# Improving the Quality of Laser-Welded Butt Joints of Metal-Polymer Sandwich Composites
## 3 Results and Discussion
Once the optimum laser spot diameter was achieved, a further increase in its size caused a decrease in temperature on the material surface and, consequently, a decrease in penetration depth. It was determined that energy control for laser welding in the conduction mode by keeping the surface conditions just below the material evaporation point allows one to achieve maximum penetration. Material melting, in this case, is a sufficiently stable process and there is a real possibility of obtaining high-quality welds without pore formation and spattering of the material. Suitable regimes for laser welding of steels must provide a combination of such parameters as qualitative weld formation, sufficient operating strength and acceptable mechanical properties in the welded joint. A necessary pre-requisite for the absence of degradation of the core polymer layer will be a partial penetration weld, i.e., a weld in which the fusion penetration is intentionally less than full penetration of the external metal layer. For regimes with significant evaporation, a more intense formation of pores and discontinuities was observed; therefore, conduction welding was carried out in such cases where the heat generation density was not high enough to cause intensive evaporation of the material. The results of experimental studies demonstrated that, for specific values of energy and pulse duration, there is a certain diameter of the laser spot at which fusion penetration reaches a maximum. By gradually increasing the diameter of the laser spot, an increase in penetration depth was observed only until evaporation of the material ceased, i.e., when the surface temperature became equal to the evaporation temperature. The parameters of the pulsed-periodic laser welding were determined. The laser pulse configuration was chosen with an extended pulse leading edge. Laser welding with pulse energy of 32 J at a pulse duration of 8 ms and a pulse frequency of 1 Hz, at which the size of the laser spot on the material surface was 0.75 mm, provided satisfactory results. The optical system had a focal distance of 7.5 inches and the focal plane was above the material surface. Figure 2 shows the top view of the joint created by welding metal-polymer samples made of MPM alternately on both sides. Figure 1: Typical cross-section of the laser butt welding zone of the upper layer of a steel–polymer three-layer composite sample at a pulse duration of 1 ms: 1—remelted zone with a developed interdendrite microheterogeneity; 2—heat-affected zone; 3—initial metal. The structure of the laser-butt-welded joints was examined on prepared samples after welding. The surface of the prepared microsections was chemically etched. The samples were examined visually and using a Neophot-30 microscope. In order to enhance the contrast in the image of the microstructure, an oblique illumination was used. Figure 3 shows the etched microsection of the welded joint in the MPM. As can be seen, the weld was performed only to the half of the thickness of each covering steel sheet. Figure 4 shows a microsection of the welded joint in the top layer of an MPM sample. The welding area consists of two zones, one being the weld metal and the other the heat-affected zone that is a part of the unmolten original metal, the microstructure of which was affected. The weld metal does not contain a coarse-grained cast structure. Microscopic cavities with external inclusions, as well as very small, single gas pores were also identified during investigation. Other weld defects were not detected in the examined samples. Figure 3: Cross-section of the joint area: 1, 3—dual-phase steel sheets with a thickness of 0.48 mm; 2—the core material with a thickness of 0.6 mm. Figure 2: Top view of the welded joint. The morphology of the laser-welded zones of covering sheets made of 1.0939/HCT500X steel was examined with a VEGA\SB, Tescan SEM.