Reliability Analysis of Process-Induced Cracks in Rotary

Home , Cold working

Journal of Mechanical Science and Technology 26 (7) (2012) 2155~2158 www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-0535-z

Reliability analysis of process-induced cracks in rotary swaged shell nose part† Jeong-hwan Jang1, Won-hee Kwon2, Se-hwan Chun3 and Young-hoon Moon1,* 1School of Mechanical Engineering/Engineering Research Center for Net Shape and Die Manufacturing, Pusan National University, Busan, 609-735, Korea 2Poongsan corporation/Angang Plant, Kyungju, Gyeongbuk 780-805, Korea 3Trinity Engineering Co., Ltd., Deagu 704-946, Korea

(Manuscript Received February 22, 2012; Revised March 16, 2012; Accepted April 10, 2012)

------Abstract

The rotary swaging process is a cold working process used to reduce the diameter, produce a taper or add point to a round workpiece. For the preform design of a swaged shell nose part by a rotary swaging process, finite element simulation and experimental verification have been carried out to obtain a shell body nose of desired quality. Reliability analysis for the occurrence of process-induced cracks is performed by fault tree analysis. The various process parameters such as initial nose thickness, feed distance and reduction diameter are applied for the fault tree in order to obtain the desired target dimensions. Through simulation, the effects of initial nose thickness on the swaged shapes are analyzed. With fault tree analysis, the risk of process-induced cracks was predicted by finite element simulation and the crack occurrence was verified by swaging experiment. The results show that a swaged shell nose part having higher reliability can be successfully produced by rotary swaging process.

Keywords: Rotary swaging process; Reliability analysis; Process-induced crack; Fault tree analysis; Finite element simulation ------

performed at each stage of process-induced, is intended to 1. Introduction identify all possible hazards with their relevant causes [8-10]. The rotary swaging process is a metal forming process for We performed a reliability analysis of the swaged shell nose the reduction of cross-sections of bars, tubes and wires. It can part by FTA. The reliability analysis results were verified by also impart internal shapes in hollow workpieces through the occurrence of process-induced cracks in rotary swaging. The use of a mandrel. Sets of two, four, or in special cases up to results of the analysis provide manufacturing conditions that eight dies perform small, high frequency, simultaneous radial can produce shell nose part without unacceptable cracks. movements [1]. In particular, a manufacturing technique has been developed to produce high performance components and 2. Preform design using fault tree analysis minimized machine work economically in the metal working 2.1 Background of the shell body nose shape industry because of its lower manufacturing cost compared to conventional metal forming processes [2]. Rotary swaging has The inner diameter of shell body nose area is reprocessed advantages such as small loading and high accuracy of the for the screw machining after plastic deformation. material. To design the rotary swaging process it is necessary Therefore, the inner diameter and the length of the nose area to understand the relations between the parameters of the should be specified. Fig. 1 shows the desired shape of the shell technological process, the microstructure and properties [3]. body. ‘L’ is the length of the nose area and ‘D’ is the inner The finite element method can provide more detailed informa- diameter. Table 1 shows the target dimensions of the nose area. tion in the analysis of the rotary swaging process [4]. Several tools are used to determine the probability of pre- 2.2 Fault tree analysis (FTA) form design failure and the probability that a preform design will operate successfully. Fault tree analysis (FTA) is a well- Fig. 2 shows the preform design for shell nose swaging. established tool for evaluating safety and reliability in process- The swaged nose thickness depends on the initial nose thick- induced, development and operation [5-7]. Safety analysis, ness. For the desired target dimensions, various initial nose *Corresponding author. Tel.: +82 51 510 2472, Fax.: +82 51 512 1722 thicknesses are applied for finite element analysis. In this E-mail address: [email protected] study, the preform design is performed by FEM, and the fea- † This paper was presented at the ICMR2011, Busan, Korea, November 2011. Recommended by Guest Editor Dong-Ho Bae sible process parameters are obtained. © KSME & Springer 2012

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Table 1. Target dimensions of the nose area. Table 2. Chemical compositions of AISI9260 (wt.%).

Parameter Inner diameter Length Fe C Si Mn P S Target dimension ≤ 12.13 mm ≥ 11.87 mm Bal. 0.600 1.860 0.950 0.008 0.004

L Table 3. Mechanical properties of AISI 9260.

D Tensile strength Yield strength Elastic modulus Elongation 770 MPa 440 MPa 200 GPa 20%

Fig. 1. The shape of shell body and nose area notation.

Shell nose area Swaging die

FEM Workpiece Application of various nose thicknesses

Axial feed direction Satisfaction of desired No dimensions Fig. 3. Proposed FE model.

Yes

Analysis of nose area with various diameter reduction

Occurrence of process- Yes induced cracks (a) (b)

No Fig. 4. The rotary swaging machine with four-die type: (a) four-split Reliability dies; (b) rotary swaging machine. Optimized preform dimension Preform design failure Fig. 2. Preform design for shell nose swaging.

Final shape Process-induced 2.3 Finite element simulation and experiment failure crakcs

A commercial finite element code, FORGETM, is used to analyze the deformation characteristics of the nose area. Fig. 3 Specimen Operating Nose area failure failure analysis shows the proposed FE model for rotary swaging analysis. The material selected for FE analysis is AISI 9260. Tables 2 Fig. 7 and 3 show the chemical compositions and mechanical prop- Initial Feed Inner Nose nose erties of AISI 9260, respectively. distance diameter length thickness The rotary swaging machine with four-dies type used for the experiment is shown in Fig. 4. Fig. 5. Fault tree of the preform design for process reliability. The swaging process actually allows the manufacturer to vary the wall thickness of the tube material to match certain requirements for weight distribution and strength. The head forming feed of 430 mm/min and spindle revolution of 200 rpm. assembly of the rotary swaging machine with outer rotor and the designed four-split dies is shown in Fig. 4(a). The four-die 3. Results and discussion rotary swaging machine is composed of spindles, dies, rollers 3.1 Effects of initial nose thickness and hammers. The rollers located between the outer spindle and inner spindle transfer forming strokes that occur by the Reliability analysis for the occurrence of process-induced rotation of the spindle to hammers. Thus, the hammer and die cracks is performed by FTA. The various process parameters are forced to move radically corresponding to the deviation of are applied for the fault tree in order to obtain the desired tar- the hammer profile from a circular arc. The rotary swaging get dimensions of nose without cracks. The first stage of pre- deformation can be achieved under the following conditions: processing is the various initial nose thicknesses. Fig. 5 shows

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Table 4. Reliability analysis for the various nose thicknesses. Nose area Initial nose Feed Nose Inner analysis thickness distance length diameter Failure (mm) (mm) (mm) (mm) 150 10.868 12.556 Risk of Experimental process-induced 2.61 157 10.918 12.152 Failure verification crack 164 11.583 12.118 150 11.558 12.596 Failure 2.67 157 11.992 12.065 - 164 12.423 12.014 Maximum Diameter strain reduction 150 11.524 12.305 Failure 2.81 157 12.149 11.797 - Fig. 7. Fault tree for the nose area analysis. 164 12.547 11.592 150 11.696 12.078 Failure Effective strain 2.95 157 12.612 11.144 1.0 - Max. 0.904 0.9 164 12.706 11.056 0.8 0.7 0.6 (a) Effective strain 0.5 1.0 Max. 1.013 0.4 Max. 0.901 Max. 0.904 0.9 0.3

0.2 0.8 0.1 (b) 0.7 0.0

0.6 Fig. 8. Maximum effective strain with 2.67 mm initial nose thickness (a) (b) 0.5 for diameter reduction: (a) 63.8%; (b) 64.0%. 0.4 Max. 1.007 Max. 1.009 0.3

0.2 the reduction diameter is directly affected by the initial nose thickness. On the other hand, the process-induced cracks due 0.1 to the concentrated compressive strain at inner area occur at 0.0 (c) (d) the case of thicker initial nose thickness. The changes of nose area for the reduction of diameter are ana- Fig. 6. Effective strain at various initial nose thicknesses: (a) 2.61 mm; (b) 2.67 mm; (c) 2.81 mm; (d) 2.95 mm. lyzed. The reduction of diameter can be obtained from Eq. (1).

Diameter reduction Φ=red(%) × 100. (1) the fault tree of the preform design for process reliability. Initial diameter Table 4 shows the reliability analysis for the various nose thicknesses. The inner diameter is directly affected by the Fig. 8 shows the variation of effective strain with respect to the initial nose thickness. Successful inner diameter of the nose diameter reduction. The maximum compressive strain at inner area occurs if the initial nose thickness is sufficiently de- side is larger than that of the outer side because the cross-sectional formed for the feed distance. At the feed distance of 150 mm, area increased at the nose area with deformation. The maximum the desired inner diameter was not obtained at all initial nose strain of the 64% diameter reduction is higher than that of 63.8%. thicknesses. In case of 2.61 mm initial nose thickness, the inner Maximum strain occurs along the thicker nose area as the forming diameter and the nose length were not satisfied. To manufac- feed increased with a maximum value of over 1. It is considered ture the initial nose thicknesses with satisfied desired target that the process-induced crack is likely to occur at inner area dimensions, the swaged nose area was analyzed in more detail. when the localized strain hardening reduces ductility. Fig. 6 shows the effective strain for various initial nose thicknesses To verify the results from reliability analysis, swaging ex- at the feed distance of 157 mm. As shown in the figure, the maxi- periments were performed. Fig. 9 shows the cross-sectional mum effective strain increases with increasing initial nose thickness. swaged specimen with 2.67 mm initial nose thickness at diameter reduction of 64.0%. The crack occurs in ‘A’ area when 3.2 Analysis of the nose area the localized strain hardening reduces ductility at the inner area. As shown in Fig. 8(b), the maximum strain at ‘A’ area is Fig. 7 shows the fault tree for the nose area analysis. The 1.013. As a result, the risk of process-induced cracks is sig- thicker initial nose thickness produced lower inner diameter nificantly increased when the diameter reduction exceeds 64% than that of the thinner initial nose thickness. In other words, and the resultant maximum strain reaches 1 or more.

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Table 5. Results of nose area analysis. tion, Science and Technology (2011-0006-257).

Initial nose Feed Diameter Maximum thickness distance reduction Failure References strain (mm) (mm) (%) [1] V. Piwek, B. Kuhfuss, E. Moumi and M. Hork, Light weight 157 63.858 0.904 - 2.67 design of rotary swaged components and optimization of the 164 64.013 1.003 swaging process, International Journal of Material Forming, 157 64.663 1.007 2.81 Risk of 3 (2010) 845-848. 164 65.276 1.013 process-induced [2] S. Yuan, X. Wang, G. Liu and D. Chou, The precision form- 157 66.618 1.009 crack ing of pin parts by cold-drawing and rotary-forging, Journal 2.95 164 66.881 1.017 of Materials Processing Technology, 86 (1999) 252-256. [3] S. J. Lim, H. J. Choi and C. H. Lee, Forming characteristics of tubular product through the rotary swaging process, Jour- nal of Materials Processing Technology, 209 (2009) 283-288. [4] X. Han and L. Hua, Comparison between cold rotary forging and conventional forging, Journal of Mechanical Science A and Technology, 23 (2009) 2668-2678. [5] S. W. Lee and H. K. Lee, Reliability prediction system based

on the failure rate model for electronic components, Journal of Fig. 9. Cross-sectional views for the verification of crack occurrence in ‘A’ area. Mechanical Science and Technology, 22 (5) (2008) 957-964. [6] R. Ferdous, F. I. Khan, B. Veitch and P. R. Amyotte, Meth- odology for computer-aided fault tree analysis, Process Safety and Environmental Protection, 85 (1) (2007) 70-80. [7] Y. C. Lee, K. S. Kim, J. H. Ahn, J. W. Yoon, M. K. Min and S. B. Jung, Effect of multiple reflows on the mechanical reliability of solder joint in LED package, Korean Journal of Fig. 10. Swaged shell part without process-induced cracks. Metals and Materials, 48 (11) (2010) 1035-1040.

[8] M. Bayraktar, N. Tahrali and R. Guclu, Reliability and fa- Table 5 shows the results of nose area analysis. At initial tigue life evaluation of railway axles, Journal of Mechanical nose thickness over 2.67 mm, the diameter reduction exceeds Science and Technology, 24 (3) (2010) 671-679. 64% and the maximum strain is higher than 1. Therefore, the [9] N. E. Kang, C. D. Yim, B. S. You and I. M. Park, Fracture behav- risk of process-induced cracks can be predicted by FTA. ior of AZ31-xCa (x=0, 0.7, 2.0 wt.%) extrudes during compres- Fig. 10 shows swaged shell nose part manufactured under sion, Korean Journal of Metals and Materials, 48 (1) (2010) 85-89. the optimized processing conditions. The results show that the [10] S. Y. Kim, S. S. Kim and H. K. Choi, Remaining life estima- swaged shell nose part without process-induced cracks can be tion of a level luffing crane component by computer simulation, successfully produced by the rotary swaging process. Korean Journal of Metals and Materials, 48 (6) (2010) 489-498.

4. Conclusions Jeong-Hwan Jang received his M.Sc. (1) With fault tree analysis, the risk of process-induced (2007) in Mechanical Engineering at cracks was predicted by finite element simulation and occur- Pusan National University, Busan, Ko- rence of cracks was verified by rotary swaging experiment. rea. Since 2007, he has been pursuing (2) The proposed preform satisfies the target dimensions of Ph.D studies at the same university. His swaged shell nose. The swaged shape of the nose area was signifi- research interests are related to direct cantly affected by the initial nose thickness and diameter reduction. laser melting process of metal powder (3) The maximum effective strain increases with increasing and material processing technology. initial nose thickness. The process-induced cracks are likely to occur at the inner side of the nose area when the localized Young-Hoon Moon received his Ph.D strain hardening reduces ductility. in Metallurgical and Materials Engineer- ing from Colorado School of Mines,

Golden, USA. He is a professor at the Acknowledgment School of Mechanical Engineering, This research was partially supported by NCRC (National Pusan National University, Busan, Korea. Core Research Center) program through the National Re- His research includes materials process- search Foundation of Korea funded by the Ministry of Educa- ing technology and reliability analysis.