A Prima Power Laserdyne post titled “Selecting and Delivering Shield Gas in Laser Welding” highlights the various types of shield gases used for laser welding and their specific applications. For example, results of applications R&D studies within Prima Power have supported conclusions of academic research that “nitrogen also tends to yield lower porosity than argon in welding steels, stainless steels, and nickel-based alloys.”
A recent study conducted by Prima Power Laserdyne has shown that the choice of shield gas is also important in welding the structural steel alloy A36.
About the alloy
ASTM A36 is one of the most commonly used hot-rolled steels. It has a low carbon content with good strength and good formability. It is easy to machine and fabricate and is easy to weld. Applications and industries in which A36 is found include tanks, fixtures, cams, gears, ornamental work, automotive and agricultural equipment, and machinery parts.
Table 1 shows the composition of the alloy as published by ASTM International. This level of carbon makes the alloy easy to laser weld. With the relatively low carbon content of the alloy, even the rapid cooling associated with laser welding produces a structure with good mechanical properties.
Table 1: Chemical compositions of A36 steel specified by ASTM (weight %)
Weld process development
Edge welds of 4.75 mm thick A36 plates (Figure 1) were produced using a Convergent Photonics CF3000 CW fiber laser. The laser beam was delivered to the weld joint using a 600-µm fiber, 140 mm focal length recollimating lens, and 150 mm focal length focusing lens. The theoretical focused beam diameter based on this configuration is 643 µm.
A range of laser and process conditions were used to document the relationship between welding speed and weld penetration for a range of focus positions and three assist gas conditions.
Weld profile and penetration
Weld penetration versus welding speed for each of the assist gas conditions was documented. From Figure 2, one can see that the greatest penetration for a given welding speed is achieved with no assist gas, presumably due to the additional energy contributed to the process by the oxygen in the ambient environment.
Figure 3 shows the profiles of welds produced with each of the three assist gas conditions. The focus of the laser beam was on the surface for the weld made with no gas while the focus was 1 mm above the surface for welds made with nitrogen and argon assist gas.
|(a) No assist gas||(b) Nitrogen assist gas||(c) Argon assist gas|
|Figure 3: Laser weld profile using 3 kW average power and welding speed of 0.5 m/min and different shielding conditions.|
Figure 3a shows the crossection of an edge weld made without a shield gas. In other words, the weld was made in the ambient air environment.
Figure 3b and 3c show crossections of welds made with nitrogen and argon respectively. The shape of the fusion zones (FZ) of these two welds is the same. However, the heat affected zone (HAZ) of the weld made using argon as the shield gas is wider than that made using nitrogen because of the greater amount of plasma formed with argon.
Another important difference between these two welds is that the one made using nitrogen as the shield gas does not show any porosity while the weld made in the presence of argon shows considerable porosity. For more information about shield gas effects in laser welding, read the article titled “Selecting and Delivering Shield Gas in Laser Welding“.
Weld microstructure and hardness
Table 2 shows Vickers microhardness of the weld fusion zone, heat affected zone, and base metal for several weld conditions.
Table 2: Microhardness of A36 welds and base metal samples
|Welding speed||Shield gas||Focus position||FZ (HV)||HAZ (HV)||BM (HV) (m/min)|
|0.5||No gas||0 mm||280.9||186.5||159.8|
The higher fusion zone hardness associated with the higher welding speed is consistent with the expectation that a higher cooling rate will yield a greater proportion of the hard martensite phase in the weld fusion zone. Conversely, the lower hardness associated with no gas (no convective cooling; presence of oxygen) compared to that for nitrogen and argon shield gases, suggest that the cooling rate for no gas is lower than that for the two conditions involving a shield gas.
Inspection of the weld fusion zones at higher magnifications revealed that the microstructure of welds produced at 0.5 m/min consists of a greater amount of bainite (Figure 4a) than those produced at 1.25 m/min. On the other hand, the major constituent of the weld produced at the higher speed (1.25 m/min) is martensite (Figure 4b).
|(b) Welding speed: 0.5m/min. (Etched in 2% Nital.)||(a) Welding speed: 1.25m/min. (Etched in 2% Nital.)|
|Figure 4: Edge joint weld of 4.5 mm thick A36 plates at different welding speeds. Both welds were produced using an average power of 3kW and nitrogen shield gas.|
Laser welds of ASTM A36 structural steel can be produced without any assist gas. This simplifies the welding process by eliminating the need to provide a means of delivering the shield gas to the weld joint.
The main conclusions from the present study are:
- There was no major difference in the weld penetration with nitrogen, argon and no gas.
- The specimens welded with argon shield gas gave the greatest porosity, while those welded with nitrogen and no gas had no porosity. The results were very consistent for all the laser parameters tested.
- Reduced porosity under nitrogen and no gas is likely due to reduced surface tension of the molten pool and the consequence that gas bubbles are more easily able to escape the weld pool.
- Microhardness results support the fact that higher cooling rates yield higher hardness and vice versa.
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