Porosity Development during Heat Treatment of Aluminum-lithium Alloys

The development of a sub-surface layer of porosity during heat treatment has been studied in a variety of A1-Li alloys. Pure binary A1-Li alloys and three commercial materials were heat treated in air, vacuum and hydrogen for various lengths of time. Subsequent metallographic sectioning and polishing revealed the presence of a band of pores in the near-surface region extending approximately 300 pn into the sample after a 16 h heat treatment. This band of porosity is thought to arise from a Kirkendall effect due to the loss of lithium during heat treatment of the sample. The evolution of the spatial distribution of these pores was characterized with a quantitative image analyzer. The effects of prior surface preparation were also determined. The results showed that structural defects influenced pore nucleation and that environmental factors were also significant. A simultaneous pore healing process also occurred immediately adjacent to the external surface.