Thermal Aspects Material Considerations and Cooling Strategies in Cryogenic Machining-Libre
Thermal Aspects Material Considerations and Cooling Strategies in Cryogenic Machining-Libre
Thermal Aspects Material Considerations and Cooling Strategies in Cryogenic Machining-Libre
107
Because the high temperatures associated with the cutting
process reduce the tool life, the machining industry has
been using oil-based or water-based emulsion cutting
fluids to cool and lubricate the machining process.
Unfortunately, conventional cutting fluids create both
health [Beattie and Strohm 1994] and environmental
problems, adding to production costs. The National Institute for Occupational Safety and Health estimates that
more than 6 million workers are exposed to mineral oil,
and that approximately 1.2 million are exposed in the
metal cutting fluid application [NIOSH 1977]. Long term
exposure to cutting fluid can cause dermatitis, a skin
disorder common in the machining industry [Bennett
1992]. Dermatitis can range from an ugly rash to malignant cancer. In Ohio, line operators in a major automobile plant reported to the author that 30% of their
machining operators had developed dermatitis of various
degrees; two required hospitalized. Inhaling cutting fluid
vapor may cause lung disorders as well [Kennedy et al.
1989]. Because conventional cutting fluids are non-biodegradable, manufacturers must comply with environmental
regulations to dispose of them. Furthermore, manufacturing materials contaminated with the cutting fluid must
be treated before being disposed. The disposal of cutting
fluids now costs at least double the purchasing price in
the United States and four times its price in Europe.
Manufacturers need an environmentally safe new
1
machining process that is energy efficient, yet able to
Introduction
Metal cutting, or machining, which represents an annual improve tool life, increase productivity, and reduce
$150 billion output in the United States, is widely used in production cost.
nearly all manufacturing industries, including the
Cryogenic machining offers a potential solution to the
aluminum, steel, automobile, and aerospace industries.
environmental and health concerns of conventional
machining. This process replaces the cutting fluid coolant
with super cold liquid nitrogen. Nitrogen, which is abundant (79% of air), is naturally recycled without damage
Received: 25 November 1998 / Accepted: 12 February 1999
to the environment. Nitrogen can be compressed to a
liquid, and as liquid it cools the cutting process, evapoS.Y. Hong, Z. Zhao
rates and becomes part of the air again. Nitrogen can
Columbia University, Department of Mechanical Engineering,
sufficiently lower the machining temperature to reduce
New York, NY 10027, USA
the tool wear and increase the tool life. However, to be
e-mail: sh2956columbia.edu
effective and economical, cryogenic machining must be
Correspondence to: S.Y. Hong
done under specific conditions.
The purpose of the machining process is to transform
This work is supported by National Science Foundation
the material into a desirable shape by using a cutting
(DMI-97-96089), the Edison Materials Technology Center
tool. The material properties of the workpiece and of the
(CT-32) and industrial support from GE Aircraft Engines,
tool determine the effectiveness in the cutting process.
GM-Delphi Chassis, Cincinnati-Milacron, Kennametal, and the
BOC Groups. The authors wish to express their gratitude to Mr. Because heat is a natural byproduct of the cutting
process, understanding the effect of temperature on the
D. Siddle and his group at Kennametal research laboratory for
material properties will help researchers and manufactheir support during the testing.
Abstract Conventional machining prolongs tool life by
using cutting oils to cool the metal cutting process.
Unfortunately, the cutting fluid contaminates the environment, and endangers the health of humans. Cryogenic
machining offers an environmentally safe alternative to
conventional machining by using liquid nitrogen, which
can be naturally recycled. However, for the cryogenic
machining process to be effective and economical, manufacturers must select the correct cooling approach. This
paper describes our experimental study to investigate the
cryogenic properties of some common cutting tool materials and five workpiece materials of industrial interest:
low carbon steel, AISI 1010, high carbon steel AISI 1070,
bearing steel AISI 52100, titanium alloy Ti-6Al-4V, and
cast aluminum alloy A390. The paper addresses the major
aspects of heat generated in metal cutting in terms of its
effects on chip formation, tool wear, and on the functional integrity of the machined component. The paper
then discusses the cooling strategies for cryogenic
machining each material based on the thermal effects and
material properties. The investigators conclude that the
cooling approach must be finely adjusted for different
materials to obtain the optimum effectiveness in cryogenic machining. The goal of our study is to provide a
basis for designing the cryogenic machining system.
108
3
Cryogenic machining and cooling strategies
Fig. 1. Heat generation zone and cutting temperature distribution using example in dry cutting low carbon steel
Shane Y. Hong, Zhibo Zhao: Thermal aspects, material considerations and cooling strategies in cryogenic machining
Table 1. Nominal compositions of the tested carbide tool materials [provided by Kennametal, Inc.]
K3109
K313
K420
K68
SP274
Cobalt
(wt%)
Ta
(wt%)
Ti
(wt%)
Other
(wt%)
Carbide
Size
12.2
6.0
8.5
5.7
5.85
0.3
0
10.2
1.9
5.2
0
0
5.9
0
2.0
0
(Cr:0.4%)
0
0
0
Large
Fine
Large
Medium
Medium
109
110
5
Properties of workpiece materials:
case studies of cooling strategies
Shane Y. Hong, Zhibo Zhao: Thermal aspects, material considerations and cooling strategies in cryogenic machining
5.1
Materials and experiments
Five workpiece materials were utilized in this study: low
carbon steel 1010, high carbon steel 1070, bearing steel
E52100, high silicon aluminum alloy A390, and titanium
alloy Ti-6Al-4V. These materials represent a spectrum of
the typical materials used in automobile and aerospace
industries of special interests to our sponsors. Their
compositions are given in Table 2. Essentially, the
specimen preparation and the procedures for these
mechanical testing meet ASTM standards. For low
temperature experiments, a tensile tester, a Rockwell
tester and an impact tester were modified so that a liquid
Fig. 4. Strength and hardness of 1010 versus temperature
container could be attached. Liquid chemicals, such as
alcohol and isopentane, that were cooled by liquid
nitrogen were employed as the cooling media.
Machining tests on some of the workpiece materials
were conducted using a CNC turning machine. The evaluated machining characteristics included cutting force,
rate of tool wear, chip breakability, and surface finish of
the machined surface.
5.2
AISI-SAE 1010
AISI-SAE 1010 is a very soft, mild steel of low strength
and high ductility. In spite of its low hardness, it has a
poor machinability due to its difficulty in chip breaking.
The adhesive wear of the cutting tool, the formation of
built-up edges at moderate cutting speeds, and its particularly poor chip breakability represent the machining
characteristics of this steel and most other low carbon
steels.
The microstructure of 1010 steel consists of two
micro-constituents, ferrite and pearlite, which accounts
for the soft, ductile, sticky nature of this material.
Figures 4 and 5 show the mechanical properties of 1010
steel at cryogenic temperatures. Hardness and strength
increase as the temperature decreases. The average room
temperature hardness is only Rgp54, (corresponding to
Rbp83). Even at liquid nitrogen temperature, the hardness of 1010 only reaches a level of Rcp27. Due to the
existence of the BCC (body center cubic) structure of
ferrite, the impact strength of 1010 exhibits a sharp transition from ductility to brittleness when the temperature
is decreased to about 50 7C. While elongation and reduc-
111
112
5.3
AISI 1070 and AISI E52100 steels
tures. Unlike the 1010 steel, 1070 and E52100 alloys have
no distinctive ductility to brittleness transition. Instead,
impact strength, elongation and reduction in area change
gradually.
Shane Y. Hong, Zhibo Zhao: Thermal aspects, material considerations and cooling strategies in cryogenic machining
113
114
chemical affinity for tool materials at operating temperatures, an effective cryogenic strategy is to cool the workpiece and cutting tool simultaneously. This is expected
to:
lower the cutting temperature,
enhance the chemical stability of the workpiece,
enhance the hardness and chemical stability of the
tool, and
reduce the friction at the work/tool interface and tool/
chip interface.
Titanium and its alloys have been a major subject of
cryogenic machining studies. Most of the work indicates
an improvement in their machinabilities by either
freezing the workpiece or cooling the tool using a cryogenic coolant. [Uehara 1968, 1970, Dillon 1990, Holis 1961,
Reed 1965, Christopher 1990]. Freezing the workpiece was
suggested as the ideal cutting condition for machining
titanium and its alloys from a technical perspective
[Dillon 1990, Rennhack 1974, Xuan 1991]. However, due
to its increased strength and hardness, frozen titanium
negatively affects the machining process. Therefore, if an
effective coolant were available for lowering the tool
temperature, it would be more appropriate to use than to
cool the workpiece.
Titanium and its alloys represent the most challenging
materials in machining. With the advancement in cutting
tool materials, many difficult-to-machine materials can
Shane Y. Hong, Zhibo Zhao: Thermal aspects, material considerations and cooling strategies in cryogenic machining
6
Summary
References
Fig. 14. The SEM Photograph of the rough side of Ti-6Al-4V
chip cut under cryogenic machining conditions
115
116