Mechanism of Selective Laser Impulse Impact on Three-Dimensional Defects in the Titanium Alloy Surface Layer

Metallurgy and Material Science


Аuthors

Safronov I. S.1, 2*, Ushakov A. I.2**

1. National University of Science and Technology "MISIS", 4, Leninsky Prospect, Moscow, 119991, Russia
2. Moscow Technical University of Communications And Informatics, 8a, Aviamotornaya Str., Moscow, 111024, Russia

*e-mail: issafronov@yandex.ru
**e-mail: ushakov_a_i@mail.ru

Abstract

Structural materials employed in aviation engineering should be of high specific strength. Titanium alloys possess such properties, and performance properties of the parts made from these alloys are being largely determined by the the surface layer properties. One of the reasons for titanium alloys strength characteristics degradation is the presence of various defects in the surface layer.
One of the most up-to-date ways of a material mechanical characteristics control is the laser impulse selective effect on defective areas. The authors revealed the effect of simultaneous of nanohardness and cracking resistance increase of the VT18u titanium alloy surface. The said effect was being achieved by nanosecond laser impulse of 532 nm wavelength and a impulse energy of 75 mJ impact on the surface of the sample under study. While processing, laser radiation was focused on the surface of the sample with edges overlapping of the laser-treated areas. The nanopores state changing while selective laser processing is being analyzed by computer modeling. No mechanical stress concentrators present in an ideal pore shaped like a ball. The pore was considered vacuumed, which allowed assume its thermal conductivity equal to zero, as well as simplifying the pore healing process computation.
Heating specifics modeling of the titanium alloy surface layer, in which the system of pores of various sizes was placed, was performed. This approach allowed identifying some heating process regularities near nanopores with different sizes, as well as the effect of their configuration on the distribution of isotherms between the pores. Based on the obtained experimental results and the results of computer modeling, a mechanism for healing pores, located in the surface layer of the sample by the impulse short-time heating of the material surface and shock pressure  was proposed. The article considers the main stages of the healing process of nanopores, located in the upper layer of the nanopore system. The metal alloy above the pore becomes more plastic and fills the pore under the action of the shock pressure. The article demonstrates that theoretical assumptions of the idealized model are consistent with experimental data. The changes in the micro- and nanopores state are being determined primarily by the laser impact conditions. Laser irradiation application allows healing selectively a part of the defects in the surface layer of the material, while the defect-free material does not change its structure and properties. Multiple impulse impact on the local segment of the defective material contributes to the proposed mechanism implementation in the deeper layers of the material. Physical process of simultaneous shock pressure and temperature impact can be employed at the stage of surface finishing of the parts. The obtained theoretical and experimental results are up-to-date for the physical materials science and expand the idea of possible laser processing methods for metal alloys with volumetric defects in a thin surface layer. This effect may find application at the stage of surface finishing of titanium parts.

Keywords:

selective laser impulse, shock compression, volumetric defects, nano-hardness, healing of defects

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