GB2274257A

GB2274257A - Method of preparing and welding zinc coated steel

Info

Publication number: GB2274257A
Application number: GB9300941A
Authority: GB
Inventors: Stewart Wynn Williams; Graeme Scott
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1993-01-19
Filing date: 1993-01-19
Publication date: 1994-07-20
Also published as: GB9300941D0

Abstract

A zinc coated steel work piece 4 is prepared for welding by scanning with laser radiation 2, to remove the zinc coating layer. The laser has a wavelength in the range of 150 nanometres to 20 micrometres, at a fluence of 0.1 to 100 Joules per square centimetre, with a pulse length in the range of 1 nanosecond to 10 microseconds and for a minimum of 1 pulse, preferably at least 10 pulses, at each laser radiation impact point on the surface. The laser is preferably an eximer laser. The prepared surface of the work piece can then be welded to a further work piece by resistance spot welding, arc welding, friction welding or laser welding. For laser welding a laser is used, preferably operating at a power of substantially 750 watts at a scan rate of substantially 240 millimetres per minute in an atmosphere of nitrogen. <IMAGE>

Description

METHOD OF PREPARING AND WELDING ZINC COATED STEEL This invention relates to a method of preparing a zinc coated steel work piece for welding and to a method of welding such a prepared work piece.

Welding of zinc coated steel (galvanised steel) by laser is made difficult by the presence of the zinc coating. The high vapour pressure and low vaporisation temperature of zinc means that when welding is attempted by laser the zinc vaporises rather than melts which gives rise to the problems of highly porous welds and instability of the melt pool due to the escaping vapours when this boils. It has been proposed to overcome these problems by allowing only a very small gap (typically 100 micrometres) between the surfaces to be welded together to allow the vapours to escape. However this small gap is very difficult to set up and has a very small tolerance.

As a consequence it is very difficult to ensure that the gap spacing is even around a large work piece such as a car body wing panel where the gap tolerance may be smaller than the size tolerance on the panel itself.

There is thus a need for a generally improved method for preparing and welding a zinc coated steel work piece.

According to a first aspect of the present invention there is provided a method of preparing a zinc coated steel work piece for welding, in which a zinc coated surface of the work piece is scanned with laser radiation having a wave length in the range of from 150 nanometres (nm) to 20 microns (cut) at a fluence of 0.1 to 100 Joules per square centimetre (3cm2), with a pulse length in the range of from 1 nanosecond (ns) to 10 microseconds (cos) and for a minimum of 1 pulse, preferably at least 10 pulses, at each laser radiation impact point on the surface, so as to remove the zinc coating from the irradiated surfaces.

Preferably the laser radiation utilised has a wavelength of substantially 308 nanometres, a fluence of substantially 5 Joules per square centimetre, a pulse length of substantially 14 nanoseconds and is effected for substantially 200 pulses at each laser radiation impact point.

Conveniently the laser radiation is effected in air, oxygen, argon or helium.

Advantageously the laser radiation is supplied by an excimer laser.

Preferably the method includes the step of wiping the surface after laser irradiation has been effected.

According to a second aspect of the present invention there is provided a method of welding a zinc coated work piece to a further work piece, in which a zinc coated surface of the work piece is prepared according to the method as hereinbefore described, and welded to the further work piece by resistance spot welding, arc welding, friction welding or laser welding.

Preferably the laser welding is carried out by an infrared laser operating at a power of substantially 750 watts at a scan rate of substantially 240 millimetres per minute at an atmosphere of nitrogen.

Conveniently the laser utilised is a CO2 or Nd/YAG laser.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying single figure drawing in which Figure 1 shows diagrammatically apparatus for carrying out the method of the present invention for preparing a zinc coated steel work piece for welding.

Basically the method of the present invention for preparing a zinc coated steel work piece for welding involves controlled local removal of the zinc layer from the work piece surface to be welded. This is effected by laser radiation of the surface. The zinc layer is removed in areas of the surface where welding temperatures are expected to be above that of the vaporisation temperature of zinc.

The laser preparation method may be carried out in any convenient manner such as by means of the apparatus shown in the accompanying Figure 1. In this apparatus the laser 1 used is preferably an excimer laser preferably producing an output of up to 300 millijoules at a maximum pulse repetition rate of 300 Hz in the ultra violet. The laser beam 2 is passed via an image projection system generally indicated at 3 to a work piece surface 4 mounted on a computer controlled X-Y table 5 whereby the work piece surface 4 can be scanned continuously or repeatedly stepped beneath the laser beam 2.

The image projection system 3 includes a mirror 6 an attenuator 7, a field lens 8, a beam homogeniser 9, an adjustable aperture mask 10, a limiting aperture mask 11, an imaging lens 12 and a computer control system 13 for the table 5. With this apparatus fluences in the range of from 1 to 10 Jim~2 can be provided on the surface 4. The dimensions of the focused image of the limiting aperture mask 11 are typically of the order of 530 micrometres by 500 micrometres.

The method of the invention for preparing a zinc coated steel work pieces for welding involves preparation of the surface 4 to be welded by controlled local removal of the zinc layer by laser radiation. Thus the surface 4 of the work piece is scanned with laser radiation having a wave length in the range of from 150 nanometres (nm) to 20 microns (pin), at a fluence of 0.1 to 100 Joules per square centimetre (3cm2), with a pulse length in the range of from 1 nanosecond (ns) to 10 microseconds (ills) and for a minimum of 1 pulse, preferably at least 10 pulses, at each laser radiation impact point on the surface, so as to remove the zinc coating from the irradiated surface 4.

Preferably the laser radiation is carried out at a wavelength of substantially 308 nanometres, a fluence of substantially 5 Joules per square centimetre, a pulse length of substantially 14 nanoseconds and is effected for substantially 200 pulses at each laser radiation impact point.

The laser radiation may be effected in air, oxygen, argon or helium atmosphere but preferably is effected in helium. Preferably the laser radiation is supplied by an excimer laser. Typically the zinc layer to be removed from the zinc coated steel has a thickness of the order of 8 to 10 microns. The method of the invention will remove all this layer. Not only does the preparation method of the present invention remove the zinc coating from the surface to be welded but it also removes contaminants such as oils, greases, aluminium and other contaminants from the galvanising process from the surface 4. This improves the surface cleaning which in turn improves the subsequent welding. Additionally in the method according to the invention of preparing the work piece for welding, the surface 4 after laser irradiation preferably is wiped for cleaning purposes in any convenient manner such as by means of a tissue drawn across the surface.

Example 1 Galvanised steel samples 15 millimetres by 10 millimetres in size having an overall thickness of 0.72 millimetres, of which the zinc layer was 12 millimicrons in thickness, were irradiated using the apparatus of Figure 1 by an excimer laser at a fluence of 5 Jim~2 for 100 laser pulses in air, in argon and in helium. The samples were visually inspected and subjected to Auger electron spectroscopy for a semi-quantitative surface analysis.

Visually the sample irradiated in air showed a light brown deposit extending around the irradiated surface region whilst the sample irradiated in Argon showed a very dark deposit extending around the irradiated surface region. On the contrary the sample irradiated in helium showed little deposit but a somewhat different surface effect on the irradiated surface. In the helium atmosphere the irradiated surface region (the melted region) was much more rippled in appearance compared to the relatively smooth irradiated region (melted region) of a sample irradiated in air or in oxygen. The sample irradiated in oxygen produced a smooth surface irradiated surface.

In the Auger Electron Spectroscopy surface analysis technique the sample surface 4 was bombarded by electrons having sufficient energy to induce inner shell Auger transitions leading to the ejection of electrons having an energy characteristic of a particular elemental species.

By measuring the total signal at this energy the concentration of each species can be estimated. The analysis enables the total zinc and iron signals to be estimated to yield the zinc to iron ratio (Zn/Fe ratio) which provides an indication of the depth of removal of the zinc coating. Thus Auger analysis was carried out on the samples after irradiation either in air, Argon or helium to determine the zinc to iron ratio in the laser irradiated surface regions.

This ratio provides an indication of the amount of zinc coating present on the steel substrate. Carrying out Auger analysis on a sample surface 4 not treated according to the method of the present invention showed the presence of zinc, oxygen, iron and surface trace elements such as aluminium, with a zinc to iron ratio of approximately 25.

The samples were then treated by laser irradiation using an excimer laser at a fluence of 5 Jim'2 for 10 and 100 laser pulses. After 10 laser pulses a layer of the surface containing trace elements such as aluminium and the oxide was removed and the zinc to iron ratio was reduced to about 20. This shows that the zinc coating thickness has not been substantially reduced. After 100 laser pulses in air the zinc to iron ratio was still high of the order of 18 showing once again that the zinc layer had not been removed. Continuing experiments with the samples indicated that after 200 laser pulses in air the zinc to iron ratio had been reduced to approximately 0.45 which is sufficiently low to indicate that the zinc layer had been removed.

Thus for treatment in air at a fluence of 5Jcm-2 it was found necessary to irradiate for 200 pulses to remove the zinc coating. Further work in helium indicated that fewer laser pulses were needed in helium to remove the zinc layer. This was achieved after 126 laser pulses in helium. Hence a preferred method according to the present invention utilises a laser radiation having a wave length of substantially 308 nanometres, a fluence of substantially 5 Joules per square centimetre, a pulse length of substantially 14 nanoseconds and a number of pulses of substantially 200 at each laser radiation impact point.

If a zinc coated steel work piece is welded to a zinc coated or uncoated steel work piece without treatment according to the method of the presence application, the welded region is very irregular with high porosity where zinc is explosively ejected from the region. Weld penetration is poor. It is considered that since the vaporisation temperature for zinc (1180E) is considerably lower than the melting temperature of iron (1808R) the power density required to achieve a melting phase of steel is sufficiently high to cause complete vaporisation of the zinc layer. Expansion of this vapour phase in the molten weld pool results in the explosive ejection of material from the weld region.

On the contrary a smooth weld region was achieved subsequent to carrying out the preparation method of the present invention on a zinc coated steel work piece to be welded when the prepared work piece was welded to a further prepared work piece or uncoated work piece, with a result similar to that obtain on ungalvanised steel. To achieve this a zinc coated steel work piece with a surface 4 treated according to the method of the present invention to remove the zinc layer therefrom was welded to a further prepared zinc coated steel work piece or to an uncoated work piece using an infrared laser such as a CO2 or Nd/YAG laser operating at a power of substantially 750 watts at a wavelength of substantially 10.6 micrometres. Using a focusing lens having a focal length of 127 millimetres and in a nitrogen atmosphere it was possible to obtain optimum weld results at a scan rate of substantially 240 millimetres per minute.

Both work pieces welded together may be zinc coated on at least one surface prepared according to the method of the invention. Both sides of the work piece to be welded may be zinc coated and one or both sides may be prepared according to the method of the invention.

Claims

1. A method of preparing a zinc coated steel work piece for welding, in which a zinc coated surface of the work piece is scanned with laser radiation having a wave length in the range of from 150 nanometres (nm) to 20 microns (pin) at a fluence of 0.1 to 100 Joules per square centimetre (Jcm~2), with a pulse length in the range of from 1 nanosecond (ns) to 10 microseconds ('its) and for a minimum of 1 pulse at each laser radiation impact point on the surface, so as to remove the zinc coating from the irradiated surface.

2. A method according to claim 1, in which laser radiation scanning is carried out for at least 10 pulses.

3. A method according to claim 1 or claim 2, in which the laser radiation utilised has a wavelength of substantially 308 nanometres, a fluence of substantially 5 Joules per square centimetre, a pulse length of substantially 14 nanoseconds, and is effected for substantially 200 pulses at each laser radiation impact point.

4. A method according to any one of claims 1 to 3, in which the laser radiation is effected in air, oxygen, argon or helium.

5. A method according to any one of claims 1 to 4, in which the laser radiation is supplied by an excimer laser.

6. A method according to any one of claims 1 to 5, including the step of wiping the surface after laser irradiation has been effected.

7. A method of preparing a zinc coated steel work piece for welding, substantially as hereinbefore described and as illustrated in Figure 1 of the accompanying drawings.

8. A zinc coated work piece prepared for welding according to the method of any one of claims 1 to 7.

9. A method of welding a zinc coated steel work piece to a further work piece, in which a zinc coated surface of the work piece is prepared according to the method of any one of claims 1 to 8 and welded to the further work piece by resistance spot welding, arc welding, friction welding or laser welding.

10. A method according to claim 9, in which laser welding is carried out by an infrared laser operating at a power of substantially 750 watts at a scan rate of substantially 240 millimetres per minute in an atmosphere of nitrogen.

11. A method according to claim 10, in which the laser utilised is a CO2 or Nd/YAG laser.

12. A method of welding together a zinc coated steel work piece preparing according to the method of any one of claims 1 to 6, to a further work piece, substantially as hereinbefore described with reference to Figure 1 cf the accompanying drawings.

13. A method according to any one of claims 9 to 12, in which both work pieces are zinc coated and prepared according to the method of any one of claims 1 to 8.

14. Two work pieces welded together according to the method of any one of claims 9 to 13.