5.1. Alloying

As already mentioned, the alloying processes are based on the melting of the surface of the alloyed material and applying alloying elements into the melt pool which are dissolved into the matrix of the based material and forming surface alloys. The alloying material can be applied previously in the form of coatings by means of other techniques (e.g., magnetron sputtering, plasma spraying, etc.) or can be incorporated into the melt pool directly in the form of a powder stream or wire.

coauthors [39] have studied the conditions of alloying of pure Ti with Al and Nb and with Al and V. The results showed that the formation of an intermetallic surface alloy by means of electron beam alloying strongly depends on the input energy of the e-beam. The same authors [39] have claimed that the melting point of the materials plays a major role in the optimization

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From the performed literature review, it is obvious that the modification of titanium with different transition metals by means of high intensity energy fluxes tends to a significant improvement in the functional properties of the discussed materials. As already mentioned, the modification of aluminum for improving of its operational characteristics is also a subject

Lazarova et al. [40] have demonstrated a modification of the mechanical properties of pure Al by incorporation of TiCN nanopowder by means of electron beam alloying. Their results show that the alloyed zone has a thickness in the range of 14–33 μm and microhardness from 562 to

The authors of [41] have studied the improvement in the surface properties of Al-Si cast alloy by means of laser beam surface alloying with Fe. Their results present an increase in the microhardness. Similarly, Almeida et al. [42] have studied laser beam surface alloying of Al with Cr using two-step process (alloying and remelting). The results show that an increase of the remelting speed points to an increase in the volume fraction of the intermetallic compound and the hardness, respectively. Almeida et al. [43–46] have studied an alloying of pure Al with Nb by means of laser alloying technique. The results obtained by the authors show that the distribution of the alloying element is not homogeneous and undissolved Nb particles surrounded by dendrites of Al3Nb exist. However, the characteristics of the alloyed layers were greatly improved after a laser remelting. Most of the structural defects were eliminated, and undissolved Nb particles have not been observed. The manufactured laser beam surface alloys have been successfully formed with an Al3Nb dendritic microstructure. The same authors [46] claimed that the microhardness increases with an increase of the scanning speed during the laser alloying technology. The reported microhardness changes from 480 to 650 HV with varying of the scanning speed from 5 to 40 mm/sec. Valkov et al. [47] have studied an alloying of pure aluminum with as-deposited Ti-Nb coatings by means of a scanning electron beam, and their results show that undissolved particles have not been observed after the alloying process, contrary to the case of laser beam alloying. The alloyed zone consists of (Ti,Nb)Al3 intermetallic fractions randomly distributed in the biphasic structure of fine (Ti,Nb)Al3 particles dispersed in the Al matrix. The increase of the speed of the specimen motion tends to more homogeneity distribution of the intermetallic phase in the soft Al matrix and much finer microstructure. Also, the measured microhardness reaches values of 775 HV and it does not depend on the speed of the specimen motion during the electron beam alloying process. Therefore, a significant difference between the properties and structure of the fabricated surface alloys by electron and laser beam exists. The authors of [48] have made a comparative study of electron and laser beam surface alloying of pure Al with Nb. The results reported in [48] have shown that the observed differences in the microstructure of the surface alloys formed by both techniques are explained by the different way of controlling the lifetime of

of the technological parameters of a selective electron beam technology.

798 HV or the alloyed zone is 16–22 times harder than the base Al substrate.

of investigations in the field of the modern materials science.

The high energy fluxes alloying techniques are widely used for fabrication of materials for the needs of the automotive and aerospace industries, for manufacturing of railway cars, space crafts, light ships, etc. Such materials are aluminum alloys due to their attractive mechanical properties and light weight. Alloying of pure aluminum with different transition metals by means of high energy fluxes is among the most promising methods for fabrication of surface alloys and for improvement of the surface properties of the materials. For that reason, many researchers are working on the formation and characterization of surface alloys by high energy fluxes.

The alloying of titanium and titanium alloys is a subject of investigations for many scientists due to the application of these materials in the field of the contemporary aviation and automotive industries, for different biomedical applications and many more.

The authors of [35] have studied a laser alloying of Ti-Si compound coating on Ti6Al4V in order to improve the bioactivity. They have reported that the microhardness increases dramatically after the alloying process. Also, the corrosion resistance of laser-alloyed Si coating is 27% improved in comparison to the base Ti6Al4V alloy. The evolution of the cell growth is the same for the case of Si alloyed layer and base Ti6Al4V materials on the first day, but in the progress, the cell growth starts to be faster on the laser-alloyed Si layer in comparison to the Ti6Al4V alloy.

Similarly, the authors of [36] have studied a laser alloying of titanium with boron and carbon. Their results show that the microstructure of the alloyed zone consists of a hard ceramic phases, namely, TiB + TiB2, TiB + TiB2 + TiC, or TiC. A significant increase in the hardness and wear resistance of all surface alloys has been observed in comparison to the commercially pure titanium.

As already mentioned, in addition to the laser alloying, the electron beams are also widely used for alloying processes. The authors of [37] have studied a cycling mixing of predeposited Al films onto Ti substrate by means of pulsed electron beam. The phase composition of the alloyed zone consists of TiAl and TiAl2 phases. The measured nanohardness of the nearsurface region is significantly greater in comparison to the base Ti substrate—11 GPa.

Similarly, Valkov et al. [38] have studied an electron beam surface alloying of pure Ti with Al films, and their results show that the alloyed zone consists of biphasic structure of Ti3Al and TiAl, which is transformed in single phase structure of TiAl in depth. This transformation is accompanied by a decrease in the hardness from the surface to the depth. Also, Valkov and coauthors [39] have studied the conditions of alloying of pure Ti with Al and Nb and with Al and V. The results showed that the formation of an intermetallic surface alloy by means of electron beam alloying strongly depends on the input energy of the e-beam. The same authors [39] have claimed that the melting point of the materials plays a major role in the optimization of the technological parameters of a selective electron beam technology.

5. Overview of the processes

78 Advanced Surface Engineering Research

of a powder stream or wire.

As already mentioned, the alloying processes are based on the melting of the surface of the alloyed material and applying alloying elements into the melt pool which are dissolved into the matrix of the based material and forming surface alloys. The alloying material can be applied previously in the form of coatings by means of other techniques (e.g., magnetron sputtering, plasma spraying, etc.) or can be incorporated into the melt pool directly in the form

The high energy fluxes alloying techniques are widely used for fabrication of materials for the needs of the automotive and aerospace industries, for manufacturing of railway cars, space crafts, light ships, etc. Such materials are aluminum alloys due to their attractive mechanical properties and light weight. Alloying of pure aluminum with different transition metals by means of high energy fluxes is among the most promising methods for fabrication of surface alloys and for improvement of the surface properties of the materials. For that reason, many researchers are

The alloying of titanium and titanium alloys is a subject of investigations for many scientists due to the application of these materials in the field of the contemporary aviation and automo-

The authors of [35] have studied a laser alloying of Ti-Si compound coating on Ti6Al4V in order to improve the bioactivity. They have reported that the microhardness increases dramatically after the alloying process. Also, the corrosion resistance of laser-alloyed Si coating is 27% improved in comparison to the base Ti6Al4V alloy. The evolution of the cell growth is the same for the case of Si alloyed layer and base Ti6Al4V materials on the first day, but in the progress, the cell growth starts to be faster on the laser-alloyed Si layer in comparison to the Ti6Al4V alloy.

Similarly, the authors of [36] have studied a laser alloying of titanium with boron and carbon. Their results show that the microstructure of the alloyed zone consists of a hard ceramic phases, namely, TiB + TiB2, TiB + TiB2 + TiC, or TiC. A significant increase in the hardness and wear resistance of all surface alloys has been observed in comparison to the commercially pure

As already mentioned, in addition to the laser alloying, the electron beams are also widely used for alloying processes. The authors of [37] have studied a cycling mixing of predeposited Al films onto Ti substrate by means of pulsed electron beam. The phase composition of the alloyed zone consists of TiAl and TiAl2 phases. The measured nanohardness of the near-

Similarly, Valkov et al. [38] have studied an electron beam surface alloying of pure Ti with Al films, and their results show that the alloyed zone consists of biphasic structure of Ti3Al and TiAl, which is transformed in single phase structure of TiAl in depth. This transformation is accompanied by a decrease in the hardness from the surface to the depth. Also, Valkov and

surface region is significantly greater in comparison to the base Ti substrate—11 GPa.

working on the formation and characterization of surface alloys by high energy fluxes.

tive industries, for different biomedical applications and many more.

5.1. Alloying

titanium.

From the performed literature review, it is obvious that the modification of titanium with different transition metals by means of high intensity energy fluxes tends to a significant improvement in the functional properties of the discussed materials. As already mentioned, the modification of aluminum for improving of its operational characteristics is also a subject of investigations in the field of the modern materials science.

Lazarova et al. [40] have demonstrated a modification of the mechanical properties of pure Al by incorporation of TiCN nanopowder by means of electron beam alloying. Their results show that the alloyed zone has a thickness in the range of 14–33 μm and microhardness from 562 to 798 HV or the alloyed zone is 16–22 times harder than the base Al substrate.

The authors of [41] have studied the improvement in the surface properties of Al-Si cast alloy by means of laser beam surface alloying with Fe. Their results present an increase in the microhardness. Similarly, Almeida et al. [42] have studied laser beam surface alloying of Al with Cr using two-step process (alloying and remelting). The results show that an increase of the remelting speed points to an increase in the volume fraction of the intermetallic compound and the hardness, respectively. Almeida et al. [43–46] have studied an alloying of pure Al with Nb by means of laser alloying technique. The results obtained by the authors show that the distribution of the alloying element is not homogeneous and undissolved Nb particles surrounded by dendrites of Al3Nb exist. However, the characteristics of the alloyed layers were greatly improved after a laser remelting. Most of the structural defects were eliminated, and undissolved Nb particles have not been observed. The manufactured laser beam surface alloys have been successfully formed with an Al3Nb dendritic microstructure. The same authors [46] claimed that the microhardness increases with an increase of the scanning speed during the laser alloying technology. The reported microhardness changes from 480 to 650 HV with varying of the scanning speed from 5 to 40 mm/sec. Valkov et al. [47] have studied an alloying of pure aluminum with as-deposited Ti-Nb coatings by means of a scanning electron beam, and their results show that undissolved particles have not been observed after the alloying process, contrary to the case of laser beam alloying. The alloyed zone consists of (Ti,Nb)Al3 intermetallic fractions randomly distributed in the biphasic structure of fine (Ti,Nb)Al3 particles dispersed in the Al matrix. The increase of the speed of the specimen motion tends to more homogeneity distribution of the intermetallic phase in the soft Al matrix and much finer microstructure. Also, the measured microhardness reaches values of 775 HV and it does not depend on the speed of the specimen motion during the electron beam alloying process. Therefore, a significant difference between the properties and structure of the fabricated surface alloys by electron and laser beam exists. The authors of [48] have made a comparative study of electron and laser beam surface alloying of pure Al with Nb. The results reported in [48] have shown that the observed differences in the microstructure of the surface alloys formed by both techniques are explained by the different way of controlling the lifetime of the melt pool. The electron beam alloying technique can be realized in different geometries of scanning (circular, linear, etc.) since the electrons can be deflected and guided due to their nature of charged particles. When using circular scanning mode, the trajectory of the e-beam overlaps which points to longer lifetime of the melt pool. Using laser beam alloying technique, such technological conditions cannot be realized and the lifetime of the melt pool is significantly shorter [48]. The authors of [49, 50] have performed detailed investigations of the microstructure and the crystallographic structure of surface alloys fabricated by electron and laser beam alloying and explain the difference in the hardening mechanism of both kind of alloys. Vilar et al. [49] have studied the crystallographic structure of laser beam–manufactured surface alloys, and their results show that the increase of the scanning speed during the alloying process reflects to formation of a preferred crystallographic orientation while such effect of electron beam–fabricated surface alloys has not been observed [50]. According to the authors of [51, 52] the formation of a preferred crystallographic orientation can significantly affect the mechanical properties which, as mentioned in [50] can be a possible reason for the observed differences in the hardening mechanism of electron- and laser-processed surface alloys.

be increased simply by increasing the number of formation passes and the beam oscillation

Similarly, Abe et al. [59] have studied WC12% Co- and Ni-base self-fluxing alloy powder with a mild steel substrate. Their results show good quality coatings with superior functional properties, including wear and corrosion resistance. The microhardness of the formed clad-

As already mentioned, the hardening process by means of high energy fluxes is based on the irradiation of the hardened surface and formation of metastable structures as well as on the

Such approach for improving of the functional properties of the materials has been used by the authors of [60]. They have analyzed the influence of the laser treatment of carbon steel on the changes of the microstructure and microhardness in depth. The results are compared with those obtained by conventional hardness, and it has been concluded that the laser treatment technique leads to 80% harder structure in comparison to the conventional processes. Similarly, Sarnet and coauthors [61] have conducted a surface treatment of alloyed steels by an excimer

In study [62] the results of investigation of the electron beam surface modification of 5CrMoMn steel are shown. The beam current in the study has been varied from 6 to 8 mA with a step of 0.5 mA. The results show that surface properties of the modified specimens are greatest at a beam current of 7 mA. The microhardness reaches from 355 HV for the base material up to 656 HV and decreases with further increasing of the current above 7 mA due to

The authors of [63] have performed similar investigation. They have studied an electron beam surface hardening of 30CrMnSiA steel as the subject of discussion is the influence of the density of the input energy of the e-beam on the possibility of formation of hardened layers. The results show that the hardness increases dramatically, from 320 to 520 HV when the input

hardness decreases because of the convective mixing of the melted zone, which effect becomes

Also, electron beam surface hardening can be combined with other methods, such as physical vapor deposition [64], plasma nitriding [34, 65], etc. Grumbt et al. [64] have studied a duplex treatment Ti1-xAlxN coatings with subsequent electron beam treatment of steel substrates. The results of study [64] show that the coating significantly enhanced the absorption properties, resulting to an increase in the hardened depth for the same parameters of the electron beam hardening process of coated and uncoated steel. Moreover, the hardened depth increases with an increase of Al content or the thickness of the coating. Also, it was demonstrated that the electron beam hardening of previously coated steel substrates is capable to form significantly

predominant after the discussed density of the input energy of the electron beam.

. By further increasing the input energy, the

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laser. As a result of the treatment, the microhardness increases with about 200%.

amplitude. The measured hardness was 791 HV.

the reduction of martensite and carbon content.

energy density reaches a value of 1.857 kJ/cm<sup>2</sup>

harder surfaces in comparison to uncoated materials.

ding layer reaches values of 1400 HV.

changes in the microstructure.

5.3. Hardening

### 5.2. Cladding

The cladding technique is widely used for manufacturing of coatings for protective purposes, against adhesive wear (when two bodies with similar mechanical properties slide against each other) and abrasive wear (when hard particles or hard body slide against softer one). Such materials which can overcome these drawbacks are alloys in the system of Co-Cr-W-C due to their high strength and resistance to corrosion and wear at high temperatures [53]. These alloys are also known as stellites.

Such coatings have been applied by [54] using laser cladding of preplaced powder on stainless steel substrates, as the authors have studied the influence of the energy density of the laser beam on the degree of dilution. Their results show that an energy density of 6.4 kJ/cm<sup>2</sup> is needed to melt the powder and wet the substrate in order to form a good quality continuous track with less than 6% dilution. An increase in the energy density leads to significantly higher and inappropriate dilution of up to 27%. Lower energy density is capable to melt the powder without wetting the substrate. The authors of [55–57] have studied the formation of stellite coating on austenitic steel substrate as a function of the scanning speed which was in the range 1.67–167 mm/s. They have reported that coatings free of crack and pores with excellent adherence have been formed. The same authors have claimed that the microstructure of the coatings does not depend on the technological conditions of the cladding and becomes similar in the range of the scanning speed. However, the same authors [57] have mentioned that stellites can be applied without preheating of the substrate, but carbon-rich coatings have higher brittleness and are capable to crack. Therefore, the substrate must be preheated in order to avoid such defects.

The authors of [58] have studied an electron beam cladding of Cr3C2/Ni-Cr powder on steel substrates. The conditions for obtaining an increased thickness and modified area of still good surface layer have been investigated. The results show that the thickness and cladded area can be increased simply by increasing the number of formation passes and the beam oscillation amplitude. The measured hardness was 791 HV.

Similarly, Abe et al. [59] have studied WC12% Co- and Ni-base self-fluxing alloy powder with a mild steel substrate. Their results show good quality coatings with superior functional properties, including wear and corrosion resistance. The microhardness of the formed cladding layer reaches values of 1400 HV.

### 5.3. Hardening

the melt pool. The electron beam alloying technique can be realized in different geometries of scanning (circular, linear, etc.) since the electrons can be deflected and guided due to their nature of charged particles. When using circular scanning mode, the trajectory of the e-beam overlaps which points to longer lifetime of the melt pool. Using laser beam alloying technique, such technological conditions cannot be realized and the lifetime of the melt pool is significantly shorter [48]. The authors of [49, 50] have performed detailed investigations of the microstructure and the crystallographic structure of surface alloys fabricated by electron and laser beam alloying and explain the difference in the hardening mechanism of both kind of alloys. Vilar et al. [49] have studied the crystallographic structure of laser beam–manufactured surface alloys, and their results show that the increase of the scanning speed during the alloying process reflects to formation of a preferred crystallographic orientation while such effect of electron beam–fabricated surface alloys has not been observed [50]. According to the authors of [51, 52] the formation of a preferred crystallographic orientation can significantly affect the mechanical properties which, as mentioned in [50] can be a possible reason for the observed differences in the hardening mechanism of electron- and laser-processed surface

The cladding technique is widely used for manufacturing of coatings for protective purposes, against adhesive wear (when two bodies with similar mechanical properties slide against each other) and abrasive wear (when hard particles or hard body slide against softer one). Such materials which can overcome these drawbacks are alloys in the system of Co-Cr-W-C due to their high strength and resistance to corrosion and wear at high temperatures [53]. These alloys

Such coatings have been applied by [54] using laser cladding of preplaced powder on stainless steel substrates, as the authors have studied the influence of the energy density of the laser beam on the degree of dilution. Their results show that an energy density of 6.4 kJ/cm<sup>2</sup> is needed to melt the powder and wet the substrate in order to form a good quality continuous track with less than 6% dilution. An increase in the energy density leads to significantly higher and inappropriate dilution of up to 27%. Lower energy density is capable to melt the powder without wetting the substrate. The authors of [55–57] have studied the formation of stellite coating on austenitic steel substrate as a function of the scanning speed which was in the range 1.67–167 mm/s. They have reported that coatings free of crack and pores with excellent adherence have been formed. The same authors have claimed that the microstructure of the coatings does not depend on the technological conditions of the cladding and becomes similar in the range of the scanning speed. However, the same authors [57] have mentioned that stellites can be applied without preheating of the substrate, but carbon-rich coatings have higher brittleness and are capable to crack. Therefore, the substrate must be preheated in order

The authors of [58] have studied an electron beam cladding of Cr3C2/Ni-Cr powder on steel substrates. The conditions for obtaining an increased thickness and modified area of still good surface layer have been investigated. The results show that the thickness and cladded area can

alloys.

5.2. Cladding

are also known as stellites.

80 Advanced Surface Engineering Research

to avoid such defects.

As already mentioned, the hardening process by means of high energy fluxes is based on the irradiation of the hardened surface and formation of metastable structures as well as on the changes in the microstructure.

Such approach for improving of the functional properties of the materials has been used by the authors of [60]. They have analyzed the influence of the laser treatment of carbon steel on the changes of the microstructure and microhardness in depth. The results are compared with those obtained by conventional hardness, and it has been concluded that the laser treatment technique leads to 80% harder structure in comparison to the conventional processes. Similarly, Sarnet and coauthors [61] have conducted a surface treatment of alloyed steels by an excimer laser. As a result of the treatment, the microhardness increases with about 200%.

In study [62] the results of investigation of the electron beam surface modification of 5CrMoMn steel are shown. The beam current in the study has been varied from 6 to 8 mA with a step of 0.5 mA. The results show that surface properties of the modified specimens are greatest at a beam current of 7 mA. The microhardness reaches from 355 HV for the base material up to 656 HV and decreases with further increasing of the current above 7 mA due to the reduction of martensite and carbon content.

The authors of [63] have performed similar investigation. They have studied an electron beam surface hardening of 30CrMnSiA steel as the subject of discussion is the influence of the density of the input energy of the e-beam on the possibility of formation of hardened layers. The results show that the hardness increases dramatically, from 320 to 520 HV when the input energy density reaches a value of 1.857 kJ/cm<sup>2</sup> . By further increasing the input energy, the hardness decreases because of the convective mixing of the melted zone, which effect becomes predominant after the discussed density of the input energy of the electron beam.

Also, electron beam surface hardening can be combined with other methods, such as physical vapor deposition [64], plasma nitriding [34, 65], etc. Grumbt et al. [64] have studied a duplex treatment Ti1-xAlxN coatings with subsequent electron beam treatment of steel substrates. The results of study [64] show that the coating significantly enhanced the absorption properties, resulting to an increase in the hardened depth for the same parameters of the electron beam hardening process of coated and uncoated steel. Moreover, the hardened depth increases with an increase of Al content or the thickness of the coating. Also, it was demonstrated that the electron beam hardening of previously coated steel substrates is capable to form significantly harder surfaces in comparison to uncoated materials.

The authors of [34, 65] have studied an electron beam surface hardening of previously nitrogen-alloyed steel. Their results show a significant increase in the microhardness as well as double improvement of the wear resistance. The authors of the discussed studies [34, 65] claimed that the reasons of these improvements of the functional properties are the refined microstructure consisting of α-solid solution (nitrous martensite) and γ-solid solution (nitrous austenite) and dispersed fine nitride precipitates. Ormanova et al. [22] have presented a combined method for surface modification of tool steels, consisting of electron beam hardening followed by plasma nitriding and subsequent electron beam hardening, and the results demonstrate a hardness of 760 HV after the electron beam treatment and plasma hardening. The application of additional electron beam treatment process tends to a slight decrease in the hardness due to structural transformation and reduction of the amount of N atoms.

Conflict of interest

Author details

Bulgaria

References

49-66

1997;48:49-50

The authors have no conflict of interest to declare.

Stefan Valkov\*, Maria Ormanova and Peter Petrov

USA: Industrial Press Inc; 1994

Materialia. 2016;117:371-392

Francis Group; 2018. pp. 133-160

born, MI, USA; 2001

\*Address all correspondence to: stsvalkov@gmail.com

Academician Emil Djakov Institute of Electronics, Bulgarian Academy of Sciences, Sofia,

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