Research On The Characterization Of Microporosity And Its Formation In Aluminum-copper Alloys

Microporosity, which is one of the most common defects in aluminum alloy castings, isdetrimental to the casting pressure tightness and mechanical properties. A lot of researcheshave been conducted on the mechanism of microporosity formation, microporosity formationprediction, relationship between microporosity and casting mechanical properties, and so on.These researches provided important theories for understanding of microporosity formation,controlling microporosity defects and getting sound aluminum castings. Three dimensional(3D) characterization of microporosity in aluminum casting alloys is research frontier and hotpoint in recent years. The domestic reports in this field are very limited. In this study,microporosities in aluminum-copper alloys with different microstructures were investigated.The effect of compositions and solidification parameters is theoretically analyzed anddiscussed.A series of Al-Cu alloys specimens were prepared by the use of Bridgman typedirectional solidification (DS) apparatus, and the pulling speeds were set to50~150μm/s. Theporosities in directionally solidified specimens were inspected by high resolution3D X-raytomography. The3D characteristics of porosities were analyzed. The effect of alloycompositions and pulling speeds on microporosity formation was discussed. It is found thatthe average pore volume in Al-15wt.%Cu alloy is smaller, while the average sphericity ofpores is higher than that of Al-4.5wt.%Cu alloy at the same pulling speed of150μm/s. Thepulling speed of DS, which actually determines the solidification velocity, has a significantinfluence on the pore formation. Generally, the lower the pulling speeds, the larger theaverage pore volume and the more uniform the pore size distribution. The interaction ofcolumnar dendrites and pore growth is clearly observed in longitudinal sections of thedirectionally solidified Al-Cu alloys.Wedge-shaped castings of Al-Cu alloys with the decreasing cooling rate from bottom totop were cast, and inspected by using the3D X-ray tomography to obtain pore morphology.The results show that microporosity distributes in a very large range from the bottom of theAl-4.5wt.%Cu casting, but densely distributes in the top part of the Al-15wt.%Cu casting. The3D morphologies of pores in equiaxed grain microstructures are much more complex than those in columnar dendrite microstructures. A linear relationship between equivalent porediameter, pore sphericity and local cooling rate in the Al-4.5wt.%Cu alloy casting is alsodeduced. As the rate increases, the diameter decreases while the sphericity increases.According to the DS characteristics, a numerical model of microporosity formation indirectionally solidified aluminum alloy was proposed, assuming that pores are nucleatedhomogeneously in the mushy zone at the hydrogen supersaturation. The growth of pores isconsidered to be controlled by hydrogen diffusion through pore/liquid interface. The modelalso tries to consider continuous nucleation to predict pore numbers. A C++program wasdesigned and developed. It has been applied to directionally solidified Al-4.5wt.%Cu alloys.The comparison of the numerical and experimental results showed that a good agreement wasobtained in terms of distribution of porosity and the trend of porosity number varies withpulling speeds. The accuracy of calculated porosity fractions should be improved further.