Laser technology, as an emerging field of science and technology, has developed rapidly and has permeated almost all areas of natural science, with an extremely broad range of applications.
Laser applications can be categorized into two main types: thermal effects of lasers, which include laser cladding, cutting, welding, shock processing, and guidance, and non-thermal effects of lasers, which encompass laser-induced material separation, chemical reactions, chemical bond cleaving, and laser chemical kinetics detection, as well as other uses like laser rangefinding, communication, and cooling.
In the 3D printing sector, both the thermal effects, such as laser cutting and shock processing, and the non-thermal effect of laser photocuring are employed.
(1) Basic Principle of Laser Cutting
The fundamental principle of laser cutting is that a laser beam is focused into a very small spot with a diameter that can be less than 0.1mm, achieving a power density at the focus exceeding 10W/mm2.
At this juncture, the heat input from the beam (converted from light energy) far surpasses the heat lost through reflection, conduction, or diffusion by the material, quickly heating the material to vaporization temperature and causing it to evaporate, creating a hole. As the beam moves linearly relative to the material, a continuous and narrow cut (about 0.1mm wide) is formed.
The heat-affected zone around the cut is minimal, resulting in virtually no deformation of the workpiece. An assist gas compatible with the material being cut is also added during the process. When cutting steel, oxygen can be used as an assist gas to oxidize the material through an exothermic chemical reaction with the molten metal, while also helping to blow away the slag in the kerf.
For cutting polypropylene and other flammable materials like cotton and paper, inert gases are used. The assist gas entering the nozzle also serves to cool the focusing lens, preventing dust from contaminating the lens holder and overheating the lens. Figure 3-3 is a schematic diagram illustrating the principle of laser cutting.
(2) Commonly Used Laser Cutting Equipment
Common laser cutting machines include CO2 laser cutters and fiber laser cutters.
① CO2 Laser Cutter
CO2 laser cutting is a predominant method offering the following advantages over methods like electrical discharge machining (EDM), flame cutting, and plasma cutting:
a. High-quality cuts. The kerf width is narrow (typically 0.1~0.5mm), precision is high (typically with a center-to-center error of 0.1~0.4mm and contour dimension error of 0.1~0.5mm), and the surface roughness of the cut is desirable (typically Ra 12.5~25μm), often making further processing unnecessary for welding.
b. Fast cutting speeds. For instance, with a 2kW laser power, the cutting speed for 8mm thick carbon steel is 1.6m/min, and for 2mm thick stainless steel, it is 3.5m/min. The heat-affected zone is small, resulting in minimal deformation.
c. Clean, safe, and pollution-free. Laser cutting is a non-contact process with no mechanical stress on the edges, no shearing burrs, and minimal debris, even when cutting materials like asbestos and fiberglass. Additionally, the processing system during laser cutting is equipped with necessary assist gas blow-off devices to remove the slag produced at the kerf, significantly improving the working environment for operators.
CO2 laser cutting technology is widely used in the processing of both metal and non-metal materials, significantly reducing processing times, lowering costs, and improving workpiece quality.
Many metal materials can be cut without deformation in industrial manufacturing, regardless of their hardness. Of course, for materials with high reflectivity, such as gold, silver, copper, and aluminum alloys, which are also good conductors of heat, laser cutting can be challenging or even impossible.
②Fiber Laser Cutting Machines
Fiber laser cutters are a new type of laser that has seen rapid development internationally. When compared with the bulky gas lasers and solid-state lasers, fiber lasers hold distinct advantages. They have become important candidates in fields requiring high precision laser processing, laser radar systems, space technology, and laser medicine.
Fiber laser cutting machines can perform both flat and beveled cutting with neat and smooth edges, making them suitable for high-precision cutting tasks such as metal sheet fabrication. Additionally, when equipped with a robotic arm, they can execute three-dimensional cutting, serving as a replacement for the previously imported five-axis laser cutting machines. Fiber laser cutters have the following advantages over CO2 laser cutters:
a. Exceptional beam quality: Smaller focal spots, finer cutting lines, higher work efficiency, and better processing quality.
b. Extremely high cutting speeds: Twice as fast as CO2 laser cutters of the same power rating.
c. Outstanding stability: Uses top-of-the-line imported fiber lasers, offering stable performance with key components lasting up to 100,000 hours.
d. Very high electro-optical conversion efficiency: The fiber laser cutter’s efficiency is about 30%, three times that of CO2 laser cutters, making it energy-saving and eco-friendly.
e. Significantly low operational costs: Power consumption is just 20%-30% of that of similar CO2 laser cutters.
f. Minimal maintenance costs: No laser gases are required; fiber transmission eliminates the need for reflective lenses, saving a considerable amount in maintenance.
g. User-friendly operation and maintenance: Fiber transmission eliminates the need for optical path adjustments.
h. Superior flexibility in light guiding: Compact and tightly structured, it easily meets flexible processing requirements.
However, compared to CO2 laser cutters, fiber lasers have a relatively narrow cutting range. Due to their wavelength, they can only cut metallic materials; non-metals are not easily absorbed, which affects their cutting capabilities.
(3)Laser Cutting Applications
Laser cutting is the most important application technology in the laser processing industry, accounting for over 70% of all laser processing. Compared with other cutting methods, laser cutting is distinguished by its high speed, precision, and adaptability. It also offers advantages such as narrow kerf, minimal heat-affected zone, high-quality cutting surfaces, noise-free operation, and easy integration into automated control systems.
Laser cutting of sheet metal eliminates the need for molds, replacing certain stamping processes that require complex, large-scale molds. This can significantly shorten production cycles and reduce costs. Consequently, laser cutting has been widely applied in industries such as automotive, construction machinery, aerospace, chemical, light industry, electrical and electronics, petroleum, and metallurgy.
It is capable of cutting various metals, such as titanium alloys, nickel alloys, chromium alloys, stainless steel, beryllium oxide, copper alloys, and non-metallic materials like high-hardness, brittle silicon nitride, ceramics, quartz, as well as organic non-metal materials such as fabric, paper, plastic plates, and rubber.
From a techno-economic perspective, for metals that are not suitable for mold manufacturing, especially those with complex contours, small batches, and thicknesses of less than 12 mm for low-carbon steel plates or less than 6 mm for stainless steel, laser cutting offers direct processing savings in both mold costs and production time.
Typical products that have employed this technology include structural components for automatic elevators, elevator panels, machine tool and grain machinery housings, various electrical cabinets, switchgear, textile machinery parts, construction machinery structures, and large motor silicon steel sheets.
In the automotive industry, laser cutting techniques have been widely used for components with spatial curves, such as car sunroofs. The Volkswagen Group in Germany utilizes a 500W laser to cut complex-shaped body panels and various curved parts.
In the aerospace sector, engine flame tubes, titanium alloy thin-walled casings, aircraft frames, titanium alloy skins, wing spars, tail wall panels, helicopter main rotor blades, and space shuttle ceramic heat shields have all been processed using laser cutting.
Laser cutting is also used for creating patterns, markings, and text for the decoration, advertising, and service industries in materials such as stainless steel (typically up to 3mm thick) and non-metallic materials (typically up to 20mm thick). Examples include patterns for art photo albums and signage for companies, institutions, hotels, shopping malls, and bilingual text for stations, docks, and public places.
Although there are some limitations to laser cutting machines, such as their inability to cut very thick steel plates and their relatively high cost, the continuous improvement in laser system quality and the gradual reduction in processing equipment costs are expected to expand the application scope of laser cutting.
Laser Shock Peening
Laser shock peening is an innovative processing method that utilizes the mechanical effects of shock waves induced by intense laser pulses to form and enhance material properties. As illustrated in Figure 3-4(a), by using different backing plates and molds, it is possible to achieve target material performance enhancement, surface smoothing, and material shaping.
During the laser peening process, a laser beam characterized by high power density (GW/cm² level) and short pulse duration (5-30 ns) first passes through a confinement layer (water or K9 glass) and strikes the absorption layer (black paint or aluminum foil) on the surface of the metal target; the absorption layer then absorbs the laser energy and vaporizes instantaneously to form a high-temperature, high-pressure plasma, which continues to absorb laser energy and expand rapidly.
As the plasma is confined by the layer above the ablation layer, it explodes, generating a high-intensity shock wave that impacts the metal surface.
When the pressure of the shockwave exceeds the dynamic yield strength of the target material, the material will undergo corresponding plastic deformation and residual stress based on the type of the backing block and mold, which can also improve its mechanical properties.
The absorption layer is used to absorb laser energy, increase peak pressure, and protect the integrity of the target material’s surface; the constraint layer confines the plasma, effectively raising the peak pressure and duration of the shockwave.
Existing research indicates that the performance enhancement of materials stems from the plastic deformation of the target material’s surface, with the key parameters affecting the extent of plastic deformation being the dynamic yield strength of the metal material and the peak pressure of the shockwave.
Therefore, with consistent material properties, the plastic deformation of the target material can be controlled by adjusting the peak pressure of the shockwave, P(t), which in turn controls the deformation and performance enhancement of the target material. The estimation formula for this parameter can be derived from the macroscopic equations of plasma motion:
In the formula, I(t) represents the laser power density; L(t) is the thickness of the plasma between the constraint layer and the target material; V(t) is the expansion velocity of the plasma; Z is the total acoustic impedance of the constraint layer (Z1) and the target material (Z2), in g/(cm2·s); a is the coefficient for the conversion of energy into plasma internal energy, typically a=0.1.
The estimation formula for the peak pressure of the shockwave, Pm, is as follows:
In the formula, I is the laser power density in GW/cm2, calculated using the simplified formula (3-5); E is the energy of a single laser pulse in joules (J); τ is the pulse width in nanoseconds (ns); D is the diameter of the laser spot irradiated on the surface of the absorption layer in centimeters (cm), as shown in Figure 3-4(b).
Additionally, the overlap rate, target material temperature, and number of impacts during the laser shock peening process all have certain effects on the target material’s post-process surface morphology, residual internal stress, surface microstructure, internal defects, and macroscopic mechanical properties.
Researchers often control the movement of the laser spot by using a laser scanner or move the sample with a high-precision platform to achieve overlap of the spots on the sample surface. The movement path and overlapping effect are depicted in Figure 3-4(b), and the overlap rate can be calculated using formula (3-6).
In the formula, RT and RL are the transverse and longitudinal overlap rates on the target material surface, respectively; d1 and d2 are the distances between the centers of two adjacent laser spots in the transverse and longitudinal directions, respectively, in centimeters (cm).
(1) Fundamental Concepts of Photochemistry
Laser photo-curing is an important branch of photochemistry, and the following basic photochemical concepts are crucial:
① Ultraviolet Light
Laser photo-curing typically employs ultraviolet (UV) light, which ranges from 40 to 400 nm in wavelength and can be further divided into vacuum UV (<200 nm), mid-UV (200–300 nm), and near-UV (300–400 nm). In general photochemical research and photo-curing applications, mid-UV and near-UV light are of practical significance.
According to their wavelengths, they can be categorized into UVA (315–400 nm), UVB (280–315 nm), and UVC (200–280 nm) bands. UVA and UVB are more commonly used in photo-curing systems, while UVC and even shorter wavelengths of UV light are utilized in photolithography for integrated circuit fabrication.
② Beer-Lambert Law of Light Attenuation
In applications with thicker coatings, such as photo-curing paints, deep curing issues must be considered because the intensity of light passing through an absorbing substance will attenuate. The degree of light attenuation can be described by the Beer-Lambert law.
In the formula, I0 represents the intensity of the incident light, and I is the intensity of the transmitted light. The molar extinction coefficient, ε, is related to the properties of the light-absorbing substance in the material being penetrated and the wavelength of the incident light. The concentration of the light-absorbing substance is denoted by c, and l is the path length of the light.
According to the equation, a higher concentration of the absorbing substance results in a lower intensity of the transmitted light and more significant light attenuation. Therefore, in practical applications, an excessively high concentration of the photoinitiator is not conducive to curing at greater depths.
③Absorbed Light Energy
The absorption of light essentially involves the transfer of light energy to the light-absorbing substance, causing its molecules to transition from a low-energy state to a higher-energy state, such as from the ground state to an excited state. The energy absorbed is related to the wavelength of the light as follows:
In the formula, ΔE represents the energy level difference between the excited and ground states of the molecule, measured in joules (J). The constant h is Planck’s constant, valued at 6.62×10-34 J·s. The frequency of light, denoted by v, is measured in s-1. The speed of light, c, is 3×108 m/s or 3×1017 nm/s. The wavelength of light, λ, is measured in nanometers (nm).
It is evident that shorter wavelengths correspond to higher energy. Ultraviolet light has a shorter wavelength than visible light, hence its higher energy can damage biological cells, which is why exposure of skin to ultraviolet light should be avoided as much as possible. Far-ultraviolet light has even higher energy and can be used for sterilization; for example, germicidal lamps typically have a primary wavelength range of 200-300 nm.
Although light absorption is a property of the entire molecule, in organic molecules, certain atoms or groups of atoms often act as units of light absorption, known as chromophores (or photophores). Typical organic chromophores include C=C, C=O, and aromatic groups. Table 3-1 lists some important organic chromophores, their maximum absorption wavelengths λmax, extinction coefficients εmax, and types of excitation.
The light-absorbing characteristics of a substance can be utilized to estimate or identify which chromophores a molecule contains. Conversely, by introducing specific chromophores into a molecule, one can alter the substance’s light-absorbing properties.
Table 3-1 Maximum absorption wavelengths (λmax), extinction coefficients (εmax), and excitation types of some important organic chromophores.
In the molecular design of photoinitiators, the operational wavelength can often be altered by modifying the structure of the chromophore group.
In a photochemical reaction at a specific wavelength, the number of product molecules generated per quantum absorbed is referred to as the quantum yield (or quantum efficiency) Φ:
Φ = Number of product molecules produced / Number of quanta absorbed
Determining the quantum yield is crucial for understanding the process and mechanism of a photochemical reaction. For instance, Φ > 1 suggests a chain reaction is occurring. Additionally, the quantum yield is an important measure of the initiating efficiency of a photoinitiator.
⑥Excited States and Electron Transitions
A molecule can obtain the activation energy required for a chemical reaction through heat; similarly, in photochemical reactions, the activation energy is acquired by absorbing light. While the fundamental chemical theories underlying both types of reactions are the same, the electron arrangements of the reacting molecules are completely different.
During thermochemical reactions, the molecules are in their ground state, whereas in photochemical reactions, they are in an excited state. After absorbing light energy, electrons in lower energy orbitals can transition to higher energy orbitals, creating an excited state molecule. Such transitions must follow specific rules; those that comply are “allowed transitions,” while those that do not are “forbidden transitions.”
Figure 3-5 uses arrows to show four possible transitions. It should be noted that “forbidden transitions” are not entirely impossible; they simply occur with low probability, as indicated by their small extinction coefficient ε values. For example, an n→π transition is a “forbidden transition” with ε ranging from 10 to 100 L/(mol·cm).
Molecules in the excited state have higher energy and are unstable compared to those in the ground state. They can lose energy through various pathways to return to the ground state, a process known as deactivation. If the molecule remains unchanged during deactivation, returning to the ground state as the original molecule, the process is called a photophysical process.
If the molecule undergoes a chemical reaction while in the excited state, the resulting ground state molecule is no longer the original molecule; this is a photochemical (reaction) process.
⑧Spin States During Electron Transitions
Excited singlet and triplet states involve electron spin. The spin direction of two electrons can be the same (i.e., spin parallel), or opposite (i.e., spin antiparallel), corresponding to triplet and singlet states, respectively.
Triplet states are typically denoted as T, while singlet states are represented as S. Nearly all molecules in their ground state have electrons paired with opposite spins, that is, in a singlet state, often indicated as S0.
However, excited-state molecules are formed when one of a pair of paired electrons jumps to a higher energy level, and these electrons may have either parallel or antiparallel spins, resulting in what is known as excited triplet states (T1) and excited singlet states (S1), respectively.
The energy levels of triplet states are usually lower than those of singlet states, but when a molecule absorbs light energy, the resulting excited electronic state is often a singlet state. This is because transitions that do not change the multiplicity of the molecule are more probable. Figure 3-6 illustrates the spin situations during electron transitions.
(2) Photo-curing reactions
The photo-curing process typically refers to the transformation of liquid photosensitive resin into a solid state upon exposure to light, with most of these reactions being light-initiated chain polymerization.
A broader definition of photo-curing also includes the conversion of soluble solid resins into insoluble solids through exposure to light, a typical example being negative photoresists, which undergo photopolymerization reactions such as the dimerization cyclization of polyvinyl cinnamate.
It’s important to note that during the linking of monomers into polymer chains, material densification and resin shrinkage occur, which can introduce stress into the resulting resin, leading to strain and cracking.
Photo-curable coatings and 3D printing typically involve converting liquid resin into a solid dry film, and the photolytic process they undergo is essentially chain polymerization.
Through polymerization, the molecular weight of the system increases, forming a cross-linked network, which results in a solid dry film. Light-initiated polymerization reactions primarily include light-initiated free-radical polymerization and light-initiated cationic polymerization, with the former being more common.
① Light-initiated free-radical polymerization
Free-radical polymerization usually comprises initiation, chain propagation, chain transfer, and termination phases. The difference between light-initiated and traditional thermal-initiated free-radical polymerization lies in the initiation mechanism. The latter uses thermal initiators that decompose upon heating to produce initiating free radicals, while the former relies on the photodecomposition of photoinitiators to generate active free radicals.
Photoinitiators (PI), upon exposure to light, absorb energy and transition from their ground state to an excited state (PI*), which then decomposes into free radicals. These radicals combine with the carbon-carbon double bonds of monomers (M), initiating chain growth and polymerizing the double bonds.
This process is accompanied by transfer and termination of chain-bound free radicals. Free-radical photo-curing systems are widely used in photosensitive resins due to their rapid curing speed and relatively low raw material costs. However, issues such as significant shrinkage and oxygen inhibition are present, with the latter being a particularly challenging problem to overcome in formulation design.
② Oxygen inhibition
The inhibitory action of atmospheric oxygen molecules manifests in two ways. Firstly, ground-state triplet oxygen can act as a quencher, extinguishing the excited triplet state of photoinitiators. Oxygen molecules are excited to the reactive singlet state while photoinitiators return to the ground state, hindering the generation of active free radicals.
Fortunately, for most type I photoinitiators with short-lived excited triplet states, the probability of bimolecular quenching with molecular oxygen is relatively low and can often be overlooked. Secondly, ground-state oxygen molecules are essentially in a triplet state, which are fundamentally double free radicals.
Thus, they exhibit strong addition activity to the active free radicals generated during the initiation process, forming peroxy radicals that do not add to vinyl monomers. This process is rapid and can compete with the addition of active free radicals to monomers, significantly impeding the polymerization process.
To overcome oxygen inhibition, various physical and chemical methods can be employed in actual production.
a. Physical Methods
Wax Flooding Method: Paraffin wax is appropriately added to the system. As the coating film spreads out, the paraffin forms a thin layer on the surface due to its incompatibility with the organic resin system, serving to hinder the diffusion of external oxygen molecules into the coating.
Film Covering Method: After the system has been coated, a layer of inert plastic film, such as polyethylene, is tightly applied on top to act as an oxygen barrier. After curing under UV light, the film is removed. However, this method affects the gloss and uniformity of the cured coating and significantly reduces production efficiency.
Intense Light Irradiation Method: Intense light irradiation leads to the simultaneous decomposition of photoinitiators, instantly generating a large number of active free radicals. These radicals can add to monomers or react with oxygen molecules. Although the reaction with monomers may not seem advantageous, the absolute rate of initiation increases.
Once polymerization begins, the coating’s viscosity rapidly rises, greatly impeding the diffusion of external oxygen molecules into the high-viscosity system, which is beneficial for the rapid progression of free radical polymerization.
In actual photopolymerization processes, irradiation light sources of thousands of watts are commonly used, with several lamps installed side by side. The light intensity in the overlapping irradiation areas from adjacent lamps is additive. Improving light source quality and increasing irradiation intensity have become conventional means to overcome oxygen inhibition.
Dual Irradiation Method: Initially, the coating is irradiated with a short-wavelength light source (e.g., 254nm), which, due to its poor penetration in organic coatings, is almost entirely absorbed in the shallow surface layer, thereby benefiting anti-oxygen polymerization. At this stage, polymerization only occurs in the shallow surface layer and once the surface is cured, it acts as an excellent oxygen barrier for the underlying layers.
Subsequent irradiation with a conventional medium-pressure mercury lamp, whose longer wavelengths (e.g., 313nm, 366nm) can penetrate the entire coating, completes the polymerization and curing. This irradiation method can also achieve special surface effects.
b. Chemical Methods
Addition of Oxygen Scavengers, such as tertiary amines, thiols, and phosphorus compounds. These compounds, as active hydrogen donors, can react rapidly with peroxy radicals, regenerating active free radicals while transforming the peroxy radicals into alkyl peroxides, which can further decompose into alkoxy and hydroxyl free radicals.
The regenerated active amine alkyl radicals initiate polymerization, and the alkoxy radicals released from the decomposition of alkyl peroxides also possess initiating activity towards vinyl monomers, but their further hydrogen abstraction reactions seem more dominant. The addition of tertiary amines has become an important means to overcome oxygen inhibition in free radical photopolymerization formulas.
However, amine-containing systems tend to yellow upon curing, and their storage stability is relatively poor, which is a significant drawback of using amines as an anti-oxygen inhibition method.
The photoinitiator system combines Type I and Type II photoinitiators. For example, Ciba’s Irgacure 500, which contains equimolar Irgacure 184 and benzophenone, exhibits superior performance in the presence of air.
This is because the excited triplet state of benzophenone effectively promotes the decomposition of hydroperoxides (ROOH), generating alkoxyl (R·O·) and hydroxyl (·OH) radicals that initiate polymerization, while the free radicals produced by the photolysis of Type I photoinitiators consume oxygen, thereby reducing the quenching effect of oxygen on the excited triplet state of benzophenone, indicating a synergistic effect between the two.
Photoinitiated cationic polymerization typically involves the generation of protonic acid by a cationic photoinitiator under light, catalyzing the ring-opening polymerization of epoxide groups or the cationic polymerization of electron-rich carbon-carbon double bonds, such as vinyl ethers.
This class of cationic photoinitiators mainly includes sulfonium and iodonium salts, which upon photolysis produce strongly acidic HPF6, leading to the ring-opening polymerization of epoxide groups. Monomers or oligomers in cationic photocuring systems can also include vinyl ethers, undergoing cationic addition polymerization of the vinyl ether double bonds under strong acid catalysis.
However, in practical applications of cationic photocurable coatings, epoxy compounds are predominantly used as monomers and oligomers.
The greatest advantage of cationic photocuring systems is the absence of oxygen inhibition, coupled with minimal curing shrinkage and strong adhesion, making them particularly suitable for photocurable adhesives. The disadvantages include slower curing speeds compared to free radical systems and higher raw material costs, which are the main reasons why cationic systems are less widely adopted than free radical systems.
Laser photocuring can achieve localized curing in two ways:
a. Vector or scanning method, which uses a UV laser beam to “write” the desired contours on the surface of the resin through a specific scanning technique, as shown in Figures 3-7 (a) and (b).
b. Masking method, which employs a strong UV light source passing through a proportionally reduced mask to form the physical layer, as shown in Figure 3-7 (c).