cavitation erosion

1. North American Die Casting Association – 114thAFS NADCA
Cavitation
σi (vapor pressure) is the
performance threshold
such that
σ > σi no cavitation
effects
σ < σi cavitation effects
such as performance,
degradation, noise and
vibration
(non dimensional engrg parameters)
po = pressure
pv = vapor pressure
U = velocity
ρ (Greek rho) = atmospheric density
σ (Greek sigma) = performance
threshold)
σi = incipient cavitation

2. North American Die Casting Association – 114thAFS NADCA
Physics of Cavitation
• Any device handling liquid is subject to
cavitation
• Local pressure that falls below saturated
vapor pressure causes cavitation to occur
• Resulting vapor cavities (void bubbles)
collapse on surface material with high
energy and heat
• Inertial cavitation is commonly destructive

3. North American Die Casting Association – 114thAFS NADCA
Collapse on the surface
Spike generation as shown in 1-
18. Must have surface
nucleation in order to collapse
Surface – note formation of
spike 10-18 as vacuum
cavity collapses with high
energy

4. North American Die Casting Association – 114thAFS NADCA
Kinds of cavitation
• Dynamic – produced by pressure variations in a
flowing liquid – i.e. die casting
• Acoustic – sound induced
• Optic – photons of high energy – i.e. laser
• Particle – elementary particle – proton bubble
chamber.
(dynamic and acoustic – liquid tension)
(optic and particle – deposition of energy)

5. North American Die Casting Association – 114thAFS NADCA
Dynamic stages
• Traveling – moves with liquid flow
• Fixed – steady state fixed (sheet or cloud)
• Vortex – in cores of vortices of high sheer
Incipient – term used to describe type and
stage where cavitation appears
Desinent – term used to describe when
cavitation disappears

6. North American Die Casting Association – 114thAFS NADCA
Stages, Cases, Types
Propeller cavitation offers excellent examples of the basic types
cloud cavitation
tip vortex cavitation
hub vortex cavitation
propeller hull vortex cavitation
sheet cavitation
bubble cavitation
Blade root cavitation

7. North American Die Casting Association – 114thAFS NADCA
Example Vortex Incipient
Dynamic liquid flow is left to right at
a low velocity. Note – the beginning
tiny stream of vortex bubbles on the
tip of the obstruction
As the velocity increases, actual
vortexes can be seen along with sheet
and some cloud cavitation – note – how
the cavitation occurs on the opposite
side of the flow direction.
Similar to how core pins in die casting exhibit deterioration and
soldering on the back side of flow

8. North American Die Casting Association – 114thAFS NADCA
Video Examples
flow decreasing
flow direction

9. North American Die Casting Association – 114thAFS NADCA
Hard to believe facts
As the cavity bubbles collapse on the surface:
• Bubbles collapse to a minute fraction of original
size with immense focused force (4500 psi)
• At total collapse a violent mechanism results
temperature of bubble 5000 Kelvin or 8540 F (4727C)
• By comparison sun’s surface temp is 5778 Kelvin)
• Sonoluminescence (light) can be produced at
collapse (Pistol Shrimp)

10. North American Die Casting Association – 114thAFS NADCA
Inertial in HPDC dies
Obstruction in a flowing water with flow from left to right - note how incipient cavitation
moves to the back side of the core obstruction in the form
of a cloud while at the top right the 150,000 rpm vortex stream is created.
This is very representative of core deterioration from molten metal flow during HPDC.
Slow motion photographs of water at high velocity
flowing past an obstruction in its path creating
pressure drops and cavitation

11. North American Die Casting Association – 114thAFS NADCA
Not all cavitation is bad
Non-inertial
Submarine torpedoes
Ultrasonics for cleaning
Chemistry i.e. water purification
Homogenization of paint or milk
Medical for kidney stone removal or IV drug
Badly soldered H-13 core on left.
After cleaning ultrasonically with NaOH on right
Courtesy of FISA

12. North American Die Casting Association – 114thAFS NADCA
Occurrences
Inertial & Non inertial
• Aquatic animals – dolphins, tuna, shrimp
• Plants – xylem vascular plant system
• Humans – cracking knuckles, bullet wound
• Geology – Scablands Mega flood
• Dams – Spillways
• Industry – Propellers, Pumps, Turbines
High Pressure Die Casting Dies

13. North American Die Casting Association – 114thAFS NADCA
Mega flood
When a 2000ft high glacier ice dam broke, the high speed flood of water rushing over
the land caused cavitation inception. The water’s massive volume (1/2 Lake Michigan),
high speed (over 100ft/sec), and incipient vortex and sheet cavitation from obstacles
along the way carved out this area of land between eastern Washington and western
Montana about 15,000 years ago.

14. North American Die Casting Association – 114thAFS NADCA
The 1964 SDCE Paper
Abstract:
Die steel is washed away by the blasting turbulent
action of the molten metal entering the cavity.
Pitting and breakout is an excessive extension of
erosion.
Conclusion:
Theory could not be proven since the erosion
occurs in the opposite location to where it should.
Cavitation is what causes the failure which is
evident by observing pressure drop locations and
flow separation of the molten metal.
Paper 25 – 01-64-03S

15. North American Die Casting Association – 114thAFS NADCA
1981 SDCE Paper
Paper G-T81-123
Abstract
Zinc die casting dies experience cavitation
which damage the dies surface due to
pressure releases at collapse
Recommendations
Control geometic factors, velocities, design, and
material strength all dictate higher process control
to reduce the effect and damage from cavitation

16. North American Die Casting Association – 114thAFS NADCA
HPDC cavitation components
• Local pressure of the metal falls below the
saturated vapor pressure of the liquid
• Occurs at sudden change of flow direction
• Occurs at obstructions to flow that
generate pressure variations
• High velocities of flow at these locations
• Two basic types: sheet and vortex

17. North American Die Casting Association – 114thAFS NADCA
Breakout from Cavitation in HPDC
Pitting, breakout, erosion, and washout commonly occur in the opposite locations of
where they would be expected - on the low pressure side of the flowing liquid
FLOW

18. North American Die Casting Association – 114thAFS NADCA
Chicken and the Egg
Does thermal fatigue cause breakout or does
breakout lead to heat checking of the die’s
surface?
Can heat check cracking lead to breakout without
incipient inertial cavitation?
YES

19. North American Die Casting Association – 114thAFS NADCA
Cavitation & Heat check
Pressure drop location
FLOW
Cavitation Effect Heat check
breakout?
Cavitation effect exacerbates itself when
more low vapor pressure develops from the
disrupted flow patterns

20. North American Die Casting Association – 114thAFS NADCA
Cavitation Exacerbated
Gate
Sheet and Possible
Vortex Cavitation
& Erosion
exhibited sheet or cloud and vortex cavitation effect

21. North American Die Casting Association – 114thAFS NADCA
How others buffer cavitation damage
• Dam Spillways - Air slot
• Diesel Engines – Coolant cylinders textured
(patent 7146939 – manganese phosphate)
• Ship Propellers & Rudders – design, pressures,
buffer elastomer paint
• Turbines, Pumps – design, operating parameters

22. North American Die Casting Association – 114thAFS NADCA
Dam spillways concrete liner – Air slot
Leap frog cavitation in the dam spillway – damages 150ft long – 32ft deepest – 40ft dia.
The air slot installed (4ft wide x 4 ft deep 253 below spillway gate) creates a
cushion of non vacuum bubbles that buffer the cavitation destructive force and
allow higher flows
Chunk of concrete
Air bubble cushion
Air slot

23. North American Die Casting Association – 114thAFS NADCA
Diesel Engine Cavitation
Bigger engines suffer from vibrations of the cylinder walls inducing alternating low and high
pressures as the pistons move up and down. The resulting cavitation (like hitting the surface
with a microscopic ball peen hammer) causes pit holes to develop in the cylinder wall that let
cooling fluid leak into the cylinder wall.

24. North American Die Casting Association – 114thAFS NADCA
Diesel Engine - Texturing
Patent 7146939 – Dec 2006
It is feasible that this same buffering
action takes place in compressive
stress textured die surfaces with the
cooling molten metal absorbing the
cavitation shock along with the
induced fatigue resistant substrate.
Drawing of the diesel cylinder liner with textured
manganese phosphate coating and resultant
textured surface. Coating retains coolant which
absorbs the cavitation impact energy of the
collapsing cavities that damage the liner.

25. North American Die Casting Association – 114thAFS NADCA
Propellers and Rudders
The Navy and Marine industry spend
billions on the prevention and cure of
cavitation damage to ships’ propellers
and rudders. Cavitation also affects
power requirements for ships to attain
their maximum speed.

26. North American Die Casting Association – 114thAFS NADCA
Propeller and Rudder
Design and Elastomer Paint
Propeller Redesign
USS ARLEIGH BURKE
Class DDG 51)
Destroyer
Twisted Rudder
Redesign
Belzona 2141
& AER 48E Elastomer
Surface

27. North American Die Casting Association – 114thAFS NADCA
Pumps and Turbines
sheet cavitation bubbles acting on a
centrifugal pump impeller surface
Impeller drawing showing discharge and
cavitation location – low pressure side
cavitation at the blade location of a
turbine caused this damage Vortex (tip) cavitation at hub of
turbine

28. North American Die Casting Association – 114thAFS NADCA
Design and Operation
Operation
• Prevent dead head operation
(occurs when the pump's discharge is closed either due to a blockage in the line or an inadvertently closed
valve. )
stationary diffuser vanes
rotating impeller
double-volute wall
Pump and turbine design changes to improve flow and reduce the possibility of cavitation
• Prevent suction recirculation
(fluid will start to recirculation or move in a reverse direction at the suction and at the discharge)

29. North American Die Casting Association – 114thAFS NADCA
Application to HPDC
Cavitation – controlled but not eliminated
Factors to control or consider
• Velocity of metal (dam spillway)
• Extreme pressure variations (all)
• Buffering the cavitation effect (diesel)
• Fatigue resistance (compressive)
• Design & Operation parameters (pumps)

30. North American Die Casting Association – 114thAFS NADCA
Considerations
• Simulation studies when designing
• Better stronger and tougher die materials
• Consideration regarding cavitation inception
• Gate designs
• Gate velocities
• Increase the fatigue resistance
• Buffering the harmful cavitation bubbles

31. North American Die Casting Association – 114thAFS NADCA
Buffer by compressive texture?
After 30,000 shots, there is no evidence of cavitation
damage in the form of breakout, pitting, or erosion.
Did the compressive texturing buffer cavitation?
macro photo of casting surface shown belowtypical compressive
finish for 47-48Rc

32. North American Die Casting Association – 114thAFS NADCA
Conclusions and Summary
1. Anything handling liquid is subject to cavitation
2. Found in nature, geology, industry and hpdc
3. Inertial cavitation (flow and pressure) is destructive
4. Occurs when the local pressure falls below vapor pressure
5. Has always been assumed that molten metal is to blame
6. Very little investigation to date for hpdc cavitation
7. There is a need to study its effects to hpdc dies
8. Considerations include parameters from design to operation
9. You will never totally eliminate cavitation
10. Are there similar phenomena in HPDC??
11. Can the cavitation effect be buffered in HPDC?