GB2087429A

GB2087429A - Improvements in or relating to electrolytes

Info

Publication number: GB2087429A
Application number: GB8035957A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1980-11-08
Filing date: 1980-11-08
Publication date: 1982-05-26

Abstract

Electrolytes are devised, which take account of the characteristics introduced into superalloys, by forging and by casting. The electrolytes comprise a permuation of Sulphuric acid, Nitric Acid, Phosphoric Acid and filtered water.

Description

SPECIFICATION Improvements in or relating to electrolytes This invention relates to electrolytes which may be used in the electrochemical machining of metals.

More specifically the invention relates to electrolytes suitable for the electrochemical machining of superalloys.

Superalloys are alloys of metal which have been developed specifically to enable parts which are made from them, to operate in conditions too hostile for other metals. Superalloys may thus be used for the manufacture of e.g. gas turbine engine components, particularly rotating components such as turbine blades. Turbine blades operate in temperatures of 1273 OK and above, in an oxidising environment and under great mechanical stress.

The superalloys themselves have created problems as regards their machinability by conventional means and it has become standard practice, to machine at least local features on parts made from superalloys, by electrochemical methods in which the electrolytes used, contain or consist of salts in solution.

The invention seeks to provide improved electrolytes for machining superalloys.

According to the present invention, there is provided an electrolyte which is suitable for use in the electrochemical machining of nickel based superalloys and comprising by volume: Sulphuric Acid 0-25% Nitric Acid 530% Phosphoric Acid 0-30% Balance filtered water and wherein the total acid content does not exceed 60% and the filtered water content does not exceed 80%.

The invention will now be described by way of example.

The following is a list of alloys, all of which are nickel based superalloys. The numerical values given for each integer of each alloy, is a nominal value, each being variable by some degree of magnitude, without materially changing the essential characteristics of each respective alloy.

All the alloys listed are known to the man skilled in metallurgy, by respective trade names e.g.

alloys D and E are known as "NIMONIC 90" and "NIMONIC 108" respectively, both names being registered trade marks. Alloy F is called "WASPALLOY" (registered trade mark). The letter 'C' indicates that parts made from the respective alloy are normally cast into shape and the letter 'F' denotes that parts made from the relevant alloys are usually forged, wrought or rolled into a desired shape.

C Cr Co Mo Ti Al W Ta Nb Zr B Hf V Ni a 0.31 22 10 10.5 0.3 0.2 0.2 - 0.25 - - - - Bal C max max max max b 0.06 20 20 5.8 2.25 0.5 Bal F c 0.15 15 10 8 3.5 4 .005 Bal C d 0.07 20 18 2.5 1.5 .10 .01 Bal F e 0.15 15 20 5 1.25 4.7 .15 .01 Bal F f 0.06 20 13.5 4.2 3 1.4 Bal F g 0.13 6 - 2 - 6 11 .13 Bal C h 0.175 9.5 15 3 4.75 5.5 .075 .015 1 Bal C i 0.07 18 14 7 2.25 2.1 Bal F j 0.15 9 10 - 1.5 5.5 10 2.5 .05 .015 1.5 Bal C

It has been found that, in order to satisfactorily electrochemically machine superalloys of the kind described hereinbefore, any electrolyte used, must be a good conductor of electricity. Moreover, the electrolyte should be saltless. In the context of the present invention, salt means the products of the process which are generated when electrical conditions cause the electrolyte to evaporate through heating, which products reach saturation point and are deposited on the workpiece to form a crust which is a poor conductor.

There is, however, another form of salt known in the electrochemical field and this is the salt which when combined with water, forms brine which is known as an electrolyte. Such electrolytes are relatively poor conductors of electricity and further, the salt tends to precipitate out onto the nozzle from which the electrolyte is ejected. Flow is thus reduced and/or deflected. The need for such electrolytes are obviated by the electrolytes of the present invention.

Experiments were made, using nitric acid, sulphuric acid and phosphoric acid in various proportions ranging from zero regarding at most, two of the acids, with filtered water added. Each acid exhibited maximum conductivity when they each formed 30% by volume of the whole. However, such a composition in which all the acids were utilised in equal proportions and the whole made up with 10% filtered water, proved so corrosive tithe machine tool structure, that the composition was rejected.

There follows hereinafter, a list of compositions of electrolytes which have proved suitable for use in electrochemically machining cast and/or wrought, nickel based superalloy parts.

Electrolytes % Sulphuric acid % Nitric acid % Phosphoric acid % Filtered water 1 20 20 20 40 2 20 5 20 55 3 - 20 20 60 4 20 20 - 60 5 20 5 - 75 6 - 20 - 80 7 10 10 30 50 8 - 20 30 50 9 20 5 30 45 10 - 30 10 60 11 10 30 - 60 12 25 4 20 51

Electrolyte 1 An easily handleable electrolyte with which drilling tests were carried out. The results showed that the hole samples were prone to "salting" because the electrolyte still has a high acid content. The only alloys for which it could be used to advantage were alloys c and j which are cast to form.

Electrolyte 2 This electrolyte contained a much reduced proportion of Nitric acid (5% being needed to inhibit chemical attack by the sulphuric acid). It was found to produce good surface finishes on all the alloys which are wrought to form and acceptable finishes in casting alloys c, g and j. This was the electrolyte used for alloy j.

Electrolyte 3 Dispensing with the sulphuric acid produced holes of acceptable but inferior surface finish in all samples except casting alloy 'a' and possibly a development of this electrolyte could be used for this material.

Electrolyte 4 Dispensing with the phosphoric acid produced good surface finishes on the wrought alloys but generally poorer ones on the cast material.

Electrolyte 5 It produces very good surface finishes on all the wrought materials tested but is acceptable for cast materials.

Electrolyte 6 Surface finishes similar to electrolyte 5 could be produced on the wrought materials but drilling conditions had to be more tightly controlled to prevent undesirable side effects such as pitting. Again surfaces on cast materials were generally unacceptable.

Electrolyte 7 This had a lower conductivity than the other electrolytes tested with the result that the drilling cycle times were increased. No improvement in surface finish could be seen.

Electrolyte 8 The increased nitric acid content produced an improvement over electrolyte 7 but low conductivity effects were still evident. Surface finishes were generally good on both wrought and cast materials. The best overal! surface finish for alloy h was produced by this electrolyte and further development of this or electrolyte 7 would probably produce good results in this material.

Electrolyte 9 This is similar to electrolyte 8 but with sulphuric acid replacing the nitric acid. It showed no general advantages but produced the best surface finish in alloy c.

Electrolyte 10 This electrolyte showed the effects of increasing the nitric acid proportion. It produced the best hole surface in alloy g and a good surface in alloy a. However, it produced a generally poorer surface finish on the remaining materials and showed a tendency to "salt" easily.

Electrolyte 11 This electrolyte gave good results with the wrought materials but is very susceptible to "salting".

It showed great promise in producing good surfaces on alloy h.

In every case, the drilling was carried out, using apparatus essentially as that described and claimed in British patent specification 1,339,544.

The acceptable finishes produced, ranged from two micro inches to seven micro inches. The unacceptable finishes were relatively rough and on closer examination may exhibit grain boundary attack. Salting was also a problem. This is represented by scab like deposits in or near the hole entrance.

Claims

1. An electrolyte suitable for use in the electrochemical machining of nickel based superalloys and comprising by volume; Sulphuric Acid 0-25% Nitric Acid 530% Phosphoric Acid 0-30% Balance filtered water and wherein the total acid content does not exceed 60% and the filtered water content does not exceed 80%

2. An electrolyte as claimed in claim 1 comprising sulphuric acid 20%, nitric acid 20%, phosphoric acid 20%, balance filtered water.

3. An electrolyte as claimed in claim 1 comprising sulphuric acid 20%, nitric acid 5%, phosphoric acid 20% balance filtered water.

4. An electrolyte as claimed in claim 1 comprising nitric acid 20%, phosphoric acid 20%, balance filtered water.

5. An electrolyte as claimed in claim 1 comprising sulphuric acid 20%, nitric acid 20%, balance filtered water.

6. An electrolyte as claimed in claim 1, comprising sulphuric acid 20%, nitric acid 5%, balance filtered water.

7. An electrolyte as claimed in claim 1 comprising nitric acid 20%, balance filtered water.

8. An electrolyte as claimed in claim 1 comprising sulphuric acid 10%, nitric acid 10%, phosphoric acid 30% balance filtered water.

9. An electrolyte as claimed in claim 1 comprising nitric acid 20% phosphoric acid 30%, balance filtered water.

1 0. An electrolyte as claimed in claim 1 comprising sulphuric acid 20%, nitric acid 5%, phosphoric acid 30%, balance filtered water.

1 An electrolyte as claimed in claim 1 comprising nitric acid 30%, phosphoric acid 10%, balance filtered water.

1 2. An electrolyte as claimed in claim 1 comprising sulphuric acid 10%, nitric acid 30%, balance filtered water.

13. An electrolyte as claimed in claim 1 comprising sulphuric acid 25%, nitric acid 4%, phosphoric acid 20%, balance filtered water.