LASER WELDING METHOD FOR WORKPIECE

In a method for laser welding of workpieces W1 and W2, a laser beam is generated and a workpiece W1 is irradiated. An irradiation point of the laser beam is swung within a predetermined heating area on the workpiece W1 encompassing a welding location where laser welding is to be executed, heating the mating surface area of the workpieces W1 and W2 corresponding to the heating area to a temperature higher than or equal to the boiling point of the coating material of the workpiece W2 and lower than the melting point of the base material of the workpiece W1, forming a gap between the workpieces W1 and W2 by vaporizing the coating material by the heating, discharging the coating material through the gap to the outside of the mating surface area, and melting and welding together the welding location of the base materials of the workpieces W1 and W2.

FIELD OF THE INVENTION

The present disclosure relates to a method of laser welding workpieces.

BACKGROUND OF THE INVENTION

There has been known a method of laser welding a pair of workpieces (galvanized steel sheets) stacked with a cover material (galvanized) interposed therebetween (e.g., Patent Literature 1).

PATENT LITERATURE

SUMMARY OF THE INVENTION

There has been known a problem in that when the cover material interposed between base materials is vaporized by heat of the laser beam to be mixed in the base materials molten, bubbles are formed inside the base materials.

A method of laser welding a first workpiece and a second workpiece stacked so as to surface-contact with each other, the first workpiece and the second workpiece each including a base material, at least one of the first workpiece and the second workpiece including a cover material interposed between the base materials of the first workpiece and the second workpiece, of one aspect of the present disclosure includes: generating a laser beam by a laser oscillator and irradiating the first workpiece with the laser beam; swinging an irradiation point of the laser beam within a heating area, which is set on the first workpiece so as to encompass a welding location on which the laser welding is to be executed, and heating a mating surface area of the first workpiece and the second workpiece, which corresponds to a heating area, to a temperature being equal to or higher than a boiling point of the cover material and lower than a melting point of the base material of the first workpiece; forming a gap between the first workpiece and the second workpiece by vaporizing the cover material in the mating surface area by the heating, and discharging the cover material to outside of the mating surface area through the gap; and melting and welding the base materials of the first workpiece and the second workpiece to each other in the welding location by irradiating the welding location with the laser beam, after discharging the cover material to the outside of the mating surface area.

With the present disclosure, the cover material can be discharged from the mating surface area through the gap. Thus, the cover material can be reliably removed from the region where the base materials melt. Thus, when the base materials are molten in the welding location, vapor of the cover material can be prevented from mixing into the base materials in a form of bubbles. No through hole for discharging the vapor of the cover material to the outside needs to be formed in the base materials, whereby processes of the welding flow can be simplified.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In various embodiments described below, the same elements are designated by the same reference numerals and duplicate description will be omitted. In the following description, an orthogonal coordinate system C1in each drawing is used as a reference for directions, and for the sake of convenience, a positive x-axis direction of the coordinate system C1is referred to as toward the right side, a positive y-axis direction is referred to as toward the front, and a positive z-axis direction is referred to as toward the upper side.

A laser welding system10according to an embodiment is described with reference toFIG.1andFIG.2. The laser welding system10is a system for welding a pair of workpieces W1and W2using a laser beam. The laser welding system10includes a laser oscillator12, a light-guiding member14, a laser irradiation device16, an irradiation device movement mechanism18, an irradiation point movement mechanism20, and a control device22.

The laser oscillator12is a solid-state laser oscillator (e.g., a YAG laser oscillator or a fiber laser oscillator) or a gas laser oscillator (e.g., a carbon dioxide laser oscillator), or the like, internally generates a laser beam LB through optical resonance in response to a command from the control device22, and emits the laser beam LB to the light-guiding member14.

The light-guiding member14includes an optical element such as an optical fiber, a light guide path made of a hollow or light-transmitting material, a reflection mirror, or an optical lens, and guides the laser beam LB generated by the laser oscillator12to the laser irradiation device16. The laser irradiation device16is a laser scanner, a laser processing head, or the like, focuses the laser beam LB incident from the light-guiding member14, and irradiates the workpiece W1with the laser beam LB.

The irradiation device movement mechanism18moves the laser irradiation device16relative to the workpiece W1and the workpiece W2. For example, the irradiation device movement mechanism18is a vertical articulated robot capable of moving the laser irradiation device16to any position in the coordinate system C1. Alternatively, the irradiation device movement mechanism18may include a plurality of ball screw mechanisms that move the laser irradiation device16along the x-y plane of the coordinate system C1and in the z-axis direction of the coordinate system C1.

The coordinate system C1is, for example, a world coordinate system defining a three-dimensional space of a work cell, a movement mechanism coordinate system (e.g., a robot coordinate system) for controlling the operation of the irradiation device movement mechanism18, a workpiece coordinate system defining the coordinates of the workpiece W1and the workpiece W2, or the like, and is a control coordinate system for automatically controlling the operation of movable components (i.e., the irradiation device movement mechanism18and the irradiation point movement mechanism20) of the laser welding system10.

The irradiation point movement mechanism20moves an irradiation point P on the workpiece W1, when the laser irradiation device16irradiates the workpiece W1with the laser beam LB, relative to the workpiece W1. Specifically, the irradiation point movement mechanism20includes an optical element such as a mirror or an optical lens, a driving device for driving the optical element, a work table for moving the workpiece W1and the workpiece W2, and the like, and operates these components, to move the irradiation point P relative to the workpiece W1.

The control device22controls the operation of the laser oscillator12, the laser irradiation device16, the irradiation device movement mechanism18, and the irradiation point movement mechanism20. Specifically, the control device22is a computer including a processor50, a memory52, and an I/O interface54. The processor50includes a CPU, a GPU, or the like, and is communicably connected to the memory52and the I/O interface54via a bus56. The processor50performs arithmetic processing for implementing various functions described below while communicating with the memory52and the I/O interface54.

The memory52includes a RAM, a ROM, or the like, and stores various types of data temporarily or permanently. The I/O interface54includes, for example, an Ethernet (trade name) port, a USB port, an optical fiber connector, or an HDMI (trade name) terminal and performs wired or wireless data communications with an external device under a command from the processor50.

The control device22is provided with an input device58and a display device60. The input device58includes a keyboard, a mouse, a touch panel, or the like, and accepts input of data from an operator. The display device60includes a liquid crystal display, an organic EL display, or the like and displays various types of data. The laser oscillator12, the laser irradiation device16, the irradiation device movement mechanism18, the irradiation point movement mechanism20, the input device58, and the display device60are connected to the I/O interface54, in such a manner as to be capable of performing wired or wireless communications.

Next, the laser irradiation device16and the irradiation point movement mechanism20according to an embodiment will be described with reference toFIG.3. The laser irradiation device16illustrated inFIG.3is a laser scanner including a main body24, a light receiver26, an optical lens28, a lens driving device30, and an emitting unit32. The main body24is hollow, and has a propagation path for the laser beam LB defined therein. The light receiver26is provided to the main body24and receives the laser beam LB propagated in the light-guiding member14.

The optical lens28has a focus lens and the like, and focuses the laser beam LB. In the present embodiment, the optical lens28is supported inside the main body24so as to be movable in the direction of an optical axis O of the laser beam LB incident on the optical lens28. The lens driving device30includes a piezoelectric element, an ultrasonic vibrator, an ultrasonic motor, or the like, and displaces the optical lens28in the direction of the optical axis O in response to a command from the control device22, and thus displaces the focal point of the laser beam LB, with which the workpiece W1is irradiated, in the direction of the optical axis O. The emitting unit32emits the laser beam LB, focused by the optical lens28, to the outside of the main body24.

The main body24further accommodates mirrors34and36and mirror driving devices38and therein. The mirror34(first mirror) is supported inside the main body24so as to be rotatable about an axis A1. The mirror34is disposed on an optical path O of the laser beam LB entered into the main body24through the light receiver26, and reflects the laser beam LB toward the mirror36.

The mirror driving device38is a servo motor for example, and rotates the mirror34about the axis A1in response to a command from the control device22. By thus rotating the mirror34, the mirror driving device38changes the orientation of the mirror34, and thus can change the direction of reflection of the laser beam LB by the mirror34.

The mirror36(second mirror) is supported inside the main body24so as to be rotatable about an axis A2. The axis A2and the axis A1are substantially orthogonal to each other. The mirror36is disposed on the optical path O of the laser beam LB reflected by the mirror34, and reflects the laser beam LB toward the optical lens28.

The mirror driving device40is a servo motor for example, and rotates the mirror36about the axis A2in response to a command from the control device22. By thus rotating the mirror36, the mirror driving device40changes the orientation of the mirror36, and thus can change the direction of reflection of the laser beam LB by the mirror36. Generally, the mirrors34and36are what are known as galvano mirrors, and the mirror driving devices38and40are what are known as galvano motors.

As described above, the laser beam LB that has entered into the main body24from the light receiver26is reflected by the mirrors34and36, and then is focused by the optical lens28. The resultant laser beam LB is to emitted to the outside through the emitting unit32and onto the workpiece W1. The control device22operates the mirror driving devices38and40, to change the orientation of each of the mirrors34and36, and thus moves the irradiation point P on the workpiece W1irradiated with the laser beam LB, relative to the workpiece W1. Thus, in the present embodiment, the mirrors34and36, and the mirror driving devices38and40form the irradiation point movement mechanism20.

Next, a method of laser welding the workpiece W1and the workpiece W2by using the laser welding system10will be described. As illustrated inFIG.1andFIG.4, the workpiece W1and the workpiece W2are flat-plate members, are stacked so as to surface-contact with each other, and are fixed by a jig (not illustrated) or the like. In the present embodiment, each of the workpieces W1and W2is positioned at a known position in the coordinate system C1so as to be substantially parallel to the x-y plane of the coordinate system C1.

The workpiece W1includes a base material100and a cover material102stacked on a surface of the base material100. The base material100is a flat-plate member made of metal (e.g., iron), and includes an upper surface104and a lower surface106on the side opposite to the upper surface104. In the present embodiment, the cover material102is stacked on the surfaces of the base material100so as to cover the entire surfaces thereof, and includes a first layer102athat covers the upper surface104of the base material100and a second layer102bthat covers the lower surface106of the base material100. The cover material102is made of metal (e.g., zinc) of a type different from the base material100.

Similarly, the workpiece W2includes a base material110and a cover material112stacked on a surface of the base material110. The base material110is a flat-plate member made of metal (e.g., iron), and includes an upper surface114and a lower surface116on the side opposite to the upper surface114. In the present embodiment, the cover material112is stacked on the surfaces of the base material110so as to cover the entire surfaces thereof, and includes a first layer112athat covers the upper surface114of the base material110and a second layer112bthat covers the lower surface116of the base material110. The cover material112is made of metal (e.g., zinc) of a type different from the base material110.

In the present embodiment, it is assumed that the base material100and the base material110are made of metal (iron) of the same type, and the cover material102and the cover material112are made of metal (zinc) of the same type (e.g., the workpiece W1and the workpiece W2are both galvanized steel sheets). A boiling point T1of the cover material102and the cover material112(about 900° C. in the case of zinc) is lower than a melting point T2of the base material100and the base material110(about 1500° C. in the case of iron).

The workpiece W1and the workpiece W2are stacked on and fixed such that the second layer102bof the cover material102and the first layer112aof the cover material112surface-contact with each other. As illustrated inFIG.4, when the workpiece W1and the workpiece W2are fixed, the second layer102bof the cover material102and the first layer112aof the cover material112are interposed between the base material100and the base material110.

As a preparation process PP for welding the workpiece W1and the workpiece W2, the operator sets a work condition CD for executing the work of welding the workpiece W1and the workpiece W2. The work condition CD includes: data on a welding location WL where the laser welding is to be executed in a full welding process WP described below; and data on a heating area HA where the workpiece W1is to be heated in a heating process HP described below. Referring toFIG.5toFIG.9, a method of setting the welding location WL and the heating area HA will be described below.

First of all, the operator sets the welding location WL for the workpiece W1in the coordinate system C1. In the example illustrated inFIG.5, the welding location WL is defined by two teaching points TP1and TP2set in the first layer102aof the cover material102of the workpiece W1, and a welding line LN connecting the teaching points TP1and TP2. The teaching points TP1and TP2are target positions in which the irradiation point P of the laser beam LB is to be positioned in the full welding process WP described below, and the welding line LN defines a target path in which the irradiation point P is to be moved from the teaching point TP1to the teaching point TP2.

For example, the operator operates the input device58while visually recognizing drawing data (CAD data) of the workpiece W1and the workpiece W2displayed on the display device60, and designates the teaching points TP1and TP2in the first layer102aof the workpiece W1. The processor50sets the teaching points TP1and TP2and the welding line LN in the coordinate system C1based on the input data from the operator.

Next, the operator operates the input device58to set the heating area HA so as to encompass the welding location WL on the first layer102a. In the example illustrated inFIG.5, the heating area HA is set as a rectangular region that includes the entire welding location WL on the inner side, has a longitudinal direction extending in parallel with the x axis of the coordinate system C1, and has a shorter side direction extending in parallel with the y axis of the coordinate system C1.

More specifically, the heating area HA has a length x1in the longitudinal direction, and has a width y1in the shorter side direction. As an example, the length x1of the heating area HA may be set in such a manner that a left side SD1of the heating area HA is at a position separated toward the left side from the teaching point TP1by a distance x2(e.g., by 1 [mm] to 2 [mm]), and a right side SD2of the heating area HA is at a position separated toward the right side from the teaching point TP2by a distance x3(e.g., 1 [mm] to 2 [mm]). Thus, in this case, the length x1of the heating area HA is longer than the length of the welding line LN in the x-axis direction of the coordinate system C1.

Also, the width y1of the heating area HA may be set to be at least three times as long as a width of a bead, formed on the welding line LN when the base material100and the base material110are welded along the welding line LN in the full welding process WP described below, in the y-axis direction of the coordinate system C1(or the width of the irradiation point P of the laser beam LB in the full welding process WP or the heating process HP). Each vertex and side of the heating area HA can be expressed by coordinates on the coordinate system C1. Thus, the heating area HA is set in the first layer102aso as to encompass the welding location WL.

Next, the operator sets a teaching point TPn for the heating process HP and an irradiation point movement path MP in the heating area HA. The teaching point TPn for the heating process HP is a target position where the irradiation point P of the laser beam LB is to be positioned in the heating process HP described below. The irradiation point movement path MP defines a target path of the intended movement of the irradiation point P from the teaching point TPn to a teaching point TPn+1.FIG.6illustrates an example of how the teaching point TPn is set.

The operator operates the input device58to set teaching points TP11, TP12, TP13, TP14, TP15, and TP16in the heating area HA. Note that inFIG.6, the welding location WL is omitted for ease of understanding. In the example illustrated inFIG.6, the teaching points TP11, TP12, TP15, and TP16are disposed at the respective vertices of the heating area HA, and the teaching points TP14and TP13are disposed at midpoints of the respective sides SD1and SD2of the heating area HA.

Next, the operator operates the input device58to set the forward path of the irradiation point movement path MP based on the teaching point TPn, as illustrated inFIG.7. In the example illustrated inFIG.7, the forward path of the irradiation point movement path MP is set as a path passing through the teaching points TP11, TP12, TP13, TP14, and TP15in this order.

Next, the operator operates the input device58to set the return path of the irradiation point movement path MP, as illustrated inFIG.8. In the example illustrated inFIG.8, the return path of the irradiation point movement path MP is set as a path passing through the teaching points TP15, TP16, TP13, TP14, and TP11in this order. Thus, as illustrated inFIG.9, the irradiation point movement path MP is set in the heating area HA, as a path that passes through the teaching points TP11, TP12, TP13, TP14, TP15, TP16, TP13, TP14, and TP11in this order.

The teaching points TP11to TP16and the irradiation point movement path MP are expressed with coordinates in the coordinate system C1. The position of the heating area HA in the coordinate system C1can be expressed with coordinates of the teaching points TP11to TP16and the irradiation point movement path MP. Thus, the heating area HA can be regarded as the region defined by the teaching points TP11to TP16and the irradiation point movement path MP.

As described above, the operator sets the welding location WL (the teaching points TP1and TP2and the welding line LN) and the heating area HA (the teaching points TP11to TP16, and the irradiation point movement path MP) for the workpiece W1. The operator may set a plurality of the welding locations WL and heating areas HA at different positions on the workpiece W1.

The position data (specifically, the coordinates of the teaching points TP1and TP2and the welding line LN in the coordinate system C1) on the welding location WL, and the position data (specifically, the coordinates of the teaching points TP11to TP16and the irradiation point movement path MP in the coordinate system C1) of the heating area HA thus set are stored in the memory52as the work condition CD.

The work condition CD further includes data such as: swinging speed V1(first speed) of the irradiation point P and laser power LP1of the laser beam LB in the heating process HP; a time period tip during which the heating process HP is executed; forward movement speed V2(second speed) of the irradiation point P and laser power LP2of the laser beam LB in the full welding process WP; a focal position FP of the laser beam LB in the heating process HP and the full welding process WP; and an operation mode OM of the laser oscillator12in the heating process HP and the full welding process WP.

For example, the operation mode OM of the laser oscillator12includes a first operation mode OM1under which the laser oscillator12generates a laser beam LB1of a first type, and a second operation mode OM2under which the laser oscillator12generates a laser beam LB2of a second type different from the first type. For example, the laser beam LB1of the first type is a pulsed oscillation laser beam, whereas the laser beam LB2of the second type is a continuous wave laser beam.

In the preparation process PP, the operator operates the input device58to set the work condition CD including the speed V1and the speed V2, the laser power LP1and the laser power LP2, the time period tip, the coordinates of the focal position FP in the coordinate system C1, and the operation mode OM. Then, the operator generates a welding program PG based on the set work condition CD (the welding location WL, the heating area HA, the speed V1and the speed V2, the laser power LP1and the laser power LP2, the time period tip, the focal position FP, and the operation mode OM).

The welding program PG is a computer program that makes the processor50execute a welding flow described below (FIG.10). Parameters of the work condition CD are defined in the welding program PG. The welding program PG generated is stored in the memory52of the control device22. Thus, in the preparation process PP, the work condition CD is set and the welding program PG is generated.

Next, the welding flow executed by the laser welding system10will be described with reference toFIG.10. The welding flow illustrated inFIG.10starts when the processor50receives a welding start command from the operator, a higher-level controller, or a computer program (e.g., the welding program PG). The processor50executes the welding flow illustrated inFIG.10according to the welding program PG that is stored in the memory52in advance.

In step S1, the processor50operates the irradiation device movement mechanism18to place the laser irradiation device16at a predetermined welding position Pw relative to the workpiece W1and the workpiece W2. When the laser irradiation device16is placed at this welding position Pw, the entirety of the heating area HA set for one welding location WL to be welded falls within the range of movement of the irradiation point P, on the workpiece W1, caused by the irradiation point movement mechanism20.

In step S2, the processor50executes the heating process HP. Specifically, the processor50first switches the operation mode OM of the laser oscillator12to the first operation mode OM1, and transmits a command for generating the laser beam LB1of the first type having the laser power LP1, to the laser oscillator12. In response to the command, the laser oscillator12generates the laser beam LB1having the laser power LP1through pulsed oscillation, and emits the laser beam LB1to the laser irradiation device16through the light-guiding member14.

In addition, the processor50operates the lens driving device30(FIG.3) of the laser irradiation device16to adjust the position of the optical lens28, to control the focal point of the laser beam LB1emitted from the laser irradiation device16to be at a focal position FP1. In the present embodiment, the focal position FP1is set to a position shifted slightly toward the upper side (or lower side) from the upper surface of the workpiece W1(i.e., the upper surface of the first layer102aof the cover material102).

Thus, the laser beam LB1having the laser power LP1is emitted onto the workpiece W1. An irradiation point P1of the laser beam LB1at this time has an area E1. The area E1is proportional to the shifted amount of the focal position FP1from the upper surface of the workpiece W1. Note that at this time point, the irradiation point P1may be disposed at the teaching point TP11of the heating area HA.

Next, the processor50operates the irradiation point movement mechanism20to swing the irradiation point P1of the laser beam LB1at the speed V1in the heating area HA. Specifically, the processor50operates the mirror driving devices38and40to respectively change the orientation of the mirrors34and36, and thus makes the irradiation point P1move at the speed V1relative to the workpiece W1.

For example, when the laser irradiation device16is placed at the welding position Pw, the irradiation point P1can be displaced along the x axis of the coordinate system C1in the heating area HA by changing the orientation of one of the mirrors34and36, and the irradiation point P1can be displaced along the y axis of the coordinate system C1in the heating area HA by changing the orientation of the other one of the mirrors34and36.

The processor50changes the orientation of each of the mirrors34and36, to make the irradiation point P1repeatedly reciprocate at the speed V1along the irradiation point movement path MP described above (path passing through the teaching points TP11, TP12, TP13, TP14, TP15, TP16, TP13, TP14, and TP11in this order), to thus make the irradiation point P1swing in the heating area HA. This speed V1is set to 200 [m/min], for example.

With the irradiation point P1thus swinging at high speed in the heating area HA, the heating area HA is entirely heated by the laser beam LB1. The heat produced in the heating area HA propagates to a mating surface area SE between the workpiece W1and the workpiece W2through the base material100. Thus, the mating surface area SE is also heated.

The mating surface area SE may be defined as a region including the lower surface of the second layer102band the upper surface of the first layer112arespectively of the cover material102and the cover material112in surface contact with each other, and a region between the lower surface106of the base material100and the upper surface114of the base material110(or the occupied region of the second layer102band the first layer112a) for example.

In the present embodiment, the processor50makes the irradiation point P1continuously swing for the time period tip in the heating area HA, to heat a mating surface area SE′ corresponding to the heating area HA in the mating surface area SE, to a temperature T that is equal to or higher than a boiling point T1of the cover material102(i.e., the cover material112) and lower than the melting point T2of the base material100(i.e., the base material110) (T1≤T<T2).

For example, the mating surface area SE′ may be defined as a region of the heating area HA projected onto the mating surface area SE in the z-axis direction of the coordinate system C1(in other words, a region, of the mating surface area SE, having the position in the x-y plane of the coordinate system C1and the area that are substantially the same as those of the heating area HA). InFIG.11, an example of the mating surface area SE′ is schematically illustrated as a gray region.

FIG.12illustrates an example of a graph of a temperature distribution of the mating surface area SE′ heated in this step S2, in the y-axis direction of the coordinate system C1. InFIG.12, a y coordinate yαcorresponds to the positions of the teaching points TP15and T16in the y-axis direction in the coordinate system C1(FIG.9), a y coordinate yβcorresponds to the positions of the teaching points TP13and T14in the y-axis direction in the coordinate system C1, and a y coordinate yγcorresponds to the positions of the teaching points TP11and TP12in the y-axis direction in the coordinate system C1.

Through step S2, as illustrated inFIG.12, the temperature T of the mating surface area SE′ is controlled to be within a temperature range (T1≤T<T2) that is equal to or higher than the boiling point T1of the cover material102and lower than the melting point T2of the base material100. In the present embodiment, the irradiation point P1passes through the path between the teaching points TP13and T14in the irradiation point movement path MP twice, and passes through the other paths only once, while reciprocating once in the forward path (FIG.7) and the return path (FIG.8) of the irradiation point movement path MP.

In other words, with the irradiation point movement path MP, the irradiation point P1more frequently passes through the center portion of the heating area HA in the y-axis direction of the coordinate system C1in step S2. Thus, the temperature of the center portion of the heating area HA is the highest. As a result, the temperature of the center portion (portion of y=yβ) is also the highest in the mating surface area SE′ as illustrated inFIG.12.

When the mating surface area SE′ is heated to the temperature T that is equal to or higher than the boiling point T1and is lower than the melting point T2, the second layer102bof the cover material102and the first layer112aof the cover material112in the mating surface area SE′ vaporize. The inflation pressure of the gas generated by the vaporization of the cover material102and the cover material112is extremely high.

Thus, the inflation pressure of the cover material102and the cover material112, produced in the mating surface area SE′ pushes the upper surface114of the base material110toward the lower side, and pushes the lower surface106of the base material100toward the upper side, resulting in slight elastic deformation of the base material100and the base material110at high temperature. This elastic deformation of the base material100and the base material110is reversible, meaning that the base material100and the base material110return to their original shapes upon being cooled.

A gap G is formed between the pair of workpieces W1and W2as illustrated inFIG.13, as a result of such vaporization of the cover material102and the cover material112, elastic deformation of the base material100and the base material110caused by the vaporization, thermal expansion of the base material100and the base material110due to heating, and the like. Note that inFIG.13, the gap G is illustrated in an emphasized manner for the sake of easier understanding. The actual size of the gap G is in the order of microns.

The vapor of the cover material102and the cover material112produced in the mating surface area SE′ is radially blown toward the outside of the mating surface area SE′, through the gap G. As a result, the second layer102bof the cover material102and the first layer112aof the cover material112in the mating surface area SE′ are discharged to the outside of the mating surface area SE′.

With the mating surface area SE′ thus heated to the temperature T that is equal to or higher than the boiling point T1and lower than the melting point T2in this step S2, the cover material102and the cover material112can be discharged from the mating surface area SE′ while maintaining the base material100and the base material110in the solid state. In other words, the work condition CD (the speed V1, the laser power LP1, the time period tip, the focal position FP1, and the operation mode OM1) used in step S2is set so that the temperature T of the mating surface area SE′ can be controlled to be within the temperature range that is equal to or higher than the boiling point T1and lower than the melting point T2.

The present inventors have performed an experiment of executing step S2on the workpieces W1and W2, which are galvanized steel sheets each having a thickness of 0.7 [mm], under the work condition CD described below.

As a result of this experiment, it was confirmed that the cover material102and the cover material112were discharged from a rectangular region of the length x≈55 [mm]×the width y≈3 [mm] encompassing the entire region of the mating surface area SE′ therein. Thus, this experiment result indicates that with the work condition CD appropriately set, the cover material102and the cover material112can be not only discharged from the mating surface area SE′ but can also be discharged from a region in the periphery of the mating surface area SE′.

When the time period tip set as the work condition CD elapses from a time point at which the swinging of the irradiation point P1in step S2has started, the processor50transmits a command to the laser oscillator12to stop the emission of the laser beam LB1, thereby terminating the heating process HP in step S2. For example, the processor50may stop the emission of the laser beam LB1by stopping the laser beam generation operation by the laser oscillator12. Alternatively, the laser oscillator12may further include a shutter that opens and closes the optical path of the emitted laser beam LB1, and the processor50may stop the emission of the laser beam LB1by closing the shutter.

Referring back toFIG.10, the processor50determines whether the base material100and the base material110have been cooled down to a temperature not higher than a predetermined threshold value T3in step S3. This threshold value T3may be set to the melting point of the cover material102and the cover material112, for example, or may be set to ambient temperature of the atmosphere.

For example, the processor50may measure an elapsed time period t1from a time point at which the heating process HP in step S2has ended, and determine that the base materials100and110are cooled down to a temperature not higher than the threshold value T3(i.e., YES) when the elapsed time period t1has reached a predetermined time period tth.

This time period tthis determined in advance by the operator (e.g., tth=20 [msec]) as a time period sufficient for the base material100and the base material110heated in step S2to be cooled down to a temperature not higher than the threshold value T3, and is stored in the memory52. The processor50proceeds to step S4upon determining YES, and loops step S3upon determining NO.

In step S4, the processor50executes the full welding process WP. Specifically, the processor first switches the operation mode OM of the laser oscillator12to the second operation mode OM2, and transmits a command for generating the laser beam LB2of the second type having the laser power LP2, to the laser oscillator12.

In response to the command, the laser oscillator12generates the laser beam LB2having the laser power LP2through continuous oscillation, and emits the laser beam LB2to the laser irradiation device16through the light-guiding member14. In the present embodiment, the laser power LP2is set to a value smaller than the laser power LP1in step S2(LP2<LP1).

In addition, the processor50operates the lens driving device30of the laser irradiation device16to adjust the position of the optical lens28, to control the focal point of the laser beam LB2emitted from the laser irradiation device16to be at a focal position FP2. In the present embodiment, the focal position FP2is set to a position (e.g., the position of the upper surface of the first layer102a) closer to the upper surface of the workpiece W1(i.e., the upper surface of the first layer102aof the cover material102) than the focal position FP1described above is.

Thus, the laser beam LB2having the laser power LP2is emitted onto the workpiece W1. An irradiation point P2of this laser beam LB2has an area E2(<E1) corresponding to the focal position FP2. Note that at this time point, the irradiation point P2may be disposed at the teaching point TP1of the welding location WL.

Then, the processor50operates the irradiation point movement mechanism20to move the irradiation point P2of the laser beam LB2with which the welding location WL is irradiated. Specifically, the processor50operates the mirror driving devices38and40to respectively change the orientation of the mirrors34and36, to make the irradiation point P2advance toward the right along the welding line LN from the teaching point TP1to the teaching point TP2at speed V2. The speed V2may be set, for example, to be 3 [m/min] (i.e., V2<<V1).

The processor50may make the irradiation point P2advance toward the right side along the welding line LN while swinging, in this step S4. Specifically, the processor50changes the orientation of the mirrors34and36, to make the irradiation point P2advance toward the right side while swinging in the y-axis direction of the coordinate system C1. This configuration can suppress production of sputtering as a result of melting the base material100and the base material110using the laser beam LB2.

When the irradiation point P2reaches the teaching point TP2, the processor50transmits a command to the laser oscillator12to stop the laser beam LB2emission. Thus, the full welding process WP in step S4ends. With the full welding process WP in step S4, the base material100and the base material110are molten by the laser beam LB2along the welding line LN, and are welded to each other in the welding location WL.

In step S5, the processor50determines whether the welding has been completed for all the welding locations WL. For example, the processor50can determine whether the welding has been completed for all the welding locations WL by analyzing the welding program PG. The processor50terminates the flow illustrated inFIG.10upon determining YES. On the other hand, upon determining NO, the processor50returns the step S1and executes steps Step S1to S5for the next welding location WL.

As described above, in the present embodiment, in step S2, the processor50makes the irradiation point P1of the laser beam LB1swing within the heating area HA to heat the mating surface area SE′ at the temperature T that is equal to or higher than the boiling point T1of the cover materials102and112and lower than the melting point T2of the base materials100and110. Thus, the cover material102and the cover material112are discharged to the outside of the mating surface area SE′ through the gap G formed between the workpieces W1and W2.

Then, in step S4, the processor50irradiates the welding location WL with the laser beam LB2. As a result, the base material100and the base material110are molten and welded to each other in the welding location WL. With the present embodiment, through step S2, the second layer102bof the cover material102and the first layer112aof the cover material112can be removed from the region in which the base material100and the base material110are molten in step S4. Thus, the vapor of the cover material102and the cover material112in a form of bubbles can be prevented from mixing into the base material100and the base material110, as a result of melting the base material100and the base material110in the welding location WL in step S4.

In the present embodiment, the gap G is formed by vaporizing the cover material102and the cover material112in the mating surface area SE′, and the vapor of the cover material102and the cover material112is discharged to the outside of the mating surface area SE′ through the gap G. Thus, a through hole, through which the vapor of the second layer102band the first layer112aproduced in step S4is discharged to the outside, does not need to be formed in the base material100or110as in known configurations. Thus, the process of the welding flow can be simplified.

In the present embodiment, the orientations of the mirrors34and36are changed to make the irradiation point P1swing in the heating area HA. With this configuration, the irradiation point P1can swing at a high speed (speed V1) relative to the workpiece W1(i.e., the speed V1can be set to a high value). With this configuration, the entirety of the mating surface area SE′ can be heated relatively uniformly in step S2.

In the present embodiment, the speed V2as the work condition CD in step S4is set to be much lower than the speed V1as the work condition CD in step S2(V2<<V1). With this configuration, the entirety of the mating surface area SE′ can be heated relatively uniformly in step S2, and the base material100and the base material110can be reliably molten in step S4.

In the present embodiment, the area E1of the irradiation point P1in step S2is larger than the area E2of the irradiation point P2in step S4(E1>E2). With this configuration, the area heated by the laser beam LB1in step S2is large. Thus, the cover materials102and112can be more effectively discharged with high inflation pressure of the cover materials102and112produced in the mating surface area SE′. In addition, in step S4, the laser power per unit area at the irradiation point P2can be increased, whereby the base material100and the base material110can be reliably molten.

In the present embodiment, the laser power LP1as the work condition CD in step S2is greater than the laser power LP2as the work condition CD in step S4(LP1>LP2). With this configuration, the mating surface area SE′ can be swiftly heated to the temperature T that is equal to or higher than the boiling point T1of the cover materials102and112and lower than the melting point T2of the base materials100and110in step S2.

In the present embodiment, the heating area HA is irradiated with the laser beam LB1(pulsed oscillation laser beam) of the first type in step S2, and the welding location WL is irradiated with the laser beam LB2(continuous wave laser beam) of the second type in step S4. With this configuration, the mating surface area SE′ can be efficiently heated while preventing excessive rise in temperature of the upper surface of the workpiece W1in step S2, and the base material100and the base material110can be efficiently molten in step S4.

In the present embodiment, the irradiation point P1swings in the heating area HA, to make temperature T′ in the center portion of the heating area HA be highest in step S2. With the temperature gradient thus formed in the temperature distribution in the heating area HA, a temperature gradient as illustrated inFIG.12is also formed in the temperature distribution of the mating surface area SE′. Thus, the vapor of the cover material102and the cover material112can be more effectively blown radially to the outside of the mating surface area SE′ in step S2.

In order to form the temperature gradient as illustrated inFIG.12, in step S2, the processor50may control the laser power LP1to be laser power LP1_1while making the irradiation point P1pass through the path between the teaching points TP13and T14in the irradiation point movement path MP, and control the laser power LP1to be laser power LP1_2(<LP1_1) while making the irradiation point P1pass through other paths. With the laser power LP1thus increased while the irradiation point P1passes through the path between the teaching points TP13and T14, the temperature gradient with the temperature being high in the center portion of the heating area HA (i.e., the mating surface area SE′) can be effectively formed.

In the present embodiment, the base material100of the workpiece W1is cooled to a temperature not higher than the threshold value T3(i.e., when it is determined YES in step S3) after step S2, and then step S4is executed. With the base material100and the base material110thus cooled after being heated, a fine material structure of the base material100and the base material110is obtained, whereby the base material100and the base material110can have higher strength. Alternatively, the processor50may omit step S3described above and execute step S4immediately after step S2.

In the above-described embodiment, the memory52may store in advance a data table DT1storing in association with each other a material MT (or thermal conductivity) of the workpiece W1and the workpiece W2, a thickness f of the workpieces W1and W2, and the parameters of the work condition CD (the welding location WL, the heating area HA, the speeds V1and V2, the laser powers LP1and LP2, the time period tHP, the focal position FP, and the operation mode OM).

As an example, the data table DT1may be generated to store in association with each other a material MTA(or thermal conductivity) of the base material100and the base material110, a material MTB(or thermal conductivity) of the cover material102and the cover material112, the thicknesses f of the workpiece W1and the workpiece W2(or the thickness of the base material and the thickness of the cover material), and the parameters of the work condition CD used in step S2(heating process HP) (e.g., the length of the welding line LN, the length x1and the width y1of the heating area HA, the speed V1, the laser power LP1, the time period tHP, the focal position FP1, and the operation mode OM1).

Then, the processor50may display the data table DT1on the display device60. In this case, the operator can search the data table DT1for the optimum work condition CD used in step S2from the material MTAof the base material100and the base material110of the workpieces W1and W2that are the work targets, the material MTBof the cover material102and the cover material112, and the thickness f of the workpieces W1and W2, while referring to the data table DT1.

Alternatively, the processor50may generate an input screen on which the material MTA, the material MTB, and the thickness f can be input and display the input screen on the display device60. Then, while visually recognizing the input screen displayed on the display device60, the operator may operate the input device58to input the information about the material MTA, the material MTB, and the thickness f to the input screen.

Then, the processor50may search the data table DT1for the work condition CD corresponding to the materials MTAand MTBand the thickness f input, and automatically set the work condition CD as the work condition CD used in step S2. With this configuration, the operation of setting the work condition CD can be automated, whereby the preparation process PP can be more easily executed.

The data table DT1may be generated to store in association with each other the materials MTAand MTB, the thickness f, and the parameters (e.g., the length of the welding line LN, the length x1and the width y1of the heating area HA, the speed V2, the laser power LP2, the focal position FP2, and the operation mode OM2) of the work condition CD used in step S4(full welding process WP). The data table DT1can be generated by collecting data through experimental techniques or simulations.

Next, a laser welding system70according to another embodiment is described with reference toFIG.14andFIG.15. The laser welding system70is different from the laser welding system10described above, in that a temperature sensor72is further provided. The temperature sensor72includes, for example, a thermocouple, a platinum temperature measurement resistor, and an infrared detection type temperature measuring device (such as a thermographic camera), and measures the temperature T′ of the heating area HA on the workpiece W1in a contact or contactless manner.

Next, a welding flow executed by a laser welding system70will be described with reference toFIG.16. The welding flow in the present embodiment is different from the flow illustrated in FIG. in step ST (heating process HP). Hereinafter, step ST is described with reference toFIG.17.

After step S2′ is started, the processor50starts generating the laser beam LB1in step S11. Specifically, as in step S2described above, the processor50switches the operation mode OM of the laser oscillator12to the first operation mode OM1, and makes the laser oscillator12generate the laser beam LB1(pulsed oscillation laser beam) of the first type having the laser power LP1. In addition, the processor50operates the lens driving device30to adjust the position of the optical lens28, and controls the focal point of the laser beam LB1to be at the focal position FP1.

In step S12, the processor50starts the operation of making the irradiation point P1of the laser beam LB1swing within the heating area HA. Specifically, as in step S2described above, the processor50operates the irradiation point movement mechanism20, to start the operation of making the irradiation point P1of the laser beam LB1swing along the irradiation point movement path MP at the speed V1in the heating area HA.

In step S13, the processor50estimates the temperature T of the mating surface area SE′. Specifically, the processor50acquires the temperature T′ of the heating area HA measured by the temperature sensor72at this time point, and estimates the temperature T of the mating surface area SE′ based on the temperature T′. For example, the memory52stores in advance a data table DT2storing in association with each other the temperature T′ of the heating area HA and the temperature T of the mating surface area SE′.

This data table DT2can be generated through experimental techniques, simulations of thermodynamics, or the like. The processor50searches the data table DT2for the temperature T corresponding to the temperature T′ acquired. Thus, the processor50can estimate the temperature T of the mating surface area SE′ at this point, from the temperature T′ of the heating area HA measured by the temperature sensor72. As another example, the temperature T of the mating surface area SE′ may be estimated by applying the temperature T′ of the heating area HA measured by the temperature sensor72to a known thermodynamic equation.

Note that the temperature sensor72may be disposed to measure the temperature T′ of the center portion of the heating area HA. In this case, the temperature sensor72measures the maximum temperature T′ of the heating area HA, and the processor50estimates the temperature T (maximum temperature) of the center portion of the mating surface area SE′ from the maximum temperature T′ in this step S13. Alternatively, the temperature sensor72may be disposed to measure the temperature T′ of any position in the heating area HA (e.g., a position of any of the teaching points TP11to TP16).

In step S14, the processor50determines whether the temperature T estimated in the most-recent step S13is lower than a predetermined threshold value Tth1(T<Tth1). The threshold value Tth1is determined by the operator in advance and stored in the memory52. For example, the threshold value Tth1may be set to the boiling point T1(or lower temperature) of the cover materials102and112, or may be set to a temperature higher than the boiling point T1and lower than the melting point T2of the base materials100and110(T1<Tth1<T2). The processor50determines YES and proceeds to step S17when T<Tth1holds, and determines NO and proceeds to step S15when T≥Tth1holds.

In step S15, the processor50determines whether the temperature T estimated in the most-recent step S13is higher than a predetermined threshold value Tth2(T>Tth2). The threshold value Tth2is determined in advance by the operator as a value higher than the threshold value Tth1described above and stored in the memory52.

For example, the threshold value Tth2may be set to the melting point T2(or a temperature not lower than the melting point T2) of the base materials100and110, or may be set to a temperature that is higher than the boiling point T1of the cover materials102and112and lower than the melting point T2(e.g., T1<Tth1<Tth2<T2). The processor50determines YES and proceeds to step S17when T>Tth2holds, and determines NO and proceeds to step S16when T≤Tth2holds.

In step S16, the processor50determines whether the time period tHPset in the work condition CD has elapsed after the start time of step S12. Specifically, the processor50measures a time period t2elapsed after the start time of step S12, and determines whether the elapsed time period t2has reached the time period tHP. The processor50determines YES, ends step S2′, and proceeds to step S3inFIG.16when the elapsed time period t2has reached the time period tHP, and determines NO and returns to step S13when the elapsed time period t2has not reached the time period tHP.

Upon determining YES in step S14or step S15, the processor50changes the work condition CD in step S17. Specifically, in step S17after determining YES in step S14, the processor50changes the work condition CD to, for example, reduce the speed V1, increase the laser power LP1, increase the time period tHP, or move the focal position FP1toward the upper surface of the workpiece W1.

The reduction in the speed V1, the increase in the laser power LP1, the increase in the time period tHP, and the movement of the focal position FP1toward the upper surface of workpiece W1all lead to an increase in the temperature of heating area HA (i.e., the mating surface area SE′). Therefore, by thus changing the work condition CD, the temperature T of the mating surface area SE′ can be increased to be equal to or higher than the threshold value Tim.

On the other hand, in step S17after determining YES in step S15, the processor50changes the work condition CD to, for example, increase the speed V1, reduce the laser power LP1, reduce the time period tHP, or move the focal position FP1away from the upper surface of the workpiece W1.

The increase in the speed V1, the reduction in the laser power LP1, the reduction in the time period tHP, and the movement of the focal position FP1away from the upper surface of the workpiece W1all lead to a reduction in the temperature of the heating area HA (i.e., the mating surface area SE′). Therefore, by thus changing the work condition CD, the temperature T of the mating surface area SE′ can be reduced to be equal to or lower than the threshold value Tth2. After executing step S17, the processor50continues step S2′ under the work condition CD after the change, and proceeds to step S16.

As described above, in the present embodiment, the processor50estimates the temperature T of the mating surface area SE′ from the temperature T′ of the heating area HA measured by the temperature sensor72, and changes the work condition CD based on the temperature T. With this configuration, the temperature T of the mating surface area SE′ can be controlled in detail while step S2′ is being executed. Thus, the cover material102and the cover material112in the mating surface area SE′ can be discharged to the outside more effectively. The work condition CD (e.g., the time period tHPfor executing the heating process HP) can be optimized.

The laser irradiation device16and the irradiation point movement mechanism20are not limited to the embodiment illustrated inFIG.3. For example, one of the mirrors34and36can be omitted from the irradiation point movement mechanism20illustrated inFIG.3. In this case, the irradiation point movement mechanism20may be configured using the other one of the mirrors34and36to make the irradiation point P on the workpiece W1reciprocate relative to the workpiece W1over the length x1in the x-axis direction of the coordinate system C1.

On the other hand, the irradiation point movement mechanism20may further include a work table to which the workpieces W1and W2are fixed, and a table driving device (e.g., a piezoelectric element, an ultrasonic vibrator, or an ultrasonic motor) that reciprocates the work table within the width y1in the y-axis direction of the coordinate system C1(both of which are not illustrated).

In this case, the irradiation point movement mechanism20can heat the entire heating area HA, by making the workpieces W1and W2swing in the y-axis direction of the coordinate system C1using the table driving device and making the irradiation point P swing in the x-axis direction of the coordinate system C1relative to the workpiece W1using the other one of the mirrors34and36. The heating area HA at this time is a substantially rectangular region of the length x1×the width y1, and is defined by movement paths of the irradiation point P relative to the workpiece W1.

The laser irradiation device16is not limited to the laser scanner as illustrated inFIG.3, and may be, for example, a laser processing head including a mirror that reflects the received laser beam and an optical lens that focuses the laser beam reflected by the mirror. In this case, the irradiation point movement mechanism20may include a rotary lens rotatably disposed inside the laser processing head.

The rotary lens is supported, on the optical path of the laser beam reflected by the mirror of the laser processing head, to be rotatable about the axis parallel to the optical path, and has a laser beam incident surface inclined relative to the optical path. The irradiation point movement mechanism20can displace the irradiation point P on the workpiece W1by rotating this rotary lens.

In the embodiment described above, a case has been described where in the preparation process PP, the operator sets the heating area HA for the workpiece W1, and then sets the teaching points TP11to TP16and the irradiation point movement path MP (FIG.5toFIG.9). Still, the process of setting the heating area HA can be omitted from the preparation process PP.

For example, the operator may set the teaching points TP11to TP16to surround the welding location WL after setting the welding location WL for the workpiece W1, and then set the irradiation point movement path MP based on the teaching points TP11to TP16. In this case, the heating area HA is uniquely determined as illustrated inFIG.9, for example, based on the teaching points TP11to TP16and the irradiation point movement path MP set.

In the preparation process PP, after the welding location WL has been set by the operator, the processor50may automatically set the heating area HA to encompass the welding location WL, based on the position data on the welding location WL. In this case, the operator may input information such as the length x1, the width y1, the distance x2, and the distance x3illustrated in FIG. in advance through the input device58, and the processor50may automatically set the heating area HA based on the input data from the operator.

At least one of the distance x2and the distance x3illustrated inFIG.5may be zero. In this case, the teaching point TP1is disposed on the left side SD1of the heating area HA, or the teaching point TP2is disposed on the right side SD2of the heating area HA. The teaching point TP1may be disposed more on the left side than the left side SD1of the heating area HA.

Alternatively, the teaching point TP2may be disposed more on the right side than the right side SD2of the heating area HA. In this case, most of the welding location WL is encompassed in the heating area HA, and both end portions of the welding location WL are disposed outside the heating area HA. As described above, as a result of the experiment performed by the present inventors, it has been found that through the heating process HP, the cover material102and the cover material112can be discharged not only from the mating surface area SE′ but also from the region in the periphery of the mating surface area SE′. Thus, even when part of the welding location WL is disposed outside the heating area HA, the cover material102and the cover material112may be dischargeable from the region where the welding location WL exists.

The irradiation point movement path MP illustrated inFIG.9is merely an example, and various other irradiation point movement paths are conceivable.FIG.18illustrates another example of the irradiation point movement path MP. In the example illustrated inFIG.18, four teaching points TP11, TP12, TP15and TP16are set to be at the vertices of the heating area HA, and the irradiation point movement path MP is set as, for example, a path that passes through the teaching points TP11, TP12, TP15, TP16, and TP11in this order. The irradiation point movement path MP may be set as a path that makes the temperature T of the mating surface area SE′ uniformly rise when the heating process HP is executed, without forming the temperature gradient as illustrated inFIG.12.

FIG.19illustrates still another example of the irradiation point movement path MP. In the example illustrated inFIG.19, two teaching points TP21and TP22are set to the heating area HA, and the irradiation point movement path MP is set as a path that reciprocates between the teaching points TP21and TP22. The teaching points TP21and TP22may be set at the same positions as the teaching points TP1and TP2in the y-axis direction of the coordinate system C1. Also with such an irradiation point movement path MP, the entirety of the heating area HA and the mating surface area SE′ can be heated, by appropriately setting the work condition CD.

In step S2described above, the laser power LP1may be changed together with the speed V1as the irradiation point P1moves along the irradiation point movement path MP from one teaching point TPαto the next teaching point TPγsubsequent to the teaching point TPα. Control to achieve this will be described below with reference toFIG.20.

InFIG.20, the horizontal axis represents two consecutive teaching points TPαand TPγin the irradiation point movement path MP and a point (e.g., a midpoint) TPβbetween the teaching points TPαand TPγ, and the vertical axis represents the speed V1and the laser power LP1. A solid line in the graph inFIG.20indicates the laser power LP1, while a dashed line indicates the speed V1.

In the example illustrated inFIG.20, when the irradiation point P1moves from the teaching point TPαto the teaching point TPγin step S2, the irradiation point P1is gradually accelerated from the teaching point TPαto the point TPβwith the speed V1increasing. The irradiation point P1is gradually decelerated with the speed V1decreasing until the irradiation point P1reaches the teaching point TPγafter passing through the point TP13.

If the laser power LP1is controlled to be constant in the above-described case where the speed V1changes while the irradiation point P1moves from the teaching point TPαto the teaching point TPγ, in the heating area HA, the temperature of the region in the vicinity of the teaching points TPαand TPγwhere the speed V1decreases can be excessively higher than the temperature of the region in the vicinity of a point TPβ. In this case, the temperature T at the end edge (e.g., the sides SD1and SD2) of the heating area HA can be excessively higher than that in the center portion.

Thus, as illustrated inFIG.20, in step S2, the processor50increases the laser power LP1together with the speed V1as the irradiation point P1moves from the teaching point TPαto the point TPβ, and reduces the laser power LP1together with the speed V1as the irradiation point P1moves from the point TPβto the teaching point TPγ. With the laser power LP1changed together with the speed V1as described above, the entirety of the heating area HA (i.e., the mating surface area SE) can be relatively uniformly heated.

In the embodiment illustrated inFIG.9, the irradiation point movement path MP from the teaching point TPαto the teaching point TPγillustrated inFIG.20may be, for example, the path from TP11to TP12, the path from TP13to TP14, and the path from TP15and TP16. In this case, the processor50may control the value (maximum value, minimum value, or average value) of the laser power LP1in the path from TP13to TP14to be LP1_1, and control the value of the laser power LP1during the passage of the other paths to be LP1_2(<LP1_1).

On the other hand, the processor50may control the laser power LP1to be constant, while the irradiation point P1passes through the path from TP12to TP13, the path from TP14to TP15, the path from TP16to TP13, and the path from TP14to TP11. Thus, in this case, the processor50changes the laser power LP1when the irradiation point movement path MP between the two teaching points TPαand TPγis relatively long, and controls the laser power LP1to be constant when the irradiation point movement path MP between the teaching points TPαand TPγis relatively short.

In the embodiment illustrated inFIG.18, the irradiation point movement path MP from the teaching point TPαto the teaching point TPγillustrated inFIG.20may be the path from TP11to TP12, the path from TP12to TP15, the path from TP15to TP16, and the path from TP16to TP11. In the embodiment illustrated inFIG.19, the irradiation point movement path MP from the teaching point TPαto the teaching point TPγillustrated inFIG.20may be the path from TP11to TP12and the path from TP12to TP11.

Note that in the work condition CD described above, a focus-point power density ρ1of the laser beam LB1with which the heating area HA is irradiated in step S2may be determined instead of (or in addition to) the laser power LP1and the focal position FP1. The focus-point power density ρ1may be defined as, for example, the laser power LP1per unit area of the irradiation point P1on the workpiece W1(i.e., ρ1=LP1/E1).

In the work condition CD described above, a focus-point power density ρ2of the laser beam LB2with which the welding location WL is irradiated in step S4may be determined instead of (or in addition to) the laser power LP2and the focal position FP2. The focus-point power density ρ2may be defined as, for example, the laser power LP2per unit area of the irradiation point P2on the workpiece W1(i.e., ρ2=LP2/E2). Here, as described above, an area E of the irradiation point P on the workpiece W1depends on the focal position FP of the laser beam LB. Thus, a focus-point power density p is controllable by appropriately selecting the laser power LP of the laser beam LB and the focal position FP of the laser beam LB.

In the work condition CD, the focus-point power density ρ1of the laser beam LB1in step S2may be set to a value smaller than the focus-point power density ρ2of the laser beam LB2in step S4(ρ1<ρ2). For example, the processor50controls the laser power LP1to be 5 [kW] in step S2, and controls the focal position FP1to be at a position10[mm] above the upper surface of the workpiece W1. In this case, the diameter of the irradiation point P1is about 0.9 [mm], and the area E1is about 0.64 [mm2]. Therefore, in this case, the focus-point power density ρ1can be controlled to be ρ1≈8 [kW/mm2].

On the other hand, the processor50controls the laser power LP2to be 2 [kW] in step S4, and controls the focal position FP2to be at the position of the upper surface of the workpiece W1. In this case, the diameter of the irradiation point P2is about 0.4 [mm], and thus the area E2is about 0.13 [mm2]. Therefore, in this case, the focus-point power density ρ2can be controlled to be ρ2≈15.4 [kW/mm2]>ρ1.

Note that the memory52may store in advance a data table DT3storing a focus-point power density p in association with the laser power LP and the focal position FP. Then, when executing step S2or S4, the processor50may search the data table DT3for the laser power LP and the focal position FP corresponding to the focus-point power density ρ set for the work condition CD, and irradiates the workpiece W1with the laser beam LB with the laser power LP and the focal position FP thus found, to control the focus-point power density ρ.

In the work condition CD described above, the time period tMPrequired for the irradiation point P1swung by the irradiation point movement mechanism20in the heating process HP to reciprocate once on the irradiation point movement path MP, may be determined instead of (or in addition to) the speed V1. The heating process HP (step S2or S2′) and the full welding process WP (S4) may be executed under the same operation mode OM (OM1or OM2). In this case, the workpiece W1is irradiated with the same type of laser beam LB (LB1or LB2) in the heating process HP and the full welding process WP.

The focal position FP may be the same between the heating process HP and the full welding process WP. In this case, the area E1of the irradiation point P1in the heating process HP and the area E2of the irradiation point P2in the full welding process WP are substantially the same. The laser power LP may be the same (LP1=LP2) between the heating process HP and the full welding process WP.

The laser welding system10may include a plurality of the control devices22each individually controlling a corresponding one of the laser oscillator12, the laser irradiation device16, the irradiation device movement mechanism18, and the irradiation point movement mechanism20. The heating area HA (teaching point TPn and the irradiation point movement path MP) is not limited to the first layer102aof the cover material102, and may be set to the base material100.

The heating area HA may be set to the workpiece W1(the first layer102aof the cover material102) and the welding location WL may be set to the workpiece W2(the second layer112bof the cover material112). In this case, the processor50may irradiate the workpiece W1with the laser beam LB1from the upper side in the heating process HP, and irradiate the workpiece W2with the laser beam LB2from the lower side in the full welding process WP.

In this case, the laser welding system10,70may further include a second laser irradiation device18B that can irradiate the workpiece W2with the laser beam LB2from the lower side and a second irradiation point movement mechanism20B that moves the irradiation point P2on the workpiece W2. When the heating area HA is set to the workpiece W1and the welding location WL is set to the workpiece W2, while the heating area HA and the welding location WL are separated from each other in the z-axis direction of the coordinate system C1, the welding location WL can be regarded as being encompassed in the heating area HA as viewed in the z-axis direction illustrated inFIG.5.

One of the workpieces W1and W2may not include the cover material102or112. For example, when the workpiece W1does not include the cover material102, the workpiece W1is formed by the base material100, and the workpieces W1and W2are stacked such that the lower surface106of the base material100surface-contacts with the upper surface of the workpiece W2(the upper surface of the first layer112aof the cover material112). In this case, the first layer112aof the cover material112is interposed between the base material100and the base material110.

The base material100and the base material110may be made of metals of types different from each other. The cover material102and the cover material112may be made of metals of types different from each other. In this case, in the heating process HP, the mating surface area SE′ may be heated to a temperature that is equal to or higher than the boiling point of the cover material102and the cover material112and lower than the melting point of the base material100(and the base material110). The cover material102and the cover material112may be made of a material (e.g., resin) other than metal.

Although the present disclosure has been described above through the embodiments, the above embodiments are not intended to limit the invention as set forth in the claims.

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