CA1165670A - Selective chemical milling of recast surfaces - Google Patents

Abstract

ABSTRACT

Disclosed is a process for machining nickel-base superalloys wherein a thermal effect process, such as laser or electric discharge machining, is first used to remove material but leaves a recast layer. Next a chemical milling process is used wherein the etchant only attacks and removes the recast layer. The etchant is comprised by volume percent of 40-60 HNO3, 5-20 HC1, and balance H2O, with which is included 0.008-0.025 moles/liter FeC13 and at least 0.016 moles/liter CuSO4.
The FeC13 improves removal rate but tends to cause unwanted pitting and intergranular attack. These tendencies are inhibited by the addition of CuSO4, preferably the molar ratio of CuSO4 to FeC13 is 2:1. The beneficial combination of FeC13 and CuSO4 is usable in other etchants.

Description

~ ~. 6s6~0 Des cription Selective Chemical Milling Of Recast Surfaces Background Art 1. The present invention relates to the machining of superalloys by chemical milling in combination with thermal effect metal remo~al processes I such as those utilizing electric discharge and lasers.

2. As a c12ss of materials, superalloys used in the manu~acture of airfoils fox gas turbine engines are quite difficult to machine by conventional metal cutting processes using tool bits and the like which convert metal into small chips. Two types of machining are particularly difficult: drilling fine holes through the walls of hollo~ airfoils, and providing a complex contoured three-dim~nsional surface shape, such as a pattern of grooves. As a resul~ many innovative processes have been developed in the last few decade~, including those utilizing steady and inte~mittent electric discharce~ lasers, electron beams, electro-chemistry and chemical attack~
However, m,-~y oS these advanced processes have their disadvantases. As a class, the electrochemical and chemical processes suffer from a lack of precision, ~5 at least to the hish tolerances required in many gas turbine engine components. Also most cast and wrought airfoil materials have some metallurgical inhomogeneities znd a multiplicity of phases with dif~erent compositions.
Resultant local variations in resistance to the chemlcal 3D attack often can lea~ to undesirable irregular surface finishes, or in the worst case, preferential an~ excessive attack of certain areas such as gr~in boun~aries resul~ing in an unusable fa isue crack prone surface. Consequently, ~-4~82 - ~ -the selection of e~chants and electrolytes ~ust ~e care-fully considered and controlled, especially in chemical m~lling where the inherent corrosion reslstance of the superalloys must be overcome with powerful etcha~ts.
The processes which utilize concentrated beam energy or electric discharge cause metal removal by ~ery co~-centra,ed melting and vaporization; they are often capable of producing the reguisite accuracy, but adverse metal workpie~e.
To describe the problems more specifically by example, in making holes by laser or electr~n beam drilling, a beam is impinged in concentrated foxm on a cast airfoil workpiece surface until it penetrates through. In the process metal is melted and vaporized by the intense beam energy, creating the hole. The intensity of these processes is such that molten and vaporized metal is expelled from the hole being created, this effect being augmented by the use of a volatilizable backer material at the workpiece exit sur~ace. However, there is usually nonetheless a small quantity of molten metal remaining at points along the pexiphery or len~th of the hole. When the beam energy is terminated this molten layer solidiies very rapidly. Thus not only is the metallurgical structure of this "recast" layer . 25 di fferent from that of the more controllably cast and slowly cc,oled airfoil, but the resolidified or recast layer is orten characteri7ed by small cracks due to shrinkage. When airfoils with holes having recast layers are used, the imperfect recast layer structure tends to cause premature cracking of the airfoil due to fa~igue, compared to the resistance to f2ti~ue which the part would have if the holes lacked the deviant metallursical struct~re. Nat~rally, a great d al of effort has been expended to modify the beam energy drilling processes to eliminate the recast layer, but i5~70 while it has been minimized it has not been able to be eliminated.
Another example involves the production on a work~
piece surface o~ a pattern of varying depth grooves and depressions. Electric discharge machining is a favored process to produce such suxface contours, much as it is favored for three~dimensional die sinXing. In electric discharge machining ~EDM) a preformed electrode is placed in close proximity to the workpiece and electric spark discharge between the electrode and the workpiece causes vaporization and expulsion of material from the workpiece surface into a surrounding dielectric fluid.
When surfaces machined by electric discharge are examined they also are found to have a xecast layer comprised of material which was momentarily melted and remains adhered to the surface. Further, EDM sur~aces are usually characterized by a certain roughness caused by the erratic nature of the spark discharge and in many instances it is desired to have a smo~ther surface than is typically producible. Of course, if a general secondary machining operation such as grindin~ is used to smooth an EDM surface the good accuracy from the EDM
process can easily be lost, or costs will be incxeased.
Thus, it is very much desired to have a process 25 which efficiently removes material but which lea~es a surface finish neaxly comparable to that of a con-ventional cast or machined surface.
' ,~ S~ y r~ h~OE~ of Invention An object of the invention is to machine a super~
alloy using a thermal effect process, but without leaving a recast layer or other imperfect surface.
According to the invention the recast la~r may be selectively re~oved using chemical milling and an etchant having the composition by vol~e percent of b~tlaoc ~
40-~0 HN~, 5-20 HCl, and ~ H20, with which is included 0.00~0.025 mole/l FeC13 and at least0.016 mole/l CuSO4. Preferably the etchant is 50 HNO3, 10 HCl and 40 H2O, with 1~3 g/l FeC13 and 2.6 g/l CuSO4. The FeC13 improves removal rate but tends to cause unwanted pitting and intergranular attack.
These tendencies are inhibited by the addition of CuSO4, preferably the molax ratio of CuSO4 to ~eC13 is 2:1. The beneficial combination of FeC13 and CuSO4 is usable in other etchants.
The etchant has a self-limiting feature that is very unique. Only the recast layer is removed and the removal of metal which is not recast is minimal.
Gas is evolved during removal (preerably done at 40-80C) and the cessation of evolution may be used as an indication of the completion of the chemical milling process.
The invention provides a rapid way for removing material from a superalloy since thermal effect pro-cesses are exceedingly fast and the chemical millin~is very selective and also rapid. Machined superalloy surfaces with surfaces free from adverse metalluxgical features are thereby provided.
The foregoing and other objects, features and advantages of the present invention will become more app~rent from the follQwing description of preferred embodiments an~ accompanying drawings.

Brief Description of Drawings Figure l(a) is a planar surace view of the entr2nce of an oblique laser drilled hole showing a cracked recast layer;
Figure l(b) shows the hole entrance with the recast layer removed after che~ical milling.

Figure 2(a) is a partial longitudinal section through the hole of Figure l(a) showing the hole wall;
Figure 2(b) shows the hole wall after chemical millina.
Figure 3(a) is a planar surface view of a EDM
surface showing the rough recast layer, Figure 3~b) is the surface after chemical milling.
Figure 4(a) is a cross section through the surface shown in Figure 3(a); -Figure 4(b) is a cross section of the surface in ~igure 3(b).

Best ~lode for Carrying Out the Invention The invention is described hereafter in application to the nickel-base superalloy ~R M-200 ~ Hf, a nickel-base alloy ha~ing the composition by weight percent of 10 Co, 9 Cr, 2 Ti, 5 Al, 12 W, 1 Cb, 2 Hf, 0.15 C, 0.015 B, 0.05 Zr, balan~e Ni. Limited experiment indicates that the process will be useful for other nickel alloys, especially the superalloys such as IN-100, IN-718 and Astroloy.
In its preferred practice the invention was used to produce both holes of improved quality in airfoil walls, and contoured surfaces on superalloys. The hole drilling will be described first. About 10 holes of 0.7 to 1.3 ~m diameter were drilled in the as-cast surface of a hollow airfoil ~-all workpiece having 2 thickness of abo~at 2.5 mm;
the holes were at different inclinations to the surface and thus ranged in length between 2.5 and 5 mm. A neo-dymium laser generated pulse radiation at 1.06 micron wavelength was applied to the workpiece entrance surface zt an intensity of about 107 watts/cm2, with a pulse

3~ duration of about o60 microseconds and rate in the ranse .

1 ~ 6 .~

0.3 to 1 pulses/second. The exit side of the woxkpiece had applied thereto a backer of epoxy resin to both absorb energy when the w211 is penetrated and preve~t dzmage to other surfaces, and to aid in the expulsion of molted metal from the drilled hole. Por the general functions and characteristics o~ desirzble backers for ~ electron beam drilling reference may be made to ~y~r~L*~-l ~ U.S. Patent No. 4,23~,954 of Howard et al;
the art for laser drilling is analogous. Figure l(a~ is a view of the entrance o~ the drilled hole on the surface 16 o~ a workpiece~ The beam has impinged on the sur ace s~ that the hole slants downward toward the left of the photogr2ph. Around the entrance of ~he hole can be seen the recast layer 10, co~taining a prominent crack 12 as well as other cracks. Sone other recast layer molten material 14 is on the surface surxo~nding the hole as well. Figure 2(a) shows a portion of a longitudinal section ~hrough the same hole. The specimen has been etched to reveal nicrostruct~re znd the recast layer 10 which is light colored and featureless compared ~LO the ~ore characteristic cast morphology of the base metal 18 which is more removed ~rom the hole. Tne xecast layer W2S non-uniform and varied in thickness from about 0.08 to O . 8 rlun.
~i~ures l(b) zn~ 2(b) are analogous views to Pigures l(a) 2nd 2 (a), showing the workpiece after chemucal mill-ing ~hlch is ~escribed in more detcil below.
~enerally convention21 ED~ ~echniques are use~ to produce 2 p2t,ern of srooves vzrying in depth from 2~4 to 2 . 9 mm and in wid,h from 1.~ to 1.8 Anm. But to better illustr2~e tne invention, z rectangular pa~211el-plped test piece ~i.h 2n entirel~ ED~ sur 2ce o, aDout 1.61 sq.cm, on one face ~zs produce~. The ED~5 condi~ions were nom~nally:
80 volts DC; 3 cmps; ~ pulse frequency o_ 3 ~ilocycles; a 3~ ca~aci.2nce o~ i microl2~2c; usinc 2 czrbon elec.ro~e wiLh ~ ~5 a mine-zl seal dielectxic fluid (Exxon Mentor No. 28, Exxon Co_p., Houston, Texas) at 27~C. The foregoing conditions are characteristic of those usea for a light roughing mode of operation. To produce a piece with grooves 2 ~uitably shaped electrode is prepared, and the ED.r~ ~zrameters adjusted according to the area and other consi~erations in a manner amiliar to those with skill in E~M.
The EDM prGduced a surface finish ( 25 measured by 0 2 surf2ce profilometer) of about 80-120 root mean square (RMS) ~.icro inches. Of course better finishes can be obtained in EDM but with undesirably slow rate of material removal. The surface condition of a portion of the r DM surface is shown in planar view in Figure 3(a) and in c~oss section in Figure 4(a). In the latter figure the lighter recast layer 20 is evident in contxzst to the ~n2~fected ~ase metal 22, similarly to the appear-~nce Or the laser drilled hvles. The reczst layer ~aried in thickness from O.08 to 0.8 mm.
Removal of material ~y either laser or EDM are design2ted herein as "thermzl effect processes". By this we mean they are processes in which metal is removed by heating 2bo~e its m~lting point and wherein there is a residu21 recast laye~ on ~e workpiece surface. Thus we 2i embrace in the scope o, our invention o,her thermal ellect processes including but not lir.lted to those mentione~ in ,he Backc_ound.
Both the workpiece with the laser drilled holes and th2t with the E~M surface were sep2rately i~mersed in a c:~e,.iczl etchant. The composition of the etchant wzs as _cllows:
Conc. ~NO3 (69-71%)1892 ml (50 v/o) Conc. HCl (32.5-38~) 375 ml (lQ ~/o) H20 liOO ml (40 v/o) FeC13 1.3 5/1 (0.008 m~le/l) C~SCj~ 2.6 c/l (0.016 mole/l) -~ale, llilclr-k 6 7 ~

The workpiece haYing the laser drilled holes was imm~rsed in the etchant at 77C; aftex initially observed ~as e~olution ceased, the workpiece was removed from the etchant and examined. As shown in ~igures l(b) and 2(b) the recast layer was completely remo~ed from the drilled holes. l~here was some small degree of general attack on the non-recast areas of the workpiece as evidenced by the Figures and examination showed the 6.55 gm workpiece had lost only about 0.118 gm or 1.8~ of its original weight.
Thus, the substantial ef~ect of the chemical milling wa~
to only the recast layer, and more uniform, smooth, and crack-free holes were provided.
The workpiece with EDM portions was immersed in the electrolyte at 66C and heavy ~as evolution was evident from the EDM areas. After about 5 minutes the gas evolu-tion substantially ceased and the workpiece was removed.
Comparati~e examination produced the data in Table 1.
Basically, only the recast layer was removed and the other parts of the test piece were not affected. The height dimension, defined at one end of the part by the sole EDM surface and at the opposing end by an ordinary machined surface, has a change indicati~e of the removal of the recast layer and smoothing. The other dimensions, length and width, are indicators of the lack of substan~
2~ tial effect of the process on non-EDM surfaces. Electxon micro probe measurement of the surface showed the concen-tration of W increased and that of Cr decre2sed sliyhtly (about 20~ change for each). This is 2 superficial effect and regarded as mlnor in consequence.

3. 1 ~5~

Table 1. Comparative Measurements on E~M
Workpiece Subjected to Chemical Milling Feature Before Millinq After Millin~ Change Surface Finish 80-120 40-60 50 ~RMS micro-inch) Length-mm11.151 11.138 -0.013 Height (EDM9.779 9.728 -0.051 surface)-mm Width-mm 5.982 5.982 -0.0000 Surface Chemistry- Tungstenrich Depleted in EDM area tungsten and chromium Surface Chemistry- ~ominally Nominally Base Metal within within specification specification A very striking aspect of the invention is the self-limiting nature of the chemical milling portion of the process. The evolution of gas (hydrogen) is evidence of substantial metal removal; thus when the gas evolution substantially ceases the quantity of metal being dissolved per unit time is substantially reduced. We have not run sufficient detailed experiments, but i~ the workpiece was maintained in the solution some further gradual and general dissolution probably will take place, given the corrosive nature of the etchant~ However, for practical purposes the process is self-limiting and the near-cessation of gas evolution gives a signal that the removal of the undesired recast material is complete.
While we made visual observation to sense the diminution of gas evolution, physical or chemical gas sensing devices may be alternately used to signal or automatically effect removal of the workpiece from the etchant, for best efficiency and avoidance of minor attack. Another desir-able aspect of the process is that the workpiece is left with an improved surface finish and that the corrosive 5~70 attack of the workpiece in the areas which are not recast is minimal.
The exact mechanism which provides the chemical milling with its self-limiting feature is not evident.
However, it is dependent on the constituents, as there are many seemingly similar electrolytes which do not produce this desired result, including that described in Canadian application No. 386,180 filed September 18, 1981, "Chemical Milling of High Tungsten Content Superalloys". (As a matter of note, tungsten segregation in normally cast MAR M-200 base metal, the effects of which the related invention overcomes, does not occur in the rapidly quenched recast layer.) The chemical diEferences between the recast layer and unaffected work-piece substrate are not very great, although they may contribute to the effect observed. Another speculation is that the rapid cooling rates associated with the thermally effected layers produce a metallurgical structure which is more susceptible to corrosion due to its structure, compared to the more slowly cooled and presumably more equilibrated workpiece structure.
Based on our experiments we believe that -the electro-lyte constituents may be varied within the following range:
by volume percent, 40~60 H~03, 5-20 HCl, halance H20, in combination with 0.016 - 0.083 moles/liter CuS04 and 0.008-0.025 moles/liter FeC13; where the acids are 69-71%
conc. nitric acid and 36.5-38% conc. hydrochloric acid~
In our etchant we include ferric chloride as an additional corrodent to speed the rate of material removal. Howeve~, the use of the acids by themselvas or in combination with the FeC13 results in pitting and uneven attack of the material being removed, especially, the grain boundaries are attacked. The addition of CuSO4 abo~e the minimum amount prevents this unwanted attack~ As indicated, our solu'ion described only slightly attacks the base metal.
However, we have noticed that in the absence of CuSO4, the ~ 1~5~70 slight attack of the base metal is accelerated prefer-entially at the grain boundaries. Preferably the molar ratios of CUS04 and FeC13 is 2:1. FeC13 should not be added beyond the indicated range, regardless of the amount of CuS04, because the inhibiting action of CuSO~ will not be sufficient. On the other hand, the amount of CuS04 may be increased beyond the indicated range since it is benign. We believe that the combination of FeC13 and CuSO~ to be novel and significant in chemical removal of nickel base superalloys.
The moderately elevated temperature we used is desirable to increase the rate of reaction; apart from our nominal best temperatureof 66C, the process is believed operable between 40-80C, and we prefer to operate in the range of 60-70C.
Our invention, as described, combines laser or EDM with uniquely selective chemical milling. Generally, our invention combines a thermal effect process with chemical milling using a specialized etchant. In its best use it provides precision machining and quality of surface condition in ni~kel alloys, but it will be applicable to other nickel alloy material processing using a thermal effect process where the recast layer - is undesirable.
While chemical milling is preferably carried out by immersion as described above, other modes o~ appli~
cation may also be utilized. Additionally, wet~ing agents,thick~ners and so forth may be included with our etchant, as the user is inclined~
Although this invention has been shown and described with respect to a preferred embodiment, it will be under-stood by those skilled in the art that various changes in form and detail thereof may be made wit~out departin~ from the spirit and scope of the claimed inventionO

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Machining the surface of a nickel-base super-alloy workpiece by the process which comprises:
removing workpiece material using a thermal-effect process which causes melting and vaporization of metal, thereby leaving on the workpiece surface a recast layer, and contacting the surface from which material is removed with an etchant comprised by volume percent of 40-60 HNO3, 5-20 HCl, balance H2O, at least 0.016 moles/liter CuSO4, 0.008-0.025 moles/liter FeC13, thereby chemically dissolving the recast layer without substantially removing other workpiece surface material.

2. The process of claim 1 wherein the thermal-effect process is one utilizing beam energy,

3. The process of claim 1 wherein the beam energy is derived from a laser or electron beam.

4. The process of claim 1 wherein the thermal-effect process is one utilizing an electric discharge.

5. The process of claim 2 wherein the removal of material produces holes in the workpiece.

6. The process of claim 1 wherein the etchant is maintained at 40-80°C and which further comprises sensing the completion of removal of the recast layer from a substantial diminution in the evolution of gas at the workpiece.

7. The process of claim 1 wherein the molar ratio of FeC13 and CuSO4 is maintained at 1:2.

8. The process of claim 1 wherein the etchant consists by volume percent of about 50 HnNO3, 10 HC1, 40 H2O, with 1.3 g/l FeC13 and 2.6 g/l CuSO4.

9. The process of claims 1 or 8 wherein the superalloy is based on the alloy consisting by weight percent of 10Co, 9Cr, 2Ti, 5Al, 12W, lCb, 0.15C, 0.015B, 0.05Zr, balance Ni.

10. An etchant for chemical milling the recast layer of a nickel-base superalloy comprised by volume percent of 40-60 HNO3, 5-20 HCl, balance H2O, and con-taining at least 0.016 moles/l CuSO4 and 0.008-0.025 moles/1 FeC13.

11. The etchant of claim 10 wherein the molar ratio of CuSO4 and FeC13 is 2:1.

12. The etchant of claim 10 consisting of about 50 HNO3, 10 HC1, 40 H2O with 1.3 g/l FeC13 and 2.6 g/l CuSO4.