Metallife and Thermallife extend die casting die life and improving performance of die casting dies and other molds.

Buffers Cavitation Breakout Effect

Almost all High Pressure Die Casting dies experience cavitation. This often occurs in or the near Gate area or other locations of metal flow where the local pressure suddenly falls below the vapor pressure of the molten metal.

Since we have processed more than 100,000 die inserts and tooling during our 28 years, we have also seen more distress failures of this nature than anyone else in the industry.

A paper on this phenomena was presented at the 2010 Casting Expo in Florida. The paper shows why cavitation occurs, and how it can be reduced and buffered. This destructive force is just now beginning to be acknowledged and studied by the die casting industry.

The cavitation force and frequency is what causes these pit marks. Usually these pit marks grow to become stress concentration points from which cracks can propagate. Some pit marks, however, stay as pits without crack lines propagating from them. So heat checking can occur with of without these pits. From our experience and observation, far too often the two go together.

These pit marks in the die steel show up as round raised rough areas on castings. People that have been in die casting for years, mistakenly still attribute exacerbated heat checking (thermal fatigue) to the creation of these pit marks.

Pitting of the die causes raised small bumps on casting

Smaller die pits, if caught early enough, can be corrected

This distress results in higher die casting costs, either through the need for casting rework (to remove the risers on the casting) or increased casting scrap rates. In the worst case, repair or retirement of the die occurs. Welding of these numerous pitted areas is not effective so many times the dies need to be re-sunk to get below the breakout.

As previously stated, the cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure of the liquid...a value given by the tensile strength of the liquid. Small low pressure vacuum bubbles collapsing on the surface of the steel with extreme force and rapidity create high energy "jackhammer" like shockwaves leading to steel fatigue and breakout. Although it is hard to conceive, the vacuum vapor in the bubble at collapse has been recorded to be 5,000 kelvin (8,540 F or 4,727 C) which is close to the surface temperature of the sun (5778 kelvin). At the same time pressures of several hundred atmospheres of pressure (4500 psi) are exerted on the nucleating surface at the same location with each shot.

MetaLLife increases the steel's fatigue strength of die steel by inducing a high value of compression that can be as much as .018" in subsurface depth. Cracks of pits do not start in a steel surface that is in a compressive state. Steel has to go into tensile stress before it yields to fatigue.

This MetaLLife surface is textured. This surface retains lubricant as well as changing the flow characteristics of the molten metal. Buffering is accomplished when the bubbles collapse, not on the steel, but on the lubricant or flowing metal that is present on the textured surface. The buffering surface is replaced with each shot. This integral renewable shield absorbs the implosion energy from the collapsing bubbles which is then quickly healed with each shot.

No other die surface engineering treatment besides MetaLLife and TooLLife can provide these two benefits.

Unfortunately cavitation cannot be totally eliminated, however, these high energy vacuum bubbles can be buffered. By increasing the steel fatigue strength and causing nucleation (collapse) on another surface other than the steel surface, the cavitation effect is buffered. Diesel engine coolant cylinder liners use the buffering method to reduce cavitation effect to cylinder walls.

Whenever the high velocity molten metal flow path is disrupted to cause the vapor pressure to drop below local pressure, cavitation results. This includes a sudden change of flow direction or obstructions to flow. Examine your dies and see if your breakout and washout are in these areas. To see papers, videos, and slide presentations of these forces and how they occur in nature and the industry, visit our section.
Resources.

The phenomenon of metal breakout is not new and occurs in other industries besides die casting. Often it is experienced in pumps, turbines, diesel engine cylinders and other products such as boat propellers. The boat propeller will literally be eaten away over time without being subjected to any type of outside abrasive or destructive environment other than water.