Standard Weight(kg) Equation y = a + b*x Weight No Weighting Residual Sum of Squares Pearson's r Adj. R-Square Value Standard Error Voltage Intercept Voltage Slope Volatage(V) -7 Voltage Linear Fit Voltage vs. Weight -8-9 Fig. 6. Load cell calibration chart Fig. 5. Load cell calibration set-up The experimental set-up for loadcell calibration is as shown in Fig. 5. Voltage output from the load cell is recorded using Lab View software. All the recorded readings are negative because of the compressive loading conditions. From the data, calibration chart has been prepared as shown in Fig. 6. Regression analysis gives equation of line as y = x (1) where, y = Output Voltage recorded during the experiment and x = Weight applied during calibration process Equation (1) is used to convert voltage readings into equivalent force during experimentation. A. Material Characterization III. EXPERIMANTAL WORK The materials used for the experimental work (Aluminum alloy - AA1100 of 0.71 mmthickness) have been characterized using thetensile test prescribed by ASTM A370/ASTM E8 [16] standard.the test results are summarized in Table I. Material is found to follow the power law where K is strength coefficient and n is strain hardening exponent. The chemical compositions of the material have been found by performing spectrometry analysis and shown in Table II. TABLE I. TENSILE TEST OF AA1100-F ALUMINIUM ALLOY Sr. no. Material property 0.71 mm thick sheet 1 Yield stress (N/mm 2 ) UTS (N/mm 2 ) Elongation (%) in 50 mm gauge length Strain hardening exponent n Strength co-efficient K Mass density (kg/m 3 ) Young modulus (MPa) Poisson s ratio TABLE II. CHEMICAL COMPOSITION OF AA1100-F ALUMINIUM ALLOY Sr. No. Alloy % Sr. No. Alloy % 1 Al Zn Si Ti Fe Cr Nil 4 Cu Nil 10 Ni Mn Le Mg Nil 12 Tin Nil

4 B. Experimental work From the documented research work and preliminary experimentation it is found that tool radius and step-depth are important factors affecting the force in ISF process and hence their effect on forming force is studied in this experimental work. The levels of the factors considered for experimental investigation are listed in Table III. Spherical tool with two different radius have been used. Step-depth is the vertical motion given to the tool after it completes one pass. TABLE III. FACTORS FOR EXPERIMENTAL INVESTIGATION Sr. No. Factors Low Level High Level 1 Tool radius (mm) Step-depth (mm) The factors which have been kept constant during the process are shown in Table IV. Aluminium grade AA1100 sheet having the thickness 0.71 mm have been used during the experimental work. Feed rate is the rate at which forming tool is given the motion along the tool path. SPIF process can be carried out with and without tool rotation. In this experimental work no rotation is given to the tool. Mineral oil is used as a lubricant to reduce the friction. Zig-Zag tool traverse strategy as described in Fig. 7 is followed in all the experiments to form the groove. Experiments have been performed in the order given in Table V. Fracture depth, forming force and forming time are taken as responses in the experimental work. Fracture depth is the depth of the groove at which fracture is observed and it is instantaneous Z-coordinate of the tool which can be derived from the part program. Forming force is measured using the force measurement set-up discussed in Section II. Forming time is noted from VMC controller. Fig. 7. Zig-Zag tool traverse strategy IV. EXPLICIT FE ANALYSIS USING LS-DYNA Explicit FE analysis of the SPIF process have been performed using LS-DYNA software. The typical finite element model of the groove forming is shown in Fig. 8. Sheet is modeled as rectangle and meshed by BT-shell element. Zero degree of freedom constraint is given to the edge of the blank. Tool is modeled as sphere with a radius equal to the radius of hemispherical ended tool. The contact between tool and blank; blank and backing plate is modeled using CONTACT_FORMING_ONEWAY_SURFACE_TO_SURF ACE. The friction between the tool and blank is considered 0.30 (dynamic frication) and 0.39 (static friction) for blankbaking plate interface. The friction co-efficients were found from the standard friction test Static co-efficient of friction is obtained by block and slide test. Material of blank is defined by MAT_POWER_LAW_PLASTICITY and tool material is defined by MAT_RIGID. Motion to the tool is assigned by BOUNDARY_PRESCRIBED_MOTION_RIGID and DEFINE_CURVE keywords. Backing plate and Clamping plate are modeled as rigid. TABLE IV. PARAMETERS AND THEIR VALUES Sr. No. Parameters Value 1 Sheet Material AA Sheet Thickness (mm) Feed (mm/min) Tool traverse Strategy Constant Z-depth 5 Tool rotation No tool rotation 6 Material type Aluminum AA1100-F 7 Lubrication oil Mineral oil Fig. 8. Typical groove geometry in LS-DYNA

5 V. RESULTS& DISCUSSION Experiments were carried out with the process parameters as shown in Table IV to form the groove. The tool traverse is controlled through part programming and the tool was traversed till the fracture was observed. Summary of experimental and FE simulation results are given in Table V. A typical force pattern recorded by LAB VIEW software during experimental (step-depth 0.3 mm and tool radius 6 mm) is shown in Fig. 9. The peaks in Fig.9 represent the force exerted when the step-depth is given to the tool at every pass. Number of passes are represented by numbers 1 to 22 on the graph. Fig. 9. Experimental forming force for step-depth 0.3mm and tool radius 6 mm TABLE V. EXPERIMENTAL AND FE ANALYSIS RESULTS Sr. No. stepdepth (mm) Tool radius (mm) Sheet thickness (mm) Fracture Depth ( mm ) Force at Fracture ( N ) Experiment FEA Experiment FEA Time (min) For the groove forming with step-depth 0.3 mm, fracture depth is found to be 6.6 mmand it is also recorded from the NC part program. Fig. 10 show the component fractured at step 22 when formed with the tool (radius 6 mm) and step-depth 0.3mm. phenomena is observed when the step-depth is more (0.3 mm). However the formability i.e. the groove depth that can be formed is more with 6 mm radius tool. The stresses responsible for incremental forming are lower with 6 mm radius tool and hence the facture is reached after 6.6 mm and 6.5 mm groove depth respectively when step-depth was 0.3 mm and 0.1 mm. The same pattern is also seen in the FE simulation results. However the forces predicted with FE analysis are found to be larger as the load cell arrangement is different in experimental set-up. Fig. 10. Fractured componant for rustep-depth 0.3mm and tool radius 6mm It has also been observed that the fracture takes place at corner where step-depth is given. This can be attributed to the development of impact force when step-depth is given. It is also seen from the experimental results that the forces developed with 5 mm radius tool are lower than that developed with 6 mm radius tool. This is due to the contact area and the deformation zone is larger in case of 6 mm radius tool. Same Fig. 11. Forming force Experiment and FE analysis graph ( step-depth 0.3 mm and tool radius 6 mm)

6 FE analysis predicts the forming load as reaction force on the tool where as in actual set-up damping of the force occurs. This leads to deviation in the force by approximately 300 N. It is very apparent that when large step-depth is given, the time required to form the groove will be less. From the results we can infer that the larger is the tool radius and the step-depth the formability in forming the groove can be enhanced. VI. CONCLUSION Following conclusions can be drawn from the experimental study: 1. Large tool radius develops higher forming forces. 2. Increasing the tool radius, enhancesformability i.e. the fracture depth. 3. The proposed methodology for FE analysis of ISF process predicts the formability and forming forces in close agreement to the experimental results. 4. The combination of larger tool radius and larger stepdepth results in higher formability and less production time. References [1] Emmens, W. C., G. Sebastiani, and A. H. Van den Boogaard. "The technology of incremental sheet forming a brief review of the history." Journal of Materials Processing Technology (2010): [2] Hagan, E., and J. Jeswiet. "A review of conventional and modern single-point sheet metal forming methods." Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture (2003): [3] Li, Yanle, William J.T. Daniel, Zhaobing Liu, Haibo Lu, Paul A. Meehan "Deformation mechanics and efficient force prediction in single point incremental forming." Journal of Materials Processing Technology 221 (2015): [4] Szekeres, Alexander, M. Ham, and J. Jeswiet. "Force measurement in pyramid shaped parts with a spindle mounted force sensor." Key Engineering Materials. Vol Trans Tech Publications, [5] Hussain, G., and L. Gao. "A novel method to test the thinning limits of sheet metals in negative incremental forming." International Journal of Machine Tools and Manufacture 47.3 (2007): [6] Centeno, Gabriel, Isabel Bagudanch, A.J. Martínez- Donaire, M.L. García-Romeu, C. Vallellano "Critical analysis of necking and fracture limit strains and forming forces in single-point incremental forming." Materials & Design 63 (2014): [7] Duflou, Joost R., Yasemin Tunckol, and Richard Aerens. "Force analysis for single point incremental forming." Key Engineering Materials. Vol Trans Tech Publications, [8] Duflou, Joost, Yasemin Tunc kol, Alex Szekeres, Paul Vanherck"Experimental study on force measurements for single point incremental forming." Journal of Materials Processing Technology (2007): [9] Al-Ghamdi, K. A., G. Hussain, and Shahid I. Butt. "Force variations with defects and a force-based strategy to control defects in SPIF." Materials and Manufacturing Processes (2014): [10] Petek, Aleš, Karl Kuzman, and Blaž Suhač. "Autonomous on-line system for fracture identification at incremental sheet forming." CIRP Annals-Manufacturing Technology 58.1 (2009): [11] Pérez-Santiago, Rogelio, Isabel Bagudanch, and Maria Luisa García-Romeu. "Force modeling in single point incremental forming of variable wall angle components." Key Engineering Materials. Vol Trans Tech Publications, [12] Ambrogio, G., L. Filice, and F. Micari. "A force measuring based strategy for failure prevention in incremental forming." Journal of materials processing technology (2006): [13] Fratini. L.,G. Ambrogio, R. Di Lorenzo, L. Filice, F. Micar "Influence of mechanical properties of the sheet material on formability in single point incremental forming." CIRP Annals-Manufacturing Technology 53.1 (2004): [14] Hagan, E., and J. Jeswiet. "Analysis of surface roughness for parts formed by computer numerical controlled incremental forming." Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture (2004): [15] Ingarao, Giuseppe, Giuseppina Ambrogio, Francesco Gagliardi, Rosa Di Lorenzo"A sustainability point of view on sheet metal forming operations: material wasting and energy consumption in incremental forming and stamping processes." Journal of Cleaner Production 29 (2012): [16] ASTM International, Standard Test Method for Determining Forming Limit Curves