Study On The Formation Of Intrusive Pores In Al-Si Castings Made In Sand Mold

With the mass production and popular use of large-size aluminum alloy castings, the intrusive pore defect has become an increasingly concerning problem. However, at present people don't know very well about intrusive pores. To have a comprehensive and detailed understanding of the stereoscopic morphology of intrusive pores, and to investigate the effect of process parameters on the formation of intrusive pores, a sand mold with a key sand-core was designed, in which intrusive pores are more likely to form.Combining XCT (X-ray computed tomography) with 3D reconstruction technology, the morphology details of intrusive pores can be analyzed automatically through the software of VG Studio MAX 2.2, however, it is only for closed pores. A new method has been found to show 3D characteristics of open pores on or pores near the surface of the sample). Through this method, the volume, superficial area and sequent sphericity of each pore was estimated and the volume fraction of intrusive pores in casting was obtained. The 3D morphologies of intrusive pores suggest they are much more complex than expected. And the formation and hence morphology of intrusive pores are dependent on three main factors:a) the competition between the solid and pores for growing space; b) the solidifying shrinkage of inter-dendritic liquid; c) the flow of melt. The 1st one is most important.Effects of resin content and sand particle size in sand mold, melt quality (whether the melt is treated by vacuum-holding or not) and composition of aluminum alloy (hypo-eutectic or near eutectic Al-Si alloys) on the formation of intrusive pores were investigated. It is found that:a) All the number, volume fraction and sphericity of intrusive pores in the near-eutectic casting are higher than that in the hypo-eutectic casting, which is mainly because the near-eutectic alloy has a lower surface tension, longer liquid time and more latent heat (poured at the same temperature), b) For hypo-eutectic alloy, resin content (3% or 5%) in the sand mold has little effect on the number, volume and sphericity intrusive pores. c) In hypo-eutectic alloy casting, sand mold with coarse particles leads to more pores and larger volume pore fraction than the one with fine particles, but in near-eutectic alloy casting, sand mold with fine particles results in more pores and larger volume pore fraction. It indicates the effect of sand particle size on the formation of intrusive pores are closely related with the thermal properties of the alloy. d) Whether for hypo-eutectic alloy melt or for near eutectic alloy melt, vacuum-holding treatment leads to dramatical drop of the number and volume fraction of intrusive pores in the castings and the pores become much more rounded. It suggests that the melt quality has an important influence on the formation of intrusive pores.