1. Process parameters of laser welding
1.1 Power density
Power density is one of the most important parameters in laser processing.
With higher power density, the surface layer can be heated to boiling point in the microsecond time range, resulting in a large amount of vaporization.
Therefore, high power density is beneficial for material removal processing, such as drilling, cutting and carving.
For lower power density, it takes several milliseconds for the surface temperature to reach the boiling point.
Before the surface layer vaporizes, the bottom layer reaches the melting point, which is easy to form a good fusion welding.
Therefore, in the conduction laser welding, the power density is in the range of 104 ~ 106 W/cm2.
1.2 Laser pulse waveform
The laser pulse waveform is an important problem in laser welding, especially for sheet welding.
When the high-intensity laser beam hits the surface of the material, 60 ~ 98% of the laser energy on the metal surface will be reflected and lost, and the reflectivity changes with the surface temperature.
During a laser pulse, the reflectivity of metal varies greatly.
1.3 Laser pulse width
Pulse width is one of the important parameters of pulsed laser welding.
It is not only an important parameter different from material removal and material melting, but also a key parameter determining the cost and volume of processing equipment.
1.4 Effect of defocusing amount on the welding quality
Laser welding usually needs a certain defocus, because the power density of the spot center at the laser focus is too high, and it is easy to evaporate into holes.
The power density distribution is relatively uniform on the plane away from the laser focus.
There are two defocusing modes: positive defocusing and negative defocusing.
If the focal plane is above the workpiece, it is positive defocusing, otherwise it is negative defocusing.
According to the theory of geometrical optics, when the positive and negative separations are equal, the power density on the corresponding plane is approximately the same.
But in fact, the shape of the molten pool is different.
When defocusing is negative, a greater penetration can be obtained, which is related to the formation process of molten pool.
The experimental results show that the material begins to melt when heated by laser for 50 ~ 200 us, forming liquid phase metal and vaporization and commercial pressure steam, which emits dazzling white light at a very high speed.
At the same time, the high concentration of vapor makes the liquid metal move to the edge of the molten pool, forming a depression in the center of the molten pool.
When the negative defocusing occurs, the internal power density of the material is higher than that of the surface.
Additionally, it is easy to form stronger melting and vaporization, which makes the light energy transfer to the deeper part of the material.
Therefore, in practical application, negative defocusing should be used when the penetration is large, and positive defocusing should be used when welding thin materials.
2. Laser welding technology
1) Sheet to sheet welding
It includes butt welding, end welding, center penetration fusion welding and center perforation fusion welding.
2) Wire to wire welding
It includes wire to wire butt welding, cross welding, parallel lap welding and T-type welding.
3) Welding of wire and block element
Laser welding can be used to connect the wire and the block element successfully, and the size of the block element can be arbitrary.
Attention should be paid to the geometric dimension of the wire element in welding.
4) Welding of different metals
Weldability and range of weldable parameters should be solved for welding different types of metals.
Laser welding between different materials is only possible for certain material combinations.
There are many ways of soldering, among which laser soldering is mainly used for PCB soldering, especially for wafer assembly technology.
3. Advantages of laser soldering
1) Due to the local heating, the element is not easy to produce thermal damage and the heat-affected zone is small, so the soldering can be carried out near the thermal element.
2) With non-contact heating, it can melt the bandwidth.
Without any auxiliary tools, it can be processed on the double-sided printed circuit board after the double-sided components are equipped.
3) Repeated operation is stable.
Flux has little pollution to welding tools, the laser irradiation time and output power are easy to control, and the laser brazing yield is high.
4) The laser beam is easy to realize beam splitting, and can be divided into time and space with optical elements such as half lenses, mirrors, prisms, scanning mirrors, etc., which can realize simultaneous symmetric welding of multiple points.
5) Laser brazing mostly uses a laser with a wavelength of 1.06 um as a heat source, which can be transmitted by optical fiber.
So it can be processed in parts that are not easy to weld by conventional methods, with good flexibility.
6) It has good focus, and is easy to realize the automation of multi-station devices.
4.1 Metallurgical process and technology theory
The metallurgical physical process of laser deep penetration welding is very similar to that of electron beam welding, that is, the energy conversion mechanism is completed through the “small hole” structure.
When the power density is high enough, the material will evaporate and form a small hole.
The hole filled with steam is like a blackbody, which almost absorbs the energy of the incident light.
The equilibrium temperature in the hole cavity is about 25000 degrees.
Heat is transferred from the outer wall of the high-temperature cavity to melt the metal surrounding the cavity.
The hole is filled with high-temperature steam generated by continuous evaporation of wall material under the irradiation of light beam.
The four walls of the hole are surrounded by molten metal, and the liquid metal is surrounded by solid material.
The liquid flow outside the hole wall and the surface tension of the wall are in dynamic equilibrium with the continuous vapor pressure in the hole cavity.
The material outside the hole is flowing continuously.
With the movement of the beam, the hole is always in a stable state.
That means that the keyhole and the molten metal surrounding the hole wall move forward with the forward speed of the leading beam, the molten metal fills the gap left by the keyhole moving away and condenses, thus forming the weld.
4.2 Influencing factors
The influencing factors of laser deep penetration welding include: laser power, laser beam diameter, material absorptivity, welding speed, shielding gas, lens focal length, focus position, laser beam position, laser power increasing and decreasing control at the beginning and end of welding.
4.3 Characteristics of laser deep penetration welding
1) High aspect ratio
Because the molten metal forms around the cylindrical high-temperature steam cavity and extends to the workpiece, the weld becomes deep and narrow.
2) Minimum heat input
Because of the high temperature of the source cavity, fast speed of the melting process, and the low heat input into the workpiece, the thermal deformation and heat affected zone are very small.
3) High density
Because the small hole filled with high-temperature steam is conducive to the weld pool stirring and gas escape, resulting in the formation of non porous penetration welding.
The high cooling rate after welding is easy to refine the weld microstructure.
4) Strengthen the weld.
5) Precise control.
6) It is non-contact, atmospheric welding process.
4.4 Advantages of laser deep penetration welding
1) Because the power density of the focused laser beam is much higher than that of the conventional method, the welding speed is faster.
The heat-affected zone and deformation are smaller, and it can also weld titanium, quartz and other refractory materials.
2) Because the beam is easy to transmit and control, and it does not need to change the torch and nozzle frequently, which can significantly reduce the downtime auxiliary time, so the load factor and production efficiency are high.
3) Due to the purification and high cooling rate, the welding seam is strong and the comprehensive performance is high.
4) Because of low heat input and high machining accuracy, the cost of reprocessing can be reduced.
In addition, the cost of laser welding is relatively low, which can reduce production cost.
5) It is easy to realize automation, and can effectively control the beam intensity and fine positioning.
4.5 Laser deep penetration welding equipment
Continuous wave CO2 laser is usually used in laser deep penetration welding.
This kind of laser can maintain enough high output power, produce “keyhole” effect.
It can penetrate the whole workpiece section, and form a strong and tough welding joint.
As far as the laser itself is concerned, it is only a device that can produce parallel beam with good directivity and can be used as heat source.
If it is directed and effectively processed and then shot to the workpiece, its input power will have strong compatibility, making it better adapt to the automation process.
In order to effectively implement welding, laser and other necessary optical, mechanical and control components together constitute a large welding system.
This system includes laser, beam transmission module, workpiece handling and moving device, and control device.
This system can be simply carried and fixed by the operator, or it can include automatic loading, unloading, fixing, welding and inspection.
The general requirement of the design and implementation of this system is to obtain satisfactory welding quality and high production efficiency.
5. Laser welding of steel materials
5.1 Laser welding of carbon steel and common alloy steel
Generally speaking, the laser welding effect of carbon steel is good, and the welding quality depends on the impurity content.
Like other welding processes, sulfur and phosphorus are the sensitive factors for welding cracks.
In order to obtain satisfactory welding quality, preheating is needed when the carbon content exceeds 0.25%.
When the steels with different carbon contents are welded to each other, the welding torch can be slightly inclined to the side of low carbon materials to ensure the quality of the joint.
Low carbon rimmed steel is not suitable for laser welding because of its high content of sulfur and phosphorus.
Due to the low impurity content, the welding effect of low carbon killed steel is very good.
Medium and high carbon steels and common alloy steels can be well laser welded, but preheating and post welding treatment are needed to eliminate stress and avoid crack formation.
5.2 Laser welding of stainless steel
In general, laser welding of stainless steel is easier to obtain high quality joint than conventional welding.
Due to the small heat-affected zone of high welding speed, sensitization is not an important problem.
Compared with carbon steel, stainless steel with low thermal conductivity is easier to obtain deep penetration narrow weld.
5.3 Laser welding between different metals
The high cooling rate and small heat-affected zone of laser welding create favorable conditions for the compatibility of materials with different structures after melting of many different metals.
It has been proved that the following metals can be welded successfully: stainless steel ~ low carbon steel, 416 stainless steel ~ 310 stainless steel, 347 stainless steel ~ hastally nickel alloy, nickel electrode ~ cold forged steel, bimetallic strip with different nickel content.