Anomalous porosity in certain aluminum alloys

Metallographic examination was used to study the formation of porosity in an aluminum alloy A1-4%Cu with grain size in ranges 1000-200 and 120-85 , in technical grade aluminum Al (grain size 80-60 ), and in type D1 alloy (grain size 30-10 ) during a thermal cycling treatment, each cycle of which included aging in water at 100 ° C. The character of porosity depends on alloy composition and grain size. In coarsely granular materials the pores are formed inside grains, have clearly defined form, and reach a size of 10–20 after 60 cycles. The pore size in finely granular materials is smaller by one order of magnitude; the pores are formed predominantly along grain boundaries with some pores formed in the grain interior. Pores formed in alloy D1 are of an irregular form and grow to sizes exceeding the grain size. Data on changes in the specimen length, diameter, and volume were obtained. The relative increase in specimen volume reaches 0.5–1.0% per cycle. In spite of differences revealed by metallographic examination, the phenomena taking place in specimens with large and small grain size are of the same nature.