# Improving the Quality of Laser-Welded Butt Joints of Metal-Polymer Sandwich Composites
## 3 Results and Discussion  
The two-sided welding was carried out for steel-polymer three-layered composite samples made from DPK 30/50+ZE 0.48 mm thick, dual-phase (DP) steel sheets (HCT500X, 1.0939 [26], ThyssenKrupp Steel Europe AG, Duisburg, Germany). The steel sheets were subjected to electrolytic galvanizing. A 0.6 mm thick thermoplastic polyolefin (polypropylene-polyethylene--PP/PE, Konrad Hornschuh AG, Weissbach, Germany) foil was used as core material [27; 28]. These three-layered samples are abbreviated further on \"MPM\". Two-sided welding of a three-layer composite material with a partial penetration of each covering steel layer on two sides of the material was carried out. The surface condition of the joint to be welded largely determines the quality of laser welding. The surface was cleaned from impurities and degreased with the acetone organic solvent. The area to be welded was prepared by mechanical machining. Even small bumps, dents and scratch marks after edge cutting, which is usually carried out after all mechanical preparation operations of samples, have a negative effect on the weld quality, contributing to the formation of hot cracks. To avoid such defects, the preparation of the samples for welding was performed by machining the edges. As the external surfaces of the samples were electrolytically galvanized, in was necessary to remove the zinc coating by grinding the surface in the area of the welded joint foreseen. The quality and reliability of laser-welded joints also depend on the accuracy of the assembling of the elements to be welded. This assembly must ensure that the edges are close together over the entire length of the weld, with a minimal gap and edge misalignment. The offset of one edge in relation to the other must not exceed a certain value from the thickness of the parts to be welded. The non-rectilinearity of the surfaces to be welded was maintained below 0.03 mm over the entire weld length. In order to eliminate the need for the so-called slope up, i.e., controlled increase in the beam power at the beginning of the welding, and slope down--controlled decrease in the beam power at the end of the welding--entry and exit strips were used. These were clamped to the specimen to obtain a good thermal contact during welding and after the joint was made, the strips were removed. The criterion for the welding quality is the strength of the joint, which is directly related to the size of the welded area. If the pulse energy is increased, then the depth and diameter of the weld penetration zone increase. It is assumed that the strength of the joint should increase. When reaching a particular value of energy, the power density becomes sufficient for ejecting a portion of the molten metal from the welding pool due to intense evaporation. In this case, the cross-sectional area of the molten zone is reduced; simultaneously, the strength of the joint decreases. For the investigated samples at a pulse duration of 1-3 ms and less, a relatively small cross-sectional area of the molten zone is explained by a relatively small penetration depth as a consequence of reaching intense evaporation early and ejection of the molten metal from the weld pool due to high power density. For example, as shown in Figure 1, at a pulse duration of 1 ms for a spot diameter of 0.45 mm, the maximum penetration depth does not exceed 0.16 mm. When the pulse duration is increased, the ultimate depth and diameter of the molten zone increases. However, the portion of the energy lost through heat dissipation due to thermal conduction increases. Therefore, the overall energy required to start the melting of the metal increases. At a pulse duration greater than 20-30 ms, a further increase in duration no longer results in an even greater increase in the size of the molten zone due to an increase in the energy loss by thermal conduction for the samples under investigation.