# Improving the Quality of Laser-Welded Butt Joints of Metal-Polymer Sandwich Composites
## 3 Results and Discussion
It is known that the initial microstructure of the material (region 1 in Figure 4) consists of elongated grains with an average size of about 5 \(\upmu\)m. These grains are stretched in the rolling direction and form a characteristic ferrite-martensite structure. As shown in Figure 5, martensite islands are dispersed in a soft ferrite matrix, resulting in medium strength at sufficient steel impact toughness. The ferrite phase with lower strength is usually continuous, which imparts the ductility required for various applications to this steel. Figure 4: (**a**) Microsection of the welded joint in the upper layer of the MPM sample, registered using a Neophot-30 microscope. To enhance the contrast in the surface image of the microstructure an oblique illumination was used; (**b**) cross-section of part A of the heat-affected zone recorded using a VEGA\SB, Tescan SEM: 1—the base metal; 2, 3—the heat-affected zone; 4, 5—the welded metal. Figure 5: The initial structure of the 1.0939/HCT500X DP steel. In the region of the heat-affected zone, close to the initial structure, laser exposure results in transient-layered and finely dispersed structures as a consequence of multiple heating and cooling cycles (region 2 in Figures 4 and 6a). At the selected processing parameters, the exposure time at high temperatures is short enough that the fibrous microstructure is no longer present in this area. The lighter regions in the immediate vicinity to the melting zone shown in Figure 6b (region 3 in Figure 4) represent small grains of ferrite, formed in a recrystallized structure. When remelting regimes are used, the recrystallisation of the grain takes place, as a result of high-temperature heating. The characteristic initial structure of the steel disappears completely. After several cycles of melting and crystallisation, a finely dispersed structure of low-carbon martensite forms in the weld (region 4 in Figures 4 and 7a). It represents the result of multiple rapid remelting and solidification in the areas where the laser spots overlap. This structure is formed as a result of a relatively rapid cooling into the depth of the material. The lighter regions in Figure 7b (region 5 in Figure 4) also represent small grains of formed ferrite. The formation of the described metallic structures in the heat-affected and remelting zones during laser treatment can be explained by the difference in the parameters of the ongoing thermal cycles of heating and cooling. It is known that the value of cooling rate is related to the size of the cast zone and, thus, depends on the welding conditions. As the volume of the weld pool increases, the cooling rate in the remelted zone and the rate of crystallization decrease. Dendritic structures with developed interdendritic microheterogeneity are not observed in the melt pool. MPM samples with a width of 16.2 mm were prepared for tensile testing of the welded joint. The results of the tensile tests performed on a Shimadzu AGS-100 kNN testing machine at an applicable test speed of 1.5 mm/min showed a maximum tensile force of 3.78 kN and maximum elongation of about 0.6 mm. The typical stress-strain diagram for welded joints in the cross-section is presented in Figure 8. The strength of the welded area of the samples was 465 MPa, which constituted 80% of the measured ultimate strength of the base material 580 MPa. Figure 6: Images of the sample cross-section in the heat-affected zone: (**a**) close to the initial structure; (**b**) in the vicinity of the remelting zone. The fractographic investigation of the fracture surface was carried out to evaluate the quality of the welded joint. This fracture surface was analyzed using an SEM and it was determined that its structure is homogeneous. The fracture surface is fibrous, with no metallic shine and without such defects as pores and non-metallic inclusions. Such a fine-grained fibrous fracture without shine indicates a sufficient plasticity and impact toughness of the metal material.