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Developments in electromagnetic induction welding of composites

For over 30 years, Sinergo has developed special electromagnetic induction machines. In the last 10 years, the focus has been on the technique of heating composite materials by means of electromagnetic changes to weld several parts. In particular, research is aimed at theoretical investigation and simulation of the welding process on different types of materials, experimental tests in laboratories to validate results, and finally the design and construction of industrial machinery. This article has been published in JEC Composites Magazine n°156.

Developments in electromagnetic induction welding of composites
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5 minutes, 50 secondes

In the last 10 years, Sinergo has developed vast knowledge of the welding process, with particular regard to the aspects of localised heating, temperature control, surface cooling, application of forces and their control. This activity has been established on different materials including thermoset and thermoplastic, as well as on combinations of composites and metals. Through research, the production of parts, field applications and machinery construction have demonstrated the excellence of induction welding technology on composite materials. It complements traditional joining methods such as autoclaving, bonding and riveting.  

SICE ARM 250 plant

The latest Sinergo system recently installed in one of the world’s largest companies in the aeronautical field is a complex one, based on robotic arm capable of processing pieces up to 1.000 mm x 2.000 mm. The uniqueness of this system lies not only in the movement along the robotic arm following various profiles but also in the adoption of an electromagnetic field generator, specifically developed for this setup. In fact, the working head is equipped with an induction antenna of over 200,000 watts reactive power (200 Kvar). It can generate very intense high frequency electromagnetic fields suitable for heating materials that are not easily susceptible.

This new system is capable of monitoring and regulating temperatures and the process. In addition to controlling movement paths, it can apply programmable forces. The temperature profile is managed by a system based on a thermal camera, closed loop, real time, programmable system with parameters and profiles calculated by the embedded PLC CPU defined by the operator. Ultimately, it is a machine of formidable power and versatility, to monitor a highly refined process. Furthermore, Sinergo has developed a sophisticated software package that provides users with numerous tools for programming and control. Moreover, a thoughtfully designed graphical interface makes this machine easy to use. 

Sinergo’s SICE ARM 250
Sinergo’s SICE ARM 250

Induction welding advantages, fields of application and facts

Induction welding applied to composite materials is one of several cutting-edge out-of-autoclave (OoA) processes in the industrial sector. When it is adopted, its adoption allows mechanical joints (rivets, screws, etc.) to be eliminated, reducing the weight of the composite structure. It also eliminates the need for bonded joints, since an additional material, such as adhesive glue, is not required apart from the matrix material.

In the case of thermoplastic matrix composites, induction welding exploits the ability of the thermoplastic matrix to flow when heated above the glass transition temperature (Tg, for amorphous polymers) or melting point (Tm, for semi-crystalline polymers). Moreover, they regain their mechanical properties after cooling. It can generally be described as the joining of two parts through the melting of the interface in contact, followed by cooling under pressure, enabling the bond to form.

The welding principle for carbon fibre-reinforced thermoplastics is schematically the following:

  • the coil carrying alternating current produces an alternating magnetic field around itself;
  • this magnetic field induces eddy currents within the conductive reinforcement fibres of the composite;
  • the induced current tends to locally heat and melt the composite matrix through Joule heating;
  • in the continuous induction welding (CIW), following the heating, welding pressure is applied through a cooled roller positioned at a distance “l” from the coil, depending on how quickly the laminate cools.
Simulation of electromagnetic field and inducted temperature

The advantages of CIW

Currently, the CIW is a technology under development for the aerospace sector, applied to thermoplastic matrix composites (e.g., PPS, PEEK, PEI, LMPAEK). Here are its advantages compared to the traditional bonding technique typically carried out in an autoclave. 

High mechanical properties

The absence of adhesive makes it possible to maximise the joint properties. This is because the same composite matrix is used as a bonding element for the parts to be welded, after being appropriately melted. Single lap shear strength values of 30 MPa to 40 MPa can be achieved, depending on the overlap width.

High production rate

Welding speeds exceeding 15 mm/sec can be achieved, enabling significantly faster processing cycles compared to those in autoclaves (which last several hours) used for curing and consolidating bonded joints.

SICE 1, the first Sinergo CIW machine. Working head detail.

Simplicity and cost-effectiveness of welding tools

The tools used in the welding process, which hold the components to weld in their final position, are significantly simpler and more economical to design and manufacture compared to the adhesive bonding tools used in the autoclave process.

High level of automation

The welding head can be installed on an anthropomorphic arm to weld complex geometries. 

Low investment costs

The acquisition and maintenance costs of an induction welding cell (anthropomorphic arm + welding head) are considerably lower than the investments required for the autoclave process. 

Absence of consumables required y for the process

Drawbacks of CIW

One of the main critical aspects is thermal gradient management across the thickness of the component. The component’s surface (closer to the coil) tends to heat up more than the welding interface (­single-lap joint).

Micrograph of the cross-sectional view of an adherend where the thermal gradient was not controlled during induction heating operations.

Heating leads to the complete fusion of the matrix of the upper adherend to achieve weldability conditions at the interface, causing defects (porosity and delamination) to occur in the superficial and internal zones of the upper adherend. During cooling, these areas are the first to drop below the melting temperature and do not consolidate optimally. 

Cetma-Sinergo solution to optimise CIW

The 2 companies have collaborated to expand the range of CIW applications in order to eliminate defects resulting from inaccurate management of the thermal gradient, as previously described.

Simulation of the thermal gradient achieved in the thickness of the superfi cial adherend; (a) possible overheating of the surface, (b) no surface overheating Cetma-Sinergo technology

Cetma focused on the scientific and testing part, while Sinergo focused on the technical and equipment side to co-develop a new induction welding head. This welding head is different from the others because it is equipped with a controlled cooling system. This system uses air to cool the laminate surface (with specific characteristics in terms of temperature and flow rate) to concentrate the heat necessary for matrix fusion exclusively on the welding interface. Optimal temperature distribution can then be achieved within the material without the edge effect.

Thermoplastic CIW

Thermosetting CIW

Another innovation patented by Cetma enables the use of this technology for welding thermoset matrix composites, not limited to thermoplastic materials. The patented solution involves hybridizing the welding surface with a thermoplastic film, which is applied to the surface of the thermoset composite before curing the resin. During the curing phase, when temperature and pressure are applied, the thermoplastic film interpenetrates with the thermoset network, functionalising it for the induction welding process. In the subsequent induction welding process, only the thermoplastic film will undergo fusion, allowing for the creation of a welded joint with another thermoset component, also surface-hybridized.

The primary sector for application of the ­Cetma-Sinergo solution to manufacture components is in aerospace. 

Welded door in the H2020 TOD (thermoplastic on door) research project; material: PPS/carbon fabric; welded components: outer skins; internal reinforcement elements.

Sinergo offers robust experience developing and building CIW machines and plants tailored specifically to client requests and application needs. Furthermore, through the Cetma consortium, it offers scientific support, feasibility study services, product development support, and laboratory tests.

Cover photo: Sinergo – Elisa Buso, Communication Manager, Sinergo S.R.L.

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JEC Composites Magazine n°156
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