A COMPARATIVE STUDY OF POROSITY AND PORE MORPHOLOGY IN A DIRECTIONALLY SOLIDIFIED A356 ALLOY

Abstract

Hydrogen in aluminum alloy melts leads to porosity, generally considered to be the most serious defect affecting both static and dynamic properties of structural aluminum castings. In order to study these effects, an investigation has been undertaken to e examine the A356 alloy that is gravity die cast and directionally solidified. Castings were produced with as-molten metal and degassed metal, and with metal up-gassed by various treatments. Hydrogen content during melt treatment was measured with ALSPEK H,(R) a device based on electrochemical principles. Porous disc filtration apparatus (PoDFA) and reduced pressure tests (RPT) were performed in parallel with casting experiments to assess melt quality. The effect of hydrogen content and Melt cleanliness on the amount, size, shape, and location of porosity was characterized, and the results show, that amongst all the up-gassing methods used, up-gassing with wood and Ar-water vapor mixture is the most efficient. Pore size and shape factor increased with increasing distance from the chill.