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Meeting Challenges of New Aero Engine Designs

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

The aerospace industry is continually challenged to improve quality, reliability, performance, fuel efficiency and lower turbine engine emissions. This is driving engine manufacturers to consider fiber laser welding and the possibility to automate their welding processes to improve consistency and part quality.

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A typical automated setup for fiber laser welding high-strength alloy aerospace components using Laserdyne 795 BeamDirector system.

Another driver is changing workforce dynamics. It is hard to find or train skilled workers who can consistently and reliably weld titanium and high-strength nickel alloys.

Given these challenges, Prima Power Laserdyne provides the latest path for engine manufactures to automate welding processes for 3D engine components.

Incremental improvements allow for highly efficient welding of a wider variety of nickel- and titanium-based alloys with and without filler wire.

New machine features and capabilities allow for welding in difficult and previously inaccessible part locations.

Another often overlooked improvement is the elimination of fasteners by welding components together. This allows weld joints to be redesigned based on function rather than method of joining, thereby reducing weight while improving joined part reliability.

By adding an element of automation to the new processes, part quality is improved, worker shortages can be alleviated with predictable productivity and greater manufacturing profitability.

More Alloy Options, More And Better Design Opportunities

For many years in the aerospace and turbine engine industry, the general trend has been to continue with Electron Beam (EB) and Tungsten Inert Gas (TIG) welding.

Recently, more manufacturers are investigating the use of fiber laser welding for joining aerospace alloys. The reason: many of these alloys, especially high strength, precipitation hardened alloys, present problems for traditional welding methods as they are prone to heat affected zone (HAZ) and strain age cracking. This limits their manufacturability as well as their repair weldability.

The latest laser welding processes from Prima Power Laserdyne now allow efficient joining of these metals. In addition, many engine manufacturers and components suppliers are seeking more automated welding processes to improve the quality, consistency, and reliability of the weld joints.

As an example, titanium alloys, such as Ti6Al4V (6% Al, 4% V), Ti6242 (6% Al, 2% Sn, 4% Zr, 2% Mo) and TiCu2 (2%Cu) are widely preferred for blades and casing structures of the compressor stages in turbine engines. Even more challenging, nickel-based super alloys—Inconel 718, Inconel 909 and Single crystal 2000—are used in aero engines where operating temperatures are very high, up to 1400 degrees C beyond the melting point for many metals. (Figure 1)

Many methods are used to weld aerospace alloys, including traditional TIG and EB. However, new fiber laser technology can be a superior and cost-effective alternative for welding complex 3D-shaped components made from these alloys. Fiber laser welding provides many advantages including:

  • Fewer manufacturing stages, less edge preparation with joint fixturing the most time-consuming stage operation.
  • Narrow, deep-penetrating weld pool, allowing through-thickness welds to be made rapidly and accurately in a single pass without the requirement of vacuum.
  • Narrow and well-controlled heat affected zone (HAZ) with limited distortion and residual stresses.
  • The laser beam can be directed into previously inaccessible locations, providing the possibility for joints to be redesigned based on their function rather than the method of joining.
    The process is easily automated for high-volume production. For aerospace applications, these advantages can be translated into improvements in productivity and joint quality, as well as new opportunities for improved designs, leading to an overall reduction in aircraft weight.
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A typical automated setup for fiber laser welding high-strength alloy aerospace components using Laserdyne 795 BeamDirector system.

TIG and EB proponents identify the major drawback for fiber laser welding as the stringent joint requirements. In a typical butt joint, the widest acceptable air gap for autogenous laser welding is usually 10% of the material thickness. This tolerance can be fulfilled when components are relatively small and manufactured with machining or laser cutting. For larger components, it may be difficult to achieve the required accuracy, fit-up when they are positioned. The solution is the addition of a wire fed filler. Filler material compensates for the fit-up creating a solid butt joint while controlling the weld geometry and thereby achieving the desired robust weld metallurgy. With the ability to easily add filler wire to the welding process, the result is more welding processes that are compatible with a wider range of components.

The improvements Prima Power Laserdyne has undertaken with its unique hardware and software “smart techniques” produce superior quality welds with these features:

  • No or minimum porosity
  • No or greatly reduced start/ stop “divot” in closed welds
  • Well controlled shield gases during the welding process
  • No cracking
  • No top and under bead undercut
  • Top bead seam width of predetermined size
  • Waist (center of the weld) of predetermined size
  • Weld interface width control
  • Bottom bead seam of predetermined size
  • Oxide free top and bottom weld

Aerospace nickel and titanium alloys are fiber laser weldable. The welds are neat in appearance, consistently low porosity, no cracking in the HAZ and have low distortion when compared with their arc-welded counterparts. The reasons—fusion zone width and the grain growth is controlledby the laser power at the workpiece and the welding speed. (Figures 2 and 3 show weld speeds for typical alloy material thicknesses.)

As with all welding processes, special attention must be given to the joint cleanliness and the gas shielding. The high-strength alloys are often highly sensitive to oxidation during the welding process, especially titanium-based alloys. The most likely contaminants are oxygen, nitrogen and hydrogen. The nitrogen and oxygen are picked up from air entrained in the gas shield (improper gas shielding) or from impure shield gas while hydrogen is introduced from moisture or surface contamination. The oxides, nitrides and hydrides that form as a result of contamination increase the weld and heat affected zone (HAZ) add porosity and lead to a brittle weld, reduced fatigue life and reduced toughness. (Figures 4, 5 , 6 and 7)

Three Different Gas Shielding Nozzles Control Weld Oxidation Levels Adding Strength And Quality To The Autogenous Welds

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Figure 4: Tapered coaxial shield gas nozzle to weld components with restricted access.
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Figure 5: Coaxial shield gas nozzle to weld 2D and 3D components.
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Figure 6: Shield gas nozzle with extended gas shielding for autogenous welding of reactive materials.

 

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Figure 7: Color of the weld metal and heat affected zone adjacent to the weld indicates the level of oxidation (ppm = parts per million), as well as quality and strength of the weld. Oxidation of less than 20 ppm produces a silvery, shiny weld, the highest strength weld with the best appearance.

Gas Shielding And Laser Parameter Control Results

Note: With optimum gas shielding and control of the laser parameters, porosity and crack-free welds are achievable in a full range of nickel- and titanium-based alloys as shown in the above three weld images. (Figures 8,9 and 10)

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Figure 8: 2.3mm thick Inconel 718 Welding speed >2.5m/min.
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Figure 9: 3.2mm-thick Hastelloy-x; Welding speed >1m/min.
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Figure 10: 3.2mm-thick Haynes 230;
Welding speed >1m/min.

Note: Acceptable autogenous butt joints of titanium-based alloys are accomplished using Argon Shield Gas at 1.8 kW average power. (Figures 11 and 12)

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Figure 11: 1.4mm-thick Ti-6Al-4V alloy;
Welding speed 2.3mmin 2m/min.
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Figure 12: 3.0mm-thick Ti-6Al-4V alloy;
Welding speed 1.7m/min.
 

Fiber Laser Welding With Filler Material

Fiber laser welding is a viable solution for flat and complex 3D-shaped components. For a typical butt joint, the widest acceptable air gap for autogenous laser weld is usually considered to be 10% of the material thickness. This tolerance can be met if the components are relatively small with mating, clean edges that are either machined or laser cut. For larger components, filler wire can be used to compensate for any fit-up and mismatch for butt joint welding. This will control the weld geometry and achieve the needed weld strength.

Wire feed rate for a given air gap and plate thickness is an important parameter and will depend on welding speed and the cross-sectional area of the gap between the joint face and cross sectional area of the filler wire. While the addition of filler wire may result in a small loss in linear welding speed at a given laser power, the process benefits include faster setup, improved part fit-up and overall process time improvement that outweigh this small offsetting loss.

Acceptable wire feed delivery angles are between 30° and 60° with 45° being the norm, as it simplifies setting the wire intersection position with the laser beam centerline. Angles greater than 60° laser wire to the laser intersection are difficult and angles less than 30° create a large area of intersection to the laser beam, causing melting and vaporization of the wire without incorporating it into the weld pool.

The spot size should be close to the filler wire diameter. A laser spot size too small compared to the wire diameter leads to welds with porosity because the filler wire has not melted properly.

Successful Fiber Laser Welds Using Gas Fill Material

Figures 13,14,15,16 highlight the transverse sections of the laser welds with joint gap of 0.15mm for both 3.2mm thick nickel- and titanium-based alloys butt joints, respectively.

The welds were fully penetrated without any cracking, porosity and no underfill/undercut of the top bead. For those aerospace alloys noted, welding the underfill of the top bead is undesirable because it reduces the cross sectional thickness of the weld. This may lead to reduced tensile strength and create stress point as well as reduced fatigue strength of the joint.

Figures 13 and 14: 3.2mm-thick Inconel 625 superalloy; butt joint with 0.15mm gap; 1.2mm-diameter Inconel 625 wire; average power 1.8 kW; nitrogen shield gas.

8200_Laserdyne_Figure_131.jpg
Figure 13: 3.0mm-thick Ti-6Al-4V alloy;
Welding speed 1.7m/min.
8200_Laserdyne_Figure_141.jpg
Figure 14: Autogenous weld, slight undercut at top bead.
 

Figures 15 and 16: 3.2mm-thick Ti-6Al-4V titanium-based alloy; butt joint with 0.20mm gap; 1.0mm-diameter wire; argon shield gas.

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Figure 15: Autogenous weld; undercut and few pores.
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Figure 16: Weld made with filler material; no pores and undercut.
 

Conclusions

Aerospace manufacturers can benefit from fiber laser welding titanium- and nickel-based alloys because it minimizes the HAZ and eliminates the strain age cracking which was a barrier in the past.

With strong demand for engines, manufacturers are finding fiber laser welding with a multi-axis system for 3D parts and shapes is enabling engineers to design lighter, more cost effect components.

The combination of fiber laser welding and machine capability is providing consistent, robust, quality welds and at higher throughput. These systems support the goals of the aerospace industry by providing significant process and quality improvements.

The key to these advances are greater control of the process parameters—energy, distance and time. This is the same philosophy used in Laserdyne hole-drilling approach, which has made it the dominant supplier in that area.

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