Design of Process Parameters in Wiping Z-bending Process using Statistical Analysis Technique

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 183 (2017) 5 – 10

17th International Conference on Sheet Metal, SHEMET17

Design of process parameters in wiping Z-bending process using statistical analysis technique Sutasn Thipprakmasa, *, Pakkawat Komolrujia a

Department of Tool and Materials Engineering, King Mongkut’s University of Technology Thonburi, 126 Bangmod, Thungkhru, Bangkok, 10140, Thailand

Abstract With the more severe requirement for the wiping Z-bent parts, in recent years, the high dimension precision is required. The major forming problem of spring-back is the main barrier faced in product quality upgrading in the precision bent parts. With the various process parameters, including bend angle, material thickness, tool radius, web height and workpiece length, these resulted in the processing difficulty in the control of spring-back feature. However, these process parameter designs for controlling the spring-back characteristic has not been researched yet. In this study, therefore, the effects of these process parameters were investigated by using the finite element method (FEM) simulation. In addition, the process parameter design was also examined by using the combination of the FEM simulation, and statistical analysis technique to determine the degree of importance of each process parameter. The experiments were carried out to validate the FEM simulation results. The results elucidated that the material thickness, bend angle and tool radius have a major influence on spring-back characteristic, respectively. ©©2017 Authors. Published by Elsevier Ltd. This 2017The The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SHEMET17. Peer-review under responsibility of the organizing committee of SHEMET17

Keywords: Z-bending; Spring-back; Analysis of variance (ANOVA); Finite element method

1. Introduction In many industrial fields such as an automotive industry and a housing-utensil industry, by using a die, a bending process is widely employed to form curved shapes in sheet-metal parts. In recent years, the more severe requirements of high dimension precision for the industrial sheet-metal parts are required. The spring-back is the principal forming problem faced in product quality upgrading in the precision bent parts. In the past, most researches

* Corresponding author. Tel.: +662-4709218; fax: +662-4709218. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SHEMET17

doi:10.1016/j.proeng.2017.04.003

6

Sutasn Thipprakmas and Pakkawat Komolruji / Procedia Engineering 183 (2017) 5 – 10

of bending process were carried out to develop with the different ways for achieving the precision bent parts via the finite element method (FEM) and experimental analysis [1-5]. However, almost all of the past researches were performed the L-, V-, and U-bending processes [1-5]. Only a few researches were done on the Z-bending process [6, 7]. Therefore, it resulted in a lack of some understanding for the improving this Z-bending process to achieve the precise bent parts. In the present research, to have much more understanding of Z-bending process, a wiping Zbending process being one type of Z-bending process was investigated. The FEM was used as a tool for investigating the effects of process parameters on spring-back characteristics and obtaining the spring-back characteristics as well as the laboratory experiments were performed to validate the FEM simulation results. Next, by using the combination of the FEM simulation and statistical analysis technique, the process parameter was also examined to determine the degree of importance of each process parameter. As the results, the statistical analysis results are able to specify the process parameters that markedly influence the spring-back characteristic and yield information about the degree of importance of each process parameter on the wiping Z-bending process. The results elucidated that the material thickness, bend angle and tool radius have a major influence on spring-back characteristic, respectively. 2. FEM-simulation and experimental procedures In the present research, as shown in Fig. 1, the investigated model of wiping Z-bending process was illustrated and the measured bend angles in the Z-shape parts was also depicted. Next, Table 1 lists the details of this model and the process parameter conditions. A two-dimensional plane strain was applied. The two-dimensional, implicit, quasi-static finite element method of a commercial analytical code, DEFORM-2D, was used for the FEM simulation. The punch, die, and blank holder were set as rigid types and the workpiece material was set as an elastoplastic type. The rectangular elements approximately 4,000 elements were generated on workpiece material. The aluminum A1100-O (JIS) was used as workpiece material in the present research and its properties were taken from tensile test data [3, 7]. To examine the degree of importance of process parameters in relation to the spring-back characteristic, as listed in Table 1, the three levels of the five parameters, including bend angle, material thickness, tool radius, web height and workpiece length were applied. In the present research, owing to the lower bend radius was set by the tool radius (Rp) and material thickness (t), the investigation of it was neglected. On the basis of the central composite design technique, the experimental design was performed and the 43 experiments, in total, were carried out as listed in Table 2. The Analysis of variance (ANOVA) technique was also applied to illustrate the degree of importance of each parameter that markedly influenced the spring-back characteristic, as depicted in the equation (1)

% Contributions = [SeqSStreatment / SStotal] x 100

…… (1)

Where %Contributions, SStreatment and SStotal represent the degree of importance of each parameter, the treatment sum of squares and the total sum of squares, respectively.

Punch Rp

Blank holder

workpiece l Hd

Rld

(θu)

Rud Die

Lower bend angle

(θl)

Fig. 1. FEM simulation model and measured bend angles.

Upper bend angle

7

Sutasn Thipprakmas and Pakkawat Komolruji / Procedia Engineering 183 (2017) 5 – 10 Table 1. FEM simulation and experimental conditions. Simulation model

Workpiece material

Plane strain model Workpiece : Elasto-plastic Punch/Die : Rigid Blank holder : Rigid A1100-O

Flow curve equation

V

Friction coefficient (μ)

0.1

Workpiece length (l)

100, 110, 120 mm

Web height ( Hd)

40, 45, 50 mm

Bend angle ( θu, θl)

90q, 105q, 120q

Tool radius (Rp, Rud)

15, 16, 17 mm

Material thickness (t)

1, 1.5, 2 mm

Lower bend radius (Rld)

Tool radius (Rp) + Material thickness (t)

Object types

153.5H 0.20  88

Punch

Workpiece

Die

Fig. 2. The punch and die components for experiments.

The laboratory wiping Z-bending experiments were done to validate the FEM simulation results. As per the experiments from past research [7], Fig. 2 shows the wiping Z-die used for the experiments. The 5-ton universal tensile testing machine (Lloyd Instruments Ltd.) was used as the press machine. Five samples from each bending condition were used to inspect the obtained bend angles. After unloading a profile projector (Mitutoyo model PJA3000) was used for the bend angle measurement. The observed bend angle and bend force were recorded and compared with those analyzed by the FEM simulation. 3. Results and discussion 3.1. The use of FEM simulation and its validation To reduce the number of experiments for examination of the degree of importance of process parameters, in the present research, the FEM simulation was used as a tool. Therefore, the validation of accuracy of the FEM simulation results is necessarily. The laboratory experiments were carried out. The analyzed bent angles and bend forces obtained by FEM simulation were compared with those obtained by the experiments. As shown in Fig. 3, as per the past research [8, 9], the FEM simulation results showed the spring-back characteristic which corresponded well with the experimental results. The compared error of the FEM simulation results with the experimental results was approximately 1%. Next, as per the past research [8] (Figure omitted), the analyzed bend force was also compared with that obtained by experiments. The FEM simulation results showed a good agreement with the experimental results, in which the error was approximately 1 %. 3.2. Stress distribution analyzed Fig. 4 shows the stress distribution analyze before unloading phase and the spring-back characteristic after unloading phase. As shown in Fig. 4(a), the stress distribution analyze illustrated the stresses generated on the upper and lower bending allowance zones (zone A and zone B), the web zone (zone C), and the leg zone (zone D). These manners of the stress distribution analysis corresponded well with the bending theory and the literature [8]. In addition, it was also observed that the generated stresses on web and leg zones was reversed comparing with that generated stress on the lower bending zone (zone B). Therefore, after unloading and compensating these stress distributions, the bend angles were examined and the results show in Fig. 4(b). The predicted upper bend angle (θu) and lower bend angle (θl) were of 90.31° and 88.83°, respectively. As aforementioned stress distribution analyze, it was also observed that, owing to the reversed bending stresses generated on web and leg zones comparing with that generated on lower bending zone (Zone B), the spring-back characteristic generated on lower bend angle was smaller than that generated on the upper bend angle. As these results, they showed that, owing to the different bending mechanism on upper and lower bend angle, the obtained upper and lower bend angles were different

8

Sutasn Thipprakmas and Pakkawat Komolruji / Procedia Engineering 183 (2017) 5 – 10

although the symmetrical Z-shape part was bent. Therefore, the more understanding of process parameter design is necessarily to control the spring-back characteristic on each bend angle.

Results

Bent parts

Bend angles θu

θl

Experiment

89.23°

88.36°

FEM

90.31°

88.83°

Fig. 3. Comparison of the bending angle between the FEM simulation and the experimental results. (Rp = 3 mm, Rud = 6 mm l = 60 mm, Hd = 20 mm, t = 3 mm)

(a)

(b) A C D

-150

-50

B

50

Mean stress (MPa) 150

θu = 90.31°, θl = 88.83°

Fig. 4. (a) Illustration of stress distribution analysis before unloading; (b) Predicted bend angles after unloading (Rp = 3 mm, Rud = 6 mm l = 60 mm, Hd = 20 mm, t = 3 mm)

3.3. Statistical analysis Table 2 shows the amounts of spring-back analyzed by FEM simulation. To investigate the degree of importance of process parameters on the spring-back characteristic, the ANOVA technique was carried out based on these amounts of spring-back prediction. In the present research, the amount of spring-back was calculated by subtracting the predicted bend angle from the required bend angle. The sums of squares due to the variations about the overall mean (SStotal) and about the mean of the process parameters (SStreatment) were calculated. On the basis of these results, to consider the degree of importance of each parameter, the percentage contributions of main effects and interactions as shown in equation (1) were calculated and its calculated values were listed in Fig. 5. The results showed that, the percentage contributions of material thickness was largest, followed by bend angle and tool radius, respectively. As these results, the statistical analysis indicated that the process parameters of material thickness had the most influence on the spring-back characteristic, followed by bend angle and tool radius, respectively.

9

Sutasn Thipprakmas and Pakkawat Komolruji / Procedia Engineering 183 (2017) 5 – 10 Table 2 The amounts of spring-back analyzed by FEM simulation. FEM No.

Bend angle (°) 90

Tool radius (mm) 15

2

90

15

3

90

4

90

5

Process parameters Material thickness (mm) 1

Spring-back (°) Web height (mm) 40

Workpiece length (mm) 100

θu

θl

7.93

5.82

1

40

120

8.13

5.27

15

1

50

100

8.77

4.67

15

1

50

120

9.17

4.17

90

15

2

40

100

2.88

2.15

6

90

15

2

40

120

3.00

1.95

7

90

15

2

50

100

3.07

2.02

8

90

15

2

50

120

3.18

1.70

.....

….

….

….

….

….

….

….

1

36

105

15

1.5

45

110

4.03

2.62

37

105

17

1.5

45

110

4.29

2.79

38

105

16

1

45

110

6.35

5.08

39

105

16

2.0

45

110

2.80

2.21

40

105

16

1.5

40

110

4.75

2.83

41

105

16

1.5

50

110

4.32

2.59

42

105

16

1.5

45

100

4.31

3.59

43

105

16

1.5

45

120

4.40

3.03

(a)

(b)

2.26%

1.12%

0.92% 0.35%

0.12%

0.01%

t

θu

9.71%

1.45% 2.07%

1.34%

0.94%

0.58%

0.32% t

4.18%

θl

θu× t θu× θu

11.11%

Rp Hd

17.25%

Rud

θl× t

θu× l 74.40%

t×t

t× l 71.87%

l

l ×l

θl×t×l

Hd

θl×l

Fig. 5. The percentage contributions. (a) Upper bend angle case; (b) Lower bend angle case

4. Conclusions In the present research, in relation to the spring-back characteristic, the FEM simulation in association with the statistical analysis of the ANOVA technique was applied to examine the degree of importance of process parameters in wiping Z-bending process, including bend angle, material thickness, tool radius, web height and workpiece length. The ANOVA results illustrated the influence of each process parameter on the spring-back characteristic, together with their calculated percentage contributions. The material thickness had the most influence, followed by

10

Sutasn Thipprakmas and Pakkawat Komolruji / Procedia Engineering 183 (2017) 5 – 10

the bend angle and tool radius, respectively. Laboratory experiments were carried out. The FEM simulation results, as validated by laboratory experiments, showed a good agreement with the experimental results that the error in the bend angle compared with the laboratory experimental results was approximately 1%. The analyzed bend forces were also compared with that obtained by experiments and they showed a good agreement with the experimental result, in which the error was approximately 1 %. Therefore, this technique could be applied as a tool for quality control of the wiping Z-bent parts based on the spring-back characteristic by optimization of the process parameter design.

Acknowledgements The authors would like to express their gratitude to the Thailand Research Fund (TRF) (MSD56I0088) and the Diamond Dimension Co., Ltd., for their financial assistance to this study. The authors also thank the postdoctoral researcher, Wiriyakorn Phanitwong, Ph.D., and graduate students,Mr. Arkarapon Sontamino and Miss Untika Boochakul, for their help in this study. References [1] K. Dilip Kumar, K.K. Appukuttan, V.L. Neelakantha, P.S. Naik, Experimental determination of spring back and thinning effect of aluminum sheet metal during L-bending operation, J Mater Des. 56 (2014) 613–619. [2] D.K. Leu, Position deviation and springback in V-die bending process with asymmetric dies, Int J Adv Manuf Technol. 79(2015)1095–1108. [3] S. Thipprakmas, U. Boochakul, Comparison of spring-back characteristics in symmetrical and asymmetrical U-bending processes, Int J Precis Eng Manuf 16(7) (2015) 1441–1446. [4] Y.Y. Zong, P. Liu, B. Guo, D. Shan, Springback evaluation in hot V-bending of Ti-6Al-4Valloy sheets, Int J Adv Manuf Technol. 76 (2015) 577–585. [5] W. Phanitwong, S. Thipprakmas, Centered coined-bead technique for precise U-bent part fabrication, Int J Adv Manuf Technol. 84(9) (2016) 2139–2150. [6] B.T. Cheok, J.Y. Li, A.Y.C. Nee, Integrated feature-based modeling and process planning of bending operations in progressive die design, Int J Adv Manuf Technol. 20 (2002) 883–895. [7] S. Thipprakmas, P. Komolruji, Analysis of bending mechanism and spring-back characteristics in the offset Z-bending process, Int J Adv Manuf Technol. 85(9) (2016) 2589–2596. [8] W. Phanitwong, P. Komolruji, S. Thipprakmas, Finite Element Analysis of Spring-back Characteristics on Asymmetrical Z-shape Parts in Wiping Z-bending Process, The 6th International Conference on Simulation and Modeling Methodologies, Technologies and Applications (SIMULTECH 2016). [9] P. Komolruji and S. Thipprakmas, Finite Element Analysis of Bending Angle Precision in Z-bending Process, International Conference on Materials Processing Technology (Proc. of MAPT2013), (2013) 41-45.