Additive Manufacturing (AM) is any of various processes of making three-dimensional solid objects from a digital file. Unlike subtractive manufacturing methods that start with a solid block of material and then cut away the excess to create a finished part, additive manufacturing builds up a part (or features onto parts) layer by layer from geometry described in a 3D design model. For many decades, AM processes have been used for rapid prototyping. Over the last few years, additive manufacturing has gained incredible interest in all industry facets: from aerospace applications to simple one-off consumer home builds. This technology has immense versatility and flexibility, due to its ability to create complex geometries with customizable material properties. Discover how the additive manufacturing processing of metals makes it possible to design and build lightweight parts in real time and understand potential of heat treatments in vacuum for 3D printed parts.

1. Fatigue and fracture behavior of additively manufactured
metals after heat treatment in vacuum
Summary
The design and development of components for structural applications in industrial sectors such as
aerospace, energy production, automotive and biomedical, requires information on the mechanical
performance actually achievable under severe conditions of use. In the case of metal additive
manufacturing (AM), a disruptive technology for custom part fabrication, such information is still
predominantly limited to static conditions. This presentation shows new experimental results on the fatigue
and fracture performance of metals such as Ti6Al4V and Inconel 718 produced with the selective laser
melting technology.
Prof. Gianni Nicoletto – University of Parma & TP Engineering - gianni.nicoletto@unipr.it

2. An interdisciplinary group at the University of Parma – Italy – is active
in the field of metal additive manufacturing.
> Gianni Nicoletto - Professor of Machine design
Univ of Parma – President and founder of academic
spin-off company TP Engineering srl
> Anellino Stocchi – TP Engineering srl – Product
development – Material testing Lab
> Giacomo Baruffaldi - TP Engineering srl – CAE
Analyst – CAE Lab
> Graduating students and PhD students develop
projects and theses in the field of AM .
> Long-term partnership with BEAM-IT, a leading
iItalian AM service company.
> On-going cooperation with universities and
international research centers in the field of AM.
> Metallurgy
Lab
> Materials
testing Lab
> CAE Lab
MEMBERS LABS

3. Our activity in the broad field of AM is focused on three specific
aspects and the close cooperation with technology partner BEAM-IT.
DESIGN: understand and exploit the benefits of AM technology in product
innovation; test and apply software design tools.
PROCESS/EQUIPMENT: collaborate in process optimization with the
BEAM IT technology partner that operates metal AM systems
MATERIALS : investigate the process / material / properties relationships;
quantify fatigue and fracture performance; develop test methods for AM parts.
QUALIFICATION: cooperate with the BEAM IT partner in the development
of design and qualification methods for the AM parts.
KNOW-HOW TRANSFER: Academic teaching to engineering students, organization
of courses and workshops on AM, know how transfer to industry
AREAS OBJECTIVES

4. Topics
Materials and technology
Ti6Al4V, Inconel 718, AlSi10Mg
DMLS EOS System; Renishaw; SLM Solutions, EBM ARCAM
Fatigue of AM metals
Influence of process, heat treatments, surface state, notch effcts, directionality
Standard specimens vs Minispecimen technology
Fracture mechanics of AM Metals
Fatigue crack growth, fracture toughness
Influence of process, heat treatments, directions
The aim of our activity is the development of know-how supporting the
qualification of metal AM parts for load-bearing applications.
AM-specific fatigue design
methods
Safe-life approach
Damage tolerant approach

5. Up-to-date metal AM systems operated by BEAM-IT were used for the
production of high quality specimens and parts for testing.
Source: BEAM-IT
> 20 years experience as service provider in rapid
prototyping and additive manufacturing
> In the last 5 years, concentration and investments in
metal powder bed fusion systems.
> Currently 20 metal AM systems are installed: DMLS
EOS M260/M290; DMLS EOS M400; Renishaw
AM250, SLM 280, SLM 500, ARCAM EBM Q10
> Largest AM service provider in Italy.
> Main industry sectors served: aerospace,
motorsport, dental, energy, mechanical, biomechanics
etc.
Facts and AM systems of BEAM-IT
FACTS ABOUT BEAM-IT AM SYSTEMS AT BEAM-IT

6. Fatigue assessment capability is key for acceptance of metal AM in
structural-bearing applications: the aerospace sector.
> The largest percentage of jet
engine failures are attributed to the
fatigue loading conditions.
> Structural design of aerospace
components combines demanding
lightweight and fatigue strength
requirements.
> Stress levels and material quality
affect the probability of fatigue
failures.
> Metal AM technology has to reach
material quality adequate for
certification
Structural failures in aerospace
ISSUES STRUCTURAL FAILURES IN AEROSPACE

7. Currently the AM process needs to be tailores to specific product
requirements in a lengthy development process.
Complexity of AM production process
Source: Roland Berger

8. Metal additive manufacturing allows the straightforward implementation of
lightweight design principles. But what about part qualification?
AM part design and qualification
PART QUALIFICATION
Design: Topological optimization.
LIGHTWEIGHT PART DESIGN
AM Metal part
> The typical metal AM part shows geometric
complexity and specific material and
technological features currently negatively
affecting qualification for structural
applications.
For example
> Surface quality of hard-to-access areas ->
as-built surface effects
> Location-specific and directional properties
> High number of Kt features (organic
shapes, lattice)
> Inspectability challenges (i.e. lattice,
cavities)

9. Two are the AM metals that have been investigated in fatigue.They both
benefit of suitable post fabrication heat-treatments.
> Ti6Al4V is widely used Ti alloy in aerospace.
This α-β titanium-based alloy accounts for 50%
of total titanium usage.
> Typical applications: airframe, fasteners, discs,
hubs, spacer, seals, compressor blades,
structural parts and complex turbine engine
components.
AM metals
Ti6Al4V
Source: Farinia
Inconel 718
> Inconel 718 is the predominant super-alloy in
the world and accounts for up to 50% of the
weight of aircraft turbojet engines.
> It is considered as a refractory super-alloy since
it can be used permanently above 600°C.
> It shows good creep and rupture strength with a
high resistance to fatigue.
Ti6Al4V

10. The fatigue testing activity generates material data in relation to many
influencing factors to be used for AM part designing .
Fatigue testing and material data
FATIGUE TESTING S/N CURVE
> The fatigue strength is a material property
graphically represented by the Woehler curve.
> The curve links the number of cycles (N) and the
applied stress amplitude (sa) according to the
relation
sa = C (N)b
where C and b are fitting paramters
> Fatigue strength is dependent on many material
(strength, surface quality, size ..) and
experimental factors (load type, geometry, etc.).
> Ferrous-based metals typically show a fatigue
limit.

11. A recent US investigation of the fatigue behavior of DMLS Ti-6Al-4V used
similar AM system, process parameters and test method.
High cycle fatigue results of DMLS Ti-6Al-4V
INFO AND COMMENTS RECENT FATIGUE RESULTS FROM MIT LAB
Source : Mower, Long, Int. J. Fatigue, 2016
> MIT Lincoln Lab/Air Force Dept.
> Test method: rotating bending
> Specimen: standard (6-mm-dia)
> Process parameters: DMLS (EOS)
laser 200W; layer thickness 30mm
> In the case of as-built surfaces very
low fatigue strength is determined
> Machined surfaces considerably
increase the fatigue strength
> The fatigue strength of wrought,
polished Titanium is very high.
As-built – no HT
HIPed - machined
Wrought - polished

12. The influence of AM process and surface conditions on the fatigue behavior
of DMLS Ti-6Al-4V, SLM In718 and SLM AlSi10Mg is investigated.
Specimen details and test method
SPECIMEN PRODUCTION FATIGUE TEST METHOD
Source: BEAM-IT
> Process: DMLS (EOS M290)
> Layer thick. 60 mm / Laser 400 W
> Direction Z
> Post fabrication heat treatment in
vacuum.
> Standard smooth geometry
> Min diameter: 6 mm
> Surface finish:
> As-build
> Machined
> Rotating bending (R=-1)
> Frequency: 50 Hz
> Runout: 107 cycles

13. A heat treatments of DMLS Ti6AL4V was performed using a vacuum furnace
installed at BEAM-IT.
Fatigue testing and heat treatment
HEAT TREATMENT AND MICROSTRUCTURE
> Process #2: DMLS
EOS 290, 400W laser
60 mm layer thickness
> Heat treatment: 2 hrs @
730°C and slow cooling
to 530°C in vacuum &
final cooling in Ar.
> 385 HV30
TAV VACUUM FURNACE
The microstructure is process-dependent
> Uniform microstructure
> Uniform grain size

14. The fatigue results of DMLS Ti-6Al-4V by MIT lab and by our work show the
influence of AM system and process.
Comparison of fatigue results for DMLS Ti-6Al-4V
COMMENTS
Source: Mower, 2016; Lutjering
> The fatigue strength of
GLAM Parma DMLS Ti-
6Al-4V is improved
compared to MIT DMLS
Ti-6Al-4V for both as-built
and machined surfaces.
> The process parameters
of two AM systems were
different
> Heat treatment was
added.
> Literature shows that
surface finish is critical
for Ti-6Al-4V
ASSESSMENT OF ALL FATIGUE RESULTS
Influence of surface
finish on wrought Ti
As-built – no HT
HIPed - machined
Wrought - polished
As-built – HT
Machined – HT

15. The fatigue strength of DMLS Ti-6Al-4V compares favorably with that of
conventional wrought Ti-6Al-4V.
Comparison of fatigue strength of DMLS Ti-6Al-4V
COMMENTS COMPARISON OF FATIGUE STRENGTHS
Source: Mower and Long 2016, Lutjering
> The AM process
parameters influence
the fatigue strength.
> The surface quality
has a key role on
fatigue strength both
AM and wrought
Ti64.
> Rapid AM
technology
evolution requires
periodical fatigue
data re-evaluation.
Influence of surface finish on
wrought Ti-6Al-4V

16. Great need for “smart” testing methods because the fatigue behavior of
SLM metals is expensive and number of factors is large.
Cost of SLM and new test method
> Break-down of SLM production costs
> Metal AM powder cost hundreds of euros per
kilogram
> SLM production and fatigue characterization
requires thousands of euros per fatigue curve.
> Many factors of the SLM process and finish
influence the fatigue strength.
STANDARD VS. MINI SPECIMENS
> Specimen dimensions are relevant as they
impact material weight and AM machine run time
costs.
SLM FABRICATION COSTS
> Mini specimen
weight is about 1/7
of std rot bend
specimen and
1/70 of push pull
specimen

17. The new fatigue test method is efficient because it drastically reduces
powder and process costs.
Smart fatigue test method
EXAMPLE OF BUILD
> Miniature specimen
> Dim: 22 x 7 x 5 mm3
> Plane bending (R=0)
> Frequency: 20 Hz
> Different directions with
respect to to build
> Surface effects
> Fatigue test machine
DETAILS OF TEST METHOD
Standard
Standard CT
Minispecimens

18. The new fatigue test method based on mini specimens was validated by
comparison against standard rotating bending specimens.
Validation of innovative fatigue test method
VALIDATION DETAILS
Source : G. Nicoletto, Inter J Fatigue, 2016
> Mini specimens oriented in the Z direction and rotating
bending specimens with axis in in Z were produced in the
same job and heat treated according to the same stress
relief HT.
> The surface quality was the same: as-built.
> The inertia properties of the minimum cross-sections of
the two specimens were similar to eliminate size effects.
> Load ratio of rotating bending specimens was R=-1 and
of mini specimens was R=0.
> The mean stress effect was managed by Haigh relation.
> Correlation of the two sets of fatigue data is good and
supports use of mini specimens.

19. Process parameters and heat treatment significantly modify the fatigue
behavior of as-built DMLS Ti-6Al-4V.
Specimen production and fatigue behavior
INFO AND COMMENTS COMPARISON OF FATIGUE BEHAVIOR – AS BUILT
> Two heat treatments of as-built DMLS Ti-
6Al-4V specimens were compared:
> Process #1: 200W laser 30 mm layer
thickness, stress relief HT in Argon
> Process #2: 400W laser 60 mm layer
thickness, HT in Vacuum
> Process #1 show a columnar microstructure
and a marked directional fatigue behavior.
> Process #2 results in a fine and
homogeneous structure and a isotropic
fatigue behavior.
> Process #2 is characterized by the
highest fatigue strength.

20. The influence of a recommended heat treatment for SLM Inconel 718 was
investigated using the new mini specimen geometry.
Specimen production
MICROSTRUCTURE
> Process : SLM
Renishaw, AM
250 200W
laser 50 mm
layer
thickness.
> Heat treament
in vacuum.
COMPOSITION AND HT DETAILS
The microstructure is highly process-dependent
> Columnar microstructure

21. SLM Inconel 718 AM with process parameters and recommended heat
treatment shows directional fatigue behavior
Fatigue behavior of IN718
ABOUT
> The microstructure of SLM IN718
showed elongated grains in the build
direction.
> An heat-treatment recommended for
SLM Inconel 718 because it induces
precipitation hardening was applied to
the as-built specimens before fatigue
testing.
> The fatigue behavior is directional.
> The worst fatigue performance when the
material is stressed in the build
direction.
Z
DIRECTIONAL FATIGUE BEHAVIOR – AS BUILT

22. The fatigue strength of SLM Inconel 718 compares favorably with that of
conventional wrought Inconel 718
Comparison of fatigue strength of Inconel 718
COMMENTS COMPARISON OF FATIGUE STRENGTHS
> The fatigue strength
of SLM IN718
depends on the
surface condition.
> Standard and mini
specimens gave
coherent fatigue
results.
> Fatigue strength of
SLM IN718
compares with that
of conventional
IN718.

23. The geometrical complexity of many metal AM parts heavily influences the
structural assessement and qualification.
Geometrical and material complexity of AM
> Although geometric complexity of AM parts is
supposed to come for free, there may be
negative effects on structural design and
fatigue assessment.
For example
> High number of Kt features (organic shapes,
lattice)
> Inspectability challenges (i.e. lattice, cavities)
> Location-specific and directional properties
> Surface quality of hard-to-access areas -> as-
built surface effects
FACTORS TO BE CONSIDERED Topological optimization.
Structural
directionality
Crack initiation at as-build surface

24. The notch fatigue behavior of as-built DMLS Ti6Al4V is readily
investigated using suitably loaded mini specimens.
Mini specimen loading types
UNNOTCHED TEST CONDITION
> Cyclic tensile loading of flat surface
> Load ratio R=0
NOTCHED TEST CONDITION
> Cyclic tensile loading of notch root
> Load ratio R=0
Applied bending load
Source: G. Nicoletto, Inter J Fatigue, 2017
> Stress concentration factor Kt = 1.63ELASTIC FEM CHARACTERIZATION

25. The combined effect of as-built surface and material direction on the
notch fatigue behavior of DMLS Ti6Al4V is determined.
Influence of notch and direction
ABOUT
Source: Design for AM, Fraunhofer IWU, 2017Source: G. Nicoletto, Int Jou Fatigue, 2017 (accepted)
> The combined effect of a rough
as-built surface and a geometrical
notch needs to be established to
enable relevant fatigue
predictions for structural parts.
> Two sets of heat treated Type B
and Type C mini specimens made
of DMLS Ti6Al4V were tested.
> The data are well-behaved with a
reduced scatter.
> The notch effect is different for
the two specimen orientations
although Kt is the same.
DIRECTIONAL NOTCH EFFECT
Unnotched
Notched
Type B
Type C

26. The quality of the as-built surface is known to depends on its
orientation with respect to build direction.
Upskin vs downskin surfaces
Source: VDI Guideline 3405 (2015); R. Mertens et al., ASME Jou Manufact Scie Eng, 2014
DEFINITION SURFACE ORIENTATION AND QUALITY
> Relationship between down-skin angle d
and roughness Ra for SLM
Up-Skin
Down-skin surfaces
d=60° d=45° d=30°
d=60°
d=30°

27. The notch of Type A specimens can be obtained by SLM in two
different ways and that may have an influence on the fatigue behavior.
Upskin vs downskin notches
Source: G. Nicoletto, Int. J Fatigue, 10/2017
NOTCH ORIENTATION AND LOADING SURFACE ORIENTATION AND FATIGUE
> Type A mini specimens were produced
with the upskin and downskin notches.
> They were tested in fatigue and compared
to the unnotched behavior
A+=Au
A- =Ad Build

28. The damage tolerant approach is based on the material resistance (in
combination with geometry) against crack growth.
Crack growth behavior
MATERIAL BEHAVIOR
> The resistance of a material (in combination with geometry) against crack
growth is a property represented by the “Paris” curve.
> A Paris curve plots the crack propagation rate (da/dN) against the stress
intensity factor (DK).
> The latter is a term which describes the stress state in front of the crack
tip. The stress intensity factor considers the applied stress range (Ds),
the crack length (a) and the geometry of the structure through the
parameter b.
> The damage tolerant design approach needs to be updated for AM
processed materials.
PARIS CURVE AND DK

29. GLAM Parma investigated the influence of AM process on the fatigue crack
growth behavior of DMLS Ti-6Al-4V
Specimen details and test method
SPECIMEN PRODUCTION TEST METHOD
> Two heat treatments of
DMLS Ti-6Al-4V
specimens were
compared:
> Process #1: 200W laser
30 mm layer thickness,
stress relief HT in Argon
> Process #2: 400W laser
60 mm layer thickness,
HT in Vacuum
> ASTM Standard CT
specimen geometry
> Pulsating cycle (R=0.1)
> Frequency: 5 – 50 Hz
> Mode I growth
> Load shedding
> Threshold rate: 10-7 mm/cycle

30. Fatigue crack growth behavior of DMLS Ti6Al4V is similar for the two heat
treatments and crack plane orientations.
FCG behavior of DMLS Ti6Al4V
> Fatigue crack growth behavior is similar for the two heat treatments and for different crack plane orientations
with respect to build.
> The scatter is very small and the data trend is linear in the log-log plot.
COMMENTS ON RESULTS
Source : Kunz, Nicoletto et al, 2017

31. The influence of the stress relieving temperature on fatigue crack growth
behavior of DMLS Ti6Al4V is quantified.
Summary of FCG data
> The as-built state exhibits the worst long crack
growth behavior.
> Increasing stress relieving temperature shifts the
crack growth curve to the right and improves the
crack growth resistance.
> In the Paris region (crack rate interval 10-6 to 10-
3 mm/cycle) the crack growth data for material
stress relived at temperatures 380, 650 and 740
°C fall in one scatter band.
> Heat treatment at 800 °C shifts the crack growth
curve in Paris region even more to the right.
> The ΔKth data are also influenced by heat
treatment.
COMMENTS ON RESULTS
Source : Kunz, Nicoletto et al, 2017
COMPARISONS

32. Questions and comments: gianni.nicoletto@unipr.it