In the casting process of copper balls, the formation of internal shrinkage cavities primarily stems from insufficient compensation for the volume shrinkage during solidification of the molten metal. This phenomenon is closely related to the casting structure design, gating system optimization, process parameter control, and material properties. Shrinkage cavities typically appear in the hot spots during the final solidification of the casting, such as thick-walled sections or areas of geometric abrupt changes. Their formation mechanism is directly related to the solidification characteristics of the molten metal.
Casting structure design is fundamental to controlling shrinkage cavities. If the cross-sectional dimensions of the copper ball vary too much, the cooling rate in thin-walled areas is much faster than in thick-walled areas, making sequential solidification difficult and prematurely interrupting the feeding channels. For example, sharp corner structures exacerbate local heat accumulation, forming hot spots; while rounded transitions can uniformly distribute the temperature field and reduce stress concentration. By optimizing the structure, such as replacing sharp corners with rounded corners, achieving smooth transitions between thick and thin sections, or using hollow structures to disperse hot spots, the risk of shrinkage cavities can be significantly reduced.
The design of the gating system directly affects the filling and feeding capabilities of the molten metal. Riseres are the core component for feeding; their size, location, and quantity must be precisely matched according to the distribution of hot spots in the copper ball. Riser should be placed in the last solidification zone to ensure that the molten metal continuously feeds towards the hot spot under pressure. If the riser size is insufficient or its position is off, the feeding channel will solidify prematurely, leading to shrinkage cavities. For example, in a magneto cover case, by extending the solidification time of the ingate and adjusting the pouring direction, the shrinkage cavity size was successfully reduced to an invisible size. Furthermore, the cross-sectional area and number of ingates need to be optimized to avoid pressure attenuation when the molten metal preferentially fills deep cavities or thin-walled areas, affecting the feeding effect.
Controlling process parameters is a key aspect of shrinkage cavity prevention. Excessively high pouring temperatures will prolong the solidification time of the molten metal, exacerbating mold wall displacement and causing feeding failure; excessively low temperatures will reduce the fluidity of the molten metal, making it difficult to fill the mold cavity. During the smelting process, the copper composition must be strictly monitored to avoid impurities or gas contamination, reducing heterogeneous nuclei formed by oxide inclusions, which can become the starting point for shrinkage cavities. Simultaneously, the strength and compactness of the molding sand must meet standards to prevent mold displacement from blocking the feeding channel. For example, insufficient molding sand strength may lead to increased casting cross-section, disrupting the sequential solidification conditions. The influence of material properties on shrinkage cavities cannot be ignored. The solidification characteristics of copper alloys directly affect their tendency to shrink. For example, high-silicon aluminum-silicon alloys shrink the solid-liquid coexistence zone, increasing solidification time and the risk of shrinkage cavities. By adjusting the alloy composition and selecting alloys with near-eutectic compositions or narrow crystallization ranges, solidification behavior can be optimized, reducing the tendency to shrinkage cavities. Furthermore, the purity of the molten copper needs to be improved through refining to reduce oxide inclusions and gas content, thus reducing the causes of shrinkage cavities at the source.
The design of the mold and cooling system needs to be optimized in tandem. The uniformity of the mold temperature field is crucial for the solidification process of the copper ball. If the mold is locally overheated, it will lead to delayed solidification of the molten metal and premature failure of the feeding channels. By adding cooling pipes or using mold materials with better thermal conductivity, cooling of hot spots can be accelerated, shortening the solidification time difference. For example, in one case, by replacing the core rod material with beryllium bronze, the solidification time of the thickest part was successfully shortened, reducing the shrinkage cavity area.
Post-processing can further eliminate residual shrinkage cavities. For structural castings, processes such as hot isostatic pressing (HIP) can compress internal cavities without damaging the shape, but this is costly and suitable for high-precision applications. Conventional castings require grinding and polishing to remove surface defects, while simultaneously inspecting internal quality to ensure shrinkage cavities do not affect mechanical properties.
Controlling shrinkage cavities in copper ball casting requires coordinated optimization across the entire process, including design, manufacturing processes, materials, molds, and post-processing. By optimizing the structure to reduce hot spots, precisely designing the gating system, strictly controlling process parameters, and selecting appropriate materials and mold designs, the risk of shrinkage cavities can be significantly reduced, improving casting quality and yield.
