Finishing processes can cause fouling that results in damage. This primarily involves surface fouling. The insides of hollow parts such as cooled hot parts or pipes can also be fouled by single particles, agglomerates, and residue. In the following, surface fouling is considered to be all loose or attached deposits that deviate from the specified surface composition. Fouling will not necessarily have a damaging effect without the presence of other factors. Often, the damaging effect only develops in subsequent process steps such as during heat treatment (Ill. 220.127.116.11-10) or in cleaning baths (Ill. 18.104.22.168-13). An especially unpleasant situation is damages that only develop during operation (Ills. 22.214.171.124-9 and 126.96.36.199-10.3) .
These can affect the operating behavior and operating safety of the parts. The damaging effect may only develop through an interaction of sequential finishing steps. Typical examples include fouling reacting with base materials during heat treatments, or in cleaning baths.
The origins and types of fouling in finishing processes are extremely varied:
There are a similarly large variety of ways in which fouling can arrive on parts (Ill. 188.8.131.52-1):
Fouling on parts can be prevented with appropriate measures (Ills. 184.108.40.206-18 and 18-5). The prerequisite for this is good housekeeping, which requires permanent motivation, awareness, and effort (Ill. 220.127.116.11-20).
Illustration 18.104.22.168-1: The damage potential of various surface foulings (Ills. 22.214.171.124-7, 126.96.36.199-8, 188.8.131.52-9, 184.108.40.206-10 and 220.127.116.11-11) can only be recognized upon closer inspection. The following text explains this using a selection of typical examples.
Smearing occurs when there is a firmly bonding connection with the base material. This can occur due to cold welding (galling) under sufficient relative movements and/or pressure. These conditions are given in the following cases:
The damaging effect primarily occurs through reactions with the base material at high temperatures during subsequent finishing processes and operation. Dangerous diffusion processes and melting lead to local strength losses and/or embrittlement. In extreme cases with sufficiently high tensile stress and wetting conditions, it can result in liquid metal embrittlement (LME, Ill. 18.104.22.168-10.1), a type of brittle cracking (Ill. 22.214.171.124-11). Additional problems include damage to coatings (e.g. diffusion coatings) that require costly reworking or shorten the oxidation life during operation.
Overlays and residue: Metallic residues such as lead tape are especially dangerous. These are markings similar to adhesive tape. They are used for marking during X-ray testing (Ill. 126.96.36.199-10.2). Al tape and lead tape are used as overlays during blasting and electroplating. Al tape is also problematic.
Form-fitting residue of low-melting cast alloys can remain in hollow spaces such as the cooling air ducts of blades. Insufficiently removed metal coatings (e.g. silver, Volume 3, Ill. 12.4-14), such as from reworking, can also be dangerous. Their potential damaging effect is similar to that of metallic smearing.
Non-metallic residue forms from dried fluids. These include etching and cleaning baths, as well as foreign contaminants from storage and transport (Ills. 188.8.131.52-2 and 184.108.40.206-6). These contaminants can especially worsen the oxidation behavior of hot parts (Ill. 220.127.116.11-8). If residue contains halogens such as chlorine, it can cause cracking in titanium parts under tensile stress at temperatures above 450°C (stress corrosion cracking, Ref. 18.104.22.168-1; Ills. 22.214.171.124-8 and 126.96.36.199-16).
Particles and dust: Non-metallic residues from transport and storage, as well as from foreign sources (Ill. 188.8.131.52-2) usually occur as dust or powders. They can also react with the base material. SiC reacts with Ni alloys in unallowable ways at high temperatures (also see Volume 2, Ill. 7.1.4-14).
Loose dust is especially capable of worsening the adhesive strength of thermal spray coatings (microsphere effect, Ill. 184.108.40.206.2-4).
Metallic powders and dusts are created during finishing processes such as grinding, thermal spraying, or laser machining. If these bond with the surface as sparks upon impact, the damage will be especially intensive (Ill. 220.127.116.11-5).
Spray powders, coating powders, and soldering powders can remain on parts in case of carelessness. The special danger of these highly reactive particles is diffusion and melting of the base material at high temperatures. These usually occur during heat treatment or welding.
Blasting media is the primary type of particle that tends to remain in hollow spaces.
Fluids, greases, and pastes: These are usually auxiliary materials (Ill. 18.104.22.168-3) such as lubricating greases and cooling lubricants (Chapter 22.214.171.124.1). Damage occurs during subsequent coating processes through poor adhesion or delamination of the coatings.
Residue from etching media and soldering pastes: These can cause corrosion damage in combination with condensation water (Ills. 126.96.36.199-15 and 188.8.131.52-17)
Illustration 184.108.40.206-2.1: An important mechanism through which fouling arrives on part surfaces during finishing is the transferral of media. The actual damaging only occurs during later finishing steps such as heat treatment.
Cleaning and degreasing baths: If the removed fouling, such as greases and oils, float on the surface of the bath similar to grease drops on soup (Ill. 220.127.116.11-12), parts dipped into the bath will be fouled again upon removal (top left diagram, Ill. 17.3.1-8). In this way, fouling can spread like an epidemic. A typical example is silicon as a cause of problems with adhesion and wetting.
Exhaust gases and ventilation systems: If parts and/or transport equipment are stored or transported without sufficient covering, particles and aggressive media can settle on them (top right diagram). Another possibility for damage is the exhaust gas from electroplating if there are no sufficient filters and separators, or if their function is limited for some reason. If acid drops are carried along or acid vapors are trapped by settling air moisture (condensation water), it can cause dangerous corrosion (e.g. intergranular corrosion, IGC). Transport equipment represents a special risk of fouling transferral. Absorbent materials, such as uncovered porous plastics, wooden boxes, or wooden pallets, are especially dangerous. If the material sucks up fouling, it can transfer them to other parts like a stamp. These other parts are then capable of passing on the fouling even further.
Using pressurized air blasts to remove dust and fluids can also be problematic if the functioning of the water/oil separators is limited for some reason.
Condensation of vapors and/or reaction with vapors, as well as dripping of melts at high temperatures: Heat treatment containers, especially those with a vacuum, can release metal vapors. Metal vapors that condense on cooler surfaces of the same part or other parts can cause damages (middle left diagram). Cadmium, lead, bismuth, silver, and aluminum (Ill. 18.104.22.168-11), as well as coating powders (Ill. 22.214.171.124-14) and solders (Ill. 126.96.36.199-5) have an especially high potential for damage. Residue such as X-ray markings (Ill. 188.8.131.52-10.2) and remnants of low-melting cast alloys (Ill. 184.108.40.206-14) on insufficiently cleaned parts can dangerously reduce the strength of the parts through diffusion, melting, or cracking (LME, Ill. 220.127.116.11-13).
Contaminants in blasting media: If equipment for peening and abrasive blasting (middle right diagram) is used in a way that contamination of the blasting media is not absolutely prevented, the fouling can be carried to subsequent part batches (Ill. 18.104.22.168-11). Dangerous wear products from equipment made from unsuitable materials can also be created and transferred to the part in the same work process (Ill. 22.214.171.124-1). A typical example is wear products from lead covering bands that are used to prevent shot particles from entering into hollow spaces.
Abrasive blasting is also used to remove coatings. This means that there is a possibility that these particles may remain on sensitive zones through a charging effect, and cause problems in later finishing steps (Ill. 126.96.36.199.1-3).
Dust deposits due to carelessness: This risk is increased through improper storage/coverings (Ill. 18-5), poor instructions, low motivation (Ill. 188.8.131.52-20) and/or insufficient work preparation. Opening the lid of a storage container, removing powder, and pouring powder can release powder. This results in dust deposits on parts in the surrounding area (bottom left diagram). These powders include coating powders, spray powders, sintering powders, and solder powders. Due to their low melting point, tendency to diffuse, and wetting characteristics, they are especially problematic for steels, Ti alloys, and Ni alloys. During heat treatment, they can damage both the surface and interior of parts.
If improper methods are used to remove the powder, it can make the problem worse. For example, if a vacuum cleaner with an overly coarse or malfunctioning filter is used, the outlet air can distribute the powder throughout the entire room (bottom right diagram).
Smearing from grinding tools and equipment: Grinding and smoothing tools, especially sanding-type tools, can store wear products. This can result in various materials (e.g. non-ferrous metals, light metals, and Ni alloys) being transferred to different part zones (e.g. coatings) or parts (bottom center diagram).
Equipment or tools (e.g. punches) made from unsuitable material (e.g. copper or brass) can smear onto the parts, causing problems in subsequent finishing processes (e.g. electroplating, etching) or unallowably influencing the material during heat treatment (Ill. 184.108.40.206-11).
If tools such as steel brushes or steel wool (!) transfer ferrous wear products onto Ti alloys, it can lead to the embrittlement of weld seams (Ill. 220.127.116.11-19).
Illustration 18.104.22.168-2.2: Fouling and damaging media from finishing can also directly influence the engine. This does not involve contamination of the parts during finishing.
Often, testing rigs are located near finishing shops. Experience has shown that contaminated exhaust air can be carried several kilometers by wind and enter the intake area of an engine. These contaminants collect and build up in the engine, where they can lead to serious damages in the long term. Usually, these damages are caused by corrosion, and especially sulfidation in the hot parts. Air contamination from electroplating should be viewed as being especially aggressive. This type of fouling includes chlorine compounds from etching baths.
In one documented case, the Ni-alloy disks of the low-pressure turbine of test engines showed pitting corrosion after long testing rig runs. It was determined that, during standing times, condensation water formed aggressive electrolytes in combination with chlorine-contaminated air that was sucked up by the engine. The fouling originated in an electroplating shop located several hundred meters away. The electrolytes dissolved silver from the threaded connections on the rotors. This silver solution was centrifuged outward when the engine was started up. It vaporized at the high operating temperatures, and the precipitated silver caused sulfidation with pitting corrosion-like damages in critical disk zones (Volume 3, Ill. 12.4-14).
In another case, lacquer vapors were sucked up by an engine on a testing rig during an acceptance test. The deposits necessitated disassembly and complete cleaning of the compressor, which resulted in a very high cost (Volume 1, Ill. 5.5-1).
Illustration 22.214.171.124-3: Many problems with surface fouling in finishing can be traced back to auxiliary materials. The damaging potential of transferred auxiliary material residue will be shown using typical examples.
Lubricants: Machine tools require lubrication with grease or oil on sliding surfaces, bearings, shafts, etc. If parts are fouled with these lubricants and not sufficiently cleaned, there is a danger
that the parts could be damaged in subsequent finishing steps, such as during coating (Ref. 126.96.36.199-5) and heat treatment. A similar problem with residue from paint marking is shown in Ill. 188.8.131.52-4. In nickel alloys under tensile stress, MoS2 additives can cause dangerous cracking and/or oxidation (sulfidation). Other oil additives can also lower the oxidation resistance and thus affect later operation.
Anti-foaming agents/silicon oils (Ill. 184.108.40.206-13): As fouling, these compounds influence the wettability. This can have serious consequences:
The adhesive strength of coatings such as lacquers and the coating buildup can be compromised. If the strength between the layers of fiber-reinforced polymers (FRP) decreases, it will result in delamination-sensitive flaws that are very difficult to detect using non-destructive testing methods.
Crack detection with the aid of penetrant (Ill. 17.3.1-8) may become less sensitive. This effect is suspected to exist, but testing results are inconclusive. However, since the suspicion exists, thorough cleaning of potentially affected parts is a necessity. This means that the risk is limited to parts that were unconsciously affected and installed in engines.
Cooling lubricants (CL, Chapter 220.127.116.11.1): Especially difficult to cut materials, such as Ni and Ti alloys, may require cooling and cutting fluids for better cutting performance. There may be a risk that these fluids could have an aggressive and damaging effect in later finishing processes. High-strength titanium alloys are especially sensitive to cracking (SCC) under the influence of chlorine. This is probably also true of other halogens such as fluorine. Because of this, if possible, cutting fluids containing chlorine and sulfur should generally be avoided in finishing processes due to the risk of them being transferred (Ref. 18.104.22.168-2). In the case of nickel alloys, sulfuric components at high temperatures (welding, heat treatment) can promote sulfidation during operation.
If oils containing sulfur act on silver-plated parts such as bolts, nuts, and sliding surfaces, dark tarnishing to the point of blackening reveals the formation of silver sulfide. This can increase the coefficient of friction considerably, and worsen the sliding properties. This means that the required preloading force of threaded connections may not be reached using the specified tightening torque. Poor sliding properties of silver coatings can promote operating damages through hot running and galling. A similar relationship was observed in the 1960s in the silver-plated slide shoes of axial piston fuel pumps. Silver sulfide created by microbacteria in turn created aggressive sulfur compounds in the fuel tanks, which caused catastrophic pump damages after short operating times.
Lubricants are also used in reforming. They are applied to the parts and/or tools to minimize wear and friction forces (Ref. 22.214.171.124-4). The lubricants make it possible to reform materials that are very difficult to deform, such as high-strength titanium alloys. During the reforming process at high temperatures (e.g. intermediate annealing), these lubricants combine with the part surface and are then very difficult to remove. Lubricant residue such as fatty acid amides or softening agents can cause flaws in subsequently applied lacquers (Ref. 126.96.36.199-5). The reforming process is often done at high temperatures, and/or heat treatment is done between the single reforming steps. This creates a risk that components of the lubricant (e.g. MoS2 ) may have a damage-promoting reaction with the surface. The selection and approval of these lubricants must take this danger into account. In some cases, suitable aftertreatment of the parts may be necessary. If the parts are to be coated, such as with lacquer, enamel, electroplating, or thermal spraying, the surface must be suitably treated (Ref 188.8.131.52-4). Experience has shown that even tiny amounts of lubricant residue (top atom layer) can lead to craters, cracks, separation, and wetting problems in lacquers.
Secondary ion mass spectroscopy (SIMS, Ref. 184.108.40.206-5) has proven effective for detecting such minor amounts of fouling.
Cleaning agents: Even remnants of these materials can compromise coatings, especially lacquers. This was seen in the case of traces of a synthetic detergent (sodium lauryl sulfate) remaining during the production process (Ref. 220.127.116.11-5).
Releasing agents usually contain PTFE (Teflon®) or silicon compounds. They are especially used in the production of fiber-reinforced synthetic resin parts. They are applied to the molds and enable easier release of the part. These media can be transferred by careless spraying, cleaning rags, or cleaning baths. This then results in poor wettability and adhesion problems during coating processes. The bonding of layers of fiber-reinforced parts is weakened, promoting delamination.
If Teflon residue is sufficiently heated, such as during heat treatment, it will create fluorine. Similar to chlorine, it can be assumed that the fluorine will pose a risk for stress corrosion cracking in titanium parts under sufficient tensile stress.
Covering materials: In finishing, masking/covering materials are necessary for processes such as soldering and diffusion coating. They prevent undesired coating of certain surfaces. If residue of these covering materials remains after the finishing step, or if covering materials are transferred to surfaces that are to be coated or other parts, it will result in flaws.
High-temperature lubricants are used on heat treatment equipment and threaded connections in hot parts in order to make damage-free removal possible later. These media contain dry lubricants such as Cr oxides and hexagonal boron nitride. It is feared that this type of fouling can change the friction conditions in friction welding (Ill. 18.104.22.168-34) in a way that results in bonding flaws. This danger could also be present in the case of lubricants containing graphite.
Hand washing pastes, skin creams, and other sanitary media: If these contain materials such as silicon, they present similar threats to the integrity of the parts as do silicon-based anti-foaming agents and releasing agents. The danger of transferral is primarily due to prints from unprotected hands.
Illustration 22.214.171.124-4: A special field of damage-relevant fouling is the marking of parts with markers, chalks, and paints. Fundamentally, the marking materials must be approved for the specific application. However, it is possible that this approval only applies to a specific process step, and the use of the marking materials in other, subsequent steps is questionable. In this case, the parts must of course be sufficiently cleaned.
The degree to which finishing processes and part characteristics are affected by markings cannot always be accurately estimated. If new methods or new auxiliary materials such as cleaning agents are used (e.g. for environmental reasons), it is possible that markings will not be sufficiently removed. In this case, unexpected consequences cannot be ruled out.
Porous materials such as coatings or metal felts can absorb the paint from stamps so that it has a damaging effect both inside and on the adhesive layer.
Example 126.96.36.199-1 (Ref. 188.8.131.52-1): The effects of marking are shown in the bottom sequence of diagrams:
During the finishing process, sheet metal parts of a combustion chamber were marked with a marking pen. During subsequent heat treatment in air, the oxidation of the marked area was considerably different. In addition to influencing the base material, very stable oxides formed that could not be sufficiently removed using the usual subsequent etching process. Intensifying the etching process is more likely to result in intergranular corrosion (Ill. 184.108.40.206-10) with unallowable strength losses. Leaving the non-protective oxidation can promote oxidation during operation. At the very least, the altered appearance may irritate the customer.
Illustration 220.127.116.11-5: A special problem is the influence of foreign metals (Ills. 18.104.22.168-10.1 to -11).
This can occur when metals with lower melting points than the base material are in a wetting contact with the metallic part surface at high temperatures. Therefore, dangerous damaging will not occur in all cases of metallic fouling on heated parts. Oxidation can prevent the necessary metallic contact. This is true for oxides on the part surface, as well as for situations in which the fouling oxidizes before melting when it is heated.
Therefore, specimen tests that do not show any damaging effects should be viewed skeptically. In this case, the test may not simulate the necessary unfavorable combination of damage-relevant influences.
The following damage mechanisms are primarily active between metallic surface fouling and the base material:
Melting in connection with the formation of low-melting phases. This creates the danger of clear, deep-acting strength losses and embrittlement.
Micro and macro cracking (liquid metal embrittlement, LME) usually require sufficiently high tensile stress while the melt acts. The melt then shoots into the grain boundaries (Ill. 22.214.171.124-13). The tensile stresses can form as thermal stress during heating or cooling. However, they may also have been present in the part as residual stresses (e.g. from forging) before heating.
Diffusion: Even before melting of the fouling, it is possible for the base material to be damaged through diffusion. Depending on the material, this can already happen at relatively low temperatures. For example, cadmium damages titanium alloys at around 260°C (Ill. 126.96.36.199-11). Diffusion can decisively reduce strength and/or cause embrittlement. If diffusion prevents a protective oxide layer from forming, then dangerous corrosion can occur in subsequent finishing processes such as etching.
It is also possible that damaging metals could be transferred as vapors, such as when a metal vapor forms from fouling in a vacuum oven, and then settles on the part (Ill. 188.8.131.52-13).
A decisive factor for damage can be temperature changes in finishing processes or operation that create high thermal stress with plastic deformation of the surface (thermal fatigue). If a protective oxide layer is torn open beneath a metal melt, it can result in melting and/or cracking in the base material. This can make it possible for metal melts to even attack oxidized parts of the type expected in operation (Ill. 184.108.40.206-14). The required metallic contact between the fouling and the base material can also occur through smearing (Ref. 220.127.116.11-12), for example, during handling. If no separating oxide layer forms during subsequent heating, then a damaging situation is given.
If heating occurs in a vacuum (heat treatment, high-temperature soldering) or in an inert (heat treatment) or caustic atmosphere (diffusion coating), protective oxide coatings cannot be expected.
Some typical metals that have considerable damage potential as fouling are (Ill. 18.104.22.168-11):
Dangerous fouling may already be present before the part enters the oven. However, it may also arrive on the part in the oven. This risk increases if the oven is used for various finishing processes such as vacuum annealing and soldering. The bottom diagram shows a case in which a pinhead-sized drop of silver solder caused through-going cracking in a forged disk one centimeter thick. The solder drop evidently originated in an earlier soldering batch and was caught in the porous graphite insulating mats of the oven or on the charging rack (top diagram). This case also demonstrates the importance of housekeeping (Ills. 22.214.171.124-6 and 126.96.36.199-20).
Illustration 188.8.131.52-6: Experience has shown that construction work must often be done in finishing halls in the context of reconfigurations, repairs, changed processes (moving machines) and the assembly of new production facilities and machines. Often, finishing must continue in the same room during the construction work. In these cases, there is an increased risk of damage-relevant fouling of parts. The remedy is especially careful housekeeping. This includes
Metal drops from welding, grinding, and cutting must be seen as being especially dangerous (Ills. 184.108.40.206-3 and 220.127.116.11-5). In this case, there is a danger of reactions and the induction of high tensile stresses resulting from rapid local heating of the metal surface. This can dangerously reduce the dynamic fatigue strength and the cyclical life span.
Ferrous grinding dust can cause embrittlement and cracking in welds on titanium alloys.
Illustration 18.104.22.168-1 describes the dangerous effects of lacquers, oils, and dust.
Illustration 22.214.171.124-7: Particles in cross-sections with a throughflow in the fuel (top left diagram), oil (bottom left diagram), and air (right diagram) systems can lead to part failure with extreme consequential damages. The damage cause is a reduction, blockage, and/or diversion of the flow.
If cooling air is required, the overheating damage on hot parts (Ill. 126.96.36.199-9) is to be expected. Roller bearings and slide seal rings are doubly affected by reduced oil flows. First of all, there is insufficient cooling, and secondly, poor lubrication results in greater heat development. Even the deflection of a jet coming out of a nozzle can affect its function (lubrication) and/or overheat part areas such as combustion chambers and housings due to the angled burning of the flame.
Particles on the bearing races lead to failure due to race fatigue in roller bearings. If abrasive particles enter between the seal surfaces of floating ring seals, it will compromise the seal and most likely result in an oil fire.
Illustration 188.8.131.52-8 and Illustration 184.108.40.206-9: The diagram shows damage possibilities on the inside and outside of a cooled turbine blade. Ill. 220.127.116.11-9 provides more detail regarding inner damages (cooling air zone). The operating behavior, especially the life-determining characteristics, can be affected by fouling in various ways.
Fouling from the finishing process can, for example, compromise cooling by blocking the cooling air bores with particles. In the depicted case, potential threats to the operating behavior from fouling are shown using the example of a cooled turbine rotor blade. Based on their damage mechanism, they can be grouped into the following categories, which can influence one another:
Damages that already occurred in the finishing process due to fouling. One type of damage is the loss of strength. Examples include unallowable corrosive attack around the fouling or reactions with the fouling during heat treatment (Ills. 18.104.22.168-1 and 22.214.171.124-5).
Increased operating temperatures: If one remembers that an increase in operating temperatures, which are always near the physical limits of the materials, of about 15°C will result in a 50% reduction in creep life, it is easy to recognize the effects of even apparently minor disruptions of the cooling. To make this even clearer, it should be pointed out that +30 °C results in a 75% reduction of creep life, while at + 45 °C creep life is reduced to one tenth.
The following influences can be expected to worsen the cooling and result in local temperature increases in the part (see right diagram in Ill. 126.96.36.199-9):
Changes in the part function and other life-determining effects: If, for example, the oxidation resistance of the diffusion protection coating on the face of a turbine blade is worsened by fouling, material removal through oxidation can change the profile significantly and at least worsen the efficiency of the blade.
Example 188.8.131.52-2 (Ill. 184.108.40.206-10, Ref. 220.127.116.11-11):
Excerpt: “Stage I turbine blades in engines of…commercial airplanes were precision cast with …Ni-base superalloy. After service for 506 h, a fracture suddenly occurred during flight, and the engine was destroyed: the incident nearly led to a catastrophic flight accident.
The (investigation) results show that the blade fatigue failure is due to embrittlement, which is induced by the contamination of a Bi-Sn low melting point alloy on the blade surface during the manufacturing process of the blades, under the effects of service temperature and stress.
…high concentrations of Bi and Sn, which are not from the melting and casting process , but are from a Bi-Sn low melting point alloy, which subsequently adheres to the blade surface. Bi and Sn not only exist on the blade fracture surface, but penetrate into the blade foil…
Investigation of the manufacturing procedures of the blades shows that, during the machining of the blade, one procedure adopts Bi (57%)-Sn(43%) alloy as a locating die. The melting point of this alloy is 123 °C, and this alloy is very brittle in the solid state. This low melting point alloy is widely used as a locating die. Since the surface of the blade extended root is not machined, and it is incompletely cleaned, it is possible that the Bi-Sn alloy will remain … after manufacture.
(In service) The centrifugal stress is about 900 MPa, and the temperature about 600°C…
… the (LMIE= LME) fracture is intercrystalline brittle…
Under dynamic stress, LMIE cracks propagate to a definite dimension. Owing to the lack of liquid metal at the crack tip, the cracks then begin to propagate by fatigue…“
Comments: LME is also referred to as LMIE (Liquid Metal Induced Embrittlement) in this excerpt. It is surprising, that this damage only occurred after longer operating times. It could be expected that the LME conditions of a wetting contact between base material and melt would be defused during the first operating run through centrifugal force, vaporization, and oxidation (Ref. 18.104.22.168-8). This case disproves this assumption and underlines the danger of this process. It is possible that smearing resulted in the necessary wetting contact over longer operating times.
Illustration 22.214.171.124-10.1 and Illustration 126.96.36.199-10.2 (Examples 188.8.131.52-4 and 184.108.40.206-5): Metallic fouling can decisively worsen the strength properties of parts and is therefore a safety hazard. The damage mechanisms are described in the following.
There are several different terms for the same damage mechanism: liquid metal embrittlement (LME), liquid metal induced embrittlement (LMIE), strain-induced stress corrosion cracking in liquid metals. There are evidently multiple requirements for these damages to occur:
Conditions for liquid metal embrittlement can occur in many different ways (bottom diagrams).
The requirement for metallic contact between the melt and the base material is met by the surfaces of nickel and titanium alloys, which are metallic and not noticeably oxidized, at least during finishing. This is also true of heat treatments and processes (e.g. coating) in vacuums or under
reducing conditions. Smearing promotes continuous metallic contact, in spite of subsequent oxidation. This type of smearing is cold welding (galling). Typical causes are tools made from unsuitable materials, joining processes with coated fitting surfaces (e.g. silver), and blasting media as fouling.
Even beneath thick coatings such as silver or Cd coatings on Ti or Ni alloys, a metallic contact can remain over longer operating times (Examples 220.127.116.11-3 and 18.104.22.168-4).
If the separating oxide layer is damaged beneath the melt by a scratch, it will result in LME. Scratches can be created by tools or relative movements between contact surfaces. Examples include thermal strain-dependent relative movements between the contact surface and the charging rack or equipment.
Another possibility is the breaking open of oxide layers and the base material surface during cyclical plastic deformation (LCF loads, thermal fatigue). This mechanism can also explain LME damages that occur after longer operating times (Example 22.214.171.124-2, top left diagram).
If a drop of melt falls onto a metallic surface with sufficient kinetic energy, thin oxide layers (e.g. during annealing in cover gas, in a vacuum, or in a reducing atmosphere) can be penetrated by the drop impact (Ill. 126.96.36.199-5), which will result in creation of the metallic contact necessary for damage to occur (top right diagram, Ill. 188.8.131.52-5).
Even small amounts of metal are sufficient to cause liquid metal embrittlement (Ref. 184.108.40.206-12). The fouling does not necessarily have to be pure metal. Even metal pigments in coating materials are sufficient.
Embrittlement through metallic contact in the solid state (solid metal induced embrittlement, SMIE): For embrittlement to occur during contact with metallic surface foulings, it is not necessary for the fouling to be in a melted state. There are also cases in which solid-state metal contact can cause embrittlement and cracking. This damage mechanism applies to the bursting of silver-plated nuts at operating temperatures below the melting point (top center diagram, Ref. 220.127.116.11-8).
Melting (Ill. 18.104.22.168-10.2): Damage can occur if a metal melt is in contact with the base material, even if the required tensile stresses are not present. This damage occurs through the formation of low-melting phases, which can also cause the base material to melt. The strength values of these melts are usually considerably worse than those of the base material. Often, the melted area is so embrittled that there is a risk of immediate cracking under operating loads in the LCF range. If, during cooling, sufficiently high shrinkage stresses are created between the solidified melt and the surrounding base material, it can cause cracking (e.g. hot cracks). Depending on the amount of fouling, even thick cross-sections (centimeters) can be irreparably damaged.
Diffusion processes: The base material can be damaged even if the tensile stresses are not high enough and the melting temperature of the fouling is not exceeded. This occurs when diffusion processes take place.
Example22.214.171.124-4 (Ref. 126.96.36.199-8: Bolts made from the titanium alloy Ti6Al4V were cadmium-plated in order to prevent element formation with the aluminum part they were to be screwed into. The temperature of the tightened bolts caused brittle fractures due to LME caused by the cadmium. The fractures originated in the transition radius of the shaft to the bolt head, which was under high tensile notch stress.
Example188.8.131.52-3 (Ill. 184.108.40.206-10, Ref. 220.127.116.11-8): Cadmium-plated nuts made from low-alloy steel with a hardness of 44 HRC were used for V-band clamps that connected air takeoff pipes on compressor housings in a military engine. The operating temperatures were around 500 °C. The nuts burst into several pieces, and the surfaces of the fragments were oxidized and coated. The surfaces were brittle and intergranular, and were coated with a thin layer of cadmium. Some of them had red, blue, and goldish yellow discoloring.
Illustration 18.104.22.168-10.3 (Refs. 22.214.171.124-16 and 126.96.36.199-10): In several similar cases, combustion chamber outer casings (CCOC) fractured during takeoff. At this point, the combustion chamber pressure and loads on the pressure casing are especially high. Investigations revealed that, contrary to regulations, there was an Ni-Cd coating in the holes for the threaded connection of the rear flange. This coating is applied to steel housings for corrosion protection, and consists of a nickel coating as an intermediate layer for the Cd coating.
However, if this protective Ni coating fails or if Cd is directly precipitated onto the steel, it will result in LME at the high temperatures of the combustion chamber casing.
This situation was given following overhauling of the flange bores. Cracking (bottom detail) and crack growth caused the housing to burst. Hot gases and the explosion pressure led to fuel escaping and igniting. However, safe evacuation of the aircraft was successful in the reported cases.
Illustration 188.8.131.52-11 (Ref. 184.108.40.206-8): This table shows metal combinations found in engine production that, under unfavorable conditions, can result in liquid metal embrittlement (LME, Ill. 220.127.116.11-8) and damage through contact with solid bodies (SMIE). These materials are primarily titanium alloys, nickel-based alloys, and high-alloy steels. Low-melting metallic surface fouling consisting of or containing cadmium (Cd), silver (Ag), lead (Pb), and bismuth (Bi) are especially dangerous.
Cadmium was often used in older engine types as corrosion protection for rusting steels (Example 18.104.22.168-3, Ill. 22.214.171.124-10.4, and Ref. 126.96.36.199-17). Cadmium was transferred to corrosion-resistant materials via baths or by smearing. In rare cases, it was part of an intentional coating (Example 188.8.131.52-4). Today, use of Cd is avoided if possible, due to its toxicity and role in hydrogen embrittlement.
Silver and silver alloys are used on threaded connections to improve the rubbing conditions (low friction torque in threads) during tightening and loosening. In operation, silver can be transferred during standstill to other parts through corrosion (Volume 3, Ill. 12.4-14). Sliding surfaces and fitting surfaces can transfer silver to other parts.
Lead and bismuth: Lead is used in a relatively pure form in coatings and lead tape for masking (electroplating, blasting) and marking (X-rays, Ill. 184.108.40.206-10.2). It can be transferred from coatings onto fitting and sliding surfaces (e.g. shafts). Blasting media fouled by lead masking bands, or remnants of these bands, can also contaminate surfaces (Ill. 220.127.116.11-10.2).
Lead and bismuth are used in low-melting alloys that are poured to fasten parts. In this case, there is a risk of residue remaining after the process, especially in hollow spaces of cooled turbine blades and as smearing (Ill. 18.104.22.168-9 and Example 22.214.171.124-2).
Copper, brass, and bronzes: These are known as causes of liquid metal embrittlement in steels. The melting of copper (e.g. electrode or coating) occurs during welding (Ref. 126.96.36.199-17).
Copper is also used as a solder for assembled compressor stator vanes and housings (Ill. 188.8.131.52-1) in older engines. Punches made from copper or bronze can create dangerous smearing.
Gold is used in engines, albeit rarely, as a reflective coating to keep operating temperatures within acceptable limits. Due to its high toughness and low soldering temperatures, gold solder is used in some assembled guide rings made from nickel alloys near the compressor outlet.
A special, if apparently indirect danger, is posed by smearing from gold jewelry. Finger rings made from gold can transfer gold to parts during handling. For this reason, finger rings should not be worn during this type of work.
Illustration 184.108.40.206-12: Surface fouling rarely has damaging effects in the finishing process in which it originates. It is more common for the damage to occur in subsequent finishing steps, and to be detected even later (top diagram). Even the compromising of a non-destructive testing method can be classified as damaging. A typical example is fouling containing chlorine on titanium parts. Only later, at processing temperatures above 450°C, does cracking occur. Therefore, it requires a great deal of expertise to be able to analyze the damage symptoms and finishing sequence and determine the cause and time (finishing step) at which the fouling occurred. The diagram shows typical process-specific problems and damages that are related to fouling from other finishing steps, handling (Ills. 220.127.116.11-16 and 18-4) and storage. It is easy to recognize that the time and location at which fouling occurs are often not identical with the damage symptoms.
The adhesive strength of coatings that are
Insufficient adhesive strength can immediately reveal itself through flaking or blistering. It may also only reveal itself after additional finishing steps. If the coating has not yet separated from the base material, non-destructive detection of adhesion problems is usually not possible (Ills. 18.104.22.168-10, 22.214.171.124-5 and 126.96.36.199-6). This type of fouling primarily results from carelessness during handling, transport, and storage. It can disrupt the coating process through the formation of oxide layers at sufficiently high temperatures (welding, heat treatments), as well as by worsening the wettability.
Coating structures: One example is the compromising of diffusion coatings. Ceramic and metallic residue or oxides can locally prevent diffusion. This influences the composition and thickness of the coating, and lessens the protective effect.
Strength and toughness of the base material Splashes of metal melts are created by welding (EB, friction welding), while hot particles are created by grinding, separating, and high-speed machining. They have damaging effects on the part surface (Ill. 188.8.131.52-4). If splashes have been removed in an uncontrolled process, subsequent non-destructive detection of damage (Ill. 184.108.40.206-5) becomes considerably more difficult.
If fouling acts like metal melts (LME, SMIE) and reacts with the base material, it can be expected to cause significant strength losses and embrittlement (Ills. 220.127.116.11-10.1 and 18.104.22.168-10.2).
Stress corrosion cracking in titanium alloys occurs in titanium alloys under the influence of fouling containing chlorine (Ill. 22.214.171.124-8).
Increased hydrogen absorption and embrittlement (hydrogen embrittlement) around fouling usually occurs in galvanic processes and in etching baths. One example is the effect of ferrous deposits during the etching of titanium alloys (Ill. 126.96.36.199-9). These deposits include grinding particles and rust from iron wear products (wire brushes, steel wool, shot peening). Smearing from tools and equipment is another source. The iron can embrittle welds on titanium alloys.
Compromising the sensitivity of non-destructive testing: A typical example is poor wettability due to fouling such as silicon compounds (e.g. from cutting fluids, hand creams, and releasing agents), which can compromise the effectiveness of penetrant testing (Ills. 17.3.1-2 and 17.3.1-3).
The detectability of segregations during etching tests such as blue etch anodizing (BEA, Ill. 15.2-16) of titanium alloys or segregation etching on Ni-based forged alloys (Ill. 15.3-12) can understandably be negatively affected by fouling.
Strength of soldered and welded joints: Examples include flaws such as pore formation through vaporizing contaminants in the welding gap (EB welding, Ill. 188.8.131.52-25) and laminar bonding flaws. Diffusion welds react especially sensitively to fouling that prevents metallic contact between the welding surfaces, thereby preventing diffusion (Ill. 184.108.40.206-38).
In friction welds, it is suspected that bonding flaws are caused by changes in the friction conditions by fouling with the properties of high-temperature lubricants (e.g. hexagonal boron nitride or graphite).
Illustration 220.127.116.11-13: In exceptional cases, damage can also enter into engines (Ill. 18.104.22.168-16). There are various reasons for this:
Damages that already occur during finishing can negatively affect both the strength and function of parts.
Cracking, such as liquid metal embrittlement, presents a direct danger of catastrophic failure (top right diagram).
Embrittlement of titanium parts during etching presents a danger of dynamic fatigue cracking in operation-specific notches (erosion). The cause is rust (top left diagram).
Formation of silver sulfide on silver-plated bolts and nuts through contact with sulfurous media (e.g. cutting oil). This affects the tribological behavior by increasing the coefficient of friction (middle right diagram).
The following considerations are limited to fouling with a damaging effect that only occurs under operating conditions (bottom frame). This means that the operating temperature is decisive for the damaging effects of fouling.
Cracking as the result of a reaction between metals (LME and SMIE, Ills. 22.214.171.124-10.1,.2 and 126.96.36.199-11) can also occur during operation. This is especially likely in the case of smearing and firmly adhering particles, when the material connection prevents the formation of a protective intermediate oxide layer.
Increased oxidation resulting from etching agent residue (Ill. 188.8.131.52-9) has a thermal insulation effect and also constricts cooling air ducts. This unfortunate combination causes the part temperature to rapidly rise, shortening the part life. Blockages with particles from the finishing process have similar effects. If these contaminants promote grain boundary corrosion (Ill. 184.108.40.206-10), it will further reduce the strength levels.
Residue from etching agents and flux (Ill. 220.127.116.11-17) can have similar effects, and can cause corrosion damage in case of alternating condensation water formation and heating (Ill. 18.104.22.168-15).
Illustration 22.214.171.124-14 (Ref. 126.96.36.199-6): This flight accident was traced back to the fracturing of several turbine blades in the first high-pressure stage (bottom right diagram). All fractured blades came from the same heat treatment batch, which had already been connected with previous similar damages. 1.4-1.9 ppm of the element bismuth was discovered in the base material of the blades. These minimal traces of bismuth were responsible for the reduced life span of the blades.
The main question was how the bismuth entered into the turbine blades. There were three likely possibilities (bottom frame):
Unfortunately, the available literature does not indicate whether the part had traces of bismuth all over, or if these were concentrated in the fracture zone. It is also unclear whether the distribution was even, or whether there were concentrations at structural characteristics such as the grain boundaries.
Material fouling limited to the damaged area would indicate smearing or residue in the cooling air ducts. However, the fact that the affected parts were apparently limited to a heat treatment batch would contradict this theory of the method by which the bismuth fouling occurred. This can hardly be plausibly explained by coincidental residue and smearing.
If the blade is fouled across a larger area, it would indicate the occurrence of diffusion in relation to vapor deposition. This type of process might occur in a vacuum or cover gas oven, in which bismuth fouling vaporizes and settles on the parts. This theory is supported by the fact that only one heat treatment batch was affected.
Illustration 188.8.131.52-15 (Ref. 184.108.40.206-13): The literature about this damage incident states:
“The first two losses (of modern military fighters) occurred in 1997 and seem to have been caused by the rupture of a flexible hose. `The hoses had been contaminated by chlorine during the manufacturing process, and this weakened the braids in the hoses once the engine entered service'.”
Details have been depicted by the author on the basis of his experience. It can be assumed that the chloric residue combined with condensation water to form a corrosive medium (also see Ill. 220.127.116.11-17) that, especially at operating temperatures, had a very concentrated effect on the stainless steel wires (usually made from CrNi18 9) of the supporting sleeve. Under a microscope, the corrosive attack probably looked like that shown in the detail.
Illustration 18.104.22.168-16: Two examples of stress corrosion cracking related to media from finishing. In the case in the top diagram, chloric residue from a degreasing bath caused stress corrosion cracking during subsequent heat treatment and high tensile stresses (welding stress).
The bottom diagram shows the origin of the blade fracture that occurred during operation as a result of stress corrosion cracking. Dried hand sweat in an area with high tensile residual stresses from forging caused the cracking. Even this minor amount of chlorine in the form of common salt residue is enough to cause cracking.
Illustration 22.214.171.124-17: Residue from etching agents and corrosive media from the finishing process, such as etching baths, fluxes, and soldering pastes, can have damaging effects during later storage and/or under operating conditions (Ill. 126.96.36.199-14).
Brazing with Cu- or silver-based solders was used on oil lines made from 18 8 type steels and, in some older engine types, on assembled compressor stator vanes made from 13% Cr steels (Ill. 188.8.131.52-1).
In the depicted case, a dynamic fatigue fracture of an oil nozzle (right diagram) occurred in an accessory gearbox (left diagram). The broken nozzle became trapped between the gears and caused extensive consequential damage. The dynamic fatigue fracture started at the transition of the soldered joint, which showed intergranular corrosion (detail) that was traced back to aggressive flux residue.
This type of corrosive attack is possible if the dried fouling combines with condensation water or increased air moisture to create a watery electrolyte (Ill. 184.108.40.206-13). Corrosion will be promoted by element formation between the solder and base material. In addition, in some materials, soldering can result in an unfavorable structural state (sensitization) that promotes grain boundary corrosion (intergranular corrosion, IGC).
Illustration 220.127.116.11-18: Preventive measures can be taken during the finishing process to deal with questionable surface fouling on parts. The effectiveness of these measures often lies in their combination. In the following, preventive measures were categorized according to individual steps of a typical finishing process.
“1”: Effective housekeeping is especially important. Housekeeping refers to cleanliness and order, as well as the necessary discipline to maintain this. It requires understanding and awareness from all workers. This includes giving all affected personnel plausible explanations for these requirements. This attitude can be fostered by suitable training courses, even on-location with memorable examples.
In order to realize the desired housekeeping conditions, a zero tolerance policy similar to that successfully implemented in large US cities (New York) is a promising approach. This is based on the experience that silently tolerating a cigarette butt being thrown on the ground will soon result in a mountain of garbage around it. Naturally, a suitable infrastructure must be in place to realize this. This includes a sufficient number of suitable garbage containers, transporting containers that cannot be used for anything but their intended purpose, proper handling, and proper storage of the parts.
“2”: Even the delivery of the parts to the individual processing steps is of decisive importance in being able to detect fouling in time (Ill. 18.104.22.168-19). There are several reasons for this:
If the personnel are sufficiently experienced, there is a high probability that they will notice unusual changes in the parts. This could be in their own best interest, since problems and rejects will otherwise be attributed to their “own” finishing processes, with unpleasant consequences. If fouling is detected sufficiently early, the risk of high scrapping costs can be minimized. In addition, there is a higher probability that the cause can be rapidly found and specific corrective measures taken.
“3”: In order to estimate the risks of potentially affected or even delivered parts, it is necessary to understand the damage that has already occurred, and also to identify the fouling. There are specific laboratory testing methods for this purpose, especially microanalysis using SEMs (Ill. 17.3.2-7). If possible, especially in the case of expensive parts, the tests are done in a non-destructive manner. The identification of an active damage mechanism, as well as the extent of damage, are prerequisites for the development and approval of any reworking and repairs (Ill. 17.1-11).
“4”: The monitoring of the finishing process, i.e. the parts and auxiliary materials, by experienced and motivated personnel is of decisive importance (Ill. 17.1-5). Unusual changes in parts (e.g. color, sheen, stains) and/or unusual behavior of baths (e.g. bubbling, discoloration, effectiveness) and machines (strange noises, wear, shavings) can be important indicators.
“5”: If fouling and/or unusual surface changes have been confirmed, an investigation by specialists is necessary in order to identify the damage for risk assessments and possible reworking. The person the specialists report to, or at least the chain of information, should be clearly determined and understood. This is especially true for finished parts that are to be delivered following reworking.
“6”: In order to prevent fouling, it may be necessary to implement cleaning processes at suitable places in the finishing sequence. These processes must be selected, tested, and approved by the responsible specialists. This is absolutely necessary in order to prevent unexpected new damage-relevant effects (e.g. etching damage or damage to coatings) from occurring.
Illustration 22.214.171.124-19: A special problem is the detection of finishing-related fouling in poorly visible hollow spaces. This includes cooling air ducts in turbine blades, integrated oil bores in gearbox housings, and branching pipes as are typical for oil systems. For this reason, special attention should be paid to fluids and particles falling or flowing out of parts during handling. Reaction zones and deposits (stains) around bores that lead into the part are also important indicators (top left diagram).
Constrictions and blockages can be detected with the aid of flow metering, perhaps in combination with thermography (middle diagram).
In order to detect internal fouling with X-rays, it is necessary to weaken the X-ray power sufficiently to match the part walls.
If fouling can be dissolved by an otherwise non-damaging bath, it may be possible to detect fouling on the parts on the basis of contaminants in the bath (right diagram). This process is used to detect extremely small amounts of residue of low-melting workholding materials. For the material bismuth (Bi), the detection limit is 0.02 ppm (Ref. 126.96.36.199-7).
Illustration 188.8.131.52-20: In order to prevent fouling from having damaging effects, it is especially important to closely visually inspect parts before finishing processes that can cause damages (e.g. heat treatment and galvanic processes). Typical signs of fouling are shown in the bottom diagram. It may be necessary to consult the specialists in the specified information chain for evaluations and solutions.
Illustration 184.108.40.206-21: Covers are necessary in order to prevent fouling. However, they may themselves be a source of fouling.
“A”, masking with rubber covers during blasting to prevent deformation of filigreed sections. Metal tape is used during abrasive blasting. Wear products or residue from metal tape can cause damages in subsequent processes (Ills. 220.127.116.11-9 and 18.104.22.168-19).
“B”, covering the blade roots of turbine blades in order to prevent coating with a brittle diffusion coating, which would reduce LCF strength (Ill. 22.214.171.124-3). Media can infiltrate under the cover. Any remaining powder can cause damage during subsequent heat treatment (Ill. 126.96.36.199-2).
“C”, covering against welding splash from electron beam, laser, and friction welding (Ills. 188.8.131.52-6 and 184.108.40.206-12). Splash can locally overheat and damage the surface (Ill. 220.127.116.11-5).
“D” in order to prevent splash from laser boring, hollow spaces in cooled turbine blades are masked with a wax filler (Ref. 18.104.22.168-7) that covers the opposing walls, and catches splash. In this case, it is possible that adhering splash may not have been completely removed, or may come loose later.
“E” masking hollow spaces. If worn or damaged covers allow blasting media to remain in hollow spaces, it can overheat hot parts or cause damages in the oil circulation system (bearings). If wear products are transferred to other areas, they can cause damages in subsequent processes (Ills. 22.214.171.124-9 and 126.96.36.199-19).
“F” covering during thermal spraying to prevent ricocheting particles from being sprayed (microsphere effect, Ill. 188.8.131.52.2-4).
“G” rubber covers used to keep baths away from certain part zones. If the cover is infiltrated, etching baths can cause corrosion, and coatings can be undercut. If the covers remain on the parts for an extended period after the process, bath residue in gaps can cause damage.
184.108.40.206-1 “ASM Handbook”, Volume 4, “Heat Treating”, ASM International, 1991, ISBN 0-87170-379-3. pages 919 and 910.
220.127.116.11-2 “Machining Titanium Alloys”, www.supraalloys.com/Machining_titanium.htm, 10.4.2004, pages 1-12.
18.104.22.168-3 “Titanium Design and Fabrication Handbook for Industrial Applications”, “Titanium Surface Treatments, Cleaning & Maintenance”, Titanium Metals Corporation (Timet) ,www.timet.com/fab-p34.htm, page 1.
22.214.171.124-4 W.Klose, “Kühlschmiermittel auf Metalloberflächen”, periodical “Oberflächentechnik”, 86 (1995) No. 6, pages 1876-1882.
126.96.36.199-5 R.D.Münster, “Fehlern auf der Spur, Haftungs- und Benetzungsstörungen an lackierten Bauteilen”, periodical “Lackiertechnik”, 49 (1995) 3, pages 190-191.
188.8.131.52-6 NTSB -AAR (Aircraft Accident Report) 72-9 “United Airlines, INC. Boeing 737-222, N9005U, Philadelphia International Airport, July 19, 1970”, pages 1-38.
184.108.40.206-7 P.Adam, “Fertigungsverfahren von Turboflugtriebwerken”,Birkhäuser Verlag, 1998, ISBN 3-7643-5971-4, pages 60 and 81.
220.127.116.11-8 AMS, Metals Handbook Ninth Edition, “Volume 11, Failure Analysis and Prevention”, M.H.Kamdar, “Liquid Metal Embrittlement”, “Embrittlement by Solid-Metal Environments”, ISBN 0-871170-007-7 (v.1), pages 225 - 244.
18.104.22.168-9 AMS, Metals Handbook Ninth Edition, “Volume 9 , Metallography and Microstructures”, R.R.Boyer “Titanium and Titanium Alloys”, ISBN 0-871170-007-7 (v.1), page 464.
22.214.171.124-10 NTSB Identification MIA88IA231, Microfiche number 36430A, July 1988.
126.96.36.199-11 Z.Peidao, Y. Hai, “A Case Study of Bi-Sn-Induced Embrittlement”, D.R.H.Jones, “Failure Analysis Case Studies” 1998 Elsevier Science Ltd. pages 360-365, reprinted from “Engineering Failure Analysis”, 3, 241-247 (1996).
188.8.131.52-12 W.Friehe, “Dehnungsinduzierte Spannungsrisskorrosion in Flüssigmetallen” (“Strain-induced stress corrosion cracking in liquid metals”) , periodical “Werkstoffe und Korrosion 29” pages 747-753.
184.108.40.206-13 S.W.Kandebo, “GE Win Signals Entree Into F-15 Business” , periodical “Aviation Week & Space Technology”, April 29, 2002, pages 27 and 30. (2338)
220.127.116.11-14 H.Simon, “Oberflächenreaktionen an Titanwerkstoffen”, periodical “Metall Oberfläche” 5-1982, pages 211-217.
18.104.22.168-15 E.U.Lee, R.G. Mahorter, J.D. Wacaser, “Fracture of Ti-8Al-1Mo-1V Alloy Fan Blade by Stress Corrosion Cracking and Fatigue”. proceedings of the symposium on May 1-6, 1977, Toronto, Canada, “Fractography in Failure Analysis”, ASTM Special Technical Publication 645, pages 128-143.
22.214.171.124-16 Aviation Occurrence Report Number 88H0001, Canadian Aviation Safety Board, 12.December 1989.
126.96.36.199-17 W.Shih, J.King, C.Raczkowski, “Liquid-Copper/Zinc Embrittlement in Alloy 718”, Welding Research Supplement, pages 219-s to 222-s, of the “Welding Journal”, June 1998.