Electric heating – Metal heating – By arc
Patent
1996-02-26
1998-03-31
Walberg, Teresa J.
Electric heating
Metal heating
By arc
21912184, B23K 2614
Patent
active
057341460
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a cutting process by means of a laser beam.
BACKGROUND ART
As it is known, laser cutting, e.g. of a metal plate, is usually performed with the aid of a jet of gas directed on to the cutting area so that the liquefied material may be easily flushed out by means of both fluid and mechanical actions originating from the impact of the gas with the material.
Oxygen is generally employed as such in cutting ferrous materials, due to the noticeable iron oxidation exothermic reaction produced at high temperatures (over 720.degree. C.) of the combustion type; if properly employed, the energy so produced, along with the fluid-thermodynamic effects, may result, for a given laser power, in higher cutting speed, improved flushout of the liquefied material and more accurate surface finish of the cutting faces.
Results have so far been unsatisfactory as to both cutting speed and quality.
Indeed, cutting processes currently in use allow for limited cutting speed (a few meters or tens of meters per minute), surface finish of the cutting faces being often unsuitable owing to undesired geometrical and/or metallurgical properties (scoring, droplets of liquified material, hardened areas).
Actually, the phenomena taking place in the cutting area are highly complex ones, due to the presence of a fluid-thermodynamic field, non stationary by nature, in that it is caused by the interaction between a gas and the material being cut; the gas is therefore subjected to intense heat by both laser beam and liquified material, as well as increased by the mass of the material in the form of liquid droplets varying in size, which may be converted into steam; the latter may in turn be energized or even ionized.
If the gas, e.g., oxygen, is reactive, the iron oxidation (combustion) exothermic reaction triggered at temperatures exceeding 720.degree. C. further emphasizes the above phenomena, causing their instability in time to become more markedly so.
Not only are commonly used laser cutting processes affected by such phenomena, they also amplify them, owing to the improper management of both gas and other process parameters.
For instance, in most known laser cutting equipment oxygen is conveyed by means of a conical nozzle on to the cutting area, with no separation between it and the laser beam, the latter being focused through said nozzle. The drawbacks connected with such method are manyfold.
Firstly, oxygen consumption is too high, most of it impinging on the surface of the material surrounding the sides of the cut, thus failing to flash out the liquified material. Moreover, the gas jet, freely directed on to the cutting surface, shows instability as to both direction and speed, which amplifies the alterations in flow caused by both internal aerodynamic factors (boundary layers and related reduction of the duct section already at its entry) and external ones (atmospheric air mix and drag).
The instability of a freely directed gas jet makes it more difficult for it to enter and penetrate the cut, thus aggravating the chocking phenomenon, of the viscous and thermal kind, typical of subsonic and supersonic jets in a duct. In the portion of the duct downstream from the chocking section, speed is significantly reduced. Traditional cutting faces, therefore, present a longitudinal line (i.e., parallel to the top and bottom surfaces of the workpiece), imputable to the chocking effect and, up-and downstream from such a line, a series of differently sloping score lines, indicative of a change in flow speed. In particular, the slope of such score lines is greater downstream from the chocking line, thus pointing to a flow speed reduction.
A significant reduction in flow speed also results in variations of scoring frequency, as well as in erosion accompanied by droplets of liquefied material. Such phenomena being caused by the uncontrolled exothermic reaction produced by greater penetration of the face by the isotherm (about 720.degree. C.) triggering said reaction.
According to the above method, oxyg
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Mills Gregory L.
Walberg Teresa J.
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