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JWJ > Volume 37(6); 2019 > Article
Nursyifaulkhair, Park, Baek, and Lee: Effect of Process Parameters on the Formation of Lack of Fusion in Directed Energy Deposition of Ti-6Al-4V Alloy

Abstract

The effect of different process parameters on the formation of lack of fusion (LOF) in Ti-6Al-4V alloy fabricated using DED was studied. The specific energy was calculated to evaluate the minimal amount of required energy to avoid LOF. The results showed that a specific energy smaller than 2700 J.g-1 led to the formation of LOF; however, a higher specific energy was able to successfully prevent the defect. A smaller amount of specific energy resulted in inadequate heat, which was too low to completely melt the metal powder, and resulted in insufficient penetration depth. Subsequently, LOF was observed between the interface of the deposited layers. Furthermore, a higher powder feed rate had a more significant effect on the formation of LOF than higher laser power.

1. Introduction

Lack of fusion (LOF) is identified as an irregular and large porosity with the sharp edges, as a result of incomplete adherence of the melt to the surrounding part1). The formation of lack of fusion (LOF) in the most utilized titanium alloy, Ti-6Al-4V, can act as a stress concentration2,3), which extremely lowered ultimate tensile strength and elongation3). Furthermore, the occurrence of this defect also yielded a large anisotropy in the strength and the elongation of Ti-6Al-4V alloy4,5). In general, the LOF can be found in the products of conventional welding and additive manufacturing (AM). In the conventional welding, the defect generally forms at the interfaces between the filler and the base metal or between the different layers of the filler metal6). Similarly, the LOF in the AM occurs at the interfaces of the deposited layers.
The AM is a promising technology to produce near- shape components and reduce materials waste7). This technology generally is distinguished into directed energy deposition (DED) and powder bed fusion (PBF). Compared to PBF, DED has a higher productivity because of its faster process8,9). In the DED, there are several process parameters, such as laser power, travel speed, and powder feed rate, which influence the presence of the LOF10). The layer by layer technique is able to introduce the LOF at the interface of layers, and as a result, it is difficult to produce a fully dense component with the consistent mechanical properties11). Therefore, in this work, we focused on investigating the effect of different DED process parameters on the formation of LOF in Ti-6Al-4V alloy. Furthermore, we also calculated the specific energy from different process conditions during DED of Ti-6Al-4V alloy in this work and the other published papers to confirm the results. Hence, the minimal required specific energy to eliminate the formation of LOF in the DED of Ti-6Al-4V alloy can be obtained.

2. Experimental Method

The Ti-6Al-4V AM specimen was manufactured using a DED process by depositing the metal powder onto a substrate of the same alloy. The metal powder with the average size of approximately 90 ㎛ was produced using plasma atomization method (Fig. 1). During the manufacture, a single mode IPG fiber laser of 700, 800, and 900 W was utilized with the focal length of 9 mm, the beam diameter of 1 mm and the zigzag scanning method. Furthermore, the powder feed rate was varied to 3, 6, and 9 g·min-1. The travel speed was set at 900 mm·min-1 and the Ar shielding gas was flowed at 3 L·min-1 to protect melt pool from the contaminant. Table 1 summarizes the process conditions applied in this DED fabrication.
Fig. 1
(a) Ti-6Al-4V powder and (b) size distribution with average size of around 90 ㎛
jwj-37-6-579-g001.jpg
Table 1
Summary of DED process parameters to build the Ti-6Al-4V alloy in this work
No. Name of specimen Laser power (W) Powder feed rate (g·min-1) Travel speed (mm·min-1)
1. 700-3 700 3 900
2. 700-6 700 6 900
3. 700-9 700 9 900
4. 800-3 800 3 900
5. 800-6 800 6 900
6. 800-9 800 9 900
7. 900-3 900 3 900
8. 900-6 900 6 900
9. 900-9 900 9 900
After the manufacture, the specimens were prepared for an optical observation. The specimens were cut in a cross-sectional direction, followed by a grinding using SiC paper and a mechanical polishing using 1-㎛ diamond suspension. An optical microscopy (OM) examination was conducted to observe the presence of LOF in the specimens. Furthermore, the specific energies from the process parameters in this work and the other works were measured to find the minimal specific energy for avoiding the LOF. The specific energy (J·g-1) was calculated using these following formulas which were used in the laser cladding process12):
(1)
Massfeedperunitlength=mv
(2)
Linearenergyinput=αPv
(3)
Specificenergy=LinearenergyinputMassfeedperunitlength
From the Eq. (1)-(2), m, v, α, and P are the powder feed rate (g·min-1), the travel speed (mm·min-1), the laser absorption coefficient of 0.2713), and the laser power (W), respectively. The specific energy is the linear energy input divided by the mass feed per unit length, as described in the Eq. (3). Moreover, the value of specific energy was used to evaluate the maximal melt pool temperature (T3) using the Eq. (4):
(4)
T3Qmc1(T2T1)Lc2+T2
where Qm is the specific energy (J·g-1), c1 and c2 are the specific heat capacities of Ti-6Al-4V alloy at room temperature and melting temperature (J·g-1·K-1), respectively. Moreover, T1 and T2 are the initial temperature of Ti-6Al-4V powder (assuming at room temperature or 298 K) and the melting temperature of Ti-6Al-4V (1877 K14)), respectively, and L is the latent heat of Ti-6Al-4V alloy (J·g-1). The maximal melt pool temperature is the peak temperature arising at the center of the heated melt pool15). This temperature is relatively higher than the melting temperature (T2) of Ti-6Al-4V. It can affect the temperature of previous layer to obtain the melting point or even higher, thus making a good fusion between the melt pool and the previous layer. On the other hand, the minimal temperature at the rear boundary of the melt pool was assumed to be at the melting temperature. Hence, the mean melt pool temperature was estimated by calculating the average of the maximal melt pool temperature and the melting temperature of Ti-6Al-4V alloy15).

3. Results

Fig. 2a shows the overview of an as-built DED Ti- 6Al-4V specimen which represents the presence of LOF between the interface of deposited layers. Furthermore, the OM images of all specimens with the different process parameters are shown in Fig. 2b. It was observed that the LOF was not formed in the specimens of 700-3, 800-3 and 900-3, however, the other specimens with the higher powder feed rates suffered from the defect. It implies that an increase in laser power did not affect extensively on avoiding the formation of LOF in this experiment. On the other hand, a higher powder feed rate contributed significantly to the formation of the defect. This might indicate that a higher volume of the metal powder cannot be fully melted to obtain a good interlayer bonding.
Fig. 2
(a) As-built condition of Ti-6Al-4V in the specimen 900-9 representing the formation of LOF between the layers, designated by the white arrows. (b-j) OM images of all specimens showing the LOF in the specimens with the feed rates of 6 and 9 g·min-1
jwj-37-6-579-g002.jpg
The values of the specific energy, the mean melt pool temperature, and the formation of LOF for the specimens are listed in Table 2. It shows the specific energies of 1260-2430 J·g-1 and the mean melt pool temperatures of 1915-2525 K resulted in the formation of the defect. On the other hand, at the specific energies of 3780-4320 J·g-1 with the mean melt pool temperatures of 3229- 3510 K, the presence of LOF was not found in the specimens. Moreover, it shows a critical range of specific energy between 2430 and 3780 J·g-1 where the LOF can still be formed.
Table 2
Results of the specific energy and the mean melt pool temperature calculations, and the presence of LOF in this work
No Name of specimen Specific energy (J·g-1) Initial temperature of Ti-6Al-4V powder (K) Melting temperature of Ti-6Al-4V14)(K) Mean melt pool temperature (K) Presence of LOF
1. 700-3 3780 298 1877 3229 No
2. 700-6 1890 298 1877 2243 Yes
3. 700-9 1260 298 1877 1915 Yes
4. 800-3 4320 298 1877 3510 No
5. 800-6 2160 298 1877 2384 Yes
6. 800-9 1440 298 1877 2009 Yes
7. 900-3 4860 298 1877 3792 No
8. 900-6 2430 298 1877 2525 Yes
9. 900-9 1620 298 1877 2103 Yes

4. Discussion

The LOF was formed in the specimens because of a smaller amount of specific energy applied during the DED process16). It caused the metal powder incompletely melted owing to lower melt pool temperature, and this condition led to an insufficient penetration depth to the previous layer4,17), consequently, introducing an interlayer porosity at the interface of deposited layers. Therefore, it is important to yield an adequately mean pool temperature for fully melting the powder and achieving an appropriate penetration depth with the pre-existing layer or substrate. Subsequently, the lack of fusion between the deposited layers can be eliminated.
Furthermore, the specific energies from the DED process conditions of Ti-6Al-4V in the other works11,18,20) were also calculated to confirm the minimal specific energy to avoid the LOF. In the Table 3, it is noticed that the LOF was formed in the specimen at the specific energy of 2661 J·g-1 with the melt pool temperature of 2646 K. Hence, it indicates that the LOF can still be formed in the critical specific energy range of 2430- 3780 J·g-1. Meanwhile, at the greater specific energy, 2700 J·g-1, the defect was not formed in the DED Ti- 6Al-4V.
Table 3
The process parameters and the formation of LOF in DED of Ti-6Al-4V from the other works11,18,20) to estimate the specific energies and the mean melt pool temperatures
Name Laser power (W) Travel speed (mm·min-1) Feed rate (g·min-1) Specific energy(J·g-1) Initial temperature of Ti-6Al-4V powder (K) Melting temperature of Ti-6Al-4V14) (K) Mean melt pool temperature (K) Presence of LOF Ref.
R 1 2000 636 8.0 4050 298 1877 3370 No 11)
R 2 250 408 2.8 1473 298 1877 2026 Yes 18)
R 3 250 510 2.8 1473 298 1877 2026 Yes 18)
R 4 300 408 2.8 1767 298 1877 2179 Yes 18)
R 5 400 635 2.4 2700 298 1877 2666 No 19)
R 6 1150 800 7.0 2661 298 1877 2646 Yes 20)
R 7 1265 800 7.0 2928 298 1877 2784 No 20)
R 8 1410 800 6.5 3514 298 1877 3090 No 20)
R 9 1440 685 15.5 1505 298 1877 2043 Yes 20)
The relationship between the specific2043 energy, the mean melt pool temperature and the formation of LOF are shown in Fig. 3. The critical specific energy of LOF formation in this work ranged between 2430 and 3780 J·g-1 (Fig. 3a), thus the minimal specific energy to utterly avoid the defect was 3780 J·g-1. However, after confirming the results with the specific energies calculated from the process parameters in the other works, the critical range was shifted to 2661-2700 J·g-1 (Fig. 3b). Hence, the minimal specific energy to successfully prevent the formation of LOF was 2700 J·g-1 with the mean melt pool temperature of 2666 K.
Fig. 3
Dependence of the LOF formation on the specific energy and the mean melt pool temperature according to the process conditions in (a) this work and (b) the other works11,18,20). The green area in graphs indicates the critical range in which the LOF can be formed
jwj-37-6-579-g003.jpg
Furthermore, in the Fig. 4, it can also be noticed that the combination of a relatively high linear energy input and a lower mass feed per unit length would minimize the probability of LOF formation. Linear energy input was calculated by dividing the absorbed laser power with the travel speed, as described in the Eq. (2). Meanwhile, the mass feed per unit length was obtained by dividing the powder feed rate with the travel speed (Eq. (3)). Since the travel speed was a constant value, the linear energy input was more influenced by the laser power (Eq. 2), and the mass feed per unit length was more affected by the powder feed rate (Eq. 1). Therefore, the process condition of a higher laser power and a lower powder feed rate should be considered to obtain a proper specific energy, and thus, eliminating the formation of LOF.
Fig. 4
Influence of mass feed per unit length and linear energy input on the formation of LOF
jwj-37-6-579-g004.jpg

5. Conclusion

The effect of process parameters on the formation of LOF in the Ti-6Al-4V using DED has been successfully investigated. From the current study, the conclusions are as follows:
1) In this work, the powder feed rate was more critical on the formation of LOF, rather than the laser power.
2) The insufficient specific energy was confirmed as the cause of the formation of LOF in the DED Ti-6Al- 4V alloy.
3) The critical specific energy was ranged between 2430 and 3780 J·g-1 in this work, in which the LOF probably can be formed. However, after calculating the specific energy from the various process parameters in the other works, the critical range was shifted to 2661 and 2700 J·g-1.

Acknowledgement

This study was supported by the Yeungnam University research grant 2018.

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