Nonconventional Technologies Review - no. 4 / 2007 ELECTROCHEMICAL REMOVAL OF UNIFORM SURFACE LAYERS UNDER MACHINING COMPRESSOR OR TURBINE BLADES

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1 ELECTROCHEMICAL REMOVAL OF UNIFORM SURFACE LAYERS UNDER MACHINING COMPRESSOR OR TURBINE BLADES P. KOCENKO 1, I. RUSICA 2, B. SAUSHKIN 3 Key word: removal, blades, electrochemical, ABSTRACT: A number of technological problems are concerned with removal of uniform thin layers out of manufacturing parts. For example such situation occurs in technologies of compressor or turbine blades production. Often it is necessary to remove so named defective layer received under precise blank stamping of this parts. In spite of the fact that there are some well-known ways to perform this operation but the electrochemical machining (ECM) process offers a number of advantages over other methods. Therefore the main objective of this paper is to discuss ECM possibilities and to underline some technological peculiarities under practical use of ECM in the blades production. The main parameters of ECM are considered in order to study their influence on surface roughness, dimensional accuracy and material removal rate INTRODUCTION Over the past few years there has been a resurgence of interest in the expansion of electrochemical technologies application. This method is used in micro machine building [1], producing the different kinds of surface relief with a help of dielectric masks and coats [2], new fields of dimensional electrochemical machining [3] and other fields [4]. It is well known that the most widely used and available anodic electrochemical technologies are based on either electrochemical polishing (ECP) or dimensional electrochemical machining (ECM) processes. The first of them is usually carried out in the current density region from 0,05 to 0,5 A/сm 2. Usual range of current density variation for ECM is approximately A/cm 2. It is seen that the range from 0,5 up to 10 A/cm 2 is practically not used. Only several publications can be found on technological investigations in this current density region. The point is that this region is not interesting for ECM because high surface roughness is arrived under using traditional water electrolytes. On the other hand classic electropolishing is also not effective in this region because some additional effects are developed negatively such as gas evolution and electrolyte heating. This paper investigates whether electrochemical process in current density region mentioned above can offer a solution to machining problems of compressor and turbine blades production. It regards to one technological stage in the production of this parts namely to the stage of defective layer removal. Such defective layer is produced under hot stamping of the compressor blade blanks or under precision casting of turbine blade blanks. The main requirements to the technology of removal of thin surface layers out of titanium and steel blades are the following: - uniformity of removal layer thickness is not more than 30-50% from nominal size of this layer; - grain boundary etching must be not observed; - size of removal layer is approximately 150 mcm; - machined surface must be smooth enough. Chemical etching is conventional method used usually for realization of this technological stage. Its disadvantages are well known: - removal rate is relatively low, approximately (5-10) 10-3 mm/min; 35

2 - working liquids used at this process are very aggressive; - grain boundary etching is not impossible. Therefore, theoretical and experimental studies were made in order to choose an appropriate producing method on the base of electrochemical process. Traditional ECM is not suitable in this case because it does not ensure required uniformity of removal layer. Classic electropolishing can t provide higher productivity than chemical machining and it is also connected with using aggressive electrolyte components. THEORETICAL BACKGROUND Theoretical studies were performed in order to working out new generation of electrolytes for electrochemical dissolution [5]. It is shown that electrochemical behavior of metals is strongly depended on the nature of solvent. So oxide films are not formed as a rule in organic solutions. Besides in organic medium low valency particles of dissolved metals are stable. So electrochemical equivalent and specific dissolution velocity of metal are higher with respect to aqueous solutions. These reasons allow recommend organic and water-organic solutions of mineral salts for electrochemical machining metals and alloys especially under low current densities. Two additional points must be also emphasized. The matter is that such electrolytes provide as a rule very good surface finish and polishing effect is often observed. Moreover the large dissipated property of organic electrolytes must provide good uniformity under removal of thick surface layers. EXPERIMENTAL METHODS Aqueous-organic solutions of mineral salts were used because good previous results were obtained under electrochemical dissolution of titanium, nickel-chromium alloys and alloyed steels [6]. The blades produced from titanium alloy (6Al- 2Mo-1,5Cr-Ti) and heat-resistant alloy (14Cr- 35Ni-Fe) was used as specimens. They were stamping by hot die forging, so thin uniform defective layer was formed around blade body. Thickness of this layer was approximately 0,15-0,20 mm. Special experimental device is presented in fig.1. Two electrodes connected to a DC supply are required with an electrically 36 conductive solution between. The blade (work piece) is the positive electrode (anode) and two conductive plates of profile form represent the negative electrode (cathode). Cathode Electrolyte feed Anode Fig.1. Scheme of experimental installation. Interelectrode gap a can be changed in the range from 1 to 10 mm with the help of dielectric gasket seals. Machined blade and cathodes are assembled in a compact electrode block. This block sinks into working chamber. Working liquid is passed through electrode block by the pump. Electrolyte flow velocity is regulated with drive valve. Tank for electrolyte is situated in the lower part of this device. Power source provides variation of voltage in the range from 10 to 100 V. Because of the small size of removal stock the machining way with fixed electrodes was used. Before and after machining of every specimen its surface was measured by special measuring device with accuracy not more than 0,02 mm. Average value of removal layer size was determined in result of statistical analysis of measurements. Also surface roughness at the different places of blade was determined with a standard device. RESULTS AND DISCUSSION a a Power sourse Working chamber Fig.2 shows that linear dissolution velocity V is practically proportional to current density. It means that electrochemical equivalent and current efficiency are not notably changed during this experiment. Operating time corresponded to machining allowance about 0,3 mm (titanium alloy) is inversely proportional to current density as in is predicted by theory.

3 The same result is got under manufacturing of Cr-Ni-Fe-alloy blades. It is noted that the change of temperature in the large range does not result in some variation of dissolution velocity under manufacturing of titanium alloys. That regards Cr-Ni-Fe-alloy noticeable changes in dissolution rate art observed upper o C. Such distinction is explained with the different properties of electrolytes applied. passed through unit of electrolyte volume does not influence on the value of dissolution velocity up to 250 A h/l for titanium alloy and 100 A h/l for Cr-Ni-Fr-alloy. This means that electrolyte compositions used in presented experiments have good operating life. Dynamic of the forming of surface roughness is illustrated with fig.3.this data are got under the following conditions: interelectrode gap, mm 4-6; electrolyte temperature, o C ; electrolyte flow rate, m/c 1-2. Dependences presented in fig.3, estimate the size of surface roughness not only under the same operating time but under the same size of machining allowance. It s impotent when they find such machining allowance under which required surface roughness may be received. It s seen that dependence of surface roughness parameter R a on operating time t 0 is described by equation R a = R a *exp(- At 0 ) + B Fig.2. Influence of current density (1,2), factor of passed quantity of electricity through the electrolyte volume unit (3) and electrolyte temperature (4,5) on linear dissolution rate (1,3-5) and operating time (2). Current density, A/cm 2 : 3, 4 1 and 5 0,5. Upper graphs are concerned of titanium alloy. where R a * - is initial size of surface roughness parameter, A and B are some constants, determined by electrolysis conditions [4]. Metallographic investigations of machined surface show that any structure changes in the surface layer are absent (fig.4). It s seen the essential reduce of surface roughness parameter. Etching effect along the grain boundary isn t observed. Micro hardness of surface layer after stamping is in 1,3-1.5 times greater than one into the middle part of a specimen. After removal this layer by electrochemical dissolution micro hardness is the same at any place of cross section of a specimen. It is also shown that quantity of electricity Fig.3. Influence of operating time on average size Z of removal layer (1-3) and surface roughness (1-4 ) under current density, A/cm 2 : 1,1-0,15; 2,2-0,3; 3,3-0,5; 4-1. Material of blade Cr-Ni-Fe-alloy. a) b) Fig.4.Microphotographs of the cross section of the specimens received from titanium alloys (upper line) and Cr-Ni-Fealloy (lower line) after hot stamping (a) and electrochemical machining (b) 37

4 Measurements of hydrogen pickup of surface layer show that this parameter is less, than 0,005 (Ti-alloy) and 0,01 mass. % (Сr-Ni-Fealloy) if only machining allowance is more than 0,1 mm. PRACTICAL APPLICATION On the base of theoretical and experimental results described above new technology and rotor electrochemical machine tool were designed. This machine tool was done and approved in machine building plant conditions (see fig.5). It consists of the following basic parts: Fig.5 Rotor machine tool for electrochemical removal of surface defective layer. - a ring bass for electrolyte provided with cooling system around itself; - main frame with rotating shaft (rotor) which is mounted axially to a ring bass; - six cross arms mounted on a rotor top; - every cross arm is equipped with special device for electrode block fixing; - power source with control system. After compressor blades (see fig.6) are fixed into electrode block rotor unit begins to rotate with working frequency. At this time the blade blanks sunken into electrolyte begin to move relatively electrolyte making required hydrodynamic. After passing of working current during operating time manufactured blades without surface defective layer are received. Fig.6. Compressor blades of the different sizes after stamping (upper part) and electrochemically manufactured. Some sample of blades with the same size was machined and measured with the purpose to determine dimensional accuracy. The following technological parameters are used: -working voltage, V current density, A/cm 2 0,6-1,0 - interelectrode gap, mm 6 - average value of electrolyte flow, m/c 0,5 - electrolyte temperature, o C Statistical analysis of measured data is shown in fig.7 It is seen that dimensional accuracy is about ±0, 05 mm. Fig.7.Results of statistical analysis of measured data: convex part (back) on the left hand and concave part on the right hand. 38

5 CONCLUSIONS The following conclusions may be formulized according to results discussed above: - anodic electrochemical dissolution is an effective way to solve a problem of removal of thick uniform layers out of surface of a part; - this technological problem is successfully solved by application of aqueousorganic electrolytes with specially selected properties; - in comparison with chemical machining electrochemical dissolution of thick uniform layers allows to increase productivity approximately in 3-5 times and to eхclude aggressive working liquids from industrial application; - in this case electrochemical technology provides size accuracy ± 0,05 mm under thickness of removal layer 0,15-0,3 mm and surface roughness R a = 0,32 mcm. Metal surface gets also high reflective property. - special equipment for electrochemical removal of uniform thick layers is designed and approved in the conditions of aircraft engine blades production; - presented process and equipment may be also used in the technologies of high effective electrochemical polishing. REFERENCES: [1 ] Tiginyanu I. Ion-implantation assisted electrochemical nanostructuring of GaP for optoelectronic applications. Electronic manufacturing of materials. 2000, 5. P [2] Keloglu O., Mustytsa A., Dikucar A. Different forms of cavities generated under presence of dielectric masks on anodic surface in the conditions of micro ECM. Modern electrotechnology in machine building production. Tula P [3] Rajurkar K., Zhu D., McGeough J. and other. New developments in ECM. Annals of the CIRP P [4] Physical and chemical manufacturing methods in production of gas-turbine engines. Edited by B.P. Saushkin. Moskow p. [5] Atanasyants A., Saushkin B. Problems of electrochemical machining of metals in nontraditional electrolytes. Proc. Intern. Symp. Electromachining (ISEM-X). Germany P [6] Saushkin B., Plarksin V., Atanasyants A. Finish electrochemical machining of the large-scale punches for hot die forging. Proc. Intern. Symp. Electromachining. Switszerland P AUTHORS 1. P. Kocenko, Chief specialist of machine building plant Topaz, Kishinev, Moldova 2. Rusica I State Technical University of Moldova, Kishinev, ion rusica@maul.md 3. B. Saushkin, Russian State Technological University MATI, Moscow, Russia, Department of Machine Manufacturing. 39