**5. Conclusions**

A numerical simulation of laser welding allowed us to estimate the parameters for the lap joint with partial penetration. By programming properly calibrated heat-source geometry and boundary conditions, accurate results can be obtained. Welding simulation based on thermo-mechanical solution with phase transformation gave realistic results with a convex face of the weld. A single-pass laser-welded lap joint based on calculated parameters was produced, and the properties of the obtained joint were investigated. Hardness in the measured trial joint was lower than that from the calculated results and did not exceed 227 HV10. Therefore, according to restrict B quality level, no additional heat treatment was applied. The tensile test results showed the joint strength was 110 MPa, and it is under nominal strength of BM. The not-uniaxial position resulted in the occurrence of tensile-shearing phenomena. Crystallographic analysis confirmed the typical ferritic–pearlitic structure of the BM, grain refining in the heat affected zone and the weld having a characteristic dendritic structure. No welding defects were detected. The energy dispersive X-ray spectroscopy analysis showed good mixture factor of alloying elements. The fusion zone had a uniform structure. Spectroscopy showed that oxides precipitates in the weld. Manganese oxides were detected in the overlap transition line. No additional shielding gas between welded sheets was used, and atmospheric oxygen affected inclusions. The vacuum atmosphere or complete shielding of the fusion zone by using inert gas could reduce the oxidation process. High-quality welding of the sealed lap joint for a gas pipe system was performed by using a laser beam.

Further research of lap joint welding will consider using twin spot-welding optics for widening the fusion zone, improving strength characteristic and producing a lap joint by using a zigzag-shaped welding trajectory. Further research will apply heat-source enhancement to decrease the differences in hardness values between numerical simulation and measured results. Development of the shielding system for overlap region is planned. Further work on laser lap-joint welding of low-carbon steel based on these assumptions can provide a comprehensive analysis of the investigated problem.

**Author Contributions:** Conceptualization, H.D. and A.S.; methodology, H.D. and A.S.; software, H.D.; validation, H.D. and A.S.; formal analysis, H.D. and A.S.; investigation, H.D.; resources, H.D.; writing—original draft preparation, H.D.; writing—review and editing, H.D.; visualization, H.D.; supervision, A.S.; project administration, H.D.; funding acquisition, H.D. All authors have read and agree to the published version of the manuscript.

**Funding:** This research was funded by NCBiR, grant number LIDER/31/0173/L-8/16/NCBR/2017: Technology of manufacturing sealed weld joints for gas installation by using concentrated energy source.

**Conflicts of Interest:** The authors declare no conflict of interest.
