Rotors are one of the most critical components in impeller machinery and equipment. These precision components rotate at extremely high speeds and must withstand significant stresses over a wide range of service times.
To achieve this level of reliability, manufacturers must ensure that the components are suitable for the application. Strict controls on composition, mechanical properties and processing ensure that parts are acceptable.
These checks, verifications and safeguards maximize service life while minimizing the risk of catastrophic failure.
However, wear and tear from normal operation will eventually cause enough damage to require repair or replacement. Accumulated damage is usually superficial, and repair provides cost and time advantages over replacing the entire rotor while increasing the minimal risk associated with the repair process.
Typical repair processes include painting, plating, arc welding, plasma welding, and laser welding. Each process has advantages and disadvantages depending on a variety of factors including the location and extent of damage, operating conditions, environment of use, substrate and repair material required, and customer acceptance.
This article focuses specifically on laser welding repairs and how the laser welding process can benefit compressor and turbine shaft repairs, including issues to be addressed.
Discussion includes the shaft areas most commonly repaired, the risks of laser welding in these locations, and the types of tests that should be required to validate the procedure.
Laser Beam Welding
Prior to the advent of laser beam welding (LBW), the most common shaft repair process was submerged arc welding (SAW), primarily because of its ruggedness and high deposition rate.
However, this process involves high heat input, which can lead to shaft distortion and high residual stresses. Due to distortion, SAW repairs often require the removal of all protruding features from the repair area, reconstruction of these features, and extensive overlay to ensure sufficient machining allowance to restore dimensions.
In addition, due to the high residual stresses generated by welding, repairs always require Post Weld Heat Treatment (PWHT) prior to final machining, which relieves the residual stresses and thus minimizes axial motion (distortion) during machining operations.
Welding (including cladding), cutting and heat treatment can be performed using focused lasers. Although LBW has been around since the 1970s, improvements in technology and affordability have expanded its industrial applications to now include impeller mechanical rotor repair.
The main advantage of LBW is that it is a high energy density process and therefore can be welded with very low heat input, thus minimizing base metal degradation, heat affected zone (HAZ) size, residual stresses and distortion, while also allowing for very fast welding speeds.
At the same time, a smaller heat affected zone is also beneficial because less of the axial volume has deleterious properties caused by the heat generated by the fusion process.
This is particularly important in the case of heat-treatable alloys, such as quenched and tempered steels, which are commonly used in impeller machinery rotors. An example of a laser welding setup is shown in Figure 1.
In addition to low heat input, the LBW process produces high quality fusion welds with metallurgical bonding (no delamination, adhesion that can occur in coatings), easy automation, consistency and repeatability, and high geometric accuracy.
For example, the laser spot sizes used in this study can range from a small weld with a diameter of 0.2 mm to a high deposition rate overlay with a diameter of 2.0 mm.
In order to take full advantage of the LBW process, the process capabilities must be matched to the application, and the other considerations outlined below must be examined before implementing LBW for rotor repair.
Metal Filling
There are two different processes for laser welding. One uses powder-filled metal (LBW-P) and the other uses wire-based filled metal (LBW-W). In LBW-P, powder is delivered from a powder feeder through a tube and one or more nozzles by an inert gas jet, which transports the powder into the weld pool.
In LBW-W, the filler metal is delivered by manually or mechanized wire feeders that feed the wire into the weld pool.
There are metallurgical and logistical differences between the two methods, which must be taken into account when determining the most appropriate process for a particular repair. This is especially true considering that ASME BPVC has not yet taken these differences into account.
Among the basic variables are details related to the powder filler metal, including powder metal size, density, and feed rate. However, there is no mention of fill line parameters.
This indicates that the current specification only considers powder laser welding applications. It also follows that process characterization is only relevant to powder-based laser welding.
This is one reason why laser beam welding may require additional process characterization requirements.
Laser Sources
A variety of laser light sources can be used for laser welding. This article focuses on the two most common welding laser sources, the Nd:YAG laser and the fiber laser.
Nd:YAG lasers consist of neodymium-doped yttrium aluminum garnet crystals that are excited by a xenon flashlight to produce a laser beam, while fiber lasers consist of a set of diodes that excite a rare-earth-element-doped fiber to produce a laser beam.
While both laser sources can be used for rotor repair, they both offer tradeoffs including beam quality, beam size, beam frequency, lifetime, cost and efficiency.
Choosing the best laser depends on the application. However, when ASME BPVC compliance is an issue, fiber lasers are a better choice.
The reason for this is the difference in how the laser beam is generated and its stability over time. In Nd:YAG lasers, the xenon flash bulb degrades over time and dims with age.
Dimming the bulb causes the excitation of the Nd:YAG crystal to diminish, which reduces the intensity of the resulting laser beam. The result is that the output power of a given laser setup decreases throughout the life of the flashlight, although the rate of decay may be unknown.
This is problematic for compliance because, according to ASME BPVC Section IX Table QW-264, laser power is a critical variable that cannot be changed during a given welding process.
Maintaining this requirement is nearly impossible for Nd:YAG lasers, although there is no mention of this fact in the code. In contrast to Nd:YAG sources, fiber laser sources do not have this problem because the excitation is performed by a diode.
Therefore, fiber lasers are superior and arguably necessary in situations where code compliance is required.
Continuous or Pulsed Lasers
Some laser systems now have the ability to operate in both pulsed mode and continuous mode of operation. The advantage of using pulsed lasers is that heat input can be reduced to minimize the size of the HAZ, residual stresses and aberrations.
In addition to the general advantages, pulsing can be useful in special cases, such as welding on finished parts where PWHT is not possible. This is due to the fact that pulsed power has a lower heat input than continuous power.
However, pulsed laser operation is largely limited to LBW-W, as LBW-P systems use continuous power to operate most efficiently. This is due to the fact that in powder-based applications, the powder is delivered continuously, which would result in a significant amount of wasted powder or lack of fusion due to insufficient heat between pulses.
For wire-based systems, the wire feeder is precisely controlled by the equipment to maintain consistent welding conditions. It is important to note that weld mode as an independent variable can also have an effect on the deposition rate of the welding process, but this is highly dependent on the type of system, as well as the conditions of the repair.
Overall, the choice of weld pattern should be based on the type of filler metal delivered, but also on the type of repair and the desired weld properties.
Weld joint design
To minimize potential defects, the joint design must be appropriate for the type of welding system being used. Wire-based welding systems are generally more tolerant of sharp corners and deep grooves than powder welding systems.
This is due to the fact that wire systems do not require a gas transportation system to carry filler material to the weld zone. In powder-based welding systems, turbulence in the carrier gas used to transport the powder to the molten pool caused by the substrate geometry (e.g., v-bevel) results in poorer powder delivery rates and poorer shielding.
Poor powder delivery rates can lead to inefficient welding and excess heat reaching the substrate, while poor shielding properties can lead to porosity and the formation of oxide inclusions. In addition, with LBW-P, excess unfused powder can accumulate in the joint.
Welding on such loose powder can lead to serious defects including lack of fusion, porosity, or cracking. As a result, powder-based filler metal delivery at the bevel requires a wider bevel angle, which creates more welded joints but also increases the volume of the bevel.
As a result, when using the LBW-P, the v-groove volume required to extract the specimen is very large compared to the typical dimensions of a laser weld, making it impractical to fabricate a specimen for process characterization.
In the case of wire-based filler metal delivery, the sloped walls of the groove present geometric challenges for shielding gas and wire delivery, which increases the likelihood of porosity and increases susceptibility to lack of fusion defects.
However, beveling is possible with LBW welding. Additionally, for most shaft repairs where LBW is applicable, the repair is often a weld overlay and does not require bevel welding.
Figure 2 shows common types of shaft repairs, including overlay, buildup, and stub repairs. Although stub repairs require bevel welding, LBW is not usually used because other processes have higher deposition rates.
Regarding filler material types, LBW-P and LBW-W can be used for general shaft repair, but caution should be taken when welding close to steps or features that may cause turbulence in the powder process.
However, welding process qualification requirements may be impossible or impractical for LBW-P, and LBW-P may also experience difficulties where porosity is unacceptable.
Cost and Availability of Filler Metals
The ability to select a filler metal depends on the availability of the material being discussed.
In general, wire and powder versions are available for a variety of materials.
However, wire-based materials tend to be limited to common welding alloys, while powder materials tend to be geared toward higher alloy steels and specialty alloys.
This is because one of the key drivers for powder production is powder-based additive manufacturing, which has the highest cost-benefit ratio for more exotic materials.
Because of this, it is difficult to find carbon and low alloy steels in powder form because these materials are cheap enough that it is not cost effective to use the powder form for most industrial applications.
Since carbon and low alloy steels are heavily utilized in the impeller machinery industry, wire based laser welding systems are often a better option due to the better availability of these materials. Additionally, filler metals in wire form are also usually cheaper than powder form.
Disadvantages
From an application standpoint, one of the main differences between powder-based laser welding and wire-based laser welding is the type of defects and the likelihood of defect formation during the welding process.
LBW-W is capable of producing fully dense, defect-free welds, while LBW-P typically has the least amount of porosity. In any case, suboptimal welding parameters, joint geometry or conditions can produce defects in either process.
Typical defects that occur in laser welding include the following, which shows defects in the LBW-P overlay:
- Porosity
- Lack of fusion
- Unfused particles
- Cracks
Porosity is characterized by the presence of voids in the weld deposit, created by the escape of gases trapped during solidification.
For LBW, there are several ways to introduce gases into the weld pool, but the main theories include the trapping of shielding gases or metal vapors, cavitation induced by unstable small-hole welds, and gases trapped in the powder particles during atomization and released during the welding process.
In addition, poor shielding gas coverage during the welding process can lead to cavitation, which is usually caused by incorrectly aligned gas lenses or turbulence near the weld pool.
This can be due to turbulence from rapid oxidation of the cured weld pool, or gassing due to combustion of oxygen in the air. Finally, lack of cleanliness of the base and filler materials can also contribute to porosity.
Welding on organic material (oil, grease, dirt, oxides, etc.) can lead to outgassing during the welding process, trapped in the weld pool as it solidifies.
Lack of fusion is characterized by the filler metal not being fused to the base metal at a location. This occurs when the heat source does not generate enough heat to bond the filler and base metals.
Typical causes of this condition include poor weld angles, high filler material feed rates, and/or insufficient laser power. Similar to unfused, unfused particles are characterized by the presence of residues of unfused powder in the weld.
This type of defect is unique to LBW-P because it involves powder, which LBW-W does not. The reason for the unfused particles is similar to lack of fusion, i.e., there is not enough heat to fully melt and fuse the filler material with the base material.
This is usually due to the laser not having enough time, power, and/or proper positioning to melt all of the filler metal in the weld area.
Cracking is characterized by the weld metal breaking off due to stress. Cracking can be caused by a variety of factors, common examples include highly constrained joint design, rapid cooling rates, filler metal susceptibility, contamination, weld shape, and/or incorrect welding parameters. Translated with www.DeepL.com/Translator (free version)
