# Weld Metal Microstructure Prediction in Laser Beam Welding of Austenitic Stainless Steel
## 2 Materials and Methods  
### Welding Process and Materials  
A CO\({}_{2}\) gas laser beam apparatus UT (United Technologies, East Hartford, CT, USA) with a maximum yield power of 25 kW, operating in robotic mode, was used to butt-weld two stainless steel plates in a flat position (Figure 1). Welding was carried out in a single pass. The plates (each one with size 1000 \(\times\) 1000 mm, thickness of 10 mm), were made of AISI 304L austenitic steel as base material (BM), which was selected for its good weldability due to the reduced carbon content [24]. They were prepared with square edges, no gap, and filler material (FM) interposed in the form of four consumable inserts (each one being a sheet with size 1000 \(\times\) 10 mm, thickness 0.4 mm), initially fixed by gas tungsten arc tack-welding. The optical device consisted of a paraboloid mirror with a focal length of 682 mm. By means of a diagnostic system, the operating parameters were checked before welding, evaluating the laser beam quality, depending on degree of focusing, diameter and position of the focus, which was located inside the plates at a distance \(\Delta\)z from their upper surface (Table 1). At the experimented power level, the incident beam energy is so high that the portion of irradiated material melts and vaporizes, forming a capillary cavity (keyhole), surrounded by molten metal. At high temperatures, a part of the vapor ionizes forming plasma, which is harmful because it absorbs energy and attenuates the laser beam effects. The control of plasma was performed by a transverse helium flow, through a nozzle with an internal diameter of 4 mm, directed above the interaction zone of the laser beam with the molten bath. The use of helium, which is technically preferable to argon although more expensive, is justified by a greater resistance offered to ionization. The compositions of FM and BM are shown in Table 2. Instead of AWS 308 L (considered commonly for welding AISI 304L stainless steel [25]), the inserts used were made of AWS 309 L, in order to diversify the composition of the FZ with respect to that of the BM and compare the levels of Ni and Cr detected experimentally in the joint with those obtained by applying the analytical model. The welded bead was subjected to visual inspections for a quality check and to macrographic observations by a stereo microscope Leica MZ 16 1 FA (Leica Microsystem, Milan, Italy) on some cross sections cut at regular intervals along the weld bead, in any case far from the ends. The cross-section surface was prepared by mechanical grinding with abrasive paper (gradation from 180 to 2400), polishing by a velvet cloth with an aqueous suspension of 0.5 um Al\({}_{2}\)O\({}_{3}\) and etching through the Glyceregia reagent (16% HNO\({}_{3}\), 42% HCl, 42% glycerol). The FZ microstructure was characterized by an optical microscope Olympus G71 (Microscope System Limited, Glascow, UK). Finally, the Ni and Cr contents in the FZ were detected performing energy dispersive X-ray spectroscopy EDS measurements on the welded section by means of a JEOL JSM-7610F apparatus (JEOL Ltd., Tokyo, Japan). The EDS measurements were performed in automatic mode, with no less than 100,000 counts in order to have a precision of 1% with a confidence level of 99%. As for the accuracy, the detector error can be estimated by the relative deviation from the expected value, that is the relative error \(\varepsilon\) between the measured composition C and the true value C\({}_{\text{true}}\) (i.e., the concentration of the elements known from \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Laser Beam Power** & **Welding Speed** & **Focus Diameter** & **Focus Position** & **Helium Flow Rate** & **Insert Width** \\
**P (kW)** & **v (m/min)** & **(mm)** & **\(\Delta\)z (mm)** & **(L/min)** & **(mm)** \\ \hline
14 & 1.2 & 0.5 & 5.5 & 20 & 1.6 (4 sheets, each width 0.4 mm) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Welding parameters. Figure 1: Scheme of the welding process.