Electric heating – Metal heating – By arc
Reexamination Certificate
1997-05-20
2003-09-16
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121720
Reexamination Certificate
active
06621040
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of lasers to cut metals, and more specifically, it relates to the use of ultrashort laser pulses for machining metal.
2. Description of Related Art
Conventional mechanical lathes and machine tools (e.g., slitting saws) are effective for cutting metals down to approximately 100 microns kerf width at depths on the order of 1 millimeter (aspect ratio <10:1). Below this level, electron beam or laser tools are typically used for cutting or high precision machining (sculpting, drilling). Both electron beam and existing industrial laser technology remove material by a conventional thermal process where the material to be removed is heated to the melting or boiling point. Laser processing by molecular dissociation in organic (and some inorganic) materials can be achieved with excimer lasers but this photodissociation mechanism is not applicable to metals.
The basic interaction in localized thermal processing, as is achieved with electron beam or current state of the art lasers, is the deposition of energy from the incident beam in the material of interest in the form of heat (lattice vibrations). Cutting efficacy and quality may differ strongly between metals dependent upon the thermomechanical properties of the metal. Laser absorption is also dependent upon the optical properties of the metal of interest. The laser energy that is absorbed results in a temperature increase at and near the absorption site. As the temperature increases to the melting or boiling point, material is removed by conventional melting or vaporization. Depending on the pulse duration of the laser, the temperature rise in the irradiated zone may be very fast resulting in thermal ablation and shock. The irradiated zone may be vaporized or simply ablate off due to the fact that the local thermal stress has become larger than the yield strength of the material (thermal shock). In all these cases, where material is removed via a thermal mechanism, there is an impact on the material surrounding the site where material has been removed. The surrounding material will have experienced a large temperature excursion or shock often resulting in significant change to the material properties. These changes may range from a change in grain structure to an actual change in composition. Such compositional changes include oxidation (if cut in air or, in the case of alloys, changes in composition of the alloy. This affected zone may range from a few microns to several millimeters depending on the thermomechanical properties of the metal, laser pulse duration and other factors (e.g., active cooling). In many applications, the presence of the heat or shock affected zone may be severely limiting since the material properties of this zone may be quite different than that of the bulk. Furthermore, devices with features on the order of a few tens of microns cannot tolerate the thermal stress induced in the material during the machining process. Even the slightest thermal stress or shock can destroy the feature of interest.
Another limitation of conventional laser or electron beam processing in high precision applications is the presence of redeposited or resolidified material. As mentioned previously, cutting or drilling occurs by either melting or vaporizing the material of interest. The surface adjacent to the removed area will have experienced significant thermal loading often resulting in melting. This melting can be accompanied by flow prior to solidification as shown in FIG.
1
A. This can result in the deposition of slag surrounding the kerf. In many high precision applications, the presence of slag is unacceptable. In the cases where the deposition of conventinal slag can be prevented, redeposition of vaporized material on the walls or upper surface of the kerf is common. This condensate often reduces the quality of the cut and decreases the cutting efficiency since the beam must again remove this condensate before interacting with the bulk material underneath.
FIG. 1A
shows a top view of stainless steel cut with a conventional infrared (1053 nm) laser operating at a pulse duration >1 nsec. The presence of resolidified molten material (slag) and poor single pass cut quality indicative of laser cutting by conventional methods is readily apparent.
Many of these limitations can be reduced by the use of secondary techniques to aid the cutting process. The most common of these are active cooling of the material of interest either during or immediately following the laser pulse, and the use of high pressure gas jets to remove vaporized or molten material from the vicinity of the cut to prevent redeposition. These techniques can be effective at improving the kerf at the cost of a significant increase in system complexity and often a decrease in cutting efficiency.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for laser cutting/machining of metals and alloys which achieves high machining speed with extreme precision, negligible heat affected zone, and no modification to the material surrounding the kerf.
The method involves the use of very short (10 femtoseconds to approximately 100 picoseconds) laser pulses delivered at high repetition rate (0.1 to over 100 kHz). Although the absorption mechanism for the laser energy is the same as in the case of long pulse lasers, the short (<100 psec) duration offers a simple and striking advantage. By adjusting the pulse duration such that the thermal penetration depth during the pulse, L
th
=2&agr;&Sgr;(&agr;=k/&rgr;c
p
is the thermal diffusivity, k is the thermal conductivity, &rgr; is the density, c
p
is the heat capacity and &tgr; is the duration of the laser pulse] is less than one micron, very small amounts of material (0.01-1 micron) can be removed per laser pulse with extremely small transport of energy either by shock or thermal conduction away from the volume of interest. This offers extremely high precision machining with a negligible (submicron) heat or shock effected zone. For example, type 304 stainless steel exhibits a thermal penetration depth of only 1.5 nm for a 100 femtosecond pulse compared to an optical penetration depth of approximately 5 nm. In this case, the electric field of the laser penetrates more deeply into the steel than the thermal wave during the pulse. Hence, the depth of material removal is determined solely by the intensity and wavelength of the laser, and the absorption and heat capacity of the metal—the effects of heat conduction and thermal shock are eliminated.
The lack of significant energy deposition beyond the volume of interest achieved by using these ultrashort pulses enables the use of high repetition (0.1-100 kHz) lasers without the need for external cooling of the part being machined. Even though only a very small depth of material is removed per pulse, the high repetition rate enables extremely high cut rates (beyond 1 mm depth per second).
Cut quality and cut efficiency with these ultrashort pulses can be significantly higher than that achievable for conventional long pulse lasers. This follows from two critical features: 1) there is little loss of energy away from the region of interest since thermal conduction during the pulse is negligible and 2) there is no vaporization or transport of material during the pulse. The second of these features may require additional explanation. During the pulse, there is insufficient time for hydrodynamic expansion of the vaporized material. As a result, the laser pulse encounters the solid surface for the duration of the pulse, depositing energy into the solid density material and raising a depth to a temperature far beyond the boiling point (typically to temperatures above the ionization point). After the pulse is over, the depth which has been raised above the boiling point leaves the surface with an expansion velocity determined by the initial temperature. Typical temperatures in the expanding plasma are between 1 and 100 eV and are determined b
Perry Michael D.
Stuart Brent C.
Heinrich Samuel M.
The Regents of the University of California
Thompson Alan H.
Wooldridge John P.
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