Welding and Composite Structures: The Ever Evolving Demands for Design and Monitoring of Composite Welding

Your chair doesn’t get carved out of a tree, your bicycle isn’t fully demolded and ready to ride – materials and parts need some joining to make an assembly of a larger structure. In general there are three main joining techniques for materials: adhesive, mechanical and welding. Through this article we will try to focus our attention on welding – the joining method that is being widely explored in aviation and automotive sectors nowadays – especially when considering the composite materials replacing the currently dominating ones (i.e. steel).

Welding is the act of fusing two material parts together through application of heat and/or force.

Composite material welding, the more common approaches:

A specific large class of composite materials known as Thermoplastic Composites are essentially plastic polymer materials that can be moulded and shaped at a certain temperature, then cooled into a finished version. This cycle of heating and cooling can be repeated which makes the final composite a material with exceptional shelf life.

Unlike thermoset composites, due to the melting and solidification feature of thermoplastics, they can be welded – making them excellent candidates for applications that cannot support bolted/riveted joining methods (I.e. the aerodynamic surface of an aircraft).

What is Induction Welding:

This process is used to transfer magnetic flux that creates Eddy Currents generating heat in the composite material particles which have conductivity. When applying el. current to induction coil – the process starts. Surely not all thermoplastics have the particles that have conductivity, however Carbon Fibres – which are the more common ones – have this ability. This process can be used in aerospace, automobile, construction, oil & gas and furniture industries alike.

What is Ultrasonic Welding:

Ultrasonic welding is one of the more time-efficient methods of composite welding. In this method a thermoplastic resin is placed between the layers of composites that will be welded; the vibrations of the sonotrode heats thermoplastic resin in seconds (with no need to generate electrical current), the parts are then clamped together to weld. Ultrasonic heads using 20 KHz signals are commonly used to weld lightweight composite thermoplastics (Gr-Ps) beams in space shuttles.

What is Resistance Welding:

Resistance welding includes the logic of placing a conductive layer (i.e. a metal mesh) in between the layers of composites which will heat when an electrical current is applied, melting the composite parts together.

Later, after the cooling and solidification – the metal particles become a part of the structure.

Other honourable mentions in the composite welding techniques is Fusion Welding which is used to join fibre reinforced thermoplastic structures by placing resistance wires or a mesh of stainless steel screen, melting them and applying pressure on the bonding location; or in the case of ceramics and/or metals welding friction welding and isostatic compaction welding methods are showing high productivity and wide use in the industry.

Challenges a Welded Composite can bring:

Welding, being a process of joining two parts together, requires certain energy (temperature, pressure, etc.) to be applied and technique to be carried out as discussed above. The levels of these applied energy and precision have been calculated over hundreds of tests and identified per specific technique, type of composite, final application.

Ensuring the precision of these applied energy and techniques is an important task during the welding and after – when maintenance strategies are implemented. The standardised monitoring methods are not easy to follow and guarantee in practice. Think about it, how are you supposed to know if for example the temperature has exactly reached the necessary level, or if the cooling of the material didn’t ever slightly deform the material, if the welding path isn't internally damaged?

Furthermore, how could you exactly ensure the above mentioned during the life-cycle of the material when they are used in the structure like a commercial aircraft, products, etc.).

In general, be it a composite material or not, welding is a material joining technique. It is widely used for repair as well – to rebuild the load path through the structure, and to match the original properties of the structure, such as the strength, stiffness and weight.

Unfortunately the welded structure is not homogeneous as a full piece of material, thus the general tradeoffs of the repair welded, or generally, welded structure can appear:

• Thermal distortion (heat can distort the welded point and the vicinity (when trying to weld large numbers of parts, especially ones that need precise fitting the distortion can bring to severe consequences).
• Matching the original strength: locally heavier and stiffer than the original.
• Matching the original stiffness: locally weak and heavier than the original.

The welding (especially repair welding) is being calculated to an acceptable strength, stiffness and weight limits. Taking into account the safety-critical application of these repairs – it is integral to control the welding process and to integrate maintenance tactics for the integrity monitoring through the rest of the life cycle of these materials.

An unfortunate result of improper welding process and lack of control was the first Starship by SpaceX – the crew of which was contracted by a water tower company – the welds were not automated, it was hard to ensure homogeneity, the welding paths were not treated and finally when one of the welds failed – the mark 1 exploded. SpaceX then made extraordinary improvements on mark 2 of Starship.

The above example proves the famous notion “If you think knowledge is expensive – try ignorance!”.

InLine Control the Composite Welding: Approach from within

There is a documented monitoring gap in the industry of composite welding that needs filling. The transition in the industry is to inline process control especially with thermal measurements that are carried out real-time instead of setting a calibrated standard for the bulk of manufactured composites.

Currently a variety of sensors and sensing assemblies are being tested for in-situ monitoring of welding which can be explored in the CompositesWorld article .

Especially to ensure the integrity of i.e. aeroplane fuselage with welded parts instead of riveted or adhesively attached ones – it is clear that the industry will need precise argumentation and data that each and every individual joining is conducted properly, and the preference here is to conduct, collect and further continue the monitoring within the welded part.

RVmagnetics is able to ensure this process, however “to good to be true” it might sound – the MicroWire passive sensors of RVmagnetics are able to be placed within the welded parts during for example Induction Welding and provide contactless real-time monitoring of temperature, pressure, vibrations, bending, cracks, delaminations, shock, solidification, etc. not only during welding inline, but also after the welding.

This is possible as the MicroWires are thin and elastic like human hair and cause no addition to weight and mechanical properties of the welded composite structure (surely not as noticeable as even the metallic mesh that can be used in resistance welding).

There are a number of added benefits of this sensor for monitoring the composite welding which are being further explored with each application. Take for example the thermocouples usage during induction welding. The Eddy Currents created during the welding can harm the signal of standard sensors, however they reduce with MicroWire sensors (extremely thin in diameter 20–70 µm), thus the effects of induction welding is not an issue for MicroWire, unlike other, currently tested sensors and sensing assemblies.

The signal from RVmagnetics MicroWires can be detected with no direct contact through the magnetic field with a sensing system also custom designed and manufactured by RVmagnetics for their clients.

The fundamental use cases of these sensors are to detect and continuously monitor the precise temperature, straining, position and the overall Structural Health (other quantities such as vibrations, el.current, etc.), and so making them immune candidates for the in-situ monitoring of composite welding and composite materials as such – so consequently these are agents for Manufacturing Process Control and once the material is manufactured, now for Structural Health Monitoring in Non Destructive Testing Methods.