Single-mode vs. Multi-mode comparison

2.1 Three elements of laser welding

Welding is a process that connects two or more metals together by heating. Welding usually involves heating the material to its melting point, and the parent material melts to fill the gap between the joints to form a strong connection. Laser welding is a connection method that uses laser as a heat source.

Take square shell power batteries as an example: the battery cell is connected by laser from multiple components. During the entire laser welding process, the material connection strength, production efficiency, and defect rate are three issues that the industry is more concerned about. The material connection strength can be reflected by the metallographic penetration depth and width (closely related to the laser light source); production efficiency is mainly related to the processing capability of the laser light source; the defect rate is mainly related to the choice of laser light source.

2.2 Laser light source comparison factors

2.2.1 Laser light source analysis

Because laser welding is essentially a light-heat conversion process, the key parameters involved are as follows: beam quality (BBP, M2, divergence angle), energy density, core diameter, energy distribution form, welding head adapter, processing window, and processable materials. The laser light source is analyzed and compared mainly from these aspects.

2.2.2 Rayleigh length

2.3 Keyhole

Related articles on keyholes: Special Topic on Laser and Material Interaction - Thermal Conductivity Welding and Deep Penetration Welding (② Keyhole Effect) I will not go into details here.

2.4 Comparison and definition of single-mode and multi-mode lasers

Single-mode: It is a single distribution mode of laser energy on a two-dimensional plane. The core of a single-mode laser is thinner and emits a typical Gaussian beam with very concentrated energy, similar to a steep mountain peak. The beam quality of a single-mode laser is better than that of a multi-mode laser, so it performs better in applications that require high precision and high quality. It is suitable for cutting stainless steel/carbon steel thin plates of 1mm and below, and for heat conduction welding, which can obtain more uniform and smooth welds. Single-mode lasers usually have low power and are suitable for small and medium power applications.

Multi-mode: It is a spatial energy distribution mode formed by the superposition of multiple distribution modes. A multi-mode laser is equivalent to a combination of multiple Gaussian beams, and the energy distribution is similar to an inverted cup, which is relatively average. The beam quality of a multi-mode laser is relatively poor, but it is more suitable in applications that require high power output. It is suitable for cutting thick plates of 2mm and above, and for deep fusion welding, which can obtain welds with a better depth-to-width ratio. Multimode lasers usually have higher power and are suitable for high-power applications, such as cutting or deep-melting welding of thick plate materials, high-speed cutting and welding of special materials, and remote processing.

The size of the beam quality M2 factor can usually be used to determine whether the fiber laser output is single-mode or multi-mode: M2 less than 1.3 is a pure single-mode laser, M2 1.3 to 2.0 is a quasi-single-mode laser (few modes), and M2 greater than 2.0 is a multi-mode laser.

2.5 Single-mode-multi-mode laser

Common single-mode lasers: IPGYLR-2000-SM, SM is the abbreviation of Single Mode. The calculation uses collimated focusing 150-250 to calculate the focal spot size, and the energy density is 2000W. The focal energy density is compared.

2.6 Comparison of single-mode and multi-mode laser welding effects

1. Single-mode fiber laser:

Small core diameter, high energy density, strong penetration ability, small heat-affected zone, similar to a sharp knife, especially suitable for welding thin plates and high-speed welding, and can be used with a galvanometer to process tiny parts and high-reflection parts (ears, connectors, etc.).

Single-mode has a smaller keyhole and a limited volume of high-pressure metal vapor inside, so it is generally less likely to have defects such as internal pores. At low speeds, the appearance is relatively rough without blowing protective gas. At high speeds, the processing quality is better with protective gas, high efficiency, smooth and flat welds, and high yields. It is suitable for stacking and penetration welding.

2. Multi-mode fiber laser:

Large core diameter, slightly inferior energy density to single-mode, blunt knife, larger keyhole, thicker metallographic structure, smaller depth-to-width ratio, and 30% lower melting depth than single-mode at the same power. It is suitable for butt welds with large assembly gaps and thick plate processing.