Effect of Laser Power and Powder Flow Rate on Dilution Rate and Surface Finish Produced During Laser Metal Deposition of Titanium Alloy
Rasheedat M. Mahamood
Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park Kingsway
Campus, Johannesburg, 2006, South Africa.
e-mail: [email protected]
Esther T, Akinlabi
Department of Mechanical Engineering Science, University of Johannesburg, Auckland Park Kingsway
Campus, Johannesburg, 2006, South Africa.
e-mail: [email protected] Moses G. Owolabi
Department of Mechanical Engineering, Howard University,
Washington DC, USA e-mail: [email protected] Abstract—The influence of processing parameters on the
resulting properties of laser metal deposition process cannot be overemphasized. In this research, the influence of laser power and powder flow rate on the dilution and surface roughness value produced are critically studied.. The laser power was set between 1.8 kW and 3.0 kW, while the powder flow rate was set between 2.88 and 5.76 g/min. The scanning speed and the gas flow rate were maintained at constant values of 0.05m/s and 4l/min respectively. The study revealed that, as the laser power was increased, the degree of dilution increases but the average surface roughness value decreases. Also, as the powder flow rate was increased, the dilution decreases and the average surface roughness increases. The study shows that it is important to keep the powder flow rare low so as to achieve a better surface finished and also not to use too high laser power as this will result in higher dilution, which is not desirable in the laser additive manufacturing process because it will affect the dimensional accuracy of the part under processing.
Keywords-component; additive manufacturing; dilution;
processing parameter; laser metal deposition; surface roughness;
titanium alloy
I. INTRODUCTION
Laser metal deposition (LMD) is an advanced manufacturing technique that is used to obtain complex metallic parts directly from a three dimensional (3D) model of the intended part. LMD belong to the directed energy deposition class of additive manufacturing technology that is fabricating fully dense metal parts directly from the 3D Computer Aided Design (CAD) solid models of the part by adding materials layer by layer [1]. LMD has currently been the focus of research because of its great importance such as the ability to produce complex part, the ability to repair high valued components which were in the past not repairable [2] and the possibility to produce composites and functionally graded materials [3-6]. The repair of component
with intricacies is possible with this important additive manufacturing process. LMD has demonstrated great potential in the fabrication of advanced composite materials which could not be achieved with other conventional processes as a result of limitation imposed by the thermodynamic laws. Laser metal deposition processing has also been demonstrated to be a promising additive manufacturing technology that has the potential to reduce the product lead time from the initial concept to the finished part and the product to market time. Despite all these exciting properties of LMD, the process stability is still an issue because of the high sensitivity of the processing parameters such as laser power, powder flow rate, gas flow rate and scanning velocity. These issues need to be resolved before this process can widely be acceptable in a large number of industries.
A number of researches has been conducted on laser metal deposition process with respect to the influence of processing parameters on the evolving properties [7-13].
Zhong eta at. [7] investigated the effects of process parameters on properties of laser metal deposition process of Inconel 718 (IN718) powder. The effects of laser power, scanning speed, and powder mass flow on porosity, track geometry, deposition rate, and powder efficiency were studied. The result revealed that the porosity decreases with an increase of laser power and decrease of powder flow rate. Campanelli et al. [8] evaluation of the properties of 18 Ni (300) laser deposited powder using numerical and experimental approach. The study was conducted by varying the laser power, scanning speed, powder flow rate and overlap percentage. They were able to achieve High density samples with moderate porosity, and a close agreement was achieved between the experiment and the mathematical model. Mahamood et al. [9] studied the influence of laser power and scanning speed on the shape, size and degree of
porosity laser metal deposited titanium alloy. The study revealed that as the laser power was increased and the scanning speed was decreased, the degree of porosity was also reduced, and the size of the porosity was reduced as the laser power was increased. Zang et al. [10] investigated the influence of processing parameters on dilution ratio in laser cladding experiment. The results showed that the scanning speed influence on the degree of porosity was very high when compared to the effect of the laser power. The study revealed that moderate dilution rate is necessary to ensure the good quality deposit with strong metallurgical bonding with the right process parameter. Mahamood et al. [11]
studied the influence of processing parameters on the resulting surface finish produced during laser metal deposition of Titanium alloy. The investigation revealed that as the laser power was increased, the surface finish was found to increase. Mahamood et al. [12] reported the influence of laser power density on the evolving properties of laser metal deposited titanium alloy. The properties studied were microstructure, microhardness and the surface roughness. The results revealed that as the laser power density was increased, the average surface roughness was found to be reduced. In a related study by Mahamood and Akinlabi [13], the influence of scanning speed and gas flow rate on surface finish was investigated. Process parameters such as Powder flow rate, gas flow rate and laser power have also been reported to influence properties as well as material utilization efficiencies in laser metal deposition process [14-16].
In this study, the influence of laser power and powder flow rate on the surface roughness was investigated. The scanning speed and the gas flow rate were kept constant throughout the experiment at values of 0.05m/s and 4l/min respectively. The laser power was varied between 1.8 kW and 3.0 kW in a separate experiment and the powder flow rate was also varied between 2.88 and 5.76 g/min in another experiment.
II. EXPERIMENTAL METHOD
The laser metal deposition process was conducted with a continuous wave Nd-YAG laser operating at 1.06 µm wave length with a maximum power of 4 kW. The laser was carried by a Kuka robot in its end effector with powder nozzle attached co axially with the laser. The focal length of the laser was maintained throughout the experiment at a distance of 195 mm above the substrate given a spot size of 2 mm. The experimental set-up also consists of an improvised glove box that was filled with argon gas to help maintain the oxygen level at not more than 10 ppm. The materials used for this experiment are Ti6Al4V powder and Ti6Al4V sheet as the substrate. Both materials are of 99.6%
purity. The substrate was sandblasted and then degreased using acetone and ethanol prior to the deposition process.
The powder is of particle size range between 150-250 µm.
The scanning speed and the gas flow rate were fixed at 0.05 m/s and 4 l/min. Two separate experiments were performed;
the first one was performed with varying laser power with the experimental matrix presented in Table 1 and the second experiment with varying powder flow rate as presented in Table 2.
Table 1. Experimental Matrix with varying laser power
Sample No. Laser power (kW) Powder flow rate (g/min)
A1 3.0 2.88
A2 2.6 2.88
A3 2.2 2.88
A4 1.8 2.88
The laser beam was used to create a melt-pool on the surface of the substrate while the powder is delivered into the melt-pool during the laser metal deposition process. A solid track of the material is produced upon solidification of the melt-pool. A single track was produced at each set of processing parameters in Tables 1 and 2. After the deposition process, the samples for the surface roughness measurement were cleaned with acetone to remove any loose powder that stick to the surface of the deposit.
Table 2. Experimental Matrix with varying powder flow rate
Sample No. Powder flow rate (g/min)
Laser power (kW)
A1 2.88 3.0
A2 4.32 3.0
A3 5.04 3.0
A4 5.76 3.0
The surface roughness of the samples were measured using the stylus surface analyzer produced by Jenoptik and equipped with Hommelmap 6.2 software. The measuring condition used was according to the ‘BS EN ISO 4288:1998’ standard [17]. The effective measuring length was maintained at 4.8 mm; the cut-off length at 0.8 mm; and the measuring range at 400 µm. A sliding speed of 0.50 mm/s was used, and five measurements were taken on each of the samples, and the arithmetic mean of the two dimensional (2D) roughness profiles (Ra) was recorded. The samples for the measurement of dilution depth were cut laterally across the deposition direction to reveal the cross
section. The cut samples were mounted, ground and polished according to the standard metallurgical preparation of Titanium and its alloy, the ASTME3-11 standard [18].
The polished samples were etched with Kroll’s reagent that consists of 100 ml of water with 1.5 ml of hydrofluoric acid and 3 ml of nitric acid. The sample was studied using Olympus BX51M Optical Microscopy (OP) equipped with an Olympus DP25 digital camera. The microscope was connected to the computer system equipped with Stream software for image analysis.
III. RESULTS AND DISCUSSION
The results of the effect of laser power on the dilution depth and the average surface roughness is presented in table 3. The graph of the dilution depth and the average surface roughness against the laser power is shown in Fig. 1.
It can be een from the graph that as the as the laser power was increasing, the dilution depth was increassing. The reason for this can be attributed to the higher large melt- pool generated high laser power which causes the melt-pool to stay longer on the surface of the melt-pool. The longer the melt-pool stays on the surface of the substrate, the more the substrate gets melted and mixed with the deposited powder. This makes more of the deposited powder to be buried into the substrate and hence the higher dilution depth seen at a higher laser power. The average ssurface roughness is seen to reduce as the laser power was increase.
This is due to the slower cooling of the melt pool created at higher laser power. Certain degree of dilution is required to create a sound metallurgical bonding between the deposited powder and the substrate. Too much of the dilution can make the laser metal deposition process to become unstable and with poor dimensional accuracy. The optimum laser power of about 2.63 kW and powder flow rate of 2.88 g/min will ressult in minimum dilusion and optimum average surface roughness.
Table 3. Results with varying laser power
Sample No. Laser power (kW)
Powder flow rate (g/min)
Dilution depth (μm)
Average surface roughness (μm)
A1 3.0 2.88 1460.00 13.16
A2 2.6 2.88 1259.52 16.70
A3 2.2 2.88 1214.01 18.70
A4 1.8 2.88 1192.11 20.21
The result of the varying powder flow rate and the resulting dilution depth and the average surface roughness are presented in Table 4. The graph of the dilution depth and the average surface roughness against the powder flow rate is shown in Fig. 2. The graph showed that as the powder flow rate is increasing, the dilution depth is seen to be decreasing. This is as a result of the more powder being available for melting at higher powder flow rate that prevent the substrate to be melted more than necessary which always cause excessive dilution .
Table 4. Results with varying powder flow rate
Sample No. Powder flow rate (g/min)
Laser power (kW)
Dilution depth (μm)
Average surface roughness (μm)
A1 2.88 3.0 1460.10 14.5
A2 4.32 3.0 1028.11 15.7
A3 5.04 3.0 0900.01 17.8
A4 5.76 3.0 0863.25 21.14
On the other hand, the average surface roughness is seen to increase as the powder flow rate is increased. The reason for this is that at low powder flow rate, the available laser power was able to properly melt the delivered powder that is responsible for the low surface roughness observed at low powder flow rate. The high surface roughness seen at high powder flow rate could be as a result of improper melting of the deposited powder. The optimum powder flow rate with minimum dilution and average surface roughness is about 4.6 g/min at a laser power of 3.0 kW.
IV. CONCLUSION
In this study, the influence of laser power and powder flow rate on the dilution depth and the average surface roughness has been thoroughly investigated. The laser power was seen to have great influence on the degree of dilution as well as the surface roughness produced. Dilution is desired for the proper bonding of the deposited powder and the substrate but too much dilution is not desirable. The dilution increases with increase in laser power and the surface roughness decrease with increase in laser power.
The increase in powder flow rate is found to result in decrease in the degree of dilution while the surface roughness increases as the powder flow rate increases.
ACKNOWLEDGEMENTS
This work was supported by University of Johannesburg research council and the L'OREAL-UNESCO for Women in Science.
REFERENCES
[1] A. Kobryn, and A.L. Semiatin, Laser Forming of Ti-6Al-4V:
Research Overview, Solid Freeform Fabrication Symposium Proceeding. 2000, pp. 58-65.
[2] J. Pinkerton, W. Wang, and L. Li, Component repair using laser direct metal deposition, Journal of Engineering Manufacture. 222 (2008) 827-836.
[3] R. M. Mahamood, E. T. Akinlabi, M. Shukla and S. Pityana.
Characterization of Laser Deposited Ti6A4V/TiC Composite. Lasers in Engineering, 29(3-4) (2014) 197-213.
[4] R. M. Mahamood, E. T. Akinlabi, M. Shukla and S. Pityana, Functionally graded material: An overview. Proceedings of the World Congress on Engineering, WCE 2012, July 4 - 6, 2012, London, U.K.
3(2012)1593-1597.
[5] R.M. Mahamood, E.T. Akinlabi, Laser metal deposition of functionally graded Ti6Al4V/TiC, Materials & Design, 84 (2015) 402-410.
[6] W. Liu, J.N. DuPont, Fabrication of functionally graded TiC/Ti composites by Laser Engineered Net Shaping, Scripta Materialia. 48 (2003) 1337–1342.
[7] A. Zhong, T. Biermann, A. Gasser and R. Poprawe, Experimental study of effects of main process parameters on porosity, track geometry, deposition rate, and powder efficiency for high deposition rate laser metal deposition, Journal of Laser Application. 27 (2015) http://dx.doi.org/10.2351/1.4923335
[8] S. L. Campanelli, A. Angelastro, C. G. Signorile, and G. Casalino, Investigation on direct laser powder deposition of 18 Ni (300) marage steel using mathematical model and experimental characterisation, The International Journal of Advanced Manufacturing Technology.
(2016) 1—11, doi="10.1007/s00170-016-9135-x",
[9] R. M. Mahamood, E. T. Akinlabi, M. Shukla and S. Pityana, Characterizing the Effect of Processing Parameters on the Porosity
Properties of Laser Deposited Titanium Alloy, International Multi- conference of Engineering and Computer Science (IMECS 2014), (2014),
[10] K. Zhang, X. M. Zhang, W. J. Liu, Influences of Processing Parameters on Dilution Ratio of Laser Cladding Layer during Laser Metal Deposition Shaping, Advanced Materials Research. 549 92012) 785-789
[11] R. M. Mahamood, E. T. Akinlabi, Effect of Laser Power on Surface Finish during Laser Metal Deposition Process, Proceedings of World Congress on Engineering and Computer Science. 2 (2014) 965-969.
[12] R. M. Mahamood, E. T. Akinlabi, M. Shukla and S. Pityana, Characterizing the Effect of Laser Power Density on Microstructure, Microhardness and Surface Finish of Laser Deposited Titanium Alloy, Journal of Manufacturing Science and Engineering. 135(6) (2013). doi:10.1115/1.4025737.
[13] R. M. Mahamood, E. T. Akinlabi, Effect of Scanning Speed and Gas Flow Rate on Surface Roughness of LMD Titanium-alloy, Proceedings of the World Congress on Engineering and Computer Science, vol. 2. (2016).
[14] S. Pityana, R. M. Mahamood, E. T. Akinlabi, and M. Shukla, Effect of powder flow rate and gas Flow Rate on Properties of Laser Metal Deposited Ti6Al4V. 2013 International Multi-conference of Engineering and Computer Science (IMECS 2013), (2013) 848-851.
[15] R. M. Mahamood, E. T. Akinlabi, M. Shukla and S. Pityana,.
Material Efficiency of Laser Metal Deposited Ti6Al4V: Effect of Laser Power. Engineering Letters, 21:1 (2016). EL_21_1_03,
available online at http://www.engineeringletters.com/issues_v21/issue_1/EL_21_1_0
3.pdf
[16] R.M. Mahamood, E.T. Akinlabi, Process Parameters Optimization for Material Deposition Efficiency in Laser Metal Deposited Titanium Alloy, Lasers in Manufacturing and Materials Processing, 3(1) (2016) 9-21. DOI 10.1007/s40516-015-0020-5
[17] BS EN ISO 4288:1998, Geometric product specification (GPS).
Surface texture. Profile method: Rules and procedures for the assessment of surface texture, BSI.
[18] E3−11 (2011). Standard Guide for Preparation of Metallographic Specimens, ASTM international Book of Standards, vol. 03.01.
Figure 1. The graph of dilution depth and average surface roughness against the laser
Figure 2. The graph of dilution depth and average surface roughness against the powder flow rate.
