# Solid-State Rotary Friction-Welded Tungsten and Mild Steel Joints
###### Abstract  
This paper is a study of the microstructure and other selected properties of solid-state, high-speed, rotary friction-welded tungsten and mild steel (S355) joints. Due to the high affinity of tungsten for oxygen, the welding process was carried out in a chamber with an argon protective atmosphere. Joints of suitable quality were obtained without any macroscopic defects and discontinuities. Scanning electron microscopy (SEM) was used to investigate the phase transformations taking place during the friction welding process. Chemical compositions in the interfaces of the welded joints were determined by using energy dispersive spectroscopy (EDS). The microstructure of friction welds consisted of a few zones, fine equiaxed grains (formed due to dynamic recrystallization) and ultrafine grains in the region on the steel side. A plastic deformation in the direction of the flash was visible mainly on the steel side. EDS-SEM scan line analyses across the interface did not confirm the diffusion of tungsten to iron. The nature of the friction welding dissimilar joint is non-equilibrium based on deep plastic deformation without visible diffusive processes in the interface zone. The absence of intermetallic phases was found in the weld interface during SEM observations. Mechanical properties of the friction-welded joint were defined using the Vickers hardness test and the instrumented indentation test (IIT). The results are presented in the form of a distribution in the longitudinal plane of the welded joint. The fracture during strength tests occurred mainly through the cleavage planes at the interface of the tungsten grain close to the friction surface.  
tungsten-steel joint; dissimilar joints; refractory metal +
Footnote †: journal: Applied  
steel and (4.3-6.0) \(\times\) 10\({}^{-6}\) K\({}^{-1}\) for tungsten [13]--which can lead to large residual stresses during cooling of the joints after the joining process, and, consequently, a high risk of cracks in the joint [14]. One of the most used process for joining materials with disparate CTE values is diffusion bonding [15], in which materials with an intermediate CTE value (V, Ta, Cu, ceramics, etc.), or functionally graded interlayers [16; 17], are used to reduce the residual stress level. In common opinion, the transient liquid phase bonding process [18] (also known as diffusion brazing [19]) is preferable because of its relative simplicity. The described methods, however, are based on heating up the entire volume of the joined objects, which, regardless of the physical properties of the joined materials, results in a high level of residual stress, which in the tungsten-steel configuration should be avoided by all possible means. Limiting the level of residual stress can be achieved by local heating with various methods only in a narrow area of the joint formation [20]. From this point of view, the friction welding process could be considered as preferable because of its advantages such as local heating [21; 22], a narrow heat-affected zone (HAZ) [23], short operating time [24] and the real possibility of precise control [25] of the amount of heat introduced into welded materials [26; 27]. Friction welding methods are often used to obtain joints of dissimilar materials [28; 29]. Therefore, in this research, rotary friction welding (RFW) was used to weld tungsten with mild steel for advanced industrial applications.