# Patent

## Title
Method and apparatus for composite YAG laser/arc welding

## Abstract

A combined YAG laser/arc welding apparatus performs a welding operation with a large depth of fusion and a deep fusion at a high efficiency. The welding apparatus includes an arc electrode which targets a point on a member being welded where a laser beam from a YAG laser is focused. While an arc power supply is connected between the arc electrode and the member being welded, the member being welded is irradiated by the YAG laser to produce plasmoid gasses and metal vapors or a plume which is effective to start an arc from the arc electrode. A combined YAG laser/arc welding operation takes place while keeping the plume within and without a keyhole. Welded surface is covered by a hood, and a shielded gas passed through a commutation metal network surrounds the welded focus to prevent an oxidation and keeps the plume maintained. A filler wire is fed to a molten pool.

## Background
This is a Divisional of application Ser. No. 10/318,032, filed on Dec. 13, 2002.
The disclosure of the prior application is hereby incorporated by reference herein in its entirety.


FIELD OF THE INVENTION
The present invention relates to the art of YAG laser welding in which the input welding heat is assisted by an electric arc, or a welding method utilizing a combination of YAG laser and an electric arc and an apparatus therefor.
BACKGROUND OF THE INVENTION
YAG laser welding has features as mentioned below and is extensively used in the automatic welding process for automobiles.
1) A focused laser radiation is obtained and enables a welding operation with a low distortion and at a high rate;
2) Laser radiation absorption coefficient for metal materials is several times higher than for a CO2 laser, thus allowing an efficient welding operation. Since the wavelength of the laser radiation is on the order of 1/10 that of CO2 laser, attenuation caused by plasmas generated during the welding operation is reduced.
3) The laser radiation can be transmitted through a flexible optical fiber, and thus a handling is facilitated and the laser can be used with a multi-joint robot. The transmission to a location which is located as far as 100 meters is possible.
4) The laser radiation can be used in a time division manner or spatially split (or power divided ) manner, and this allows the distribution of the laser radiation to a plurality of machining stations to achieve a high utilization efficiency.
On the other hand, the welding operation by using YAG laser alone involves the following issues:
1) YAG laser is used in the welding operation by condensing the beam radiation to achieve an increased power density. However, the condensed spot has a very small diameter on the order of φ0.3 to φ0.1 mm, and this causes the following inconveniences;
For butt welding, the presence of a gap between members to be welded which are disposed in abutment against each other (see FIG. 4a, for example) allows the laser radiation to pass through the gap, causing weld defects. This imposes a severe demand on the cutting of an end face of a member to be welded where a weld is to be formed as well as on the manner of constraining parts of the members located adjacent to the weld.
For lap welding, the width of faying interface of a bead which extends from a top member to a bottom member (see FIG. 4b, for example) is limited, preventing a satisfactory bonding strength from being obtained.
For fillet welding, a satisfactory depth of fusion (see FIG. 4c. for example) cannot be obtained in the similar manner as mentioned above, preventing a satisfactory bonding strength from being obtained.
Where a filler wire is concurrently supplied to a region subject to YAG laser welding for purpose of improving excess weld metal or surface bead, the weldability is degraded because the power is consumed in melting the filler wire.
2) YAG laser apparatus requires a high equipment cost or initial cost, and therefore, where a plurality of equipment must be provided as in an automobile production line, a prohibitive installation cost results.
Japanese Utility Model Laid-Open No. 82,194/1987 discloses a nozzle structure which is used where it is considered that the presence of plasmoid gasses and metal vapors or a plume within and above a keyhole produced in a member being welded as a result of the irradiation from a laser stands in the way to achieving a deep fusion as a result of absorption of the laser beam by the plume, the nozzle structure serving to blow a shield gas into the keyhole to drive the plume to an exhaust path formed outside a laser radiation path to be withdrawn and exhausted. This represents an approach to improve the efficiency of supplying the input laser heat to the member being welded.
Japanese Patent Publications No. 49,195/1981 and No. 51,271/1992 and Japanese Patent Applications Laid-Open No. 137,044/1978 and No. 287,060/2001 disclose a combined use of TIG and a laser where a TIG torch melts a member to be welded, and a laser beam is projected to a resulting molten pool to increase the depth of fusion. These represent an approach in which TIG forms a shallow, but an extensive molten pool while the laser achieves a narrow, but deep fusion. The TIG produces an extensive molten pool to increase the width of the molten surface of the member being welded 41, 42 to fill a butt welding gap, as illustrated in FIG. 4a, for example, while the laser produces a deep fusion to increase the depth of fusion between members being welded 45, 46 as illustrated in FIG. 4c. As a consequence, the welding efficiency is improved as compared with the use of the laser alone, while improving the welding quality.
However, it will be seen that only the fusion produced by the laser prevails at a deep position, limiting the width of the fusion and resulting in a reduced welded strength. As illustrated in FIG. 4b, for example, the width of faying interface between members being welded 43, 44 is limited in the butt welding operation, leaving much to be improved.
SUMMARY OF THE INVENTION
It is an object of the invention to perform a welding operation with an increased width of fusion and an increased depth of fusion at a high efficiency.
The present invention relates to a composite YAG laser welding method in which an arc electrode is disposed close to a machining head of a YAG laser (or laser radiation condenser system) with a potential applied between the electrode and the member being welded so that the welding operation takes place thorough a combination of the YAG laser and the electric arc. According to the composite YAG laser welding method, the irradiation with the YAG laser radiation produce dissociated gasses and vapors of weld metal which are effective to excite and maintain an arc between the arc electrode and the member being welded and an atmosphere (plume) comprising plasmoid gasses and vapors of weld metal is maintained as close to a region which is irradiated by the laser by the combination of the YAG laser radiation and the arc.
Specifically, according to the composite YAG laser/arc welding method of the present invention, a spot on a member being welded which is irradiated by the YAG laser is targeted by the arc electrode, and while an arc power supply is connected across the arc electrode and the member being welded, the member being welded is irradiated by the YAG laser to produce plasmoid gasses and metal vapors which are effective to start an arc from the arc electrode, thus maintaining the plasmoid gasses and metal vapors which are produced by the YAG laser within and without the keyhole, thus completing the combined YAG laser/arc welding operation.
In a normal TIG process, a high frequency spark is created between the electrode and member to dissociate gasses in order to produce an arc. However, according to the present invention, there is no need to create such a high frequency spark. The plume, that is, the plasmoid gasses and metal vapors which are produced by the YAG laser and the arc assume an elevated temperature and a high pressure to depress a molten metal, and such heat transfer achieves a welding rate and a bead width which are greater than those achieved by the laser alone, thus allowing a greater depth of fusion.
This allows an increase in the gap margin in the butt welding operation, an increase in the width of faying interface in the lap welding operation, and an increase in the width of fusion in the fillet welding operation to be obtained, securing a bonding strength and providing a stable performance which is free from weld defects.
It will also be noted that when the invention is applied in the welding operation in which the filler wire is concurrently supplied, the heat input from the arc contributes to melting the filler wire, thus preventing the weldability from being degraded. It will be noted that the heat input from the arc generated allows a YAG laser of a low output to achieve a welding performance which is comparable to a YAG laser of a greater output. It is a simple matter to split a laser radiation from a single YAG laser source to a plurality of laser irradiation heads, thus allowing the installation cost per laser irradiation head to be simply reduced.
Above and other objects and features of the present invention will become apparent from the following description of several embodiments thereof with reference to the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal section of a welding head of a composite YAG laser/arc welding apparatus according to a first embodiment of the present invention;
FIG. 2a is a schematic section, to an enlarged scale, around a focus 8 shown in FIG. 1;
FIG. 2b is an enlarged section schematically showing a weld bead representing a result of composite YAG laser/arc welding operation which employed the welding apparatus shown in FIG. 1;
FIG. 3a is a longitudinal section of a welding head of a composite YAG laser/arc welding apparatus according to a second embodiment of the present invention;
FIG. 3b is an enlarged section schematically illustrating a weld bead representing a result of composite YAG laser/arc welding operation which employed the welding apparatus shown in FIG. 3a; 
FIG. 4a is an enlarged section illustrating a butt welding gap formed between members 41, 42 which are subject to a butt welding operation;
FIG. 4b is an enlarged section illustrating the width of a faying interface formed between members 43 and 44 in a lap welding operation; and
FIG. 4c is an enlarged section illustrating the width of fusion formed between members 45 and 46 in a fillet welding operation.



DESCRIPTION OF PREFERRED EMBODIMENTS
First Embodiment
FIG. 1 shows a composite YAG laser/arc welding apparatus according to one embodiment of the present invention. YAG laser beam 9 is supplied from a YAG laser source, not shown, through an optical fiber cable, not shown, to a YAG irradiation head 10. The head 10 includes a lens 11 through which the laser radiation is passed to converge to a focus 8 (a point being irradiated) through an objective sleeve 12 and a relay sleeve 13.
An objective hood 15 is secured to the bottom of the relay sleeve 13. A gas commutation metal network 16 is disposed within the hood 15 to divide the interior of the hood into an upper space or a gas supply space and a lower space or welding space. The hood 15 and the metal network 16 are formed with openings 17 and 18, respectively, which allow the laser radiation 9 to be directed toward the focus 8.
An electrode base 19 formed of ceramics is secured to the hood 15, and a tungsten electrode 20 is mounted on the electrode base 19 and is oriented to be directed toward the focus 8. The tungsten electrode 20 is secured to the electrode base 19 by a bolt 21 which presses against the electrode and which is connected through a power switch 22 to an arc welding power supply 23, the positive electrode of which is connected to a member 32 being welded.
A wire guide 24 extends through the hood 15 and the metal network 16 and is fixedly connected to the hood 15. A filler wire 25 is fed from a wire feeder, not shown, and is passed through the wire guide 24 and is brought by a wire feed tip located at the lower end of the wire guide 24 to the vicinity of the laser focus 8.
Wheels 26 and 27 are disposed in the bottom of the hood 15 to provide a small gap between the member 32 being welded and the bottom of the hood 15.
In FIG. 1, members 31 and 32 which are to be welded together comprises sheets, and when these sheets are subject to a lap welding operation, a shield gas is blown into the relay sleeve 13 through a pipe 14 which is mounted on the relay sleeve 13, the power switch 22 is closed to apply a no-load voltage across the electrode 20 and the sheet 31 which conditions for an arc starting, and then the irradiation of the YAG laser beam 9 is initiated. The plume which is produced by the irradiation causes an arc to be generated between the electrode 20 and the member 32. As soon as the arc is generated, the YAG irradiation head 10 begins to be driven in y direction and the filler wire 25 begins to be fed. This provides a stable welding condition. In other words, the laser irradiation welding and the arc welding run in parallel.
Under this welding condition, the shield gas which is blown into the relay sleeve 13 through the pipe 14 passes through the opening 17 to enter the gas supply chamber located above the commutation metal network 16 disposed within the hood 15 where it is diffused in both x and y directions to be commutated to be substantially vertical through the meshes of the commutation metal network 16 to descend on the surface of the member 32 being welded. After flowing along the surface, the shield gas passes through a gap provided at the bottom of the hood 15 to be exhausted to the outside of the hood. Such a flow of the shield gas allows the interior of the hood 15 to be filled with the shield gas, and accordingly, in the welding space disposed below the metal network 16, the shield gas provides a vertically commutated gentle flow which surrounds the focus 8, namely, the keyhole formed by the laser irradiation, the plume disposed within and without the keyhole, the arc and the tungsten electrode.
The surrounding gas isolates the molten pool around the focus 8 and the tungsten electrode 20 from air (oxygen) to prevent their oxidation and also prevent the plume located within and without the keyhole from being scattered, thus maintaining the plume within and without the keyhole.
FIG. 2a schematically shows a keyhole. As a result of the irradiation by the YAG laser, or as a result of the injection of a high density power, the plume within the keyhole assume a high temperature and a high pressure explosively to pressurize the molten metal within the keyhole, melting the bottom of the keyhole to provide a deep fusion. It is to be noted that if the shield gas stream (which strongly displaces or blows away) the plume is allowed to be directed to the focus 8, the fusion will be extremely inhibited, and accordingly, in order to achieve a deep fusion, it is essential that the plume be maintained as much as possible in the keyhole. This result has been confirmed by the inventor by an experiment.
A lap welded bead as shown in FIG. 2b is obtained by the described welding operation. In this Figure, 33 represents a region of the member being welded which is once melted and is then solidified, and 34 represents a bead which is formed by the filler wire 25. It will be noted that the width of the faying interface is large and the fusion is deep. In this manner, a high rate welding operation of a stabilized quality is possible. The welding conditions used are given below.

    Welded member 31: material A5052, 2 mm thick
    Welded member 32: material A5052, 2 mm thick
    YAG laser: output of 4 KW, without assist gas (or center gas)
    Arc


Current: 200 A
Tungsten electrode: 2.4 mm in diameter

    Shield gas: Ar, 25 liters/min
    Filler wire: rating A5356, φ1.2 mm, feed rate 2 m/min
    Welding rate: 4 m/min


Second Embodiment
FIG. 3a shows a composite YAG laser/arc welding apparatus according to a second embodiment. In the second embodiment, there is no filler wire feeding, and thus the wire guide 24 of the composite welding apparatus of the first embodiment is omitted. In this embodiment, because no power is dissipated in melting the filler wire, the arc current is reduced. As a consequence, a lap welding bead as shown in FIG. 3b is obtained. Again, the width of faying interface is large and the fusion is deep. A high rate welding operation with a stabilized quality is possible. Welding conditions used are given below.

    Welded member 31: material A5052, 2 mm thick
    Welded member 32: material A5052, 2 mm thick
    YAG laser: output 4 KW, without assist gas (center gas)
    Arc


Current: 150 A
Tungsten electrode: 2.4 mm in diameter

    Shield gas: Ar 25 liters/min
    Welding rate: 4 m/min


As described above, since the plume generated by the laser irradiation starts an electric arc, there is no need to use a high frequency high voltage in order to start an arc. The arc welding added to the YAG laser welding provides an increased heat input to the surface layer of the welded member, permits a welding operation to be performed in a stable and reliable manner if there is a degree of butt welding gap, also permitting a high rate welding operation. Since the plume is kept within and around the key hole which is produced by the irradiation by the YAG laser, a welding operation is made possible which has a deep fusion and a large width of faying interface.
While a preferred embodiments of the present invention have been shown and described above, a number of changes and modifications therein are possible. Accordingly, it is not intended that the invention be limited to the specific construction or arrangement disclosed herein, but that the right is reserved to all changes and modifications coming within the scope of the invention defined by the appended claims.

## Claims

1. A composite YAG laser/arc welding method comprising the steps of
disposing an arc electrode so that it targets a point on a member being welded which is irradiated by a laser beam from a YAG laser;
connecting an arc power supply of a no-load voltage between the arc electrode and the member being welded,
irradiating the member being welded with the laser beam from the YAG laser to produce plasmoid gasses and metal vapors which are effective to start an arc from the arc electrode which is connected to the arc power supply of a no-load voltage;
and maintaining the plasmoid gasses and metal vapors which are produced by the irradiation by the YAG laser within and around a keyhole in a member being welded created by the YAG laser to provide a combined use of the YAG laser and the arc to perform a welding operation,
wherein a welded surface of the member being welded is covered by a hood, the interior of which is partitioned by a gas commutating body having a multitude of openings into a welding space located toward the welded surface and a gas supply space, and in which the member is irradiated by the YAG laser through openings formed in the hood and the gas commutating body, a shield gas being supplied to the gas supply space and passed through the gas commutating body to provide a shield gas atmosphere around a point irradiated by the laser and the tip of the arc electrode while suppressing a disturbance of plasmoid gasses and metal vapors within the keyhole by the shield gas.

2. A composite YAG laser/arc welding method according to claim 1 in which a filler wire is fed to a molten pool located adjacent to a point irradiated by the YAG laser.