*3.3. Metallographic Analysis*

Optical and electron microscopes were used for crystallographic structure analysis. The structure of the base material (Figure 13) was examined by using a HiroxKH-8700 confocal digital microscope at a magnification of ×800. It showed a typical low-carbon structure. The microstructure of BM was characterized as fine-grained ferritic–pearlitic (dark areas: ferrite and bright: pearlite), with no explicitly shown banding, which is characteristic of S235JR steel. *Materials* **2020**, *13*, x FOR PEER REVIEW 11 of 18 of BM was characterized as fine-grained ferritic–pearlitic (dark areas: ferrite and bright: pearlite), with no explicitly shown banding, which is characteristic of S235JR steel. *Materials* **2020**, *13*, x FOR PEER REVIEW 11 of 18 of BM was characterized as fine-grained ferritic–pearlitic (dark areas: ferrite and bright: pearlite), with no explicitly shown banding, which is characteristic of S235JR steel.

**Figure 13.** Microstructure of BM. **Figure 13.** Microstructure of BM. **Figure 13.** Microstructure of BM.

The low-carbon S235JR steel is not a typical hardening material, but phase transformation during laser welding at the speed rate of 1 m/min affected the crystallographic structure. The microscopic examination showed a good-quality weld with no defects (Figure 7b). The low-carbon S235JR steel is not a typical hardening material, but phase transformation during laser welding at the speed rate of 1 m/min affected the crystallographic structure. The microscopic examination showed a good-quality weld with no defects (Figure 7b). The low-carbon S235JR steel is not a typical hardening material, but phase transformation during laser welding at the speed rate of 1 m/min affected the crystallographic structure. The microscopic examination showed a good-quality weld with no defects (Figure 7b).

The HAZ structure analysis using an optical microscope with a magnification of ×140 was performed (Figure 14). In the weld interface, assuming a direction from the fusion (I) toward the BM (V), three characteristic areas were identified: II—overheated zone, III—normalization zone and IV—partial recrystallization zone. The HAZ structure analysis using an optical microscope with a magnification of ×140 was performed (Figure 14). In the weld interface, assuming a direction from the fusion (I) toward the BM (V), three characteristic areas were identified: II—overheated zone, III—normalization zone and IV—partial recrystallization zone. The HAZ structure analysis using an optical microscope with a magnification of ×140 was performed (Figure 14). In the weld interface, assuming a direction from the fusion (I) toward the BM (V), three characteristic areas were identified: II—overheated zone, III—normalization zone and IV—partial recrystallization zone.

**Figure 14.** Microstructure of HAZ. **Figure 14.** Microstructure of HAZ. **Figure 14.** Microstructure of HAZ.

The weld-structure investigation was performed by using an optical microscope at a

The weld-structure investigation was performed by using an optical microscope at a

typical for the laser-welding process.

typical for the laser-welding process.

The weld-structure investigation was performed by using an optical microscope at a magnification of ×400. The weld exhibited a coarse-grained dendritic structure (Figure 15), which is typical for the *Materials*  laser-welding process. **2020**, *13*, x FOR PEER REVIEW 12 of 18 REVIEW *Materials* **2020**, *13*, x FOR PEER REVIEW 12 of 18

**Figure 15.** Microstructure of the weld. **Figure 15.** Microstructure of the weld. The structure, precipitations and oxides in the weld across the interspace line between the

The structure, precipitations and oxides in the weld across the interspace line between the welded sheets were determined by using energy-dispersive X-ray spectroscopy performed on a JSM-7100F scanning electron microscope. The chemical composition in the cross-section of HAZ to FZ (Figures 16 and 17) and weld (Figures 18 and 19) was analyzed [58]. The structure, precipitations and oxides in the weld across the interspace line between the welded sheets were determined by using energy-dispersive X-ray spectroscopy performed on a JSM-7100F scanning electron microscope. The chemical composition in the cross-section of HAZ to FZ (Figures 16 and 17) and weld (Figures 18 and 19) was analyzed [58]. **15.** Microstructure of the The structure, precipitations and oxides in the weld across the interspace line between the by spectroscopy performed a JSM-7100F scanning electron microscope. The chemical composition in the cross-section of HAZ to FZ (Figures and weld analyzed [58]. welded sheets were determined by using energy-dispersive X-ray spectroscopy performed on a JSM-7100F scanning electron microscope. The chemical composition in the cross-section of HAZ to FZ (Figures 16 and 17) and weld (Figures 18 and 19) was analyzed [58].

**Figure 16.** HAZ-to-FZ measurement line for chemical composition analysis. **Figure 16.** HAZ-to-FZ measurement line for chemical composition analysis.

**Figure 17.** Ferrite and manganese content along the HAZ-to-FZ measurement line. **Figure 17.** Ferrite and manganese content along the HAZ-to-FZ measurement line. measured line (Figure 17). **Figure 17.** Ferrite and manganese content along the HAZ-to-FZ measurement line.

A chemical composition analysis of HAZ to FZ, based on ferrite and manganese distribution,

measured line (Figure 17).

A chemical composition analysis of HAZ to FZ, based on ferrite and manganese distribution, was performed (Figure 16). A uniform mixture of ferrite and manganese was found across the measured line (Figure 17). *Materials* **2020**, *13*, x FOR PEER REVIEW 13 of 18 *Materials* **2020**, *13*, x FOR PEER REVIEW 13 of 18 Using the ferrite and manganese distribution to identify fusion-zone uniformity, the weld in the overlap area was analyzed (weld transition) (Figure 18).

*Materials* **2020**, *13*, x FOR PEER REVIEW 13 of 18

the overlap area was analyzed (weld transition) (Figure 18).

*Materials* **2020**, *13*, x FOR PEER REVIEW 13 of 18

Using the ferrite and manganese distribution to identify fusion-zone uniformity, the weld in

Using the ferrite and manganese distribution to identify fusion-zone uniformity, the weld in

Using the ferrite and manganese distribution to identify fusion-zone uniformity, the weld in the overlap area was analyzed (weld transition) (Figure 18). Using the ferrite and manganese distribution to identify fusion-zone uniformity, the weld in the overlap area was analyzed (weld transition) (Figure 18). the overlap area was analyzed (weld transition) (Figure 18).

 **Figure 18.** Measure line of fusion zone in overlap transition. 

 **Figure 19.** Ferrite and manganese amount along the overlap transition measurement line. **Figure 19.** Ferrite and manganese amount along the overlap transition measurement line.

 **Figure 19.** Ferrite and manganese amount along the overlap transition measurement line. Uniform distribution of the measured elements showed a high mixing factor. Precipitation analysis of fusion zone was performed, and some inclusions were detected (Figures 20 and 21). Uniform distribution of the measured elements showed a high mixing factor. Precipitation analysis of fusion zone was performed, and some inclusions were detected (Figures 20 and 21).

**Figure 20.** Spectroscopy analysis of identified aluminum oxide in the weld. **Figure 20.** Spectroscopy analysis of identified aluminum oxide in the weld.

 Analysis of weld transition zone revealed the presence of aluminum oxide (Figure 20). Analysis of weld transition zone revealed the presence of aluminum oxide (Figure 20).

**Figure 21.** Spectroscopy analysis of identified oxide in the weld.

Further analysis revealed other oxides (Figure 21). In addition to aluminum oxide (Figure 20), manganese oxides in significant quantities were detected in the overlap fusion zone. No oxides or

inclusions were found in the face and root of the weld. No impurities affecting the mechanical properties of the weld, such as phosphorus or sulfur, were detected.

#### **4. Discussion**

The numerical simulation of laser welding in lap joint specimens was performed, and welding parameters for obtaining partial penetration for the sealed joint were estimated. According to simulation results, 4 kW of output power with a speed ratio equal to 1 m/min gave enough linear power density to obtain a partial joint penetration weld of 4.38 mm. The programmed simulation provided realistically accurate results. The difference in weld face width between the simulation and experimental results was 0.16 mm, approximately 0.06 mm in the overlapping zone, and 0.05 mm was the depth difference. Therefore, the programmed heat-source geometry and boundary conditions can be assumed to be accurate [59]. Results of the stress–strain analysis showed that the maximum value of principal stress was equal to 1120 MPa and the total displacement was equal to 0.32 mm. The calculated total displacement maximum value was related to the face of weld geometry; however, for the determined measurement points, the displacement was more than 0.15 mm (Figure 9b, point 3). The maximum principal stress at the measurement points exceeded 250 MPa (Figure 10b, point 4) and was related to sheet restraint, energy dumping factor, thermal gradient resulting from the heat absorbed by welded materials, and the material thermo-mechanical properties [60,61]. The highest displacement value occurred in the fusion zone. The maximum principal stress was related to the fixed geometry.

The simulation and trial joint analysis indicated differences in the cross-sectional hardness distribution, with higher values in the simulation results. The hardness values in BM from the simulation ranged from 181 to 220 HV and varied from the measured values. Moreover, the weld and HAZ achieved higher hardness values in the simulation. The highest value in the weld from the simulation was 237 HV, with the measured value of 231 HV10. In the HAZ, hardness was between 222 and 249 HV, and the measured values ranged from 200 to 218 HV10. The differences may result from the thermal gradient and phase-transformation velocity factor [62]. Moreover, in the numerical simulation, the load applied during the hardness test was not defined. The measured hardness of the trial joint was lower, and the weld zones showed smaller differences in hardness compared to the values calculated in the numerical simulation. The maximum measured value for the trial joint did not exceed 350 HV10, and no additional post-weld heat treatment was carried out.

The tensile strength of the tested joint was 110 MPa, and compared to the BM, it is lower by about 250 MPa. Both tested specimens failed along the fusion zone line at the maximum load of 11.5 kN. During the performed tensile-strength test, according to specimen configuration, both tensile and shear phenomena occurred (Figure 6). Not-uniaxial complex-force distribution affected the test results and the joint strength obtained was related to the tensile–shear strength of the weld. The stress–strain curve did not have the serrated flow region that is characteristic of low-carbon steels, and the plastic–elastic joint character was observed [63,64].

The crystallographic structure of the base material was identified as fine-grained ferritic–pearlitic. The material structure in the fusion zone changed during melting and solidification processes. The metallographic analysis showed a coarse-grained dendritic structure of the weld. Separate dendrite groups formed pillar crystals, with the growth direction related to the fusion line. No impurities or welding defects were detected in the pillar crystals' contact area. HAZ consist of three areas: the overheated area with a characteristic coarse-grained structure, the normalization area with a uniform fine-grained structure and the partial recrystallization area (incomplete annealing) heated to Ac<sup>1</sup> ÷ Ac<sup>2</sup> transformation point during the welding process. The partial recrystallization area consisted of non-transformed ferrite grains and a fine-grained ferritic–pearlitic structure established from the austenite range [65].

The uniform weld structure was analyzed by using ferrite and manganese distributions. The quantity analysis of the distribution of alloying elements in the joint showed a uniform weld structure. Lack of differences along the measurement line of overlap transition (Figures 18 and 19)

confirmed the obtainment of a weld of high quality. Ferrite and manganese distribution from the BM to the weld line confirmed a uniform chemical composition of the laser-welded trial lap joint (Figures 16 and 17) [66].

The energy-dispersive X-ray spectroscopy analysis showed some precipitation. No porosity defects were detected in the obtained weld; nevertheless, some oxides in the transition zone were observed. Precipitation in the form of aluminum oxide was detected (Figure 20), which is typical of low-carbon steels and probably related to steel deoxidizing in the metallurgical process, not to the welding process. Further investigations showed other inclusions, in the form of manganese oxide precipitations (Figure 21). The presence of oxides in the weld is related to the absence of shielding gas between the welded steel plates, and the types of oxides are related to the composition of the welded material.
