Influence of Nickel Content on the Formation of an Interaction Zone during Contact Melting of Titanium with Copper-Nickel Alloys

Abstract

The diffusion processes during the contact melting at the boundary of explosively welded VT1-0 titanium with CuNi19 (melchior) and CuNi45 (constantan) alloy composites were studied. Heat treatment of composites led to the formation of the interaction zone at the joint boundary. The interaction zone in VT1-0 + CuNi19 consists of TiCuNi and αTi + Ti₂Cu(Ni) continuous layers as well as a mixture of TiNi(Cu) + TiCu(Ni) + Ti₂Cu(Ni) intermetallics. It has been shown that an increase in the nickel content in the case of VT1-0 + CuNi45 composite leads to a decrease in the temperature of contact melting, a change in its mechanism, an increase in the titanium content in the interaction zone, and the appearance of additional Ti₂Ni(Cu) intermetallic in its composition.

1. Introduction

Increasing the service life and reliability of products operated in extreme conditions (under contact conditions, at high temperatures, and large, including cyclic, loads) is an urgent problem of modern technology. Binary intermetallic compounds of titanium with copper have high hardness and unique tribological properties combined with high electrical conductivity [1,2,3]. However, along with high hardness, they have low ductility [4,5], which limits their practical use for protecting the surface of titanium, copper, and alloys based on them.

It is possible to provide a more favorable combination of strength, ductility, and fracture toughness of coatings based on Ti-Cu intermetallic compounds by introducing additional phases into the coating composition and thus dispersing the structure [6,7,8,9,10]. For example, it is known that alloying Ti-Cu intermetallic compounds with Ni helps to increase their ductility, and when its content is 15–20%, a shape memory effect appears. Additionally, the addition of Ni in alloys of the Ti-Cu system helps to reduce the melting point of the alloy, thereby facilitating the processing of the material. In addition, Ni promotes solid solution strengthening in Ti-Cu alloys and increases their corrosion resistance. The work [11] shows that Cu/Ni coatings on the surface of titanium after heat treatment have high adhesive strength. Due to their strength characteristics, the alloys of the Cu-Ni-Ti system have found wide use as brazing materials for joining titanium alloys [12,13].

There are many known methods for producing Ti-Cu coatings: pack cementation [14,15], self-propagating high-temperature synthesis [16], laser processing [17], electrolytic deposition [18], and electric spark alloying [19,20]. The most promising method is one that includes explosive welding followed by heat treatment via contact melting (CM) [21,22,23,24]. Explosive welding makes it possible to obtain plastic metal composites of various dimensions, which can be subjected to pressure treatment in order to give semi-finished products the required shape and size. Subsequent heat treatment under contact melting conditions leads to the formation of an interaction zone (IZ) at the joint boundary as a result of reactive diffusion. The formation of the coating is achieved by dissolving one of the metal layers completely in IZ or by mechanically separating the unreacted part [22,23,24]. Using this technology, it is possible to obtain defect-free coatings of various thicknesses, chemical and phase compositions in a short time. The structure and phase composition of the Cu-Ti coatings can be controlled by the time and temperature of the CM process.

As is known [21,25], contact melting is a phenomenon in which two solid metals in contact begin to melt at temperatures below their melting points. A necessary condition for CM is the presence of a minimum in the phase diagram of the contacted crystals. That is, the CM phenomenon occurs in systems with a minimum point on the liquidus curve, on eutectic-type diagrams, and on diagrams with the chemical interaction of components. Typically, the CM temperature coincides with the eutectic temperature. In systems with chemical interaction of components, the CM temperature can be lower than the eutectic temperature (the phenomenon of the ΔT effect).

Contact melting in the Cu-Ti system at the boundary of an explosively welded composite begins above the eutectic L↔TiCu₄ + TiCu₂ at a temperature of about 900 °C [22]. In the Ni-Ti system, contact melting begins above the eutectic L↔βTi + Ti₂Ni at a temperature of about 970 °C [24].

The doping of a binary system can lead to a change in CM temperature, as well as the formation of a different phase composition of IZ. It is known [21] that doping of binary systems can lead to both an increase and a decrease in the contact melting temperature. In the first case, this may be due to a decrease in the diffusion mobility of atoms, and in the second, due to the formation of new phases and ternary eutectics.

There is a lot of work on the issues of diffusion interaction in the Ti-Cu-Ni system. Most studies are devoted to the description and refinement of phase equilibria data in the Ti-Cu-Ni ternary system [26,27,28]. In particular, these works describe the structure of individual binary and ternary compounds, data on the temperatures of phase transitions, and so on.

In the work [29], the process of diffusion interaction at the boundary of titanium and copper-nickel alloy was considered in detail. The authors of [29] critically studied the partial isothermal section of the Cu-Ni-Ti ternary system at 800 °C and showed the layered structure of the interaction zone formed at this temperature. However, the presented results do not take into account contact melting temperatures. The same temperature range is considered in [11].

Studying the features of the Ti-Cu-Ni system protective coatings formation is of great interest since such coatings are promising for protecting titanium alloys from external impacts and aggressive environments. To designate optimal conditions for producing coatings, it is necessary to understand the sequence of formation and growth of the interaction zones at the alloy interface. Despite the fact that diffusion processes in the Ti-Cu binary system have been studied in detail, there is practically no information in the literature about the influence of Ni on the features of diffusion processes at the interface between titanium and copper alloy via CM.

Based on this, the purpose of this work was to study the structure and phase composition of the Ti-Cu-Ni system IZ obtained using explosive welding and subsequent heat treatment via CM.

2. Materials and Methods

Studies were carried out on samples obtained from explosively welded VT1-0 titanium + CuNi19 alloy and VT1-0 titanium + CuNi45 alloy (2 + 1 mm) composites. The structures of the joint boundary after explosive welding are shown in Figure 1.

CuNi19 (Melchior, LLC SisbMetallTorg, Novosibirsk, Russia) and CuNi45 (Constantan, LLC SibMetallTorg, Novosibirsk, Russia) copper-nickel alloys–cladding plates (1 mm thick) and titanium (VT1-0, LLC SibMetallTorg, Novosibirsk, Russia)-base plate (2 mm thick), were used. The chemical compositions of the initial materials are presented in

Table 1. Chemical composition of VT1-0 titanium (wt.%).

Ti	Fe	C	Si	N	O	H
99.24–99.7	≤0.25	≤0.07	≤0.1	≤0.04	≤0.2	≤0.01

,

Table 2. Chemical composition of the CuNi19 alloy (wt.%).

Cu	Ni + Co	Fe	C	Si	Mn	S	P	As	Pb	Mg	Zn	Sb	Bi
78.5–82	18–20	≤0.5	≤0.05	≤0.15	≤0.3	≤0.01	≤0.01	≤0.01	≤0.005	≤0.05	≤0.3	≤0.005	≤0.002

and

Table 3. Chemical composition of the CuNi45 alloy (wt.%).

Cu	Ni + Co	Fe	C	Si	Mn	S	P	As	Pb	Mg	Zn	Sb	Bi
53–56	44–46	≤0.6	≤0.05	≤0.06	≤0.05	≤0.005	≤0.01	≤0.002	≤0.005	≤0.03	≤0.002	≤0.005	≤0.002

.

Explosive welding was carried out according to a parallel scheme, in which the thrown plate (titanium) was located parallel to the stationary one (copper-nickel alloy) at a strictly defined stand-off distance. The explosive charge of a strictly defined height was located on the throwing plate. The explosion is initiated from the edge of the plate. Explosive welding regimes are presented in

Table 4. Explosive welding regimes.

Explosive Type	Height of the Explosive Charge, mm	Stand-off Distance, mm	Contact Velocity, Vc, m/s	Impact Velocity, Vi, m/s
VT1-0 + CuNi19
Ammonite N°6ZhV	20	1.5	2900	560
VT1-0 + CuNi45
Ammonite N°6ZhV	20	1.6	2900	570

.

Heat treatment of explosively welded samples to form IZ was performed in an SNOL 8.2/1100 furnace in the air. Heat treatment was carried out at 850 and 950 °C. The first temperature below the eutectic for the Ti-Cu-Ni system [26] was chosen to implement solid-phase interaction. The second temperature is above the eutectic for the Ti-Cu-Ni system, which was chosen for CM. The holding time at 850 °C was 50 h, and at 950 °C, it was 1, 2, 3, and 15 min. After heat treatment, the samples were retrieved from the furnace and cooled in still air.

Metallographic studies were carried out using an optical microscope (Olympus BX-61, Olympus Corporation, Tokyo, Japan) and scanning electron microscope (SEM–Versa 3D) (FEI Company, Hillsboro, OR, USA).

Bruker D8 Advance ECO X-Ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) (CuKα radiation (λ = 0.15406 nm)) was used to identify the phase composition. Layer-by-layer grinding was carried out to obtain the diffraction patterns at different IZ depths. The chemical composition was determined using energy-dispersive X-ray spectroscopy (EDS–EDAX Trident XM 4, EDAX, Mahwah, NJ, USA).

3. Results and Discussion

3.1. Diffusion Processes in VT1-0 + CuNi19 Composite

Heat treatment of the VT1-0 + CuNi19 composite at 950 °C for 1 min led to the formation of IZ with a layered structure at the joint boundary. Its structure (Figure 2) included five interlayers. According to the results of energy dispersive analysis (

Table 5. Point EDS analysis results (950 °C, 1 min) (see Figure 2a).

Element	Content, at.%
	1	2	3	4	5
Ti	31.99	49.54	49.8	65.07	89.35
Cu	38.25	29.78	47.09	30.14	8.74
Ni	29.75	20.69	3.11	4.79	1.91
Phase	TiCuNi	TiNi(Cu)	TiCu(Ni)	Ti2Cu(Ni)	αTi + Ti2Cu(Ni)

), four of which corresponded to the compounds TiCuNi, TiNi(Cu), TiCu(Ni), and Ti₂Cu(Ni). The fifth interlayer, on the titanium side, consisted of αTi solid solution and Ti₂Cu(Ni) secondary phase, which is a product of the βTi decomposition upon cooling.

Increasing the holding time to 2 min led to a significant increase in the thickness of IZ. This is due to the onset of the CM process, as evidenced by the formation of a eutectic structure (Figure 3a). This structure is a mixture of TiNi(Cu), TiCu(Ni), and Ti₂Cu(Ni) (Figure 3b). Thin continuous layers were formed at the CuNi19 side (TiCuNi and TiNi(Cu)) and at the VT1-0 side (αTi + Ti₂Cu(Ni)). The formation of such layers is typical for solid-state diffusion interaction. This indicated that the onset of the CM process (appearance of the liquid phase) is preceded by solid-state diffusion.

To study the initial stages of contact melting, IZ was previously formed at the VT1-0 + CuNi19 joint during heat treatment under solid-state diffusion conditions (850 °C, 50 h). IZ with a layered structure had a total thickness of 80–100 μm. Next, heat treatment was carried out at 950 °C with a holding time of 1 to 3 min. The structure and chemical composition of IZ are presented in Figure 4 and Figure 5 and

Table 6. Point EDS analysis results (850 °C, 50 h + 950 °C, 1 min) (see Figure 4).

Element	Content, at.%
	1	2	3	4	5	6
Ti	32.06	49.12	50.04	67.27	48.82	50.18
Cu	44.79	31.05	47.68	30.62	44.95	41.94
Ni	23.14	19.83	2.28	2.11	6.23	7.58
Phase	TiCuNi	TiNi(Cu)	TiCu(Ni)	Ti2Cu(Ni)	TiCu(Ni)	TiCu(Ni) + TiNi(Cu)

.

After holding for 1 min, the thickness of the zone remained virtually unchanged. Along the boundary of the TiNi(Cu) and TiCu(Ni) interlayers, the formation of extended heterogeneous areas perpendicular to the joint boundary is observed (Figure 4b,c). According to point energy dispersive analysis data, these areas correspond to a mixture of TiCu(Ni) + TiNi(Cu) (Figure 4d and Table 6).

An increase in time led to the formation of a continuous layer of eutectic structure, which, along with TiCu(Ni) and TiNi(Cu), contains dispersed inclusions of Ti₂Cu(Ni) and Ti₂Ni(Cu) (Figure 5).

The results obtained allow us to conclude that the CM process begins at the TiCu-TiNi interface (Figure 4). According to CM theory [30], the liquid phase formation at the TiCu-TiNi interface can be caused by the melting of the surface layer of one of the contacting crystals. Which, in turn, is due to two processes of the interatomic interaction of crystals:

-

the occurrence of stresses in the crystal lattice in a thin boundary layer caused by the dimensional mismatch of conjugating atom groups that are located on the surfaces of contacting substances;

-

weakening of interatomic bonds in the surface layer of one of the contacting crystals.

TiCu and TiNi intermetallics have different crystal structures. Tetragonal (a = 0.3118 nm and c = 0.5921 nm) for TiCu and body-centered CsCl type (B2, a = 0.3015 nm) for TiNi. The interatomic interaction between TiCu and TiNi shifts the phase equilibrium temperature at the boundary layer to a critical point, which is the eutectic temperature. This temperature corresponds to the temperature at which CM begins. Analysis of the TiCu-TiNi quasi-binary section (Figure 6) on the Ti-Cu-Ni phase diagram showed that it is equal to ≈924 °C [31]. The dissolution of TiNi in the TiCu melt occurs via spontaneous dispersion, that is, a dispersal system is formed. The structure of the formed disperse system is shown in Figure 4d. CM process develops due to the diffusion of dispersed TiNi particles via the melt to the surface of solid TiCu.

Metallographic studies of IZ after heat treatment at 950 °C, 15 min (Figure 7,

Table 7. Point EDS analysis results (950 °C, 15 min) (see Figure 7b).

Element	Content, at.%
	1	2	3	4
Ti	25.61	48.02	47.52	42.45
Cu	37.25	35.93	48.68	52.83
Ni	37.14	16.04	3.79	4.72
Phase	TiCuNi	TiNi(Cu)	TiCu(Ni)	Ti2Cu3(Ni)

) without preliminary heat treatment at 850 °C showed that on the side of the CuNi19 alloy, a layer of TiCuNi ternary intermetallic compound with a thickness of 3–4 μm was formed. In place of the dissolved CuNi19 alloy, a matrix structure based on the TiNi(Cu) intermetallic compound with dispersed Ti₂Cu₃(Ni) and needle-shaped TiCu(Ni) inclusions was formed (

Table 8. Point EDS analysis results (950 °C, 15 min) (see Figure 7c).

Element	Content, at.%
	1	2	3
Ti	49.31	66.23	49.37
Cu	36.96	19.7	48.05
Ni	13.73	14.07	2.58
Phase	TiNi(Cu)	Ti2Cu(Ni)	TiCu(Ni)

). In place of the dissolved titanium VT1-0, a mixture of intermetallic compounds TiNi(Cu), TiCu(Ni), and Ti₂Cu(Ni) was formed (

Table 9. Point EDS analysis results (950 °C, 15 min) (see Figure 7d).

Element	Content, at.%
	1	2	3	4	5
Ti	47.73	65.59	49.36	86.03	100
Cu	36.28	30.2	47.36	10.84	-
Ni	15.99	4.21	3.28	3.13	-
Phase	TiNi(Cu)	Ti2Cu(Ni)	TiCu(Ni)	αTi + Ti2Cu(Ni)	αTi

). On the titanium side, an αTi + Ti₂Cu(Ni) layer was formed.

The results of the X-ray diffraction analysis are presented in Figure 8. The following phases were reliably identified: TiCu, Ti₂Cu, Ti₂Cu₃, TiNi, TiCuNi. It should be noted that most of the phases formed in the Ti-Cu-Ni system have close 2θ reflection angles, which causes the corresponding reflections to overlap in the diffraction patterns.

The structure analysis of IZ formed at the VT1-0 + CuNi19 joint after heat treatment at 950 °C and holding time of 2 and 15 min showed that its thickness varied from ~0.05 to ~1.3 mm. Measuring the thickness of the original alloys before and after heat treatment made it possible to calculate the volume fraction of chemical elements in IZ. In the time range of heating at 950 °C for up to 15 min, the volume fraction of elements lay in the following range: 60–67% Ti, 27–32% Cu, 6–8% Ni. When recalculated into mass percentages, it was found that at 950 °C, the concentration of elements in IZ tended to be 43 wt.% Ti, corresponding to 50 at. % Ti. The obtained data correlated well with the results of energy dispersive analysis: 42–62 at. % Ti, 30–50 at. % Cu and 2–15 at. % Ni (Figure 7), which, in terms of stoichiometric composition, corresponded to an area of the TiNi(Cu), TiCu(Ni), and Ti₂Cu(Ni) phases formation.

3.2. Diffusion Processes in VT1-0 + CuNi45 Composite

For the VT1-0 + CuNi45 composite, preliminary heat treatment was carried out under solid-state diffusion conditions (850 °C, 50 h). After heat treatment, on the side of the CuNi45 alloy, three layers were formed in stoichiometric composition corresponding to the TiCuNi, TiNi(Cu), and Ti₂Ni(Cu) intermetallic compounds (Figure 9,

Table 10. Point EDS analysis results (550 °C, 50 h) (see Figure 9).

Element	Content, at.%
	1	2	3	4	5
Ti	33.08	49.65	66.87	73.87	99.59
Cu	27.22	21.83	9.97	14.85	0.41
Ni	39.7	28.52	23.16	11.28	-
Phase	TiNiCu	TiNi(Cu)	Ti2Ni(Cu)	Ti2Cu(Ni)	αTi

) with a total thickness of ~20 μm. The fourth layer, with a thickness of ~550 μm, located on the titanium side, consisted of αTi with inclusions of the Ti₂Cu(Ni) intermetallic compound (Figure 9, Table 10, points 4, 5). The formation of the last layer was due to the decomposition of the βTi solid solution upon cooling.

Metallographic studies of IZ after heat treatment at 850 °C, 50 h + 950 °C, 1 min showed that on the side of the CuNi45 alloy, interlayers of TiNi₃(Cu), TiCuNi, TiNi(Cu) and Ti₂Ni(Cu) were formed, and on the side of titanium VT1-0 solid solution based on αTi was formed (Figure 10,

Table 11. Point EDS analysis results (850 °C, 50 h + 950 °C, 1 min) (see Figure 10).

Element	Content, at.%
	1	2	3	4
Ti	32.48	32.22	47.53	66.78
Cu	24.11	29.55	4.02	8.8
Ni	43.41	38.55	48.39	24.43
Phase	TiNi3(Cu)	TiCuNi	TiNi(Cu)	Ti2Ni(Cu)

). In this case, no CM process is observed. After 2 min of holding time, an eutectic structure was formed at the site of the Ti₂Ni(Cu) interlayer. It is a mixture of Ti₂Ni(Cu), TiNi(Cu), and Ti₂Cu(Ni) intermetallics (Figure 11,

Table 12. Point EDS analysis results (850 °C, 50 h + 950 °C, 2 min) (see Figure 11).

Element	Content, at.%
	1	2	3	4	5	6
Ti	33.02	32.65	46.93	66.65	55.65	65.78
Cu	24.13	37.04	4.44	18.73	4.35	23.7
Ni	42.85	30.31	48.64	14.62	39.99	10.52
Phase	TiNi3(Cu)	TiCuNi	TiNi(Cu)	Ti2Ni(Cu)	TiNi(Cu)	Ti2Cu(Ni)

). Changing the region of the eutectic structure formation and its phase composition allowed us to conclude that the CM process was ongoing according to a mechanism similar to that described in [22,24].

In accordance with [22,24], an increase in the nickel content in β-Ti to a certain critical concentration led to the “blocking” of its individual small regions by “partitions” of nickel atoms and the formation of nano-sized volumes (clusters) on which size effect of melting was realized [21]. According to [21], the chaotically distributed clusters subsequently merged upon contact with each other, forming a liquid matrix with individual solid β-Ti particles (microcrystals). The solid particles surrounded by the liquid dissolve, and the entire region of the solid solution turns into a thin film of liquid. Since it is energetically beneficial for the system to form a liquid phase at the lowest temperature of its stable existence, the temperature of the thin liquid film should quickly decrease, and the concentration of nickel in it should increase to eutectic volume. According to the calculated reactions presented in [26,32,33] and energy dispersive analysis data (Figure 11, Table 12), the temperature of the thin liquid film should be ~860 °C, and the eutectic reaction should look like this: L↔Ti₂Cu + TiCu + Ti₂Ni.

The change in the CM mechanism in the Ti-Cu-Ni system with increasing Ni content is indicated by a number of factors. Firstly, the formation of the Ti₂Ni intermetallic was noted in the VT1-0 + CuNi45 composition at the solid-phase diffusion stage, the formation of which was not noted in the VT1-0 + CuNi19 composition. Secondly, a change in the phase composition of IZ created favorable conditions for the eutectic transformation to occur at a lower temperature. Third, change in localization of the eutectic structure to the region at the boundary with the titanium layer. The above factors, together with an increase in the time until the formation of the liquid phase from 1 min to 2 min, indicated a change in the CM mechanism from non-diffusion to diffusion, described in [30].

A further increase in holding time at 950 °C to 15 min (Figure 12) actually has no effect on the distribution of chemical elements in IZ (Figure 12) and its phase composition. On the side of the CuNi45 alloy, in the process of solid-phase interaction, a layer of TiCuNi ternary intermetallic compound with a thickness of 3–4 μm was formed (Figure 12,

Table 13. Point EDS analysis results (950 °C, 15 min) (see Figure 12b).

Element	Content, at.%
	1	2	3	4
Ti	46.5	66.26	51.99	66.11
Cu	21.08	21.8	18.61	16.61
Ni	32.42	11.94	29.41	17.28
Phase	TiCuNi	Ti2Cu(Ni)	TiNi(Cu)	Ti2Ni(Cu)

). The IZ was a finely dispersed structure consisting mainly of intermetallic compounds TiNi(Cu), Ti₂Ni(Cu), and Ti₂Cu(Ni). On the titanium side, an αTi + Ti₂Ni(Cu) layer was formed (

Table 14. Point EDS analysis results (950 °C, 15 min) (see Figure 12c).

Element	Content, at.%
	1	2	3	4
Ti	66.56	66.65	90.28	90.71
Cu	9.76	22.11	4.5	3.33
Ni	23.68	11.24	5.22	5.96
Phase	Ti2Ni(Cu)	Ti2Cu(Ni)	βTi	βTi

).

Diffraction patterns obtained during layer-by-layer analysis of IZ are presented in Figure 13. The following phases were reliably identified: TiCuNi, TiNi, TiCu, Ti₂Ni, and Ti₂Cu.

The structure analysis of IZ formed at the CuNi45+VT1-0 joint after heat treatment at 950 °C and holding time range from 2 to 15 min showed that its thickness varied from ~0.02 to ~1.5 mm.

In the time range of heating at 950 °C for up to 15 min, the volume fraction of elements (determined using the proportion of reacted layers) lay in the following range: 67–80% Ti, 11–18% Cu, 9–15% Ni, which corresponds to 64 wt.% or 71 at.% Ti and correlates well with energy dispersive analysis data (Figure 12).

A comparison of the experimental results showed that replacing the CuNi19 alloy with the CuNi45 alloy in the composition of a layered titanium-copper-nickel composite led to an increase in the proportion of titanium in IZ from ~50 at.% to ~65–70 at.%. This change, in turn, is caused by a change in the temperature at which the liquid is in equilibrium with the solid phase from 924 °C (L↔TiCuNi + TiNi + CuTi) to 860 °C (L↔Ti₂Cu + TiCu + Ti₂Ni). Thus, a change in the mechanism of CM has been established in the case of an increase in the proportion of Ni in the Cu-Ni-Ti system.

Thus, doping the Ti-Cu binary system with Ni led to an ambiguous effect on the CM temperature. At 19% Ni, CM temperature increases relative to the Ti-Cu system from 900 to 924 °C. At 45% Ni, CM temperature, on the contrary, decreases to 860 °C. Based on this, we can conclude that the temperature of contact melting in ternary systems is influenced not only by the constituent elements but also by their percentage content at the point of the crystals’ contact. Changing the percentage of elements can change the reactions that are energetically favorable for the formation of liquid. As the study showed, a change in the reaction can be either towards lower temperatures or towards higher ones.

It is important to keep in mind that if Cu/Ni protective coatings are applied to the surface of titanium alloys (rather than pure titanium), CM temperature may also change. Establishing the features of the CM process in such systems requires additional research.

4. Conclusions

Contact melting in the VT1-0 + CuNi19 explosively welded composite begins after the formation of a liquid phase film at the TiCu-TiNi interface. The resulting interaction zone consists of TiCuNi and αTi+Ti₂Cu(Ni) continuous layers formed as a result of solid-state diffusion as well as from a mixture of TiNi(Cu) + TiCu(Ni) + Ti₂Cu(Ni) intermetallics.

The contact melting process in the VT1-0 + CuNi45 explosively welded composite begins after the transformation of the nickel in β-Ti solid solution into a film of the liquid phase. The resulting interaction zone consists of TiCuNi and αTi + Ti₂Ni(Cu) continuous layers formed as a result of solid-state diffusion as well as from a mixture of TiNi(Cu) + Ti₂Ni(Cu) + Ti₂Cu(Ni) intermetallics.

Replacing the CuNi19 alloy with the CuNi45 alloy leads to a decrease in the temperature at which the liquid (formed during contact melting) is in equilibrium with the solid phases from 924 °C (L↔TiCuNi + TiNi + CuTi) to 860 °C (L↔Ti₂Cu + TiCu + Ti₂Ni), to a change in the mechanism of contact melting, and to an increase in the proportion of titanium in the interaction zone.

The results of studying the contact melting process for the Ti-Cu-Ni system compositions with different Ni content are of great interest for the development of technology for producing protective coatings on titanium alloys. Taking into account the obtained results, the influence of elements in the composition of titanium alloys on the process of contact melting will be studied in the future.