WO2003022508A1

WO2003022508A1 - Process and device for the measurement and regulation of a laser welding process

Info

Publication number: WO2003022508A1
Application number: PCT/NL2002/000571
Authority: WO
Inventors: Sjoerd Postma; Johannes Meijer; Ronald Godfried Karien Marie Aarts
Original assignee: Netherlands Institute For Metals Research
Priority date: 2001-08-31
Filing date: 2002-08-30
Publication date: 2003-03-20
Also published as: NL1018861C2; WO2003022508A8

Abstract

The present invention relates to a method and a device 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 function of the difference between the measured value and the determined reference value. According to the invention, 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 or 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 per unit length, 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.

Description

PROCESS AND DEVICE FOR THE MEASUREMENT AND REGULATION OF A LASER WELDING PROCESS

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.

During welding, it is known in the prior art to measure various characteristics of the welding process, 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.

According to the known method, first of all data is collected about the quantity of infrared light which is released during welding with a defined penetration depth. During the formation of a weld, the quantity of infrared light emitted 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.

However, 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. In view of the above, 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.

According to the invention, 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.

According to the present invention, use is made of the insight that, during welding of a workpiece with the aid of a laser, the light intensity above the weld pool changes suddenly at the moment at which, during welding, a state of incomplete penetration changes into a state of complete penetration.

During the state of incomplete penetration, a relatively high light intensity will prevail above the weld pool. When complete penetration is reached, the energy will find its way to the underside of the worlφiece, i.e. at that moment energy flows away at the underside of the workpiece and the light intensity which can be measured at the top decreases suddenly.

For numerous welding processes, in particular for welding tailor made blanks (TMBs), it is very important for a weld which has been made to achieve complete penetration. Tailor made blanks are used, inter alia, in the automotive industry. 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.

With 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.

The term "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".

It should be noted that the prior art has disclosed methods in which, during the welding of a workpiece with the aid of a laser, so much energy per unit length is added to the workpiece that one can be certain that complete penetration has been achieved. The quality of the weld which is to be made is ensured in this way. A significant advantage of the method according to the invention over the method in which excess power is added to the workpiece is mat with the method according to the present invention welding can be carried out with minimal amounts of energy and at a maximum speed.

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.

Moreover, according to the invention 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.

According to the invention, 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. By using a response time which is as short as possible, it is possible to ensu e that the entire weld has complete penetration. During laser welding, there is what is known as a keyhole present at the location of the laser. If it is detected that there is a risk of this keyhole closing, the quantity of energy supplied to the workpiece per unit length of the workpiece is increased. The changes in the size of the keyhole take place very quickly. The molten material is located next to the keyhole. The conditions in the melt change much more slowly than the conditions in the keyhole_* i.e. when it is established that there is a risk of the keyhole closing and that incomplete welding is occurring, 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,

According to the invention, it is possible that, when the relatively low level of the light intensity is measured, 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,

As has already be stated above, it is important to be certain of complete penetration of the weld while the weld is being made. However, to allow welding to be carried out with the lowest possible production of energy and at the highest possible speed, it is advantageous for the quantity of energy supplied to be allowed to decrease periodically in small steps or gradually. If it is measured that the weld is no longer achieving complete penetration, additional energy is then added to the weld pool in one relatively large step. This relatively large step ensures, as described above, that complete penetration is achieved.

According to a first aspect of the invention, it is possible to change the quantity of energy to be supplied to the weld pool per unit length by changing the quantity of power supplied to the laser.

If it is measured that the welding parameters need to adjusted, the quantity of power added to the laser can be changed. In the case of a high light intensity being measured, this means, for example, an extra-high consumption of energy by the laser which is to be used and an associated extra high quantity of energy which is transmitted to the weld pool by the laser.

Alternatively, it is possible to change the quantity of power to be supplied to the weld pool per unit length by changing the rate of advance of the laser with respect to the workpiece.

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.

According to the invention, it is possible to use a plurality of sensors. In this case, it is possible, in a first embodiment, to use coaxially positioned sensors,

Furthermore, it is possible to use a sensor which is specifically suitable for light with a wavelength of 400-600 nm. Moreover, it is possible to use a sensor which is specifically suitable for light with a wavelength in the vicinity of 800 nm. As soon as a signal exhibits a sharp, step change between complete weld penetration and incomplete weld penetration, it is in principle possible to employ the present invention.

During the formation of a weld with the aid of a laser, there is what is known as a plume, which comprises hot metal vapour, between the laser and the workpiece. 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. Moreover, the molten material will emit infrared light with a wavelength of approximately 800 nm and above.

By using a plurality of sensors which are specifically designed to measure a specific portion of the light which is emitted, it is possible for the step change from complete penetration to incomplete penetration to be recorded more successfully and more quickly.

According to the invention, it is possible to use an Nd:YAG laser. In this case, it is possible to use 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.

In the second aspect, 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.

In addition to the method described above, 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. The device according to the present invention is characterized in that 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_th (heat input) as a function of the difference determined by the said calculation means. In this case, 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.

The invention will be explained in more detail with reference to the appended figures, in which:

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.

Figure 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. During the welding process, 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. Via a semi-transparent mirror 4, 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. Furthermore, a camera 9 may also be added to the arrangement. In addition to these fitted sensors 7 and 8, 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.

In addition to the components illustrated in Figure 2, the device according to the present invention will be provided with control means which can process the signals intercepted by the various sensors 7, 8 and 10 into input signals for the laser. As has been extensively described above, according to the present invention the quantity of power supplied to the laser head 1 and therefore the quantity of energy supplied to the worlφiece is varied as soon as the light intensity above the weld pool undergoes a step change. These further control means are shown in Figure 1.

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.

In accordance with Figures 3a and 3b, over a first section from 0 to 50 millimetres, welding is carried out at a rate of 100 millimetres per second. Over the subsequent section from 50 to 90 millimetres, welding is carried out at a rate of 120 millimetres per second. In the experiment, welding was carried out with a laser power of 875 W,

It can be seen from Figures 3 a and 3b that the weld which is formed, over the section from 0 to 50 millimetres, has complete penetration over part of the length and incomplete penetration over part of the length. When the speed is increased from 100 millimetres per second to 120 millimetres per second, a weld which has incomplete penetration over the entire length is formed.

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. In 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. In Figure 4b, the measured light intensity is once again plotted against the length of the weld.

Once again, over a section from 0 to 50 irullimetres, welding was carried out at a speed of 100 millimetres per second. Over a section from 50 to 90 millimetres, welding was carried out at a speed of 120 millimetres per second. The experiment shown in Figures 4a and 4b does not use a fixed quantity of power for the laser, but rather, after a start-up section of 10 mm, the method according to the present invention is used, i.e. the quantity of power which is supplied to the laser is dependent on the measured light intensity above the weld pool. It can be seen from Figure 4b that, after an initial period, which is approximately 11 millimetres long, complete penetration is achieved during the welding. 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. When 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.

Figures 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.

As has already been stated above, 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. Furthermore, 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.

Fmlhermore, 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.

Furthermore, 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.

Claims

1. 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 function of the difference between the measured value and the determined reference value, characterized in 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 or 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 per unit length, 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 tight intensity.

2. Method according to Claim 1, characterized in that the light intensity is measured periodically, and the quantity of energy to be supplied per unit length is correspondingly adjusted periodically.

3. Method according to Claim 2, characterized in that the period between two successive adjustments is selected to be between 0.01 and 0.10 s, preferably 0.03 and 0.07 ms, more preferably 0.05 ms.

4. Method according to one of the preceding claims, characterized in that during the measurement of the relatively low level of the Hght intensity, the quantity of power which is to be supplied to the weld pool per unit length is 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 is allowed to increase in relatively large steps.

5. Method according to one of the preceding claims, characterized in that the quantity of energy to be supplied to the weld pool per unit length is changed by changing the amount of power fed to the laser.

6. Method according to one of the preceding claims, characterized in that the quantity of energy to be fed to the weld pool per unit length is changed by changing the rate of advance of the laser with respect to the workpiece,

7. Method according to one of the preceding claims, characterized in that a plurality of sensors are used.

8. Method according to Claim 7, characterized in that coaxially positioned sensors are used.

9. Method according to one of Claims 7 or 8, characterized in that a sensor which is specifically suitable for light with a wavelength of 400-600 nm is used,

10. Method according to one of Claims 7-9, characterized in that a sensor which is specifically suitable for light with a wavelength in the vicinity of 800 nm is used.

11. Method according to one of the preceding claims, characterized in that a Nd:YAG laser is used.

12. Method according to Claim 11, characterized in that a sensor which is specifically suitable for light with a wavelength of 1064 nm is used.

13. Method for welding a tailor made blank (TMB), characterized in that use is made of the method for measuring and controlling a laser welding process according to one of the preceding claims.

14. Device for measuring and controlling a laser welding process, provided with a laser, such as an NdrYAG laser, and control means for controlling the said laser, characterized in that the said control means comprise an optical sensor for measuring the light 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 length as a function of the difference determined by the said calculation means.

15. Device according to Claim 14, characterized in that the control means are designed to increase the quantity of energy which is to be fed to a workpiece per unit length when the quantity of light measured by the optical sensor is higher than the reference value.