المعرفة العامة حول اللحام بالليزر

الوقت المقدر للقراءة: 27 الدقائق

About laser welding is an efficient and precise welding method that uses a high-energy-density laser beam as a heat source for welding. With the rapid development of science and technology and the continuous development of new materials, the performance requirements of welded structures are getting higher and higher. About laser welding has attracted attention for its advantages of high energy density, deep penetration, high precision, and strong adaptability. Laser welding plays a very important role in the welding of some special materials and structures. This welding method has been applied in high-tech fields such as aerospace, electronics, automobile manufacturing, nuclear power, etc., and has received increasing attention from industrialized countries.

Laser is a kind of monochromatic, strong directivity and bright light beam produced by using stimulated radiation to realize the principle of light amplification. After focusing by a lens or mirror, an energy beam with a diameter of less than 0.01mm and a power density of up to 10¹²W/m² can be obtained, which can be used as a heat source for welding, cutting, and surface cladding of materials.

Principle and classification of laser welding

Principle of Laser Welding

Laser welding is a welding method that uses laser energy (visible light or ultraviolet) as a heat source to melt and connect workpieces. Laser welding can be achieved not only because the laser itself has extremely high energy, but more importantly because the laser energy is highly focused at one point, which makes its energy density very large.

During laser welding, the laser irradiates the surface of the material to be welded, and it acts on it. Part of it is reflected, and part is absorbed and enters the material. For opaque materials, the transmitted light is absorbed, and the linear absorption coefficient of metal is   10⁷~10⁸m^-1. For metals, the laser is absorbed in the thickness of 0.01-0. 1 чm on the metal surface and converted into heat energy, which causes the temperature of the metal surface to rise sharply, and then transmits to the inside of the metal.

The working principle of CO₂ laser is shown in Figure 3.1. The optical system composed of mirror and lens focuses and transmits the laser to the work piece to be welded. Most laser welding is done under computer control. The work piece to be welded can be moved by a one-dimensional or three-dimensional computer-driven platform (such as a CNC machine tool); the work piece can also be fixed, and the welding process can be completed by changing the position of the laser beam.

The principle of laser welding is that photons bombard the metal surface to form a vapor, and the evaporated metal can prevent the remaining energy from being reflected by the metal. If the welded metal has good thermal conductivity, it will get a larger penetration depth. The reflection, transmission, and absorption of laser light on the surface of the material are essentially the result of the interaction between the electromagnetic field of light waves and the material. When the laser light wave enters the material, the charged particles in the material vibrate according to the step of the light wave electric vector. The radiant energy of the photon becomes the electron’s kinetic energy. After a substance absorbs the laser light, it first produces excess energy of certain particles, such as the kinetic energy of free electrons, the excitation energy of bound electrons, or excess phonons. These original excitation energies are converted into heat energy after a certain process.

In addition to being an electromagnetic wave like other light sources, lasers also have characteristics that other light sources do not possess, such as high directivity, high brightness (photon intensity), high monochromaticity, and high coherence. During laser welding, the conversion of light energy absorbed by the material to heat energy is completed in a very short time (about 10^-9s). During this time, the heat energy is only limited to the laser-irradiated area of ​​the material, and then through heat conduction, the heat is transferred from the high-temperature area to the low-temperature area.

The absorption of laser light by metal is mainly related to factors such as laser wavelength, material properties, temperature, surface condition, and laser power density. Generally speaking, the absorption rate of metal to laser increases with the increase of temperature and increases with the increase of resistivity.

Lasers used for laser welding include CO₂ lasers, YAG lasers, semiconductor lasers, and fiber lasers. The following lasers are mainly used in the welding field: YAG solid-state laser (Yttrium-Aluminum-Garnet with Nd³⁺, YAG for short); CO₂ gas laser; fiber laser.

During the laser welding process, the workpiece and the beam move relative to each other. Due to the strong driving force generated by the violent evaporation, the molten metal in the front of the small hole is accelerated at a certain angle, and the near-surface behind the small hole is formed as shown in Figure 3.2. Melt flow (major vortex). After that, the temperature of the liquid metal behind the small hole drops rapidly due to the effect of heat transfer, and the liquid metal quickly solidifies to form a continuous weld.

Laser welding classification

According to the way the laser acts on the workpiece and the output energy of the laser, it can be divided into continuous laser welding and pulsed laser welding. Continuous laser welding forms a continuous weld during the welding process. The energy input to the workpiece by pulsed laser welding is intermittent and pulsed, and each laser pulse forms a circular welding spot during the welding process.

There are two basic modes of laser welding. According to the different power density of the spot on the workpiece after laser focusing, laser welding is generally divided into thermal conduction welding (power density less than 10⁵ث / سم²) and deep penetration welding (also called small hole welding, power The density is greater than 10⁶ث / سم²).

Laser thermal welding (heat transfer welding)

Under lower laser power density and longer laser irradiation time, the material gradually melts from the surface layer. With the input energy and heat conduction, the liquid-solid interface migrates to the inside of the material, and finally, the welding process is realized, similar to the tungsten electrode. In argon arc welding (TIG), the surface of the material absorbs the laser energy, transfers it to the inside through heat conduction and melts it, and forms a solder joint or weld after solidification.

Figure 3.3 shows a schematic diagram of the melting process of laser thermal conduction welding. When the laser spot power density is less than 10⁵ث / سم², the laser heats the metal surface to between the melting point and the boiling point. When welding, the surface of the metal material converts the absorbed light energy into heat energy, so that the temperature of the metal surface rises and melts, and then the heat energy is transferred to the inside of the metal through thermal conduction so that the melting zone gradually expands, and the solder joint or weld is formed after solidification. Therefore, thermal conductivity welding is also called heat transfer welding.

In the process of laser thermal conduction welding, the temperature change caused by laser heating changes the surface tension of the molten pool, which produces a greater stirring force in the molten pool, so that the liquid metal in the molten pool flows in a certain direction. Since there is no vapor pressure, non-linear effect and pinhole effect during laser thermal conduction welding, the penetration depth is generally shallow. The comparison between laser thermal conduction welding and deep penetration اللحام is shown in Figure 3.4.

During laser thermal conduction welding, the surface temperature of the workpiece does not exceed the boiling point of the material. The light energy absorbed by the workpiece is converted into heat energy and then the workpiece is melted by heat conduction. The shape of the molten pool is approximately hemispherical. The characteristic of thermal conduction welding is that the power density of the laser spot is small, a large part of the laser is reflected by the metal surface, the absorption rate of the laser is low, the welding depth is shallow, the solder joint is small, and the heat-affected zone is small, so the welding deformation is small and the accuracy is high. The welding quality is also very good, but the welding speed is slow. Thermal conduction welding is mainly used for the precision welding of thin plates (thickness δ<1mm) and small workpieces such as instrumentation, battery shells, electronic components, etc.

Whether the laser welding is performed by thermal conduction welding depends on the process parameters of laser welding. In essence, when the laser spot power density is less than 10⁵ث / سم², the surface of the material is heated to between the melting point and the boiling point to ensure that the material is fully melted without vaporization, and the welding quality is easy to guarantee.

Laser deep penetration welding (small hole welding)

Small hole welding is similar to electron beam welding. The high-power density laser beam causes the material to melt locally and form small holes. The laser beam penetrates the small holes into the molten pool and forms with the movement of the laser beam Continuous weld. When the spot power density is high, the small holes produced will penetrate the entire plate thickness to form deep penetration welds (or solder joints). In continuous laser welding, the small hole advances along the welding direction with the beam relative to the workpiece. The metal melts in front of the small hole, and after the deposited metal flows around the small hole to the back, it solidifies again to form a weld.

The laser beam of deep penetration welding can penetrate deep into the weldment, thus forming a weld with a relatively large depth and width. If the laser power density is large enough and the material is relatively thin, the small hole formed by laser welding penetrates the entire plate thickness and the back surface can receive part of the laser. This method can also be called thin plate laser pinhole effect welding.

Figure 3.5 shows the heating phenomenon of laser beams with different power densities. The small hole is surrounded by molten pool metal. The gravity and surface tension of the molten metal has a tendency to bridge the small hole, while the continuous metal vapor tries to maintain the small hole. With the movement of the laser beam, the small hole will move with the light, but its shape and size are stable.

An oblique ablation front is formed in front of the small hole. In this area, there is a pressure gradient and a temperature gradient around the small hole. Under the action of the pressure gradient, the sintered material flows along the periphery of the small hole from the front to the back. The temperature gradient means that small surface tension is established around the small hole, which further drives the molten material to flow around the small hole from the front to the back, and finally solidifies behind the small hole to form a weld.

As far as the absorption of laser light by metal materials is concerned, the appearance of small holes is a dividing line. Before the appearance of small holes, whether the surface of the material is in a solid phase or a liquid phase, the absorption rate of laser light only changes slowly with the increase of surface temperature. Once the material vaporizes and forms plasma and small holes, the material’s absorption rate of the laser will undergo a sudden change, and its absorption rate is almost no longer related to the laser wavelength, metal characteristics, and material surface state, but mainly depends on the plasma and laser factors such as interaction and small hole effect.

The absorption rate of the laser will undergo a sudden change, and its absorption rate is almost no longer consistent with the laser wavelength, metal properties, and surface shape of the material. The state is related, and mainly depends on factors such as the interaction between the plasma and the laser and the pinhole effect.

Figure 3.6 shows the actual measurement of the reflectivity of the workpiece surface to the laser during the laser welding process as a function of the laser power density. When the laser power density is greater than the vaporization threshold (10⁶ث / سم²), the reflectivity R suddenly drops to a very low value due to the generation of small holes, and the material’s absorption rate of the laser increases sharply.

The small hole effect

Laser deep penetration welding is also called laser keyhole welding, and its essential feature is laser welding with a keyhole effect. The laser beam can radiate to the deep layer of the material through the small hole, complete the energy transfer and conversion in the small hole, realize the deep penetration welding, and obtain the deep and narrow weld with a large aspect ratio.

When the power density of the laser spot is large enough (>10⁶ث / سم²), the metal surface is rapidly heated under the irradiation of the laser beam, and its surface temperature rises to the boiling point in a very short time (10^-8~10^-6s), To melt and vaporize the metal. The generated metal vapor leaves the molten pool at a certain speed, and the overflowing vapor generates additional pressure on the molten liquid metal, which makes the metal surface of the molten pool sink downward, creating a small hole under the laser spot. When the laser beam continues to heat the bottom of the small hole, the metal vapor generated on the one hand presses the liquid metal at the bottom of the hole to further deepen the small hole, on the other hand, the steam flying out of the hole squeezes the molten metal to the periphery of the molten pool. An elongated hole is formed in the liquid metal, as shown in Figure 3.7.

When the recoil pressure of the metal vapor generated by the laser beam energy is balanced with the surface tension and gravity of the liquid metal, the small hole does not continue to deepen, forming a deep and stable small hole for welding (small hole effect).

The sidewall focusing effect produced during the development of the pinhole has an important influence on the welding process. When the small hole is formed, when the laser beam entering the small hole interacts with the sidewall of the small hole, a part of the light is absorbed by the sidewall, and the other part of the light beam is reflected by the sidewall surface to the bottom of the small hole and re-converges, as shown in Figure 3.8.

Due to the sidewall focusing effect, the laser beam with a certain divergence angle will not significantly diverge and expand the small hole even if it enters the deep part of the material but is reflected and focused on the bottom of the small hole to maintain a small spot size, making the small hole The depth keeps increasing. When the laser is reflected and focused once in the small hole, its energy is reduced by a part, until the laser energy attenuates to a certain value, the depth of the small hole no longer increases, and finally, a deep and narrow weld is obtained.

During the welding process, the sidewall of the small hole is always in a highly fluctuating state, and the thinner layer of molten metal in the front wall of the small hole flows downward with the fluctuation of the wall [Figure 3.9(a)]. Any bumps on the front wall of the small hole will evaporate strongly due to the irradiation of the high-power density laser beam, and the generated steam will be sprayed back to impact the molten pool metal on the back wall, causing the oscillation of the molten pool and promoting the solidification process of the molten pool. The overflow of gas.

Put tungsten particles with a diameter of 0.1~0.4mm in the molten pool, and the flow state of the molten pool under the action of the small holes can be clearly observed by X-ray irradiation, as shown in Figure 3.9(b). There is a rotating eddy current in the molten pool and the energy is large, which has a strong stirring force. Figure 3.8 The sidewall of the small hole drops rapidly on the front wall of the small hole at a speed of about 0.4m/s. When it reaches the bottom of the small hole, a vortex is formed behind the small hole by the downwardly moving liquid flow. At this time, the tungsten movement speed of particles is 0.2~0.3m/s, which is much faster than normal natural convection. The movement of tungsten particles can basically represent the flow of liquid metal in the molten pool. The larger bubbles generated at the bottom of the molten pool do not completely rely on buoyancy to drain out of the molten pool but are brought out of the molten pool by the liquid flow of metal.

The vapor in the molten pores is composed of high-temperature metal vapor and the protective gas drawn in by the pulsation of the pores and is partially ionized to form a charged plasma. The steam flow from the small holes is fast (close to the speed of sound), and chaotic noises can be heard. The strong evaporation of the metal in the small holes even forms a jet. This irregular evaporation causes the rapid vibration of the liquid metal and causes the fluctuation of the small holes.

Characteristics of Laser Welding Penetration State and Weld Seam Formation

Penetration state characteristics of laser welding

The penetration depth of laser welding refers to the thickness of the workpiece that is melted by the laser during the welding process. Generally, the depth of the small hole is considered to be the penetration depth, so the penetration of the small hole through the workpiece is often equivalent to the penetration. In fact, because there is a certain thickness of liquid metal layer around the small hole, there may be situations where the small hole does not penetrate the workpiece but the workpiece has been melted through. Through the analysis of the laser welding process and the penetration state of the back of the weld, it can be determined that the laser deep penetration welding has the following penetration states, as shown in Figure 3.10

Not melted through

During the welding process, the small hole and the liquid metal below it did not penetrate the base material (workpiece), and no trace of the metal being melted can be seen on the back of the work piece (Figure 3.10(a)).

Only weld pool penetration

During the welding process, the small hole is close to the lower surface of the workpiece, but has not penetrated the workpiece, and the liquid metal under the small hole penetrates the back of the workpiece. Although the back of the workpiece is melted, the molten liquid metal cannot form a wide molten pool on the back of the workpiece due to the effect of surface tension. Therefore, the back of the weld shows a slender continuous or discontinuous pile height after solidification. Although this state is also in the range of penetration, the penetration of the entire weld is unreliable and unstable due to the narrow width of the back side (Figure 3.10(b)), especially when the weld is butt welded. If there is a slight deviation, there will be no fusion.

Moderate penetration (small hole penetration)

During the welding process, the small hole just penetrates the workpiece. At this time, the metal vapor inside the small hole will spray below the workpiece, and its recoil pressure will cause the liquid metal to flow around the small hole, resulting in a significant increase in the width of the back of the molten pool, which is formed after welding. Weld shape with uniform and moderate welding width on the back side and basically no build-up [Figure 3. 10(c)]

Over-penetration

Due to the excessive heat input during the welding process, the small hole not only penetrates the workpiece, but the diameter of the small hole and the thickness of the liquid metal layer around it increase significantly, resulting in an excessively wide molten pool (significantly larger than the backside melting in a moderate penetration state Wide), and even cause the weld surface to dent and so on [Figure 3. 10(d)].

Among the above four penetration states, the moderately baked (small hole penetration) state is the ideal penetration state, because the small hole penetrates the workpiece at this time to ensure that the weld is completely penetrated, and the molten pool is not too wide. This leads to dents on the surface of the weld. Therefore, the state of moderate penetration (small hole penetration) can be used as a benchmark for penetration detection and control.

Microscopic analysis showed that only the weld section in the state of penetration of the molten pool presents a more obvious inverted triangle, while the section of the weld in the state of moderate penetration presents an inverted trapezoid or hyperbolic shape. That is to say, the proper penetration state should be expressed as the welding seam’ s front and back sides are both formed and flat, without dents and no obvious pile height, and have a certain backside melting width.

Characteristics of weld formation in laser welding

The weld of laser thermal conduction welding has the characteristics of conventional fusion welding (such as arc welding, gas shielded welding, etc.). The formation of the weld seam during laser deep penetration welding is shown in Figure 3.11. The molten pool of laser welding has the characteristic of periodic change, the reason is the self-oscillation effect in the process of laser and material interaction. The frequency of this self-oscillation is generally 100~10000Hz, the amplitude of temperature fluctuations is 100~500Hz, and the amplitude of temperature fluctuations is 100-500K.

Due to the self-oscillation effect, the small holes and metal flow in the molten pool undergo periodic changes. The formation of the small hole allows the laser to radiate to the depth of the small hole, strengthens the absorption of laser energy by the molten pool, and further increases the depth of the original small hole. The vaporization of the molten metal allows the small hole to be maintained, forming an aspect ratio Large continuous welds.

Since the heat input of laser deep penetration welding is 1/10~1/3 of arc welding, the solidification process is very fast. Especially in the lower part of the weld, because it is very narrow and has good heat dissipation conditions, it has a fast cooling rate, so that fine equated crystals are formed inside the weld, and the grain size is about 1/3 of that of arc welding.

Using laser welding, “As long as you can see, you can weld.” Laser welding can be carried out in a faraway station, through a window, or in the interior of three-dimensional parts where electrodes or electron beams cannot penetrate. Like electron beam welding, laser welding can only be performed from a single side, so single-side welding can be used to weld laminated parts together. This advantage of laser welding opens up a new way for welding joint design. With laser welding, not only the welding quality is significantly improved, but the productivity is also higher than that of traditional welding methods.

Characteristics and applications of laser welding

Features of laser welding

Laser welding is a fusion welding method that uses a high-energy density laser beam as a heat source. With laser welding, not only the productivity is higher than the traditional welding method, but the welding quality is also significantly improved. Compared with general welding methods, laser welding has the following characteristics.

• The focused laser has a high power density (10⁵~10⁷ث / سم² or higher), and a fast heating speed, which can realize deep penetration welding and high-speed welding. Due to the small laser heating range (the spot diameter is less than 1mm), it is at the same level. Under the conditions of power and weldment thickness, the welding heat-affected zone is small, and the welding stress and deformation are small.
• Astigmatism can be emitted and transmitted and travel a considerable distance in space with very small attenuation. It can be transmitted and deflected by bending optical fibers, prisms, etc., and is easy to focus on. It is especially suitable for focusing on micro parts in small, inaccessible parts or far away. Distance to be welded.
• It belongs to non-contact welding, no electrode is needed, and there is no electrode contamination or wear. One laser can be used for different processing on multiple workbenches. It can be used for welding, but also cutting, cladding, alloying and surface heat treatment, etc. One machine has multiple uses.
• The laser beam has little attenuation in the atmosphere and can pass through transparent objects such as glass. It is suitable for welding highly toxic materials such as beryllium alloys in a sealed container made of glass; the laser is not affected by electromagnetic fields (arc welding and electron beam welding are affected), can accurately align the weldment; there is no X-ray protection, and no vacuum protection is required.
• It can weld materials that are difficult to weld by conventional welding methods, such as high melting point metals and non-metallic materials (such as ceramics, organic glass, etc.). Materials that are sensitive to heat input can also be laser welded. No heat treatment is required after welding, and various types of welding can be performed. Heterogeneous materials.

Compared with electron beam welding, the biggest feature of laser welding is that it does not require a vacuum chamber (welding can be carried out in the atmosphere) and does not produce X-rays.

The main obstacles currently affecting the expansion of laser welding are as follows.

• Lasers (especially high-power continuous lasers) are expensive. At present, the maximum power of industrial lasers is about 25KW, and the maximum thickness of weldable workpieces is about 20mm, which is much smaller than electron beam welding
• The processing, assembly, and positioning requirements of the weldment are very high. The position of the weldment must be very accurate, and it must be within the focus range of the laser beam.
• The laser’s electro-optical conversion and overall operating efficiency are low, and the beam energy conversion rate is only 10% to 20%. It is difficult for laser welding to weld metals with high reflectivity.

Laser welding example

Manufacturing

Japan replaces flash butt welding with CO₂ laser welding to connect rolled steel coils. Welding of ultra-thin plates (such as foils with a thickness of less than 100 pm) cannot be welded, but YAG laser welding with a special output power waveform can be successfully welded, which shows the broad prospects of laser welding. Japan’s Kawasaki Heavy Industries Corporation changed the traditional spot welding process to laser welding in the manufacturing of railway vehicles, which improved the car body’s strength, rigidity, and airtightness, and the production efficiency was also significantly improved. Figure 3.12 shows a schematic diagram of the honeycomb structure of the laser-welded high-speed rail car body. Japan has also successfully developed the use of YAG laser welding for the welding and maintenance of thin steam generator tubes in nuclear reactors.

Automotive industry

In the late 1980s, kilowatt-level laser welding was successfully applied to industrial production. Nowadays, laser welding production lines have appeared on a large scale in the automobile manufacturing industry. European automobile manufacturers such as Audi, Mercedes-Benz, Volkswagen in Germany, and Volvo in Sweden took the lead in using laser welding technology to weld roofs, bodies, and side frames as early as the 1980s. In the 1990s, GM, Ford, and Chrysler also competed to introduce laser welding into automobile manufacturing. Although it started late, it developed rapidly. The Italian Fiat company uses laser welding in the welding and assembly of most steel plate components. Japan’s Nissan, Honda, and Toyota also use laser welding and cutting processes in the manufacture of body panels.

Laser tailored welding technology is widely used in foreign car manufacturing. As early as 2000, there were more than 100 laser tailor-welded production lines for tailored blanks worldwide, with an annual output of 70 million pieces of tailor-welded blanks for car components, and continued to grow at a relatively high rate each year. The domestically produced imported models Passat, Buick, Audi and others have also adopted some cut blank structures.

High-strength steel laser welding assembly parts are increasingly used in automobile body manufacturing due to their excellent performance. According to the characteristics of large batches and high automation in the automobile industry, laser welding equipment is developing in the direction of high power and multi-channel. On the one hand, Sandia National Laboratory in the United States and Pratt Whitney have jointly conducted research on adding powder and metal wire in the laser welding process. The Institute of Applied Beam Technology in Bremen, Germany has conducted a lot of research on the use of laser welding of aluminum alloy body frames. Adding filler metal to the weld can help eliminate hot cracks and increase the welding speed. The developed production line has been put into production in Mercedes-Benz.

At present, laser welding technology has been widely used in automobile production lines and has been used in chassis, body, roof, door, side frame, engine cover, engine frame, radiator frame, luggage compartment, instrument panel, variable speed gearbox, valve lifter Structures and components such as rods and door hinges. The large-scale application of laser welding technology has significantly improved the level of automobile manufacturing, product quality, and performance, and created conditions for the realization of lightweight, high-strength, and flexible design and manufacturing.

Aviation industry

The application of laser welding technology in the aviation manufacturing industry has attracted the attention of developed countries in the world. For example, in Europe, the Airbus A330/340 fuselage wall structure is a laser welded overall structure. The fuselage skin (6013-T6 aluminum alloy) and ribs (6013-T6511) are welded to form an integral fuselage wall using laser welding technology. The board replaces the original riveted sealing wallboard, reducing the weight by 15% and reducing the cost by 15%. For another example, a CO₂ laser with a rated power of 10KW is used to weld the T-shaped joint of aluminum alloy wall panels (6013, thickness 2mm) and ribs (6013, thickness 4mm), and AISi12 welding wire is added, and the welding speed is 10m/min. Below, the actual welding power is 4KW, the width of the overall welded wall is about 2m, and the application effect of the laser welding structure is good. The small-cell honeycomb core manufactured by the scientific and technological personnel of our country using laser welding technology provides a technical guarantee for improving the performance of aero-engines.

The above several typical examples show that laser welding technology has a very broad application prospect in the manufacture of aircraft structures. In my country, the application of 5KW industrial CO₂ laser welding equipment in the aviation industry has gradually become popular, and lasers above 10KW have also entered engineering applications.