Laser Macroprocessing

Laser macroprocessing is defined as the machining of metal parts with thicknesses greater than a mm to produce large features with multi-mm dimensions. The process uses high power lasers with typical average powers ranging from a few hundred W to many kW. Key applications in macroprocessing are metal cutting, metal welding, and additive manufacturing of metals. The laser cutting industry is expected to grow significantly over the next few years. Much of the growth is driven by the fact that the application space is constantly increasing. For example, the automotive industry is producing laser-cut and laser-welded car body parts, and the construction industry is using lasers to cut construction components. The plastics industry uses lasers to cut textiles and packaging materials. The biggest advantage to laser use is that any arbitrarily-shaped cut can be achieved, including 3D shapes, without the typical limitations of mechanical tools. Unlike mechanical processes, producing well-defined cutting edges is easily achievable because the laser process is non-contact and therefore wear-free. Hence, even the hardest or most abrasive materials can be processed without the need for tool replacements. Because the laser process is non-contact, no force or mechanical stress is applied to the processed part. This is especially important for the processing of brittle or soft materials as well as in high-speed cutting applications where material movement exceeds 100 m/s, e.g. when cutting paper. Additionally, laser systems reduce production time and tool costs because there is minimal set-up time and no need to produce individual tools when punching metal sheets.

In the metal cutting arena, lasers compete with plasma- and water jet cutting. However, the edge quality and ability to control power in lasers is superior to that of plasma cutters. Water jet has the best edge quality on very thick metal sheets and takes the lead when material thicknesses exceed 20 mm. With thinner materials, the greater flexibility of the laser is exploited. In the past, the CO2 laser was the workhorse for metal cutting. This was primarily due to its price advantage compared to solid-state lasers, e.g., the Nd:YAG laser, despite the fact that the latter delivers a better material absorbance. However, this has changed with the introduction of cost-effective disc and fiber lasers, causing CO2 lasers to be supplanted in many applications. Nonetheless, for organic materials like plastics or wood, there is no alternative to the CO2 laser because of the strong absorption at the FIR operational wavelength of 10.6 µm.

Laser welding is ubiquitous and has become the preferred technology in many industries over the past few years. It can be used to interconnect a wide variety of organic and inorganic materials. Production sectors dealing with mobility in the broadest sense, such as the automotive, shipbuilding, and aerospace industries are increasingly replacing bolted assemblies with welds. Laser welds provide permanent connections that save weight and reduce risks associated with nuts and bolts that can loosen or break over time. Welding is also commonly used in other applications including gas-tight welding of heart pacemakers, welding fine jewelry, and welding stainless steel in heat exchangers for white goods/appliances or in heater/cooler systems. Also, in contrast to traditional welding processes relying on electrical discharges, lasers can produce a minimal HAZ because it is possible to control the laser beam more precisely.

In laser metal welding, three major processes are currently in use. One involves conventional welding optics which have a rather short focal length, e.g., 100 to 200 mm. With the introduction of a filling wire, the joined parts are melted at a joint to allow both metals to mingle before cooling and becoming one solid part. It is important that there are no gaps in the joint between the parts to ensure the weld is effective. When using a filling wire, there are three components interacting. The two loose parts and the filling wire are melted in the focal spot and joined. The wire interacts with each of the materials and helps facilitate the connection between them. Another welding process, known as remote welding, uses a three-axis galvanometric scanning system with a long focal length, e.g., 0.8 to 1.5 m, that can be located far away from any obstructing parts. This is important for welding large parts like door panels where clamping fixtures are needed to ensure proper alignment but tend to interfere with the motion system used to position the laser. Such scanning systems allow for fast beam steering and generate many welding dots over a long distance in short time. Remote welding allows manufacturers to save valuable production time and achieve a higher throughput. Fiber lasers and disc lasers are typically used for laser welding and remote welding applications, while CO₂ laser welding is preferred for certain applications because of the special weld seam characteristics provided by these lasers.

The third welding process, the welding of plastics, has different laser requirements. Diode lasers and CO₂ lasers dominate this application process. CO₂ laser wavelengths are absorbed by any plastic material regardless of whether the plastic is transparent or not. Diode laser wavelengths are only absorbed by colored plastics. This difference in absorbance leads to a welding strategy where one transparent part and one colored part can be joined by steering the laser beam through the transparent material onto the colored plastic to melt the colored plastic and join them together. One major use of laser welding in the automotive industry is for welding headlight and taillight assemblies. Laser welded housings for electronic components are also found in many applications.

Additive manufacturing is often a confusing topic because different technologies are combined in this application. The processes can involve melting materials and delivering them through a nozzle and depositing the melt layer by layer. This process is typically referred to as 3D printing. Processes that use a laser to melt or fuse powder are typically referred to as selective laser melting or laser sintering. Laser sintering can be used with different materials such as sand, polymers, or metal powder. Sintering sand is used for mold making in mold casting processes. Powder polymers are typically used for either mold making or to create components in rapid prototyping applications. Liquid polymers are used in laser stereolithography with a UV laser curing selected areas of the plastic to build a solid part. Model building for technical design and medical device applications are the main areas of interest for laser stereolithography processes. Metal powders were originally used with additive manufacturing processes to manufacture tools for molds in injection molding applications. However, metal powders are now also used in direct manufacturing of many functional parts, including parts for vehicles, bionic designs, and medical implants.

Today, large additive metal manufacturing machines can use up to four lasers at a time to produce either one large part or to produce multiple parts in parallel. While initially used to produce prototypes, additive manufacturing is increasingly being used by companies to efficiently produce complicated 3D structures. The benefit of this approach over traditional metal manufacturing techniques that selectively remove metal to produce a structure is the significant reduction in waste material.

For macroprocessing applications, high power IR lasers with kW output powers are needed to perform metal cutting, welding and additive manufacturing processes. Most of these lasers are CW lasers, i.e., not pulsed, while some are quasi-CW where the laser is pulsed in ms timescales to increase the peak output power for a given average power. One particularly important parameter in these applications is the brightness of the laser beam as represented by its BPP (beam parameter product). BPP is the product of the beam diameter in mm and beam divergence in mrad (see Laser Beam Spatial Profiles for details). Higher brightness levels (or lower BPP) are necessary for metal cutting applications, while metal welding applications require less brightness (or higher BPP). Figure 1 maps laser power versus BPP for various macroprocessing applications.

The beam delivery section of a laser machining system transfers the laser beam from the laser cavity to the workpiece. For CO₂ laser-based 2D-machines, at least two moveable mirrors are required for guiding the laser beam to any point on the workpiece. In modern machines, and especially in 3D machines, the beam delivery section requires additional mirrors due to the more complicated design of the structure being manufactured and to enable other features incorporated in the system. The cutting head of a CO₂ laser-based 2D-machine includes a ZnSe focusing lens, which focuses the laser beam on the workpiece. For fiber-laser-based machines, the fiber core itself guides the beam to the cutting head, which includes a collimator and a focusing lens. Carbon dioxide molecules are excited by a gas discharge in the cavity of a CO₂ laser. Mirrors are placed at both ends of the discharge tube such that the laser beam is reflected many times to build up the laser beam intensity. Each mirror has some transmittance: the output coupler transmits the usable laser beam while the rear mirror transmits a very small portion of the beam for power measurements and beam diagnostics. In order to gain sufficient intensity to produce several kW of output laser power, the total length of the laser cavity needs to be several meters. Covering this distance with one discharge tube is very problematic and impractical. Therefore, the laser cavity is split up into several discharge tubes working in series. To reduce the mechanical footprint as much as possible, the optical axis of the laser beam within the cavity is "folded" several times by using highly reflective mirrors.

In most CO₂ laser machines, several mirrors are used to deliver the laser beam from the cavity to the cutting head. Reflectance of these mirrors should be as high as possible, i.e., minimum absorption and scattering, to minimize laser power losses. In addition to laser power and beam mode, beam polarization also affects cutting quality. For the best cutting quality, circular polarization is required. This is achieved by converting the linear polarization emitted from the cavity to circular polarization using a series of phase-shift mirrors. Zero-phase-shift mirrors guide the beam to the cutting head while maintaining its linear polarization. The addition of one 90¡ phase-shift mirror at a specific orientation converts the linear polarization to circular.

In contrast to CO₂ lasers, fiber lasers do not require cavity optics nor beam delivery optics. These functions are performed by the fiber itself. The output coupler and the rear mirror are embedded in the fiber core, and the beam is delivered by the fiber to the cutting head. Due to the relatively small core diameter of a fiber (typically 50 to 100 µm), beam divergence is quite significant. Therefore, a collimator lens is used to collimate the beam after it emerges from the fiber. The cutting head of a fiber laser system includes a lens that focuses the beam on the workpiece. The diameter of the laser spot size on the workpiece depends on the focal lengths of the collimator and focusing lenses. It is necessary to adjust the laser spot diameter to achieve the optimal cutting conditions for a particular application. Sheet material and thickness, laser power, and other laser system parameters determine the optimized spot diameter required for cutting quality and speed. Typically, the spot size should be increased as sheet thickness increases. In CO₂ laser-based machines, the operator is tasked with replacing the focusing lens in order to adjust the beam size. In fiber-laser-based machines, replacing the focusing lens is not recommended because of the extreme sensitivity of these machines to dust particles, which may penetrate the cutting head while switching lenses. The best solution for varying the fiber laser spot size is to use a continuous zoom lens in which the focal length varies by moving internal optical elements. In this manner, the cutting head remains sealed and free of dust particles. Another advantage of using a zoom lens is the ability to adjust the spot size continuously across the full zoom range. Only when performing optimally can laser macroprocessing systems guarantee the most cost-effective production of high-quality components. Even the smallest deviations in beam adjustment or focal position may lead to reduced part quality, massive cost increases, and pollution of the environment in a variety of ways, including increased energy consumption and use of process gases. See Real-Time Laser Power Measurement for additional information.

Ensuring a consistently high-quality weld in a proactive manner is preferred and reduces overall costs. The ability to measure the laser beam in a non-contact manner offers key advantages. A non-contact method developed by MKS Ophir is based on Rayleigh scattering. See Non-Contact Laser Beam Measurement for additional

Laser macroprocessing has expanded to many facets of metal manufacturing from cutting and welding to newer processes in additive manufacturing. As laser diode cost per watt of power continues to improve (similar to Moore's Law for semiconductors), laser machining will become increasingly compelling over conventional processes in many applications. One can envision laser macroprocessing becoming the dominant process for the full range of industrial manufacturing in sectors ranging from automotive to shipbuilding.