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16.2.2.6 Local Overheating: Sparks, Splashed Metal, Arcs

In the following, local overheating (Ill. 16.2.2.6-1) is understood to be a concentrated application of energy, often limited to a micro-area, that leads to damaging temperatures. This results in negative effects such as (Ill. 16.2.2.6-2):

  • Strength losses related to structural changes such as soft
  • or solution annealing
  • The induction of dangerous tensile residual stresses.
  • Embrittlement during melting and re-solidification, in some cases together with absorption of embrittling elements (oxygen, carbon). Creation of geometric notches (craters).
  • Cracking in the re-solidified layer and base material. The cracking in the base material is often hard cracks (steels) or hot cracks.

Typical causes for damaging, locally concentrated overheating are:

  • Sparks (glowing, non-melted metallic particles): grinding sparks, welding and cutting sparks (autogenous, mechanical, Ill. 16.2.2.6-2), high speed milling
  • Splashed metal (liquid drops of melt): splashed solder (in furnaces), welding splash (Ills. 16.2.2.6-3 and 16.2.2.6-4)
  • Arcs: in the case of insufficient electrical contacts (Ills. 16.2.2.6-7, 16.2.2.6-8, and 16.2.2.6-9) during electroplating, welding, and ECM due to accidental short-circuits between the current-conducting tool (ECM, welding, EDM; Ill. 16.2.2.6-4) and the part
  • High-energy beams: lasers, electron beams, plasmaAccidental local overheating can especially dangerously lower the dynamic fatigue strength (LCF and HCF; Ill. 16.2.2.6-10) and lead to operating fractures (Ill. 16.2.2.6-8). Understandably, this especially applies to highly-stressed parts such as rotor components and pressurized housings.

The detection and verification of these damages can be difficult, and requires considerable experience (Ills. 16.2.2.6-11 and 16.2.2.6-13).

Illustration 16.2.2.6-1: In order to prevent local overheating or identify it in time, a specific search is the primary chance of success. This requires an understanding of the relevant situations of the finishing processes, which makes it more likely that the following questions will be asked and answered in a specific manner:

  • Which part areas were probably affected?
  • At which point during the finishing process would controls/tests have the greatest chance for success?
  • What do the damages look like?
  • What solutions/remedies are possible?

Machining and abrasive cutting: The particles are created primarily during work that is not done on the part. Experience has shown that this includes cutting work that is not done during the finishing of engine parts, but is done in the context of construction work around the finishing area (top left diagram). Sparks can also come from work on other, neighboring parts. Large surfaces of blades and disks are especially exposed. Depositing is especially likely if large part surfaces are unprotected and stored horizontally (Ill. 18-5).
Machining work on the part itself can also produce dangerous sparks. Examples of this include hot shavings from high-speed milling, which can threaten certain part zones, depending on the location of the work surface on the part and the direction of machining.

Welding and thermal cutting processes: The resulting melt drops and sparks deposit after flying through the air (Ill. 16.2.2.6-6). Their likely point of impact can usually be predicted and should be protected. Subsequent inspections should also be especially concentrated on these part areas (Ill. 16.2.2.6-12). The middle left diagram shows this using a friction weld seam on a compressor rotor as an example. In this case, the threatened zones are the disk membrane and hub bore.

Electric processes such as electroplating, electrochemical machining, and electro-welding: If the electrical contact is poor, it will result in arcs (electrical sparks) with high damage potential, at least in the micro-zone (Ill. 16.2.2.6-8). Therefore, the likely damage location will be the specified electrode fastening (bottom right diagram, Ill. 16.2.2.6-10). If a deviating fastening point was selected, prohibited hub and bolt bores are likely candidates. These zones are predestined for unwanted electric currents, e.g. through a fastening device (top right diagram). If there are any doubts, these areas should be examined most closely. Damage in these areas is especially dangerous because they are life span-determining and subjected to high LCF loads.
Additional part zones that are especially threatened by arcs are those that are in contact with surfaces that are carrying electrical currents (welding tables, etc.). One can easily recognize that in typical rotor disks, ring zones around the hub bore or the faces of ring racks (seal carriers, centering bands) are especially threatened by electrical sparks.

Illustration 16.2.2.6-2: The following text examines the damaging local overheating effect of various processes. Usually, this damage is related to deviations from prescribed procedures and process steps. Any reworking should be oriented along the recommendations in Ill. 16.2.2.6-14. Most local overheating during finishing can be traced back to the following causes:

  • Beam energy (EB, laser): In these boring and welding processes, it is possible that the beam may be deflected or have too much energy. This will allow it to penetrate a protective cover or cross-section and damage the surface behind it through overheating. This situation can occur during adjustment or malfunctioning during the process.

Expected damages: see “Arcs”.

  • Arcs are especially dangerous due to the many possible ways they can be created. They form at the contact points of an electrical supply (Ills. 16.2.2.6-7, 16.2.2.6-8, and 16.2.2.6-9, Ref. 16.2.2.6-3) and at unintended striking points during welding. Damages caused by poor contacts or short-circuits can not always be recognized by tarnishing. Often, these processes will occur in a cooling bath (electroplating) or in an electrolyte flow (ECM). The heat removal of the fluid and the part (Ref. 16.2.2.6-3), combined with the restricted access of oxygen, are sufficient to prevent tarnishing. Expected damages: Local melting with cracking and/or high tensile stresses in a brittle re-solidified layer (also see Ill. 16.2.1.2-3). Deep influencing of the structure, significant strength losses, and tensile residual stresses (Ill. 16.2.2.6-3) are to be expected, as is always the case with heating that creates thermal stresses in the plastic range. The depth of the effect can make reworking difficult.
  • Glowing hard particles usually cool off directly after they are created, and have little energy when they strike a part surface. These include sparks consisting of particles from grinding, cutting, or high-speed machining (Ill. 16.2.2.6-1).Expected damages: Flat structural changes with strength losses and a tensile stress zone. Usually, reworking can be accomplished with acceptable dimensional changes.
  • Melt droplets: These are created by exothermic reactions (burning) with the surrounding atmosphere, giving them a great amount of energy, and they strike part surfaces in a liquid state. This usually involves droplets of titanium alloys. During the flight of these droplets, they experience intensive burning in combination with the oxygen in the air (Ill. 16.2.2.6-1).
  • Expected damages: Melting and alloy changes with embrittlement and cracking. During forcible removal of firmly bonding particles, there is a risk of sharp-edged breakouts with high notch-effects being created. Structural changes and tensile stresses with deep effects can be expected. This makes potential reworking difficult.
  • High-frequency fields: It is possible that overheating can occur due to high frequencies. This mechanism is used to heat parts when soldering or when induction-hardening steels.

Expected damages: Relatively flat structure influences with strength losses. In cases with minor effects, reworking may not be necessary.

Illustration 16.2.2.6-3: A simple model (bottom left diagram) can plausibly explain the processes that occur in parts during local heating.

Two solid walls act as a model for the deformation-restricted process of local heating. They simulate the large, cold, and stiff mass around the heated area (top diagram). Between the walls, there is a prismatic rod that is bonded to the walls. The rod represents the heated volume. When it is heated, it tries to expand but is restricted by the walls. This creates compressive heat stress in the rod, i.e. the heated zone (bottom right diagram). If these compressive stresses exceed the flow limit, it will result in plastic compression. After cooling, the rod will be shortened by this length. However, because it is firmly bonded to the walls, correspondingly high tensile residual stresses are created.

One can recognize that every sufficiently intensive local heating will result in tensile residual stresses after cooling. These raise the mean stress, thereby lowering the dynamic fatigue strength of the part (Ill. 16.2.2.4-4). Therefore, on the basis of the tensile residual stresses, it must be assumed that damage will occur during local overheating. Whether or not damage actually occurs must be estimated on the basis of the results of a sufficiently specialized appraisal (Ill 16.2.2.6-13). It must also be remembered that tensile residual stresses only represent one form of damage (Ill. 16.2.2.6-4).

Illustration 16.2.2.6-4: The damage potential of hot particles in contact with the part surface is highly varied. On the basis of the probable thermal energy that is available to heat the contact surface, the following categorizations are used:

Glowing hard particles: These sparks primarily occur as shavings from grinding and cutting processes or high-speed machining. A special case is sparks that can occur during friction welding (Ill. 16.2.2.6-6). Spark particles are generally very small and therefore carry little thermal energy.
Heating occurs during the machining process. Oxidation occurs during the particles` flight through the air. The higher the temperature, the brighter the spark. Bright, rapidly extinguishing sparks indicate that the oxidation during the flight is not sufficient for keeping the temperatures high. Sparks of titanium alloys, especially, glow white across long flight distances. Heavy oxidation during flight leads to higher temperatures in the particle when it strikes a part surface. This gives these particles a greater damage potential. If, on the other hand, an oxide coating has formed during flight, it will hinder fusion with the part surface considerably.
In this case, the damage potential of the sparks will be very low, similar to dust particles. If particles adhere more firmly (e.g. titanium particles), so that they resist removal using fingernails (Ill. 16.2.2.6-13), then the surface must be examined closely for signs of damage (breakouts, signs of reactions; Ill. 16.2.2.6-2).

Burning melt droplets: This effect can occur during fusion welding and thermal cutting, during boring and cutting processes that use energy beams (electron beams, lasers) as well as during extreme separating and grinding (Ref. 16.2.2.6-1 and Example 16.2.2.6-1) in atmospheric conditions (Ill. 16.2.2.6-5). It can be assumed that melt droplets will have a greater mass than the sparks discussed above. This alone makes their heat energy relatively high, and gives them a corresponding potential for damage. If the droplet burns, this highly exothermic process will at least prevent the droplet from solidifying, and perhaps even heat it further. This process cannot be expected with nickel alloys in normal atmosphere. With titanium alloys, however, this is entirely possible. One indicator is the pronounced light emission of white glowing particles and spark formation upon impact.
Because the droplets are molten, there is a high probability that they will bond with the material surface upon impact (Ill. 16.2.2.6-5). The droplet impact will destroy an oxidation layer around the droplet, as well as the thin oxide layer on the part. This creates a metallic contact, resulting in alloying and good thermal conductivity. In this case, dangerous local damage (Ill. 16.2.2.6-5) to a significant depth (Ill. 16.2.2.6-3) can be expected.

Metal droplets without significant oxidation: This situation can be expected when droplets are created in a vacuum or in cover gas (Ill. 16.2.2.6-6). Typical examples are EB welding, cutting, and boring, as well as fusion welding processes in inert gas (laser, WIG). In a vacuum, the melt droplet only loses energy through radiation. Cover gas has a cooling effect through heat dissipation. In these conditions, the melt droplet will not have a significant oxide coating, and metallic contact will occur even at low impact speeds. Experience confirms this model, since droplets do adhere to part surfaces and alloy with them.
A pronounced heat-influenced zone forms around the droplet (Example 16.2.2.6-1).
This results in a significant potential for damage (Ills. 16.2.2.6-3 and 16.2.2.6-5) and dangerously reduces dynamic fatigue strength.

Illustration 16.2.2.6-5: Metal droplets can be created (Ill. 16.2.2.6-4) by fusion welding and extreme separating and grinding processes (top diagram, Example 16.2.2.6-1). If the cooling process is sufficiently slow and/or burning (oxidation) in the atmosphere creates sufficient heat, a partially or completely melted drop can strike the part surface. It can be assumed that thin protective oxide coatings, such as are found on new parts made from Ni and Ti alloys, will be penetrated by the impact (model in middle left diagram). This results in metallic contact between the droplet and the part surface. The local heating can induce high tensile residual stresses, especially in a radial direction around the edge of the melt droplet (bottom left diagram, Ref. 16.2.2.6-2 and Ill. 16.2.2.6-3). In addition, an intensive notch effect can be expected from a dimensional notch at the droplet edge, while structural changes (tempering, diffusion, alloying) and embrittlement (bottom right diagram) can alter the strength and hardness of the material. If these damaging influences combine, the dynamic strength reduction around the melt droplet should be especially pronounced.

Example 16.2.2.6-1 (Ref. 16.2.2.6-1):

Excerpt: “Several high-pressure compressor blades used in the first stage of an aeroengine exhibited low fatigue life…
The blades were made of titanium alloy…and were manufactured by the closed-die forging process. Targeting bosses at the extremities of the blades serve as reference points for dimensional control and are later removed by grinding…
During fatigue testing, a few blades were found to have unusually low …(fatigue strength. The reason) was associated with spherical beads. The fatigue cracks had initiated in the vicinity of these beads…The beads were essentially of the same composition as that of the blade material….they were in the fused condition at the time of impact on the blade. These particles had sufficient velocity at the time of impact to become welded to the blade surface.
It was learned that the targeting bosses of the forged blades were removed in the final stage by a grinding operation…(before fatigue testing).
The possibility of ground particles …being thrown onto the blade surface, while still in the molten or semisolid condition existed if grinding was severe and coolant was unsufficient….(The beads had on) the blades …a localized embrittlement effect, leading to initiation of fatigue cracking.”

Comments: It is surprising that even grinding and separating processes can lead to molten droplets if they are sufficiently intense. This means that the shavings heat up sufficiently after they are created. It indicates further heat development in an exogenous process (burning) of the type observed in titanium in connection with titanium fires.

Illustration 16.2.2.6-6: Welding is used on many highly-stressed components in modern engines (top diagram). This creates melt beads and sparks. These are especially dangerous for HCF strength (Example 16.2.2.6-2) and cyclical life span under LCF loads. Typical affected parts are compressor stators and rotors. Unprotected highly-stressed part zones on blades (bottom left diagram) and disks (hub area and hub bore, middle and right diagrams) are predestined for being struck by melt beads and high-energy sparks. It is also problematic if the blades are arranged so closely together in the rotor that it is not possible to cover the threatened areas. There is an additional problem if the threatened areas cannot be inspected by sight or touch (bottom center and right diagrams, Ill. 16.2.2.6-13). This prevents important control methods from being applied.

Example 16.2.2.6-2: Electron beam-welded integral compressor stators are common in the fans of military engines. They are made from a high-strength titanium alloy and showed dynamic fatigue cracks after test runs. The cracks ran along the trailing edge of single guide vanes. The cracks evidently originated in small fused melt beads made from the same titanium alloy as the blades. This was determined by a close investigation that examined their surface structure, which was typical for re-solidification. It is well-known that these melt beads can seriously reduce dynamic fatigue strength (Ill. 16.2.2.6-5). This allows high-frequency vibrations with a nodal line along the rear blade edge to cause dynamic fatigue cracks.
It was discovered that the melt beads were created during welding of the individual blades with an electron beam (EB). The required protective covers were evidently insufficient.

Illustration 16.2.2.6-7 (Ref. 16.2.2.6-3): If the passage of current is blocked, it will create “electrical sparks”. Actually, this is a small arc that radiates intense heat. Depending on the transferred electrical energy and duration of the arc, this can intensely heat a part surface, causing damage to the surface and accordingly reducing its dynamic fatigue strength. There are several ways in which damage can occur, depending on the material (Ill. 16.2.2.6-2, Ref. 16.2.2.6-9). Some indicators of damage are (top diagram, Ill. 16.2.2.6-11):

  • Melted craters
  • Breakouts
  • Strength losses resulting from structural changes together with losses in hardness and alloying of the backing electrode
  • Embrittlement in the re-solidified zone
  • Cracking
  • Tensile residual stresses

Fundamentally, damaging arcs are possible in all finishing processes that use significant electrical currents. Some of the typical finishing processes that can result in dangerous arcs are discussed in the following (not in order of importance):

“A” Magnetic crack detection: Magnetic parts such as toothed gears are flooded with powerful, low-voltage, direct current for testing. At the same time, elastically formable electrodes, usually cushions made from copper wire, are pressed against the parts. If this connection is not good enough, it can cause electrical sparks and local heat development. If temperatures above the annealing temperature are reached in heat-treated or hardened steel surfaces, they will cause hardness losses in the form of reduced dynamic fatigue strength (Ill. 16.2.2.4-24).

“B” Marking parts with spark pens (electro etching pens) or electrochemical processes (Ill. 16.2.2.6-9): The part is necessarily the opposite polarity of the marking tool. If the part is not in sufficient contact with the electrical supply, it can result in small arcs and damages. If these sparks are created on a highly-stressed part zone such as the fir tree root of a turbine blade, it can dangerously reduce the cyclical life span of the part.

“C” Insufficient contact during electroplating (Ills. 16.2.2.6-8 and 16.2.2.6-9): The part must be electrically charged for electroplating. A current flows between the electrolyte and the part. If the contact is poor, it will result in arcs and local heat development. If the contact is located in a highly-stressed, i.e. prohibited, part zone (Ill. 16.2.1.8.3-6) it will threaten operating safety (Ill. 16.2.2.6-10). These conditions are also present if current accidentally flows through a fastening device on a bore.

“D” Contact of electric welding processes: These problems are everpresent in electro-welding because of the high currents and significant voltages. If the contact between the ground and the part being welded is poor, intense arcs with high damage potential may form (Ill. 16.2.2.6-9).
Another typical problem is striking points outside of the weld. These are created through carelessness or violation of specifications.

“E” Electrochemical machining (ECM, Ill. 16.2.1.2-8): This process requires high currents and significant voltages. This means that arcs will have high energy and can very quickly damage the part if it comes into contact with the tool (Ref. 16.2.2.6-3). Light arcs can also be created at the electric flow into the part. Although modern ECM machines have fast shut-offs that activate if there is a short-circuit between the tool and part, the high electrical energy levels mean that there is a danger of damage, even with a very short acting time.

“F” Accidental contact with electrically charged cables or sensors (Ill. 16.2.2.6-9): As discussed earlier, many finishing processes require an electrical supply, which is usually connected with cables. If the insulation of the cables is damaged or the live end is not sufficiently protected, arcs can be created when handling the already connected parts (Ill. 16.2.2.6-8). The damage potential is further increased if the arc is dragged across the part along with the cable.

Illustration 16.2.2.6-8 (Refs. 16.2.2.6-4 and 16.2.2.6-5): In this case, an unnoticed electrical spark was created during reparatory chrome plating. The spark seems to have damaged the disk so seriously that it burst and flew off. Unfortunately, the available literature does not provide more information regarding the specific conditions. However, this example impressively demonstrates the danger posed by electrical sparks and arcs created by contact problems and/or accidental contact between electrodes and parts.

Note:
Unwanted electrical sparks on a part surface are always a serious warning sign. It may be necessary to have a specialist conduct a risk assessment if sparks have occurred.

Illustration 16.2.2.6-9 (Refs. 16.2.2.6-6 and 16.2.2.6-7): During a cyclical part test, a low-pressure turbine disk (LPT disk) failed due to LCF. A subsequent laboratory investigation revealed that the LCF crack originated at a striking point (electrical arc-out), which was caused by an arc at a poor electrical contact. This flaw was caused by careless handling of an electrochemical sensor while marking the position of the rotor disk during installation. If a disk damaged in this way was installed in the engine, it could be expected to fracture after a sufficient number of operating cycles.

Suspect parts included various disk types in the low-pressure turbine and the fan disks in a large number of engines of the same “family”. The corrective measures implemented by the responsible authorities were visual inspections and eddy current testing. This combined inspection must be done after a set number of cycles; in LPT disks this is every 3,100 cycles. Otherwise, the disks must be scrapped.

The special problem in this case was that the damage occurred during installation, meaning that sufficient testing steps could not be inserted into the process.

Illustration 16.2.2.6-10 (Ref. 16.2.2.6-8): This case affected disks from several compressor stages in many variants of the same engine type in different aircraft, which were very widely used in large numbers (top diagram). It was suspected that bores were damaged by spark formation (bottom diagram). Because the bores in disks are fundamentally highly-stressed, life span-determining areas, they must not be used for introducing electric current. This not only threatens the bore, but also the ring area around the inlet and outlet. The compressor disks of this older engine type are made from a heat-treated steel. This material is not sufficiently corrosion-resistant and is therefore protected by a galvanic Ni-Cd-Be coating. The bores in the disks serve various purposes: threaded connections, air flow, and stress relief of the area around the circle. They seem ideally suited for attaching electrical current sources (bolts, hooks). If the electroplating personnel are not sufficiently trained and/or there are no guidelines prohibiting this sort of current introduction, the conditions for spark formation with extensive consequential damages are given.

Note:
Never attach contact points for the introduction of electrical current in or around disk bores!

Illustration 16.2.2.6-11: Tarnishing can be an important first indication of dangerous temperature influences and their causes. For this reason, it is important that unusual tarnishing is not removed through reworking (local polishing, etc.) before it has evaluated by a specialist. The analysis of tarnishing to determine its causes can be very problematic. It must be remembered that tarnishing is caused by thin oxide layers. These oxide layers can also form at low temperatures, such as in the form of anodic oxidation. For this reason, tarnishing is not necessarily a sign of damage. However, it can be a sign that the affected part area should be closely examined for suspicious phenomena. Blue etch anodizing (BEA, Ill. 15.2-16) of titanium alloys uses tarnishing to find structural irregularities. The thickness of the oxide coating affects the color and is determined by many different factors, including:

  • Material, tendency to oxidize
  • Reactivity of the surface
  • Atmosphere: oxygen supply
  • Time in oxidation conditions, such as increased temperature.

Titanium alloys react especially sensitively to minor fouling on the surface. This means that tarnishing can only provide very limited information regarding its cause. More important aspects are the shape (extent) and position of unusual tarnishing on the part (top diagram).

Straight edges indicate contact with another part that left a pattern along its edge. This may have occurred during the period of increased temperature, but may also have occurred before (contamination). It is also possible that the straight edge may be due to the fluid level of a bath into which the part was partially inserted before heating.

Spot-like tarnishing indicates splashed contaminants. These are usually fluids, metal droplets, or dust (Ills. 16.2.2.3-1 and 16.2.2.3-2) that affect oxidation. A tangible raised area centered on the tarnished area (Ill. 16.2.2.6-13) indicates splashed metal (melt beads, shavings). Melt craters, on the other hand, are the result of an arc forming during the passage of current (Ills. 16.2.2.6-7 and 16.2.2.6-8).

Flow marks: Tarnishing with the pattern of a standing or flowing liquid, usually around edges or in concave depressions, indicates contamination occurring before heating.

Orientation along machining marks indicates that the machining process (e.g. grinding, high-speed machining) was the cause of heating.

Concentric tarnishing around bores can indicate overheating of the drill (Ills. 16.2.1.1-9.1 16.2.1.1-9.2, 16.2.1.1-9.3, 16.2.1.1-9.4 and 16.2.1.1-9.5).

The location of tarnishing on the part can also provide important clues regarding its cause. On contact surfaces, fouling is a likely cause. Small discolored areas at edges indicate the passage of electrical current (e.g. accidental electrical contact). Tarnishing on certain part zones can indicate locally applied machining procedures.

Beneath tarnishing, there can be very different influences and even damages (bottom diagram, Ill. 16.2.2.6-1). A special task is determining the type and extent of these damages (Ill. 16.2.2.6-14). If sufficiently high material temperatures were reached, damage can primarily be attributed to structural changes with strength losses. This can be due to a combination of various influences such as embrittlement, loss of hardness, or notch effects. In addition, there is the negative influence of possible tensile residual stresses on the dynamic fatigue strength of the part.

Illustration 16.2.2.6-12: Prevention of metallic splashes and melt beads is a better approach than later detection. It is more reliable and less costly than scrapping or reworking. These splashes are typical for certain processes, so they can be expected and targeted countermeasures taken. Dangerous metal splashes occur not only during electron beam and laser welding, but also during friction welding. In this case, a cover gas atmosphere in an enclosed space of the part (e.g. inside a rotor) can have a damage-promoting effect, since no protective oxide coatings form. If splashes strike a part surface, a significant decrease in dynamic fatigue strength can be expected in the contact area (Ills. 16.2.2.6-5 and 16.2.2.6-6). In order to eliminate this risk, direct contact between the metal splashes/droplets and the contact surface must be prevented. This can be done with the aid of intermediate layers, coatings, or protective covers. This protection must fulfill several requirements:

  • Protection against direct metallic contact and overheating
  • No reactions with the base material in the impact area of the droplets
  • No interference with the welding process (e.g. no vaporization in the vacuum)
  • Cost-effective and non-damaging application
  • Cost-effective and non-damaging removal and disposal.

Safe prevention of residue that could have a damaging effect in subsequent finishing steps (heat treatment, etc.).In practice, the following measures have proven to be effective protection against welding splash:

Adhesive aluminum foils and tapes (top left diagram): Experience has shown the protective effect of these foils to be good. Depending on the part geometry and complexity, their application can be very time-consuming. This is especially true when spherical forces require cupping of the foils. Later removal also requires considerable manual work.

Temperature-resistant coatings (top center diagram): Inorganic high-temperature lacquers with suitable fillers are promising. These are usually Al powder in a ceramic binding medium (also see Ill. 16.2.1.5-2 “L” ) or boron nitride slurry. The safe function of this type of coating must be verified in suitable tests. In addition to even quality and a minimum thickness that covers all surfaces and edges, problem-free removal is essential. Avoid coating components that could introduce damaging residue into subsequent finishing steps. These include halogens such as fluorine and chlorine compounds (“Teflon”) that can cause stress corrosion cracking during heat treatments (Ill. 16.2.1.7-8). This also applies for coating removal baths. Contamination from process media, such as baths (Ill. 16.2.1.7-2) and/or abrasive blasting media (Ill. 16.2.2.3-2), must also be considered as it may spread to other steps of the finishing process.

Loose covers made from metal sheets or other suitable materials (top right diagram): This protection can be seen as safe for all surfaces to be protected. The prerequisite is that there are no gaps through which splash could reach the part. In suitable cases, this should be the most cost-effective option. Protective covers are often reusable. Problems can arise when applying preformed metal covers to parts with complex shapes, and there is a risk that the part could be damaged during application and removal of the cover. It must also be ensured that no wear products from the covers remain on the parts that could cause damage in later process steps (e.g. iron on titanium, Ills. 16.2.1.3-19 and 16.2.1.7-9).

Experience has shown that unfavorable part geometries make it very difficult to ensure safe anti-splash protection of surfaces that will be subjected to high dynamic loads during operation. One example of this is welded compressor rotors. The bottom right diagram shows a case involving friction welding. In this case, covers and reliable non-destructive testing for damages are rendered practically impossible by the limited accessibility.
During electron beam welding in the bottom right diagram, an additional ring rack in the root area prevents splashes from entering the rotor drum. It is vital that the electron beam does not penetrate this ring rack, due to mistaken welding parameters, etc.

Illustration 16.2.2.6-13: There are several clues that can indicate possible local overheating to specialists conducting visual and physical inspections.

Visual: Experienced inspectors can quickly recognize surprisingly minor signs of local overheating or suspicious rework (top diagram). The surface must be well-illuminated in brightness, color, and direction, and it may also require positioning and/or moving in various angles to the light.
Naturally, unusual tarnishing is an indicator of unallowable local overheating. However, it is easy to remove before it has been controlled, leaving potential damages undetected. For this reason, special attention must be paid during reworking. Even small, local, soft unevenness with a changed structure relative to the surrounding surface (e.g. turned surface) may be an indicator of undocumented reworking. Subsequent polishing and grinding are often marked by tracks that do not run in the same direction as the initial work. This can necessitate inspections to find any unallowable hidden overheating that may have occurred.
Local discoloration following etching or heat treatment can also indicate overheating-related material changes.
Dark points can identify small material breakouts on a part surface. The success of this depends on the surface structure, which is preferably smooth (e.g. polished). With the naked eye, breakouts can best be identified at a low viewing angle. Because these breakouts are indicators of firmly-adhering melt beads that were present earlier, a close examination to determine any residual damages, such as overheating, may be necessary.

Manual: Touching titanium parts with bare hands is problematic if there is any possibility that the parts will later be heated above 450°C (Ill. 16.2.2.3-16). If in doubt, cotton gloves that allow sufficient feeling should be used. Experience has shown that for geometrically complex and/or poorly visible surfaces, such as drum rotors or integral stators, feeling them with ones fingertips is a fast, sensitive, and therefore relatively safe method. This is especially true for raised signs of overheating (middle diagram). These are primarily adhering or simply lying particles (Ills. 16.2.2.6-4 and 16.2.2.6-6). Suspicious particles that are found on a part surface in the form of loose dust should also be examined under a binocular microscope to determine whether they have characteristics of machining chips or melt droplets. Depending on accessibility, this examination can also be done without completely opening packages. Naturally, this type of indicator must be followed up by further tests, such as thorough visual inspections. Because discoloration and other signs of overheating can obviously not be found by feeling, the reliability of this method is understandably relativized.

Fingernail testing (bottom diagram): This targeted test is only practical after a visual inspection and/or feeling the part with hands. The adhesive strength of a particle indicates the degree of damage (Ill. 16.2.2.6-4). The adhesion of metallic particles on a metallic surface is probably due to interlocking roughness tips (form locking) and/or a material connection (fusion). The more firmly a particle adheres to a surface, the more likely it is that damage has occurred, and the more serious its negative effects on the material characteristics may be.

Illustration 16.2.2.6-14: The decision regarding usability of a locally overheated part with or without reworking requires a systematic approach.

“1” Recognition of overheating: There is relevant information in Ills. 16.2.2.6-11 and 16.2.2.6-13. “Unauthorized” reworking can cause problems. This usually occurs during attempts to recognize possible damage or restore a surface appearance. In this case, it is assumed that no damage occurred, which may be a mistake. In general: a surface should not be altered before the responsible specialists have decided how to proceed. The personnel should be instructed accordingly. In addition, it is recommended to conduct training in order to sensitize personnel to overheating damage. Firstly, the causes for overheating should be understood. Secondly, the characteristics of typical overheating should be known. If in doubt, the process for reporting situations to the responsible authorized specialists should be specified and understood by all personnel.

“2” Non-destructive testing: If there is a suspicion of local overheating on a sufficiently costly and/or deadline-related part, it must be non-destructively tested by the responsible specialists. This may allow continued use or reworking. There are several available processes for this, which can be used individually or in combination, depending on the specific situation. These can verify conclusions regarding the type, size, and effects of damage.

“3” Extent of damage: The depth of the damage must be determined in order to estimate the possibility of continued use or reworking. There are also non-destructive methods for this. Etching will indicate the diameter and possible structural changes (Ill. 16.2.2.6-2), which specialists can use to estimate the probable depth. With sufficient experience, eddy current testing can also be used to draw conclusions regarding the damage depth. The depth of damage can also be determined iteratively during reworking. A typical approach is incremental removal (polishing, etc.) of the damage. The surface is then etched and metallographically inspected with the aid of an impression (Ill. 17.3.2-8). If multiple parts are affected, it may be useful to destroy a typical, representative part for an inspection.

“4” Determining rework: This is done within the framework of specified (design drawings) part properties and the inspection results discussed above (points “1”, “2”, “3”). This determination defines the limits of the rework and the testing of the desired end result. If this is not achieved, the part must be scrapped. Naturally, the responsible departments and specialists, especially those that were involved in developing the instructions, must all agree to the documented and verifiable rework. It may also be necessary to obtain the agreement of authorities and customers in the proper format.

“5” Execution of rework: The rework must be done by trained specialist personnel in accordance with the outlined procedure (e.g. work plan).

“6” Verification of the success of rework: This usually requires a subsequent inspection (metallographic impression, etc.). It confirms and documents the end result. If the rework has been done iteratively, each step must be dealt with accordingly.

“7” Documented release: The responsible departments and authorities must be notified of the end result determined in “6” in accordance with regulations.

References

16.2.2.6-1 S.Radhakrishnan, A.C.Raghuram, R.V.Krishnan, V. Ramachandran, “Fatigue Failure of Titanium Alloy Compressor Blades”, ASM “Handbook of Case Histories in Failure Analysis, Volume 2”, Chapter “Rotating Equipment Failures”, pages 299 and 300.

16.2.2.6-2 F.Rotvel, “The Non-Destructive Measurement of Residual Stresses”, AGARD-AG-201-Vol.II, AGARDDograph No. 201 on “Non-Destructive Inspection Practices”, page 481.

16.2.2.6-3 Guy Bellows, “Surface Integrity of Eletrochemical Machining”, ASME-Paper 70 GT 117 of the “Gas Turbine Conference & Products Show”, Brussels, Belgium, May 24-28, 1970, pages 1-16.

16.2.2.6-4 Australian Transport Safety Bureau, “In-flight uncontained engine failure and air turn-back, Boeing 767-219ER.ZK-NBC”,Air Safety Investigation Report 200205780, December 8, 2002, ISBN 1877071 83 8, page 40.

16.2.2.6-5 NTSB Report Identification: LAX911A376, Microfiche number 46630, August 29, 1991.

16.2.2.6-6 Airworthiness Directive for “General Electric Company CF34-3A1 and 3B1 Series Turbofan engines”, Docket No. FAA-2004-18648, pages 1 and 2.

16.2.2.6-7 Airworthiness Directive of the Commonwealth of Australia, Civil Aviation Safety Authority, for “General Electric Turbine Engines- CF34 Series” 8/2001DM.

16.2.2.6-8 Airworthiness Directive of the FAA, 14 CFR Part 39, Docket No. 81-ANE-03 , Amendment 39-9327, AD 95-16-07 and Docket 97-ANE-45-AD.

16.2.2.6-9 L.Engel, H.Klingele, “Rasterelektronische Untersuchungen von Metallschäden”, Carl Hanser Verlag, ISBN 3-446-13416-6, 1982, page 239.

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