Lasers Manuf. Mater. Process. (2016) 3:9–21 DOI 10.1007/s40516-015-0020-5
Processing Parameters Optimization for Material Deposition Efficiency in Laser Metal Deposited Titanium Alloy Rasheedat M. Mahamood 1,2
& Esther
T. Akinlabi 1
Accepted: 14 October 2015 / Published online: 9 November 2015 # Springer Science+Business Media New York 2015
Abstract Ti6Al4V is an important Titanium alloy that is mostly used in many applications such as: aerospace, petrochemical and medicine. The excellent corrosion resistance property, the high strength to weight ratio and the retention of properties at high temperature makes them to be favoured in most applications. The high cost of Titanium and its alloys makes their use to be prohibitive in some applications. Ti6Al4V can be cladded on a less expensive material such as steel, thereby reducing cost and providing excellent properties. Laser Metal Deposition (LMD) process, an additive manufacturing process is capable of producing complex part directly from the 3-D CAD model of the part and it also has the capability of handling multiple materials. Processing parameters play an important role in LMD process and in order to achieve desired results at a minimum cost, then the processing parameters need to be properly controlled. This paper investigates the role of processing parameters: laser power, scanning speed, powder flow rate and gas flow rate, on the material utilization efficiency in laser metal deposited Ti6Al4V. A two-level full factorial design of experiment was used in this investigation, to be able to understand the processing parameters that are most significant as well as the interactions among these processing parameters. Four process parameters were used, each with upper and lower settings which results in a combination of sixteen experiments. The laser power settings used was 1.8 and 3 kW, the scanning speed was 0.05 and 0.1 m/s, the powder flow rate was 2 and 4 g/min and the gas flow rate was 2 and 4 l/min. The experiments were designed and analyzed using Design Expert 8 software. The software was used to generate the optimized process parameters which were found to be laser power of 3.2 kW, scanning speed of 0.06 m/s, powder flow rate of 2 g/min and gas flow rate of 3 l/min.
* Rasheedat M. Mahamood
[email protected] 1
Department of Mechanical Engineering Science, University of Johannesburg, Johannesburg 2006, South Africa
2
Department of Mechanical Engineering, University of Ilorin, Ilorin 23400003, Nigeria
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Lasers Manuf. Mater. Process. (2016) 3:9–21
Keywords Additivemanufacturing . Deposition efficiency . Design ofexperiment . Laser metal deposition . Processing parameters . Titanium alloy
Introduction Titanium and its alloys are characterized by their exciting properties which include: high strength-to-weight ratio, high corrosion resistance and they retain these properties even at elevated temperature [1]. Ti6Al4V is the most widely used Titanium alloy which is often referred to as the workhorse in the Titanium industry [2]. Ti6Al4V finds their applications in many industries which include: aerospace, petrochemical and as medical implants in medical industry [3]. The use of Ti6Al4V is still prohibitive in most industries because of the high cost of the material. This burden can be removed and the application area of Ti6Al4V can be expanded by cladding it with less expensive material and using efficient manufacturing method. This will reduce the cost of the part and the part can benefit from the exciting properties of the Ti6Al4V. Laser Metal Deposition (LMD) process is an additive manufacturing process that is capable of producing parts directly from its three-Dimensional (3-D) Computer Aided Design (CAD) model of the part, no matter the complexity, simply by adding material in a layer-wise manner and in one single step [4]. LMD is also a flexible manufacturing process that can handle multiple materials and can be used to make parts with functionally graded material. LMD is also very useful in the repair of high valued parts [5] with minimum heat affected zone. A lot of research work on LMD has appeared in the literature [5–16]. Some of these research focus on studying the processing parameters influence on the microstructure and properties of the deposited part [5–8, 11]. Considering the fact that some materials are recoverable in LMD process, Titanium has high affinity for oxygen at high temperature [17] and so, it cannot be re-used because of contamination. It is therefore important to minimize material wastage in LMD process, as this will affect the economy of the process. This is only possible with proper combination of processing parameters. The effect of processing parameters on the economy of the material was less explored in the literature. The few researches done in this area were done qualitatively [9, 10, 12]. Most of these studies did study few processing parameters and statistical design of experiments were not employed, which would help in the developing a model where statistical inference could be drawn. In this study, full factorial design of experiment was used to investigate the influence of processing parameters on the material deposition efficiency in the cladding of engineering materials. The processing parameters that achieve this, forms the lower levels of the processing parameters used in this study. Each factor was set at high and low levels and the experiment was designed using, a powerful statistical software Design Expert 8. Four processing parameters were studied namely: laser power, scanning speed, powder flow rate and gas flow rate (powder carrier gas flow rate). The software generated a total of sixteen (16) experiments and the results were also analyzed using the software. The experiments were run twice, and a total of thirty two (32) samples were produced and the average results are reported. The initial studies that were conducted informed the high and low process parameters setting used in this study [9, 10]. In these previous studies, single process parameter was investigated in each of
Lasers Manuf. Mater. Process. (2016) 3:9–21
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the studies, in order to establish the process window that achieve sound deposit with good metallurgical integrity with the substrate and with no porosity and with minimum dilution. The processing parameters that achieve these were used as a guide for the low and high level settings of the processing parameters used for this study. This paper investigates a combination of four process parameters as against one, in the previous studies so as to draw a valid statistical inference from this present study. The results are presented and explained in detail.
Experimental Procedure Ti6Al4V powder of particle size ranging between 150 and 200 μm and of 99.5 % purity was used in this investigation. The substrate material is also Ti6Al4V of 99.6 % purity and with a dimension of dimension 72 mm×72 mm and 5 mm thick sheet. Before the deposition process, the substrate was sandblasted and cleaned with acetone to degrease the surface and also to aid the laser absorption. Argon gas was used to transport the powder from the powder hopper through to the nozzle. The SEM micrograph of the Ti6Al4V powder is shown in Fig. 1a. The laser metal deposition process was achieved by a Kuka robot carrying the Nd-YAG laser head and the coaxial powder nozzles on its end effector. The laser used was an NdYAG laser of maximum power of 4.0 kW. The laser sport size was kept at 2 mm at a focal length of 195 mm above the substrate. The laser beam creates a melt-pool on the surface of the substrate and the Ti6Al4V powder was then delivered onto the melt- pool through the co-axial powder nozzles. Upon solidification, a solid track of the Ti6Al4V was seen on the laser path on the substrate. Figure 1b shows the schematic of the LMD process. The processing parameter used in this work was generated using the setting in Table 1. The settings were entered in to the Design Expert 8 software for two-level full factorial design and the software generated the sets of processing parameters (see Table 2). A length of 0.060 m track was deposited at each set of the processing parameters. The substrate was weighed using digital weighing balance before each deposition began. And after each deposition process using the set of processing parameters, the deposited track and the substrate were cleaned with wire brush to remove any stung unmelted Ti6Al4V powder particles and re-weighed. The difference in mass is the mass of the deposited Ti6Al4V powder. The experiment was performed twice at each settings and the average result is reported. The Actual mass of the delivered powder through the powder nozzle was determined using Eq. 1. M po ¼ P f r xT D
ð1Þ
Where Mpo is the mass of powder delivered in (g), Pfr is the powder flow rate in (g/s) and TD is the deposition time in (sec). The deposition time is also given in Eq. 2 as TD ¼ L=Ss
ð2Þ
Where L is the length of the deposited track in (m), and Ss is the scanning speed in (m/s).
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Fig. 1 a SEM micrograph of Ti6Al4V powder [18] b Schematic of the laser metal deposition process (adapted from Mahamood et al. [19]) c Picture of some of the deposited samples
Percentage material loss was determined using Eq. 3. % material loss ¼
Mpo −Mp f =Mpo 100
Where Mpf is the mass of the deposited powder
ð3Þ
Lasers Manuf. Mater. Process. (2016) 3:9–21
Table 1 Processing parameters settings
13
S/N
Factor
High level
Low level
1
Laser Power (kW)
3.0
1.8
2
Scanning Speed (m/s)
0.1
0.05
3
Powder Flow Rate (g/min)
4
2
4
Gas Flow Rate (l/min)
4
2
Results and Discussion The picture of some of the deposited track of the samples at different sets of processing parameters is shown in Fig. 1c. The results are presented in Table 2. The results were analyzed using the Design Expert 8 software. A regression model was built using the software. The analysis of variance (ANOVA) for the selected factorial model is presented in Table 3. The Model’s F-value of 9373.44 and the p-value of 0.0081 imply that the model is significant. And that there is only a 0.81 % chance that an F-value this large could occur due to noise. Values of BProb>F^ less than 0.0500 indicates that the model terms are significant. From Table 2 A, B, C, D, AB, AC, BC, BD, ABD are significant model terms. Values greater than 0.1000 indicate the model terms are not significant. The standard deviation and RSquare values are presented in Table 4.
Table 2 Results Gas flow TD (s) Mpo (g) Mpf Std. Laser Scanning Powder Powder order power speed flow rate flow rate rate (l/min) =0.06/Ss =Pfrx TD (kW) (m/s) (g/min) (Pfr) (g/s)
% loss =((Mpo - Mpf)/ Mpo)×100
1
1.80
0.05
2
0.03
2
1.2
0.04
0.030 25
2
3.00
0.05
2
0.03
2
1.2
0.04
0.035 12.5
3
1.80
0.1
2
0.03
2
0.6
0.02
0.010 50
4
3.00
0.1
2
0.03
2
0.6
0.02
0.016 20
5
1.80
0.05
4
0.07
2
1.2
0.08
0.042 47.5
6
3.00
0.05
4
0.07
2
1.2
0.08
0.060 25
7
1.80
0.1
4
0.07
2
0.6
0.04
0.013 68
8
3.00
0.1
4
0.07
2
0.6
0.04
0.028 30
9
1.80
0.05
2
0.03
4
1.2
0.04
0.028 30
10
3.00
0.05
2
0.03
4
1.2
0.04
0.036 11
11
1.80
0.1
2
0.03
4
0.6
0.02
0.011 45
12
3.00
0.1
2
0.03
4
0.6
0.02
0.017 17
13
1.80
0.05
4
0.07
4
1.2
0.08
0.045 43.75
14
3.00
0.05
4
0.07
4
1.2
0.08
0.060 25
15
1.80
0.1
4
0.07
4
0.6
0.04
0.016 60
16
3.00
0.1
4
0.07
4
0.6
0.04
0.030 25
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Table 3 Analysis of variance (ANOVA) Source
Sum of squares
df
Mean square
F value
P-value Prob>F
Model
4613.49
14
329.54
9373.44
0.0081
A-laser power
2855.57
1
2855.57
81225.00
0.0022
B-scanning speed
454.22
1
454.22
12920.11
0.0056
C-powder flow rate
957.13
1
957.13
27225.00
0.0039
D-gas flow rate
7.91
1
7.91
225.00
0.0424
AB
145.50
1
145.50
4138.78
0.0099
AC
75.47
1
75.47
2146.78
0.0137
AD
3.75
1
3.75
106.78
0.0614
BC
29.57
1
29.57
841.00
0.0219
BD
59.10
1
59.10
1681.00
0.0155
CD
0.32
1
0.32
9.00
0.0948
ABC
1.41
1
1.41
40.11
0.0997
ABD
19.69
1
19.69
560.11
0.0269
ACD
0.098
1
0.098
2.78
0.0740
BCD
3.75
1
3.75
106.78
0.0614
Residual
0.035
1
0.035
Cor. Total
4613.53
15
significant
The BPredicted R-Squared^ of 0.9980 is in reasonable agreement with the BAdjusted R-Squared^ of 0.9999; i.e., the difference is less than 0.2. BAdeq Precision^ measures the signal to noise ratio. A ratio greater than 4 is desirable. A ratio of 313.970 indicates an adequate signal and this model can be used to navigate the design space. The model terms properties are presented in Table 5. The Model in terms of the coded factor is presented in Eq. 4 and the model in terms of the actual factors is presented in Eq. 5. % Material loss ¼ þ 34:05–13:36*A þ 5:33*B þ 7:73*C−0:70*D−3:02*AB ð4Þ −2:17*AC−0:48*AD−1:36*BC−1:92*BD −0:14*CD þ 0:30*ABC þ 1:11*ABD þ 0:078*ACD−0:48*BCD
Where A=Laser Power, B=Scanning Speed, C=Powder Flow Rate, D=Gass Flow Rate
Table 4 Model standard deviation and R-squared
Std. Dev.
0.19
R-squared
1.0000
Mean
34.05
Adj R-Squared
0.9999
C.V. %
0.55
Pred R-Squared
0.9980
PRESS
9.00
Adeq Precision
313.970
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Table 5 Model terms and properties Factor Intercept
Coefficient estimate
df
Standard error
95 % CI low
95 % CI high
VIF
34.05
1
0.047
33.45
34.64
−13.36
1
0.047
−13.95
−12.76
1.00
B-scanning speed
5.33
1
0.047
4.73
5.92
1.00
C-powder flow rate
7.73
1
0.047
7.14
8.33
1.00
D-gas flow rate
−0.70
1
0.047
−1.30
−0.11
1.00
AB
−3.02
1
0.047
−3.61
−2.42
1.00
AC
−2.17
1
0.047
−2.77
−1.58
1.00
AD
−0.48
1
0.047
−1.08
0.11
1.00
BC
−1.36
1
0.047
−1.95
−0.76
1.00
BD
−1.92
1
0.047
−2.52
−1.33
1.00
CD
−0.14
1
0.047
−0.74
0.45
1.00
ABC
0.30
1
0.047
−0.30
0.89
1.00
ABD
1.11
1
0.047
0.51
1.70
1.00
ACD
0.078
1
0.047
−0.52
0.67
1.00
1
0.047
−1.08
0.11
1.00
A-laser power
BCD
−0.48
Final Equation in Terms of Actual Factors: % Material loss ¼ −88:81250 þ 28:35938*Laser Power þ 1590:00000*Scanning Speed þ 21:06250*Powder Flow Rate þ 17:31250*Gas Flow Rate −482:29167*Laser Power*Scanning Speed −5:49479*Laser Power*Powder Flow Rate ð5Þ −6:74479*Laser Power*Gas Flow Rate −43:75000*Scanning Speed*Powder Flow Rate −196:25000*Scanning Speed*Gas Flow Rate þ 1:00000*Powder Flow Rate*Gas Flow Rate þ 19:79167*Laser Power*Scanning Speed*Powder Flow Rate þ 73:95833*Laser Power*Scanning Speed*Gas Flow Rate þ 0:13021*Laser Power*Powder Flow Rate*Gas Flow Rate −19:37500*Scanning Speed*Powder Flow Rate*Gas Flow Rate The graph of predicted versus actual experimental values is shown in Fig. 2. It is seen from the graph that, the model is a good representation of the actual experimental data. The main effects plot of laser power is shown in Fig. 3a. The laser power has a negative slope which means that it has a negative effect on the percentage material loss. That is, the higher the laser power, the lower the material loss. The reason for this is that, at higher laser power there was more melting of the Ti6Al4V powder taken place, thereby minimizing the unmelted powder particles that resulted in low material wastage. The main effect plot of scanning speed is shown in Fig. 3b. The scanning speed
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Lasers Manuf. Mater. Process. (2016) 3:9–21
Fig. 2 The graph of predicted versus actual
Fig. 3 The main effect plots of: a laser power b scanning speed c powder flow rate d gas flow rate
Lasers Manuf. Mater. Process. (2016) 3:9–21
17
has a positive slope, which means that the scanning speed has a positive effect on the material loss. It indicates that at lower scanning speed, there was more time for the laser material interaction and for the proper melting of the Ti6Al4V powder thereby resulting in lower material wastage. On the other hand, at higher scanning speed, there was no sufficient time for the powder to properly melt thereby producing more unmelted powder that results in high material wastage. The main effect plot of the powder flow rate is shown in Fig. 3c. The powder flow rate also has a positive slope meaning a positive effect on the material loss. The lower the powder flow rate, the lower the material loss. This is because; the available laser power could effectively melt the delivered powder. Thus, minimizing the quantity of unmelted powder and reducing the material wastage. The higher the powder flow rate, the higher the material loss. This is because, as the powder flow rate was increased, the available laser power was not sufficient to properly melt the powder and hence more unmelted powder particles which resulted in high material wastage. Figure 3d shows the main effect plot of the gas flow rate. The gas flow rate has the least significant effect on material loss, although at higher gas flow rate, the material loss was a bit lower than at lower gas flow rate. It may be attributed to the fact that, at high gas flow rate, the gas helps to deliver enough powder to the melt-pool area on time, thereby causing most of the powder particle to take part in the melting process. Hence, this would result in a bit higher material efficiency. The interaction plot of laser power and scanning speed is shown in Fig. 4a. There is a very strong interaction between the laser power and the scanning speed. At high laser power and low scanning speed, the percentage material loss was at its lowest value. This is because, at high laser power and low scanning speed, there was sufficient laser power and the interaction time of the laser and material was also higher thereby providing enough time for the proper melting of the powder thereby minimizing material wastage. On the other hand, at low laser power and high scanning speed, there was insufficient laser power as well as insufficient laser material interaction time which caused improper melting of the Ti6Al4V powder and resulted in higher material loss. A similar interaction was also observed in the interaction plot of the laser power and powder flow rate as shown in Fig. 4b. The lower percentage material loss that was observed at high laser power and low powder flow rate was as a result of sufficient laser power for the available Ti6Al4V powder and that resulted in the proper melting of the powder; thereby reducing the material wastage. At low laser power and high powder flow rate there was insufficient laser power for proper melting of the high volume of powder delivered into the melt pool thereby resulting in large quantity of unmelted powder. There was no significant interaction between the laser power and the gas flow rate, as seen in the interaction plot of laser power and gas flow rate shown in Fig. 4c. There was interaction between the scanning speed and the powder flow rate as shown in Fig. 4d. At low scanning speed and low powder flow rate, the percentage material loss was low because there was a low quantity of powder delivered and the laser material interaction time was high for the proper melting of the Ti6Al4V powder, thereby resulting in the low percentage material loss. On the other hand, the material loss was higher at high scanning speed and high powder flow rate. This is because, the volume of powder delivered to the melt-pool was higher and that the laser material interaction time was low making it impossible for the proper melting of the Ti6Al4V powder which resulted in large quantity of unmelted powder and hence higher material wastage. The surface plot is shown in Fig. 5.
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Lasers Manuf. Mater. Process. (2016) 3:9–21
Fig. 4 Interaction plots between: a laser power and scanning speed, b laser power and powder flow rate, c laser power and gas flow rate and d scanning speed and powder flow rate
The surface plot of % loss against scanning speed and laser power is shown in Fig. 5a. The material loss is found to be very high at low laser power and high scanning speed because there is low interaction time for the Ti6Al4V powder and the laser at high scanning speed couple with low laser power resulted in improper melting of the powder and hence powder wastage. Material wastage is minimal at high laser power and low powder flow rate as shown in Fig. 5b. This is because proper melting of the Ti6Al4V powder occurs at these settings. Also, material wastage is minimal at high laser power and low gas flow rate as shown in Fig. 5c. The unmelted powder is reduced at high power and there is less powder blown away from the melt pool at low gas flow rate. Figure 5d shows the surface plot of % loss against scanning speed and powder flow rate. The material wastage is found to be increased as the scanning speed and the powder flow rate was increased. This is because at low scanning speed, the lasermaterial-interaction time was low thereby resulting in improper melting of the powder. Also the high powder flow rate does not permit proper melting of the powder, as the available laser power becomes insufficient to properly melt the delivered powder. The Design Expert software was used to obtain the optimized process parameters that minimizes material loss and found the optimized settings as:- laser power of 3.2 kW, scanning speed of 0.06 m/s, powder flow rate of 2 g/min and gas flow rate of 3 l/min. To demonstrate the robustness of the developed model, a set of experiment was performed at processing parameter settings outside those settings used in the model
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Fig. 5 Surface plots of % loss against a scanning speed and Laser power b powder flow rate and laser power c laser power and gas flow rate d scanning speed and powder flow rate
development. The experimental results are compared with those predicted by the model and are presented in Table 6. The results show that there is good agreement between the predicted values and the actual experimental results, the validation results show that the model is robust and it can be used to improve material efficiency in laser metal deposition process of Ti6Al4V.
Conclusion The material utilization efficiency in LMD process has been investigated using Ti6Al4V powder and Ti6Al4V substrate. Full factorial design of experiment was used in this Table 6 Validation results
S/N Laser Scanning Powder Gas flow Predicted Actual power speed flow rate rate (l/min) % loss % loss (kW) (m/s) (g/min) 1
2.4
0.05
2
2
18.75
18.02
2
2.00
0.075
3
4
42.58
43.21
3
2.80
0.05
3
3
21.82
21.14
4
1.60
0.05
2
3
30.13
29.86
5
3.20
0.06
2
3
10.03
11.02
6
3.00
0.06
3
3
19.3
19.65
20
Lasers Manuf. Mater. Process. (2016) 3:9–21
investigation to be able to capture all the main effects and all the interaction effects of the processing parameters. The experiment was designed and analyzed using the Design Expert 8 software. The processing parameters investigated are: laser power, scanning speed, powder flow rate and gas flow rate. The effect of these processing parameters on the percentage material loss was thoroughly studied. A model was developed to predict the percentage material loss as a function of the processing parameters. The model was validated by performing more experiment at setting different from those considered in the model development. The model was found to be in good agreement with the experimental data. The study revealed that the laser power, the scanning speed and the powder flow rate have significant main effect on the percentage material loss. While the gas flow rate has the least significant effect on the percentage material loss. Also, strong interactions were observed between the laser power and the scanning speed; between the laser power and the powder flow rate; and between the scanning speed and the powder flow rate. It can be concluded that, to minimize material wastage in laser metal deposition process, there is need for proper combinations of processing parameters. The model developed in this study can be used in order to reduce the material wastage in cladding of engineering material. The use of Ti6Al4V can be expanded by cladding it on less expensive materials and the model will still be valid since in cladding such materials, a number of layers will be required to be deposited. As it will be expected, the first few layers will be an alloy of the base material and the titanium alloy because of the dilution that is required for bonding. the subsequent layers will imitate the cladding on the same material that is similar to what was used to develop the model in this study. The future research will be to study the influence of these processing parameters on the deposition efficiency of three dimensional object.
Nomenclature
Mpo Pfr TD L Ss Mpf
Mpo is the mass of powder delivered The powder flow rate The deposition time Length of the deposited track The scanning speed Mass of the deposited powder
Acknowledgments This work is supported by the Council of Scientific and Industrial Research (CSIR) National Laser Centre, Rental Pool Program, Pretoria, South Africa.
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