Laser Welding of NiTi Shape Memory Alloys

Candidate:
Mehrshad Mehrpouya, Ph.D.
Sapienza University of Rome
Department of Mechanical and Aerospace Engineering
Laser welding of NiTi shape memory sheets:
Experimental analysis and numerical modeling
Case Study

Outlines:
Chapter 1. introduction
Chapter 2. Shape Memory Alloys (SMAs)
Chapter 3. Laser Welding NiTi Shape Memory Alloys
Chapter 4. Materials and Methods
Chapter 5. Experimental Study
Chapter 6. Numerical Modeling
Chapter 7. Conclusion

Laser Welding of NiTi Shape Memory Alloy
Aim and Scopes
1- The effect of the diode laser welding process on the mechanical
and microstructural of the welded components including alloy
crystallography, composition, and the transformation temperature
and microhardness.
2- To achieve the optimum laser parameters, including laser power
and scan speed, for laser welding of NiTi sheets.
3- To obtain a reliable numerical model for predicting the principle
welding issues including the transient temperature, welding
penetration, and the dimension of the Heat Affected Zone (HAZ)
and the Fusion Zone (FZ).
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What are Shape Memory Alloys?
Shape Memory Alloys (SMAs) are metallic alloys that undergo a solid-to-
solid phase transformation (Martensite to Austenite), which can exhibit
large recoverable strains.
 NiTi or Nitinol is the most common SMA (90% of the market).
 Cu-Based SMA such as CuZnAl and CuAlNi.
 Transformation Temperature (TT) has the most effect on the
functionality of SMAs.
 Their functional properties, which include shape memory effect
(SME) and superelasticity (SE), offer a particular flexibility to
design many smart components.
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Superelastic
(SE)
Shape Memory
Effect (SME)
Engineering
Effects of SMAs

Superelastic behavior (SE)
SMAs deformed above a critical
temperature show a large reversible elastic
deformation (recoverable strains up to 10%.
much exceeding the elasticity) as a result of
stress-induced martensitic transformation.
 Frames for eyeglasses
 SMA Actuators
 Cardiovascular Stents
 Antennas for cellular phones
8/67

Shape Memory Effect (SME)
Shape memory effect defines as the recovery
of the apparently permanent deformation
during martensitic transformation in a stress-
free situation.
 Orthodontic archwires
 Engines
 Actuators for smart systems
 Couplings
9/67

Machining of Nickel-Titanium Alloy
 Poor workability due to their high ductility and work-
hardening.
 Therefore, it is necessary to have an appropriate joining
technique to make complex shape components.
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Laser welding of NiTi SMA
 Laser welding process is the most applied technique for joining NiTi alloys
among the other approaches.
 It tends to obtain narrow welds and a better arc welding.
 Because of Low heat input and High energy density.
 Leads to achieving a finer microstructure with a lower thermal stress and
strain from the welding cycle.
Illustration of a fusion weld 12/67

• CO2, Nd:YAG, Diode or fiber laser with continuous- or
pulsed-wave modes can produce a welded joint with
different characteristics.
• In particular, Nd:YAG and fiber lasers are the most applicable
laser sources for the welding of low thickness SMAs
compared to CO2 laser.
• Because they have a lower wavelength which enhances the
laser absorption due to using less energy for the laser
welding process.
• laser with pulse wave mode is preferred to be used for
joining of wires or thin sheets, while the continuous wave
mode is applied for thick sheets with superior mechanical
properties in the welded zone.
Laser welding of NiTi SMA
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Laser type
Material
type
Joint materials NiTi Composition Thickness Laser power Author Year
CO2 laser Sheet NiTi-NiTi Ni50Ti Ni50.9Ti 2 mm 2200-2500 W Hsu [119] 2001
Fibre laser
Thin foil,
Wire
NiTi-NiTi Ni55.91Ti
0.25 mm
0.5 mm
63-77 W
Chan [40, 74, 75, 104, 120-
124]
2011, 2012, 2013,
2015, 2017
Fibre laser Wire NiTi-NiTi NA 90-100 μm 65 W [98] 2016
Nd:YAG laser Sheet NiTi-NiTi Ni51Ti 1.15 mm 850 W Falvo [35, 125, 126] 2005, 2007, 2008
Nd:YAG laser Wire
NiTi-NiTi
NiTi - stainless steel
Ni50.5Ti 0.1 mm 100 W Gugel [97] 2008
Nd:YAG laser Wire
NiTi-NiTi
Ni55Ti 0.36 mm 1000 W Mirshekari [127] 2013
Nd:YAG laser Wire NiTi-NiTi Ni54Ti 0.22 mm 1000 W Zamani [39] 2016
Nd:YAG laser Sheet NiTi-NiTi Ni55.8Ti 0.37 mm 600-900 W Khan [115, 128] 2008, 2010
Nd:YAG laser Sheet NiTi-NiTi Ni50.6Ti 0.2 mm 80 w Gong [117] 2011
Nd:YAG laser
Sheet,
Wire
NiTi-NiTi Ni49.4Ti Ni50.6Ti
1 mm
0.5 mm
λ=1.064 μm Yan [129-132] 2006, 2007, 2014
Nd:YAG laser Sheet
NiTi-NiTi
Ni56.4Ti 0.34 mm λ=1.064 μm Pouquet [107] 2012
Nd:YAG laser Sheet NiTi-NiTi Ni51.5Ti 0.5 mm λ=1.064 μm Schlossmacher [69] 1997
Nd:YAG laser Wire NiTi-NiTi Ni50Ti Ni50.9Ti 1 mm λ=1.064 μm Song [99] 2008
Nd:YAG laser Sheet NiTi-NiTi Ni49Ti 0.54 mm 312 w Tuissi [114] 1999
Nd:YAG laser Sheet NiTi-NiTi Ni50.8Ti 1 mm 910 W Vieira [108] 2011
Nd:YAG laser
Wire,
Tube
NiTi-NiTi
Ni49.6Ti
Ni50.8Ti
0.4, 0.6 mm λ=1.064 μm Zeng [64] 2016
Nd:YAG laser Sheet NiTi-NiTi Ni49.2Ti 0.75 mm 850 W Maletta [133] 2008
Nd:YAG laser Sheet NiTi-NiTi Ni50.8Ti 1 mm 990 W Oliveira [116, 134, 135] 2016, 2017
Nd:YAG laser Wire NiTi-NiTi NA 0.37 mm λ=1.064 μm Panton [136] 2016
Nd:YAG laser Sheet
NiTi-NiTi
Ni50.8Ti 0.34 mm λ=1.064 μm Quintino [100] 2012
Nd:YAG laser Wire
NiTi-NiTi
NiTi - NiTiCu
Ni49.85Ti Ni51.2Ti 0.45 mm λ=1.064 μm Sevilla [19] 2008
Nd:YAG laser Wire NiTi-NiTi Ni44.2Ti 0.41 mm λ=1.064 μm Tam [101] 2011
Yb:YAG laser Sheet NiTi-NiTi Ni55.64Ti 1 mm λ= 1030 nm Sathiya [137, 138] 2016, 2017
ListofpublicationsonNiTilaserweldingprocess

Laser welding of NiTi Shape Memory Alloy: A Review
15/67

• HAZ is the area of base material, either a
metal or a thermoplastic, which is not melted
and has had its microstructure and properties
altered by welding or thermal process.
• The dimension of FZ and HAZ has a substantial
influence on the thermal stress and strain and
tends to increase by expanding its volume
during the heating process.
BM-HAZ-FZ
Fusion Zone (FZ) and Heat-Affected Zone (HAZ)
16/67

• Laser power and wavelength, scan speed, focus shape, and size,
shielding gas and its flow rate are the most effective parameters in the
welding process.
• Developing an appropriate technique can reduce HAZ and FZ regions,
and consequently improve the weldability of NiTi alloy.
The effect of Laser welding parameters
Micrograph of NiTi welded sheet in different zones including BM, HAZ, and FZ
17/67

Example: The Effect of Laser
Power on the Transformation
Temperatures
18/67

High-Power Diode Laser (HPDL)
Laser head (top) and power unit (down)
Laser parameters specification
Laser parameters Value
Maximum Power 1500 W
Wavelength 940 nm
Spot Shape Ellipse
Dimension of the Spot 1.2 mm × 3.8 mm
Efficiency 30-50 %
Wight 8 Kg
20/67

A schematic of the scanning patterns
Designed Fixture
Argon shield
 Ni 54.76 wt.% Ti sheets (Memry GmbH, Germany) were chosen with a
dimension of 15 × 20 mm and thickness of 0.50 (±0.05) mm.
 High-Power Diode Laser (HPDL) source (ROFIN-SINAR, DL 015, Hamburg,
Germany) was employed for this laser welding process.
21/67

The experimental Plan
Designed Fixture
The design of (A) clamping system, (B)
Argon shield
A schematic of the clamping system and
Argon shield
22/67

A schematic of experimental equipment
The experimental Plan
Preheatin
g power
(W)
Main
power
(W)
Scan
speed
(mm/s)
Path
length
(mm)
Argon
pressure
(bar)
1 100 300 3 20 1.5
2 100 350 3 20 1.5
3 100 450 5 20 1.5
4 100 500 5 20 1.5
 The experimental plan was based on two main
variables of laser processes that are the scan speed
and the power of laser beam.
 A preheating treatment was considered for all
strategies with a laser power of 100 W.
 since it represents a powerful tool for recovering
the mechanical and microstructural properties of
the welded components.
23/67
 It can reduce the impurity and
metastable elements during the
solidification process in HAZ and FZ
regions.

A schematic of the optical microscopy system
The experimental Analysis Systems
A schematic of the microhardness system
 In order to study the microstructure of
welded zone and remove the surface oxide,
the polished surface chemically etched
with an acid liquid in an HF: HNO3: H2O
solution with a dilution of 1:5:10.
 The optical microscope system
(DMI5000M, Leica, Italy) which was
employed for observing the etched
surfaces.
 The micro-hardness measurement was
performed using Vickers Harness-tester
(Leitz, model Dialux 22-RZD-DO) and with a
load of 981 mN.
24/67

The experimental Analysis Systems
A schematic of SEM/EDS systems
A schematic of the DSC system
 Scanning Electron Microscope (SEM, S2500,
Hitachi, Germany) was applied for the surface
analysis and the elemental study was performed
using energy dispersive X-ray spectroscopy (EDS)
(NORAN System Six, Thermo Fisher Scientific, UK)
to assess nickel and titanium concentration at the
FZ and HAZ regions.
 The phase transformation temperatures were
measured by Differential Scanning Calorimetry
(DSC, 200 PC, Netzsch Group, Selb, Germany)
 The temperature of the laser affected region (HAZ)
was recorded by a thermometer (FLUKE, Model
#54-2-B) during the welding process.
25/67

Visual appearance of some welded
joints of NiTi sheets
Micrograph of NiTi welded sheet in different zones
including BM, HAZ, and FZ
 Samples 9NA and 10NA appear a uniform and homogeneous weld.
 the choice of a range for laser power of 350-450W and scan speed of
3-5 mm/s, has allowed to obtain the best welded joints.
 Therefore, the following analyses were considered for the comparison
between samples 9NA and 10NA.
Optical microscopy analysis
27/67

Overall cross-section view of NiTi welded sample (above), and
microstructure of A) FZ, B) HAZ/FZ, C) HAZ regions
 The fusion zone has a coarser grain structure than the heat affected zone due to
the higher temperature of laser welding process in the WZ and the effect of the
solidification process.
 Site A represents a closer investigation in the fusion zone (FZ) with really big grains.
 The grains growth is essentially due to the temperature gradient in the welding
center, The heat spreads very little because of the low heat conductivity of NiTi
alloys.
28/67

Overall cross-section view of NiTi welded sample (above), and
microstructure of A) FZ, B) HAZ/FZ, C) HAZ regions
 The fusion boundary beside the recrystallized HAZ region is shown in the site
B. An epitaxial growth occurred in this zone. The grains are formed in small
columnar oriented 45° to the centerline.
 Site C shows the microstructure of the HAZ region with a fine equiaxial
crystal zone.
 The grain structure at the interfacial region significantly depended on the
grain structure of the base material and the welding conditions.
29/67

(a) SEM scan of the laser-welded NiTi sample
9NA, (b) EDS results of the FZ, and (c) the
HAZ regions
(d) SEM scan of the laser-welded NiTi sample
10NA, (e) EDS results of the FZ, and (f) the
HAZ regions
• The microstructure of the HAZ and FZ regions are similar to micrograph results.
• The FZ (right side) shows some significant coarse grains, while fusion boundary (left
side) indicates epitaxial crystallization of the grains induced by the laser welding process
SEM/EDS analysis
30/67

Weight % Atom %
Position Section Ni Ti Ni Ti
(b) FZ 9 52.53 47.47 47.45 52.55
(c) HAZ 9 53.07 46.93 47.99 52.01
(e) FZ 10 52.46 47.54 47.38 52.62
(f) HAZ 10 52.85 47.15 47.77 52.23
The composition of NiTi welded samples at
different points
 For sample 9NA, the amount of the Ni element in BM has reduced from 54.76 wt.% to 52-
53 wt.% for the welded samples.
 It is due to the generated high temperature in the HAZ and FZ regions during the laser
welding process.
 For the sample 10NA, the obtained results are also very close to the reference material.
However, the composition of the welded sample 9NA is slightly closer to the BM
compared to the sample 10NA for both FZ and HAZ regions
 The lower laser power and scan speed can have a better influence on the material
composition and also it can effectively preserve the functionality of the welded NiTi
material. 31/67

DSC measurement of the base material (BM)
• All samples are in the martensite state to display
the shape memory effect at room temperature.
• the transform from martensite (B19’ monoclinic
lattice structure) to austenite (B2 body-centered
cubic structure) during heating and cooling
processes.
• Rhombohedral phase (R-phase trigonal
structure) transformation is obvious in the graph
during the cooling process before the martensite
transformation, while the reverse martensitic
transformation appears during the heating
process.
Transformation temperature
32/67

DSC measurement of the welded NiTi sheets (left) sample 9NA, and (right) sample 10NA.
• The combination of laser parameters results in various transformation temperatures
which affect directly on the functionality of the welded sample.
• Sample 9NA show a typical DSC graph of NiTi alloy in the cooling and heating
process, while sample 10NA represents the R-phase in the cooling process similar to
the based material.
33/67

Cooling Curve Heating Curve
Sample Mf (˚C) Ms (˚C) As (˚C) Af (˚C)
As-received 17.51 69.15 71.43 96.96
9NA 18.13 70.29 71.60 109.62
10NA 26.97 75.30 63.73 118.62
Transformation temperature of the welded NiTi samples
 This temperature for samples 9NA and 10NA noticeably increases
in order to 109.62˚C and 118.62˚C after the laser welding process.
 The sample 9NA shows a better convergence to the reference
material than sample 10NA.
34/67

Microhardness of the weldment in
different zones (FZ, HAZ and BM)
Microhardness measurement
 The BM has the highest amount of hardness
while it sharply reduces in the HAZ region,
then it increases in the FZ region.
 This variation is mainly due to the change in
the grain structure, formation of some
precipitation and recrystallization in these
regions and it meaningfully depends on the
cooling rate.
 Sample 9NA shows remarkably a higher
hardness values compare to sample 10NA. It
might be due to the higher laser power (450
N) of sample 10 NA which leads to enlarging
the grain size and subsequently decrease the
hardness rate.
35/67

Finite element modeling
The flow chart of the FM modelling
 Developed a three-dimensional (3D) thermal
model.
 The first heat transfer model for SMAs.
 The welding simulation was carried out by
ABAQUS 6.11.
 Using subroutine code (DEFLUX).
 Obtaining the optimum parameters for this
process.
 To reduce the FZ and HAZ dimensions, and
consequently enhancing the weldability of
NiTi alloys.
37/67

𝑞 𝑓 𝑥, 𝑦, 𝑧 =
6 3 𝑓𝑓 𝑄
𝑎𝑏𝑐𝑓 𝜋 𝜋
. 𝑒
−3
𝑥2
𝑎2
. 𝑒
−3
𝑦2
𝑏2
. 𝑒
−3
𝑧2
𝑐 𝑓
2
𝑞 𝑟 𝑥, 𝑦, 𝑧 =
6 3 𝑓𝑟 𝑄
𝑎𝑏𝑐 𝑟 𝜋 𝜋
. 𝑒
−3
𝑥2
𝑎2
. 𝑒
−3
𝑦2
𝑏2
. 𝑒
−3
𝑧2
𝑐 𝑟
2
Heat flux in front
Heat flux in rear
The moving heat source model
Goldak’sdouble-ellipsoidmodel
38/67

Mesh Geometry
• A multi-layer mesh for two pieces of NiTi
sheets.
• The mesh geometry is dense in the laser
path for achieving a more accurate result in
this region.
• 3D eight-node linear heat transfer
hexahedron (C3D8R)-type elements with
7105 elements.
• The thermal boundary conditions show that
there are heat losses to the surroundings
by natural convection and radiation during
this laser process
39/67

Simulation results
Simulation of NiTi laser welding when laser
power and scan speed are 500 W and 5 mm/s.
Isotherm evolution during laser welding process, first preheating
pass (a-b), and the second pass (c-e) (P1=100W, P2=350 W, V=3
mm/s).
40/67

3D map of maximum temperature-laser power-scan speed
based on experiment results
Experimental result
• The temperature is minimum around
123˚C in the heat-affected region, when
laser power and scan speed are set to
300 W and 5 mm/s respectively. While
the maximum temperature is about
490˚C when laser sources work with 500
W as laser power and 3 mm/s as scan
speed
• The linear trend of the graph expresses
that the temperature reduces with
increasing the scan speed and
decreasing the laser power.
41/67

3D map of maximum temperature-laser power-scan
speed achieved from the simulation results
Simulation result
 The simulation result has a similar trend
like the experimental outcomes and
shows the minimum temperature at a
laser power of 300 W and a laser velocity
of 5 mm/s around 146˚C. While the
temperature increases meaningfully to
538˚C, when the laser power grows to
500 W.
 The maximum temperature resulted by
the simulation has a similar trend like the
experimental outcomes.
 These results show that, compared to
scan speed, laser power has a very strong
influence on the maximum temperature.
42/67

The comparison of HAZ and FZ dimension based
on the simulation and experiment results
(P1=100W, P2=450 W, V=5 mm/s)
Dimension of weld bead
 The predicted FZ and HAZ size is in good agreement with the presented
micrograph.
 Overall, this graph shows the size of FZ and HAZ that slightly decrease when the
combination of laser power and scan speed increase.
 The simple ratio power/speed, this is equal to 90 in the first case (450W – 5mm/s )
and 117 in the second case (350W – 3mm/s ).
The comparison of the FZ and HAZ dimension between
the simulation and the experimental results
43/67

Conclusion
This study has addressed the following results.
1- The most significant results, obtained from the experimental tests, showed that
samples treated with laser powers in the range 350W- 450W and scan speeds in
the range of 3-5 mm/s showed a good quality, with a weld bead uniform and
homogeneous and free from any cracks or other defects;
2- The microstructural investigations reveal that laser welding process induced the
formation of coarse grains in the FZ and some precipitation such as NiTi2, Ni3Ti,
Ni4Ti3, while an epitaxial crystallization of the grains is evident in fusion boundary,
between the FZ and HAZ. Lastly, fine equiaxial crystals are observable in the heat
affected zone, near the base material;
3- The composition of the welded samples is quite similar to the base material.
However, the employ of lower laser power and scan speed may result in a minor
modification of the percentage composition of the two elements, Ni and Ti. This
means that functionality of the welded NiTi material can be better preserved;
45/67

Conclusion
4- The transformation temperature analysis reveals that for the laser-welded samples
there is not a significant variation when compared with the base material. However,
also, in this case, a better convergence is obtainable by employing of lower laser
power and scan speed;
5- The Vickers microhardness analysis indicates that the highest values of hardness are
achieved near the base material, with almost 320 HV0.1. A sharply decrease it is
evident in the heat affected zone since the values are in the range of 250-270 HV0.1.
Lastly, it increases again in the fused zone, and the values are in the range 270-300
HV0.1;
6- This study also has developed a numerical model to predict the optimum
parameters for laser welding of NiTi shape memory alloy for obtaining the ideal joints.
The three-dimensional map primarily showed the effect of the operational parameters;
46/67

Conclusion
7- The numerical model also predicted the dimension of the FZ and HAZ according
to various beam powers and laser speeds. The results showed the dimensions of the
fusion zone and the heat affected zone slightly reduced when the combination of
laser power and scan speed were increased;
8- The modeling results had a good agreement with the experimental achievements.
Therefore, this model can be applied as an effective tool to predict the optimum
laser parameters for laser welding of NiTi shape memory alloy since it can reduce
the heat affected regions, and subsequently provides a greater guarantee on the
shape memory effect of the welded area.
47/67

Thank You Everyone
For Your Kind Attention 48

Laser Welding of NiTi Shape Memory Alloys

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