technical report on electroc

1

Production Engineering Department, Jadavpur University

1. Introduction

Electrochemical micromachining (EMM) has emerged as a versatile process for

machining and surface structuring of metallic materials for biomedical and

microsystems applications. From a fundamental point of view EMM presents

many similarities with electrochemical machining (ECM) and electro polishing

(EP) provided one takes into account the scale dependence of phenomena.

Surface Structuring is the process of manipulating the material surface in such

a way so that the tribological properties of the surface get enhanced. Tribology is

the science and engineering of interacting surfaces in relative motion. It includes

the study and applications of the principles of lubrication, friction and wear. We

all know that tribology studies the ways to maintain the surface characteristics of a

material at its best conditions so that the production process reaches its highest

efficiency. As for example, making the surface better introduced to lubricating

fluids so that the lubricants assume the correct positions to the correct extents.

And the application of EMM process in surface structuring has opened a new

direction and a scope of thought for the scientists. In this paper we’ll discuss scale

dependent surface structuring phenomenon. Scale resolved electrochemical

surface structuring of titanium leads to well-defined topographies on the

micrometer and nanometer scales, which are of interest for biomedical

applications.

In this report we shall concentrate our discussion to surface structuring of

Titanium.

2


1. i. Surface Structuring Present Status

Surface Structuring is the process of manipulating the material surface in such

a way so that the tribological properties of the surface get enhanced Currently in

the field of manufacturing science, surface structuring is opening new windows

for higher study and research. The various advantages and disadvantages of this

surface structuring process are must know for further study purposes.

Advantages: i. This helps in developing the surface properties of the material.

ii. Frictional wear can be reduced up to great extent.

iii. The surface can be manipulated in such a way so that lubricants get their

full functionality properly.

iv. Ensure an excellent surface finish to the final product for precision

manufacturing processes.

Disadvantages: i. The initial cost of the instruments is a bit high.

ii. This process needs high level of expertise and hence only professional

people are eligible to operate and perform it.

iii. If the process isn’t fully correct, then negative results may appear which

means the surface quality can go down further.

Every coin on this earth has two flip sides. So as similar to all other processes,

this surface structuring process has also some disadvantageous sides. But despite

all of them, for higher precision and perfection in the field of manufacturing, it has

been necessary to research more and more on this science.

3


1. ii. An Overview Of ECM

Electrochemical machining (ECM) is a method of removing metal by

an electrochemical process. It is normally used for mass production and is used for

working extremely hard materials or materials that are difficult to machine using

conventional methods. Its use is limited to electrically conductive materials. ECM

can cut small or odd-shaped angles, intricate contours or cavities in hard and

exotic metals such as titanium aluminides, Inconel, Waspaloy and high nickel,

cobalt, and rhenium alloys. Both external and internal geometries can be

machined using this process.

ECM is often characterized as "reverse electroplating", in that it removes

material instead of adding it. It is similar in concept to electrical discharge

machining (EDM) in that a high current is passed between an electrode and the

part, through an electrolytic material removal process having a negatively charged

electrode (cathode), a conductive fluid (electrolyte), and a conductive work piece

(anode); however, in ECM there is no tool wear. The ECM cutting tool is guided

along the desired path close to the work but without touching the piece. Unlike

EDM, however, no sparks are created. High metal removal rates are possible with

ECM, with no thermal or mechanical stresses being transferred to the part, and

mirror surface finishes can be achieved.

In the ECM process, a cathode (tool) is advanced into an anode (work piece).

The pressurized electrolyte is injected at a set temperature to the area being cut.

The feed rate is the same as the rate of "liquefication" of the material. The gap

between the tool and the work piece varies within 80–800 micrometers (0.003–

0.030 in.) As electrons cross the gap, material from the work piece is dissolved, as

the tool forms the desired shape in the work piece. The electrolytic fluid carries

away the metal hydroxide formed in the process.

As far back as 1929, an experimental ECM process was developed by W.

Gussef, although it was 1959 before a commercial process was established by the

Anocut Engineering Company. B.R. and J.I. Lazarenko are also credited with

proposing the use of electrolysis for metal removal.

Much research was done in the 1960s and 1970s, particularly in the gas turbine

industry. The rise of EDM in the same period slowed ECM research in the west,

although work continued behind the Iron Curtain. The original problems of poor

dimensional accuracy and environmentally polluting waste have largely been

overcome, although the process remains a niche technique.

4


The ECM process is most widely used to produce complicated shapes such

as turbine blades with good surface finish in difficult to machine materials. It is

also widely and effectively used as a deburring process.

In deburring, ECM removes metal projections left from the machining process,

and so dulls sharp edges. This process is fast and often more convenient than the

conventional methods of deburring by hand or nontraditional machining

processes.

Advantages and Pitfalls:

Because the tool does not contact the work piece, there is no need to use

expensive alloys or tempering procedures to make the tool tougher than the work

piece. As a result, the tools can be made from any cheap and easily machined,

cast, or engraved electrically conductive substance. Along with this, there is less

tool wear in ECM, and less heat and no stress are produced in processing that

could damage the part. Fewer passes are typically needed, and the tool can be

repeatedly used. Also this method works even on very hard and brittle work

pieces.

The saline (or acidic) electrolyte poses the risk of corrosion to tool, work piece

and equipment. Only electrically conductive materials can be machined. High

Specific Energy consumption.

Applications:

Some of the very basic applications of ECM include:

• Die-sinking operations

• Drilling jet engine turbine blades

• Multiple hole drilling

• Machining steam turbine blades within close limits

• Jet engines

• Micro Machining (EMM)

5


1. iii. An Overview Of EMM

This objects to removal of material of very small dimensions. This ranges from

several microns to millimeters. It has a promising future for production of

miniaturized parts and components.

Several difficulties have been noted while machining of miniature parts and

components with the help of conventional machining processes. Also the huge

requirement of precision manufacturing process has led to the wide application of

electrochemical machining process or EMM.

When electrochemical machining process (ECM) is applied to micro

machining range i.e. 1 micron to 999 micron, for manufacturing of ultra-precision

shapes then this is termed as electrochemical micro machining or EMM process.

Several fundamental observations in this EMM process are that- a very small

inter electrode gap is used. Rate of dissolution or material removal rate depends

upon atomic weight of the material (a), valance (v) of ions produced, machining

current (I), and the time (t) for which the current passes. Though the shape of

electrodes remain unaltered like ECM. Material removed can be calculated as

MRR= Iaη/Fvρ

Where η= metal dissolution efficiency, ρ= density of material and F=

Faraday’s constant= 96500 coulomb.

The various process parameters which influence the machining process are:

• Nature of power supply and machining pulse- DC power supply and DC

pulsed power supply.

• Inter- electrode gap

• Electrolyte type- Passivity and Non-Passivity Electrolyte

• Electrolyte concentration and flow

• Size shape and material of the tool

6


Various applications of EMM for micro fabrication are:

• Nozzle plate for ink jet printer head

• Production of high accuracy holes

• 3 dimensional micro machining

As the days are passing by there are numerous no. of things developing in

EMM. The recent advancements of these EMM process are magnificently noted

in:

• Micro Electrochemical Milling

• Wire Electrochemical Machining

• Solid Electrochemical Drilling

• Oxide Film Laser Lithography (OFLL)

• Micro and Nanometer scaled surface structuring

• Laser Electrochemical Micro Machining (LECMM)

7


2. Different Techniques Of Surface Structuring

The different techniques of surface structuring can be broadly classified

into two categories viz.

i. Conventional techniques:

Conventional techniques are those techniques which uses conventional

machining processes such as grinding, honing, lapping etc. for surface

structuring purpose. Also it can be stated that these processes use mechanical

energy for machining. Frankly speaking, surface structuring is the process of

developing surface in miniaturized level. But these processes are often used

mainly for surface finishing which is imperative in precision manufacturing

process. So these processes can be termed as conventional processes for

surface structuring.

The major advantages of these conventional surface structuring processes

are mentioned below:

a) These processes are suitable for almost all types of materials.

b) These processes are very effective to economy i.e. they’re very economical.

c) Capital cost of performing these operations are low.

d) Easy setup of equipment are noted.

e) Can be operated by anyone, be him or her high skilled or low.

However there are some disadvantages of these conventional processes

which has led to applications of non-conventional techniques. The major

disadvantages are:

a) Lower accuracy and poor surface finish is obtained sometimes.

b) Higher wastage of material due to high wear rate.

c) Noisy operation cause sound pollution.

d) Tool life is less due to high wear.

e) Cannot be used to operate on very small parts, like in micro and Nano range.

f) The processes are manual, hence there’re chances of human error.

8


ii. Non-Conventional techniques:

Non-Conventional techniques are those techniques which uses non-

conventional machining processes such as Electric Discharge Machining etc. for

surface structuring purpose. There is no direct contact between tool and work

piece in most cases, and hence it uses some kind of energy like thermal energy,

chemical energy etc. material removal purposes.

The major advantages of these non-conventional machining processes are

mentioned below:

a) Higher accuracy and surface finish can be obtained here with compared to

the conventional processes.

b) Tool life is far more than the former.

c) Lower wastage of material due to low or no wear.

d) Operation is quiet, mostly no sound is produced.

e) Fully automated process; hence there’re no chances of human error.

f) Can operate into small ranges, even in the micro or Nano range. Prototypes

can be machined here too.

Just like all other processes, the non-conventional processes have some pitfalls

too. These are mentioned below:

a) Capital cost is high.

b) Not suitable for all types of material e.g. EDM or ECM processes require

electrically conductive materials to be operated on.

c) These processes are economical mainly for precision manufacturing

processes.

d) Complex setup of equipment.

e) Only experts can perform these operations.

It has been seen that though non-conventional techniques have some

disadvantages, their weight of advantage is much more than the other sides. Hence

for the surface structuring process, non-conventional techniques are more used

than the former. Among all the non-conventional techniques, ECM, more

specifically EMM has shown excellent performance to be operated under

predefined suitable conditions for this surface structuring purposes. That is why

ECM or EMM has got most attention for this operations.

9


3. Fundamental Aspects Of Electrochemical Surface Structuring

Electrochemical processes for shaping and surface structuring of metals by

controlled anodic dissolution include electrochemical machining (ECM),

electrochemical polishing (EP) and electrochemical micromachining (EMM).

These processes find many applications in the manufacturing industry and several

reviews discussing fundamental and applied aspects are available.

Among the named processes EP has been known for the longest time. The

invention of EP as an industrial surface treatment process is usually attributed to

Jacquet who took a patent in 1930, but the first recorded publication dealing with

electro polishing (EP) goes back even further. EP is widely used for surface

finishing of metallic objects of any shape and for deburring. Overviews of

electrolytes commonly used in practice are given in references. EP involves

levelling and brightening of surfaces. The former is usually associated with a

decrease in roughness in the micrometer or larger range, the latter with achieving

specular reflectivity by surface smoothing in the sub micrometer range.

ECM was developed in the late fifties and early sixties for the shaping of

difficult to machine metals and alloys. In the ECM process the shape of the

cathode is reproduced on the anode by high rate anodic dissolution. During

dissolution the cathode is progressively advanced to maintain a constant electrode

spacing, typically on the order of one or several tenth of a millimeter. Extremely

high current densities, up to 100 A/cm2 or more, are applied to achieve high

machining rates. Such high current densities require flow rates of several meters

per second in the inter-electrode gap in order to evacuate the reaction products and

the heat produced in the process. Mostly neutral salt solutions such as NaCl or

NaNO3 are used as electrolytes. ECM was originally developed for the aerospace

industry, but has found applications also in other industries, especially in the

automobile industry, which in the sixties and early seventies developed an active

research program in this field. Subsequently, due to the development of computer

controlled electric discharge machining (EDM), the industrial importance of ECM

has decreased somewhat. However, the process still presents advantages for

certain applications, especially when a smooth surface finish is required.

EMM is a relatively recent method originally developed as an environment

friendly alternative to chemical milling for the electronics and micromechanics

industry. An important advantage of EMM compared with chemical milling is that

it requires no oxidizing agent that must be disposed of with the spent solutions. In

addition, EMM is faster and permits machining of chemically resistant metals

10


such as super alloys, stainless steels or titanium. Similar as in chemical milling, in

EMM a photosensitive polymer (photoresist) is applied to the metal. Irradiation of

the photoresist through a suitably designed mask leads to formation of a pattern.

The irradiated (positive resist) or the non-irradiated (negative resist) parts of the

photoresist are then chemically dissolved in a suitable solvent. Finally, the

exposed metal is subjected to electrochemical dissolution forming the desired

shapes. Depending on the application, thin metal sheets are machined by one sided

or two sided attack.

From a fundamental point of view EP, ECM, and EMM have much in

common. They can be understood and theoretically modelled based on the same

principles provided one takes duly into account scale effects resulting from

differences in the rate of material removal, the size of the system and the relevant

geometry. To be focused into our topic, recent results on EMM and surface

structuring of titanium will be presented here only.

11


4. New Development Of EMM For Titanium Surface Structuring

The recent researches on EMM on titanium has opened new directions in the

field of mechanical engineering, more specifically manufacturing processes. The

various new developments mainly comprise of:

• Oxide film laser lithography: This does not much differ with

original lithography process except the fact that the layer by layer

development is made with typical laser spot diameter used for Oxide film

laser lithography or OFLL.

• EMM of multilevel structures: EMM using the photoresist

technique is usually applied to single step etching of flat surfaces, using one

sided or two sided attack of thin sheet samples. Different examples of such

applications have been given in the literature. More recently, the application

of EMM to the fabrication of two level structures has been explored.

• Scale dependent surface structuring: Control of surface

topography on the micrometer and the nanometer scale is of importance in

many applications and for that purpose this is a recently developing

technique. Our discussion is focused on this matter.

12


5. Scale Dependent Surface Structuring

Control of surface topography on the micrometer and the nanometer scale is of

importance in many applications. Electrochemical etching can for example be

used for producing polished or grained surfaces of aluminum. The surface

topography of biomedical implants plays an important role for cell attachment and

differentiation. For example, the surfaces of dental implants made of titanium are

subjected to physical and chemical treatments that produce finely textured

surfaces in order to reduce the time of recovery for the patients. In our laboratory

we develop electrochemical methods for the fabrication of titanium surfaces of

defined micro- and nanostructure in order to produce model surface topographies

that can be tested for cell response by biologists. Typically, EMM using the

photoresist technique serves for the fabrication of micrometer sized hemispherical

cavities of different size and separation. Since biological testing requires large

numbers of samples of identical surface structure the EMM process had to be

optimized. Using an ethanol cooled sample holder and an optimized voltage

function well defined regular surface structures on 1.5 cm diameter polished disks

were made as illustrated by the SEM micrograph of the below figure.

The sample surface outside the cavities is the original mechanically polished

surface, whereas the inside of the cavities has an electro polished surface finish.

Varying the cavity size and distance one, therefore, also varies the ratio between

electro polished and mechanically polished surfaces. The effect of these variables

on cell response is being tested at present. Surface topography at the nanometer

scale is thought to be at least as important for cell response as micrometer scale

topography. For titanium, chemical etching in hot sulphuric and hydrochloric acid

13


based electrolytes can produce roughness on a sub micrometer scale. By

superposing this type of nano-roughness with electrochemical micro structuring

one can produce surfaces with controlled roughness at two different scales. An

example of such a surface is given in the below figure.

Another way to produce nanometer topography is based on the formation of

porous oxides by anodisation. Porous oxide films are well known for anodized

aluminum where regularly shaped pores can be formed with well-controlled pore

size. Apparently, in the sulphuric acid electrolytes typically used for aluminum

anodisation, the pores form as soon as oxide film starts to grow and their size

depends on the applied voltage. For titanium a similar pore formation mechanism

has very recently been reported in presence of fluoride at very long anodisation

times. Anodisation in sulphuric or phosphoric acid, on the other hand, leads to the

formation of a compact film, which at sufficiently high voltage undergoes

dielectric breakdown leading to formation of a multitude of pores, the size and

number of which depends on experimental conditions. The breakdown events

manifest themselves by marked current fluctuations. As an illustration, the SEM

picture of the below figure shows the surface of a porous anodic oxide film

formed in 1 M H2SO4 by sweeping the voltage up to 125 V.

14


Interestingly, on mechanically polished samples the pores were quite regularly

distributed, almost independent of the grain structure of the underlying substrate.

On the other hand, pore formation on flat electro polished surfaces was in general

less regular. By porous anodizing of electrochemically structured titanium

surfaces containing 30 mm size cavities, scale resolved surface structuring on the

micrometer and the nanometer scales could be achieved. An example of such a

surface is shown in the below figure.

15


The ability to fabricate well-defined surface topographies will be useful for

reaching a better understanding of the complicated interactions of living cells with

implant materials.

16


6. Conclusion

In the present paper a brief overview has been given first of the basic principles

governing the performance of ECM and further the discussion has been

concentrated to EMM. Several characteristics of ECM, EMM and further EP

process are discussed. Most important are the consideration of current distribution,

mass transport and the behavior of passive oxide films which are not discussed

here due to the fact this will be beyond the scope of this topic. Taking into account

the relevant scales ECM, EP and EMM exhibit many similarities. Recent studies

on EMM of titanium with and without photoresist discovered, which demonstrate

the many possibilities of the process for biomedical and micromechanics

applications. In particular, EMM can be used for scale resolved surface structuring

on the micrometer and nanometer scales. Using oxide film laser lithography

instead of conventional photoresist technique, the usefulness of EMM can be

extended to non-planar surfaces and to the machining of multilevel structures for

device fabrication. These developments together with a better theoretical

understanding of the critical electrochemical parameters are expected to

eventually open new fields of application for EMM. Future work should be aimed

at finding ways to accelerate the different steps involved in resist-free EMM in

order to improve the economic attractiveness of the process. Another possible

development could be to use other means of oxide sensitization such as electron or

ion beams, which would permit to extend OFLL to machining of nano size

features. The techniques described here for titanium can probably be extended to

other valve metals such as tantalum or niobium, although the specifics of oxide

sensitization may require further study and optimization of the interaction of laser

radiation with oxide films.

17


7. References

1. “Advancement in Electrochemical Micro Machining” by Dr. B.

Bhattacharyya, J. Munda, M. Malapati.

2. “Electrochemical micromachining, polishing and surface structuring of metals: fundamental aspects and new developments” by D. Landolt, F.

Chauvy, O. Zinger.

3. “Electrochemical Micro Machining” by Todkar Mahesh.

----------THE END----------

technical report on electroc

Documents