GB2155175A

GB2155175A - Method and device for controlling welding processes by analysing the light generated during welding

Info

Publication number: GB2155175A
Application number: GB08505538A
Authority: GB
Inventors: Giancarlo Alessandretti
Original assignee: Centro Ricerche Fiat SCpA
Current assignee: Centro Ricerche Fiat SCpA
Priority date: 1984-03-02
Filing date: 1985-03-04
Publication date: 1985-09-18
Also published as: IT8467198A1; IT1180008B; IT8467198A0; GB8505538D0; FR2560696A1; DE3507299A1

Abstract

A method of controlling the welding of two metal parts (1,2), an area (8) of which is subjected to the energy (7) emitted by a thermal energy source such as a laser, consists in gauging a preset characteristic of the light radiated by the vapour given off from the said area (8) during welding, for example the intensity of the light of a wavelength characteristic of an element of the metal parts. A device (11) for carrying out the said method comprises a photoelectric sensor (12), a photometer (13) and a display (14). <IMAGE>

Description

SPECIFICATION Method and device for controlling welding processes by analysing the intensity of the light generated during welding The present invention relates to a method and device for controlling welding processes, whereby the parts being welded are supplied with energy by an external source (e.g. electric arc, laser, electron gun, etc.). In particular, the present invention relates to a process and device for controlling the heat generated in the weld area.

When welding with an external source (e.g. electric arc, laser, electron gun, etc.) for locally melting an area to form a weld bead, the formation of the latter may prove uneven, particularly as regards welding depth, on account of varying power output from the source, varying travelling speed of the parts being welded, incorrect positioning of the said parts or even on account of dirt, oxidation or corrosion on the weld surfaces. Furthermore, when welding with an electron gun or laser beam, the said lack of uniformity may even be caused by ill-focusing of the beam. On a standard mass-production welding set-up, such lack of uniformity may result in a relatively high percentage of rejects.

A number of known weld control methods are based on gauging the temperature in the weld bead areas, such gauging being performed by optical devices for analysing the intensity of infrared radiation emitted by the said area subsequentto being welded.

The employment of devices of the aforementioned type poses a number of drawbacks, owing to the high thermal gradients involved and the presence of incandescent material which may damage the said devices during use. As infrared radiation detectors must perforce be located close to the welding area, where they may easily be damaged by splash from the melting material or by the heat generated on the same, provision must be made for protection and cooling systems which, besides being cumbersome, also involve considerable cost.

Furthermore, infrared radiation is known to depend, not only on the final temperature of the weld part, but also on a number of other parameters, such as the nature of the weld surface, its surface finish and the colour of the metal the weld part is made of.

Consequently, a change in radiation intensity detected by the said known devices does not necessarily correspond with a change in temperature.

Further devices have also been proposed, based on the acoustic energy emitted from the weld area on the part. Though highly efficient from the design standpoint, such devices are difficult to employ in production plants owing to the errors caused by background noise in the plants themselves.

The aim of the present invention is to provide for uniform welding by controlling the energy absorbed by the smelted material on the two parts being welded, and so avoid the current high percentage of rejects already mentioned. With this aim in view, the present invention relates to a method of controlling the welding of at least two metal parts, one area of which is subjected to the energy emitted by a thermal energy source, characterised by the fact that it provides for gauging at least one preset chracteristic of the light radiated by the vapour given off from the said interacting area during the weld process.

The present invention also relates to a device for controlling welding processes, in particular, for conrolling the interaction between the energy emitted by a thermal source and two metal parts, characterised by the fact that it comprises: means for detecting the light radiated by the vapour given off during welding from the area on the weld parts subjected to the said energy; gauging means designed to generate a signal proportional to the light radiation detected by the said detecting means; and means for displaying the said signal generated by the said gauging means.

Further characteristics and advantages of the present invention will now be described in detail with reference to the attached drawings, provided by way of a non-limiting example in which: Figure 1 shows a view in perspective of a control device according to the present invention; Figure 2 shows a graph of a parameter detected by a part on the Figure 1 device.

Figure 1 shows two parts, 1 and 2, being welded by locally melting an area of the material the two parts are made of. In the specific example shown, the two parts, 1 and 2, are supported on a base 10 and consist of two parallelepiped parts contacting along the two longitudinal edges to be welded.

Numbers 3 and 4 in Figure 1 indicate the top surfaces of parts 1 and 2 respectively, while number 5 indicates the contacting edge of surfaces 3 and 4.

In the example shown, the weld area consists of a strip in the plane of surfaces 3 and 4 and extending along the entire length of parts 1 and 2 round contacting edge 5.

The said weld area is subjected to the energy emitted by a known source (not shown) housed inside supporting structure 6.

The said source emits a fixed energy beam to which the entire length of the said weld area is subjected by moving the said parts 1 and 2 continuously in the direction of arrow Z.

Energy beam 7 is therefore focused on interacting area 8, so as to melt the material of parts 1 and 2 locally and produce weld bead 9. The material of area 9 subjected to energy beam 7 has been found to evaporate and emit light. According to the present invention, weld quality is controlled essentially by gauging the light radiated by the vapour given off from interacting area 8 subjected to energy beam 7.

In more detail, the said light radiation is gauged by device 11 essentially comprising a photoelectric sensor 12, a photometer 13, a display 14 and a control unit 15.

Photoelectric sensor 12 is arranged so asto pick up radiation from the vapour given off from interacting area 8. In more detail, in the example shown, the body of sensor 12 is supported on an arm 16 projecting from pillar 17 connecting base 10 to structure 6. In a manner not shown, the photosensitive element on sensor 12 is conveniently preceded by one or more filters, each of which is designed to select a given wave length band of the light radiated by the said vapour.

Photometer 13 generates an electric signal proportional to the mean value of the signals from sensor 12, and consequently also proportional to the mean intensity of the light radiated by the vapour given off from interacting area 8 subjected to the said energy beam.

A graph of the signal (in Volts) detected by photometer 13 is plotted against the welding depth (im millimetres) on parts 1 and 2 is shown in Figure 2.

Display 14 may be of any type (analogue, numerical, etc. or even an oscilloscope, for example) for displaying the electric signal generated by photometer 13.

Before describing the weld control method according to the present invention, it should be pointed out that the present Applicant has discovered that, when two parts are heated for welding and evaporation of material on the said parts occurs resulting in the formation of so-called "key-holes", the material vapours radiate light the spectrum of which comprisyes the atom and ion emission lines ofthevapourized material, the wave lengths of which fall mainly within the visible and near-ultraviolet zone, particularly between 200 and 800nm.

Laboratory tests and research conducted by the present Applicant, particularly in connection with laser beam welding, have provided useful information concerning the relationship between welding depth and light radiation in appropriate spectrum regions, such information enabling the quality of the weld process to be controlled.

The possibility therefore exists of correlating the intensity of the light radiated by the vapour with the energy absorbed by the part being welded, and so operating on weld parameters for providing optimum weld conditions.

The present weld control method is based essentially on detecting and subsequently gauging the light radiated by the material vapour at interacting area 8 subjected to energy beam 7. On the basis of the information shown on display 14, the operator may adjust unit 15 for appropriatly modifying one or more welding parameters, the latter essentially being the power of the energy source, the travelling speed of the parts being welded and, in the case of a source consisting of a laser beam generator or electron gun, focusing of the beam emitted by the same.

For example, when welding parts involving straightforward geometry and travel, power is the most appropriate regulating parameter to work on.

When welding other than flat parts involving straightforward travel (e.g. corrugated sheet metal), the most appropriate reglating parameter to work on would appear, in the case of a laser beam or electron gun, to be focusing of the beam. In other cases, involving curves and/or complicated geometry, the travelling speed of the weld parts may be the best regulating parameter to work on. To ensure correct results, therefore, the light radiated by the vapour must perforce be equal to a preset value corresponding to required welding conditions. As shown on the graph in Figure 2, for example, the operator could work on one of the aforementioned parameters for obtaining a uniform weld bead at least as far as welding depth is concerned.

Tests and research conducted by the present Applicant have led to the conclusion that light is radiated by the vapour at specific wave lengths for each type of material. The aforementioned device may be used for analysing radiation from any metal material of the same type, such as steel, cast iron, aluminium, etc. The Figure 2 graph is relative to a laser beam weld in which the material of parts 1 and 2 is Fe 52, and photoelectric sensor 12 is fitted with a filter for selecting a light radiation wave length of 515 nm.

The aforementioned method, however, may be employed in a fairly wide range of applications. It may be used for any type of welding operation (with or without weld material) and, in the case of laser beam welding, with or without welding gas.

Furthermore, the present method of controlling weld processes by analysing the intensity of the light generated during the said process may be employed in all cases involving the emission of vapour having specific light characteristics resulting from the interaction between the energy supply and the material being welded.

To those skilled in the art it will be clearthat changes may be made to an arrangement and details of the present invention as described herein purely by way of a non-limiting example, without, however, departing from the scope of the same.

For example, an optical fibre beam may be employed for conveying the light radiated by the material vapour towards the sensitive element on photoelectric sensor 12, so as to enable the latterto be located well away from the weld area.

The aforementioned method provides for "open loop" control and adjustment of the energy source.

In the case of weld operations enabling an optimum trend of one of the weld parameters to be defined beforehand, an alternative "closed loop" system may be adopted for providing negative feedback. In this case, control unit 15 could be designed for automatically correcting the said preestablished parameter (e.g. source power, part travelling speed or beam focus, should the energy source be a power laser or electron gun) according to the amplitude of the output signal from the photometer, in such a manner as to maintain constant the interaction between the energy source and parts 1 and 2 during welding.

Claims

1. Method of controlling the welding of at least two metal parts, one area of which is subjected to the energy emitted by a thermal energy source, characterised by the fact that it provides for gauging at least one preset characteristic of the light radiated by the vapour given off from the said interacting area during the weld process.

2. Method according to Claim 1, characterised by the fact that it provides for regulating at least one parameter conrolling the said weld process, according to the detected said light radiation values and until the intensity of the said light radiation equals a preset value.

3. Method according to Claim 2, characterised by the fact that the said parameter controlling the said welding process is selected from among: the power of the thermal energy source, the relative travelling speed of the said parts in relation to the energy source, and focusing of the energy beam in the case of a laser or electron gun energy source.

4. Method according to any one of the foregoing Claims, characterised by the fact that light radiation is gauged selectively by filtering means the pass band of which falls within a wave length range of 200 to 800 nm.

5 Device for controlling welding processes, in particular, for controlling the interaction between the energy emitted by a thermal source and two metal parts characterised by the fact that it comprises: means for detecting the light radiated by the vapour given off during welding from the area on the weld parts subjected to the said energy; gauging means designed to generate a signal proportional to the light radiation detected by the said detecting means; and means for displaying the said signal generated by the said gauging means.

6. Device according to Claim 5, characterised by the fact that the said detecting means consist essentially of a photoelectric sensor.

7. Device according to Claim 6, characterised by the fact that it comprises at least one filtering element located between the said interacting area and the photosensitive element on the said photoelectric sensor

8. Device according to any one of the foregoing Claims from 5 to 7, characterised by the fact that it comprises a control unit connected to the output of the said gauging means and designed to regulate automatically at least one parameter controlling the said welding process, in such a manner as to maintain the intensity of the said light radiation within a preset range.

9. Method and device for controlling welding processes, essentially as described and illustrated with reference to the attached drawings.