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Development of a laminar flow local shielding device for wire + arc additive manufacture
Journal of Materials Processing Technology 226 (2015) 99–105
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Development of a laminar flow local shielding device for wire + arc additive manufacture J. Ding a,∗ , P. Colegrove a , F. Martina a , S. Williams a , R. Wiktorowicz b , M.R. Palt c a b c
Welding Engineering and Laser Processing Centre, Cranfield University, Cranfield MK43 0AL, UK Air Products PLC, Crockford Lane, Chineham, Basingstoke, RG24 8FE, UK Air Products Management SA./NV., Ch. de Wavre 1789, B-1160 Brussels, Belgium
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 12 June 2015 Accepted 10 July 2015 Available online 19 July 2015 Keywords: Local shielding Trailing shield Laminar flow Additive manufacture Titanium CFD
a b s t r a c t A shielded environment is required during the wire + arc additive manufacture (WAAM) of titanium alloys to prevent oxidation. Applying local shielding can increase the flexibility of the WAAM process, however conventional devices do not provide adequate protection due to entrainment of the surrounding air. In this study, a new local shielding device based on laminar flow was developed and compared with a conventional device. The laminar local shielding device showed up to three orders of magnitude improvement with contamination levels below 2000 ppm being achieved with a stand-off distance of 30 mm. The performance was also assessed along a mock-up WAAM wall which showed that it could be protected up to 30 mm from the top. Finally, computational fluid dynamics models provided insight into the device performance and enabled the performance of an argon knife to be evaluated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Additive manufacture (AM) techniques are receiving interest in the manufacturing community. As summarised by Frazier (2014), AM can potentially reduce production costs and can reduce lead times because it allows just-in-time manufacturing. This is especially the case for AM of structural materials, and titanium in particular. There are a variety of high-deposition rate processes that may be used for manufacturing large-scale AM parts where the heat may be provided by laser, electron beam or arcbased processes. The latter are often referred to as wire + arc additive manufacture (WAAM) and have the advantage of high build rates with comparatively low equipment cost. Martina et al. (2012) applied the WAAM process to titanium and showed that build rates of 2 kg/h could be achieved whilst maintaining high fidelity. Fig. 1 shows a typical WAAM system. As described by Williams et al. (2015), the WAAM process utilises an electric arc as a heat source and wire as feedstock. The arc-based welding process can be based on either metal inert gas (MIG) system, tungsten inert gas (TIG) systems, or plasma arc welding (PAW) system. The motion can
∗ Corresponding author. E-mail address: jialuo.ding@cranfield.ac.uk (J. Ding). http://dx.doi.org/10.1016/j.jmatprotec.2015.07.005 0924-0136/© 2015 Elsevier B.V. All rights reserved.
be provided either by robotic systems or by computer numerical controlled (CNC) systems. Parts are manufactured by depositing layers of metal with the welding-based deposition process. Babish (2007) claimed that when AM or welding titanium, the component needs to be shielded where the temperature exceeds 427 ◦ C to avoid oxidation of the surface. Contamination of titanium occurs primarily from oxygen which can reduce the ductility and toughness of the material. Baufeld et al. (2011) achieved this by performing the deposition process within an airtight chamber filled with argon (99.999%); however this becomes increasingly difficult to implement as the size of the part increases. The time required for evacuation of the chamber becomes long, problem solving is difficult and integrating other manufacturing processes such as machining or rolling becomes prohibitively expensive. Local or trailing shielding overcomes most of the shortcomings of a fixed chamber or tent and was used by Almeida and Williams (2010). However the devices developed for welding have an important shortcoming: they can only be used with a short stand-off distance from the workpiece. Beyond this, flow turbulence results in entrainment of the surrounding air which gives poor shielding and increases contamination—particularly on the side walls of the WAAM part. Therefore it was necessary to construct an additional structure beside the wall being built to reduce contamination. This involved considerable manual work which slowed the process, and constrained the shielding device to simple structures.
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Fig. 1. Robotic WAAM system. Fig. 3. Laminar flow local shielding device design.
Fig. 2. Cross-section of a conventional local shielding device (Babish, 2007).
A schematic diagram of a typical shielding device design used for welding from Babish (2007) is shown in Fig. 2. The device is packed with bronze or stainless steel wool as a diffusing medium which slows down and distributes the inlet gas. A metal mesh or porous plate made of copper or stainless steel is used at the outlet to support the wool and further diffuse the gas. Wada et al. (2012) investigated the performance of such devices with a computational fluid dynamics (CFD) model and found that the contamination level was critically dependent on the stand-off distance—the distance from the bottom of the shielding device to the workpiece. To improve the protection, it is critical to control the turbulence. Kulkarni et al. (2011) described in detail of using different honeycomb-screen combinations for turbulence management in a wind tunnel. And Bloch (1992) proposed a laminar flow water jet cutting nozzle which also used a combination of honeycomb flow straighteners with screens. This concept can be potentially borrowed in the design of the local shielding device for the WAAM processes. In this paper we describe the design of a new local shielding device for WAAM based on laminar flow. A preliminary experiment is performed to determine the contamination levels required by the device before evaluating the device performance. Finally, a computational model is used to aid understanding of the device performance. 2. Local shielding device design The new shielding device design is shown in Fig. 3 and consists of three parts. The first part is a diffusion chamber which is used to uniformly distribute the inlet gas. At the bottom of the diffusion chamber, two layers of 5 mm thick metal foam with an average hole size of 0.5 mm were placed 10 mm from the two gas inlets. The second part of the device is the honeycomb flow straightener. Kulkarni et al. (2011) stated that honeycomb can straighten
the flow and reduce large scale turbulence by reducing the lateral velocities. In addition, the length-to-diameter ratio had to be greater than eight to reduce the turbulence. The honeycomb used in this study had regular hexagonal cells with a size of 6 mm and a height of 90 mm giving a length-to-diameter ratio of 15. The third part of this device is a setting chamber followed by one layer of metal mesh. Kulkarni et al. (2011) stated that although honeycomb can significantly constrain the gas flow in the lateral direction the flow at the outlet fluctuates from near zero at the wall to a maximum in the centre of the honeycomb as shown in Fig. 3. This non-uniformity decreases with increasing downstream distance from the honeycomb end and Kulkarni et al. (2011) recommended using a downstream distance of three times the honeycomb cell size. Therefore the setting chamber height was 15 mm. Finally a layer of metal mesh was used at the end of the setting chamber to further improve the uniformity of the flow. In this study, a stainless steel square woven mesh was used with wire diameter of 0.2 mm and an aperture of 0.647 mm, which gave a mesh porosity of 0.42. The selection of this mesh was based on the pressure drop coefficient described by Chong et al. (2009). Overall, the device was 200 mm long and 50 mm wide. 3. Experimental procedure 3.1. Titanium WAAM wall with different oxygen levels in shielding gas Before evaluating the new shielding device performance, the environmental contamination level requirements were determined. Four titanium walls were manufactured in a rigid, sealed chamber with environmental contamination levels of ca. 30 ppm, 1000 ppm, 2000 ppm and 4000 ppm oxygen. This was achieved by filling the chamber with a combination of argon and oxygen through a gas mixing station which allowed the ratio between the two gases to be adjusted. A Migatronic plasma power supply was used to deposit 20 layers onto a 300 mm × 150 mm × 7 mm Ti-6Al-4V substrate. Ti-6Al-4 V wire with diameter 1.2 mm, whose composition is shown in Table 1, was used to deposit layers that were 250 mm long. The parameters for the plasma deposition process are shown in Table 2. The oxygen fractions of the four samples were measured by the inert gas fusion technique following ASTM E1409-13 (2013). Cross sections were extracted from each wall and the hardness was mea-
J. Ding et al. / Journal of Materials Processing Technology 226 (2015) 99–105
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Table 1 Chemical composition of Ti-6Al-4V wire used in the experiments. Ti
Al
V
Fe
O
C
N
H
TOE
Y
Others
89.397
6.14
3.96
0.18
0.14
0.02
0.011
0.001
<0.1
<0.001
<0.005
Table 2 Parameters for the plasma deposition process. Wire feed speed Travel speed Current Plasma gas flow-rate Shielding gas flow-rate (through the torch) Torch stand-off
2.4 m/min 4.5 mm/s 160 A 1 l/min 10 l/min 8 mm
Fig. 5. New laminar shielding device and setup for the contamination trials along the wall.
the novel laminar flow device described previously and a commercially available shielding device. The Argweld Typical Flat Trailing Shield whose length and width were identical to the laminar shielding device (200 × 50 mm). Standard (industrial grade 4.0) argon (99.995% purity) was used as the shielding gas and the shielding devices were placed above a flat plate as shown in Fig. 4(a). Within the plate there was a 5 mm diameter hole that was connected to a fast response zirconia oxygen analyser for measuring the contamination level. The experimental conditions are summarised in Table 3. In the first experiment the measurement hole was located under the centre of the shielding device as shown in Fig. 4(a) and the stand-off distance and flow-rate were varied. In the second and third experiments the shielding device was moved laterally in the x (in and out of the page) and y directions to produce a map of the oxygen contamination underneath the shielding devices for a fixed stand-off distance and flow-rate. All the experiments were performed in an environment where drafts were minimised. 3.3. Shielding device Trial 2–evaluation of performance using a mock-up wall
Fig. 4. Schematic diagrams of experiment setups for: (a) Trial 1 where the contamination levels across a flat plate were determined; and (b) Trial 2 where the contamination across a mock-up WAAM wall was determined.
sured along the centreline of the samples with a load of 500 g. For each sample, a total of 19 points were measured from the bottom of the wall to the top with a spacing of 1 mm.
To investigate the effectiveness of the laminar shielding device when used on a WAAM wall structure, a mock-up wall was constructed which is shown in Fig. 4(b). There were nine Ф2 mm measuring holes on the wall which are shown on the schematic diagram: one was located on the top and the remaining 8 were located along the side of the wall at various distances from the top. Internal holes were drilled from the measuring hole to a tapping hole at the base of the wall which was connected to the oxygen analyser. When doing a measurement from a particular hole the shielding device was moved so that the hole was aligned with the middle of the shielding device in the x direction. Argon flow-rates of 20, 25, 30 and 35 l/min were used. This experiment was not repeated with the conventional shielding device The laminar shielding device and setup for the contamination trials along the wall are shown in Fig. 5. Note that even though the picture shows a welding torch attached to the shielding device, subsequent work has shown that it is necessary to modify the shielding device design to avoid high oxygen levels at the interface between the torch and trailing shield.
3.2. Shielding device Trial 1–evaluation of performance across a flat plate
3.4. CFD modelling
A schematic diagram showing the setup for the first trial is shown in Fig. 4(a). Two shielding devices were used for this trial:
To further understand the performance of the shielding gas with different flow rates and different stand-off distances, com-
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Table 3 Summary of the experiments performed for Trial 1. Shielding device
Stand-off distance (mm)
Argon flow-rates (l/min)
Oxygen sampling location (s)
Laminar and conventional Laminar and conventional Laminar
10, 15, 20 and 30 15 25
10, 15, 20 and 25 15 25
Centre of shielding device Map under device with a spacing of 10 mm in the x and y directions
Fig. 7. Hardness and oxygen content of specimens built with different environmental contamination levels. The error bars indicate the 95% confidence interval of the mean.
Fig. 6. Schematic diagram of the CFD model of the laminar shielding device: (a) Trial 1 where the contamination levels across a flat plate were determined; and (b) Trial 2 where the contamination across a mock-up WAAM wall was determined.
putational fluid dynamics (CFD) models were created using Fluent v14.0. 2D models were created in the y–z plane (see Fig. 4) and the main boundary conditions are shown in Fig. 6. An unstructured quadratic mesh was generated using ICEM CFD software. A small mesh with a size of 0.5 mm × 0.5 mm was used near the inlet and the size of the mesh was increased gradually near the outlet. A steady-state Species Transport model was used to determine the oxygen contamination at different locations under the shielding device. A k-epsilon turbulence model with the enhanced wall treatment option was used in this study and the model was solved with SIMPLE algorithm. Two different geometries were developed for the two trials described in the previous section. Only half the device was modelled due to the symmetric geometry. The models used the same parameters as those used in the experiments. All the parameters in this model were defined and calculated at room temperature (20 ◦ C). In both models an argon inlet was used to represent the flow from the laminar shielding device. The species fractions of the inlet were defined as 99.995% argon, 0.002% oxygen (measured by oxygen sensor) and 0.003% nitrogen by volume. A pressure outlet was defined on the exterior walls of the model. Along this boundary, the pressure was atmospheric and the composition was 20.95% oxygen, 78.08% nitrogen and 0.97 % argon by volume. Although there are trace amounts of other gases in practice, their existence was
ignored in the model. A turbulence intensity of 1% was used in this model at the inlet and outlet. The effect of an argon knife on the shielding performance was investigated with the CFD model. This was done to see whether it helped to reduce entrainment of the surrounding air underneath the shielding device. In the model the argon knife was located at the periphery of the laminar shielding device with the geometry for Trial 2 only (see Fig. 6(b)). This argon knife was 1 mm wide and 5 mm long and was positioned at three angles relative to vertical: −30◦ , 0◦ and 30◦ and had flow-rates from 0 l/min to 25 l/min. These parameters were used with flow-rates of 10, 20 and 30 l/min in the main shielding device. 4. Results and discussion 4.1. Evaluation of effects of oxygen contamination in the shielding gas on titanium WAAM samples The oxygen measured within the sample and the hardness for the different environmental contamination levels within the chamber are shown in Fig. 7. The amount of oxygen detected in each sample ranged from 1622 ppm to 1825 ppm and was marginally affected by the quantity of oxygen present in the atmosphere during deposition. When building with 4000 ppm oxygen in the atmosphere, the increase in the oxygen level in the deposited wall was only 200 ppm, compared to an environment with negligible oxygen. Oxygen was present in the wire as an alloying element (Table 1) and this is the source of most of the oxygen in the final part—the oxygen content of the wire was 1400 ppm. There was no significant difference in the hardness for the different oxygen levels measured in the specimens. This result is consistent with the study by Li et al. (2005) who investigated the effect of oxygen contamination in the argon shielding gas during laser welding of titanium. They found that there was little change in the mechanical properties provided the oxygen within the shielding gas was below 5000 ppm. Although this study showed the hardness of the deposited wall was not affected by the envi-
J. Ding et al. / Journal of Materials Processing Technology 226 (2015) 99–105
Fig. 8. Comparison of the oxygen contamination levels between the laminar and conventional shielding devices for Trial 1.
ronmental contamination level provided it was below 4000 ppm, a more stringent (and conservative) requirement of 2000 ppm was selected for the subsequent work on the local shielding device. 4.2. Trial 1–evaluation of shielding device performance across a flat plate Fig. 8 shows the results from the first trial which compared the two shielding devices at different stand-off distances from a flat plate. It clearly shows that the new laminar shielding device performed much better than the conventional one for all the conditions. For the stand-off distance of 20 mm the laminar shielding device can provide a protection of as low as 20 ppm with high flow rates, which is about three orders of magnitude improvement compare with the conventional device. A 2000 ppm line was added to this figure which represents the highest acceptable oxygen content from the previous experiment. It can be seen that the commercial local shielding device provided inadequate shielding when the stand-off distance was above 10 mm and the oxygen level was insensitive to the flow-rate. The higher levels of oxygen contamination with this device indicated that the gas flow from the outlet
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was not constrained and the velocity magnitude and direction varied in a random and chaotic way. The device was only effective for small stand-off distances from the workpiece. While this may be adequate for most welding applications, it is unsuitable for WAAM due to the additional demands of high stand-off distances and less side protection due to the part geometry. The laminar shielding device significantly improved the protection. For higher stand-off distances (>20 mm), the oxygen level exceeded the 2000 limit when the flow-rate was low, but the contamination reduced significantly when the flow rate was increased to 25 l/min. The oxygen content can be as low as ∼20 ppm with the high flow rates, which is comparable to the level in the inlet argon gas. Fig. 9 shows the map of the oxygen level under the shielding devices. The conventional shielding device (Fig. 9(a)) cannot provide the required oxygen content of 2000 ppm at any location under the device. In addition, the coverage was far from uniform with higher values being recorded adjacent to the argon inlet which is indicated with a red circle. It is possible that the metal mesh inside the shielding device did not efficiently diffuse the inlet gas which resulted in a higher velocity adjacent to the inlet. This may have induced additional turbulence in the flow which increased entrainment of the surrounding air. The laminar shielding device provided greatly improved shielding at all locations under the device as shown in Fig. 9(b and c) for the two conditions analysed. The white area indicates the region that is below 2000 ppm and the large size of this region indicated that most of the area under the shielding device was adequately protected. At the higher stand-off distance it was necessary to use a higher flow-rate to provide adequate coverage, confirming the results from Fig. 8.
4.3. Trial 2–evaluation of shielding device performance using a mock-up wall Fig. 10 shows the oxygen across the mock-up wall for the different argon flow-rates. The oxygen level increased significantly with increasing distance from the top of the wall—note the logarithmic scale. Nevertheless a flow-rate of 35 l/min enabled the wall to be
Fig. 9. Oxygen map underneath the shielding devices for Trial 1: (a) conventional shielding device with a stand-off distance of 15 mm and flow-rate of 15 l/min; (b) laminar shielding device with the same stand-off distance and flow-rate and (c) laminar shielding device with a stand-off distance of 25 mm and a flow rate of 25 l/min. Note that the minimum value of the colour-map is 2000 ppm and the argon inlets to the shielding device are shown with a red half circle.
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Fig. 10. Oxygen content across the mock-up wall from Trial 2.
effectively protected up to 30 mm from the top of the wall which could enable out-of-chamber WAAM of active materials. 4.4. CFD modelling results Fig. 11 shows the comparison between the predictions from the model and those from the experiments. Although this simplified 2D model overestimated the oxygen level, it correctly predicted the trends in the experiments: namely the improvement in shielding with higher flow-rates and lower stand-off distances; and the increase in oxygen contamination along the wall with increasing distance from the shielding device. The reasons for the discrepancy between the model and the experiments include: the 2d simplification of the model; and as reported by Khan and Joshi (2015) the k– model used for capturing the flow which is robust and computationally cheap, but has limited accuracy. To better understand how the stand-off distance and flow-rate affected the contamination levels, 2D plots of the oxygen content were produced for selected conditions in Fig. 12. In all cases there is a low pressure just underneath the inlet which sucks the surrounding air into the argon stream. The effect of this varies with stand-off distance. With increasing stand-off distance (Fig. 12 (b) and (c)) more air is entrained into the argon steam which results in ineffective shielding. However, by increasing the argon flow-rate there is less time for the surrounding air to diffuse to the centreline. Therefore contamination levels under the device are reduced.
Fig. 11. Comparison of CFD and experimental results for (a) Trial 1 and (b) Trial 2.
Finally, the performance of the argon knife is shown in Fig. 13 which shows the protected wall height vs. the argon knife flowrate. The protected wall height refers to the distance from the top of the wall where the oxygen level is below 2000 ppm. There was a small benefit with low argon knife flow-rates of around 5 l/min. Beyond this there was a significant reduction in the protected wall height. As shown in Fig. 14 when a high flow rate was used for the argon knife (25 l/min) a large turbulence was generated near the argon inlet which greatly increased mixing with the surrounding air. Using an angle of 0◦ provided marginally better protection that −30◦ and +30◦ . Although there is a beneficial effect of the argon knife, it is worth noting that a similar improvement in protected wall height can also be achieved by increasing the flow-rate in the main shielding device (as shown in Fig. 13). Since there appeared
Fig. 12. Oxygen distribution for (a) stand-off distance of 10 mm and argon flow rate of 10 l/min; (b) stand-off distance of 15 mm and argon flow rate of 10 l/min; (c) stand-off distance of 20 mm and argon flow rate of 10 l/min and (d) stand-off distance of 20 mm and argon flow rate of 20 l/min.
J. Ding et al. / Journal of Materials Processing Technology 226 (2015) 99–105
Fig. 13. Protected wall height as a function of argon knife flow-rate with the geometry from Trial 2. Note that the protected wall height refers to the distance from the top of the wall where the oxygen level is below 2000 ppm.
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4000 ppm. Therefore 2000 ppm was selected as a conservative limit for the environmental contamination level. • CFD models were able to capture the trends in the experimental results. They demonstrated the improved performance with increased flow-rate and the reduction in performance with higher stand-off distances. Analysis of the 2D contamination profile under the device showed how the surrounding air was entrained due to a low pressure region just under the device. • It was demonstrated that the protection can be improved by increasing the Argon flow of the new local shielding device or by using an argon knife at the periphery of the device. • The laminar shielding device could potentially enable out-ofchamber WAAM of active materials. This device may also be suitable for welding applications which involve welding on a curved surface that result in a large stand-off distance for the trailing shielding device. Acknowledgements The authors would like to thank Air Products, U.K. for their financial support. They would also like to thank Brian Brooks for his technical assistance and Timet for providing the analysis of the oxygen content within the titanium deposits. References
Fig. 14. Vector velocity plot for the shielding device with argon knife (flow rate: 25l/min).
to be very little benefit of the argon knife this was not investigated experimentally. 5. Conclusions In this work we have demonstrated: • A laminar shielding device that had significantly improved (up to 3 orders of magnitude) performance compared to a conventional shielding device. This was demonstrated by comparing the performance across a flat plate as well as a mock-up WAAM wall. • The mechanical properties (hardness) were not affected by the oxygen level in the environment provided it was below
Almeida, P.S., Williams, S., 2010. Innovative process model of Ti-6Al-4V additive layer manufacturing using cold metal transfer (CMT). In: Proceedings of the Twenty-First Annual International Solid Freeform Fabrication Symposium, Austin, USA, pp. 25–36. Babish, F., 2007. Guide for the Fusion Welding of Titanium and Titanium Alloys, AWS. G2.4-G2.4M-2007. Baufeld, B., Brandl, E., Van Der Biest, O., 2011. Wire based additive layer manufacturing: comparison of microstructure and mechanical properties of Ti-6Al-4V components fabricated by laser-beam deposition and shaped metal deposition. J. Mater. Process. Technol. 211, 1146–1158. Bloch, C.J., 1992. Method and apparatus for water jet cutting including improved nozzle. US Patent 5169065A. Chong, T.P., Joseph, P.F., Davies, P.O.A.L., 2009. Design and performance of an open jet wind tunnel for aero-acoustic measurement. Appl. Acoust. 70, 605–614. Frazier, E.W., 2014. Metal additive manufacturing: a review. J. Mater. Eng. Perform. 23, 1917–1928. Khan, Z., Joshi, J.B., 2015. Comparison of k–, RAM and LES models for the prediction of flow pattern in jet loop reactor. Chem. Eng. Sci. 127, 323–333. Kulkarni, V., Sahoo, N., Chavan, S.D., 2011. Simulation of honeycomb-screen combinations for turbulence management in a subsonic wind tunnel. J. Wind Eng. Ind. Aerodyn. 99, 37–45. Li, X., Xie, J., Zhou, Y., 2005. Effects of oxygen contamination in the argon shielding gas in laser welding of commercially pure titanium thin sheet. J. Mater. Sci. 40, 3437–3443. Martina, F., Mehnen, J., Williams, S., Colegrove, P., Wang, F., 2012. Investigation of the benefits of plasma deposition for the additive layer manufacture of Ti-6Al-4V. J. Mater. Process. Technol. 212, 1377–1386. Wada, Y., Inoue, S., Tsukamoto, H., Yamaguchi, T., Nishio, K., 2012. Numerical simulation of shielding gas behaviour in tungsten inert gas welding of titanium plate. Sci. Technol. Weld. Join. 17, 116–121. Williams, S.W., Martina, F., Addison, A.C., Ding, J., Pardal, G., Colegrove, P., 2015. Wire + arc additive manufacturing. Mater. Sci. Technol., http://dx.doi.org/10. 1179/1743284715Y.0000000073.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Development of a laminar flow local shielding device for wire + arc additive manufacture J. Ding a,∗ , P. Colegrove a , F. Martina a , S. Williams a , R. Wiktorowicz b , M.R. Palt c a b c
Welding Engineering and Laser Processing Centre, Cranfield University, Cranfield MK43 0AL, UK Air Products PLC, Crockford Lane, Chineham, Basingstoke, RG24 8FE, UK Air Products Management SA./NV., Ch. de Wavre 1789, B-1160 Brussels, Belgium
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 12 June 2015 Accepted 10 July 2015 Available online 19 July 2015 Keywords: Local shielding Trailing shield Laminar flow Additive manufacture Titanium CFD
a b s t r a c t A shielded environment is required during the wire + arc additive manufacture (WAAM) of titanium alloys to prevent oxidation. Applying local shielding can increase the flexibility of the WAAM process, however conventional devices do not provide adequate protection due to entrainment of the surrounding air. In this study, a new local shielding device based on laminar flow was developed and compared with a conventional device. The laminar local shielding device showed up to three orders of magnitude improvement with contamination levels below 2000 ppm being achieved with a stand-off distance of 30 mm. The performance was also assessed along a mock-up WAAM wall which showed that it could be protected up to 30 mm from the top. Finally, computational fluid dynamics models provided insight into the device performance and enabled the performance of an argon knife to be evaluated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Additive manufacture (AM) techniques are receiving interest in the manufacturing community. As summarised by Frazier (2014), AM can potentially reduce production costs and can reduce lead times because it allows just-in-time manufacturing. This is especially the case for AM of structural materials, and titanium in particular. There are a variety of high-deposition rate processes that may be used for manufacturing large-scale AM parts where the heat may be provided by laser, electron beam or arcbased processes. The latter are often referred to as wire + arc additive manufacture (WAAM) and have the advantage of high build rates with comparatively low equipment cost. Martina et al. (2012) applied the WAAM process to titanium and showed that build rates of 2 kg/h could be achieved whilst maintaining high fidelity. Fig. 1 shows a typical WAAM system. As described by Williams et al. (2015), the WAAM process utilises an electric arc as a heat source and wire as feedstock. The arc-based welding process can be based on either metal inert gas (MIG) system, tungsten inert gas (TIG) systems, or plasma arc welding (PAW) system. The motion can
∗ Corresponding author. E-mail address: jialuo.ding@cranfield.ac.uk (J. Ding). http://dx.doi.org/10.1016/j.jmatprotec.2015.07.005 0924-0136/© 2015 Elsevier B.V. All rights reserved.
be provided either by robotic systems or by computer numerical controlled (CNC) systems. Parts are manufactured by depositing layers of metal with the welding-based deposition process. Babish (2007) claimed that when AM or welding titanium, the component needs to be shielded where the temperature exceeds 427 ◦ C to avoid oxidation of the surface. Contamination of titanium occurs primarily from oxygen which can reduce the ductility and toughness of the material. Baufeld et al. (2011) achieved this by performing the deposition process within an airtight chamber filled with argon (99.999%); however this becomes increasingly difficult to implement as the size of the part increases. The time required for evacuation of the chamber becomes long, problem solving is difficult and integrating other manufacturing processes such as machining or rolling becomes prohibitively expensive. Local or trailing shielding overcomes most of the shortcomings of a fixed chamber or tent and was used by Almeida and Williams (2010). However the devices developed for welding have an important shortcoming: they can only be used with a short stand-off distance from the workpiece. Beyond this, flow turbulence results in entrainment of the surrounding air which gives poor shielding and increases contamination—particularly on the side walls of the WAAM part. Therefore it was necessary to construct an additional structure beside the wall being built to reduce contamination. This involved considerable manual work which slowed the process, and constrained the shielding device to simple structures.
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J. Ding et al. / Journal of Materials Processing Technology 226 (2015) 99–105
Fig. 1. Robotic WAAM system. Fig. 3. Laminar flow local shielding device design.
Fig. 2. Cross-section of a conventional local shielding device (Babish, 2007).
A schematic diagram of a typical shielding device design used for welding from Babish (2007) is shown in Fig. 2. The device is packed with bronze or stainless steel wool as a diffusing medium which slows down and distributes the inlet gas. A metal mesh or porous plate made of copper or stainless steel is used at the outlet to support the wool and further diffuse the gas. Wada et al. (2012) investigated the performance of such devices with a computational fluid dynamics (CFD) model and found that the contamination level was critically dependent on the stand-off distance—the distance from the bottom of the shielding device to the workpiece. To improve the protection, it is critical to control the turbulence. Kulkarni et al. (2011) described in detail of using different honeycomb-screen combinations for turbulence management in a wind tunnel. And Bloch (1992) proposed a laminar flow water jet cutting nozzle which also used a combination of honeycomb flow straighteners with screens. This concept can be potentially borrowed in the design of the local shielding device for the WAAM processes. In this paper we describe the design of a new local shielding device for WAAM based on laminar flow. A preliminary experiment is performed to determine the contamination levels required by the device before evaluating the device performance. Finally, a computational model is used to aid understanding of the device performance. 2. Local shielding device design The new shielding device design is shown in Fig. 3 and consists of three parts. The first part is a diffusion chamber which is used to uniformly distribute the inlet gas. At the bottom of the diffusion chamber, two layers of 5 mm thick metal foam with an average hole size of 0.5 mm were placed 10 mm from the two gas inlets. The second part of the device is the honeycomb flow straightener. Kulkarni et al. (2011) stated that honeycomb can straighten
the flow and reduce large scale turbulence by reducing the lateral velocities. In addition, the length-to-diameter ratio had to be greater than eight to reduce the turbulence. The honeycomb used in this study had regular hexagonal cells with a size of 6 mm and a height of 90 mm giving a length-to-diameter ratio of 15. The third part of this device is a setting chamber followed by one layer of metal mesh. Kulkarni et al. (2011) stated that although honeycomb can significantly constrain the gas flow in the lateral direction the flow at the outlet fluctuates from near zero at the wall to a maximum in the centre of the honeycomb as shown in Fig. 3. This non-uniformity decreases with increasing downstream distance from the honeycomb end and Kulkarni et al. (2011) recommended using a downstream distance of three times the honeycomb cell size. Therefore the setting chamber height was 15 mm. Finally a layer of metal mesh was used at the end of the setting chamber to further improve the uniformity of the flow. In this study, a stainless steel square woven mesh was used with wire diameter of 0.2 mm and an aperture of 0.647 mm, which gave a mesh porosity of 0.42. The selection of this mesh was based on the pressure drop coefficient described by Chong et al. (2009). Overall, the device was 200 mm long and 50 mm wide. 3. Experimental procedure 3.1. Titanium WAAM wall with different oxygen levels in shielding gas Before evaluating the new shielding device performance, the environmental contamination level requirements were determined. Four titanium walls were manufactured in a rigid, sealed chamber with environmental contamination levels of ca. 30 ppm, 1000 ppm, 2000 ppm and 4000 ppm oxygen. This was achieved by filling the chamber with a combination of argon and oxygen through a gas mixing station which allowed the ratio between the two gases to be adjusted. A Migatronic plasma power supply was used to deposit 20 layers onto a 300 mm × 150 mm × 7 mm Ti-6Al-4V substrate. Ti-6Al-4 V wire with diameter 1.2 mm, whose composition is shown in Table 1, was used to deposit layers that were 250 mm long. The parameters for the plasma deposition process are shown in Table 2. The oxygen fractions of the four samples were measured by the inert gas fusion technique following ASTM E1409-13 (2013). Cross sections were extracted from each wall and the hardness was mea-
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Table 1 Chemical composition of Ti-6Al-4V wire used in the experiments. Ti
Al
V
Fe
O
C
N
H
TOE
Y
Others
89.397
6.14
3.96
0.18
0.14
0.02
0.011
0.001
<0.1
<0.001
<0.005
Table 2 Parameters for the plasma deposition process. Wire feed speed Travel speed Current Plasma gas flow-rate Shielding gas flow-rate (through the torch) Torch stand-off
2.4 m/min 4.5 mm/s 160 A 1 l/min 10 l/min 8 mm
Fig. 5. New laminar shielding device and setup for the contamination trials along the wall.
the novel laminar flow device described previously and a commercially available shielding device. The Argweld Typical Flat Trailing Shield whose length and width were identical to the laminar shielding device (200 × 50 mm). Standard (industrial grade 4.0) argon (99.995% purity) was used as the shielding gas and the shielding devices were placed above a flat plate as shown in Fig. 4(a). Within the plate there was a 5 mm diameter hole that was connected to a fast response zirconia oxygen analyser for measuring the contamination level. The experimental conditions are summarised in Table 3. In the first experiment the measurement hole was located under the centre of the shielding device as shown in Fig. 4(a) and the stand-off distance and flow-rate were varied. In the second and third experiments the shielding device was moved laterally in the x (in and out of the page) and y directions to produce a map of the oxygen contamination underneath the shielding devices for a fixed stand-off distance and flow-rate. All the experiments were performed in an environment where drafts were minimised. 3.3. Shielding device Trial 2–evaluation of performance using a mock-up wall
Fig. 4. Schematic diagrams of experiment setups for: (a) Trial 1 where the contamination levels across a flat plate were determined; and (b) Trial 2 where the contamination across a mock-up WAAM wall was determined.
sured along the centreline of the samples with a load of 500 g. For each sample, a total of 19 points were measured from the bottom of the wall to the top with a spacing of 1 mm.
To investigate the effectiveness of the laminar shielding device when used on a WAAM wall structure, a mock-up wall was constructed which is shown in Fig. 4(b). There were nine Ф2 mm measuring holes on the wall which are shown on the schematic diagram: one was located on the top and the remaining 8 were located along the side of the wall at various distances from the top. Internal holes were drilled from the measuring hole to a tapping hole at the base of the wall which was connected to the oxygen analyser. When doing a measurement from a particular hole the shielding device was moved so that the hole was aligned with the middle of the shielding device in the x direction. Argon flow-rates of 20, 25, 30 and 35 l/min were used. This experiment was not repeated with the conventional shielding device The laminar shielding device and setup for the contamination trials along the wall are shown in Fig. 5. Note that even though the picture shows a welding torch attached to the shielding device, subsequent work has shown that it is necessary to modify the shielding device design to avoid high oxygen levels at the interface between the torch and trailing shield.
3.2. Shielding device Trial 1–evaluation of performance across a flat plate
3.4. CFD modelling
A schematic diagram showing the setup for the first trial is shown in Fig. 4(a). Two shielding devices were used for this trial:
To further understand the performance of the shielding gas with different flow rates and different stand-off distances, com-
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Table 3 Summary of the experiments performed for Trial 1. Shielding device
Stand-off distance (mm)
Argon flow-rates (l/min)
Oxygen sampling location (s)
Laminar and conventional Laminar and conventional Laminar
10, 15, 20 and 30 15 25
10, 15, 20 and 25 15 25
Centre of shielding device Map under device with a spacing of 10 mm in the x and y directions
Fig. 7. Hardness and oxygen content of specimens built with different environmental contamination levels. The error bars indicate the 95% confidence interval of the mean.
Fig. 6. Schematic diagram of the CFD model of the laminar shielding device: (a) Trial 1 where the contamination levels across a flat plate were determined; and (b) Trial 2 where the contamination across a mock-up WAAM wall was determined.
putational fluid dynamics (CFD) models were created using Fluent v14.0. 2D models were created in the y–z plane (see Fig. 4) and the main boundary conditions are shown in Fig. 6. An unstructured quadratic mesh was generated using ICEM CFD software. A small mesh with a size of 0.5 mm × 0.5 mm was used near the inlet and the size of the mesh was increased gradually near the outlet. A steady-state Species Transport model was used to determine the oxygen contamination at different locations under the shielding device. A k-epsilon turbulence model with the enhanced wall treatment option was used in this study and the model was solved with SIMPLE algorithm. Two different geometries were developed for the two trials described in the previous section. Only half the device was modelled due to the symmetric geometry. The models used the same parameters as those used in the experiments. All the parameters in this model were defined and calculated at room temperature (20 ◦ C). In both models an argon inlet was used to represent the flow from the laminar shielding device. The species fractions of the inlet were defined as 99.995% argon, 0.002% oxygen (measured by oxygen sensor) and 0.003% nitrogen by volume. A pressure outlet was defined on the exterior walls of the model. Along this boundary, the pressure was atmospheric and the composition was 20.95% oxygen, 78.08% nitrogen and 0.97 % argon by volume. Although there are trace amounts of other gases in practice, their existence was
ignored in the model. A turbulence intensity of 1% was used in this model at the inlet and outlet. The effect of an argon knife on the shielding performance was investigated with the CFD model. This was done to see whether it helped to reduce entrainment of the surrounding air underneath the shielding device. In the model the argon knife was located at the periphery of the laminar shielding device with the geometry for Trial 2 only (see Fig. 6(b)). This argon knife was 1 mm wide and 5 mm long and was positioned at three angles relative to vertical: −30◦ , 0◦ and 30◦ and had flow-rates from 0 l/min to 25 l/min. These parameters were used with flow-rates of 10, 20 and 30 l/min in the main shielding device. 4. Results and discussion 4.1. Evaluation of effects of oxygen contamination in the shielding gas on titanium WAAM samples The oxygen measured within the sample and the hardness for the different environmental contamination levels within the chamber are shown in Fig. 7. The amount of oxygen detected in each sample ranged from 1622 ppm to 1825 ppm and was marginally affected by the quantity of oxygen present in the atmosphere during deposition. When building with 4000 ppm oxygen in the atmosphere, the increase in the oxygen level in the deposited wall was only 200 ppm, compared to an environment with negligible oxygen. Oxygen was present in the wire as an alloying element (Table 1) and this is the source of most of the oxygen in the final part—the oxygen content of the wire was 1400 ppm. There was no significant difference in the hardness for the different oxygen levels measured in the specimens. This result is consistent with the study by Li et al. (2005) who investigated the effect of oxygen contamination in the argon shielding gas during laser welding of titanium. They found that there was little change in the mechanical properties provided the oxygen within the shielding gas was below 5000 ppm. Although this study showed the hardness of the deposited wall was not affected by the envi-
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Fig. 8. Comparison of the oxygen contamination levels between the laminar and conventional shielding devices for Trial 1.
ronmental contamination level provided it was below 4000 ppm, a more stringent (and conservative) requirement of 2000 ppm was selected for the subsequent work on the local shielding device. 4.2. Trial 1–evaluation of shielding device performance across a flat plate Fig. 8 shows the results from the first trial which compared the two shielding devices at different stand-off distances from a flat plate. It clearly shows that the new laminar shielding device performed much better than the conventional one for all the conditions. For the stand-off distance of 20 mm the laminar shielding device can provide a protection of as low as 20 ppm with high flow rates, which is about three orders of magnitude improvement compare with the conventional device. A 2000 ppm line was added to this figure which represents the highest acceptable oxygen content from the previous experiment. It can be seen that the commercial local shielding device provided inadequate shielding when the stand-off distance was above 10 mm and the oxygen level was insensitive to the flow-rate. The higher levels of oxygen contamination with this device indicated that the gas flow from the outlet
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was not constrained and the velocity magnitude and direction varied in a random and chaotic way. The device was only effective for small stand-off distances from the workpiece. While this may be adequate for most welding applications, it is unsuitable for WAAM due to the additional demands of high stand-off distances and less side protection due to the part geometry. The laminar shielding device significantly improved the protection. For higher stand-off distances (>20 mm), the oxygen level exceeded the 2000 limit when the flow-rate was low, but the contamination reduced significantly when the flow rate was increased to 25 l/min. The oxygen content can be as low as ∼20 ppm with the high flow rates, which is comparable to the level in the inlet argon gas. Fig. 9 shows the map of the oxygen level under the shielding devices. The conventional shielding device (Fig. 9(a)) cannot provide the required oxygen content of 2000 ppm at any location under the device. In addition, the coverage was far from uniform with higher values being recorded adjacent to the argon inlet which is indicated with a red circle. It is possible that the metal mesh inside the shielding device did not efficiently diffuse the inlet gas which resulted in a higher velocity adjacent to the inlet. This may have induced additional turbulence in the flow which increased entrainment of the surrounding air. The laminar shielding device provided greatly improved shielding at all locations under the device as shown in Fig. 9(b and c) for the two conditions analysed. The white area indicates the region that is below 2000 ppm and the large size of this region indicated that most of the area under the shielding device was adequately protected. At the higher stand-off distance it was necessary to use a higher flow-rate to provide adequate coverage, confirming the results from Fig. 8.
4.3. Trial 2–evaluation of shielding device performance using a mock-up wall Fig. 10 shows the oxygen across the mock-up wall for the different argon flow-rates. The oxygen level increased significantly with increasing distance from the top of the wall—note the logarithmic scale. Nevertheless a flow-rate of 35 l/min enabled the wall to be
Fig. 9. Oxygen map underneath the shielding devices for Trial 1: (a) conventional shielding device with a stand-off distance of 15 mm and flow-rate of 15 l/min; (b) laminar shielding device with the same stand-off distance and flow-rate and (c) laminar shielding device with a stand-off distance of 25 mm and a flow rate of 25 l/min. Note that the minimum value of the colour-map is 2000 ppm and the argon inlets to the shielding device are shown with a red half circle.
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Fig. 10. Oxygen content across the mock-up wall from Trial 2.
effectively protected up to 30 mm from the top of the wall which could enable out-of-chamber WAAM of active materials. 4.4. CFD modelling results Fig. 11 shows the comparison between the predictions from the model and those from the experiments. Although this simplified 2D model overestimated the oxygen level, it correctly predicted the trends in the experiments: namely the improvement in shielding with higher flow-rates and lower stand-off distances; and the increase in oxygen contamination along the wall with increasing distance from the shielding device. The reasons for the discrepancy between the model and the experiments include: the 2d simplification of the model; and as reported by Khan and Joshi (2015) the k– model used for capturing the flow which is robust and computationally cheap, but has limited accuracy. To better understand how the stand-off distance and flow-rate affected the contamination levels, 2D plots of the oxygen content were produced for selected conditions in Fig. 12. In all cases there is a low pressure just underneath the inlet which sucks the surrounding air into the argon stream. The effect of this varies with stand-off distance. With increasing stand-off distance (Fig. 12 (b) and (c)) more air is entrained into the argon steam which results in ineffective shielding. However, by increasing the argon flow-rate there is less time for the surrounding air to diffuse to the centreline. Therefore contamination levels under the device are reduced.
Fig. 11. Comparison of CFD and experimental results for (a) Trial 1 and (b) Trial 2.
Finally, the performance of the argon knife is shown in Fig. 13 which shows the protected wall height vs. the argon knife flowrate. The protected wall height refers to the distance from the top of the wall where the oxygen level is below 2000 ppm. There was a small benefit with low argon knife flow-rates of around 5 l/min. Beyond this there was a significant reduction in the protected wall height. As shown in Fig. 14 when a high flow rate was used for the argon knife (25 l/min) a large turbulence was generated near the argon inlet which greatly increased mixing with the surrounding air. Using an angle of 0◦ provided marginally better protection that −30◦ and +30◦ . Although there is a beneficial effect of the argon knife, it is worth noting that a similar improvement in protected wall height can also be achieved by increasing the flow-rate in the main shielding device (as shown in Fig. 13). Since there appeared
Fig. 12. Oxygen distribution for (a) stand-off distance of 10 mm and argon flow rate of 10 l/min; (b) stand-off distance of 15 mm and argon flow rate of 10 l/min; (c) stand-off distance of 20 mm and argon flow rate of 10 l/min and (d) stand-off distance of 20 mm and argon flow rate of 20 l/min.
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Fig. 13. Protected wall height as a function of argon knife flow-rate with the geometry from Trial 2. Note that the protected wall height refers to the distance from the top of the wall where the oxygen level is below 2000 ppm.
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4000 ppm. Therefore 2000 ppm was selected as a conservative limit for the environmental contamination level. • CFD models were able to capture the trends in the experimental results. They demonstrated the improved performance with increased flow-rate and the reduction in performance with higher stand-off distances. Analysis of the 2D contamination profile under the device showed how the surrounding air was entrained due to a low pressure region just under the device. • It was demonstrated that the protection can be improved by increasing the Argon flow of the new local shielding device or by using an argon knife at the periphery of the device. • The laminar shielding device could potentially enable out-ofchamber WAAM of active materials. This device may also be suitable for welding applications which involve welding on a curved surface that result in a large stand-off distance for the trailing shielding device. Acknowledgements The authors would like to thank Air Products, U.K. for their financial support. They would also like to thank Brian Brooks for his technical assistance and Timet for providing the analysis of the oxygen content within the titanium deposits. References
Fig. 14. Vector velocity plot for the shielding device with argon knife (flow rate: 25l/min).
to be very little benefit of the argon knife this was not investigated experimentally. 5. Conclusions In this work we have demonstrated: • A laminar shielding device that had significantly improved (up to 3 orders of magnitude) performance compared to a conventional shielding device. This was demonstrated by comparing the performance across a flat plate as well as a mock-up WAAM wall. • The mechanical properties (hardness) were not affected by the oxygen level in the environment provided it was below
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