# Solid-State Rotary Friction-Welded Tungsten and Mild Steel Joints
## 2 Materials and Methods  
Rotary friction welding was performed on bars made of technically pure tungsten (\(\upvarepsilon\) mm) and S355 steel (\(\upvarepsilon\) on a length of 20 mm). The chemical composition of S355 steel, in accordance with the EN 10204, is presented in Table 1.  
A high-speed RSM210 friction welding machine by Harms & Wende GmbH (Hamburg, Germany) was used for processing the friction-welded joints of tungsten and S355 steel (Figure 1a). The machine consists of a few main components, including a precise control system equipped with a welding cycle parameter recorder and a high-speed rotation spindle with a pneumatic piston for creating pressure in the friction phase and the upsetting (forge) phase of the joint formation cycle. The main advantages of the welding machine include the wide operating range of the parameters that can be set--special attention should be paid to achieving a rotational speed above 20,000 RPM--and the ability to set the friction time in milliseconds, which is significantly supported by the efficient braking system. Figure 1b shows the assembly of the friction welding rods. The rod in the spindle (tungsten) (1) rotates in relation to a common axis with the fixed rod (355 steel) mounted in a vice with a handle (2). The whole friction system of the specimens is enclosed by a shielding gas chamber (3), which enables friction welding in an argon shielding atmosphere (<10 ppm of O\({}_{2}\)). Table 2 presents a set of high-speed friction welding parameters as results of multi-stage tests of welding and, finally, optimization according to the criterion of interface strength. Cracks of the welded joints during tensile strength tests were located in the tungsten specimen not far (100-300 \(\upmu\)m) from the interface. The welding parameters were set according to the criterion of the minimum friction (heating) time, which in turn leads to a large temperature gradient in the narrow zone of the joint formation. The narrow window of parameters was set at a level close to the limit of the possibility of dynamic torsion of a tungsten rod. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**C.** & **Si** & **Mn** & **P** & **S** & **Cr** & **Ni** & **Mo** & **Al** & **V** & **Cu** & **Ti** & **Fe** \\ \hline
0.171 & 0.247 & 1.005 & 0.026 & 0.0059 & 0.038 & 0.020 & 0.003 & 0.032 & 0.003 & 0.010 & 0.0026 & rest \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (wt %) of S355 steel based on manufacturer specification—3.2 type quality certificate according to EN 10204. Characterization of the microstructure of the friction joint was performed using an optical (OM) and a scanning (SEM) microscope on the cross-section (along the rod axis). The metallographic observations were made under the optical microscope Olympus BX51M coupled with a digital camera and a computer with Olympus Stream Essentials software, with magnification from 50\(\times\) to 1000\(\times\). SEM studies were carried out on a JEOL JSM-7600F scanning electron microscope equipped with a Schottky emission gun (FEG). Measurements of the hardness distribution across the friction-welded joint in two characteristic areas were carried out using the Vickers method on a LEITZ MINILOAD 8375 hardness tester with a load of 100 g and a test period of 15 s. The sample, after inclusion in epoxy resin, was wet-ground with discs with grain size from 80 to 2500 until the axis of the bars was reached and polished on a polishing cloth with Al\({}_{2}\)O\({}_{3}\) suspension. Finally, the samples were etched with Nital 5%, and in a second step, by Murakami reagent. Hardness measurements were also performed on the sample prepared in this way. For this joint, the instrumented indentation test (IIT) was conducted using Hystitron Ti 950 TribolCenter with Berkovich tip. Load-depth curves were obtained for points across the friction-welded joint.