Qigui Wang

Investigation on Formation Mechanism of Irregular Shape Porosity in Hypoeutectic Aluminum Alloy by X-Ray Real Time Observation

The formation mechanism of irregular shape porosity in hypoeutectic aluminum silicon alloy (A356) was investigated by X-ray real time observation on porosity evolution during solidification and re-melting. Porosity in the hypoeutectic aluminum A356 alloy with high hydrogen content (>0.3xa0mL/100xa0gxa0Al) first forms in the liquid as small spherical gas bubbles, then expands along with the pressure drop in the mushy zone due to shrinkage and lack of feeding, and finally deforms into irregular morphology by the impingement of aluminum dendrite network. Degassing is a key to eliminate porosity in aluminum alloy castings.

Effect of Oxide Level on Pore Formation in A356 Alloy by X-Ray Imaging and Direcctional Solidification Technology

Effect of oxide level on porosity formation in an A356 alloy was investigated using micro-focus X-ray imaging and directional solidification technology. The increase of oxide level in liquid aluminum was achieved by violently stirring molten metal at elevated temperature. During solidification, the increased oxide content in melt significantly increases the amount of active nucleation sites for porosity and thus raises the nucleation temperature of pores. The fast growth of those early formed pores further restrains the succeeding nucleation operations of new pores in local regions and results in a considerable reduction in pore density. It was also found that the melt with high oxide content shows less dependency of growth rate reduction with local temperature.

Characterization of Pore Formation in A356 Alloy with Different Oxide Levels During Directional Solidification

Characterization of porosity formation in an A356 alloy with different oxide levels during directional solidification was investigated using micro-focus X-ray imaging and directional solidification technology. Stirring melt is thought to provide more active nucleation sites for pore formation, thus lead to a remarkable rise in the nucleation temperature of pores. The fast growth of those pores formed at higher temperatures further restrains the succeeding nucleation operations in local regions, and results in a considerable reduction in the pore volume density but a significant increases in pore volume fraction. Fluctuations of pore volume fraction and pore volume density along solidification length is thought to be closely related to a competition mechanism of pore nucleation with pore growth for hydrogen supplement. The increase in oxide content by stirring melt completely changes the pore size distribution and considerably increases the average size of pores formed.

Effect of Solidification Velocity and Hydrogen Content on Porosity in Directionally Solidified A356 Castings

Micro-focus X-ray technology was utilized to evaluate the influence of solidification velocity and hydrogen content on the volume fraction, number density and sizes of pores in the directionally solidified A356 castings. The results indicate that hydrogen content has a significant influence on porosity formation. When hydrogen content is low, few small irregular-shape pores were observed indicating the dominate impact of solidification shrinkage. While in high hydrogen specimens, many large spherical pores were found. The pore size distribution also shows dual populations. The group of large pores is formed in the liquid far from the solidification front. The group of small pores is formed near the solidification front. When hydrogen content is high, increasing solidification velocity (from 0.1mm/s to 0.2mm/s) not only decreases volume fraction of porosity but also significantly reduces the maximum pore sizes of large pores