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North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Cavitation
σ i (vapor pressure) is the performance threshold such that
σ > σ i no cavitation effects
σ < σ i cavitation effects such as performance, degradation, noise and vibration
(non dimensional engrg parameters) po = pressurepv = vapor pressureU = velocity ρ (Greek rho) = atmospheric density σ (Greek sigma) = performance threshold)σi = incipient cavitation
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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)
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Video Examples
flow decreasing
flow direction
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Hard to believe factsAs 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 resultstemperature 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 CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Inertial in HPDC dies
Obstruction in a flowing water with flow from left to right - note how incipient cavitationmoves 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Not all cavitation is badNon-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 CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
OccurrencesInertial & 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 CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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.
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
1981 SDCE Paper Paper G-T81-123
AbstractZinc die casting dies experience cavitation which damage the dies surface due to pressure releases at collapse
RecommendationsControl 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 CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Cavitation ExacerbatedGate
Sheet and PossibleVortex Cavitation
& Erosion
exhibited sheet or cloud and vortex cavitation effect
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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.
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Diesel Engine - TexturingPatent 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.
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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.
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Propeller and Rudder Design and Elastomer Paint
Propeller RedesignUSS ARLEIGH BURKE
Class DDG 51) Destroyer
Twisted Rudder Redesign
Belzona 2141& AER 48E Elastomer
Surface
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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)
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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)
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS 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
North American Die Casting Association – 114th Metalcasting CongressCastExpo’10, March 20-23, 2010 Orlando, Florida USA
AFS NADCA
Conclusions and Summary
1. Anything handling liquid is subject to cavitation2. Found in nature, geology, industry and hpdc3. Inertial cavitation (flow and pressure) is destructive4. Occurs when the local pressure falls below vapor pressure5. Has always been assumed that molten metal is to blame6. Very little investigation to date for hpdc cavitation7. There is a need to study its effects to hpdc dies8. Considerations include parameters from design to operation9. You will never totally eliminate cavitation
10. Are there similar phenomena in HPDC??11. Can the cavitation effect be buffered in HPDC?