US3389068A - Electrochemical machining using a chloride electrolyte including 2-mercaptobenzothiazole or 2-benzimidazolethiol - Google Patents

Description

United States Patent 0 3,389,063 ELETROHEMHAL MACHINING USING A (IHLUTRHDE ELECTRQLYTE llNCLUDING 2- Mllli.CAPZTQBENZGTHHAZOLE OR Z-BENZ- EVHBAZULETHEGL Mitchell A. La iioda, East Detroit, and Warren E. Duty,

Royal Oak, Mich assignors to General Motors Corporation, Detroit, Mich, a corporation or Delaware No Drawing. Filed (let. 23, 1965, Ser. No. 504,147 12 Claims. (Cl. 204-143) This invention relates to electrochemical machining processes and more particularly to electrolytes for use therewith.

In recent years electrolytic machining procedures for generating shapes, cavities and contoured surfaces have been developed and are generally classified into one of two basic categories, the first being electrochemical machining and the second electrolytic grinding, a specialized application of the first. Electrolytic grinding is essentially an electrochemical dcplating process which can be used on virtually any electrically conductive material. It is generally adapted to metal removel operations comparable to those performed by cutofi wheels, saws, and grinding or milling machines and the like and uses equipment similar in appearance to conventional grinders except for the electrical accessories. About 95% of the metal removal results from electrolytic rather than mechanical action.

A particular version of an electrolytic grinding process is characterized by a flow of electrolyte between the workpiece and a rotating grinding cathode wheel. The rotating cathode wheel consists of a conductive metal matrix having a plurality of nonconducting abrasive particles imbedded therein to provide nonconductive spacing between the workpiece and the cathodic matrix. Electric current is passed through the workpiece, electrolyte and cathode to dissolve the anodic surfaces of the workpiece, and the imbedded particles of the wheel abrade the surface to remove any irregularities resulting from nonuniform erosion or reaction product buildup.

While aqueous solutions of individual inorganic salts, such as nitrates, cyanides, carbonates, hydroxides and nitrites have been used as electrolytes in electrochemical machining and grinding processes, none has offered any significant advantage over the now well accepted aqueous sodium chloride solution most commonly used today. However, regardless of What salt is chosen, an inherent problem with electrochemical machining and grinding processes using these salts singularly or in combination is overcut or wild-cutting which is the uncontrolled anodic dissolution of the workpiece in unwanted areas resulting in undesirable tapering of holes, rounding of edges, and the like. Such anodic dissolution can occur" even in areas which are fairly Well removed from the cathode. This wildcutting, or cutting in low current density areas which are bathed in the electrolyte, but substantially removed from the cathode, has been substantially reduced in the prior art by the use of costly and time consuming masking operations which isolate the areas to be machined by protecting the surrounding areas from the erosive effect of the electrolyte. These masking operations are frequently quite involved and require a high degree of skill to insure a satisfactory product. Likewise, additional steps subsequent to the machining steps are required to strip the workpiece of the mask. Additionally, the prior art has attempted to reduce wildcutting by designing special purpose electrodes and machines to meet individual and specialized machining requirements.

By our invention we have at least reduced, and in most 3,389,068 Patented June 18, 1968 cases actually eliminated, the need for recourse to the prior arts attempted resolutions.

It is therefore an object of our invention to provide a self-masking electrolyte.

It is a further object of our invention to provide an additive for existing ECM electrolytes, which selectively inhibits anodic dissolution in unwanted areas.

It is a further object of our invention to effect a sharply contoured machining utilizing a basic aqueous electrolyte containing known salts and improving same by adding thereto a compound which upon reaction with the workpiece forms a film thereover, which film inhibits or stops off electrolytic action or anodic dissolution in unwanted areas.

It is a further object of our invention to efiect a sharply contoured machining by utilizing electrolytesv contain ing known salts and an additive from the group consisting of electrochemical erosion inhibiting film forming thiazoles and thiols and particularly Z-mercaptobenzothiazole (C H SCSl-LN) and 2-benzimidazolethiol (C H NC(SH) :N) respectively.

Further objects and advantages of the present invention will become apparent from the following detailed description of the invention.

Our invention, briefly stated, involves adding certain electrochemical erosion inhibiting film forming thiazoles or thiols to basic known ECM electrolytes. When added to these electrolytes the additives of our invention, and especially 2-mercaptobcnzothiazole or 2-benzimidazolethiol, create an inhibited solution which effectively forms a heavy adherent film over the surface of the workpiece. The film retards or substantially eliminates electrochemical erosion in those areas protected by the film. In a particular application of our invention a chloride electrolytic grinding electrolyte is modified by adding thereto either Z-mercaptobenzothiazole or Z-benzimidazolethiol. The film formed is subsequently abraded away in those areas where electrochemical machining is to continue, hence presenting a limited uninhibited surface to unrestricted electrochemical action.

Our experience has been that using the additives of our invention, we can successfully machine metal samples ranging from the softer lower carbon steels (SAE 1008) to the harder low alloy steels (SAE 5160H). The additives of our invention are likewise applicable and effective for ferrous alloys wherein the iron content is 50% or more.

While the preferred electrolytes comprise 80 grams per liter of Z-mercaptobenzothiazole or 60 grams per liter of Z-benzimidazolethiol, respectively, with a 108 grams per liter aqueous sodium chloride solution, we have found that effective electrolytes can be compounded using from 21 to 101 grams per liter of either Z-benzimidazolet-hiol or Z-mercaptobenzothiazole with dilute up to saturated solutions of sodium chloride. In this connection the lighter alkali metal (Li, Na and K) chlorides are preferred because they produce relatively neutral pHs, do not plate out or have a deleterious elTect upon the cathode and represent a source of inexpensive material. Likewise, while electrolytes dilute as to chloride ion are operative, as a practical matter there are no significant advantages to operating at the lower concentrations and, in fact, it is less desirable to do so when considering such factors as solution conductivity and the like.

We have been successful in operating these electrolytes at voltages up to 40 volts and anode current densities from 5 to 500 amperes per square inch. However, it is to be expected that in some circumstances even higher current densities can be used. Though room temperatures are generally preferred for manufacturing processes, we

assaees have successfully produced good results at higher temperatures.

Inasmuch as neither 2-benzimidazolethiol or Z-mercaptobenzothiazole are readily soluble in water or sodium chloride solutions, it is necessary first to prepare the respective compounds for introduction into the respective solutions. A mole-weight to mole-weight ratio of Z-mercaptobenzothiazole or Z-benzimidazolethiol is mixed with sodium hydroxide and subsequently dissolved in a limited amount of water which is maintained at near its boiling point. We have successfully obtained concentrations up to 0.15 gram per milliliter using this procedure. In order to make up the preferred electrolyte the appropriate volume of said 0.15 gram per milliliter solution is added to the appropriate aqueous sodium chloride solution. Owing to the ready availability of acidic Z-mercaptobenzothiazole and Z-benzimidazolethiol the aforementioned procedure is preferred though it is recognized that the sodium salt of the respective compounds is formed and would be equally effective as an additive in that form.

Generally speaking, tests were conducted utilizing a system wherein anodic steel tube samples were brought up to a cathodic rotating sintered bronze diamond impregnated wheel. A gravity feed system kept the samples at the face of the wheel at all times. The feed system was such that an adjustable weight provided the capability of varying the pressures at which the samples would en gage the wheel. It was found that to properly evaluate the inhibitive efforts of our additives a minimum workpiece-to-wheel pressure should be employed in order to reduce the mechanical cutting component of the abrasive wheel. The electrolyte was pumped at a pressure of 9 pounds per square inch through a bore in the workpiece and into the gap between the cathode and the workpiece at a rate of 0.25 gal. per minute. This gap was held constant at about 0.0025 inch by the spacer effect of the nonconductive diamond chips. Tube length decrease per unit time was used to determine metal removal rates. The tube ends Were compared with those produced by electrochemically grinding similar samples under the same conditions but with additive-free electrolytes.

The following are some specific examples encompassed within the scope of our invention:

Example 1 A room temperature electrolyte comprising 179 grams per liter of sodium chloride, 33 grams per liter of 2-mercaptobenzothiazole and the balance water, was used to machine a sample of an SAE 1020 steel. A voltage of 14.5 volts was applied and a current density of 400 amperes per square inch was maintained resulting in the production of a sample exhibiting a strong inhibiting film formed over the workpiece.

Example 2 A room temperature electrolyte comprising 197 grams per liter of sodium chloride, 21 grams per liter of Z-mercaptobenzothiazole and the balance water, was used to machine a sample of an SAE 1020 alloy. A voltage of 8.9 volts was applied and a current density of 425 amperes per square inch was maintained, resulting in the production of a sample displaying a strong inhibiting film over the surface thereof.

Example 3 A room temperature electrolyte comprising 111 grams per liter of sodium chloride, 77 grams per liter of Z-mercaptobenzothiazole and the balance water, was used to machine a sample of an SAE 5160H alloy. A voltage of 4.5 volts was applied and a current density of 165 ampercs per square inch was maintained, resulting in the production of a sample displaying a sharply cut machining, an incident of the production of a highly inhibitive film over the surface of the workpiece.

(.1. Example 4 A room temperature electrolyte comprising 189 grams per liter of sodium chloride and 26.35 grams per liter of Z-benzimidazolethiul and the balance water, was used to machine a sample of an SAE 5l6OH alloy. A voltage of 9 volts was applied and a current density of 360 amperes per square inch was maintained, resulting in the production of a sample displaying excellently machined characteristics with no overcut.

Example 5 A room temperature electrolyte comprising 137 grams per liter of sodium chloride, 60 grams per liter of 2- benzimidiazolethiol and the balance water, was used to machine a sample of an SAE SlOH alloy. A voltage of 4 volts was applied and a current density of 150 amperes per square inch was maintained, resulting in the production of a sample displaying a strongly adherent inhibitive film formed over the surface thereof.

Example 6 A room temperature electrolyte comprising 103 grams per liter of sodium chloride, 83 grams per liter of 2- benzimidazolethiol and the balance water, was used to machine a sample of an SAE 5160H alloy. A voltage of 9 volts was applied and a current density of 400 amperes per square inch was maintained, resulting in the production of a sample displaying excellent machined characteristics. I

While metal removal rates as high as 0.103 inch per minute were noted at relatively high current densities, entirely satisfactory results were consistently produced at lower current densities. Likewise, though electrolytes were compounded using as much as 101 grams per liter of either Z-mercaptobenzothiazole or 2-benzimidazolethiol, no appreciable advantages over the somewhat lesser concentrated solutions were readily apparent save for the somewhat faster formation of a stronger film. It is doubt ful that the marginal extra benefits to be derived from the highly concentrated solutions would ever economically justify their use over the preferred ran es.

Other nonabrasive means for the local removal of the inhibiting film of our invention may be employed, such as washing away with localized increased electrolyte flow, and/or variety of sophisticated variations of these and others. Therefore, though our invention has been described in terms of certain preferred embodiments, it is to be understood that others may be adopted and that the scope of our invention is not limited except by the appended claims.

We claim:

1. An aqueous electrochemical machining electrolyte consisting essentially of an alkali metal chloride and from about 21 grams per liter to about 101 grams per liter of an additive from the group consisting of Z-benzimidazolethiol and Z-mercaptobenzothiazole.

2. An electrolyte in accordance with claim 1 wherein said alkali metal is from the group consisting of lithium, sodium and potassium.

3. An electrolyte in accordance with claim 1 wherein said additive is Z-oenzimidazolethioi.

4. An electrolyte in accordance with claim 3 wherein the concentration of the Z-oenzimidaz lethiol is from about 21 grams per liter to about 60 grams per liter.

5. An electrolyte in accordance with claim 1 wherein said additive is Z-mercaptobenzothiazole.

6. An electrolyte in accordance with claim 5 wherein the concentration of said Z-mercaptobenzothiazole is from about 21 grams per liter to about 80 grams per liter.

7. A process for electrochemically machining ferrous metals comprising the steps of establishing said metal as the anode in an electrochemical cell oricrting an clectrolytic grinding cathode adjacent tosaid metal, bathing the junction between said cathode and said metal in an aqueous electrolyte consisting essentially of an alkali metal asaaaaa chloride and an additive from the group consisting of 2-benzimidazolethiol and Z-mercaptobenzothiazole, passing an electric current through said metal, electrolyte and cathode whereby an electrochemical erosion inhibiting film is formed on the metal, and selectively removing said film whereby electrochemical machining can thus selectively continue.

8. A process in accordance with claim 7 wherein the concentration of said additive is from about 21 grams per liter to about 101 grams per liter.

9. A process in accordance with claim 7 wherein said additive is 2-benzimidazolethiol.

10. A process in accordance with claim 9 wherein the concentration of said Z-benzimidazolethiol is from about 21 grams per liter to about 60 grams per liter.

References Cited UNITED STATES PATENTS 2,939,825 6/1960 Faust et a1. 204-442 3,058,895 10/1962 Williams 204-143 3,130,138 4/1964 Faust et al. 204143 3,284,327 11/1966 Maeda et al 204-143 r ROBERT K. MIHALEK, Primary Examiner. O

Claims (1)

1. 7. A PROCESS FOR ELECTROCHEMICALLY MACHINE FERROUS METALS COMPRISING THE STEPS OF ESTABLISHING SAID METAL AS THE ANODE IN AN ELECTROCHEMICAL CELL ORIENTING AN ELECTROLYTIC GRINDING CATHODE ADJACENT TO SAID METAL, BATHING THE JUNCTION BETWEEN SAID CATHODE AND SAID METAL IN AN AQUEOUS ELECTROLYTE CONSISTING ESSENTIALLY OF AN ALKALI METAL CHLORIDE AND AN ADDITIVE FROM THE GROUP CONSISTING OF 2-BENZIMIDAZOLETHIOL AND 2-MERCAPTOBENZOTHIAZOLE, PASSING AN ELECTRIC CURRENT THROUGH SAME METAL, ELECTROLYTE AND CATHODE WHEREBY AN ELECTROCHEMICAL EROSION INHIBITING FILM IS FORMED ON THE METAL, AND SELECTIVELY REMOVING SAID FILM WHEREBY ELECTROCHEMICAL MACHINING CAN THUS SELECTIVELY CONTINUE.