Laser Welding Machine

Learn The Truth About Laser Welding

Learn The Truth About Laser Welding

Estimated reading time: 31 minutes

Laser welding equipment and technical parameters

Composition of laser welding equipment

Laser welding equipment mainly includes laser, beam transmission, focusing system, gas source (shielding gas), nozzle, welding machine, workbench, operating panel, power supply, control system, etc. The equipment’s core is a laser composed of an optical oscillator and a medium placed between the mirrors at both ends of the oscillator cavity. When the medium is excited to a high-energy state, the welding machine produces light waves of the same phase and reflects back and forth between the mirrors at both ends, forming a photoelectric string junction effect, amplifying the light waves obtaining sufficient energy to start emitting laser light.

According to different laser working materials, the equipment is divided into YAG solid equipment and CO2 gas equipment; according to different laser working methods, it is divided into continuous laser welding equipment and pulse laser welding equipment. No matter which kind of equipment, the basic composition is roughly similar. The composition of the equipment and the welding torch is shown in Figures 1.1 and 1.2.

The composition of laser welding equipment
Figure 1.1 The composition of laser welding equipment
1-Laser, 2-Beam detector, 3-Deflection focusing system, 4-Workbench, 5-Control system
Welding torch device
Figure 1.2 Welding torch device
Laser

The laser is the core part of the laser equipment. The characteristics of the welding laser are shown in Table 3.1. According to the air cooling method, the CO2 gas laser is divided into cross-flow type, axial flow type (high speed and low speed), and diffusion cooling type. The performance characteristics of different CO2 lasers are shown in Table 3.2.          

Compared with CO2 lasers, YAG lasers have shorter laser wavelengths and can be transmitted through optical fibers, which greatly simplifies the light guide system and is suitable for three-dimensional welding; it is beneficial to the absorption of metal surfaces and is more suitable for high reflectivity materials (such as aluminum alloys, etc.) Welding.

LaserWave length/чmWorking mode  Repetition frequency /HzOutput power or energy range  The main purpose
Ruby laser0.69Pulse0-11-100JSpot welding, drilling
Neodymium glass laser1.06Pulse0-0.11-100JSpot welding, drilling
YAG laser1.06Pulse Continuous0-4001-100J 0-2KWSpot welding, drilling Welding, cutting, surface treatment  
Enclosed CO2 laser10.6Continuous0-1KWWelding, cutting, surface treatment
Cross-flow laser10.6Continuous0-25KWWelding, surface treatment
High-speed axial flow CO2 laser10.6Continuous Pulse0-50000-6KWWelding, cutting
Table 3.1 Features of welding laser
ItemCross flow typeAxial flow typeDiffusion cooling type  
Output power level3-45KW1.5-20KW0.2-3.5KW
Pulse capacityDCDC-1kHzDC-5kHz
Beam modeAbove TEM02TEM00, TEM01TEM00, TEM01
Beam propagation coefficient≥0.18≥0.4≥0.8
Gas consumptionSmallBigVery small
Electric-optical conversion efficiency≤15%≤15%≤30%
Welding effectGoodGoodBetter
Cutting effectPoorGoodBetter
Transformation hardeningGoodNot so badNot so bad
Surface coatingGoodNot so badNot so bad
Surface claddingGoodNot so badNot so bad
Table 3.2 Performance characteristics of different CO2 lasers

Compared with traditional gas and solid-state lasers, fiber lasers developed in recent years have the following characteristics.

  • Glass optical fiber has low manufacturing cost, mature technology, and the flexibility of optical fiber brings advantages of miniaturization and intensiveness.
  • The optical fiber has a very high surface-to-volume ratio, fast heat dissipation, low loss, high conversion efficiency, and low laser threshold.
  • There is no optical lens in the fiber laser resonant cavity, which has adjustment-free, maintenance-free, and high stability characteristics.
  • With high power and high photoelectric efficiency, the comprehensive photoelectric efficiency of 10KW fiber laser reaches more than 20%.
  • Small size, long life, easy to integrate the system, easy to achieve long-distance laser transmission, and can operate normally in harsh environments of high temperature, high pressure, high vibration, and high impact.

It is precisely because of the above-mentioned advantages of high-power fiber lasers that its application in the field of material processing is continuously expanding and has extremely broad application prospects.

Beam transmission and focusing system

The beam transmission and focusing system are also called the external optical system. It is composed of a circular polarizer, a beam expander, a mirror or an optical fiber, a focusing mirror, etc., used to transmit and focus the laser beam on the workpiece, and its end installation provides protection or assistance airflow torch.

The main material of the focusing lens is ZnSe, which has good transmission and focusing performance and is cheap. However, the focusing lens is easily contaminated by smoke and metal splash during the welding process. When the laser power is low (<2KW), the focusing lens is often used, and the reflective focusing lens should be used for high-power (>2KW) welding. The reflective focusing mirror is made of metal with high reflection to the laser. In laser welding, copper parabolic mirrors with different coatings are usually used. This type of focusing mirror is stable and can be used in conjunction with water-cooled components. It has small thermal deformation and is not easy to be polluted. However, the focusing performance is not as good as that of the lens focusing mirror and the relative position of the incident laser. It requires high precision, difficult to adjust, and is easy to cause astigmatism in the focus spot, and the price Higher.

The focal length of the focusing lens has an important influence on the focusing effect and welding quality, generally 127-200mm. Reducing the focal length can get a small focus spot and higher power density, but if the focal length is too small, the focusing lens is susceptible to contamination and damage. Once the mirror surface is contaminated, the absorption of the laser will increase significantly, thereby reducing the power density to the workpiece and easily causing the lens to break.

Gas source (protective gas)

Shielding gas is necessary for it. In most processes, the shielding gas is delivered to the laser radiation area through a special nozzle. At present, most CO2 lasers use He, N2, CO2, and mixed gas as the working medium and protective gas. The ratio is 60%: 33%: 7%. He is expensive, so the high-speed axial flow CO2 laser has a higher operating cost. Should consider its cost.

Nozzle

The nozzle is generally designed to be placed coaxially with the laser beam. It is commonly used to feed the shielding gas from the side of the laser beam into the nozzle. The typical blow-up aperture is 4-8mm, and the distance from the nozzle to the workpiece is 3-10mm. Generally, the pressure of the shielding gas is lower. The gas flow rate is 8~30L/min, Figure 1.3 and 1.4 shows the nozzle structure which is widely used for CO2 laser and YAG laser.

Nozzle structure of CO2 laser
Figure 1.3 Nozzle structure of CO2 laser
Nozzle structure of YAG laser
Figure 1.4 Nozzle structure of YAG laser

In order to protect the optical components of laser welding from welding fumes and splashes, several horizontal jet nozzle designs can be used. The basic idea is to consider allowing the airflow to pass through the laser beam vertically, according to different technical requirements, or for blowing Welding fumes, or use high kinetic energy to divert metal particles.

Laser welding machine

The laser welding machine includes a workbench and a control system. It is mainly used to realize the relative movement between the laser beam and the workpiece and complete the welding. It is divided into two types: special welding machine and general welding machine. The latter commonly used numerical control systems, there are right-angle two-dimensional, three-dimensional welding machines or articulated welding robots, servo motor-driven worktables can be used to place workpieces to achieve welding. The control system mostly adopts a numerical control system.

Power supply

In order to ensure the stable operation of the laser, solid-state electronic control power supplies with fast response and high stability are used.

The main technical parameters of laser welding machine

Table 3.3 lists the main technical parameters of some domestic laser welding equipment. When purchasing equipment, comprehensive consideration should be given to the size, shape, material, and characteristics of the equipment, technical indicators, scope of application, and economic benefits.

A low-power laser welding machine can be used to welder micro parts and precision parts, and the machine with higher power should be used for a welder and thick parts. Spot welding can choose a pulse laser welding machine, to obtain a continuous weld, you should choose a continuous machine or a high-frequency pulse continuous laser welding machine. In addition, attention should be paid to whether the machine has functions such as monitoring and protection.

Model  NJH-30  JKG  DH-WM01GD-10-1  
Name  Neodymium glass pulse laser welding machineNeodymium glass CNC pulse laser welding machineAutomatic battery shell YAG laser welding machineRuby laser spot welding machine  
Laser wave length /чm1.061.061.060.69
Maximum output energy /J130974013
Repetition rate1-5Hz30times/min(at rated output)1-100Hz(seven gears)16 times/min
Pulse width /ms0.5 (at maximum output) 6 (at rated output)2-80.3-10(seven gears)6(maximum)
Laser working material sizeΦ12×350Φ10×165
UsesElectric welding and perforationIt is used for butt welding, lap welding and overlap welding of thin wires, thin plates, and the welding penetration depth can reach 1mTable 3.2 Main technical parameters of some domestic laser welding equipment.Welding battery shells. Double workbench, the welding process is fully automatedElectric welding and drilling. Suitable for plate thickness less than 0.4mm, wire diameter less than 0.6mm  
Table 3.3 Main technical parameters of some domestic laser welding equipment

The low-power pulse laser welding machine is suitable for spot welding between metal wire and wire, wire and plate (or film) with a diameter of less than 0.5mm, especially for spot welding connection of micron-level filament and foil film. Continuous laser welding machines, especially high-power continuous, are mostly CO2 laser welding machines, which can be used to form continuous welds and deep penetration welding of thick plates.

Characteristics and parameters of pulse laser welding process

Process characteristics of pulse laser welding

About laser welding is a kind of fusion welding, which uses laser beams as an energy source to impinge on the weldment joints. The laser beam can be guided by a flat optical element (such as a mirror), and then a reflective focusing element or lens is used to project the beam on the weld. It is non-contact welding. No pressure is required during the operation, but an inert gas is required to prevent oxidation of the molten pool, and sometimes filler metal is also used.

Pulsed laser welding is similar to spot welding. Its heating spots are very small, in the order of micrometers. Each laser pulse forms a welding spot on the metal part. Mainly used for the welding of micro, precision components and microelectronic components. It is carried out by spot welding or seam welding by lap joints. Commonly used lasers for pulsed laser welding include ruby lasers, neodymium glass lasers and YAG lasers.

Process characteristics of pulse laser welding

Pulse laser welding has four main welding parameters: pulse energy, pulse width, power density and defocus.

Pulse energy and pulse width

In pulse laser welding, the pulse energy determines the heating energy and mainly affects the amount of metal melting. The pulse width determines the welding heating time and affects the penetration depth and the size of the heat-affected zone. Figure 3. 5 shows the effect of pulse width on penetration. When the pulse is widened, the penetration depth gradually increases. When the pulse width exceeds a certain critical value, the penetration depth decreases instead. When the pulse energy is constant, there is an optimal pulse width for different materials, and the welding penetration is the largest at this time. The best pulse width for steel welding is 5-8ms.

The effect of pulse width on penetration
Figure 3.5 The effect of pulse width on penetration

The pulse energy mainly depends on the thermophysical properties of the material, especially the thermal conductivity and melting point. Metals with good thermal conductivity and low melting point are easy to obtain greater penetration depth. There is a certain relationship between pulse energy and pulse width during welding, and it varies with the thickness and properties of the material.

The average power P of the laser is determined by equation (3.1):

P=E/τ (3.1)

In the formula, P is the laser power, W; E is the laser pulse energy, J; τ is the pulse width, s.

In order to maintain a certain power, as the pulse energy increases, the pulse width must be increased accordingly to get better welding quality.

Power density

When the power density of the laser spot is small, the welding is carried out by thermal conduction welding, and the diameter and penetration of the welding spot are determined by the heat conduction. When the power density reaches a certain value (106W/cm2), a pinhole effect is produced during the welding process, forming a deep penetration solder joint with an aspect ratio greater than 1. At this time, although a small amount of metal evaporates, it does not affect the formation of the solder joint. However, when the power density is too high, the metal evaporates violently, resulting in too much vaporized metal, forming a small hole that cannot be filled with liquid metal, and it is difficult to form a firm solder joint.

During pulse laser welding, the power density is determined by equation (3.2);

Pd=4E/πd2τ (3.2)

In the formula, Pd is the power density on the laser spot, W/cm2; E is the laser pulse energy, J; d is the spot diameter, cm; τ is the pulse width, s.

Figure 3.6 shows the relationship between pulse energy and pulse width during pulse laser welding of materials with different thicknesses. The pulse energy E and pulse width τ have a linear relationship. As the thickness of the weldment increases, the laser power density increases accordingly.

 The relationship between pulse energy and pulse width during pulse laser welding of materials with different thicknesses
Figure 3.6 The relationship between pulse energy and pulse width during pulse laser welding of materials with different thickness
Defocus

Defocus refers to the distance between the surface of the weldment and the smallest spot of the focused laser beam during welding (also called the focus). There are two defocusing methods: positive defocus and negative defocus. The focal plane above the workpiece is called positive defocus, otherwise, it is called negative defocus. After the laser beam is focused by the lens, there is a minimum spot diameter. If the surface of the weldment coincides with it, the defocus amount F=0; if the surface of the weldment is below it, F>0, which is a positive defocus amount; otherwise, F<0, is the negative defocus amount.

Changing the amount of defocus can change the size of the laser heating spot and the beam incident condition. But too much defocus will increase the diameter of the spot, reduce the power density on the spot, and reduce the penetration depth.

In pulse laser welding, the metal with low reflectivity, large thermal conductivity, and small thickness is usually selected as the top sheet; before the thin wire and the film are welded, a small ball with a diameter of 2 to 3 times the wire diameter can be welded at the end of the wire. To increase the contact surface and facilitate laser beam alignment. Pulse laser welding can also be used for thin plate seam welding. At this time, the welding speed v=df(1-K), where d is the diameter of the welding spot, f is the pulse frequency, and K is the overlap coefficient (0.3~0.9 according to the thickness of the plate).

The process parameters of pulse laser welding of various material weldments are shown in Table 3.4. Table 3.5 shows the process parameters and joint performance of wire-to-wire pulse laser welding.

MaterialThickness (diameter)/mmPulse energy / JPulse width /ms  Laser category  
Gold-plated phosphor bronze + foil aluminum0. 3+0.2  3.54.3Neodymium glass laser
Stainless steel sheet0.15+0.151.213.7Neodymium glass laser
Pure copper foil0.05+0.052.34.0Neodymium glass laser
Nickel chromium wire + copper sheet0.10+0.151.03.4
Stainless steel sheet + Ni-Cr wire0.15+0.101.43.2Neodymium glass laser
Silicon aluminum wire + stainless steel sheet0.10+0.151.43.2Neodymium glass laser
Table 3.4 Process parameters of pulse laser welding of weldments of various materials
MaterialDiameter/mmJoint FormProcess ParametersProcess ParametersJoint PerformanceJoint Performance
   Output power/JPulse width/msMaximum unloading/NResistance/Ω
301 stainless steel(1Cr17Ni7)0.33Docking83.0970.03
  Overlap83.01030.03
  Cross83.01130.03
  T-shaped83.01060.03
 0.79Docking103.41450.02
  Overlap103.41570.02
  Cross103.41810.02
  T-shaped113.61820.02
 0.38+0.79Docking103.41060.02
  Overlap103.41130.03
  Cross103.41160.03
  T-shaped113.61200.01
 0.79+0.40T-shaped113.6890.01
Copper0.38Docking103.4230.01
  Overlap103.4230.01
  Cross103.4190.01
  T-shaped113.6140.01
Nickel0.51Docking103.4550.01
  Overlap72.8350.01
  Cross93.2300.01
  T-shaped113.6570.01
Tantalum0.38Docking83.0520.01
  Overlap83.0400.01
  Cross93.2420.01
  T-shaped83.0500.01
 0.63Docking113.5670.01
  Overlap113.5580.01
  T-shaped113.5770.01
 0.65+0.38T-shaped113.6510.01
Copper and tantalum0.38Docking103.4170.01
  Overlap103.4240.01
  Cross103.4180.01
  T-shaped103.4180.01
Table 3.5 Process parameters and joint performance of wire-to-wire pulse laser welding

Continuous laser welding process and parameters

Different metal reflectivity, melting point, thermal conductivity and other parameters, the output power required for continuous laser welding varies greatly, generally from several kilowatts to tens of kilowatts. The difference in output power required for continuous laser welding of various metals is mainly caused by the difference in absorptivity. Continuous laser welding mainly adopts CO2 laser and fiber laser, and the welding seam shape is mainly determined by the laser power and welding speed. The CO2 laser is widely used in continuous laser welding because of its simple structure, large output power range, and high energy conversion rate.

Joint form and assembly requirements

The common form of laser welding head is shown in Figure 3.7. Laser welding mostly uses butt joints and lap joints, and the assembly dimensional tolerance requirements of butt joints and lap joints are shown in Figure 3.8.

Laser welding has high requirements for the assembly quality of weldments. During butt welding, if the amount of misalignment of the joint is too large, the incident laser will be reflected at the corner of the board, and the welding process will be unstable. When welding thin plates, if the gap is too large, the welding will be done after welding. The seam surface is not fully formed, and perforations are formed in severe cases. When lap welding, the gap between the plates is too large and it is easy to cause poor fusion between the upper and lower plates. The assembly requirements of various types of laser-welded joints are shown in Table 3.5, which allows to increase the assembly tolerance of the joints and improve the undesirable state of the laser-welded joint preparation. Experience has shown that if the gap exceeds 3% of the plate thickness, the self-fluxing weld will not be full.

Figure 3.7 Common laser welding head forms
Butt joint and lap joint assembly dimensional tolerance requirements
Figure 3.8 Butt joint and lap joint assembly dimensional tolerance requirements

During laser welding, the weldment should be clamped to prevent welding deformation. The deviation of the light spot from the center of the welding seam perpendicular to the welding movement direction should be less than the radius of the light spot. For iron and steel materials, the surface of the weldment needs to be degusted and degreasing treatment before welding; when the requirements are stricter, it needs to be pickled before welding, and then cleaned with ether, acetone, or carbon tetrachloride.

Laser deep penetration welding can perform all-position welding, the gradual transition of starting and ending welding, which can be realized by adjusting the increase and attenuation process of laser power and changing the welding speed. It can realize a smooth transition from the beginning to the end when welding the girth seam. The use of internal reflection to enhance the laser absorption of the weld can improve the efficiency and penetration of the welding process.

Joint formMaximum allowable gapMaximum allowable upper and lower side deviation  
Butt joint0.10δ0.25δ
Angle joint0.10δ0.25δ
T joint0.25δ
Lap joint0.25δ
Crimping joint0.10δ0.25δ
Table 3.6 Assembly requirements for various types of laser welding heads

Filler metal

It is suitable for self-fusion welding. Generally, no welding material is added, and the joint is formed by the melting of the welded material itself. But sometimes in order to reduce the assembly accuracy, improve the weld formation and improve the adaptability of the welded structure, it is also necessary to add filler metal. Adding filler metal can change the chemical composition of the weld, so as to achieve the purpose of controlling the weld structure, improving the shape, and improving the mechanical properties of the joint. In some cases, it can also improve the ability of the weld to resist crystal cracks

Figure 3.9 shows a schematic diagram of laser filler wire welding. Filler metal is often added in the form of welding wire, which can be cold or hot. During deep penetration welding, the amount of filler metal should not be too large to avoid destroying the pinhole effect.

Schematic diagram of laser filler wire welding
Figure 3.9 Schematic diagram of laser filler wire welding

The welding wire for laser filler wire welding can be introduced from the front of the laser or from the rear of the laser, as shown in Figure 3.10. The pre-wire feeding method is often used. The advantage is that the reliability of dragging the welding wire is high, and the butt groove has a guiding effect on the welding wire. The post-wire feeding method has finer ripples on the surface of the weld and has a better appearance. The disadvantage is that once the wire feeding accuracy is reduced, the welding wire may stick to the weld. The centerline of the welding wire and the centerline of the welding seam must coincide, and the angle with the laser optical axis is generally 30°~75°. The welding wire should be accurately fed into the intersection of the optical axis and the base metal so that the laser first heats the welding wire and melts to form a droplet. Later, the base metal is also heated and melted to form a molten pool and small holes, and the wire droplets then enter the molten pool. Otherwise, the laser energy will penetrate through the joint gap and cannot form small holes, making the welding process difficult.

Two wire feeding methods of low light fill wire welding
Figure 3.10 Two wire feeding methods of low light fill wire welding

The welding wire also absorbs and reflects laser energy. The degree of absorption and reflection is related to factors such as laser power, wire feeding method, wire feeding speed, and focal length. When the pre-wire feeding method is adopted, the combined action of laser radiation and plasma heating will melt the welding wire, which requires a large amount of energy, so the welding process is unstable. When the post-wire feeding method is adopted, the heat of the molten pool also participates in heating the welding wire, so that the energy of heating by laser radiation is reduced, and the laser energy can be used to heat the base material to form small holes.

Wire feeding speed is an important process parameter of laser wire filler welding. The joint width and weld height increase during laser wire filler welding is mainly formed by the welding wire deposited metal. The wire feeding speed is determined by the welding speed, joint gap, welding wire diameter, and other factors. The wire feeding speed is too fast or too slow, resulting in excessive molten metal. More or less, all affect the interaction between the laser, base metal, and welding wire and the weld formation.

Laser filler wire welding is beneficial to the welding of brittle materials and dissimilar metals. For example, due to the difference in carbon and alloying elements during laser welding of dissimilar steel or steel and cast iron, brittle structures such as martensite or white mouth are easily formed in the weld. The mismatch of the linear expansion coefficient will also lead to greater welding stress. The combined effect of this will cause welding cracks. The filler wire can adjust the weld metal composition, reduce the carbon content and increase the nickel content, and inhibit the formation of brittle structures. Laser multi-layer filler wire welding can also use smaller power equipment to realize the welding of large thickness plates and improve the adaptability of laser welding to thick plates.

Process parameters

The process parameters of continuous laser welding include laser power, welding speed, spot diameter, defocus amount, type and flow rate of shielding gas, etc.

Laser power P

Laser power refers to the output power of the laser, without considering the loss caused by the light guide and focusing system, it is one of the most critical parameters of continuous laser welding. Continuously working low-power lasers can produce limited heat transfer welds on thin plates at low speeds. For high-power lasers, small holes can be used to produce narrow welds on thin plates at high speed, or small holes can be used to produce welds with relatively large depth and width at low speeds (but not less than 0.6m/s) on medium and thick plates. In heat transfer laser welding, the laser power range is 104-106W/cm2. Laser welding penetration is closely related to output power. For a certain spot diameter, the welding penetration increases with the increase of laser power. Figure 3.11 shows the relationship between laser power and penetration in continuous laser welding of different materials.

The relationship between laser power and penetration
Figure 3.11 The relationship between laser power and penetration

Welding speed V

The welding speed will affect the heat input per unit time. If the welding speed is too slow, the heat input will be too large, causing the work piece to burn through; if the welding speed is too fast, the heat input will be too small, causing the work piece to be incompletely welded. Under a certain laser power, increase the welding speed, the heat input will decrease, and the weld penetration will decrease. Appropriately reducing the welding speed can increase the penetration depth, but if the welding speed is too low, the penetration depth will not increase, but will increase the penetration width. The effect of welding speed on stainless steel weld penetration is shown in Figure 3.12. It can be seen that when the laser power and other parameters remain unchanged, the weld penetration decreases as the welding speed increases.

The effect of welding speed on stainless steel weld penetration
Figure 3.12 The effect of welding speed on stainless steel weld penetration

Using different power laser welding, the relationship between welding speed and penetration is shown in Figure 3.13. As the welding speed increases, the penetration depth gradually decreases. The influence of laser welding speed on carbon steel penetration and the penetration depth obtained at different welding speeds are shown in Figure 3.14 and Figure 3.15, respectively.

The influence of welding speed on weld penetration under different laser power
Figure 3.13 The influence of welding speed on weld penetration under different laser power
The effect of laser welding speed on carbon steel penetration
Figure 3.14 The effect of laser welding speed on carbon steel penetration

The relationship between penetration depth, laser power and welding speed can be expressed by equation(3.3):

Penetration depth obtained at different welding speeds (P=8.7KW, plate thickness 12mm)
Figure 3.15 Penetration depth obtained at different welding speeds (P=8.7KW, plate thickness 12mm)

h=βP1/2v-r (3.3)

In the formula, h is the welding penetration depth, mm; P is the laser power, W; v is the welding speed, mm/s: β and r are constants that depend on the laser source, focusing system and welding material.

In laser deep penetration welding, the main driving force to maintain the existence of the small hole is the recoil pressure of the metal vapor. After the welding speed is low to a certain level, the heat input increases, and more and more molten metal. When the recoil pressure generated by the metal vapor is not enough to maintain the existence of the small hole, the small hole not only no longer deepens, but even collapses, welding The process degenerates into heat transfer welding, so the penetration depth will not increase. With the increase of metal vaporization, the temperature of the small hole area increases, the plasma concentration increases, and the absorption of laser light increases. For these reasons, the penetration depth of laser welding has a maximum value when welding at low speed.

Spot diameter do

According to the theory of light diffraction, the minimum spot diameter d of the focused laser. It can be calculated by formula (3.4):

do=2.44fλ(3m+1)/D (3. 4)

In the formula, do is the minimum spot diameter, mm; f is the focal length of the lens, mm; λ is the laser wavelength, mm; D is the beam diameter before focusing, mm; m is the order of the laser vibration mode.

For a beam of a certain wavelength, the smaller the f/D and m values, the smaller the spot diameter. In order to obtain a deep penetration, weld during welding, a high power density on the laser spot is required. In order to conduct small-hole heating, the power density at the laser focus during welding must be greater than 106W/cm2.

There are two ways to increase the power density: one is to increase the laser power P, which is proportional to the power density; the other is to reduce the spot diameter, and the power density is inversely proportional to the square of the spot diameter. Therefore, the effect of reducing the spot diameter is more obvious than increasing the power. To reduce the spot diameter do, you can use a short focal length lens and reduce the order of the transverse mode of the laser beam, and a smaller spot can be obtained after the low-price mode is focused.

Defocus F

The amount of defocus not only affects the size of the laser spot on the surface of the weldment, but also affects the incident direction of the beam, which has a greater impact on the weld penetration, weld width and weld cross-sectional shape. When the defocus amount F is large, the penetration depth is very small, which belongs to heat transfer welding; when the defocus amount F is reduced to a certain value, the penetration depth increases leaps and bounds, which marks the occurrence of pinholes.

According to geometric optics theory, when the distance between the positive and negative defocus planes and the welding plane is equal, the power density on the corresponding planes is approximately the same, but in fact the shape of the molten pool obtained is different. When the defocus is negative, a greater penetration depth can be obtained, which is related to the formation process of the molten pool. Because when the defocus is negative, the internal power density of the material is higher than that of the surface, which is easy to form stronger melting and vaporization, so that the beam can be transmitted to the deeper part of the material. In practical applications, when welding thicker plates, when the penetration depth is greater, the appropriate negative defocus can be used to obtain the maximum penetration; when welding thin materials, the positive defocus should be used.

The effect of defocusing amount on weld penetration, penetration width and cross-sectional area
Figure 3.16 The effect of defocusing amount on weld penetration, penetration width, and cross-sectional area

Figure 3.16 shows the effect of the defocus amount on the penetration depth, weld penetration width and weld cross-sectional area. It can be seen that after the defocus amount is reduced to a certain value, the penetration depth changes abruptly, that is, the penetration hole is established. Necessary conditions. In laser deep penetration welding, the focal position when the penetration depth is the largest is below the surface of the weldment, and the weld formation is best at this time.

Protective gas

The use of shielding gas in laser welding has two functions: one is to protect the weld metal from harmful gases, prevent oxygen contamination, and improve the performance of the joint; the other is to affect the plasma during the welding process and inhibit the formation of plasma clouds. During deep penetration welding, the high-power laser beam causes the metal to be heated and vaporized, forming a metal vapor cloud above the molten pool, which dissociates under the action of the electromagnetic field to form plasma, which acts as a barrier to the laser beam and affects the laser beam to be welded. absorb.

In order to eliminate plasma, high-speed nozzles are usually used to spray inert gas to the welding area to force the plasma to deviate, and at the same time protect the molten metal from the atmosphere. Shielding gas is mostly Ar or He. He has excellent protection and plasma suppression effect, and has a large penetration during welding. If a small amount of Ar or O2 is added to He, the penetration can be further increased. Figure 3.17 shows the influence of shielding gas on the penetration of laser welding

Figure 3.17 The influence of shielding gas on penetration

The gas flow rate also has a certain influence on the penetration depth. The penetration depth increases with the increase of the gas flow rate. However, excessive gas flow rate will cause the surface of the molten pool to sink, and even burn through in severe cases. The weld penetration depth obtained under different gas flow rates is shown in Figure 3.18. It can be seen that after the gas flow rate is greater than 17. 5L/min, the weld penetration depth no longer increases. The distance between the blowing nozzle and the weldment is different, and the penetration depth is also different. Figure 3.19 shows the relationship between the distance from the nozzle to the weldment and the weld penetration.

Weld penetration depth under different gas flow rates
Figure 3.18 Weld penetration depth under different gas flow rates
The relationship between the distance from nozzle to weldment and weld penetration (P=1.7KW, Ar protection)
Figure 3.19 The relationship between the distance from nozzle to weldment and weld penetration (P=1.7KW, Ar protection)

Note: The percentage in the figure is the percentage adjusted to the distance between the normal nozzle position and the work piece.

The relationship between laser welding process parameters (such as laser power, welding speed, etc.) and penetration, weld width, and welding material properties has a large amount of empirical data and established a regression equation for the relationship between them:

P/vh=q+b/r (3.5)

In the formula, P is the laser power, KW; v is the welding speed, mm/s: h is the welding penetration, mm; a and b are parameters; r is the regression coefficient

The values ​​of parameters a, b and regression coefficient r in formula (3.5) are given in Table 3.7.

MaterialLaser typea/k]*mm-2b/k]*mm-1Regression coefficient r
SUS304 stainless steel (OCr18Ni9)CO20.01940.3560.82
Mild steelCO2 YAG0.016 0.0090.219 0.3090.81 0.92
Aluminum alloyCO2 YAG0.0219 0.00650.381 0.5260.73 0.99
Table 3.7 Values ​​of a, b, r for several materials

The process parameters of continuous CO2 laser welding are shown in Table 3.8.

MaterialThickness/mmWelding Speed /cm*s-1Seam Width /mmAspect RatioPower /kw
321 stainless steel (1Cr18Ni9Ti)0.133.810.45Full penetration5
 0.251.480.71Full penetration5
 0.420.470.76Partial penetration5
17-7 stainless steel(0Cr17Ni7Al)0.134.650.45Full penetration5
302 stainless steel(1Cr18Ni9)0.132.120.5Full penetration5
 0.201.270.50Full penetration5
 0.250.421.00Full penetration5
 6.352.140.7073.5
 8.91.271.0038
 12.70.421.00520
 20.321.11.00520
 6.358.476.516
Inconel 6000.106.350.25Full penetration5
 0.251.690.45Full penetration5
Nickel alloy 200.131.480.45 5
Monel 4000.250.600.60 5
Industrial pure titanium0.135.900.38 5
 0.252.120.55 5
Mild steel1.190.320.630.65
Table 3.8 The process parameters of continuous CO2 laser welding

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4 thoughts on “Learn The Truth About Laser Welding

  1. Suresh says:

    Thanks for your article

  2. Alcira says:

    Wow, I have learn much from this article.

    1. Sandy says:

      Thank you

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