WO2007060479A1

WO2007060479A1 - Laser welding of zinc coated steels with no gap using a supermodulated solid state laser

Info

Publication number: WO2007060479A1
Application number: PCT/GB2006/050398
Authority: WO
Inventors: Mohammed Naeem
Original assignee: Gsi Group Limited
Priority date: 2005-11-22
Filing date: 2006-11-20
Publication date: 2007-05-31
Also published as: GB0523693D0

Abstract

A method for laser welding a lap joint between two or more zinc coated steel sheets (35, 36) , comprising holding the sheets together so that there is subsequently no gap at the desired weld location (34) and applying an output from a supermodulated solid state laser along the desired weld line, thereby to weld the joint.

Description

Laser Welding of Zinc Coated Steels

This invention relates to laser welding of zinc coated steel sheets.

The majority of steel used in the automotive industry (eg for car body panels) is zinc coated. In comparison to uncoated steel, zinc coated steel needs extra care during overlap welding, which is used extensively to connect panels together. Laser lap welding of zinc coated steel sheets is a very appealing joining method for such automotive assembly applications. Laser welding has significant advantages over conventional resistance spot welding techniques. These include smaller flange (overlap) requirements, the possibility of new design and single sided access, superior weld mechanical performance, improved dimensional control and less distortion, high speed and productivity, shorter production line, zero electrode wear, and so on.

However, a barrier to applying this process more widely in high volume vehicle and other types of volume production is zinc expulsion at the interface, which results in spattering and porosity in the welds.

During a welding process, the welding heat can vaporise the zinc coating at approximately 900⁰C, which is significantly lower than the melting point of the steel. The low boiling point of zinc causes a vapour to form during the so-called key-hole process, which needs to escape from the weld pool. In most cases, the zinc vapour can become trapped in the solidifying weld pool resulting in excessive undercut and weld porosity. A common type of joint is a lap joint wherein two, three or more layers are joined and this effect is particularly critical in such lap joints where two layers of zinc are present at the interface between the sheets. However, producing a controlled gap of around 0.1 to 0.2 mm at the sheet interface can circumvent these problems and systems achieving this have already been installed in car production for the welding of double or triple layer sheets for roof welding.

Various techniques are currently used to produce a controlled gap between the sheets and these include careful joint design, the use of dimples, metal shim, controlled clamp pressure and fixed design in addition to trials of different types of zinc coatings. The use of dual laser spot has also been proposed and the use of knurling.

The best results so far have been achieved by proper manipulation of the laser beam and optimisation of the laser parameters. Techniques such as elliptical beam welding, dual elliptical beam welding and dual beam welding have been used with various degrees of success. These are shown in respectively Figures 2(a), (b) and (c) which show various laser beam configurations for laser welding of zinc coated steel without a gap at the interface. An elliptical beam, as shown in Figure 2(a), can achieve good weld quality on certain thickness combination but duty cycle is critical to weld quality and problem arises when trying to weld galvannealed steels. A dual elliptical beam system, as shown in Figure 2(b), attempts to overcome this problem by combining a parabolic elliptical beam mirror with a wedge shaped roof top mirror. The aim of this is to further lengthen the key-hole for zinc gas escape and success has been achieved with this approach for galvannealed steels. To improve the process further, it has been proposed to deliver a laser beam (eg a Nd:YAG laser) via an optical fibre in combination with a beam splitting lead to generate two generally circular beams as shown in Figure 2(c). With this beam configuration, a series of stacks of galvanised and galvannealed steels have been successfully welded. However, galvannealed steels are still more difficult to weld than galvanised steels. In addition, the dual beam inter-beam distance parameter is confined in a very narrow range for a robust process and, when the two beams separate too far, the rear beam generates a concavity in the rear key-hole wall. This results in a non-stable structure for the liquid metal in the weld pool at the rear key-hole wall and a result fluid flow may lead to spattering and porosity in the weld.

None of the above approaches are entirely satisfactory. The present invention arose in an attempt to provide an improved method and apparatus for lap joint welding of zinc coated steels.

According to the present invention in a first aspect, there is provided a method for laser welding a lap joint between two or more zinc coated steel sheets, comprising holding the sheets together so that there is substantially no gap between them along the desired weld line and applying an output from a supermodulated solid state laser along the desired weld line, thereby to weld the joint.

The term 'supermodulated' means a laser in which a laser beam is modulated to have a peak power greater than the rated CW power of the laser producing the beam. Techniques for this are described in applicant's co-pending application WO 03/071639, which is incorporated herein by reference and to which the reader is referred.

The method may comprise using an optically pumped solid state laser apparatus having a rated CW output power of around 100 watts or more, the method comprising: modulating an optical pump to directly generate a modulated laser output beam having peak power greater than the rated CW output power, the pump being controllable with a time varying electrical signal so as to provide average output power up to the rated CW power; focusing the output beam and applying the output beam to the lap joint to be welded.

In preferred embodiments, the output of the laser is a sine wave or square wave.

Preferably, the method is such that the laser output has a beam off and a beam on period and wherein energy is stored in the power supply during the beam off time and this energy is applied to a laser medium forming part of the laser during turn on time of the laser, resulting in a short duration of high peak power. The high peak power may be, for example, up two times the CW rating of the laser.

The invention further provides apparatus for welding a lap joint between two or more zinc coated steel sheets, comprising a laser having an active laser medium and a pumping means, means for modulating the pumping means so as to obtain a supermodulated output from the laser, means for directing the laser to a joint to be welded and means for clamping the two or more zinc coated steel sheets so as substantially prevent gaps between the sheets at the desired weld location. According to the present invention in a further aspect, there is provided laser welding apparatus for laser welding zinc coated steel, the system comprising a solid state laser apparatus with a rated CW output power, the system comprising a solid state resonator comprising at least one solid state laser medium, at least one reflector and at least one output coupler; a pump and super modulating power supply for modulating the pump so that the pumps the at least one solid state laser medium to generate a modulated laser output beam having peak power greater than the rated CW output power; an optical system for focusing the output beam means and directing the beam to the site to be welded.

The invention also provides a method for welding lap joints between zinc-coated steel surfaces, comprising providing weld energy from a supermodulated laser.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures l(a) and (b) show typical prior art welds;

Figures 2(a), (b) and (c) show different spot configuration of previous;

Figure 3 is a schematic view of a laser pumping chamber;

Figure 4 shows a laser resonator;

Figure 5 is a graph of power over time for a CW input; Figure 6 is a graph of power over time for a supermodulator sine wave input;

Figure 7 is a graph of power over time for a square wave input;

Figure 8 is a schematic view of zinc -coated steel being welded;

Figures 9(a) and (9b) show welds made with embodiments of the present invention; Figure 10 shows schematically a clamping mechanism; Figure 11 shows a clamping mechanism; and Figure 12 shows a lever clamp.

Referring to Figure 3, a CW laser typically comprises a laser resonator 1 including at least one laser rod 2 (which may be a Nd: YAG element or other element) which is mounted between two flat mirrors 3 and 4. Mirror 3 is a high reflectivity rear mirror and mirror 4 is a partially transmissive front mirror 4 known as the output coupler. The laser rod 2 is pumped by one or more pumping element such as lamps 5 which are powered by an electrical source 6 (eg AC source that generally includes a resonant circuit). Typically, the source 6 is designed to produce an output of about 15 KW average power and 30 KW peak power.

CW lasers have a rated average power and this is shown in Figure 5 at level "CW". This level may be 1000 W as shown in the figure or more a different level. In previously proposed lasers used for steel sheet welding, the CW laser is generally modulated by altering the power supply to the one or more pumping lamps 5 up to the CW level, depending upon the power requirement at any time during the welding operation, as shown in Figure 5. The level L may be dynamically varied up to the CW level to control the welding operation. Alternatively, a DC power supply may be used. The CW output is one that can be maintained for 100% of the time as an average level and the laser power L is varied up to this to control a welding operation.

Figure 4 shows a typical laser resonator. This resonator may include two flooded ceramic cavities 10 and 11, each housing a respective Nd: YAG rod 12, 13. Alternatively, dry gold cavities might be used, for example. Each chamber includes two arc lamps fitted with electrodes for AC excitation, typically up to 8 KW per lamp at 25 to 30 KHz. The resonator may be formed between two flat mirror 14 and 15 which are spaced a desirable amount apart to give predetermined resonator pitch. The space may for example 1500 mm to give a resonator pitch of 750 mm. The cavities 10 and 11 are arranged to give a symmetrical periodic resonator controlled by three apertures 16, 17 and 18 placed at the centre point and close to the respective mirrors 14 and 15.

The resonator output through output coupler 15 (which has 50% reflectance) is directed via turning mirrors 21 and 22 and imaged to an appropriate size (eg nine times) using optical system 19, 20, then launched into an optical fibre 23 of desired length (eg 5 m). The fibre is then directed so that the beam output from it can be used to weld steel joints. The beam may be applied direct from the output of the fibre 23 or via focusing or beam shaping optics, as is shown schematically in Figure 8.

As is described in International Patent Application No WO 03/071639, lasers have recently been developed which can pulse or modulate the laser output power with peak power up to two times the CW rating. This is shown in the following Table 1, where various lasers manufactured by GSI Lumonics Ltd are used: Table 1

Note that the average power is the average power at the workpiece at the end of the lamp life.

This modulation or pulsing is usually accomplished by storing some energy in the power supply during the beam off time, as is clearly described in the aforesaid International patent application WO 03/071639. Extra energy is then sent to the lasing medium during turn on of the laser and results in a short duration of high peak power.

Such lasers can produce three different outputs, ie CW, sine wave and square wave. For CW operations, the only parameters that is used is the demand range. By demand range is meant the range between 0 to 100% of available input power and from simmer to maximum power. The laser begins to produce power at about 50% demand, approximately half its rated power at about 70% demand and full rated power at approximately 90 to 100% demand.

Sine wave operation is illustrated schematically in Figure 6. In this, the laser output is super modulated as described in the aforesaid International patent application to produce output 30. For such sine wave operation, the parameters used are demand range and frequency. In the schematic example shown in Figure 6, in which a JK 1002 laser made by GSI Lumonics Limited is used, the configuration is one of 100% mean power and 50% depth. The frequency is set between the values of 100 to 1000 Hz or thereabouts. The demand range varies between 0% and 100%, with 40% being the approximate threshold for laser operation. 70% produces approximately half the laser rated average power and 90 to 100% produces full rated power. The depth values ranges from 0 to 100%. If the depth value is set to 0% then the laser operates in CW mode with sinusoidal output.

If the depth value is set to 100% with a demand range 100% then the peak power of the laser will be 200% of the rated average power and the minimum value will be 0 W at the trough of the sinusoidal wave form. If the depth value is set to 50% with a demand range of 100% (ie as in the figure) then the peak power will be 150% of the average power and the minimum power at the trough of the sinusoidal output will 50% of the rated average power. Adjusting the demand range changes the peak at minimum powers of the sinusoidal output but the waveform shrinks in proportion to the mean power that the demand ranges calls for, so there is no 'clipping' of the sinusoidal waveform at 0% or 200%. Note that adjusting the frequency does not affect the depth value of the laser, simply the frequency of the sinusoidal output.

Figure 7 illustrates square wave operation. In this case, the parameters used are demand range, peak power and frequency. The frequency is typically set between the values of 100 to 1,000 Hz. The percentage on-time or duty cycle of the laser operating in square wave mode is determined by the ratio of the demand range to the peak power. The duty cycle and frequency determine the actual on-time or pulse size.

For example, if the laser parameter on a JK 1002 laser made by GSI Lumonics Limited are set at 200% peak power and 100% demand range and 500 Hz, the duty cycle is 100/200 or 50% on-time. Because the laser is operating at 500 Hz, the pulse period is 1/500 or 2 ms. 50% of 2 ms is 1 ms so that a laser operating on these parameters will be operating at 500 Hz with pulse-width (or on-time) of 1 ms and a peak power of 200% of the rated power (ie 2 KW). This is shown by the square wave plot 31 of Figure 7. Note that these parameters are produced at the full 1,000 W rated average power of the laser since the demand range is set 100%.

Figure 8 shows schematically an apparatus for applying a super modulated laser beam to a joint for welding it. The beam is applied via an output fibre 23 which may be of 600 μm diameter. In trials, a JK 1002 CW laser of Lumonics Limited was fitted with such a fibre. The fibre terminates in an output housing 32 including a set of focusing optics (not shown). Preferably, with zinc -coated steels, no gas shielding is used. The housing and focusing optic focuses the laser down to the topsheet surface 33 of a lap connection 34 between two sheets of zinc -coated steel 35, 36.

The two sheets are clamped tightly together to ensure that there is no gap between them. The laser beam is focused at the topsheet surface. In experiments, the welding has been performed with CW and square wave output, although sinusoidal output is equally useful.

In typical zinc-coated steel sheets, the steel is of about 0.8 mm thick depth and this is coated on both sides with a zinc layer of approximately 10 μm.

In experiments, various welding parameters including welding speed, modulation, frequency and peak power are monitored and the results shown in Table 2 were achieved.

Table 2

Visual examination of the weld surfaces from both sides was carried out. The percentage of the good portion of the weld out of the total length was recorded for each weld on both sides. A welding window is considered good when the weld has a good portion that exceeds 95% of the total weld length. The welds were then cross-sectioned at selected locations from a metallurgical evaluation procedures. Figures 9(a) and (b) shows cross-sections of welds made with a modulated laser beam in which the parameters used were laser power of 1,000 W used at 150% peak power (super modulation) with a modulation frequency of 600 Hz, a spot size of 0.48 mm and at a speed of 2.5 m per minute. Figure 9(a) shows a top bead and Figure 9(b) shows a side cross-section. No porosities or pin holes were observed in these weld cross-sections.

Welds made with higher peak percentages, such as 170% and 200% had excessive under fill at the top and bottom of the weld surface. However, there was again no porosity in the weld. Results were very similar for all the modulation frequencies tested. Results have shown that by optimising welding speed and the peak output percentage, it is possible to produce welds with very little under-fill at the top and bottom of the weld.

Hence, lap joints in tightly clamped specimens of zinc -coated steel sheet can be made with a square wave modulated laser output. By optimising laser and processing parameters, welds can be obtained which have highly acceptable visually sound appearance with no internal cracks and no zinc gas blowholes or pitting on the top surface of the material. The reason for the success of welding is mainly due to venting of the zinc vapour through the stable keyhole.

In contrast, the use of a CW laser output on joints with no clearance, can lead to excessive spatter and potential porosity formation in welds. During welding, the only route for exhausting zinc vapour is through the weld pool along with the iron vapour formed in the keyhole. The high pressure of zinc at the leading edge of the weld tends to distort the location of the key-hole forward. A modulated laser beam however produces a more stable key-hole that helps to produce defect free welds. The main parameters affecting the welding process are percentage peak power, laser output and modulation frequency. The best results have been achieved so far with square wave output at 150% peak power, 400 to 600 Hz and a speed of around 2.5 m per minute for 1,000 W average power. However, by adjustment of any of these parameters and corresponding adjustment of other parameters, similar results may be achieved for other parameter combinations.

Use of a supermodulated laser enhances both the weld speed, weld penetration and the weld quality compared to a standard CW laser.

Super modulation techniques using square waves, sine waves or other waves, may be used for different speeds of welding, for example and for lap joints of more than two layers.

Figure 10 to 12 show non-limiting examples of clamping mechanisms. In a generalised form shown in Figure 10, a sample 100, comprising overlapping sheets 101a, 101b, overlapped at the welding line, is clamped between fixed upper plates 102, 103 and a floating lower plate 104. Alternatively, the lower plate may be fixed and the upper plates floating. The floating plate is movable towards and away from the fixed plates as shown by arrow A, to clamp the sample tightly between the plates. Plates 102 and 103 are in line, with as small a gap G between them where the weld line is positioned. By having gap G as small as possible (perhaps a few mm or a few cm) the samples are clamped as near to the weld line as possible.

Figure 11 shows a test jig in which the floating plate 104 is acted upon by a plurality of lever clamps 105, shown in more detail in Figure 12. In addition to the fixed and floating plates, side walls 106 are provided which include cut-outs 107 and 108. Respective projections 109 from the floating plate 104 are retained with these cut-outs. Thus, the cut-outs act as guides ensuring the plate 104 floats only vertically and does not tilt. A base 109 is also provided.

Other guide means ensuring this effect will be apparent. The term vertically is used relatively, in practice the sheets may be clamped horizontally, vertically or at any other angle, and the clamping mechanism arranged accordingly.

Figure 12 shows schematically a lever clamp 105. This comprises a clamp arm 110 threadally mounted on a bolt 111 against a spring 112. The floating plate is pushed up by the lever (against the base) and the spring tension allows the plate 104 (and therefore the sample) to float.

Claims

1. A method for laser welding a lap joint between two or more zinc coated steel sheets, comprising holding the sheets together so that there is subsequently no gap at the desired weld location and applying an output from a supermodulated solid state laser along the desired weld line, thereby to weld the joint.

2. A method as claimed in Claim 1, wherein the laser is an optically pumped solid state laser apparatus having a rated CW output power and the method comprising modulating an optical pump to directly generate a modulated laser output beam having peak power greater than the rated CW output power.

3. A method as claimed in Claim 2, wherein the laser apparatus has a rated CW output power of 100 watts or more.

4. A method as claimed in Claim 3, wherein the laser has a rated CW output power of around 1,000 watts.

5. A method as claimed in any of the preceding claims, wherein the output of the laser is a square wave.

6. A method as claimed in any of Claims 1 to 4, wherein the output of the laser is a sine wave.

7. A method as claimed in any preceding claim, wherein the two or more zinc coated steel sheets are clamped together.

8. A method as claimed in any preceding claim, wherein the laser is supermodulated to have a peak power of around 150% of its rated CW output.

9. A method as claimed in any preceding claim, wherein the laser is moveably directed to weld the joint at a speed of around 2.5 m per minute.

10. A method as claimed in any preceding claim, wherein the laser is modulated as a square wave with a modulation of around 400 to 600 Hz.

11. A method as claimed in any preceding claim, wherein the laser output is a square wave modulation so as to produce a peak output of between 150% and 200% its rated CW output.

12. A method as claimed in any preceding claim, wherein the laser output is directed to a welding spot size of 0.48 mm.

13. Apparatus for welding a lap joint between two or more zinc coated steel sheets, comprising a laser having an active laser medium and a pumping means, means for modulating the pumping means so as to obtain a supermodulated output from the laser, means for directing the laser to a joint to be welded and means for clamping the two or more zinc coated steel sheets so as substantially prevent gaps between the sheets at the desired weld location.

14. Apparatus as claimed in Claim 13, wherein the supermodulated laser output is applied by an optical fibre delivery means.

15. Apparatus as claimed in Claim 13 or Claim 14, including delivering and receiving an output from the optical fibre and focus the output beam to a desired spot size at the welding point.

16. Apparatus as claimed in any of Claims 13 or Claim 14, including means for moving the sheets to be welded relative to the laser to achieve a desired speed.

17. A method for welding lap joints between zinc -coated steel surfaces, comprising providing weld energy from a supermodulated laser.