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Badger Metal Tech
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fixing other modes of die casting (diecasting)
failures
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Cavitation
videos - strong destructive
forces to die casting dies.
In hydraulic pumps, it occurs, not on the high pressure side but rather on the low pressure side and was misunderstood for years. Unfortunately this effect also occurs too frequently during die casting operations but is not recognized for what it is. Breakout (which usually occurs before major heat checking) should be attributed more to cavitation effect than thermal fatigue. This may be a new science for die casting, but it has already been documented in the fluid dynamic studies of rotary hydraulic pumps for impact hammers used to break up rock and concrete. NADCA does recognize cavitation as a mode of failure but has not understood the devasting result of not addressing it.
There are a number of contributing factors that lead to cavitation including: trapped gas bubbles, local change in flow, and varying pressures all of which create voids or trapped gas bubbles. The die casting process has all 3 of these present along with exacerbated high temperature extremes. Any one or a combination of these can lead to cavitation effect.
When Badger Metal examines castings from dies that come to us for initial treatment in a USED condition, most all of these show varying degrees of premature surface failure (breakout, pitting, erosion) in the gated area of the casting. We also see this condition where flow fill dramatically changes direction . Molten metal, being at extremely high temperatures, behaves somewhat like boiling water. As a liquid is close to turning into a gas state, many bubbles are present. Even after prudent degassing, molten metal still contains these bubbles with new volumes introduced every time the die is filled with molten metal. These bubbles prevent the liquid (mroten metal) from being in a sought after ideal "incompressible" state. As this gaseous liquid enters the die, the bubbles collapse (implode) on the die's surface. As a bubble implodes, the nature of this implosion creates millions of cycles of "void to positive" impact pressures on the surface of the die even within extremely short durations of thousands of a second (figure 1). These millions of cycles, even though they are at relatively low pressure, result in higher cycle fatigue locally at the die surface. Extremely small particles begin to separate from the die's surface during each shot. The small particle separations are initially not visible to the human eye (figure 2). Very rapidly, however, and after a relatively few number of shots and billions of bubble implosions, the continued separations exhibit themselves as visible pits in the surface commonly referred to as breakout and still mistakenly blamed on thermal heat checking (figure 3). How can the implosion of these small pressure bubbles do so much damage?
Even though the pressures are very low, they see billions of these cycles in a very short period of time. These billions of cycles, in a short period of time, start with small microscopic die material particles being removed from the face of the tool which eventually exhibit themselves as missing metal from the tools surface and teats on the casting surface. If there is an absence of these gases being induced, such as in Case Western's dip tank test, there is also an absence of breakout which further supports confirmation of the cavitation fluid dynamic effect. The cavitation effect continues further weakening the die steel surface as more tensile stresses are developed. The stresses build until the fatigue strength of the steel is exceeded at which point heat checking begins. Sometimes both breakout and heat checking appear to start at the same time. This creates a recipe for disaster and accelerated die failure. See photos below.
back to Sep 07 newsletter Addressing The Issue - Apply MetaLL* ifeÒ Before Sampling To Both the Cover & Ejector! Badger's validation has always been MetaLL ifeÒ's ability to close minor heat checks in USED tooling in an encapsulating layer of compression thereby increasing the fatigue strength of the steel and preventing propagation or initiation of further cracks. We cannot, however, put back missing metal. The fix for missing metal is to apply weld to the voids which only further decreases the steel's integrity leading to more problems. In the early 1990's, realizing that it is always better to prevent rather than to fix a problem, we began to recommend doing MetaLL ifeÒ to NEW tooling but after sampling and on both sides of the die. Some customers only do the MetaLL ifeÒ to the problem side which is WRONG since metal flows and cavitation occurs on both sides of the die. Another problem with doing MetaLL ifeÒ after sampling is that many tools go straight into production so they never get the benefit of being done except as a Band-Aid fix.
Research
has also shown than only a relatively small number of shots starts to weaken
unprotected steel
by starting the generation of the unwanted tensile stresses and micro cracking
on corners. Advancements in EDM and heat treatment allow many tools today, to be built and run without any or sometimes many major engineering corrections and changes. Because of these advances and what the Residual Stress study showed, Badger now advocates doing tooling when NEW but BEFORE sampling. If some changes are necessary, we are willing to reprocess the tooling if changes are required and do so at a substantial reduced or on a "No Charge" basis. If you do MetaLL an ifeÒ to the tool (cover & ejector) in this manner and again at the normal half life of the tool, we will GUARANTEE a 25% increase in tool life* beyond what you customarily would get. Think of it this way. When your car is new, do you wait until the vehicle has 75,000 miles and the engine shows problems before you change the oil for the first time? Of course not. Equally so, with this new knowledge, die casters must be even more proactive regarding tool preventative maintenance.
back to newsletters
What Will
MetaLL*
ifeÒ
Do For My
Tool Life and Die
Performance?
Simple explanation: In addition to creating an encapsulating protective layer of compression, the MetaLL ifeÒ process takes the smooth surface finish at critical locations of a die's surface and adds micro uniform topography with alternating high and low wave shapes. As the molten metal liquid passes over the gate and runner surfaces, these summits and valleys quickly disrupt cavitation bubbles, thereby eliminating or drastically reducing the destructive eroding and pitting from vortex or sheet cavitation on the die's surface (Figure 4).
How we stop the bubbles:
The
MetaLL
ifeÒ
compressive surface is effective to intentionally create a very thin stagnant
layer of liquid metal adherent to the outer die surfaces. Typically, this layer
of stagnant cooling liquid measures anywhere from 2 to 20 μm thick, depending
upon the composition and viscosity of the cooling medium. At this order of
magnitude, adhesive forces strongly bind the molten substance to a surface,
especially if the fluidized metal is polar in nature (sharing electrons) like
water (H2O). Also at this magnitude, surface tension effects
become very pronounced. Adhesion and surface tension effects are thus
leveraged by the surface texture. Thus, the cavitation bubbles are held,
by this stagnant layer, away from the outer surface of the die. Moreover,
the impinging jets from imploding bubbles will have a longer path to travel and
have to overcome the uniform film formed by the stagnant fluid layer. This
shielding action rapidly dissipates the incoming high kinetic energy from the
imploding bubbles on the die's surface. In addition the metal cools more
uniformily thus eliminating laminar fill problems. The topography range (breadth and height), coupled with the tight uniform compressive spacing, enables the adhesion and surface tension effects within the liquid cooling medium to couple and act as capillary action to constitute the stagnant fluid layer on the surface.
back to Sep 07 newsletter When flow is inefficient and gases are trapped between two layers of molten metal, laminations or laminar fill can occur. This also is referred to as blistering. Metal rushes over a flat surface of the die, and as heat is transferred, the metal closest to the die surface cools first. The gas bubbles trying to escape, however, are trapped by a thin layer of already solidified metal. As the rest of the metal further solidifies, this may create a thin layer with trapped gas between them (Figure 5). This lamination can cause rejected castings because of appearance or internal porosity damage as in the case of a transmission or other fluid sealed dependent parts. Eliminate or reduce the gas bubbles (see cavitation effect) and eliminate the laminations/porosity (Figure 6). MetaLL ifeÒ accomplishes this by its topography change that functions to:
1. Create turbulent flow which allows more uniform solidification since the
metal does not cool in layers.
Summary Die cast tools often show heat checking and cavitation breakout, especially in the runner/gate areas, or low pressure voids when filling the casting. Both can occur, however, one usually leads to the other but in themselves are not interdependent. Heat checking is due to the fatigue yield strength of the die being exceeded by thermal, chemical, and flex stresses (tensile), while breakout is the result of vortex or sheet cavitation on the steel's surface. MetaLL ifeÒ provides benefits in both of these areas by increasing the fatigue resistance and reducing cavitation effect, thereby extending tooling life, countering breakout effect and stopping lamination or blistering problems. |
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