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= Welding of Advanced Thermoplastic Composites = 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. 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 decrease 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) 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[3]. With optimize welding parameters and joint design weld strength, up to 80% of the base material can be retained for advanced thermoplastic composites. 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.

Laser welding
Laser welding involves two parts being pressed together as a low powered defocused laser moves along the bondline. Pigments such as carbon or dies are added to help increase the laser absorptivity. Advanced thermoplastic composites that are commonly welded with laser are polyphenylene sulfide-carbon fiber and polyphenylene sulfide-glass fibers. The use of lasers welding is pretty limited, because there are other welding techniques that are less expensive and can do the same joints. Most of the laser welding currently being done is still under investigation, but shows promising results.

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)

Draft for Welding of Advanced Thermoplastic Composites
Thermoplastic and thermoset 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 are much less common thermoset composites, despite having higher damage tolerance, excellent corrosion, high fracture toughness, high impact resistance, good fatigue resistance, low storage cost, and infinite shelf life. Another big advantage over traditional thermoset composite is their ability to be formed and reformed, reparible and fusion welded. To join thermoset composite, fasteners and adhesives are used and reduce the performance and increases the cost. This is why fusion bonding can help reduce the manufacturing time, thus reducing cost.

Fusion bonding fundamentals
Fusion bonding requires the melting of the joint interface, which decrease the viscosity of the polymer and allows for intermolecular diffusion. These long polymer chains then diffuse across the joint interface then 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 different classifications for their ways of generating heat; such as frictional heating, external heating and electromagnetic heating. However, some of these techniques can be very limited and only used for specific joints and geometries.

Friction welding techniques
Friction welding is best used for parts that are typically 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 is 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 not flat.

Spin welding
Spin welding is not a very common welding technique for advanced thermoplastic because this can be done with parts that have a circular geometry. This is done by one part remaining stationary and the other is continuously rotated with apply pressure to the weld interface. Rotational velocity will be different throughout different radius 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 attributed to high residual stress and decrease 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) 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 asperites 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. With optimize welding parameters and joint design weld strength up to 80% can be retained for advanced thermoplastic composites. [2] However, during 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 techniques
Thermal welding can produce good weld quality although extra precautions need to be taken to prevent high residual stress, warping, and deconsolidation.

Laser welding
Laser welding involves two parts being pressed together as a low powered defocused laser moves along the bondline. Pigments such as carbon or dies are added to help increase the laser absorptivity. Advanced thermoplastic composites that are commonly welded with laser are polyphenylene sulfide-carbon fiber and polyphenylene sulfide-glass fibers. The use of lasers welding is pretty limited, because their are other welding techniques that are less expensive that can do the same joints. Most of the laser welding currently being done is still under investigation, but shows promising results.

Electromagnetic welding techniques
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 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, embedded with conductive material such as metal or carbon fibers. An induction coil is then place near the weld joint, which introduces a current in embedded in material used to generate heat..

Welding carbon fiber, carbon and graphite fiber mats with higher electrical resistance are used 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 coefficient of thermal expansions, which introduces the residual stress. Things such as heat input, cooling rates, volume fraction of the fibers, and matrix material will influences 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 used for welding
-Carbon Fiber polyetherimide (CF/PEI)

-Carbon Fiber polyphenylene sulfide (CF/PPS)

-Carbon Fiber polyetheretherketone(CF/PEEK)

Potential Topic for Projects
-Applications: On the wiki page for "Plastic Welding" theirs lots of information on processes, but not a lot of information on where these process are used.

-Weldability of thermoplastics: On the wiki page for "Weldability" their is only information on metals. I think there could be a section on the weldability of plastics.

-Plastic and Composite Welding used in Aerospace: Their is little to no information on this topic.

Instructor Comments
Adding applications with examples for each process in the "Welding of Plastics" article would be good, but may be difficult.

Weldability of thermoplastic is a complex topic that is process dependent, so it may also be difficult.

Plastics and Thermoplastic Composite Welding for Aerospace would be an excellent topic, but it may too broad. You may want limit your article to just Welding of Advanced Thermoplastic Composites for Aerospace Applications.