HK1178486B - Fiber-optic laser-machining equipment for etching grooves forming incipient cracks - Google Patents

Description

Fiber laser processing equipment for etching grooves forming incipient cracks

Technical Field

The present invention relates to the field of laser machining grooves in the wall or surface of a mechanical part to define incipient cracks for breaking said mechanical part into at least two pieces. In particular, the machined mechanical component is a connecting rod for a spark ignition engine. These links have a main bore and are initially formed from a single part. Two diametrically opposed trenches are etched into the circular sidewalls of the main hole. The connecting rod is then broken into two pieces by pressure using a mechanical device. This technique is well known to those skilled in the art.

The use of a laser beam for etching the trench to form the incipient crack has significant advantages. In particular, laser techniques can be used to prepare relatively narrow and deep grooves, resulting in clean fractures along a plane containing the central geometric axis of the main bore of the connecting rod.

Background

In particular, fiber laser processing equipment is known from DE patent No. 102007053814. As mentioned in paragraphs 3 and 4 of this document, various types of lasers are used for the specific application in question, but the profile of the trench obtained is not optimal. The quality of the machined groove is a determining factor in obtaining a clean break along a predetermined geometrical plane and breaking with less force. The above-mentioned german patent document aims at using a specific type of laser device, called a fiber laser, in the processing equipment. Preferably, the fiber laser comprises a set of diodes for pumping the active medium (doped fiber). The use of a diode pumping arrangement increases the frequency of the pulses provided by the laser device in the proposed pulse mode. According to paragraph 31 of DE patent No.102007053814, the group of diodes forming the pumping means operates in a pulsed manner. The frequency of the generated laser pulses may lie in the operating range of 10 and 100 kHz. This prior art document therefore proposes to use a fibre laser operating in pulsed mode, i.e. a mode in which the pumping means is powered in pulsed mode to modulate the laser beam produced at the output of the laser device. The pulse mode therefore consists in modulating the continuous power (CW) of the laser so that the maximum pulse power is equal to the CW (maximum average power) of the laser.

The laser device proposed in DE patent No.102007053814 cannot produce a trench with a sufficiently optimum profile. The use of fiber lasers is an advantageous solution because a higher quality laser beam is obtained, which is required for machining narrow, deep trenches. In fact, the fiber laser can generate a high quality laser beam that is not degraded during transmission to the laser processing equipment in a low mode fiber (preferably single mode). However, the pulse patterns mentioned in the aforementioned patent documents greatly limit the potential resulting from the use of fiber lasers, in particular with respect to the depth/width ratio of the trench, the metallurgical quality of the walls and the radius at the bottom of the trench.

The object of the present invention is to improve the method of machining trenches for forming incipient cracks by proposing a laser device that can reduce stray thermal stresses that have several damaging consequences for the formation of the trench and the area surrounding said trench (for example, a change in the metallurgical structure in the two walls of the trench and the occurrence of micro-cracks in the walls). Another object of the present invention is to improve the efficiency of machining grooves in the side wall of a connecting rod by allowing the use of simplified equipment and/or equipment capable of increasing machining speed and/or limiting the movement of the machining head for machining two directly opposing grooves in the main bore of the connecting rod.

Disclosure of Invention

In the laser machining method developed according to the present invention, the inventors found that a fiber laser device operating in pulsed mode cannot provide a groove with a profile having a smaller ratio between the width and the depth of the groove and a smaller radius at the bottom of the groove (which is a determinant of reducing the force required to fracture the mechanical part and also of ensuring fracture in a defined geometrical plane). The disadvantages of the laser device mentioned in the aforementioned prior art document mainly result from the drawbacks of the two proposed modes of operation. First, the proposed pulse mode modulates only the generated laser beam to provide pulses with a maximum or peak power equal to the CW power that the laser can provide. In the aforementioned prior art document, the pulsed power is comprised between 10 and 100 watts (W) and preferably between 40 and 60W. For large links, it is intended to use a more powerful laser providing a peak power of about 200W. (note that higher power CW lasers exist but they are not economically feasible in the industry). Note that in pulsed mode, as suggested, for example, in the aforementioned prior art documents, a more powerful laser limited to current industrial laser technology must be used to obtain pulses with a peak power of about 200W.

Laser pulses having a maximum power substantially equal to 200W or less, with relatively long pulse durations for providing sufficient energy for ablation of the material, are likely to result in melting of the material forming the machined mechanical part. This molten material causes problems with obtaining high quality grooves. In particular, the material must be ejected using a high pressure jet of air. The molten material also causes problems with clean grooves and machined surfaces and protective glass at the output of the machining head. In addition, although a high-quality laser beam is generated by the fiber laser apparatus, the reduction of the radius of curvature at the bottom of the groove is limited by the melted material and the discharge thereof using the compressed gas.

Next, another problem caused by the operation mode mentioned in the aforementioned document comes from the fact that the duration of the applied pulses is generally in the range of microseconds (μ s), i.e. more than one microsecond. In particular, a diode operating in a pulsed mode may provide pulses having a duration between 5 and 10 microseconds. Contrary to the discussion of the preceding document, an increase in frequency does not necessarily cause a decrease in the amount of energy per pulse. The amount of energy contained in each laser pulse is determined by both the power and duration of the pulse. For a given average power, the high frequency generally leads with certainty to a decrease in pulse duration, but the pulse patterns proposed in conventional devices do not allow the pulse duration to be reduced below 1 μ s, which leads to adverse secondary thermal effects problems. In fact, the heat diffusion in the material being processed depends on the pulse duration. The longer the pulse duration, the greater the secondary thermal effect will be and in particular the more conduction of thermal energy in the region of the machined groove. This leads firstly to an increase in the molten material, which leads to a larger width of the groove and a relatively large mean radius of curvature at the bottom of the groove. Thus, although the total energy per pulse substantially corresponds to a power of about 100W and a pulse duration of a few microseconds, the profile of the trench obtained is not optimal.

In the development of the present invention, the inventors demonstrated that a substantial increase in the peak power of the applied laser pulses can reduce the amount of melted material by increasing the amount of sublimed material. In addition, by increasing the pulse power, the required energy per pulse can be provided with a shorter pulse duration, which reduces the thermal stress at the trench periphery and thus the radius of curvature of its bottom.

The invention therefore relates to a method for etching at least one groove in a side wall or surface of a mechanical part by means of laser pulses provided by a fiber laser device, said groove defining an incipient fracture for the subsequent breaking of said mechanical part into at least two pieces. The etching method is characterized in that the fiber laser device is controlled so that the laser pulse has a peak power greater than 400W and at least twice the maximum average power of the laser device and in that the duration of the laser pulse is in the nanosecond range (1 ns to 1000 ns) or less.

According to a preferred embodiment, the fiber laser device is controlled with quasi-continuous wave (QCW). The fibre laser device controlled in QCW mode may, for example, produce pulses with a maximum power or peak power ten times the average maximum power of the laser device.

According to a preferred embodiment, the fiber laser device is controlled in a Q-switched mode. According to another preferred embodiment, the fiber laser device comprises a seed laser source (e.g. a diode providing pulses in the nanosecond range) and at least one fiber amplifier medium providing laser processing pulses at the output.

As a result of the features of the present invention, extremely narrow and relatively deep trenches can be machined that have a very small radius at the bottom of the trench. The laser pulses provided by the fiber laser device used in the method of etching trenches according to the invention reduce the amount of melted material and also significantly limit the stray thermal effects that lead to a degradation of the quality of the obtained trenches.

Drawings

The invention will be described in more detail in the following description with reference to the attached drawings, given by way of non-limiting example, and in which:

figure 1 shows a schematic diagram of a connecting rod and fibre laser device in communication with a trench machining head;

figure 2A shows a perspective view (partial view) of a groove machined using a prior art apparatus.

Fig. 2B shows a schematic view similar to fig. 2A but showing a groove obtained by the machining method according to the invention.

Fig. 3 shows a schematic cross-sectional view of a particular first embodiment of a processing head of a laser device according to the invention.

Fig. 4 shows a schematic cross-sectional view of a particular second embodiment of a machining head of a laser device according to the invention.

Fig. 5 shows a schematic cross-sectional view of a particular third embodiment of a processing head of a laser device according to the invention.

Detailed Description

Fig. 1 is a schematic view of a laser machining apparatus 2 for making grooves 8 and 9 in the side wall of a main bore 6 of a connecting rod 4. These grooves are oriented along the central geometric axis of the main bore. The apparatus comprises a fibre laser device 12 connected to a machining head 22 by a flexible optical cable 24. Machining head 22 and connecting rod 4 are associated with motor means (not shown) for etching the grooves, with relative movement along said central geometric axis. The laser device 12 comprises an active fiber medium 14 known to those skilled in the art and a pumping device 16 formed by a light emitting diode coupled to the active medium. The device comprises a control unit 18 which controls the power and other parameters of the optical pumping means in accordance with the selected mode of operation. Generally, a series of laser pulses is provided.

The use of fiber lasers has several advantages related to the quality of the laser beam obtained. In addition, the laser beam can be introduced into the processing head through a low mode optical cable while still maintaining good beam quality, which simplifies the apparatus. Beam quality is important to allow proper focusing (even with an incident laser beam on an optical focusing system having a relatively small diameter) and thereby reduce the diameter of the beam at the focal point. This results in the formation of narrow trenches. However, manufacturing a narrow and sufficiently deep trench with walls defining an acute angle and a small radius at the bottom of the trench involves parameters other than beam quality. As mentioned above, controlling the supplied energy and in particular the luminous intensity, i.e. the power density, is a determining factor in the processing of this type of trench with an optimal profile. In obtaining this optimal profile, the manner in which the material in the wall of the connecting rod is ablated is important.

The use of a fiber laser operating in pulsed mode by modulating the pump power of the active medium, as suggested in the prior art, results in pulses with a peak power equal to the nominal (nominal) power of the laser (typically less than 200W for industrial fiber lasers) and with a duration greater than 1 μ s. This relatively low power does not provide sufficient luminous intensity to prevent a large portion of the material receiving the laser pulse from melting and thereby becoming liquid. The molten liquid material causes an emptying problem and tends to remain partially at the bottom of the trench. This results in a relatively large average radius of curvature R1 at the bottom of the trench, as shown schematically in fig. 2A. In addition, the relatively long pulse duration also produces secondary thermal effects or stresses in the material as the thermal energy propagates further into the edge region 26 of the machined groove 28. Thus, the melted material, the amount of energy provided by each laser pulse and the duration of each pulse, contributes to widening the trench and creating a relatively large average radius R1 at the bottom of the trench.

The results found in the development of the present invention led to the selection of specific controls for the fiber laser device. According to the invention, the fiber laser device is controlled such that the laser pulses have a peak power of more than 400W and at least twice the maximum average power of the laser device and such that the duration of the laser pulses is in the nanosecond (ns) range, i.e. between 1ns and 1000ns or less.

According to a first mode of operation of the fiber laser device of the present invention, the laser device is controlled in a quasi-continuous wave (QCW) mode. For a laser having a power between 50W and 150W, a peak power of about 1000W (1kW) can be easily obtained. Depending on the variant and application, the laser device is set up to obtain a peak power or maximum pulse power of between 400W and about 3000W (3 kW). Those skilled in the art of fiber lasers know how to implement QCW modes and the particular diodes required to obtain such laser pulses.

In order to obtain short pulses with very high power peaks in the nanosecond range, in particular between 50ns and 400ns, two main variants described below are envisaged.

According to a second mode of operation, the fiber laser device is controlled in a Q-switched mode. This second adjustment mode is preferred as it facilitates obtaining a significantly shorter pulse duration and a higher peak power, e.g. about 10kW, than the QCW mode of the proposed first operation mode. Thus, an extremely high luminous intensity can be obtained to sublimate (i.e., directly convert a solid state into a gaseous state) the material of the machine part being processed. For example, the fiber laser device is controlled to provide laser pulses at a frequency between 10kHz and 200 kHz. Because the duration of the pulses is short, the amount of energy provided by each pulse is also limited. The amount of energy can be adjusted to optimize the laser machining method according to the invention, in particular between 0.1mJ and 2 mJ. Because the duration of the pulse is short, the secondary thermal effects and the thermal energy conducted into the material are greatly limited. This allows to obtain an extremely narrow and relatively deep trench with an optimal profile, as schematically shown in fig. 2B. The perforations made are narrower than that obtained in the prior art. The width/depth ratio of the machined trench 30 is lower than that obtained with prior art laser devices and the average radius of curvature R2 at the bottom of the trench is significantly lower than that of fig. 2A (R1). All this results in a clean trench with minimal jetting of material onto the sidewalls of the opening at the edge of the trench and also improves the initial crack for subsequent fracture of the mechanical part into two pieces.

According to a third, also preferred embodiment, the fiber laser device comprises a seed laser source and at least one fiber amplifier medium providing laser processing pulses at the output. The seed laser source forms low power pulses that are generated with very short duration and very high frequency, such as 10 MHz. These seed pulses are introduced into the input of a fiber amplifier medium that substantially preserves the duration and frequency of the seed pulses and greatly amplifies the pulse power. The amplification device can easily achieve peak powers of more than 1000W. It is well known to those skilled in the art how to construct fiber laser devices of this type.

The machining method according to the invention and the laser machining apparatus for carrying out the method have further advantages. First, the generation of very high power pulses makes it possible to envisage the simultaneous machining of two diametrically opposed grooves in the connecting rod, in particular by splitting the energy from each main laser pulse into two secondary pulses of half the power of the main laser pulse, while maintaining the other advantageous effects of the invention. The particular machining head shown in figure 3 is a particular embodiment that uses this additional advantage. Second, because melted material is greatly limited or avoided in the laser apparatus according to the present invention, the high pressure gas used in the prior art is no longer necessary. Thus, it is no longer necessary to use a nozzle with a small orifice for injecting high-pressure gas at the location where the laser light is applied to the wall of the machined machine part, however, the gas may continue to be used in order to keep the machining head clean, but this gas may be a low-pressure gas and spread over a wide area. The two particular machining heads shown in fig. 4 and 5, respectively, are embodiments that benefit from this additional advantage.

The processing head 32 shown in fig. 3 is connected to the optical cable 24 and receives at an input a laser beam 34 formed by laser pulses according to the invention. The input to this machining head also comprises a collimator 44 with a large aperture for the laser beam leaving the fibre, a first semi-transparent mirror for splitting the main beam 34 into two secondary beams 40 and 42, and a second mirror 38 for reflecting the secondary beam 42 in a substantially axial direction. Each of the two secondary beams is associated with optical focusing means, schematically represented by convex lenses 48 and 50, respectively, to focus the two secondary beams on the side walls of the connecting rod 4. In order to adjust the respective focal points of the two secondary beams, the lenses 48 and 50 may preferably be moved vertically. It should be noted that due to the high quality of the laser beam produced by the fiber laser, good focusing of the incident laser beam can be achieved on a relatively small diameter focusing device. Therefore, the convex lens can have a relatively small diameter and the processing head can be kept in a compact form. The end 52 of the machining head, which leads into the hole in the connecting rod 4, comprises a mirror 54 with two inclined reflecting surfaces for shifting the two secondary beams in a substantially perpendicular direction to the side of the hole of the connecting rod and simultaneously machining two directly opposite grooves 8 and 9 in two opposite directions, respectively. The two secondary laser beams propagate out of the end portion 52 in the same geometrical plane through two diametrically opposed end apertures defined by the mouths of the two nozzles 56 and 58, respectively. In a variant, the reflecting surface of each prism 54 has a slope, which reflects the secondary incident beam obliquely to the machined side surface. In a known manner, gas can be provided in the processing head and discharged through the two conventional nozzles. A single relative vertical movement between the connecting rod 4 and the machining head 32 enables the two grooves 8 and 9 to be machined in parallel.

The processing head 60 shown in fig. 4 is connected to the optical cable 24 and receives at an input the laser beam 34 formed by the laser pulses according to the invention. The input to this machining head comprises a collimator 66 with a large aperture for the laser beam leaving the fibre, a movable mirror 62 associated with motor means 64 allowing linear movement thereof, which movable mirror 62 reflects the beam in a direction parallel to the longitudinal axis of the machining head, and focusing means 68 (represented by a convex lens) integrated with the movable mirror. Note that in a variant, the focusing means are advantageously arranged behind the movable mirror, close to the machined side surface. Preferably, the focusing device is movable relative to the mirror 62 to adjust the position of the focal point. Finally, the beam terminates in a tilted mirror 70 provided at the end of the processing head. The mirror 70 is oriented so that the incident plane of the laser beam is parallel to the direction of movement of the mirror 62. The light beam reflected by the mirror 70 exits through an end slot 72 having a height at least equal to the length of the groove 8, i.e. equal to or slightly greater than the height of the connecting rod. By moving the movable mirror 62, the laser beam is caused to scan the hole wall of the link 4 vertically. Thus, the beam moves stepwise along the end slot 72 to machine the groove 8 without any relative vertical movement between the machining head 60 and the connecting rod 4. Thus, during laser machining of the groove, the machining head remains in a fixed position. Here, the end slot does not cause any particular problem because high-pressure gas is not used for discharging the molten material. However, a low pressure gas may be injected through the slot 72 to protect the cover glass disposed adjacent the slot. In a variant, the injection gas is injected between the wall of the connecting rod and the end slot from above or below, depending on the direction of machining. The processing head is formed with two sections 60A and 60B, the top section 60A being fixed (or able to move in a horizontal linear direction) and the bottom section 60B being able to rotate to allow processing of directly opposed channels without having to rotate the top section connected to the optical cable 24. For rotating the bottom part, a torque motor 76 is provided having its stator part 78 connected to the stationary part 60A and its rotor part 79 connected to the rotating part 60B. The motor has a central opening for the laser beam to pass through and is actuated by a programmable control unit 80 which is normally supplied with electrical power.

Fig. 5 shows a further embodiment of a machining head 82 according to the invention. The head includes a vertically movable top portion 82A and a likewise rotatable bottom portion 82B. The above-described torque motor 76 for rotating the bottom portion is provided to allow two diametrically opposed grooves to be machined without having to rotate the top portion 82A connected to the optical cable 24. At the output of the cable is provided a means 66 of collimating the incident laser beam, which is incident on a mirror 84 arranged obliquely to reflect the laser beam 34 in the axial direction. Bottom portion 82B includes an objective lens 86 that can be moved vertically to adjust the focus, a first tilting mirror 88 and a second mirror 89 that ultimately reflect the light beams obliquely. The exit angle of the laser beam may vary depending on the orientation of the mirrors 88 and 89. This particular arrangement makes it possible to make two diametrically opposed grooves in the aperture without having to move the top portion 82A of the machining head. Therefore, the optical axis of the top portion coincides with the central axis of the hole of the mechanical member 84. A specific bottom portion may be provided for each different diameter. Preferably, the vertical position of the lens 86 is adjustable to adjust the focus according to the diameter of the rod. Note that the bottom portion 82B of the processing head is entirely located above the portion to be processed 84. This makes the method according to the invention unnecessary for the gas for discharging the molten material formed in the groove and M²Is sufficiently small to have a relatively small focal point relatively far from the focusing means to produce the trench. There is relative vertical movement between the part 84 and the machining head 82 or the scanning is generated by changing the position of at least one of the two mirrors 88 and 89.

Claims (13)

1. A method of etching at least one groove (8, 9, 30) in a side wall or surface of a mechanical part (4; 84) by a laser pulse provided by a fiber laser device, the groove defining an incipient crack for subsequently breaking the mechanical part into at least two pieces, characterized in that the fiber laser device is controlled such that the laser pulse has a peak power of more than 400W and at least twice the average maximum power of the laser device, and in that the duration of the laser pulse is in the nanosecond range from 1ns to 1000ns or less.

2. The etching method according to claim 1, wherein the fiber laser device is operated in a quasi-continuous wave (QCW) mode.

3. Etching method according to claim 2, characterized in that the laser pulses have a peak power between 400W and 3000W (3 kW).

4. Etching method according to claim 1, characterized in that the fiber laser device is operated in Q-switched mode.

5. Etching method according to claim 1, characterized in that said fiber laser device comprises a seed laser source and at least one fiber amplifier medium providing said laser pulses at the output.

6. Etching method according to claim 4 or 5, characterized in that the laser device is controlled such that the duration of the laser pulse is between 50ns and 400 ns.

7. Etching method according to claim 4 or 5, characterized in that the fiber laser device is controlled so as to provide the laser pulses with a peak power greater than 1000W (1 kW).

8. Etching method according to claim 4 or 5, characterized in that the fiber laser device is controlled so as to provide the laser pulses with a frequency between 10kHz and 200 kHz.

9. Etching method according to any one of claims 1 to 5, wherein a low mode optical cable is provided between the laser device and a processing head supplied with the laser pulses.

10. Etching method according to any one of claims 1 to 5, characterized in that the mechanical part is a tie rod, two diametrically opposed grooves being etched simultaneously in the main bore of the tie rod.

11. Laser machining device for simultaneously machining two grooves defining two directly opposite incipient cracks in the wall of a bore of a connecting rod, the laser machining device being adapted to perform a method according to any one of claims 1 to 10, characterized by comprising a machining head (32) with a semi-transparent mirror (36) for dividing an incident main laser beam into two different secondary laser beams (40,42) and a mirror (54) with two inclined reflective surfaces on which the two secondary laser beams are respectively incident, the two secondary laser beams then being respectively emitted through two directly opposite end bores in the machining head by propagating in the same geometrical plane.

12. Laser machining device for machining at least one rectilinear groove defining an incipient crack in the wall of a hole of a mechanical part, the laser machining device being adapted to carry out the method according to any one of claims 1 to 10, characterized by comprising a machining head (60) having an end slot (72), the height of the end slot (72) being at least equal to the length of the machined groove, the machining head comprising a movable mirror (62) and focusing means (68) integrated with the linearly movable mirror so that the laser beam exits through the end slot by gradually moving along the end slot, the machining head being held in a fixed position during the laser machining of the rectilinear groove in the mechanical part.

13. Laser machining device for the continuous machining of two grooves defining two directly opposite incipient cracks in the wall of a hole in a connecting rod, the laser machining device being adapted to perform the method according to any one of claims 1 to 10, characterized in that a bottom portion (82B) of a machining head (82) has a width greater than the diameter of the hole of the connecting rod, the bottom portion of the machining head remaining entirely above the connecting rod during the machining of the two grooves.