GB2185816A

GB2185816A - Ultrasonic weld monitoring

Info

Publication number: GB2185816A
Application number: GB08630114A
Authority: GB
Inventors: Dr Francis Alan Wedgwood; Dr Christopher Brian Scruby
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1986-01-27
Filing date: 1986-12-17
Publication date: 1987-07-29
Anticipated expiration: 2006-12-17
Also published as: GB8630114D0; GB8601873D0; GB2185816B

Abstract

A method and apparatus (30) are provided for monitoring a weld (10), in particular to detect failures of fusion at the sidewall (21) between the weld (10) and an object (12). Pulses of ultrasonic energy are generated in the weld (10) by a focussed laser beam (34), and ultrasonic waves propagating in the object (12) are detected by an electromagnetic non-contact probe (40). The method can be used on a hot weld (10) straight after its formation, and is particularly advantageous in monitoring a multi- pass weld. <IMAGE>

Description

SPECIFICATION Ultrasonic weld monitoring The invention relates to a method and an apparatus for monitoring the quality of a weld using ultrasonic techniques.

It is known to inspect welds after fabrication of an object using ultrasonic or radiographic techniques. These techniques enable major defects to be detected, so that the defects can be located and rectified, but inspection must take place after the weld is completed.

Where a weld consists of several beads formed by successive passes of a welding apparatus, then any defect (such as a failure of sidewall fusion) would be easier to rectify if detected immediately after its creation and before being buried by the next bead; but if the weld must be allowed to cool before inspection can occur then such detection will be very time consuming.

According to the present invention there is provided a method of monitoring a weld comprising a bead on an object, the method comprising generating ultrasonic waves in the weld bead with a non-contact wave generator, and detecting the waves propagating through the object.

Preferably the wave generator is a pulsed laser beam. The wave detector may be an electromagnetic non-contact probe.

By generating ultrasonic waves within the weld bead itself and detecting the waves in the object a relatively simple transmission path for the waves can be achieved, passing through the sidewall region of the weld where lack of fusion is most likely to occur. Inspection of this region by pulse-echo techniques would be geometrically complex. The use of a non-contact wave generator and in particular a pulsed laser beam enables monitoring of the weld bead to be performed on a hot weld bead straight after welding; furthermore the monitoring may be performed on a weld as the object moves past the wave generator.

In a further aspect the invention provides an apparatus for monitoring a weld comprising a bead on an object, the apparatus comprising a non-contact ultrasonic wave generator arranged to generate ultrasonic waves within the weld bead, and a detector arranged to detect ultrasonic waves propagating through the object. The apparatus may be supported adjacent a welding head so as to monitor the weld straight after welding while the bead is still hot.

Preferably the wave generator is a pulsed laser beam, and the beam is desirably focussed onto a line extending in the direction of the weld, so that the ultrasonic energy is directed in a plane perpendicular to the weld direction and so approximately perpendicular to any defect in the sidewall.

The invention will now be further described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a sectional view through a partly completed multipass weld; and Figure 2 shows a diagrammatic view of an apparatus for monitoring the weld of Fig. 1.

Referring to Fig. 1, a multi-pass weld 10 is shown joining two adjacent objects 12, 14 at a stage where the first five passes have been made. On each successive pass, a weld bead 15, 16, 17, 18, 19 is produced, gradually filling the gap 20 between the objects 12, 14.

The first bead 15 bridges the gap 20 as its narrowest point while each successive bead 16, 17, 18 or 19 fuses onto one or other of the sidewalls 21, 22 of the weld 10. Any failure of a bead 15, 16, 17, 18 or 19 to fuse to the respective sidewall 21 or 22 is a serious defect which can significantly weaken the weld 10.

Referring to Fig. 2, an apparatus 30 is shown for monitoring the weld 10, and in particular for monitoring fusion between the bead and the respective sidewall 21 or 22 after each successive pass. The apparatus 30 comprises a Q-switched neodymium-doped yttrium-aluminium-garnet laser 32 whose light beam 34 is focussed by a cylindrical lens 36 to form a line image 38 on the surface of the most recently formed bead 19 of the weld 10 (the individual beads are not indicated in Fig.

2 for clarity). The image 38 is about 0.5mm wide in the plane of the Figure and about 4mm long. Each pulse of the laser 32 is of energy 100mJ and duration 25 nanoseconds and sets up thermal stresses within the weld 10, so that a pulse of ultrasonic energy propagates principally as a compression wave pulse through the weld 10 and into each object 12, 14. Because the waves originate along the line image 38, most of the wave power is directed parallel to the plane of the Figure.

An electromagnetic ultrasonic detector 40 is arranged next to the lower surface of the object 12 and spaced away from the weld 10 in such a position as to receive ultrasonic waves from the line image 38 which have passed through the sidewall 21 of the weld 10. The detector 40 is connected to a signal recorder 42.

In operation of the apparatus 30 the laser 32 is energised to produce pulses at a rate of 1 KHz, and the apparatus 30 is scanned along the length of the weld 10. Any variation in the degree of sidewall fusion between the bead 19 and the sidewall 21 will be indicated by a variation in the ultrasonic signal strength received by the detector 40, and defects of this nature can thus readily be detected. It will be appreciated that the scanning may be brought about by movement either of the apparatus 30 or of the objects 12 and 14. It will also be understood that the laser 32 can be used as described above to generate ultra sound even in a very high temperature weld 10, so that the apparatus 30 can be supported adjacent to a welding head (not shown) to monitor the weld 10 straight after the bead 19 is formed and before it has cooled much below its melting point.The laser 32 has the further advantage that the ultrasonic pulses produced are of substantially constant energy so that there is no need to monitor the energy of the ultrasonic pulse.

In an alternative arrangement of the apparatus 30, the detector 40 is arranged next to the upper surface of the object 12 and further from the weld 10 so as to receive ultrasonic waves which have passed through the sidewall 21 and have then undergone specular reflection at the lower surface of the object 12. This has the advantages that all the apparatus 30 is on the same side of the object 12 or 14, and that the detector 40 is further from the heat of the weld 10, but the signals will not be as large.

Although the detector 40 described above is an electromagnetic detector, other types of detector may equally be used, for example a piezo-electric transducer provided with liquid couplant, or a piezo-electric transducer coupled by a wheel probe and compliant tyre with or without a liquid couplant.

Claims

1. A method of monitoring a weld comprising a bead on an object, the method comprising generating ultrasonic waves in the weld bead with a non-contact wave generator, and detecting the waves propagating through the object.

2. A method as claimed in Claim 1 wherein the wave generator is a pulsed laser beam arranged to be incident on the surface of the weld beam.

3. A method as claimed in Claim 2 wherein the laser beam is brought to a line focus on the surface, the line being at least approximately parallel to the orientation of the weld bead.

4. A method as claimed in any one of the preceding Claims wherein the method is performed on a hot weld bead straight after welding.

5. A method as claimed in any one of the preceding Claims wherein the waves are detected by an electromagnetic non-contact probe.

6. A weld monitoring apparatus for monitoring a weld comprising a bead on an object, the apparatus comprising a non-contact ultrasonic wave generator arranged to generate ultrasonic waves within the weld bead, and a detector arranged to detect ultrasonic waves propagating through the object.

7. An apparatus as claimed in Claim 6 wherein the wave generator comprises means to generate pulsed laser beam.

8. An apparatus as claimed in Claim 7 also including means to bring the laser beam to a line focus.

9. An apparatus as claimed in any one of Claims 6 to 8 wherein the detector comprises an electromagnetic non-contact probe.

10. A method of monitoring a weld substantially as hereinbefore described with reference to the accompanying drawings.

11. A weld monitoring apparatus substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.