Investigating Alumina-Silicate Bauxite and Phenol-Formaldehyde Resin Embedded TiH₂ as Foaming Agents for Producing A356 Foam

Abstract

The melt foaming process has become the most widely used method in closed-cell aluminum foam manufacturing in large dimensions. This process creates pores by adding a foaming agent to the molten metal. Therefore, selecting appropriate foaming agents is vital, and it controls pore sizes and their distribution in producing a homogeneous foam. In the current research, as cost-effective foaming agents, the Bauxite and Phenol-formaldehyde resin (PFR) embedded TiH₂ were successfully produced and then investigated by SEM and EDX analysis. It can be concluded that in the presence of the Bauxite ceramic phase and silica gel formation in Bauxite-embedded TiH₂ and the formation of a carbon layer due to the burning of resin in PFR-embedded TiH₂, heat-resistant protective layers are formed around TiH₂ powders, and thus it delays (120 s) the gas release. The delay in the decomposition of H₂ gas is equal to/higher than in the literature, and it gives the gas’ bubbles enough time to establish pores in the metallic matrix; thus, foams with uniform distribution of pores were produced. A quantitative examination of the cross-section of the produced foams shows that the number of cells with smaller sizes in the foams produced with the modified foaming agent is more, and the distribution of pores or cells is more homogeneous.

1. Introduction

Cellular materials (CM) comprise an interconnected network of solid struts or plates that form the edges and faces of cells [1]. Metal foams are a class of CM inspired by nature, e.g., wood, bones, and sea sponges [2,3]. Aluminum (Al) foam, as a metallic foam, because of its physical and mechanical properties, could be widely used in various strategic areas such as aerospace, automotive, petroleum, transportation, construction, etc. [4,5,6].

Al foams are produced with various methods, e.g., powder metallurgy, sintering, adding gas in melt injection, using the agent in melt foaming, and investing casting [7]. Due to its simple operation and low cost, the melt foaming process has become the most widely used method in closed-cell Al foam manufacturing in large dimensions [8,9,10]. In the melt foaming process, pores are created by adding a foaming agent to the molten metal, which can release gas due to the reaction with the melt [11]. Since pore sizes and their distribution in producing a homogeneous foam with controllable porosity are critical in the industrialization of metal foams, some research has been conducted on foaming agents and their modification and improvement [12]. Therefore, the foaming agent and its dispersion state play an essential role in the melting method and the fabrication of small pore-size foams [8,13,14].

Titanium hydride (TiH₂) is widely and industrially used as a foaming agent with high specific hydrogen content and reasonable economic cost [15]. Nevertheless, TiH₂ has low thermal decomposition temperature properties mismatched with Al’s melting point and leads to poor foam structure and low mechanical and physical properties in Al foam [16]. Therefore, it is necessary to improve the decomposition temperature of TiH₂ to improve the structure and mechanical properties of Al foam [17].

Kennedy et al. [18] and Matijasevic-Lux et al. [19] used heat treatment to prevent the decomposition of TiH₂ during the Al melting by creating a dense layer of oxide on TiH₂. Fang et al. [20] created a homogeneous and dense SiO₂/Al₂O₃ coating on the TiH₂ surface by the sol-gel method, which delayed the release time of the initial hydrogen gas for up to 2 min at 700 °C. Proa-Flores et al. [21] used Ni-coated TiH₂ in the production of Al foam to create a significant delay in the gas release. Jiang et al. [22] used SiO₂/TiO_X and observed that the dense SiO₂/TiOx composite layer could delay the primary hydrogen gas release time due to TiH₂ decomposition by more than 180 s at 650 °C. Thus, engineering the TiH₂ surface with a uniform and dense layer and having high thermal resistance is a helpful way to improve the decomposition temperature and delay the release time of hydrogen gas [23].

Most published research has focused on the principle of creating and modifying a uniform and dense layer. However, there are still challenges to developing a practical, simple, cost-effective, and mass-producible approach to modifying the TiH₂ foaming agent [24,25]. Zhou et al. [14] introduced a new foaming agent for producing Al foam with a simple, low-cost, industrialized production process. They used the AlMg35-TiH₂ composite foaming agent and declared an Al foam with an average pore size of 0.6 mm.

Another theory is intended not to delay the decomposition and gas release of the dense layer but to introduce alumina with lower thermal conductivity and high porosity to control the release of TiH₂ gas [10].

In this research, at first, the alumina coating-modified TiH₂ (Al₂O₃-TiH₂) was successfully produced as a foaming agent using the slurry method on an industrial scale. The foam morphology and structure and Al₂O₃-TiH₂ thermal properties were assessed. Additionally, the TiH₂ embedded in Phenol-formaldehyde resin (PFR) as an innovative foaming agent was successfully investigated. Therefore, the novelty of our work is the use of two foaming agents that have been modified cost-effectively and makes it feasible to produce foams in these techniques on an industrial scale.

2. Materials and Methods

2.1. Modified Al₂O₃-TiH₂ Preparation

The raw materials used include bauxite with 80% alumina with an average particle size of 50 μm (Spinel Refractories Company), 99.99% micro silica with an average particle size of 60 μm (Khomein Ferrosilicon Co., Khomeyn, Iran), TiH₂ powders with an average particle size of 45 μm, and lubricant. The chemical analysis of the raw materials was based on X-ray fluorescence spectrometry (XRF) and inductively coupled plasma optical emission spectrometry (ICP-OES) to determine oxides. The chemical analysis of the used raw materials is presented in

Table 1. Chemical analysis (wt.%) of Bauxite used in this research.

	Al2O3	SiO2	CaO	BaO	MgO	MnO	Fe2O3	TiO2	K2O	Na2O	P2O5	SO3	LOI
Bauxite	77.07	13.41	0.27	<0.05	0.55	<0.05	2.79	4.48	0.05	0.05	<0.05	<0.05	0.27

and

Table 2. Chemical analysis (wt.%) of micro-silica powder used in this research.

	SiO2	Al2O3	Fe2O3	C	Na2O	K2O	MgO	S	CaO
Micro silica	86–94	0.4–1.0	0.2–1.5	0.5–2.5	0.4–1.5	1.0–3.0	0.5–2.0	0.1–0.4	0.1–0.5

via weight percent, and the main characteristics of the TiH₂ powder are given in

Table 3. Physical and chemical characteristics of TiH₂ powder.

Material	Purity (%)	Average Grain Size (μm)	Density (gm/cm3)	Decomposition Temperature (℃)
TiH2	99	45	3.7	$~$400

. Additionally, the morphology of the non-coated TiH₂ powders and its EDX analysis are presented in Figure 1a,b.

Figure 1a shows that the particle size is around 15 μm. Additionally, the chemical analysis by the EDX method of non-coated TiH₂ powder is shown in Figure 1b. As is clear from Figure 1b, only the Ti element can be detected, and no specific impurity is observed.

First, 800 gm of bauxite and 190 gm of micro silica with 10 gm of additive were mixed, and then some water was added to prepare a homogenous and stable slurry. The prepared slurry was stirred for 30 min by a magnet stirrer. After that, 100 gm TiH₂ powder was added to the slurry and stirred for 1 hour. Then, the slurry dried at 110 °C for 24 h in the oven. In the next step, the dried slurry was heat treated at 350 °C for 5 h in an electric furnace, and subsequently, it was fast-milled to achieve fine powders. Figure 2 demonstrates the preparation steps of the bauxite-embedded TiH₂ as a foaming agent.

2.2. Modified PFR-TiH₂ Preparation

The raw materials used include Phenolic resin (VP Instruments, Delft, The Netherlands, 99 wt.% purity), TiH₂ powder (Spinel company, 99.7 wt.% in purity, <45 µm on average), and Ethanol (Zanjan Chimie Co., Tehran, Iran, 99.8 wt.% in purity). First, a 0.1 molar solution was prepared with formaldehyde resin mixed with Ethanol. In the next step, 30 gm of TiH₂ was added to the solution and stirred for 2 h with a magnetic stirrer. The precipitated TiH₂ solution was placed in the open air for 24 h (Figure 3a). Next, it was placed in the oven at 150 °C for 24 h to prepare a jelly-like and soft mixture that contained resin and TiH₂ (Figure 3b). Then, it was heat treated at 220 °C for 6 h, and the combination of resin and TiH₂ turned into a fragile and glassy material (Figure 3c). This fragile material was fast-milled, and a dark yellow powder was finally obtained (Figure 3d).

2.3. Al Foam Production

The casting process to produce Al closed-cell foam by the TiH₂ foaming agent was used. The A356 aluminum-silicon alloy used in this research is according to the chemical composition shown in

Table 4. Chemical composition (wt.%) of A356 aluminum-silicon alloy used in this research.

Al	Si	Mg	Cu	Fe
Bal.	7.31	0.29	0.04	0.12

.

Figure 4 shows the equipment used for producing the foam. First, the A356 alloy was melted in a cast iron crucible at a temperature of 700 °C to produce the foam (see Figure 4a,b), then 2% weight percent of calcium granola was added to the melt as a thickener and viscosifier. Then, 1 wt.% titanium hydride powder was added and stirred at 1000 rpm for 10 min (Figure 4c). Next, the melt was poured into a steel mold in the electric furnace shown in Figure 4d to create foam at a temperature of 680 °C for 3 min. With the completion of the foaming process, the melt was cooled, the solidification was complete, the possibility of gas exit was eliminated, and the foam was formed (Figure 4e).

2.4. Characterization

The surface structure of TiH₂ powders before and after modification by bauxite and BPR was investigated by Scanning Electron Microscopy (SEM) equipped with an Energy-Dispersive Spectrometer (EDS). Color mapping of the distributed elements for bauxite-modified TiH₂ was prepared. The thermal decomposition of H₂ gas was studied for PFR-modified TiH₂ by thermal gravimetric analysis (TGA). Additionally, the produced foam was examined by SEM.

3. Results and Discussions

3.1. Investigation of Bauxite Embedded TiH₂

Figure 5a,b illustrates the SEM micrograph with a magnification of 1000× and EDX analysis of TiH₂ powder dispersed inside the ceramic phase (bauxite). Additionally, Figure 5c displays the color mapping of the distributed elements for the bauxite-modified TiH₂. According to Figure 5b, the presence of Ti, Al, Si, O, and Ca elements proves that Al₂O³.SiO₂ (bauxite) embeds TiH₂ powders. Additionally, according to Figure 5c, a TiH₂ powder grain with dark blue color is surrounded by particles including Si (pink), Al (Purple), and O (light blue), where these elements represent the presence of bauxite. Therefore, the embedding process of TiH₂ powders has been successfully carried out.

3.2. Investigation of Phenol-Formaldehyde Resin Embedded TiH₂

Figure 6a–c demonstrates the SEM micrograph of the TiH₂ powder dispersed inside the polymeric phase (phenol-formaldehyde resin) with magnifications of 100×, 1000×, and 3000×, respectively. EDX analysis of PFR-embedded TiH₂ powder was carried out from the three points of Figure 6c indicated by A, B, and C, and the results are presented, respectively in Figure 6d–f.

Figure 6d presents the EDX analysis of point A. It could be found that the peak of the C element is more significant than other elements, e.g., Ti, N, and O. Thus, at this point, the significant peak of C proves that the C phase is dominant in this region. Figure 6e represents the EDX analysis of point B. It is clear that the peak of the Ti element is more significant than the other elements, e.g., C, N, and O. Therefore, at this point, the intensive peak of Ti, as evidence of the TiH₂ phase, proves that the Ti phase is dominant in this region. Figure 6f displays the EDX analysis of point C. The Ti and C peaks indicate that the C point is a transition region between the TiH₂ and burnt carbon phases. Finally, these EDX analyses prove that TiH2 is trapped in the resin matrix and acts as a protective coating layer. This protective layer can delay the release of hydrogen and the decomposition of TiH₂.

The TGA curve of the TiH₂ powder embedded in formaldehyde resin is presented in Figure 7. According to the diagram, the hydrogen gas release starts at almost 120 s. The determination of the gas release time in the foam was obtained from the visual and direct measurement of the gas release time in the melt after adding the foaming agent. The reason for this decomposition delay is the polymer layer’s presence because the layer decomposes and burns first.

3.3. Quantitive Investigation of Cells Sizes and Their Distribution

Figure 8a shows the produced foam and its SEM micrographs (Figure 8b,c). It is clear that the pore sizes are small, and their number is high. Additionally, the pore distribution is homogenous.

Figure 9 quantitatively investigates the distribution and size of pores in foams produced by TiH₂ and modified TiH₂ foaming agents. According to Figure 9a,b, It can be seen that the foam produced by the modified TiH₂ as a foaming agent has a more uniform distribution than the foam produced by TiH₂. Figure 9c,d illustrates the normal distribution by frequency curves for the produced foams by TiH₂ and modified TiH₂ foaming agents. By comparing these two curves, it is clear that the foam produced by the TiH₂ has a sharper peak than that produced by the modified TiH₂. Therefore, the produced foam by modified TiH₂ has a more homogenous distribution than TiH₂. With a comparison of Figure 9e,f, it can be concluded that the maximum frequency of cell size for modified TiH₂ is around 1.26–1.76 mm, and for TiH₂, it is around 2.47–3.46 mm. Therefore, the cell size of the foam produced by the modified TiH₂ is smaller than that of TiH₂. Additionally, by comparing Figure 9e,f with Figure 9g,h, it is evident that the graphs for modified TiH₂ are more symmetrical than TiH₂, which shows the foam produced by modified TiH₂ is more homogenous than TiH₂.

From these results, it could be concluded that the delay in the decomposition of H₂ gas in the produced foams by modified TiH₂ has successfully occurred, and the gases had enough time to establish pores in the metallic matrix.

The foams produced according to the need and application should be investigated. For example, considering that the mechanical properties of the produced foams depend on the cell size and shape, and relative density, the foams produced by modified TiH₂, which have smaller cell size and more homogeneous gas pores, will have a more homogeneous and better mechanical response when a compressive load is applied [26,27].

3.4. Mechanism for Delaying the Release of TiH₂ Gas

The reasons for the delay in the gas release can be mentioned that in the presence of the Bauxite ceramic phase and silica gel formation, a heat-resistant protective layer is formed around TiH₂ and can delay the release up to 120 s. In the case of resin, the formation of a carbon layer due to the burning of resin at a high temperature can be the reason for the delay in the release and the possibility of a more uniform release and the more homogeneous properties of aluminum foams. Schematics of these mechanisms are displayed in Figure 10.

4. Conclusions

In this research, TiH₂ powders were modified cost-effectively with bauxite and PFR, and then the A356 foams were successfully produced by these foaming agents. The modified TiH₂ powders were investigated, and the following results were achieved:

(1)

The EDX analysis and color map of bauxite-embedded TiH₂ powders show the presence of Ti, Al, Si, O, and Ca elements, proving that Al₂O₃.SiO₂ (bauxite) embeds TiH₂ powders.

(2)

EDX analysis of PFR-embedded TiH₂ powder was carried out from the three points, A, B, and C, indicated in the SEM micrograph. The results show a transition zone between TiH₂ powder and the polymeric matrix (resin), proving TiH₂ powders are embedded in the PFR matrix. According to the TGA results, this protective layer can delay the release of hydrogen and the decomposition of TiH₂.

(3)

The heat-resistant protective layer is the mechanism of the delay in TiH₂ decomposition in the presence of the Bauxite ceramic phase, silica gel formation, and a carbon layer due to the burning of resin.

(4)

The delay in the decomposition of H₂ gas (120 s) gives the gas bubbles enough time to establish pores in the metallic matrix, making the pore sizes of produced foams by modified TiH₂ small. Additionally, it makes more homogenous pores and size distribution in the produced foams by modified TiH₂ compared with TiH₂.

These results prove that the foams produced with two modified foaming agents can have favorable mechanical properties, which will be studied in the future. Additionally, considering the cost-effectiveness of producing aluminum foams using these foaming agents, the possibility of their production on an industrial and large scale will be investigated.