Modeling and Optimization of Powder Based Additive Manufacturing (AM) Processes

204 Pages·2013·6.14 MB·English

Modeling and Optimization of Powder Based Additive
Manufacturing (AM) Processes

A dissertation submitted to the
Graduate School of the University of Cincinnati
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
in Mechanical Engineering
in the School of Dynamic Systems
in the College of Engineering and Applied Sciences

By
RATNADEEP PAUL
M.S.M.E University of Cincinnati
B.Tech National Institute of Technology, Warangal

2013

Committee Chair: Dr. Sam Anand
ABSTRACT
Metal powder based Additive Manufacturing (AM) processes are increasingly being
accepted across several industries such as aerospace, automobile, medical, consumer products
and electronics systems. However, the most pressing issues faced by AM technology today are
concerns in achieving part accuracy and excessive material and energy utilization. Therefore, a
comprehensive part-level approach analyzing the relation between the different physical
phenomena occurring during the manufacturing of a metal AM part is required.
A comprehensive virtual manufacturing model has been developed for simulating the
surface of parts manufactured using metal powder based AM processes. The virtual
manufacturing model simulates the geometry of the AM part using computational geometry
techniques. Points are sampled from the surface of the simulated parts simulated and used for
calculating the Geometric Dimensioning and Tolerancing (GD&T) errors and also for
correlating these errors to the input parameters. The geometric virtual manufacturing model is
further augmented by material shrinkage and thermal deformation error models. The shrinkage
model estimates the effect of material shrinkage in AM part errors while the thermal deformation
model analyzes the combined effect of shrinkage and thermal deformation on part errors.
The Stereolithography (STL) file format approximates part surfaces with planar
triangular facets resulting in errors in the manufactured part. Another file format, called Additive
Manufacturing File (AMF) format uses recursive sub-division of curved triangles into planar
triangles which however, leads to the same approximation error. This research introduces a new
format which uses curved Steiner patches for approximating part surfaces and generating the
different slices. Several test surfaces and parts are virtually manufactured using the Steiner
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format and the GD&T errors of the manufactured parts are calculated and compared with those
parts manufactured using the STL and AMF representations.
Support structures are additional material sintered during the manufacturing of an AM
part for supporting internal cavities and overhangs. This study presents a voxel based algorithm
for calculating the location and the volume of support structures used in AM processes and
correlating them to the process parameters. Two parametric energy models have also been
developed in this research: one for calculating the total process energy in AM processes which
do not use supports and another for those processes which require supports for overhangs and
holes. The first energy model uses a convex hull approach while the second model applies the
voxel algorithm to develop a predictive model for calculating the total energy expended in AM
processes.
Finally, optimization models have been developed which minimize part errors, part
strength, material utilization and energy expenditure in metal powder AM process by varying the
process parameters and calculating their optimal values. The optimization algorithms will assist
AM practitioners in producing error free parts the first time ("first part right"), thereby
eliminating the trial and error process currently associated with manufacturing an AM part. They
will also reduce the material utilization and the energy footprint of powder based AM processes,
thereby driving down the costs associated with producing parts by metal powder based AM
processes.

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ACKNOWLEDGEMENTS
I would like to take this opportunity to thank Dr. Sam Anand, my advisor and my mentor, for his
support and guidance during my graduate studies, especially while working on my Doctoral
research and dissertation. I will always remember our association fondly for the rest of my life.

I would also like to thank Dr. Sam Huang, Dr. Dave Thompson, Dr. Murali Sundaram and Dr.
Todd Rockstroh for graciously agreeing to serve as members of my defense committee. I would
like to acknowledge the help and support received from Dr. Frank Gerner for the various
discussions we had during the development of the dissertation.

I dedicate this thesis to my mother and father, Mrs. Kalyani Paul and Mr. Anil Ranjan Paul.
Without their upbringing, support, guidance and belief, I would not have made so far in my life. I
will always be indebted to them. Thank you, Maa and Baba.

Also, I would like to thank my wife, Mrs. Dipanwita Paul for always having unwavering faith in
me and my abilities, even when my mind was filled with doubts. It is hard for me to imagine
how I could have finished my research without her presence in my life.

A special “thank you” goes out to my friends Neeraj, Gaurav, Nandkumar, Santosh, Kunal,
Anish and Abhinav. They were the ones who made my life a whole lot easier during the course
of my stay in UC.

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TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION ................................................................................................... 1
1.1 Background ........................................................................................................................... 1
1.2 Motivation and Research Objectives..................................................................................... 5
1.3 Significance and Impact of Research .................................................................................. 10
1.4 Outline of Dissertation ........................................................................................................ 11
CHAPTER 2: REVIEW OF RELEVANT LITERATURE .......................................................... 12
2.1 AM Process Parameters and Part Errors ............................................................................. 12
2.2 Metal Shrinkage and Thermal Deformation........................................................................ 14
2.3 Input File Format ................................................................................................................. 17
2.4 AM Material Utilization ...................................................................................................... 18
2.5 AM Process Energy ............................................................................................................. 20
2.6 AM Part Strength ................................................................................................................ 21
CHAPTER 3: DEVELOPMENT OF VIRTUAL AM MODEL FOR MINIMIZING PART
ERRORS ....................................................................................................................................... 23
3.1 Methodology to Develop Virtual Manufacturing Model .................................................... 23
3.1.1. Simple Geometric Model ............................................................................................. 24
3.1.2. Simulation of Manufactured Surface using CAD Model ............................................. 24
3.1.3. Simulation of Manufactured Surface using STL Model............................................... 28
3.2 GD&T Error Formulations .................................................................................................. 30
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3.3 Results of Part GD&T Error Calculations........................................................................... 35
3.4 Minimization of GD&T Errors ........................................................................................... 37
3.4.1 Graphical Optimization of AM Processes .................................................................... 38
3.4.2 Analytical Optimization of AM Processes .................................................................... 42
CHAPTER 4: MODELING EFFECT OF SHRINKAGE AND THERMAL DEFORMATION IN
AM PART ERRORS .................................................................................................................... 52
4.1 Modeling Material Shrinkage in AM Parts ......................................................................... 52
4.1.1 Shrinkage Model based on First Principles ................................................................. 54
4.1.2 Shrinkage Model based on Experimental Results ........................................................ 57
4.1.3 Modeling Effect of Shrinkage on Part Geometry ......................................................... 58
4.2 AM Part Errors due to Material Shrinkage ......................................................................... 60
4.3 Predictive Equations for AM Part Errors ............................................................................ 66
4.4 Modeling Thermal Deformation in AM Parts ..................................................................... 72
4.4.1 Modeling of Shrinkage and Thermal Distortion........................................................... 73
4.4.2 Validation of Thermal Deformation Model .................................................................. 82
4.4.3 Superimposition of Thermo-Mechanical and Geometric Model .................................. 84
4.5 Part Errors Calculated from Combined Geometric and Thermo-Mechanical Model ......... 85
CHAPTER 5: STEINER AM FILE FORMAT ............................................................................ 96
5.1 Steiner Surface .................................................................................................................... 98
5.2 Converting CAD surface to Uniform Steiner Format ....................................................... 101
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5.3 Adaptive Tessellation based Steiner Patch Format ........................................................... 106
5.4 Virtual Manufacturing of Steiner Files ............................................................................. 112
5.5 Chordal Error Comparison between Steiner Format and STL File ................................... 117
5.6 Chordal Error Comparison between Steiner Format and AMF File ................................. 119
5.7 Chordal Error Comparison between Adaptive and Uniform Steiner Formats .................. 122
5.8 GD&T Error Comparison between Adaptive Steiner and STL Files................................ 124
CHAPTER 6: MATERIAL AND ENERGY UTILIZATION IN METAL AM PROCESSES . 127
6.1 Calculation of Support Structures ..................................................................................... 127
6.2 Process Energy Estimation in AM Processes without Supports ....................................... 132
6.3 Process Energy Estimation in AM Processes with Supports ............................................ 146
CHAPTER 7: OPTIMIZATION OF AM PROCESS ................................................................. 154
7.1 Minimization of GD&T Errors and Volume of Support Structures.................................. 154
7.2 Overall Optimization Model ............................................................................................. 158
7.2.1 Calculation of Optimal Slice Thickness...................................................................... 158
7.2.2 Calculation of Optimal Part Orientation ................................................................... 160
CHAPTER 8: CONCLUSIONS AND FUTURE SCOPE FOR RESEARCH ........................... 165
CHAPTER 9: REFERENCES .................................................................................................... 169

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