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What is laser radiation?

What is laser light?

Laser Light: It travels at 299.792.458 meters per sec. Its visible spectrum ranges from 400 to 700 nanometers. Its smallest unit is a packet of energy, a photon. It is light, and its use is growing in tube and pipe production and fabrication facilities throughout the world.
In most of its natural and artificial forms, light has little power. However, a groundbreaking invention in the latter half of the 1950s increased its power and concentrated it in a small area.
Thus was born a modern and revolutionary concept: Light Amplification by Stimulated Emission of Radiation, or laser.

Fiber Laser Welding

Fiber laser welding, like other laser-machining methods, is a non-contact technology that has a limited, heat-affected zone (HAZ), which is why the technology is a preferred method for welding delicate products at high speeds. Laser welding is also a more repeatable and consistent process than other welding methods, and is capable of producing high-strength bonds without the need for filler material, flux, prepping, or secondary cleaning and finishing processes. Fiber Laser Welding has enabled many applications, such as energy storage with lithium ion batteries and implantable medical devices, to be manufactured in extreme scales, at much lower costs, with greater consistency, at greater speeds, and with much less waste and quality-control issues. Moreover, a laser welding manufacturing process is much more reliable than other welding technologies, as the latest laser-welding machines require little to no maintenance, and virtually no downtime.

The laser as a heating source

Laser welding requires that the laser raise the temperature of the material to be welded. The fiber laser light must be absorbed by the material to induce a temperature rise. In effect, the fiber laser light beam is focused onto the material similar to the way the sun can be focused by a magnifying glass. The difference is that the laser’s power density is many orders of magnitude higher. 

Laser light photons, packets of light energy that make up the laser, impinge onto the material and are partly or wholly absorbed. The energy of the photon is absorbed in the metal material and causes a heat waves within the metal. Repeated absorption of photons eventually leads to metal surface breakup and melting.

Even for metals that absorb well, such as steel, the laser is initially reflected. A small percentage of the laser is absorbed, heating the metal surface. The increased surface temperature increases the absorption of the fiber laser light photons. This creates a snowball effect, in which the material is rapidly heated by the laser, leading to melting and formation of the weld.
 

Fiber Laser Welding

Unlimited possibilities

Laser Welding is a welding technology used to join several metal components. A laser produces a beam of high-intensity that is concentrated into one spot. This concentrated heat source enables fine, deep welding and high welding speeds.  
Traditional laser welding technologies, such as continuous-wave CO2 welding lasers are limited in terms of accuracy and undesired, high heat input into the weld and the traditional pulsed Nd:YAG welding lasers are limited by the maximum welding speed, the minimal spot size that can be achieved and the electrical to optical energy conversion efficiency that is very bad. 
With fiber laser welding, the output power and the oscillation form of the laser beam is possible to change.  Fiber Laser Welding is also very suitable for welding materials with a high melting point or with high thermal conductivity due to a very low thermal effect during welding.  The energy conversion rate is very high and all this makes fiber laser highly adaptable to various applications for use in various welding assembly processes. 
The fiber laser beam used for welding can be adapted as follows and characterized by different laser oscillation modes :

 

 Pulsed laser beam welding (ideal for spot welding) 

and 

Continuous laser beam welding (ideal for seam welding)

 
Even more applications are demanding a higher precision control, lower heat input and lower electrical energy consumption.

Fiber Laser Welding is a technology that offers ALL those features.

A weld can be created either as an individual spot or a seam weld.

The laser output that creates these welds can be achieved in one of two ways:

A pulsed laser produces a series of pulses, discrete packets of energy, at a certain pulse width and frequency until stopped. The “pulsed” descriptor refers to a laser that can produce a peak power that is greater than its average power.

A continuous wave (CW) laser produces extended output – the laser remains on continuously until stopped.

For example, a 25 W pulsed Nd:YAG laser can produce peak powers of up to 5 kW for a few milliseconds. This means it can produce a spot weld that would require a CW laser sized at 5 kW! With pulsed lasers a seam weld is created by a series of overlapping spot welds. For a continuous weld the laser remains on for the duration of the seam. A CW laser can also produce discrete pulses of laser light – known as gated or modulated output. In this case the CW laser peak power does not exceed the laser’s rated average power.

An Nd-YAG laser operates only in pulsed mode
 Diode lasers operate in continuous wave 
Fiber lasers can operate in both modes. 


Choosing when to use pulsed, continuous wave, or modulated output is determined by the specific application. Spot welding typically uses pulsed operation. For seam welding, the selection is made based on heat input and cycle time. For instance, when seam welding an implantable device, a pulsed laser is used to minimize heat input and maintain a uniform weld around a complex geometry with varying welding speeds. In contrast, for airbag initiators, welding at high speed using CW operation is
 

Laser welding does not operate on the same principles as other types of welding like TIG, MIG, MAG,..

Laser welding uses a beam of light, instead of electricity, to join two pieces of metal together through a melting and cooling process.  

Another key difference with laser welding is the intensity and ability to focus the heat source: the laser. The much higher, focused heat than, say, the electricity of a MIG welder or TIG welder, means that the weld occurs much more quickly. What’s more, the ability to narrowly pinpoint the weld area leads to much greater precision and more accurate and attractive weld joints.

What does this mean for you?

  • Higher speeds: till 10x faster than MIG welding, and till 40x faster than TIG welding.
  • Minimal/no finishing: The accuracy of the laser welding process means that little to no grinding or finishing is needed.
  • Visually superior: Laser welding is ideal for straight line joints in furniture and other consumer products, since there is a much smaller heat-affected zone and a much tighter weld.
  • Greater strength: A smaller heat-affected zone also means less weakening of the material.

So when wouldn’t you want to use laser welding? Thicker materials and parts where the weld joint construction does not allow the fit-up to be consistently maintained generally are not good candidates for laser welding.

Laser welding can be divided into the following processes:

Heat conduction welding and Keyhole (deep penetration) welding

Fiber laser welding is a high power density process that provides a unique welding capability to maximize penetration with minimal heat input. The weld is formed as the intense laser light rapidly heats the material – typically in fractions of milliseconds. There are two types of welds, based on the power density contained within the focus spot size: conduction modeand penetration/keyhole mode. A third type named as transition keyhole mode is the combination of the conduction mode and penetration/keyhole mode. 

Conduction mode laser welding

Conduction mode welding is performed at low energy density, typically around 0.5 MW/cm², forming a weld nugget that is shallow and wide. The heat to create the weld into the material occurs by conduction from the surface. Typically this can be used for applications that require an aesthetic weld or when particulates are a concern, such as certain battery sealing applications.

Heat conduction welding is a laser welding method that features a low power output laser beam. This makes for a penetration depth of no more than 1 to 2 mm. With the ability to handle a relatively wide power range, heat conduction welding can be adjusted to the ideal power level, and the shallow penetration makes it possible to weld materials that are susceptible to heat effects under optimal conditions.
This welding type is used for butt joints, lap joints, and other welding applications for thin plates, and can also be used for welding hermetic seals and other seals. Heat conduction welding is also suitable for volatile alloys such as magnesium and zinc, for which keyhole (deep penetration) welding is not suitable.

Transition mode laser welding

Transition mode laser welding occurs at medium power density, around 1 MW/cm2, and results in more penetration than conduction mode due to the creation of what is known as the “keyhole.” The keyhole is a column of vaporized metal that extends into the material; its diameter is much smaller than the weld width and is sustained against the forces of the surrounding molten material by vapor pressure. The depth of the keyhole into the material is controlled by power density and time. Because the optical density of the keyhole is low it acts as a conduit to deliver the laser power into the material.
 

Keyhole mode laser welding or penetration mode laser welding

Keyhole or penetration mode welding – Increasing the peak power density beyond 1.5MW/cm2 shifts the weld to keyhole mode, which is characterized by deep narrow welds with an aspect ratio greater than 1.5. The penetration depth rapidly increases when the peak power density is beyond 1 MW/cm2, transitioning the weld mode from conduction to keyhole/penetration welding. 
Penetration or keyhole mode welding is characterized by narrow welds. This direct delivery of laser power into the material maximizes weld depth and minimizes the heat into the material, reducing the heat affected zone and part distortion. In this keyhole mode, the weld can be either completed at very high speeds – in excess of 500mm per second with small penetration typically under 0.5 mm – or at lower speed, with deep penetration up to 12 mm.

Keyhole welding (deep penetration welding) uses a high power output laser beam for high-speed welding. The narrow, deep penetration allows for uniform welding of internal structures. Because the heat-affected zone is very small, distortion of the base material, due to the heat from the welding, will be minimized.
This method is suitable for applications requiring deep penetration or when welding multiple base materials stacked together (including for butts, corners, Ts, laps, and flange joints).

Laser Welding Process Parameters

The key to success

Power density

Power density is one of the most critical parameters in laser processing.  With a higher power density, the surface layer can be heated to the boiling point in the microsecond time range, resulting in a large amount of vaporization.

Therefore, high power density is advantageous for material removal processing such as punching, cutting, and engraving. For lower power densities, the surface temperature reaches the boiling point and takes several milliseconds.  Before the surface layer is vaporized, the bottom layer reaches the melting point, and it is easy to form a good fusion weld. 

Therefore, in conduction laser welding, the power density is in the range of 10^4~10^6W/CM^2.

Laser pulse waveform

Laser pulse waveforms are an important issue in laser welding, especially for sheet welding.  When a high-intensity laser beam is incident on the surface of the material, the metal surface will be reflected by 60 to 98% of the laser energy and the reflectivity will vary with the surface temperature.  During a laser pulse action, the metal reflectivity changes greatly.

Laser pulse width

Pulse width is one of the important parameters of pulse laser welding.  It is an important parameter that is different from material removal and material melting, and is also a key parameter that determines the cost and volume of processing equipment.

The effect of the defocus amount on the weld quality

Laser welding usually requires a certain amount of defocus because the power density at the center of the spot at the laser focus is too high and it is easy to evaporate into holes.  The power density distribution is relatively uniform across the planes exiting the laser focus.  

There are two ways of defocusing: positive defocusing and negative defocusing.

The focal plane is located above the workpiece for positive defocusing, and vice versa for negative defocus.  According to the theory of geometric optics, when the distance between the positive and negative defocus planes and the welding plane are equal, the power density on the corresponding plane is approximately the same, but the shape of the molten pool obtained is actually different.  In the case of negative defocusing, a greater penetration can be obtained, which is related to the formation of the molten pool.  

Experiments have shown that the laser heating 50~200us material begins to melt, forming liquid phase metal and partially vaporizing, forming high pressure steam, and spraying at a very high speed, emitting dazzling white light.  At the same time, the high concentration vapor moves the liquid metal to the edge of the bath and forms a depression in the center of the bath.

When negative defocusing, the internal power density of the material is higher than the surface, and it is easy to form a stronger melting and vaporization, so that the light energy is transmitted to the deeper part of the material.  Therefore, in practical applications, when the penetration depth is required to be large, negative defocusing is used; when welding thin materials, positive defocusing is preferred.

Welding speed

The speed of the welding speed will affect the heat input per unit time.  If the welding speed is too slow, the heat input is too large, causing the workpiece to burn through.  If the welding speed is too fast, the heat input amount is too small, causing the workpiece can’t be welded well.

Industrial Laser Welding Systems

Industrial laser welding has had a dramatic impact in the Machine Tool Industry over the years and has become the answer to many problems brought about by traditional welding methods. Many metalworking companies have turned to laser spot and laser seam welding due to these advantages. Automation, high welding speeds and contact-free processing help laser welding users achieve economic advantages very quickly. 

Since laser welding produces pure, concentrated energy, it creates deeper welds while leaving high production output. This is one of the many reasons laser spot and seam welding systems are used in manufacturing in this industry. Items we use every day in our gadgets, electronics and parts have gone through laser welding technology in the course of their production.

Laser Spot Welding & Laser Seam Welding

Laser spot welding, laser seam welding, and direct welding are highly useful and slightly differing applications of laser welding technology. Laser spot and seam welding refer to welding functions applied to a single point or along a line. By setting a laser welding system to a high speed and extremely narrow weld geometry, the laser welder can produce extremely fine spot welding. Also, you can adjust the system to weld on continuous wave mode, welding with several kilowatts of power. The ideal laser speed for laser spot or seam welding projects varies according to the particular laser model, the laser power setting, and the material being laser welded.