Abstract: In order to improve the wear resistance performance laser cladded Ni + TiB2 composite coating prepared on Ti6Al4V surface，the influence of powder ratio on microstructure and mechanical properties is studied． The laser cladding coating is mainly composed of TiB，TiB2，α-Ti，β-Ti，NiTi alloy solid solution and TiO2 ． The cladding layer is mainly com- posed of black elliptic phase，elongated needle-like phase and surrounding cell crystal phase． The black elliptic phase，nee- dle-like phase and surrounding cell crystal phase are TiB2，TiB，NiTi，respectively． When the TiB2 additive amount content increases，the TiB content increases，the TiB metallographic particles become coarse． The highest microhardness of the cladding layer reaches 920． 8 HV1． 0，which is about 3 times that of the Ti6Al4V alloy，the increased microhardness improves the wear resistance properties of the cladding coating． The brittle spalling becomes more serious with the load increasing，and the composite coating is not suitable for high load conditions．
Keywords: laser cladding; Ni + TiB2 composite coating; Ti6Al4V; wear resistance property

1. Introduction

Titanium alloys have excellent properties such as high strength, low density and good corrosion resistance, and are often used in aerospace, marine engineering, automobile manufacturing and other fields [1]. However, the low hardness and poor wear resistance of titanium alloys limit their wide application. In surface modification technology, laser cladding with high energy density, small heat-affected zone and strong metallurgical bonding has always attracted much attention [2].

Different material systems have been introduced into the laser cladding of titanium alloys, among which composite material system is a more popular and effective method [3]. In the composite material system, TiB2 reinforcement phase is used as a feasible way to improve hardness and wear resistance. Qi K. et al. [1] prepared TiB2/metal composite coating on Ti6Al4V alloy by laser cladding Fe, Co, Cr, B and C mixed powders, and studied the effect of magnetic field on the mechanical properties and wear properties of the coating. Lin Y. H. et al. [4] used pure TiB2 powder to prepare TiB2/TiB gradient coating on titanium alloy. The microhardness showed a gradient decrease trend, but the fracture toughness showed a gradient increase trend. Kumar S. et al. [5] studied the powder mixture of Ti6Al4V, CBN and TiO2 laser cladding coating, and found different structures such as needle-shaped, cylindrical rod-shaped and short-length dendrite-shaped. The metal matrix composite material (TiN, TiAlN, AlN and TiB2) of nitride and boride was used as the main structural phase of the coating to improve the hardness and wear resistance.

Nickel or nickel-based alloy is an ideal matrix with good structural stability, high temperature resistance, corrosion resistance, high strength and good wettability. Laser cladding particle reinforced composite coating was prepared by directly adding reinforcing agent or related elements to the optimized alloy powder, and laser cladding coating with at least two phases with different mechanical properties will become an important demand for surface strengthening in the future [6]. Xu S. Y. et al. [7] prepared TiC/Ni60 composite coating on the surface of Ti6Al4V alloy by laser cladding. Yu X. L. et al. [2] prepared nickel-titanium carbide composites on 20 steel substrate by laser cladding. The large amount of TiC particles in the Ni/40TiC composite hindered the growth of nickel crystals, resulting in a finer microstructure of the Ni/40TiC composite. The average microhardness of the Ni/40TiC composite was about 851HV, and the friction coefficient was 0.43. Wang Q. et al. [8] studied the microstructure and properties of Ni-based gradient composite coatings. The coatings consisted of Ni matrix, WC and multiple carbide and boride hard phases. The maximum microhardness reached 1053.5HV0.2, and the friction coefficient and wear loss values ​​were lower than those of Q345 steel.

In order to study the microstructure and wear resistance of Ti6Al4V alloy, Ni and TiB2 mixed powders were selected to prepare Ti6Al4V alloy laser cladding layers.

2 Experimental materials and methods
2. 1 Experimental materials
A 100mm × 100mm × 10mm Ti6Al4V alloy plate was selected as the substrate, and its chemical composition and mechanical properties are shown in Table 1 and Table 2, respectively. Since Ni powder can improve the heat source distribution and concentrate heat during laser cladding, Ni powder and TiB2 powder were selected to prepare a composite coating with TiB2 as the reinforcement phase. The metallographic morphology of Ni powder and TiB2 powder is shown in Figure 1.

2. 2 Experimental methods
In order to make the powder and the base plate tightly bonded, mechanical grinding was used to remove the surface oxide layer of the titanium alloy plate, and 5% HF + 15% HNO3 acid solution was used to remove oil stains. A YSL-3000 continuous fiber laser was used to provide continuous laser, and the Ti6Al4V plate with preset powder was placed in a 200mm × 200mm × 50mm plastic box, and argon gas was continuously injected into the plastic box. During the laser cladding process, the spot diameter is 1.8 mm and the scanning speed is 7 mm/s. When the ratio of Ni + TiB2 is 40%, the laser powder parameters are 700W, 900W and 1100W respectively, and the effect of laser powder on microstructure and mechanical properties is studied; when the laser powder mass is 900W, the powder ratios are Ni + 20% TiB2, Ni + 30% TiB2, Ni + 40% TiB2 respectively, and the effect of powder ratio on laser powder mass is studied. The samples with laser cladding layer can be marked as S-1 (P = 700W), S-2 (P = 900W), S-3 (P = 1100W), S-4 (R = Ni + 30% TiB2), S-5 (R = Ni + 40% TiB2).

The X-ray diffractometer (XＲD) specimens, scanning electron microscope (SEM) specimens and performance test specimens were prepared by electric spark cutting, and the specimens were mechanically ground, mechanically polished and corroded by 5% HF + 15% HNO3 acid solution. The phase composition of the laser cladding layer was characterized by Brooker D8-advance micro-area X-ray diffractometer (XＲD), and the microstructure of the laser cladding layer was observed by optical microscope (OM) and scanning electron microscope (SEM). The HV-5 Vickers hardness tester was studied to measure the hardness along the surface depth of the laser cladding layer. The HＲS-2M high-speed reciprocating friction and wear tester was selected for friction and wear tests. The friction auxiliary material was Si3N2 ceramic grinding ball with a diameter of 4mm. The friction and wear parameters were reciprocating speed of 200r/min and radial load of 20/40/60N.

3 Results and discussion
3.1 XRD phase composition
The XRD phase composition of the five samples is shown in Figure 2. Each sample contains a small amount of TiN in its chemical composition, which is the reason why N atoms penetrate into the laser cladding layer to cause the nitriding reaction. During the flow of the molten pool, a small amount of vanadium dissolves in the titanium alloy matrix material, and in this process, the α phase transforms to the β phase, so β-Ti appears in Figure 2. TiB2 has a dissolution-precipitation characteristic during the laser cladding process. A small amount of TiB2 can be completely dissolved, and some TiB2 can combine with Ti to form TiB, and the remaining TiB2 can recrystallize. Ti can react with Ni to form NiTi, Ni3Ti and NiTi2, but Ti and Ni have the same chemical bond energy, which makes it easier to form a stable NiTi metal inert compound, and Ti atoms have a strong diffusion rate, so Ti and Ni react to form only NiTi［9］. As can be seen from Figure 2, the laser cladding layer is mainly composed of TiB, TiB2, α-Ti, NiTi alloy solid solution, TiO2, etc., and the XRD results also show a small amount of β-Ti.

According to the average Gibbs free energy, three reactions can occur: see (1), (2), and (3) in the figure. During the laser cladding process, Ni and B atoms can react with Ti atoms to generate TiB2, NiTi, and TiB. The average Gibbs free energy ΔG2 < ΔG1 < ΔG3, so the order of material formation is TiB > NiTi > TiB2.

When the proportion of TiB2 powder increases to 30%, the thermochemical reaction formula (2) proceeds to the right. The TiB phase in the laser cladding layer increases and the Ti phase decreases. When the proportion of TiB2 powder continues to increase to 40%, the content of TiB and TiB2 phases increases further. In addition, Ni and Ti have a strong affinity and gradually form NiTi metallization. Therefore, the final main products of Ni + 40% TiB2 laser cladding layer are NiTi, TiO2, TiB, TiB2 and Ti.

3.2 Microstructure
The SEM structure of Ni + 20% TiB2 laser cladding layer is shown in Figure 3. The cladding layer is mainly composed of black elliptical phase, elongated needle phase and surrounding cellular phase. The average diameter of the most distributed micro-particle phase is 0.5 ~ 3.0μm. Since the atomic number of B element is 5, ordinary energy spectrum analyzer cannot accurately measure the content of elements with atomic number less than 10. Electron probe X-ray microanalysis (EPMA) is used to measure the distribution and content of each element in the cladding layer [10, 11]. The EPMA results at different positions in Figure 3 are shown in Table 3.

It can be seen from Table 3 that the chemical composition of the cladding layer is mainly composed of Ti, B, Ni elements, and contains a small amount of Al and V elements. The content of Ti and Ni elements at position a is basically the same, there is no B element, and NiTi solid solution may exist. The main elements at position b are Ti and B, and the content of both elements exceeds 40%. It can be inferred that the needle-like phase at position b is TiB.

According to Gibbs’s thermodynamic law, B-B bond energy > B-Ti bond energy > Ti-Ti bond energy [12], which makes the growth rate of TiB in its own height direction faster and faster than the growth direction perpendicular to its own height, which makes the needle-like phase easy to appear. The content of B element at position c is about twice that of Ti element. The XRD spectrum in Figure 2 shows that the intensity of the diffraction peak of TiB2 is relatively high. The black elliptical phase at position c is likely to be TiB2.

The SEM microstructure of laser cladding layers with different powder ratios is shown in Figure 4. It can be seen that when the TiB2 addition content is small, the TiB content in the cladding layer decreases and its distribution is also more dispersed. When the TiB2 addition content increases, the TiB content increases, the TiB metallographic particles become coarser, and the distribution is dispersed. This phenomenon is caused by the increase of B element promoting the reaction between B and Ti element.

In order to study the microstructure of the coating, the SEM microstructure of the top, middle and bottom of the coating is shown in Figure 5.

The evolution of the cladding layer structure with the depth gradient is very obvious. A large number of two-phase particles are synthesized in situ at the top of the coating, many of which are finely crushed, and there are a small number of needle-shaped and shaped structures. At the same time, TiB and TiB2 hard reinforcement particles can prevent excessive temperature loss at the top of the molten pool. After melting and destruction, the grains in the cladding layer grow non-directionally in an irregular direction and re-nucleate. The size of the new phase after nucleation is small, which makes the phase particles refined [13]. The middle of the coating can be affected by alternating heat convection from top to bottom, and a large number of elements are concentrated in the middle, so EPMA cannot detect boron elements, and the top of the coating is composed of black petal-shaped phases, black fine needle-shaped phases and white herringbone phases.

As shown in Figure 6, the results of plane scanning of the microstructure show that there is a rich eutectic structure. The black petal-shaped phase may be the TiB/TiB2/TiNiB eutectic phase, the white herringbone phase is NiTi, and the other phases are derivatives of the titanium martensitic phase transformation. The BES microstructure in the middle of the 20% TiB2 laser cladding coating is shown in Figure 7, with phases of different colors, namely bright white, black and dark gray. The bright one is the NiTi intermetallic compound, the black one is the titanium-boron mixed phase, and the dark gray one is the mixed phase of martensitic titanium and titanium oxide. The herringbone phase at the bottom of the laser cladding coating gradually increases, the area of ​​the dark gray layer begins to increase, and the black petal-shaped phase and the black fine needle-shaped phase are significantly reduced.

3.3 Microhardness
According to the microhardness test, the hardness of Ti6Al4V alloy is 349.2HV1.0. The microhardness distribution of laser cladding layers prepared with different powder ratios along the depth is shown in Figure 8. It can be seen that the microhardness of laser cladding layers with different powder ratios is higher than that of Ti6Al4V alloy. With the increase of TiB2 powder ratio, the microhardness gradually increases. When the TiB2 powder ratio is 40%, the highest microhardness of the cladding layer reaches 920.8HV1.0, which is about 3 times that of Ti6Al4V alloy.

With the increase of the depth of the laser cladding layer within a certain range, the microhardness of the layer shows a rapid decline trend, and the cross-section layer above the bonding surface of the substrate and the coating shows a fluctuation phenomenon of microhardness. The cross-section layer with a depth of 0.7 to 0.8 mm is in the heat affected zone. The microhardness of this area is about 400HV1.0, and the upward trend of microhardness is very slow. The microhardness of the cross-section layer at a depth of 0.7 to 0.8 mm is relatively high because the harder TiB2 grains in the laser cladding layer have a strong impact resistance, and the laser cladding process can promote the formation of fine TiB and prevent grain boundary dislocation slip, thereby improving the microhardness of the laser cladding layer prepared by the laser cladding process [14].

Under the influence of the molten pool flow, the surface TiB2 begins to diffuse, and there will be some residual TiB2 in the middle of the cladding layer, but the concentration will not be too high, and the microstructure [15] will also decrease slightly. The bottom edge of the cladding layer is the heat-affected zone. A large amount of Ti elements float up after melting, resulting in a large dilution rate of the parent material to the molten pool, without sufficient strengthening phase, and the heat-affected zone has the lowest microhardness [16]. The results show that the addition of TiB2 powder significantly improves the hardness of the cladding layer.

3.4 Wear resistance
The wear rate of the laser cladding layer with the same powder ratio varies with load as shown in Figure 9. The wear rates of Ti6Al4V and laser cladding layers increase with the increase of load, and the wear rate of laser cladding layers is much lower than that of Ti6Al4V substrate materials, indicating that the wear resistance of cladding layers is very excellent. The wear rate of cladding layers is closely related to the hard phase content. When the TiB2 powder ratio increases from 20% to 30%, the TiB hard phase content increases and the wear rate decreases; when the TiB2 powder ratio increases from 30% to 40%, the TiB hard phase content further increases, and TiB2 appears, resulting in the minimum wear rate of only 1.5 × 10-4 mm3/s.

The SEM wear morphology of Ti6Al4V under different loads is shown in Figure 10. As can be seen from Figure 10a, the titanium alloy produces very little wear debris under a load of 20 N, and the wear zone is irregular, curved, and diamond-shaped (see area A in Figure 10a), indicating that the Ti6Al4V substrate material is severely damaged during reciprocating motion. When the load increases to 40N, the depth of the gully increases (see area B in Figure 10b), the abrasive particles increase rapidly, and wear and deviation occur during the substrate wear process, so the abrasive wear and adhesive wear are very serious. When the load is 60N, some large pits are generated on the wear surface (see area C in Figure 10c), and abrasive particles accumulate on the scratch surface (see area D in Figure 10c). Therefore, the increased load will accelerate the peeling of the titanium alloy material during the friction and wear process, and the friction and wear performance of the titanium alloy is very poor. Li J. N. et al. [17] and Weng F. et al. [18] also found similar wear surfaces of titanium alloys.

The Ni + 40% TiB2 cladding layer has the highest microhardness and the best wear resistance. Therefore, the Ni + 40% TiB2 cladding layer on the titanium alloy surface was selected to study the wear mechanism of the laser cladding layer. The SEM wear morphology of the laser cladding layer under different loads is shown in Figure 11. The microhardness of the laser cladding layer is significantly improved, so the wear performance of the cladding layer is much better than that of the titanium alloy. As can be seen from Figure 11a, the number of abrasive particles has been greatly reduced and the size has also become much smaller (see area A in Figure 11a). This is due to the wear of the hard NiB, TiB2 and TiO2 hard phases [5]. Some collapsed structures appear in the worn cladding layer (see area B in Figure 11b). The structure is likely to be hard phase particles. The tiny metal chips are striped due to their high load-bearing capacity, avoiding the formation of grooves and scratches. When the load increases to 40 N, lamellar spalling is more likely to occur, the abrasive dust of the Ni + 40% TiB2 cladding layer increases significantly, micropores appear on the worn surface (see area C in Figure 11b), and abrasive wear and adhesive wear occur at the same time. As the load increases further, the abrasive dust of the cladding layer begins to spread to the entire worn surface, and the depth and width of the micropores increase (see area D in Figure 11b). These phenomena all indicate that with the increase of load, brittle spalling becomes more serious, and the composite coating is not suitable for high load conditions.

4 Conclusion

In order to improve the wear resistance of Ti6Al4V alloy, laser cladding coating was prepared on the surface of titanium alloy by using Ni and TiB2 mixed powder. The results are shown below.

(1) XRD results of laser cladding layer show that the laser cladding layer is mainly composed of TiB, TiB2, α-Ti, β-Ti, NiTi alloy solid solution and TiO2, and with the increase of TiB2 powder ratio, the TiB2 phase content increases further.

(2) The cladding layer is mainly composed of black elliptical phase, elongated needle-like phase and surrounding cellular phase. The black elliptical phase is TiB2, the needle-like phase is TiB, and the surrounding cellular phase is NiTi. With the increase of TiB2 addition, the TiB content increases and the TiB metallographic particles become coarser.

(3) When the TiB2 powder ratio is 40%, the microhardness of the cladding layer reaches a maximum of 920. 8HV1. 0, which is about 3 times that of Ti6Al4V alloy. The increase in microhardness improves the wear resistance of the cladding layer. As the load increases, the brittle peeling of the composite coating becomes more and more serious, which is not suitable for high load conditions.