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LASER WELDING OF ADVANCED THERMOPLASTIC COMPOSITES

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.

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.

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. 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.

Affect 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 affect 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..

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. 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. By the same token, amorphous polymers lack this trend with thickness.

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. 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.

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.

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.

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.

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.

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. This methodology initially was applied only to plastics, but has recently been applied to composites such as carbon fiber reinforced PEEK.

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

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. Greater strength is imparted because full fusion is achieved without damaging the surface of the transparent material.