Figure 4 presents the value of melt pool depth at different values of ED obtained using experimental studies and numerical simulation. An increasing trend is observed for the melt pool depth with an increase in the ED. This is mainly due to the availability of more amount of laser energy for melting. A comparison of the simulated melt pool geometry with experimental value as presented in Figure 4 indicates a maximum deviation of 17.2%. The variation can be due to the various effects such as spattering, ablation and evaporation phenomenon, which are not accounted in the simulation. Figure 5 presents the typical temperature distribution obtained from numerical analysis on the rough surface of the sample obtained by DED process at ED of 13.3 J/mm2 and 8.3 J/mm2. It can be seen from the temperature distribution that the temperature is maximum at the centre of the laser beam and it decreases away from the centre of the laser beam. This is mainly due to the Gaussian energy distribution of the laser source and faster heater conduction at the edges of the melt pool.
Table 2 presents the roughness values of samples after laser polishing. Shallow surface melting (SSM) and surface over melt (SOM) are the two major processes that impact surface roughness. The SSM area is a partially melted metal surface formed by the capillary pressure created by the shallow melting of micro peaks that fills the valleys with molten metal. The molten metal thickness is smaller than the peak and valley distance in this partially melted metal area. However, if the energy density is higher, molten metal flows from peaks to valleys in SSM [8], leading to a reduction in surface roughness as a result of the capillary pressure. The melt pool thickness would be more than the peak to valley distance if the laser energy density is raised. This results in a decreased frequency of peak and valley but an increase in their amplitude, which enhances surface roughness. This region in which the over melting of the peak occurs is known as SOM [21, 22]. The roughness value depends upon the scan speed and laser power. Figure 6 represents surface periodic formation during SOM mechanism.
Table 2: Roughness values of different samples after laser polishing at different values of laser polishing parameters.
Sample
|
Laser power (W)
|
Scan speed
(mm/s)
|
Hatch Spacing
(mm)
|
ED
(J/mm2)
|
Average Roughness (Ra)
(µm)
|
Root mean square roughness (Rq)
(µm)
|
Sample 1
|
100
|
300
|
0.03
|
11.11
|
11.18
|
15.50
|
Sample 2
|
100
|
500
|
0.06
|
3.33
|
13.38
|
18.57
|
Sample 3
|
100
|
700
|
0.09
|
1.58
|
16.17
|
21.50
|
Sample 4
|
150
|
300
|
0.06
|
8.33
|
12.06
|
21.57
|
Sample 5
|
150
|
500
|
0.09
|
3.33
|
14.77
|
20.25
|
Sample 6
|
150
|
700
|
0.03
|
7.142
|
9.00
|
12.62
|
Sample 7
|
200
|
300
|
0.09
|
7.407
|
14.32
|
19.80
|
Sample 8
|
200
|
500
|
0.03
|
13.33
|
13.34
|
17.52
|
Sample 9
|
200
|
700
|
0.06
|
4.76
|
17.52
|
22.70
|
As-Built Sample
|
|
|
|
|
21.37
|
28.39
|
Usually, it is preferred to partially melt the material than completely melting it. Therefore, it can be concluded that the surface of the sample undergoes through 3 phases when the energy density is increased which are:
- When laser energy density is not able to melt the surface, i.e., incomplete melting zone,
- When laser energy density is suitable enough to melt the material so that it can form a SSM region, and
- When laser energy density is high such that it over-melt the material, i.e., SOM region.
By comparing the results of the average roughness of the as-built sample and macro polished samples, it can be noted that the roughness decreased by macro polishing. The optical profilometer readings of as-built and the best macro polished samples are given in Figures7(a) and 7(b), respectively.
As-built samples show the highest roughness, whereas other laser polished samples show lower values of surface roughness as compared to that of as-built samples. The surface profile of the as-built sample, sample 6, sample 3, and sample 8 are shown in Figures 7(a), 7(b),7(c) and 7(d), respectively. Sample 6 shows the lowest roughness value among all the samples. Sample 3 (ED =1.58 J/mm2) and sample 8 (ED= 13.33 J/mm2) show higher roughness than sample 6 (ED=7.142 J/mm2). Thus, it can be noted that the surface roughness increases with an increase in the energy density up to a certain limit and then by further increasing the energy density the surface roughness value decreases.
Figure 8 shows average roughness (Ra) variation with energy density. It can be seen that when the ED is low, the variation in the roughness is minimal. This is mainly due to the lower amount of energy available to melt the peaks and thus leading to minimal change in roughness. When the laser energy density reaches 7.142 J/mm2, the surface roughness is reduced to the minimum values i.e., SSM occurs. However, when the energy density is increased above 7.142 J/mm2, over melting of peaks starts and thus surface over melting region is reached and the surface roughness starts increasing beyond 7.142 J/mm2.
Figure 9 presents the effect of laser polishing on surface topography. Surface topography analysis using SEM shows that the unmelted powder particles spread on the as-built LDED sample surface are melted by laser polishing, which primarily leads to the reduction in the surface roughness of the polished parts. The balling effect also plays a role in increasing the roughness of the as-built sample. The balling effect is caused by powder particles that have not melted or have just partially melted and are trapped inside the built layer. The creation of a balling effect on the surface of the produced components is caused by insufficient wetting. In Figure 9(b), a high amount of energy density is available due to the lower scan speed caused by the presence of irregularities in the melt pool. In Figure 9(c), no significant effect of laser polishing can be seen since parameters were not optimized, which resulted in an unmelted regime. However, in Figure 9(d), a sound melting region is formed with no existence of melt drop and crack. From SEM images, it can be concluded that the samples with lower energy density had a dominant balling effect and unmelted regime, whereas with higher energy density over melting and melting irregularities are dominant. At optimum energy density, a smoother surface is obtained due to shallow surface melting.
Figures 10 (a) and (b) present the elemental mapping of laser polished and as-built surfaces. It is seen that the segregation is more evident in the laser polished samples as opposed to as-built samples. The segregation of Si, Mn and C is more evident from the samples as they have more chances of segregation due to lower diffusion coefficient. In addition, the presence of Cr oxide is also evident in the as-built and laser polished samples. In Figure 11(a), XRD analysis indicates the presence of peaks at angles 44.42°, 51.58°, 75.47°, and 91.43°corresponding to γ-Fe (111), γ-Fe (200), γ-Fe (220), and γ-Fe (311) [23, 24], respectively in as-built samples. In addition, it is seen that there is no extra peak on the laser polished samples by polishing. It can also be seen from Figure 11 (b) that a peak shift is observed between the as-built sample and the laser polished sample. This can be due to the variation in the lattice spacing as a result of the variation in the lattice strain. In addition, it can also be seen that there is a peak shift between samples polished at different ED values, which indicates the variation of stress pattern in the material at different ED values.