1. Laser welding gas
Shielding gas will be used in laser welding to improve the welding effect and avoid deposition on laser tools.
According to different usage, it can be divided into:
- Auxiliary gas (MDE gas)
- Shielding gas
- Jet gas
2. Why need shielding gas?
Continuous laser welding is the use of high-energy laser beam as a heat source to irradiate the surface of the workpiece, so that the workpiece can be melted and connected to achieve excellent welding joint.
In the process of high power laser welding, the laser irradiates the surface of the material to melt the workpiece, but the high temperature is accompanied by the gasification of the metal, forming the metal vapor plasma.
The formed metal vapor plasma will absorb, refract and reflect the laser, which will weaken the energy actually reaching the surface of the workpiece and affect the stability of the molten pool.
Therefore, it is necessary to blow shielding gas with high ionization energy to suppress the generation of plasma.
At the same time, the shielding gas can also isolate the air in the welding process, so that the molten pool will not be oxidized;
It can also reduce the welding spatter and make the weld surface even and smooth.
3. The functions of protective gas
In laser welding, shielding gas will affect the weld formation, weld quality, weld penetration and weld width.
In most cases, blowing shielding gas will have a positive effect on the weld, but it may also bring adverse effects.
3.1 Positive effects
1) The correct blowing of shielding gas can effectively protect the weld pool and reduce or even avoid oxidation;
2) The correct blowing of shielding gas can effectively reduce the spatter in the welding process;
3) The correct blowing of shielding gas can make the weld pool spread evenly during solidification, and make the weld shape uniform and beautiful;
4) The proper blowing of shielding gas can effectively reduce the shielding effect of metal vapor plume or plasma cloud on laser, and increase the effective utilization rate of laser;
5) The correct blowing of shielding gas can effectively reduce the weld porosity.
As long as the gas type, gas flow rate and blowing mode are selected correctly, the ideal effect can be achieved.
However, incorrect use of shielding gas will also bring adverse effects to welding.
3.2 Negative effects
1) Incorrect blowing of shielding gas may cause weld deterioration;
2) Choosing the wrong gas type may lead to cracks in the weld, and may also lead to the decrease of mechanical properties of the weld;
3) The wrong selection of gas blowing flow rate may lead to more serious oxidation of weld (whether the flow rate is too large or too small), or the weld pool metal may be seriously disturbed by external force, resulting in weld collapse or uneven forming;
4) Choosing the wrong way of gas blowing will lead to the weld not achieving the protection effect, or even basically no protection effect, or have a negative impact on the weld forming;
5) The penetration of welding seam will be affected by blowing protective gas, especially when welding thin plate.
4. Types of shielding gas
The commonly used shielding gases for laser welding are N2, Ar and He.
Their physical and chemical properties are different, so the effect on the weld is also different.
The ionization energy of N2 is moderate, higher than that of AR and lower than that of he.
The ionization degree of N2 is general under the action of laser, which can reduce the formation of plasma cloud and increase the effective utilization of laser.
Nitrogen can react with aluminum alloy and carbon steel at a certain temperature to produce nitride, which will improve the weld brittleness, reduce the toughness and have a great adverse effect on the mechanical properties of the weld joint.
Therefore, it is not recommended to use nitrogen to protect the weld of aluminum alloy and carbon steel.
The nitride produced by the chemical reaction between nitrogen and stainless steel can improve the strength of the weld joint, which is conducive to the improvement of the mechanical properties of the weld.
So it can be used as shielding gas when welding stainless steel.
The ionization energy of AR is relatively low, and the ionization degree is relatively high under the action of laser, which is not conducive to the control of the formation of plasma cloud.
It will have a certain impact on the effective utilization of laser.
However, low activity of Ar makes it difficult to react with common metals, and the cost of AR is not high.
In addition, the density of Ar is large, which is conducive to sink above the weld pool, and can better protect the environment.
Therefore, it can be used as conventional shielding gas.
The ionization energy of he is the highest, and the ionization degree is very low under the action of laser, which can well control the formation of plasma cloud.
Laser can well act on metal, and He activity is very low, basically does not react with metal, so it is a good shielding gas for weld.
However, the cost of He is too high, so it will not be used in general mass production products.
He is generally used in scientific research or products with very high added value.
5. Blowing mode of shielding gas
At present, there are mainly two ways to blow shielding gas: one is side shaft side blowing shielding gas, as shown in Figure 1; the other is coaxial shielding gas, as shown in Figure 2.
Fig. 1 Side shaft side blowing protective gas
Fig. 2 Coaxial shielding gas
How to choose the two ways of blowing is a comprehensive consideration in many aspects.
In general, it is recommended to use the way of side blowing shielding gas.
6. Selection principle of shielding gas blowing mode
First of all, it needs to be clear that the “oxidation” of weld is just a common name.
Theoretically, it refers to the chemical reaction between the weld and the harmful components in the air, which leads to the deterioration of the weld quality.
The common reason is that the weld metal reacts with oxygen, nitrogen, hydrogen in the air at a certain temperature.
To prevent the weld from being “oxidized” is to reduce or avoid the contact of such harmful components with the weld metal at high temperature.
This high temperature state is not only the molten pool metal, but also the whole process from the time when the weld metal is melted to the time when the weld metal solidifies and its temperature drops below a certain temperature.
For example, titanium alloy welding can rapidly absorb hydrogen when the temperature is above 300 ℃, absorb oxygen when the temperature is above 450 oC, and absorb nitrogen when the temperature is above 600 oC.
Therefore, titanium alloy weld after solidification and the stage under 300 oC needs to be effectively protected, otherwise it will be “oxidized”.
It is not difficult to understand from the above description that the blowing shielding gas not only needs to protect the weld pool in time, but also needs to protect the just solidified area that has been welded.
Therefore, the side shaft side blowing shielding gas shown in Figure 1 is generally used.
Compared with the coaxial protection in Figure 2, the protection scope of this method is wider, especially for the area where the weld has just solidified.
For engineering application, not all products can adopt the way of side shaft side blowing protective gas.
For some specific products, only coaxial shielding gas can be used, and specific choices need to be made from the product structure and joint form..
7. Selection of specific blowing mode of shielding gas
7.1 Straight weld
As shown in Figure 3, the weld shape of the product is linear, and the joint form can be butt joint, lap joint, internal corner joint or overlap welding joint.
It is better for this product to adopt the side shaft side blowing shielding gas method as shown in Figure 1.
Fig. 3 Straight weld
7.2 Plane closed graph weld
As shown in Fig. 4, the weld shape of the product is plane circular shape, plane multilateral shape, plane multi segment linear shape and other closed graphics, and the joint form can be butt joint, lap joint, overlap joint, etc.
So, the coaxial shielding gas method shown in Fig. 2 is preferred for this type of product.
Fig. 4 Plane closed graph weld
The selection of shielding gas has a direct impact on the quality, efficiency and cost of welding production.
However, due to the diversity of welding materials, the selection of welding gas is more complex in the actual welding process.
It is necessary to comprehensively consider the welding material, welding method, welding position and required welding effect.
Only by welding test can choose more suitable welding gas and achieve better welding results.
8. Effect of shielding gas on weld morphology
In addition to choosing suitable shielding gas according to welding material, it is necessary to study the influence of blowing angle, direction and flow rate of shielding gas on weld morphology.
Based on the same welding conditions, the influence of different blowing angles of shielding gas on the weld is studied.
Through the experimental test, under the same control of other variables, the influence trend of weld morphology is the same under different flow rates.
However, the greater the flow rate is, the more obvious the effect on the weld penetration is, and the less the effect on the weld surface and lower weld width is.
Therefore, when the shielding gas flow rate is 5 L/min and other variables are controlled, only the blowing angle is changed to study the blowing angle.
The test results are shown in Fig. 5, and the metallographic diagram of weld morphology cross section is shown in Fig. 6.
Fig. 5 Effect of different blowing methods on weld penetration and width
Fig. 6 Weld morphology at different blowing angles
According to the experimental data, the weld penetration first increases and then decreases with the increase of blowing angle.
At 0° or more than 45°, the penetration decreases rapidly.
When the blowing angle is 30°, the weld penetration reaches the maximum.
The weld width is determined by the attenuation of laser by plasma and the effect of gas flow on weld pool.
When the blowing angle is 0°, the width of the melt is the smallest.
With the increase of blowing angle, the melting width increases.
When the angle is greater than 45°, the width of the weld has little change.
The influence of shielding gas on the weld morphology is mainly through controlling the size of plasma to determine the power density of laser reaching the workpiece surface.
By observing the metallographic diagram of the weld cross-section, it can be seen that the weld morphology tends to thermal conduction welding mode at 0° or 75° and deep penetration welding morphology is obvious at 30° and 45° respectively.
To sum up, under the same welding process parameters, it is suggested that the blowing angle of shielding gas should be 30° for larger penetration.
If the surface width is large, it is recommended to use 45° blowing angle.
It is suggested to use 0° or 75° blowing angle if the lower melting width is larger.
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