To
estimate the power density (
) needed to obtain the temperature Tm on
the surface, the one-dimensional model of heating of semi-infinite body
by a heat source with the intensity constant in time can be used [4]:
(1)
where
a and l
are thermal diffusivity and conductivity respectively.
The
critical value of heat power density (
), which leads to evaporation, can be estimated from the following
equation [4]:
(2)
where r
is the density of workpiece material
The
time 
required to reach a given
temperature on the surface of the material, for example, to boiling point
Tb, increases with an increasing boiling temperature,
heat conduction l,
and specific heat of the material C, and decreases with increasing
heat power density q. To estimate the value of 
, we can use the expression obtained from the one-dimensional
model of heating of a semi-infinite solid:
(3)
The
above expressions (1-3) can be used to estimate the characteristics of heat
sources for removal processes in EDM, LBM, EBM and PBM.
| Fig.
1 Temperature dependence of the yield strength Rp0.2
and ultimate tensile strength Rm of two Ni-base
superalloys [5] |
|
The resistance of workpiece material to mechanical removal
process by cutting edge or abrasives (random oriented edges) depend on
the high temperature (below melting point) generated at the machining
zone. It is known that stress-strain curves and the flow and fracture
properties derived from the tension test strongly depend on the temperature
at which the test is conducted. In general, the strength decreases and
ductility increases as the temperature is increased. For example, the
temperature dependence of the strength for heat resistant Ni-base superalloys
is illustrated Figure 1 [5]. Therefore, it is quite logical to assume
that high temperatures reduce the cutting forces and the energy consumption,
and increase the machinability of workpiece material by mechanical removing
processes. Some cross processes has been developed according to this idea,
such as laser assisted ECM (Section 3)
and laser or plasma assisted machining of ceramics (Section
5).
In chemical or electrochemical interactions, an increase
in temperature leads to acceleration
of kinetic reactions on heated surfaces, such as anodic dissolution or
cathodic deposition. The effect of temperature on the rate of these processes
(i.e. on current density iL, which determines the rate
of dissolution/deposition) is described by following Arrhenius equation:
(4)
where ,
To, and Ts are initial and final (after
heating) surface temperatures respectively, 
is the increment in temperature,
io is current density without heating i.e. at 
= 0, E0 is the activation energy, and R
is the gas constant. The
increase in current density (rate of processes) as the result of thermal
assistance is shown in Figure 2 [6].
 |
| Fig.
2 Effect of temperature on the current density of ECM electrode [6] |
The value
of iL/i0 at a temperature increase
of DT =
70, i.e., near the boiling point of the electrolyte, depends on the workpiece
material. For steel, copper, etc, the value of iL/i0
changes in a range from 30 to 570!
Therefore,
local heating by laser beam can be used to assist the chemical or electrochemical
removal/deposition processes.
Chemical
and Electro-chemical Interactions
Chemical
milling (CM), etching (E), electrochemical machining (ECM), pulse electrochemical
machining (PECM), electropolishig (EP) are shaping
and finishing processes based on the dissolution of materials.
Chemical
milling is the selective and controlled metal removal by chemical action.
It is especially useful for removing metals from sheet components to reduce
weight, and it can be employed after parts have been formed and heat-treated.
Any metal that can be chemically dissolved in solution can be chemically
milled.
Electrochemical
shaping and finishing is based on controlled
anodic electrochemical dissolution process of the workpiece (anode) with
a cathode tool in an electrolytic cell. Being a non-mechanical metal removal
process, ECM is capable of machining any electrically conductive materials
with high stock removal rates regardless of their mechanical properties,
such as hardness, elasticity and brittleness. Shaping
by electrochemical dissolution is described by the distribution of the dissolution
velocity, Vn, on the anode-workpiece surface,
(5)
where KV
is the electrochemical machinabilty coefficient, which
is defined as the volume of material dissolved per unit electrical charge,
and ia is the current density on anode-workpiece surface.
The assistance of other actions during electrochemical machining
is determined by their effect on KV and ia,
such as the effect of heating on electrochemical processes described by
equation (4).
The
interaction of chemical and electrochemical processes with
other removal processes is mainly based on their influence on the physical
properties of the workpiece surface layer and the working media.
The effect of electrochemical and chemical processes contributes in changing
the dislocation density in surface layer of the workpiece material. And
due to the reduction in surface potential energy during dissolution, plastic
deformation is easier and the level of work hardening is decreased. Fig.
3 illustrates the change of surface microhardness (HV)
during anodic dissolution[7]. As seen in the figure, even with relatively
low current densities, significant reduction in microhardness occurs. For
example, in the case of 3H13 steel, surface microhardness reduces from 220
MPa at i = 0A/cm2 to 168 MPa at i = 1A/cm2.
 |
|
Fig.
3 Effect of current density of anodic dissolution on the microhardness
of machining surface (electrolyte: water solution of NaNO3
– 50 g/l, and NaNO2
– 10 g/l; material: Armco-curves 1, 3H13 steel-curve 2) [7]
|
Chemical
and electrochemical action can influence the physical conditions in the
machining zone. For example, generation of gas bubbles during electrochemical
reaction is the major factor in electrical discharges in an electrolyte.
Chemical and electrochemical actions may increase
the radiation absorptivity of metals and thus decrease the radiation energy
thresholds for melting and vaporization during laser action.
There is good reason to combine these beneficial effects in different processes.
Mechanical
Interactions
Traditional cutting, grinding and abrasive finishing methods
are based on mechanical interactions of the cutting edge with the workpiece
materials. Mechanical interactions also play important roles in water
jet machining and ulttrasonic machining. Freedom of movement can be added
to the various machining processes which increase the flexibility of the
porcesses. The mechanical assistance in
the thermal and/or electrochemical removal processes is based on their
influences on the properties of the workpiece and the working media.
Mechanical interactions
can improve the working conditions in the anodic dissolution processdue
to mechanical depassivation of the surface. By removing a thin layer of
oxides and other compounds from the anode, the surface dissolution is
intensified and the smoothness of the machined surface can be increased.
Mechanical interactions can increase the machining rate but may leave
substantial residual stresses which may be undesirable. While chemical
processes can relieve the residual stresses. Thus it's attractive to combine
these beneficial effects in one process. It
has been found that as a result of mechanical depassivation, material
removal rate by dissolution process (electrochemical grinding) has
been increased by 100-142% when grinding T15KG carbides, by 21-44% for
H18N10T steel, and by 32-71% for Armco iron [7].
Significant
effects of mechanical action can be generated by ultrasonic wave. The cavitation
generated by ultrasonic vibration can enhance the EDM, ECM and LBM processes.
In ultrasonically assisted EDM or LBM drilling, it is recognised that the
role of the acoustic wave and cavitation is to improve the flushing and
the material removal from the surface craters.
These process condition improvements are significant for micro drilling
and grooving.
(6)
