Considerations in Welding Nickel-Based Aerospace Alloys

Over the past two years, Prima Power Laserdyne Applications Engineers have invested much time in developing laser welding processes that meet the stringent requirements of aerospace, both for airframe as well as for aero-engine applications.

These efforts have led to the demonstration of high quality butt and lap joint welds in a number of common nickel-based aerospace alloys, some examples pictured below. These show the desirable crown (convexity) of the weld on both the top and bottom sides of the weld, ability to control weld dimensions with or without filler metal, and no observable porosity.

Hastelloy X, 3.2 mm thick	Inconel 625, 0.8 mm thick	Inconel 625, 3.2 mm thick	Haynes 230, 3.2 mm thick

The correct laser process parameters must take into account the relatively large solidification temperature range of nickel based aerospace alloys and that brittle phases can form when solidification rates are low. If these phases are unable to withstand the stresses associated with the welding process, cracking may occur. However, applying this knowledge during process development can, and has, led to crack-free welds.

In addition to laser process parameters, there are two other important considerations with laser welding nickel-based aerospace alloys: (1) joint cleaning and (2) fixturing.

Joint cleaning

Some argue that proper cleaning of the material to be heated and melted during welding is the most important requirement for successful joining of nickel alloys. At high temperatures, nickel alloys are susceptible to embrittlement by sulfur, phosphorus, lead, and other low-melting-point substances that are often present in materials used in everyday manufacturing processes. For example, grease, oil, paint, cutting fluids, marking crayons and inks, processing chemicals, machine lubricants, and temperature-indicating sticks, pellets, or lacquers may be found on materials to be welded. In situations where it is impractical to avoid the use of these materials during processing and fabrication of the alloys, it is necessary that the metal be thoroughly cleaned prior to welding.

Oxides, particularly those visible on the surface of the materials to be welded, can lead to porosity and other weld defects if the oxides are incorporated into the weld. The light oxide that forms when clean material is exposed to normal atmospheric temperatures will not cause difficulty. However, the heavy oxide that forms during exposure to high temperatures (hot-working, heat-treating, or high-temperature service) must be removed. Failure to remove heavy oxides can lead to porosity and lack of fusion since the oxides melt at temperatures significantly higher than the base metal.

Fixturing

Laser welding fixture for butt and overlap welding of plate and sheet.

Since the thermal expansion characteristics of nickel-based aerospace alloys are similar to those of carbon steels, one can expect similar forces to be generated and similar distortion to be produced during welding. The restraint provided by a properly designed fixture, such as that shown in the picture at the right, can be used to control stresses in the weld. For example, if an appropriate clamping force is used to restrain the material near the weld joint, the expansion created in the weld joint will lead to a compressive force in the weld. This compressive force will in turn lead to upsetting of the weld metal and a corresponding reinforcement, or crown, of the top and bottom of the weld, even without filler metal.

In addition to providing proper clamping of the materials during welding, the fixture must also provide proper shielding of the top and back sides of the weld. Shield gas is provided to the top of the weld using the SmartShield™ welding nozzle. Shield gas is provided to the back side of the weld through a groove in the fixture beneath the weld.

Prima Power Laserdyne Applications Engineers have many years of experience in laser welding with fundamental knowledge in welding metallurgy, laser process development, fixture design, and programming. In other words, we know how to develop an application for laser welding and move it into production.