Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
  • B23K26/03—Observing, e.g. monitoring, the workpiece
  • B23K26/032—Observing, e.g. monitoring, the workpiece using optical means
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K2101/00—Articles made by soldering, welding or cutting
  • B23K2101/18—Sheet panels
  • B23K2101/185—Tailored blanks

Definitions

• the present invention relates to a method for measuring and controlling a laser welding process, in which - a workpiece is welded with the aid of a laser, the light intensity above the weld pool is measured with the aid of an optical sensor, the value measured by the optical sensor is compared with a defined reference value, and the energy supplied to the weld pool per unit length is adjusted as a Junction of the difference between the measured value and the determined reference value.
• US Patent 5,674,415 has disclosed a method and a device for measuring the quantity of emitted infrared light which is released during the welding process during the welding. Then, the values collected during welding are used to monitor the quality of the weld which is formed. For example, there is a relationship between the quantity of TR light emitted and weld depth achieved.
• the quantity of infrared light which is released during welding with a defined penetration depth is collected again and is compared with the predetermined set data. Then, by making a comparison between the predetermined data and the data obtained during the welding, it is possible to check whether the weld which has been formed satisfies quality requirements,
• a significant drawback of the method according to the prior art is that this known method can be used to carry out a retrospective check. If a defect occurs in the weld which is formed during welding, this defect can be detected and the weld can be rejected. For example, in the device according to the prior art there is an alarm which can detect irregularities in the weld pool.
• the known method and the known device are not designed to adjust the welding parameters in order to prevent welding defects while the weld is being formed.
• the object of the present invention is to provide a method of the type described in the preamble in which, during welding, the welding parameters used can be adjusted in real time, i.e. while the weld is being formed, in order as far as possible to prevent the occurrence of welds of lower quality.
• this object is achieved by the fact that the reference value is determined by allowing the energy supplied to the weld pool per unit length to increase from a relatively low starting level to decrease from a relatively high starting level until a step change in the measured light intensity from a relatively high level to a relatively low level or vice versa is measured after which the intensity measured by the optical sensor is used as a control signal for the quantity of energy which is to be supplied to the weld pool, the energy which is to be supplied to the weld pool per unit length being allowed to increase when the optical sensor measures a relatively high level of the light intensity and the energy to be supplied to the weld pool per unit length being allowed to decrease when the optical sensor measures a relatively low level of the light intensity.
• TMBs welding tailor made blanks
• the tailor made blanks comprise, for example, various plates of different wall thicknesses which are welded to one another. In a subsequent process, the plates may be exposed to a high mechanical load, for example in a high-pressure press. These mechanical operations represent a heavy test of the welds which have been formed in the material, i.e. high quality demands are imposed on the welds.
• the method according to the present invention it is possible to continuously measure the light intensity above the weld pool. If it is noticed, while the quantity of energy which is to be fed to the workpiece per unit length is being increased, that the light intensity falls suddenly, it will be evident that the state of complete penetration has been reached. Then, the quantity of energy which is to be fed to the workpiece per unit length can be allowed to decrease slightly. At the moment at which the state of complete penetration changes back to the state of incomplete penetration, the measured light intensity of the weld pool will increase suddenly. This step turn increase can be used as an input signal for the system used in order for the quantity of energy to be supplied to the workpiece per unit length to be increased again.
• energy per unit length is used many times in the present text. This term is to be understood as meaning the quantity of energy which is supplied to the workpiece by the laser per unit length of the weld which is to be formed. The term “energy per unit length” can also be understood as meaning the term "heat input”.
• the method according to the present invention results in a weld being applied to the workpiece with complete penetration with just enough power being supplied to the workpiece, Te. either the quantity of energy used can be kept relatively low or the speed applied to the system can be kept relatively high, while at the same time the optimum quality of the weld is still ensured, According to the invention, it is advantageous for the light intensity to be measured periodically and for the amount of energy to be supplied per unit length to be correspondingly adjusted periodically.
• a significant aspect of the present invention is that the process conditions can be adjusted in real time, i.e. while the weld is being formed. To be able to achieve this, it is important for the light intensity above the weld pool to be measured periodically. The welding parameters can then be adjusted if necessary as a function of the measured value.
• the production rate can be optimized by using the method according to the invention to determine the optimum quantity of energy which needs to be supplied to the workpiece per unit length.
• the processing speed of the workpiece can then be optimized by setting the power of the laser installation to a value which is close to the maximum value and then adjusting the rate at which the workpiece is processed to this set value of the laser. This "maximum value" is the maximum power which the laser can supply.
• the adjustment of the processing speed can also take place in real time.
• the period between two successive adjustments is selected to be between 0.01 and 0.10 ms, preferably 0.03 and 0.07 s, more preferably 0.05 ms.
• the quantity of energy supplied per unit length of the workpiece is increased instantaneously.
• the molten material has not yet had sufficient time to react (melt to a greater or lesser extent), and the additional quantity of energy supplied means that complete welding over the entire length of the weld is ensured,
• the quantity of energy to be supplied to the weld pool per unit . length can be allowed to decrease in relatively small steps, after which, at the transition to the relatively high light intensity, the quantity of energy to be supplied to the weld pool per unit length can be allowed to increase in relatively large steps,
• the quantity of power added to the laser can be changed.
• the effect of this measure is that there is no need to adjust the settings of the laser.
• the weld can be made with a fixed setting of the laser and the associated equipment.
• the quantity of energy which is to be supplied to the weld pool per unit length can easily be controlled by determining the speed of the laser with respect to the workpiece.
• a second aspect of this measure is that the weld can be made to move over the workpiece at a maximum speed. This means that relatively little time is needed to form a weld and that time can be saved during the formation of a weld.
• a plume which comprises hot metal vapour
• This plume and the weld pool emit light in the spectrum which is visible to humans.
• This radiation has a wavelength of 400-600 nm.
• the molten material will emit infrared light with a wavelength of approximately 800 nm and above.
• Nd:YAG laser it is possible to use an Nd:YAG laser.
• a sensor which is specifically suitable for hght with a wavelength of 1064 nm.
• a weld for example on a tailor made blank, can be formed using an Nd.YAG laser. This laser emits light with a wavelength of 1064 nm. During welding, the reflection of the light of this specific wavelength can be measured using a third, specially designed sensor. This further helps with accurate determination of the transition from complete penetration to incomplete penetration.
• the present invention relates to a method for welding a tailor made blank (TMB) in which the method according to the present invention is used.
• TMB tailor made blank
• the present invention also relates to a device for measuring and controlling a laser welding process, provided with a laser, such as an Nd: YAG laser and control means for controlling the said laser.
• a laser such as an Nd: YAG laser
• control means for controlling the said laser.
• the said control means comprise an optical sensor for measuring the Hght intensity above the weld pool and calculation means for instantaneously determining the difference between the light intensity measured by the optical sensor and a defined reference value, the said calculation means being operatively connected to the said control means of the laser in order to increase or decrease the quantity of energy which is to be fed to a workpiece by the laser per unit leng t h (heat input) as a function of the difference determined by the said calculation means.
• the control means it is advantageous for the control means to be designed to increase the quantity of energy to be supplied to a workpiece per unit length when the quantity of light measured by the optical sensor is higher than the reference value.
• Figure 1 diagrammatically depicts a device which can be used in the method according to the present invention
• Figure 2 shows a more detailed view of the laser head 1 according to the present invention
• Figures 3a and 3b show the result of the measured light intensity above the weld pool and the associated degree of penetration during welding with a fixed value for the set power of the laser, without using the method and device according to the present invention
• Figures 4a and 4b show a similar experiment to that shown in Figures 3a and 3b, but Figures 4a and 4b involve direct control of the laser power used, in accordance with the method according to the present invention.
• Figure 1 diagrammatically depicts the device according to the present invention which can be used to carry out the method according to the present invention.
• the device comprises a laser head 1, with the aid of which a workpiece 2 can be welded.
• the laser head is powered by means of an optical cable 11.
• This optical cable 11 is connected to a laser installation 12. hi use, there will be a plume 3 between the laser head 1 and the workpiece 2.
• Light which is released during the welding of the workpiece can be partially diverted by means of a mirror 4 which is arranged in the laser head 1.
• Figure 1 shows a first possible configuration for the positioning of sensors 17, 18 and 19 for picking up the light deflected via the mirror 4.
• the light reflected by the sensors can be fed to control means 13.
• These control means 13 can then act on the laser installation in order to adjust the settings of the laser installation in such a way that complete weld penetration of the workpiece 2 continues to be ensured during welding. The precise way in which the installation operates is explained in more detail below.
• FIG. 2 diagrammatically depicts a laser head 1 which is used to weld a workpiece 2.
• This workpiece 2 is, for example, a tailor made blank.
• a so-called plume 3 is formed at the bottom of the welding head 1, between the welding head 1 and-the workpiece 2, During welding, the presence of the plume 3 means that light With a high intensity will be present above the workpiece 2,.
• the quantity of light which is present above the workpiece 2 is partially reflected and picked up in the laser head 1 in accordance with Figure 2.
• the reflected light is diverted via what are known as beam splitters 5 and 6 in order to allow the reflected light to be fed to a first and a second sensor 7, 8.
• a camera 9 may also be added to the arrangement.
• a separate sensor 10 it is possible for a separate sensor 10 to be added in the vicinity of the laser process.
• This sensor may be designed, for example, as a camera and may be adapted to capture a defined quantity of light of a predetermined wavelength. It is possible to arrange a further sensor, which measures the light coming from the weld pool via the optical cable, in the laser installation.
• the laser light is conveyed via an optical cable 11 (fibre) to the welding head. It is possible for a further sensor to be arranged in the cable 11 in order to measure light which is emitted by the weld pool.
• control means which can process the signals intercepted by the various sensors 7, 8 and 10 into input signals for the laser.
• the quantity of power supplied to the laser head 1 and therefore the quantity of energy supplied to the worl ⁇ iece is varied as soon as the light intensity above the weld pool undergoes a step change.
• Figures 3a and 3b shown the results of a welding experiment, in which a weld is formed in a workpiece at various speeds, The length of the weld is plotted in millimetres on the x-axis. The power consumed by the laser is plotted on the y-axis. In Figure 3b, the length of the weld in millimetres is once again plotted on the x-axis. In this case, the light intensity above the weld pool which is measured by one of the sensors is plotted on the y-axis. The various peaks and valleys of the measured light intensity indicate that in the event of a change from complete penetration to incomplete penetration during welding, the measure light intensity changes suddenly.
• Figures 4a and 4b show the results of a second welding experiment, in which once again a weld with a length of 90 millimetres is formed.
• Figure 4a the total length of the weld which is formed is plotted on the x-axis.
• the associated quantity of power is plotted on the y-axis.
• Figure 4b the measured light intensity is once again plotted against the length of the weld.
• the associated quantity of energy which is sufficient to achieve complete penetration is, per unit length, lower than 1000 watts, as can be seen from Figure 4a.
• the speed is increased (after approximately 50 millimetres)
• the power which is fed to the laser and therefore the energy which is fed to the workpiece has to increased in order to be able once again to ensure complete penetration.
• FIGS 4a and 4b show that the method according to the present invention can be used effectively to adjust the welding parameters in real time, so that complete penetration is ensured while the weld is being formed.
• the method according to the present invention has the advantage that a workpiece can be welded at an optimum speed. If the quantity of power which is added to the laser is kept constant, it is sufficient to change the speed of the laser with respect to the workpiece in order to supply an optimum quantity of energy to the workpiece per unit length.
• the present invention uses a number of terms. Firstly, the word "reference value" is used.
• the reference value is a value which is selected between the high intensity of a signal (associated with incomplete weld penetration) and the low intensity (associated with complete weld penetration) of a signal. There is no single value associated with the transition from incomplete to complete weld penetration. This is a step change, and ' the reference value is selected at this transition.
• the present invention refers to a workpiece.
• the word workpiece is to be understood as meaning any object which can be welded with the aid of the method according to the present invention.
• the present invention places the emphasis on measuring the light intensity above a workpiece. It will be clear that, in a corresponding way, it is also possible for other welding parameters to be used as input signal for adjusting the quantity of energy which is supplied to a workpiece per unit length.
• the present invention refers to the fact that measurement is carried out at the top of a workpiece. It is equally possible not to measure the intensity differences at the top side, but rather to determine intensity differences in the measured light intensity at the underside of the workpieces. These signals too could be used in the present method without departing from the inventive idea of the present invention.

Landscapes

• Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Engineering & Computer Science (AREA)
• Plasma & Fusion (AREA)
• Mechanical Engineering (AREA)
• Laser Beam Processing (AREA)
• Photometry And Measurement Of Optical Pulse Characteristics (AREA)

Applications Claiming Priority (2)

Publications (2)

Family

ID=19773947

Family Applications (1)

Country Status (2)

Cited By (4)

Citations (2)

• 2001
  • 2001-08-31 NL NL1018861A patent/NL1018861C2/nl not_active IP Right Cessation
• 2002
  • 2002-08-30 WO PCT/NL2002/000571 patent/WO2003022508A1/en not_active Application Discontinuation

Patent Citations (2)

Also Published As

Similar Documents

Legal Events