*Metals* **2020**, *10*, 484

The optical system used in this experiment has a spot diameter of 400 μm. The focus focal length was 148.8 mm, and the depth of focus was 6 mm.

Functionally controllable parameters in laser welding are approximately four parameters (welding speed, laser power, focus position and polarization), and most are optimized for the four conditions and used according to the situation. In addition, the alignment condition (thickness, gap and mismatch) of the welding material is also one of influences on the welding quality. The effect of each welding variable on welding quality is as follows:


In this study, laser power and laser welding speed, which greatly affect the penetration, are controlled, and the penetration shape is observed after BOP.

Experiments are conducted to analyze the effect of welding conditions on the formation of the molten part and the shape of the bead through the BOP test. The penetration depth that can be seen through this experiment provides data that can correspond to the thickness of the base metal. The data on the width of the bead indicate the welding conditions that can be applied to open-gap welds. In order to achieve the above goal, experiments are performed with 2 parameters (laser power and welding speed) and collect data of bead geometry, penetration, micro image and effect of parameters. Each experimental BOP condition for A553-1 using the laser welding process is shown in Table 3.


**Table 3.** Experimental conditions of laser Bead on Plate (BOP).

In order to analyze the laser-welding characteristics of A553-1, reference data for general laser welding are obtained through the welding experiments of the A36 (low carbon steel) which chemical composition is shown in Table 4 with some of same welding conditions. In this study, the laser weldability of carbon steel is not discussed in detail, but is used only as data to compare welding characteristics of simple A553-1.


**Table 4.** The chemical composition of A36 (low carbon steel).
