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Hollow Blades from Titanium Alloys

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Development of Elements of the Technology Producing Hollow Blades from Submicrocrystalline Titanium Alloys by Combining Molding and Diffusion Welding under Low-Temperature Superplasticity, Including Non-Destructive Quality Control over Parts Produced

Tech Area / Field

  • MAN-MCH/Machinery and Tools/Manufacturing Technology
  • MAN-MAT/Engineering Materials/Manufacturing Technology

Status
8 Project completed

Registration date
02.08.2000

Completion date
12.07.2005

Senior Project Manager
Ryzhova T B

Leading Institute
Russian Academy of Sciences / Institute of Metals Superplasticity Problems, Russia, Bashkiria, Ufa

Supporting institutes

  • VNIITF, Russia, Chelyabinsk reg., Snezhinsk

Collaborators

  • GE Corporation Research & Development, USA, NY, Schenectady

Project summary

The State of the Art in the Field and the Impact of the Proposed Project on the Progress in the Field.

The technique combining superplastic forming and diffusion welding (SPF/DW) seems to be a prominent technological progress in titanium alloys machining. Due to significant expansion of the existing technological potentialities and creation of principle new ones the SPF/DB process concurrent with decreased labor cost and the significant material saving provides producing complex profile weight reduced structures which application is most urgent in aero-space engineering. This opens up advances for creation of the most up-to-date structures which production by using other conventional technological processes is rather problematic. At present there are available data on research and development activities relating to the commercial implementation of the SPF/DW technology by aircraft and rocket-building companies of USA, United Kingdom, France, Germany and Japan. However, in spite of the evident advantage and attractiveness of the SPF/DW technological approach (increase in a coefficient of metal used from 0.25 to 0.7, decrease in structure weight up to 30% and labor cost up to 40%) there are serious problems of real implementation connected with fabrication of load-bearing elements of critical application structures, for example, such elements as aircraft engine hollow blades. The main reasons responsible for retarding full-scale commercial implementation of the SPF/DW method are as follows: the absence of scientifically substantiated technological regimes for obtaining sound solid state joint of intricate shape sheets promoting ensured quality of parts produced and the absence of reliable methods for controlling quality of parts produced and, first of all, methods of non-destructive control.

It is apparent that the key to solution of the first problem lies in proper understanding of the nature of formation of solid state materials joining under structural superplasticity condition. In this connection, the Institute for Metals Superplasticity Problems RAS has got a significant 10 year research and procedure experience in conducting fundamental investigations of solid state welding of superplastic materials (titanium and aluminum alloys, interemetallics and ceramics). The conducted investigations were first to distinguish a leading role of superplastic (SP) deformation in solid state joint formation and to reveal a mechanism of its formations. The obtained scientific results are characterized by novelty and almost completely change the well-accepted concepts on the nature of solid state joints. This provides a principle opportunity to develop new high technologies. The main results were published in leading international journals and collections such as Acta Metall. & Mater., Materials Science Forum, Scripta Met. Et Mat., Proc. Of 4-th Intern. Conf. On Numerical Methods in Industrials Forming Processes – Numiform,92. A.A.Balkema/ Rotterdam/Brookfield/1992, Transactions Tech Publications, Switzerland, (1997), J. Mater. Eng. Performance (1999).

The second important problem affecting, to a large extend, on the possibility of commercial implementation of the SPF/DW technology is the radical solution of the task of non-destructive quality control of parts produced. This question is most urgent in aero-space industry being the main potential user of the SPF/DW technology. Accepting the extreme importance of the reliable non-destructive quality control of the parts produced including all technological stages of their processing one should note that real results of such investigations are rather limited in literature.

It is known that the defects of solid state joint can be macropores (non-continuosity, incomplet fusion), chains of micropores, oxide films, presence of flat interfaces in a joint area, etc. All these defects exert a specified effect on properties of solid state joint making them weak.

Systematic studies in this direction, as applied to parts fabricated by the SPF/DW method, are almost absent. This complicates the statement of problems for developing universal non-destructive control. The existing known methods of non-destructive control are not almost adapted to concrete structures processed by the SPD/DB technology. Reasoning from the available in IMSP and VNIITF results of joint investigations, one can assert that the most advanced method of non-destructive control for quality evaluation of parts produced by the SPF/DW technology is a method of laser holographic interferometry, characterized by significant advantages as compared to methods of ultrasound flaw detection and acoustic emission.

Unfortunately, there is not much information about foreign developments of devices and systems for holographic non-destructive quality control over multilayer structures processed by means of the SPF/DW. It is known the following foreign companies manufacturing holographic equipment (lasers, systems of hologram recording, vibration protective tables, systems of interferogram processing, optical mechanical accessories, etc.): Newport corp., Holosource corp., Spectratec corp. in USA, Nalko Marketing, Oriel GmbH & Co. KG in Germany. It should be noted that, as a rule, Western companies develop, manufacture and sell holographic equipment based on modulus principle. The given moduli can be used for solving a very wide spectrum of tasks, including holographic control of sandwich structures. However, due to wide applicability of these moduli, a number of questions, namely, specific choice of modulus composition, attachment of the system to immediately solving problems, furnishing with additional equipment (for example, with a system of object loading), and, naturally, all the methodology of the furnished equipment application are to be solved by the user himself. As a rule, only highly skilled personal can work with holographic equipment. It may be either research workers who use special devices or their own designed apparatus, or specially trained operators, working on holographic devices fulfilling some definite direct task. There are known many examples of such application of holographic equipment by western companies. For example, NASA has been using holography for pilot design development of its most demanding articles for more than 25 years, General Motors has been using a specialized device for holographic quality testing of motor car tires already for 15 years.

In the framework of the ISTC Project No. 796-98, the scientific results available in IMSP RAS were sampled for investigations of elements of the pilot technology for production of a reduced component of a hollow blade from submicrocrystalline titanium alloy Ti-6Al-4V sheets based on application of the SPF/DW method. The results of the activities fulfilled in the framework of ISTC Project No.796-98 have confirmed that the traditional method of diffusion welding even by virtue of the commonly acceptable definition of this process can hardly be used for creation of a technology ensuring structure control. Such situation cannot provide obtaining solid state joint of desired quality. On the other hand, the hollow cellular structure processed by the SPF/DW method has solid state joint of a significant long range area and its quality, to a great extent, determines operating properties of an item as a whole. In this connection, within the framework of the Project No.796-98, on the basis of the available fundamental scientific results, a principally new approach for creation of a technology for producing a hollow blade component on the basis of the SPF/DW method has been developed and tested. The essence of this approach is creation of a structure controlling technological process on the basis of superplastic deformation. Unlike diffusion welding, the strictly regulated and controlled parameters promoted by this technology are as follows: strain rate, strain value, temperature, and mode of a stress-strain state. Put otherwise, contrary to the known diffusion welding, the realization of an effect of superplastic deformation promoted by the SPF/DW technology provides a unique opportunity to regulate the quality of the obtained solid state joint. This activity results in filing an application entitled “Method for producing a multi-layer cellular structure”; its priority number is 99113931.

During the Project No.796-98 fulfillment the original pilot technology for processing of submicrocrystalline titanium alloy VT6 sheets with a grain size of 0.5 mm was developed. Using the submicrocrystalline sheets for the first time allowed to create an experimental technology for producing hollow blade components from titanium alloy VT6 by means of the SPF/DW method, the procedure being realized at a temperature no higher than 800 °C. The strength of the material of the hollow blade component produced from a submicrocrystalline alloy at a temperature not above 800 °C exceeds by more then 15% the strength of the material of the hollow blade component produced from conventional microcrystalline sheets (a grain size is 5-10 mm) at T=900 °C by means of the SPF/DW method. In this case the shear strength of the solid state joint obtained by the technology proposed is 97% from the shear strength of the basic material whereas in the case of usual microcrystalline alloy its value is 90%. Another important result obtained within the framework of Project No.796-98 fulfillment was successful approbation of laser-holographic methods of non-destructive testing by the example of investigation of hollow blade elements produced by the SPF/DW technology. It has been established that on the basis of definite interferograms of a blade one can make justified evaluation of the quality of performance of welding and forming of internal cells within the whole blade interior (shape and deviations from a cell shape, thickness and deviations of cell wall thickness, etc.), and determine the quality of diffusion welding of a filler rib. It has been shown that data obtained by laser-holographic methods provide real evaluation of a reaction of a given structure to various loads (bending, torsion, vibration etc.). The necessity exists of improving the package of applied programs for interferograms analysis, more comprehensive in consideration of the peculiarities of surface projection of the part under study. However, for forecasting an effect of mechanical loading on operability of the element of hollow blade during laser-and-holographic control additional work on correlation between the results of mechanical tests and ones of holographic studies, and testifying of the technique of holographic control on specially created testified defects are required.

The scientific and technical potentialities and specialized equipment created in the process of the Project No.796-98 realization have provided favorable conditions for optimization of the technology for producing titanium alloy VT-6 (Ti-6Al-4V) hollow blade components and make it more attractive for commercial implementation. This becomes possible due to the additional decrease in the temperature of the SPF/DW process by 30-50 °C more. Such a task can be realized by using the equipment available in IMSP and is possible due to optimization of the technology of processing SMC VT6 alloy sheets and obtaining a ensured grain size no more than 0.3 mm. The decrease in the temperature of the SPF/DW process by using a more disperse SMC structure creates conditions providing conservation of a structure close to SMC in the parts processed. Analysis of the results of Project No.796-98 activities has shown that the growth of grains to a value of 2-3 µm observed in titanium alloy at the temperature 800 °C during the whole SPF/DW cycle retards realization of the unique properties in the articles produced unlike the cases with SMC alloys. It seems actually possible to preserve a disperse granular structure with a grain size of no more than 1 m in the articles processed. This surely provides saving electrical energy and improving quality of articles, in particular, strength, damping properties and multiple cyclic fatigue. Concurrently, as an additional and important element of the technology, it is proposed to develop a method of laser-holographic non-destructive control as a method of concrete flaw detection of a hollow blade reinforced by such non-dependent methods as ultrasound detection and acoustic emission.

The objective of this Project: complex development and optimization of the unique technology in case for producing a hollow blade component out of submicrocrystalline titanium alloys at low temperatures of about 750-770 °C including creation of science-technique basis and development of a reliable method of non-destructive testing by using laser-holographic interferometry.

The proposal includes optimization of the pilot technology for processing SMC sheets with an ensured grain size of 0.3 µm, optimization of the technology of superplastic deformation and diffusion bonding by using an effect of low temperature superplasticity and development of the technique of several non-dependent methods of non-destructive quality control of hollow blade components, development of techniques for forecasting results of mechanical loading of hollow blade components by parameters of stress concentrators at laser-holographic testing and development of proposals on industrial application of unique laser-holographic methods of non-destructive control.

In parallel with this work, operability tests, aimed at the evaluation of strength, damping, elastic, plastic, fatigue, corrosion properties, as well as heat tolerance and electric conductivity within the wide temperature range (20 °C - 800 °С) are provided. Creation of the technique concept is called for development of the equipment and technological supply means intended to realization of the integrated technology.

Expected Results and their Application

The Proposal belongs to development of new technologies for processing materials integrally combined with the technology for producing sandwich structures of an intricate shape orientated for air-space industry.

The Project realization will allow to make comparison evaluations of limiting potentialities of the technology combining superplastic forming and diffusion welding for producing complex shape titanium alloy parts of critical application such as hollow blades by using submicrocrystalline materials. Here one should refer technologically possible low and high temperature limits of processing submicrocrystalline titanium alloy, attaining maximum high mechanical, physical and operating properties of a structure and a material from which it will be fabricated by the technology developed. It will bring to reality the evaluation of the unique technological procedure for control of microstructure and properties in the producing hollow blade component. It’s realization will also result in presentation of theoretical and experimental techniques for forecasting mechanical behaviour of the hollow cellular structure component, as well as laser-holographic and other non-dependent techniques of non-destructive control of quality adapted to the hollow blade component with regard to typical characterised defects. The result of the Project will also be a creation of basis for development of an industrial sample of a holographic control-measuring system, which promotes conducting non-destructive testing of multilayer hollow structures fabricated by means of the SPF/DB technique on the basis of laser and holographic interferometry.

The Project realization will result in development and optimization of pilot technology components providing the following:

– increase of a metal utilization factor from 0.12-0.25 up to 0.7-0.85;


– decrease of a temperature of SPD/DB process by 150-200 °С;
– reduction of a weight by 30-40% and of labor costs of multilayer hollow structure manufacture by 20-30% as compared to conventional technologies;
– save electrical energy by 1.5-2 times;
– increase of structures quality due to improvement of their mechanical and physical properties;
– expansion of technological potentialities of the SPD/DB technique.

In the framework of the present proposal it is planned to file a PCT application on developed and optimised elements of the technology in economically developed countries.

Industrial application of the technology comprises study of possibilities of its application and identification of results expected. The final result of this Project will be creation of the base technology for producing structures from submicrocrystalline titanium alloy sheets for commercial implementation ensured by the reliable method of non-destructive control. The Project possesses (is characterized by) high commercial potentiality since its results can successfully be used in other branches of industry that stimulates (initiates) transition of weapon technologies to new technologies of civil products operating during a long period under conditions of cyclic loading, corrosive medium which are used in gas- and oil pumping stations, power and transport machines.

Scope of activities

The following activities will be performed in the course of realization of the submitted Project:

1. Development and optimization of the pilot technology for producing sheets, 0.8×200×300 mm in dimension, from VT6 alloy with a grain size of 0.3 µm and manufacture of a batch of sheets on an isothermal rolling mill LIS 6/200.

2. Optimization of regimes of superplastic forming of sheets from submicrocrystalline titanium alloy VT-6 under low temperature superplasticity.

3. Optimization of regimes of solid state welding of sheets from submicrocrystalline titanium alloy VT-6 under low temperature superplasticity.

4. Development and optimization of regimes of contact welding of submicrocrystalline titanium alloy VT 6 as applied to the SPF/DB process.

5. Technological work-out on obtaining of lock joint of the hollow blade.

6. Optimization of the temperature regime of production of a hollow blade in a low temperature area.

7. Testifying of specific (typical) defects of multilayer cellular structure with respect to their influence on mechanical properties of the material and the structure and manufacture of standard samples with characterized defects for their detection by methods of non-destructive control.

8. Development of non-dependent techniques of non-destructive control of multilayer cellular structures.

9. Testing of standard structures having defects for determination of their real influence on structure fitness for work.


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