Laser Welding Process: A Comprehensive Guide

The laser welding process includes laser heat conduction welding, laser deep fusion welding, laser welding with filler material, laser spot welding, pulsed laser welding, laser brazing, and laser-arc hybrid welding.

Based on the different ways of laser output energy, laser welding can be divided into pulsed laser welding and continuous laser welding (including high-frequency pulsed continuous laser welding).

According to the different power densities on the focused spot after laser focusing, laser welding can be divided into heat conduction welding (power density less than 105 W/cm2) and deep fusion welding (power density greater than or equal to 105 W/cm2).

Based on the joint form, deep fusion welding is further divided into butt welding, fillet welding, end welding, and overlap welding, among others. Laser brazing is suitable for connecting the car body roof with the left and right side panels, connecting the upper and lower parts of the luggage compartment, and connecting the water channel.

It belongs to laser heat conduction welding, requires filler material, and can achieve a smooth weld surface with high sealing properties. The schematic diagrams of heat conduction welding and deep fusion welding are shown in Figure 2-4.

Figure 2-4: Schematic Diagram of Thermal Conduction Welding and Deep Melting Welding
Figure 2-4: Schematic Diagram of Thermal Conduction Welding and Deep Melting Welding

Laser Heat Conduction Welding

(1) Principle of Laser Heat Conduction Welding

When the laser converges at a point, it generates a very high temperature (related to the energy density). When the temperature reaches 1490°C, steel will melt. The welding method that uses this heat effect is called heat conduction welding. The process is as follows: first, the laser heats the surface of the workpiece to the melting point.

After the metal melts, it forms a hemispherical molten pool. The radius and depth of the molten pool gradually increase. When the absorbed laser energy and the heat diffusing from the molten pool reach a balance, the molten pool stops expanding. By moving the laser beam along a predetermined path, the molten pool also moves, continuously melting the metal in front of it while the metal behind it cools, thus forming a weld seam.

The advantages of heat conduction welding are reflected in smooth weld seams with minimal spattering, a speed of 1 to 3 m/min, and a weld depth-to-width ratio of less than 1.

During heat conduction welding, the laser heats the metal surface to between the melting point and the boiling point. The absorbed laser energy is converted into heat energy on the metal surface, raising the temperature and causing melting. The heat is then conducted into the interior of the metal, gradually enlarging the molten area. After solidification, this forms a weld spot or weld seam with a wide and shallow fusion contour.

The characteristic of heat conduction welding is that the power density of the laser spot is small, with some of the light being reflected by the metal surface and a low light absorption rate, resulting in shallow weld depth and slow welding speed. Heat conduction welding is mainly used for welding and processing thin plates (less than 1mm thick) and small workpieces.

Based on these characteristics, this welding method is commonly used for flat panel assembly in automobile production.

(2) Applications of Laser Heat Conduction Welding

Taking a laser marking machine as an example, its equipment consists of:

1) Main control unit: for editing marking graphics and setting marking parameters.

2) Laser power supply: provides energy to the pump source.

3) Cooling system: dissipates the heat generated by the laser during operation, ensuring stable operation of the laser.

4) Worktable: used to locate the laser focus and place the object to be marked.

5) Optical system: the core component.

6) Core component: optical system (main beam).

The structure of a solid marking machine is shown in Figure 2-5.

Figure 2-5: Structure of the solid marking machine.
Figure 2-5: Structure of the solid marking machine.

Laser Deep Fusion Welding

(1) Metallurgical Process and Process Theory

The metallurgical and physical process of laser deep fusion welding is very similar to electron beam welding, in which the energy conversion and coupling mechanism are mainly achieved through the “keyhole” structure. When the material is irradiated with a sufficiently high power density beam, it produces vapor to form a keyhole.

This vapor-filled keyhole acts like a black body, absorbing almost all the energy of the incident light, and the equilibrium temperature inside the keyhole reaches about 25,000°C. Heat is transferred from the outer wall of this high-temperature keyhole, causing the surrounding metal to melt. The schematic of laser deep fusion welding is shown in Figure 2-6.

Figure 2-6: Laser Deep Fusion Welding Illustration

The keyhole is filled with high-temperature vapor continuously evaporated from the wall material under the irradiation of the beam. The molten metal surrounds the walls of the keyhole, with liquid metal surrounded by solid material. The flow of liquid on the outer wall and the surface tension of the wall layer maintain a dynamic equilibrium with the continuous vapor pressure inside the keyhole.

The beam continuously enters the keyhole, while the material outside the keyhole flows continuously. As the beam moves, the keyhole remains in a stable state of flow. In other words, the keyhole and the molten metal surrounding the walls move forward with the advancing beam, filling the void left after the keyhole moves and then solidifying to form the weld seam.

(2) Factors Affecting Laser Deep Fusion Welding

Factors affecting laser deep fusion welding include: laser power, laser beam diameter, material absorption rate, welding speed, shielding gas, lens focal length, focal point position, laser beam position, and the gradual increase and decrease of laser power at the start and end points of welding to control the appearance of the weld and suppress spatter.

(3) Characteristics of Laser Deep Fusion Welding

1) High depth-to-width ratio. Because the molten metal surrounds the deep fusion keyhole and extends towards the workpiece, the weld becomes deep and narrow.

2) Minimal heat input. Due to the high source cavity temperature, the melting process occurs very rapidly, resulting in very low heat input to the workpiece, minimal thermal deformation, and a small heat-affected zone.

3) High density. The vapor-filled keyhole facilitates mixing of the weld pool, resulting in a pore-free fully penetrated weld seam. The high cooling rate after welding also leads to fine microstructure of the weld.

4) Strong weld seam.

5) Precise control.

6) Non-contact, atmospheric welding process.

(4) Advantages of Laser Deep Fusion Welding

1) Due to the much higher power density of the focused laser beam compared to conventional methods, it enables fast welding, with minimal heat-affected zone and deformation, allowing the welding of difficult-to-weld materials such as titanium and quartz.

2) Because the beam is easily transmitted and controlled, and there is no need for frequent replacement of welding torches and nozzles, it significantly reduces downtime and auxiliary time, resulting in high load factors and production efficiency.

3) Due to the purification effect and high cooling rate, the weld seam has high strength and overall performance.

4) Due to the low balanced heat input, high processing accuracy, and the ability to reduce rework costs. Additionally, the operating costs of laser welding are relatively low, which can reduce production costs.

5) Easily achieves automation and effective control of beam intensity and precise positioning.

(5) Laser Deep Fusion Welding Equipment

Laser deep fusion welding typically uses continuous wave CO2 lasers or high-power solid-state lasers. These types of lasers can maintain sufficiently high output power to produce the “keyhole” effect, penetrating the entire cross-section of the workpiece to form a strong and tough weld joint.

As for the laser itself, when combined with a “telescope” system, it is simply a device that can produce a directional parallel beam of light that can be used as a heat source.

When it is directed and effectively processed to the workpiece, its input power has strong compatibility, making it better suited for automated processes. In order to effectively carry out welding, the laser and some necessary optical, mechanical, and control components together form a large welding system. This system includes the laser, beam delivery components, workpiece loading and moving devices, and control devices.

This system can be as simple as manually handling and fixing the workpiece by the operator, or it can include automatic loading, unloading, fixing, welding, and inspection of the workpiece. The overall requirement for the design and implementation of this system is to achieve satisfactory welding quality and high production efficiency.

Factors affecting light absorption: wavelength, material properties, material surface condition, and their relationships are shown in Figure 2-7.

The relationship between material absorption rate and temperature is shown in Figure 2-8.

The relationship between the absorption rate of 10.6μm wavelength material and temperature is shown in Equation 2-1:

Where A10.6 is the absorption rate of 10.6μm wavelength material; T is the temperature; Kρ is the extinction coefficient (corresponding to resistivity); ρ20 is the DC resistivity at 20°C.

Figure 2-7: Relationship between wavelength, material properties, and material surface condition.
Figure 2-8: Relationship between material absorption rate and temperature
Figure 2-8: Relationship between material absorption rate and temperature

(6) Application of Laser Deep Fusion Welding

1) Laser welding of carbon steel and ordinary alloy steel. In general, laser welding of carbon steel yields good results, and the welding quality depends on the impurity content. Similar to other welding processes, sulfur and phosphorus are sensitive factors in producing welding cracks. To achieve satisfactory welding quality, preheating is required when the carbon content exceeds 0.25% (mass fraction).

When steels with different carbon contents are welded together, the laser focal point may slightly lean towards the side of the low carbon material to ensure joint quality.

Low carbon boiling steel is not suitable for laser welding due to its high sulfur and phosphorus content. Low carbon calm steel, with low impurity content, yields good welding results. Medium and high carbon steels and ordinary alloy steels can be effectively laser welded, but preheating and post-weld treatment are necessary to eliminate stress and prevent crack formation.

2) Laser welding of stainless steel. Generally, laser welding of stainless steel yields higher quality joints compared to conventional welding. Due to the high welding speed, the heat-affected zone is minimal, and sensitization is not a significant issue. Compared to carbon steel, stainless steel has a lower thermal conductivity, making it easier to achieve deep narrow welds.

3) Laser welding between different metals. The extremely high cooling rate and minimal heat-affected zone in laser welding create favorable conditions for the compatibility of materials with different structures after melting.

It has been proven that the following metals can be successfully laser deep fusion welded: stainless steel to low carbon steel, 416 stainless steel to 310 stainless steel, 347 stainless steel to HASTELLOY nickel alloy, nickel electrode to cold-forged steel, and bimetallic strips with varying nickel contents.

Pulse Laser Welding

Pulse laser welding is commonly used for the welding of small electronic components and thin metal. It includes several welding types:

1) Welding between sheets. This involves four process methods: butt welding, end welding, center penetration fusion welding, and center perforation fusion welding.

2) Wire-to-wire welding. This includes four process methods: wire-to-wire butt welding, cross welding, parallel lap welding, and T-shaped welding.

3) Welding of metal wire to bulk components. Laser welding can successfully achieve the connection between metal wire and bulk components of any size. Attention should be paid to the geometric dimensions of the wire-like components during welding.

4) Welding of different metals. Welding different types of metals requires addressing welding properties and weldable parameter ranges. Laser welding between different materials is only possible for specific material combinations. The application of pulse lasers is illustrated in Figure 2-9.

Figure 2-9: Application of Pulse Laser

Laser Brazing

Brazing is a welding method that uses a brazing material with a melting temperature lower than that of the base material. It involves heating the parts to be joined to a temperature lower than the solidus line of the base material but higher than the liquidus line of the brazing material.

When the parts and the brazing material are heated to the melting point of the brazing material, the liquid brazing material wets, spreads, melts, and diffuses with the base material, achieving the connection between the parts through wetting, capillary flow, filling, melting, and diffusion in the gap between the base materials. Brazing is divided into hard brazing and soft brazing, with temperature being the primary distinguishing criterion.

Brazing carried out with a brazing material liquidus line temperature above 450°C is considered hard brazing, while brazing carried out with a temperature below 450°C is considered soft brazing. Brazing is a solid-phase connection method that differs from fusion welding methods.

Because the base material does not melt, the temperature is low, resulting in minimal deformation, enabling the joining of dissimilar materials and allowing for disassembly. Therefore, robotic laser brazing is widely used in automotive body production.

(1) Laser Soft Brazing

Laser brazing is used for connecting certain products where laser fusion welding is not suitable, but where the use of laser as a heat source can be employed for soft brazing and hard brazing, yielding results similar to laser fusion welding.

There are various ways to apply brazing, among which laser soft brazing is mainly used for soldering printed circuit boards, especially suitable for the assembly of sheet-like components. The characteristics of laser soft brazing are as follows:

1) Due to localized heating, components are less prone to heat damage, and the heat-affected zone is small, allowing for soft brazing near heat-sensitive components.

2) Non-contact heating, wide melting zone, no need for any auxiliary tools, and processing can be carried out after the assembly of double-sided components on double-sided printed circuit boards.

3) Stable repeatability. The brazing material has minimal contamination of the welding tool, and the laser irradiation time and output power are easy to control, resulting in a high yield in laser brazing.

4) The laser beam is easy to split, enabling simultaneous symmetrical welding at multiple points using optical elements such as half mirrors, reflectors, prisms, and scanning mirrors for time and space division.

5) Laser brazing mostly uses a 1.06μm wavelength laser as a heat source, which can be transmitted through optical fibers, allowing for processing in areas where conventional methods are difficult, offering good flexibility.

6) Good focusing, easy to achieve automation for multi-station devices.

Commonly used soft brazing materials include tin-based brazing materials, lead-based brazing materials, cadmium-based brazing materials, zinc-based brazing materials, gold-based brazing materials, and low-melting-point brazing materials such as bismuth-based brazing materials, bismuth-based brazing materials, and indium-based brazing materials.

The most widely used is tin-lead brazing material. When the mass fraction of tin in tin-lead alloy is 61.9%, a eutectic with a melting point of 183°C is formed, with the highest strength and hardness.

(2) Laser Hard Brazing

Hard brazing, due to its high strength, can be used for brazing stressed components and is widely applied. It includes aluminum-based brazing materials, silver-based brazing materials, copper-based brazing materials, manganese-based brazing materials, nickel-based brazing materials, gold-based brazing materials, and palladium-based brazing materials.

Aluminum-based brazing materials are based on aluminum-silicon alloys and can also incorporate elements such as copper, zinc, and germanium to meet process performance requirements for brazing aluminum and aluminum alloys.

Silver-based brazing materials are primarily based on silver-copper and silver-copper-zinc alloys, and can also incorporate cadmium, tin, manganese, nickel, lithium, and other elements to meet different brazing process requirements, and are the most widely used hard brazing materials. Copper-based brazing materials have been widely used in the brazing of steel, alloy steel, copper, and copper alloys.

Manganese-based brazing materials can meet the requirements of different processes, with good ductility, good wetting ability for stainless steel and heat-resistant steel, high room temperature and high-temperature strength of the brazed joint, moderate oxidation resistance and corrosion resistance, and no significant erosion of the base metal.

Nickel-based brazing materials often incorporate elements such as chromium, silicon, boron, iron, phosphorus, and carbon, and have excellent corrosion resistance and heat resistance, commonly used for brazing austenitic stainless steel, duplex stainless steel, and martensitic stainless steel. Gold-based brazing materials often incorporate elements such as copper and nickel.

Gold-based hard brazing materials have a minimal effect on the base metal and are commonly used for brazing jewelry. Palladium-based brazing materials have strong wetting ability, low vapor pressure, good ductility, high strength, and minimal tendency for base metal erosion, suitable for brazing stainless steel, nickel-based alloys, and other materials. The welding joint forms and wire feeding methods for laser brazing are shown in Figure 2-10.

Figure 2-10: Laser Brazing Welding Joint Forms and Wire Feeding Methods

a) Butt joint (T-shaped end joint)
b) Lap joint

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