- Email: [email protected]
Experimental and numerical investigations on structural thinning, thinning evolution and compensation stratagem in deformation machining stretching mode
Journal of Manufacturing Processes 26 (2017) 216–225
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Experimental and numerical investigations on structural thinning, thinning evolution and compensation stratagem in deformation machining stretching mode Arshpreet Singh, Anupam Agrawal ∗ Department of Mechanical Engineering Indian Institute of Technology Ropar, Rupnagar, 140001, Punjab, India
a r t i c l e
i n f o
Article history: Received 14 July 2016 Received in revised form 4 January 2017 Accepted 16 February 2017 Keywords: Thin section machining Incremental forming Thinning
a b s t r a c t Deformation machining (DM) is a combination of thin structure machining and single point incremental forming/bending. This process enables creation of monolithic structures with complex geometries employing conventional tooling and equipment. In the present work, a comprehensive experimental and numerical (finite element) investigations on structural thinning, evolution of thinning across the forming depth in DM stretching mode has been performed. Structural thickness was found to be highly non uniform along the forming depth across all the investigated fixed and variable forming angles profiles. Structural thickness of the formed structure influences the strength and stiffness of the formed component. A theory behind non-uniform and reducing thickness profile has been proposed from the analysis of thinning evolution of the same formed profile at varied forming depths. Finally, a compensation strategy in thin structure machining has been proposed to obtain uniform structural thickness encompassing variable profiles in incremental forming. In this strategy, a relationship to machine an initial non-uniform section thickness radially taking the uncompensated formed thickness profile into consideration is obtained in order to achieve a uniform formed thickness profile. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Complex shapes and free form sheet metal and thin monolithic structures have wide range of applications in automotive, aviation, marine and construction sectors. Thin monolithic structures are now widely used in aviation, marine and automotive industries, owing to their increased strength, safety and light weight. Quality and inexpensive fabrication of monolithic structures with complex thin features is a challenge. This requires complex and intricate dies and tooling, making the process expensive and inflexible. Smith et al. proposed Deformation machining (DM) as a solution, a combination of two processes- thin structure machining and single point incremental bending and forming [1]. In this process firstly, thin structures are machined in the desired orientation and size from the bulk and then incrementally bent or formed into the desired shape depending upon the application. This process can create lighter weight monolithic components with novel and complex
geometries, in one setup, employing simple tooling and equipment. Therefore, enabling cost reduction in equipment, fabrication, assembly and weight of the components. The two aspects of DM, firstly the thin structure machining [2–4] followed by single point incremental bending and forming [5–7], their challenges, strategies, advantages and drawbacks has been discussed in detail in the previous published literature about the process [8–11]. The potential applications of deformation machining and thin monolithic parts with complex geometries are in aerospace industry: mold lines of fuselage, avionic shelf, impellers, and pressurized bulk heads, biomedical engineering (cranial plate, bone and joint support, prosthetics) [12], heat transfer and dissipation (irregular, curved fins). Deformation Machining is classified into two modes: (i) Bending and (ii) Stretching, based upon the orientation of the deforming tool and the component [8–11].
1.1. Deformation machining bending mode ∗ Corresponding author at: Room No. 224, Administrative Block, IIT Ropar, Rupnagar, Punjab, 140001, India. E-mail addresses: [email protected] (A. Singh), [email protected] (A. Agrawal).
In Deformation Machining Bending Mode the deformation is perpendicular to the axis of tool resulting in bending of thin vertical structure. Firstly, thin vertical sections are machined from the bulk material and then bent incrementally using a single point tool to the
http://dx.doi.org/10.1016/j.jmapro.2017.02.013 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
217
Nomenclature Incremental bending angle Incremental depth Maximum bent angle Inclination of bent structure along length Forming angle Tool diameter Floor size Forming feed rate Wall height to length ratio Instantaneous radial distance from base circumference tb Wall thickness Floor thickness tf tc(at R = zcot) Instantaneous compensated initial thickness at instantaneous radial distance ‘R’ Final required section thickness tr ta Actual uncompensated formed thickness Instantaneous forming depth z a z ␣ d D ff h/l R
Fig. 1. Schematic of DM bending mode [9–12].
desired shapes. Fig. 1 shows conceptually Deformation Machining of a thin wall with all the vital process parameters. 1.2. Deformation machining stretching mode In Deformation Machining Stretching Mode the deformation is along the axis of tool resulting in stretching of thin horizontal structure. Firstly, thin horizontal sections are machined from the bulk material and then stretch formed using a single point tool to the desired shapes. Fig. 2 shows conceptually Deformation Machining of a thin floor with all the vital process parameters. The phenomenon of thinning is a big challenge in conventional and incremental stretch forming of sheets. The surface area of formed profile increases at the expense of the thickness of the sheet as volume of the sheet is constrained unlike in deep drawing process. Sheet or structural thinning is also one of the indicators of process formability and onset of fracture [13,14]. It adversely affects the strength and stiffness of the formed component. Previous findings pertaining to thickness evaluations in incremental forming reported non uniform formed profile thickness with decreasing trends across the forming depth [15,16], clearly in violation to the theoretical cosine law prediction of the final formed thickness. The present work is an extension to the previous study pertaining to
Fig. 2. Schematic of DM stretching mode [8–11].
thinning evaluations in incremental forming of thin monolithic structures [11]. The thickness profiles of formed structures at constant forming angles for DM stretching mode components along the forming depth both experimentally and through finite element simulations were evaluated. Based on the results from experiments and simulations, a compensation strategy was proposed and realized towards achieving uniform thickness distribution for the
Fig. 3. Fixture for holding DM stretching mode samples.
218
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
Table 1 Properties of AA-6063. Properties
Density
Melting Point
Poisson’s ratio
Modulus of elasticity
Tensile strength
Shear Strength
Proof stress
Magnitude
2.7 gm/cc
600 ◦ C
0.3
70 GPa
215 MPa
150 MPa
170 MPa at 0.2%
using a single point hemispherical tool. The tool path for incremental forming was generated through a standard machining module of Catia V5. A fixture was designed and fabricated, consisting of a holding plate and clamps for holding thick components, a backing plate for supporting the components during high speed machining, and a base plate for overall stability of the fixture (Fig. 3). During forming, the backing plate was removed from the fixture and the components were formed through a circular orifice in the holding plate. Section thickness of the formed profiles was measured on a coordinate measuring machine (CMM) (Make: Accurate with Renishaw probes). The thickness profile was evaluated by subtracting outer and inner measured radii along the depth. Fig. 4 shows the inspection outer and inner radii. The thickness is measured as a function of the difference between the outer and inner radii ‘x’ and the instantaneous forming angle ‘’ along the depth. Fig. 5(a) is the schematic depicting thickness measurement. Fig. 4. Inspection external and internal radii on CMM.
constant forming angle profiles. The present work extends it further towards generalizing the same strategy towards achieving uniform thickness profiles at varied angles profiles. Moreover, a reasoning to non-uniform thickness profile in incremental forming has also been proposed and conformed through experimental and FE analysis on thinning evolution of the same profile at varied forming depths. 2. Methodology 2.1. Experimental procedure The material used in the present study is AA 6063-T6, a commonly used alloy in aerospace and aviation. Table 1 depicts mechanical properties of the alloy. A 12 mm × 100 mm aluminium flat was used as the raw material. The samples were fabricated on a 3 axis CNC vertical milling machine (Make: BFW, Model: VF 30 CNC VS). Samples were firstly machined to a thin floor structure using a tungsten carbide end mill tool and then formed incrementally
2.2. Experimental plan Table 2 depicts the fixed level of incremental forming parameters. Thickness of the formed structure in incremental stretch forming is primarily a function of forming angle ‘’ and is given by cosine law (Eq. (1)). t f = t i cos
(1)
where, tf is the final formed thickness, ti is the initial thickness and is the forming angle. Therefore, influence of fixed forming angles (30◦ , 45◦ , 60◦ , 75◦ ) and continuously varying forming angle with each increment (30◦ to 75◦ and 75◦ to 30◦ ) conical profiles on the section thickness were evaluated. 2.3. Numerical simulation Incremental forming process associated with deformation machining has been simulated for structural thinning at varied forming angles corresponding with the experimental trials.
Fig. 5. (a) Schematic of thickness measurement; (b) Image of profile measurement across the depth.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
219
Table 2 Fixed levels of process parameters. Process Incremental Forming
Process parameters
Dimensional attributes
Parameters
Value
Spindle RPM Feed Rate ‘Ff ’ (m/min) Tool Diameter ‘d’ (mm) Incremental Step Depth ‘z’ (mm) Initial Machined Thickness ‘tf ’ (mm) Base Radius ‘R’ (mm)
500 0.25 10 5 1, 1.5 35
Fig. 6. Mean anisotropic stress strain curves for raw materials.
2.3.1. Part and material model The material for the considered workpiece to be bent has elastoplastic properties obtained from the tensile test results of the AA 6063 T6 machined extrusions (Fig. 6). The dimensions of workpiece considered as a thin structure to be formed were 150 × 150 mm with variable opening depending upon the size of the formed structured. The thickness of the thin structure was also varied according to the requirement.
Fig. 8. Cross sectional images of thin formed components.
2.3.2. Interactions and contact properties Apart from the same tool-workpiece interaction employed for incremental bending, In case of DM stretching mode hard contact interaction between the deformable thin structure and backing plate was considered. This was done to make region of the
Fig. 7. Meshing and boundary conditions in incremental forming.
220
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
deformable thin structure in contact with the backing plate rigid (to be considered as monolithic thick region). 2.3.3. Mesh control Shell elements with global size of 0.5 mm were used for the workpiece were used in structural thickness evaluations. Solid hexahedral elements with global size 1.0 mm were used for the deforming tool. 2.3.4. Boundary conditions and degrees of freedom Thin structure ends and the backing plate was considered to be fixed in all three directions (encastred) in incremental forming. Rotational and translational degree of freedom was provided to the tool for corresponding tool path generated on CATIA V6 machining module determining the shape of the formed structure and incremental step size (Fig. 7). Fig. 9. Experimental results of variation of section thickness along the forming depth.
3. Results and discussion 3.1. Section thickness at constant forming angle profiles Fig. 8 shows the cross sectional images of thin formed components. The variation of section thickness along the forming depth at varying forming wall angles experimentally is depicted in Fig. 9. From the results it can be observed that the section thickness is highly non uniform with a decreasing trend along the forming depth, unlike uniform thickness predicted by cosine law. A sharp reduction in section thickness after a certain depth was also observed owing to transition from bending zone to actual stretch
forming especially at steeper forming angles. The formability limits, geometrical accuracy and overall strength of the fabricated components are compromised owing to non-uniform structural thinning. The minimum section thicknesses were 0.756 mm, 0.537 mm, 0.319 mm and 0.175 mm for wall angles 30◦ , 45◦ , 60◦ and 75◦ respectively for 1 mm initial section thickness. A ductile fracture was observed at depth of 11.5 mm for section formed at 75◦ . The minimum section thickness at all the tested forming angles was less than the theoretical final formed thickness predicted by the cosine law (Eq. (1)). Figs. 10 and 11 shows the corresponding thick-
Fig. 10. Finite element modelled section thickness profiles at varied forming angles.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
Fig. 11. F E. simulated results of variation of section thickness along the forming depth.
ness profiles and values obtained from FE simulations respectively. The results from finite element modelling are in close agreement with the experiments, though having slight variations owing to the shearing effect (Fig. 12) and consequent material removal, as the process incremental forming is a combination of stretching and shearing [15]. This shearing effect in the process has not been considered in finite element modelling.
221
Fig. 12. Micro image of the formed surface.
3.2. Evolution of structural thinning in incremental forming Results obtained from experimental and numerical investigations of section thickness in DM stretching mode reveal highly non-uniform and constantly diminishing thickness profile along the depth of the formed components across all the investigated forming angles. These results are violating the theoretical assump-
Fig. 13. Experimental section thickness at varying forming depths.
222
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
tion about uniform final thickness (tf ) in relation to the initial thickness (ti ) and cosine of the forming angle () (Eq. (1)) irrespective of other process parameters involved in incremental stretch forming process. To find the exact reason for such non-uniform thickness profiles, experimental and numerical (finite element) measurements and analysis of thickness profiles have been carried out for a constant forming angle (45◦ ) at varying forming depths of 5 mm to 30 mm (Each profile has been formed 5 mm in depth extra to accommodate the tool radius in each case). Both the experimental (Fig. 13) and numerical (Fig. 14) outcomes reveal significant variations in thickness profiles with varying forming depths with certain degree of similarity. The slight variation in experimental and numerical outcomes could be because of shear between tool and sheet surface resulting in material removal, which has not been considered in the numerical simulations. At the same instantaneous forming depth, the thickness of the formed sheet varies significantly at varying final forming depths, keep all the other process parameters constant. The minimum thickness changes from 0.903 mm to 0.783 mm at instantaneous forming depth of 5 mm for the final formed depth of 5 mm to 30 mm respectively. It has been assumed and proven (numerically and experimentally) with certain degree of confidence that deformation in incremental forming is of highly localized nature. The local region in instantaneous contact with the single point tool is primarily stretch formed in incremental forming. The formed region not in contact with the tool, which is under plane strain condition, remains largely unaffected (Fig. 15). But according the results of thickness distribution with varying forming depth, the formed region not under the tool contact undergo considerable amount of stretching resulting in reduced section thickness. Moreover, apart from stretching, some amount of shearing between tool and thin structure interface in incremental forming lead to possible material removal and added to the reduced section thickness than the theoretical formed thickness predicted by cosine law [15].
Fig. 15. Schematic showing forming regions in incremental forming.
3.3. Compensation strategy for uniform thickness Prior machining of thin structures to be incrementally formed in DM, gives the flexibility and opportunity to alter the initial thickness (ti ) in order to achieve a desired final thickness after forming. The strategy is to achieve a final uniform thickness (tr ) through-
Fig. 14. Modelled section thickness at varying forming depths.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
223
Fig. 16. Compensated machined thickness profiles for different forming angles. Fig. 18. Compensated thickness profiles at various forming angles.
out the forming depth is to radially vary the initial thin machined structure thickness. The compensated (instantaneous) thickness (tc ) depends upon the uncompensated section thickness (ta ) at varying forming angles and is given by Eq. (2).
thickness is observed, mainly owing to material removal because of shearing action along with the stretching. In order to achieve larger desirable structural thickness (tr ), machine capabilities must be taken into the consideration.
t c(atR = zcot) = t r /cos -t a /cos + t r =(machined thickness required for uniform formed thickness) − (machined thickness of the obtained non uniform thickness at instantaneous depth z) + (required uniform forming thickness) Therefore,t c(atR = zcot) = 1/cos[t r (1 + cos) − t a(atz) ]
(2)
Fig. 16 depicts the compensated machined thickness for uniform forming thickness profile at different forming angles obtained from the strategy. Fig. 17 shows the actual unaltered and compensated initial machined thin structure thickness. Figs. 18 and 19 shows the results from altered machining approach resulting in highly uniform section at desirable thickness (1 mm), negating thinning of formed section at all the forming angles. Though at steeper angles (60 and 75◦ ), reduced section
3.4. Section thickness at variable forming angle profiles Confirming the applicability of the compensation strategy of uniform forming thickness for random forming profiles is important for the overall generality of the concept. For this two profiles with varying forming angles with each increment have been formed experimentally and modelled through finite element methodology. The forming angles vary from 30◦ to 75◦ and 75◦ to 30◦ along the forming depth for the two profiles. Fig. 18 depicts the modelled, non-compensated and compensated section thickness of the varying angle profiles. The images depicted in Fig. 20 and corresponding results of non-compensated experimental and modelling of section thickness shown in Fig. 21 reveal highly non uniform thickness profile for both the cases, with variation up to 0.5 mm for the initial
Fig. 17. (a) Unaltered uniform machined section; (b) Compensated non-uniform machined section.
224
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
Fig. 19. Compensated section thickness values across the forming depth.
Fig. 20. Modelled, non-compensated and compensated section thickness of the varying angle profiles.
thickness of 1.5 mm. The result for compensated section thickness shows highly uniform section thickness across the forming depth for both the profiles (Fig. 22). Therefore, the compensation strategy of non-uniform machining is also valid for random profile formed through variable forming angles. 4. Summary and conclusions Experimental and finite element investigations on structural thinning in DM stretching reveal considerable thinning in the formed section along with highly non uniform thickness profiles across the forming depth. A reasoning to non-uniform thickness profile in incremental forming has also been proposed and a con-
formed through experimental and FE analysis on thinning evolution of the same profile at varied forming depths. Finally, a compensation strategy for uniform thinning in DM stretching for fixed and variable angle profiles has been proposed and realized in this work. Desired formed thickness along with considerable uniform profile across the forming depth was achieved by employing a varying thin section machining compensation strategy, prior to incremental forming. This would probably enhance the formability limits (especially at steeper forming angles) and strength of the formed monolithic components.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
225
References
Fig. 21. Non-compensated section thickness along the forming depth.
Fig. 22. Compensated section thickness along the forming depth.
Acknowledgement The authors acknowledge DST project SB/FTP/ETA-254/2012 funded by DST-SERB, India for providing the financial support and IIT Ropar, India for providing the basic facilities.
[1] Smith S, Woody B, Ziegert J, Huang Y. Deformation machining − A new hybrid process. CIRP Ann. Manuf. Technol 2007;56(1):281–4. [2] Tlusty J, Smith S, Winfough W. Techniques for the use of long slender end mills in high-speed milling. Ann. CIRP 1996;45(1):393–6. [3] Smith S, Dvorak D. Tool path strategies for high speed milling aluminium work pieces with thin webs. Mechatroni. J 1998;8(3):291–300. [4] Smith S, Wilhelm R, Dutterer B, Cherukuri H, Goel G. Sacrificial structure preforms for thin part machining. CIRP Ann. Manu. Technol 2012;61(1):379–82. [5] Jeswiet J, Micari F, Hirt G, Bramley A, Duflou J, Allwood J. Asymmetric single point incremental forming of sheet metal. Ann. CIRP 2005;54(2):623–50. [6] Duflou JR, Tunckol Y, Szekeres A, Vanherck P. Experimental study on force measurements for single point incremental forming. J Mater Process Technol 2007;189:65–72. [7] Echrif S, Hrairi M. Research and progress in incremental sheet forming processes. Mater Manuf Processes 2011;26(11):1404–14. [8] Singh A, Agrawal A. Investigation of surface residual stress distribution in deformation machining process for aluminum alloy. J Mater Process Technol 2015;225:195–202. [9] Singh A, Agrawal A. Comparison of deforming forces, residual stresses and geometrical accuracy of deformation machining with conventional bending and forming. J Mater Process Technol 2016;234:259–71. [10] Singh A, Agrawal A. Experimental investigation on elastic spring back in deformation machining Bending mode. Proceedings of the 10th ASMEManufacturing Science and Engineering Conference 2015. [11] Singh A, Agrawal A. Investigations on structural thinning and compensation stratagem in deformation machining stretching mode. Manuf. Lett 2016. Accepted for publication. [12] Ambrogio G, De Napoli L, Filice L, Gagliardi F, Muzzupappa M. Application of incremental forming process for high customized medical product manufacturing. J. Mater. Process. Technol 2005;162:156–62. [13] Allwood J, Shouler D, Tekkaya A. The increased forming limits of incremental sheet forming processes. Key Eng Mater 2007;344:621–8. [14] Filice L, Fratini L, Micari F. Analysis of material formability in incremental forming. CIRP Ann. Manuf. Technol 2002;51(1):199–202. [15] Jackson K, Allwood J. The mechanics of incremental sheet forming. J Mater Process Technol 2009;209(3):1158–74. [16] Young D, Jeswiet J. Wall thickness variations in single-point incremental forming. J. Eng. Manuf. Part B 2004;218:1453–9.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Experimental and numerical investigations on structural thinning, thinning evolution and compensation stratagem in deformation machining stretching mode Arshpreet Singh, Anupam Agrawal ∗ Department of Mechanical Engineering Indian Institute of Technology Ropar, Rupnagar, 140001, Punjab, India
a r t i c l e
i n f o
Article history: Received 14 July 2016 Received in revised form 4 January 2017 Accepted 16 February 2017 Keywords: Thin section machining Incremental forming Thinning
a b s t r a c t Deformation machining (DM) is a combination of thin structure machining and single point incremental forming/bending. This process enables creation of monolithic structures with complex geometries employing conventional tooling and equipment. In the present work, a comprehensive experimental and numerical (finite element) investigations on structural thinning, evolution of thinning across the forming depth in DM stretching mode has been performed. Structural thickness was found to be highly non uniform along the forming depth across all the investigated fixed and variable forming angles profiles. Structural thickness of the formed structure influences the strength and stiffness of the formed component. A theory behind non-uniform and reducing thickness profile has been proposed from the analysis of thinning evolution of the same formed profile at varied forming depths. Finally, a compensation strategy in thin structure machining has been proposed to obtain uniform structural thickness encompassing variable profiles in incremental forming. In this strategy, a relationship to machine an initial non-uniform section thickness radially taking the uncompensated formed thickness profile into consideration is obtained in order to achieve a uniform formed thickness profile. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Complex shapes and free form sheet metal and thin monolithic structures have wide range of applications in automotive, aviation, marine and construction sectors. Thin monolithic structures are now widely used in aviation, marine and automotive industries, owing to their increased strength, safety and light weight. Quality and inexpensive fabrication of monolithic structures with complex thin features is a challenge. This requires complex and intricate dies and tooling, making the process expensive and inflexible. Smith et al. proposed Deformation machining (DM) as a solution, a combination of two processes- thin structure machining and single point incremental bending and forming [1]. In this process firstly, thin structures are machined in the desired orientation and size from the bulk and then incrementally bent or formed into the desired shape depending upon the application. This process can create lighter weight monolithic components with novel and complex
geometries, in one setup, employing simple tooling and equipment. Therefore, enabling cost reduction in equipment, fabrication, assembly and weight of the components. The two aspects of DM, firstly the thin structure machining [2–4] followed by single point incremental bending and forming [5–7], their challenges, strategies, advantages and drawbacks has been discussed in detail in the previous published literature about the process [8–11]. The potential applications of deformation machining and thin monolithic parts with complex geometries are in aerospace industry: mold lines of fuselage, avionic shelf, impellers, and pressurized bulk heads, biomedical engineering (cranial plate, bone and joint support, prosthetics) [12], heat transfer and dissipation (irregular, curved fins). Deformation Machining is classified into two modes: (i) Bending and (ii) Stretching, based upon the orientation of the deforming tool and the component [8–11].
1.1. Deformation machining bending mode ∗ Corresponding author at: Room No. 224, Administrative Block, IIT Ropar, Rupnagar, Punjab, 140001, India. E-mail addresses: [email protected] (A. Singh), [email protected] (A. Agrawal).
In Deformation Machining Bending Mode the deformation is perpendicular to the axis of tool resulting in bending of thin vertical structure. Firstly, thin vertical sections are machined from the bulk material and then bent incrementally using a single point tool to the
http://dx.doi.org/10.1016/j.jmapro.2017.02.013 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
217
Nomenclature Incremental bending angle Incremental depth Maximum bent angle Inclination of bent structure along length Forming angle Tool diameter Floor size Forming feed rate Wall height to length ratio Instantaneous radial distance from base circumference tb Wall thickness Floor thickness tf tc(at R = zcot) Instantaneous compensated initial thickness at instantaneous radial distance ‘R’ Final required section thickness tr ta Actual uncompensated formed thickness Instantaneous forming depth z a z ␣ d D ff h/l R
Fig. 1. Schematic of DM bending mode [9–12].
desired shapes. Fig. 1 shows conceptually Deformation Machining of a thin wall with all the vital process parameters. 1.2. Deformation machining stretching mode In Deformation Machining Stretching Mode the deformation is along the axis of tool resulting in stretching of thin horizontal structure. Firstly, thin horizontal sections are machined from the bulk material and then stretch formed using a single point tool to the desired shapes. Fig. 2 shows conceptually Deformation Machining of a thin floor with all the vital process parameters. The phenomenon of thinning is a big challenge in conventional and incremental stretch forming of sheets. The surface area of formed profile increases at the expense of the thickness of the sheet as volume of the sheet is constrained unlike in deep drawing process. Sheet or structural thinning is also one of the indicators of process formability and onset of fracture [13,14]. It adversely affects the strength and stiffness of the formed component. Previous findings pertaining to thickness evaluations in incremental forming reported non uniform formed profile thickness with decreasing trends across the forming depth [15,16], clearly in violation to the theoretical cosine law prediction of the final formed thickness. The present work is an extension to the previous study pertaining to
Fig. 2. Schematic of DM stretching mode [8–11].
thinning evaluations in incremental forming of thin monolithic structures [11]. The thickness profiles of formed structures at constant forming angles for DM stretching mode components along the forming depth both experimentally and through finite element simulations were evaluated. Based on the results from experiments and simulations, a compensation strategy was proposed and realized towards achieving uniform thickness distribution for the
Fig. 3. Fixture for holding DM stretching mode samples.
218
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
Table 1 Properties of AA-6063. Properties
Density
Melting Point
Poisson’s ratio
Modulus of elasticity
Tensile strength
Shear Strength
Proof stress
Magnitude
2.7 gm/cc
600 ◦ C
0.3
70 GPa
215 MPa
150 MPa
170 MPa at 0.2%
using a single point hemispherical tool. The tool path for incremental forming was generated through a standard machining module of Catia V5. A fixture was designed and fabricated, consisting of a holding plate and clamps for holding thick components, a backing plate for supporting the components during high speed machining, and a base plate for overall stability of the fixture (Fig. 3). During forming, the backing plate was removed from the fixture and the components were formed through a circular orifice in the holding plate. Section thickness of the formed profiles was measured on a coordinate measuring machine (CMM) (Make: Accurate with Renishaw probes). The thickness profile was evaluated by subtracting outer and inner measured radii along the depth. Fig. 4 shows the inspection outer and inner radii. The thickness is measured as a function of the difference between the outer and inner radii ‘x’ and the instantaneous forming angle ‘’ along the depth. Fig. 5(a) is the schematic depicting thickness measurement. Fig. 4. Inspection external and internal radii on CMM.
constant forming angle profiles. The present work extends it further towards generalizing the same strategy towards achieving uniform thickness profiles at varied angles profiles. Moreover, a reasoning to non-uniform thickness profile in incremental forming has also been proposed and conformed through experimental and FE analysis on thinning evolution of the same profile at varied forming depths. 2. Methodology 2.1. Experimental procedure The material used in the present study is AA 6063-T6, a commonly used alloy in aerospace and aviation. Table 1 depicts mechanical properties of the alloy. A 12 mm × 100 mm aluminium flat was used as the raw material. The samples were fabricated on a 3 axis CNC vertical milling machine (Make: BFW, Model: VF 30 CNC VS). Samples were firstly machined to a thin floor structure using a tungsten carbide end mill tool and then formed incrementally
2.2. Experimental plan Table 2 depicts the fixed level of incremental forming parameters. Thickness of the formed structure in incremental stretch forming is primarily a function of forming angle ‘’ and is given by cosine law (Eq. (1)). t f = t i cos
(1)
where, tf is the final formed thickness, ti is the initial thickness and is the forming angle. Therefore, influence of fixed forming angles (30◦ , 45◦ , 60◦ , 75◦ ) and continuously varying forming angle with each increment (30◦ to 75◦ and 75◦ to 30◦ ) conical profiles on the section thickness were evaluated. 2.3. Numerical simulation Incremental forming process associated with deformation machining has been simulated for structural thinning at varied forming angles corresponding with the experimental trials.
Fig. 5. (a) Schematic of thickness measurement; (b) Image of profile measurement across the depth.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
219
Table 2 Fixed levels of process parameters. Process Incremental Forming
Process parameters
Dimensional attributes
Parameters
Value
Spindle RPM Feed Rate ‘Ff ’ (m/min) Tool Diameter ‘d’ (mm) Incremental Step Depth ‘z’ (mm) Initial Machined Thickness ‘tf ’ (mm) Base Radius ‘R’ (mm)
500 0.25 10 5 1, 1.5 35
Fig. 6. Mean anisotropic stress strain curves for raw materials.
2.3.1. Part and material model The material for the considered workpiece to be bent has elastoplastic properties obtained from the tensile test results of the AA 6063 T6 machined extrusions (Fig. 6). The dimensions of workpiece considered as a thin structure to be formed were 150 × 150 mm with variable opening depending upon the size of the formed structured. The thickness of the thin structure was also varied according to the requirement.
Fig. 8. Cross sectional images of thin formed components.
2.3.2. Interactions and contact properties Apart from the same tool-workpiece interaction employed for incremental bending, In case of DM stretching mode hard contact interaction between the deformable thin structure and backing plate was considered. This was done to make region of the
Fig. 7. Meshing and boundary conditions in incremental forming.
220
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
deformable thin structure in contact with the backing plate rigid (to be considered as monolithic thick region). 2.3.3. Mesh control Shell elements with global size of 0.5 mm were used for the workpiece were used in structural thickness evaluations. Solid hexahedral elements with global size 1.0 mm were used for the deforming tool. 2.3.4. Boundary conditions and degrees of freedom Thin structure ends and the backing plate was considered to be fixed in all three directions (encastred) in incremental forming. Rotational and translational degree of freedom was provided to the tool for corresponding tool path generated on CATIA V6 machining module determining the shape of the formed structure and incremental step size (Fig. 7). Fig. 9. Experimental results of variation of section thickness along the forming depth.
3. Results and discussion 3.1. Section thickness at constant forming angle profiles Fig. 8 shows the cross sectional images of thin formed components. The variation of section thickness along the forming depth at varying forming wall angles experimentally is depicted in Fig. 9. From the results it can be observed that the section thickness is highly non uniform with a decreasing trend along the forming depth, unlike uniform thickness predicted by cosine law. A sharp reduction in section thickness after a certain depth was also observed owing to transition from bending zone to actual stretch
forming especially at steeper forming angles. The formability limits, geometrical accuracy and overall strength of the fabricated components are compromised owing to non-uniform structural thinning. The minimum section thicknesses were 0.756 mm, 0.537 mm, 0.319 mm and 0.175 mm for wall angles 30◦ , 45◦ , 60◦ and 75◦ respectively for 1 mm initial section thickness. A ductile fracture was observed at depth of 11.5 mm for section formed at 75◦ . The minimum section thickness at all the tested forming angles was less than the theoretical final formed thickness predicted by the cosine law (Eq. (1)). Figs. 10 and 11 shows the corresponding thick-
Fig. 10. Finite element modelled section thickness profiles at varied forming angles.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
Fig. 11. F E. simulated results of variation of section thickness along the forming depth.
ness profiles and values obtained from FE simulations respectively. The results from finite element modelling are in close agreement with the experiments, though having slight variations owing to the shearing effect (Fig. 12) and consequent material removal, as the process incremental forming is a combination of stretching and shearing [15]. This shearing effect in the process has not been considered in finite element modelling.
221
Fig. 12. Micro image of the formed surface.
3.2. Evolution of structural thinning in incremental forming Results obtained from experimental and numerical investigations of section thickness in DM stretching mode reveal highly non-uniform and constantly diminishing thickness profile along the depth of the formed components across all the investigated forming angles. These results are violating the theoretical assump-
Fig. 13. Experimental section thickness at varying forming depths.
222
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
tion about uniform final thickness (tf ) in relation to the initial thickness (ti ) and cosine of the forming angle () (Eq. (1)) irrespective of other process parameters involved in incremental stretch forming process. To find the exact reason for such non-uniform thickness profiles, experimental and numerical (finite element) measurements and analysis of thickness profiles have been carried out for a constant forming angle (45◦ ) at varying forming depths of 5 mm to 30 mm (Each profile has been formed 5 mm in depth extra to accommodate the tool radius in each case). Both the experimental (Fig. 13) and numerical (Fig. 14) outcomes reveal significant variations in thickness profiles with varying forming depths with certain degree of similarity. The slight variation in experimental and numerical outcomes could be because of shear between tool and sheet surface resulting in material removal, which has not been considered in the numerical simulations. At the same instantaneous forming depth, the thickness of the formed sheet varies significantly at varying final forming depths, keep all the other process parameters constant. The minimum thickness changes from 0.903 mm to 0.783 mm at instantaneous forming depth of 5 mm for the final formed depth of 5 mm to 30 mm respectively. It has been assumed and proven (numerically and experimentally) with certain degree of confidence that deformation in incremental forming is of highly localized nature. The local region in instantaneous contact with the single point tool is primarily stretch formed in incremental forming. The formed region not in contact with the tool, which is under plane strain condition, remains largely unaffected (Fig. 15). But according the results of thickness distribution with varying forming depth, the formed region not under the tool contact undergo considerable amount of stretching resulting in reduced section thickness. Moreover, apart from stretching, some amount of shearing between tool and thin structure interface in incremental forming lead to possible material removal and added to the reduced section thickness than the theoretical formed thickness predicted by cosine law [15].
Fig. 15. Schematic showing forming regions in incremental forming.
3.3. Compensation strategy for uniform thickness Prior machining of thin structures to be incrementally formed in DM, gives the flexibility and opportunity to alter the initial thickness (ti ) in order to achieve a desired final thickness after forming. The strategy is to achieve a final uniform thickness (tr ) through-
Fig. 14. Modelled section thickness at varying forming depths.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
223
Fig. 16. Compensated machined thickness profiles for different forming angles. Fig. 18. Compensated thickness profiles at various forming angles.
out the forming depth is to radially vary the initial thin machined structure thickness. The compensated (instantaneous) thickness (tc ) depends upon the uncompensated section thickness (ta ) at varying forming angles and is given by Eq. (2).
thickness is observed, mainly owing to material removal because of shearing action along with the stretching. In order to achieve larger desirable structural thickness (tr ), machine capabilities must be taken into the consideration.
t c(atR = zcot) = t r /cos -t a /cos + t r =(machined thickness required for uniform formed thickness) − (machined thickness of the obtained non uniform thickness at instantaneous depth z) + (required uniform forming thickness) Therefore,t c(atR = zcot) = 1/cos[t r (1 + cos) − t a(atz) ]
(2)
Fig. 16 depicts the compensated machined thickness for uniform forming thickness profile at different forming angles obtained from the strategy. Fig. 17 shows the actual unaltered and compensated initial machined thin structure thickness. Figs. 18 and 19 shows the results from altered machining approach resulting in highly uniform section at desirable thickness (1 mm), negating thinning of formed section at all the forming angles. Though at steeper angles (60 and 75◦ ), reduced section
3.4. Section thickness at variable forming angle profiles Confirming the applicability of the compensation strategy of uniform forming thickness for random forming profiles is important for the overall generality of the concept. For this two profiles with varying forming angles with each increment have been formed experimentally and modelled through finite element methodology. The forming angles vary from 30◦ to 75◦ and 75◦ to 30◦ along the forming depth for the two profiles. Fig. 18 depicts the modelled, non-compensated and compensated section thickness of the varying angle profiles. The images depicted in Fig. 20 and corresponding results of non-compensated experimental and modelling of section thickness shown in Fig. 21 reveal highly non uniform thickness profile for both the cases, with variation up to 0.5 mm for the initial
Fig. 17. (a) Unaltered uniform machined section; (b) Compensated non-uniform machined section.
224
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
Fig. 19. Compensated section thickness values across the forming depth.
Fig. 20. Modelled, non-compensated and compensated section thickness of the varying angle profiles.
thickness of 1.5 mm. The result for compensated section thickness shows highly uniform section thickness across the forming depth for both the profiles (Fig. 22). Therefore, the compensation strategy of non-uniform machining is also valid for random profile formed through variable forming angles. 4. Summary and conclusions Experimental and finite element investigations on structural thinning in DM stretching reveal considerable thinning in the formed section along with highly non uniform thickness profiles across the forming depth. A reasoning to non-uniform thickness profile in incremental forming has also been proposed and a con-
formed through experimental and FE analysis on thinning evolution of the same profile at varied forming depths. Finally, a compensation strategy for uniform thinning in DM stretching for fixed and variable angle profiles has been proposed and realized in this work. Desired formed thickness along with considerable uniform profile across the forming depth was achieved by employing a varying thin section machining compensation strategy, prior to incremental forming. This would probably enhance the formability limits (especially at steeper forming angles) and strength of the formed monolithic components.
A. Singh, A. Agrawal / Journal of Manufacturing Processes 26 (2017) 216–225
225
References
Fig. 21. Non-compensated section thickness along the forming depth.
Fig. 22. Compensated section thickness along the forming depth.
Acknowledgement The authors acknowledge DST project SB/FTP/ETA-254/2012 funded by DST-SERB, India for providing the financial support and IIT Ropar, India for providing the basic facilities.
[1] Smith S, Woody B, Ziegert J, Huang Y. Deformation machining − A new hybrid process. CIRP Ann. Manuf. Technol 2007;56(1):281–4. [2] Tlusty J, Smith S, Winfough W. Techniques for the use of long slender end mills in high-speed milling. Ann. CIRP 1996;45(1):393–6. [3] Smith S, Dvorak D. Tool path strategies for high speed milling aluminium work pieces with thin webs. Mechatroni. J 1998;8(3):291–300. [4] Smith S, Wilhelm R, Dutterer B, Cherukuri H, Goel G. Sacrificial structure preforms for thin part machining. CIRP Ann. Manu. Technol 2012;61(1):379–82. [5] Jeswiet J, Micari F, Hirt G, Bramley A, Duflou J, Allwood J. Asymmetric single point incremental forming of sheet metal. Ann. CIRP 2005;54(2):623–50. [6] Duflou JR, Tunckol Y, Szekeres A, Vanherck P. Experimental study on force measurements for single point incremental forming. J Mater Process Technol 2007;189:65–72. [7] Echrif S, Hrairi M. Research and progress in incremental sheet forming processes. Mater Manuf Processes 2011;26(11):1404–14. [8] Singh A, Agrawal A. Investigation of surface residual stress distribution in deformation machining process for aluminum alloy. J Mater Process Technol 2015;225:195–202. [9] Singh A, Agrawal A. Comparison of deforming forces, residual stresses and geometrical accuracy of deformation machining with conventional bending and forming. J Mater Process Technol 2016;234:259–71. [10] Singh A, Agrawal A. Experimental investigation on elastic spring back in deformation machining Bending mode. Proceedings of the 10th ASMEManufacturing Science and Engineering Conference 2015. [11] Singh A, Agrawal A. Investigations on structural thinning and compensation stratagem in deformation machining stretching mode. Manuf. Lett 2016. Accepted for publication. [12] Ambrogio G, De Napoli L, Filice L, Gagliardi F, Muzzupappa M. Application of incremental forming process for high customized medical product manufacturing. J. Mater. Process. Technol 2005;162:156–62. [13] Allwood J, Shouler D, Tekkaya A. The increased forming limits of incremental sheet forming processes. Key Eng Mater 2007;344:621–8. [14] Filice L, Fratini L, Micari F. Analysis of material formability in incremental forming. CIRP Ann. Manuf. Technol 2002;51(1):199–202. [15] Jackson K, Allwood J. The mechanics of incremental sheet forming. J Mater Process Technol 2009;209(3):1158–74. [16] Young D, Jeswiet J. Wall thickness variations in single-point incremental forming. J. Eng. Manuf. Part B 2004;218:1453–9.