As advanced welding techniques such as remote and hybrid welding become more popular, the issue of how to deliver the shielding gases must be addressed carefully
Mark Faerber, Joe Berkmanns, and Wolfgang Danzer
High-quality products demand suitable welding processes and conditions such as adequate weld pool atmosphere. Avoiding shielding gases and thus welding in an air atmosphere is not sufficient.
Based on traditional welding methods, we know that nitrogen, oxygen, and water vapor in the air can be detrimental to the seam and weld zone. If air comes in contact with annealed material, oxides, nitrides, and pores may form, affecting seam appearance and its technical properties such as formability and strength. Note that oxidation starts at approximately 250°C (5250K). This applies to steels-and not just to sensitive materials, such as magnesium or zirconium, which are materials that do not permit any air at all in the welding zone, otherwise they become very brittle.
In contrast, when it comes to laser welding of steels, more customers believe air serves as a highly effective-or, at least, acceptable-shielding gas. So, what has changed compared to traditional welding methods and enclosed material tests? One factor is definitely seam width: laser seams are much smaller than traditional welding seams. The enclosed weld pool is also smaller, as is the active surface of the melt pool that comes in contact with the shielding gas. In addition, filler wire is often not used; the droplets it produces react with air and help transfer foreign gases and reaction products to the weld pool. A second factor is the high welding speed of laser welding, which limits the time foreign gases have to react with the weld pool. High cooling rates also limit the heat-affected zone (HAZ) and post-weld reactions. A third factor in laser welding is plasma formation. Gases require energy and time to become active, that is, to become dissociated and ionized. An energy source could be, for example, an arc in MIG/MAG welding or a laser beam and the related plasma in laser welding.
Gas in the weld zone
The shielding gas, regardless of type, is exposed to high temperatures in excess of 10,000K in the weld zone. All gases are affected by the high temperatures and become dissociated, ionized, and thus activated. However, the extent to which the gas becomes active depends greatly on the type of gas, temperature, and amount of time it is exposed to the high temperature.
Nitrogen effects
In the case of steel welds, CO2 laser welding with an argon-nitrogen mixture results in nitrogen levels up to equilibrium nitrogen solubility, according to Kokawa1. Nitrogen pick-up in Nd:YAG laser welds, on the other hand, was considerably lower; pick-up in TIG welds was considerably higher. Nitrogen pick-up takes place at a temperature interval between the melting and boiling point of iron and is thus concentrated on the liquid iron phase, the melt pool surface, the edges of the keyhole, and the keyhole walls in laser welding. During TIG welding, spectral images indicate a high concentration of dissociated nitrogen in the high-temperature arc plasma. This nitrogen was extensively distributed over the large weld pool. In contrast, the pool in laser welding is small. Considerably less dissociated nitrogen was detected in the high-temperature laser-induced plasma. Differences in nitrogen content in CO2 and Nd:YAG welds are assumed to be related to the plasma temperature, which is lower in Nd:YAG laser welding. This means partial pressure is lower and less nitrogen is absorbed. Down the keyhole, high metallic vapor partial pressure reduces nitrogen partial pressure, which is considered to be the cause of the decrease in nitrogen deep inside the material.1
Nitrogen becomes embedded in the iron matrix. Even small amounts cause hardness and volume to increase and formability to decrease.2 For this reason nitrogen must be excluded, for example, in laser welding of coil material where an enlarged seam volume would trigger an error message of the automatic quality inspection system. This process is characterized by very high welding speeds of thin steel material. However, nitrogen pick-up would still be unacceptably high for this application.
Nitrogen pick-up has also been reported in laser welding of automotive grade sheet material, resulting in clearly reduced formability (see Figure 1) compared to helium, argon/CO2, and argon.3 Furthermore, physical properties do not change if nitrogen is diluted with other gases such as argon or helium.4
In solid-state laser welding, nitrogen pick-up seems to be considerably lower, as will be shown below. Fatigue test results of laser welds made with argon or nitrogen gas shielding are similar due to the different plasma conditions as compared to CO2 laser welding. The plasma in solid-state laser welding is considerably less dense and markedly cooler than CO2 laser plasma, which clearly exhibits lower nitrogen activation efficiency.
Effects of air
Air, which basically consists of nitrogen and oxygen, becomes dissociated in the high-temperature welding zone. The effect of nitrogen was discussed above. Here, we will take a look at the effect of oxygen, which is easier to activate than nitrogen. Note that oxygen already forms oxides at temperatures above approx. 250° C or 525K. Oxygen is known to refine the iron matrix. However, if there is too much oxygen, oxides are formed in and on the material such that the surface area of the weld pool and the HAZ are covered with an oxide layer. High melting temperatures of the oxide layer clearly reduce wetting properties and impair metal-to-metal bonding mechanisms. Consequently, weld imperfections are created that may act as crack initiators (see Figure. 2). In static tensile strength tests these cracks seem to have only a limited impact on fatigue strength. In contrast, in dynamic load tests, the fatigue strength of samples welded in air with solid-state laser is markedly lower than that of samples welded with nitrogen or argon shielding (see Figure. 3).
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Effects of gas feeding
If, at this point, you have decided to eliminate air from your weld zone, you may want to take a look at the gas supply device. Blowing shielding gas towards the weld pool does not automatically mean that it will have the desired effect on the weld pool atmosphere. In the early days of laser welding, a “plasma jet” was developed to basically blow away the plasma cloud in CO2 laser welding. This requires a high-velocity gas stream, which is turbulent and injects ambient air based on the Venturi principle. With this system, the weld pool and HAZ are probably covered with only air. The next step was to develop a larger nozzle that blows from the side with a considerably lower gas stream velocity. The plasma is controlled by a portion of helium in the laminar gas stream. This configuration requires exact alignment and adjusted gas flow rates to guarantee an air-free shielding gas atmosphere.
Nitric oxide (NO) tests have been used to examine gas shielding with this type of side nozzle and the resulting weld area atmosphere. NO is generated if any air is present in the welding zone. Figure 4 explains why a minimum required argon flow rate of 25 l/min (0.9ft3/min) is required to exclude air and enclosed NO from weld zone.
The classical coaxial nozzle as a third alternative is effectively used with solid-state lasers. In CO2 laser welding, on the other hand, it seems to be difficult to get the shielding gas down to the welding area against the hot plasma rising from the weld pool. If the gas velocity is too high, then it disturbs weld pool motions; if it is too low, then it does not provide any shielding.
Another hidden problem is the “cross jet.” This nozzle is meant to blow away spatter and smoke from the optics. However, its set-up is also crucial for the welding area atmosphere. If not “neutrally” aligned, it will either blow towards the welding area or draw gas away from the welding area. In this case, the perfectly aligned shielding gas nozzle is not able to operate effectively.
Most suitable welding gases
In CO2 laser welding, a laminar shielding gas supply of argon-helium has been reported to be most effective. This basic mixture can be doped with, for example, CO2 for specific applications such as welding of tailored blanks. In Nd:YAG welding, helium is not required; argon and even nitrogen seem to be suitable shielding gases. Fiber lasers are much more powerful than Nd:YAG lasers so that helium seems to be the best choice. However, please note that these lasers as well as the enclosed studies are still in the very early stages.
Dr. Mark Faerber is marketing manager laser applications; Dr. Wolfgang Danzer is head of the Laser Technology Department; Dr. Joachim Berkmanns is program manager laser applications, all with Linde AG. For more information, contact [email protected].
References
- H. Kokawa, “Nitrogen absorption and desorption by steels during arc and laser welding,” Welding International 18, 2004, 227-287.
- H. Sinclair, K. Nilsson, “Influence of shielding gas type in CO2 laser welding of carbon steels,” Techniska Hogskolen I Lulea, Laserforskning, 1993.
- J.C. Ion, “Evaluation of the mechanical performance of steel CO2 laser welded with different shielding gases,” TWI, 1994.
- C.J. Dawes, S.C. Kennedy, R.C. Crafer, “Shielding gases for CO2 laser welding-an experimental case study on lap welds in HSLA steel,” TWI, 1989.