Abstract. The paper presents the results of mechanical properties study of the material produced by direct metal laser deposition of VT6 titanium powder. The properties were determined by the results of stretching at tensile testing machine, as well as compared with the properties of the same rolled material. These results show that obtained samples have properties on the level or even higher than that ones of the samples obtained from the rolled material in a certain range of technological regimes.
IntroductionTitanium alloys have already well approved themselves in chemical, marine, aerospace industries. They are very strong, able to withstand heavy loads and have reduced mass. In addition, titanium alloys are used for manufacturing implants and prostheses. Tissues around such implants are not subjects to change. They are resistant to corrosion in aggressive environment of the human body, and oxide coating on their surface prevents the escape of the implant ions into the body. However, the manufacture of titanium parts and implants is strictly individual task, as it is a material, which is difficult to process. This fact significantly increases the cost of a product. There is a significant interest in using different additive technology for manufacturing titanium parts in recent times. It is necessary to provide wide complex of researches and tests. Titanium alloys are used in the following additive technology: melting layer by layer powder by electron beam (EBM) [1,2] or laser (SLM, SLS methods) [3,4], feeding melted powder or wire by plasma arc [5,6] or laser (DMLD) [7,8]. We suggest using laser metal deposition technology for manufacturing of titanium parts. This technology involves melting of metallic powder feeding through the nozzle coaxially with the radiation. When the nozzle moves along the surface, new metal layer will be formed after solidification. Thus, it is possible to produce parts of the desired shape. The research presents the results of the mechanical properties study of the samples obtained by laser metal deposition technology by different technological modes.
The failure of reactor steel AISI 316 under tension was investigated by structural analysis and scanning contact potentiometry (SCP). In real time surface potentiograms were plotted by the change of which the growth of crack nucleus was tracked from its initiation till the stage of cup fracture formation. The nucleus of the microscopic crack on the potentiogram was first detected at the end of the next to the last cycle of testing at a load of 525 MPa in the yield region and then was persistently reproduced on potentiograms in subsequent tests up to the failure. The most noticeable changes in the parameters of dynamic waviness and roughness occurred at the moments of sharp crack growth. Using the results of the SCP method, three main stages of crack development were identified. In the first one, under loads corresponding to the conditional yield point, a nucleus was formed in the region of the maximum tangential stresses. This region of localization arises as a result of macroscopic loss of stability due to the significant mechanical energy accumulation by the system, which leads to an increase in the magnitudes of the plastic strain fluctuations. The second stage, the one of a progressive growth of the crack nucleus, is the longest in time and lasts from the moment of detection of the nucleus to the sample failure. The amplitude of the electrical potentials in the central part of the sample increased with the rise in intensity of the applied load which is associated with an increase in the inhomogeneity of the internal stress field around the nucleus, as well as the accelerating creep process and the growth of plastic strain in this region. At the pre-failure stage in the hardened surface layer, a macro-groove appears in the form of a standing deformation wave along the fracture line. Under the microscope, shear strain bands on the sample surface are also visible.
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