Electric heating – Metal heating – By arc
Reexamination Certificate
2000-03-03
2001-11-13
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
C219S121850, C372S058000, C372S061000
Reexamination Certificate
active
06316744
ABSTRACT:
The invention relates to a machining head and to a process for the surface machining of workpieces by means of a laser beam, in which coating, alloying in the area close to the surface or dispersion of a peripheral zone of the base material with powder particles can be carried out using a pulverulent filler which is supplied, so that the geometry and/or the surface properties of a workpiece can be influenced in a controlled manner. The invention is particularly suitable for applications in which a relative movement takes place between the laser beam used and the workpiece, and this relative movement exhibits frequently changing directions. The solution according to the invention can be used for coating and regenerating tools, dies and components, such as for example for foundrywork, metal forming, cutting engineering and in the motor and turbine construction sector. Moreover, it is suitable for the machining processes which are covered by the terms “Rapid Prototyping” and “Rapid Tooling”.
In “Oberflachenbe-handlung mit Laserstrahlung” [Surface Treatment using Laser Radiation]; Springer-Verlag; 1998, pages 300 and 301, E. Beyer and K. Wissenbach have pointed out possible ways of pneumatically conveying powders and, inter alia, have also pointed out so-called coaxial nozzles, in which a laser beam can be directed onto a workpiece through such a partly hollow nozzle in order to heat the surface of this workpiece, and a powder-gas flow can also be directed onto the surface of a workpiece through such a nozzle, in the form of a conically tapering hollow jet.
In addition, U.S. Pat. No. 4,724,299 has disclosed a corresponding machining head which has a two-part housing. The two parts of this housing can be displaced telescopically with respect to one another, in order to make it possible to adapt the focal position of a powder jet to the focal position of the laser beam, so that this powder jet is heated even before it strikes the surface of a workpiece. In this case, both the powder jet and the laser beam have to pass together through a nozzle-like opening. However, this requires a suitable diameter of such a nozzle-like opening, and accordingly a cylindrical powder jet of appropriate size is directed onto the surface of a workpiece, so that the powder is supplied in excess and accordingly high powder losses are experienced.
In this solution, the powder is introduced together with a gas stream via powder inlets and it passes into a relatively large chamber through holes which are formed in a perforated disk. Turbulence is generated in the chamber, and particularly in the case of powders whose individual constituents have different densities, this may lead to segregation. From this chamber, the powder passes directly into a conical annular gap, the reduction in the cross section of which results in an increase in the conveying speed, which has an adverse effect on the utilization of the powder and the layer-forming operation. The turbulence in the chamber may result in changing pressure conditions and consequently also changing conveying speeds, so that the rate at which powder is conveyed may change over the course of time; this has an adverse effect particularly when applying layers or forming desired contours on such workpieces.
A cooling system is attached to the outside of the device described in that document, but is not necessarily able to cool sufficiently the particularly critical areas, and in addition the entire wall of the housing component lies between the cooling means and the interior.
The object of the invention is to provide a means which makes it possible to carry out surface machining by means of laser beams, with which means a powder is supplied and the mass flow rate of powder supplied can be kept virtually constant irrespective of direction and the hollow powder jet can be maintained at a uniform level.
According to the invention, this object is achieved by means of the features of claim
1
. Advantageous configurations and refinements of the invention result from the features given in the dependent claims.
A machining head according to the invention can carry out a relative movement with respect to the surface of a workpiece in at least two axes, and it accordingly is possible to move such a machining head on its own, the workpiece on its own or the machining head and the workpiece together, so that it is possible to achieve rapid, locally controlled surface machining by laying tracks, even with frequent changes of direction.
The machining head is constructed in such a way that it can be used to guide a laser beam so as to form an adjustable beam spot on the surface of a workpiece. A housing is designed suitably for this purpose, so that a cavity is present parallel to the longitudinal axis of the laser beam, by means of which cavity the laser beam can be directed onto the workpiece surface in a suitably focused manner.
Inside the housing, a turbulence chamber is formed in a rotationally symmetrical manner around the cavity through which the laser beam is guided and into which a powder-gas flow is introduced through at least one entry opening, preferably in the upper part of the turbulence chamber. In the turbulence chamber, the powder is distributed uniformly in the form of a cloud of powder which forms coaxially with respect to the laser beam, and the powder is mixed intensively. In its lower area, the turbulence chamber leads, preferably via an inlet funnel which tapers conically, into a plurality of stabilizing passages which are arranged radially symmetrically and parallel to the longitudinal axis of the laser beam, and are distributed at regular intervals over the circumference. The stabilizing passages then lead into an annular gap which tapers conically in the direction of the surface of the material and in which there are no ribs or other vortex-forming elements. The powder then leaves the annular gap, also as a conically tapering hollow powder jet, the focal point of this conically tapering hollow powder jet lying outside the housing and consequently also outside the machining head. Advantageously, the annular gap is designed in such a way, and has a corresponding angle of inclination, that, taking into account the distance to the surface of the workpiece, it forms a conically tapering hollow powder jet whose focal point lies directly on the surface of the workpiece.
In this case, the size of the focal point of the hollow powder jet should at least approximately correspond to the size of the beam spot of the laser beam on the surface of the workpiece, so that optimum utilization of the powder can be achieved.
The stabilizing passages arranged between the turbulence chamber and the annular gap, in particular their alignment parallel to the longitudinal axis of the laser beam and also their shape and dimensions, enable powder to be conveyed uniformly with very low fluctuations in the mass flow rate of powder of less than 5%, so that when the surface of a workpiece is correspondingly machined it is substantially possible to dispense with additional measures, such as controlling the laser power, and yet obtain a desired layered structure or to influence surface areas as desired by performing alloying or dispersion on a workpiece.
The length of the stabilizing passages should be at least 10 mm in order to achieve the desired effect.
Advantageously, there may also be two entry openings at the turbulence chamber, which are preferably arranged on radially opposite sides of the turbulence chamber. In this way, it is also possible to use powders of different compositions, which are then mixed homogenously in the turbulence chamber and can then be directed onto the surface as a homogenous mixture, via the stabilizing passages and the annular gap.
The stabilizing passages are expediently dimensioned in such a way that the sum of their clear cross-sectional areas is at least no less than the clear cross-sectional area of the one or more entry openings. This allows virtually constant pressure conditions and flow velocities to be achieved in the turbulenc
Nowotny Steffen
Scharek Siegfried
Barnes & Thornburg
Dunn Tom
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Stoner Kiley
LandOfFree
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