13-09-2013, 04:51 PM
The Effects of Cold and Cryogenic Treatments on the Machinability of Beryllium-Copper
Alloy in Electro Discharge Machining
The Effects of Cold and Cryogenic.docx (Size: 40.2 KB / Downloads: 29)
Abstract
In this study, beryllium-copper alloy was subjected to around -150° F for cold treatment and to around -300° F
for cryogenic treatment and the effects of these cold and cryogenic treatments on the machinability of
beryllium-copper workpieces in electro discharge machining have been investigated. Experimental results
showed about 20-30 % increase in material removal rate of cold and cryogenic treated workipeces. Variations
in electrode wear rate, surface roughness and average white layer thickness were found to be marginal.
INTRODUCTION
Advanced engineering materials often pose machinability
challenges for traditional processes such as turning and
milling. Nontraditional processes like electro discharge
machining (EDM); due to their unique mechanism of
material removal are useful alternatives in such cases [1].
Conversion of electrical energy to thermal energy through
repeated occurrence of sparks between the tool and the
workpiece in EDM results in the material removal from
workpiece as well as tool by melting and evaporation.
Numerous studies on the improvement of EDM
performance have identified major process parameters
such as electrical parameters (voltage, current, pulse
on/off time, polarity etc.), flushing (pressure, flushing
direction and method) and material characteristics
(electrode, work-piece, dielectric). However, the effects of
cryogenics on EDM performance have not yet been
adequately explored. Limited literature available on this
topic indicate the potential for the application of
cryogenics for the improvement of several processes
such as turning [2, 3], milling [4], drilling [5] and EDM [6,
7]. In these studies, cryogenic temperatures have been
used for treatment of cutting tools or cooling purposes. In
an earlier study, it was reported that material removal rate
of EDM process was improved by employing
cryogenically treated copper electrodes [6].
COLD AND CRYOGENIC TREATMENTS
Cryogenics is a branch of low-temperature physics
concerned with the effects of very low temperatures less
than about 123°K (-150°C) and it extends down to
absolute zero -273°C (-459°F). Historically, the
development of cryogenic science occurred primarily in
the years from 1900 to 1950 with liquefaction technology for cryogens. Applications of cryogenics in industry vary
from space research to food handling. The effects of
cryogenic temperatures on properties of materials have
been examined extensively in terms of mechanical,
thermal and electrical properties. It is reported for several
engineering materials that mechanical properties such as
the yield strength, tensile strength, fatigue strength,
impact strength, hardness and elastic modulus increase
as the temperature decrease [11]. Another study reports
that the thermal conductivity decreases as the
temperature is lowered for certain alloys such as titanium
alloy-TC4 and impure metals such as magnesium-AZ31B
[12]. Positive effects of low temperatures on mechanical,
thermal and electric properties of materials has lead to
the cold/sub-zero and cryogenic treatments of wide
variety of cutting tools and mechanic parts in
manufacturing and automotive industry to increase their
strength, hardness and wear resistance and thus
substantial savings were recorded. As the name
suggests, cold treatment or sub-zero treatment involves
temperatures below zero but higher temperatures than
the cryogenic temperatures (down to about -80 ºC).
Cryogenic treatment can be characterized by its
application temperature, below 123°K or at about liquid
nitrogen (LN2) temperature (-196ºC).
Cold and Cryogenic Treatment Cycles
Cold and cryogenic treatment processes in this study
consist of three main periods and soaking time, the most
important stage for the final properties, was kept same for
both treatment cycles for a valid comparison. The thermal
cycle for the cold treatment was as follows: Linear ramp
from room temperature to -150 ºF in about 4 hours, dwell
at -150 ºF for 8 hours (soaking stage), warm by ambient
heat gain into closed chamber to near room temperature
over approximately 10 hour, linear ramp up to +300 ºF in
1 hour, dwell at +300 ºF for 1 hour, allow to cool in open
chamber back to room temperature. And, the thermal
cycle for the cryogenic treatment was as follows: linear
ramp from room temperature to -300 ºF in 6 hours, dwell
at -300 ºF for 8 hours (soaking stage), warm by ambient
heat gain into closed chamber to near room temperature
over approximately 52 hour
CONCLUSIONS
In this study, Be-Cu alloy workpieces were subjected to
around -150° F for cold treatment and to around -300° F
for cryogenic treatment and the effects of cold and
cryogenic treatments on their machinability in EDM have
been investigated. Experimental results showed about 20-
30 % increase in material removal rate by cold and
cryogenic treatment processes. Variations in electrode
wear rate, surface roughness and average white layer
thickness were found to be marginal. Supplemental
electrical/thermal conductivity tests can be utilized in the
future attempts. In addition, different combination of
processes including deeper cryogenic temperatures down
to -273 °C and variable tempering stages can be
evaluated. Different kind of electrode-workpiece material
pairs should be subjected to cold and cryogenic
processes for the future EDM machinability researches.