Experimental Studies in Multi Stage Incremental Forming of Steel Sheets

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ScienceDirect Materials Today: Proceedings 4 (2017) 4116–4122

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5th International Conference of Materials Processing and Characterization (ICMPC 2016)

Experimental Studies in Multi Stage Incremental Forming of Steel Sheets KurraSuresha*, NasihHRa*, JastiNVKb, MaheshwarDwivedyc a

BITS-Pilani Hyderabad Campus Hyderabad, Telangana, India b BITS-Pilani Pilani Campus, Rajasthan, India c BML Munjal University, New Delhi, India

Abstract Incremental Sheet Forming (ISF) is a relatively new sheet metal forming process. It hasshown great diversity in its applications ranging from automotive to biomedical fieldsHowever, the geometries having steep walls cannot bemanufactured in single stage incremental forming process due to the fact that the maximum wall angle that can be formed is limitedfor a given sheet material and thickness. This limitation can be overcome with Multi Stage Incremental Forming (MSIF) process. This paper presents some of the experimental studies in multi stage incremental forming of steel sheets to get steep wall angles.The effect of process parameters in forming cylindrical, square and spherical cups in MSIF process has been discussed. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016). Keywords:Incremental Forming; CNC Milling; Tool Path, MSIF

1. Introduction Many new processes have been developed in the last decade to meet the demands from manufacturing sector. Among them, incremental forming process seems to be promising technology for sheet metal prototyping and low volume production of complex parts.The major advantage with incremental forming is its die less nature and simple tooling. The formability of material in ISF is also far better than the conventional deep drawing and stamping processes * Corresponding author. Tel.;040-66303570 E-mail address:[email protected] 2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).

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The process can be used for aluminum, steel, magnesium and titanium alloys.Limited geometric accuracy, low surface quality, difficulties in forming steep wall angles and long processing times are some of the limitations of the process. However, the researchers are providing different solutions to overcome these limitations. The geometric accuracy of the formed part can be improved by modifying the tool path, adopting two point incremental forming process or by providing the partial support below the blank during the forming process [1]. Kurra et al. [2,3] studied the the effect of process parameters on surface quality and has been modeled using response surface methodology and different soft computing techniques. The developed mathematical models have been used further for optimization to get optimum process settings to enhance surface quality and to minimize the processing time. Maximum formable wall angle is the primary parameter to measure the formability of material in ISF process. Suresh et al. [4,5] formed varying wall angle conical and pyramidal frustums with different generatrices to measeure the limiting wall angle for Extra Deep Drawing (EDD) steel. The maximum wall angle that can be formed without fracture with conical and pyramidal frustums was found to be 75±20 and 73±20. Further, it was observed that the material in varying wall angle conical frustums and faces of the pyramid is under plane strain conditionswhile the material at corners of the pyramidal frustums is towards equi bi-axial stretching [5]. The process parameters are having significant effect on formabilty. The formability of the material is decreasing with increasing in tool diameter and step depth [6]. Many studies have carried out to simulate the ISF process to understand the process mechanics without requiring time consuming experimental trials [7,8]. The problem of long computational time due to long tool paths associated with ISF process can be overcome with time scaling, mass scaling and adaptive re-meshing techniques without sacrificing the accuracy of numerical simulation results [9,10]. The first attempts, to utilize multi-stage SPIF were made by Kitazawa et al. [11, 12] whoproduced hemiellipsoidal axis-symmetric parts by employing two sequential stages. The first stage was utilized to shape an intermediate conical geometry that was subsequentlyformed into the desired hemi-ellipsoidal shape. Later, Kim et al. [13] and Young etal. [14] developed a two-stage strategy to improve the final thickness distribution for thegeometries with steep areas. Their results show that the occurrence of a thinning bandin the single-stage forming process can be delayed in the two-stage process. Thereforeit can be used to produce geometries with steep walls. Duflou et al. [15] explored amulti-step tool path strategy to manufacture parts with vertical walls in order to avoidpart failure. They observed that the final thinning in multi-stage forming process canexceed the maximum thickness reductions in the single-stage process. By this they inferredthat there is a formability shift. Manco et al. [16] studied the effect of tool trajectory onthe final thickness distribution and formability. Four different multi-stage strategieshave been implemented to manufacture the same shape and evaluate thickness distributions.Bambach et al. [17] improved a pyramid's geometric accuracy by changing the size ofround corners gradually. Hirt et al. [18] made a pyramid with a wall angle (α) of 810basedon a preformed shape (α = 450) with an increase in angle of 30 or 50 at each stage, whichachieved a more homogeneous thickness distribution. Skjoedt et al. [19] investigated amulti-stage strategy to produce cylindrical cups with vertical walls. They pointed out thatthe movement of the forming tool in multi-stage SPIF has a great influence on thicknessdistribution. Table 1 provides an overview on the research studies related to MSIF. 2. MSIF Strategies There were different strategies used by researchers for the multi-stage forming of sheets. Some of these are listed below: The strategies 1,2, &3 were used by Bambach et al. [17]. 1.“small corner radius" (1): The authors created a pre-form in which the corner radius was smaller than that of the final shape. The corner radius was increased in each step from the pre-form to the final shape. 2. “large corner radius" (2): In this strategy, the pre-form corner radius is larger than that of the final shape. On its bottom, the pre-form assumes a circular shape. 3.“in-plane radius" (3): Authors started with a conical pre-form to form a pyramid as the final shape. This strategy leads to excessive thinning in the corners of the final pyramidal part. To alleviate this problem, a blending

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between the pre-form and the final part was realized on each z-level, i.e. instead of forming all stages from the preform through the intermediate stages to the final part sequentially, blending between the stages was performed for each z-level of the tool path. 4. "down, down, down, and down" (DDDD): This strategy is used by most of the authors. The strategy consists of four downward movement of the tool. 5. “down, down, down, and up" (DDDU): This strategy was proposed by Silva et al.[20], who named this strategy DDDU. The strategy consists of four stages, in which there are three downward movements of the tool followed by an upward movement in the fourth and final stage.

Table 1 Literature review The next three strategies were used by Liu et al. [21]. 6.“incremental diameter" (A): In this strategy the part diameter is increased from D1 to Dn gradually. The reason of using this strategy is that material in the central area is expected to create more bending deformation rather than stretching deformation. 7. “incremental part angle" (B): The draw angle of the part is increased gradually to reach the final wall angle 900. The aim of using this strategy is to regulate as much material as possible to involve deformation. 8. “incremental part height and draw angle" (C): The formed part height and draw angle are increased simultaneously in steps to reach the final shape. In addition to these strategies, Liu et al. also used a combination of different strategies viz. A+B and A+C A relationship betweenthe thickness thinning Ri(i = 1,2, ………, n) of each forming stage and the total thicknessthinning R0 has been proposed by J Li [22] as follows. This relation can be used for calculating number of stages. (1) ln(1- R1) + ln(1- R2) + _ _ _ + ln(1 - Rn) = ln(1 - R0) Assuming R1 = R2 = _ _ _ = Rn = R0, the total number of forming stages is found to be: n =ln(1 - R0) / ln(1 - R0)

(2)

3. Experimental Setup The experiments were carried out on a 3-axis CNC milling machine from HardingeBridgeport. This machine uses Fanuc series Oi-MD controller. The machine has a spindlespeed range of 1-8000 rpm and maximum feed rate of 12000 mm/min. In the present work, the feed rate was fixed at 1500 mm/min and the spindle speed at 300 rpm. EDD qualitysteel sheets of 250 mm X 250 mm were used as the blank material. These were either1mm or 1.5mm thick. EDD has good formability and dent resistance and widely used inautomotive industry. A benchmark shape having initial conical frustum geometry was used. The final shape to be obtained is a cylindrical cup. Another benchmark shape is the squarecup. This shape has an initial geometry of a pyramidal frustum. The blanks were formedto

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required shape using hemispherical headed tools. These were either made of EN-36steel hardened to 60HRC by heat treatment, or of High Speed Steel. The tool diameters used were8mm, 10mm and 14mm. The 8mm had a taper shank with a taper of 70. The other tools have a straight shank. The forming tools were polished with fine grade abrasive paperand lapping paste to minimize the friction and ploughing action between the tool andblank interface. 4. Experiments This section describes the experiments done for the multi-stage incremental forming. Sincethere are only guidelines for the selection of the number of stages, the experiments wereperformed on a trial and error basis. The starting angle for the preform was chosen to be600. This was incremented in steps of 100upto 800. Then either 850 or 900 was formed inthe final step. Three different benchmark shapes were chosen for the experimental study.These were circular cups with a diameter of 110mm, square cups having sides 110mm anda dome shape having a diameter of 110mm. The dome shape was chosen because it has avery steep initial wall angle. This cannot be formed using a single pass forming. The initial shape for the circular cup was chosen to be a frustum of a cone with a wallangle of 600. For the square cups, pyramidal frustums were the initial shapes. The startingangle in this case was also chosen to be 600. In the case of the dome shape, conicalfrustum was formed having initial angle 600. As the angle was increased, the depth of the frustum was reduced to fit inside the hemisphere. Initial experiments for the circular andsquare cups were performed upto the depth of 40 mm. Later this was reduced to 30mm.The toolpaths were generated using either Pro-E manufacturing or Master CAM. All theexperiments with their details are given in Table 2 Ex. No. 1

Tool Diameter (mm)

Sheet Thickness (mm)

Step Depth (mm)

Tool Path

Shape

8

1.5

1

z-level

Circular

2

8

1.5

1.5

Spiral

Circular

3

14

1.5

1.5

Spiral

Circular

4

14

1.5

1

Spiral

Circular

5

14

1.5

0.5

Spiral

Circular

6

10

1.5

1

Spiral

Circular

7

10

1.5

1

Spiral

Circular

8

10

1.5

0.5

Spiral

Circular

9

10

1

0.5

Spiral

Circular

10

10

1

0.5

Spiral

Circular

11

10

1

0.5

Spiral

Circular

12

10

1

0.5

Spiral

Circular

13

10

1

0.5

Spiral

Circular

14

10

1

0.5

Z-level

Square

15

14

1

1

Z-level

Square

16

10

1

0.5

Spiral

Square

17

10

1

0.5

Spiral

Dome

18

10

1

0.5

Spiral

Dome

19

10

1

0.5

Spiral

Dome

.Table 2. Experimental Plan

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5. Results and Discussion In the first experiment , the inner side of the formed part shows transition marks which areclearly visible. These are caused by the tool rotating at a specific point for some timebefore moving to the next arc segment resulting in increased wear at these points. Cracksin this experiment originated at some of these transition points. The surface finish of theformed part is very less as can be seen from the picture. The constant z-level toolpath wastherefore considered to give unsatisfactory results for the multi-stage incremental forming. The 8mm diameter tool had a taper shank. Therefore it could not be used for angles higherthan 800. The 14mm diameter tool suffered from lots of vibrations and thus the surfaceof the part was very rough. This led to the formation of cracks both in the horizontaland the vertical direction. The fracture is the outcome of in-plane stretching caused bymeridional tensile stresses. Hence both these tools were not used for further experiments. The 10mm tool leads to the formation of vertical walls in four stages but upto a limiteddepth of 22.5mm. 850wall was formed successfully upto a depth of 30mm. Strategy A+Bwhich was reported to be successful by Liu et al. [21], failed in the experiments conducted.Both cylindrical and square cups were formed successfully upto a depth of 30mm using10mm diameter tool and 0.5mm step-depth. During the formation of square cups, cracks were developed at the corners. This wasattributed to the springback effect which is more pronounced at the corners. As two sidesintersect their springback gets coupled and instead of a corner, we get an arc of someradius. When the tool tries to form the corner in the subsequent stages, it has to deform larger amount of material. This ultimately results in fracture at the corners. Hence, aradius of 180% of the tool diameter was introduced in place of the corners. Ex. No. 2

Ex. No. 12

Ex. No. 5

Ex. No. 13

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Ex. No. 10

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Ex. No. 16

Ex. No. 19

Fig. 1 Parts formed in different experiments

For producing the dome shape, single stage strategies are insufficient because of the steepangle at the sheet surface to be formed. Two different strategies were attempted. In the first strategy, the path was created from the inside of the dome to the outside. In thesecond strategy, the tool-path originated from the outer contour and proceeded inwards. The first strategy fails since there is no support underneath the metal forming area. Thisleads to the bending of the sheet from areas lying in between the backing plate openingand the contact point of the tool with the sheet. Hence instead of the part of the sheetwhere contact with the tool takes place going down, the entire sheet bends. Therefore,when the tool reaches further outside, it has to deform the material to a large depth,resulting in failure. The second strategy fails, because, the initial angle is very large. Such large angles cannotbe formed using single stage as predicted by the sine law. Hence multi-stagestrategy was adopted.Some of the parts formed in MSIF is shown in Fig. 1. All the formed part have extrusions which were not part of the original design. Thesearise as there is no support underneath the metal forming area. When the forming tooltravels in each stage, it will squeeze the material downwards [21]. This results in theelongation of the material. Hence the extrusions form. To overcome these, the parts haveto be designed with an increasing depth, with the part reaching the desired depth in the final stage. 6. Conclusions Multi-stage SPIF allows rapid prototyping of complex geometries which may have verticalwalls. Fracture happens during ISF process when excessive thinning occurs in the criticalareas of a part. In this study, by revisiting multi-stage methodology, cups with verticalwall surfaces have been formed with proper strategies and strategic combinations by material ow control. The findings of experiments are summarized as follows: 1. For the investigated cylindrical cup, in the present study, it could not be formedusing strategy A+B, although this strategy has been adopted to form a cylindricalcup successfully in article [21]. In this study, the strategy DDDD

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provided successfulresults for the formation of the vertical walls. The reason may be because of thediameter of the tool which was one-third of that used by Liu et al. This may alsobe caused because of the difference in the materials. 2. For the formation of square cups, corners had to be avoided. Fracture occurred atthe corners due to the springback effect, which makes it necessary for the tool todeform larger amount of material in the subsequent stages. A radius of 180% of thetool diameter when applied to the geometry gave successful results. 3. The dome shape cannot be formed using single-stage strategy. Both the outwardand the inward tool-paths failed to create the dome shape in a single stage. UsingMSIF it was possible to create the desired dome shape. However this shape sufferedfrom the extrusion defect. This can be corrected by changing the design of thestages. The depth of the first stage has to be small, and the depth must increment with each stage. The final stage must be given the full depth. Additionally, the results in this study can also provide insights into forming complexgeometries with steep wall surfaces by controlling material flow to avoid excessive thinning. References [1] Jeswiet J, Micari F, Hirt G, Bramley A, Duflou J, Allwood J. CIRP Annals-Manufacturing Technology. 54(2) 200588-114. [2] Kurra S, Nasih HR, Regalla S, Gupta AK. Proceedings of the IMechE, Part B: Journal of Engineering Manufacture. 22(2015) 0954405414564408. [3] Kurra S, Rahman NH, Regalla SP, Gupta AK. Journal of Materials Research and Technology.4(3) (2015) 304-13. [4] Kurra S, Regalla SP. Journal of Materials Research and Technology. 3(2) (2014) 158-71. [5] Suresh K, Bagade SD, Regalla SP. Materials and Manufacturing Processes. 30(10) (2015)1202-9. [6] Kurra S, Regalla SP. International Journal of Materials Engineering Innovation. 6(1) (2015) 74-90. [7] Suresh K, Khan A, Regalla SP. Procedia Engineering. 64 (2013) 536-45. [8] Kurra S, Regalla SP, Pérez-Santiago R. Advances in Materials and Processing Technologies. 1(1-2) (2015) 201-9. [9] Suresh K, Regalla SP. Applied Mechanics and Materials 612 (2014) 105-110. [10] Suresh K, Regalla SP. Procedia Materials Science. 6 (2014) 376-82. [11] K. Kitazawa, A. Wakabayashi, K. Murata, and K. Yaejima, Japan Institute of Light Metals. 46(2) (1996) 65-70. [12] K. Kitazawa and M. NakaneJapan Institute of LightMetals. 47 (1997) 440-445. [13] Kim TJ, Yang DY. International Journal of Mechanical Sciences. 42(7) (2000) 1271-86. [14] Young D, Jeswiet J. Proceedings of IMechE Part B: Journal of Engineering Manufacture. 218(11) (2004) 1453-9. [15] J. Duou, J. Verbert, B. Belkassem, J. Gu, H. Sol, C. Henrard, and A. Habraken,CIRP Annals-Manufacturing Technology. 57(1) (2008) 253256 [16] Manco L, Filice L, Ambrogio G. Proceedings of the IMechE, Part B: Journal of Engineering Manufacture. 225(3) (2011) 348-56. [17] Bambach M, Araghi BT, Hirt G. Production Engineering. 3(2) (2009)145-56. [18] Hirt G, Junk S, Bambach M, Chouvalova I. InMaterials Science Forum 426 (2003) 3825-3830. [19] Skjødt M, Silva MB, Martins PA, Bay N. The Journal of Strain Analysis for Engineering Design.45(1) (2010) 33-44. [20]Silva MB, Skjoedt M, Bay N, Martins PA. CMNE 2011. [21]Liu Z, Li Y, Meehan PA. Materials and manufacturing processes.28(5) (2013) 562-71. [22]Li J, Hu J, Pan J, Geng P. The International Journal of Advanced Manufacturing Technology.62(9-12) (2012) 981-8. [23]Centeno G, Silva MB, Cristino VA, Vallellano C, Martins PA. International Journal of Machine Tools and Manufacture.31 (2012) 46-54. [24] Li J, Bai T, Zhou Z. The International Journal of Advanced Manufacturing Technology.74(9-12) (2014) 1649-54.