Field of the Invention

The present invention relates in general to so-called spot laser welding of overlapping metal workpieces. In particular, the present invention relates to a laser welding system, laser welding head and a laser welding method which enable an efficient spot welding of overlapping metal workpieces. It should be noted that the expressions spot laser welding and point laser welding can be used interchangeably in the context of the present invention.

Background of the Invention

Several techniques are applied in the sheet metal welding industry, in particular the automobile industry, to join two or more overlapping pieces of metal materials, especially of overlapping metal sheets. For example, resistance spot welding (RSW) is a type of electric resistance welding for welding various sheet metal products, through a process in which contacting metal surface points are joined by the heat obtained from resistance to electric current. In this process, the sheet meal workpieces are held together under pressure exerted by electrodes. Forcing a large current through the spot between the electrodes melts the metal and forms a spot weld or a point weld. An illustration of an exemplary RSW system in shown for example in Fig. 1A.

However, the resulting surface of the spot weld is usually rough, such that the welded spot can be prone to oxidation which is a concern in the automotive industry. Further, the motion sequence of electrodes, namely opening and closing of electrodes, slow down the RSW process. Moreover, a typically bulky design of the welding head in RSW systems prevents an easy access to welding spots. For example, body panels of an automobile are transported to a welding station where a clamping system is imposed to hold the body panels, while the welding operations are performed. Due to the configuration of some of the parts to be welded, certain clamping and welding apparatuses cannot be utilized as spatial restrictions may limit the amount of space i available for the necessary maneuvering and the proper functioning of the clamping and welding apparatuses.

In order to improve the quality of spot welds, laser spot welding techniques have been developed. Laser spot welding (LSW) is a welding process that uses the power of a laser beam to join two metal surfaces at a spot or point. The laser beam targets a small spot and transfers enough energy to melt and fuse metal surfaces together. For example, European Patent Application No. EP 2 701 875 A1 describes a so-called laser screw welding process in which a high-power laser beam incident on a plurality of metal sheets is scanned such that the laser beam is rotated about an axis perpendicular to a surface of the sheets, thereby forming a melt pool in the plurality of sheets. However, in order to ensure sufficient proximity or contact between metal sheets, the metal sheets need to be pressed against each other by separate supplementary devices.

An example of a LSW system according to the prior art is illustrated in Fig. 1 B.

In order to avoid the use of such separate pressing or clamping devices, for example, International Patent Application No. WO 2014/063151 A1 belonging to the same applicant as the present invention describes a so-called laser seam stepper (LSS) equipped with a so-called C-gun structure. Unlike in laser spot welding, surfaces of the overlapping workpieces are welded over a long, continuous surface in laser seam welding. Such a laser seam stepper has similar disadvantage as the aforementioned RSW systems. Namely, the opening and closing of the C-gun parts slows doen the welding process. Similarly, a bulky design of the weld head in the laser seam stepper also prevents an easy access to welding locations. Moreover, the laser welding devices described in WO 2014/063151 A1 are not suitable for a so-called “on-the-fly” welding mode which is faster than said RSW and laser welding stepper with C-gun applications. Problem to be Solved by the Invention

The present invention intends to overcome the problems of the prior art and provide a laser welding system and a laser welding method which allows to perform contact welding of metal sheets without damaging the surface of the workpiece. Furthermore, it is an object of the present invention to increase the speed of the welding process.

Solution According to the Invention

According to a first aspect, the present invention provides a laser welding system according to claim 1 . According to a second aspect, the present invention provides a laser welding method according to claim 12. Further aspects of the invention are set forth in the dependent claims, the drawings, and the following description of preferred embodiments.

The inventive laser welding system, also called laser seam pointer (LSP) is a welding tool with an laser adapted for producing point welding connections as a direct alternative to a resistance spot welding connection or a laser screw welding process. However, a laser welding system according to the present invention may also be adapted to carry out a laser screw welding process.

The LSP can be used, for example, for producing overlapping sheet metal connections in applications of the automotive industry as well as in the rest of the sheet metal processing industry. Achievable advantages of the LSP according to this invention are the significantly better accessibility to the welding point compared to the laser seam stepper (LSS) welding head. In direct comparison with a resistance spot welded (RSW) connection, the welding time can be reduced and, if the components permit, the down and up movement of a shaft from the welding tool can be dispensed. In direct comparison with the laser screw welding process (LSW), it is possible with the laser seam pointer (LSP) to press the components together without a gap in order to achieve an optimal welding result. In terms of size, appearance and durability, an RSW and the LSW point weld can be compared to an LSP point weld. Due to the combination of the faster welding process and the potential savings in the movement sequence (opening/closing), the total time for the welding and movement cycle can be reduced, for example from approximately 3.5 seconds for an RSW point weld to approximately 1.5 seconds for the LSP point weld.

Due to the design of the inventive LSP tool and associated closed beam guidance up to the welding point or just before the welding point, the dirt and emissions produced during the welding can be extracted directly at the point or origin with the help of an adaptable suction.

The inventive system, head and method can be used in places where connections are made with resistance welding guns or laser for screw welding processes today. Coated sheet metal combinations can be welded as well as uncoated sheets. Due to the one- side accessibility, sub-floor assemblies (sheets on profile) can be welded.

The following describes parameters of an exemplary LSP laser welding system.

LSP-3500 Welding-System

Base

- Producing round point welding connections like in a spot-welding process or in a laser screw welding process

- In terms of size, appearance and durability an LSP point can be compared 1 :1 to an RSW or an LSW point

Advantages

- Accessibility to the component is significantly better than with an LSS-System.

- More than twice as fast as the RSW process.

- Intact component surface. No surface damage caused by the electrode sliding effect at RSW process.

- Smooth surfaces of the components deliver significantly better results in the corrosion test. - In contrast to the LSW process, the components can be pressed together with the LSP. Thus, no additional, expensive clamping unit is required.

- No additional fixation points are necessary. In the LSW process, additional fixation can be necessary as before with RSW-welding tools.

- Welding process is optionally possible without pressing, therefore allowing to perform remote welding as well.

- First fix the components are with pressure, then they are welded on the fly.

- No dust and dirt due to the connected suction instead of the laser screw welding process

- Compact 19” rack laser with full control of laser, welding tool and optional fumator and/or cleaning station

- Only one HMI interface for program creation, visualization, log data.

The laser welding system according to the invention is configured for producing point welding connections, in particular between overlapping workpieces. For example, at least parts of the workpieces overlap, such that by performing a screw welding process (SWP) on one or more surfaces of the workpieces generates a spot weld between the workpieces to join the workpieces.

In the present description, the expression “a surface of the workpieces” can mean that when two or more overlapping metal sheets are welded together, the laser beam only directly hits one surface.

A connection unit of the laser head is mechanically connected to a positioning device of the laser welding system for moving the laser welding head with respect to the workpieces. The positioning device may be configured for moving the laser head in space with three, four, five or six degrees of freedom. For this purpose, the positioning device may comprise plural translation and/or rotation stages, in particular for linear movements in three orthogonal directions and three degrees of rotation.

A mechanical shaft surrounds at least a portion of the output laser beam. A pressure piece is connected to the mechanical shaft and comprises a front surface for applying a pressure onto the surface of the workpieces. In other words, the front surface of the pressure piece can be pressed against a surface of the workpieces.

In a first operating state, the laser welding head is configured for simultaneously applying a pressure onto the surface of the workpiece with the pressure piece and carrying out a screw welding process.

In a second operating state, the laser welding head may be configured for carrying out the screw welding process (SWP) without applying pressure onto the surface of the workpieces. In particular, the SWP may be performed with a distance between the laser welding head and the surface of the workpieces. In this operating state, no pressure is applied onto the surface of the workpieces. In other words, the pressure piece may not contact the workpieces in the second operating state. Thus, the laser welding head can be operated in at least two different operating states, with and without applying pressure onto the workpieces.

For example, when welding without applying pressure by the pressure piece, the workpieces may even be welded together with a gap between the workpieces.

In the first operating state, the optics unit may be configured for adjusting the focus of the output laser beam in a plane of the front surface of the pressure piece. As a result, the SWP can be performed on the same surface which is contacted by the pressure piece. As a result, a contact welding connection between the workpieces can be generated by the SWP.

In the second operating state, the optics unit may be configured for adjusting the focus of the output laser beam in a plane of the surface of the workpieces having a distance from the pressure piece.

The mechanical shaft may be configured for extending and/or retracting its length along the axis of the output laser beam. By extending the mechanical shaft, the pressure on the workpieces may be applied and/or controlled. The motion of extending and/or retracting the mechanical shaft can be achieved my means of an electric motor such as a servo motor or the like or by a hydraulic or pneumatic actuator. As a result, the generated pressure may be controlled with high speed and high precision. Moreover, large pressures can be exerted onto the workpieces in order to ensure contact between the workpieces, thus allowing to generate a strong connection between the workpieces.

The mechanical shaft may comprise a proportional valve controlled by the control unit for extending and/or retracting the length of the mechanical shaft. In this case, the proportional valve may control the hydraulic or pneumatic actuator.

The mechanical shaft may comprise a pressure sensor for measuring the pressure applied to the surface of the workpieces. A signal generated by the pressure sensor may be fed to the control unit of the laser head which may feedback-control the motion of the mechanical shaft, for example by controlling the proportional valve.

The control unit may be configured for controlling the pressure applied by the mechanical shaft to the surface of the workpieces based on a pressure sensor signal provided by the pressure sensor. As a result, the pressure may be controlled with high precision to ensure sufficient contact between the workpieces while avoiding damage of the workpieces.

A protective glass may be provided between the mechanical shaft and the optics unit. The protective glass may prevent weld fumes and other contaminants to enter the optics unit. Thus, contamination of the optics and possible damage of the optics may be prevented.

The mechanical shaft may comprise an over jet nozzle for providing a gas flow onto the protective glass. As a result, it can be prevented that dust, weld fumes or other particles and/or contaminants adhere to the protective glass. Thus, damage of the protective glass and/or a deterioration of the laser beam quality may be prevented.

The pressure piece may comprise a fume extraction unit for extracting weld fumes generated during the welding process. As a result, contamination of optics, the pressure piece, the protective glass, the workpieces and/or other components of the laser welding system can be efficiently prevented. Moreover, contamination of the surrounding ambient air may be reduced in order to avoid health risks for operators of the welding system or other workers in the vicinity of the laser welding system.

The laser welding system may comprise a high-power laser source providing a high- power laser output beam such as a fiber laser which may output several kilowatts of laser power in single mode or multimode laser operation at a suitable laser wavelength.

Preferably, the laser welding system comprises a fixture for holding the workpieces. The fixture may be positioned on one or more translation and/or rotation stages for positioning the workpieces relative to the laser welding head.

The laser welding system may comprise a control system for controlling operation of the laser welding system. The control system may act as a higher-level control system which also controls the control unit of the laser welding head. Moreover, the control system may control the motion of the positioning system. Furthermore, the control system may control operation of the high-power laser source.

Brief Description of the Drawings

The invention is illustrated in greater detail with the aid of schematic drawings.

Figure 1A: Fig. 1A illustrates an example of the known resistance spot welding (RSW) tool.

Figure 1 B: Fig. 1 B illustrates an example of the known laser screw welding (LSW) system.

Figure 2: Fig. 2 shows an example of the laser beam movement pattern on the surface of the workpieces to be welded. Figure 3: Fig. 3 is an external view of a laser head in a laser seam pointer (LSP) system according to the present invention.

Figure 4: Fig. 4 is an exploded view of the laser head in a laser seam pointer (LSP) system according to the present invention.

Figure 5: Fig. 5A is an external view and Fig. 5B is cross-section along line A-A of the laser head in a laser seam pointer (LSP) system according to the present invention.

Figure 6: Fig. 6 shows an example of the inventive laser seam pointer (LSP) system in a first operating state.

Figure ?: Fig. 7 shows an example of the inventive laser seam pointer (LSP) system in a second operating state.

Figure 8: Fig. 8 shows an example of the point (spot) welding connections manufactured by the laser seam pointer (LSP) system according to the present invention.

Detailed Description of Embodiments

Fig 2 shows an exemplary spiral trace which is scanned by the laser beam spot on the workpiece surface when performing laser screw welding. Such a trace in a screw welding process can be used to create a point-like spot weld between sheet metal workpieces as shown e.g. in Fig. 8.

Welding Process

In order to obtain the round shape similar to a resistance welding point or laser screw welding point, an optical system is used that can position the laser beam, for example by means of scanning mirrors which may be controlled using actuators which may comprise piezo elements or servo motors. According to the art, a trace of a laser screw weld usually resembles a spiral as depicted in Fig. 2.

The shape, direction of movement, laser power adjustment during movement and speed can be freely selected in order to optimize the resulting spot weld.

Basic process data:

Exemplary laser power may range from several Watts to several Kilowatts, for example 3,5 kW maximum peak power. The scanning speed of the laser beam may range between for example 60 mm/s to 250 mm/s. The resulting diameter of the spot-weld may range between for example 1 mm to 7mm.

Figs. 3 to 5 illustrate an embodiment of a laser welding system 1 according to the present invention for producing point welding connections on overlapping workpieces 2 (not depicted in Figs. 3 to 5). The laser welding system 1 comprises a laser welding head 3. Fig. 3 shows an outside perspective view of the laser head 3. Fig. 4 shows an exploded view of the laser welding head 3. Fig. 5A shows front plan view and Fig. 5B shows a cross section along line A-A.

The laser head 3 comprises a housing 14, a control unit 4 (see Fig. 5B), a connection unit 5, a laser beam delivery unit 7, an optics unit 10, a mechanical shaft 12, and a pressure piece 13. On the backside of the housing 14, a connection unit 5 is provided to attach the laser head 3 to a positioning device which can translate and rotate the laser head 3.

Control electronics of the control unit 4 are visible in the cross-sectional view of Fig. 5B. The control unit 4 controls operation of the laser welding head 3 including scanning optics such as mirrors of the optics unit 10 and a motion of the mechanical shaft 12. The connection unit 5 can mechanically connect the laser head 3 to a positioning device 6 of the laser welding system 1 for moving the laser welding head 3 with respect to the workpieces 2. The laser beam delivery unit 7 is connected to an optical fiber 8 providing an input laser beam 9. In particular, the laser beam delivery unit 7 comprises a fiber coupler for connecting the optical fiber 9 and a collimation lens for collimating the input laser beam.

The optics unit 10 comprises a plurality of optical components for receiving the input laser beam 9 and delivering an output laser beam 11 to a surface of the workpieces 2. Here, the optics unit 10 may comprise at least two scanning mirrors for scanning the output laser beam in two directions across the surface of the workpieces. Furthermore, the optics unit 10 may comprise at least one focusing lens to focus the output laser beam onto the surface of the workpieces.

Furthermore, cameras and sensors may be provided to analyze the beam quality. Moreover, polarizing optics may be used to control a polarization of the output beam.

The optics unit 10 is configured for focusing the output laser beam 11 onto the surface of the workpieces 2 and for scanning the output laser beam 11 across the surface of the workpieces 2.

The mechanical shaft 12 surrounds at least a portion of the output laser beam 11. A pressure piece 13 is connected to the mechanical shaft 12. The pressure piece 13 comprises a front surface 13a for applying a pressure onto the surface of the workpieces 2.

In a first operating state, the laser welding head 3 can simultaneously apply a pressure onto the surface of the workpiece 2 with the pressure piece 13 and carry out a welding process such as a screw welding process by scanning the laser beam across the surface of the workpieces, for example following a spiral trace as illustrated in Fig. 2. Of course, also traces with other suitable shapes may be used.

In a second operating state, the laser welding head can carry out the screw welding process without applying pressure onto the surface of the workpieces 2. In this second operating state, a distance welding process may be carried out. In the first operating state, the optics unit 10 can adjust the focus of the output laser beam 11 to lie in a plane of the front surface 13a of the pressure piece 13. Since the front surface 13a of the pressure piece 13 contacts the surface of the workpieces, the laser focus will lie on the surface of the workpieces, such that the surface is heated and melted to create the weld. in the second operating state, the optics unit 10 can adjust the focus of the output laser beam 11 such, that the focus lies in a plane of the surface of the workpieces 2 having a distance from the pressure piece 13.

The mechanical shaft 12 can extend and retract its length along the axis of the output laser beam. By extending the mechanical shaft, the force of the applied pressure can be controlled with high precision. Alternatively, the pressure may be controlled by controlling a translation stage of the positioning device 6 to which the laser head 3 is connected.

The mechanical shaft 12 comprises a proportional valve controlled by the control unit 4 for extending and/or retracting the length of the mechanical shaft 12. Preferably, the mechanical shaft 12 comprises a pressure sensor for measuring the pressure applied to the surface of the workpieces 2.

The control unit 4, see Fig. 5B, is configured for controlling the pressure applied by the mechanical shaft 12 to the surface of the workpieces 2 based on a pressure sensor signal provided by the pressure sensor. The control unit 4 can be connected to a higher-level control system of the laser welding system 1 via the control line connection 15 which is provided on the outside of housing 14.

A protective glass 17 is provided between the mechanical shaft 12 and the optics unit 10 and protects the optics of the optics unit 10 from dust, weld fumes and other contaminants. The mechanical shaft 12 comprises an over jet nozzle for providing a gas flow onto the protective glass. The pressure piece 13 comprises a fume extraction unit 18 for extracting weld fumes generated during the welding process.

Reference sign 16 indicates a connection line for providing hydraulics or pneumatics for the valve or suction for the fume extraction unit.

A large portion of the laser beam is guided through the mechanical shaft 12, which takes on several tasks at the same time. By surrounding the laser beam, the mechanical shaft 12 acts as a protection device which prevents unwanted interactions between the laser beam and other objects. Thus, the mechanical shaft 12 can improve a safe operation of the laser welding system 1 .

Furthermore, the mechanical shaft 12 can be extended in length by up to 12 mm using a proportional valve. This value is only an example and not intended to limit the scope of the invention. In other embodiments the length of the mechanical shaft 12 may be varied within a range of ±10 mm, ±15 mm, ±20 mm, ±25 mm, ±30 mm, or up to ±50 mm.

On the other hand, the mechanical shaft 12 can be used as a pressure tool to join sheet metal pairings of the workpieces together. When using the press function, an air cushion may be pressed with a maximum force of up to, for example, 1 kN. For pressing the workpieces, the mechanical shaft 12 comprises the pressure piece 13.

The mechanical shaft 12 also comprises a so-called over jet, which protects the protective glass of the optics from dirt and smoke from the welding process. The compressed air consumption can be reduced to a minimum due to the design of the overjet nozzle and can be lowered by a factor of two compared to standard high-power scanner optics with large beam exit surfaces, which are used in laser screw welding processes.

The pressure piece 13 can be connected with an optional fume extraction and/or an overjet nozzle. In this case the surface of the welded part of the workpiece and the welding area is much cleaner compared to the LSW process. The emission load from welding fumes and the environment can be significantly reduced. The pressure piece 13 touches down with its smooth surface 13a without damaging the surface of the workpieces 2 or leaving a sliding mark as can be case in the RSW process.

Figs. 6 and 7 illustrate the first and second operating state of the welding head 3, respectively. As can be seen in Fig. 6, the front surface 13a of the pressure piece 13 contacts the surface of the workpieces 2. On the other hand, Fig. 7 shows the second operating state in which a distance welding process is carried without contact between the pressure piece 13 and the surface of the workpieces.

The features described in the above description, claims, and figures can be relevant to the invention in any combination. Their reference numerals in the claims have merely been introduced to facilitate reading of the claims. They are by no means meant to be limiting.

List of Reference Numerals

1 . laser welding system

2. workpiece

3. laser welding head

4. control unit

5. connection unit

6. positioning device

7. laser beam delivery unit

8. optical fiber

9. input laser beam

10. optics unit

11 . output laser beam

12. mechanical shaft

13. pressure piece

13a front surface of the pressure piece

14 housing

15 control line connection

16 connection to hydraulics/pneumatics/suction

17 protective glass

18 fume extraction unit