Aeroengine Safety

Figure "Electric discharge machining features": One must assume that during the EDM process the micro-arc will have considerable damaging effects on the properties of the work surface. A potential loss of dynamic strength can always be expected with EDM, and must be incorporated into the constructive design. Experience has shown that even reworking will not be able to completely restore surface integrity (Fig. "Danger of insufficient EDM recast layer rework")

Fundamentally, two main effects occur:
A zone briefly melts and then solidifies again (recast layer). With normal process data, this zone is thinner than 0.01 mm (top detail). The melted metal absorbs carbon from the cracked dielectric. This leads to hardening, due to martensite formation in steels and due to carbide formation in Ni and Ti alloys. Hardening is accompanied by embrittlement. As a consequence of the shrinkage during solidification and cooling, the recast layer has high tensile stresses that can cause micro-cracking. These cracks can also spread into the unmelted base material (Ref. 16.2.1.2-6).
Below the recast layer, there is a heat-influenced zone of roughly the same thickness.

The temperatures in this heat-influenced zone can lead to structural changes such as tempering effects in steels and solution annealing in Ni alloys. This loss of hardness should be accompanied by a loss of strength. In addition, an increase in ductility is to be expected.
Structural changes in the heat-influenced zone can influence the corrosive attack on the base material in combination with strongly deviating corrosion behavior in the recast layer. The oxidation behavior may also worsen around the oxidation-sensitive recast layer, or if the formation of an optimal Al diffusion layer is prevented. Even in galvanic coatings, remnants of recast layers should result in coating flaws.
Micro-craters and material breaking out of recast layers can act as notches.
The pronounced roughness of EDM surfaces can unallowably influence the flow in cooling air bores with sensitive functionality. An example is the bores for the extremely sensitive air film in combustion chambers (Fig. "Disturbance by burrs in pipe lines"). The air film, which protects and supports the combustion process, can lose some of its effectiveness.
If particles of the recast layer break off and enter into the oil or fuel systems, they can block gliding systems with tight tolerances, such as regulators or axial piston pumps. It is less likely that the expected very fine particles would cause bearing damage.

Figure "Fatigue strength of machined titanium parts" (Ref. 16.2.1.2-2): Machining processes have a considerable influence on the dynamic strength of a part. This is due to both positively and negatively acting effects. It is also due to the frequently occurring flexural stresses that are especially strong at the surface. The diagram shows some typical process-specific dynamic strengths in the high-strength titanium alloy Ti-6Al-4V.
The dynamic strength is characterized by the endurance limit (HCF range). This is more sensitive to damages than loads in the LCF range. A relatively good endurance limit can be attained with a careful grinding process (machining direction perpendicular to the dynamic stress), sufficient cooling, and very moderate grinding parameters, such as circumferential speed of the disk and infeed/cutting performance (Ill. 16.2.1.1-3). However, as soon as grinding becomes more intensive, there is a dramatic decrease in dynamic strength. At the same time, unlike in other material-removing processes, it is evidently no longer relevant whether the process is unusually intensive beyond this level.

The machining processes with defined cutting edges, i.e. milling and turning, show somewhat similar behavior. In general, even with high machining data, good dynamic strength can be attained. Even high machining performance can have a positive effect. This should be due to the increase of dynamic strength-increasing effects such as compression residual stresses and hardening.
In EDM, however, the typical melting that occurs during the machining process evidently causes serious damage, even if material removal is done carefully. The dynamic strength can only be raised through subsequent reworking with sufficient material removal. However, this is problematic (Fig. "Removing problem of EDM recast layer"). Only in wire-EDM can this damage be limited, which is why this process is used for the finishing of test parts and prototypes with very limited life spans.

Chemical milling (CHM) is similar to electrochemical machining with regard to their results. Although the expected dynamic strength is lower than that after turning or milling, this effect does not have a pronounced dependence on the process parameters. The somewhat lower dynamic strength can be explained by the lack of hardening and missing compression residual stresses. Therefore, the attainable dynamic strength corresponds to that of the undamaged base material.

Illustrations 16.2.1.2-3 and 16.2.1.2-4: Bores are part zones with stress concentrations. They are potentially under high loads. In addition, their accessibility for non-destructive testing is often complicated (Fig. "Inspection of bores in disks during overhaul"). For this reason, the quality, especially of small bores, is highly dependent on optimal finishing parameters and their strict adherence. If bore shapes are not circular (Fig. "Removing problem of EDM recast layer") and/or there are many small bores in a material that is difficult to machine, the melting processes EDM, electron beams, and lasers are used (middle diagram). An alternative is electrochemical processes (right).
Typical examples with a large number of small bores are air-cooled hot parts such as combustion chambers or blades.

Machined bores (left in the diagram in Fig. "Quality of boring processes"): Special unwanted characteristics are burrs at the inlet and outlet. Their influence on the dynamic strength and flow is treated in Ills. 16.2.2.2-6 and 16.2.2.2-7. The consequences of damage to a bore wall in a part under high LCF-stress, related to unsuitable boring parameters, are shown in 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, 16.2.1.1-9.5 and 16.2.1.1-9.6.

Laser boring, electron beam welding, EDM (middle in the diagram in Fig. "Quality of boring processes"): These processes influence the work surface in various ways. Fig. "Electric discharge machining features" shows this using the example of EDM. This process negatively affects the dynamic strength and throughflow. Fused splashed material (top frame in Fig. "Boring methods for cooling air") restricts the flow by increasing roughness and/or constricting the cross-section of the bore. If this splashed material breaks off, it can block cooling air bores, such as those in cooled turbine blades (Ills. 16.2.2.3-13 and 16.2.2.3-19). The resulting overheating shortens the part life considerably.
It must be assumed that the splashed melted material will damage the part surface (Ills. 16.2.2.6-2 to 16.2.2.6-5). This can dangerously lower the dynamic strength.
Recast layers have a similar effect (middle frame in Fig. "Boring methods for cooling air"). These coatings are usually extremely hard, brittle, and under high tensile stresses (Fig. "Effects of residual stresses") that can cause micro-cracking (Fig. "Electric discharge machining features").

Electrochemical boring (EC, left in Fig. "Quality of boring processes"): This term includes several boring processes (Ref. 16.2.1.2-1):

• Electro-stream drilling = ESD,
  
• Shaped tube electrochemical machining = STEM
  
• Electrochemical fine boring = ECF,
  
• Electro-jet boring = EJ.
  

Ills. 16.2.1.2-7 and 16.2.1.2-8 show the influence on the work surface. Unsuitable process parameters can cause intergranular attack and thus reduce the dynamic strength. In hot parts, galled grain boundaries are attacked by oxidation. These flaws act as crack initiators in case of thermal fatigue.
If the bore walls have pronounced ridges (bottom frame in Fig. "Boring methods for cooling air") due to poor process conditions, it will affect the throughflow. Although this will reduce the volume of throughflow, heavy turbulance in the cooling air can contribute to improved heat removal.
If a cooling configuration was developed in the prototype stage with ridged bores, and the boring process was later changed to create smooth walls during serial finishing, this could result in considerable differences in behavior relative to the testing stage.
Galling on the surface around a bore opening under high dynamic strength (Fig. "Problemzones of the ECM process"), such as in a rotor part, may preclude use of the part without reworking (Fig. "Minimizing scrap rates throuch reworking").

Figure "Removing problem of EDM recast layer": EDM can be used for cost-effective finishing of special bore cross-sections. Oil drainage bores in a spacer ring of the rotor were optimized for positive stress distribution and given an oval shape. The location of the bore takes into account the direction of the high tangential stresses (top left diagram).
In the case of cooling air bores in a turbine blade, the unfavorable angle of the borer and the poor machinability of the cast Ni alloy are the primary reasons for the selection of EDM for the finishing of this part (right diagram).
In both cases, the bores are in zones under high dynamic stress. In order to ensure sufficient dynamic strength, the bore walls were reworked with abrasive processes such as abrasive flow machining or oxide blasting. This was done in order to attempt to remove the potentially damaged zone in the recast layer.
In the testing phase, dynamic cracks originating in the bores occurred in both reworking processes. It was determined that the reworking had not sufficiently reliably removed the EDM damages (Fig. "Fatigue strength of machined titanium parts"). A similar case is shown in Fig. "Danger of insufficient EDM recast layer rework".
These experiences let us conclude that EDM should not be used in part zones under high dynamic stress, even with reworking. In these areas, the higher costs of other processes cannot be avoided.

Figure "Danger of insufficient EDM recast layer rework" (Ref. 16.2.1.2-5): The damage incident described here is an example of the dangers related to a problematic application of EDM.
As expected from the typical temporal stress progression in turbine disks, the damage occurred with a dull bang roughly two minutes after takeoff. The right engine overtemperature warning light in the cockpit lit up, even though the other monitoring instruments stayed in the normal range. Only after about 10 seconds did the engine lose power and shut down. The aircraft turned back and landed safely.
The first inspection of the engine revealed typical signs of an uncontained failure, with a 25 x 12 mm hole near the gas generator turbine. Smaller damages were found on the nacelle and wing.
After the engine was disassembled, it was discovered that the cooling air-carrying aft cover plate (ACP, also see cover plate) on the second gas generator turbine disk (bottom right diagram) had fractured at one of the three cooling air bores. A segment covering about 15 cm of the circumference was missing. The rotor disk had about 11,450 startup/shutdown cycles and had been installed as a new part. Records of the new part production and maintenance did not show any abnormalities. This kind of damage is alarming because it indicates that a large number of additional parts may be affected by the problem.
Subsequent laboratory inspections of the fracture surface revealed that the fracture had been caused by LCF cracks spreading radially inward and outward from the bore. There were cracking zones both in the bore wall and at the bore edges at the disk surface (bottom left diagram). In all of the bore walls, including the other two that were not involved in the fracture, axial notches up to 0.5 mm deep ran along their entire length. Signs of micro-cracking were found in the notch base. Remnants of a recast layer were found on two neighboring cracks. The base material did not show any abnormalities or deviations that would be evaluated as responsible for the damage.
Further research at the OEM revealed that the three cooling air bores in the cover plate were produced with EDM in accordance with the design specifications. This was followed by an abrasive flow process in which abrasive particles in a highly viscous medium are pressed through the bores (Ref. 16.2.1.2-1). It was not determined how the axial notches remained in the bores, but it is speculated that the cause was an undetected deviation of the EDM process that was not completely removed. The investigation revealed that small cracks in the axial notches were evidently already present in the new part. Under the cyclical operating loads, LCF cracks originated in these flaws. Parts from earlier production lots were made using ECM, but this process was later replaced with EDM.

The following lessons can be drawn from this incident:

Illustrations 16.2.1.2-7 and 16.2.1.2-8: Electrochemical machining (ECM) dissolves material. Under optimal conditions in accordance with specifications, this occurs without thermal or mechanical action. This avoids many of the damages typical for chipping and EDM. An ECM surface that is not influenced by hardening or residual stresses can have advantages or disadvantages relative to surfaces produced by chip removal (Fig. "Fatigue strength of machined titanium parts"). This depends on whether damaging effects (tensile stresses) or positive characteristics (hardening, compressive stresses) were removed. In many cases, ECM is combined with a subsequent hardening process, usually shot peening. In spite of this, under unfavorable conditions, there are process-specific potential damages that can remain (Ref. 16.2.1.2-5).

There is a certain uncertainty resulting from the fact that ECM remains a somewhat exotic process for many manufacturers even today. In addition, many of the process parameters and the energy forms that act to cause material removal (electric, mechanical, chemical, hydraulic) influence one another in a nonlinear fashion. Several 10¹ atmospheres of pressure in the electrolyte with flow speeds of up to 100 m/s may act in the gap between the tool and part. If this electrolyte flow is rapidly blocked or disrupted, the high flow rate can result in a water hammer. This places extreme forces on the tool and part, and their deflection can bridge the work gap and cause a dangerous short circuit.

The high flow rate and increased electrolyte temperatures can lead to local vaporization of the electrolyte and cause cavitation. This promotes uneven material removal and vibration of the tool and/or part. Depending on the size of the machine, the acting flow forces can reach levels of up to 10⁵ A. This means that short circuits become dangerous damage mechanisms. The electrolyte is influenced both by the direct electric current and the electric heating of the part being machined. The resulting thermal strain differences can change the work gap and therefore also the finishing measures. For this reason, they must be accounted for in the design of the tool.
As opposed to the usual classifications for the intensity of material removal in chipping processes such as smoothing (finishing, gentle) or rough machining (abusive), ECM uses the terms standard and off standard. If process parameters deviate too far from the standard, short circuits and spark formation are more likely. These cause dangerous, deeply acting damages through overheating and melting (Fig. "Dynamic fatigue lowered by electric arcs").

The following problems can be expected when process conditions are not optimal (off standard):
Intergranular attack (IGA): The wrong electrolyte concentration or insufficient current density can cause intergranular attack. These conditions are present at surfaces outside of the ones actually being machined. Primarily, this occurs around the insulated tool surface or in finished side surfaces that the tool has already passed. A dangerous deep intergranular attack usually only occurs when there is a material condition with sensitive grain boundaries. The depth of damage is material-dependent, and is usually in the range of several hundredths of a millimeter. These conditions (sensitization) can occur, for example, in connection with unsuitable heat treatment, weld seams, or temperature control during forging. Therefore, it is important to determine and ensure the proper material conditions for optimal ECM.

The electrochemical attack can concentrate in specific areas and cause pitting or pitting corrosion. If certain structural components dissolve more rapidly, it will result in selective attack. These notches can be avoided through increased current density and higher material removal rates. The high current density balances the small chemical differences between neighboring grains or inhomogeneities such as micro-segregations. On the other hand, lower current intensity and related low voltages promote selective attack. This means that the width of the gap in the active zone (tool face, acting surface) has a special importance. Minor pitting can occur in part zones that are farther removed from the active surface. As the distance to the active surface of the tool increases, various roughnesses can occur, from polished to high roughness. It is not clear how much small pits affect dynamic strength. They should have a less damaging effect than a crack-like intergranular attack.

In connection with corrosion, the potential danger of stress corrosion cracking in titanium alloys must be mentioned. If electrolyte residue containing common salt remains on a part surface that is under sufficently high tensile stresses, it is possible that cracking will occur at temperatures above 450°C (Fig. "Stress corrosion cracking by process baths and hand sweat"). Because of this, one must be sure to sufficiently clean the parts after ECM and before heat-treatment or welding. The ECM process reacts very sensitively to scattering of the material properties of the part that influence the dissolving process. Even differences in the grain orientation (lattice location) or deformation/hardening can act as changes in roughness, even if they are not outside of the allowable tolerances.

There are structural components that act as non-conductors and/or are not dissolved by the ECM due to their high chemical stability. These include carbides and intermetallic phases in nickel alloys. These carbides (spikes), which often have very sharp edges, can often protrude from the ECM surface. However, these flaws are so small that they will not cause a short-circuit with the tool, nor considerably change the operating behavior of the part. It is possible, though, that they may affect any diffusion coatings, such as those used to protect Ni alloys in hot parts from oxidation. For example, these oxide spikes can penetrate the diffusion coating and reduce the oxidation life of the part.

In cast alloys with their typical macro- and micro-segregations (e.g. around dendrites, Fig. "Types of hollow material flaws"), the problem of selective attack is more pronounced. In this case, areas with lower dissolving rates can lead to contact with the tool and cause short-circuits.

The consequences of short-circuits between the tool and part are especially damaging to the safety of parts under high cyclical stresses. Local overheating leads to melt craters, recast layers, tension residual stresses, and strength losses (Fig. "Causes of local overheating"). The result is an extreme decrease in dynamic strength. This situation can be traced back to elastic tools, flawed infeed pressure, and/or insufficient local material removal. The resulting intensive arcs are not sufficiently safely controllable, even with electronic emergency shut-downs, to completely prevent unallowable damage.
Another form of damaging short-circuit occurs in electrical connections to the part.
If the contact surface is deformed, concentrating the current flow in a local area, it can result in heat development acting deep into the surface. Due to the restricted air access, dangerous heat levels will not necessarily reveal themselves through tarnishing. These heated zones can create “overtemperature lenses” below the part surface. The remedy for this is proper, sufficiently massive design of the contacts, clean and even contact surfaces, and, if applicable, the use of a suitable contact grease.

Experience has shown that there is no danger of hydrogen embrittlement during ECM, even though hydrogen is released by the material removal process. This can be explained by the removal of the hydrogen in the very fast electrolyte flow.

Stress corrosion cracking is a real danger, especially in high-strength titanium alloys. If electrolyte residue containing chlorine remains on the part even after flushing, it can cause cracking if there are sufficient tension (residual) stresses and temperatures above 450°C (heat treatment, welding; Fig. "Titanium cracks by chlorine during production").

Flat (only a few mm deep) oriented surface structures (flowlines, Fig. "Influence of ECM at the processed surface") can occur on the ECM surface. Their development is related to the process parameters. The wave-like soft structures with minimal depth should not have any noticeable effect on dynamic strength, although a suitable verification of this must be conducted in each case.