Laser Welding: Characteristics and Techniques

The birth of the laser

In 1960, American physicist Theodore Maiman created the first ruby laser in his laboratory, giving rise to the phenomenal light known as the laser. Its operation was initially limited by the crystal’s thermal capacity, producing only brief pulse beams at low frequencies. Despite the instantaneous pulse peak energy reaching up to 106 watts, it was still considered a low-energy output.

The term “laser” stands for Light Amplification by Stimulated Emission of Radiation.

Research into laser welding commenced shortly after the invention of lasers in the 1960s. It has evolved over 50 years from welding thin and small parts to extensive industrial applications of high-power laser welding.

Laser welding, with its advantages of high energy density, minimal deformation, narrow heat-affected zone, high welding speed, ease of automation, and no need for post-processing, has become an increasingly important method in metal material processing and manufacturing in recent years.

It is now widely used in automotive, aerospace, defense industry, shipbuilding, marine engineering, nuclear power equipment, and other fields, encompassing nearly all metallic materials.

Laser welding has excellent welding characteristics

Laser welding is a highly technical advanced manufacturing process. Generally, the power density and pulse width of the laser used are determined based on the metal’s optical properties (such as reflection and absorption) and thermal properties (such as melting point, thermal conductivity, thermal diffusivity, latent heat of fusion, etc.).

For ordinary metals, the absorption coefficient of the light intensity is approximately in the range of 105 to 109 W/cm². If the power density of the laser is between 105 to 109 W/cm², the penetration depth on the metal surface is on the order of micrometers.

To avoid metal spattering or pitting during welding, it is necessary to control the laser power density to maintain the metal surface temperature near its boiling point. For general metals, the laser power density is often taken as 105 to 106 W/cm².

Laser welding is an efficient and precise welding method that uses high-energy density lasers as a heat source. With the rapid development of industries such as aerospace, automotive, and microelectronics, the complexity of product part structures has increased. There is a growing demand for product processing precision, surface integrity, production efficiency, and working environment.

Traditional welding methods are unable to meet these requirements, leading to the widespread application of high-energy welding methods, represented by laser welding. Laser welding is highly valued by various manufacturers due to its advantages such as high energy density, focusability, deep penetration, high efficiency, high precision, and strong adaptability.

Laser welding directs a high-intensity laser beam onto the metal surface. Through the interaction between the laser and the metal, the metal absorbs the laser energy, converting it into heat, melting the metal, and then cooling and crystallizing to form a weld seam. Depending on the laser output energy mode, laser welding can be divided into pulsed laser welding and continuous laser welding (including high-frequency pulsed continuous laser welding).

Based on the power density on the focused spot after laser focusing, laser welding can be divided into conduction welding and deep penetration welding. In deep penetration welding, it can further be divided into butt welding (brazing) and lap welding, the former requiring filler material for aesthetic appearance.

The characteristics of laser welding technology

The advantages of laser welding mainly include: a small laser focus spot, high power density, capable of welding high melting point and high-strength alloy materials; laser welding is a non-contact process, free from tool wear and tool change issues; adjustable laser energy and travel speed enable various forms of welding processing;

high level of automation, computer-controlled, fast welding speed, high efficiency, and easy welding of any complex shape; minimal heat-affected zone and material deformation, eliminating the need for subsequent processing; laser can pass through glass, enabling welding of workpieces inside vacuum containers and within complex structures; easy to guide and focus, facilitating directional changes;

compared to electron beam processing, laser welding does not require strict vacuum equipment systems, making it convenient to operate; high production efficiency, stable and reliable processing quality, and good economic and social benefits.

Highly concentrated laser can provide functions such as welding, cutting, and heat treatment.

Figure 1-1 Laser Welding (Plate Butt Joint)

The process characteristics of laser welding include: high power density, fast welding speed; narrow weld, high weld strength; small heat-affected zone; minimal welding deformation; good suppression performance; good sprayability; and low assembly error. The laser welding mechanism is as follows:

1) Laser welding belongs to fusion welding, using the laser beam as the energy source, impacting the weld joint.

2) The laser beam can be guided by flat optical elements (such as mirrors), and then projected onto the weld seam using reflective focusing elements or lenses.

3) Laser welding is a non-contact welding process, requiring no pressure during operation, but inert gas is used to prevent oxidation of the weld pool, and filler metal is occasionally used. The thermal process of laser processing goes through the four stages of “heating, melting, vaporization, and solidification”, as shown in the schematic diagram of laser welding in Figure 1-2.

Figure 1-2: Schematic diagram of laser welding (penetration welding)

a) Light absorption
b) Formation of molten pool
c) Mixing through heat conduction
d) Re-solidification

In the mid-1980s, laser welding, as a new technology, gained widespread attention in Europe, the United States, and Japan. The use of neodymium (Nd) as the excitation element for yttrium aluminum garnet crystal rods (Nd:YAG) can produce a continuous single-wavelength laser beam of 1 to 8 kW – YAG laser, with a wavelength of 1.06 μm, which can be connected to the laser processing head through flexible optical fibers.

The equipment layout is flexible and suitable for welding material thicknesses of 0.5 to 6 mm. Using CO2 as the excitation material, CO2 lasers (wavelength 10.6 μm) with an output energy of up to 25 kW can achieve full penetration welding of 20 mm thick plates, and they have been widely used in metal processing in the industry.

In 1985, ThyssenKrupp and Volkswagen successfully used the world’s first laser-welded panel on the Audi100 body. In the 1990s, major car manufacturers in Europe, North America, and Japan began to use laser-welded panels on a large scale in car body manufacturing. Both laboratory and car manufacturing plant experiences have proven that laser-welded panels can be successfully used in car body manufacturing.

Laser welding uses laser as the energy source to automatically join and weld various materials such as steel, stainless steel, and aluminum alloys with different thicknesses and coatings to form a single integrated plate, profile, sandwich panel, etc., to meet the different requirements of material properties for components, achieving lightweight equipment with the lightest weight, optimal structure, and best performance.

In developed countries such as Europe and the United States, laser welding is not only used in the transportation equipment manufacturing industry but also extensively used in the welding production of building construction, bridges, household appliance panels, and steel plate welding in rolling mills (steel plate connections during continuous rolling), among other fields.

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