# Experimental Analysis of Welded Rods with a Functionally Graded Material Approach
## 3 Results and Discussions  
Functionally grading is often observed in continuous structures. Rods with different properties are produced by a welding method and non-homogeneous structures are created. As previously mentioned and applied, in the production of these types of structures, friction welding methods are used. Parts of friction welding strength and other mechanical properties are not examined by some researchers [33; 34; 35]. Optimized friction welding conditions 70-80% hardness and poor mechanical properties are obtained by welding component friction-like materials [37]. Some examples of friction induced parts are: drill bits, pump shafts, drive shafts, piston rods, hydraulic cylinders and high torque drive shafts. Several products manufactured by friction welding are shown in Figures 10-12. These products are used in many different fields from the aerospace industry to the automotive industry. As can be seen in Figure 11, friction welding is made by placing a fixed axle in the middle of two different moving parts. This is an application of the automotive industry. In Figure 12, there is a turbine wheel used in aerospace applications. It can be clearly seen that turbine blades are produced by using friction welding. In 1987, in order to develop information about thermal stresses in functionally graded materials, the Research on the Basic Technology for the Development of Functionally Gradient Materials for Relaxation of Thermal-Stress Project was started. The goal of this project is thermal-shielded structural for advance aerospace programs. Many advanced materials have been produced by this project. As a result of this project, thermo-mechanical properties of FGMs (such as thermal shock, fatigue resistances, etc.) have been developed [44]. In addition, in a study conducted in recent years, functional graded tools are created with specimens obtained from friction welding [45]. In our article, the microstructure and mechanical properties of the materials affected by the friction and welding effect are examined by considering them as FGM design. The mechanical properties (specific to the elasticity module) of the heat-affected areas of the specimens are shown in Figure 13. These mechanical properties (specifically for the modulus of elasticity) are designed as functionally graded and Figure 14 is obtained. In this design, as can be seen in the tensile test specimens, it is assumed that the rods are welded exactly in the middle. When these three specimens are examined, it is seen that the functionally grading occurs exponentially in the electric arc welded specimen and linearly in the friction welded specimen. As the reason for this situation, in the electric arc welding, the temperature of the welding spot (welding pool) rises up to 3000 \({}^{\circ}\)C. That is, the melting temperature of the steel is doubled. Therefore, in electric arc welding, the mechanical properties of the welding spot (welding pool) are adversely affected. Figure 13: Heat affected zone properties. Based on Figure 15, the elasticity modules of these three specimens are calculated and the results are given in Table 3. The value of elasticity modules given in Table 3 obtained with the help of experimental analysis methods reveals the negative effect of high temperature on mechanical properties when compared with theoretical values. Since the distance of the specimen to the welding pool is inversely proportional to the temperature parameter, it is assumed that this proportion will also have a functionally grading effect on the modulus of elasticity and the design is carried out within the framework of this assumption. Figure 14: Functionally linear and exponential grading of the heat affected zone, (**a**) Arc welded specimen, (**b**) steel–steel friction welded specimen, (**c**) aluminum–steel friction welded specimen. Figure 15: (**a**) Determination of the elasticity moduli of welded rod and (**b**) toughness of welded rods. The data obtained as a result of tensile tests are: 181 GPa, 195 GPa and 45 GPa, respectively.