US 3844859 A
A composition and a process for chemical milling titanium utilizing an aqueous nitric-hydrofluoric acid base etchant having a nitric acid content ranging from about 0.2 - 1.2 weight percent.
Description (OCR text may contain errors)
United States Patent 1 Roni [ TITANIUM CHEMICAL MILLING ETCHANT  Inventor: Jack C. Roni, Seattle, Wash.
 Assignee: The Boeing Company, Seattle,
 Filed: June 20, 1972  Appl. No.: 264,594
Related US. Application Data  Continuation of Ser. No. 885,598, Dec. 16, 1969,
 US. Cl 156/18, 252/79.3, 252/79.4  Int. Cl. C231 1/00  Field of Search 252/79.3, 79.4; 156/18,
[ Get. 29, 1974  References Cited UNlTED STATES PATENTS 2,981,609 4/1961 Acker et al. 252/79.3 2,981,610 4/1961 Snyder et a1 252/79.3 3,048,503 8/1962 Foote et a1 156/18 X 3,598,741 8/1971 Shozo Kanno 252/79.3
Primary ExaminerWilliam A. Powell Attorney, Agent, or Firm-Morris A. Case; Glenn Orlob 5 7 ABSTRACT A composition and a process for chemical milling titanium utilizing an aqueous nitric-hydrofluoric acid base etchant having a nitric acid content ranging from about 0.2 1.2 weight percent.
2 Claims, 1 Drawing Figure Amie/2' A00 [aware/Mam, n f
TITANIUM CHEMICAL MILLING ETCIIANT This is a streamlined continuation of prior copending application, Ser. No. 885,598. filed Dec. 16, 1969, and since abandoned.
BACKGROUND OF THE INVENTION Commercially used titanium chemical milling solutions are generally of a nitric-hydrofluoric acid solution or of a chromic-hydrofluoric acid solution. The nitrichydrofluoric acid solution is most often used as it has a longer work life, lower material cost, is easier to control, has less smut block problems, and gives about the same milling characteristics.
Milling characteristics of these etchants leave much to be desired. The nitric-hydrofluoric acid etchant has a non-uniform rate of metal removal. Formed parts having both horizontal and vertical surfaces undergo a faster rate of metal removal on the vertical surfaces. This etch rate differential may vary by as much as 40 percent. In addition, considerable taper is produced on vertical surfaces, with surface areas higher in the etching tank having the metal removed faster than surface areas lower in the tank. A transition zone, which is defined as the distance from the edge of a milled cut to the point where nominal thickness is reached, is about ten times the depth of the cut. Channels and ridges are formed in this transition zone at the base of the uppermost fillets of vertically milled areas, and a fillet overhang is created at the edge of the cut. The ridges and the overhang must be mechanically removed after completion of the chemical milling.
It has become common practice to partially compensate for non-uniform milling characteristics by periodically removing the part from the etchant and turning the part down side up before re-immersing.
It was discovered that uniform milling characteristics can be obtained, the transition zone can be reduced to about three times the depth of cut, and mechanical removal after chemical milling becomes unnecessary by using a nitric-hydrofluoric acid solution or etchant bath wherein the nitric acid content is reduced to a concentration of about 0.2 1.2 weight percent. Ammonium bifluoride may be added to further reduce or to completely remove channeling and ridging. It was further discovered nitric acid in concentrations as low as 0.2 percent by weight will still limit hydrogen absorption by titanium. Below that concentration excessive hydrogen absorption does take place.
An object of this invention is to obtain uniform chemical milling characteristics for titanium.
Another object of this invention is to obtain uniform chemical milling characteristics on all surfaces of titanium parts having both horizontal and vertical surfaces.
Another object of this invention is to chemical mill titanium without imparting a taper on vertical surfaces.
Still another object is to reduce the length of the transition zone when chemical milling titanium.
Yet another object is to chemical mill titanium without leaving extraneous metal or creating a smut block which must be removed by mechanical means.
DESCRIPTION OF DRAWING The FIGURE is of a graph depicting optimum nitric acid concentration versus concentration of dissolved titanium in the disclosed chemical etchant.
DETAILED DESCRIPTION The nitric acid content of an aqueous nitrichydrofluoric acid solution or etchant was discovered as controlling the uniformity of removal of titanium during chemical milling. Tests were performed using a solution containing nitric acid and sufficient hydrofluoric acid to effect an etch rate of 0.0008 to 0.0010 inches per minute per side, and dodecylbenzene sulfonic acid as a surface active agent in quantities sufficient to control surface tension of the solution at 30 35 dynes per cm. Formed titanium parts having horizontal and vertical surfaces were immersed in the etchant for 40 minutes with the etchant bath at a temperature of 1 15F 3F. A series of tests were performed in which the nitric acid concentration was varied. Table I below shows the effect of varying the nitric acid concentration.
Table I Nitric Acid Concentration Percent difference refers to rate of metal removal on a horizontal surface with respect to rate of removal from a vertical surface.
A negative figure is obtained when metal is removed from a horizontal surface faster than from a vertical surface.
Conversely a positive figure is obtained when metal is removed from a vertical surface faster than from a horizontal surface.
The percent difference is represented by the equation:
Percent Difference (vertical surface-horizonal surface)/(vertical surface) When titanium is removed it goes into solution in the etchant bath, and will remain in solution until the titanium concentration reaches about 8.7 weight percent at which point it starts to precipitate out. As the dissolved titanium concentration builds up the hydrofluoric acid and the nitric acid concentration must be increased to maintain rate of metal removal and uniformity of removal. In Table I, above, the dissolved titanium concentration ranged from about 1.8 to 2.2 weight percent. Table II shows the effect of varying nitric acid concentration as the dissolved titanium concentration varies.
Table II Dissolved Titanium Nitric Acid Weight Percent Dissolved Titanium Weight Percent Percent Difference 4.9 0.64 +l.9 6.4 0.82 --3.0 6.9 L08 -l.0
dissolved titanium concentration to effect uniform rate of metal removal, reduce the transition zone, and to obtain etching which does not require mechanical removal of metal after chemical milling. Above the optimum range, area 2, metal is removed from a vertical surface faster than from a horizontal surface and a taper is produced on a vertical surface with the metal being removed more rapidly on the surfaces which are located higher in the chemical milling bath. Below the optimum range, area 3, the metal removal characteristics are slightly reversed. The horizontal surface is milled faster than the vertical surface and a taper is produced on a vertical surface with the metal being re moved more rapidly at the lower part of the surface in the chemical mill bath. Difficulties may also be experienced below about 0.20 weight percent nitric acid due to excessive hydrogen ion absorption. Above about 8.7 weight percent of dissolved titanium the titanium precipitates out as a fluoride.
A preferred chemical milling solution or etchant may be 0.2 1.2 weight percent nitric acid, 4 18 weight percent total fluoride and a surface active agent in amounts sufficient to control surface tension at from 28 60 dynes per cm. with about 33 dynes per cm. preferred.
ln another embodiment the hydrofluoric acid concentration may be sufficient to effect an etch rate of from 0.0004 to 0.0015 inches per side per minute, but with a preferred etch rate of 0.0007 to 0.001 1 inches per side per minute The addition of small quantities of ammonium bifluoride from about 0.07 to 2.9 weight percent will further reduce or completely eliminate channeling and ridging in the fillet area.
The surface active agent may be selected from the group consisting of dodecylbenzene sulfonic acid or linear alkyl sulfonic acid.
A preferred process may use any of the nitrichydrofluoric acid etchants with a low concentration of nitric acid as herein disclosed and immerse titanium parts in the etchant at from about 90F to 130F but with about F 3F preferred, and holding in the etchant bath until the desired depth of cut is obtained.
In another preferred embodiment for chemical milling titanium utilizing a nitric-hydrofluoric acid etchant the nitric acid concentration may be controlled at a percentage range such as to effect a uniform rate of metal removal on both horizontal and vertical surfaces.
I. A method of chemical milling titanium to remove metal from the horizontal and vertical surfaces of the titanium at essentially the same uniform rate, the steps comprising:
a. immersing the titanium in an aqueous etching fluid containing a concentration of hydrofluoric acid sufficient to effect an etch rate on titanium of from about 0.0004 to 0.0015 inches per side per minute, a surface active agent selected of materials consisting essentially of dodecylbenzene sulfonic acid and linear alkyl sulfonic acid in amounts sufficient to control surface tension of the etching solution at from about 28 to 60 dynes per cm., and about 0.2 weight percent of nitric acid, said aqueous etching fluid uniformly removes titanium metal which when removed goes into solution in the etching fluidf varying the nitric acid concentration in the etchant as the dissolved titanium concentration increases such that the nitric acid concentration increases from 0.2 to about 1.2 weight percent as the dissolved titanium weight percent in the solution increases up to about 8.7; and
c. holding the titanium in the etchant until the desired depth of cut is obtained.
2. A method of chemical milling titanium as recited in claim 1, further comprising: adding from about 0.07 to 2.9 weight percent of ammonium bifluoride to the