|
New Page 1
Analysis
of the Heat Affected Zone of Marble and Limestone Tiles Cut By Laser
Researched,
compiled and written by World Lasers, Inc. © 2007
The Marble Industry:
From time immemorial ornamental stones have been considered
a noble material in building and construction. Traditionally, Portugal, among
some other countries, is recognized for the high quality resources of these
ornamental stones, especially marble and limestone. Quarrying and transformation
of these stones is major industry for the Portuguese economy making sizable
contribution to its GNP. Two main groups of marble can be distinguished as
Calcitic and Dolomitic (their technical names). Marble is a crystalline rock
with a high level of metamorphism. Limestone has, basically, the same chemical
composition but a lower degree of crystallinity. While traditional marble
cutting and shaping is a much matured industry utilizing latest gadgets and
equipment for transforming the quarried stone, this industry still looks to new
improved ways that will make operations more efficient and reduce wastages. It
is in this connection that laser cutting is being investigated to evaluate its
potential in ornamental stone processing field. It is found that marble and
limestone can be cut by commercially available CO2 laser, the cut surface
quality being much dependent on the chemical compositions of the stone as also
on the degree of crystallinity the rock has been able to achieve.
Laser Cutting of
Marble:
Laser cutting of marble is a thermal process with rapid
localized heating of the stone with the formation of blue colored plasma in a
melt shearing process. The plasma forms a cavity and particles resulting from
stone’s decomposition absorb the energy radiation. Separation of stone parts
results when plasma and molten pool are blown away by the assist gas (high
pressure air) injected coaxially with the laser beam. Besides, the effect of
expelling the molten material, the assist gas also has a cooling effect on the
cut surfaces which decreases the extent of the heat affected zone. When the
laser beam is removed, rapid solidification occurs and a kind of glassy waste
substance (called vitrified dross) is seen attached to the cut surfaces edges.
This is due to the vapor pressure effect that ejects the molten material to the
surfaces.
The Laser Cutting
Experiments Using CO2 Laser:
Three different Portuguese stones were tested under the
commercial marks Moca Creme, Rosa de Borba and Branco Pardais.
Petrographic analysis (Analysis
of microscopic fluid inclusions within mineral grains)
of the stones was performed using optical microscopy for identification
of constituents and description of their morphology. Other petrographic
characteristics measured were the average grain size, the total porosity and the
average pore diameter. Average grain size was measured. Also, the hardness which
indicates the material’s mechanical behavior during processing, was measured.
Compressed air was used as coaxial gas at a pressure
of 5 bars. The laser beam and the co-axial gas passed through a conical copper
cutting nozzle.
Important
Observations:
Given below are some of the major observations from the
research experiments conducted when ornamental marble tiles samples were cut
using CO2 laser.
- High
energy CO2 lasers were found to be adequate for the thicknesses used
industrially.
- It was
found that the cutting speed with laser is very much dependent upon the laser
output power and on the tiles thickness. Increasing the laser power increases
the cutting speed. And increasing the tile thickness decreases the cutting
speed. For thicknesses of industrial interest, those between 10 and 30 mm, laser
power should not be below 2 kW. For linear cuts, the laser cutting speed was
found to be lower than the one obtained with diamond saws. However, if other
than straight cuts the cutting speeds can be comparable.
- It can
be concluded that marble and limestone tiles in industrially used thicknesses
can be cut by CO2 laser. Rock hardness and porosity affect the maximum cutting
speed and the surface quality. Moca Crème, a popular marble, exhibited the
highest cutting speed and the best cut surface quality. Cut marble surfaces
showed vitrified dross especially at the bottom cut edge made of a rapidly
solidified molten material.
- For
Moca Crème and Branco Pardais tiles of 10 mm thickness, the cutting speed of
about 25 inches/minute is observed. This is halved when the plate thickness is
doubled. However, when the plate thickness increases, two major observations
were made: the first one is that there is a small difference in the cutting
speed from 20 to 30 mm thick plate suggesting that there is a critical thickness
above which the speed is almost constant. The second is that Rosa de Borba type
exhibits lower cutting speeds than Branco Pardais. This can be explained due to
the fact that Rosa de Borba has a lower total porosity than Branco Pardais, that
is, the total amount of material to melt is smaller. This effect is specially
noticed in thicker tiles.
- Another
major observation was that tested samples exhibited a color change along the
plane of cutting, that is, the cut surface presented a lighter color. Cracks
form as a consequence of the sharp thermal cycle.
- There
is formation of pores on the surface of the cut fronts. Both pores and micro
cracks can negatively influence the physic-mechanical properties of the stones.
- One
interesting observation was that tiles cut by laser were seen to degrade with
time, that is, after a certain time in air environment, the cut surfaces were
more brittle and cracked.
- When the
heat input is very high, that is when laser beam power is high and cutting speed
is low, decomposition of CaO (burning of calcite) occurs and formation of
metallic calcium as observed.
|
