What is the best soldering method to use for copper?
Can fiber lasers be used for welding of copper?
What needs to be aware of during the welding process?
We tested four sets of test plans in an attempt to find a suitable solution for welding copper within 1mm.
Copper has excellent ductility, electrical conductivity, thermal conductivity and a certain degree of corrosion resistance.
In physical structure, it has good elasticity, friction resistance, abrasion resistance and other characteristics.
It is mainly used in the production of electrical equipment such as generators, bus bars, cables, switchgear, transformers and other electrical equipment as well as heat exchangers, pipes, solar heating devices, such as plate collectors and other thermal equipment.
Therefore, it is widely used in the construction industry, electrical, machinery manufacturing, defense industry and other industries.
According to the research results in the thesis Copper Laser Welding Process and Application Research, the current share of copper and copper alloys in industrial applications is shown in Figure 1.
Fig. 1 Current share of copper and copper alloys
The CPU, for example, which we are familiar within our lives, generates a lot of heat when it works.
If the heat does not dissipate quickly, it will affect the performance of the CPU or even damage it.
The common heat dissipation method as shown in Figure 2 is to use a fan on one side and a copper heat sink on the other side, where the copper heat sink needs to be soldered.
Fig.2 CPU Cooling structure
As the widespread use in the application of copper, the quality of the copper connection is required to be higher, and welding connection is the commonly used method.
At present, copper welding is mainly concentrated in the traditional friction stir welding, argon arc welding, ultrasonic seam welding, brazing, laser welding and other methods.
Because copper has high thermal conductivity, large expansion coefficient and high reflectivity to infrared laser, the absorption rate of copper to the infrared laser is about 5% at room temperature, and the absorption rate increases sharply to about 20% after heating to near the melting point.
Therefore, in the fiber laser welding of copper, the energy at the bottom of the hole is likely to be too large and lead to over-expansion at the bottom of the small hole causes destabilization of the hole, resulting in weld defects.
In general, there are several problems in fiber laser welding of copper:
(1) Need high laser energy density.
(2) Prone to porosity and splashing.
(3) Poor stability and molding of the welding process.
4) The poor performance of weld joints.
The main controls or welding methods currently used to address these problems are as follows.
Laser fillet welding
Compared to conventional laser welding, laser fillet welding is more adaptable to the workpiece processing assembly gap, while the weld area can be controlled by adjusting the composition of the filler wire.
The welding pattern is shown in Figure 3.
Fig.3 Schematic diagram of laser fillet welding
Laser-arc hybrid welding
Laser-arc hybrid welding combines the respective advantages of two independent heat sources, laser and arc.
For example, laser heat source has a high energy density, excellent directivity, and transparent medium conduction characteristics; while the arc plasma has a high thermal-electrical conversion efficiency, low operating costs of equipment costs, technology development and other advantages.
It greatly avoids the shortcomings of both, such as the high reflectivity of metal materials on the laser caused by laser energy loss, laser equipment, high equipment costs and low electrical-optical conversion efficiency, low energy density of arc heat source and poor discharge stability when moving at high speed etc.
At the same time, the organic combination of the two derived a lot of new features: high energy density, high energy utilization, high arc stability, low accuracy of the tooling preparation and surface quality of the workpiece to be welded etc., which make it become a new welding heat source with great prospects for application.
The welding form is shown in Figure 4.
Fig.4 Schematic diagram of laser-arc hybrid welding
After the surface treatment, the absorption rate of the fiber laser by the copper will be significantly increased, thereby improving the weldability of the copper.
Currently, the main treatment methods include blackening of the surface and the addition of additional light-absorbing material coatings etc.
At room temperature, the absorptivity of copper for lasers with a wavelength of 532nm is 30%-40%.
Based on previous scholars’ research on the application of lasers in copper welding:
(1) Focusing the 532nm laser and the 1030nm laser on the same position on the surface of the workpiece to weld copper, it was found that spatter can be significantly suppressed, the weld surface shape is significantly improved, and the process repeatability is greatly improved.
(2) Carry out 400Hz～600Hz sine modulation to the laser output power, and then weld the copper, which is found that the stability of the welding process is significantly improved, the weld shape is regular, and the number of pores in the weld is greatly reduced.
Although the above-mentioned treatment methods have different good effects, they have different degrees of complexity in equipment or procedures, and the cost performance is not high when welding some relatively simple workpieces.
The process is too complicated? PASS!
High equipment cost? PASS!
For tailor welding and fillet joint welding of copper plates with a thickness of less than 1mm, is there a simpler welding process?
One test is better than a hundred questions.
Let’s start the welding test now:
Raycus laser RFL-C6000 with 100μm core diameter fiber and KUKA robot
Precitec YW52 laser welding head, collimator lens 125mm, focusing lens 250mm
Material and joint form:
copper, corner joint, as shown in Figure 5.
Fig. 5 Joint type and welding method
Specific welding parameters were tested using the following four types.
According to the above welding equipment and four groups of welding parameters for welding, we get four parameters for welding effects:
As can be seen from the diagram, the appearance of the weld formed by the different welding parameters varies:
- Weld No. 1 has a very severe blowout and is poorly formed.
- Weld No. 2 has a small number of blow holes and is poorly formed.
- Weld No. 3 was not found to be popped and was well-formed, but there was some unevenness.
- Weld No. 4 seam has a better formation effect, uniformity and stability.
It can be concluded that the weld formation is greatly influenced by the welding speed, the faster the welding speed, the better and more stable the weld formation.
When the fiber laser is welding copper, the reason for the weld defect is that the energy at the bottom of the small hole is easy to be too large, which causes the excessive expansion at the bottom of the small hole, which causes the small hole to become unstable.
This also can explain why the faster the welding speed, the more stable the weld seam becomes: the high welding speed avoids excessive heat input, and the existence time of small holes generated in a certain fixed position of the joint during the welding process is also greatly reduced, avoiding excessive expansion of the bottom of the small hole due to long-term accumulated heat input, which causes the small holes to be unstable, and then produce weld defects.
The parts processed in accordance with the above welding schemes hardly need to undergo post-welding treatment.
In order to achieve such a welding effect, Raycus laser technical engineers suggest that during the welding process, when paying attention to controlling the joint clamping gap, it is generally recommended to control it within 0.05mm to achieve the ideal welding effect.