By Dr. Mohammed Naeem
1. Welding cover gases
Aerospace materials (nickel & titanium based alloys) and stainless steels are readily laser weldable, however, special attention must be given to the joint cleanliness and gas shielding. These materials are highly sensitive to oxidation as well as interstitial embrittlement (via oxygen, hydrogen, nitrogen, and carbon), therefore it requires the use of an inert shield gas to provide protection against oxidation and atmospheric contamination. The most frequently used cover gas for nickel based alloys and stainless steels is oxygen free nitrogen. However, nitrogen should not be used for austenitic stainless steels alloyed with titanium and niobium. Nitrogen forms nitrides with these elements so there is less free titanium and niobium available for prevention of chromium carbide formation and corrosion. For titanium based alloys only argon cover gas should be used.
2. Types of porosity
There are two types of porosity that can occur during laser welding of nickel base alloys, i.e. fine scale porosity (typically <0.2 mm diameter) and large scale porosity (typically >0.5 mm diameter).
Fine scale porosity is present in most fusion welds often close to fusion boundary. It is spherical in form and has no significant effect on the mechanical properties of the weld, unless present in a large numbers and clustered, when it may reduce the load bearing capacity of the weld. This type of porosity is therefore likely to be influenced by the shielding gas (i.e. type, flow rate, etc.) used in the welding process.
Large scale porosity is defined as pores greater than 0.5 mm in diameter and is often found along the center line of the weld in the lower half of the weld bead. Due to its size, it may have a much more marked effect on the weld properties and may be more damaging. The mechanism of this type of porosity formation is probably explained in terms of instabilities in the welding keyhole since the pores are generally irregular in shape and occur deep in the weld. The keyhole instabilities can occur either due to insufficient power density at the workpiece or formation of plume above the workpiece during laser welding.
On average, the level of porosity with argon shield gas is significantly higher than the welds made with nitrogen shield gas. It is believed that reduced porosity under nitrogen shielding gas is due to reduced surface tension of the molten pool and hence the bubbles are more easily able to escape the weld pool.