Additive manufacturing for repairing: from damage identification and modeling to DLD processing

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Additive manufacturing

for repairing:

from damage identiﬁcation

and modeling to

DLD processing

Matteo Perini

Supervised by Prof. Paolo Bosetti

School of Materials, Mechatronics

and Systems Engineering

Faculty of Industrial Engineering

University of Trento

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To Mirela. . .

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v

Acknowledgements

Firstly, I would like to express my deepest gratitude to my advisor Prof. Paolo Bosetti, for having given me the opportunity to undertake this doc-toral research and for his ongoing and valuable support during these in-credible three years.

Then, I would like to thank the each and every one of the members of my thesis committee – Assoc. Prof. Ladislav Moroviˇc, Prof. Antonino Squillace and Prof. Massimo Pellizzari, – for their insightful comments and encouragement.

An important recognition is due to Trentino Sviluppo and in particular to the ProM Facility, for giving me the opportunity to use one of the first hybrid machines in the world – the DMG MORI Lasertec 65 3D Hybrid. I will forever consider this as one of the greatest professional gifts that I have received in my entire career.

I am extremely indebted to Mr. Paolo Gregori, ProM Facility CEO. With-out the enormous support of the entity he leads and his personal guidance it wouldn’t have been possible to conduct this research. I am extremely grate-ful not only for Mr. Gregori’s thought partnership, but also for the excellent example he has provided as a manager and leader.

A special mention and word of recognition for his precious insights also go to Amos Collini, CTO of the ProM Facilty lab.

Prof. Dr. Nicolae Bâlc provided me with the wonderful opportunity of joining his team of researchers as a foreign Ph.D student and he granted me access to valuable facilities at the Technical University of Cluj-Napoca, in Romania. His wise counsel and guidance helped me tremendously in refining my thinking during the time I spent in this beautiful city in the heart of Transylvania. Thank you, professor Bâlc, from the bottom of my heart!

The "MDPI Machines" Journal has supported my work with the 2019 travel award and I feel grateful for that, as I also feel grateful to all the members of the DII department of the University of Trento. I thank my fellow labmates of the ProM Facility for the stimulating conversations and debates, for the constant support and for all the fun we had together in all

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vi

these years. This group has been a source of friendship as well as excel-lent advice and collaboration. I am especially grateful to Luca, Amedeo, Gianluca, Maurizio and Sasan.

Also I would like to express my appreciation to my friends of the Tech-nical University of Cluj Napoca (UTCN). In particular, I am grateful to Adrian, Alex, Alina, C˘at˘alin, Cosmin, Cristian, Cristina, Glad, Mariana, Vasile and Vlad. They warmly welcomed me and made me feel like home.

Last but not the least, I would like to thank my family for all their love and encouragement, in particular my parents who supported me in all my pursuits. I am grateful to my beloved Mirela: without her I wouldn’t have started this amazing journey. Thank you!

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List of Figures

1.1 An example of kintsugi art1. . . 2 2.1 Direct Laser Deposition schema. . . 17 2.2 The DMG MORI Lasertec 65 3D Hybrid machine used for this research. 20 2.3 Some infill strategies. a) Stripes, b) Meanders, c) Spiral, and d)

Con-centric. All of them can be executed with or without the contour (in red). . . 23 3.1 Example of geometry digitization. a) The damaged part digitized by

a structured-light scanner, b) The output that is a triangulated surface (mesh) and c) The same model rendered to highlight the damage. . . 29 3.2 Working principle of stereovision. Observers Oland Orsee the point

P respectively at the projection points Pl and Pr. Points El and Er are

the epipoles and are used to rectify the view. . . 30 3.3 The Creaform Metrascan 3D scanner used in this research1. . . 31 3.4 An example of alignment obtained using Meshlab. The color map

highlight the distance between the mesh and the undamaged solid model. . . 34 3.5 The DUOADD result, deliberately kept rough, that represent a object

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List of Figures xi

4.1 Two octrees of the same propeller. The resolution obtained is selected by the user. a) Rough resolution and b) The resulting model using an

higher discretization level. . . 39

4.2 Octree data structure . . . 40

4.3 Maximum number of nodes at each level of depth. . . 42

4.4 Densification of the octree around the faces of the mesh. . . 44

4.5 Sectioned octree. a) All the nodes, b) Only the leaves and c) The filled octree. . . 45

4.6 Number of ray–skin intersection. a) Inside points show an odd amount of intersections. b) Outside points always present an even number in-tersections. . . 46

4.7 Octree depth can be increased for specific sub–volumes. . . 47

4.8 The XOR operation. a) The octree of the original CAD model, b) The octree of the 3D scan (with damages) and c) The new octree that rep-resents the difference between the firsts two (in red). . . 50

4.9 A result of XOR operation subjected to the proposed filtering pro-cedure. By changing the threshold number of nearby cubes all the isolated and spurious nodes can be removed from the resulting set. The final volume is marked in yellow, while the removed nodes are the reds. From (a) to (f) the number of close nodes change from 0 to 8. 50 5.1 CAD tools to subdivide or merge 3D models (screenshot of Siemens NX window). . . 56

5.2 Types of deposition strategies (screenshot of Siemens NX). . . 58

5.3 Some of the parameters that affect the toolpaths. a) The path over-hang and the stepover, b) The feed , speed and power settings and c) The geometrical description of the nozzle (screenshots of Siemens NX windows). . . 60

5.4 The Lasertec 65 3D Hybrid simulated inside Siemens NX (screenshot of Siemens NX window). . . 61

5.5 Toolpaths palette (screenshot of Siemens NX window). . . 62

5.6 The Touching Probe1. . . 64

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xii List of Figures

5.8 New material added to a free-form damage. . . 68 5.9 The original shape restored by milling. . . 69 6.1 The mold that needs to be restored. a) Placed on the working table

and b) The detail of the damaged spots . . . 72 6.2 Meshes of the two models. a) The one generated from the original

CAD file and b) The 3D scan of the damaged mold. . . 73 6.3 Color map of the alignment distance. . . 74 6.4 The mesh of the damaged component (left part of the model)

con-verted in octree (green part). . . 75 6.5 The octree of the mold. a) The tree structure highlighted and b) Only

the set of leaves. . . 76 6.6 The enhancement of resolution on the damaged area. Everything else

in the model has been deliberately kept coarser. . . 77 6.7 The octree of the damaged spot. a) The cavity left by the missing

material and b) The obtained CAD model of the volume of the damage. 78 6.8 The solid models of the damaged spots. a) Over-imposed to the

orig-inal CAD file and b) During the simulation of the additive operations inside Siemens NX. . . 78 6.9 The additive and subtractive operations. a) New material applied to

the damaged area and b) The same spot after finishing operations. . . 79 6.10 The DLD Tool during an additive operation. . . 81 7.1 The octree with the tree structure on the left side and only the set of

leaves on the right side. . . 87 7.2 Powders for additive manufacturing. a) Rounded shape and b)

Irreg-ular shape with satellites. . . 88 7.3 DOE of Laser Power VS Powder Feed Rate (the scanning speed used

for the tests was 1000 mm/min). a) Top view of the single tracks and b) All the track sectioned. . . 89 7.4 Analysis of a single track section. . . 90

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List of Figures xiii

7.5 Example of single tracks data obtained depositing stainless steel pow-der on two different substrate materials. The powpow-der feedrate was set to 8 , 12 and 16 g/min. a) Stainless steel substrate and b) Mild steel substrate. . . 91 7.6 An example of porosity between each adjacent track. . . 92 7.7 Steel over Cu substrate. The adhesion between them is negligible. . . 92 7.8 Stepover examples. . . 93 7.9 Tensile test to check the adhesion between Ferro 55 and AISI 316L. . 98 7.10 Hardness profile at the interface between Ferro 55 and AISI 1.1189. . 99 7.11 Porosity in a DLD specimen. . . 99 7.12 Stress-Strain test on AISI 316L specimens. . . 100 7.13 An example of dilution at the interface between AISI 1.2343 and

Cuprum-Berillium. . . 101 7.14 Dilution example between Ferro55 and Stainless Steel AISI 316L. . . 101 8.1 Time spent to compute the octree of a mesh composed by 2325000

faces. The black line represents the speed of DUOADD before start-ing to optimize it while red line is traced after the introduction of the ray/triangle algorithm. The blue line depict the current performance of the software. . . 107

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List of Tables

2.1 DED technologies. . . 14 4.1 XOR truth table . . . 48 7.1 Parameters influencing DLD . . . 85 7.2 Example of process parameters to deposit different shape. Stainless

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List of Abbreviations

AM Additive Manufacturing . . . 5

B-Rep Boundary Representation . . . 51

CAD Computer Aided Design . . . 5

CAM Computer Aided Manufacturing . . . 5

CE Circular Economy . . . 3

CMM Coordinate Measuring Machine . . . 26

CNC Computer Numerical Control . . . 5

CPU Central Processing Unit . . . 41

CS Coordinate System . . . 79

CT Computed Tomography . . . 27

DED Direct Energy Deposition . . . 14

DLD Direct Laser Deposition . . . 6

DMD Direct Metal Deposition . . . 14

DOE Design Of Experiment . . . 88

DUOADD DUOADD Uses Octree As Damage Detector . . . 7

EBAM Electron Beam Additive Manufacturing . . . 15

EDXS Energy Dispersive Xray Spectroscopy . . . 86

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xvi List of Abbreviations

FDM Fused Deposition Modeling . . . 14

FEM Finite Element Method . . . 102

HAZ Heat Affected Zone . . . 91

ICP Iterative Closest Point . . . 33

IGES Initial Graphics Exchange Specification . . . 51

LDMD Laser Direct Metal Deposition . . . 102

LMD Laser Metal Deposition . . . 14

LENS Laser Engineered Net Shaping . . . 16

MAG Metal Active Gas . . . 15

MIG Metal Inert Gas . . . 15

NC Numerical Control . . . 64

OCC Open CasCade . . . 51

PAW Plasma Arc Welding . . . 15

PLC Programmable Logic Controller . . . 21

RAM Random Access Memory . . . 41

RE Reverse Engineering . . . 5

RPM Rotation Per Minute . . . 20

SD Standoff Distance . . . 19

SEM Scanning Electron Microscope . . . 86

SLM Selective Laser Melting . . . 58

STL STereo Lithography interface format . . . 42

STEP STandard for the Exchange of Product model data . . . 10

TIG Tungsten Inert Gas. . . .15

UK United Kingdom . . . 6

UCS Universal Coordinate System . . . 10

USA United States of America . . . 4

UTS Ultimate Tensile Strength . . . 97

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List of Abbreviations xvii

XRF X-Ray Fluorescence . . . 86

Yb Ytterbium . . . 19

WAAM Wire Arc Additive Manufacturing . . . 15

WC Tungsten Carbide . . . 108

WCS Workpiece Coordinate System . . . 63

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1 Introduction

The point is that things can be repaired.

That they are even more beautiful for having been repaired. Sonali Dev – A Change of Heart

1.1 |

The Value and the Politics of Repairing

The art of repairing broken objects has been a source of wonder and awe for Western civilizations, as the Japanese had a truly inspiring idea when decid-ing to give a second – and even more valuable – life to their broken objects. In the Western world, kintsugi [1], this particular Japanese idea of repairing ob-jects, was indeed unheard of and, to a certain extent, even inconceivable, as the Japanese used the most expensive material to do the repair: gold. This metal was “wasted” by the Japanese to repair their broken objects. Totally unbeliev-able, if not. . . if not for the Japanese wisdom and sense of value, that collectively agreed that a broken object repaired in this “unreasonable” way would become a totally new, more valuable and more desirable object. How far are we, now, in the whole world from the Japanese way of giving a second, more valuable life to our objects. How far and how regretfully wrong! This thesis makes the case for a modern form of the kintsugi art to be restored, by means of the most recent technology now available. For centuries and decades we had many excuses to

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Chapter 1. Introduction 1.1. The Value and the Politics of Repairing

Figure 1.1: An example of kintsugi art1.

dispose of our broken objects. But not any longer today, as superb technology is now available. Machines that can print objects in a way that 10 or 15 years ago would have been impossible. Of course, the existence of these printing machines would hardly be enough for fine repairing, but this research brings a “connect-ing the dots” innovation. A new technique that makes a sort of kintsugi possible for a large number of objects and different materials.

Beyond kintsugi, in the current industrial world there is also a very practical value in repairing objects. Tragically, in the last half century, manufacturing activities alone have led to a 65% increase in the use of raw materials. The world is now becoming aware of the unsustainability of our industrial habits that have a major impact on our planet resources, along with a dramatic increase in energy consumption and staggering levels of pollution [2]. As environmental activists are warning us: we are consuming the planet at accelerated rate, that might even destroy it sooner rather than later.

In such a deem scenario, saving raw materials becomes not only a necessity, but also a moral imperative to avoid both an environmental overload and a lack of raw materials in the near future, not to mention a consequent increase in

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Chapter 1. Introduction 1.1. The Value and the Politics of Repairing

the prices of the raw materials themselves. In academic circles and beyond the concept of sustainable development has been proposed.

One of the more effective ways used to make products more environmentally friendly is to extend their life by using re-manufacturing or repair techniques following principles of Circular Economy (CE) [3]. As it is becoming increas-ingly know, harsh corporate policies and the lack of sound institutional control, regulation and monitoring have led to practices such as planned-obsolescence and other such practices that discourage the component repair and reuse – i.e. if the repair process costs more than the new component.

Equally important, in recent years the European Union Commission has also intervened with a series of measures aimed to tackle unfair and environmentally unsustainable industrial policies1.

In the second half of the last century, there was an ever-increasing push and pull around our unsustainable throwaway culture. The vigorous development and automation of industrial processes, combined with the massive relocation of production facilities to more “convenient” areas of the world, made the prices of certain types of products to drop significantly, to levels that would have been unimaginable in other historical times so we have reached the paradoxical state where it is more convenient to use an object only one time rather than foresee a way to reuse it.

Too often, in fact, it happens, even in mechanical systems, that countless resources are wasted both in terms of raw materials and money, by replacing the whole components when they are damaged. Unfortunately, very often the possibility of repair is not even considered. It is clear that extending the life of such components can be particularly important both from an environmental and economic point of view. This could be extended to many different realities but in this study only mechanical components will be dealt with.

In the logic of this work, repairing an object is undeniably more efficient than recycling given that repairing makes it possible for a significant part of the component to be maintained, thus reducing the need to build a new component every time [4]. Today, repair is considered to be one of the highest value-added

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Chapter 1. Introduction 1.2. Repair Is a Challenge

strategies for the treatment of end-of-life parts. The ability to restore the object’s original performance is also closely linked to the design of the component: fol-lowing the sustainable design approach it is possible to take into account the whole life of a component [5].

One of the main factors that nowadays makes industry choosing to repair instead of replacing a component is the price. For very expensive mechanical parts, repair becomes very important. The repair process is quite complex and, in most cases, is currently carried out manually. This makes this operation par-ticularly expensive and prone to errors of the workers.

The repair of damaged objects is still an open challenge in industry [6]. In spite of the problem of repair is very challenging, recently, there has been a growing interest in repair and its total value in the real economy is clearly rising. For example, in 2016 the value of remanufacturing in the USA alone was about 75 billion USD [7]. Many countries are increasing the amount of resources employed to enhance repair capacity. Among them the most important players are: the United States of America (USA), the European Union (EU), China and Japan.

One of the aims of this research is to help fight the waste of energy and raw materials by proposing a new and innovative system for the automatic repair of mechanical components that could make repair attractive and make it preferable to complete replacement. Three aspects of this system – reduced cost, reliability and speed – can make the repair process more cost-effective and consequently it can also increase the number of objects that can afford a repair. A greater application of repair techniques leads to an extension of the products life with consequent savings in raw materials, money and energy [8, 9].

1.2 |

Repair Is a Challenge

The infinite variety of ways in which an object can be damaged has always been an obstacle in the automation of repair operations. Until recently, only a human being had the capacity to detect the damaged area and add new material where

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Chapter 1. Introduction 1.2. Repair Is a Challenge

it was missing. The component was then restored to its original shape with manual finishing operations. The addition of material was usually done through welding operations that were strongly influenced by both, the weldability of the materials and the skills of the workers.

In general, a repair workflow consists of 3 basic phases:

Detecting the damage spot on the object; Adding new material where it is missing; Restoring the original shape of the part.

These operations are relatively simple to perform for a human being, with their flexibility and adaptability, but rather complicated for an automatized pro-cess. Understanding where the damage is located and how to fill it with new material is the most difficult part.

Looking for an automatization of the process, at the time being, the three op-erations listed above need different technologies to be performed: Reverse En-gineering (RE) to detect the damaged shape, an Additive Manufacturing (AM) technique to add new material and a subtractive manufacturing technology to refurbish the part.

Until recently, there were only two alternatives to carry out the repair using a Computer Numerical Control (CNC) machine.

The damaged area is pre-machined removing by milling a well known

ge-ometry that is then filled in with new material;

The Computer Aided Design (CAD) model of the damaged volume is

cre-ated and used within the Computer Aided Manufacturing (CAM) software to compute the toolpath.

The repair process used to involve several different machines and many manual operations, as machines were unable to understand automatically how to perform the repair.

At the time being, Additive Manufacturing is deemed as the solution in en-hancing the remanufacturing process. In the repair procedure AM is intended to

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Chapter 1. Introduction 1.3. The Idea Behind This Work

be used to substitute the manual processes and to make the restoration cheaper, faster and reliable [10, 11]. Direct Laser Deposition is an Additive Manufactur-ing technology that can be suitable for repairManufactur-ing metal objects due to its flex-ibility and reliability. Direct Laser Deposition (DLD) is described in detail in Section 2.1.

Only recently, a few machine tool manufacturers developed the idea to in-sert a Direct Laser Deposition tool inside their milling machine creating a “hy-brid machine” capable to both, add and remove material [12, 13]. Having such a machine, it is foreseeable that the industrial world will try to replace man-ual operations with automated digital restoration processes [6]. At the time of writing only a few models of hybrid machines exist: for this study the DMG MORI Lasertec 65 3D Hybrid was used. Figure 2.2 shows a picture of the hybrid machine.

There is a limited number of papers that deal with the use of DLD for reman-ufacturing and all of them follow the approach of removing the damage in ad-vance. For example, in Manchester in United Kingdom (UK) Pinkerton [14] de-veloped a solution to deposit tool steel AISI 1.2344 (H13) in well-shaped grooves, while Wilson et al. [15] in Indiana in USA removed the defect from turbine blades – following a precise geometry – before adding the new material.

Only Zhang [16] from Dalian University of Technology, China has tried an approach which is similar to the one proposed in this research but, as reported in Chapter 4, since he didn’t use any discretization techniques, he experienced many robustness issues which made the system unsuitable for real cases.

1.3 |

The Idea Behind This Work

To automate the repair process, the three technologies needed – i.e. Reverse Engineering to detect the damage, Additive Manufacturing (Direct Laser Depo-sition) to add new material and a CNC Subtractive Manufacturing (milling) to restore the original shape – must be integrated together.

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Chapter 1. Introduction 1.3. The Idea Behind This Work

The hybrid machine already integrates a CNC Subtractive Manufacturing and Additive Manufacturing, while there is a missing link between these two and the Reverse Engineering.

Unfortunately, the CAM capable of handling the hybrid machine – Siemens NX – requires a three-dimensional solid model of the damage to create toolpaths. As described in Chapter 3, using RE and in particular 3D scanning techniques, a mesh of the damaged object can be obtained, but not a solid model. Translating a triangled surface – i.e. a mesh – into a solid model is very difficult. Moreover, it is a task that does not solve the problem created by the fact that the damaged component can be detected while the volume of the damage can not. This made the automatic repair impossible until now.

This research has placed the focus on this most critical part of the process cre-ating a system able to detect the solid model of the damage and make it suitable for the CAM. The aim is to automate as much as possible this operation which represents the current bottleneck in the repair procedure.

The most important aspect of the current work, as well as the most origi-nal and innovative, pertains to the fact that a new way of repairing an object without the need for any preventive machining has been imagined. Unlike the methods listed above, this research aims to automatically detect the damages present on the object. In theory, the damage is represented by the differences be-tween the theoretical CAD model and the real model – i.e. its 3D representation. The ability to obtain the 3D models of the volume of the damage paves the way for a more automatic and much faster approach.

The most important contributions of this research project are represented by the efforts to create and implement an innovative software called DUOADD Uses Octree As Damage Detector (DUOADD) [17, 18], that allows the automatic recognition of the damage. It succeeded where other projects failed [16] be-cause it transforms the meshes in octrees [19] before executing their compari-son. As explained in Chapter 4, DUOADD compares the 3D CAD model of the component and the 3D scanned mesh, obtaining the volume of the difference – i.e. the volume of the damage. This application, developed in C++, returns a discretized representation of the volume of the damaged zone and it is able to export it as a solid CAD model that can be used by Siemens NX to create the

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Chapter 1. Introduction 1.4. A Novel Repair Workﬂow

toolpaths. So, the “hybrid repair” paradigm is presented as involving the use of a hybrid machine – i.e. with additive and subtractive capabilities – in the repair of damaged objects.

DUOADD fills the gap between the Reverse Engineering of the damaged component and the CAM software. The case study presented in Chapter 6 demonstrates that the remanufacturing of high-value components such as a in-jection moulding die is not only possible but also advisable [20].

In addition, the fact that, by using DUOADD, the damaged area does not need to be machined – i.e. expanded and pre-milled – before being filled with new material makes the repair of metal components even more cost-effective because machining is skipped and less material needs to be added. This means that both less energy and less raw materials are used.

1.4 |

A Novel Repair Workﬂow

A typical repair process in the industrial environment consists of six phases: disassembly, cleaning, inspection, repair, reassembly and testing [21].

This research puts the focus on the repair task creating a novel method to restore a damaged component using an hybrid machine. Chapter 2 describes the pros and cons of several Additive Manufacturing technologies and how a hybrid machine works. The procedure devised to accomplish the repair follows these steps:

1. 3D Scanning of the damaged component; 2. Repair/closing mesh (make it watertight);

3. Alignment between 3D scan and the original model; 4. Computing octree for mesh and solid model;

5. Fill the empty space inside the octrees;

6. Performing the difference between the octrees; 7. Filter results to delete isolated nodes;

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8. Converting the resulting volumes into a solid model file;

9. Manage bodies to build up CAM additive operations and toolpaths; 10. Finishing the component by milling operation.

The first three steps listed above represent preliminary operations in the Re-verse Engineering phase. As explained in Chapter 3, these steps are fundamen-tal for performing an automatic analysis of three-dimensional models. Opera-tions 4 to 8 are the phases conducted by DUOADD and represent the main con-tribution of this research. Without these steps the repair could not be carried out. Chapter 4 describes in details how DUOADD faces all those phases. The reso-lution of the octree is limited by the number of calculations needed to execute the process. Finally, the last two steps describe how the solid model obtained running DUOADD is used by CAD/CAM to carry out the repair. An in-depth discussion on these points can be found in Chapter 5.

Although the heart of this work has been the development of the DUOADD software, in order to perform a real repair, it is necessary to also consider a num-ber of aspects which are less related to the software or procedural aspects. Chap-ter 6 is a case study of the entire repair process. In this chapChap-ter the fieldwork of repairing a die-injection mold is reported and many practical aspects that need to be considered in order to complete the repair procedure are also highlighted. Chapter 7 describes some of these aspects focusing on the development of the process parameters and on how those parameters may affect the properties of the restored component.

1.4.1 |

Detecting the Damaged Spot on the Object

The first step in the repair procedure is to identify the damage on the component surface. A large part of this research focuses on this issue and how to solve it. The solution proposed by this research for the automatized identification of the damaged areas has four main phases:

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Comparison with the original CAD model;

Detection and creation of the volume model representing the damage; Export of the 3D CAD model which can be used by CAM.

Three-dimensional scanning can be performed using several different sys-tems. Chapter 3 lists some of the main technologies and highlights the pros and cons of using each of them.

As already mentioned there are only a few CAM software that can handle DLD processes using a 5-axis hybrid machines. None of them can use meshes to compute the toolpaths needed to add the new material. A solid CAD model of the volume to be added to work on is needed for performing a repair.

This is why this research has designed a system that uses an octree discretiza-tion to voxelize the two meshes and make them comparable. The algorithms used to perform the discretization are explained in Chapter 4, while the compar-ison between the two volumes and the further elaboration of the data generated to obtain a consistent model are explained in Sections 4.2 and 4.3 respectively. The three-dimensional solid model obtained using DUOADD is re-elaborated and exported in STandard for the Exchange of Product model data (STEP) for-mat so it can be used directly within the CAM.

1.4.2 |

Adding New Material Where It Is Missing

Using a solid model of the damage, referenced with respect to the same refer-ence system – i.e. a Universal Coordinate System (UCS) – as the original CAD model, makes it possible to use the CAM to create the toolpath that the laser-head must follow to add the material where it is necessary. Chapter 5 presents the whole workflow that needs to be followed to obtain the toolpaths. There are several deposition strategies that can be used to perform the repair but the most important parameters to keep in consideration are related to the quality of the deposited material and its adhesion to the substrate. Chapter 7 presents the workflow used to find the optimal deposition parameters. The standard procedure is composed by three main steps, namely single track, single layer

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and 3D structure [22]. In any case, the additive material needs to undergo vari-ous mechanical tests and microstructural characterizations – e.g. tensile, impact, hardness, adhesion, etc. – to guarantee performances comparable with the ones of the original undamaged component.

1.4.3 |

Restoring the Original Shape of the Object

The last step to complete the repair process consists in removing the excess of material and restoring the original shape of the component. This operation is al-ways carried out inside the same machine – i.e. it is one of the advantages of the hybrid CNC machine – and it is performed by milling. One of the advantages of the procedure developed by this research is that for CNC milling operations the original 3D solid model can be used as reference model. Therefore, there is no loss in resolution. An important aspect that must be taken into account concerns the presence of residual stress and/or thermal deformations. In Chapter 7 this topic is extensively discussed and possible solutions to address the issue are also highlighted.

As described in the conclusions – i.e. Chapter 8 – this is the first time that the repair process has been addressed in this way. The results of this approach are promising and they should be validated in the long term, as this research might pave the way for the cheap, reliable and fast repair of the damaged objects.

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2 Hybrid Machines for Repairing

Overview

In the last few years the technologies able to print metal have grown in the industrial market and the quality of the objects obtained has signif-icantly improved. The possibility of using some of these technologies in synergy with the subtractive capabilities of a milling CNC machine gives rise to a hybrid machine. This chapter will provide an overview of the technologies needed for repair and their pros and cons. Section 2.1 describes the main 3D metal printing techniques with an emphasis on the DLD technology (see Section 2.1.3) used in this research to perform the repair. In Section 2.2 a description of the hybrid machine, the DMG MORI Lasertec 65 3D Hybrid, is reported. Lastly, Section 2.3 presents the software that manages the machine: Siemens NX which is used to perform both additive and subtractive operations.

2.1 |

Metal AM Processes

One of the main aspects that distinguishes this approach from other approaches is the fact that a hybrid machine was used to carry out the repair process. The term

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Chapter 2. Hybrid Machines for Repairing 2.1. Metal AM Processes

“hybrid” in this case means that the machine has the ability both to add material – using Additive Manufacturing technologies – and to remove it by milling.

Only recently this kind of machine became available on the market, open-ing up new perspectives in the industrial field, when a couple of machine tool manufacturers introduced an additive tool into 5-axis milling machines, thus ob-taining a machine able to add and remove material without needing to change the machine. The additive technology used in these machines is called by the manufacturers Direct Laser Deposition (DLD) while the international name is Laser Metal Deposition (LMD). Often this technology is also called Direct Metal Deposition (DMD) or Direct Energy Deposition (DED). The latter is actually a definition that includes all processes where a source of concentrated energy generates a molten pool where the feedstock is deposited. This process can use various types of energy such as laser, electric arc or electron-beam. The feed-stock is usually in the form of powder or wire. Depending on the combinations we obtain, different technologies and nomenclatures as shown in table 2.1

Table 2.1: DED technologies.

Power Source feedstock

Powder Wire

Arc . . . WAAM

Laser DLD/LENS LMD

E-Beam EBAM

The origins of these technologies originate from the welding process. The automation of this process, at the beginning of the twentieth century, has given rise to wire welding machines that can work continuously using processes in which the arc is stable and protected by a protective gas. This technology was patented in 1920 when additive manufacturing and 3D printing did not yet exist.

2.1.1 |

Wire Arc Additive Manufacturing

Recently, exploiting the analogy of this process with other additive technologies such as Fused Deposition Modeling (FDM), these machines have also been used for the construction of three-dimensional objects (layer by layer) [23]. The DED

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technique that uses the electric arc and a wire as a filler material has taken the name of Wire Arc Additive Manufacturing (WAAM) [24, 25]. The “Arc-based AM” has significant potential in the industrial field due to the possibility of re-ducing time and costs in the production of medium/large size objects. This is due to the considerable capacity of material that can be processed (between 1 and 10 kg/h). There are several ways in which the material is melted and de-posited.

Among the most important of them we can list:

Metal Inert Gas (MIG); Metal Active Gas (MAG); Tungsten Inert Gas (TIG); Plasma Arc Welding (PAW).

In the MIG/MAG technologies [26] the filler material also acts as an electrode and the arc occurs between the base metal and the wire, consuming it. In TIG the electrode is fixed and made of tungsten. The filler material is introduced between the electrode and the base material [27]. WAAM is not a net-shape technique because the dimensional accuracy is low (±1 mm) and normally the parts obtained must be machined. It is therefore necessary to foresee a surplus of material to be removed during the finishing phases. The materials that can be used are those that are normally weldable. The chemical/physical properties of the materials used influence the process considerably.

2.1.2 |

Electron Beam Additive Manufacturing

Electron Beam Additive Manufacturing (EBAM) is a 3D printing/cladding pro-cess that uses an electron beam as energy source while the adding material can be either in powder form or as a skein of wire. An electromagnetic field (in vacuum) directs the free electrons in a controlled way toward the working spot. When the electron beam impacts the base material and the filler material, heat is produced in a highly localized manner.

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The wire used typically ranges from 1 to 3 mm in diameter and powder par-ticle sizes are similar to those used in powder metallurgy processes, between 50 and 150 µm.

The electronic beam must be produced and used in a vacuum. The difficulty of producing and maintaining a vacuum is quite high and usually limits the size of the machine and/or makes the required equipment very expensive. While from one point of view this is a disadvantage, from another point of view the vacuum allows the printing of reactive materials with high standards of purity – e.g. aluminium and titanium.

In terms of electron-beam DED systems, EBAM is a technology commercial-ized by Sciaky Inc1. Usually, the applications are very specific and belong to the fields of aeronautics, aerospace, defence or medical.

2.1.3 |

Direct Energy Deposition: LMD, DLD, LENS

LMD, DLD [28] and Laser Engineered Net Shaping (LENS) are Direct Energy Deposition techniques that use a fiber laser as energy source. In this case the laser generates a molten pool on the base material. At the same time, the adding material in the form of powder or wire is injected. The process is very similar to the one described above but in this case the process can be performed in with-out making vacuum. This makes the process cheaper and easier than EBAM. In most cases the working area is covered by a localized shield of protective gas. When working with reactive metal powders – e.g. aluminium and titanium – the whole working chamber must be saturated of inert gas to prevent powder explosion.

Today, the market counts quite a few manufacturers of DED laser-beam based 3D printers. The main manufacturer players in this field include BeAM, DMG MORI, Trumpf, Optomec, InssTek, Relativity and only few others.

A hybrid machine, the DMG MORI Lasertec 65 3D hybrid, was used for this work. This machine combines a 5-axis milling machine with the Direct Laser Deposition technology which uses a fiber laser as power source and metal pow-der as input material. In this work this machine will only be called “Lasertec”.

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Figure 2.1: Direct Laser Deposition schema.

Figure 2.1 shows a schema of how this technology works while in Figure 2.2 a picture of the machine is shown.

Among the most important advantages of the DLD technology we can list:

The deposition speed of the material is quite high if compared to other

Additive Manufacturing processes;

It allows a reduced time-to-market that can also avoid the need of

ware-housing spare-parts;

It can produce components with internal geometries (machined);

Very hard or difficult to process materials can be used (for some materials

it is very difficult to obtain a wire to be use as input material, using powder avoids this problem);

Electrical conductivity properties do not affect the process;

The laser and the powder come out of the same nozzle together with an

inert gas that avoids oxidation;

Different powders can be mixed together to obtain alloys and/or gradients

of composition [29, 30].

These advantages are further increased when Direct Laser Deposition tech-nology is combined with a CNC milling machine. This, as in the case of Lasertec,

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results in a hybrid machine that has the ability to machining the deposited ma-terial alternating the two operations until the desired geometry is obtained. The Lasertec is based on a CNC milling machine that operates simultaneously moving all its 5-axes. This allows it to perform very complex milling operations without requiring a change of positioning of the workpiece on the machine table. At the same time, 5-axes can also provide flexibility in the deposition process, avoiding (at least in part) the problem of undercuts and reachability [31].

This type of machine is perfect for operations such as:

Complete production of a new component; Adding features to an existing object; Repair a damaged or worn part; Cladding a functional surface.

Like any technology, DLD has its pros and cons[32, 33]. Undoubtedly the pros listed above and the variety of operations that can be performed using this technique enhance the freedom of designers to create objects that are at the same time complex but highly functional. Objects that are very often impossible to produce with other available technologies. On the other hand, we must also take into account the issues raised by the Direct Laser Deposition and that inevitably affect in some way also the design of the components. The design can determine if a part is producible or not.

In particular, we must take into account that with the DLD:

A large amount of thermal energy is introduced into the component which

must be dissipated and controlled in some way;

High temperature gradients are created which can generate stress and

de-formation in the produced parts;

Sometimes, the resolution of the deposition is not good enough and it

re-quires subsequent processing (also in the internal features);

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Chapter 2. Hybrid Machines for Repairing 2.2. The Milling Machine

When working with different materials a good compatibility between them

is required.

The hardware of the additive part of the hybrid machine and in particular the laser part is developed by Sauer GmbH, which has more than 25 years of experience in the development of precision laser technology. In addition to this part DLD requires a powder supply system which provides the feedstock for the process. The powder unit controls the amount of material transported to the nozzle by adjusting the outflow of powder from the hopper and the carrier gas that transports it through the pipes. That way it is possible to obtain flow rates up to 1.5 Kg/h and even more.

The laser source is composed by high power and high brightness diodes doped with rare-earth elements – i.e. Ytterbium (Yb) in our case. Doping the diodes using Ytterbium produces laser with a wavelength between 1030-1100 nm that is appropriate to melt metals. The wavelength generated by the fiber laser is within the so-called “near-infrared” spectrum and it is not visible. The power that can create such a laser source is equal to three thousand watts. The laser beam produced is delivered to the DLD head, where mirrors point it toward the axis of the nozzle. Before exiting the nozzle, the laser beam passes through a lens that makes the light converging to a point, called the Standoff Distance (SD), which is located 13 mm outside the nozzle outlet. This point is the same as the one where the powder flow is delivered and where the molten pool will be cre-ated. The working principles are shown in Figure 2.1.

2.2 |

The Milling Machine

The additive part of the Lasertec machine is integrated on a standard milling machine: the DMG MORI DMU 65 monoBLOCK. Figure 2.2 shows the Lasertec machine installed by the ProM Facility Laboratory2.

This machine is equipped with 5 independent axes: three of them are ded-icated to the spindle head movement, while two of them are used to move the worktable. In particular, the X,Y and Z axes are three linear movements that

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Chapter 2. Hybrid Machines for Repairing 2.2. The Milling Machine

Figure 2.2: The DMG MORI Lasertec 65 3D Hybrid machine used for this re-search.

move the tool inside the working volume. The axes named A and C represent two rotations of the worktable. The C axis is the rotation around the Z axis while the A axis is directed along the X vector. In this latter case, the rotation is lim-ited by the presence of the spindle and ranges from−120 to+120 degrees. Such an amplitude of oscillation of the tilting table ensures that even hard-to-access areas can be machined with ease.

The spindle is equipped with a standard HSK-A63 holder that can mount both the laser head and standard tools. The rotation of the tools is guaranteed by a powerful electro-spindle able to provide 13 KW of power at a maximum speed of 14000 Rotation Per Minute (RPM). Lasertec is able to change tools au-tomatically and has an integrated tool magazine with sixty places. The laser head is also treated as a tool, but it has a dedicated storage area which also

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con-Chapter 2. Hybrid Machines for Repairing 2.2. The Milling Machine

tains the laser source and the powder supply cabinet. In Figure 2.2 the right part of the picture shows the AM tool storage.

The working volume is variable depending on whether Additive Manufac-turing processes or milling operations are being carried out. This is due to the fact that the additive head (usually) has a larger size than the milling tools. The maximum working volume that can be machined in the case of AM opera-tion is a cylinder with a diameter of 500 mm and a height of 400 mm. The axes are powerful enough to handle objects weighing up to 600 kg.

The Lasertec, like all CNC machines, needs a “brain” that translates the in-formation from the part program into signals that drive the actuators. To do this the Lasertec is equipped with a set of SIEMENS 840D Programmable Logic Controller (PLC): controllers that can manage up to 10 channels and 31 axes each.

The user interface on board the machine is a software developed by DMG MORI called Celos®. This software acts as an interface between the machine tool and the other divisions of the company and creates the prerequisites for a system in line with industry 4.0 needs. Celos® includes various apps that simplify the work of the operator of the machine – e.g. drawings/documents management, messenger, on-line connection, tool management, remote assistance, machine status and much more.

Celos® also offers an interface to SIEMENS ShopMill, a software that is use-ful for programming simple milling jobs directly on board the machine. By us-ing this tool, one is allowed to program the millus-ing machine graphically without needing to write a part-program from scratch or without using a CAD/CAM software.

The operations that can be performed using ShopMill do not allow the si-multaneous use of the 5-axes. Even if this restricts the potential of the machine, with only few steps on a touch screen one can create a part-program that con-tains the most important machining operations for chip removal – i.e. roughing, finishing, drilling, threading, etc. ShopMill also allows to visualize a graphic simulation of the milling process before the actual execution of the machining. Unfortunately, at the time of writing, no functions have yet been implemented to simulate the additive manufacturing process.

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Chapter 2. Hybrid Machines for Repairing 2.3. CAD/CAM

2.3 |

CAD/CAM

To perform more complex operations than those listed in the previous section, a CAD/CAM software must be used, which allows to create part programs in an easier way. A CAM is a computer program that performs the calculation of toolpaths starting directly from the solid model of the component to be pro-duced. These software allow to calculate the coordinates that a tool must follow in order to remove the exceeding material. The Direct Laser Deposition works in the opposite way – i.e. adding material. For now, most commercial CAMs do not allow to compute operations for adding new material to a model.

To manage the Lasertec effectively, SIEMENS has developed a completely new part of its SIEMENS NX CAD/CAM software. With this solution the tool-paths for both removal and deposition operations can be calculated. NX is also able to simulate the process by displaying the three-dimensional model of the operations performed. An advantage of this approach is that it is also possi-ble to simulate hybrid processes – i.e. processes consisting of both subtractive and additive tasks. The material added will become part of the existing three-dimensional model and can be re-machined with other types of processing in a rather easy way.

It is not the purpose of this work to describe all the material removal opera-tions that can be carried out with Siemens NX. However, the operaopera-tions concern-ing addition operations will be briefly described to make the next parts of this research clearer. More details about the AM strategies are described in Chap-ter 5.

The strategies for adding material are classified according to both the number of axes involved and the geometry to be obtained. In addition, the operations are also differentiated by the use of a single material or the simultaneous use of two different types of powder. The types of structures are: solid body, thin wall and tube.

Each one of these strategies differs significantly from the others. Each kind of structure can be deposited either on a flat surface using 3-axes or on a free-form surface – i.e. using all the 5-axes at the same time. Finally, to generate

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(a) (b) (c) (d)

Figure 2.3: Some infill strategies. a) Stripes, b) Meanders, c) Spiral, and d) Con-centric. All of them can be executed with or without the contour (in red).

axial-symmetrical shapes by adding material in radial direction with respect to the revolution axis, there is a dedicated function described in Section 5.2.

The CAM software allows users to set all the parameters needed to define the strategy used to create the new part. For example in the case of the “solid body” it will be necessary to define how each layer is deposited which means to set the layer thickness, the hatching distance, the deposition speed, the laser power and many other parameters. The most common deposition strategies are the following:

Contour profile + stripes infill; Contour profile + meander infill; Spiral path;

Concentric (offset from the contour).

Figure 2.3 shows an example of the strategies listed above. After the last CAM update – i.e. Siemens NX 12.0 – some new building strategies have been implemented in the software to avoid some issues of reachability of the working area and other geometric consistencies. For example, the simultaneous construc-tion of elements that are close to each other has been made possible. This was a challenge before, because the construction of one part made impossible the realization of the others. The problem has been solved generating a part pro-gram that will alternatively deposit a layer for each part to be built. This way, the parts grow simultaneously avoiding that one of them obstructs the access

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to the others. Other features are described in Section 5.2 while the procedure to determine the best process parameters is explained in Chapter 7

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3 3D Models Digitization

Overview

One of the biggest barriers in automating the repair process is that it is difficult to obtain a solid model of the volume to be repaired. This re-search focuses on solving this challenge. The present chapter addresses the first step in the repair process, which consists in the digitization of the damaged geometry. In Section 3.1 several digitization techniques are described and their pros and cons are listed and discussed. Once the three-dimensional model is obtained using one of the technologies, the mesh obtained must be corrected and made “watertight”. Section 3.2 ex-plains how to align the mesh obtained to a 3D CAD model or to another mesh. The digitizing process is not always easy so Section 3.3 presents practical solutions that can make both, the scanning procedure and the subsequent operations necessary to obtain an effective repair, easier.

3.1 |

Digitization Techniques

To repair an object, the first essential step is to identify the damage. This can be done in many ways such as visual inspection, by dimensional probing ma-chines, by tomographic scanning and many others. In the case of manual

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re-Chapter 3. 3D Models Digitization 3.1. Digitization Techniques

pair, the identification of the damaged area and the size of the damage is dele-gated to a specialized worker who, with his experience, analyses the component and decides where to put the new material. This has both pros and cons. The decision-making flexibility of a human being is much better compared to that one of a computer algorithm. On the other hand, a worker is more prone to errors and is influenced by external factors in his or her evaluation. The whole evaluation process depends on the initial analysis and how that is interpreted by the person who will physically perform the repair.

The automation of repair operations is the main purpose of this work. The C++ software called DUOADD has been developed specifically for the auto-matic identification of the volumes that need to be repaired and, later, to make the results available to the CAM software that produces the toolpaths.

It is therefore clear that the main prerequisite for an automated repair process is to have digital information of the geometry of the component to be repaired. In addition, in order to know how much material is required, it is essential to know what the original shape of the component was. The CAD model of the part can be used to obtain this information. This can also be used as reference for the subsequent reworking of the component by milling.

The digitization of an object can be done with various technologies. Among the most important the following can be listed:

Mechanical probing; Computed tomography;

Three-dimensional scanning by optical methods.

3.1.1 |

Mechanical Probing

Machines that rely on mechanical probing – e.g. Coordinate Measuring Ma-chine (CMM) – use a touching probe to touch the object at multiple points (see Figure 5.6). Each time the probe hits a surface, it returns the position of the points hit to the software. The CMM is usually used for dimensional and ge-ometric inspection of objects due to its accuracy and precision. The points

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col-Chapter 3. 3D Models Digitization 3.1. Digitization Techniques

lected by the machine can be used in several ways: for distance measurement, for creating and measuring geometric features, for reverse engineering and also for free-form surface reconstruction.

In the latter case the touching probe maps the (external) surface of an object by detecting multiple points on the entire surface. This method is very time-expensive especially when the surface needs to be digitized with a good reso-lution. Only to give an example if one tries to discretize a plane of 1 cm2 with a resolution of 0.1 mm we should touch the surface ten thousand times. It is therefore evident that mapping large objects is extremely time-consuming. An-other negative aspect of this technique is that the touching probe has a given dimension that will inevitably affect the measured values. Furthermore, since physical contact between the measured surface and the probe is necessary, many geometries are impossible to map – e.g. narrow cracks or concave edges. An-other limitation of this technique is that it is not possible to investigate soft or deformable objects because the pressure applied by the probe – although small – influences the measurement.

The points collected during the process must then be converted to a triangu-lar surface – i.e. a mesh. Because of this limitations, there are technologies that allow to perform a three-dimensional reconstruction in an easier and faster way.

3.1.2 |

Computed tomography

Developed in principle for medical purposes, Computed Tomography (CT) has quickly established itself as an essential tool for research and diagnosis. Thanks to its enormous potential, this technology found immediately the interest of the military and defence sector. It was only later, and after considerable investment, that this technology found application in industry.

In recent years, the exponential increase in computing power and the im-provement of related technologies has significantly increased the resolution, reaching micro-metric scales. Today, the large size that can be inspected and the flexibility of the tomographic systems, together with the very high defini-tion of the analysis, make tomography one of the most advanced techniques in non-destructive testing. The materials that can be inspected are the most varied,

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Chapter 3. 3D Models Digitization 3.1. Digitization Techniques

from solid and massive metal components to polymeric assemblies and com-posite materials, while applications cover the most disparate fields. For exam-ple, a Computed Tomography that uses a 225 KV X-ray radiation source – with a 225 W of target performance – can inspect a cylindrical volume which has a di-ameter of 300 mm and a hight of 350 mm. Inside that working volume the max-imum permissible workpiece weight is 50 Kg. Such kind of machine can inspect a component with a linear accuracy up to 10 µm depending from its size and weight.

Using a Computed Tomography, one is allowed to perform the internal and external dimensional inspections achieving micrometric resolutions [34]. It is also possible to compute density, volume and continuity analysis of an object.

The working principle is to collect and record multiple radiographic projec-tions of the component under inspection. This is achieved by placing the object on a rotary table and radiographing it while it is rotating. The thousands of X-rays acquired by the digital detector are then processed by the reconstruction software that returns the volumetric image of the component, including all in-ternal details.

The CT is an outstanding tool that can directly return the mesh of the investi-gated object including all the internal features, not observable directly from the outside or difficult to access [35, 36]. The resulting model can be exported as a mesh and it is the ideal input for DUOADD.

The negative aspects of this technology are few but not negligible. The ma-chine uses an X-ray source that is potentially very dangerous for the operator. In addition, Computed Tomography is very expensive and hardly available for ordinary applications.

3.1.3 |

Three-Dimensional Scanning by Optical Methods

For the reasons and limitations described in the previous sections, the solution adopted for the three-dimensional reconstruction of the object was a 3D optical scanner. In particular a structured light 3D scanner was used. There are many types of 3D scanners that use light to reconstruct three-dimensional geometries. An incomplete list of the main technologies is the following:

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(a) (b) (c)

Figure 3.1: Example of geometry digitization. a) The damaged part digitized by a structured-light scanner, b) The output that is a triangulated surface (mesh) and c) The same model rendered to highlight the damage.

Lidar;

Structured light scanner; Stereovision.

The time-of-flight laser called “lidar” is an active scanning device that uses a laser beam to measure distances and detect three-dimensional morphology of laser-reflective surfaces [37]. The laser sensor calculates the distance between the instrument and a surface by accurately timing the travel time of a light impulse. Since the speed of light c is a known constant, the traveling time determines the length of the light travel, which is twice the distance between the scanning device and the reflecting surface.

Then, the laser scans the investigated area one point at a time, changing the direction of inspection to explore different points. The laser beam can be rotated using mirrors that direct it towards the point to be investigated or the laser-emitting diode can be rotated directly.

Time-to-flight scanners [38] can measure up to hundred of thousand points per second, with an accuracy ranging from a few millimeters to a few centime-ters. It is interesting to note that the distance from the center of the instrument to the scanned surface can reach several hundred meters. The output of this type of scanner is usually a point cloud obtained from a single observation point.

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O

l

O

r

P

_l

_P

_r

P

E

r

E

l

Figure 3.2: Working principle of stereovision. Observers Ol and Orsee the point

P respectively at the projection points Pland Pr. Points Eland Erare the epipoles

and are used to rectify the view.

The resolution of this type of scanner and its detectable dimensions make it hardly suitable for the reconstruction of a mechanical component.

Much more appropriate for this purpose is the structured light scanner which uses a light source – to project patterns onto the object to be reconstructed – and a camera to “see” how the light warps on the surface of the piece [39, 40]. The projected patterns can be continuous or discrete. The difference is usually due to the source that projects the pattern. If a projector is used the pattern is continuous while if the source is a laser the pattern will be discrete.

The points detected by the camera are transferred to a software that, through triangulation operations – i.g. in a similar way to how the human eyes work – can understand the depth to the scene. For this reason there must always be a certain misalignment between the camera and the source. The process is usu-ally very fast and precise and allows to capture a considerable number of points every second. One of the negative aspects of this technique is that some surfaces can not be reconstructed. In fact areas that are not “seen” at the same time from both the camera and the light source can not be investigated. Furthermore, re-flective or dark pieces can compromise the measurement because they can make the pattern recognition difficult. The points transmitted to the software are usu-ally automaticusu-ally transformed into mesh.

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Figure 3.3: The Creaform Metrascan 3D scanner used in this research1. Lastly, stereovision is a technique that uses epipolar points on images of the same object taken from different angles [41]. Figure 3.2 shows the working prin-ciple used by stereovision to define the distance of a point from the cameras. This technique allows to identify the spatial position of all the points of interest of the object under consideration – i.e. points that can be uniquely identified on the object. To acquire a greater number of points in some cases, makers are ap-plied to the object to be reconstructed. These markers are sometimes projected on the surface to be reconstructed using specific patterns. The projection has the advantage to not add thickness to the investigated objects as happen with physical markers. Whatever the technology used, the output to be obtained is a complete mesh of the damaged component.

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Since no computed tomography was available for this research, it was de-cided to reconstruct the geometry of the damaged object using a structured light scanner: the Creaform Metrascan 3D. The system shown in Figure 3.3 allows the reverse engineering and dimensional inspection of assemblies or final products. The scanner can reconstruct components with dimensions ranging from a few cubic millimeters up to 16 m3of volume with an accuracy of 0.064 mm.

This system does not require a rigid measurement setup and maintains the same level of performance despite environmental instabilities. The scanner con-sists of two parts, one fixed and one mobile. Figure 3.3 shows only the mobile part. This device can be held by an operator who, by pointing the object to be scanned from different directions, acquires a huge amount of points from the surface. The scanner is able to detect and store 480 thousand points per second thanks to the use of more than one laser beam. The Metrascan creates seven intersections on the object surface at the same time. This improves both the scanning speed and the accuracy. The scanner software – i.e. VXScan – directly exports a mesh of the scanned object. Before exporting the model, it is possi-ble to perform a surface repair – e.g. close holes, remove unwanted parts, etc. This fact is important because the 3D model obtained, in order to be used by DUOADD, must have some fundamental characteristics: it must be watertight, consistently oriented and aligned with regards to the original CAD model.

Watertight – or manifold – means that the mesh obtained is closed – i.e. with-out holes, it is free of geometric inconsistencies such as double vertexes, overlap-ping triangles, intersecting triangles or other broken geometries. This condition is mandatory when voxelizing meshes into occupancy grids. If a mesh is error-free all the faces of the triangles that compose it are consistently oriented so that an inner and an outer side can be unequivocally identified. In a triangular sur-face with this characteristic all the triangles adjacent to any sur-face must have the same orientation of the face itself.

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Chapter 3. 3D Models Digitization 3.2. Meshes Alignment

3.2 |

Meshes Alignment

To effectively discretize the two meshes – i.e. the original CAD model (trans-formed into a mesh) and the model of the damaged component (obtained by 3D scanning) – it is essential to provide to DUOADD two aligned models. The two meshes, having slight differences – i.e. due to wear, damage or scale – need to be aligned using algorithms that take this aspect into account. There is a large variety of software that can perform the alignment of the two models. To carry out this research, three of them have been tested:

Meshlab1;

CloudCompare2; VXModel3.

Meshlab [42] is a great open-source project dedicated to the manipulation of surfaces. This software is equipped with many tools for inspection, cleaning, repair, modification and rendering of not-so-small unstructured 3D models that arise in the 3D scanning pipeline. Meshlab uses an algorithm called Iterative Closest Point (ICP) [43] to perform the alignment. This algorithm minimizes the distance between two “points cloud” consisting each of the vertices of the two meshes. Figure 3.4 shows an alignment example obtained using Meshlab. The overlap of the two models is very good on all the undamaged areas.

CloudCompare is another very interesting open-source project that offers tools similar to those already listed for Meshlab but with an important differ-ence in the alignment between two objects. This software offers the possibility of using different types of algorithms to minimize the distance between models. There are ICP, point-cloud to mesh and mesh to mesh algorithms. This gives the user the freedom to select the most appropriate method. In addition, Cloud-Compare allows to align surfaces while neglecting parts of the mesh – i.e. in-complete parts, with few vertices or the damaged area. The fact that a portion

1_{http://www.meshlab.net/} 2_{http://www.danielgm.net/cc/} 3_{https://www.creaform3d.com/}

Additive manufacturing for repairing: from damage identification and modeling to DLD processing