Welding of advanced thermoplastic composites

Last updated November 08, 2023

Advanced thermoplastic composites (ACM) have a high strength fibres held together by a thermoplastic matrix. Advanced thermoplastic composites are becoming more widely used in the aerospace, marine, automotive and energy industry. This is due to the decreasing cost and superior strength to weight ratios, over metallic parts. Advance thermoplastic composite have excellent damage tolerance, corrosion resistant, high fracture toughness, high impact resistance, good fatigue resistance, low storage cost, and infinite shelf life.^[1] Thermoplastic composites also have the ability to be formed and reformed, repaired and fusion welded.

Fusion bonding fundamentals

Fusion bonding is a category of techniques for welding thermoplastic composites. It requires the melting of the joint interface, which decreases the viscosity of the polymer and allows for intermolecular diffusion. These polymer chains then diffuse across the joint interface and become entangled, giving the joint its strength.

Welding techniques

There are many welding techniques that can be used to fusion bond thermoplastic composites. These different techniques can be broken down into three classifications for their ways of generating heat; frictional heating, external heating and electromagnetic heating. Some of these techniques can be very limited and only used for specific joints and geometries.

Friction welding

Friction welding is best used for parts that are small and flat. The welding equipment is often expensive, but produces high-quality welds.

Linear vibration welding

Two flat parts are brought together under pressure with one fixed in place and the other vibrating back-and-forth parallel to the joint. Frictional heat is then generated till the polymers are softened or melted. Once the desired temperature is met, the vibration motion stops, the polymer solidifies and a weld joint is made. The two most important welding parameters that affect the mechanical performance are welding pressure and time. Developing parameters for different advance thermoplastic composite can be challenging because the high elastic modulus of the material will have a higher heat generation, requiring less weld time. The pressure can affect the fiber orientation which also greatly impact the mechanical performance. Lap shear joints tend to have the best mechanical performance from the higher volume fraction of fibers at the weld interface. Overall linear vibration welding can achieve high production rates with excellent strength, but is limited to the joint geometries that are flat.

Spin welding

Spin welding is not a very common welding technique for advanced thermoplastic composites because this can only be done with parts that have a circular geometry. This is done by one part remaining stationary while the other is continuously rotated with pressure applied to the weld interface. Rotational velocity will vary throughout different radii of the Interface. This will result in a temperature gradient as a function of the radius, resulting in different shrinkage for the fibers causing high residual stresses. The orientation of the fibers will also contribute to high residual stress and reduction in strength.

Ultrasonic welding

Ultrasonics welding is one of the most commonly used technique for welding advanced thermoplastic composites. This is due for its ability to maintain high weld strength, hermetic sealing, and high production rates. This welding technique operates at high vibrational frequencies (10–70 kHz)^[2] and low amplitude. The direction of vibration is perpendicular to the joint surface, but can also be parallel to the joint for hermetic application. Heat is generated from the surface and intermolecular friction due to the vibrational. On the surface of the joint there are small asperities called energy directors, where the vibrational energy concentrates and induces melting. Design of the energy director and optimized parameters can be critical to improve the quality of the weld to reducing any fiber disruption during welding. Energy directors that are triangular or semi-circle often achieve the highest strength.^[2] With optimize welding parameters and joint design weld strength, up to 80% of the base material can be retained for advanced thermoplastic composites.^[3] However, welding can cause damage to the fibers, which will result in premature failure. Ultrasonic welding of advanced thermoplastic composites is used for making automotive parts, medical devices and battery housing.

Thermal welding

Thermal welding can produce good weld quality although extra precautions need to be taken to prevent high residual stress, warping, and decohesion. Other thermal welding techniques are not commonly used due their high heat input, which can damage the composite.

Laser welding

Laser welding of advanced thermoplastic composites is a process by which the LASER (Light Amplification of Simulated Emission of electromagnetic Radiation), a highly focused coherent beam of light melts the composite tin various ways. Taking advantage of joint design and material properties, lasers can be applied either directly or indirectly to create the welded joint. There are processing methods that take advantage of material structure/properties to create the weld joint. Welding variables affect weld quality in both positive and negative ways depending on how they are manipulated.

Laser heating mechanism in matter

When a laser beam impinges on a material, it excites electrons in the outer most shell of the atom. The return of those electrons to the relaxed state induces thermal heating through conversion to vibrational states which propagate to the surrounding material.^[4]

Joining methods for laser welding

Surface heating

This method involves using infrared radiation to heat the surfaces the composites to be welded and then clamping until and holding the parts together.^[5]

IR/Laser stacking

This method involves laser melting a polymer post and pressing a die into the molten post to create a rivet-like button to joint materials like metals.^[5] This process can be used to join metallic joints to composite structures.

Through Transmission IR welding (TTIr)

This method utilizes one laser transparent (LT) and one laser absorbing (LA) material. Typically, the components are layered as a sandwich with the laser beam passing through the LT layer and irradiating the surface of the LA. This creates a melt layer at the interface of two components leading to a weld.^[4]

Effect of Constituent Properties on Weldability

To understand how the properties of a composite affect is weldability, the effects of the individual constituents (fiber, matrix, additives, etc.) need to be understood. The effect of each will be noted separately and then the combined effects will be discussed.

Matrix

Electromagnetic radiation interaction

A laser beam can interact in one of three ways when it contacts the polymer matrix. It can be absorbed, transmitted, or reflected. The amount of absorption determines the amount of energy available for welding. The reflectivity is affected by the index of refraction according to this relation: $R={\frac {(n-m)^{2}}{(n+m)^{2}}}$ , where n is the index of refraction of the polymer and m is the index of refraction of air.^[5]

Absorption can be affected by the following structural characteristics of the polymer to be discussed below: crystallinity, chemical bonding, and concentration of additives.

Crystallinity

Increased crystallinity tends to cause lower laser beam transmission because of scattering caused by changes in the index of refraction encountered when going from one phase to the next or because of changing crystallographic orientation.^[5] Increased crystallinity can cause the transmission to increase monotonically as a function of polymer thickness. The relationship follows the Lambert-Bouuger's Law: $I_{t}=I_{0}e^{-(\alpha t)}$ , where $I_{t}$ is the intensity of the laser beam at a given depth or thickness, t. $I_{0}$ is the intensity of laser beam at its source. $\alpha$ is the absorption constant of the polymer.^[5] By the same token, amorphous polymers lack this trend with thickness.^[5]

Chemical bonding

Polymers absorb EMR (Electro Magnetic Radiation) in a specific wavelength of light depending on what functional groups are present on the polymer. For instance, bending of the C-H bond on the $-CH_{2}-$ at 6800 nm.^[5] Many polymers have vibrational modes at wavelengths greater than 1100 nm, so to apply methods such as TTIr, laser sources must produce photons at wavelengths shorter than that. Therefore, Nd:YAG lasers (1064 nm) and diode lasers (800-950 nm) can pass through the LT until they impinge on the intended modified polymer or additive that results in absorption, whereas ${\ce {CO_2}}$ lasers (10,640 nm) will be absorbed too easily as it passes through the LT.^[5]

Reinforcements

Reinforcements such as fibers or short particles. Reinforcing fibers can be added to increase the strength of a composite.

Some reinforcements like carbon fibers have high thermal conductivity and can dissipate the heat of welding, thus requiring more energy input than with other reinforcement materials such as glass. Glass reinforcements can cause scattering of the beam.^[6]

The orientation of the continuous fibers can affect the width of welds being made. When the welding direction is parallel to the orientation of the fibers, the weld width is usually narrower due to heat being channeled through the fibers to the front and the rear of the weld.^[6]

Increased volume fraction of reinforcements such as glass can scatter the laser beam, thus allowing less to be transmitted to the weld joint. When this happens, the amount of energy necessary to fuse the joint may increase. The increase if not done carefully can cause damage to the transparent part of a TTIr weld joint.^[6]

Additives and Fillers

Some additives can be intentionally added to absorb laser energy. This technique is especially useful in concentrating the weld joint to the mated surfaces of two materials that are relatively transparent to the laser beam. For example, carbon black increases absorption of the laser beam. There can be some unintended consequences of using these absorbing additives. Increasing the concentration of carbon black in a polymer can decrease the depth of heating and increase the peak temperature at the weld joint. Surface damage can occur if the concentration of carbon black becomes excessive.^[5]

Some additives such as the highly selective materials used in the Clearweld process are applied only to the mating surfaces between the plastics to be joined. Some of the chemicals such as cyanines only absorb in a narrow wavelength band centered around 785 nm.^[4] This methodology initially was applied only to plastics, but has recently been applied to composites such as carbon fiber reinforced PEEK.^[7]

Other additives called clarifiers can do the opposite of carbon black by increasing laser beam transmission by reducing crystallinity in polymers.^[5]

Despite the fact that both pigments and dyes can both add color to a polymer, they behave differently. A dye is soluble in a polymer, whereas a pigment is not.

Welding technique comparisons

Contour Welding (CW) vs quasi-simultaneous (QS)

During TTIr, although it takes more energy per unit length to achieve fusion with QS than with CW, QS offers the advantage of achieving higher weld strength and weldability of low transmissive materials such as continuous glass fiber thermoplastics.^[6] Greater strength is imparted because full fusion is achieved without damaging the surface of the transparent material.

Electromagnetic welding

Electromagnetic welding is capable of welding complex parts with also the possibility of reopening welds for replacement or repair. To achieve good welds the design of the coil and implant is important for uniform heating.

Implant resistance welding

Implant resistance welding can be a low cost solution for welding parts that are flat or with curved surfaces. The heating element used is often a metal mesh or carbon strips, which provides uniform heating. However, advanced thermoplastic composites that contain conductive fibers can’t be used due to unwanted power leakages.

Implant induction welding

Induction welding uses a implant or susceptor that is placed at the weld interface and embedded with conductive material such as metal or carbon fibers. An induction coil is then place near the weld joint, which induces a current in embedded in the material used to generate heat. When welding carbon fiber, carbon and graphite fiber mats with higher electrical resistance are used to concentrate the heat at the weld interface. This has the ability to weld complex geometry structures with great weld strength.

Challenges of welding advanced thermoplastic composites

The heat generated during welding thermoplastic composite, induces residual stresses in the joint. These stresses can greatly reduce the strength and performance of the part. Upon cooling from welding the matrix and fibers will have different coefficients of thermal expansion, which introduces the residual stress. Things such as heat input, cooling rates, volume fraction of the fibers, and matrix material will influence the residual stress. Another important factor to consider is the orientation of the fibers. During the molten state of welding, fibers can reorient themselves in a manner that reduces weld strength.

Advanced thermoplastic composites commonly used for welding

Carbon fiber polyetherimide (CF/PEI)
Carbon fiber polyphenylene sulfide (CF/PPS)
Carbon fiber polyetheretherketone (CF/PEEK)

Related Research Articles

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as brazing and soldering, which do not melt the base metal.

A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions.

Induction welding is a form of welding that uses electromagnetic induction to heat the workpiece. The welding apparatus contains an induction coil that is energised with a radio-frequency electric current. This generates a high-frequency electromagnetic field that acts on either an electrically conductive or a ferromagnetic workpiece. In an electrically conductive workpiece, the main heating effect is resistive heating, which is due to induced currents called eddy currents. In a ferromagnetic workpiece, the heating is caused mainly by hysteresis, as the electromagnetic field repeatedly distorts the magnetic domains of the ferromagnetic material. In practice, most materials undergo a combination of these two effects.

Ultrasonic welding is an industrial process whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together. When used to join metals, the temperature stays well below the melting point of the involved materials, preventing any unwanted properties which may arise from high temperature exposure of the metal.

Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat. Welding of thermoplastics is accomplished in three sequential stages, namely surface preparation, application of heat and pressure, and cooling. Numerous welding methods have been developed for the joining of semi-finished plastic materials. Based on the mechanism of heat generation at the welding interface, welding methods for thermoplastics can be classified as external and internal heating methods, as shown in Fig 1.

Laser beam welding (LBW) is a welding technique used to join pieces of metal or thermoplastics through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume and precision requiring applications using automation, as in the automotive and aeronautics industries. It is based on keyhole or penetration mode welding.

Filler materials are particles added to resin or binders that can improve specific properties, make the product cheaper, or a mixture of both. The two largest segments for filler material use is elastomers and plastics. Worldwide, more than 53 million tons of fillers are used every year in application areas such as paper, plastics, rubber, paints, coatings, adhesives, and sealants. As such, fillers, produced by more than 700 companies, rank among the world's major raw materials and are contained in a variety of goods for daily consumer needs. The top filler materials used are ground calcium carbonate (GCC), precipitated calcium carbonate (PCC), kaolin, talc, and carbon black. Filler materials can affect the tensile strength, toughness, heat resistance, color, clarity, etc. A good example of this is the addition of talc to polypropylene. Most of the filler materials used in plastics are mineral or glass based filler materials. Particulates and fibers are the main subgroups of filler materials. Particulates are small particles of filler that are mixed in the matrix where size and aspect ratio are important. Fibers are small circular strands that can be very long and have very high aspect ratios.

Polymer engineering is generally an engineering field that designs, analyses, and modifies polymer materials. Polymer engineering covers aspects of the petrochemical industry, polymerization, structure and characterization of polymers, properties of polymers, compounding and processing of polymers and description of major polymers, structure property relations and applications.

X-ray welding is an experimental welding process that uses a high powered X-ray source to provide thermal energy required to weld materials.

Carbon fiber-reinforced polymers, carbon-fibre-reinforced polymers, carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic, also known as carbon fiber, carbon composite, or just carbon, are extremely strong and light fiber-reinforced plastics that contain carbon fibers. CFRPs can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness (rigidity) are required, such as aerospace, superstructures of ships, automotive, civil engineering, sports equipment, and an increasing number of consumer and technical applications.

Hot plate welding, also called heated tool welding, is a thermal welding technique for joining thermoplastics. A heated tool is placed against or near the two surfaces to be joined in order to melt them. Then, the heat source is removed, and the surfaces are brought together under pressure. Hot plate welding has relatively long cycle times, ranging from 10 seconds to minutes, compared to vibration or ultrasonic welding. However, its simplicity and ability to produce strong joints in almost all thermoplastics make it widely used in mass production and for large structures, like large-diameter plastic pipes. Different inspection techniques are implemented in order to identify various discontinuities or cracks.

Rheological weldability (RW) of thermoplastics considers the materials flow characteristics in determining the weldability of the given material. The process of welding thermal plastics requires three general steps, first is surface preparation. The second step is the application of heat and pressure to create intimate contact between the components being joined and initiate inter-molecular diffusion across the joint and the third step is cooling. RW can be used to determine the effectiveness of the second step of the process for given materials.

Hot-gas welding is a manual plastic welding process for joining thermoplastic materials. A hot-gas torch is used to direct hot air to both the joint surface and weld rod, heating the materials to their softening temperature. Application of pressure on the heated weld rod to the joint surface bonds the materials together to form a completed weld. This technique is not easily automatized and is primarily used for repairs or individual manufacturing needs of small or complex components.

Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding. Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength.

Laser welding of polymers is a set of methods used to join polymeric components through the use of a laser. It can be performed using CO₂ lasers, Nd:YAG lasers, Diode lasers and Fiber lasers.

Spin welding is a form of friction welding used to join thermoplastic parts. The parts to be welded must be round, and in plane with each other. Like all other welding methods this process utilizes heat, time, and pressure to create a weld joint. Heat is generated via internal friction generated between the two parts when rotating and subjected to a load normal to the weld joint. This frictional heat causes the plastic to melt and a bond to be created.

IR welding is a welding technique that uses a non-contact heating method to melt and fuse thermoplastic parts together using the energy from infrared radiation. The process was first developed in the late 1900s, but due to the high capital cost of IR equipment the process was not commonly applied in industry until prices dropped in the 1990s. IR welding typically uses a range of wavelengths from 800 to 11,000 nm on the electromagnetic spectrum to heat, melt, and fuse the interface between two plastic parts through the absorption and conversion of the IR energy into heat. Laser welding is a similar joining process that applies IR radiation at a single wavelength.

Implant resistance welding is a method used in welding to join thermoplastics and thermoplastic composites.

Implant induction welding is a joining method used in plastic manufacturing. The welding process uses an induction coil to excite and heat electromagnetically susceptible material at the joint interface and melt the thermoplastic. The susceptible material can be contained in a gasket placed between the welding surface, or within the actual components of a composite material. Its usage is common for large, unusually shaped, or delicate parts that would be difficult to weld through other methods.

Laser polishing, also referred to as laser re-melting, is a type of micro-melting process employed for improving surface quality of materials. As opposed to other conventional polishing processes, this process does not involve removal of materials from the workpiece surface. In this process, the laser is made incident on the workpiece to melt the surface down to a certain depth, thus enabling subsequent betterment of surface parameters due to re-solidification of the melted material.

References

↑ Yousefpour, A., M. Hojjati, and J.-P. Immarigeon, Fusion Bonding/Welding of Thermoplastic Composites. Journal of THERMOPLASTIC COMPOSITE MATERIALS, 2004. 17(4) p. 303-341
1 2 Liu, S. , Chang, I. and Hung, S. (2001), Factors affecting the joint strength of ultrasonically welded polypropylene composites. Polym Compos, 22: 132-141. doi:10.1002/pc.10525
↑ Benatar, A. and T.G. Gutowski, Method for fusion bonding thermoplastic composites. SAMPE Q.;(United States), 1986. 18(1).
1 2 3 Steen, William. Laser Material Processing. pp. 364–370.
1 2 3 4 5 6 7 8 9 10 Benatar, Avraham (2003). Plastics and Composites Welding Handbook. pp. 271–309.
1 2 3 4 Jaeschke, Peter (2015). "Advanced laser welding of high-performance thermoplastic composites". Journal of Laser Applications. 27 (S2): S29004. Bibcode:2015JLasA..27S9004J. doi: 10.2351/1.4906379 .
↑ Vacogne, C. (2011). "Joining of high performance carbon fibre/PEEK composites". Science and Technology of Welding and Joining. 16 (4): 369–376. doi:10.1179/1362171811Y.0000000027. S2CID 137394756.

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[1] Yousefpour, A., M. Hojjati, and J.-P. Immarigeon, Fusion Bonding/Welding of Thermoplastic Composites. Journal of THERMOPLASTIC COMPOSITE MATERIALS, 2004. 17(4) p. 303-341

[:0-2] 1 2 Liu, S. , Chang, I. and Hung, S. (2001), Factors affecting the joint strength of ultrasonically welded polypropylene composites. Polym Compos, 22: 132-141. doi:10.1002/pc.10525

[3] Benatar, A. and T.G. Gutowski, Method for fusion bonding thermoplastic composites. SAMPE Q.;(United States), 1986. 18(1).

[:22-4] 1 2 3 Steen, William. Laser Material Processing. pp. 364–370.

[:12-5] 1 2 3 4 5 6 7 8 9 10 Benatar, Avraham (2003). Plastics and Composites Welding Handbook. pp. 271–309.

[:02-6] 1 2 3 4 Jaeschke, Peter (2015). "Advanced laser welding of high-performance thermoplastic composites". Journal of Laser Applications. 27 (S2): S29004. Bibcode:2015JLasA..27S9004J. doi: 10.2351/1.4906379 .

[7] Vacogne, C. (2011). "Joining of high performance carbon fibre/PEEK composites". Science and Technology of Welding and Joining. 16 (4): 369–376. doi:10.1179/1362171811Y.0000000027. S2CID 137394756.

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