3.1. Microstructural analysis
Figure 2 shows the optical microstructures of the laser-treated samples. The surface morphology of the laser-treated zone differs; depends upon parameters and the composition of the SiC-Ti-Ni was indicated by the microstructures. The microstructure of the laser-treated region for samples Q1, Q2, and Q3 (Fig. 2) shows a smooth surface with uniform melting and distribution of powders. It is observed that laser melted regions show two different microstructural characteristics; one is intensely bright layers shown in Fig. 2a and the other one is white layer appears over the dark layer with low intensity is shown in Fig. 2b. The energy density of 30–40 J/mm2 leads to the formation of a dark layer. The microstructure reveals homogeneous clad with crack-free and less porosity. A good bonding was achieved between the clad surface and substrate which attributes to the selection of process parameters for the above samples.
The microstructure of samples K is shown in Fig. 3. It reveals that for samples K1, K2, and K3, by a significant amount of porosity, roughness, and surface irregularity the clad layer is characterized due to excessive porosity and brittleness. The spilling of the alloyed region in the clad surface also observed, which leads to residual thermal stresses generated, the coefficient of low thermal expansion on the substrate, and the alloying powder was responsible for the poor cladding of the surface.
3.2. Microhardness testing
Overall, Vickers microhardness was measured; values were obtained between 1300 HV. Specific factors include the transformation characteristics of the material and the relative amount of the phases present and changes in the process parameters, which will contribute to the overall heat input which will control the cooling rate. The interface region exhibited lower hardness, principally attributed to re-melted and re-heated at least twice; this will give a tempering effect on the material which plays a role in reducing the hardness. The metal becomes softer towards the metal substrate, and the material is hardest at the top surface. The region near the substrate is softer than the top layers because they had a more extended temperature history, as layers of cladding were added to the sample, the previous layer was heated up again which will result in the superimposition of the heat this will increase the overall heat input, this reduces the cooling rate effectively. An improvement in the surface hardness of 316L was achieved.
3.3. Scanning electron microscopy analysis
In general, Fig. 4 shows the microstructure of the cross-sectional laser cladding of specimen Q1 with a laser scanning speed of 2 m/min and 2 kW (laser power). It is concluded that the clad layer is about 380 µm thick and it shows that there are no pores and cracks. A strong adhesive between the laser clad layer and substrate is observed. The samples Q2 and Q3 also exhibit similar behaviour other than a few cracks.
When SiC-Ti-Ni (40-40-20)wt%, a high-quality layer on the substrate with good metallurgical bonding with minimal dilution is formed between the substrate and the coating, the microstructure of the clad layer depicts gray and white-colored fine structures. These revealed that the alloy TiC particles were dispersed in the matrix, as the Si element is confirmed from EDS analysis. It is known that Ti reacts with carbon and forms the TiC compound below 2350°C. The temperature generated during laser surface cladding leads to Ti and SiC particles dissolving in the melted pool thereby forms TiC compounds.
In the laser melted region, the coating and the substrate are heated to a temperature above the melting point and then rapidly re-solidified. The melting pool was mixed enough, gas and other impurities in the pool were thereby formed a fine layer. Besides, this ensured the quality of the layer. Figure 5 shows the cross-sectional SEM image of laser cladding of sample K1 at a scanning speed of 2 m/min and laser power of 2 kW. The significant increase in silicon carbide percentage and thus the alloyed layer is characterized by a significant number of pores and cracks. During sample preparation for metallographic study, alloyed layers have worn out due to porosity and brittleness.
The alloyed regions were found in some regions of a hard region. The hard regions were brittle and exhibited cracks due to stress generation. Thermal stresses have been developed due to high thermal gradients during cooling which originates crack. It is reduced by preheating the substrate.
The high degree of thermal expansion of the solidified clad materials and their substrate leads to cracking, as well as the delaminating of the clad from the substrate. There is no enough time for gas escape; a large number of pores and voids are due to low heat input and high cooling rate. The top region exhibits a small flower-like structure, just above the bottom of the alloyed laser region. Further, this indicates that the solidification starts with fine columnar grains from the bottom and ends with fine network grains.
3.4. XRD phase analysis for the first proportion
By comparing XRD and EDS results, particular phase formations in the laser-treated zone have been identified. The presence of unique chemical and physical metallurgical characteristics of the coating materials in the laser-generated melt region favors thermodynamic and dynamic conditions that form various phases by the reaction between the coating elements as well as the substrate.
The XRD spectrum of the laser clad layer of sample Q1 shown in Fig. 6. From the plane view samples of the clad, XRD spectra were taken. The spectra show the formation of Fe3C, SiC, TiC, NiC, Si5C2, which ensures the mixing of coating material with the substrate, which is the reason for the good metallurgical bonding between clad layer and substrate. Thus partial decomposition of SiC into Si and C and. Mixing of the molten substrate and the preplaced material, mostly by convection, were observed by the Fe bearing phases detected during XRD analysis as well as the reasonably homogeneous microstructure were observed. Since SiC is excellent oxidation and creep resistance materials at high temperatures, the formation of its carbides widely applied in industries at solid-state as it strongly reacts with most transition metals and molten metallic substrate.
3.4.1. EDS analysis of sample Q1
EDS analysis of sample Q1 shown in Fig. 7. The white and gray particles in sample Q1, which exhibit high hardness lacking any pore or crack. The brighter region in the Q1 sample is found to have a large amount of Si and network grain region Fe, Ni predominantly. Silicon carbide phase was found in the Q1 sample that confirms the Si-rich zone. The other elements have been confirmed in XRD that indicate the presence of Fe and Ni-rich region γ-Fe and NiC phases comparing with the XRD analysis, sample K1 also exhibits the similar phases found in sample Q1.
3.4.2. EDS analysis of sample Q1
Figure 8 depicts the EDS analysis of specimen K1. Further, the EDS analysis of sample K1 shows that Fe, Ti, and Si were predominantly present. The laser-exposed area is predominantly reinforced with Ti and Si phases, which was shown by the above results. Thus the formed new phases increase the corrosion resistance and wear as exhibited in the EDS profile. In general, the retained austenitic phase due to Ni and Cr, corrosion resistance is evident.