Scientists uncover behaviour of aluminium

In a groundbreaking experiment, the scientists discovered that during the annealing process, aluminium grains grow in the bulk. Up to now, there was a general assumption that the grains grow smoothly and in a regular pattern.

In the experiment, the researchers followed the grain as it grew after annealing the metal.

"Individual grains don't behave like average and having a look at the local scale will help to create a better model,"​ explained Lawrence Margulies, one of the authors of the paper.

The results have been published today in the journal Science.​ The experiment, say scientists, offers industry a new insight into this metal.

The ESRF uses the simple aluminium can to illustrate its findings. Every can is processed before taking the shape of a cylinder. In a first stage, the aluminium is deformed, and the energy is concentrated in its bulk.

The material then goes through a process of annealing to achieve the shape of the can. In the annealing process, grains grow in the bulk. Up to now, there was a general assumption that the grains grow smoothly and in a regular shape.

The experiment is a real breakthrough in the field, since previous studies on metals have tended to be in 2D, and focused just on the surface. The science team from Risø National Laboratory and the ESRF​ however have achieved measurements that go into the bulk, which has a very different structure than the surface.

By identifying this behaviour, aluminium processors and packagers can now work out how to best process the material to achieve certain properties, such as more strength.

The in situ measurements were done using the 3D X-ray diffraction microscope at the ESRF. The scientists filmed in 3D the changes in the bulk of deformed aluminium after annealing. This is the first time an experiment such as this has taken place.

The sample had a pre-annealing period of one hour at 260°C. Afterwards, it was put in a furnace that rose the temperature from 270° C to 290°C.

Researchers took 73 snapshots of the grain during almost 30 hours and made a movie where one can clearly see the irregular growth of the grain in a micrometre spatial resolution.

The discovery is of importance to the food and beverage industry because usage of aluminium in packaging has been increasing in recent months. Shipments of foil for the first quarter of 2004 have been promising, with sales increasing by nearly one per cent compared with the first three months of 2003.

"Aluminium is a metal widely used in industry; therefore the more that is known about it, the more effectively it can be used,"​ said the ESRF in a statement.

European Aluminium Foil Association (EAFA) executive director Stefan Glimm told FoodProductionDaily.com: "Consumption has been good, largely because of the increasing quality of the product. There has also been a general trend towards flexible packaging, which is substituting rigid packaging."​

Glimm contends that as the demand for low-weight, flexible packaging increases, the market of aluminium is set to expand further. One perceived advantage of flexible packaging is that it offers excellent barrier protection, and a recent scientific study conducted by Fabes appears to support this.

The study concluded that aluminium foil of a thickness of at least 6 micron acts as an absolute barrier in food contact applications.

"Based on the theoretical evaluation of the experimental data, especially those obtained from permeation experiments performed at 100°C, it could be demonstrated for the laminates containing an Alufoil with a thickness of at least 6 µm that at 100°C the migration limit of 10 ppb will not be exceeded before 30 hours,"​ the study concluded.

Figures from the EAFA show that in total, 208,000 tons of aluminium foil was sold in the first quarter of 2004, which marks a strong recovery from the slow down in sales during the fourth quarter of last year. Sales were mainly driven by exports outside the EU, with a growth of 5.6 per cent. The usage inside the EAFA region was stable.

Approximately three quarters of aluminium foil is used in packaging where its characteristics of strength, formability and barrier properties have made it an essential part of many flexible packaging and container applications. The number of new products that feature innovative uses of alufoil based packaging indicates a healthy future for the material.