Aug 06, 2020
The versatility of nanosecond (ns) pulsed infrared fiber lasers is well-known, and they are ideal for most industrial marking and engraving applications. Usually when the pulse energy is less than a few millijoules and the average power does not exceed 100W, it is effective in high pulse repetition frequency, continuous wave (CW) and quasi-continuous wave modulation (QCW) modes. Recently, they have begun to be used in a variety of micromachining and laser surface texturing, and even remote microcutting applications. Most of these applications involve material removal.
For users, the benefits of lasers are obvious, such as multi-tasking laser sources and compact integrated forms that are often air-cooled to achieve seamless integration. Globally, industries such as consumer electronics, energy storage, and medical equipment need to add more functions to smaller and smaller volumes and high-density packaging. Therefore, more efficient manufacturing technology is needed to help these products become a reality. This article will focus on laser welding technology. Therefore, this production technology realized by industrial (nanosecond infrared fiber) lasers can provide the required high repeatability, accuracy and production capacity, as well as low cost, so it can meet the needs of the market.
According to different application areas, various types of lasers have different advantages, including pulsed YAG lasers, disc lasers, fiber (CW and QCW) lasers, and even diode lasers. So far, nanosecond pulsed lasers have only been used in a few cutting-edge applications, but the situation is changing, and nanosecond fiber lasers have recently begun to be applied to material connections.
SPI is the pioneer in introducing master controlled oscillator power amplifier (MOPA) to nanosecond fiber lasers, and this has also proven to be a very versatile tool because it can control and adjust the pulse according to the requirements of the application. parameter. This is mainly achieved by changing the pulse duration and pulse frequency. They can also be switched between pulsed and continuous wave modes, which is also a very important feature because of their availability in a range of light sources with different beam qualities, so that different tools can be provided according to the current task. In the range of average power and peak power, this laser can be modulated in the millisecond range to be suitable for applications that require millisecond pulses with low average power.
Take plastic welding as an example. In some applications that require high precision, such as microfluidic devices, the use of fiber lasers will gain more advantages than other laser sources. Sometimes the energy distribution of the spot will have an impact. For example, in a complex medical device, a transparent polymer is welded to a black polymer, and a 40W laser beam is set to CW mode (Figure 1). "It allows me to control the spot size, energy distribution, and depth of field as needed." commented Joe Lovotti, director of laser technology at Okay Industries (New Britain, Connecticut).
Figure 1: Plastic welding of insulin syringes with 40W HS-H laser.
In terms of metal welding, micro-connection applications in the medical device industry are becoming more and more common, which is a challenge even for the best application engineers. The connection of thin metal wires is just such an example, and CW fiber lasers have been widely used in this field. However, as the metal wire becomes thinner, the problems related to heat input become more difficult. When welding 50μm diameter coiled wire, using 20W, M2 <1.6 laser to achieve concentrated focus will bring good results. The challenge is to suppress the peak power of the pulse by operating the laser at a higher repetition rate, creating more pulses with <0.1mJ and quasi-continuous wave modulation (QCW) with a frequency greater than 250kHz, which in the final analysis is to create short pulses string.
Some applications need to connect an outer covering or braid to a wire (Figure 2). We found that in this case, a pulse with a wider energy distribution can achieve better wettability between the two components. In this example, the 40W, M2=3 laser used has a larger spot and slightly higher pulse energy (greater than 1.25mJ), which can help bridge the gap.
Figure 2: Using 250 kHz, 20W EP-Z laser to weld 50μm diameter metal wires (a and b),
A 20W EP-Z laser is used to weld the thermocouple (c), and a 40WHS-H laser is used to weld the braid and wire (d).
On the other hand, using a single-mode laser with 20W and M2 <1.3 can achieve extremely high accuracy. An example is the welding of fine dissimilar metal wires with a diameter of 12 μm and successfully welding them to form a thermocouple. To achieve the desired effect in this application, jig selection and vision systems are as important as lasers.
Tin soldering application
For tin soldering, CW or direct diode lasers are usually used, but in applications where heat input is critical, pulsed lasers can be considered. By using long pulses at high repetition rates, energy utilization efficiency can be improved and the risk of thermal damage can be reduced. By using the beam scanning transmission system, the laser energy can be deposited on a larger target area, so that the gold/tin solder paste in this example will only melt at the contact location (Figure 3).
Figure 3: Soldering with 500 kHz, 40W HS-H laser.
Metal welding and joining
Using nanosecond pulsed lasers to weld metals requires careful adjustment of pulse parameters and process settings, so as to truly achieve a good connection. After all, these types of pulses are mainly used to remove material, not to melt and resolidify. Optimizing the pulse can get the maximum peak power and pulse energy, but these characteristics can be modified by using them at higher frequencies. This reduces the peak power and makes the output closer to the QCW strobe pulse while maintaining average power.
At this higher frequency, the effect of the pulse on the material changes from ablation to closer to melting. The effect is remarkable. In the example below, we can see the use of a 70W laser to surfacing a ring with a diameter of 6mm on a thin stainless steel plate. Using a 250ns, 1mJ pulse at 70kHz, the result is extremely rough and highly oxidized. Keep all the parameters unchanged, just increase the pulse frequency, you can see a significant improvement. By doubling the frequency to 140 kHz and halving the pulse energy to 0.5mJ, it can be seen that the roughness and oxidation are greatly improved. By increasing the frequency to 500kHz and reducing the pulse energy to below 0.15mJ, a brighter The welding seam does not even need shielding gas. By using this technique, it is possible to achieve a 250μm lap weld (Figure 4).
Figure 4: Overlay welding on the board realized by 250ns pulses at different frequencies.
By using wobbling technology to expand the weld and improve the weld penetration (affecting the welding speed), the weld shape can be further improved.
A test was conducted on a weld between stainless steel and stainless steel, and the shear strength of two 1mm welds in a complete 0.5mm lap weld exceeded 224 pounds. In a 180-degree peel test performed on a linear weld of 5 mm long and 1 mm wide, the component yielded at 241 pounds (Figure 5).
Figure 5: The stainless steel weld (a) and its peel test (b) microscopic image produced by 70W EP-Z laser.
In fact, this technology can be applied to various other metals, such as steel, aluminum, and even copper. When this technology is used to connect highly reflective materials, the pulse needs to be adjusted to provide enough pulse energy to couple into the material. Spot welding of any size can be achieved by using a spiral technology. For example, a 70W EP-Z laser can be used to create three 1mm welding points to connect two 150μm thick copper foils. Then, the next challenge is to connect different metals.
Dissimilar metal welding
The most common in electronic and battery applications is to connect thin copper foil to aluminum. In order to meet this challenge, SPI's application team has invested in intense research. They quickly announced progress, and when asked how strong the weld was, they showed a picture of the weld under a static load of 12 pounds (Figure 6).
Figure 6: Copper-aluminum spot welding (a) and its tensile test results (b), and the picture (c) under a static load of 12 pounds.
In fact, a recent test conducted on a tensile test showed that in a 90-degree peel test, the copper-aluminum connection failed under a shear force of 26 pounds, and yielded at 6 pounds. The weld is intact.
More challenging welding of dissimilar materials can be used in actual commercial applications (Figure 7). "Using only a 20W single-mode nanosecond pulsed laser allows us to well control the heat input and weld geometry, especially in very challenging micro welding applications, such as 0.1mm stainless steel foil and 0.25mm titanium foil Connection.” said Dr. Geoff Shannon of Amada Miyachi (Monrovia, California).
Figure 7: Welding stainless steel and titanium foil with 20W HS-S laser.
The introduction of this kind of multi-purpose tool can simplify the production line, so it has been rapidly developed in industries such as medical equipment manufacturing. "These fiber lasers are a revolutionary tool that can use nanosecond pulses to cut, etch, and grind, and can use multi-millisecond pulses to weld. All these processes can be completed with the same laser in the same production activity." Commented Mark Brodsky of Laser Mark's (San Jose, California).
This article introduces a variety of applications from plastic joining and soldering to metal sheet welding and wire welding. In these applications, the use of nanosecond pulsed fiber lasers is an excellent alternative to conventional millisecond pulse YAG and modulated CW laser sources. solution. The good performance of these lasers in multi-process manufacturing further proves their versatility.