simulation and visualisation of wire-arc additive … thermal plasma in lte ... momentum transfer in...
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Simulation and Visualisation of
Wire-Arc Additive Manufacture
Ioannis Bitharas
Jialuo Ding
Andrew Moore
Stuart Williams
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Overview
Background previous work, WAAM process
MHD flow modelling Plasma arc welding torch model
Schlieren imaging
Validation through optical diagnostics
Ongoing work Trail shield, torch optimisation
Pressure validation
P. Bidare, I. Bitharas et al., Fluid and particle dynamics in LPBF,
Acta Materialia 142 (2018)
I. Bitharas et al., Visualisation and optimisation of shielding gas
coverage during GMAW, Journal of Materials Processing Technology
(2018)
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Wire – Arc Additive Manufacture (WAAM)
HiVE Chamber @ Cranfield University: gantry-based motion
Plasma arc welding torch
Ar supply
(up to 195 lt/min)
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PAW torch simulation layout
Ar - air mixture properties 𝒇(𝑻,𝝎)
Tu
ng
ste
n
Co
pp
er
no
zzle
Sh
rou
d
Ar-air mixture
Radia
l sy
mm
etr
y
Plasma gas inlet
Shielding gas inlet
Ti-6Al-4V workpiece (Wall/substrate) Ar-air thermal plasma in LTE Highly non-linear multiphysics problem
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Plasma arc welding: Magneto-hydrodynamics
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Simulating MHD flow with COMSOL
ρ(𝐮 ∙ ∇𝐮) = ∇ ∙ −p + μ(∇𝐮 + (∇𝐮)𝐓) + 𝐅 (1)
∇ ∙ (ρ𝐮) = 0 (2)
−∇ ∙ (k∇T) + ρcp(𝐮 ∙ ∇T) = 𝐉 ∙ 𝐄 +5kb
2e𝐉 ∙ ∇T − 4πεN (1)
Compressible flow
(SST turbulence model)
Heat transfer
(Solids & Fluids)
Transport of concentrated species
(Ar & air)
Electric & Magnetic fields
(gauged)
∇ × 1
μ0∇ × 𝐀 + σ∇V = 0 (1)
∇ ∙ σ∇V = 0, (1)
−∇ ∙ 𝜌Dio ∇ωi + 𝜌ωiDi
o∇Mn
Mn
+ DiT∇T
T + 𝜌(𝐮 ∙ ∇)ωi = 0, (1)
Implicit & explicit physics couplings
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Plasma jet – shield gas stream interaction
Temperature field partly constricted by nozzle
Arc pinches shield gas flow
Convective recirculation
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Steady-state air entrainment
100 A 200 A
Bo
dy fo
rce
s
(N/m
3)
O2 c
on
ce
ntra
tion
(pp
m)
10,000 + ppm
O2
Lorentz force (FL= J × B) high near electrodes
Stronger pull but similar air levels with higher current
~4k ppm air transported near melt pool (!)
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Deposited wall geometry: Temperature
Heat transfer to wall influenced by convective action
Side jet inclination changes with wall width
Also relevant to torch positioning during builds
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Heat transfer to wall
Flow along wall enhances heat transfer Inner and outer vortex structure
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Wall geometry – O2 concentration
Inert environment changes with wall geometry
O2 co
nce
ntra
tion (p
pm
)
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Schlieren imaging
Light collimated between M1 & M2
Flow information = Refracted rays
Cut-off highlights
Band pass filter at 633 ± 10 nm
illumination
M1
M2
Camera & filters
𝜕𝑛
𝜕𝑥 ∝
𝜕𝜌
𝜕𝑥
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PAW torch – schlieren video
Torch gas shield gradients
Plasma jet gradients
Side jet gradients
Nozzle position relative to wall determines side jet angle
𝜕𝑛
𝜕𝑥 ∝
𝜕𝜌
𝜕𝑥
Turbulence increases with current & plasma gas flowrate
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Ongoing work
Pre
ssure
(Pa)
Distance from arc (mm)
Velo
city (m
/s)
Velo
city (m
/s)
100 A 200 A
Pre
ssure
(Pa)
Pre
ssure
(Pa)
100 A 200 A Pressure measurements to
further validate model
Momentum transfer in arc critical
in understanding interaction with
melt pool
No steady-state level set!
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Results summary
MHD flow features validated from schlieren
Schlieren interpretation facilitated by simulation
Allows optimisation of WAAM process
Torch – Shielding – Manufactured part
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HiVE local shielding system
Inner vortex
Outer vortex
Trail shield bulk flow
air
Fast boundary flow along wall, substrate
Travel speed only affects thermal gradient
Gas recirculation between side jet and built wall (inner vortex)
Torch gradients interact with trail shield flow creating outer vortex
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MHD modelling: Turbulent jets
J. R. Fincke, R.L. Williamson, et al. Entrainment in
high-velocity, high-temperature plasma jets. Parts I &
II , Int. J. Heat Mass Transf., 2003
• Enthalpy
probes
• Thomson
scattering
• Laser-induced
fluorescence
Comparison with simulation, including
k-ε turbulence model
K. Cheng, X. Chen, Prediction of the entrainment of ambient air into a
turbulent argon plasma jet using a turbulence-enhanced combined-
diffusion-coefficient method, Int. J. Heat Mass Transf., 2004
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200 A 100 A
Peak T ~13kK Peak T ~19kK
100 A 200 A
Umax
60 m/s
Umax
260 m/s
TIG torch in WAAM wall
Temperature plot Velocity plot
• Step towards more representative plasma torch model • MHD physics identical with PAW, steady-state flow patterns similar • 2D axisymmetric geometry: ~15 mins solution time per case
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4 l/min 9 l/min
Increased accuracy in boundary layer due to wall functions (SST turbulence model)
Air contamination
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Parametric sweeps
Pressure (Pa) Air mass fraction (1)
Distance from arc (mm) Distance from arc (mm)
• Stagnation pressure increases non-linearly with arc current • Air entrainment doubled for 200 A compared to 100 A
• Theoretical analyses to complement future measurements
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Turbulence intensity
• As current increases, the side jets contract but also push out and downwards with more momentum
• Overall greater turbulence levels
• Relatively higher air content on top of solidifying metal expected
200 A
Rv: torch + trail shield/PAW_trail_shield_7
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The high wall problem
• As the physical constraint of the substrate is no longer there, the inner area becomes more exposed
• The outer vortex stretches to the extent that it loses effectiveness
• Air contamination increases proportionally to standoff from substrate
Air in
Air in
Rv: torch + trail shield/PAW_trail_shield_9
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Background: MIG welding process optimisation
15 l/min
6 l/min
𝜕𝑛
𝜕𝑟 ∝
𝜕𝜌
𝜕𝑟
12 l/min
Observed flow features predicted
by simulation
Qualitative validation through
schlieren
𝑚3 𝑠
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Arc welding simulations
Air entrainment more
severe at 6 l/min
Availability for air
absorption on weld
surface
15 l/min nozzle flow
constrains jet circa
3000 K isotherm
Arc plasma
temperature,
velocity similar
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Radiographic cross-examination
Ar Flowrate (l/min)
6 9 12 15 10 SW7 SW4 SW1 SW9
15 SW8 SW5 SW2 SW10
20 X SW6 SW3 SW11
10 SW15 SW14 SW13 SW12
10 SW16 SW17 SW18 SW19
6 l/min
9 l/min
Representative films &
bead on plate welds
Porosity Acceptable welds
Film
DH
36 S
am
ple
X-ray source
No
zzle
sta
nd
off
(m
m)
0 o
10 o
20 o
No
zzle
an
gle