Aluminium-copper alloys are widely used in industry, particularly in aerospace and aircraft applications because of their strength and corrosion resistance. However, pores that form during manufacture have disastrous consequences for the mechanical properties of the alloy, decreasing their strength and durability. Minimising pore formation is highly desirable.

Pores form in aluminium-copper alloys because of the presence of hydrogen atoms in the molten liquid. As the liquid solidifies, the hydrogen is no longer soluble and gets forced into the 'mushy zone' consisting of a mixture of solid and liquid. Here, the hydrogen atoms combine to form hydrogen gas, forming a pore when the liquid solidifies.

To understand pore formation, samples of different copper/aluminium compositions were exposed to different conditions such as varying the concentration of hydrogen in the atmosphere. The samples made with a low concentration of hydrogen resulted in small pore formation.

Pore formation cannot be completely eliminated because solidification shrinkage will always occur since the solid eutectic is denser than liquid eutectic, and because hydrogen in the atmosphere will dissolve into the liquid metal and generate hydrogen pores. However, from the experiment, it was found that limiting the partial pressure of hydrogen in the atmosphere by surrounding the sample with Argon limited the size of the pores, producing very small pores in the metal.

The pores that are observed are elongated in one direction since dendritic growth was uniaxial due to a temperature gradient. Liquid eutectic occurring between the dendrite branches is much denser than solid eutectic, hence during eutectic solidification, solidification shrinkage occurs and pores are formed.

Elizabeth Wann is a Part II in Natural Sciences specialising in Mineral Sciences.