Nanostructured surfaces 
Nanocrystructured surface layers in tool steel AISI D2 
formed by combined laser-ultrasonic surface processing
Lesyk D.A.1, Martinez S.2, Mordyuk B.N.3, 
Dzhemelinskyi V.V.1, Lamikiz A.2, Prokopenko G.I.3
1 Laser Systems and Physical Technologies Department, National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute". Peremohy ave., 37, Kyiv-03056, Ukraine. 
E-mail: lesyk_d@ukr.net
2 Mechanical Engineering Department, University of the Basque Country, Alameda Urquijo s/n, Bilbao-48013, Spain.
3 Physical Principals for Surface Engineering Department, Kurdyumov Institute for Metal Physics of the NAS of Ukraine. Academician Vernadsky blvd., 36, Kyiv‑03680, Ukraine. 

Today, nanocrystalline grain structures in materials with high operational properties and surface technologies for obtaining them are of considerable interest. It is well known that a number of surface mechanical processing, such as shot peening, cavitation peening, ultrasonic shot peening, laser shock peening, etc. has been applied, particularly, in order to improve tribological properties and to prevent mechanical failures of the end-products in the automotive and aerospace industries [1, 2]. As a result, the generated nanocrystalline structure elements in the surface layer can lead to significant improvement in mechanical properties of the whole part/construction.
The laser transformation hardening process combined with ultrasonic strain hardening process is an attractive approach for the improvement of tribological characteristics of the end-products via formation of the surface layers containing a high amount of lattice defects and nanoscale structural elements [3].
In the present work, a ultra-fine grained structure with the grain boundaries fixed with fine secondary carbides was produced by means of combined laser-ultrasonic processing in a tool steel AISI D2. 
The original specimens of the AISI D2 steel (nominal chemical composition: 11.5% Cr, 1.5% C, 0.83% Mo, 0.72% V, 0.46% Mn, 0.42% Si, 0.15% Ni, 0.06% Cu, and balance Fe) had the following mechanical properties: tensile yield strength 0.2 ≥ 320 MPa, tensile ultimate strength UTS ≥ 710 MPa, elongation δ ≥ 16 %.
The laser transformation hardening process was implemented using a solid-state fiber laser Rofin Sinar FL010 of 1 kW power output and 2D scanner [3]. The ultrasonic strain hardening process was realized using a machine tool with numerical control, ultrasonic generator with a power output of 0.3 kW, piezoceramic vibration system and electromotor [4]. The laser hardened surfaces were subjected to ultrasonic strain hardening taking into account that the ultrasonic processing produced the track completely covered the laser induced one.
Analysis of the microstructure in the near-surface layers was carried out using both optical and transmission electron microscopes (TEM) and X-ray diffraction (XRD) analysis. XRD studies and TEM observations show that the combined laser-ultrasonic surface processing leads to the formation of ultrafine-grained structure (~80…250 nm) with grain boundaries fixed with fine secondary carbides (~20 nm), and some areas of martensitic feathers. The existence of a large number of highly misoriented grains in the structure of the surface layer is confirmed by the significant azimuthally dispersion of the ferrite diffraction spots in the selective area electron diffraction pattern. The increased dislocation density inside the grains/subgrains plays a role of an additional factor for hardening. Thus, the main features of the nanostructuring mechanism are revealed to be the reversible α↔γ↔α’ phase transformations and partial dissolution of primary carbides accompanied with the precipitation of ultrafine secondary carbides both induced by quick heating/cooling at the laser treatment and severe plastic deformation. 
Compressive residual stress was also revealed by XRD analysis, and the surface microhardness was increased in 2.5 times in comparison with that of the original specimens. Additionally, the surface roughness was significantly decreased after the combined hardening and finishing process.
As a result, the nanocrystructured surface layer formed in tool steel AISI D2 by combined laser-ultrasonic surface processing provides markedly decreased magnitudes of the wear loss tested in dynamic conditions for 15 and 45 min. The observed reduction in the wear loss of the steel AISI D2 after the combined laser-ultrasonic process is 4 times in comparison with that of the original specimens.

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