Laser Welding Monitor

FME Co., Ltd.
? ? Shinichi Nakayama
Unitek Miyachi International Greg Bates

1. Introduction
Laser welding is utilized in various fields of industries such as automobile, iron and steel, electronic parts, semiconductor. Mostly-used laser are CO2 laser of 10.6μm wavelength, and YAG laser of 1.064μm wavelength. High-power continuous wave oscillation of no less than 1 kW is employed to weld relatively large parts such as auto parts, thicker plates, and pulse oscillation of 10 W to 50 W is employed to weld smaller electronic parts. Pulsed laser is used to weld thinner sheet because average input heat must be lowered to reduce heat influence and output energy has to be easily controlled.

Figure 1. Example laser welding

Recently, laser welding has come to be well received by manufacturing factory to achieve high quality and high productivity; meanwhile, requirements to quality of laser welding has become severer. Conventionally, such as visual inspection, tensile strength test are needed after laser welding has been done. For strength test, destruction test such as pull test, cross section test are needed; therefore, not all welds can be tested. To solve these issues, monitoring of weld quality is being tried, in which signals simultaneously issued from portion being welded are detected and monitored.

Successful welding were reported that high-power laser satisfactorily welded workpiece in auto and iron steel industries1). Most of them are seam welding with continuous oscillation laser. Here, the possibility of monitoring for low output spot welding is introduced.

2. Lap joint welding and monitoring signal
While welding, various signals such as light, sound radiate (Figure 2). How to detect defective weld accurately using these signals is expected2)-4). However, the relation between such as light, sound, and welding is too complicated to explain. This is one of the causes that hinders high-accuracy monitoring instrument from being developed.
The possibility of monitoring lap joint welded by laser spot welding was studied. Practically, what are required for weld quality of thinner plate welding such as electronic part are weld strength and laser penetration. Using light signals as detective means and practical welding conditions, the relation between signal and weld quality is studied.

Figure 2. Signals from laser weld


Figure 3. Makeup of laser welding monitor

2.1. Weld strength
To investigate the correlation between weld strength of lap-welded joint and infrared ray radiated from weld, an experiment was conducted using SUS 304, in which laser output was varied by changing YAG laser pulse width with peak power kept constant. Strength of weld was measured using tensile pull test procedure. For detailed test conditions, refer to Figure 4 and 5, and Table 1.
The correlation was observed between infrared light signal integrated with respect to time and weld strength (Figure 6). It is seen that the strength of weld increases as the integrated infrared light signal becomes larger.

Table 1. Laser welding condition

Laser equipment	ML-2350A (Maximum output 50 W)
Peak power	1.4 kW
Pulse width	0.7 - 1.9 ms
Laser output	1.0 - 2.7 J
Optical fiber	Type SI, 400μm core diameter
Output head	Image formation ratio = 1:1 f100
Workpiece	SUS304 Upper 0.635 mm Lower 1 mm
Monitoring signal	Infrared light > 1400 nm

Figure 6. Weld strength vs. Infrared light

2.2 Penetration
The correlation between the penetration of melted metal formed by projected laser and the amount of visible light from weld was studied. The laser projection energy was varied by changing peak power with pulse width fixed. This laser was directly projected onto SUS304 of 5 mm thick, and visible light was monitored using Pin photo diode. Table 2 shows welding condition of the laser equipment and the result is shown in Figure 7. Figure 8 is photo of cross section of the weld. There is strong correlation between intensity of visible light and depth of penetration.

Table 2. Laser welding condition

Laser equipment	ML-2050A (maximum output: 15 W)
Peak power	1.0 - 1.8 kW
Pulse width	1 ms
Laser output	1.0 - 1.8 J
Optical fiber	Type SI, core dia.: 400μm
Output head	Image formation ratio = 1:1 f 70
Workpiece	SUS304 5 mm
Monitoring signal	Visible light 400 - 700 nm

Figure 8. Photo of cross section of weld


Figure 7. Amount of visible light vs. depth of penetration

3. Conclusions
The results of this study are not sufficient to apply to laser welding monitor that requires extremely high level of reliability. However, it indicates that there is a possibility of managing such as weld strength of lap joint, depth of penetration measuring such as infrared light, amount of visible light from weld. Further study is necessary to look into the correlation between signals and weld quality under various conditions.
Laser welding monitor will increasingly be expected because its reliability in repetition and accuracy lead to that of quality of laser welding. In future, feedback of laser energy based on the monitoring will be possible. These technologies will yield success of laser welding monitor, and bring profit to most of users.

4. References
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