A PROPOSED SOLUTION TO THE CRACK-LIKE FLAWS IN HOT FORGING PROCESS BASED ON NUMERICAL SIMULATIONS WITH LS-DYNA
A thesis submitted in fulfilment of the requirement for the degree of Master of Science (Mechanical Engineering)
Kulliyyah of Engineering
International Islamic University Malaysia
An important concern in metal forming process is whether the desired deformation can be accomplished without defects in the final product. These defects which may occur on the surface or within the product are due to the complex nature of the hot forging processes. The hot forging process is controlled by many parameters which include the temperature of the workpiece, the dies geometry, and the compression rate. As a result, experimental approach is ineffective in the investigation and elimination of the defects. Various theoretical fracture criteria have been developed and experimentally verified for a limited number of cases of forging processes. These criteria are highly dependent on the geometry of the workpiece and cannot be utilized for complicated shapes without prior experimental verification. However, experimental work is a resource hungry process. This study proposes the usage of the finite element analysis (FEA) software LS-DYNA to pinpoint the crack-like flaws in bulk metal forming products. Two different approaches named as the Arbitrary Lagrangian Eulerian (ALE) and smooth particle hydrodynamics (SPH) formulations were adopted. The results of the numerical simulations agree well with the experimental work as the final geometry and dimensions of the workpiece were accurately achieved. A comparison between the two formulations has been carried out to investigate the pros and cons of each method. Both formulations successfully predicted the flow of workpiece material and the plastic deformation during hot forging. However, only ALE method was able to approximate the location of the flaws. The numerical simulations reveal that the uneven thickness of the product disturbs the plastic flow of the material which increases the stress levels and results in the formation of the flaws. A parametric study was carried on to obtain the optimum wall thickness, compression rate, and process temperature. The application of these suggested values in the numerical simulations eliminated the occurrence of the flaws in the product.
Finite Element Method
I certify that I have supervised and read this study and that in my opinion, it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a thesis for the degree of Master of Science (Mechanical Engineering).
Qasim Hussain Shah Supervisor
I certify that I have read this study and that in my opinion, it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a thesis for the degree of Master of Science (Mechanical Engineering).
Meftah Hrairi Examiner (Internal) ...
Wahyu Kuntjoro Examiner (External) This thesis was submitted to the Department of Mechanical Engineering and is accepted as a fulfilment of the degree of Master of Science (Mechanical Engineering).
Head, Department of Mechanical Engineering This thesis was submitted to the Kulliyyah of Engineering and is accepted as a fulfilment of the degree of Master of Science (Mechanical Engineering).
Md. Noor bin Salleh Dean
Kulliyyah of Engineering
I hereby declare that this thesis is the result of my own investigation, except where otherwise stated. I also declare that it has not been previously or concurrently submitted as a whole for any other degrees at IIUM or other institutions.
INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA
DECLARATION OF COPYRIGHT AND AFFIRMATION OF FAIR USE OF UNPUBLISHED RESEARCH
A PROPOSED SOLUTION TO THE CRACK-LIKE FLAWS IN HOT FORGING PROCESS BASED ON NUMERICAL
SIMULATIONS WITH LS-DYNA
I declare that the copyright holder of this thesis/dissertation is jointly owned by the student and IIUM
Copyright © 2016 Ameen Topa and International Islamic University Malaysia. All rights reserved.
No part of this unpublished research may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder except as provided below.
1. Any material contained in or derived from this unpublished research may be used by others in their writing with due acknowledgement.
2. IIUM or its library will have the right to make and transmit copies (print or electronic) for institutional and academic purposes.
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By signing this form, I acknowledged that I have read and understand the IIUM Intellectual Property Right and Commercialization policy.
Affirmed by Ameen Topa
First and foremost, All praise, gratitude, and thank are to Allah, the Most Merciful and Most Compassionate for granting me a precious opportunity to do this work and the strength and spirit to complete it.
I would like to convey my sincere thanks and gratitude to my supervisor, Associate Professor Dr. Qasim Hussain Shah for bearing with me throughout my MSc journey, and for his scholarly guidance and remarkable assistance. Truthfully this work shall not be completed without his continuous support and encouragement.
I would like to extend my acknowledgement to the Research and Management Centre of the International Islamic University Malaysia for providing the research grant [EDW B12-407-0885] for the completion of this project.
Not to forget all of my colleagues in the Department of Mechanical Engineering as well as all other academic and non-academic staff for their assistant and help without any hesitation.
Last but not least, my deepest gratitude towards my mother, Ramlah and father, Abdul Latif, for their unconditional love and continuous prayers for my success in completing this work.
TABLE OF CONTENTS
Abstract ... ii
Abstract in Arabic ... iii
Approval Page ... iv
Declaration ... v
Copyright Page ... vi
Acknowledgements ... vii
List of Tables ... x
List of Figures ... xi
List of Abbreviations ... xiv
List of Symbols ... xv
CHAPTER 1: INTRODUCTION ... 1
1.1 Background ... 1
1.2 Problem Statement And Its Significance ... 3
1.3 Research Objectives ... 4
1.4 Research Methodology ... 4
1.5 Scope of the Dissertation ... 6
1.6 Dissertation Outline ... 8
CHAPTER 2: LITERATURE REVIEW ... 9
2.1 Introduction ... 9
2.2 Analytical Work ... 9
2.2.1 Ductile Fracture Mechanism ... 9
2.2.2 Ductile Fracture Criteria ... 10
2.3 Experimental Work ... 16
2.4 Numerical Analysis ... 19
2.4.1 Ductile Fracture Criteria in Numerical Simulations ... 22
2.4.2 Arbitrary Lagrangian Eulerian Formulation ... 26
2.4.3 Smooth Particles Hydrodynamics Formulation ... 31
2.4.4 Material Selection for Numerical Simulations ... 39
2.5 Summary ... 44
CHAPTER 3: EXPERIMENTAL WORK ... 46
3.1 Introduction ... 46
3.2 Experimental Procedure ... 47
3.3 Experimental Results ... 50
3.4 Statistical Analysis ... 56
3.5 Summary ... 59
CHAPTER 4: NUMERICAL SIMULATIONS OF HOT FORGING PROCESS .... 60
4.1 Introduction ... 60
4.2 Simulation Preprocessing ... 60
4.2.1 Material Properties of the Workpiece ... 60
4.2.2 Tools Geometry, Material Properties, and Motion ... 62
4.2.3 Simulations Control and Output Commands ... 64
4.4 ALE Formulation ... 65
4.4.1 Pre-Processing the Simulations with ALE Formulation ... 68
4.4.2 Results of Numerical Simulations with ALE Formulation ... 68
4.4.3 Validation of Numerical Simulations with ALE Formulation ... 72
4.5 SPH Formulation ... 74
4.5.1 Pre-Processing the Simulations with SPH Formulation ... 74
4.4.2 Results of Numerical Simulations with SPH Formulation... 76
4.4.3 Validation of Numerical Simulations with SPH Formulation ... 80
4.5 ALE vs SPH Formulations ... 81
4.5.1 Pre-Processing the Model ... 81
4.5.2 Accuracy of the Results ... 83
4.5.3 Capability to Describe the Material Flow ... 85
4.6 Summary ... 87
CHAPTER 5: PARAMETRIC STUDY ON HOT FORGING PROCESS ... 88
5.1 Introduction ... 88
5.2 Parametric Study on Hot Forging Processs ... 88
5.2.1 Minimum Wall Thickness ... 88
5.2.2 Temperature of the Process ... 96
5.2.3 Strain Rate ... 98
5.3 Proposed Solution to the Cracks Problem ... 101
5.4 Summary ... 103
CHAPTER SIX: CONCLUSIONS AND RECOMMENDATIONS ... 104
6.1 Conclusions... 104
6.2 Major Contribution ... 105
6.3 Recommendations for Future Studies ... 105
REFERENCES ... 107
LIST OF PUBLICATIONS ... 116
APPENDIX A: SAMPLES OF LS DYNA CODE ... 117
LIST OF TABLES
Table 2.1: Theoretical Ductile Fracture Criteria 15
Table 3.1: The experimental wall thickness of the specimen 53 Table 3.2: The coordinates of the cracks locations in each specimen 56
Table 4.1: Input parameters for brass 60
Table 4.2: Input parameters for the tools 62
Table 4.3: The input parameters of coupling card definition 67 Table 4.4: Measured wall thickness with numerical simulations (ALE 71 Table 4.5: Measured wall thickness with numerical simulations (SPH 77 Table 4.6: Comparison between the experimental and numerical results 80
LIST OF FIGURES
Figure 1.1: Flow diagram of the research methodology 7
Figure 2.1: Johnson-Cook triaxiality diagrams 11
Figure 2.2: A 3D sketch of fracture envelope in the space of the principal stresses 13 Figure 2.3: Strain rate effect on stress triaxiality, Lode angle and plastic strain 18 Figure 2.4: Stress strain curve representation in plastic kinematics model 40 Figure 2.5: Stress strain curve representation in power law plasticity model 41 Figure 2.6: Algorithm of piecewise linear plasticity material model 44
Figure 3.1: The dimensions of the specimen 47
Figure 3.2: Position of the specimen in the backward extrusion machine 48 Figure 3.3: The experimental procedure of the manufacturing process 49
Figure 3.4: The product of the hot forging process 50
Figure 3.5: A visible crack in the flash area before machining the product 51 Figure 3.6: The specimen after the removal of the flash 51 Figure 3.7: The region where the cracks commonly appear 52 Figure 3.8: Top view of the specimen reveals the material uneven distribution 53 Figure 3.9: Difference in the wall thickness of the design 54
Figure 3.10: Locating the position of a crack 55
Figure 3.11: Microscopic view of the crack (×50 55
Figure 3.12: Reference system for the coordinates of the cracks locations 56
Figure 3.13: The average location of the cracks 58
Figure 4.1: Stress-strain curves for brass at 800 0C and different strain rates 61 Figure 4.2: Isometric, top, side and front views of the punch 62 Figure 4.3: Isometric, top, side and front views of the die 63
Figure 4.4: Velocity profile of the punch 64
Figure 4.5: The ALE mesh of the workpiece and void (half) 66
Figure 4.6: The die and punch are submerged in the void 66
Figure 4.7: Coupling Algorithm for ALE/Lagrangian Interaction 68
Figure 4.8: Hot forging simulation with ALE formulation 70
Figure 4.9: The formation and closure of voids during the process 71 Figure 4.10: Horizontal section view of the numerical simulations results (ALE) 72 Figure 4.11: Comparison between simulation results and experimental data 73
Figure 4.12: Specimen representation with SPH particles 75
Figure 4.13: Contact definition between SPH particles and tools surface 76
Figure 4.14: Hot forging process with SPH formulation 77
Figure 4.15: The vertical plane section shows the flow restriction 79 Figure 4.16: The horizontal plane section view of the results with SPH 80 Figure 4.17: (a) Experimental (b) ALE, and (c) SPH results 84 Figure 4.18: Comparison of the geometrical of the specimen in ALE and SPH 85 Figure 4.19: Comparison of the flow prediction of ALE (right) and SPH (left 86 Figure 5.1: Modification in die geometry to increase the critical wall thickness 89 Figure 5.2: ALE simulations with increased wall thickness, d = 3.5 mm 90 Figure 5.3: ALE simulations with increased wall thickness, d = 4 mm 91 Figure 5.4: ALE simulations with increased wall thickness, d = 4.5 mm 92 Figure 5.5: Comparison of material flow in the new and original geometries 93 Figure 5.6: The material density in the new and original geometries 95 Figure 5.7: The effect of different minimum wall thickness on triaxiality factor 96 Figure 5.8: Stress-strain diagrams of the brass at different temperatures 97 Figure 5.9: The plastic strain contours at different temperature 97 Figure 5.10: The effects of temperature on the triaxiality factor 98 Figure 5.11: The effects of temperature on punch maximum force 99
Figure 5.12: The node 2147767 path of deformation 99
Figure 5.13: The effects of varying the punch velocity on the triaxiality factor 100 Figure 5.14: The relationship between punch velocity and energy absorbed 101 Figure 5.15: Effects of forging temperature and punch velocity on triaxiality factor 101 Figure 5.16: ALE simulations of hot forging process with proposed solution 102
LIST OF ABBREVIATIONS
ALE Arbitrary Langrangian Eulerian FDM Finite Difference Method FEA Finite Element Analysis FEM Finite Element Method FSI Fluid Structure Interaction
GPa Giga Pascal
LSTC Livemore Software Technology Corporation
MPa Mega Pascal
SPH Smooth Particle Hydrodynamics
LIST OF SYMBOLS
Initial yield stress Hydrostatic stress
Cauchy stress tensor
Von-mises equivalent stress
Updated deviatoric stress
Trial updated deviatoric stress Strain rate tensor
Effective plastic strain Strain rate effects parameter Von Mises flow rule
Triaxiality factor Hardening function
Plastic hardening modulus
Components of body force per unit mass Pressure at particle b
Mass of particle b
Components of heat flow Specific internal energy
Position vector of particle b Velocity vector of particle b Density of particle b
Rate of heat addition by a source
Rate of rotation of the neighbourhood
CHAPTER ONE INTRODUCTION
Metal forming is a manufacturing process in which an initially simple part is plastically deformed between tools to obtain the desired geometry. The complex geometry is "stored" in the tools and achieved when the tools impart pressure of the deforming material. The metal flow, the friction at the tool and material interface, the heat generation and transfer during plastic deformation are complicated and difficult to predict and analyze. Thus, the physical phenomena constituting metal forming processes are not easy to express with quantitative relationships (Kobayashi et al., 1989 and Reddy et al., 2000).
In metal forming, the material is subjected completely or partially to large plastic deformation. There is little or no material to be removed after the forming operation. Compared to other manufacturing processes such as casting or machining, the products of metal forming have better mechanical properties as the microstructure of the material is broken down and refined by the plastic deformation during the metal forming process. Moreover, in cold metal forming, the mechanical properties will increase during cold deformation due to the accumulation of the plastic strain in the workpiece (Valbreg, 2010).
Metal forming can be categorized into two major types. The first type is called sheet metal forming in which the workpiece is a sheet or a part manufactured from a sheet. The workpiece is forming against a die and commonly subjected to tensile forces. In some occasions, the magnitudes of the plastic and elastic deformation are similar. Thus, springback phenomenon may be considerable (Kobayashi et al., 1989).
There is a large limitation in the workability and deformation of the workpiece due to the tensile stresses in sheet metal forming. Excessive forming would results in material instabilities such as necking and rupture. The second type is bulk metal forming which involves a high volume to surface ratio. The workpiece is shaped into useful products by means of compressive forces. The most important bulk metal forming processes are drawing, rolling, extrusion, and forging. (Valbreg, 2010 and Wifi et al., 1995)
Forging is an important bulk metal forming process in which metals are shaped into useful complicated structures (Wifi et al., 1996). In this process, the workpiece is compressed between two dies and undergoes large plastic deformation. The material flow is confined in the cavity between the dies and the extra material flows out through the gap between them (Kobayashi et al., 1989 and Wagoner et al., 1997). In bulk metal forming, surface defects might appear as a result of plastic instabilities associated with the localization phenomena. On the other hand, internal cracks might result from the accumulation of damage during the deformation which depends on the stress and strain paths (Reddy et al., 2000).
The main objective of conducting finite element analysis of metal forming processes is to provide necessary information for proper control and design of these processes (Oh et al., 1979). The development time and cost are reduced as the number of the required experimental trials is minimized. Nowadays, the development of inexpensive powerful computers technology and the application of Finite Element Method (FEM) into user friendly programs have brought this technology forward.
This evolution has more or less revolutionized the metal forming analysis (Clift et al., 1990 and Valberg, 2010).
The work presented in this thesis describes an approach to identify and eliminate the cracks-like flaws in bulk metal forming through extensive numerical simulations. Two main techniques are used: Smooth Particles Hydrodynamics (SPH) and Arbitrary Lagrangian Eulerian (ALE) with the commercial finite element software LS-DYNA. A parametric study is conducted to investigate the effects of punch velocity, temperature of the process, and workpiece thickness on the formation of the cracks. The optimum values of these parameters are proposed based on the results.
1.2 PROBLEM STATEMENT AND ITS SIGNIFICANCE
In metal forming processes, the term workability refers to the degree of deformation that can be achieved in the workpiece without the occurrence of a defect i.e. the appearance of surface or internal cracks. Failure usually occurs as ductile fracture in metalworking and rarely as brittle fracture. The propagation of the cracks is of little interest as the main objective is to avoid their initiation. These cracks usually appear within regions that are highly strained due to extensive plastic flow of the material during the metal forming process (Gouveia et al., 2000).
Flaws or defects in the metallic products manufactured by using bulk metal forming pose a severe problem for the manufacturing industry because the defective products are inappropriate to fulfil their intended utility. In the suggested research the source of cracks in the formed products would be identified and corrective measures would be suggested for improved product manufacturing.
The results from this research work would be applicable to the wide ranging of industrial manufacturing processes. The expertise in the usage of commercial finite element software to predict the bulk metal forming process would be able to reduce
the number of rejected parts. Many manufacturing industries can benefit from the results of the present study.
1.3 RESEARCH OBJECTIVES
The objectives of the proposed research are as follows:
To perform a three dimensional numerical simulations for a case study of hot metal forming process using ALE and SPH approaches.
To identify the sources of crack-like flaws in the metal-formed products with the help of numerical simulations.
To investigate the effects of different parameters on the hot forging process.
To suggest corrective measures to eliminate the flaws and improve manufacturing process.
1.4 RESEARCH METHODOLOGY
The methodology in this study is summarised in Figure 1.1 and it is as follows:
1) Literature Review
This research begins with a detailed review of the literature in the relevant studies of the experimental approach in predicting the cracks, numerical simulation of metal forming processes, and the presently available ductile failure criteria.
2) Industrial Visits
The experimental work involves a case study of hot forging process. A number of industrial visits is carried out to analyse the environment of the metal forming process and to collect samples of the products. The products are thoroughly examined to pinpoint the location of cracks. The final
dimensions of the products are recorded. Statistical analysis is performed to obtain the average location of the cracks and to investigate the probability of the cracks appearance.
3) Three Dimensional Numerical Simulations
Computer simulations are performed to assist the prediction of cracks locations. A detailed three dimensional nonlinear numerical simulations of the hot forging process is developed with both SPH and ALE approaches in LS-DYNA. The material properties of the workpiece, the punch velocity and the boundary conditions are inserted in the input file of LS-DYNA.
4) Validation of Numerical Simulations
The validation is carried out by comparing the results of the numerical simulations to the data obtained from the industrial visits. The dimensions of the final product and the location of the cracks are evaluated. If the validation is not successful, the numerical simulations are conducted again.
5) Parametric Study and Solution Proposal
The effects of varying the workpiece minimum wall thickness, process temperature, and punch velocity on the formation of the cracks are discussed. A parametric study on these three variables is performed and their optimum values are proposed as a solution to the crack like flaws problem.
6) Verification of the Proposed Solution with Numerical Simulations
The proposed solution is assessed by implementing it in the numerical simulations. If the cracks are eliminated in the LS-DYNA simulations, then the proposed solution is numerically verified. However, if the problem still exists, the parametric study is repeated for a better solution.
7) Validation of the Proposed Solution Experimentally
Once the proposed solution is verified with numerical simulations, it is suggested to the industry. Should the industry accept the solution;
modifications in the experimental set up of the hot forging process are made to match the proposed values of the punch velocity, temperature, and minimum wall thickness. The final product of the process is investigated to examine if the cracks is completely eliminated. If the flaws still exists in the product, the parametric study is repeated to find a better solution.
1.5 SCOPE OF THE DISSERTATION
This work involves the investigation and prevention of the crack-like flaws in hot forging process. The process was carried out at an elevated temperature and the workpiece was made from brass. Present research consists of a three dimensional simulation of a hot forging process where the workpiece is compressed between a stationary die and a moving punch. The numerical simulations are conducted with the commercial finite element software LS-DYNA to find out the causes of the flaws and to pinpoint their locations. Two techniques available in LS-DYNA are utilized in the numerical simulations and comparisons are drawn between the results of each approach. The numerical simulations are validated by the experimental results obtained from the industrial visits. Similarities between the experimental and numerical simulations results are observed. The effects of wall thickness of the workpiece, punch velocity, temperature of the process are examined and their optimum values are proposed. The formation of the cracks is prevented in the numerical simulations by applying the proposed values.
Figure 1.1: Flow diagram of the research methodology.
Verifcation of the Solution with Numerical Simulations
Application of the Proposed Solution Experimentally Validation of Numerical Simulations
3-D Numerical Simulations Industrial Visits
Topic Overview and Literature Review
Parametric Study and Solution Proposal
Proposed the Solutions to the Industry
YES YES YES
8 1.6 DISSERTATION OUTLINE
The dissertation starts with the first chapter which presents the introduction of the research and consists of the background, problem statement and its significance, objectives and methodology of the research, scope and outline of dissertation. The second chapter is a review of the relevant numerical and experimental work. The experimental work on a case study of the hot forging process is presented in chapter three. The subsequent chapter discusses the pre-processing, results, and validation of a three dimensional numerical simulation of a hot forging process. It is followed by the fifth chapter which includes the parametric study on three variables of hot forging process and the proposed solution to the crack like flaws. The dissertation concludes with the sixth chapter in which the conclusions derived from the study, major contributions and recommendations for the future work are presented.
CHAPTER TWO LITERATURE REVIEW
Due to the importance of ductile fracture and failure prediction in metal forming processes, tremendous efforts were made by different researchers from both academic and industrial research institutions, to develop a method to predict the surface and hidden flaws. The ability to pinpoint the flaws locations possesses great importance for metal forming industries since the detection and elimination of failure in prototyping phase would reduce the costs significantly. Different methods were attempted by researchers to predict the ductile fracture such as developing a valid criterion and failure model through analytical work, performing extensive laboratory experiments, and developing a robust three dimensional simulation models. In the following subsections, the relevant reported studies of ductile fracture are discussed in details.
2.2 ANALYTICAL WORK
In this subsection, relevant literatures about analytical work in ductile fractures are presented.
2.2.1 Ductile Fracture Mechanism
The understanding of the ductile fracture mechanism is essential for the analytical approach of developing a valid failure criterion. The initiation of ductile fracture is a multistep process consisting of micro-void nucleation, growth and their subsequent coalescence (Goijaerts et al., 2000). Micro-voids usually nucleate due to the