The components of the cutting head of a laser cutting machine include a nozzle, a focusing lens, and a focusing tracking system.
In laser material processing, it is usually necessary to guide the focused laser beam onto the workpiece, and the device used for this purpose is called a laser processing head. According to different application scenarios, it can be divided into cutting heads, welding heads, brazing heads, cladding heads, etc.
Beam focusing
In many cases, laser light must be focused on a small point on the workpiece in order to achieve sufficient optical intensity, and the focus of the beam is generally located on or near the surface of the workpiece. Usually, laser light is sent to the processing head in an approximately collimated form (i.e. with a relatively large beam diameter and low beam divergence), and then focused inside the head through a lens or curved mirror. The distance from the focusing element to the focal point of the beam is roughly equal to its focal length. In ome cases, the focusing element can be replaced to use different focal length values, which not only affects the beam radius at the focal point, but also affects its distance from the machining head outlet, i.e. the working distance.
The processing head is usually designed for a specific wavelength range and coated with corresponding anti reflective coatings on the mirrors and lenses used, resulting in good beam focusing performance. Common machining heads work at 1.06 μ M (for YAG laser and fiber laser) or 10.6 μ The wavelength range of m (CO2 laser) can also be used for green light (532nm, 515nm) or 2 μ Processing head with m wavelength range. For free space beam systems (such as common solutions for CO2 lasers and excimer lasers), the input beam should usually have an appropriate collimated beam radius and beam quality factor M2.
For transmission using high-power fiber optic cables (commonly used in solid-state lasers such as fiber lasers and diode lasers), it is usually only related to the core diameter and incidnt angle of the fiber. In fact, the distribution of optical power on the fiber optic mode also has an impact, ultimately determining the size of the focused spot obtained. The laser power cannot exceed the rated value of the processing head, and in addition to the average power of the laser, the peak power may also be limited for pulse lasers.
Beam shutter
Many lasers cannot be frequently turned on or off, so beam shutters are used to block the beam. In some cases, the beam shutter is integrated into the processing head.
Multi beam/guiding light
In most cases, the processing head only has a single output beam. However, sometimes multiple beams are used:
Multiple beams with the same power and other parameters can simultaneously process multiple points on the workpiece.
For certain applications, it is possible to switch between two different beams, such as having different optical wavelengths and/or beam parameters.
In certain special configurations, some processes may use a combination of several beams of light. For example, two-point technology uses two closely arranged laser beams with equal power and diameter, for example, for welding, which can achieve a front to back distribution or parallel distribution, resulting in different welding processes or weld pool morphologies.
Some laser heads can generate visible wavelength guided laser, which is used to display the focus position and facilitate teaching the welding trajectory.
Working distance
The working distance is the distance between the output end of the machining head and the surface of the workpiece. In some cases, the working distance needs to be relatively small in order to effectively focus or blow at close range. In most other cases, larger working distances are required, such as remote laser welding (which usually does not require process gas), specialized processing heads need to be developed, and laser sources with higher beam quality are likely to be required. For situations where the effective Rayleigh length of the beam is small (i.e. strong focusing and/or poor beam quality), it may be necessary to use an automatic feedback system to accurately stabilize the working distance, using distance sensors (usually capacitive sensors or CMOS sensors)
Beam positioning and scanning
Accurate laser beam positioning is crucial during the machining process, especially in laser micro machining where the tolerance can be much lower than 1mm. The positional parameters may involve the distance from certain features on the workpiece (such as solder pads or welded seams), or the distance from the edge of the workpiece.
Some laser processing heads have fixed beam paths; At this point, the beam can only be moved by moving the entire laser head, using Gantry+servo mechanisms or robots, or by moving the workpiece to a fixed beam position. Some processing heads are equipped with integrated laser scanners, known as galvanometer systems, which contain movable optical components used to scan the direction of the output beam in one or two or even three dimensions. There are usually two different types of technical solutions for laser head galvanometer systems:
A movable scanning mirror is installed behind the focusing or reflecting mirror and works with the already focused beam. The fixed beam position on the focusing element has the advantage that the element does not need to be so large and its optical aberration is not as critical. This configuration is usually used for situations where the working distance is large and the beam direction is variable.
First, send the beam to the movable mirror, and then use the focusing element to use f- θ A lens through which a constant beam direction can be achieved at a variable position on the workpiece. However, the diameter of such lenses is limited, so their working area is also limited, and the working format is generally not large.
Fast beam scanning is often used as a means of flexibly generating various beam cross-sections. When the beam is scanned at a sufficiently high frequency, the average intensity cross-section obtained can be easily modified by scanning parameters without the need to change the optical components. There are also some laser drilling heads that contain a special type of scanner that quickly moves the beam focus around the circle. This is very useful for cutting holes with larger diameters.
Sometimes, it is necessary for the laser head to have particularly high flexibility, not only to change the position of the laser point on the plane, but also to change the direction from which the beam enters the workpiece. Sometimes, the laser head is installed on the robot arm and can achieve five or six degrees of freedom. For example, sometimes the laser head can be rotated around one joint, tilted through another joint, and moved in any direction. Due to the multiple requirements of high precision and high-speed, precision motor technology is usually used to achieve this.
Process gas
In most cases, the gas used in laser processing is usually an inert gas, such as nitrogen or argon, which can protect the surface from oxidation. In laser cutting, sometimes air or even concentrated oxygen is used to blow out slag or oxides. Another function of process gases (whether inert or oxygen) is to blow off materials and accelerate the cutting process. Due to the need for a normal and high-pressure gas jet flow, it is necessary to apply it at close range. However, there are also some remote cutting processes that do not require process gas.
In some cases, the nozzle of the laser head is integrated and can simultaneously send a laser beam and blow process gas. For example, cutting, drilling, or welding of non-ferrous metals. Sometimes the process gas is blown sideways and then flows along the surface of the workpiece, which is commonly used for welding or surface hardening. Sometimes the workpiece does not need to be supplied with protective gas, but instead uses an air curtain cross jet inside the processing head to prevent contamination of the protective lens, commonly used in laser welding processes.
Feeding
Laser brazing requires some solid solder to be provided in the form of wires, balls, or powders. Therefore, a wire feeding system (sometimes with a resistance heated hot wire mechanism) or powder conveying device is equipped on one side of the processing head. When using laser soldering, small solder balls are applied through capillaries. Some laser surface modification processes also require pre laying of some powder. Sometimes, these processed materials are coaxial with the laser beam, and sometimes the materials are supplied from the side.
Protecting optical components
Usually, a protective gas nozzle is integrated to form an air curtain to prevent splashing, dust, and oil vapor from polluting the lens during laser processing. Sometimes, optical windows with anti reflective coatings or detachable protective lenses are used to protect internal optical components. These are replaced regularly as consumables or spare parts, and the advantage is that the beam does not need to be adjusted.
Power adaptation
Many laser processing processes require considerable power, thus requiring the processing head to be able to adapt to the corresponding power level. In addition to the risk of damage, lens thermal effects may occur at high power, and the focus position may shift as the laser power increases or after the high-power laser beam is opened. The use of water-cooled metal lenses can adapt to thousands of watts of CO2 laser, while for shorter wavelengths, high reflectivity lenses with high thermal conductivity sapphire as the substrate can be used.
For pulsed lasers, their peak power is usually several orders of magnitude higher than the average power, and there may be peak power limitations that can be set by a combination of the optical damage threshold and beam diameter of the optical element.
When the high-power beam is not calibrated, the machining head may be damaged. Therefore, caution must be exercised when installing the processing head or operating the laser. The particularly sensitive component is the scanning galvanometer, which is generally lightweight to achieve rapid movement, but also needs to ensure good thermal contact, making it difficult to handle high power. The lenses used require a very high reflectivity to reduce heat accumulation.
Laser head suitable for ultrafast lasers
Some applications of laser material processing require the use of ultrafast lasers, with pulse durations ranging from picoseconds to femtoseconds. These lasers are mainly used in 1 μ Working within the wavelength range of m, there are also some frequency doubling green light and ultraviolet lasers. Although the beam transmission system of the lens group can usually be used with such lasers, ordinary high-power multimode fiber optic cables cannot be used, Mainly due to the presence of strong mode dependent group delay (Group delay refers to the time delay caused by the difference in propagation speed of different frequency components in a laser pulse. In the frequency domain, a laser pulse can be decomposed into multiple frequency components, each corresponding to different wavelengths or frequencies of light. Due to the variation of the refractive index of a material with frequency, light waves of different frequencies will experience different phase velocities when propagating in the material) This will result in obtaining a series of multiple pulses, each with different time delays and spatial distributions, each carrying only a small portion of the total light energy. On the other hand, due to peak power, ordinary single-mode fibers cannot be used, as this can cause excessive nonlinear effects or instantaneous damage. Therefore, special hollow core optical fibers are needed to transmit ultra short pulses with hundreds of microjoules of peak power and pulse energy. The dispersion needs to be compensated through other methods, and the chromatic aberration of the focusing lens may be another issue to consider; Based on the optical bandwidth and focusing details of the pulse, it may be necessary to use achromatic optical elements.
Dedicated and multi-purpose laser heads
Many laser heads have been specifically optimized based on specific processes and product types, especially for industrial mass production.
There are also multi-purpose processing heads that can be configured and adjusted by adjusting optical components, replacing optical and other components, or adding accessories such as wire feeding devices. Some processing heads can even meet different processes, such as welding and brazing, with appropriate configurations.
Beam diagnosis
Many laser based processing processes are very sensitive to the intensity distribution of the laser beam, and the intensity distribution may change due to technical issues in the laser source or transmission system. Therefore, some laser processing heads are equipped with integrated laser beam diagnostic functions. Usually, by using a beam splitter, a small portion of the light power in the beam is directed to the diagnostic camera, and the recorded beam intensity distribution can be automatically processed or displayed on the screen.
Process diagnosis
For applications with high quality requirements, laser heads are usually equipped with process diagnosis or quality monitoring systems, such as:
Cameras are sometimes equipped with optical filters to filter light from specific spectral areas, providing clear images of the processing area, which can be used to monitor various details such as keyholes in welding processes, plasma clouds, etc. It can also be used for precise positioning of parts before machining, and for checking machining results, such as weld morphology and size. The information collected by the camera can be used for process monitoring and quality control in automation software.
Photoelectric sensors collect plasma clouds, temperature fields, and laser reflection signals generated during laser processing, monitor the processing process in real time, and achieve judgment or prediction of processing results based on big data analysis, machine learning, or neural networks, or pre alarm the status of processing equipment. Please refer to my column - Laser Welding - Quality Monitoring System for details. Some welding joints will add an external photoelectric sensor to detect the diffuse reflected light intensity on the protective lens. Due to the presence of dirt or foreign objects, the diffuse reflected light intensity will be increased, thereby achieving warning for wiping or replacing the protective lens.
Spectrometers can provide valuable information for spectral analysis of laser induced plasma clouds in certain situations. For example, the presence of certain chemical elements can be detected and the process can be automatically stopped under specific conditions.
Processing head software
Laser processing heads usually need to provide appropriate software interfaces for
(a) Remote control of the entire system, including the laser head,
(b) Obtain various data related to the workpiece (before and after processing) and
(c) Obtain detailed information about the laser processing process. Monitoring the health status of machine systems can include aspects such as laser power and beam quality, beam distortion caused by optical components, misalignment, temperature conditions, and gas flow.
