Loughborough team helps pioneer ultrasonic additive process

Upcoming advanced-manufacturing processes will enable engineers to design and create products that have passive or active devices embedded within them. The characteristics of such products will not only enable them to be interrogated in the field, but also modified while in use.

One such manufacturing process is ultrasonic consolidation, an additive-manufacturing technique that is based on the use of ultrasonic technology to weld a sequence of metal foils.

In practice, the technique exploits both surface-friction and volume-plasticity softening effects to create a join between two layers of metal foils through the use of a rotating cylindrical sonotrode.

When the sonotrode is excited by a piezoelectric transducer, it oscillates at ultrasonic frequencies as it rolls across the strips of metal foil, imparting energy to the top strip and causing it to vibrate by tens of microns.

The combination of the pressure imparted onto the foils by the clamping force of the sonotrode and the ultrasonic vibration of the foil results in friction, which creates the solid-state weld between the two strips. The process is then repeated layer by layer until a solid component has been created. CNC contour milling is then used to create the shape that is required from the material.

According to Dr Russell Harris, a senior lecturer at Loughborough University who has worked on refining such systems over a number of years together with US firm Solidica, ultrasonic consolidation is not entirely dissimilar to other additive-manufacturing techniques in as much as it also builds up a product in a series of layers. ’While additive manufacturing is traditionally used to build many similar layers of material, the ultrasonic-consolidation technique can also be used to bond dissimilar metallic materials such as aluminium and titanium. Since the process is performed while the material is in a solid state, no material is actually melted and little pressure needs to be applied to manufacture a part,’ said Harris.

That means that the low-temperature solid-state ultrasonic process, which can be performed at a fraction of the metal materials’ melting temperature, is not only fast compared with other additive-manufacturing processes, but has a lower energy consumption too.

Harris said that three distinct metallurgical stages are traditionally believed to take place in the ultrasonic bonding process: asperity collapse, partial oxide disruption, and localised softening and adhesion. In reality, in ultrasonic-consolidation production machines such as the ones developed by Solidica, the bonding process occurs in a fraction of a second.

The technique can also be used to bond dissimilar metallic materials such as aluminium and titanium

’The combination of the ultrasonic frequency applied to the upper foil coupled with the pressure applied by the sonotrode leads to a collapse in the asperities or roughness or harshness of the surfaces between the two foils, producing an intimate contact. That happens along the whole length of the foil as the sonotrode rolls over the surface,’ he said.

Harris said that the process has also been shown to manipulate the oxide layer that is naturally present on the surface of such metals as aluminium-oxide layers that could potentially hamper the effectiveness of the joint between the two foils.

This is due to a phenomena unique to ultrasonic processing that has been termed the ’volume effect’ or ’acoustic softening’ resulting in a high level of metal plastic flow at low temperature and pressure.

The same phenomena also allows different elements to be embedded between the layers of metal. Even elements that would be impossible to incorporate in a dense metal object by other techniques due to their sensitivity to temperature and pressure can be encapsulated by the metal that flows in a low-temperature plastic state around them, allowing them to retain their functionality.

Harris said that, since the process can be interrupted and modified at any point, many components can be augmented into any metal matrix to form composite structures. These advanced materials can be stiff and lightweight the perfect combination for high-stress, weight-critical environments.

The technique has been used to manufacture parts that incorporate reinforcing meshes and fibres into metal matrices for strengthening materials, single-mode optical fibres for sensing applications, and shape-memory alloy fibres that allow the physical properties of finished parts to be manipulated while in use.

“It has the potential to allow the creation of composite materials that are lighter than current composites”

RUSSELL HARRIS, LOUGHBOROUGH

Harris believes that the process can be refined further to offer even more opportunities for designers. Laser-etching channels in the foils prior to laying in any fibre elements would, for example, accurately retain their position and eliminate the possibility that they might move during the ultrasonic-consolidation process. He also said that such a concept would help when building parts that demand a greater packing volume of fibres, enabling enough metal to plastically flow through the spaces between them.

As flexible as the present manufacturing technology might be, Harris acknowledged that the process parameters such as the power applied to the sonotrode or its surface roughness may need to be optimised depending on the characteristics of the fibre that is to be embedded in the material.

’For example, while silicon-carbide fibres are relatively easy to embed, a single-mode optical fibre is a more brittle object and these different characteristics may mandate a different set of process parameters in order to be successfully integrated in unison,’ said Harris.

Nevertheless, he sees the process as an important one, especially for engineers looking to build extremely complex multifunctional parts. He believes that ultrasonic consolidation will free them from traditional economics such as accounting for tooling costs and volumes of production.

Harris said that the earliest developments and applications for the technology have been in aerospace and defence, but he thinks it will eventually be exploited in automotive and high-end consumer applications too.

’Ultrasonic consolidation has the potential to allow the creation of multi-functional smart composite materials that are lighter than current composites, allowing designers to create different geometries, as well as monitor dynamic loads within a structure in real time. One of the advantages of the technique is that it is relatively low cost no exotic material preparation or prolonged heating at high temperatures of bulk materials is required. Consequently, it has the potential to become more widespread,’ he said.