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Simulation of a high pressure die casting filling process.
This is essential part of aluminum casting design process. Both part geometry and gate design can be optimized on this stage.
Shrinkage porosity
Shrinkage porosity is a major cause of defects in castings. Volumetric shrinkage occurs when a material solidifies and the density of the solidified material is greater than the density of the liquid.
Porosity forms in castings as a direct result of the volumetric shrinkage. Solidification progresses from the outside surfaces towards the center. With narrow freezing-range alloys, such as die-cast aluminum or zinc, the directional solidification creates a skin effect and forces concentration of shrinkage voids toward these section centers. Variations in shape cause regions of converging and diverging heat flow paths. Since heat will flow out of the casting quickly through diverging paths and slowly through converging paths, solidification will not progress at the same rate from each cavity surface. Uneven heat flow moves the neutral axis toward the "hot" side of the casting. The shift of the neutral thermal axis towards one surface reduces the "skin effect" on the one side and results in an excessive "skin effect" on the other.
Porosity can be moved around, within limits, minimizing it in critical areas of the casting. For this reason, thermal analysis, in the early design phase, is important to establish the thermal gradient at the cavity surface.
Die cycling simulations
Die cycling simulations are essential for high pressure die casting since the same die is used repeatedly to produce many thousands of castings. These simulations must provide accurate temperature distributions in a die through the duration of the process.
Defect tracking
An obvious goal of die casting process is to eliminate defects in the cast parts. Defects can arise from a variety of causes. Air and oxide entrainment is the root cause for over 83% of defects in rejected aluminum high-pressure die-castings. These defects may result in a mechanical weakness or loss of pressure integrity in the final part. Because high-pressure die-casting involves very high-speed metal flow it is difficult to avoid some entrapment of air during the die filling process. Effective "atomization" of the incoming metal streams is critical to more uniformly disperse the entrapped gasses over the entire part as well as producing the best material grain structure (best mechanical properties).
Simulation is one of the tools we are using to achieve this goal. It offers a relatively cheap and simple way to optimize gating and part designs for minimizing air entrapment.
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References:
- V.Adams and A.Askenazi, Building Better Products with Finite Element Analysis, OnWord Press, Santa Fe, 1998
- E.A.Herman, Heat Flow in the Die Casting Process, Society of Die Casting Engineers Inc., Rever Grove, 1983
- Flow Science Newsletter, Flow Science, Inc., 1996
- M.Koehler, Getting Started in Metal Casing Simulation, Desktop Engineering, September 2001
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