Sustainability Assessment of Additive Manufacturing Processes

211 Pages·2017·9.52 MB·English

by  Vincenzo Lunetto

Sustainability Assessment of Additive
Manufacturing Processes
Vincenzo Lunetto
Master's Thesis
Submission date: July 2017
Supervisor: Knut Sørby, MTP
Norwegian University of Science and Technology
Department of Mechanical and Industrial Engineering

Table of contents
Abstract
Acknowledgements
Glossary of terms
Table of contents
Chapter 1: Introduction
Chapter 2: Human safety risks due to different routes of exposure to metal powders
2.1 Human safety risks
2.1.1 Animal experiments
2.1.2 Case studies of human exposure
2.2 explosions, fires, protection, safety rules
Chapter 3: Process phenomena models
3.1 Cost estimation and energy demand models
3.2 Footprints emission models
3.3 Safety indexes
Chapter 4: Case study
4.1 Topology optimization study
4.2 Experimental study
4.3 Application of the models
4.3 Considerations on the obtained results
Appendix A: Respiratory protection for some metal substances exposed on Chapter 2
Reference list

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Abstract
Additive Manufacturing (AM) processes were developed in the 1980s to reduce the time for the
realization of prototypes. Nowadays, AM processes are considered as real manufacturing techniques
suitable to build end-use products. As for any new technology, research efforts aiming to process
planning and optimization within a sustainable development framework are needed. In particular, the
application of the sustainable manufacturing principles requires the creation of products that use
processes that minimize negative environmental impacts, conserve energy and natural resources, are
safe for employees, communities, and consumers, and are economically sound. In this work, the three
aspects of sustainability, namely environmental, economic and social sustainability are critically
analyzed for AM. For each topic, an analysis of models that can evaluate the consumption of energy
and CO2 emissions, costs and the impact on workers' health is carried on. The functional redesign of
a mechanical object, with the aim of reducing its mass and trying to keep unchanged the constraint
conditions imposed is accounted for. A critical analysis of the impacts on workers' health focusing
on the hazardous aspects related to the animal and human exposure to metal powders is presented.
Considerations on body weight, cancer, cardiovascular, dermal, endocrine, gastrointestinal,
hematological, hepatic, musculoskeletal, neurological, ocular, rheumatologic, renal, reproductive,
respiratory effects are reported for different elements as Cobalt, Chrome, Nickel, Titanium and for
each kind of exposure (inhalation, oral, dermal). Methods for the comparison and evaluation of an
inventory’s dissimilar pollution loads have been proposed and are critically analyzed. Considerations
on the possible workplace maintenance techniques to be followed for handling metal powders (such
as those according to ASTM, NFPA and OSHA standards) are also evaluated. Whatsoever, it is
possible to consolidate a new technology in the industrial scenario only if it is economically
sustainable and profitable. A critical study of the evolution of the main economic models is reported.
In addition, approaches that can take into account also the pre-process and post-process phases as
well as the production of different objects in the same built are presented. Moreover, in this work,
several examples and comparisons of the sustainable performance of different AM techniques are
discussed. A critical study of models that can evaluate the amount of CO2 emissions during the
production of part via AM techniques is studied. A model for the calculation of CO2 emissions from
the consumption of electricity is also detailed. The above mentioned models have been applied to
selected case studies before and after the redesign phase. NX and ANSYS software were used to
perform the geometrical optimization. An experimental study was then conducted to validate the
results.

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Acknowledgements
When I received my Bachelor's degree after studying in the Università degli Studi di Palermo, I
decided to move to Turin in order to continue my master studies. I had different feelings about that
experience: I was certainly excited about living in a multicultural context, but at the same time aware
that I left behind the place where I had lived until then, my home Sicily.
Since my arrival at the Politecnico di Torino, I found a dynamic and enriching reality. I immediately
realized how much this would have helped me and I tried to work hard to take advantage from this
new life. In Prof. Luca Settineri and in Dr. Paolo C. Priarone, I found two guides who accompanied
me in my studies, until to become my supervisors for my master's thesis. They have been teaching
me so much about overcoming various problems in my university path. I want to thank them for the
opportunities they have been giving me, but above all for their ever-present availability, even when
it had not to be, which went beyond the simple student-supervisors relationship.
I developed my thesis at the Norges Teknisk-Naturvitenskapelige Universitet, through the Eramus
program. In this way, I had the opportunity to live in Norway, a country rich of culture and amazing
landscapes. I was immediately welcomed by the people of the place, but moreover I found in the
person of my Prof. Knut Sørby, a warm welcome and a great guide for my work. I admit that I often
asked for more than how much I should have received, but my Supervisor has always been available
and patient in helping me.
Finally, I do thank my parents, Gioacchino and Anna, and my dearest friends. Both gave me the
ambition and the desire to improve always myself, the tenacity to work hard, the patience to overcome
obstacles.

Vincenzo Lunetto
Trondheim, 07th of July 2017

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Glossary of terms
AHP - Analytical Hierarchy Process HMILD - Hard Metal Interstitial Lung
Disease
AM - Additive manufacturing
HPDC - High Pressure Die Casting
BAL - Broncho-Alveolar Lavage
HRCT - High Resolution Computed
CDC - Centers for Disease Control
Tomography
CERs - Cost-Estimation Relationships
HTP - Human Toxicity Potential
CES - Carbon Emission Signature
IM - Injection Moulding
CF - Cutting Fluid
LCA - Life Cycle Assessment
CI - Confidence Interval
LDH - Lactate Dehydrogenase
COD - Chemical Oxygen Demand
LOAEL - Lowest Observed Adverse Effect
CM - Conventional Manufacturing Level
DFE - Design For Environment LS - Laser Sintering
DMLS - Direct Metal Laser Sintering MCV - Mean Connectivity Value
EBM - Electron Beam Melting MDHS - Methods for the Determination of
Hazardous Substances
EIM - Emissions Inventory Module
MIPS - Material Input Per Service-unit
ELU - Environmental Load Unit
MIT – Minimum Ignition Temperature
EMCL - Emulsion Mist Cooling/Lubrication
MMAD - Mass Median Aerodynamic
EPS - Environmental Priority System
Diameter
FDM - Fused Deposition Modelling
MMEF - Maximum Midexpiratory Flow
FEV1 - Forced expiratory volume in the 1st
MQL - Minimum Quantity Lubrication
second
MRL - Minimal Risk Level
FM - Finish Machining
MRR - Mean Reciprocal Rank
FVC - Forced Vital Capacity
MSDS - Material Safety Data Sheet
GDP - Gross Domestic Product
MTD - Maximum Tolerable Dose
GHG - Greenhouse Gas
NEL - No-Effect Level
GIP - Giant Cell Interstitial Pneumonitis
NOAEL - No Observed Adverse Effect Level
GM - Geometric Mean
OR - Odds Ratio
GSD - Geometric Standard Deviation
PEF - Peak Expiratory Flow Rate
GWP - Global Warming Potential
PEL - Permissible exposure limit
HIP - Hot Isostatic Pressing
RM - Rapid Manufacturing
HHS - Health Hazard Score
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RP - Rapid Prototyping SLM - Selective Laser Melting
RTECS - Registry of Toxic Effects of SLS - Selective Laser Sintering
Chemical Substances
SMR - Standardized Mortality Ratio
SEP - Swiss Eco-Point
SMs - Subtractive Methods
SGOT - Serum Glutamic Oxaloacetic
SPI - Sustainable Process Index
Transaminase
STL - Stereolithography
SGPT - Serum Glutamic Pyruvic
Transaminase TM - Toxicity Measure
SL - Stereolithography UPLCI - Unit Process Life Cycle Inventory

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Chapter 1: Introduction
Early additive manufacturing equipment and materials were developed in the 1980s. In 1984, Chuck
Hull of 3D Systems Corporation developed a prototype system based on a process known as
stereolithography (STL), in which layers are added by curing photopolymers with ultraviolet light
lasers. Hull defined the process as a "system for generating three-dimensional objects by creating a
cross-sectional pattern of the object to be formed". Hull's contribution is the design of the STL file
format widely accepted by 3D printing software as well as the digital slicing and infill strategies
common to many processes today. Additive manufacturing processes (AM) were created to reduce
the time for the realization of prototypes and for many years have assumed the name of "rapid
prototyping techniques". Depending on which stage of product development is interested, it is
possible to distinguish the following types of prototypes: conceptual, functional, technical, pre-series;
the goals of each are obviously different as well as the material and the manufacturing technology
used for the construction. Subsequently it was recognized the potential of this technology, and today
AM processes are considered as real manufacturing techniques that are able to build end-use products.
Moreover additive technologies can also be effectively used for rapid tooling and rapid casting
operations. Figure 1 presents a possible classification of AM processes, according to the type of the
raw material used.

At the current state of the art, technologies that work powders are the only ones that are able to realize
metal products. Selective Laser Sintering (SLS) is an AM technique that uses a laser as the power
source to sinter powdered material (typically metal), aiming the laser automatically at points in space
defined by a 3D model, binding the material together to create a solid structure. It is similar to Direct
Metal Laser Sintering (DMLS); the two are instantiations of the same concept but differ in technical
details. Selective Laser Melting (SLM) uses a comparable concept, but in SLM the material is fully
melted rather than sintered, allowing different properties (crystal structure, porosity, and so on).
Electron Beam Melting (EBM) is an AM technique where metal powders can be consolidated into a
solid mass using an electron beam as the heat source. EBM technology manufactures parts by melting
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metal powder layer by layer under vacuum, which makes it suited to manufacture parts in reactive
materials with a high affinity for oxygen, e.g. titanium. Compared to SLM and DMLS, EBM has a
generally superior build rate because of its higher energy density and scanning method. All these
processes work by fusing a powder bed in each scan cycle, with a thickness of a few hundreds of
micrometers.
Some advantages of additive techniques are the ability to make objects of complex shape, in order to
have less assembly problems, less spare parts in stock, less complexity in business because of less
parts to manage. There is also the advantage that no production of tools is necessary. However,
compared to other manufacturing technologies the available software is a limiting factor, machines
need a high calibration effort, a rework of parts is often necessary, often it is necessary to design the
support structures, the building time depends on the height of the part in the building chamber
(Lindemann et al., 2012). It is possible to understand the peculiarities of this technique by comparing
it with other manufacturing techniques. Additive techniques realize the final product by progressive
addition of material, on the contrary machining operations are subtractive techniques. AD products
have lower density with equal volume, because of the internal porosity. It is also possible a functional
redesign of the part by minimizing the weight where loads are minor, maintaining the overall
mechanical properties comparable to the ones of the pieces made for machining; one example of this
new approach is the production of lightweight structural parts for robotic applications reported in
Manfredi et al., (2013). The achieved surface quality is not usually adequate to meet the strict
requirements of the aerospace and automotive industry (Priarone et al., 2012). Comparative studies
present some differences between a traditional manufacturing route (injection moulding) with layer
manufacturing processes (stereolithography, fused deposition modelling and laser sintering) in terms
of the unit cost for parts made in various quantities. Figure 2 illustrates the breakeven analysis
comparing conventional High Pressure Die Casting (HPDC) technique with SLS for the selected
landing gear structure. Especially the breakeven point is estimated for a production of about 42
assembly components made of aluminum alloy (21 aircraft models). SLS of aluminum landing gear
structures appears economically convenient for this application. But the convenience is more than
economical; landing gears are produced within about 2.5 days from the availability of the 3D CAD
model, while time to produce moulds for impression die forging and to get started the production is
on the order of weeks (Atzemi et al., 2012).

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This shows how AM techniques are able to create niche objects belonging to the biomedical sector
and particular products realized in small-scale to be used in the automotive and aerospace industry.
Additive manufacturing techniques are spreading thanks to their potentialities, however like any new
technology, it is necessary to study them in order to be able to structure and optimize these processes.
There are several critical issues in these processes, especially with regard to topics such as toxicity,
safety and environmental problems. Among the additive fabrication processes that ones that use metal
powders as raw materials are particularly sensitive to these issues and the first part of this work is a
literature review focused on the hazard aspect concern this kind of AM processes. Laser sintering and
melting technologies normally use fine particle size distribution powders (10-45 μm) that offer good
flowability and purity, instead Electron Beam Melting technology requires coarser powders (45-106
μm) with exceptional flowability. In any case the size of the powders are very small and it is very
easy for workers to be exposed to them by inhalation. Obviously an oral and dermal exposures is also
possible. Because this technology is relatively new, in literature there are a few experiments only on
animals according to different routes of exposure, but there are not enough documents on workers in
these areas. However, the possible routes of exposure and the type of materials used suggest that the
effects on human health could be similar to those of workers in the metallurgical industries such as
production of alloys and powders, workers subject to fumes from welding and electrolysis processes.
It is therefore necessary to use appropriate equipment for the cleaning operations of the work areas
and to carry out the operations of adduction of the new powder in the machine stock. Other problems
are connected to the high possibility of fire related to the use of sensitive powders to combustions
reactions such as aluminum and titanium powders. Because of the very small size of these powders,
the ratio between the exposure surface to the entire volume is very high and the reactions may occur
in a very violent manner such as to generate powerful explosions. That is why many organizations
are working to standardize the safety rules to be followed and provide the threshold parameters of
exposures for the workers in these manufacturing sectors. It is possible to consolidate a new
technology in the industrial scenario if it is economically sustainable and profitable. This is one of
the challenges that the additive processes have to win, and there are many studies in the literature
about cost models to better characterize these processes. Continuing the comparison between additive
processes and subtractive processes it can be observed as when complex-geometries have to be
manufactured, the additive manufacturing approach could be the best strategy, if it enables a larger
amount of material savings than conventional machining. Vice versa, when a small amount of
material has to be machined-off, the high energy intensity of an AM process has a negative effect on
the performance of the process. The same authors consider the environmental impact of the additive
and subtractive manufacturing approach, focusing their attention to the whole life cycle of their case
study product. Although AM and SM offer several unique advantages, there are technological
limitations such as tolerance and surface finish requirements; tooling and fixturing, etc. that cannot
be met by a single type of manufacturing. Some authors are going to study economic models for a
new hybrid method where additive manufacturing and subtractive methods are integrated through
composite process planning in order to be able to obtain the best advantages from each one of the two
techniques (Manogharan et al., 2015).

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