Both the metallurgy and geometry of these parts are complex and each type requires a unique set of repair parameters. Even parts from the same family exhibit minute differences that can influence the repair process. The combination of these variable requirements, coupled with high repair volumes and stringent industry standards, create a need for adaptive automation.

Liburdi Group, a provider of specialized technologies and systems for turbine and aerospace applications, designed a patent-pending blade repair technology utilizing a laser and solid wire filler metal.

Industrial laser manufacturer GSI Lumonics worked with Liburdi on this project since its inception, a little more than a year ago, supplying several models of Nd:YAG, continuous-wave (CW) modulated lasers.

Laser and Filler Metal Combination
In its basic form, this new metal deposition process involves directing a laser beam onto the substrate to form a weld pool. A wire filler metal is then introduced into the weld pool, causing the wire to melt. Relative motion between the substrate and the laser beam results in solidification of the molten metal forming a continuous weld bead - Fig. 1.

The primary benefits of this technology include decreased heat input, which eliminates the need for complex cooling chills; increased processing speed; and a level of weld bead shaping that allows the achievement of "near-net-shape" buildup - Fig. 2.

The System
The two system features that have the greatest influence on the process are the filler metal delivery and the welding power source.

The choice of solid welding wire over metal powder was dictated by weld acceptance criteria imposed by the industry. One of the problems associated with metal powders is their susceptibility to oxidation caused by the increased surface area of the small particles. This can lead to unacceptable porosity levels, especially when welding titanium alloys. The powder delivery method is also susceptible to "overspray." Under the best of circumstances, only about 80% of the powder actually melts in the weld pool. The rest of it ends up on the tooling or sticks to the part, creating unpleasant working conditions and potential damage to the part.

Wire filler metal provides a greater level of control than powder feeders, which typically require at least five seconds to stabilize at a particular feed rate. Due to this limitation, they are used exclusively as On/Off devices, making it difficult to create a varying weld bead geometry. In addition, powder feed systems can exhibit poor control and stability at low flow rates, making the process less reproducible.

Modern wire feeders can be controlled very accurately. The feeders chosen for this technology are closed-loop devices that utilize optical encoders for feedback. This level of control allows not only for precise and repeatable wire delivery, but also permits fully synchronized wire feed pulsing with other process parameters. For critical applications, a wire diameter measurement module can be integrated into the systems. The module measures minute deviations from the nominal wire size and automatically compensates wire feed rates.

The next advance in technology was to use a laser as the energy source. Using lenses, the laser can focus onto the workpiece to a spot diameter of a fraction of a millimeter. This spot size can be controlled by the use of appropriate optics, and the high-power density provided by the laser minimizes the overall heat input. Lower heat input minimizes distortion and eliminates the need for complex chills to cool the part. The focused laser light provides a consistent energy source that is not affected by electrode and orifice wear.

The Nd:YAG laser was chosen because its beam wavelength allows the use of glass optics to steer and focus the beam to the workpiece using fiber-optic delivery. Direct viewing of the area before and during processing is also possible.

Dealing with Back Reflection
This wire-and-laser welding technology is essentially a metal deposition process, requiring relatively shallow weld penetration and a smooth, highly profiled deposit of filler metal over the compressor, turbine, or impeller blade being repaired. Characterized as a high-power, nonkeyhole process, the parameters typically result in a larger-than-normal proportion of the beam not being coupled into the workpiece. The resultant light can easily be reflected back onto the fiber-optic system and potentially cause significant damage. This is because at the onset of laser beam welding, especially on highly reflective materials and metals, as much as 80­90% of the laser's light can be bounced back onto the fibers until the welding surface breaks down and begins to form the weld pool.

This potential problem with the laser-and-wire welding technique was minimized with patented terminations at each end of the fiber-optic cable. This system allows welding at full power without the risk of back reflection damage by safely channeling the reflected laser light via a capillary tube safely away from the fibers and into the beam dump - Fig. 3. The light routed to the beam dump is continuously monitored and a detector lets the user know if back reflection is being generated at a dangerous level. If the amount of back reflection exceeds acceptable limits, this closed-loop monitoring system triggers an automatic shutdown response, thus protecting the fibers from unacceptable and expensive damage.

This design enhancement makes it realistic and cost-effective to do such high-power welding applications with fiber-optic beam delivery systems.

Technology for the Future
To date, production trials have shown impressive process improvements with both turbine compressor blade repair as well as geometrically more complex impellers. For the future, anything requiring material buildup to specific geometry and profile might be best served by laser-and-wire technology.