Micro-structure
The micro-structure of the whole cladding area is discovered by observing and is illustrated in Fig. 3. As Fig. 3(a) shows, it is relatively easy to differentiate between sub-regions and from bottom to top, the three sub-regions are the substrate, the zone that is affected by heat and the cladded layer. As shown in Fig. 3(b), the substrate mainly consists of white ferrite and black pearlite. The grains of the ferrite and pearlite are relatively fine. The existence of the zone that is affected by heat is due to the effect of high-energy jets. The jets brings about major change in the substrate’s structure and properties. Grains merged, which results in the formation of coarse and striated grains. This is illustrated in Fig. 3(c). The structure of the cladded layer is shown in Fig. 3(d).
The powder was scanned by the plasma jets and integrated with the substrate in a metallurgical way to form the cladded layer. This was a heating and cooling process, which happened quickly. As the ratio of temperature gradient to solidification rate changed constantly, planar crystal, cytocrystalline, columnar crystal and equiaxial crystal appeared respectively from the bottom to the top of the cladded layer, which are shown in Fig. 4.
As shown in Fig. 5, it can be clearly seen through the scanning electronic microscope that the substrate and the cladded layer are separated by a bright white stripe. The formation of the bright white stripe was because the ratio of the temperature gradient to solidification rate met the condition in which the planar crystal moved when the mixture started to solidify.
Based on the results of chemical element spectral scanning analysis, it can be seen that the distribution of six chemical elements namely Fe, Ni, Cr, Si, B, and C in the cladder layer is in a gradient form with the bonding zone separating them. From the zone that is affected by heat to the central part of cladded layer, the amount of Fe decreases gradually while the amount of both Ni and Cr increases. This means in the process of plasma cladding, melted mixture powder fully integrated with the molten area of the surface of the 42CrMo steel substrate and Fe and Ni spread at the interface. A good metallurgical bond forms during the solidification following the plasma cladding, which ensures that the combination of the Ni-based cladded layer and the substrate is strong enough.
Physical Phase Analysis
The XRD patterns of the substrate and the cladded layer are shown in Fig. 6. In the figure, it can be seen that the main physical phases of the substrate are -(Fe,Cr), -Cr, and CrC. The composition of the Ni-based cladded layers is -Ni dendrites, inter-dendrites of -(Fe,Ni), -Cr, carbides, borides, silicides etc. Compared with the substrate, the structure of the Ni-based cladder layers is more stable and the layers are more resistant to wear. The chromium carbide is a type of the hard metal ceramics. The chromium carbide’s linear coefficient of thermal expansion is close to that of Ni and it is compatible with Ni-based substrate, so it is not easy to crack in the cladding process.
Microhardness Analysis
The hardness of Ni-based cladded layer and the substrate was measured by the Vickers hardness tester. The results are illustrated in Fig. 7. It can be seen that the hardness of the cladded layer is much higher than that of the substrate: The hardness of the cladded layer is approximately 516 HV0.2 while the hardness of the substrate is approximately 282 HV0.2. The reason why the hardness of the cladded layer is nearly twice as high as the hardness of substrate is that the plasma cladding is a non-equilibrium solidification crystallization process in which a certain amount of supersaturated solid solution, for example -(Fe,Ni), is generated. The hardness of the cladded layer is increased due to the effect of solid solution. In addition, with the effect of the plasma jets, the metallurgical chemical reaction of the cladded layer generated enhanced phases, such as Cr7C3, Ni4B3, Ni3Si2. The enhanced phases strengthened phase transition and consequently affected the hardness of the cladder layer greatly. Furthermore, due to the effect of the heat source of the plasma jets such as laminar flow, it can be noticed that the hardness of the zone that is affected by heat is relatively high, which is around 350HV0.2.
Friction and Wear Performance Analysis
Both the friction and wear of the substrate and cladded layer was simulated by the reciprocating friction and wear tester. As illustrated in Fig. 8, the friction coefficients of the substrate and the cladded layer climb rapidly during the first 15 minutes and then fluctuate with an upward trend. The friction coefficient of the substrate is around 0.910 while that of the cladded layer is around 0.569. By calculation, the wear amounts of the substrate and the cladded layer are 5.39E-6 mm3·N-1·m-1, 2.24E-6 mm3·N-1·m-1 respectively, and the wear volumes of the substrate and the cladded layer are 3.88E-3 mm3 and 1.61E-3 mm3 respectively. These figures are given in Fig. 9. The wear amount of the cladded layer was approximately half of that of the substrate. This is due to the much weight loss of the substrate, which is resulted in by the substrate’s low hardness and weak resistance to abrasion. The type of wear of the substrate is adhesive wear. However, affected by the solid solution effect of the plasma jet flow, the structure of the cladded layer changes. The formations of carbides, borides, silicides, and other phases could be used as hard point obstacles. The type of wear, abrasive wear, could greatly reduce the abrasion caused by abrasive particles on the surface, which means the resistance to abrasion was increased. Based on the observation of both substrate and the cladded layer, which is illustrated in Fig. 10, the scratches on the surface of the substrate are deep, wide and dense. The overall look of the scratches is like a furrow. But there are not many scratches on the surface of the cladded layer. The scratches are shallow and not continuous. So, it is concluded that the resistance to abrasion has been increased significantly.
Molten Pool Temperature Simulation
Fig. 11 illustrates the temperatures of the molten pool horizontally and vertically. It can be found that the powder completed melted both horizontally and vertically. A small portion of the substrate that touched the powder melted. The widths and depths of the molten pool are shown in Fig. 12. When the fixed speed was 60 mm/min, the effect of current on temperature was studied. As shown in Fig. 13, with a fixed speed, the temperature of the molten pool grows when the current increases and the temperature goes up from 2932℃ to 3220℃. Furthermore, the width and the depth both becomes bigger gradually. As illustrated in Fig. 14, with a fixed current of 120A, the effect of speed on temperature was researched. The width and depth of the molten pool drop gradually when the speed increases. And the temperature also goes down from 3332℃ to 3220℃.
Fig. 15 illustrates the geometry of the molten pool when the cladding experiment was completed. It can be easily seen that both the width and depth of the molten pool grow as the current goes up. Nevertheless, the molten pool narrows when the speed increases. Table 4 is the comparison of the figures generated by simulation with the figures obtained in the experiment. The figures of simulation and experiment are roughly the same. Although errors exist, they are not significant and are acceptable. The existence of errors could result from simplification of the model during the simulation process and appropriate assumptions made.
At the same time, the simulated morphology of the molten pool of the Ni-based cladded layer is compared with the actual morphology of the molten pool, as shown in Fig. 16. It can be found that the temperature partitions of the simulated morphology and the actual morphology of the two cladding materials are roughly consistent, the cladded layer area is basically consistent, and the heat-affected area is slightly consistent. Consequently, the reliability of experiment can be verified by simulation and simulation can be utilized to find out how process parameters affect temperature of the molten pool. This is meaningful in terms of plasma technology. The method of simulation may be adopted in the field of remanufacturing.
Table 4 Comparison of figures of the simulation and experiment of the molten pool
No.
|
Width
|
Depth
|
Simulation figures
|
Experiment figures
|
Simulation figures
|
Experiment figures
|
1
|
10.56
|
9.66
|
3.86
|
3.32
|
2
|
10.78
|
10.54
|
4.12
|
3.86
|
3
|
11.23
|
11.58
|
4.54
|
4.34
|
4
|
12.05
|
12.35
|
5.21
|
4.78
|
5
|
11.78
|
11.46
|
4.98
|
4.32
|
6
|
11.23
|
10.42
|
4.54
|
3.72
|