OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 Introduction Titanium (Ti) and titanium alloys are widely used in biomedicine due to its biocompatibility, corrosion resistance, and high specific strength. In the case with titanium prosthetic implants, its fatigue strength, tensile strength, and elongation are important in a substitution of load-bearing hard tissues [1]. Strength and hardness of parts fabricated by conventional techniques can be controlled rather easily, as its mechanical properties are almost the same as forgings it is obtained from. However, for example, when milling parts, some of materials go to waste. That is why additive manufacturing (AM) becomes more preferable both in medicine and other production activities based on expensive and hard-to-machine materials [2]. While, AM process parameters such as heat source power and velocity, surface power density, scanning mode, affect the melt pool shape and dimensions during the process. This determines the thermal cycle, cooling rate, temperature gradient, and solidification rate affecting the structure formation and properties of printed parts [3]. Mechanical properties of the material fabricated by selective laser melting (SLM) or directed energy deposition (DED), depending on the structure formation, determined by thermal conditions, are widely discussed by research teams. These studies are focused on the understanding of AM processes and its optimization [4–9], since the properties of fabricated products should satisfy standard requirements [10]. Both methods of material physics and mechanical strength testing accompanied by specimen disintegration are widely used. And interest in applying non-destructive testing, capable to detect and measure strength properties of the material, is understandable. Among mechanical properties that are most often measured by non-destructive testing methods, the elastic modulus and hardness, measured by ultrasonic testing [11–14], and elastic modulus, measured by instrumental indentation techniques [15–18], should be highlighted. When ultrasonic method is used to control the quality, the specimen retains its integrity. But the determination of the elastic modulus requires specific specimen geometry due to the structural performance and sensor dimensions. Only indentation techniques can therefore be really discussed as a prospect application of nondestructive testing method. A comparison of the elastic modulus, measured by indentation techniques and ultrasonic gauging, is very useful and informative [19].Although GOST R 8.748-2011 gives the requirements for the macro- and micro-indentation loads, the obtained test results require thorough discussion and comparison [20]. It should be noted that elastic modulus is a key parameter in the material design and engineering. According to Zolotarevsky [21], the elastic modulus of pure metals is a low-sensitive parameter of the structure. In works [22, 23] it is found that this parameter changes during the transition of pure metals from coarse- to nano-crystalline state. Of great importance is the problem of the elastic modulus stability after different thermal treatment of Ti alloys, most of which consist of two phases [24]. According to numerous studies, elastic modulus, for example, for the VT6 (Ti-6Al-4V) alloy, ranges between 90 and 145 GPa [24]. It is shown that it depends on many factors, namely structure, its homogeneity, forging shape, and size of area to be measured. Elastic modulus of Ti alloys used in medicine, is an important parameter, which determines biocompatibility of implants. Its reduction to the elastic modulus of bone tissue is gained by additional doping of alloys, which leads to significant changes in its structure and phase composition [25, 26]. Controlling the values of the elastic modulus of alloys, especially at the stage of technology development, is of great importance. Ti alloys for additive manufacturing are exposed to a specific influence leading to the formation of inhomogeneous and anisotropic structures and phases. SLM or electron-beam additive manufacturing (EBAM) provide the formation of products with required properties [27]. The improvement of the economic efficiency of additive manufacturing, for example, increasing the wire-feed 3D printing performance, is associated with a complicated control for thermal conditions, and the alloy acquires a specific structure and phase composition [28, 29]. In the literature on AM-fabricated Ti alloys, information about the elastic modulus is obtained after processing tensile/compressive strain curves or after nanoindentation [29] and, to a lesser extent, after ultrasonic gauging [30]. In studying alloys with a complex structure and phase composition, it is expedient to apply several methods to measure the elastic modulus [31].
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