1. 1
Computational Weld
Mechanics Simulation
of
Hot Crack Nucleation
Ahmed Nasser
under the supervision of
Prof. John Goldak
Starting off we already knew the capability of CWM to determine
the evolution of stress-strain-temperature state.
And we knew that hot cracking is often observed during welding.

2. 2
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hot crack nucleation?hot crack nucleation?hot crack nucleation?hot crack nucleation?hot crack nucleation?hot crack nucleation?hot crack nucleation?hot crack nucleation?hot crack nucleation?
C.J. Huang, C.M. Cheng, C.P. Chou and F.H. Chen. Hot Cracking in AZ31 and AZ61 Magnesium
Alloy. J. Mater. Sci. Technol., 27(7):633-640, 2011.
Predicting the risk of hot cracking during welding is of the utmost
importance, in order to minimize the risk of occurrence.
The purpose is to reduce the risks associated with welding and
facilitate the use corrosion resistant materials that maintain
strength/stiffness at high temperatures, which can also be
susceptible to hot cracking.
This was driven by the increasing demand from the energy,
aerospace, and other industries.
Eliminating the risk hot cracking may be an impossible task, but
understanding the causes of hot cracking should clarify the options
available to reduce the risk of hot cracking.

3. 3
Hot
Cracking
Ductility
Dip
SolidificationLiquation
C.M.Cheng, C.P.Chou, I.K.Lee and I.C.Kuo. Susceptibility to Hot Cracking and Weldment
Heat Treatment of Haynes 230 Superalloy. J. Mater. Sci. Technol., (5):685-690, 2006.
One point of view is that the cause is a combination of both a tensile
strain and reduced material resistance to cracking.
The reduced material resistance means deformation cannot
compensate for a localized tensile strain.
This reduced material resistance is only experienced within certain
temperature ranges.
There are different reasons for the reduction in material resistance
and accordingly hot cracking can be classified.
Types:
DDC in heat affected zone
LC in partially melted zone
SC in fusion zone

4. 4
Ductility Dip Cracking
`
Matt Collins, Nathan Nissley and Antonio Ramirez. Ductility-Dip Cracking in Ni-base Alloys. March 2012.
· http://www.matsceng.ohio-state.edu/wjmg/ductilityDip.htmlx
DDC occurs in a completely solid phase.
e.g. For austentic alloys DDC is observed at 1/2 the absolute
solidus temperature.
The cause of reduced material resistance is not agreed upon, but
some research has attributed this to:
-segregation of elements to grain boundaries
-an accumulation of voids at the grain boundaries
-the size, shape, and orientation of grains
-a combination
Material resistance to DDC is improved:
-below that temperature range, due to hardening
-above the temperature range, due to recrystallization

5. 5
Liquation Cracking
S. Kou. "Solidication and liquation cracking issues in welding." JOM Journal of the Minerals, Metals and Materials
Society 55, 37-42 (2003).
At a higher temp range, some metallic materials are at risk of
liquation cracking.
This type of cracking occurs in a solid-liquid coexistent region.
Resistance to LC drops, because above the local solidus
temperature the GB melt, but the grain centre remains solid.
For Inconel 600
Maximum 1655 K
Minimum 1557 K

6. 6
Solidification Cracking
S. Kou. "Solidication and liquation cracking issues in
welding." JOM Journal of the Minerals, Metals and
Materials Society 55, 37-42 (2003).
Ploshikhin, V., Prikhodovsky, A., Makhutin, M., Ilin, A.
and Zoch, H.-W.. Integrated Mechanical-Metallurgical
Approach to Modeling of Solidification Cracking in
Welds. 2005.
Like liquation, solidification cracking occurs in the solid-liquid
coexistent region.
Resistance drops because below local liquidus, dendrites growing in
completely liquid phase surrounded by a liquid film.
For Inconel 600
Maximum 1655 K
Minimum 1557 K

7. 7
Material Resistance to
Cracking
Morgan Leo Gallagher. An Investigation of the Elevated
Temperature Cracking Susceptibility of Alloy C-22 Weld-
Metal . PhD thesis, The Ohio State University, 2008.
Strain to
Fracture
Decreasing
Henry Tan. MACE 61058: Plasticity – Lab. March 2012. ·
http://www.abdn.ac.uk/~eng907/teaching/plasticity/lab.htm
Vast majority of research on the subject of hot cracking is based on a
metallurgical approach.
But Limited research has been done using a mechanical approach
such as that conducted here.
The concept of mechanical approach was greatly assisted by the
work of Prokhorov in the 1950's. He quantified resistance to hot
cracking by measuring the strain to fracture within material specific
temperature.
But measurements taken by Prokhorov were not localized.
And hot cracking nucleates at a point, as a direct result of evolution
of state in the immediate neighbourhood.
In the 1980's, Matsuda improved the technique to measure localized
resistance by developing the MISO technique.
Material resistance to DDC is measured using a tensile test
conducted at various temperature within the susceptible
temperature range.

8. 8
Measuring Material
Resistance
MISO technique
Fukuhisa Matsuda, Hiroji Nakagawa, Kazuhiro Nakata, Hiroaki Kohmoto and Yoshiaki Honda. Quantitative Evaluation of
Solidification Brittleness of Weld Metal during Solidification by Means of In-Stu Observation and Measurement (Report I).
Transaction of JWRI, 12:65-72, April 1983.
On the other hand, resistance to solidification cracking is accurately
measured using the Measurement by means of In-Situ
Observation (MISO) technique.
The MISO technique measures the local strain and strain rate across
cracks behind the weld pool, where the material is a combination
of liquid and solid phase, using high-speed photography. Strain is
determined by visually measuring the distance between any two
surface marks in the desired direction. The surface marks used,
naturally appear, as a result of the welding process.

9. 9
Driving Force versus
Resistance
Yanhong H. Wei, Zhibo B. Dong, R. P. Liu and Z. J. Dong. Three-Dimensional Numerical Simulation of Weld Solidification
Cracking. Modelling and Simulation in Materials Science and Engineering, 13:437-454, 2005.
The risk of hot crack nucleation in any situation can be predicted,
using the experimentally determined resistance.
This is based on a competition between resistance and driving force.
The driving force is the tensile strain increment in the susceptible
temperature range.

10. 10
Motivation
demonstrate the potential of CWM as
an effective design tool for the
prediction of the risk of hot crack
nucleation in a welded structure
This concept is based on the work of:
-Chihoski (1972) - effect of deformation pattern on weld cracking
-Feng (1993) - CWM to determine driving force
We wanted to enhance the limited research conducted using a
mechanical approach.
Since 2004, 3 international workshops, dedicated to hot cracking,
have been held and the overwhelming majority of the presented
research on the topic has been based on a metallurgical approach

11. 11
Objective
estimate risk of hot crack based on a
mechanical approach, using the evolution
of the stress-strain-temperature state
A mechanical approach to compliment the metallurgical approach.
The evolution of the temperature-stress-strain state determined the
driving force.
We wanted to identify whether a certain area is at risk of hot cracking
nucleation.
--------------------------------------------------------------------------------------------
In contrast, the metallurgical approach explains hot cracking by:
-conditions of solidification
-grain size
-presence of low-melting eutectic films

12. 12
Method
VrWeld software
transient 3-D CWM analysis
Ductility Dip
Cracking
Solidification
Cracking
The evolution of state was based on transient 3-D analysis
conducted using a CWM software called VrWeld.
We conducted two tests. Both were bead on plate welds, but for two
different nickel-based alloys.
One of the tests was only for DDC and the other test was only for
solidification cracking.
The design of the tests used data from the literature.
In both tests, we identified the risk of hot cracking nucleation at
specific locations.

13. 13
Ductility Dip CrackingTest
Variable:
welding speed
sub-model
fine
mesh
coarse
mesh
The first test was for ductility dip cracking.
Welding speed is varied to observe its effect on the risk of DDC.
(localized strain and strain rate)
Material: Filler metal 82 (used to weld Inconel alloys)
Dimensions:100x100x2mm plate (test coupon)
Boundary conditions: Free body motion restraint
Mesh: 8-node brick
Refined along weld path because of high thermal gradients
Sub-model
The entire model was solved with a coarse mesh, then the results
were mapped onto the boundary of small section with a finer
mesh. This is the sub-model feature. This way we can get higher
accuracy, while maintaining low computational time.

14. 14
Ductility Dip CrackingTest –Temperature Results
2 mm/s
5 mm/s
Here we have a visualization of the thermal results for the two
welding speeds at the instant the heat source reaches halfway
across the weld path.
The size of the weld pool is same, but the area susceptible to DDC is
larger with the higher welding speed.

15. 15
`
Ductility Dip CrackingTest – Strain Results
2 mm/s
5 mm/s
However, the tensile stain increment is smaller with a higher welding
speed. Therefore, the likelihood of DDC nucleation is less with a
higher welding speed. According to the results, hot cracking is not
likely to occur with both welding speeds. However, there is a
relatively higher risk of DDC nucleation at distance of 4 mm from
the centre-line on the top surface of the plate, with a welding
speed of 2 mm/s.
Measurements for temperature and plastic strain increment is
computed for all principal directions at Gauss points that are in
between the markers. Shown is their location, behind the weld
pool and relative to the temperature distribution, in the sub-model
with a 2 mm/s welding speed, at t= 32.5 s.
The plots are for the maximum tensile principal plastic component,
which are almost parallel to the welding direction. The tensile
plastic strain increment is thought to be responsible for the
irreversible damage that leads to ductility dip cracking
In `ddc_pps_gp' video, the arrows show directions of the eigen
vectors for the principal plastic strains and their relative
magnitudes at a Gauss point 3 mm from weld centre.

16. 16
Ductility Dip CrackingTest -Validation
5 mm/s
2 mm/s
Jingqing Chen and Hao Lu. Investigation on ductility dip cracking susceptibility of filler metal 82 in welding.
Transaction of JWRI, 39(2):91-93, 2010.
The results obtained by the DDC test for the equivalent plastic strain
increment versus temperature are not similar to the results of
Chen and Lu.
The point with the maximum plastic strain increment was at 4 mm
from the centre line, rather than the 3 mm observed by Chen and
Lu.
The most probable cause for the apparent discrepancies is the lack
of detail in Chen and Lu's paper.
The locations, at which strain and temperature were
recorded, were not clearly identified.
In addition, the restraints applied to welded plate were not
specified. The lack of values for arc efficiency and the size of
the weld pool or heat source are also a concern.

17. 17
Solidification CrackingTest
Variable: cross head speed
sub-model
coarse
mesh
fine
mesh
The second test was for solidification cracking.
Cross-head speed is varied to observe its effect on the risk of DDC
(localized strain and strain rate) - 20, 2, 0.2, and 0.1 mm/s
Welding speed: 2mm/s
Material: Inconel 600
Dimensions: 300x50x2mm plate
Boundary conditions: Free body motion restraint
Cross head speed – a displacement rate is
applied at opposite end of the plate once the
heat source reaches halfway across the weld
path
Mesh: 8-node brick
Refined along weld path
Sub-model

18. 18
Solidification CrackingTest –Temperature Results
Here we have the visualization of the temperature results, when the
arc is halfway across the weld path
The transient temperature results are shown in video `ddc_temp'.
The video is for the sub-model, located at the centre of the plate

19. 19
Solidification CrackingTest – Strain Results
`
The total strain increment behind the weld pool is measured using a
virtual gauge of 1 mm length to measure, perpendicular to the
welding direction.
Plastic strain is not used, only to best reproduce the experiment from
literature on which this test is based.
The risk of solidification cracking can be greatly reduced, with a
lower localized strain rate rather than the magnitude of the strain
alone. The localized strain strain was controlled by the CHS.
According to the results, an applied CHS of 0.2 or 0.1 mm/s will most
likely not cause solidification cracking.
A CHS of 2 mm/s will likely lead to solidification cracking with the
lowest magnitude of strain, also referred to as minimum ductility,
which is at 1.2% and the temperature of 1645 K. Solidification
cracking will likely nucleate with a CHS of 2% with an applied CHS
of 20 mm/s.
Determining the risk of hot cracking for all points is a tedious
process. The solution is automation.

20. 20
Solidification CrackingTest
Hot Cracking
Post-Processor xy
z
Automation of the process of determining the risk of hot cracking was
made possible by the post processor developed by Goldak
technologies inc.
The risk of hot cracking is determined at all Gauss points. This is
based on the criteria that the principal plastic strain increment, in
the susceptible temperature range, exceeds critical values and the
rate of the principal plastic strain increment exceeds a critical
value, as well.
These critical values are obtained from experimentation that was
available in literature. These are an input to the library for material
properties.
The relative risk of hot cracking at the Gauss points depends on the
magnitude of the principal plastic strain increment.
The arrows show directions of the eigen vectors for the principal
plastic strains and their relative magnitudes at a single Gauss
point, after applying the CHS. The largest tensile component is
near the direction of the CHS (x-axis).

21. 21
Solidification CrackingTest
Hot Cracking
Post-Processor

22. 22
 review state of the art
 meshing / sub-model
Challenges
The main challenges faced to reach our objective included:
Studying and Understanding prior work and state of the art techniques used in
predicating hot cracking (Kuo for an overview, Nishimoto for state of the art,
Zhilli Feng and Zacharia for prior CWM.)
Another challenge was the meshing process - Refining to determine localized
strain maintaining low computational time.
We also use the Sub-model feature, to get higher accuracy, while maintaining
low computational time. This is achieved by solving the entire mesh with a
coarse mesh, then mapping the results onto the boundary of small section
with a finer mesh.
This is particularly useful for the stress analysis, which takes a longer CPU
time compared to the thermal analysis.

23. 23
Challenges
 lack of detail in literature
– optimize welding parameters
– stress Dirichlet boundary conditions
 validation
Since we attempted to emulate tests from literature, the main
problem is that people rarely provide sufficient information, to
accurately reproduce an experiment.
This was particularly a problem as we tried to optimize welding
parameters and stress Dirichlet boundary conditions, to best
represent experimental set up.
Validation / Interpreting results – we studied and compared our
results to those from literature, understanding the cause of any
discrepancy.

24. 24
Recommended
Future Work
couple micro to macro
– microstructure evolution
– composition gradient across grains
– shape and flow of liquid film
In the analysis we conducted, the macroscopic analysis alone is an
adequate approximation for the nickel-based alloys. Because
nickel alloys largely stay in the face-centred cubic (FCC) crystal
structure in the solid state.
However, in the future, a possible improvement to the CWM
simulation would be coupling a microscopic analysis to the
macroscopic constitutive equations, used in the CWM analysis.
The microscopic analysis should take into account:
the evolution of the microstructure
the composition gradient across grains(segregation)
the shape and flow of the liquid film on solidifying grains