Fundamentos da soldagem a laser: 8 conceitos essenciais

Laser Welding Fundamentals: 8 Essential Concepts

April 30, 2024 Roberto Magalhães

1. Features of laser welding

Advantage

(1) A small processing range can provide better control over energy input, leading to reduced thermal stress, a smaller heat-affected zone, and less thermal deformation.

(2) Narrow, smooth welds require fewer or no post-weld treatment processes.

(3) The fast cooling speed and fine structure of the weld result in excellent performance of the welded joint.

(4) The process has high processing speed and short working cycle.

(5) Microwelding and long-distance transmission can be performed without using a vacuum device, making it ideal for automatic mass production.

(6) Laser welding is easy to integrate with other processing methods such as bending, punching and assembly, and is suitable for automatic production.

(7) The production process is easily controlled as the sensor system monitors the process in real time to ensure welding quality.

(8) Laser welding does not require contact with the workpiece, thus avoiding any contact voltage.

Disadvantage

Although laser welding has many advantages and is a promising welding method, it also has certain limitations.

(1) The welding thickness is limited and is mainly suitable for thin materials.

(2) The workpiece must be clamped with high precision and the clearance must be kept to a minimum. This often requires precision welding fixtures, which can be relatively expensive.

(3) Accurate positioning is critical and programming requirements are relatively high.

(4) Welding materials with high reflectivity and high thermal conductivity, such as aluminum and copper alloys, can be challenging.

(5) Rapid solidification of the weld can lead to gas entrapment and result in porosity and brittleness.

(6) The equipment is expensive and, for small batch production or production with complex positioning and processes, the cost-benefit ratio may not be ideal.

2. Classification of laser welding

I asher penetration

Deep penetration laser welding requires the laser beam to have a high energy density, typically greater than 10 kW/mm ² . This results not only in the melting of the metal, but also in the formation of metallic vapor.

The pressure created by the metallic vapor generated in the weld pool causes it to displace the liquid metal. As the metal continues to melt and the metal vapor decreases, a narrow, thin hole of metal vapor is formed.

The hole is surrounded by liquid molten metal, and as the laser beam advances, the hole moves with it. The liquid metal behind the hole continues to solidify, forming the weld.

1. Keyhole
2. Molten metal
3. Welds
4. Laser beam
5. Welding direction
6. Metallic vapor
7. Workpiece

Laser welding is characterized by its narrow and thin shape, and its depth/width ratio can reach up to 10:1.

3. Laser heat conduction welding (edge welding)

The laser beam is directed along the edge of the material, causing the molten material to fuse and solidify, forming a weld. The depth of the weld can vary from close to zero to one millimeter, and the thickness of the material normally does not exceed 3mm, and is generally less than 2mm.

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

Solid-state laser heat conduction welding is mainly used to weld the corners of thin plates such as battery casings, pacemaker casings and some machine tool covers. This welding method results in a smooth, clean fillet weld that does not require any additional processing.

4. Welding head shape

Butt welding

Lap

Overlay welding

Fillet welding

Crimp welding

5. Laser welding specifications

Laser power
Fiber core diameter
Collimation and focal distance of the welded joint
welding speed
Focal depth
Shielding gas
Material absorption value (material reflectivity)

6. Materials suitable for laser welding

(1). Carbon steel and common alloy steel

In general, carbon steel is suitable for laser welding and the quality of the weld depends on the level of impurities present. High levels of sulfur and phosphorus can cause welding to crack, making laser welding unsuitable for materials with high levels of these elements.

Both medium and high carbon steels and common alloy steels can be effectively laser welded; however, preheating and post-weld treatment are necessary to relieve stress and prevent crack formation.

(two). Stainless Steel Laser Welding

In general, laser welding of stainless steel is easier to produce high-quality joints compared to conventional welding methods.

Stainless steel with low thermal conductivity is more conducive to achieving deep and narrow weld penetration.

Stainless steel can be divided into four main categories: ferritic stainless steel (which can result in brittle joints), austenitic stainless steel (prone to hot cracking), martensitic stainless steel (known for its poor weldability) and duplex stainless steel ( which may be prone to embrittlement in the area affected by welding).

(3). Aluminum alloy laser welding

The high reflectivity and thermal conductivity of aluminum alloy surfaces make laser welding difficult.

For laser welding of highly reactive materials, the energy limit performance becomes more pronounced.

The welding properties of different series and grades of aluminum alloys vary.

Aluminum alloy welding difficulties:

Aluminum has a strong oxidation capacity and is prone to oxidation in air and during welding. The resulting alumina has a high melting point and is highly stable.

Removing the oxide film is challenging and has a significant proportion, making it difficult to separate from the surface. This can result in defects such as slag inclusion, incomplete melting and incomplete penetration.

The oxide film on the surface of aluminum can also adsorb a significant amount of water, leading to the formation of pores in the weld.

There are high requirements for the cleanliness of the workpiece.

Aluminum has higher thermal conductivity and specific heat capacity.

To weld aluminum effectively, it is recommended to use energy sources with high concentration and power. Additionally, preheating can sometimes be used as a process measure.

Generally, the required laser power is relatively large.

Aluminum has a large coefficient of linear expansion and undergoes significant volume contraction during solidification, leading to high deformation and stress when welding. This can result in shrinkage cavities, shrinkage porosity, thermal cracks and high internal stress.

Aluminum has a strong ability to reflect light and heat.

There is no noticeable change in color during the solid-liquid state transformation, making it difficult to judge during the welding process.

High-temperature aluminum has low strength and has difficulty withstanding the weld pool, making it prone to welding.

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

Pore formation is common when welding aluminum. Aluminum and its alloys can dissolve a significant amount of hydrogen in the liquid state, but almost none in the solid state.

During the solidification and rapid cooling of the welding pool, hydrogen cannot escape in time, leading to the formation of hydrogen pores.

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

Evaporation and burning of alloying elements during welding can result in a decrease in weld performance.

(4). Copper alloy welding

The welding process of mirror copper is similar to that of aluminum alloy, but mirror copper has stronger reflection ability.

The most commonly used types in industry are T1, T2 and T3, which have a distinct purple appearance and are therefore also known as red copper.

Easily generated welding defects:

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

7. Welding shielding gas

Use welding shielding gas to protect the welding effect:

Some welding processes require the use of a welding shielding gas to form a thin protective layer over the weld surface. This layer helps prevent the surrounding air from affecting the weld.

The main purpose of shielding gas is to prevent molten metal from reacting with oxygen in the air, water vapor, or carbon dioxide.

Common shielding gases include helium, argon, nitrogen, or a mixture of gases. The type of gas used is normally determined by the material being welded.

Shielding gas is supplied to the weld surface through a shielding gas tube or through the fixture itself.

Note that the use of shielding gas for welding three-dimensional parts may increase the difficulty of robot movement.

8. Welding Requirements

Before welding, it is essential to clearly define welding specifications, which typically cover weld strength (such as penetration requirements, pore control requirements, crack control requirements, etc.), appearance (including weld flatness, level oxidation, depth-width ratio, etc.) and air tightness (withstanding air pressure).

(1). Weld seam

Quality inspection:

The purpose of any quality inspection is to verify that the part's performance meets usage requirements.

For welding, the quality standard for laser welding mainly focuses on the weld and the heat-affected zone produced during welding.

(two). Basic requirements:

Welds must meet the following two recent quality requirements:

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

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

The following figure shows different welding defects:

Weld quality defects

(3). Weld internal defects:

Typical internal welding defects:

Incomplete fusion: excessive gap in the weld

Air hole: small amount of air or bubbles mixed in the weld; crack: on the surface or within the weld

Quality defects in overlapping joints

(4). External welding defects:

Irregular weld shape: e.g. microcracks caused by weld collapse

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

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

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

Spatter: spatter from objects falls on the surface of the weld or workpiece, reduces surface quality and increases 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 deformation. If a part needs to have a large number of welds, the welding sequence should be reasonably selected.