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49PHYSICAL SCIENCES
LASER SURFACE ALLOYING METALS AND ALLOYS
Yurov V.
Candidate of phys.-mat. sciences, associate professor
Guchenko S.
PhD student
Karaganda University named after E.A. Buketov Kazakhstan, Karaganda Salkeeva A. Associate Professor
Kusenova A.
Associate professor Karaganda Technical University Kazakhstan, Karaganda
Abstract
Surface laser alloying has become relevant in recent years. Since this alloying is associated with the surface layer, the thickness of the surface layer and its properties come to the fore.
In this work, we proposed an empirical model of the surface layer and how this model affects surface laser doping. An empirical model of the surface layer was obtained by us on the basis of the thermodynamic approach. The equation that describes this layer depends on one fundamental parameter - the atomic volume of the element.
Traditional methods of laser surface alloying of metals and alloys are due to the diffusion of chemical elements from a gas or liquid medium. The diffusion mobility of chemical elements from the molten zone to the thermal zone does not exceed 10 ^m. We have proposed an empirical model of the surface layer of solids. The thickness of the surface layer of some metal oxides can reach up to ~ 1 ^m. Porosity can increase the thickness of the surface layer by a factor of 5-6, which is ~ 5-6 ^m for metal oxides and is close to diffusion mobility in surface laser doping. The diffusion mobility in the nanostructure depends on the surface tension c. It has been shown that the thermal conductivity of metals with a size of 1 nm is reduced by 2 times compared with bulk samples and at sizes of 50 nm they already differ little from the latter.
Keywords: laser alloying, surface energy, surface layer thickness.
Introduction
We raised the issue of laser alloying in [1]. The advantages of laser surface alloying are as follows [2]:
1. Properties of the surface layer and its reproduction.
2. The highest speed of the process.
3. Local zones are available anywhere.
4. An expensive alloying element is available.
5. No further heat treatment.
6. Laser surface alloying is environmentally friendly.
A very large number of works [2-7] are devoted to laser surface alloying of steels, in which their morphology, physical and chemical composition of surface layers, mechanical properties, changes in surface energy, surface charge and surface topography are studied. These questions were raised in foreign monographs [811]. At the end of 2013, in [12], a review was given on laser surface alloying of steel products, and in 2020 the
same conclusion was reached in [13]. Briefly, their conclusion was as follows: laser alloying is of scientific and practical interest, but at present there are no specific technologies from the applied point of view due to the lack of general laws governing changes in the properties of the surface layer upon laser irradiation. Despite this, the latest dissertations in various metals and alloys [14-16] have been devoted to laser surface alloying, and the term "super-alloying" has even appeared [17]. In this work, we will discuss the properties of the surface layer from our proposed model [18, 19].
Laser surface alloying
Traditional methods of laser surface alloying of metals and alloys are caused by the diffusion of chemical elements from a gas or liquid medium. Diffusion can be carried out chemically from the gas phase or from pre-deposited coatings on steel or alloy. As a result, a change in the chemical composition occurs in the surface layer (Fig. 1) [20].
a)
b)
1 - doped layer, 2 - transition layer, 3 - alloy base
Figure 1 - Distribution of a chemical element inside the alloy (a), the structure of the surface layer (b) [20]. The surface layer includes two zones. The alloyed and when processing with a continuous laser, 0.3-1.0
zone is in a fused state during laser irradiation. The second zone, when irradiated with a laser, remains solid, but significantly heated. The doped volume during processing with a pulsed laser lies in the range of 0.4 mm,
mm. The alloyed volume, in contrast to quenching, is saturated with chemical elements. The section of a steel sample is shown in Fig. 2 [21].
Figure 2 - Formation of the modified layer on a steel surface [21]
Under laser irradiation, convective flows are formed in the surface layer due to the large value of temperature gradients (Fig. 3) [2].
a)
Figure 3 - Scheme of laser alloying with reflow (a), (b) change in the surface layer of a sample made of steel
Kh12MF [2].
The diffusion mobility of chemical elements from the melted zone to the thermal zone does not exceed 10 microns.
Pre-coating for surface laser alloying.
In addition to the methods of laser alloying discussed above, alloying from coatings to steel or alloys
previously deposited by plasma, magnetron, galvanic and other methods can be done. In fig. 4 shows the photographs we obtained of coatings of the FeTiCrNi alloy before and after surface laser alloying.
Figure 4 - Sample surface before and after laser alloying (magnification 600 on the Epiquant microscope)
Figure 5 - Alloy coating thickness measured by Quanta 200 3D.
Figure: 1-5 confirm what was said above about In [18, 19], we proposed an empirical model of the
surface laser alloying. surface layer of solids (Fig. 6).
Surface layer thickness
vacuum
■u
transition layer
///////////¡/////Z //////
//////////V> volume phase
/// z*® 17/////
mmmm,
Al» A, 5 J Afh)
Figure 6 - Schematic representation of the surface layer [18]
The surface layer of an atomically smooth metal consists of two layers - d(I) and d(II). A layer with a thickness of h = d is called layer (I), and a layer with
h~10d is called layer (II). At h~10d, the dimensional dependence of the physical properties of the material begins to appear. At h = d, a phase transition occurs in the surface layer. To determine the thickness of the surface
layer of various compounds, we used the size dependence of the physical property A(r) [18]:
A d ^
1 — , r >> d
s. r)
A(r) = Ao A(r) = Ao
1
d ^ d + r j
(1)
r < d.
The parameter d is related to the surface tension c by the formula [18]:
, 2au
d =-, (2)
RT
Here c is the surface tension of a massive sample; u is the volume of one mole; R is the gas constant; T -
temperature. It was shown in [18] that the following relation is fulfilled with high accuracy:
ct = 0.7-10"3 • Tm,. (3)
where Tm is the melting point of the solid. The ratio is fulfilled for all metals and for other crystalline compounds. If we substitute it in (2), then at T=Tm we get:
d(I) = 0.17 • 10-9 u. (4)
Equation (4) shows that the thickness of the surface layer d(I) is determined by one fundamental parameter - the molar (atomic) volume of the element (u = M/p, M is the molar mass (g/mol), p is the density (g/cm3). As an example, in table. 1 shows the thickness of the surface layer d(I) for d-metals, and table. 2 for lanthanides [22].
Table 1
Thickness of the surface layer of d - metals [22].
Me d(I), Me d(I), Me d(I), Me d(I),
nm nm nm nm
Sc 2,6 Ta 1,8 Fe 1,2 Pd 1,5
Y 3,4 Cr 1,2 Ru 1,4 Pt 1,6
Ti 1,7 Mo 1,6 Os 1,4 Cu 1,6
Zr 2,2 W 1,6 Co 0,7 Ag 1,7
Hf 2,3 Mn 1,3 Rh 1,4 Au 1,8
V 1,7 Tc 1,4 Ir 1,4 Zn 2,0
Nb 1,8 Re 1,3 Ni 1,1 Cd 2,2
Table 2
The thickness of the surface layer d (I) of lanthanides [22].__
Me d(I), nm Me d(I), nm Me d(I), nm
Ce 3.8 Eu 5.8 Er 5.5
Pr 4.2 Gd 5.3 Tm 5.2
Nd 4.5 Tb 5.3 Yb 4.6
Pm 4.4 Dy 5.3 Lu 5.7
Sm 4.4 Ho 5.5 - -
From table. 1 and 2, it can be seen that even the transition layer in lanthanides d(II) ~ 10 d(I) does not exceed 100 nm, which is typical for nanostructures according to Glaitor [23]. Table 3 shows the thickness of the surface layer of some metal oxides, from which it
follows that the more complex the composition of the oxide, the greater the thickness of the surface layer d(I), and even more d (II), which exceeds the limit according to Glaitor [23]. For thicknesses d(II), it can reach ~ 1 ^m.
Table 3
Thickness of the surface layer d(I) of some metal oxides
Metal oxide Molar mass, g/mol Density, g/cm3 d(I), nm d(II), nm
NaCu4(AsO4)3 686,0 4,74 24,6 246
KsCusAlO2(SO4)4 751,1 3,10 41,2 412
K4Cu4OCl10 781,1 2,78 47,8 478
NaCusO2(SeO3)2Cl3 1324,5 4,15 53,4 534
Ca12Al14O32Cl2 1440,0 2,85 86,0 860
To this must be added the dependence of the density on the porosity characteristic of steels and alloys. The existing nomenclature, adopted by the International Union of Theoretical and Applied Chemistry IUPAC, distinguishes three categories of pore size depending on their diameter: microporous < 2 nm, meso-porous 2-50 nm, and macroporous > 50 nm [24]. An important characteristic of silicon (Si) is the degree of its porosity P, defined as:
P = 1-pIÊ /pSl, (5)
where is the density of porous silicon (nK), psi is the density of a single crystal. If we substitute equation (5) into equation (4), we get:
d(I)iB = d(I)si /(1 - P). (6)
Typical porosity is 40-70%, and with supercritical drying [25] it reaches 95%. From equation (6) the following table follows table 4 [26]. From table 4 it follows that the porosity can increase the thickness of the surface layer by 5-6 times, which for metal oxides is ~ 5-6 ^m and is close to the diffusion mobility with surface laser alloying (~ 10 ^m, see above).
Thus, with surface laser alloying, it is necessary to the better), and the density and porosity of steel or alloy monitor the molar mass of steel or alloy (the more M, (the less p, the better).
Table 4
P, % 40 50 60 70 80 90
d(I)nK, nm 3,5 4,2 5,25 7,0 10,5 21
d(II)nK, nm 35 42 52,5 70 105 210
Diffusion mobility during laser alloying
The thickness of the surface layer, as we have seen, is mainly a nanostructure, the physical processes in which are different from the bulk. The same goes for diffusion. Of the huge number of publications, we will focus on review [27], where it is shown that the diffusion rate in nanostructures between agglomerates along their interfaces is much higher than the diffusion for nanograin boundaries. This means that the activation energy for such a process should be much lower. For simplicity and comparison, we restrict ourselves to the stationary case of one-dimensional diffusion. Then the diffusion equation (c is the concentration of atoms, ions, or molecules) will have the form:
d
= 0.
(7)
dx t dx In the classics D(x) = const, but in ours -
D = D0 (l - a/a + x) (formula 1). Here we have
replaced d with a to avoid confusion with the differentiation sign. Taking into account the size effect, equation (7) is reduced to the form:
d2c a dc _
----= 0. (8)
dx2 x2dx
Here we took into account that (1-a/a+x) ~ exp (-a/a+ ). If D = const in (7), then we have a classical solution to the problem:
ci(x) = Qx + C2. (9)
The final solution to problem (8) will take the form:
c(x) = (N1 - N2)(o - dlnx) (10)
Equation (10) differs from the classical solution (9) by the presence of the parameter d = 2cu/RT (formula 2), i.e. diffusion mobility in a nanostructure depends on surface tension (surface energy). Determination of the surface energy of solids presents great difficulties, since the surface atoms are immobile [28-30]. However, it is possible to estimate c using formula (3): c = 0.7^10-3 Tm, where Tm is the melting temperature of steel or alloy, which is easy to measure.
Thermal conductivity of steel and alloys during laser alloying
Since a laser is used in this case, problems arise in the thermophysical properties of nanostructures. Heat transfer through nanostructures will differ significantly from the corresponding processes inside macroscopic bodies. Various research methods are applied to such objects, both theoretical and experimental [31-33].
Equations (1) obtained by us, in addition to the universality of describing the properties of nanostruc-
tures, including thermophysical ones, are of exceptional importance for the analysis of subtle effects in thermal physics. In [34], we analyzed the thermal conductivity of nanostructures. For metallic nanostructures, the issues of thermal conductivity are essentially similar to the mechanism of electrical conductivity of the metal. The kinetic approach to the electrical conductivity of a metal is based on the Boltzmann equation. Moreover, the metal is considered as a gas of free electrons, the scattering of which by phonons, defects, etc. and ultimately leads to the appearance of electrical conductivity. Electrons are also responsible for thermal conductivity in metals, so there is a relationship between these quantities, which is given by the well-known Wiedemann-Franz law. Equations (1) have a universal character and are valid for the size dependence of many properties of nanostructures, including thermophysical ones. A was calculated using a formula similar to (1). It can be seen from the calculations that the thermal conductivity coefficients of metals with a size of 1 nm decrease by a factor of 2 in comparison with bulk samples, and at a size of 50 nm they already differ little from the latter [34].
Conclusions and further research prospects
The properties of laser surface alloying are directly related to the thickness of the surface layer, which is a nanostructure. Therefore, all its properties -the diffusion mobility of alloying elements, thermophysical properties, etc. - are determined by the properties of the nanostructure, which is different from the properties of the macrostructure. Further research prospects are associated with the study of physical and chemical processes in nanostructures of various compositions.
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