Laser wire filling welding is similar to ordinary wire filling welding. While the laser irradiates the weld, feed in the corresponding welding wire.
Laser wire filling welding solves the problem of strict clamping requirements for weldments, and can realize the welding of thick weldments with low-power laser.
More importantly, proper wire filling can improve the weld quality and obtain welded joints with good hardness and plasticity.
Fig. 1 Principle of laser wire filling welding
1. Principle of laser wire filling welding
The principle of laser wire filling welding is shown in fig. 14. In this process, the focused laser spot does not directly irradiate the surface of the weldment but irradiates the surface of the welding wire.
The welding wire metal melts and then enters the welding area.
In order to protect the welding area and control the photoinduced plasma, it is necessary to blow protective gas and auxiliary gas to the action parts of laser beam, welding wire and weldment.
Axial gas is added along the axis of the laser beam to protect the focusing lens of the laser from the contamination of molten droplets;
The second gas is added from the side of the welding gun and blown out in the form of side axis to protect the surface of the molten pool and compress the plasma generated by laser excitation;
The third gas is added into the gas chamber of the welding fixture from the back of the fixture, which can directly protect the back of the weld.
The key to this process is the feeding mode and equipment of welding wire.
Because the laser is a heat source of gathering point, the spot diameter is very small, generally less than 1mm.
In order to make the welding wire always under the irradiation of focused laser spots during welding, the welding wire must have good directivity.
In the narrow gap welding of large thickness plate, the extension length of the welding wire is very long, and the requirement for the directivity of the welding wire is higher.
The wire feeding system of filler wire laser welding must have a better wire straightening function than the traditional welding method.
Laser-induced plasma will inevitably be produced during laser deep penetration welding.
The absorption and scattering of laser energy by plasma will significantly reduce the processing efficiency.
The thermal effect of plasma during narrow gap welding will also lead to groove collapse, which makes the welding process impossible.
Therefore, the plasma formed in the welding process must be controlled, The plasma control technology of filler wire laser welding is another key to realize the process.
CO2, He, Ar or their mixture can be used to eliminate the influence of plasma according to the laser power, the properties of the material to be welded and the application requirements.
2. Characteristics of laser wire filling welding
In the welding of thin plates without filler wire, when the weld metal is welded together by the self melting of the base metal, the weld metal collapse is easy to occur, especially under the condition of high power and large heat input.
In addition, in the case of no wire filling, the accuracy requirements for the processing and assembly of the base metal are high. If the butt gap is too large or uneven, the weld quality will be unstable.
Laser wire filling welding has the following characteristics:
1) Add useful alloy composition, change and control the composition of weld, and improve the quality of joint.
In laser welding with filler wire, the chemical composition and metallurgical properties of the weld are determined by the composition of base metal and welding wire according to a certain fusion ratio.
Adjusting the composition of welding wire and welding parameters can realize the control of weld composition and metallurgical properties.
Laser welding with filler wire can also directly realize the butt joint of various dissimilar metals. Since the welding wire with any alloy composition can be selected as the best weld transition alloy, it can ensure the best performance of the connection of base metals on both sides, and the butt joint of dissimilar metals can be realized only by one welding.
2) The processing and assembly accuracy requirements of base metal are reduced and the cost is saved.
The preparation requirements for the groove of butt weldment in laser welding without filler wire are very high, and the groove gap is required to reach the order of 0.1mm.
After using filler wire, laser welding can be carried out for both laser cutting groove and ordinary cutting groove, and the weld formation is good.
The use of filler wire laser welding not only reduces the requirements of groove processing accuracy of laser welding butt weldment, but also reduces the requirements of assembly accuracy of butt weldment.
Good welding results can still be obtained when the assembly gap reaches 1mm.
3) Thicker materials can be welded, which is easy to realize multi-layer welding.
When i-groove single pass laser welding without filler wire, the welding penetration is limited, and the laser connection of large thickness weldments cannot be realized.
For example, when 6kwCO2 laser is used and the welding speed is very slow at 0.2mm/min, the penetration is only about 10mm.
Due to the slow welding speed, the increase of heat input and the obvious increase of weld width and heat affected zone, the characteristics and advantages of laser welding are lost.
Using filler wire, low-power laser welding of large thickness weldments is realized, and the process difficulties that cannot be solved by conventional laser welding are solved.
The weld depth width ratio is as high as 5 ∶ 1 ~ 7 ∶ 1, and the heat affected zone is small and the welding quality is high.
Therefore, laser wire filling welding has broad application prospects.
3. Welding parameters of laser wire filling welding
The main welding parameters of laser wire filling welding include laser power, welding speed, wire feeding speed, groove gap, wire feeding angle, etc. due to the increase of welding parameters of laser wire filling welding, the matching problem of welding parameters must be solved.
(1) Laser power
The laser power should be large enough so that the welding wire can be heated and melted well. Otherwise, the welding wire melting is poor and the weld formation is not good.
(2) Wire feeding speed and welding speed
Excessive wire feeding speed will increase the weld reinforcement or the welding wire will not be able to melt. If the wire feeding speed is too small, irregular weld formation will occur.
The welding speed shall not be too fast, otherwise the fusion between the molten welding wire and the base metal is poor.
If the wire feeding speed and welding speed match each other well, qualified welds can be obtained.
(3) Wire diameter
The diameter of welding wire must be appropriate.
On the premise of meeting the directivity of welding wire, fine welding wire shall be used as far as possible.
Good weld can be obtained when the diameter of welding wire is between 0.8 ~ 1.6mm.
(4) Position of laser beam
The position of the welding wire relative to the laser beam is an important welding parameter. The offset of the end of the welding wire to the laser axis should be controlled within 0.8mm. When the offset is greater than this value, the welding wire will not completely melt and touch the molten pool, resulting in discontinuous weld.
(5) Wire feeding mode
Since the focal spot diameter of the laser beam is 0.3mm and the diameter of the welding wire is no more than 2.0mm, it is difficult for the welding wire to enter the molten pool.
Choosing a reasonable wire feeding method plays an important role in the weld formation.
There are two wire feeding modes: front wire feeding and rear wire feeding, as shown in fig. 2.
Front wire feeding refers to that the welding wire is fed from the front of the molten pool at a certain wire feeding angle, so that the end of the welding wire is on the laser focusing spot.
The end of the welding wire is irradiated by the laser, quickly melted into the molten pool and fused with the metal in the molten pool.
The post wire feeding mode is that the welding wire is fed from the rear of the molten pool at a certain wire feeding angle, so that the end of the welding wire is on the laser focusing spot.
After melting, the end of the welding wire enters the tail of the molten pool and solidifies rapidly.
The front wire feeding method has better weld formation because the welding wire is directly irradiated by laser, which makes the welding wire melt more fully;
Then, the welding wire is heated by the plasma above the molten pool and the thermal radiation and heat conduction of the molten pool, and the heat is not enough to fully fuse the welding wire with the base metal;
Therefore, the front wire feeding mode is generally selected.
(6) Wire feeding angle
The wire feeding angle is the included angle between the filler wire and the surface of the weldment.
Whether the wire feeding angle is appropriate or not will affect the reflection of the filler wire to the laser, thus affecting the heating effect of the laser on the welding wire.
The wire feeding angle is too small, on the one hand, the extension length of the welding wire becomes longer, resulting in the decrease of the directivity of the welding wire, and sometimes the welding wire will deviate from the laser beam;
Moreover, it will affect the reflection of the filler wire to the laser.
The wire feeding angle is too large. Although the reflection of the welding wire to the laser is reduced, it brings difficulties to the adjustment of the welding wire, because a small position deviation will greatly change the contact point between the spot and the welding wire in the vertical direction.
Generally, the extension length of welding wire shall not be greater than 8mm, and the wire feeding angle shall be controlled at 20 ° ~ 35 °.
See table 1 for welding parameters of AZ31 magnesium alloy by CO2 laser wire filling welding.
a) Front wire feeding b) rear wire feeding
Fig. 2 Schematic diagram of two wire feeding modes
Table 1 Welding parameters of AZ31 magnesium alloy welded by CO2 laser wire filling welding
|Sample No||Laser power P / W||Welding speed Vr / (mm / min)||Wire feeding speed Vs/ (mm / min)||Front shielding gas flow Q1 / (L / min)||Back shielding gas flow Q2 / (L / min)|