*2.2. Experimental Setup and Material*

The experiment was conducted using a 4-kW disk laser welding system (model No.: TRUMPF HLD 4002, procured from Trumpf GmbH + Co. KG, Ditzingen, Germany). Figure 2 shows a diagram of the experimental setup, which was constructed by reference to the study of You and Katayama [27]. The scanner laser head was equipped with a six-axis robot arm, and was connected with two optical fibers. One of them was the round-to-linear fiber optic bundle connected to the spectrometer, and the other was the optical fiber for laser beam input transmission from the laser power source. In the former case, the scanner laser head and the fiber optic bundle were assembled with a connector, shown in Figure 3, which also has a function to condense the light entering the scanner laser head. The light emission of visible light and laser light from the welding area was transmitted to the spectrometer, through the scanner laser head (having a partially transmitting mirror) and optical fiber. The transmitted visible light emission and laser beam reflection were collimated by the collimator, and detected by the line camera. The image data obtained by the CMOS linear image sensor were sampled and digitalized at a frequency of 5 kHz. Subsequently, the processed data were collected, displayed, and stored by the computer software. Figure 4 shows a plot of the spectrum intensity variation, in LBW of lap joint configuration, with a welding speed of 1.5 m/min and a laser power of 3500 W. To test the weld quality and collect data for development of weld quality prediction models, the experiment was carried out using typical automotive high strength steel (uncoated 780 MPa-grade dual-phase (DP) steel) sheets of 1.2 mm thickness. The workpiece was cut to dimensions of 100 × 30 mm, and welding was conducted in the overlap joint configuration.

**Figure 2.** Schematic of experimental setup.

**Figure 3.** Experimental setup and installation of the spectrometer.

**Figure 4.** Spectrum intensity variation, in the LBW of a lap joint configuration, with a welding speed 1.5 m/min and a laser power of 3500 W (thickness of sheet is 1.2 mm).

*2.3. Experimental Procedure*

In this study, we tried to classified the weld quality into four types as shown in Figure 5:

(Class 1) Unwelded: No weld is formed between the workpieces

(Class 2) Incomplete penetration: Weld is formed between the workpieces but not full penetration (Class 3) Full penetration: Weld fully penetrates all workpieces

(Class 4) Unwelded by a gap, between the workpieces in lap joint configuration


**Figure 5.** Schematics of four weld qualities defined in this study.

To evaluate the weld quality and as well as to obtain experimental data, which can be qualified according to the predefined types and are enough to develop a quality prediction model, the welding parameters were initialized with values presented in Table 1. For the laser power and the welding speed, welding was carried out twice (2 replicates) in all possible conditions (21 combinations), at a gap of 0 mm. At a gap of 0.8 mm, welding was carried out only at laser powers of 3000 and 4000 W for all welding speeds, which means that welding was performed under six conditions for two replicates. All welding runs were carried out with a length of 50 mm in the lap joint configuration. The diameter of a focused laser beam was 0.6 mm, and the wavelength of disk laser was 1030 nm. The defocused distance was fixed at 0 mm. Argon (Ar) inert gas was used as shielding gas, and the flow rate of the shield gas was set to 20 L/min. The metal back plate was set under the workpieces as shown in Figure 1.

**Table 1.** Experimental factors and their values.

