Shrinkage in aluminum gravity die casting primarily manifests as defects such as porosity, surface depressions, incomplete filling, and cold shuts.
The causes are closely related to process parameters, mold design, and material characteristics.
Porosity Defects
Porosity is characterized by smooth pore walls, distributed individually or in clusters beneath or on the surface of the casting. Causes include:
High gas content in the molten aluminum
Gas entrapment during pouring.
Gas generation from the reaction between the molten aluminum and the mold coating.
Gas attached to inclusions.
Improvement measures: Optimize coating thickness, lower core temperature (e.g., from 480℃ to a reasonable range), improve the funnel structure (change outlet diameter from φ36 to φ28) and increase the number of filter layers.
Surface Depressions
These appear as depressions on the smooth surface of the casting, commonly found in hot spots or areas with insufficient feeding. Reasons include:
Large local hot spots
Improper gate location or insufficient cross-sectional area.
Mold temperature too high or too low.
Insufficient feeding pressure.
Improvement measures: Adjust the core cooling system (increase water cooling channels, optimize vent hole layout), and ensure unobstructed feeding channels in the partition.
Incomplete Filling and Cold Shuts
Incomplete filling is characterized by incomplete filling and unclear contours, while cold shuts are smooth joint lines at the junction of molten metal flows. Common causes:
Poor fluidity of the alloy (e.g., high gas content, low pouring temperature).
Too low pouring speed or poor venting.
Unreasonable gate design (excessively long flow path).
Improvement measures: Increase pouring temperature, optimize gate location, and enhance venting (e.g., increase the number of vent grooves).
Influence of Process Parameters.
Mold temperature significantly affects shrinkage defects:
Too high: Leads to slow solidification of hot spots and failure of feeding.
Too low: Premature solidification of the partition, hindering feeding.
Sequential solidification needs to be achieved through adjustment of the cooling system (e.g., water cooling channel spacing ≥10mm).
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