Process planning for robotic wire ARC additive manufacturing

175 Pages·2017·8.34 MB·English

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University of Wollongong Thesis Collection
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Donghong Ding
University of Wollongong
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Ding, Donghong, Process planning for robotic wire ARC additive manufacturing, Doctor of Philosophy
thesis, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, 2015.
https://ro.uow.edu.au/theses/4613
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PROCESS PLANNING FOR ROBOTIC WIRE ARC
ADDITIVE MANUFACTURING

A dissertation
submitted to the University of Wollongong in partial fulfilment of the
requirements for the degree of

Doctor of Philosophy

September 2015

By
Donghong Ding

School of Mechanical, Materials and Mechatronic Engineering
University of Wollongong, Australia
1
Abstract

Robotic Wire Arc Additive Manufacturing (WAAM) refers to a class of additive
manufacturing processes that builds parts from 3D CAD models by joining materials layer-
upon-layer, as opposed to conventional subtractive manufacturing technologies. Over the past
half century, a significant amount of work has been done to develop the capability to produce
parts from weld deposits through the additive approach. However, a fully automated CAD-to-
part additive manufacturing (AM) system that incorporates an arc welding process has yet to
be developed. The missing link is an automated process planning methodology that can
generate robotic welding paths directly from CAD models based on various process models.
The development of such a highly integrated process planning method for WAAM is the
focus of this thesis.
The major steps in process planning for robotic WAAM includes generation of 2.5D
layers, generation of deposition paths for these layers, the determination of welding
parameters (e.g. wire-feed rate, travel speed, and stick-out) associated with the deposition
paths, and an optional post-process machining. A more detailed explanation for each of these
steps is provided as follows.
In WAAM, the CAD model is sliced into a set of 2.5D layers along the build direction.
Unlike existing slicing methods, which divide models into parallel slices of either equally or
varying thickness, the newly-developed multi-direction slicing algorithm decomposes CAD
models into slices in multiple directions based on geometric evaluations. This allows direct
deposition of complex models without the need to introduce additional support structures. In
addition, in multi-direction deposition, the accuracy of the deposited part is increased through
minimising the “stair-stepping” effect.
Deposition paths are subsequently generated for the 2D cross-section of the obtained
2.5D layers. Path generation for processes that have fine, statistically distributed particles in
powder-based AM system are somewhat independent of geometric complexity. However,
AM systems based on arc-wire welding produce much larger deposits than powder-based
systems, so the deposition paths are significantly influenced by geometric complexity and the
material selected. Innovative MAT-based (medial axis transformation) path planning
algorithms are proposed to generate void-free deposition paths irrespective of the complexity
of the geometry.
I
Welding parameters associated with the generated deposition paths are selected to
produce weld deposits with the desired width and height to produce the necessary layer
thickness together with complete material infill between paths. Generally, welding variables
such as wire-feed rate and travel speed are manually set by welding experts depending on the
deposition process and the material selected. In this study, artificial neural network (ANN)
single bead models are established for the gas metal arc welding (GMAW) process of mild
steel and aluminium alloy materials, respectively. A multi-bead overlapping model is also
developed to determine the appropriate overlapping distances (step-over distances) between
the paths in order to obtain a smooth multi-pass surface. Both the ANN model and multi-bead
overlapping model are integrated into the process planning methodology allowing the
automatic selection of optimum welding parameters corresponding to the generated
deposition paths.
As a result, near-net shapes are fabricated using the generated deposition paths and
welding parameters. The shapes are then machined to produce the finished parts with the
required dimensional tolerances through a series of post-process robotic machining
operations. The absence of re-fixturing problems and tool accessibility problems helps in
fully automating the generation of robotic machining paths. Several parts were built
automatically using this process planning and the results are discussed.

II
Acknowledgements

I take this opportunity to express my sincere appreciation to my principal supervisor Dr.
Zengxi Pan, and associate supervisors Prof. Huijun Li and Dr. Dominic Cuiuri, for their
support and guidance in pursuing this research. They kept me free of financial worries and
provided the much needed assistance in my program over the last three years. The excellent
research atmosphere that provided in our group was very helpful for the successful
completion of this work.
Many thanks to Prof. Weihua Li, and A/Prof. Shu-Qing Yang for their guidance during
my early PhD life (12.2011~6.2013). Weihua’s encouragement allowed me to feel more
confident to start my new research topic.
I would like to thank Mr. Nathan Larkin for his support during laboratory experiment. I
would also like to thank the other members of our group, Dr. Stephen Van Duin and Prof.
John Norrish for their valuable comments and useful suggestions.
A special thanks to my colleague Andrew Short for his supporting in experiments. Also I
am grateful to my other colleagues Chen Shen, Alexander Visser, and Yan Ma for exciting
and useful discussions.
Finally, I wish to express my special thanks to all my friends, to my parents, to my wife,
and to my lovely daughter, for their support and encouragement during this work.

III
Table of Contents

Abstract .................................................................................................................................................. I
Acknowledgements ............................................................................................................................ III
List of Figures .................................................................................................................................... VII
Chapter 1 ............................................................................................................................................... 1
Introduction ........................................................................................................................................... 1
1.1 Motivation .................................................................................................................................... 1
1.2 Literature review ........................................................................................................................ 2
1.2.1 AM vs. CNC Machining ...................................................................................................... 2
1.2.2 Powder-based vs. wire-feed AM process ............................................................................ 3
1.2.3 Wire-feed AM processes ...................................................................................................... 6
1.2.4 Wire arc additive manufacturing ....................................................................................... 8
1.3 Research gaps ............................................................................................................................ 14
1.4 Present work .............................................................................................................................. 15
1.5 Thesis outline ............................................................................................................................. 17
Chapter 2 ............................................................................................................................................. 19
Process Planning Overview ................................................................................................................ 19
2.1 The WAAM process .................................................................................................................. 19
2.2 Process planning ........................................................................................................................ 20
2.3 User interface ............................................................................................................................ 22
2.4 Summary .................................................................................................................................... 23
Chapter 3 ............................................................................................................................................. 25
Bead Modelling.................................................................................................................................... 25
3.1 Single bead geometry ................................................................................................................ 25
3.1.1 Single bead empirical models ............................................................................................ 26
3.1.2 Experimental set-up ........................................................................................................... 26
3.1.3 Curve fitting results ........................................................................................................... 28
3.2 Multi-bead overlapping model for mild steel ......................................................................... 30
3.2.1 Principle of the tangent overlapping model (TOM) ........................................................ 31
3.2.2 Overlapping of two beads .................................................................................................. 33
3.2.3 Multi-bead overlapping ..................................................................................................... 35
3.2.4 Experimental verification for the overlapping model ..................................................... 35
3.3 Artificial neural network bead models .................................................................................... 41
IV
3.3.1 Forward computing ........................................................................................................... 41
3.3.2 Error backpropagation learning ...................................................................................... 42
3.3.3 Training of neural network ............................................................................................... 44
3.3.4 Bead geometry measurement ............................................................................................ 45
3.3.5 Testing of neural network ................................................................................................. 46
3.4 Summary .................................................................................................................................... 47
Chapter 4 ............................................................................................................................................. 48
3D CAD Slicing ................................................................................................................................... 48
4.1 Uni-direction slicing .................................................................................................................. 48
4.1.1 The tolerant uni-direction slicing algorithm.................................................................... 49
4.1.2 Implementation .................................................................................................................. 52
4.2 Multi-direction slicing .............................................................................................................. 54
4.2.1 Related works of multi-direction slicing .......................................................................... 55
4.2.2 Strategy overview ............................................................................................................... 57
4.2.3 Multi-direction slicing algorithms .................................................................................... 59
4.2.3 Implementation and results ............................................................................................... 71
4.3 Summary .................................................................................................................................... 73
Chapter 5 ............................................................................................................................................. 75
2D Deposition Path Planning ............................................................................................................. 75
5.1 Existing path planning methods .............................................................................................. 75
5.2 Path planning challenges in WAAM ....................................................................................... 78
5.3 MAT-based path planning ....................................................................................................... 83
5.3.1 Overview of MAT-based path ........................................................................................... 83
5.3.2 Definition and computation of the MAT .......................................................................... 86
5.3.3 MAT-based path generation ............................................................................................. 88
5.3.4 Discussions .......................................................................................................................... 95
5.4 Adaptive MAT path planning .................................................................................................. 99
5.4.1 Essential of adaptive MAT path ....................................................................................... 99
5.4.2 Algorithm for adaptive MAT path generation .............................................................. 101
5.4.3 Validation of the proposed algorithm ............................................................................ 106
5.4.4 Discussions ........................................................................................................................ 108
5.5 Summary .................................................................................................................................. 109
Chapter 6 ........................................................................................................................................... 111
Post-process Machining .................................................................................................................... 111
V
6.1 Top region machining ............................................................................................................. 111
6.2 Side region machining ............................................................................................................ 112
6.3 Base region machining ............................................................................................................ 113
6.4 Summary .................................................................................................................................. 114
Chapter 7 ........................................................................................................................................... 115
Applications ....................................................................................................................................... 115
7.1 Mild steel components with complex geometries ................................................................. 115
7.2 Aluminium thin-walled structure with finished quality ...................................................... 118
7.3 Adaptive MAT paths with minimum post milling ............................................................... 122
7.3.1 ANN model of mild steel .................................................................................................. 122
7.3.2 Determination of 2D path planning variables ............................................................... 124
7.3.3 Determination of welding process parameters .............................................................. 126
7.3.4 Case studies ....................................................................................................................... 127
7.3.5 Discussions ........................................................................................................................ 131
7.4 Summary .................................................................................................................................. 132
Chapter 8 ........................................................................................................................................... 134
Summary and Future Work ............................................................................................................ 134
8.1 Contributions........................................................................................................................... 134
8.2 Future work ............................................................................................................................. 136
Appendix ............................................................................................................................................ 139
A. Continuous path generation algorithm .................................................................................. 139
B. Smallest enclosing crowns ....................................................................................................... 147
References .......................................................................................................................................... 150
Selected Publications ........................................................................................................................ 157

VI