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Fiber Laser Welding as a Change Agent

Dr. Mohammed Naeem Senior Manager, Applications Engineering & Technology Development Prima Power Laserdyne
By Mohammed Naeem Senior Manager, Applications Engineering & Technology Development, Prima Power Laserdyne

Fiber laser welding is gaining attention in aerospace manufacturing. Manufacturers are looking for ways to automate manual arc-welding processes and to upgrade CO2 and lamp-pumped Nd:YAG laser welding processes to ensure greater consistency and to improve their productivity.

Laser welding is being used for both aero-engine and airframe applications. Within the aero-engine, there are applications for welding of both hot section and non-hot section components. During a recent discussion with a process engineer at a leading aircraft engine manufacturer, we learned of initiatives to replace current Tungsten Inert Gas (TIG) welding of sheet metal components.

Candidates for fiber laser welding include components made of nickel and titanium-based aerospace alloys requiring control of the weld geometry and weld microstructure, particularly minimizing porosity and oxidation of the weld microstructure.

Challenging TIG and EB Welding

The growth rate of fiber laser welding is second only to additive manufacturing among the many applications for high-power laser materials processing, according to a recent report of a study by Strategies Unlimited.

In aerospace applications, fiber laser welding has potential to significantly add to this growth rate because of its many advantages over Tungsten Inert Gas (TIG) and Electron Beam (EB) welding processes for welding exotic aerospace alloys.

Reasons for the growth of laser welding over these traditional welding methods include:

  • Fiber laser welding involves fewer manufacturing stages, with edge preparation and joint fixturing being the most time-consuming operations
  • The high beam power density of fiber laser welding creates a narrow, deeply penetrating weld pool, producing robust, through-thickness welds rapidly and accurately in a single pass without the presence of vacuum.
  • The low heat input of fiber laser welding creates a narrow heat affected zone in the workpiece material with minimal distortion and residual stresses. This improves quality and reduces the need for reworking.
  • Filler material may be added to the fiber laser welding process to compensate for poor component fit-up and mismatch. Apart from compensating for less than ideal fit-up, filler material improves the weld geometry by eliminating top and bottom bead undercut.
  • Laser welding is an automated process in which the main factors affecting quality are precisely controlled.

Most important, fiber laser welding systems have become considerably more capable over the last decade, inspiring aerospace manufacturers to reconsider laser welding.

Specifically, today’s laser welding systems with high power fiber laser sources offer a combination of high average power, high electrical efficiency, and a wavelength that contributes to stable welding processes and consistent weld quality. They also offer the benefits of precise, high speed control of the laser that results from the intrinsic characteristics of these solid state devices.

Combined with the advantages of CNC, automated operation, fiber laser welding clearly stands out as a welding process now and for the future.

Unique Requirements

Historically, fiber laser welding has found its greatest application in industries like automotive, medical devices and electronics because of its ability to provide miniature welds with negligible distortion. The requirements of aero-engine, as well as airframe component welding, can be far different.

Due to stringent quality and strength requirements of aerospace components, the demands of welds made of titanium and nickel-based alloys are greater. To meet these challenges, today’s laser welding technology provides superior control of the weld geometry and weld strength essential in welded aerospace assemblies.

In many aerospace applications, the fatigue properties of the weld are a critical design criteria. For this reason alone, designers nearly always specify that the weld surfaces be convex, or slightly crowned, to create a reinforcement of the weld made possible with the Laserdyne 795XL fiber laser system.

Consider Nickel-Based Alloys

Engine hot section components require the use of certain nickel-based alloys for the heat resistant characteristics of these materials. With engine temperatures often exceeding the material’s melting point, choice of the material itself along with design of part cooling holes allows for a cooler operating engine that delivers maximum thrust and lower emissions.

Of primary concern for these nickel-based alloy parts and assemblies when fiber laser welded is the microstructure of the weld. There can be no porosity, negligible oxidation of the structure, and no microcracking. While it is possible to meet these quality requirements and many of the geometry requirements using an autogenous (no material added) process, it is difficult to consistently produce welds with a crowned (convex or reinforced) top bead and bottom bead, even with a perfect fit-up of the joint.

As part of the preparation for welding, the edges of the segments of this contoured Inconel 625 component are machined for accurate part fit up. A 1.2 mm diameter filler wire is added to the weld joint as part of an automated welding process. Addition of a precise amount of filler wire to the butt joint leads to a consistent crown of the required dimensions on both the top and bottom weld bead.

Since Inconel 625 has been proven to be reliably welded by laser, selection of Inconel 625 wire contributes to robust mechanical properties by ensuring a sound weld microstructure.

For full-penetration butt welding, a continuous wave beam is used to achieve the highest speed. Where required to control heating and cooling rates, a pulsed or modulated beam is used though at the sacrifice of welding speed and, therefore, cycle time. Laser beam modulation has also been proven to minimize porosity in partial penetration overlap welds since modulating the beam facilitates escape of gases within the weld fusion zone.

This successful aerospace fiber laser welding application has been in operation at Airbus for more than a year with hundreds of welded assemblies having been fabricated using the Laserdyne 795XL BeamDirector system. A continuous ramping up of quantities is ongoing.

Consider Titanium Alloys

Titanium alloys are used in aircraft and aero-engine applications because of their high strength to weight ratio. However, these parts are not used in aerospace parts that reach a temperature over 350°C while in service. Above this temperature, pronounced oxidation occurs which can lead to a reduction in weld quality and weld strength over time.

Typical aerospace parts made from titanium alloys include turbine disks, compressor blades, airframe and space capsule structural components, rings for jet engines, pressure vessels, rocket engine cases, helicopter rotor hubs, and fasteners. Among the most commonly used alloy for these parts is Ti-6Al-4V, a two-phase alpha-beta (α-β) alloy noted for its high strength to weight ratio and its corrosion resistance.

To weld titanium aerospace components, the relationship between laser and process parameters, weld geometry, and weld microstructure for each type weld needs to be understood. While the weldability of Ti-6Al-4V alloy, whether with or without (autogenous) filler material, is in general very good, the atmosphere around the weld must be controlled for effective results.

Titanium and its alloys react strongly with oxygen, nitrogen, and hydrogen to create compounds that increase the hardness and reduce the impact strength of the weld and its adjacent heat affected zone.

For this reason, one of the fundamental challenges in welding of titanium alloys is to eliminate atmospheric contamination by proper shielding.

The color of the surface gives a good indication of the degree of atmospheric contamination of the weld. Under ideal shielding conditions, the weld is bright and silvery in appearance.

As the level of atmospheric contamination increases, the color of the weld and surrounding area changes from a silvery, metallic appearance to straw/tan, then to blue and finally, to a powdery white. With increased contamination comes increased hardness of the weld fusion zone from brittle nitrides, oxides, and hydrides within the weld. While increasing hardness, these compounds reduce weld fatigue properties and toughness.

To minimize atmospheric contamination and maintain maximum toughness of the titanium alloy welds, a gas shielding shoe is commonly used.

The shoe provides inert gas coverage over a relatively wide area of the weld, shielding the area of melting, as well as protecting the welded material, as it cools to a temperature at which it will not react adversely with the atmosphere. Welds with the silver appearance or, in the worst case, a straw (light gold) color indicate that proper shielding has been provided.

As with nickel alloys, another important challenge in welding titanium alloys relates to joint fit up or joint gap. Given the requirements for reinforcement or a crown to the weld profile, wire of the same alloy as the base material is commonly added to the weld.

More Control

Fiber laser welding with the addition of filler metal provides greatly enhanced control of the process, including improved weld geometry (size, shape) and microstructure.

This process not only is used for welding the same alloys but provides flexibility for welding dissimilar metal and alloy combinations.

Applying this fiber laser welding process in the CNC automated setup provides aerospace manufacturers with the means to upgrade existing manual welding processes with their inherent variability.

Fiber laser welding, with a wavelength more favorable to welding and ability to precisely control key laser parameters at high speeds, also enables design and process engineers to achieve more robust processes and more consistent weld properties within the limits of the alloy and filler wire alloy being welded.

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