Table of Contents
16. Finishing
In this chapter, finishing refers to processes that follow the manufacturing of blanks and semi-finished parts discussed in Chapter 15. Finishing and repair processes are sufficiently explained in comprehensive technical literature. Some sources are provided in the list of reference works at the end of the chapter (Ref. 16.1-…). They are intended to provide information concerning the processes in order to enable the reader to understand the causes and mechanisms of typical finishing flaws. This application area is not described nearly as throughly and comprehensively in the accessible literature.
The following chapter is primarily concerned with problems in the finishing of turbine engine parts. This does not only mean the finishing processes themselves, but includes the influence of transportation, cleaning, storage, and work preparation. A description of the effects on operating behavior is intended to provide the necessary understanding for optimal damage prevention. With regard to the fatigue behavior of the parts, influences that act on the surface are especially important. The concept of surface integrity overwrites this subject and is treated in a separate chapter, 17.2. The human factor is decisive for safety. Human flaws and weaknesses are often related to safety considerations, even if the problem seems to be procedural at first glance. Examples include imprecise dimensions and mistakes in diagrams, technical drawings, specifications, and regulations.
The influence of corporate culture on the finishing process should not be underestimated (Ills. 17-6, 17.2-1, and 17.5-2). Group pressure and “personal” measures in case of rejects and damages can have a counterproductive effect. Experience has shown that believably objective approaches with a primary focus on the analysis and solution of technical problems (Ill. 17-11) will have a higher probability of success.
Experience, motivation, and training of the finishing personnel (Ill. 17-5) must not be undervalued, especially from the point of view of cost reduction while maintaining the necessary quality. Motivations also include the work environment, including the available space and machinery.
Rising demands on aerodynamics, as well as increasing thermal and mechanical loads on the parts require ever tighter limitations on weak points. The higher strength, usually combined with a higher sensitivity of materials to finishing processes, exacerbates problems with safety (Fig. "Testing of high strength material").
If there is a use of finishing processes that have characteristic flaws that cannot be sufficiently safely detected with serially implementable non-destructive testing methods, this fault must be compensated by process monitoring and process stability.
An additional problem occurs when processes that are traditionally used to ensure part properties (such as shot peening or thermal barrier coatings) become an integral part of the life span design (Volume 3, Ill. 14-24). This means that flaws and deviations in the processes can have a dangerous influence on the safety of the parts. With regard to this aspect, this chapter is closely intertwined with Chapter 17 on quality assurance.
The finishing of engine parts has reached a high level as an important component of engine safety (Ill. 17-1). However, the increase in engine numbers resulting from the growth of the global aircraft fleet demands additional efforts to raise part safety (Volume 1, Ill. 2-7). Because the currently-used processes and strategies for ensuring flawless parts have been largely exhausted, there are questions regarding a new practicable approach. This approach may lie in specific, actively quality-assuring utilization of human resources in the field of finishing. This includes motivation, experience, and technical understanding on the part of the personnel (Ills. 17-2, 17-6, and 17.5-2).
The major portion of problems and damages that have their causes in the finishing process can be viewed positively from certain angles. Each of them can provide new insights. This understanding is the basis for the two-part arrangement of Chapter 16. In one section, the problems are grouped according to the various finishing processes (Chapter 16.2.1), while in Chapter 16.2.2 they are categorized according to similar influences from different finishing processes (e.g. burring) in the context of operating safety.
Finishing is located in a stress field. For design/construction, the primary considerations are function and safety. The operator is especially interested in availability and life span at low operating costs. Minimal finishing costs must guarantee the financial success of the finishing. This demands optimal finishing technologies, but also requires adjusting the production processes to demands for the realization of new technologies, such as blisks.
Comprehensible analyses and estimates should consider all possible combinations of influences and demands on finishing in order to provide an overview of the high cost risks of development and investment in equipment for new finishing techniques, and to help minimize these risks wherever possible. Fig. "HC Hub ratio as design characteristic" shows an attempt to formulate principles for a compromise. Naturally, the individual weighting of the influences plays a decisive role.
If the costs of the parts or the new engine are the primary consideration, one will attempt to minimize the number of parts and their elaborate production processes. This includes the transition to new concepts such as the shift from disks with inserted blades to single integral parts, i.e. blisks. This trend can also be seen in the use of different types of compressor stators, from built (soldered, welded) stators to integral cast versions. These different types necessitate suitable finishing processes, which may be considerably more demanding than those for the “traditional” parts.
If the fuel costs increase considerably, the engine procurement costs will be relativized. This can lead to markedly different production processes. This demonstrates the long-term effects of strategic decisions.
Figure "HC Hub ratio as design characteristic": The procurement costs of an engine are the main concern for customers when the fuel costs are comparatively low. This type of strategy influences the design characteristics and finishing considerably, and can be decisive for the existence of a manufacturer.
Naturally, there have been attempts to develop algorithms that make it possible to make decisions with the aid of computers.
The result of such an attempt is shown as an example.
It was observed that the so-called hub ratio of the rotor components represented a parameter that influenced both the finishing costs and the most important operating characteristics of the engine. The “key area” of this analysis was the inlet of the high-pressure compressor (Ref. 16.1-4). The hub ratio is the ratio of the disk diameter (hub) to the diameter at the top of the blade tips.
If the fuel costs rise, more elaborate engine concepts with considerably higher production costs may be advantageous for the customer. If, on the other hand, low initial costs are the primary concern, this will first affect the production of blades and disks. Production costs can obviously be minimized by reducing the number of stages and blades as much as possible. The goal is to find a compromise between production costs, efficiency, weight, size, mechanical loads, and design requirements. The range of possible compromises can be seen in the half-sections of two successful competing civilian engines in a lower thrust class. The strategy of lower part numbers requires extremely exact blade profiles with special measures to minimize the blade clearance gap (e.g. armor) and to control high dynamic loads. This implies the highest strength-relevant surface quality. In order to minimize costs, less stages with a smaller number of wider blades (wide chord) were chosen. This also affected the extremely highly stressed blade roots. The high peripheral loads led to massive disks under extremely high LCF stress. In order to minimize the problems in this zone, the design was changed to a blisk construction. This demands more machining work (with tight tolerances, Fig. "Safe operating due to dimensionalaccuracy") and possibly welding (e.g. linear friction welding, Fig. "Flaws of friction welding") in dynamically highly-stressed blade zones. The minimal damping properties of this design exacerbates the problem of dynamic fatigue.
The trend is also leading towards integral stators, usually connected through welding or soldering (Fig. "Brazing applications in engines"), especially in compressors.
Therefore, it can be seen that a change to an engine concept will also require changes to the production processes. This means that optimization and development of production processes must occur in parallel with cost-reducing strategies. This also means that potential problems in the production processes will increase, at least initially.
Figure "Finishing flaws at part surface": In the following, finishing flaws are understood to be flaws or deviations beyond specifications that can be attributed to the process of production of a finished part from a raw part. Weak points, on the other hand, are within the specifications and are limited by the properly defined, prescribed finishing processes.
Flaws and weak points can also be deviations such as residual stresses and de-hardened areas.
The finishing process primarily affects the surface of a part. This is usually subjected to a combination of especially intense operating loads from vibrations, wear, oxidation, and corrosion. Finishing flaws in the part are primarily related to bonding processes such as welding and soldering. Heat treatments can lead to flaws such as thermal cracks, unfavorable structures, and residual stresses (Ills. 15.2-11 and 15.2-13).
