Aluminium toil

A new manufacturing technique ensures microcrystalline structures inside high-performance materials remain intact when processed.

Researchers at the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research in Dresden developed the technique for aluminium alloys but it could also be used for other materials.

Aluminium alloys built with microcrystalline structures are stronger and firmer than regular aluminium. The alloy’s properties, which are mainly a result of the tiny size of its crystals, are ideal for parts such as automobile screws and pistons that undergo high stress and temperatures.

Traditionally, manufacturers have faced a challenge when processing microcrystalline-structured aluminium alloy into tools or components. When pressed or joined, the alloy must be heated and this heating-up process expands the material’s crystals, which causes the microcrystalline properties to gradually fade away.

Ronny Leuschner, manager of the Fraunhofer research project, said that his team aimed to maintain the material’s microcrystalline structure throughout the entire component-manufacturing process.

In order to accomplish this, the research team produced a special aluminium alloy and then cooled it down dramatically.

He added that the process begins by developing an aluminium alloy melt. A spraying device then pours the melt onto a water-cooled rotating roller.

The melt, which can be as hot as 1,700°C, rapidly loses heat as it hits the roller. Leuschner said this leads to the formation of a filmy metal ribbon thinner than 100 micrometres.

‘Metal ribbons made by this procedure have an even distribution of the alloy components and a homogeneous ultra-fine-sized structure,’ he added.

Once the strips have solidified, they are compacted and pressed in a punch-die-sample assembly into the desired shape. The researchers developed a way to preserve the fine microstructures during this stage, using a technique known as Spark-Plasma-Sintering- Process (SPS).

Leuschner explained that the process is a modified version of hot pressing. With SPS, the sample is heated by a pulsed electric current that flows through the punch-die-sample assembly using a high current and low voltage.

‘The direct heating of the die-punch-sample assembly allows very high heating rates of more than 300°C per minute and short sintering times in the range of a few minutes,’ he added.

The entire process runs much cooler than other alloy-tool forming techniques. ‘The temperature of the die is not as high as compared to other compacting technologies such as hot pressing or hot isostatic pressing,’ said Leuschner .

The result is a strong, lightweight alloy with a fine microstructure in tact. ‘We observed the microstructure with light optical microscopy and scanning electron microscopy and did not observe any grain growth,’ he added.

The Fraunhofer team believes microcrystalline-structured aluminium alloys made with this process could be used for a range of lightweight aluminium parts that need greater strength and improved wear and corrosion resistance.

Leuschner suggested the material could be formed into plates for the body shells of aeroplanes or it could be used for screws, pistons and turbocharger wheels in automobiles. The material could also find applications with hydrogen storage tanks and energy production with thermoelectric materials.

‘We are still looking for partners with new applications,’ he added.