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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

Collapse on the surface Surface – note formation of spike 10-18 as vacuum cavity collapses with high energy Spike generation as shown in 1-18. Must have surface nucleation in order to collapse North American Die Casting Association – 114th Metalcasting Congress CastExpo’10, March 20-23, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

Video Examples flow direction flow decreasing North American Die Casting Association – 114th Metalcasting Congress CastExpo’10, March 20-23, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

The 1964 SDCE Paper Abstract: Conclusion: Paper 25 – 01-64-03S 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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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 Congress CastExpo’10, March 20-23, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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. North American Die Casting Association – 114th Metalcasting Congress CastExpo’10, March 20-23, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

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, 2010 Orlando, Florida USA

Reference items Aluminum Vapor Pressure = 2.42E-06Pa@660°C (1220°F) 101.325 kPa 14.696 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 www.safl.umn.edu North American Die Casting Association – 114th Metalcasting Congress CastExpo’10, March 20-23, 2010 Orlando, Florida USA