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Predicting Castability for Thin-Walled HPDC Parts

Posted by Hill and Griffith Company on Jun 17, 2020 2:43:30 PM

Current simulation tools account for the physics of fluidity, and should be capable of predicting castability too. Investigating thermal resistance shot sleeve heat loss effect of conventional HTC values.

Excerpt from the December 2014 article from Foundry Management & Technology by R. Bhola and S. Chandra

The term "thin-wall castings" for the HPDC process has been investigated for well over two decades.  However, the perception of what is considered a thin-walled part has changed over the years and continues to change.  Historically, 3-mm wall thickness was considered a thin HPDC part and that number decreased over the years to 2 mm, and further to 1 mm. Currently, there is a push for even thinner walled parts in the electronics industry, with thickness demands as low as 0.6 mm.  The automotive industry has been looking to make ultra large castings (ULC) using various processes, including semi-solid, permanent mold, and HPDC processes, with target thickness in the range of 1 to 2 mm[2].

Thin-walled parts are difficult to cast because the melt cools rapidly upon contact with the relatively cold die steel and can solidify quickly before die filling is complete.  The distance a given molten material travels before it freezes and stops moving is commonly referred to as "fluidity".  The dominant variables affecting fluidity are: thermophysical properties of the melt; the temperature of the melt above liquidus (superheat); and mold coating release agent[3,4].  Much of this work on quantifying fluidity was based on experiments under low pressures, generally not exceeding 15 psi to force the liquid metal through a passage. 

Further, die temperature and the heat-transfer coefficient at the die surface do not seem to be considered properties that define fluidity, though Dewhirst[4] acknowledged that the heat-transfer coefficient at the mold surface does play a significant role in the measured flow lengths for a given alloy type under given test conditions.

High Pressure Die Casting

The focus of this work is to determine if computer models can realistically predict the castability of a given ultra thin-walled part and to test the sensitivity of dominant variables that affect this castability prediction.  We confined our investigation to one cast material (A383) in a die made of H13 steel, for the HPDC process.  Our goal is to determine if we can use simulation at the concept phase of product design to specify the foundry settings necessary to successfully cast that given product.  The variables accounted for in this study are: superheat; die temperature; fill time; and die surface condition, including any "contaminants."

Thermal resistance melt/die interface. When the melt suddenly contacts the relatively cold die surface, its initial rate of heat loss is strongly controlled by this "thermal contact resistance" (Rc) at the microscopic interface that separates the melt from the die.  The value of this thermal resistance depends on the die surface roughness, die lube deposits, oxide layer on the die and oxide film on the liquid metal itself[5,6].  The die casting industry commonly refers to a heat-transfer coefficient (HTC) instead of a thermal resistance (Rc).  This HTC is simply the inverse of thermal resistance (HTC = 1/ Rc).

Since we are exploring heat loss during filling, we are concerned with the insulating characteristics at the die surface for time scales ranging from (0.001s to 0.1s).  The perception of what the heat-transfer coefficient is at the die surface when contacted by the melt varies significantly in HPDC, with values seen to range from ~5,000 W/m2K to ~120,000 W/m2K.  We are careful to select this value as accurately as possible because this HTC strongly affects the prediction of the heat loss during fast shot. 

Measuring the contact resistance of the melt suddenly contacting the die surface is a challenge because the surface temperature sensor must have a response time of less than 1/10th the timescale that we wish to know this value for [6].  So if we are concerned with knowing this value during die filling, we must have sensors with response times in the sub-millisecond range.

A detailed investigation of the influence of surface roughness, die lube deposit, and melt oxide was conducted by Y. Hichal for A383 alloy.  We selected measurements from Hichal's work because he used a surface temperature sensor with a response time of 40 nanoseconds!  This sensor made it possible to determine the thermal resistance during millisecond timescales that occurs in the die casting process.

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Tags: Die Casting, Die Casting Release Agents, Die Casting Lubricants, Release Agents for Die Casting, Aluminum Die Casting Process, Die Casting Defects, Die Casting Mold Lubricants, Die Casting Lubricant, Aluminum Die Casting, Die Casting Lubrication and Application, die casting lubricant management, Foundry Management & Technology magazine

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