8 Basic Theoretical Knowledge of Laser Welding

1. Characteristics of laser welding


(1) Small processing range can control energy input: small thermal stress, small heat affected zone, and small thermal deformation.

(2) Narrow and smooth weld: post weld treatment process can be reduced or eliminated.

(3) The cooling speed is fast and the weld structure is fine, so the performance of the welded joint is good.

(4) High processing speed and short working cycle

(5) Micro welding or long-distance transmission can be realized without vacuum device, which is conducive to mass automatic production.

(6) Easy function integration: laser welding is easy to cooperate with other processing means, such as bending, punching, assembly, etc., and it is easy to realize automatic production

(7) The production process is easy to control: the sensor system monitors the processing in real time to ensure the welding quality

(8) Laser welding does not need to contact the workpiece to avoid contact stress


Although laser welding is a new welding method with good prospects, it also has its own limitations.

(1) The welding thickness is limited, which is suitable for thin material welding.

(2) The clamping requirements of the workpiece are relatively high, and the clearance shall be as small as possible. And the cost of precision welding fixture is relatively high.

(3) The positioning should be very accurate, and the requirements for programming are relatively high.

(4) The weldability of high reflective and high thermal conductivity materials such as aluminum, copper and their alloys will be affected;

(5) Rapid solidification of the weld is not conducive to gas discharge, and there may be pores and embrittlement;

(6) The equipment is expensive. For small batch production or production with complex positioning and complex processes, the cost performance is not high.

2. Classification of laser welding

Laser penetration

Laser deep penetration welding requires that the beam has a high energy density, which is generally greater than 10 kW/mm2.

In this case, the laser not only melts the metal but also leads to a certain metal vapor.

The pressure of the generated metal vapor in the molten pool disappears and continues to take the position of liquid metal.

At the same time, the metal is melting and the metal vapor continues to decline, finally forming a narrow and fine metal vapor hole.

The pool hole is surrounded by liquid molten metal.

The pool hole moves forward with the laser beam, and the liquid metal continues to solidify behind the key hole to form a weld.

Laser penetration

1. Key hole
2. Molten metal
3. Welds
4. Laser beam
5. Welding direction
6. Metal steam
7. Workpiece

laser weld

The laser weld is narrow and thin, and the depth width ratio can even reach 10:1

3. Laser heat conduction welding (edging welding)

The laser beam is along the edge of the material, and the molten materials fuse with each other and solidify to form a weld.

The weld depth ranges from close to zero to one millimeter. The thickness of the material will not exceed 3mm, usually less than 2mm.

Laser heat conduction welding

1. Molten material
2. Weld
3. Laser beam
4. Welding direction
5. Workpiece

Solid-state laser heat conduction welding is mainly used for thin plate corner welding, such as battery shell, pacemaker shell and some machine tool covers.

Heat conduction welding can produce a smooth and clean fillet weld without subsequent processing.

Heat conduction welding

4. Form of the welding head

Butt welding

Butt welding



Overlap welding

Overlap welding

Fillet welding

Fillet welding

Crimping welding

Crimping welding

5. Laser welding specifications

  • Laser power
  • Fiber core diameter
  • Collimation and focal length of welded joint
  • welding speed
  • Focal depth
  • Shielding gas
  • Absorption value of the material (reflectivity of material)

6. Materials suitable for laser welding

(1). Carbon steel and ordinary alloy steel

Generally speaking, the laser welding effect of carbon steel is good, and its welding quality depends on the impurity content.

Sulfur and phosphorus in materials are sensitive factors for welding cracks. If the content of sulfur and phosphorus is high, it is not suitable for laser welding.

Medium and high carbon steels and ordinary alloy steels can be well laser welded, but preheating and post weld treatment are required to eliminate stress and avoid crack formation.

(2). Laser welding of stainless steel

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

Stainless steel with low thermal conductivity is easier to obtain deep penetration narrow weld.

Stainless steel is mainly divided into ferritic stainless steel (joint embrittlement), austenitic stainless steel (hot crack), martensitic stainless steel (poor weldability) and duplex stainless steel (easy embrittlement in welding affected zone).

(3). Laser welding of aluminum alloy

Due to the high reflectivity and high thermal conductivity of aluminum alloy surface, laser welding of aluminum alloy is difficult.

For laser welding of high reaction materials, the performance of the energy threshold is more obvious.

Different series of aluminum alloys, different grades, welding properties are also different.

Aluminum alloy welding difficulties:

Strong oxidation capacity of aluminum: aluminum is easily oxidized in the air and during welding. The generated alumina has a high melting point and is very stable.

It is not easy to remove the oxide film, has a large proportion, is not easy to float out of the surface, and is easy to produce slag inclusion, incomplete fusion, incomplete penetration and other defects.

The oxide film on the surface of the aluminum and adsorb a large amount of water, which is easy to produce pores in the weld.

There are high requirements for the cleanliness of the workpiece.

Larger thermal conductivity and specific heat capacity.

Energy with concentrated energy and high power should be used as much as possible, and process measures such as preheating can also be used sometimes.

Generally, the required laser power is relatively large.

Large linear expansion coefficient, large volume shrinkage during aluminum solidification, large deformation and stress of weldment, and shrinkage cavity, shrinkage porosity, thermal crack and high internal stress are easy to occur during aluminum welding pool solidification.

Aluminum has a strong reflection ability to light and heat.

There is no obvious color change during solid and liquid state transformation, so it is difficult to judge during the welding operation.

The strength of high-temperature aluminum is very low, it is difficult to support the molten pool and easy to weld through.

The laser used for welding must be resistant to high reflection.

It is easy to form pores. Aluminum and aluminum alloys can dissolve a large amount of hydrogen in liquid state and almost no hydrogen in solid state.

In the process of solidification and rapid cooling of the welding pool, hydrogen can not overflow in time, which is very easy to form hydrogen pores.

There are high requirements for the cleanliness of the workpiece, including the drying of the workpiece and its surrounding environment.

Alloy elements are easy to evaporate and burn, resulting in the decline of weld performance.

(4). Welding of copper alloy

It is similar to the welding of aluminum alloy, but the mirror copper has a stronger reflection performance.

The most commonly used brands in the industry are T1, T2 and T3, which are purple in appearance, so they are also called red copper.

Easily generated welding defects:

  • Incomplete fusion and incomplete penetration (high energy density laser beam)
  • Welding deformation
  • Thermal crack (in the crystallization process, copper and copper alloys have obvious thermal brittleness due to the distribution of low melting point eutectic between dendrites or grain boundaries. In addition, thermal crack is very easy to occur due to the effect of welding stress).
  • Pores (pores in red copper welds are mainly hydrogen pores).

7. Welding shielding gas

Use welding shielding gas to protect the welding effect:

Some welding applications require the use of welding shielding gas, which forms a thin gas protective layer on the weld surface to prevent the influence of surrounding air on the weld.

This protective gas layer is mainly used to prevent molten metal from reacting with oxygen, water vapor or carbon dioxide in the air.

The commonly used shielding gas mainly includes helium, argon, nitrogen or mixed gas. What kind of gas to use is usually determined by the type of welding material.

The welding protective gas is sent to the weld surface through the protective gas pipe, or through the setting of the fixture itself.

Generally, for the welding of three-dimensional parts, if protective gas protection is required, it will increase the obstacle of robot movement.

8. Welding requirements

Before welding, it is required to clarify the welding requirements, generally including the strength of the weld (penetration requirements, pore control requirements, crack control requirements, etc.), appearance (flatness of the weld, oxidation degree, depth width ratio, etc.) and air tightness (bearing air pressure).

(1). Weld seam

Quality inspection:

The purpose of any quality inspection is to confirm whether the performance of the workpiece can meet the requirements of use.

For welding, the quality standard of laser welding mainly focuses on the weld and welding heat affected zone.

(2). Basic requirements:

Welds need to meet the following two recent quality requirements:

  • The weld width and depth must meet the requirements of welding specifications and welding strength.
  • Weld crystal image quality: the internal structure of the weld shall be as uniform as possible, and the grains shall be fine and uniform.

The welding procedure specification also contains some other welding standards and welding defects.

The following figure shows different welding defects:

Quality defects of welds

Quality defects of welds

(3). Weld internal defects:

Typical weld internal defects:

Incomplete fusion: excessive weld gap

Air hole: a small amount of air or bubbles mixed into the weld; crack: on the surface or inside of the weld

Quality defects in lap joints

Quality defects in lap joints

(4). External welding defects:

Uneven weld shape: for example, microcracks caused by weld collapse

Molten metal splash: pits are formed on the weld surface due to the “explosion” of molten metal, which reduces the weld strength and even forms pores

Weld surface collapse and weld bottom depression: reduce the effective stress area of the weld and reduce the weld strength

Misalignment: in butt welds, misalignment results in a reduction in the effective weld area

Arc crater: reduce the effective stress area of the weld

Oxidation: reduce the oxidation resistance of stainless steel

Splash: splash objects fall on the weld or workpiece surface, reduce the surface quality and increase the follow-up treatment

Welding deformation caused by heat input: In the welding process, the heat inside the weld will be transmitted to the workpiece around the weld, resulting in a small amount of deformation. If a workpiece needs to have a large number of welds, the welding sequence shall be reasonably selected.

How to Choose Laser Welding Machine

For carbon steel and stainless steel:

  • 3mm plate, at the speed of 2m/min, at least 2kW is required;
  • 4mm plate, at the speed of 2m/min, at least 3KW is required;
  • 5mm plate, at a speed of 2m/min, requires a minimum of 4kw.

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