How to Use "Gas" Correctly During Laser Welding

1. The Role of Protective Gas

In laser welding, the shielding gas will affect the weld shape, weld quality, weld penetration depth and width. In most cases, blowing in shielding gas will have a positive effect on the weld, but it may also have negative effects.

Positive Effects

1) Correctly blowing in the shielding gas will effectively protect the weld pool and reduce or even avoid oxidation;

2) Correctly blowing in protective gas can effectively reduce spatter generated during the welding process;

3) Correctly blowing in the shielding gas can cause the weld pool to spread evenly when solidifying, making the weld shape uniform and beautiful;

4) Correctly injecting protective gas can effectively reduce the shielding effect of metal vapor plumes or plasma clouds on the laser and increase the effective utilization of the laser;

5) Correctly blowing in protective gas can effectively reduce weld porosity.

As long as the gas type, gas flow rate, and injection method are selected correctly, the ideal effect can be achieved.

However, incorrect use of shielding gas can also have adverse effects on welding.

Negative Effects

1) Incorrect blowing of shielding gas may cause the weld to deteriorate;

2) Choosing the wrong gas type may cause cracks in the weld and may also reduce the mechanical properties of the weld;

3) Choosing the wrong gas injection flow rate may lead to more serious oxidation of the weld (whether the flow rate is too large or too small), and may also cause the weld pool metal to be seriously disturbed by external forces, resulting in weld collapse or uneven formation;

4) Choosing the wrong gas injection method will cause the weld to fail to achieve the protective effect or even have no protective effect at all, or have a negative impact on the weld formation;

5) Blowing in protective gas will have a certain impact on the weld penetration, especially when welding thin plates, it will reduce the weld penetration.

 2.Type of Protective Gas 

Commonly used laser welding protective gases mainly include N2, Ar, and He. Their physical and chemical properties are different, and therefore their effects on the welds are also different.

1. N2

The ionization energy of N2 is moderate, higher than that of Ar and lower than that of He. The degree of ionization under the action of laser is average, which can better reduce the formation of plasma cloud, thereby increasing the effective utilization of the laser.

Nitrogen can react chemically with aluminum alloys and carbon steel at a certain temperature to produce nitrides, which will increase the brittleness of the weld and reduce the toughness.

It will have a great adverse effect on the mechanical properties of the weld joint, so it is not recommended to use nitrogen to protect aluminum alloy and carbon steel welds.

2. Ar

The ionization energy of Ar is relatively the lowest, and the degree of ionization under the action of laser is high, which is not conducive to controlling the formation of plasma clouds and will have a certain impact on the effective utilization of the laser.

However, Ar activity is very low and it is difficult to chemically react with common metals.

Moreover, the cost of Ar is not high. In addition, the density of Ar is relatively large, which is beneficial to sinking above the weld pool and can better protect the weld pool, so it can be used as a conventional protective gas.

3. He

He has the highest ionization energy, and the degree of ionization under the action of laser is very low. It can well control the formation of plasma clouds. The laser can act well on metals. Moreover, He has very low activity and basically does not react chemically with metals. It is a very good weld protective gas. However, the cost of He is too high, and this gas is generally not used in mass-produced products. He is generally used for scientific research or products with very high added value.

3. Insufflation Method of Protective Gas 

There are currently two main methods of blowing protective gas: one is side-axis blowing of protective gas, as shown in Figure 1; the other is coaxial protective gas, as shown in Figure 2.

The specific choice between the two blowing methods depends on comprehensive considerations. Generally, it is recommended to use the side blowing protective gas method.

Figure 1   Protective gas is blown to the side of the rangeshaft

Figure 2  Coaxial protective gas

3. Principles for Selecting Protective Gas Injection Methods 

First of all, it needs to be made clear that the so-called "oxidation" of the weld is just a common name. Theoretically, it means that the chemical reaction between the weld and harmful components in the air causes the quality of the weld to deteriorate. It is common that the weld metal reacts chemically with oxygen, nitrogen, hydrogen, etc. 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 temperatures. This high temperature state is not just the molten pool metal, but from when the weld metal is melted until the molten pool metal solidifies, And its temperature drops below a certain temperature throughout the entire period of time.

4. Example 

For example, titanium alloy welding can quickly absorb hydrogen when the temperature is above 300°C, oxygen quickly when the temperature is above 450°C, and nitrogen quickly when the temperature is above 600°C. Therefore, titanium alloy welds must be effectively protected after solidification and when the temperature drops below 300°C, otherwise they will be "oxidized".

It is easy to understand from the above description that the blown protective gas not only needs to protect the weld pool in a timely manner, but also needs to protect the newly solidified area that has been welded. Therefore, the side-shaft side-blown protective gas shown in Figure 1 is generally used, because this method of protection has a wider range of protection than the coaxial protection method in Figure 2. Especially the area where the weld has just solidified is better protected.

Side-shaft blowing For engineering applications, not all products can use side-shaft side-blowing protective gas. For some specific products, only coaxial protective gas can be used, and specific selections need to be made from the product structure and joint form.

5. Selection of Specific Protective Gas Blowing Methods

1. Straight Line 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.

This type of product is best to use the side-shaft side-blow protective gas method shown in Figure 1.

Figure 3  Straight line weld

6. Flat Closed Graphic Weld

As shown in Figure 4, the weld shape of the product is a closed shape such as a flat circular shape, a flat polygonal shape, a flat multi-segment linear shape, etc., and the joint forms can be butt joints, lap joints, overlap welding joints, etc. This type of product is best to use the coaxial protective gas method shown in Figure 2.

Figure 4  Plane closed figure shape weld 

The selection of shielding gas directly affects the quality, efficiency and cost of welding production. However, due to the diversity of welding materials, the selection of welding gas is also more complicated in the actual welding process. Welding materials, welding methods, and welding positions need to be comprehensively considered. As well as the required welding effect, more suitable welding gas can be selected through welding testing to achieve better welding results.