Laser Surface Modification of Bandage Steels with Tungsten Carbide
Article received for editing 29.11.2018
Article accepted for publication 20.01.2019
For laser surface modification, such processes as surface doping and laser surfacing are most widely used . The emergence of highly efficient sources, such as fibre and diode lasers, has given new impetus to the development of these technologies, as well as the emergence of new processing processes.
The use of laser processing allows to achieve a higher steel hardness due to the grinding of the martensitic structure, however, when applied, the depth of the treated layer is relatively small (up to 1.5 mm when processing without melting the surface) , and to achieve high performance, special optical systems are required and high-power lasers .
A common disadvantage of all these methods is the limitation of the maximum attainable processing results by the material properties of the product. A further increase in the properties of the surface layer can be achieved by surfacing or doping, in both cases the application of a laser heat source is effective for localizing the impact.
Laser surfacing of metal-ceramic composite layers allows achieving record-breaking indicators of hardness and wear resistance [10, 11], but makes a significant change in the product geometry and requires the use of large volumes of expensive filler materials. At the same time, the surface composite layer of the steel-tungsten carbide system can be obtained without using powder to form a matrix using a process related to doping, namely, by introducing reinforcing particles directly into the molten bath on the surface of the product. In particular, the possibility of hardening by using plasma-powder [12, 13] and laser-powder [14, 15] processing of steel parts was shown.
As shown in , laser-powder processing makes it possible to obtain a composite layer with spherical WC particles. The most rational use of particles with a diameter of 90–150 microns. However, unlike the processes of surfacing, while the processing requires that laser radiation is primarily used to melt the substrate metal. At the same time, the development of the metal evaporation process is undesirable, since it can lead to difficulty in the introduction of powder . Excessive heating can melt or even destroy powder particles, which will reduce its role as a reinforcing component of the composite layer, and can also lead to changes in the chemical composition of the steel .
Thus, when developing the technology of laser-powder modification of the surface in order to form a composite layer of steel-tungsten carbide, it is necessary to evaluate the decrease in the intensity of laser radiation during the passage of the gas-powder flow. Moreover, it is necessary to evaluate the heating of the particles, as well as determine the possibility of changing the chemical composition of the steel matrix.
In this work, steel of grade 2 according to GOST 398–2010 was used as the matrix material, and spherical tungsten monocarbide powder containing particles with a diameter of 50 to 200 microns with a predominant fraction of about 100 microns was used as reinforcing particles. For experimental studies, a robotic technological complex based on LS‑5 fibre laser (NTO IRE-Polyus LLC) was used, which generates continuous laser radiation with a power of up to 5 kW at a wavelength of 1070 nm. The powder was supplied coaxially with the use of a technological optical head for laser surfacing and a feeder, ensuring continuous flow of the powder in a stream of carrier gas (argon). The mutual arrangement of the laser beam and the powder flow is shown in Fig. 1.
The particle flux passing through the area of propagation of the laser beam should affect the intensity of the radiation falling on the steel substrate.
When estimating the fraction of radiation passing through a powder stream, the following assumptions can be used:
- particles are affected by laser radiation only on the final part of the path, near the focus of the powder spot;
- within the final section of the path, the diameter of the laser beam and the diameter of the powder flow can be considered constant, and, taking into account the optimization of the flow rate of the latter, equal;
- distribution of power density over the cross section of the laser beam in the same zone can be considered uniform;
- powder flow consists of particles of the same diameter, moving at the same speed.
Since the diameter of the powder grains (minimum fraction is 50 μm) is substantially larger than the wavelength of the fibre laser radiation (1.07 μm), the attenuation coefficient in the region containing particles can be calculated by the method proposed and studied in [18–19], the following expression can be used for the attenuation coefficient (1):
where kλ is the attenuation coefficient of the radiation, m‑1;
ρfWC is the relative density of the content of opaque particles in the region of propagation of the beam;
dWC is the diameter of tungsten carbide particles, m.
The formula takes into account the loss of radiation due to absorption and scattering on particles and makes it possible to estimate the drop in the intensity of a beam passing through a stream of particles.
Particle density can be calculated from geometrical considerations. In the last section of the path, the shape of the beam and the powder flow can be considered cylindrical and approximately equal in size over the length of hWC, the volume of the region containing particles and affecting the passage of the laser beam can be calculated using formula (2):
where Vl is the volume of the propagation region of the beam containing tungsten carbide particles, m3;
hWC is the path length of a particle through the learning site, it is assumed to be 0.002 m in this calculation based on the experimental data;
dl is the diameter of the laser beam, it is 0.003 m in this calculation.
Suppose that all WC powder grains have an equal diameter of 100 μm, then the volume occupied by them in the studied area depends on their quantity, which in turn depends on the mass flow of powder and the speed of movement of individual powder particles. The relative density of particles can be determined by the formula (3):
where MWC is the mass flow rate of the powder, kg · s–1; vWC is the average particle velocity, m · s–1;
γWC is the density of tungsten carbide powder, which is 15 670 kg · m–3.
The intensity of laser radiation propagating in a medium with a damping coefficient kλ is determined by formula (4):
where I0 is the power density of the laser radiation emerging from the optical system, W · m–2; Il is the power density of the laser radiation incident on the substrate, W · m2.
Based on expressions (1) – (4) for the fraction of radiation reaching the substrate, we can write down the expression (5):
The results of the calculation by formula (4) for mass flow rates in the range from 0.005 to 0.05 kg · min–1 and particle speeds from 5 to 10 m · s–1 are presented in Fig. 2
From the graphs it can be seen that the proportion of radiation reaching the substrate is from 0.90 to 0.99 with practically implementable powder supply parameters. The results obtained are in qualitative agreement with the experimental data presented in  for a similar process installation. Since the amount of energy input affects the volume of the molten bath, the laser treatment should be adjusted depending on the powder feed parameters. While maintaining the laser power at the same level, with an increase in the powder feed, a decrease in the volume of the molten bath and a decrease in the depth of the treated layer should be expected.
Calculations of the heating of particles under the action of laser radiation showed that their temperature depends on the size of the particles, and fundamentally different scenarios can be realized for powder particles of the minimum and maximum size present in the selected filler material. In particular, particles with a diameter of more than 100 μm can ingress into the melt, having a temperature below the melting point of steel, due to which they can potentially become crystallization grains. Particles less than 60 μm in diameter may partially melt. It is known from  that at high temperatures tungsten diffusion into steel is possible, which leads to a change in the properties of the latter.
Fig. 3 shows the results of measuring the content of iron and tungsten near the WC particle in the hardened layer on an electron microscope using the EDS method.The presence of this element in the steel matrix is observed, which leads to the heterogeneity of its properties.Thus, laser modification of the surface of the banding steel with tungsten carbide can be implemented in modes that provide incomplete melting of the powder and diffusion of tungsten into the steel matrix. Under these conditions, mechanical surface properties and wear resistance increase, as shown in .
- Gas-powder flow, used to introduce WC particles into the surface layer during laser processing, reduces the intensity of the transmitted radiation by up to 10%, which should be taken into account when optimizing the mode parameters;
- Heating the powder with laser radiation causes an intensification of the diffusion of tungsten, first of all from small particles, into the steel matrix of the composite layer;
- Changing the content of tungsten leads to local changes in the surface properties of steel.
The reported study was funded by RFBR according to the research project № 17-20-03230.