JP4873588B2 - Laser welding method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The invention of this application relates to a laser welding method using a laser beam machine, and in laser welding, the cyclic flow of molten metal is actively activated by providing a pulse output with periodic fluctuations of a waveform-controlled output. Thus, the present invention relates to a new laser welding method capable of effectively preventing the occurrence of welding defects.
[0002]
[Prior art and solutions]
In recent years, the output of laser oscillators has increased dramatically, and application to deep penetration and high-speed welding is expected.
[0003]
In such an environment, the inventors have studied small heat input deep penetration welding with a high-power laser for the purpose of constructing a high-strength and high-toughness welded structure.
[0004]
The biggest problem in achieving the above objective is that it is difficult to stably maintain the keyhole as the deep penetration depth increases, and defects such as porosity, blowholes and cracks are likely to occur. Met.
[0005]
As a result, in order to prevent these defects, it was found that it is necessary to appropriately control the phenomenon occurring in the keyhole and its surroundings.
[0006]
And as one means to realize this, from the examination of the influence of the pulse waveform and the ratio of the peak output and the base output on the suppression of defects, by applying modulation to the laser output with an appropriate frequency and waveform, Attempts were made to control the flow of molten metal, and it was found that the occurrence of welding defects can be greatly suppressed.
[0007]
As a pulse welding method using waveform control of output fluctuation, there is an example in which blow holes, cracks and spatters are prevented in a YAG laser. However, the frequency of the output fluctuation does not use the natural vibration frequency of the molten pool, and therefore does not activate the periodic flow of the molten metal, and is not sufficient for preventing defects.
[0008]
There is also an example in which the porosity of a galvanized steel sheet is to be prevented by pulse vibration. However, even in this case, it was completely different from the generation mechanism of defects generated due to the high vapor pressure of zinc, and the target material and the penetration depth were unique.
[0009]
Therefore, the invention of this application is based on the knowledge obtained by the inventor, which eliminates the problems of the prior art, prevents the occurrence of melting defects, and can provide a high-quality welded joint. It is an object to provide a new laser welding method that enables deep penetration welding technology to be applied to a wide range of material processing.
[0010]
[Means for Solving the Problems]
In order to solve the above problems, the invention of this application firstly provides keyhole welding as a pulse output in which periodic fluctuation is given to the laser output and the frequency is made to match the natural frequency of the molten pool. The laser welding method is characterized in that a gradient is given to rising fluctuation or falling fluctuation of a pulse output waveform .
[0011]
The invention of this application, in the second, in the first laser welding method, the ratio of the base output pulse output (WB) and the peak output (WP) (WB / WP) of 0.6 or less and A laser welding method is also provided.
[0012]
Of As described above, according to the invention of this application, welding defects in keyhole welding using a laser processing machine, by applying the variation of the laser output with a controlled waveform, positive Roshiti, like blowhole and cracks Can be effectively prevented. The occurrence of welding defects can be prevented more effectively by making the frequency of the pulse output coincide with the natural frequency of the weld pool.
[0013]
Embodiments of the invention of this application will be described below.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Regarding the invention of this application, the effect of the pulse waveform, base output (WB), etc. on the control of porosity under the peak power (WP), defocus distance, shield gas conditions, welding speed constant, First, the effects of the rise time (t _U ) and the peak output time (t _P ) on the occurrence of porosity will be described.
[0015]
Along with the output frequency, the pulse rise time (t _U ), peak output time (t _P ), pulse fall time (td), base output time (tb), peak output (WP), and base output (WB) shown in FIG. The defect can be controlled by controlling as follows.
(A) The pulse frequency of the laser output is matched with the natural vibration frequency of the molten pool. An example of the pulse waveform at this time is illustrated in FIG. By matching the pulse frequency with the natural vibration frequency of the molten pool, the periodic flow of the molten metal can be activated.
(B) About the ratio (WB / WP) of the base output (WB) and the peak output (WP), by reducing this value as much as possible, the amplitude of the weld pool vibration is increased, and the cyclic weld metal flow Can be active.
(C) With respect to the pulse rise time (t _U ), if WB / WP is reduced, sputtering is likely to occur during sudden output fluctuations.
[0016]
Also, since too small a rising gradient periodic flow becomes slow, t _U, it is necessary to shorten the extent to which spatter does not occur.
(D) With respect to the pulse drop time (td), the keyhole size decreases rapidly with the change from the base output (WB) to the peak output (WP). Therefore, in order to prevent the occurrence of defects due to rapid inflow of the weld metal to the interior keyhole, it is necessary to apply a gradient descent. The pulse drop time (td) is preferably set to a time that can prevent this defect.
(E) Regarding the peak output time (t _P ), if t _P is too long, intermittent ejection occurs during t _P (peak output time), and the periodic flow given by the output fluctuation is given. Disturbed. Therefore, t _P is a preferably short to Turkey the extent that intermittent ejection does not occur.
[0017]
In normal CW (continuous) welding, the weld metal is ejected spontaneously from the keyhole intermittently at a random frequency.
(F) Regarding the base output time (tb), since the response of the molten metal to the output fluctuation is not fast, if tb is too short, the amplitude of the vibration of the molten metal applied due to the output fluctuation becomes small. For this reason, a time that does not affect the amplitude is required.
[0018]
The invention of this application is configured based on the above points. And for "gradient" in the invention of this application, the base output (WB) peak output from the (WP), or vice base output from peak power (WP) to (WB) a straight-line manner, or substantially linearly changes The state of the change of the output with respect to time is shown.
[0019]
Then, an Example is shown below and it demonstrates in detail. Of course, the following examples do not limit the invention.
[0020]
【Example】
Partial penetration welding of general welded structural steel SM490C was performed using a pulse-modulated CO ₂ laser. He was used as the shielding gas, and side shielding was performed at a flow rate of 50 L / min.
[0021]
Output waveform using a trapezoidal waveform as shown in FIG. 1, set 20kW peak power W _P, the base output W _B to 8kW, changing the rise time t _U and the fall time td. Further, bead-on-plate welding was performed with a duty of 50%. The penetration depth at this time is about 20 mm.
[0022]
The detection of the welding defect was performed by X-ray inspection in which the laser beam axis and the direction perpendicular to the welding line were irradiated from the side surface of the weld specimen. The ratio (%) of the total defect area detected to the weld metal area was defined as the defect occurrence rate Pr, and the effect of suppressing defects was evaluated by Pr.
[0023]
FIG. 2 shows the influence of the pulse rise time t _U on the defect generation amount under the condition that the pulse drop time td is 0 second (td = 0 ms).
[0024]
If the pulse elevational rising time t _U a smaller areas is a short time, the incidence of sputtering significantly, along with this, became occurrence of underfill and porosity is significant.
[0025]
Further, when the pulse rise time t _U is increased, the occurrence of the above-described spatter is suppressed, and a normal bead can be formed, and the defect is most suppressed when t _U = 10 ms. Thus, it can be seen that the increase in t _U is one of the factors for effective defect suppression.
[0026]
However, increasing the addition t _U beyond t _U = 10 ms, becomes gentle is cyclic vibration applied to the molten pool, porosity was made of easily generated again.
[0027]
In addition, Pr in a figure represents a defect incidence rate, and a defect incidence rate shows the ratio (%) of the sum total of the detected defect area with respect to the area of a weld metal.
[0028]
Than the experiment, since it was found that defects in t _U = 10 ms is suppressed most, then under the condition that the pulse rise time t _U = 10 ms constant, defect rate, varying fall time td The impact on
[0029]
FIG. 3 shows the result.
[0030]
FIG. 3 shows that Pr is decreased by increasing the descent time td, and the defect is most suppressed when td = 20 ms. This is because when the output suddenly changes from WP to WB, the molten metal around the keyhole suddenly flows into the keyhole and the porosity tends to remain. It is because it can prevent.
[0031]
It can be seen that an increase in the fall time td is also a factor for effective defect suppression. As for the fall time td, an optimum value exists as in the rise time t _U, and the defect is suppressed most when the fall time td = 20 ms.
[0032]
However, if the descent time td is longer than 20 ms, the defect suppressing effect is reduced again. This is because the base output time tb is shortened as the fall time td increases, the laser output rises before the keyhole is completely reduced, and the amplitude of vibration applied to the molten pool at this time is reduced. .
[0033]
FIG. 4 shows a comparison of the X-ray inspection results of defect occurrence in the case of CW (continuous) welding and in the case of pulse welding using waveform control under the optimum conditions according to the invention of this application.
[0034]
In CW (continuous) welding, in the case of welding with an output of 18.3 kW, the defect occurrence rate was Pr = 1.5%. On the other hand, the optimum conditions obtained in the embodiments of the invention of this application, that is, when WB = 8 KW, WP = 20 KW, t _U = 10, and td = 20 ms, the occurrence of defects is Pr = 0.1%. It was.
[0035]
As a result, the invention of this application is very effective in that the defect occurrence rate is reduced to 1/15 compared to CW (continuous) welding when the laser welding method is used under optimum conditions. The occurrence of defects was suppressed.
[0036]
In the above embodiment, when the ratio of base output to peak output (WB / WP) is 0.4, an effective defect generation suppressing effect can be obtained, but (WB / WP) is 0. In the case of .6, the same effect could be obtained. In this case, the rectangle is the most effective in suppressing defects.
[0037]
【Effect of the invention】
According to the invention of this application, in keyhole welding using a laser processing machine, the fluctuation of the laser output with a frequency matched to the natural vibration of the molten pool and the controlled waveform is imparted, whereby the molten metal is periodically generated. Therefore, the occurrence of weld defects such as porosity, blowholes and cracks can be effectively prevented.
[0038]
In addition, according to the invention of this application, it can be provided that it can be applied also to thick plate welding that requires a welding defect prevention method which is a particularly serious problem.
This enables high-quality laser welding of thick plates, which has been difficult in the past, and is expected to expand the application field of laser welding.
[0039]
Furthermore, according to the invention of this application, high-efficiency welding of thick plates is possible, and cost reduction in the production line can be expected.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an example of an output waveform used for preventing occurrence of a defect.
FIG. 2 is a diagram showing the influence of the pulse output rise time t _U on the defect occurrence rate Pr under the condition that the pulse output fall time is zero.
FIG. 3 is a diagram showing the influence of the fall time t _d on the defect occurrence rate Pr under the condition that the rise time t _{U of the} pulse output is constant at 10 ms.
FIG. 4 is a comparison diagram of X-ray inspection results of defect occurrence status, (a) shows X-ray inspection results in normal CW (continuous) welding, and (b) shows X-ray inspection results under appropriate conditions of the embodiment. Are shown respectively.
[Explanation of symbols]
t _U rise time t _P peak output time td fall time tb base output time WP peak output WB base output Pr defect occurrence rate

Claims (2)

Grant periodic variation in the laser output, a laser welding method for performing a keyhole welding the frequency as a pulse output to match the natural frequency of the molten pool, the rising change or drop variation of the pulse output of the waveform A laser welding method characterized by providing a gradient. The laser welding method according to claim 1, wherein a ratio (WB / WP) of a pulse output base output (WB) to a peak output (WP) is 0.6 or less .

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