Influence of parameter on the characteristics and morphology of Ni60/WC coatings

Due to its unique excellent performance, titanium alloys have a broader set of uses. With the increasing use of titanium alloys, there are also certain problems when used in some special environments, for example, poor wear resistance and oxidation resistance. To address the wear resistance and ensure its application is not limited by conditions, this article uses the laser method to prepare Ni60+35% WC composite coverings on Ti6Al4V alloy using different scanning speeds (8, 12, 14 mm/s). Findings indicated the laser-clad wall coverings at three different speeds achieved strong metallurgical adhesion between the substrates. The phenomenon of mutual diffusion between Ti6Al4V alloy and coating materials was observed, and the coating’s microhardness grew as the scanning speed rose. When compared with the substrate, the surface coating’s anti-wear properties were increased. At 14 mm/s scanning, the covering exhibited the highest durability to wear.


Introduction
Titanium alloys possess exceptional strength and resistance to corrosion, as they are a combination of titanium alongside various materials, low density, and good low-temperature performance.It can still maintain its mechanical properties, good biocompatibility, and other special properties at low and ultra-low temperatures.It is rapidly being utilized within the aeronautical industry, marine, automotive, armor, and biomedical equipment [1][2][3].Titanium alloys are applied in the field of high-precision, cutting-edge technology.However, in certain working environments, it also has certain drawbacks and problems, for example, poor hard wear resistance and poor antioxidant ability.The development and adhibition of titanium alloys are also restricted on account of these shortcomings [4][5][6].At present, the commonly used methods to solve the limited adhibition are ion implantation, thermal spraying, and laser surface modification technology.Among them, laser technology is widely used to enhance the wear resistance due to its high energy and high bonding strength.
Adding a certain number of ceramic particles such as WC or TiC to self-fluxing alloy powder can obtain laser cladding layers with different functions, which has attracted widespread attention [7,8].Ni-based self-fluxing alloy powder has received much research in laser cladding due to excellent corrosion resistance and self-lubrication.Because of the great wettability and high hardness of WC and Ni-based alloys, the use of WC particle-reinforced Ni-based alloy powder to prepare a laser cladding layer to improve material surface properties has attracted extensive research enthusiasm from scholars.Liu et al. [9] prepared two composite coverings that withstand wear, Ni60+50% WC and deloro22 powder primer+(Ni60+50%WC).The conclusion indicates that the resisting abrasion, hardness, and fusion power of Ni60+50% WC covering are better than those of the deloro22 powder primer+(Ni60+50% WC) coating.This is for the weak bonding strength of the coating caused by the thick deloro22 powder primer.Xu et al. [10] prepared a 90% TiC and 10% Ni60 powder coating on the surface of TC4, mainly studying the effects of scan velocity on wear and structure resistance on coating.
A composite powder composed of WC hard phase and alloy bonding phase has great hardness and wear resistance [11][12][13].Through the different weight ratios of the two, various functional composite powders can be derived, which is currently one of the most widely used materials for laser cladding.There are differences in the thermal conductivity between WC and the base metal in applications.Therefore, preparing great-performance WC coatings on titanium alloy surfaces remains a challenge, especially in studying the influence of speed on structure properties by cladding Ni60+35% WC coverings.
This article reports the preparation with Ni60+35% WC coatings via laser cladding on Ti6Al4V alloy, exploring the effect of rate on coating structure, depth, and wear resistance, as well as the mechanism of coating wear.In the future, this study can broaden the application of titanium alloys in wear-resistant situations and provide a theoretical basis for promoting the abrasive resistance of titanium alloys.

Material
The substrate is Ti6Al4V alloy, in a size of 50×50×5 mm; before experiment, the surface oxide layer was removed by grinding with sandpaper with different mesh numbers.After that, it was cleaned and dried with alcohol.The chemical composition of Ti6Al4V alloy was 5.8-6.3wt%Al, 3.5-4.2wt% V, 0.05-0.2wt% C, 0.15wt% Fe and Ti balance.The experimental cladding material used was a mixture of Ni60 (composition: 4.31wt% Fe, 3.2wt% B, 4wt% Si, 15.9wt% Cr, 0.78wt% C and Ni balance) and WC powder, with a mass fraction of 35% WC.The cladding powder was dried for 2 hours before use.

Preparation of experimental samples
The laser equipment used in the laser experiment is a DL-HL-T200 transverse flow CO2 laser, having an ultimate output power of 2000 W. The laser spot is 3 mm.The entire experiment was conducted under argon shielding gas.The experiment used prefabricated powder and laid a layer of 1 mm mixed powder on the Ti6Al4V alloy surface.The experimental power remains unchanged at 1400 W, and the laser scanning speed is changed (8, 12, 14 mm/s).The results of different speeds on properties of the coating layer are examined.After that, cross-section of the sample is cut then embedded, ground, and polished.The cross-section is corroded with a corrosive solution for 30 seconds.

Microstructure and performance testing
The covering layer's morphology and constituent parts were used to study JSM-6490LV SEM and built-in EDS.Using a 500 g loading and a 10-second hold time, the MH-500 microhardness testing was applied to assess the covering layer's hardness.MGW-02 high-speed reciprocating fatigue wear evaluation device was applied to estimate the abrasive resistance of the surface.

Macroscopic morphology
Morphology of laser multiple films is revealed in Figure 1.From the macroscopic morphology, it is obvious as well as speed increases, the coating becomes flat and uneven, with protrusions.This indicates that due to growth with velocity, laser energy to act on the substrate surface's time is

Microstructure
SEM pictures of coverings at various scanner rates and levels of magnification are displayed in Figure 2. From the figure, we can see that the depth of the cladding coating rises in proportion to scanning velocity, which is 775.24 in sequence μ m, 727.24 μ m, and 721.17 μ m.The coating depth decreases sequentially, indicating that the rate increases, the duration of beam operation on the substrate area reduces, leading to a decrease in laser power density and a decrease in the cladding coating's thickness.The line scanning energy spectrum analysis of the heat-affected zone is revealed in Figure 3.It can be seen that there is indeed a phenomenon of mutual diffusion between the matrix Ti6Al4V alloy and the coating material.A fusion line can be seen in the coating, indicating well-metallurgical fusion between the coating and substrate.The mutual diffusion between Ti6Al4V alloy and coating material with the operation of great laser beam resulted in formation good metallurgical bonding.Concurrent with the area impacted by heat, needle-like martensite was observed in the matrix, which was caused by a heataffected zone's cooling speed exceeding its essential cooldown speed.Similar results were also proposed in [14].

Microhardness
The microhardness curves of coatings prepared at various speeds are illustrated in Figure 4, and the average microhardness of substrates at various scanning rates is illustrated in Figure 5.The picture illustrates the coating strength is noticeably greater when compared to the substrate under three different speeds.The average microhardness of the coating is 1001.43HV 0.5 , 1037.67 HV 0.5 , and 1141.61HV 0.5 with speed.The mean substrate strength is 364.78HV 0.5 , which is 2.75-3.13times greater compared to Ti6Al4V alloy.The reason why hardness increases when it increases is because laser energy decreases, temperature gradient of layer decreases, surface cooling rate increases.Rapid cooling leads to an increase in surface hardness.

Tribological properties
Figure 6 shows the lubrication factor of a coating with weight reduction at different scanning speeds.Figure 6(a) illustrates that the coating friction coefficient is the smallest at stable speed 12 mm/s, but total stability of friction coefficient is better at speed 8 mm/s.This may be due to the relatively uniform coating structure at this speed.The weight loss of the covering is substantially smaller in comparison with substrates at all three speeds, as seen in Figure 6(b).The weight loss first increases and then decreases, considering the rise in scanning speed.Figure 7 displays the substrate and coating wear morphology at various scanning speeds.From the figure, wear width of coating is the largest, and coating's wear breadth is less compared with that of the substrates.Moreover, the wear width at speed 14 mm/s is smallest, and weight loss of coating at that speed is also the smallest, indicating wear resistance is better than others at other speeds.Wear morphology of substrate has many deep furrows and plastic deformation.Wear morphology of coating has shallow furrows, while there are also certain abrasive particles present.During the coating wear process, the coat that resists wear increases with the wear of the hard phase.There are also different degrees of adhesive wear in the coating wear morphology.

Conclusion
This article uses the laser method to generate Ni60+35% WC composite coverings on Ti6Al4V alloy using different speeds of 8, 12, 14 mm/s.Main research findings are as follows: (1) Ti6Al4V alloy was coated with a composite covering of Ni60+35% WC by the laser cladding process.It exhibited good metallurgical bonding at three different speeds.The phenomenon of mutual diffusion between the matrix Ti6Al4V alloy and the coating material was also observed.
(2) The microhardness of the coating increases sequentially at various rates of scanning, with the maximum hardness occurring with speed 14 mm/s.Average microhardness is 1141.61HV 0.5 , which is 3.13 times substrate.
(3) Wear resistance of coating under various rates of scanning is more elevated than the Ti6Al4V alloy matrix, among which the coating with speed 14 mm/s has smallest weight loss and wear width, exhibiting better wear resistance.