Lulu Song, Shenzhen JPT Opto-electronics
Laser technology has developed progressively over the last 60 years. It is commonly used for traditional processing applications, for example, cutting, marking and welding, especially in fields such as aerospace, which exploit the use of advanced materials. Laser welding is used to join pieces of metal or thermoplastic. A high-density beam is focused on the surface of the materials, causing them to melt and interfuse. The materials crystallise after cooling down and a tight join is formed.
Laser welding has many advantages over conventional welding processes. First, laser beam irradiation causes the crystal grain to grow unidirectionally and exhibit a very fine dendrite structure, resulting in excellent mechanical properties and therefore exceptional anti-cracking and anti-porosity capabilities. Second, the small size of the laser beam affords much lower thermal stress and thermal effects, so the joining of refractory materials and dissimilar metals is increasingly feasible. Moreover, laser welding emits less harmful substances.
There are various types of laser for different applications on the market. For laser welding alone, there are many types of laser for different techniques. This article focuses on the welding performance of three types of laser, namely the master oscillator power amplifier (MOPA) nanosecond (ns) pulsed fibre laser, quasi continuous wave (QCW) fibre laser and yttrium-aluminium-garnet (YAG) laser. The ns pulsed fibre laser is based on the MOPA structure and features eight adjustable pulse widths. The parameters of these lasers are compared in table 1.
Table 1. Specifications for laser models used for welding evaluation
Shenzhen JPT Opto-electronics undertook two welding tests to assess the performances of the three lasers. The first test was carried out using the ns pulsed fibre laser and QCW fibre laser on steel. The 3D profiles of the molten pools are shown to be different in figure 1, which is due to different welding principles. The welding spot of the ns pulsed fibre laser is spiral and therefore its molten pool is more uniform than that of the QCW fibre laser. Furthermore, the drawing force of the ns pulsed fibre laser is more significant. However, the thermal effect of the QCW fibre laser is much smaller than that of the ns pulsed fibre laser due to a short interaction time. The welding spot is generated by one emission and the molten pool is close to cone-shaped.
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Figure 1 Left
3D profiles of the molten pools produced using the ns pulsed fibre laser and QCW fibre laser on steel.
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Figure 1 Right
3D profiles of the molten pools produced using the ns pulsed fibre laser and QCW fibre laser on steel.
The second test was carried out using the ns pulsed fibre laser, QCW fibre laser and YAG laser on thin dissimilar metals, namely aluminium-brass, brass-steel, brass-copper, copper-steel and titanium-steel. The parameters of the three lasers for use on these materials are compared in figure 2a.
Figure 2a
Parameters of the ns pulsed fibre laser, QCW fibre laser and YAG laser on dissimilar metals.
The focused beam diameter of the ns pulsed fibre laser is smaller than those of the QCW fibre laser and YAG laser. The welding spots for the QCW fibre laser and YAG laser are generated using one emission, but the welding spot for the ns pulsed fibre laser is spiral so that it is similar in size to the other two lasers. The pulse energy is 1.5 mJ for the ns pulsed fibre laser, and it is 15 J for the QCW laser and YAG laser. Therefore, it takes the pulsed fibre longer to achieve the same-size welding spot. Observing the cross-section, the molten pool is serrated using the ns pulsed fibre laser, whereas it is made hollow using the QCW fibre laser and YAG laser. Furthermore, the beam quality of the QCW fibre laser is far superior to that of the YAG laser, meaning the cross-section is close to cone-shaped.
Detailed metallographic examination results for the ns pulsed fibre laser and QCW fibre laser on copper-steel are shown in figure 2b. The aspect ratio of the welding area used by the QCW fibre laser is close to 1:1, while the drawing force is more significant for the ns pulsed fibre laser.
Figure 2b
Metallographic data for the ns pulsed fibre laser and QCW fibre laser on copper-steel.
The welding performance results for the three lasers are summarised in table 2. The welding performance is different for different types of lasers. The ns pulsed fibre laser is recommended for dissimilar metal welding, especially for thin materials. It achieves a delicate balance between melt and vapourisation. The energy has to be sufficient to melt the material, but not so as to vapourise it. Furthermore, the MOPA structure affords the benefit of flexible pulse control. The ns pulsed fibre laser is widely used for the welding of 3C products because precise process parameters can be satisfied.
The beam quality of the QCW fibre laser is well-controlled and larger than that of the YAG laser, therefore affording better spot finishing, thermal effect and welding surface.
It is important to choose the right laser for the application as this leads to a higher welding success rate. Shenzhen JPT Opto-electronics has a specialised application lab, highly trained engineers and various equipment for different applications (cutting, marking, welding, etc.) to help customers evaluate laser processing characteristics.
Shenzhen JPT Opto-electronics