Equipment for laser metal welding: technology and devices

Technical feature

Laser welding arose according to the developments of N.G. Basov, H. Towns, A.M. Prokhorov.
Specialists managed to obtain pulsed and continuous action devices. Their advantages include an increased concentration of the energy beam. The melting process is carried out at high power, which makes it possible to process dissimilar metals up to several centimeters thick.

Technical features:

• high melting rate;
• saving properties and geometry;
• minimum residual stress indicator;
• no need for filler materials or special chambers with a protected environment.

The precision of laser welding makes it possible to process products of complex configurations.

These nuances make this type of welding one of the most advanced in modern enterprises. The disadvantages include the cost of installation; for some manufacturers the purchase is unprofitable.

Features of laser metal welding

Among the common energy sources used for welding, laser radiation has the highest degree of energy concentration in a single small area. Laser radiation is tens of times more concentrated than other heat sources. Such high concentration rates are determined by the unique characteristics of the laser beam, primarily its monochromaticity and coherence.

Electron beam welding, like laser welding, also provides a high concentration of energy, but the advantage of the latter is that it does not require special vacuum chambers. Laser welding can be carried out both in air and in shielding gases. This is welding in argon, helium, or welding in carbon dioxide CO2 and others. This type of welding is suitable for joining workpieces of any size.

Due to the coherence and monochromatic nature of the laser beam, it has a low divergence, which makes it possible to achieve a high degree of focusing of large amounts of energy in a small area. As a result of this, local heating occurs on the surfaces being welded, providing high rates of heating and cooling. These parameters are much higher than with other arc welding methods.

Other features of laser welding are the small volume of molten metal and the small size of the heat-affected zone, as well as effective melting of the metal at high welding speeds, about 20-40 mm/s, which ensures high productivity.

Classification by characteristics

By energy

Laser welding methods are classified according to several criteria. The varieties differ in technical characteristics and economic aspects. These features are taken into account when choosing a specific installation.

Each type differs in power density. The process is carried out at E=1-10 MW/cm2. If you reduce this indicator, then another type of welding will be more economically suitable, one of which is electric arc. Three main modes are used, which differ in several parameters:

1. t>10-2 s, E=1-10 MW/cm2 . This mode involves the use of continuous lasers. It is suitable for machining structural steels.
2. t<10-3 s, E=1-10 MW/cm2 . This type uses pulse-periodic installations. The combination of high power and process duration acts on metals with less energy consumption compared to the previous type.
3. t=10-3 -10-2 s, E=1-10 MW/cm2 . The mode also uses a pulse-periodic installation; it is suitable for processing material of small thickness.

The workspace layout includes several important elements. The laser beam exits the nozzle, the filler wire provides strength to the weld, and the shielding gas resists adverse environmental conditions.

A rational method is selected according to specific conditions, which will allow obtaining the desired result with minimal economic costs.

Important! Laser welding modes for structural steels are selected individually; this directly depends on the specific conditions and tasks assigned.

According to economic

The first economic aspect is the welding speed. It regulates performance. Continuous laser systems are used at speeds that are 15 times more intense than simple types of welding.

The next economic factor is reducing metal costs. For example, processing of a part with a thickness of 30 mm is carried out in 1 pass without preparatory measures and the use of additives. Manual welding requires a couple of passes.

A concentrated laser beam with local action is the last factor. Thanks to this action, it is possible to obtain welded fastening in areas with a small area and difficult to reach places.

According to technological

According to the first, laser welding is divided into a method of small thicknesses and deep processing.
The latter type, as a rule, is used without additives, although to improve the degree of penetration and improve the quality of weldability, filler material is supplied to the affected zone. Deep penetration is carried out in a protected environment .
Objects of minor thickness are welded using continuous and pulse-periodic installations. The former have proven themselves well in seam welding, and the latter - for spot welding.

It is not necessary to use additives and special media, since they do not have much effect on the penetration of small thicknesses. The treatment is carried out in a gas environment if it is necessary to reduce the likelihood of oxidation of the seams.

Oscillating Welding Head Technology

FIGURE 1 demonstrates the concept of a two-dimensional dynamic beam motion or head with oscillating technology, showing the four main programmable shapes available from a standard weld head, such as the D30 from IPG Photonics. Independent control of vibration amplitude and frequency is achieved using a galvo mirror controller, providing greater flexibility in stabilizing the fusion channel melt during welding processes with typical frequencies up to 300 Hz used in most applications. Processing power of commercial beam oscillation welding heads is now available up to 12 kW.

FIGURE 1. Examples of waveforms from commercially available welding heads with independently adjustable amplitude and frequency up to 300 Hz.

Melt stability of the fusion channel is a critical factor when laser welding makes it difficult to use highly reflective materials such as copper and aluminum. This is due in part to a tendency to spatter and, in the case of some aluminum alloys, exhibit high levels of porosity due to the viscosity and surface tension of the melt, making these materials difficult to weld using more traditional laser welding methods. Recent studies [1-4] have shown the reduction or elimination of these problems by beam vibration techniques, including a recent systematic study with and without filler wire on automotive aluminum alloys [2].

In general, the oscillation method allows for better temperature control of the part as the beam passes multiple times at any point on the weld. The temperature gradient and cooling rate are slower than traditional laser welding, which helps eliminate defects and control spatter. Additionally, this welding method is compatible with typical welding accessories, such as auxiliary gas ports and coaxial nozzles, which provide plasma suppression and can help control spatter, which may not be easily compatible with scan heads used in remote welding.

In addition to stabilizing the fusion channel melt and reducing porosity in the subsequent weld, the beam swing method has proven valuable in easing the fitting requirements of laser welded parts, as listed in TABLE. Using one of the programmable shapes (the infinity sign in this case) and optimizing the amplitude and frequency of vibrations, you can see the 3X increase in allowable seam gap that is achieved with conventional laser welding.

TABLE. Brief description of wobble head welding with oscillations in the process window for seam gap and offset, where a factor of 2-3 increase in both process parameters can be achieved compared to conventional laser welding.

Application area

The greatest welding efficiency is observed in products with a thickness of up to 10 mm . The method is not widely used for economic reasons, since the cost of installation and additional equipment is high.

This processing is used in cases where other types of welding cannot be used and precise preservation of the structure of the part is required after all manipulations. The concentrated impact of the energy beam guarantees minimal changes in the properties and geometry of the product. This is an excellent solution for joining difficult-to-weld metals, without the need for additives, vacuum chambers and other additional elements.

Technology

The essence of the method is to direct the laser beam to a focus, where the beam cross-section decreases. When concentrated energy hits a part, it changes the structure of the metal, the temperature rises instantly, which leads to melting and the formation of a weld. The process is realized by partial and complete penetration, regardless of the position in space. To process products with small thickness, the beam is defocused.

Pulsed radiation is characterized by the formation of a weld in the form of dots. The units are equipped with solid-state lasers; due to their high technical characteristics, the welding speed is 5 mm/s. Additionally, filler materials are used, which can be tape, wire, or special powder. They improve the quality of adhesion by increasing the cross-section of the seam.

What causes laser welding?

Laser welding is the process of melting the edges of metal with a special beam. The latter is obtained from a light source in which excited atoms emit photons - exact copies of their prototypes, without absorbing them. The energy difference between the levels of these atoms amplifies the light. This phenomenon is called stimulated emission.

The resulting narrowly directed stream of converted light is distinguished by a constant wavelength and a given vector oscillation (polarization). It is with them that it is possible to melt the edges of metals. Such a glow can be pulsed into the welding zone when the energy reaches its peak, or constantly, but with less force.

To concentrate and direct the beam, special optics are used, consisting of transparent and translucent mirrors. Welding can occur by melting the edges of the material, or by adding filler wire. In hybrid versions of welding, the filler material can also create an electric arc that melts the tip of the wire, which is placed into the weld by a focused beam of laser energy. The weld pool is protected by an inert gas, which in this case is helium and its mixtures with argon. The video shows all the main elements of the process: a radiation source, a channel for feeding wire from the side, a nozzle for blowing gas.

Advantages and disadvantages

The relevance of this technique lies in the presence of a large number of advantages. These include:

• Precise concentration of energy, due to which it is possible to obtain high-quality products, and the size of the parts can be insignificant in radius.
• High-performance gas units allow the melting of narrow seams, which reduces the area of ​​thermal action and reduces the degree of deformation and surface stress.
• Welding work is carried out with a laser located at some distance from the working area, which is a cost-effective solution.
• Optical fiber and a system of mirrors make it possible to adjust the position, which allows welding work of any complexity, for example, for large diameter pipes.
• You can simultaneously fasten several parts using splitting prisms.

The negative qualities of the laser include the high cost of the equipment, so this option is only suitable for large enterprises.

Conditions and methods of the process

High beam concentration is achieved through a series of reflections from mirrors that have a hemispherical shape. When the critical value is reached, the beam overcomes the central zone of the first mirror and penetrates through the prisms directly into the working area.

Laser cutting and welding of metals are carried out with different localization of workpieces . The melting depth can be adjusted in a wide range, from surface to through. Treatment is carried out with a constant or intermittent beam. The efficiency of laser technology is low and requires high qualifications from the worker.

The process is divided into several types:

• Butt. It is used without additives and powders, but a protective environment is required for processing.
• Overlapping. The joined edges are installed one on top of the other. It is necessary to ensure reliable fastening of the workpieces.

There are compact models for home use that allow you to weld metal products with your own hands.

Devices

The equipment is presented in the form of large-sized machines or mobile devices:

• LAT-S is a machine designed for surfacing and welding of metal products. The device shows high technical characteristics; it is equipped with automatic coordinate machines, which increases the processing speed of complex structures.
• CLW120 is a laser welding machine that has pinpoint accuracy. Used for processing ferrous and non-ferrous alloys, stainless steel and titanium. The device operates on 220 V, so it is suitable for domestic use from an electrical panel.

Welder of the highest category Ivleev A.V.: “The vast majority of models for laser welding are equipped with a binocular; the element protects vision from the negative effects of a bright beam and improves the visibility of the part.”

With solid active element

The operating principle is based on the following aspects:

• A solid rod-shaped element is the source of the beam, it is located in a special chamber.
• The pumping lamp generates flashes of light that activate the working fluid.

Solid state laser circuit

The solid part is made from ruby; this material has shown high technical characteristics, safety and impeccable efficiency.

With elements based on a gaseous medium

These are high-performance machines that work in combination with gas protection . The active medium is a mixture of nitrogen, oxygen, and helium; it is supplied under high pressure, reaching more than 10 kPa. The working gases are excited by an electrical discharge. The efficiency of the device does not exceed 15%.

Nitrogen and helium transfer energy to carbon dioxide, which creates ideal conditions for obtaining a discharge.

Classification of gas lasers

Based on the cooling method, installations are divided into two large groups: with convective (intensive) and diffuse (slow) pumping. The latter is used in low-power single-beam lasers. It is advisable to install convective in powerful devices.

On the side of gas movement relative to the electrodes of the resonator mirrors and the gas chamber, convective lasers are divided into transverse and longitudinal pumping. The mixture is excited by a high-frequency or direct current discharge. A dual-circuit cooling system is responsible for cooling the resonator and optical elements; the working mixture is cooled by a water-gas heat exchanger.

Beam transport and focusing systems

This system includes protective beam guides, a mirror and a focusing element. The mirror is designed to change the trajectory of the beam and moves it into the working area. Low-power solid-state lasers are equipped with special prisms and refractive mirrors, which consist of a multilayer dielectric coating. Gas lasers have copper mirrors; more powerful devices use mirrors with a water cooling system.

The focusing element (tube) makes movements relative to the workpiece. The lens is fixed in it. Solid-state lasers are equipped with glass optical lenses; gas lasers use prisms made of zinc selenide or potassium chloride. Air curtains protect lenses from melting products.

The focal length to obtain high power should be about 100-150 mm. a decrease in this indicator leads to difficulty in removing harmful products.

When laser welding of carbide metal, the distance from the energy source to the working area is determined by the tabular method.

Gas protection

The purpose of a gas protection system is to reduce the likelihood of oxidation in and around the weld. It includes nozzles of different designs. These elements eliminate spatter and fumes that appear during welding. The nozzle is selected depending on the level of chemical activity of the materials, power, and melting depth. The gas with the most suitable composition is supplied to the working area.

Moving the beam and product

The welded products and the energy beam are moved by a CNC manipulator, which has several degrees of freedom, this indicator depends on the complexity of the process. Travel speed can reach 400 m/h.

When processing large parts with a large mass, it is more advisable to move the beam rather than the part. This process is realized through movable mirrors. The most promising system is to secure the tool in an automatic manipulator.

Hybrid installations

Hybrid arc welding is great for creating straight welds.
The main advantage of such installations is the complete fusion of all kinds of profiles without special preparation. The peculiarity of the method is the combination of an electric arc with an energy beam . It is used for fastening parts of large thickness at high speed in automatic mode and low heat transfer. The quality of the seams is at a high level.

Classification of techniques and methods of laser welding with fiber lasers will make it possible to more clearly organize the existing variety of technological processes.

Introduction

The use of lasers in industry is constantly growing [1]. Laser cutting and marking machines can be found in many metalworking plants. The benefits of using laser technologies are obvious: high speeds, accuracy and quality of processing, low costs and a high degree of labor automation make the laser processing process cost-effective. The appearance in the early 2000s of a new generation of lasers with radiation powers ranging from 1 kW and higher [2] based on active fiber doped with ytterbium ions, aroused interest in their widespread use for other types of metalworking: welding, heat treatment, surfacing, additive technologies. High power of laser radiation up to 100 kW [3], the possibility of transmitting radiation through a transport fiber, high efficiency (up to 35%), stability and ease of operation, no need to use gases and other consumables have increased the economic feasibility of using a laser as a heating source for various technological processes. Since the 2000s, in Europe, Japan, and the USA, laser welding technologies based on fiber lasers have been actively developed and introduced into industry. In the automotive, carriage building, mechanical engineering and aviation industries, laser (hybrid) welding is used, providing a high level of production automation, and the resulting connections made by laser are of high quality.

Laser welding is actively researched and periodically introduced at factories by such organizations as MSTU. N. E. Bauman (Moscow), Polytechnic University (St. Petersburg), IPLIT RAS (Shatura) and others, but laser welding technology has not yet become widespread in Russia. This is largely due to the current economic situation in the country and the inability of enterprises to introduce new technologies, as well as the lack of awareness of technologists and chief welders of enterprises about the modern capabilities of laser welding. The purpose of this work is to show all available laser technologies and methods today, to structure and classify the available information.

Classification of laser welding technology

With the advent of fiber lasers, laser welding technology received a second wind. Those areas where its use was difficult and seemed impossible became accessible. Over the past 10 years, laser welding technology has developed significantly and been supplemented with technological techniques and methods. Thus, the following classification is possible: 1. According to the shape (geometry) of the resulting weld; 2. By the number of passes; 3. According to the type of focal spot; 4. By type of filler material; 5. By the presence of additional heating sources; 6. According to the type of welds; 7. According to the type of optical welding heads; 8. By type of weld protection. The schematic classification of laser welding is presented in Fig. 1 .

Rice. 1. Classification of laser welding technology

Classification according to the form of penetration

By shape, three types of laser welds can be distinguished: with deep penetration, medium and wide. The penetration coefficient K, the ratio of the weld depth b to the width a, for welds with deep penetration is more than two, for medium ones it is at the level of 1 to 2, and for wide ones it is equal to or less than 1 ( Fig. 2 ).

Rice. 2. Types of welded joints made by laser welding

Rice. 3. Sample of laser welding with deep penetration k = 4: P = 5.4 kW, V = 0.9 m/min, δ = 12 mm [5]. Obtaining a weld with deep penetration ( Fig. 3 ) is possible due to the phenomenon of a vapor-gas channel that occurs in a liquid melt bath [4]. Such welds are usually performed at speeds of 1 m/min, the focus is closer to the surface of the parts being welded or slightly recessed. As a result of obtaining maximum power density on the surface of the product, the weld acquires deep “dagger” penetration in Fig. 2 . Laser welding technology with the formation of deep “dagger” penetration can be used to ensure gap-free assembly between the welded edges. Such stringent requirements can only be met with smooth (milled) edges and precise workpiece geometry. The presence of even a minimal gap between the welded edges (0.1–0.2 mm) will introduce defects into the geometry of the weld.

a) b) c) Fig. 4. Sample of laser welding with medium and wide seam formation, 6 mm thick: a) laser welding in a shielding gas environment K = 1.4, b) laser welding with filler wire, c) example of a weld with wide seam formation K = 1

To reduce the requirements for the assembly of welded edges, technological modes have been developed, which typically produce wider welds ( Fig. 4 ) with lower requirements for the quality of edge assembly. One of the easiest ways to obtain a wide seam is to weld out of focus. In this case, the required penetration depth is regulated by the welding speed and by increasing the laser radiation power. The phenomenon of the vapor-gas channel remains in this mode, but the width of the seam increases. Such welded joints reduce the requirements for the size of the gap between the edges and amount on average to 10% of the thickness of the material being welded. An increase in the diameter of the spot on the surface and the volume of the liquid melt pool allows welding with the supply of filler wire; the requirements for the gap size are reduced to 15–20% of the material thickness. Another way to increase width is to use various specialized focal spots, which will be discussed below. To obtain the widest possible weld, the welding speed is further reduced, the diameter of the spot on the surface is increased due to defocusing, while the laser radiation power is increased. For such seams, the penetration depth may be less than the width of the seam. Such welded joints can be used to obtain non-through, facing welds, as well as to compensate for inaccuracies in the assembly of parts before welding or the influence of leads that arise during the welding itself.

Classification by number of passes

Laser welding is usually performed in one pass, without cutting edges, with high process speeds, and this is undoubtedly considered an advantage. This technology is used for welding, for example, steels up to 10 mm thick without causing any technological problems. An increase in the thickness of the welded products (over 12 mm) leads to an increase in the likelihood of defects in the weld, such as dips, undercuts, pores, cavities, and cracks. The reasons for the appearance of defects are as follows: with an increase in the thickness of the welded products (over 16 mm), the power of laser radiation increases in direct proportion to ensure through penetration; the welding speed also decreases, as a result of which the volume of the liquid melt pool becomes larger and the processes occurring in it become less controllable. Thus, when a certain volume of liquid metal is reached, surface tension forces can no longer hold the molten pool and the liquid metal flows out in drops from below, forming a lack of weld material from above. To eliminate such defects, the authors of [6] propose to hybridize the process and use copper and flux pads. With an increased volume of the liquid melt bath, the transfer of metal to the rear part of the melt occurs not in one cycle, but in several, before the metal has time to crystallize. Periodic rocking of the liquid metal in the melt pool back and forth reduces the stability of the welding process; the metal, falling under the laser beam, collapses the vapor-gas channel and leads to the appearance of defects. It should also be noted that single-pass welding of large thicknesses leads to the formation of median cracks (cavities) regardless of the tendency of the material to crack ( Fig. 5 ) due to thermal shrinkage of the material and high cooling rates.

Rice. 5. Melting through the body of a plate with a depth of 20 mm, performed with a fiber laser at a power of 30 kW, at a speed of 1 m/min, material steel 3 [5]

Rice. 6. Welded joint made in two passes on both sides [5] For such cases, technologists are developing special laser welding methods using additional technological techniques and methods: in a horizontal position, on a copper lining, with blowing of a vapor-gas channel, with the preliminary introduction of compensation stresses into the design.

Another such method is welding in two passes on both sides. During the first pass, the main seam is formed with high-quality formation of the root of the seam; during the second pass, the top of the seam is formed ( Fig. 6 ). The welding speed remains high, so performing a second pass does not significantly reduce productivity. When the thickness of the welded products increases above 20 mm, the technology of multi-pass laser welding into a narrow groove can be used [7]. This welding technology has been actively researched in recent years and is beginning to be used in various industries. The advantages of the technology in relation to arc welding of large thicknesses in a wide groove are the following: increased productivity by 5–8 times, reduced milling volume by 10 times, reduced residual stresses and deformation by 3 times [8], reduced heat-affected zone, saving filler material and electricity. The advantages over single-pass laser welding are the following: high laser radiation power is not required, the likelihood of defects, especially in the form of cracks, is reduced, and process stability is increased.

Classification according to the type of focal spot

Laser welding technology is usually carried out using a single laser beam, which is focused into the desired spot diameter using a convex lens installed in the optical welding head. In the case of fiber lasers, a collimating lens is used to collect the diverging laser radiation into a parallel beam. This design is used in most laser welding machines and produces a circular focal spot that is suitable for most applications. However, in some cases, specialized focusing systems can be used to solve specific technological problems. Thus, to reduce the requirements for gaps, scanning systems can be used that allow the laser beam to oscillate around the axis of radiation propagation. The key parameters in specifying oscillations are the amplitude, frequency and phase of oscillations, in particular, harmonic and circular ones ( Fig. 4, 8, 9 ).

A)

b)

V)

G)

Rice. 7. Macro-section of a welded joint obtained by multi-pass laser welding: a) steel 09 G2 C with a thickness of 25 mm; b) aluminum alloy AMg3 30 mm thick, c) titanium alloy VT‑1 20 mm thick, d) titanium alloy VT‑1 40 mm thick [5]

A)

b)

Rice. 8. Types of laser scanning

A)

b)

Rice. 9. The influence of circular scanning on the geometry of the weld: a) without scanning; b) with circular scanning [5]

Rice. 10. Appearance of the IPG FLW D50 welding head with dual focus module [5]

In addition to scanning, systems have been developed for splitting laser radiation (TwinFocus, Dual Focus - Fig. 10 ). Double focal spots are used both to reduce gap requirements by increasing the width of the weld [8], and to stabilize processes in the vapor-gas channel and reduce the number of pores [9]. In the first case, the focal spots are located transversely to the weld, in the second case linearly along the weld. In addition to specialized optical systems, two or more lasers are used, the radiation of which is focused on the surface of the welded products according to various patterns. Dual beam laser welding is a more flexible tool compared to laser beam splitting systems. The relative arrangement of focal spots and laser radiation supply angles make it possible to regulate various processes occurring in the vapor-gas channel, liquid melt pool during welding and control the rate of crystallization and cooling of the weld [10].

Classification according to the use of filler material

Filler material in the form of wire in laser welding can be used to form medium-width welds. The filler wire is usually fed into the front of the liquid melt pool either cold or hot. The wire is heated by resistive heating and is regulated by the amount of current flowing in the wire. The addition of filler material makes it possible to form reinforcement on the front surface of the joint, as well as to alloy the weld metal. Another way to carry out the additive is by using thin plates (spacers) made of a certain material, which are fixed between the edges to be welded during the preparation of the joint for welding ( Fig. 11 ). The method is preferable when joining difficult-to-weld steels and dissimilar materials. Depending on the thickness of the insert (up to 1 mm), it is possible to form a weld of excellent composition over the entire depth, consisting of a mixture of base and filler materials.

Rice. 11. An example of a welded joint made by laser welding on an insert with a fiber laser: steel grade 40, thickness 8 mm, welding speed 1 m/min, laser power 9.5 kW [5]

A)

b) Fig. 12. Laser welded joint produced by a fiber laser with filler powder and line scanning: welding speed 1.5 m/min, laser power 7 kW. a) macrosection with microhardness measurement HV0.1 b) microstructure of the fusion zone, magnification 50x [5] It should be noted that laser welding can be carried out with filler material in powder form. This method can be used to eliminate floating gaps between welded edges when the powder is first poured into the existing gap. The gap in this case can be 1.5–2 mm. The powder can also be used to alloy the weld. So, in Fig. 12 shows a welded joint in which the weld is composed of a nickel alloy and has an austenitic structure, while the base metal has a ferritic structure.

Classification according to the presence of additional heating sources

Since the 80s, laser heating sources began to be used simultaneously with arc ones, calling the technology a combined method of laser welding and consumable arc welding (MIG) [11]. Now this technology has become quite popular and is called hybrid laser welding technology. The hybridization process can be with arc process, plasma and other heat sources. If the two sources are close enough to each other, then the two energy sources are combined in one liquid melt bath and a synergistic effect occurs, i.e., an increase in the penetration depth. If two sources are separated by a certain distance, then the synergistic effect does not occur, that is, the process becomes combined. In this case, the addition of a second heat source has other specific functions. For example, the presence of an arc source that goes ahead of the laser radiation makes it possible to preheat the product and increase the absorption capacity of the material. If the arc source goes behind the laser radiation, then the arc process affects the cooling temperature of the liquid metal, making the thermal softer, as a result of which laser welding can be used for difficult-to-weld materials, and the arc process can also be used to eliminate geometric defects in the top of the weld. In addition to the arc, plasma, a light spot, and induction heating can be added to the laser source to solve specific technological problems. The latter allows for preheating of the welded edges, which makes it possible to weld difficult-to-weld steels with a high carbon equivalent [12].

Classification by type of welds

It is known that laser welding can be used to make various types of welded joints: butt, overlap, corner, T, etc. However, in addition to the type classification, laser welded joints can also be classified according to the type of welds: continuous, spot and short-seam. High-power continuous-wave fiber lasers are usually used to perform continuous longitudinal welds. Such welded joints are most often used for welding critical welded structures to ensure the tightness of structures. For welding less critical structures, laser spot welding is used; this technology is used when welding with pulsed solid-state lasers. Short-seam laser welding is used for welding thin sheet material in the automotive industry. In particular, a specialized tong welding head has been developed for welding thin-walled structures, which is already used in a number of automobile factories [13] ( Fig. 13 ).

Rice. 13. Device for pincer laser short-seam welding type [5]

In comparison with resistance welding, this welding method makes it possible to reduce the weight of the car body, which is achieved by changing the width of the flanges for welding from 16 to 8 mm, as well as by using new types of welded joints [14]. It should also be noted that laser welding has minimal impact on the galvanized coating around the weld and, in some cases, allows welding without the weld coming out of the face.

Classification by type of optical welding heads.

Welding heads are usually distinguished by the type of optical elements: through-type and mirror type. But for the laser welding technology itself, the parameters of the optical system, the focal lengths of the collimating and focusing lenses, which are selected for the welding process specifically to solve a particular problem, are of greater importance. Thus, depending on the main parameter of the optical system - the focal length of the focusing lens, three types of welding heads can be distinguished: short-focus, medium-focus and long-focus. Short-focus systems include systems with a focal length of up to 200 mm. For medium focal lengths - from 200–600 mm. For long-focus ones - from 600 mm and above. Short-focus systems are advantageous to use for welding thin-walled products (up to 2–3 mm), which do not require high laser radiation powers. The welding process in this case proceeds without the formation of significant splashes and plasma. Mid-focus systems are used for welding thicknesses greater than 3 mm; in this case, laser sources with higher power are used and increasing the focal length reduces the risk of splashes and sparks hitting the optical elements of the welding head. Long-throw systems are used for welding small thicknesses using fiber lasers with powers up to 10 kW, or for welding ultra-large thicknesses using lasers with powers up to 100 kW. Thus, the technology of remote laser welding has become widespread ( Fig. 14 ). For these purposes, powerful scanners have been specially developed that allow, independently of the manipulator, programmed movements with a laser beam, which allows increasing the productivity of the welding process. The technology has found application in the automotive industry for welding various stamped products.

Rice. 14. Remote laser welding device [5]

Classification according to the type of protection of the weld from the environment

Typically, laser welding with fiber lasers is carried out in an argon shielding gas environment, as the cheapest inert gas. The wavelength of fiber lasers 1065–1085 nm is not absorbed by argon. However, near-surface plasma is still present, because during welding, argon mixes with metal vapor and ionization occurs. As the laser radiation power increases (over 5 kW), the near-surface plasma increases, and the transparent plasma for laser radiation from a fiber laser decreases due to the presence of various impurities, which introduces visible distortions into the process of focusing laser radiation. To suppress plasma and increase the stability of laser welding, it is possible to use various mixtures of inert gases: argon + helium. The addition of helium makes it possible to reduce the size of the surface plasma, lower its temperature and increase transparency for laser radiation, thereby increasing the stability of the welding process. To solve highly specialized problems, for example, to increase the penetration depth, productivity, or to reduce the likelihood of defects in the weld, active gases are added: carbon dioxide, oxygen, hydrogen or nitrogen. Thus, oxygen, entering in large quantities into the weld, definitely worsens the strength of the weld, since oxides appear that precipitate along the boundaries of the crystal grains, which ultimately increases the likelihood of hot and cold cracks. However, adding oxygen to the shielding gas in a limited amount makes it possible to stabilize the vapor-gas channel, resulting in a reduction in the number of internal defects in the form of pores. Carbon dioxide has a similar effect [15]. A small amount of nitrogen in the shielding gas on some steels that contain alloying elements such as manganese, titanium, and molybdenum has a positive effect. The formation of nitrides increases the strength of the weld with a decrease in ductility. Another method of protecting a weld is the use of welding flux, which can be used on top to protect the weld from interaction with the environment, and on the bottom in the form of a flux pad to maintain a liquid melt pool and to protect against oxidation. Also, shielding gas is not used on non-critical structures during laser welding. Thus, for welding low-carbon steels used in the automotive industry, protection is not used to reduce the cost per linear meter of welding. It should also be noted that laser welding, like electron beam welding, can be performed in a vacuum. Machines for laser welding of automobile transmissions in a vacuum are available on the market [16]. The laser source in this case works as a full-fledged replacement for an electron beam gun. There is no need to provide a deep vacuum.

Conclusion

1. The proposed classification of laser welding according to eight criteria makes it possible to organize the variety of technological processes available in the field of laser welding. 2. Laser welding based on fiber lasers is a flexible, diverse process, and depending on the tasks, new welding methods can be used and developed.

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Features of working with thin-walled materials

Welding of parts of medium and large dimensions is carried out by melting throughout the entire thickness. For these purposes, high concentration sources are used. The main nuance when processing thin-walled products is the risk of burning through the sheet. To avoid such a result, it is necessary to control the following indicators:

• power;
• focusing;
• speed of movement of the energy beam.

To connect thin-walled workpieces, the installation should be set to the minimum power setting . A continuous type installation must have an increased speed of movement of the contact spot.

In pulse mode, the pulse duration is reduced and the duty cycle is increased. If the flux density is too high, then they resort to defocusing the beam, which reduces the useful effect, but eliminates the possibility of burning and splashing of liquid metal.

Differences in technology

The technology of joining parts by welding for each metal and alloy has a number of distinctive features. For example, processing parameters for steel products of grade 30KhGSA require preliminary cleaning from scale and traces of corrosion. The part must be dried, which will reduce the likelihood of the appearance of an oxide film, porous structure and seam cracks. The contact area must be treated with degreasing agents.

Steel

Processing of steel products is carried out only after careful preparation ; it consists of removing dirt and moisture. Otherwise, there is a high risk of defects forming in the area that was exposed to thermal stress. The distortion and gap between the welded edges of the products should be minimal. The gap size is no more than 7% of the melting thickness.

It is recommended to use potholders only in case of emergency. For steel parts, it is better to use the butt welding method. The locking and lap varieties are highly sensitive to concentrated stress. The process is carried out in an argon atmosphere with carbon dioxide in a ratio of 3:1. Low-carbon steels are processed without a special protective environment.

Aluminum and magnesium alloys

Melting of magnesium, aluminum and alloys of these metals is complicated by their activity; they come into contact with the environment and various alloying elements. Plus, the welded edges are covered with an oxide film. These nuances can be overcome due to a concentrated energy beam.

Preparatory activities do not differ from those that must be carried out before arc welding . These include mechanical processing with cleaning, etching, hot water washing and stripping, which reduces the risk of oxide film formation. Welding is carried out in a protective environment of helium or argon.

Titanium and titanium alloys

At elevated temperatures, titanium and alloys based on it, for example, titanium technology VT1 VT20, exhibit excessive activity. Heating above 300 degrees provokes grain growth, and there is a tendency for cold cracks to form if the hydrogen level increases. The edges are prepared by mechanical or shot blasting with etching with chemical reagents, brightening, and cleaning. The protective medium is purified helium, and it is advisable to cool the products in argon.

Content

1. Features of laser metal welding
2. Classification of laser welding methods

• Classification by energy characteristics
• Classification by technological characteristics
• Classification according to economic characteristics

• Scope of laser welding
• Laser welding technology
• Welding steels
• Welding of aluminum and magnesium alloys
• Welding titanium and titanium alloys

• Solid State Lasers
• Gas lasers
• Laser beam transport and focusing systems
• Gas protection system
• Beam and product movement system

Manual

The connection of products can be carried out using manual laser welding. A small-sized machine can be easily purchased even for domestic use. Moreover, at an affordable price with high technical characteristics. Such equipment is intended for:

• repair of products with small overall dimensions, for example, jewelry, eyeglass frames;
• surfacing;
• polymers;
• butt spot welding;
• carrying out welding work in the field of microelectronics;
• mold corrections;
• processing of medical devices.

Laser welding is a method of joining products made of various materials, which has become widespread in various fields. Non-contact technology allows interaction with metals of various electromechanical properties . The work is carried out in a small area with high power, which allows you to penetrate hard-to-reach places. The application of the method is limited by economic aspects due to the high cost of installation.

Design and types of equipment used

The structure of the unit depends on the type of emitter included in its composition.

Solid state devices

The design includes elements made of ruby ​​and neodymium-doped glass. They are activated by the luminous flux emitted by powerful arc lamps. The units operate in constant radiation mode. They are characterized by high frequency, low power and efficiency. Solid-state machines are used for welding small-sized parts.

Gas welding devices

Such units are suitable for welding thick workpieces made of steel and other metals. Radiation generated in a gas environment is characterized by high power. The installation is capable of connecting parts up to 2 cm thick. It has a fairly high efficiency. The operation of the device is complicated due to the introduction of a fragile glass tube into the design.

Hybrid installations

Such devices were created for joining metal workpieces of large thickness. Together with the laser head, the device circuit includes an electric arc torch. Additionally, a feeding mechanism is installed that removes consumables into the weld pool.

Manual models

Small devices operate on the principle of standard units. The need to use compact parts when assembling makes the devices expensive. They are used to create miniature metal structures and solder microcircuits.