The consumer electronics and automotive industries are an important driving force for the increasing use of copper in industrial processing and product manufacturing.
With the development of new battery technology and higher battery capacity, the demand for corresponding connection technology is also increasing.
Although soft soldering is still the main technology for low-power applications of consumer electronics, it must be applied in the case of high transmission current or high load and dynamic load stress of joints.
This trend is particularly driven by the electric vehicle industry, which and its suppliers are looking for robust and efficient processes for mass production operations such as power storage and line transmission applications.
In the past, laser technology was limited in welding copper and copper alloys due to the physical properties of materials.
Nowadays, the emergence of high-power and high brightness fiber lasers overcomes these limitations one by one.
Through new and appropriate processing technology, stable and defect-free joints can be created in efficient welding process.
Near-infrared laser, challenge of welding copper
The challenge of laser welding copper is related to two main physical properties of the material: low absorptivity for most high-power industrial lasers and high thermal conductivity in the process.
The absorptivity of copper increases with the decrease of wavelength, which means that visible band lasers (e.g. green laser with the wavelength of 532 nm) will have significant advantages for copper welding, but these lasers are not available or have not been industrially verified for the power range required for most welding applications.
Fig. 1: Defect-free copper welding through high-speed beam swing process
The infrared laser will produce absorption problems when dealing with solid materials.
If the material melts or even evaporates through deep penetration welding, its absorption rate will increase significantly.
The absorption rate of solid copper is less than 4%, while the absorption rate of copper steam (keyhole deep penetration welding) is higher than 60% (see the table below).
This absorption problem can be overcome by a very high power density, which greatly speeds up the melting and evaporation of copper and therefore increases its absorption.
Table: absorptivity of copper to near-infrared laser radiation under different states.
|State||Absorption rate (%)|
High-speed video evaluation shows that a stable process can be established in less than 1ms.
For continuous wave (CW) welding, this obstacle must be overcome at the beginning of welding.
After the keyhole welding process is established, it will provide a constant high absorption rate. For pulse operation, it must be overcome at the beginning of each pulse.
The high power density required for welding can be obtained by using a single-mode fiber laser.
Compared with other solid-state lasers, this kind of laser has excellent beam quality and focusing performance.
IPG can provide high-power single-mode lasers up to 10 kW and high brightness multi-mode lasers with more than 10 kW. The products have solid and industrially proven designs.
Using these single-mode fiber lasers and low order mode high brightness lasers, higher than 108W/cm² can be achieved. Reliable coupling can be achieved even at a power of hundreds of watts.
Compared with ordinary multimode lasers with the same power, the intensity of these lasers is up to 50 times.
IPG provides YLR series single-mode fiber lasers with power ranging from 100W to 1000W, and is equipped with a 19″ compact frame;
In addition, the company also provides YLS series fiber lasers with power up to 10 kW (Fig. 2).
The overall efficiency of both series reaches 40%.
Fig. 2: High power single-mode fiber laser: air cooled rack type YLR-1000-SM (left) and 3KW system type YLS-3000-SM (right).
Another problem of the copper welding process is the instability of low-speed welding.
Generally, welding speed less than 5m/min will face the problem of welding instability, such as spatter, porosity and irregular weld surface.
With the acceleration of welding speed, this instability gradually disappears.
In the welding speed range of 5-15m/min, the quality reaches an acceptable level.
If the welding speed is higher than 15m/min, the resulting weld is basically free of defects (Fig. 3).
This means that the optimal welding parameters are within the limits of traditional motion systems (such as robots).
Fig. 3: Effect of processing speed on weld quality and weld depth.
This must be achieved with higher laser power.
The new process research has shown that the process stability can be realized not only by improving the speed of welding direction but also by the dynamic position change of the beam guiding lens.
This swing technology enables it to form stable solder joints at relatively low welding speed and significantly reduce the weld depth.
Through this swing technology, a high-quality copper weld with a welding depth of up to 1.5mm can be realized by using only a single-mode fiber laser with a power of 1KW.
The same technology can also be applied to high brightness multimode lasers.
A fiber laser with a power of 6kW and a beam quality of 2 mm mrad is used. The test shows that high-quality welding with a weld depth of 5mm is realized.
Fig. 4: FLW-D30 and FLW-D50 series swing welded joints launched by IPG
The dynamic control of the beam can be realized by the traditional scanning galvanometer or the new oscillating head, which combines the performance advantages of the verified welded joint and the scanning galvanometer.
The two galvanometers can flexibly use various pre-programmed graphics and shapes, such as circle, line or “8-shape”, as well as freely programmable graphics and shapes within a certain size.
One of its main advantages is that it can use standard focusing lens instead of f-theta field mirror, and can bear higher power density at lower focus offset level.
At the same time, the use of conventional transverse air curtain and protective window reduces the cost of consumables.
FLW-D50 and FLW-D30 series swing welding joints launched by IPG can work at swing frequencies up to 1 kHz and can be easily integrated into various processing systems (Fig. 4). These welded joints can withstand laser power up to 12 kW.
When welding a complex path with varying welding directions, the circular swing motion shows the best results.
The final beam velocity can be easily controlled by the swing frequency and swing diameter (VC = πD f).
In most cases, the welding speed vector VW used to dynamically locate the circular beam speed VC is negligible because the beam speed is much higher than the welding speed VW.
Fig. 5: Effect of swing amplitude on weld width and quality
The frequency setting that provides the best results depends on the spot size, swing diameter (and the resulting circular beam speed vc) and linear welding speed.
Fig. 5 shows the weld surface under constant welding speed, laser power and frequency, but different swing diameters.
The spot size is about 30 μm at the focal length f = 300mm.
The laser power is kept constant at 1kW, while the linear welding speed is set to 1m/min.
The laser power is kept constant at 1kW, while the linear welding speed is set to 1m/min.
If there is no swing motion, these parameters will lead to very unstable processes, such as overheated molten pool and pores.
With the increase of swing diameter and the corresponding improvement of circular beam velocity, the surface quality becomes more and more stable.
Depending on the swing parameters and spot size, the beam and the formed keyhole usually move in the metal bath or in solid and re-solid materials.
In both cases, the process can achieve stability.
The following weld cross-section reveals another advantage of this technology: the swing diameter can be used to customize the shape of the weld cross section.
A small swing diameter will form a typical V-shaped cross section of laser welding, while a larger diameter can deform the weld from V to U-shaped or very regular rectangle (Fig. 6).
Fig. 6: Effect of swing amplitude on weld cross section
If the energy input per unit length of the weld is constant (E = P vw), the weld cross-section remains almost unchanged.
This technology enables it to meet the requirements of weld cross-section for specific application requirements.
For the overlap welding of electrical contacts, the resistance can be reduced by increasing the contact area, and the welding depth and heat input should be controlled.
In the overlap welding of dissimilar materials such as copper and aluminum, the fusion ratio of materials can be controlled by controlling the welding depth.
Through the shallow melting of the lower metal plate, the amount of molten material can be minimized, and the intermetallic compound can be reduced by controlling the dilution ratio.
Pulse, continuous or both?
In the past few years, long pulse fiber lasers with pulse duration of several milliseconds have been introduced to the market.
They replace the traditional flash lamp pumped Nd: YAG lasers in a wide range of applications.
Such lasers include single-mode lasers with an average power of 250W and a peak power of up to 2.5kW.
The problem of pulse welding of copper was mentioned earlier.
It is important to overcome the problem of weak absorption at the beginning of the pulse and the subsequent energy input control caused by sudden changes in absorptivity and heat conduction.
By using a single-mode laser to reduce the spot size, the absorption problem can be bypassed, but at the same time, the concentrated energy input will lead to small and weak solder joints on the one hand and melt overheating on the other hand.
The solution to this problem is as simple as the process used for continuous lasers, and the same swing technology can be used on quasi continuous (QCW) lasers.
The high-frequency beam movement makes the laser beam move a relatively long distance in a relatively short pulse time.
This means that quasi continuous welding is realized during one pulse, for example, 20ms long pulse at 600Hz swing frequency realizes circular solder joint or short wire welding composed of twelve rotating beams.
By adding pulses to linear welds one by one, copper welding can have high welding quality, low average power and corresponding low investment cost.
Solidification and remelting between pulses will not produce welding defects such as pores, strong spatter or uneven penetration depth.
The swing diameter determines the weld size and weld depth. In addition, the heat input is much smaller, so it is easy to weld key electrical components with pulsed fiber laser.
Experiments show that high brightness fiber laser can overcome all known problems in copper welding applications.
The high power density can realize the instantaneous coupling and formation of keyholes, and can achieve stability and high absorptivity even at the wavelength of 1070nm.
Through high dynamic beam swing, the welding process is very stable, so as to reduce or avoid porosity and splash, and finally produce high-quality welds.
The process parameters set for beam swing can make the welding geometry controllable, resulting in very shallow welds in the deep penetration welding process.
Using long pulse quasi continuous fiber laser, spot welding can be completed by high-speed dynamic movement of beam in a single pulse.
In this way, high-quality welds can be made by increasing pulses one by one at a very low average power.