# Improving the Quality of Laser-Welded Butt Joints of Metal-Polymer Sandwich Composites
## 4 Conclusions  
As a result of the conducted investigations, the quality of butt joints of metal-polymer sandwich composites created by welding with redistribution of energy density of laser irradiation with an increasing laser spot diameter was improved. The task of increasing the joint strength of MPM samples made of DPK 30/50+ZE DP steel cover sheets with a thickness of 0.48 mm and thermoplastic polyolefin PP/PE as core material with a thickness of 0.6 mm was solved on condition of minimal degradation of the polymer core layer. Using a Rofin StarWeld Performance pulsed Nd:YAG laser, the two-sided welding of the MPM material was successfully carried out, resulting in partial penetration welds on each of the two sides of the material. An increase in the depth of melting zones to at least 50% of the thickness of the cover metal layers was achieved. Instead of previously obtained dendritic structures with developed interdendritic microheterogeneity in the heat-affected zone and the remelting zone, finely dispersed structures with better application prospects were created. Figure 9: The surface of the fracture in the central area of the weld: 1—the zone of the fracture; 2—the non-fusion zone during laser welding; 3—the core polymer material. The experimental results showed that for certain values of energy and pulse duration, there is a certain diameter of the laser spot at which the penetration depth reaches a maximum. By gradually increasing the diameter of the laser spot, an increase in the penetration was observed only until the evaporation of the material ceased. Maximum penetration was achieved by laser welding in conduction mode, when the heat input density is not high enough to cause the evaporation of the material. When the pulse duration is in the range of a few milliseconds, the relatively small cross-sectional area of the melted zone can be attributed to the early reaching of intense evaporation due to the high power density in the beam. With increasing pulse duration, the maximum depth and diameter of the molten zone increase. However, once the pulse duration reaches two to three tens of milli-seconds, the energy losses due to thermal conduction increase to the extent that a further increase in duration no longer results in an increase in the size of the melt pool. The parameters of the pulse-periodic mode of laser welding at which satisfactory results were obtained were identified as: pulse energy 32 J, pulse duration 8 ms and pulse frequency 1 Hz. The diameter of the laser spot on the material surface was 0.75 mm. The structure of laser butt welds was investigated using a Neophot-30 polarised optical microscope and a VEGA\SB, Tescan scanning electron microscope. Metallographic studies of the samples showed that the welding area consisted of two zones: a melting zone and a heat-affected zone. Repeated heating and cooling cycles during laser exposure resulted in a transient-layered and fine-dispersed structure in the heat-affected zone. After multiple cycles of melting and crystallization, a fine-dispersed, low-carbon martensite structure was formed in the weld. Minor defects represented by micropores could be observed closer to the boundary of the metal remelting zone. It should be noted that the important condition during the assembly of a butt joint is to avoid or maintain a minimum gap between the parts to be joined, and edge misalignment should be avoided. A significant improvement in welding conditions is achieved by machining the edges of the materials to be welded. At the same time, the required precision of the assembly is achieved. Tensile tests performed on metal-polymer samples with a width of 16.2 mm revealed a maximum tensile force of 3.78 kN and maximum elongation of about 0.6 mm. As a result of the fracture surface analysis, it was determined that its structure is homogeneous, without pores and non-metallic inclusions. The obtained fine-grained fibrous fracture without shine indicates a sufficient plasticity and impact toughness in the metallic material.