The aerospace industry is continually challenged to improve quality, reliability, performance and fuel efficiency while also lowering 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. Another driver is changing workforce dynamics. It’s 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 manufacturers to automate welding processes for 3-D engine components. Incremental improvements allow for highly efficient welding of a wider variety of titanium and nickel-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 and worker shortages can be alleviated – all with predictable productivity and greater manufacturing profitability.
For many years, the general trend in the aerospace and turbine engine industry has been to continue with electron beam (EB) and TIG welding. Recently, however, 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 zones (HAZ) and strain age cracking. This limits their manufacturability as well as their repair weldability.
In response to these issues, new laser welding processes from Prima Power Laserdyne allow efficient joining of these metals. In addition, many engine manufacturers and component 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, Ti6242 and TiCu2 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 aerospace engines where operating temperatures are very high, up to 1,400 degree C beyond the melting point for many metals (see 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 3-D shaped components made from these alloys. Fiber laser welding provides many advantages including:
- Fewer manufacturing stages, including less edge preparation with joint fixturing, which is often the most time-consuming 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 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.
TIG and EB proponents identify stringent joint requirements as the major drawback for fiber laser welding. However, for a typical butt joint, the tolerance can be fulfilled when components are relatively small and manufactured with machining or laser cutting. For larger components, filler wire can be used. 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 quality welds with the following features:
- No or minimum porosity
- No or greatly reduced start/stop “divot” in closed welds
- Well-controlled shielding gas during the welding process
- No cracking
- No top and under bead undercut
- Top bead seam 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 titanium and nickel-based alloys are fiber laser weldable. The welds are neat in appearance, consistently low in porosity, have no cracking in the HAZ and have low distortion when compared with their arc-welded counterparts. The reasons – fusion zone width and grain growth – are controlled by 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. High-strength alloys are often highly sensitive to oxidation during the welding process, especially titanium alloys. The most likely contaminants are oxygen, nitrogen and hydrogen. The nitrogen and oxygen are picked up from air entrained in the gas shielding (improper gas shielding) or from impure shielding gas. Hydrogen is introduced from moisture or surface contamination. The oxides, nitrides and hydrides that form as a result of contamination increase the HAZ, add porosity and ultimately lead to a brittle weld, reduced fatigue life and reduced toughness (see Figures 4, 5, 6 and 7).
Fiber laser welding is a viable solution for flat and complex 3-D shaped components. For a typical butt joint, the widest acceptable air gap for autogenous laser weld is usually considered to be 10 percent of the material thickness. This tolerance can be met if the components are relatively small with mating and have clean edges that are 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 which outweighs this small offsetting loss.
Acceptable wire feed delivery angles are between 30 and 60 degrees with 45 degrees being the norm, as it simplifies setting the wire intersection position with the laser beam centerline. Angles greater than 60 degrees to the laser intersection are difficult to work with, and angles less than 30 degrees 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 also 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.
Figures 8, 9 and 10 highlight porosity and crack-free welds that were achieved with optimum gas shielding and control of the laser’s parameters. Figures 13, 14, 15 and 16 highlight the transverse sections of the laser welds with a joint gap of 0.15 mm for 3.2-mm-thick nickel-based and titanium alloy butt joints, respectively. The welds were fully penetrated without any cracking or porosity and with no underfill/undercut of the top bead. For the 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 points as well as reduced fatigue strength of the joint.
Overall, aerospace manufacturers can benefit from fiber laser welding titanium and nickel-based alloys because it minimizes the HAZ and eliminates strain age cracking, which was a barrier in the past. With strong demand for engines, manufacturers are finding that fiber laser welding with a multi-axis system for 3-D parts and shapes is enabling engineers to design lighter, more cost-effective components.
The combination of fiber laser welding and machine capability is providing consistent, robust, quality welds and at a higher throughput. These systems support the goals of the aerospace industry by providing significant process and quality improvements. The keys to these advances are greater control of the process parameters – energy, distance and time.