CA2322531A1

CA2322531A1 - Testing a weld seam

Info

Publication number: CA2322531A1
Application number: CA002322531A
Authority: CA
Inventors: Daniel Wildmann
Original assignee: Elpatronic Ag; Daniel Wildmann
Current assignee: Elpatronic AG
Priority date: 1998-03-02
Filing date: 1999-02-26
Publication date: 1999-09-10
Also published as: AU2508799A; KR20010041402A; BR9908490A; CN1291928A; RU2194601C2; WO1999044784A1; JP2002505199A; EP1060051A1

Abstract

The testing of weld seams obtained by laser welding of a butt joint poses considerable difficulties. According to the invention, to detect welding defects the weld seam length is scanned continuously along the weld seam and welding defects are detected on the basis of excessively raised areas of the weld seam.

Description

TESTING A WELD SEAM
The present invention relates to a method and an apparatus for inspecting a weld seam of a weld joint between sheet metal parts which is produced as a butt joint by deep welding with a laser beam; according to the precharacterising clause of claim 1.
Sheet metal parts which are cut and welded for specific uses, and which are termed "tailored blanks"; are increasingly being used in the automobile industry in particular. They firstly enable savings in raw material to be achieved and secondly enable process planning 1 Q and process implementation to be simplified. "Tailored blanks" such as these are fabricated by forming butt joints by laser welding. In the course of this procedure, the edges of two sheet metal parts which are cut so that they are complementary to each other are abutted flush against each other, are fixed by means of a clamping device and are passed through a welding station equipped with a laser. High-power lasers, with which deep welding can be effected, are used in particular for this purpose.
Apart from melting, high-power lasers also result in the evaporation of the metal and in the formation of a plasma. The vapour pressure of the plasma, which is situated above and in the region of the melt, opens up a deep, narrow capillary in the melt pool.
The plasma

2 o cloud or plasma flare which is formed above the melt pool, which is termed the weld pool, can become too hot and too dense, however, which can result in screening of the laser beam and interruption of the welding process. As a rule, a process gas, mostly helium or argon, is therefore supplied in order to cool the plasma and thereby to reduce the density thereof. During deep welding using a laser beam, liquid metal flows continuously into the capillary, and under some circumstances can be explosively ejected. Most of this spitting or spatter flies within the "welding plane" which is determined by the weld seam and the laser beam.
The stability and thus the weld seam quality of a deep welding process depends on numerous parameters, e.g. on the speed of welding, on the properties of the laser and on the nature of the workpieces, particularly the edge surfaces and the course of the edges thereof. The relationships pertaining to the gas dynamics, i.e. the type of process gas feed, the removal by suction of the welding fume and the aeration of the laser beam tube, are of considerable importance in order to obtain a reproducible, uniform welding result. A
complex, labile equilibrium exists between all these parameters during the welding process.
If the position of equilibrium is disturbed by random changes in the influencing variables, this can result in transient interruptions of the deep welding operation and in the non-uniform ejection of melt from the weld pool. If the amounts of material ejected are small, the metal which is lacking locally is replaced again by the liquid weld pool.
However, if substantial amounts are ejected, no replenishment occurs, which results in drop-like deposits of material on the weld seam. The deposits of material can extend to a length of 0.5 mm to 5 mm over the entire width of the weld seam, and have a height of about 1 mm.
Since this ejected, deposited material no longer flows back into the weld capillary, a crater, channel or gap is formed in the weld seam in front of a drop-like deposit such as this. Weld defects of this type generally occur when welding is carried out using high-power lasers, and are generally not tolerated according to the specifications for the weld seam quality of weld joints produced by laser beam which are defined in ISO
13919-1.
Various methods are therefore known, by means of which the quality of a weld seam can be inspected. Methods of inspecting weld seams produced by deep welding to form a butt joint with a laser beam, or methods which are known from Patent ............
(Soudronic), are known in particular to one skilled in the art. The method disclosed in the latter document relates to the detection of the weld seam profile by means of an optical system. Evaluation devices can then assess whether or not the weld seam profile of a weld joint between sheet metal parts complies with the quality requirements. In this method, weld profile measurements are made at about every 15 mm.
Other known methods of weld seam inspection, such as that described in US-4,827,099 for example, monitor the welding process itself and are based on the detection of the UV
light emitted by the plasma cloud and of the IR radiation emitted by the glowing weld

-3-spatter. In this method, weld defects are detected by comparing the measured spectral values with stored reference values. A method of this type for monitoring a welding process necessitates the costly compilation of reference values and requires a complicated detection system. Unfortunately, false readings are always obtained in a method such as this, since this method essentially takes into consideration only two measured quantities of the complex welding process.
GB-A-2 359 269 relates to the identification of weld defects based on the acoustic signal emitted by the plasma.
The object of the present invention is therefore to create a method and an apparatus for the inspection of weld seams for the production of butt joints by deep welding with a laser beam, which method and apparatus do not exhibit the disadvantages of known methods and apparatuses, and which are capable in particular of inspecting weld seams in a simple and reliable manner.
The object in particular is to create a method and an apparatus by means of which weld defects, particularly craters, channels, gaps and/or accumulations of material along the weld seam can be detected in a simple manner and by means of which compliance with the specifications defined in ISO 13919-1 can thus be checked.
According to the invention, this object is achieved by a method of inspecting weld seams according to claim l and by an apparatus comprising the features of claim 6.
In order to detect said weld defects in a weld joint between sheet metal parts which is produced as a butt joint by deep welding with a laser beam, means are provided in particular with which the weld seam height can be continuously scanned along the weld seam. Craters, channels and gaps can be detected therewith, as can accumulations of material on the weld seam such as those described above.
Since weld defects during deep welding by means of a high-power laser are always accompanied by adjacent drop-like deposits of material, which result in a considerable

-4-excessive height of the weld seam over the entire width thereof, in a preferred embodiment of the method according to the invention it is merely these regions of excessive height of the weld seam which are detected.
It should be understood that the scanned height values are transmitted to a display and/or evaluation device. In particular, the evaluation device is capable of comparing the scanned values with the various grades of quality of ISO 13919-l, and is capable of indicating the exact positions of weld defects and/or of storing the measured data.
The method according to the invention is therefore distinguished by an extremely simple principle of measurement, which permits simple apparatus construction and which constitutes a reliable, uncomplicated and cost-effective method of inspecting weld seams and of detecting weld defects.
This simple principle of measurement enables very different means to be used for continuously scanning the weld seam height along the weld seam. It should be understood that all the means for the measurement of distance which are known to one skilled in the art can be used. Suitable means include optical distance measuring devices with which the weld seam can be continuously scanned.
In a further embodiment of the method according to the invention, a stylus is provided which is capable of executing mechanical excursions, which can be converted into electrical signals by means of piezoelectric crystals, for example.
Commercially available devices for measuring distance can be used as means for scanning the weld seam height along the weld seam.
The invention is explained in more detail below with reference to an example of an embodiment and with reference to the Figures, where:
Figure 1 is a schematic view of a defect-free weld seam;

-5-Figure 2 is a schematic illustration of a deep welding process;
Figure 3 is a schematic view of a weld seam with the formation of craters;
Figure 4 is a schematic view of a weld seam with a gap; and Figure 5 is a schematic illustration of a preferred embodiment of the apparatus according to the invention.
Figure 1 is a schematic view of a weld joint between two sheet metal parts 12 and 13 of different thicknesses, which is produced as a butt joint by deep welding with a laser beam.
The weld seam width 14; the height profile 15 perpendicular to the weld seam and the surface structure 16 along the weld seam are dependent on the weld parameters, e.g. on the properties of the laser and on the nature of the workpieces, particularly the edge surfaces, the edge progressions and the edge preparation, and are also dependent on the thickness of the sheet metal edges and the speed of welding. Thus, for example, the weld seam width 14 can vary from 0.3 to 1.3 mm. In a weld joint between sheet metal parts which complies with the relevant standard, the weld seam 11 generally has an imbricated surface structure 16, the variations of the weld seam height of which along the weld seam fall within the range of a few hundredths of a millimetre, for example 0.02 mm.
The expression "sheet metal part" which is used here refers to a flat metal product which is preferably fabricated from steel, e.g. metal strips or metal plates of any size, thickness and surface area. The weld joint between sheet metal parts which is described here refers to a joint between two sheet metal edges which is produced by welding, where the sheet metal edges can be of different thicknesses and do not necessarily have to originate from two separate sheets, but can also form part of one-piece metal sheet which has previously been rolled. The term "butt joint" is used here to denote the position of sheet metal parts, the sheet edges of which are abutted and fixed flush against each other. The term "weld seam height" comprises both depressions as well as regions of excessive height in the weld seam.

-6-Figure 2 is a schematic illustration of the situation at the location of the welding process.
A high-power laser beam 21 impinges on the sheet metal parts 22 and 33 (the latter is not illustrated) which are to be welded. The incident energy of the laser beam 21 results in the melting of the sheet metal parts 22, 23 and forms a weld pool 27. In addition, the heat generated by the laser beam 21 gives rise to a plasma 24, which forms a deep, narrow capillary 25 in the weld pool 27. The relative movement R of the laser beam 21 and the sheet metal parts 22, 23 results in a zone 26 behind the weld pool 27, which zone comprises solidified weld material and which forms the weld seam. It can be seen from Figure 2 that the liquefied metal 28 situated in front of the laser beam 21 becomes relocated at the back 29 of the laser beam 21 during the welding process due to the relative movement R, and is preferentially deposited on the upper surface of zone 26.
This results in the formation of an imbricated surface structure. With the aid of this Figure, it can also be understood that if the equilibrium of the plasma 24 is disturbed, the capillary 25 is filled with liquid weld material and that therefore no weld is produced or only a partial weld is produced. If the laser beam 21 impinges on an already existing weld pool 27, larger amounts of liquid weld material can be explosively ejected from the liquid weld material.
These ejected amounts are generally deposited on the weld seam behind the welding beam and can clearly be recognised as drop-like formations.
The weld seam 31 of a weld joint between two sheet metal parts 32 and 33 of the same thickness which is shown in Figure 3 comprises a weld defect in form of a crater 34, with a drop-like deposit of material 35 directly adjacent thereto. Craters of this type, which are also known as pinholes with a diameter of about 0.1 mm or more, can be of different depths, and typically do not extend over the entire weld seam width but are situated preferentially in the middle of the weld seam. The drop-like deposits of material 35 result in regions of excessive height in the weld seam, which can extend over the entire weld seam width and which have a length from about 0.5 mm to about 5 mm and a height of several tenths of a millimeter.
The schematic view of a weld seam 41 between two sheet metal parts 42, 43 of identical _'j-thickness which is illustrated in Figure 4 comprises a weld defect in the form of a weld seam gap 44, with a drop-like deposit of material 45 situated directly behind said gap 44.
Weld seam gaps of this type generally have a length ranging from 0.05 to 0.2 mm and typically do not extend over the entire weld seam, but have a width of about 0.2 mm. Weld seam gaps 44 are often present in the form of holes with a diameter of about 0.3 mm. The drop-like deposits of material 45 result in regions of excessive height in the weld seam, which can have a height of several tenths of a millimeter.
Figure 5 is a schematic illustration of an apparatus for carrying out the method according to the invention. Optical means 54 are preferably used for scanning the weld seam height along the weld seam S 1 of a weld joint between two sheet metal parts 52, 53 with different thicknesses. In this embodiment, said optical means 54 comprise three optical distance measuring devices, which are preferably disposed at an angle of inclination of 30° to the normal to the surface, and which are equipped with semiconductor lasers.
In a further embodiment, a detector stylus which is guided along the weld seam 51 is used instead of optical detectors. By means of piezoelectric crystals, the excursions of this detector stylus can be converted into electrical signals, which in turn can be processed by corresponding threshold value circuits. It should be understood that one skilled in the art can also employ other distance measuring devices which appear suitable to him, for example ultrasonic sensors. The construction of these detectors does not form part of the present invention and will not be explained in detail here. Regarding the selection of suitable detectors, it is essential that they are capable of determining differences in height of the order of several tenths of a millimeter in a reliable and simple manner. In particular, they should be capable of detecting regions of excessive height of this order of magnitude in the weld seam.
The advantages of the method according to the invention and of the apparatus according to the invention are the simplicity of detection of weld defects - in particular, solely by the detection of regions of excessive height in the weld seam - during deep welding with high-power lasers. Moreover, the continuous scanning of the weld seam results in said weld defects being detected reliably.

Claims

1. A method of inspecting a weld seam of a weld joint between sheet metal parts which is produced as a butt joint by deep welding with a laser beam, characterised in that the weld seam height along the weld seam is continuously scanned in order to detect weld defects.

2. A method according to claim 1, characterised in that weld seam defects are detected by means of regions of excessive height of the weld seam.

3. A method according to either one of claims 1 or 2, characterised in that the weld seam height is scanned by a mechanical stylus.

4. A method according to either one of claims 1 or 2, characterised in that the weld seam height is scanned by an optical distance measuring device.

5. A method according to either one of claims 1 or 2, characterised in that the weld seam height is scanned by an acoustic distance measuring device.

6. An apparatus for inspecting a weld seam of a weld joint between sheet metal parts which is produced as a butt joint by deep welding with a laser beam, characterised in that the apparatus comprises a scanning device (54) which is constructed for location above the weld seam (51) and which is designed as a distance measuring means for measuring the height of said scanning device above the weld seam surface, and that said apparatus also comprises means for continuously guiding the scanning device in the direction of the weld seam.

7. An apparatus according to claim 6, characterised in that the scanning device comprises a detector which is designed for recognising differences in height of the order of several tenths of millimetres from the measured height values.

8. An apparatus according to either one of claims 6 or 7, characterised in that the scanning device comprises at least one mechanical stylus.

9. An apparatus according to either one of claims 6 or 7, characterised in that the scanning device comprises at least one optical distance measuring device.

10. An apparatus according to either one of claims 6 or 7, characterised in that the scanning device comprises at least one acoustic distance measuring device.