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Effect of surface morphology on the melt pool geometry in single track selective laser melting
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Effect of surface morphology on the melt pool geometry in single track selective laser melting Ashish Kumar Mishra, Arvind Kumar ⇑ Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e
i n f o
Article history: Received 18 November 2019 Received in revised form 21 December 2019 Accepted 26 December 2019 Available online xxxx Keywords: Surface Morphology Melt Pool Geometry Single Track SLM AlSi10Mg Surface absorptivity
a b s t r a c t Single track selective laser melting (SLM) experiments are done quite often to understand the building process, attendant thermo-physical transport phenomena as well as to gather the experimental data for computational model development. Therefore, mimicking the base surface morphology as close to the reality as possible is of paramount importance in such experiments. Since the surface morphology strongly affects the radiation properties, the local powder bed thickness and the focal plane distance, using a polished base may risk the results’ reliability. Hence in this study, single track SLM of AlSi10Mg has been studied for three different surfaces – polished, shot blasted and laser deposited. The surface morphologies have been examined using 3D Optical Profilometer. Cross-section measurement of the single track reflected the effect of base surface morphology on the melt pool dimensions. A wide variation has been observed in the melt pool dimensions even for the same set of process parameters and the same surface. The average melt pool dimensions are the largest for the shot blasted while smallest for the laser deposited base surface. Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the First International conference on Advanced Lightweight Materials and Structures
1. Introduction Selective laser melting (SLM) is a powder bed fusion based metal additive manufacturing process used widely to manufacture complex parts servicing aerospace, automotive and biomedical industries. It uses laser energy to essentially weld metal powder particles together and makes the object in a layered fashion. The process has been acclaimed for handling the design and material complexity extremely well due to the layered manufacturing while offering mechanical properties better than or at par with conventional manufacturing methods like casting etc. due to the very fine microstructure resulted by the extremely rapid solidification process [1]. The experimental investigations of SLM are opulent [2– 8]. Computational simulations are also being employed widely to study the attendant thermal-physical-metallurgical phenomena that shape the end product microstructure and mechanical properties [8–13]. While the majority of the experimental studies use bulk samples manufactured through SLM, single tracks have been deposited
⇑ Corresponding author.
and studied to understand the building process as well [5,6]. Single-track studies identify crucial results such as the average width of a single track, the penetration depth and metallurgical aspects which prove helpful in process parameter optimization for bulk manufacturing. The single track results are also used in the calibration of computational models developed to understand the complex thermo-physical and thermo-mechanical aspects of SLM. Hence, the reliability of such experiments is of utmost significance. Usually, for single-track experiments, a polished surface is used to control the powder bed thickness as precisely as possible. However, this does not replicate the actual morphology of the base surface that is encountered in real life SLM. In reality, the base surface is either shot blasted (when depositing the first layer) or previously deposited material (after the first layer). The base surface morphology plays a key role in the SLM process, as it does affect the radiation properties of the base [14,15], the local distance of the laser focal plane to the surface [16] and the local thickness of the powder bed. It is observed that the effective absorptivity varies widely with the effective angle of incidence and the surface feature dimensions. Hence, the actual melt track dimensions may differ from those obtained using polished surfaces. This may have an adverse impact on the selection of process parameters for the bulk
E-mail address: [email protected] (A. Kumar). https://doi.org/10.1016/j.matpr.2019.12.357 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the First International conference on Advanced Lightweight Materials and Structures
Please cite this article as: A. Kumar Mishra and A. Kumar, Effect of surface morphology on the melt pool geometry in single track selective laser melting, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.357
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sample manufacturing. In addition, such single track SLM results when used for the computational model development can put their credibility at risk. Hence, in current study effort has been made to shed a light on the surface morphology – SLM melting characteristics by performing single track SLM experiments on three different base surfaces, viz. Polished, Shot blasted and Laser deposited. The surface morphologies of the three bases have been probed using linear as well as 3D optical Profilometer and single tracks of AlSi10Mg have been deposited. The cross-section imagery and dimension measurements are done to understand the effect of surface morphology on the melt pool dimensions qualitatively as well as quantitatively. 2. Experimental details AlSi10Mg plates of dimension 30 20 3 mm are manufactured through SLM using Concept Laser MLab Cusing R machine supplied by GE additive, to be used as base plates for single track melting experiments. The AlSi10Mg powder used is gas atomized, spherical with particle size ranging between 10 and 25 mm supplied by GE additive. One of the plates is polished using 600 grit size paper. The second plate is first polished and then shot blasted using coarse sand particles. The third plate is deposited with a thick layer of AlSi10Mg through SLM at the same parameters as was used in building the plates. Next, the three plates are heattreated according to the process given in Maskery et al. [17]. The heat treatment was done to alter the microstructure so that differentiation becomes possible during cross-sectional measurements of single tracks deposited. The surface roughness of the three plates is measured using linear surface roughness tester (Mitutoyo SJ 301) as well as 3D Optical Profilometer (Contour GTK, Bruker, USA). 14 single tracks were scanned for each set of process parameters. In making such tracks, only laser power is varied keeping the other parameters constant since the study is focused on understanding the effect of surface morphology. The process parameters have been selected through a set of experiments aimed at residual porosity minimization, conducted on bulk solid samples. The process parameters offering the minimum residual porosity are used in this study. The process parameters are given in Table 1. The samples are cross-sectioned, mounted and polished. The polished samples are etched using Keller’s etchant (H2O:HCl:HNO3: HF::95:2.5:1.5:1) and then viewed at 100X magnification with Optical Microscope (Nikon Eclipse LV100). The image processing has been done using Image J. 3. Results 3.1. Surface morphology Fig. 1 shows the enlarged view of the linear roughness profile, measured over a length of 4 mm. Fig. 2 shows the 3D optical Profilometer results in 2D as well as 3D views. The corresponding surface roughness values are listed in Table 2. The polished surface is nearly flat with a feature size of < 1 mm. It, therefore, has a high degree of specular reflection of the beam as the sub-wavelength structures reduce multiple reflections. While
Table 1 SLM process parameters for the single track experiments. Laser power Scanning speed Powder bed thickness Laser beam spot radius
80, 90, 100 W 500 mm-s 1 15 mm 25 mm
there will be a powder bed spread over it, as the powder denudation occurs during SLM [18] the energy absorption by the bed will play the key role. The specular reflection will result in lower absorption and hence less melting in such cases. The shotblasted surface has a random and larger variation in the surface profile and hence causes more trapping of the incident photons through multiple reflections. The relative feature size in the shotblasted surface is about 10 mm, roughly similar to the powder particle size and an order higher than the laser wavelength. Hence, the multiple reflection effect will increase the optical path length and considerably enhance the absorptivity. While the Ra value for the laser deposited surface is greater than that for shot blasted, the features are not rough and a smooth wavy structure is seen. This, therefore, resulted in more increase in the reflection and hence a slight reduction in the absorptivity than the polished surface. 3.2. Melt pool cross-section The melt pool cross sections for different surface morphologies are shown in Figs. 3–5. As shown, multiple single track crosssections for a set of process parameters show how melting characteristics change even when parameters are kept constant. For polished surface shallow melt tracks are formed mostly, even at laser power as high as 100 W. The melt pool is generally uniform as shown in Fig. 3. However, some randomness is seen occasionally. The maximum and minimum values of melt pool dimensions are listed in Table 3. Some melt tracks have shown defects at the melt pool boundary that are quite large. The defects are typically seen in the deeper melt pools. The melt tracks on the shot blasted surface are more random, as shown in Fig. 4. The melt pools are deeper and wider than those for the polished surface, indicating the increase in absorptivity of the surface. While the melt pool cross-sections are more or less semi-elliptic in polished, elliptic or cylindrical melt tracks are formed more frequently in case of the shot-blasted surface. The melt pool becomes larger as the incident laser power increases, yet a high degree of variation is seen at constant parameters. Some cross-sections show porosity formation as well. The melt pool cross-sections on the laser deposited surface are wide and shallow but retain a high degree of randomness as well. Interestingly, small satellite formations are also observed (Fig. 5). The randomness of the melt pool cross-sections is shown quantitatively in Fig. 6a–b. The scatter plots show the distribution of the melt track width and melt track height for different surface morphologies at constant process parameters. As shown, the three plots corresponding to the three laser power values are stacked together to provide a comprehensive picture. The widest distribution is seen with the shot blasted surface, confirming the qualitative findings of Figs. 3–5. While the polished and laser deposited surfaces have more close distribution, point singularities are seen in polished surface results. The scatter plot gives a rough estimation of the resultant effect, i.e. the melt pool dimensions are the largest for shot blasted surface and smallest for laser deposited surface. 3.3. Average melt pool dimensions The as-built single tracks are sectioned at multiple planes and polished. The melt pool dimensions are then measured using an optical microscope and are used to calculate the average values. The variation of these quantities with surface morphologies and laser powers are shown in Fig. 7. The mean melt track width and height are the smallest for polished surface and increase with increasing laser power from 80 W to 100 W. The mean value of melt pool depth paints a different picture as the values are the smallest for laser deposited surface. An interesting trend is that
Please cite this article as: A. Kumar Mishra and A. Kumar, Effect of surface morphology on the melt pool geometry in single track selective laser melting, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.357
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Fig. 1. Enlarged view of surface profile measured through a linear surface roughness tester.
Fig. 2. Surface morphology measured by 3D Optical Profilometer.
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pool dimensions remain the largest for the shot blasted surface (nearly twice) and keep increasing as the laser power is increased.
Table 2 Surface roughness measurement. Roughness metric (mm)
Polished
Shot Blasted
Laser Deposited
Ra Rz Rq
0.16 1.22 0.20
4.57 26.80 5.66
5.32 26.22 6.33
as the laser power is increased, the mean melt pool dimensions for laser deposited surface gradually decrease and become smaller than those corresponding to the polished surface. The average melt
4. Discussion The surface features affect the laser beam interaction with the base surface and hence affect the melt track geometry in SLM. The single track studies are done using a polished base surface, hence not suitable for process parameter optimization or computational model calibration. The basic fact that the base surface during
Fig. 3. Melt track cross-section over the polished surface.
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Fig. 4. Melt track cross-section over the Shot Blasted surface.
SLM will be either shot blasted or previously melt laser deposited material disqualifies the use of the polished base surface for single track studies. The wide variation in the geometry and dimensions of melt tracks built at the same parameters is an important result though. The variations are seen in the case of the polished surface as well. It shows that even when all the process parameters and processing conditions are kept the same, the melting and material deposition process can vary at different locations. One possible reason for this
can be given as the variations in the local distance of the laser focal plane to the base surface as the surface profile changes. The variation in the distance from the focal plane affects the incident laser energy density and hence causes local alteration in the melting process. Another possible reason can be the material structure of the base plate. As the base plate contains Al- and Si-rich zones as shown in Figs. 3–5 by bright and dark patches, the difference in the thermo-physical properties plays its role too in determining the quality and quantity of melting.
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Fig. 5. Melt track cross-section over the Laser Deposited surface.
While the deviations in the melt track made on the polished surface are not too large, it is significantly larger for the shot blasted surface. Here, along with the reasons mentioned earlier, the surface morphology-radiation property relationship plays a key role. The sharp features created during the shot-blasting process enhance effective absorptivity through multiple internal reflections and capturing, thereby increasing the energy absorbed by the material. The wide variation in the surface profile also keeps
changing the local powder bed thickness and hence causes substantial scattering in melt track dimensions as shown in Fig. 6. The laser deposited surface has the variations occurring in the surface over a long-range and lacks the local sharp features like those observed in the shot-blasted surface. The smooth surface created by laser deposition enhances the outward reflections of the laser beam and hence decreases the effective absorptivity. The melt tracks are even smaller than those made on the polished sur-
Please cite this article as: A. Kumar Mishra and A. Kumar, Effect of surface morphology on the melt pool geometry in single track selective laser melting, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.357
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A. Kumar Mishra, A. Kumar / Materials Today: Proceedings xxx (xxxx) xxx Table 3 Minimum and maximum values of melt track dimensions. Surface
Polished
Shot Blasted
Laser Deposited
Laser power (W)
80 90 100 80 90 100 80 90 100
Melt track width (mm)
Melt track height (mm)
Minimum
Maximum
Minimum
Maximum
61.23 61.83 67.12 53.97 70.15 70.3 62.29 51.25 67.28
90.86 96.61 99.33 96.15 119.58 127.26 76.5 84.96 80.43
21.47 25.25 26 25 48.98 42.71 26 22.68 25.7
63.95 61.98 52.91 94 90.64 125.36 76.65 52.61 36.13
Fig. 6. Melt track width and height for the three surfaces at different laser power.
face and the distribution is narrower in comparison. This can be a result of the smoothness of the profile caused by the laser melting and the waviness of the surface too, aiding in reflecting the beam away from the surface. While the polished surface has very low Ra value, it does contain small but sharp features that have their slight but significant effect on the absorptivity. A noticeable point is that as the laser power is increased, the variations in the melt track geometry tend to reduce for the tracks built over laser deposited surface. At 100 W, the scatter becomes the least. From this study, it can be concluded that the surface morphology too plays a significant role in the SLM process physics. While using a polished surface can ensure better control over the process and has become a trend, it is imperative to use a surface that better represents the actual process as the single track findings are used for process parameter optimization. The single track geometry determines the scan spacing which is an important parameter for porosity elimination. Moreover, single track results are usually used in the development and calibration of computational models
which have become a new tool in understanding the SLM process physics. In such cases, a more realistic experimental approach should be opted since the process will take place over the laser deposited surface for almost all of the time.
5. Conclusion Single-track experiments performed over three different surfaces reveal the effect of surface morphology on the laser-matter interaction and associated alteration in the melt track geometry. Wide distribution is seen in the melt track geometries built at the same process parameters on the same surface as well. The tracks built upon the shot blasted surface were most random, owing to the frequent local morphology and hence absorptivity changes. The laser deposited surface has a smooth surface with long-range waviness and hence the melt tracks built upon are more coherent.
Please cite this article as: A. Kumar Mishra and A. Kumar, Effect of surface morphology on the melt pool geometry in single track selective laser melting, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.357
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References
Fig. 7. Comparison of mean width and height of the melt track for the three surface morphologies.
CRediT authorship contribution statement Ashish Kumar Mishra: Conceptualization, Methodology, Data curation, Writing - original draft, Visualization, Investigation. Arvind Kumar: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support of DST-FIST, Department of Science and Technology, India (Grant No. SR/FST/ETII-066), and Science and Engineering Research Board, Department of Science and Technology, India (Grant No. IMP/2018/000293).
[1] W. Yu, S.L. Sing, C.K. Chua, X. Tian, influence of re-melting on surface roughness and porosity of AlSi10Mg parts fabricated by selective laser melting, J. Alloys Compd. 792 (2019) 574–581. [2] H. Ali, H. Ghadbeigi, K. Mumtaz, Effect of scanning strategies on residual stress and mechanical properties of selective laser melted Ti6Al4V, Mater. Sci. Eng. A 712 (2018) 175–187. [3] Z. Hu, H. Zhu, H. Zhang, X. Zeng, Experimental investigation on selective laser melting of 17–4PH stainless steel, Opt. Laser Technol. 87 (2017) 17–25. [4] E. Liverani, S. Toschi, L. Ceschini, A. Fortunate, Effect of selective laser melting (SLM) process parameters on microstructure and mechanical properties of 316L austenitic stainless steel, J. Mater. Process. Tech. 249 (2017) 255–263. [5] J.J.S. Dilip, S. Zhang, C. Teng, K. Zeng, C. Robinson, D. Pal, B. Stucker, Influence of processing parameters on the evolution of melt pool, porosity, and microstructures in Ti-6Al-4V alloy parts fabricated by selective laser melting, Prog. Addit. Manuf. 2 (2017) 157–167. [6] C. Kusuma, S.H. Ahmed, A. Mian, R. Srinivasan, Effect of laser power and scan speed on melt pool characteristics of commercially pure titanium (CP-Ti), J. Mater. Eng. Perform. 26 (2017) 3560–3568. [7] F. Li, Z. Wang, X. Zeng, Microstructures and mechanical properties of Ti6Al4V alloy fabricated by multi-laser beam selective laser melting, Mater. Lett. 199 (2017) 79–83. [8] X. Ding, L. Wang, Heat transfer and fluid flow of molten pool during selective laser melting of AlSi10Mg powder: simulation and experiment, J. Manuf. Processes 26 (2017) 280–289. [9] A. Foroozmehr, M. Badrossamay, E. Foroozmehr, S. Golabi, Finite element simulation of selective laser melting process considering optical penetration depth of laser in powder bed, Mater. Des. 89 (2016) 255–263. [10] T. Heeling, M. Cloots, K. Wegener, Melt pool simulation for the evaluation of process parameters in selective laser melting, Addit. Manuf. 14 (2017) 116– 125. [11] H.C. Tran, Y.L. Lo, Heat transfer simulations of selective laser melting process based on volumetric heat source with powder size consideration, J. Mater. Process. Technol. (2010), https://doi.org/10.1016/j.jmatprotec.2017.12.024. [12] Y. Zhang, J. Zhang, Finite element simulation and experimental validation of distortion and cracking failure phenomena in direct metal laser sintering fabricated component, Addit. Manuf. 16 (2017) 49–57. [13] Ashish Kumar Mishra, Arvind Kumar, Numerical and experimental analysis of the effect of volumetric energy absorption in powder layer on thermal-fluidic transport in selective laser melting of Ti6Al4V, Opt Laser Technol. 111 (2019) 227–239, https://doi.org/10.1016/j.optlastec.2018.09.054. [14] M.S. Brown, C.B. Arnold, K. Sugioka, Laser precision microfabrication, in: Springer Series in Materials Science, Springer, Berlin, 2010, https://doi.org/ 10.1007/978-3-642-10523-4__4, pp 91-120. [15] C. Niu, T. Zhu, Y. Lv, Influence of surface morphology on absorptivity of lightabsorbing materials, Int. J. Photoenergy (2019), https://doi.org/10.1155/2019/ 1476217. [16] A. Aggarwal, S. Patel, A. Kumar, Selective laser melting of 316L stainless steel: physics of melting mode transition and its influence on microstructural and mechanical behavior, JOM 71 (2019) 1105–1116. [17] I. Maskery, N.T. Aboulkhair, M.R. Cornfield, C. Tuck, A.T. Clare, R.K. Leach, R.D. Wildman, I.A. Ashcroft, R.J.M. Hague, Quantification and characterization of porosity in selectively laser melted Al-Si10-Mg using X-ray computed tomography, Mater. Charact. 111 (2016) 193–204. [18] M.J. Matthews, G. Guss, S.A. Khairallah, A.M. Rubenchik, P.J. Depond, W.E. King, Denudation of metal powder layers in laser powder bed fusion processes, Acta Mater. 114 (2016) 33–42.
Please cite this article as: A. Kumar Mishra and A. Kumar, Effect of surface morphology on the melt pool geometry in single track selective laser melting, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.357
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Effect of surface morphology on the melt pool geometry in single track selective laser melting Ashish Kumar Mishra, Arvind Kumar ⇑ Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e
i n f o
Article history: Received 18 November 2019 Received in revised form 21 December 2019 Accepted 26 December 2019 Available online xxxx Keywords: Surface Morphology Melt Pool Geometry Single Track SLM AlSi10Mg Surface absorptivity
a b s t r a c t Single track selective laser melting (SLM) experiments are done quite often to understand the building process, attendant thermo-physical transport phenomena as well as to gather the experimental data for computational model development. Therefore, mimicking the base surface morphology as close to the reality as possible is of paramount importance in such experiments. Since the surface morphology strongly affects the radiation properties, the local powder bed thickness and the focal plane distance, using a polished base may risk the results’ reliability. Hence in this study, single track SLM of AlSi10Mg has been studied for three different surfaces – polished, shot blasted and laser deposited. The surface morphologies have been examined using 3D Optical Profilometer. Cross-section measurement of the single track reflected the effect of base surface morphology on the melt pool dimensions. A wide variation has been observed in the melt pool dimensions even for the same set of process parameters and the same surface. The average melt pool dimensions are the largest for the shot blasted while smallest for the laser deposited base surface. Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the First International conference on Advanced Lightweight Materials and Structures
1. Introduction Selective laser melting (SLM) is a powder bed fusion based metal additive manufacturing process used widely to manufacture complex parts servicing aerospace, automotive and biomedical industries. It uses laser energy to essentially weld metal powder particles together and makes the object in a layered fashion. The process has been acclaimed for handling the design and material complexity extremely well due to the layered manufacturing while offering mechanical properties better than or at par with conventional manufacturing methods like casting etc. due to the very fine microstructure resulted by the extremely rapid solidification process [1]. The experimental investigations of SLM are opulent [2– 8]. Computational simulations are also being employed widely to study the attendant thermal-physical-metallurgical phenomena that shape the end product microstructure and mechanical properties [8–13]. While the majority of the experimental studies use bulk samples manufactured through SLM, single tracks have been deposited
⇑ Corresponding author.
and studied to understand the building process as well [5,6]. Single-track studies identify crucial results such as the average width of a single track, the penetration depth and metallurgical aspects which prove helpful in process parameter optimization for bulk manufacturing. The single track results are also used in the calibration of computational models developed to understand the complex thermo-physical and thermo-mechanical aspects of SLM. Hence, the reliability of such experiments is of utmost significance. Usually, for single-track experiments, a polished surface is used to control the powder bed thickness as precisely as possible. However, this does not replicate the actual morphology of the base surface that is encountered in real life SLM. In reality, the base surface is either shot blasted (when depositing the first layer) or previously deposited material (after the first layer). The base surface morphology plays a key role in the SLM process, as it does affect the radiation properties of the base [14,15], the local distance of the laser focal plane to the surface [16] and the local thickness of the powder bed. It is observed that the effective absorptivity varies widely with the effective angle of incidence and the surface feature dimensions. Hence, the actual melt track dimensions may differ from those obtained using polished surfaces. This may have an adverse impact on the selection of process parameters for the bulk
E-mail address: [email protected] (A. Kumar). https://doi.org/10.1016/j.matpr.2019.12.357 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the First International conference on Advanced Lightweight Materials and Structures
Please cite this article as: A. Kumar Mishra and A. Kumar, Effect of surface morphology on the melt pool geometry in single track selective laser melting, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.357
2
A. Kumar Mishra, A. Kumar / Materials Today: Proceedings xxx (xxxx) xxx
sample manufacturing. In addition, such single track SLM results when used for the computational model development can put their credibility at risk. Hence, in current study effort has been made to shed a light on the surface morphology – SLM melting characteristics by performing single track SLM experiments on three different base surfaces, viz. Polished, Shot blasted and Laser deposited. The surface morphologies of the three bases have been probed using linear as well as 3D optical Profilometer and single tracks of AlSi10Mg have been deposited. The cross-section imagery and dimension measurements are done to understand the effect of surface morphology on the melt pool dimensions qualitatively as well as quantitatively. 2. Experimental details AlSi10Mg plates of dimension 30 20 3 mm are manufactured through SLM using Concept Laser MLab Cusing R machine supplied by GE additive, to be used as base plates for single track melting experiments. The AlSi10Mg powder used is gas atomized, spherical with particle size ranging between 10 and 25 mm supplied by GE additive. One of the plates is polished using 600 grit size paper. The second plate is first polished and then shot blasted using coarse sand particles. The third plate is deposited with a thick layer of AlSi10Mg through SLM at the same parameters as was used in building the plates. Next, the three plates are heattreated according to the process given in Maskery et al. [17]. The heat treatment was done to alter the microstructure so that differentiation becomes possible during cross-sectional measurements of single tracks deposited. The surface roughness of the three plates is measured using linear surface roughness tester (Mitutoyo SJ 301) as well as 3D Optical Profilometer (Contour GTK, Bruker, USA). 14 single tracks were scanned for each set of process parameters. In making such tracks, only laser power is varied keeping the other parameters constant since the study is focused on understanding the effect of surface morphology. The process parameters have been selected through a set of experiments aimed at residual porosity minimization, conducted on bulk solid samples. The process parameters offering the minimum residual porosity are used in this study. The process parameters are given in Table 1. The samples are cross-sectioned, mounted and polished. The polished samples are etched using Keller’s etchant (H2O:HCl:HNO3: HF::95:2.5:1.5:1) and then viewed at 100X magnification with Optical Microscope (Nikon Eclipse LV100). The image processing has been done using Image J. 3. Results 3.1. Surface morphology Fig. 1 shows the enlarged view of the linear roughness profile, measured over a length of 4 mm. Fig. 2 shows the 3D optical Profilometer results in 2D as well as 3D views. The corresponding surface roughness values are listed in Table 2. The polished surface is nearly flat with a feature size of < 1 mm. It, therefore, has a high degree of specular reflection of the beam as the sub-wavelength structures reduce multiple reflections. While
Table 1 SLM process parameters for the single track experiments. Laser power Scanning speed Powder bed thickness Laser beam spot radius
80, 90, 100 W 500 mm-s 1 15 mm 25 mm
there will be a powder bed spread over it, as the powder denudation occurs during SLM [18] the energy absorption by the bed will play the key role. The specular reflection will result in lower absorption and hence less melting in such cases. The shotblasted surface has a random and larger variation in the surface profile and hence causes more trapping of the incident photons through multiple reflections. The relative feature size in the shotblasted surface is about 10 mm, roughly similar to the powder particle size and an order higher than the laser wavelength. Hence, the multiple reflection effect will increase the optical path length and considerably enhance the absorptivity. While the Ra value for the laser deposited surface is greater than that for shot blasted, the features are not rough and a smooth wavy structure is seen. This, therefore, resulted in more increase in the reflection and hence a slight reduction in the absorptivity than the polished surface. 3.2. Melt pool cross-section The melt pool cross sections for different surface morphologies are shown in Figs. 3–5. As shown, multiple single track crosssections for a set of process parameters show how melting characteristics change even when parameters are kept constant. For polished surface shallow melt tracks are formed mostly, even at laser power as high as 100 W. The melt pool is generally uniform as shown in Fig. 3. However, some randomness is seen occasionally. The maximum and minimum values of melt pool dimensions are listed in Table 3. Some melt tracks have shown defects at the melt pool boundary that are quite large. The defects are typically seen in the deeper melt pools. The melt tracks on the shot blasted surface are more random, as shown in Fig. 4. The melt pools are deeper and wider than those for the polished surface, indicating the increase in absorptivity of the surface. While the melt pool cross-sections are more or less semi-elliptic in polished, elliptic or cylindrical melt tracks are formed more frequently in case of the shot-blasted surface. The melt pool becomes larger as the incident laser power increases, yet a high degree of variation is seen at constant parameters. Some cross-sections show porosity formation as well. The melt pool cross-sections on the laser deposited surface are wide and shallow but retain a high degree of randomness as well. Interestingly, small satellite formations are also observed (Fig. 5). The randomness of the melt pool cross-sections is shown quantitatively in Fig. 6a–b. The scatter plots show the distribution of the melt track width and melt track height for different surface morphologies at constant process parameters. As shown, the three plots corresponding to the three laser power values are stacked together to provide a comprehensive picture. The widest distribution is seen with the shot blasted surface, confirming the qualitative findings of Figs. 3–5. While the polished and laser deposited surfaces have more close distribution, point singularities are seen in polished surface results. The scatter plot gives a rough estimation of the resultant effect, i.e. the melt pool dimensions are the largest for shot blasted surface and smallest for laser deposited surface. 3.3. Average melt pool dimensions The as-built single tracks are sectioned at multiple planes and polished. The melt pool dimensions are then measured using an optical microscope and are used to calculate the average values. The variation of these quantities with surface morphologies and laser powers are shown in Fig. 7. The mean melt track width and height are the smallest for polished surface and increase with increasing laser power from 80 W to 100 W. The mean value of melt pool depth paints a different picture as the values are the smallest for laser deposited surface. An interesting trend is that
Please cite this article as: A. Kumar Mishra and A. Kumar, Effect of surface morphology on the melt pool geometry in single track selective laser melting, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.12.357
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3
Fig. 1. Enlarged view of surface profile measured through a linear surface roughness tester.
Fig. 2. Surface morphology measured by 3D Optical Profilometer.
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pool dimensions remain the largest for the shot blasted surface (nearly twice) and keep increasing as the laser power is increased.
Table 2 Surface roughness measurement. Roughness metric (mm)
Polished
Shot Blasted
Laser Deposited
Ra Rz Rq
0.16 1.22 0.20
4.57 26.80 5.66
5.32 26.22 6.33
as the laser power is increased, the mean melt pool dimensions for laser deposited surface gradually decrease and become smaller than those corresponding to the polished surface. The average melt
4. Discussion The surface features affect the laser beam interaction with the base surface and hence affect the melt track geometry in SLM. The single track studies are done using a polished base surface, hence not suitable for process parameter optimization or computational model calibration. The basic fact that the base surface during
Fig. 3. Melt track cross-section over the polished surface.
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Fig. 4. Melt track cross-section over the Shot Blasted surface.
SLM will be either shot blasted or previously melt laser deposited material disqualifies the use of the polished base surface for single track studies. The wide variation in the geometry and dimensions of melt tracks built at the same parameters is an important result though. The variations are seen in the case of the polished surface as well. It shows that even when all the process parameters and processing conditions are kept the same, the melting and material deposition process can vary at different locations. One possible reason for this
can be given as the variations in the local distance of the laser focal plane to the base surface as the surface profile changes. The variation in the distance from the focal plane affects the incident laser energy density and hence causes local alteration in the melting process. Another possible reason can be the material structure of the base plate. As the base plate contains Al- and Si-rich zones as shown in Figs. 3–5 by bright and dark patches, the difference in the thermo-physical properties plays its role too in determining the quality and quantity of melting.
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Fig. 5. Melt track cross-section over the Laser Deposited surface.
While the deviations in the melt track made on the polished surface are not too large, it is significantly larger for the shot blasted surface. Here, along with the reasons mentioned earlier, the surface morphology-radiation property relationship plays a key role. The sharp features created during the shot-blasting process enhance effective absorptivity through multiple internal reflections and capturing, thereby increasing the energy absorbed by the material. The wide variation in the surface profile also keeps
changing the local powder bed thickness and hence causes substantial scattering in melt track dimensions as shown in Fig. 6. The laser deposited surface has the variations occurring in the surface over a long-range and lacks the local sharp features like those observed in the shot-blasted surface. The smooth surface created by laser deposition enhances the outward reflections of the laser beam and hence decreases the effective absorptivity. The melt tracks are even smaller than those made on the polished sur-
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A. Kumar Mishra, A. Kumar / Materials Today: Proceedings xxx (xxxx) xxx Table 3 Minimum and maximum values of melt track dimensions. Surface
Polished
Shot Blasted
Laser Deposited
Laser power (W)
80 90 100 80 90 100 80 90 100
Melt track width (mm)
Melt track height (mm)
Minimum
Maximum
Minimum
Maximum
61.23 61.83 67.12 53.97 70.15 70.3 62.29 51.25 67.28
90.86 96.61 99.33 96.15 119.58 127.26 76.5 84.96 80.43
21.47 25.25 26 25 48.98 42.71 26 22.68 25.7
63.95 61.98 52.91 94 90.64 125.36 76.65 52.61 36.13
Fig. 6. Melt track width and height for the three surfaces at different laser power.
face and the distribution is narrower in comparison. This can be a result of the smoothness of the profile caused by the laser melting and the waviness of the surface too, aiding in reflecting the beam away from the surface. While the polished surface has very low Ra value, it does contain small but sharp features that have their slight but significant effect on the absorptivity. A noticeable point is that as the laser power is increased, the variations in the melt track geometry tend to reduce for the tracks built over laser deposited surface. At 100 W, the scatter becomes the least. From this study, it can be concluded that the surface morphology too plays a significant role in the SLM process physics. While using a polished surface can ensure better control over the process and has become a trend, it is imperative to use a surface that better represents the actual process as the single track findings are used for process parameter optimization. The single track geometry determines the scan spacing which is an important parameter for porosity elimination. Moreover, single track results are usually used in the development and calibration of computational models
which have become a new tool in understanding the SLM process physics. In such cases, a more realistic experimental approach should be opted since the process will take place over the laser deposited surface for almost all of the time.
5. Conclusion Single-track experiments performed over three different surfaces reveal the effect of surface morphology on the laser-matter interaction and associated alteration in the melt track geometry. Wide distribution is seen in the melt track geometries built at the same process parameters on the same surface as well. The tracks built upon the shot blasted surface were most random, owing to the frequent local morphology and hence absorptivity changes. The laser deposited surface has a smooth surface with long-range waviness and hence the melt tracks built upon are more coherent.
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References
Fig. 7. Comparison of mean width and height of the melt track for the three surface morphologies.
CRediT authorship contribution statement Ashish Kumar Mishra: Conceptualization, Methodology, Data curation, Writing - original draft, Visualization, Investigation. Arvind Kumar: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support of DST-FIST, Department of Science and Technology, India (Grant No. SR/FST/ETII-066), and Science and Engineering Research Board, Department of Science and Technology, India (Grant No. IMP/2018/000293).
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