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Foam stabilization by aluminum powder
Journal Pre-proofs Foam stabilization by aluminum powder S. Sasikumar, K. Georgy, M. Mukherjee, G.S.Vinod Kumar PII: DOI: Reference:
S0167-577X(19)31774-4 https://doi.org/10.1016/j.matlet.2019.127142 MLBLUE 127142
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Materials Letters
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4 October 2019 23 November 2019 8 December 2019
Please cite this article as: S. Sasikumar, K. Georgy, M. Mukherjee, G.S.Vinod Kumar, Foam stabilization by aluminum powder, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.127142
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Foam stabilization by aluminum powder S. Sasikumar1, K. Georgy2, M. Mukherjee2, G.S.Vinod Kumar3,* 1Department
of Mechanical Engineering, SRM IST, Kattankulathur, Chennai 603203,
India 2Department
of Metallurgical and Materials Engineering, Indian Institute of
Technology Madras, Chennai 600036, India 3Department
of Mechanical Engineering, SRM University-AP, Amaravati 522502, India
*Corresponding author, Email address: [email protected] Abstract In this study, it was shown that aluminum powder can be used as stabilizing particles for the fabrication of aluminum foams by melt route. When Al powder was mixed with the TiH2 before adding into the melt, it also acted as dispersing agent for the TiH2 thus further improving the structure of the foams. Stirring during powder mixing also contributed towards foam stability by introducing oxides into the melt. The oxides were examined using SEM/EDS and oxygen analyzer. Keywords: Porous materials; Oxidation; Aluminum powder; Stabilization; Dispersion; Melt route. Introduction Melt route is the most preferred method for the fabrication of closed-cell metal foams because it is a simple and cost-effective process, and also well suited for batch production [1]. Melt route foaming necessitates stabilizing particles [2], which is either added into the melt (ex-situ: SiC) or produced inside the melt (in-situ: TiC, TiB2, and MgAl2O4) [24]. Particle addition, ex-situ into the melt is challenging due to the non-wetting nature of the particles with the melt. While the in-situ formation of particles
1
in the melt demands larger processing step(s) and hence a significant time. Stabilization can also be achieved by adding Mg ingots into Al melt [5, 6]. This is due to the in-situ formation of MgAl2O4 and transition oxide particles [6]. Körner et al. [7] exploited -phase particles as a stabilizing agent for foaming a semisolid mass of Mg melt. Yang et al. produced foams from AlSi7Mg0.5 alloy melt by first agitating the melt with 5 wt.% Al powder for 20 min in air before adding TiH2. An increase in viscosity due to oxide particle (alumina) formation was attributed to foam stabilization [8]. Zhao et al. demonstrated foamability of pure Al melt agitated with 3 wt.% of Al powder for 10 min [9]. In this study, however, CaCO3 was used as blowing agent which is known to cause stabilization through the formation of several types of oxides depending on the alloy composition [10]. In these studies, Al powder indirectly contributes to stabilization by forming oxide particles. Some studies used Al powder as a dispersing agent with TiH2 [3, 11]. While the role of Al powder as dispersing agent is clear, any direct role as stabilizing particle has not been established. In this work, we show the role of Al powder both as a dispersing agent and as stabilizing particles. Materials and methods Commercial pure aluminum ingots (99.6 % pure), Al powder (99.7 % pure, D50=45 μm) and TiH2 powder (98% pure, D50=13 μm) as blowing agent were used to produce foams. Foaming experiments conducted in this study are summarized in Table 1. First, Al ingot about 100–110 g was melted in a U-shaped alumina crucible (bottom inner diameter = 39 mm, top inner diameter = 82 mm, height = 90 mm) inside a furnace at 680 °C. Al powder (6 wt.%) and TiH2 (1.5wt.%) were added into the melt as per the sequence mentioned in Table 1. After adding the powder (or powder blend), the melt was stirred
2
using a graphite impeller coated with boron nitride paste. Then, the holding period inside the furnace allowed the foaming to occur. Subsequently, the crucible containing the liquid foam was taken out of the furnace and cooled by passing compressed air towards the outer wall of the crucible. Table 1 Designation, process sequence and expansion factor values listed for foams. ‘S’ and ‘H’ stand for stirring (at 1200 rpm) and holding period, respectively. The duration of each of them is 1 min. In “Type-1”, a blend of Al powder (Alp) and TiH2 powder (mixed using a tumbling mixer and alumina balls for 40 min) was added to Al melt. Expansion Oxygen Foam (#)
Type-1 (3) Type-2 (5) Type-3 (2) Type-4 (2) #Number
Processing steps
𝐴𝑙 𝑚𝑒𝑙𝑡 + (𝐴𝑙𝑝 + 𝑇𝑖𝐻2) 𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝐴𝑙𝑝
𝑺
+ 𝑇𝑖𝐻2
𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝑇𝑖𝐻2 𝐴𝑙 𝑚𝑒𝑙𝑡
𝑺
𝑺+𝑯
𝑺+𝑯
+ 𝑇𝑖𝐻2
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
factor
content$
(hf /h0)
(wt.%)
3.12±0.02
0.74±0.51
3.11±0.03
1.44±0.99
1.69±0.01
1.23±0.72
2.71±0.01
0.59±0.40
of foaming trials; $Average of three measurements
The structure of the foams was characterized by analyzing their central vertical sections. The sectioned samples were painted with matt black color to enhance the contrast and then ground with SiC emery papers. Samples were imaged using an optical scanner (HP Officejet Pro) at a resolution of 600 DPI. Image analysis was performed using ImageJ software to determine cell size distribution and mean cell diameter (Dmean). The foam expansion factor (hf /h0) was calculated from the height (hf) of solidified foam and the height of liquid melt (h0). To measure h0, prior to foaming a stainless steel wire (Ø = 2 mm) was dipped vertically inside the melt (after skimming of the oxide skin) till it touches the bottom of the crucible. The wire was taken out from the melt after about 5 seconds and the length of the Al coated wire was taken as h0. In each case the
3
measurement was repeated five times, and the average value of h0 was considered. The values hf and h0 are indicated as solid and dashed line, respectively, in Figs. 1 and 2. The microstructure of the foams was examined by scanning electron microscopy (SEM, FEI-Quanta-400) equipped with energy dispersive spectroscopy (EDS). The oxide content was measured using LECO ONH836 analyzer. All the errors mentioned here represent the standard deviation. Results Fig. 1 and Table 1 show that the expansion of the Type-1 and Type-2 foams are the same. However, Type-1 foam exhibits a narrow cell size distribution with finer cells compared to Type-2 foams. Between the Type-3 and Type-4 foams shown in Fig. 2, the latter one has higher expansion as seen in Table 1.
Figure 1 Vertical section of (a) Type-1 and (b) Type-2 foams. Cell size distribution of (c) Type-1 and (d) Type-2 foam. R2 is goodness of fit.
4
Figure 2 Vertical section of (a) Type-3 and (b) Type-4 foams. Fig. 3a and 3c show the morphology of oxide clusters present in the cell wall of Type-1 foam. On the contrary, such oxides were not found in Type-2 foam, see Fig. 3b and 3d. Fig. 3d shows that the gas-solid interface is covered with oxygen. The same was observed in the case of Type-1 foam (not shown here).
Figure 3 Back-scattered SEM images of the cell wall of (a) Type-1 and (b) Type-2 foams and their corresponding EDS elemental maps of oxygen in (c) and (d). The oxygen content of the ingot and Al powder was 0.13±0.05 wt.% and 0.74±0.13 wt.%, respectively. Table 1 indicates that during the processing of each type of foam,
5
the melt underwent oxidation. A high standard deviation in oxygen content implies that the oxygen level in each foam could be considered comparable. Since only 6 wt.% of Al powder was added in the Type-1 and Type-2 foams, its contribution, which is only about 0.05 wt.%, in the total oxygen content of foams can be ignored. Discussion In this study, Al powder was chosen because of two reasons. The primary reason is that the oxide layer on Al powder surface is continuous due to a favorable Pilling-Bedworth ratio of 1.29 [12]. Therefore, the metallic part of the Al powder does not come in contact with Al melt unless the oxide layer is damaged. In this way, the Al powders remain as solid-like particles in the Al melt and could stabilize the foam. The secondary reason is that in the case when the oxide layer on Al powder surface is damaged (e.g., due to stirring), its metallic content would mix with the melt without leading to any intermetallic formation which would otherwise alter the mechanical properties of the matrix. The results clearly show that pure Al can be foamed by the addition of Al powder in the melt. The foam expansion achieved in the present study using Al powder is about 210% (Type-1 and Type-2 foams, see Table 1), which is comparable to the expansion of other foams produced by other methods about 280% [6], 300% [4], and (220% 290%) [13].The pore size (3.11 mm) of the Type-1 foams obtained in the present study is comparable with the pore size of the foams produced using other stabilizing particles such as SiC (24 mm) [3], MgAl2O4 (1.5 mm) [4], graphene nanoflakes (2 mm) [11], etc. The role of Al powder in foam stabilization is elucidated in this section. Distribution of gas source and stabilization of bubbles influence the final foam structure. Al powder
6
addition has a positive contribution either in both or one of them depending on when it is added during the process. Since Al powders are solid particles, they can act as stabilizing particles similar to the -phase particles reported by Körner et al. [7]. When a blend of Al powder and TiH2 powder is added into the melt, it results in better dispersion of gas source in the melt [11]. It is known that a better distribution of gas source leads to a more uniform foam structure [13]. As seen in Fig. 1, Type-1 foams exhibited the best structure because both the contributions of Al powder are applicable for these foams. Whereas, since Al powder is added separately in the case of Type-2 foams, Al powders contribute only towards stabilization. Therefore, although Type-2 foams attain similar expansion, their cell size distribution is inferior due to a non-uniform dispersion of gas sources. Aluminum melt exposed to air readily oxidizes because of the entrained air during melting [14] and stirring [15]. These oxides are in the form of bi-films, particles and oxide layer that contribute towards foam stabilization [14, 15]. Even though Al powder was not added, oxidation of the melt alone led to the partial stabilization of the Type-3 and Type-4 foams, a similar observation reported by Kadoi et al. [15]. The importance of oxygen or oxidation for stabilization of foams was also reported in other methods for foam production. For instance, Heim et al. and Babcsán et al. showed that the oxide skin formed at the gas/liquid interface place a crucial role in stabilization of foams produced by gas injection method [16, 17]. As seen in Table 1, an additional 1 min stirring does not increase the oxidation in the Type-4 foams. However, it is expected that a better distribution of oxides and the formation of a larger number of oxide particles take place with increasing stirring [8]. Consequently, Type-4 foams show a higher expansion than Type-3 foams, see Fig. 2. Therefore, the stabilization achieved in the Type-1 and
7
Type-2 foams cannot be attributed solely to the role of Al powder, the oxidation caused due to stirring also plays a role in foam stabilization and the presence of oxides in Fig. 3 confirms this. Al powder particles contain a nanometer-thick outer oxide layer [18]. During stirring, this oxide layer of some of the Al powder particles may rupture and subsequently the metallic part of these powder particle dissolves in the melt while the oxide layer disintegrates into oxide particles of nanometer size. The Al powder particles that disintegrate do not directly stabilize the foam. However, the oxide particles resulted from their oxide layer can in turn agglomerate and form 3D oxide clusters as mentioned in Refs. [18, 19]. If all the Al powder particles remained intact, such oxide clusters should only be oxide bi-films. Since both the powder and the cell wall matrix are pure Al, it is not possible to detect the Al powder particles in the cell wall microstructure. Therefore, it remains unclear whether or not the Al powder particles disintegrate. Some complimentary but similar experiments were performed using Zn powders instead of Al powders. The results suggest that Zn powders disintegrate in the melt. The details are provided in the supplementary materials. However, Zn being a lower melting point metal, the results from these tests cannot be directly correlated with the study using Al powders. Conclusions It was demonstrated that pure Al can be foamed by adding Al powder alone in the melt and no thickening agent or ceramic particles are necessary for stabilization. Addition of Al powder and TiH2 powder mixture in Type-1 foam resulted in the best structure than when they were added separately in Type-2 foam. This is because, in the former, Al powders acted both as stabilizing particles and dispersing agent, whereas in the latter
8
only stabilizing effect of Al powders was present. SEM/EDS analysis and oxygen content data showed that during the foaming process melt also experienced oxidation, which are present as oxide layer and clusters in the cell walls. These also contributed towards stabilization. References [1] J. Banhart, Adv. Eng. Mater. 15 (2013) 82–111. [2] N. Babcsán, G.S. Vinod-Kumar, B.S. Murty, J. Banhart, Trans. Indian Inst. Met. 60 (2007) 127–132. [3] M.C. Gui, D.B. Wang, J.J. Wu, G.J. Yuan, C.G. Li, Mater. Sci. Eng. A. 286 (2000) 282–288. [4] G.S.V. Kumar, M. Chakraborty, F. Garcia-Moreno, J. Banhart, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 42 (2011) 2898–2908. [5] T. Fukui, Y. Nonaka, S. Suzuki, Procedia Mater. Sci. 4 (2014) 33–37. [6] S. Bhogi, M. Mukherjee, Mater. Lett. 200 (2017) 118–120. [7] C. Körner, M. Hirschmann, V. Bräutigam, R.F. Singer, Adv. Eng. Mater. 6 (2004) 385–390. [8] C.C. Yang, H. Nakae, J. Mater. Process. Technol. 141 (2003) 202–206. [9] W.M. Zhao, H. Zhang, H.P. Li, Z.F. Wang, Y. Zhao, R. Zhao, B.Y. Hur, Adv. Mater. Res. 214 (2011) 70–74. [10] J.D.Bryant, J.A.Kallivayalil, M.D.Crowley, J.R.Genito, L.F. Wieserman, D.M.Wilhelmy, W.E.Boren, U.S. Patent 20060243094 A1, (2008). [11] Y. An, S. Yang, E. Zhao, Z. Wang, H. Wu, Mater. Lett. 212 (2018) 4–7. [12] C.Xu, W.Gao, Mater. Res. Innovat. 3.4 (2000) 231–-235. [13] M. Mukherjee, F. Garcia-Moreno, C. Jiménez, J. Banhart, Adv. Eng. Mater. 12 (2010) 472–477. [14] V. Pamidi, M. Mukherjee, Materialia. 4 (2018) 500–509. [15] K. Kadoi, N. Babcsán, H. Nakae, Mater. Sci. Forum. 649 (2010) 385–390. [16] K. Heim, G.S. Vinod-Kumar, F. García-Moreno, A. Rack, J. Banhart, Acta Mater. 99 (2015) 313–324. [17]N. Babcsán, D. Leitlmeier, H.P. Degischer, J. Banhart, Adv. Eng. Mater. 6 (2004) 421–428. [18] A. Dudka, F. Garcia-Moreno, N. Wanderka, J. Banhart, Acta Mater. 56 (2008) 3990–4001. [19] C. Körner, M. Arnold, R.F. Singer, Mater. Sci. Eng. A. 396 (2005) 28–40.
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Conflict of interest and authorship statement o
All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
o This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. o The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript
Authors
Signature
Date
Mr.S. Sasikumar
23-11-2019
19
Mr.K. Georgy 23-11-2019
Dr. M. Mukherjee 23-11-2019
Dr. G. S. Vinod Kumar
23-11-2019
Highlights
Pure Al can be foamed by the addition of Al powder in the melt.
Al powder addition improves stabilization as well as uniform dispersion of TiH2.
Oxide layer and clusters located in the cell wall were resulted from Al powder addition.
These oxides contributed towards foam stabilization.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Foam Stabilization by Al powder)
Authors
Signature
Date
Mr.S. Sasikumar
23-11-2019
20
Mr.K. Georgy 23-11-2019
Dr. M. Mukherjee 23-11-2019
Dr. G. S. Vinod Kumar
23-11-2019
Figure 2
21
Figure 2
22
Figure 3
23
Figure S1
24
Figure Captions Figure 3 Vertical section of (a) Type-1 and (b) Type-2 foams. Cell size distribution of (c) Type-1 and (d) Type-2 foam. R2 is goodness of fit.
25
Figure 2 Vertical section of (a) Type-3, and (b) Type-4 foams.
Foam (#)
Type-1 (3) Type-2 (5) Type-3 (2) Type-4 (2)
Processing steps
𝐴𝑙 𝑚𝑒𝑙𝑡 + (𝐴𝑙𝑝 + 𝑇𝑖𝐻2) 𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝐴𝑙𝑝
𝑺
+ 𝑇𝑖𝐻2
𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝑇𝑖𝐻2 𝐴𝑙 𝑚𝑒𝑙𝑡
𝑺
𝑺+𝑯
𝑺+𝑯
+ 𝑇𝑖𝐻2
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
Expansion
Oxygen
factor
content$
(hf /h0)
(wt.%)
3.12±0.02
0.74±0.51
3.11±0.03
1.44±0.99
1.69±0.01
1.23±0.72
2.71±0.01
0.59±0.40
Figure 3 Back-scattered SEM images of the cell wall of (a) Type-1 and (b) Type-2 foams and their corresponding EDS elemental maps of oxygen in (c) and (d). Figure S1 (a) Back-scattered SEM image of the cell wall of Al foam (equivalent to Type-2 foam) produced by adding Zn powder, and (b) its corresponding EDS elemental maps of Al, oxygen, Zn, and Ti. The arrows in (b) indicate Zn inside the Al matrix.
Table 1
26
Table Caption: Table 1 Designation, process sequence and expansion factor values listed for foams. ‘S’ and ‘H’ stand for stirring (at 1200 rpm) and holding period, respectively. The duration of each of them is 1 min. In “Type-1”, a blend of Al powder (Alp) and TiH2 powder (mixed using a tumbling mixer and alumina balls for 40 min) was added to Al melt.
Table Footnotes: #Number
of foaming trials; $Average of three measurements
27
S0167-577X(19)31774-4 https://doi.org/10.1016/j.matlet.2019.127142 MLBLUE 127142
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
4 October 2019 23 November 2019 8 December 2019
Please cite this article as: S. Sasikumar, K. Georgy, M. Mukherjee, G.S.Vinod Kumar, Foam stabilization by aluminum powder, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.127142
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Foam stabilization by aluminum powder S. Sasikumar1, K. Georgy2, M. Mukherjee2, G.S.Vinod Kumar3,* 1Department
of Mechanical Engineering, SRM IST, Kattankulathur, Chennai 603203,
India 2Department
of Metallurgical and Materials Engineering, Indian Institute of
Technology Madras, Chennai 600036, India 3Department
of Mechanical Engineering, SRM University-AP, Amaravati 522502, India
*Corresponding author, Email address: [email protected] Abstract In this study, it was shown that aluminum powder can be used as stabilizing particles for the fabrication of aluminum foams by melt route. When Al powder was mixed with the TiH2 before adding into the melt, it also acted as dispersing agent for the TiH2 thus further improving the structure of the foams. Stirring during powder mixing also contributed towards foam stability by introducing oxides into the melt. The oxides were examined using SEM/EDS and oxygen analyzer. Keywords: Porous materials; Oxidation; Aluminum powder; Stabilization; Dispersion; Melt route. Introduction Melt route is the most preferred method for the fabrication of closed-cell metal foams because it is a simple and cost-effective process, and also well suited for batch production [1]. Melt route foaming necessitates stabilizing particles [2], which is either added into the melt (ex-situ: SiC) or produced inside the melt (in-situ: TiC, TiB2, and MgAl2O4) [24]. Particle addition, ex-situ into the melt is challenging due to the non-wetting nature of the particles with the melt. While the in-situ formation of particles
1
in the melt demands larger processing step(s) and hence a significant time. Stabilization can also be achieved by adding Mg ingots into Al melt [5, 6]. This is due to the in-situ formation of MgAl2O4 and transition oxide particles [6]. Körner et al. [7] exploited -phase particles as a stabilizing agent for foaming a semisolid mass of Mg melt. Yang et al. produced foams from AlSi7Mg0.5 alloy melt by first agitating the melt with 5 wt.% Al powder for 20 min in air before adding TiH2. An increase in viscosity due to oxide particle (alumina) formation was attributed to foam stabilization [8]. Zhao et al. demonstrated foamability of pure Al melt agitated with 3 wt.% of Al powder for 10 min [9]. In this study, however, CaCO3 was used as blowing agent which is known to cause stabilization through the formation of several types of oxides depending on the alloy composition [10]. In these studies, Al powder indirectly contributes to stabilization by forming oxide particles. Some studies used Al powder as a dispersing agent with TiH2 [3, 11]. While the role of Al powder as dispersing agent is clear, any direct role as stabilizing particle has not been established. In this work, we show the role of Al powder both as a dispersing agent and as stabilizing particles. Materials and methods Commercial pure aluminum ingots (99.6 % pure), Al powder (99.7 % pure, D50=45 μm) and TiH2 powder (98% pure, D50=13 μm) as blowing agent were used to produce foams. Foaming experiments conducted in this study are summarized in Table 1. First, Al ingot about 100–110 g was melted in a U-shaped alumina crucible (bottom inner diameter = 39 mm, top inner diameter = 82 mm, height = 90 mm) inside a furnace at 680 °C. Al powder (6 wt.%) and TiH2 (1.5wt.%) were added into the melt as per the sequence mentioned in Table 1. After adding the powder (or powder blend), the melt was stirred
2
using a graphite impeller coated with boron nitride paste. Then, the holding period inside the furnace allowed the foaming to occur. Subsequently, the crucible containing the liquid foam was taken out of the furnace and cooled by passing compressed air towards the outer wall of the crucible. Table 1 Designation, process sequence and expansion factor values listed for foams. ‘S’ and ‘H’ stand for stirring (at 1200 rpm) and holding period, respectively. The duration of each of them is 1 min. In “Type-1”, a blend of Al powder (Alp) and TiH2 powder (mixed using a tumbling mixer and alumina balls for 40 min) was added to Al melt. Expansion Oxygen Foam (#)
Type-1 (3) Type-2 (5) Type-3 (2) Type-4 (2) #Number
Processing steps
𝐴𝑙 𝑚𝑒𝑙𝑡 + (𝐴𝑙𝑝 + 𝑇𝑖𝐻2) 𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝐴𝑙𝑝
𝑺
+ 𝑇𝑖𝐻2
𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝑇𝑖𝐻2 𝐴𝑙 𝑚𝑒𝑙𝑡
𝑺
𝑺+𝑯
𝑺+𝑯
+ 𝑇𝑖𝐻2
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
factor
content$
(hf /h0)
(wt.%)
3.12±0.02
0.74±0.51
3.11±0.03
1.44±0.99
1.69±0.01
1.23±0.72
2.71±0.01
0.59±0.40
of foaming trials; $Average of three measurements
The structure of the foams was characterized by analyzing their central vertical sections. The sectioned samples were painted with matt black color to enhance the contrast and then ground with SiC emery papers. Samples were imaged using an optical scanner (HP Officejet Pro) at a resolution of 600 DPI. Image analysis was performed using ImageJ software to determine cell size distribution and mean cell diameter (Dmean). The foam expansion factor (hf /h0) was calculated from the height (hf) of solidified foam and the height of liquid melt (h0). To measure h0, prior to foaming a stainless steel wire (Ø = 2 mm) was dipped vertically inside the melt (after skimming of the oxide skin) till it touches the bottom of the crucible. The wire was taken out from the melt after about 5 seconds and the length of the Al coated wire was taken as h0. In each case the
3
measurement was repeated five times, and the average value of h0 was considered. The values hf and h0 are indicated as solid and dashed line, respectively, in Figs. 1 and 2. The microstructure of the foams was examined by scanning electron microscopy (SEM, FEI-Quanta-400) equipped with energy dispersive spectroscopy (EDS). The oxide content was measured using LECO ONH836 analyzer. All the errors mentioned here represent the standard deviation. Results Fig. 1 and Table 1 show that the expansion of the Type-1 and Type-2 foams are the same. However, Type-1 foam exhibits a narrow cell size distribution with finer cells compared to Type-2 foams. Between the Type-3 and Type-4 foams shown in Fig. 2, the latter one has higher expansion as seen in Table 1.
Figure 1 Vertical section of (a) Type-1 and (b) Type-2 foams. Cell size distribution of (c) Type-1 and (d) Type-2 foam. R2 is goodness of fit.
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Figure 2 Vertical section of (a) Type-3 and (b) Type-4 foams. Fig. 3a and 3c show the morphology of oxide clusters present in the cell wall of Type-1 foam. On the contrary, such oxides were not found in Type-2 foam, see Fig. 3b and 3d. Fig. 3d shows that the gas-solid interface is covered with oxygen. The same was observed in the case of Type-1 foam (not shown here).
Figure 3 Back-scattered SEM images of the cell wall of (a) Type-1 and (b) Type-2 foams and their corresponding EDS elemental maps of oxygen in (c) and (d). The oxygen content of the ingot and Al powder was 0.13±0.05 wt.% and 0.74±0.13 wt.%, respectively. Table 1 indicates that during the processing of each type of foam,
5
the melt underwent oxidation. A high standard deviation in oxygen content implies that the oxygen level in each foam could be considered comparable. Since only 6 wt.% of Al powder was added in the Type-1 and Type-2 foams, its contribution, which is only about 0.05 wt.%, in the total oxygen content of foams can be ignored. Discussion In this study, Al powder was chosen because of two reasons. The primary reason is that the oxide layer on Al powder surface is continuous due to a favorable Pilling-Bedworth ratio of 1.29 [12]. Therefore, the metallic part of the Al powder does not come in contact with Al melt unless the oxide layer is damaged. In this way, the Al powders remain as solid-like particles in the Al melt and could stabilize the foam. The secondary reason is that in the case when the oxide layer on Al powder surface is damaged (e.g., due to stirring), its metallic content would mix with the melt without leading to any intermetallic formation which would otherwise alter the mechanical properties of the matrix. The results clearly show that pure Al can be foamed by the addition of Al powder in the melt. The foam expansion achieved in the present study using Al powder is about 210% (Type-1 and Type-2 foams, see Table 1), which is comparable to the expansion of other foams produced by other methods about 280% [6], 300% [4], and (220% 290%) [13].The pore size (3.11 mm) of the Type-1 foams obtained in the present study is comparable with the pore size of the foams produced using other stabilizing particles such as SiC (24 mm) [3], MgAl2O4 (1.5 mm) [4], graphene nanoflakes (2 mm) [11], etc. The role of Al powder in foam stabilization is elucidated in this section. Distribution of gas source and stabilization of bubbles influence the final foam structure. Al powder
6
addition has a positive contribution either in both or one of them depending on when it is added during the process. Since Al powders are solid particles, they can act as stabilizing particles similar to the -phase particles reported by Körner et al. [7]. When a blend of Al powder and TiH2 powder is added into the melt, it results in better dispersion of gas source in the melt [11]. It is known that a better distribution of gas source leads to a more uniform foam structure [13]. As seen in Fig. 1, Type-1 foams exhibited the best structure because both the contributions of Al powder are applicable for these foams. Whereas, since Al powder is added separately in the case of Type-2 foams, Al powders contribute only towards stabilization. Therefore, although Type-2 foams attain similar expansion, their cell size distribution is inferior due to a non-uniform dispersion of gas sources. Aluminum melt exposed to air readily oxidizes because of the entrained air during melting [14] and stirring [15]. These oxides are in the form of bi-films, particles and oxide layer that contribute towards foam stabilization [14, 15]. Even though Al powder was not added, oxidation of the melt alone led to the partial stabilization of the Type-3 and Type-4 foams, a similar observation reported by Kadoi et al. [15]. The importance of oxygen or oxidation for stabilization of foams was also reported in other methods for foam production. For instance, Heim et al. and Babcsán et al. showed that the oxide skin formed at the gas/liquid interface place a crucial role in stabilization of foams produced by gas injection method [16, 17]. As seen in Table 1, an additional 1 min stirring does not increase the oxidation in the Type-4 foams. However, it is expected that a better distribution of oxides and the formation of a larger number of oxide particles take place with increasing stirring [8]. Consequently, Type-4 foams show a higher expansion than Type-3 foams, see Fig. 2. Therefore, the stabilization achieved in the Type-1 and
7
Type-2 foams cannot be attributed solely to the role of Al powder, the oxidation caused due to stirring also plays a role in foam stabilization and the presence of oxides in Fig. 3 confirms this. Al powder particles contain a nanometer-thick outer oxide layer [18]. During stirring, this oxide layer of some of the Al powder particles may rupture and subsequently the metallic part of these powder particle dissolves in the melt while the oxide layer disintegrates into oxide particles of nanometer size. The Al powder particles that disintegrate do not directly stabilize the foam. However, the oxide particles resulted from their oxide layer can in turn agglomerate and form 3D oxide clusters as mentioned in Refs. [18, 19]. If all the Al powder particles remained intact, such oxide clusters should only be oxide bi-films. Since both the powder and the cell wall matrix are pure Al, it is not possible to detect the Al powder particles in the cell wall microstructure. Therefore, it remains unclear whether or not the Al powder particles disintegrate. Some complimentary but similar experiments were performed using Zn powders instead of Al powders. The results suggest that Zn powders disintegrate in the melt. The details are provided in the supplementary materials. However, Zn being a lower melting point metal, the results from these tests cannot be directly correlated with the study using Al powders. Conclusions It was demonstrated that pure Al can be foamed by adding Al powder alone in the melt and no thickening agent or ceramic particles are necessary for stabilization. Addition of Al powder and TiH2 powder mixture in Type-1 foam resulted in the best structure than when they were added separately in Type-2 foam. This is because, in the former, Al powders acted both as stabilizing particles and dispersing agent, whereas in the latter
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only stabilizing effect of Al powders was present. SEM/EDS analysis and oxygen content data showed that during the foaming process melt also experienced oxidation, which are present as oxide layer and clusters in the cell walls. These also contributed towards stabilization. References [1] J. Banhart, Adv. Eng. Mater. 15 (2013) 82–111. [2] N. Babcsán, G.S. Vinod-Kumar, B.S. Murty, J. Banhart, Trans. Indian Inst. Met. 60 (2007) 127–132. [3] M.C. Gui, D.B. Wang, J.J. Wu, G.J. Yuan, C.G. Li, Mater. Sci. Eng. A. 286 (2000) 282–288. [4] G.S.V. Kumar, M. Chakraborty, F. Garcia-Moreno, J. Banhart, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 42 (2011) 2898–2908. [5] T. Fukui, Y. Nonaka, S. Suzuki, Procedia Mater. Sci. 4 (2014) 33–37. [6] S. Bhogi, M. Mukherjee, Mater. Lett. 200 (2017) 118–120. [7] C. Körner, M. Hirschmann, V. Bräutigam, R.F. Singer, Adv. Eng. Mater. 6 (2004) 385–390. [8] C.C. Yang, H. Nakae, J. Mater. Process. Technol. 141 (2003) 202–206. [9] W.M. Zhao, H. Zhang, H.P. Li, Z.F. Wang, Y. Zhao, R. Zhao, B.Y. Hur, Adv. Mater. Res. 214 (2011) 70–74. [10] J.D.Bryant, J.A.Kallivayalil, M.D.Crowley, J.R.Genito, L.F. Wieserman, D.M.Wilhelmy, W.E.Boren, U.S. Patent 20060243094 A1, (2008). [11] Y. An, S. Yang, E. Zhao, Z. Wang, H. Wu, Mater. Lett. 212 (2018) 4–7. [12] C.Xu, W.Gao, Mater. Res. Innovat. 3.4 (2000) 231–-235. [13] M. Mukherjee, F. Garcia-Moreno, C. Jiménez, J. Banhart, Adv. Eng. Mater. 12 (2010) 472–477. [14] V. Pamidi, M. Mukherjee, Materialia. 4 (2018) 500–509. [15] K. Kadoi, N. Babcsán, H. Nakae, Mater. Sci. Forum. 649 (2010) 385–390. [16] K. Heim, G.S. Vinod-Kumar, F. García-Moreno, A. Rack, J. Banhart, Acta Mater. 99 (2015) 313–324. [17]N. Babcsán, D. Leitlmeier, H.P. Degischer, J. Banhart, Adv. Eng. Mater. 6 (2004) 421–428. [18] A. Dudka, F. Garcia-Moreno, N. Wanderka, J. Banhart, Acta Mater. 56 (2008) 3990–4001. [19] C. Körner, M. Arnold, R.F. Singer, Mater. Sci. Eng. A. 396 (2005) 28–40.
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All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.
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Authors
Signature
Date
Mr.S. Sasikumar
23-11-2019
19
Mr.K. Georgy 23-11-2019
Dr. M. Mukherjee 23-11-2019
Dr. G. S. Vinod Kumar
23-11-2019
Highlights
Pure Al can be foamed by the addition of Al powder in the melt.
Al powder addition improves stabilization as well as uniform dispersion of TiH2.
Oxide layer and clusters located in the cell wall were resulted from Al powder addition.
These oxides contributed towards foam stabilization.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(Foam Stabilization by Al powder)
Authors
Signature
Date
Mr.S. Sasikumar
23-11-2019
20
Mr.K. Georgy 23-11-2019
Dr. M. Mukherjee 23-11-2019
Dr. G. S. Vinod Kumar
23-11-2019
Figure 2
21
Figure 2
22
Figure 3
23
Figure S1
24
Figure Captions Figure 3 Vertical section of (a) Type-1 and (b) Type-2 foams. Cell size distribution of (c) Type-1 and (d) Type-2 foam. R2 is goodness of fit.
25
Figure 2 Vertical section of (a) Type-3, and (b) Type-4 foams.
Foam (#)
Type-1 (3) Type-2 (5) Type-3 (2) Type-4 (2)
Processing steps
𝐴𝑙 𝑚𝑒𝑙𝑡 + (𝐴𝑙𝑝 + 𝑇𝑖𝐻2) 𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝐴𝑙𝑝
𝑺
+ 𝑇𝑖𝐻2
𝐴𝑙 𝑚𝑒𝑙𝑡 + 𝑇𝑖𝐻2 𝐴𝑙 𝑚𝑒𝑙𝑡
𝑺
𝑺+𝑯
𝑺+𝑯
+ 𝑇𝑖𝐻2
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
𝐹𝑜𝑎𝑚
𝑺+𝑯
𝐹𝑜𝑎𝑚
Expansion
Oxygen
factor
content$
(hf /h0)
(wt.%)
3.12±0.02
0.74±0.51
3.11±0.03
1.44±0.99
1.69±0.01
1.23±0.72
2.71±0.01
0.59±0.40
Figure 3 Back-scattered SEM images of the cell wall of (a) Type-1 and (b) Type-2 foams and their corresponding EDS elemental maps of oxygen in (c) and (d). Figure S1 (a) Back-scattered SEM image of the cell wall of Al foam (equivalent to Type-2 foam) produced by adding Zn powder, and (b) its corresponding EDS elemental maps of Al, oxygen, Zn, and Ti. The arrows in (b) indicate Zn inside the Al matrix.
Table 1
26
Table Caption: Table 1 Designation, process sequence and expansion factor values listed for foams. ‘S’ and ‘H’ stand for stirring (at 1200 rpm) and holding period, respectively. The duration of each of them is 1 min. In “Type-1”, a blend of Al powder (Alp) and TiH2 powder (mixed using a tumbling mixer and alumina balls for 40 min) was added to Al melt.
Table Footnotes: #Number
of foaming trials; $Average of three measurements
27