# Improving the Quality of Laser-Welded Butt Joints of Metal-Polymer Sandwich Composites
## 2 Experimental Equipment  
The welding of the metal-polymer three-layer composite material was performed using a pulsed Nd:YAG Rofin StarWeld Performance laser (Rofin-Baasel Lasertech GmbH & Co. KG, Headquarters Laser Micro, Gilding, Germany) in a basic configuration, equipped with a Sweet Spot resonator for welding materials that are sensitive to changes of laser parameters. This mobile unit is equipped with a laser having a wavelength of 1.06 \(\upmu\)m providing a nominal output power of 50 W and having a system of motorized laser spot expansion in a working plane in a range from 0.25 to 2 mm. The maximal achievable parameters are the pulse energy of 100 J and the peak power of 9 kW, while the pulse duration can be varied between 0.5 and 50 ms, at a frequency of a single pulse in the range of 0.5-50 Hz. The laser system is also equipped with an internal water-IR heat exchanger cooling unit and a connection for shielding gas and cooling air. The main processing operations of parts using the pulse shaping system were performed in the working chamber, which is equipped with fixed and flexible nozzles through which cooling and shielding gas (e.g., argon) can be supplied. An exhaust unit with filter is integrated in the working chamber, which ensures a 99.997% degree of air purification. A Leica microscope offers the possibility to magnify the viewing area by 24-times during material welding and 2.5D motorized device is a part of the CNC control module. The maximum traverse speed is 10 mm/s, with a linear displacement resolution of 2.5 \(\upmu\)m and a positioning accuracy of less than 20 \(\upmu\)m, resulting in a high-quality welded joint.  
A VEGA\(\backslash\)SB, Tescan (Tescan, a.s., Brno, Czech Republic) analytical scanning electron microscope (SEM) with a tungsten thermal emission cathode as an electron source was used to examine the welded joints obtained by pulse-periodic laser exposure. The microscope is equipped with two integrated detectors: an Everhart-Thornley type secondary electron detector (SE) for imaging with topographical contrast and a back-scattered electron detector (BSE) for imaging with compositional contrast, which allows one to record both the surface morphology and the local elemental composition of the sample. The range of electron beam energy is between 200 eV and 30 keV. An electromagnetic lens is used as a device for changing apertures to adjust the beam current. The magnification is continuous in a range from 2 to 10\({}^{6}\)-times. Electron microscope resolution in high-vacuum mode is 3 nm at 30 keV using an SE detector. Polarization-optical microscope Neophot-30 (Carl Zeiss Jena GmbH, Jena, Germany) with magnification range of 10-2000-times was used for metallographic investigation of the sample surface structure. The principle of its operation is based on the analysis of light reflected from the surface.  
The Shimadzu AGS-100 kNX testing machine (Shimadzu Europa GmbH, Duisburg, Germany) with a maximum measured force of 100 kN was used to perform tensile tests in order to evaluate the mechanical properties of the welded joint. This represents an electro-mechanical testing system performing a direct high-precision loading method and using a precision drive with a screw on a ball bearing with constant control of the deformation of the material under study.