1. A New Approach to the Cause of High Pressure Die Steel Erosion, Pitting, and Breakout
J.V. Skoff
Badger Metal Tech, Inc.
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

2. Cavitation i (vapor pressure) is the performance threshold such that
 > i no cavitation effects
 < i 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  (Greek sigma) = performance threshold)
i = incipient cavitation
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

3. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

4. Collapse on the surface
Surface – note formation of spike as vacuum cavity collapses with high energy
Spike generation as shown in Must have surface nucleation in order to collapse
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

5. 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)
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

6. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

7. Stages, Cases, Types
cloud cavitation
tip vortex cavitation
hub vortex cavitation
propeller hull vortex cavitation
sheet cavitation
bubble cavitation
Blade root cavitation
Propeller cavitation offers excellent examples of the basic types
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

8. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

9. Video Examples
flow direction
flow decreasing
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

10. As the cavity bubbles collapse on the surface:
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)
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

11. Inertial in HPDC dies
Slow motion photographs of water at high velocity flowing past an obstruction in its path creating pressure drops and cavitation
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.
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

12. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

13. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

14. 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.
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

15. The 1964 SDCE Paper Abstract: Conclusion:
Paper 25 – S
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.
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

16. 1981 SDCE Paper Paper G-T81-123 Abstract Recommendations
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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

17. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

18. Breakout from Cavitation in HPDC
FLOW
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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

19. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

20. Cavitation & Heat check
Pressure drop location
Cavitation Effect
Heat check breakout?
FLOW
Cavitation effect exacerbates itself when more low vapor pressure develops from the disrupted flow patterns
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

21. Cavitation Exacerbated
Gate
Sheet and Possible Vortex Cavitation & Erosion
exhibited sheet or cloud and vortex cavitation effect
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

22. How others buffer cavitation damage
Dam Spillways - Air slot
Diesel Engines – Coolant cylinders textured
(patent – manganese phosphate)
Ship Propellers & Rudders – design, pressures, buffer elastomer paint
Turbines, Pumps – design, operating parameters
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

23. Dam spillways concrete liner – Air slot
Chunk of concrete
Leap frog cavitation in the dam spillway – damages 150ft long – 32ft deepest – 40ft dia.
Air bubble cushion
Air slot
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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

24. 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.
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

25. Diesel Engine - Texturing
Patent – 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.
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

26. 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.
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

27. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

28. cavitation at the blade location of a turbine caused this damage
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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

29. Design and Operation Operation Prevent dead head operation
stationary diffuser vanes
rotating impeller
double-volute wall
Pump and turbine design changes to improve flow and reduce the possibility of cavitation
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. )
Prevent suction recirculation (fluid will start to recirculation or move in a reverse direction at the suction and at the discharge)
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

30. Application to HPDC Factors to control or consider
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)
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

31. 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
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

32. Buffer by compressive texture?
typical compressive finish for 47-48Rc
macro photo of casting surface shown below
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?
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

33. Conclusions and Summary
Anything handling liquid is subject to cavitation
Found in nature, geology, industry and hpdc
Inertial cavitation (flow and pressure) is destructive
Occurs when the local pressure falls below vapor pressure
Has always been assumed that molten metal is to blame
Very little investigation to date for hpdc cavitation
There is a need to study its effects to hpdc dies
Considerations include parameters from design to operation
You will never totally eliminate cavitation
Are there similar phenomena in HPDC??
Can the cavitation effect be buffered in HPDC?
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA

34. Reference items
Aluminum
Vapor Pressure = (1220°F)
kPa psi = Standard atmospheric pressure for earth sea level
62.4 cu.ft./32.2 cu.ft. specific density = slugs 1.94
French studied cavitation in - liquid Na and Hg
Oakridge Lab concern - cyclotron - nutrons and Hg
North American Die Casting Association – 114th Metalcasting Congress
CastExpo’10, March 20-23, Orlando, Florida USA