Method for determining the cut quality of a laser cutting process using a simulation program

Patent metadata

Field	Value
Publication number	US-9002683-B2
Application number	US-201113521804-A
Country	US
Kind code	B2
Filing date	Jan 25, 2011
Priority date	Feb 8, 2010
Publication date	Apr 7, 2015
Grant date	Apr 7, 2015

Abstract

The application relates to a method for determining the cut quality of a laser cutting process, said quality being assessed on the basis of the formation of solidification ridges along the cut face and/or burr formation on the lower edge of the cut face. In said method, a virtual laser cutting machine in a simulation program can be virtually operated with a set of values P0 from a parameter space P.; In a step a), the parameter set P0 is entered in the virtual cutting machine ( 103 ), then in step b), a cut is made in the virtual workpiece by calculating, from partial differential equations with initial and boundary values, the progression of the melt film thickness over time and the position of the melt front at the apex of the cutting front according to the depth of the cut and the time, and then in step c), the spatial distribution of the ridge amplitude on the cut face is specified by projecting the progression of the absorption front onto the cut face and/or a spatial distribution of the burr is calculated from the progression of the melt film thickness over time and the discharge speed on the lower edge of the cut face, and in step d), a virtual cut quality is provided ( 104 ) for further analysis.

First claim

The invention claimed is:

1. A method for determining a cut quality of a laser cutting process, said quality being assessed on a basis of a formation of solidification ridges along a cut face of a sheet metal workpiece and/or a burr formation on a lower edge of the cut face, whereby in a simulation program a virtual laser cutting machine is provided that can be operated virtually with a set of values P 0 from a parameter space P,
wherein the parameter space P is defined by
P =(λ Laser ,l 0 ( t ), f ( x,z,t ), s ( x,z,t ), p ( x,z,t ), P g ( x,z,t ),τ g ( x,z,t ), v 0 ( t ), d,P Material ), where

P Material =(ρ s ,c ps ,λ s ,ρ l ,c pl ,λ l ,H m ,H v ,T m ,T v ,η,σ,n cs ,n cl ,

where
λ Laser represents a wavelength of the laser radiation,
l 0 (t) the maximum value of the laser radiation intensity over time,
f(x,z,t) represents a spatial distribution of the laser radiation intensity over time,
s (x,z,t) represents a spatial distribution of the direction of the laser radiation over time,
p (x,z,t) represents a spatial distribution of the polarization state of the laser radiation over time,
P g (x,z,t) represents a cutting gas pressure over time,
τ g (x,z,t) represents a shear stress of the cutting gas over time,
v 0 (t) represents a feed rate, defined as the relative velocity between the laser beam axis and the workpiece over time,
d represents a thickness of the sheet metal to be cut,
ρ s represents a density of a workpiece material to be cut in the solid state,
c ps the specific heat capacity of the material in the solid state,
λ s represents a thermal conductivity of the material in the solid state,
ρ l represents a density of a laser cut melt of the workpiece,
c pl represents a specific heat capacity of the melt,
λ i represents a thermal conductivity of the melt,
H m represents a melting enthalpy of the material to be cut,
H v represents a evaporation enthalpy of the material to be cut,
T m represents a melting temperature of the material to be cut,
T v represents a evaporation temperature of the material to be cut,
η represents a dynamic viscosity of the melt,
σ represents a surface tension of the melt,
n cs represents a complex refractive index of the material in the solid state,
n cl represents a complex refractive index of the melt,
and where
x represents a coordinate in the direction of the relative movement between the material and the laser beam axis and
z represents a coordinate perpendicular to the top of the material, and
t represents a time,
said method comprising the following steps in order to start the virtual cutting machine:
(a) specifying a starting point P 0 by acquiring the machine parameters of a current, real cutting machine, wherein the set of values P 0 is selected from a parameter space P, as defined above, and the parameter set P 0 is entered into the virtual cutting machine for the sequence of the simulation program,
and thereafter starting the simulation program by
(b) making a virtual cut with the virtual cutting machine that may be based on real values, wherein the progression of melt film thickness over time h=h(z,t) and the position M=M(z,t) of the melt front at the apex of the cutting front is calculated according to the depth of the cut z (0<z<d, d sheet metal thickness) and the time t from partial differential equations PDE normalized to v 0

∂
h

∂
t

+

G
⁡

(

z
,

t
;
h

,

P
0

)

⁢

∂
h

∂
z

+

D
⁡

(

z
,

t
;
h

,

P
0

)

=

v
p

,

∂
M

∂
t

=

v
p

-
1

with initial and boundary values
h ( z,t= 0)= h 0 ( z ), M ( z,t= 0)= M 0 ( z )
h ( z= 0, t )=0, M ( z= 0, t )= m 0 ( t;P 0 )

where h 0 (z) and M 0 (z) represent any initial distributions, m 0 (t;P 0 ) the position of the upper edge of the cutting front, v p =v p (z,t) the velocity of the melt front, v s =G(z,t; h, P 0 ) the flow velocity at the surface of the melt and D(z,t;h,P 0 ) a damping of the melt film dynamics, for a given parameter set P 0 is calculated from P,
and thereafter
(c) by a projection of the time course of the absorption front, defined as the position M(z,t)−h(z,t), onto the cut face with a transfer function determined by the feed rate v 0 that depicts t on x, specifying the spatial distribution of the ridge amplitude R(x,z) on the cut face and calculating a spatial distribution of the burr B(x) at the lower edge of the cut face from the time course of the melt film thickness h(z=d,t) and the outflow velocity G(zd,t; h(z=d,t), P 0 ) at the lower edge of the cut face, and
(d) providing a virtual cutting result comprising at least one of R(x,z) and B(x), for further assessment,
whereby in an additional step values of the parameters from the parameter space P of the laser cutting process are determined for the purpose of achieving defined cut faces and/or for determining the cut faces that can be achieved with a specified laser cutting machine and/or for configuring components of an optimised laser cutting machine that meets or at least approximates the specifications regarding stated requirements on the cut face, by repeating steps a) to d) each time with changed parameters from the parameter space P,
and wherein rules are derived for the continued and/or new development of components of a real laser cutting machine from the values of the parameters from the parameter space P of the laser cutting process for obtaining defined cut faces that meet or at least approximate the specific

Assignees

• Schulz Wolfgang
• Schuettler Jens
• Vossen George
• Niessen Markus
• Fraunhofer Ges Forschung
• Rwth Aachen

Inventors

• Schulz Wolfgang
• Schuettler Jens
• Vossen George
• Niessen Markus

Classifications

• B23K26/38

  by boring or cutting · CPC title

• G06F2119/08

  Thermal analysis or thermal optimisation · CPC title

• G06F30/20

  Design optimisation, verification or simulation (optimisation, verification or simulation of circuit designs G06F30/30) · CPC title

• B23K26/06 Primary

  Shaping the laser beam, e.g. by masks or multi-focusing · CPC title

• G06F2119/06

  Power analysis or power optimisation · CPC title