Laser Welding: The Basic Guide

Before we dive into the analysis of laser welding, let’s understand what a laser is.

What Is a Laser?

Laser is “Light Amplification by Stimulated Emission of Radiation”, which is the release of energy in the form of photons when the electrons in an atom absorb energy and jump from a low energy level to a high energy level, and then fall back to a low energy level.

The induced (excited) photon beam (laser), in which the optical properties of the photons are highly uniform.

Lasers are better monochromatic, brighter and more directional than ordinary light sources.

It is used in a wide range of applications in the automotive industry.

Laser light-emitting principle

The 3 elements of laser generation:

  • Excitation source
  • Medium
  • Resonant cavity

The medium is excited to a high energy state, and the light wave is amplified due to the reflection of the excited absorption leap light back and forth between the two end mirrors, and enough energy is obtained to start emitting the laser.

The 4 properties of laser.

  • Monochromaticity
  • Coherence
  • Directionality
  • High brightness

The highly focused laser can thus provide welding, cutting and heat treatment functions.

Laser light-emitting principle

Classification of lasers

According to the light-emitting medium, lasers can be divided into:

Gas Lasers

The structure is simple, low cost, and can work continuously and stably, such as CO2 laser, 10.6μm.

Liquid Lasers

Dye lasers are commonly used, and in most cases, organic dyes are dissolved in solvents (ethanol, acetone, water) .

Solid-State Lasers

Nd:YAG laser, Nd (neodymium) is a rare earth group element, YAG stands for Yttrium Aluminum Tsuge Garnet, the main advantage is that the generated beam can be transmitted through the fiber, 1.06μm, the intensity of the excitation beam can reach 106W/cm2.

Semiconductor Lasers

Commonly used materials include gallium arsenide (GaAs), cadmium sulfide (CdS), indium phosphide (InP), zinc sulfide (ZnS), etc.

Laser welding introduction

Laser welding introduction

Laser welding is a melt welding process in which a laser beam is used as the energy source to impact on the weld joint.

Laser welding is a non-contact welding process that does not require pressure, but requires the use of inert gas to prevent oxidation of the molten pool, which is occasionally used for filler metals.

Laser welding is capable of precise energy control, thus enabling the welding of precision micro devices, and it can be applied to many metals, especially for some difficult metals and dissimilar metals.

Features of laser welding

With the research and development of high power lasers, laser welding technology is widely used in many fields, mainly because of its following characteristics:

1) Deep depth, fast speed and small deformation.

When using laser welding for workpiece connection, the connection gap of the welded workpiece is almost non-existent, and at the same time, the depth-to-width ratio of welding is large, the deformation after welding is small, the heat-affected zone is small, and the precision is high.

2)The welding device is simple and flexible, can be welded at room temperature or under special conditions, and does not have high requirements for the welding environment.

3) High power density

Laser welding has a considerable depth of melt and high power density, which can weld refractory materials, such as titanium alloys, etc.

4) High energy density and suitable for high speed welding

5) No wear and tear consumption of electrodes, tools, etc.

6) No pollution to the environment.

7) It is possible to achieve long-distance, hard-to-reach parts, multi-way simultaneous or time-sharing welding by optical fiber.

8) It is easy to change the laser output focal length and welding spot position.

9) It is easy to be mounted on a robot device.

Features of laser welding

Classification of laser welding

1. Laser brazing

Laser brazing welding principle.

Using the laser beam as a heat source, the focused beam is directed at the surface of the filled wire. The wire is continuously heated by the beam energy and melts to form a high-temperature liquid metal. The liquid metal is infiltrated into the joint of the part to be welded and, under appropriate external conditions, results in a good metallurgical bond with the workpiece.

Laser brazing process applied to welding not only makes the product more beautiful and improves the sealing, but also significantly improves the strength of the welded area and enhances the safety performance of the whole vehicle.

Laser brazing welding principle.

Note that the connection between the workpieces is achieved by brazing the molten metal and that the base material itself should not be damaged by severe laser fusion.

Advantages

High energy density, small deformation, very narrow heat-affected zone, high depth to width ratio of the weld seam, high welding speed and easy automatic control.

Disadvantages

The focal point diameter is very small, so the welding seam bridging ability is very poor; in addition, the energy conversion efficiency of the laser is low.

Laser brazing system mainly consists of laser generator and cooling system, laser brazing head, wire feeding mechanism, robot, fixture, dust removal system, control system, etc.

Laser brazing system

Laser generator and cooling system

The laser generator is the device that generates the laser light and is the equipment that provides the welding energy in the laser brazing system.

Laser brazing head

It mainly consists of collimation module, control module, focus module, weld seam tracking module, air curtain module and other parts.

Wire feeding system

The wire feeding system is responsible for stable wire feeding during welding. A push-pull wire feeding mechanism is used to ensure good wire alignment and stable wire feeding speed. If preheating is required, a hot wire power source is added.

Robotics

The motion system, which realizes the travel of the welding trajectory, also carries the laser brazing head and ancillary devices, water and gas circuits, etc.

The robot is responsible for executing the welding process and talking to the automation system to call the welding process parameters in the system.

Control system

The autonomous process cabinet controls the timing of the robot, laser, brazing head, wire feeder and preheating system via the industrial bus to complete the weld. The main production line PLC exchanges signals with the robot via the industrial bus.

2. Laser fusion welding

Laser fusion welding is a welding method that uses a laser as the heat source to melt the base material of two plate parts at the corner of each plate part (while melting the nearby welding wire to fill the corner of the two plates) to form a liquid metal, and after it cools, a reliable connection is formed.

The process principle is shown in Figure 2.

Laser fusion welding principle

Laser fusion welding can be divided into laser penetration welding, laser fusion welding (without wire filling) and laser fusion wire filling welding, etc., which are mostly used for welding of automobile roof and floor, etc.

3.Laser remote welding

Laser remote welding is to install an oscillating mirror scanning head on the sixth axis of the robot to achieve laser trajectory movement only by the oscillating reflection of the lens, without the robot arm following the movement.

The laser remote welding system is highly flexible and efficient, and one system can replace 6 to 9 sets of ordinary robotic spot welding.

The distance between the laser head and the workpiece is more than 500mm, which can extend the service life of the lens protection glass.

The biggest advantage of laser remote welding over conventional welding is increased productivity.

The fast lens movement of the scanning head allows for a significant reduction in the time spent positioning the robot, which in turn drastically reduces manufacturing time.

Compared to the average speed of 0.5 joints/s for resistance spot welding, the laser remote welding speed is 3 to 4 joints/s, allowing the laser beam to be fully utilized.

In mass production tests, laser remote welding has reduced the time by 80% compared to conventional resistance welding.

Another important aspect of the high flexibility of the scanning lens is the versatility of the weld shape.

If the weld seam is C-shaped, it has a significantly higher welding speed compared to a linear weld seam.

Laser remote welding is mostly used on automotive body-in-white sub-assemblies, and Figure 3 shows a part of the actual laser remote welding.

Laser remote welding

4.Laser composite welding

Laser composite welding mainly refers to laser and MIG arc composite welding.

In this process, the laser and the arc interact and complement each other, and the process principle is shown in Figure 4.

Laser composite welding principle

Laser composite welding is more economical than laser welding.

Laser-MIG welding uses a laser beam and an electric arc to work together, resulting in high welding speed, a stable welding process and high thermal efficiency, while allowing for a larger weld assembly gap.

The melt pool of laser-MIG composite welding is smaller than that of MIG welding, resulting in low heat input, a small heat-affected zone and low workpiece distortion, which greatly reduces the work of correcting weld distortion after welding.

The effect of laser composite welding bonding is shown in the below Figure.

effect of laser composite welding bonding

Working principle of laser welding

Laser welding can be achieved using the continuous or pulsed laser beam, the principle of laser welding can be divided into heat conduction welding and laser deep fusion welding.

The power density is less than 104~105 W/cm2 for thermal conduction welding, when the melt depth is shallow and the welding speed is slow.

Power density greater than 105 ~ 107 W / cm2, the metal surface under the effect of heat into a “hole”, the formation of deep fusion welding.

Features of fast welding speed and large depth-to-width ratio.

Heat transfer type laser welding principle.

The laser radiation heats the surface to be processed and the surface heat diffuses internally by heat conduction, causing the workpiece to melt and form a specific melt pool by controlling the laser parameters such as the width, energy, peak power and repetition frequency of the laser pulse.

Laser welding machines for gear welding and metallurgical sheet welding mainly involve laser deep fusion welding.

The following highlights the principles of laser deep fusion welding.

principles of laser deep fusion welding

The principle of laser deep fusion welding

Laser deep fusion welding generally uses a continuous laser beam to join materials. The metallurgical-physical process is very similar to electron beam welding, i.e., the energy conversion mechanism is accomplished through a “key-hole” structure.

Under sufficiently high power density laser irradiation, the material evaporates and a small hole is formed.

This vapor-filled hole is like a black body that absorbs almost all of the incident beam energy, and the equilibrium temperature inside the hole cavity reaches about 2500℃. Heat is transferred from the outer wall of this high-temperature hole, causing the metal surrounding the hole cavity to melt.

The small hole is filled with high-temperature vapor generated by the continuous evaporation of the wall material under the irradiation of the light beam.

The four walls of the small hole surround the molten metal and the liquid metal is surrounded by the solid material (whereas in most conventional welding processes and laser conduction welding, the energy is first deposited on the surface of the workpiece and then transported to the interior by transfer).

The liquid flow and wall surface tension outside the hole walls are held in dynamic equilibrium with the continuously generated steam pressure inside the hole cavity.

The beam continuously enters the small hole and the material outside the hole is continuously flowing. As the beam moves, the small hole is always in a stable state of flow.

That is, the hole and the molten metal surrounding the hole wall move forward with the forward speed of the leading beam, and the molten metal fills the void left by the moving hole and condenses with it, and the seam is formed.

All of this happens so quickly that the welding speed can easily reach several meters per minute.

principle of laser deep fusion welding

Main process parameters of laser deep fusion welding

1) Laser power

There is a laser energy density threshold in laser welding, below which the depth of melt is shallow and once it is reached or exceeded, the depth of melt increases substantially.

Only when the laser power density on the workpiece exceeds the threshold value (material dependent), plasma is generated, which marks a stable deep fusion weld.

If the laser power is below this threshold, only surface melting of the workpiece occurs, i.e. the welding proceeds in a stable heat transfer type.

When the laser power density is near the critical condition of small hole formation, deep fusion welding and conduction welding alternate and becomes unstable welding processes, resulting in large fluctuations in the melt depth.

In laser deep fusion welding, the laser power controls both the depth of penetration and the welding speed.

The weld depth of melt is directly related to the beam power density and is a function of the incident beam power and beam focal spot.

In general, for a given diameter of the laser beam, the depth of fusion increases as the beam power increases.

Laser power

2) Beam focal spot

Beam spot size is one of the most important variables in laser welding, as it determines the power density.

However, its measurement is a challenge for high power lasers, although many indirect measurement techniques are available.

The beam focal diffraction limit spot size can be calculated from light diffraction theory, but the actual spot is larger than the calculated value due to the presence of focusing lens aberration.

The simplest real measurement method is the isothermal profile method, which is to measure the focal spot and perforation diameter after burning and penetrating a polypropylene plate with thick paper.

This method should be measured by practice, mastering the laser power size and the time of beam action.

3) Material absorption value

The absorption of the laser by the material depends on some important properties of the material, such as absorption rate, reflectivity, thermal conductivity, melting temperature, evaporation temperature, etc. The most important one is the absorption rate.

The factors affecting the absorption rate of a material to a laser beam include two aspects:

  • Firstly, the resistivity of the material, which was found to be proportional to the square root of the resistivity coefficient, which in turn varies with temperature, after measuring the absorbance of the polished surface of the material.
  • Secondly, the surface condition (or finish) of the material has a more important effect on the beam absorbance and thus has a significant effect on the welding effect.

The output wavelength of CO2 laser is usually 10.6μm.

Non-metals such as ceramics, glass, rubber and plastic have high absorption of it at room temperature, while metallic materials have poor absorption of it at room temperature until it increases sharply once the material melts or even vaporizes.

The use of surface coating or surface generation of oxide film method to improve the absorption of the material to the beam is very effective.

4) Welding speed

Welding speed has a large impact on the depth of melt, increasing the speed will make the depth of melt shallow, but if the speed is too low and will lead to excessive melting of the material, and the workpiece weld through.

Therefore, certain laser power and a certain thickness of a particular material have a suitable welding speed range, and in which the maximum depth of melt can be obtained at the corresponding speed value.

The following figure shows the relationship between welding speed and depth of melt for 1018 steel.

the relationship between welding speed and depth of melt for 1018 steel

5) Protective gas

Laser welding processes often use inert gases to protect the melt pool.

However, for most applications, helium, argon and nitrogen are often used to protect the workpiece from oxidation during the welding process.

Helium is not easily ionized (ionization energy is high), allowing the laser to pass through and the beam energy to reach the surface of the workpiece unimpeded.

It is the most effective shielding gas used in laser welding but is more expensive.

Argon is cheaper and more dense, so it protects better.

However, it is susceptible to high-temperature metal plasma ionization and as a result shields part of the beam from being directed to the workpiece, reducing the effective laser power for welding and also impairing the welding speed and depth of melt.

The surface of weldments protected with argon is smoother than when protected with helium.

Nitrogen is the least expensive as a shielding gas, but it is not suitable for certain types of stainless steel welding, mainly due to metallurgical problems, such as absorption, which sometimes produces porosity in the lap zone.

The second role of using a shielding gas is to protect the focusing lens from metal vapor contamination and sputtering of liquid molten droplets.

This is especially necessary in high power laser welding, where the ejecta become very powerful.

The third function of the shielding gas is to disperse the plasma shielding produced by high power laser welding.

The metal vapor absorbs the laser beam and ionizes into a plasma cloud, and the protective gas around the metal vapor is also ionized by the heat.

If too much plasma is present, the laser beam is consumed by the plasma to some extent.

The presence of plasma as second energy on the working surface makes the depth of melt shallower and the weld pool surface wider.

The rate of electron complexation is increased by increasing the number of electron-ion and neutral-atom three-body collisions to reduce the electron density in the plasma.

The lighter the neutral atom, the higher the collision frequency and the higher the compounding rate; on the other hand, only the protective gas with high ionization energy does not increase the electron density due to the ionization of the gas itself.

Table  Atomic (molecular) weight and ionization energy of common gases and metals

MaterialsHeArNAlMgFe
Atomic (molecular) Qty.44028272456
Ionization energy (eV)24.4615.6814.55.967.617.83

As can be seen from the table, the plasma cloud size varies with the protective gas used, with helium being the smallest, followed by nitrogen, and the largest when argon is used.

The larger the plasma size, the shallower the melting depth.

The reason for this difference is firstly due to the different degrees of ionization of the gas molecules and also due to the difference in the diffusion of the metal vapor caused by the different densities of the protective gases.

Helium is the least ionized and the least dense, and it quickly disperses the rising metal vapor from the molten metal pool.

Therefore, the use of helium as a shielding gas maximizes the suppression of plasma, thereby increasing the depth of melt and improving the welding speed.

It is not easy to cause porosity because of the light mass and can escape.

Of course, from our actual welding results, the effect of protection with argon gas is not bad.

The effect of plasma cloud on the melt depth is most obvious in the low welding speed zone.

When the welding speed increases, its influence diminishes.

The protective gas is injected through the nozzle at a certain pressure to the surface of the workpiece.

The hydrodynamic shape of the nozzle and the size of the outlet diameter are important.

It must be large enough to drive the sprayed shielding gas to cover the welding surface, but in order to effectively protect the lens, to prevent metal vapor contamination or metal spatter damage to the lens, the nozzle size should also be limited.

The flow rate should also be controlled, otherwise the laminar flow of shielding gas becomes turbulent and the atmosphere becomes involved in the molten pool, eventually forming porosity.

In order to improve the protection effect, additional lateral blowing can also be used, that is, through a smaller diameter nozzle will protect the gas at an angle directly into the deep fusion welding of small holes.

The shielding gas not only suppresses the plasma cloud on the surface of the workpiece, but also exerts an influence on the plasma in the hole and the formation of the small hole, further increasing the depth of fusion and obtaining a deeper and wider weld seam than is desirable.

However, this method requires precise control of the gas flow size and direction, otherwise it is easy to produce turbulence and damage the melt pool, resulting in difficulty in stabilizing the welding process.

6) Lens focal length

Welding is usually used to focus the laser, generally choose 63~254mm (2.5″~10″) focal length of the lens.

Focused spot size is proportional to the focal length, the shorter the focal length, the smaller the spot.

But the focal length also affects the focal depth, that is, the focal depth increases simultaneously with the focal length, so the short focal length can improve the power density, but because of the small focal depth, the distance between the lens and the workpiece must be accurately maintained, and the melting depth is not large.

Due to the influence of the spatter generated during the welding process and the laser mode, the shortest depth of focus used in actual welding is mostly focal length 126mm (5″).

When the seam is large or the spot size needs to be increased by increasing the weld seam, a lens with a focal length of 254mm (10″) can be selected, in which case a higher laser output power (power density) is required to achieve a deep fusion small hole effect.

When the laser power exceeds 2kW, especially for the 10.6μm CO2 laser beam, due to the use of special optical materials to form the optical system, in order to avoid the risk of optical damage to the focusing lens, often choose the reflection focusing method, generally using the polished copper mirror as a reflector.

Due to the effective cooling, it is often recommended for high power laser beam focusing.

7) Focus position

When welding, the focal point position is critical in order to maintain adequate power density.

Changes in the position of the focal point relative to the workpiece surface directly affect the weld width and depth.

The figure below shows the effect of the focal point position on the depth of melt and seam width of 1018 steel.

the effect of focal point position on the depth of melt and seam width of 1018 steel

In most laser welding applications, the focal point is typically positioned approximately 1/4 of the desired depth of fusion below the workpiece surface.

8) Laser beam position

When laser welding different materials, the laser beam position controls the final quality of the weld, especially in the case of butt joints which are more sensitive to this than lap joints.

For example, when hardened steel gears are welded to mild steel drums, proper control of the laser beam position will facilitate the production of a weld with a predominantly low carbon component, which has better crack resistance.

In some applications, the geometry of the workpiece to be welded requires the laser beam to be deflected by an angle, and the absorption of laser energy by the workpiece is not affected when the deflection angle between the beam axis and the joint plane is within 100 degrees.

9) Welding starting and ending point of the laser power gradual rise, gradual decline control

When laser deep fusion welding, the phenomenon of small holes is always present, regardless of the depth of the weld.

When the welding process is terminated and the power switch is turned off, a crater will appear at the end of the weld.

In addition, when the laser welding layer covers the original weld, there will be excessive absorption of the laser beam, resulting in overheating or porosity of the weld.

In order to prevent the above phenomena, the power starts and stop points can be programmed so that the power start and stop times become adjustable, i.e. the starting power is electronically increased from zero to the set power value in a short period of time and the welding time is adjusted, and finally the power is gradually reduced from the set power to the zero value when the welding is terminated.

Laser deep fusion welding features and advantages and disadvantages

(1) Characteristics of laser deep fusion welding

1) High depth to width ratio

Because the molten metal forms around the cylindrical high-temperature vapor cavity and extends toward the workpiece, the weld becomes deep and narrow.

2)Minimum heat input

Because the temperature inside the small hole is very high, the melting process occurs very quickly, the heat input to the workpiece is very low, and the heat distortion and heat affected zone are very small.

3)High denseness

Because the small hole filled with high-temperature steam is conducive to welding pool stirring and gas escape, resulting in the generation of non-porous fusion penetration weld.

The high cooling rate after welding makes it easy to refine the weld organization.

4) Reinforced weld seam

Because of the incandescent heat source and the full absorption of non-metallic components, the impurity content is reduced, the size of the inclusions and their distribution in the melt pool are changed.

The welding process does not require electrodes or filler wire, and the melt zone is less contaminated, making the weld strength and toughness at least equal to or even exceed that of the parent metal.

5) Precise control

Because the focused spot is so small, the weld can be positioned with high precision.

The laser output has no “inertia” and can be stopped and restarted at high speeds, making it possible to weld complex workpieces with CNC beam movement technology.

6) Non-contact atmospheric welding process

Because the energy comes from the photon beam, there is no physical contact with the workpiece, so there is no external force applied to the workpiece. In addition, both magnetism and air have no effect on the laser.

(2) Advantages of laser deep fusion welding

1) Because the focused laser has a much higher power density than conventional methods, resulting in fast welding speed, heat-affected zone and deformation are very small, but also can weld difficult to weld materials such as titanium.

2) Because the beam is easy to transmit and control, and does not require frequent replacement of the welding gun, nozzle, and no electron beam welding required vacuum, significantly reducing downtime assistance time, so there is a load factor and high productivity.

3) High weld strength, toughness and overall performance due to purification and high cooling rate.

4) High processing accuracy due to low average heat input reduces reprocessing costs; in addition, laser welding running costs are lower, thus reducing workpiece processing costs.

5) The beam intensity and fine positioning can be effectively controlled, and the operation can be easily automated.

(3) Disadvantages of laser deep fusion welding

1) Limited welding depth.

2) High workpiece assembly requirements.

3) High one-time investment in laser system

Laser deep fusion welding equipment

Laser deep fusion welding usually uses continuous wave CO2 lasers, which can maintain a high enough output power to produce a “small hole” effect, melting through the entire cross-section of the workpiece and forming a strong welded joint.

As far as the laser itself is concerned, it is simply a device that produces a parallel beam with good directionality that can be used as a heat source.

Advantages and disadvantages of laser welding

Advantages of laser welding

  • High-quality joint strength and large depth-to-width ratio can be obtained with laser welding, and the welding speed is relatively fast.
  • Since laser welding does not require a vacuum environment, remote control and automated production can be achieved through lenses and optical fibers.
  • The laser has a large power density, which makes it possible to weld difficult materials such as titanium and quartz, and to apply welding to materials with different properties.
  • Micro welding can be performed. The laser beam is focused to obtain a very small spot and can be precisely positioned, which can be applied in the group welding of micro and small workpieces for high-volume automated production.

Disadvantages of laser welding

  • The price of the laser and the accessories of the welding system is more expensive, so the initial investment and maintenance costs are higher than the traditional welding process, and the economic efficiency is poor.
  • The conversion efficiency of laser welding is generally low (usually 5% to 30%) due to the low absorption of laser light by solid materials, especially after the emergence of plasma (plasma has an absorption effect on laser light).
  • Due to the small focused spot of laser welding, the equipment accuracy of the workpiece joint is required to be high, and a small equipment deviation can produce a large processing error.
  • Laser welding requires high workpiece assembly accuracy due to the small laser-focused spot size and narrow weld seam.

The position of the welded part must be very precise, requiring that the position of the beam on the workpiece must not be significantly shifted and must be within the focus range of the laser beam.

If the accuracy of the workpiece assembly or beam positioning does not meet the requirements, it is easy to cause welding defects.

The requirements of laser welding for the shape of the weld seam are shown in the below Figure.

requirements of laser welding for the shape of the weld seam

  • Laser fillet welding is a difficult process to control. Laser filler welding is a fusion welding process in which a focused spot is shone onto the workpiece and the wire respectively. The melt pool is small, and accurate control of the relative positions of the filaments is very important to achieve uniform melting of the continuously fed wire.
  • The weld channel solidifies relatively quickly and may have porosity and embrittlement defects.
  • Due to the large spatter, the weld seam of penetration welding is rougher compared to brazing, but much stronger than ordinary spot welding.
  • Compared to other welding methods, the cost of the laser and its associated systems is higher and the upfront one-time investment is larger.

Laser welding lasers

The main types of lasers used for welding are CO2 lasers, Nd:YAG lasers, fiber lasers, and semiconductor lasers.

CO2 laser

CO2 laser is a gas laser with a far-infrared beam and a wavelength of 10.6μm.

It generally works in a continuous manner, has a high output power and is widely used in high-power laser welding.

When the CO2 laser is used to weld at a high power of 10 kW or more, the use of argon shielding gas often induces a very strong plasma, making the melt depth shallow.

Therefore, helium, which does not produce plasma, is often used as the shielding gas for high-power CO2 laser welding.

Fiber laser

Fiber lasers are mainly used for overlap welding of thin materials with high requirements for stability of the weld joint.

With lap welding it is possible to obtain weld seams with a depth of melt of 0.01in or even higher at higher speeds.

A 200W single-mode fiber laser can achieve a depth of melt of 0.004in at speeds up to 50in/s.

Nd: YAG laser

The Nd: YAG laser is a solid-state laser that produces a beam of mainly near-infrared light with a wavelength of 1.06 μm.

The thermal conductor has a high absorption rate of light at this wavelength and can output in both continuous and pulsed modes, making it competitive in the field of welding of critical components.

Semiconductor laser

Semiconductor lasers, with their small mass, high conversion efficiency, low operating cost and long life, are one of the important directions for future laser development.

Scholars at home and abroad have begun to use high-power semiconductor lasers for welding research on aluminum alloys.

Due to the short wavelength of the semiconductor laser, the absorption rate of metal is much higher than that of CO2 laser and Nd: YAG laser, so it has a good application prospect in the field of welding.

However, the low power density of the semiconductor laser irradiating to the material surface makes it more suitable for thin plate welding and electronic component welding, etc. when performing laser welding.

4 forms of laser-arc composite heat source welding

1. Laser – TIG composite welding

The characteristics of laser and TIG compound welding are.

  • Using arc to enhance the laser action, a low power laser can be used instead of a high power laser to weld metal materials.
  • High speed welding is possible when welding thin parts.
  • It can increase the depth of melt, improve the weld formation, and obtain high-quality welded joints.
  • It can moderate the accuracy requirement of the interface of the base material end face.

For example, when the CO2 laser power is 0. 8kW, the TIG arc current is 90A, and the welding speed is 2m/ min, it is equivalent to the welding capacity of a 5kW CO2 laser welder. The depth of melt obtained at a welding speed of 0.5 to 5 m/min with a 5kW CO2 laser beam is 1.3 to 1.6 times greater than with a 5kW CO2 laser beam alone.

2. Laser – plasma arc composite welding

Laser plasma composite welding is performed in a coaxial manner as shown in Figure 3.

The plasma arc is generated by an annular electrode and the laser beam passes through the middle of the plasma arc.

The plasma arc has two main functions.

On the one hand to provide additional energy for laser welding, increasing the welding speed and thus the efficiency of the whole welding process.

On the other hand, the plasma arc surrounds the laser, which can produce a heat treatment effect and prolong the cooling time, which also reduces the susceptibility to hardening and residual stresses and improves the microstructural properties of the weld.

3. Laser – MIG composite welding

The basic principle of laser-MIG composite welding is shown in Figure 4.

basic principle of laser-MIG composite welding

In addition to the energy input to the weld zone from the arc, the laser also inputs heat to the weld metal.

Laser composite welding technology does not act in sequence between the two welding methods, but rather both methods act on the weld zone simultaneously.

The laser and the arc affect the performance of the composite weld in different degrees and forms.

In laser-MIG composite welding, volatilization occurs not only on the surface of the workpiece, but also on the filler wire, allowing more metal volatilization and thus easier energy transfer from the laser.

MIG welding is characterized by low power source cost, good weld bridging, good arc stability and easy improvement of the weld structure by filler metal.

Laser beam welding, on the other hand, is characterized by large melt depth, high welding speed, low heat input and narrow weld seam, but welding thicker materials requires a more powerful welding laser.

At the same time, the melt pool of laser composite welding is smaller than that of MIG welding, which results in less deformation of the workpiece and greatly reduces the work of correcting the welding deformation after welding.

With laser-MIG composite welding, two separate pools are created, and the heat input from the arc behind acts as a simultaneous post-weld tempering treatment, reducing the hardness of the weld (especially in welded steel).

Due to the very high welding speed of laser composite welding, production time and production costs can be reduced.

4. Dual laser beam welding technology

In the laser welding process, the high laser power density makes the welded base material is rapidly heated to melt and vaporize, generating high-temperature metal vapor.

Under the continued action of high power density laser, it is easy to generate a plasma cloud, which not only reduces the absorption of the laser by the workpiece, but also makes the welding process unstable.

If, after the formation of a large deep molten hole, reduce the laser power density to continue irradiation, and has formed a larger deep molten hole on the absorption of laser light, the result of laser action on the metal vapor is reduced, the plasma cloud can be reduced or disappear.

Thus, a pulsed laser with high peak power and a continuous laser beam, or two pulsed lasers with a large difference in pulse width, repetition frequency and peak power is used to compound the workpiece for welding

In the welding process, the two laser beams together irradiate the workpiece, periodically forming a large deep melt hole, and then stop the irradiation of a laser beam at the right time, can make the plasma cloud is small or disappear, improve the absorption and utilization of laser energy, increase the welding depth, improve the welding capacity.

Commonly used equipment for laser welding

Laser welding head

This is a series of optical processing of laser light to obtain a characteristic beam suitable for laser applications.

According to the welding application, there are fusion welding head, brazing head, and laser welding head.

Laser welding head

Robotics

Robots are more common, with enough precision and weight to apply.

Currently, the world ABB, FANUC, MOTOMAN, KUKA, etc. have laser applications.

Robotics

Important parameters of laser welding

Power density.

Power density is one of the most critical parameters in laser processing.

With a high power density, the surface layer can be heated to the boiling point in a microsecond time frame, producing a large amount of vaporization.

Therefore, high power density is very beneficial for material removal processes such as punching, cutting and engraving.

For lower power densities, it takes several milliseconds for the surface layer temperature to reach the boiling point, and the bottom layer reaches the melting point before the surface layer vaporizes, making it easy to form a good melt weld.

Laser pulse waveform.

When the high-intensity laser beam is shot to the surface of the material, the metal surface will have 60-98% of the laser energy reflected and lost, especially gold, silver, copper, aluminum, titanium and other materials reflect strong, fast heat transfer.

In the process of a laser pulse signal, the reflectivity of the metal changes with time.

When the surface temperature of the material rises to the melting point, the reflectivity decreases rapidly, and when the surface is in a melted state, the reflection is stabilized at a certain value.

Laser pulse width.

Pulse width is an important parameter for pulsed laser welding. The pulse width is determined by the melt depth and heat-affected zone, the longer the pulse width the larger the heat-affected zone, and the melt depth increases with the 1/2 power of the pulse width.

However, an increase in pulse width decreases the peak power, so increasing the pulse width is generally used for heat conduction welding methods, forming a wide and shallow weld seam size, especially for lap welding of thin and thick plates.

However, lower peak power results in excess heat input, and each material has an optimum pulse width that maximizes the depth of melt.

Off-focus volume.

Laser welding usually requires a certain amount of defocus, because the power density at the center of the spot at the laser focal point is too high and tends to evaporate into a hole.

The power density is relatively evenly distributed in all planes away from the laser focal point.

There are two types of defocusing.

  • Positive defocusing
  • Negative defocusing

The focal plane is located above the workpiece for positive defocus, and vice versa for negative defocus.

According to geometric optics theory, when the positive and negative out of focus plane and the welding plane distance is equal, the corresponding plane of power density is approximately the same, but in practice, the shape of the molten pool obtained has some differences.

Negative defocusing, greater depth of melt can be obtained, which is related to the formation process of the melt pool.

Welding speed.

Welding speed has a large impact on the depth of melt, increasing the speed will make the depth of melt shallow, but the speed is too low and will lead to excessive melting of the material, and the workpiece weld through.

Therefore, there is a suitable welding speed range for certain laser power and a certain thickness of a particular material, and the maximum depth of melt can be obtained at the corresponding speed value in it.

Protective gas.

The laser welding process often uses inert gases to protect the melt pool, and for most applications helium, argon and nitrogen are often used for protection.

The second function of the shielding gas is to protect the focusing lens from metal vapor contamination and sputtering of liquid droplets.

In high power laser welding, the ejected material is very powerful, when the protection of the lens is more necessary.

The third role of the shielding gas is to effectively disperse the plasma shielding generated by high power laser welding.

The metal vapor absorbs the laser beam and ionizes into an equal plasma, and if too much plasma is present, the laser beam will be consumed by the plasma to some extent.

Laser welding process methods

1. Sheet-to-sheet welding

It includes 4 types of process methods:

  • Butt Weld
  • End welding
  • Center penetration fusion welding
  • Center piercing fusion welding

2. Wiretowire welding

It includes 4 types of process methods:

  • Wire-to-wire butt welding
  • Cross welding
  • Parallel lap welding
  • T-welding

3. Welding of metal wire and block components

The connection of wire to lumped elements can be successfully achieved using laser welding, where the lumped elements can be of any size.

Attention should be paid to the geometry of the wire element during welding.

4. Welding of different metals

Welding of different types of metals has to address the range of weldability and weldability parameters.

Laser welding between different materials is only possible with certain combinations of materials.

Laser brazing is not suitable for the connection of some components, but the laser can be used as a heat source for soft and hard brazing, which also has the advantages of laser brazing.

There are various ways of using brazing, among which laser soft brazing is mainly used for the soldering of printed circuit boards and is particularly useful for chip component assembly technology.

Welding of different metals

Factors affecting the quality of laser welding

Laser welding is a process in which a high-energy beam of laser light irradiates a workpiece, causing a dramatic increase in working temperature, and the workpiece melts and rejoins to form a permanent joint.

Laser welding has better shear strength and tear strength.

There are many factors that affect the quality of laser welding.

Some of them are extremely volatile and have considerable instability. How to properly set and control these parameters to keep them within the right range for the high-speed continuous laser welding process to ensure the weld quality.

The reliability and stability of weld forming is an important issue related to the practical and industrialization of laser welding technology.

The main factors affecting the quality of laser welding are divided into three aspects: welding equipment, workpiece condition and process parameters.

1)Welding equipment

The most important quality requirements for lasers are beam pattern and output power and its stability.

The lower the beam pattern order, the better the beam focusing performance, the smaller the spot, the higher the power density at the same laser power, and the greater the weld depth and width.

Generally required base mode (TEM00) or low-order mode, otherwise it is difficult to meet the requirements of high-quality laser welding.

Currently, China’s lasers are still difficult to use for laser welding in terms of beam quality and power output stability.

From the foreign situation, the laser beam quality and output power stability has been quite high, will not become a problem for laser welding.

The optical system is the biggest factor affecting the quality of welding is the focus mirror, the focal length used is generally between 127mm (5in) to 200mm (7.9in), the focal length is small to reduce the focus beam waist spot diameter is beneficial, but too small is easy to be contaminated and spatter damage in the welding process.

Optical system

The shorter the wavelength, the higher the absorption.

Generally, materials with good conductivity have high reflectivity.

For YAG laser, the reflectivity is 96% for silver, 92% for aluminum, 90% for copper and 60% for iron.

The higher the temperature, the higher the absorbance, in a linear relationship.

General surface coating phosphate, carbon black, graphite, etc. can improve the absorption rate.

2)Condition of workpiece

Laser welding requires that the edges of the workpiece being processed and assembled with high precision, that the spot be strictly aligned with the weld seam, and that the original assembly precision and spot alignment of the workpiece not change during the welding process due to welding heat distortion.

This is because the laser spot is small, the weld seam is narrow, and generally no filler metal is added.

If the assembly gap is too large, the beam can pass through the gap and cannot melt the base material, or cause obvious nibbling, depression, such as spot to seam deviation is slightly larger may cause unfused or not welded through.

Therefore, the general plate butt assembly gap and spot seam deviation should not be greater than 0.1mm, the wrong side should not be greater than 0.2mm.

In actual production, sometimes the laser welding technology cannot be used because these requirements cannot be met.

To obtain good welding results, the allowable butt gap and lap gap should be controlled within 10% of the thin plate thickness.

Successful laser welding requires close contact between the substrates being welded.

This requires careful tightening of the parts to achieve the best results.

3) Welding parameters

(1) The impact on the laser welding mode and weld forming stability pieces, the most important of the welding parameters is the power density of the laser spot, which affects the welding mode and weld forming stability as follows.

With the laser spot power density from small to large in order for stable thermal conduction welding, mode instability welding and stable deep fusion welding.

Laser spot power density, in the case of a certain beam pattern and focus mirror focal length, is mainly determined by the laser power and beam focus position.

The laser power density is proportional to the laser power.

while an optimum value exists for the effect of the focal point position.

When the focal point of the beam is at a certain position under the surface of the workpiece (within the range of 1 to 2 mm, depending on the plate thickness and parameters), the most ideal weld seam can be obtained.

Deviation from this optimal focal position, the workpiece surface spot that becomes larger, causing the power density to become smaller, to a certain range, will cause changes in the form of the welding process.

The impact of welding speed on the form of the welding process and stable parts is not as significant as the laser power and focal position, only when the welding speed is too large, due to the heat input is too small to maintain a stable deep fusion welding process.

In practice, welding should be based on the requirements of the welded parts on the depth of melt to choose stable deep fusion welding or stable heat conduction welding, and to absolutely avoid mode instability welding.

(2) In the deep fusion welding range, the influence of welding parameters on the depth of melt:

In the stable deep fusion welding range, the higher the laser power, the greater the melt depth, about 0.7 times the relationship.

And the higher the welding speed, the shallower the depth of melt.

Under certain laser power and welding speed conditions, the focal point is in the best position when the depth of melt is maximum. If it deviates from this position, the depth of melt decreases and even becomes a mode of unstable welding or stable thermal conduction welding.

(3) The effect of protective gas

The main role of the shielding gas is:

Protection of the workpiece from oxidation during the welding process.

protection of the focusing lens from metal vapor contamination and sputtering of liquid molten droplets.

Dispersing the plasma generated by high power laser welding.

Cooling the workpiece and reducing the heat-affected zone.

The shielding gas is usually argon or helium, or nitrogen if the apparent quality is not high.

Their tendency to produce plasma is significantly different: helium, due to its high ionization body and fast thermal conductivity, has less tendency to produce plasma than argon under the same conditions, thus allowing for a greater depth of melt.

In a certain range, as the flow of protective gas increases, the tendency to suppress the plasma increases, thus increasing the melt depth, but increases to a certain range that tends to smooth out.

(4) Analysis of the monitorability of each parameter.

Among the four welding parameters, welding speed and shielding gas flow rate belong to the parameters that can be easily monitored and kept stable, while laser power and focal position are parameters that may fluctuate during the welding process and are difficult to monitor.

Although the laser power output from the laser is highly stable and easy to monitor, the laser power reaching the workpiece will change due to losses in the light guide and focusing system, and this loss is related to the quality of the optical workpiece, time of use and surface contamination, so it is not easy to monitor and becomes an uncertainty in welding quality.

The focal position of the beam is one of the most difficult factors to monitor and control among the welding parameters that have a great impact on the quality of the weld.

Currently in production, manual adjustment and repeated process tests are required to determine the appropriate focal position in order to obtain the desired depth of melt.

However, due to deformation of the workpiece, thermal lens effect or multi-dimensional spatial curves during the welding process, the focal position can change and may be out of the allowable range.

For the above two cases,

  • On the one hand, the use of high-quality, highly stable optical components, with frequent maintenance to prevent pollution and keep them clean.
  • On the other hand, it is required to develop real-time monitoring and control methods for the laser welding process in order to optimize parameters, monitor changes in the laser power and focal point position reaching the workpiece, and achieve closed-loop control to improve the reliability and stability of the laser welding quality.

Laser welding of steel materials

1. Laser welding of carbon steel and common alloy steel

In general, laser welding of carbon steel works well and its welding quality depends on the impurity content. As with other welding processes, sulfur and phosphorus are sensitive factors for weld cracking.

In order to obtain satisfactory weld quality, preheating is required for carbon contents above 0.25%.

When different carbon content of steel welded to each other, the torch can be slightly biased towards the low carbon material side to ensure the quality of the joint. Low-carbon boiling steel is not suitable for laser welding due to the high content of sulfur and phosphorus.

Low-carbon quelling steel is good for welding due to its low impurity content.

Medium and high carbon steels and common alloy steels can be laser welded well, but require preheating and post-weld treatment to relieve stress and avoid crack formation.

2. Laser welding of stainless steel

In general, laser welding of stainless steel is easier than conventional welding to obtain quality joints.

Due to the high welding speed heat affected zone is very small and sensitization does not become an important issue.

Compared to carbon steel, the low thermal conductivity of stainless steel makes it easier to obtain deep fusion narrow weld seams.

3、Laser welding between different metals

The extremely high cooling rate and the small heat-affected zone of laser welding create favorable conditions for the compatibility of materials with different structures after welding and melting of many different metals.

The following metals have been shown to be successfully laser deep-fusion welded:

  • Stainless Steel ~ Mild Steel
  • 416 stainless steel ~ 310 stainless steel
  • 347 stainless steel ~ HASTALLY nickel alloy
  • Nickel electrodes ~ cold forged steel
  • Bimetallic strips with different nickel content

Application of laser welding

1. Manufacturing applications

Tailored Bland Laser Welding technology is widely used in car manufacturing.

Japan has replaced flash butt welding with CO2 laser welding for the joining of rolled steel coils in the steel making industry.

In the study of ultra-thin plate welding, such as plate thickness of less than 100 microns of foil, can not fusion welding, but by having a special output power waveform of YAG laser welding has been successful, showing the broad future of laser welding.

In Japan, YAG laser welding has been successfully developed for the first time in the world for the repair of thin tubes of steam generators in nuclear reactors, and some companies have also carried out laser welding technology for gears.

2. Powder metallurgy field  

With the continuous development of science and technology, many industrial technologies on the special requirements of materials, the application of smelting and casting methods of manufacturing materials can no longer meet the needs.

Due to the special properties and manufacturing advantages of powder metallurgical materials, they are replacing the traditional smelting and casting materials in certain fields such as automobile, aircraft, tool and tool manufacturing.

With the increasing development of powder metallurgical materials, it is increasingly prominent with other parts of the connection problem, so that the application of powder metallurgical materials are limited.

In the early 1980s, laser welding with its unique advantages into the field of powder metallurgical materials processing, for the application of powder metallurgical materials opened up new prospects

Such as the use of powder metallurgical materials commonly used in the connection of brazing method of welding diamond, due to low bond strength, heat-affected zone is wide, especially can not adapt to high temperature and high strength requirements and cause the brazing material melting off, the use of laser welding can improve the welding strength and high-temperature resistance.

3. Automotive industry

In the late 1980s, kilowatt-class lasers were successfully applied to industrial production, and today laser welding lines have appeared on a large scale in the automotive manufacturing industry, becoming one of the outstanding achievements of the automotive industry.

Germany’s Audi, Mercedes-Benz, Volkswagen, Sweden’s Volvo and other European car manufacturers as early as the 1980s, the first to use laser welding roof, body, side frames and other sheet metal welding

In the 1990s, the United States General Motors, Ford and Chrysler companies have gone so far as to introduce laser welding into automotive manufacturing, although it started late, the development of rapid.

Italy’s Fiat used laser welding in the welded assembly of most steel sheet components.

Japan’s Nissan, Honda and Toyota Motor Company in the manufacture of body coverings are used in the laser welding and cutting process

Laser welded assemblies of high-strength steel are increasingly used in auto body manufacturing because of their excellent performance.

According to the U.S. metal market statistics, by the end of 2002, the consumption of laser-welded steel structures will reach 70,000 tons, a threefold increase from 1998.

According to the characteristics of the automotive industry batch, high degree of automation, laser welding equipment to high-power, multi-path type direction.

In terms of process,

Sandia National Laboratories in the United States and PrattWitney joint research in the laser welding process to add powdered metal and metal wire.

Germany Bremen Institute of Applied Beam Technology in the use of laser welding of aluminum alloy body skeleton has conducted a lot of research, that the addition of filler residual metal in the weld helps to eliminate thermal cracking, improve welding speed, solve the problem of tolerances, the development of the production line has been put into production in the Mercedes-Benz factory.

4. Electronics industry

Laser welding has been widely used in the electronics industry, especially in the microelectronics industry.

Due to the small heat-affected zone of laser welding, rapid heating concentration, low thermal stress, and therefore are integrated circuits and semiconductor device housing packaging, showing unique superiority.

Laser welding has also been used in the development of vacuum devices, such as molybdenum focusing pole with stainless steel support ring, fast heat cathode filament assembly, etc.

Sensors or temperature controllers in the elastic thin-walled corrugated sheet whose thickness is 0.05-0.1mm, the use of traditional welding methods are difficult to solve, TIG welding is easy to weld through, plasma stability is poor, there are many influencing factors, and the use of laser welding effect is very good, and is widely used.

5. Biomedical

Laser welding of biological tissues began in the 1970s, and the successful welding of fallopian tubes and blood vessels and the demonstrated superiority led more researchers to try welding various biological tissues and extended to welding of other tissues.

Research on laser welding of nerves at home and abroad has focused on laser wavelength, dose and its effect on functional recovery as well as the selection of laser welding materials.

Compared with traditional suturing methods, laser welding has the advantages of fast anastomosis, no foreign body reaction during the healing process, maintaining the mechanical properties of the welded area, and the growth of the repaired tissue according to its original biomechanical properties will be more widely used in future biomedicine.

6. Other areas

In other industries, laser welding is also gradually increasing, especially in special materials welding China has conducted many studies, such as laser welding of BT20 titanium alloy, HEl30 alloy, Li-ion batteries, etc. The German glass machinery manufacturer GlamacoCoswig has developed a new technology for laser welding of flat glass in cooperation with IFW Joining Technology and Materials Experimental Institute.

Laser welding machine vs. conventional welding machines

The first feeling of many entrepreneurs for laser welding machine is that a welding machine to be so expensive, the previous traditional welding machine (argon arc welding) only cost about 10,000RMB.

In fact, not, because the bosses have not yet understood the advantages of laser welding machine.

I believe that each company is dependent on the strength of survival by strong growth, factory production is to give people to buy (customers), and customers are smart to compare, like a piece of jade, the same place of production.

Some prices a few hundred, some up to hundreds of thousands.

This reflects the beauty, fine, rare advantages.

And currently people are increasingly picky, the pursuit of beauty, fine.

So the factory should produce products with enough beauty and delicacy to improve competitiveness.

Increase the market digestion! So the traditional process and manufacturing machines must be replaced with advanced technology equipment to produce new and sophisticated products, so that the company goes farther and higher!

Okay, so let’s say that the laser welding machine is not a product that can enhance the profits and sales.

And what are the advantages compared to the previous equipment process.

Laser welding machine

Laser welding machines have the following advantages compared to other conventional welding machines.

  1. The laser welding machine is classified as non-touch processing, and will not damage the processed parts during the welding process. The welding speed is fast, the welding strength is high, the weld seam is flat, and the deformation is small. It can be welded under special conditions (such as closed spaces). machine.
  2. The laser welding machine can weld special materials such as refractory materials of high melting point metals, and can even be used for welding machines of non-metal materials such as ceramics. It has good effect on welding special materials and has great flexibility. The non-touch long-distance welding machine is applied to the parts that are difficult to access by the welding machine.
  3. The laser beam can be focused to obtain a small spot. Because it is not affected by the magnetic field and can be accurately positioned, it can be used for micro welding machine, which is suitable for mass welding of micro and small workpieces produced automatically.

Laser welding machines Laser welding machines

Laser welding machines

Conclusion

Laser welding is a combination of modern technology and traditional technology. Compared with traditional welding technology, laser welding is particularly unique and its own application fields and application levels are more extensive, which can greatly improve the efficiency and accuracy of welding.

Its power density is high and energy is released quickly, thereby better-improving work efficiency.

At the same time, its own focal point is smaller, which undoubtedly makes the adhesion between the stitched materials better, and will not cause material damage and deformation, so there is no need for subsequent processing after welding.

As a result, it itself is mainly used in high-tech fields, and in the future as people’s understanding and mastery of this technology continue to deepen, it will inevitably be applied to more industries and fields.

It can be said that the emergence of laser welding technology has realized the application fields that traditional welding technology cannot.

It can easily achieve various welding requirements of different materials, metals and non-metals, and because of the penetrability and refraction of the laser itself, it can achieve random focus within 360 degrees according to the trajectory of the speed of light itself. This is undoubtedly unimaginable under the development of traditional welding technology.

In addition, because laser welding can release a large amount of heat in a short period of time to achieve rapid welding, it has lower environmental requirements and can be carried out under general room temperature conditions without the need to be in a vacuum environment or gas protection.

After decades of development, people have the highest degree of understanding and recognition of laser technology, and it has gradually expanded from the initial military field to the modern civilian field, and the emergence of laser welding technology has further expanded the application range of laser technology.

In the future, laser welding technology can not only be used in fields such as automobiles, steel, and instrument manufacturing, but it will inevitably be applied in military, medical, and other fields, especially in the medical field, with the help of its own high heat and high temperature. The characteristics of integration and hygiene are better applied in clinical diagnosis and treatment such as neuromedicine and reproductive medicine.

And its own precision advantages will also be applied in more precision instrument manufacturing industries, thereby continuously benefiting the development of mankind and society.

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