# Improving the Quality of Laser-Welded Butt Joints of Metal-Polymer Sandwich Composites
## 1 Introduction  
Sandwich panels are promising composite materials, combining the advantages that mono-materials often cannot offer, such as high bending resistance, good energy absorption, as well as high load-carrying capacity combined with low specific weight [1, 2, 3]. However, the possibilities for thermal joining for producing long-length tailor-made multilayer composite components based on polymers, e.g., for the automotive industry, are limited due to the instability of the polymer core at elevated temperatures. The degradation of the polymer during the welding of cover metal sheets leads not only to corrosive effects that should be avoided [4]. High temperature can negatively affect the properties of the central layer and lead to a significant local deterioration in the damping and acoustic properties of the sandwich material. In Refs. [5, 6], laser welding was proposed as a technique for accomplishing the task of joining such parts. A two-sided welding scheme, whereby the sandwich material must be welded on both sides to completely eliminate or significantly reduce the thermal impact on the polymer core, was recommended in [7, 8]. However, these Refs. [5, 6, 7, 8] describe the results of one-sided welding only, whereby it was not possible to obtain a weld without completely removing the polymer layer. Considering the results of these studies, it was reported in [9] that laser welding cannot be applied to multilayer materials without damaging the central polymer layer. One of the first successful two-sided welding procedures was achieved in [10]. It was determined that two-sided pulsed-periodic laser welding could be accomplished for each of the two metal layers without significant degradation of the middle polymer layer using specific treatment parameters. A thermal cycle with high heating and cooling rates ensured a significant reduction in the heat-affected zone during laser welding. It was established that the reduced volume of molten metal and the specific shape of the weld can influence the crystallization conditions, which, in turn, will provide an opportunity to in-crease the technological strength and mechanical properties of the welded joints. Only a limited weld depth and a relatively low strength during tensile tests, associated as well with the relatively small cross-sectional area, were achieved. Therefore, improving the quality of the welded joint was considered to be the main prospective research objective. It was assumed that the structure of the welded joint can be improved by redistributing the power density and energy of the laser beam. There are various possibilities for improving the quality and strength of a laser-welded joint. The influence of power distribution of a continuous-wave laser on the weld quality and geometry, including the welded joint microstructure, the penetration depth, the weld width and porosity, were investigated in [11; 12; 13; 14; 15; 16]. It was demonstrated that one of the methods for obtaining a relatively wide weld with lower requirements for edge alignment is welding using a defocused beam [17; 18]. When using a pulsed-periodic beam, the redistribution of energy density can also provide an opportunity to improve the structure and properties of the welded joint. For example, in [19; 20], a method was studied where overlap conduction mode welding was used to produce a shaped joint of sheet materials. In this case, it was possible to adjust the penetration depth by changing the pulse energy and duration. In [21], the authors investigated the influence of the process parameters on the quality and strength of the welded joint during pulse-periodic butt welding of 1.5 mm thick stainless-steel sheets. The most important parameters that were evaluated were the angle of incidence of the laser beam, the laser power and the welding speed. Although during welding, the energy redistribution took place by varying the angle of incidence, the effect of this parameter was not investigated separately.