Laser cladding of Ni60/WC composite coatings on Ti6Al4V alloy and its effect on microstructure and properties

Among its strong properties, titanium alloy is corrosion resistant, low density, and performs well at low temperatures. Aerospace, marine, automobile, armored, and biomedical equipment utilize it extensively. Titanium alloy also has some problems when used in some special environments, such as poor wear resistance and inadequate resistance to oxidation. By laser coating titanium alloy with Ni60/WC composite coating, the performance of titanium was better. To investigate impact of processing on cladding layer microstructure and properties, three different energy densities of 33.3 J/mm2, 46.7 J/mm2, and 53.3 J/mm2 were used for experiments. Between substrate coatings, there is strong metallurgical bonding at three laser energy densities. The coating’s microhardness first rises and then falls with increasing energy density. The performance of friction and wear coatings is superior to that of the substrate. Laser energy density at 33.3 J/mm2 has the best wear resistance coating.


Introduction
Titanium alloy is a kind of alloy metal made of titanium and other metals.Among its strong properties, titanium alloy is corrosion resistant, low density, and performs well at low temperatures.At low temperatures and ultra-low temperatures, it can still maintain its mechanical properties, good biocompatibility, and other special properties.It has become an important structural material since its inception.It has been more and more used in aerospace, marine, automotive, armored, and biomedical equipment [1][2][3].Although titanium alloy has many special properties, it also has some disadvantages.For example, it has poor tribological properties and cannot meet the wear resistance under strong load, so its application is limited [4].At present, the common method to solve the limited application of titanium and titanium alloy is surface modification technology, which includes shot peening, electrochemical treatment, and laser cladding [5].
Shot peening treatment is to impact numerous 0.25-1 mm hard steel balls on the surface of the material at a certain speed, plastic deformation occurs on the surface, and a grain-refined surface modification layer can be obtained [6,7].The hardness of titanium and titanium alloys and the wear resistance strengthened by shot peening have also been improved.
Electrochemical treatment is developed based on chemical reactions occurring on the surface characteristics of the alloy of titanium.Anodic oxidation is the electrochemical oxidation of metals and alloys, and it is a common method for preparing different oxide films on metal materials [8,9].
Laser cladding has high metallurgical bonding strength, reduces material cost and energy consumption, does not need to contact the workpiece during processing, and is green and pollution-free [10,11].It is the most widely used surface coating technology in recent years and numerous academics researched this subject.Weng et al. [12] showed that TC4 was improved by NiCoCrAlY.Laser cladding produced a coating twice as hard as Ti-6Al-4V.Mahamood et al. [13] studied how laser velocity affected Ti-6Al-4V/TiC composites.Results demonstrate that the wearability of Ti-6Al-4V/TiC composites rises as scanning speed rises, however, scanning speed has a specific upper limit value.Only under the appropriate process parameters, titanium alloy can obtain excellent coating.Liu et al. [14] prepared Ni60 + 50 % WC and deloro22 powder backing + (Ni60 + 50%WC) two kinds of wear-resistant composite coatings on TC4 alloy.The outcomes show that resistance to wear, hardness and bonding strength of Ni60 + 50 % WC coating are better than those of deloro22 powder backing + (Ni60+50%WC) coating, which may be due to the thick backing of deloro22 powder.
The primary process parameters that impact the performance of surface coating are the energy of the laser, scanning speed, point diameter, etc.Because the impact of these parameters on coating quality is related, multiple factors on the cladding layer are analyzed the influence, and the laser energy density is introduced as a comprehensive examination index.The expression is : Where  is the laser energy density (J/mm 2 );  stands for power (W);  stands for spot diameter (mm);  stands for scanning speed (mm/s).
In paper, pre-coated powder Ni60+WC laser cladding Ti-6Al-4V was used, with three various laser energy densities of 33.3 J/mm 2 , 46.7 J/mm 2 , and 53.3 J/mm 2 were introduced to investigate performance of cladding layer.

Experimental material
The Ti6Al4V alloy with size of 50 × 50 × 5 mm was accepted as an experimental base.To begin the test, the oxide layer was removed by grinding at different mesh emery paper first, then cleaned and dried with anhydrous ethanol.The chemical composition of Ti6Al4V alloy is 5.8-6.3wt% Al, 3.5-4.2wt% V, 0.05-0.2wt% C, 0.15 wt% Fe and Ti balance.The cladding materials used in the experiment are Ni60 (composition: 4.31 wt% Fe, 3.2 wt% B, 4 wt% Si, 15.9 wt % Cr, 0.78 wt% C, and Ni balance) and WC mixed powder, in which the mass fraction of WC is 35 %.

Preparation of experimental samples
The laser equipment used in the laser experiment is DL-HL-T200 cross-flow CO 2 laser with 3 mm beam focus size.The whole experiment is acted upon under argon protection gas.In the experiment, the preplaced cladding powder was 1 mm, and the cladding coating was prepared by using three different laser energy densities of 33.3 J/mm 2 , 46.7 J/mm 2 , and 53.3 J/mm 2 .

Microstructure and performance testing
Metallographic structure of cross-sectional microstructure of the sample after corrosion in the corrosive solution was observed by a metallographic microscope.Microstructure elements of cladding coatings were analysed by JSM-6490LV SEM.With an overload of 500 g and a residence duration of 10 s, the MH-500 microhardness instrument was utilized to evaluate the degree of hardness of coating layer.The coating friction and wear weight loss were trialed MGW-02 high-speed reciprocating friction tester.

Microstructure
Figure 1 depicts metallographic structure of specimens in different laser energy.The diagram demonstrates the strong metallurgy bond, as well as presence of a clear metallurgy bond line.The breadth of the heat-impacted zone narrows as the laser energy density increases.There are a large number of hard phase WC unmelted particles at low energy density, while the hard phase particles at high energy density are small and less, indicating that WC is decomposed at high laser energy density.Because the powder of the three cladding samples is the same, but the energy density is different, the scanning electron microscope microstructure of the three samples is similar.The middle area of the coating has a certain amount of hard phase and the matrix diffuses into the cladding layer in Figure 2. Table 1 shows the EDS elemental composition analysis of Figure 2. It can be seen that some of the matrix Ti diffuses into the cladding layer, and there are Ni, W, and C elements.1201.34HV0.5 , and 1017.79HV0.5 , respectively.The average hardness of the substrate is 364.78HV0.5 .The cladding coating's hardness is 2.79 to 3.29 times greater than that of the Ti6Al4V alloy used as the substrate.

Tribological properties
Figure 4 shows the laser energy density of 33.3 J/mm 2 , 46.7 J/mm 2 , 46.7 J/mm 2 , and 53.3 J/mm 2 coating coefficient and loss weight.From Figure 1(a), coating coefficient of energy density of 53.3 J/mm 2 is the smallest.The intensity of the laser is 33.3 J/mm 2 , which may be due to the dense and uniform coating structure under this energy density.From Figure 1(b), it is evident that the weight loss of coating under three energy densities is 4.8 mg, 6 mg, and 5.1 mg, respectively, which is much smaller than the weight loss of the substrate 27.6 mg.Weight loss increases initially, then declines as laser energy density increases.
Figure 5 shows wear morphology of substrate and the coating different energy densities.The figure shows that surface of substrate has the largest worn bandwidth, while the coating has a smaller worn bandwidth than the substrate.The coating's wear width with energy density 33.3 J/mm 2 has least wear width while the coating's wear width of 46.7 J/mm 2 has biggest wear width, which corresponds to the result of wear loss weight, showing that wear of the coating with energy 33.3 J/mm 2 is better than that of others.The wear morphology of the substrate has deep furrows and plastic deformation, and there is certain adhesive wear.

Conclusions
For this paper, the laser-clad Ni60/WC coated composites were created on Ti6Al4V alloy pre-laid powder method with different laser energy densities of 33.3 ,46.7 and 53.3 J/mm 2 .The following is results.
(1) By applying three different energy densities to the surface of the Ti6Al4V alloy, Ni60/WC composite layers were created.The outcomes display that the samples have fine metallurgical bonding and good metallurgical bonding at three different speeds, and the heat breadth of the impacted zone reduces as the laser increases.
(2) The microhardness with varying energy levels, the outcomes display that microhardness increases then decreases with energy density increasing.The maximum hardness is 46.7 J/mm 2 .The mean microhardness is 1201.34HV0.5 , which is 3.29 times that of the matrix.
(3) Under three energy densities, the cladding coating's wear qualities are superior to those of the base, and the loss of wear is minimal.The coating with laser energy of 33.3 J/mm 2 has smallest wear loss and the smallest wear width, showing better wear resistance.
(4) Comparing the three cladding samples from the hardness and tribological properties of coating, comprehensive performance with energy density 33.3 J/mm 2 is better.

Figure 1 .
Figure 1.Metallographic structure (a) E = 33.3J/ mm 2 ; (b) E = 46.7 J/ mm 2 ; (c) E = 53.3J/ mm 2 .Because the powder of the three cladding samples is the same, but the energy density is different, the scanning electron microscope microstructure of the three samples is similar.The middle area of the coating has a certain amount of hard phase and the matrix diffuses into the cladding layer in Figure2.Table1shows the EDS elemental composition analysis of Figure2.It can be seen that some of the matrix Ti diffuses into the cladding layer, and there are Ni, W, and C elements.

Figure 2 .
Figure 2. The middle area of the cladding layer.

Table 1 .
EDS results of middle region of cladding coating.
MicrohardnessFigure3illustrates the microhardness of specimens at various energy densities.As energy density increases, microhardness increases then decreases.If energy density is too large, energy absorbed by workpiece will increase, which will increase the decomposition of the WC hard phase.Meanwhile, the temperature gradient of cladding layer will increase, resulting slow cooling rate and reduced surface hardness.Average microhardness of cladding layer of three cladding samples is 1141.63HV0.5 ,