Table of Contents
16.2 Finishing and the Operating Behavior of the Parts
Finishing must be considered in combination with design/construction, technology, and operation. All of these fields influence one another. They are responsible for the ultimate safe guarantee of the desired operating behavior. In order for this to occur, the results, i.e. the parts, must be within the requirements of the designs and specifications. In addition, there may also be an unconscious influence of unspecified characteristics that are based on experience with the operating behavior. An example is the effect of machining on a surface, which can cause hardening and residual stresses.
The operation of the engine must be sufficiently safely guaranteed over the projected life span while maintaining the specified performance data. One prerequisite for this is that the aerodynamic quality of the blade profiles is as specified. In this context, an important factor is the roughness of the surfaces in the flow (Volume 3, Ills. 11.2.1.1-9.3 and 11.2.1.1-10). The blades must also safely endure the mechanical loads, especially vibrations. This requirement is referred to with the term surface integrity (undamaged surface; Chapter 17.2).
A prerequisite for realizing the parts is that the required technology is available for serial implementation. This applies to both the production process and the materials technology. For example, blisks require adapted and/or newer production processes than does the separate production of blades and disks. Linear friction welding and/or extensive milling become the processes of choice. The trend towards increasing gas temperatures around the hot parts necessitates more complex cooling configurations and insulating coatings (thermal barrier coatings) made from ceramics. This places special demands on the production process.
Experience shows that minimizing costs and weight at the highest possible efficiency leads to correspondingly high mechanical, aerodynamic, and thermal loads. This must be taken into account by the construction and design and leads to ever greater demands on the production quality. In order to minimize weight, the volume (Fig. "Segregation in forged disk") and the surface of parts must be especially well utilized by the high operating loads. This is especially true for rotor blades. High-performance computers and programs aid this trend. This also increases the influence of machining on the part safety. For example, the importance of surface integrity (Chapter 17.2) also rises. If the dynamic loads (RPM, vibrations) increase, scattering of surface quality that was previously tolerated becomes unallowable. This constricts the parameters of the production processes accordingly. If this is no longer possible with the required degree of safety, serially-implementable processes that fulfill the demands must be developed.
A unique effect that is not necessarily recognizable at first glance occurs when serial processes that were previously used to ensure the guaranteed operating behavior become an integral part of the design (Volume 3, Ill. 14-7). A typical example is shot peening. Previously, it was intended to safely control the effects of any small weak points in the surface area on the cyclical strength. Another example is applying thermal barrier coatings to hot parts. These are used to ensure the guaranteed life span despite a certain operator-specific scattering of the thermal operating loads. These cases are similar to a belt providing insurance for a pair of suspenders. However, if these processes become an integral part of the design, it must be assumed that procedural problems will cause the part to suddenly fail catastrophically. In order to safely prevent this, these now “conventional” processes must be subjected to a considerably more demanding quality assurance process. In the case of shot peening, even coverage and the required Almen intensity across the entire highly-stressed part surface must now be strictly guaranteed. Of course, this is not only a production problem, but also an issue of quality control.
Figure "Influences of finishing flaws": Finishing-specific flaws affect the part behavior during operation through the usual damage mechanisms. Flaws that originated in very different ways can activate the same damage mechanism. For example, cracks can occur during welding under the influence of fouling (e.g. liquid metal embrittlement, Fig. "Embrittlement by contact with solid metal and melts") and corrosion (e.g. stress corrosion cracking, 16.2.2.4-12). Ultimately, almost every damage can be essentially reduced to a loss of the usable part strength.
This includes especially:
- unfavorable structures
- embrittlement
- flaws that influence the supporting cross-section (e.g. welding cracks, bonding flaws)
- notches (grooves, imprints, galling marks)
- tension residual stresses
- constriction of cooling air ducts
In contrast, effects on the part function, such as due to geometric changes (e.g. deviations in blade profiles), are subordinate.
One problem, especially for the typical technicians working in finishing, is to recognize which of the potential mechanisms come into effect in the processes they are using. Understanding this is a prerequisite for detecting damages in time, undertaking targeted measures and remedies, and testing their effectiveness with sufficient sureity.
Figure "Effects of finishing flaws on operation": Although there is no claim to completeness, this diagram is intended to provide an overview of the influences of finishing-related flaws (top frame) on the behavior of engine parts. The effects can be generally grouped into the categories strength, function, and cost. These main categories reciprocally influence one another through the factors safety and life span.
Strength: finishing flaws can be grouped into two categories based on their effects on the part strength: flaws which lower the material strength and/or flaws which raise the (local) operating loads (usually notches).
These flaws have an especially dangerous influence on the safety of the parts and the engine as a whole. Their effects are usually closely connected to a reduction in operating life.
The material strength is reduced by finishing processes that change the structure in an undesirable or unallowable manner. This can be caused by parameter deviations during heat treatment, reactions with foreign materials such as fouling (e.g. liquid metal embrittlement, Fig. "Damages by metallic surface fouling from finishingfinishing"), undesired brittle coatings (remnants of anti-diffusion coatings), and depleted alloy components. These material influences especially affect creep resistance and thermal fatigue. In these cases, there may be a chance to estimate the remaining life span and to undertake inspections and replacements with a tolerable risk level.
Typical examples of the effects of parameter deviations on operating behavior are corrosion cracks in threaded connections for pipes and inserts made from high-strength Al alloys. In this case, even minor deviations in the heat treatment temperature during the finishing process can cause dangerous damages (Volume 1, Ill. 5.4.2.2-1). Similarly, excessive strength caused by insufficient heat treatment can lead to the failure of steel bolts through corrosion cracking.
The operating loads are increased by stress concentrations resulting from notches (Volume 3, Ill. 13-18). This primarily affects the dynamic strength of the part. Notches locally increase the loads above the tolerable limit, resulting in dynamic fatigue fractures (HCF, LCF). There is a risk of the part spontaneously failing following crack growth, affecting flight safety. Stress concentrations are present at cracks, scratches, and impressed foreign objects (Ills. 16.2.2.5-5 and 16.2.1.1-9). If there is a danger of high-frequency vibrations, as opposed to LCF loads, the crack growth will be too fast to sufficiently safely predict the time of failure and undertake intermediate inspections or later measures.
Function: finishing damages can also unallowably affect part function. If material remnants from finishing, such as melt drops or shot, block cooling air bores, it can lead to overheating of hot parts such as turbine blades (Ills. 16.2.2.3-9 and 16.2.1.6-18). Dimensional deviations in compressor blades can worsen their aerodynamic behavior to the point that it causes stalls with serious consequences. Excessively hard abradable coatings can damage blades and ultimately cause them to fracture. Unfavorable structures in ceramic sprayed coatings can dangerously reduce the effectiveness of titanium fire-proofing on compressor housings.
Costs: costs are also included in operating behavior, for they are closely related to the safety and functioning of the parts. Fundamentally, the earlier a finishing flaw is found, the lower the costs will be. In this context, costs resulting from additional quality assurance measures, reworking, or scrap are a relatively minor issue. Naturally, the cost increases as finishing stages proceed.
If flawed parts have been installed, costs for remedies can increase dramatically. It will be especially costly if affected engines have to be taken out of service.
A loss of prestige occurring, for example, due to publications or directives from authorities, is also relevant to cost factors. This is due to the reduction of market opportunities relative to competitors.
A compilation of selected part-specific finishing
In many cases, part-specific questions such as “What are the problems affecting turbine blades in the finishing process?” can be expected. These are compiled in the following text for two of the main components of engines: blades and disks. This is intended to establish a connection to Chapter 16.2.1, which gives an overview of process-specific problems, and Chapter 16.2.2, which gives an overview of damage-relevant effects of finishing technology. For this reason, the problems are not individually treated in great depth here, and the reader is instead directed to the chapters that deal with them in greater detail.
Figure "Damages in turbine blades by finishing": This diagram shows an overview of finishing problems and damages in cooled turbine blades. These are cross-referenced with illustrations, pages, and chapters that deal with the topic from various perspectives.
Figure "Finishing flaws in disks": This diagram is an overview of finishing problems and damages in rotor disks. A turbine disk is used as an example. These are cross-referenced with illustrations, pages, and chapters that deal with the topic from various perspectives.
