Performance Assessment on the Manufacturing of Zn-22Al-2Cu Alloy Foams Using Barite by Melt Route

Abstract

A barium-rich Celestine (Sr,Ba)SO₄ concentrate from the primary Mexican ore production was used as a thickening agent to produce closed-cell Zn-22Al-2Cu alloy foams, while calcium carbonate was used as a foaming agent. The microstructure and mechanical properties of the foams were analyzed by optical microscopy, scanning electron microscopy, and compression tests, respectively. The Zn-22Al-2Cu alloy foams showed a typical lamellar eutectic microstructure, constituted by a zinc-rich phase (η) and a (α) solid solution that was richer in aluminum, while a copper-rich (ε) phase was formed in the interdendritic regions. The SEM micrographs show the presence of small particles and aggregates that are randomly scattered in the cell walls and correspond to unreacted calcite and Celestine–Barian particles, especially for the higher barite addition. The compressive curves showed smooth behavior, wherein the particles at the cell walls did not affect the foam’s compressive behavior. The trial containing 1.5 wt. % of BaSO₄ and 1.0 wt. % of CaCO₃ showed a higher energy absorption capacity of 5.64 MJ m⁻³ because of its highest relative density and lowest porosity values. The Celestine–Barian concentrate could be used as a foaming agent for high melt-point metals or alloys based on the TGA results.

1. Introduction

Closed-cell metal foams are materials with outstanding properties, such as low density, high capacity to absorb kinetic energy, low thermal conductivity, and high toughness [1,2]. The methods to produce metallic foams are well-documented [2,3,4]. Among these, the foaming of liquid metal is carried out by introducing a gas into the melt [5], or by adding a foaming agent that releases gas in the molten metal, obtaining, in both cases, a porous metal [6]. Calcium carbonate was proposed as a cheaper and safer foaming agent instead of the conventional agent, titanium hydride [5,6]. To obtain a uniform distribution of cells, bubbles should be retained within the melt until the foam can be solidified [7]. Deficiencies during the foaming process due to random disturbances can cause flow, drainage, rupture, and coarsening of the cells; thus, they can produce foams with a low quality. Aluminum and its alloys are by far the main metal used for producing metallic foams [8,9].

The attractive combination of mechanical and physical properties of metallic foam makes it suitable for applications in components of the automotive, aerospace, and machinery sectors, among others. The lightest and most stiff components made of metallic foams are required in these industries. Furthermore, it is predicted that the use of metallic foam material will increase in the automobile industry. The production of metallic foam materials with higher properties has been developed using various metallic foams from the following metals and alloys: Al, Al-Si, Al-Mg, Cu, Pb, Fe, Steels, Ni₃Al, Zn, Mg, and metallic glasses, etc. [10].

Zinc and its alloys have a lower melting point than aluminum alloys, exhibit a good capacity to absorb energy, have good mechanical damping, and are compatible with steel regarding corrosion resistance, which could be suitably used as filling for hollow steel sections to improve their stiffness [11].

In this sense, zinc alloys have a superior damping capacity than aluminum alloys in compact form. The damping capacity and Young modulus of a synthetic zinc foam produced using the Alporas^® method were analyzed using a modal test based on the forced vibration and bending vibration modes. The Young modulus of the zinc foam was closer to an aluminum foam with the same density [12].

The production of new types of Zn alloys, combined with the foaming metallic technology, has led to a new class of materials, with an optimal combination of mechanical properties and functionality at the minimum weight for a wide diversity of enterprises.

The use of electric vehicles has increased the development of batteries for energy storage. Lithium batteries exhibit high lithium dendrite growth, which affects the electrochemical performance of the battery. The use of zinc instead of silver or gold in a porous Ni-Zn alloy inhibits lithium dendrite growth at a considerably lower cost [13].

On the other hand, the levels of electromagnetic radiation have been rapidly increasing due to the advancement of modern electronic devices and communication technologies, leading to electromagnetic pollution. Foam materials have emerged as promising candidates to enhance electromagnetic wave attenuation, whereas Zn has been used as a sacrifice template in the manufacturing of graphene foams [14].

In addition, green building materials have been produced through the development of foamed geopolymers that are prepared by chemical foaming, using aluminum and zinc powders as foaming agents. The foaming with aluminum is more effective than zinc powder, since larger volumes of gases are released during its chemical action [15].

The melt route used to produce closed-cell metallic foams requires a thickening process before the foaming process to increase the melt viscosity by the addition of ceramic particles such as Al₂O₃ or SiC, which increase the foam stability by increasing the cell wall thickness. An alternative method of foam generation involves adding calcium or magnesium to an aluminum alloy and stirring it in the presence of air, which leads to an in situ creation of solid particles in the melt, increasing the viscosity of the melt and suppressing the drainage of the aluminum melt during solidification [16,17]. The interaction between the stabilizing particles and the melt forms by-products or precipitates, such as Al₂CaO₄, Al₂Ca₃O₆, Al₂CaSi₂, and CaAl₄, which contaminate the cell wall and affect the macroscopic mechanical response of closed-cell aluminum foams [18,19,20,21]. In this sense, a thermodynamic analysis was carried out to determine the stability compounds formed by the interaction between the A356 aluminum alloy and calcite and alumina as foaming and thickening agents, respectively, during the production of closed-cell aluminum alloy foams by the melt route [22]. It is well known that the strength of a metallic foam is dependent on its structure. Thus, closed-cell A356 aluminum alloy foams that were refined and modified were successfully fabricated by using a melt treatment consisting of adding master alloys of Al-5Ti-1B and Al-10Sr for refining the dendritic microstructure and modifying the primary eutectic silicon, respectively. Lower secondary dendritic arm spacing (SDAS) values and the highest mechanical properties were obtained for the closed-cell foams by produced with a refining and modification treatment [23]. On the other hand, a new approach involving the utilization of ultrasonic microbubbles was carried out to manufacture porous metals of the Pb-Sn-Cu alloy, lead-free solder alloy, and zinc. It was determined that the higher the melting point, the larger the pore diameter and the lower the porosity [24]. González et al. [25] showed the feasibility of producing A356 aluminum closed-cell foams by using barite and wollastonite minerals as thickening agents, instead of the commercial alumina thickening agent, via the melt route. The Zn-22Al-2Cu alloy is a kind of superplastic material that is gaining further attention in the field of porous metallic structures [26]. In this sense, open-cell Zn-22Al-2Cu foams with spherical shape porosity and different pore sizes from 0.85 to 0.42 mm were manufactured by a centrifugal-infiltration process [27]. However, there is a scarcity of work related to the production of zinc-base alloy closed-cell foams via the melt route. Mass foam production is too expensive, and there seems to still be some potential for an improvement of properties by optimizing the foaming processes and materials selection. In this sense, México has important barite (BaSO₄) deposits, which are largely used by the oil and gas industry during drilling operations as a drilling mud weighting agent, used to balance the reservoir pressure and to prevent blowouts. Since melts without a solid phase are difficult to foam, the use of barite is considered in closed-cell foam production. Therefore, this work aims to evaluate the foamability of the Zn-22Al-2Cu alloy by the liquid melt route, using barite (BaSO₄) as a thickening agent and calcium carbonate as a foaming agent. Barite ore was characterized by X-ray diffraction, thermal gravimetrical analysis, and scanning electron microscopy with microanalysis, while the morphology, structure, and mechanical properties of the produced foams were determined by microscopy techniques and compression tests.

2. Experimental Procedure

2.1. Barite Characterization

A barite concentrate sample was obtained from Mexican ore production. Barite is a solid mineral that combines barium and sulfur. It is composed of barium sulfate (BaSO₄). Barite ores of different grades vary from one location to the other, and within deposits, and they are often associated with magnetite, celestine, fluorite, calcite, siderite, quartz, galena, dolomite, etc. The barite was analyzed in a Jeol SEM 6300, with energy dispersive spectra (EDS), to establish the element distribution in the ore sample while the mineralogical compounds were analyzed by X-ray powder diffraction in an X-ray Bruker D8 Focus diffractometer (Bruker, Billerica, MA, USA). A thermogravimetric analysis was carried out in a thermal gravimetric analyzer Mettler Toledo TGA/DSC1 Instrument (Mettler-Toledo, Columbus, OH, USA). to determine mass loss due to thermal decomposition of barite.

2.2. Zinc-Base Alloy Foams

The Zn-22Al-2Cu alloy was manufactured from pure metals in a gas furnace at 750 °C. Zinc was added at the end of melting due to its low melting point, to minimize oxidation losses. The chemical composition of the alloy produced was 76.26 wt. % Zn, 21.92 wt. % Al, and 1.79 wt. % Cu. Figure 1 shows a scheme of the experimental process followed to obtain a Zn-22Al-2Cu closed-cell foam. Furthermore, 500 g of the zinc-base alloy was set in a bipartite stainless-steel mold and melted at 700 °C in an electric furnace. The furnace has a stirring system to carry out the thickening and foaming stages. The thickening stage was developed by adding 0.5, 1, and 1.5 wt. % of barite of the mass charge at 1200 rpm for 2 min. Afterward, the foaming stage consisted of the addition of 1 wt. % of the mass charge of reagent grade CaCO₃ (Fermont, 98.5% purity and 14 µm of particle mean size) to the melt at 1200 rpm for 100 s. Later, the stirring system is removed, and the melt expands to obtain the metallic foam. After the foam reaches the highest expansion, the mold is extracted and cooled with sprayed water.

2.3. Foams Characterization

A zinc-base alloy foam is shown in Figure 2. It shows an external oxidation of the foam (Figure 2a), where particles and dross are mainly located on the top surface. A cross-section of the foam (Figure 2b) exhibits the cell structure and pore morphology. The top foam surface is removed, and samples are obtained to evaluate density, cell structure, and compression resistance. Figure 2b shows the foam regions where the cylindrical samples (20 mm diameter and 30 mm height) were extracted for compression trials and microstructural characterization (Figure 2c).

The cylindrical samples were measured and weighted to determine the foam densities, while the relative density was calculated using the ratio of ρ_foam/ρ_alloy. The foam porosity (%) was obtained using Equation (1).

$$\mathrm{Pr} \left(\%\right)=\left[1-\left({\displaystyle \frac{{\rho }_{foam}}{{\rho }_{alloy}}}\right)\right]·100$$

(1)

where Pr is the porosity, ρ_foam is the foam density, and ρ_alloy = 5.4 g cm⁻³. The microstructure of the Zn-22Al-2Cu alloy and foams produced were analyzed using the optical microscopy and SEM-EDS techniques from at least five different foam regions that are closer to the extraction zone of the cylindrical samples. The phases in the base alloy were determined by X-ray diffraction measurements. Uniaxial compression tests of the cylindrical samples were carried out in a Shimadzu universal testing machine of 100 kN capacity, at a strain rate of 2 × 10⁻² s⁻¹. The mechanical parameters of the closed-cell foams were obtained from the stress–strain curves, and the standard method for the compression of porous metals (ISO 13314:2011).

3. Results

3.1. Barite

The barite sample was analyzed using SEM-EDS techniques to determine the distribution of the main elements in particles, as it is shown in Figure 3.

Barite was supplied with a particle mean size of 20 microns. The barite sample is mainly constituted by particles containing Sr, Ba, and S, which indicates that barite (BaSO₄) is associated with Celestine (SrSO₄), which belongs to the barite group, with similar properties. In addition, some particles are formed by Ca, which indicates the calcite (CaCO₃) presence and corresponds to the main impurity in the barite ore. A low iron concentration in the particles was detected, which indicates that the presence of siderite (FeCO₃) and magnetite (Fe₃O₄) is negligible.

The X-ray diffraction pattern of the barite sample is reported in Figure 4, and it is confirmed using the results obtained from the SEM-EDS techniques, where the Celestine–Barian compound (Ba_0.25Sr_0.75SO₄) was detected as the main compound, while calcite (CaCO₃) was detected in lower concentrations as the main impurity in the barite sample.

The barite sample is considered to act as a thickening agent in the production of Zn-base alloy foams. However, the calcite content may aid in the foaming stage, releasing CO_2(g) during the metallic foam production.

In addition, a release of SO_2(g) is expected during the thermal decomposition of the Celestine–Barian sample. Figure 5 shows a thermal gravimetrical analysis of the barite sample. Three mass loss events are observed in the TGA curve, which is associated with the loss of water (from 75 to 230 °C), the release of CO_2(g) due to the calcium carbonate decomposition (from 655 to 780 °C), and the release of SO_2(g) due to the Celestine–Barian decomposition (from 1360 to over 1500 °C).

The TGA curve shows two slight mass increases, which are common for barite ores. The first event (from 300 to 600 °C) is related to water contained as an occlusion (the most primitive form of water in minerals). The included water fills the cavities, decreasing its volume, and in the course of heating, the liquid expands again to fill the whole cavities; then, with further heating, its volume increases, and the crystals burst. In addition, the high sulfur content in the Celestine–Barian sample promotes a massive SO_2(g) release at higher temperatures (from 750 to 1300 °C), promoting the fact that the sample sputtered from the sample holder [28].

The TGA results show that the barite sample could be used as a foaming agent for metals or alloys of a high melting point. In this work, the barite sample was used as a thickening agent; however, the calcite content in the sample could aid in the foaming process too. For this work, the thermal decomposition of CaCO₃ occurs in the range from 655 °C to 780 °C, wherein CO₂ gas is released. In addition, the melting point of the Zn-22Al-2Cu alloy was determined to be 670 °C. Therefore, the foaming process temperature was attained at 700 °C to release CO₂ gas into the molten alloy and avoid excessive oxidation of the alloy.

3.2. Zn-22Al-2Cu Alloy

Figure 6a shows a SEM micrograph of the Zn-base alloy, which is constituted of a eutectic microstructure, showing dark and bright areas belonging to the aluminum-rich (α) and zinc-rich (η) phases, respectively. The microstructure shows a fine inter-lamellar structure, characterized by regions of α and η phases. A small copper amount was detected by the SEM-EDS technique, and it was confirmed by the XRD pattern shown in Figure 6b. Furthermore, Figure 6b shows the aluminum-rich (α), zinc-rich (η), and intermetallic (ε) phases corresponding to CuZn₄. It has been reported [29] that the copper addition to the Zn-Al alloy originates new intermetallic phases (t’ and ε), which increases the strength without affecting the superplasticity of the alloy.

3.3. Foams Produced

Figure 7 shows the Zn-22Al-2Cu alloy foams produced by adding barite as a thickening agent and calcite as a foaming agent. The effect of increasing the amount of the thickening agent (BaSO₄) from 0.5 to 1.5 wt. % is observed, attaining the constant foaming agent (CaCO₃) at 1%. For these trials, an increase in the foaming expansion is observed, reaching a higher expansion at 1.0 wt. % of BaSO₄. In general, the foams produced under the proposed experimental conditions showed a good foaming expansion and a higher homogeneity in their metallic structure. Based on the experimental parameters evaluated in the foam’s production, it is recommended to use 1 wt. % of the thickening and foaming agents to obtain an adequate foam performance.

Table 1. Experimental densities and porosity of the Zn-22Al-2Cu alloy foams.

Sample No.	Thickening Agent (wt. %)	Foaming Agent (wt. %)	Foam Density (g cm−3)	Relative Density	Pr (%)
1	0.5 BaSO4	1 CaCO3	0.543	0.100	89.94
2	1 BaSO4	1 CaCO3	0.593	0.109	88.28
3	1.5 BaSO4	1 CaCO3	1.011	0.187	81.80

summarizes the results of density, relative density, and porosity for the foams produced.

Table 1 shows that with the increase in the barite amount, the porosity of the foam decreases, obtaining higher values for the additions of 0.5 and 1.0 wt. % of BaSO₄. As expected, the highest relative density was obtained for the foams containing the lowest porosity values, where the average cell-strut thickness is the largest. Lower porosity values were reported for Zn-base alloy foams produced by the replication-centrifugation [27] and utilization of ultrasonic microbubbles [24] methods, reaching porosity values of 68 and 48%, respectively.

Figure 8 shows representative cell structures of the Zn-22Al-2Cu alloy foams manufactured with barite as a thickening agent. It is evident from Figure 8c that the foam produced with the highest BaSO₄ addition exhibits the thicker strut cell, and, hence, the highest relative density with the smallest porosity value.

Figure 9 shows representative SEM micrographs of the microstructures obtained for the closed-cell Zn-22Al-2Cu alloy foam at different regions of the foam samples. The microstructure is composed of dendrites, as well as fine lamellar eutectic structures. The water spray-cooled foam showed a similar microstructure, identified in the as-cast alloy condition for the different regions evaluated. The phases observed as bright regions correspond to the zinc-rich phase (η), while the darker gray zones are related to the (α) solid solution richer in Al. The copper addition in the zinc-base alloy allows the formation of a copper-rich (ε) phase in the interdendritic regions.

At higher magnifications, an interdendritic region of the laminar structure of two phases, α and η, is shown in Figure 9a. A white region is observed, immersed in the eutectic structure, which corresponds to the zinc-rich phase (η), as well as small dark regions that correspond to the aluminum-rich phase (α). The qualitative results showed that the phase (η) corresponding to the white region was Zn-1.16Al-2.41 Cu, while the phase (α) corresponding to the dark region was Zn-19.94Al-2.40 Cu. Figure 9b shows a foam region where the phase (η) that is rich in zinc and the phase (α) that is rich in aluminum are immersed in a eutectic structure. The qualitative results show that the phase (η) is made up of Zn-1.4Al-3.81 Cu, and the phase (α) is made up of Zn-29.28Al-2.15Cu. The rate of cooling imposed by the water spraying did not show any appreciable effect on the final microstructure of the Zn-22Al-2Cu foam, obtaining a typical lamellar eutectic microstructure. Similar behavior was reported in the manufacturing of aluminum-base alloy foams, where the mold was cooled by a fast-blowing airflow [23]. For both cooling mediums, a solidification rate of 2.0 K/s was estimated in the middle region of the mold. In addition, it must be considered that there are different solidification rates at the different zones of the mold, where the highest cooling rate is expected in the upper part of the mold, as well as higher oxidation and particles formed by the interaction of the thickening and foaming agents with the alloy.

To develop a superplastic behavior of the Zn-Al-Cu alloy, a heat treatment is required, wherein homogenizing and subsequent quenching are required to promote the formation of nanometric grains from the α and η phases [30].

Figure 10 shows the SEM micrographs and EDS analysis of different magnifications of the matrix and particles placed on the metal walls of the foam produced with 1.5 wt. % of barite. The SEM micrographs show the presence of small particles and aggregates that are randomly scattered. The qualitative chemical analysis obtained by EDS for the foams manufactured to 1.0 wt% of CaCO₃ and 1.5 wt. % of BaSO₄ shows that these foreign particles in the aluminum foams that are produced primarily correspond to the Celestine–Barian (Ba_0.25Sr_0.75SO₄) and unreacted calcite (CaCO₃) compounds. However, it has been reported that a high concentration of particles attached to the cell wall is observed when the foaming agent is increased, which corresponds to unreacted calcium carbonate. In addition, the presence of aluminum in the alloy promotes the formation of compounds such as Al₄O₄C, Al₄C₃, and CaAl₄O₇ [22]. It detected the presence of elements such as Zn, Al, and Cu, which belong to the matrix alloy.

3.4. Compressive Behavior

Figure 11 shows the stress–strain compression curves for the Zn-22Al-2Cu alloy foams produced with the addition of barite. The stress–strain curves of closed-cell metallic foams exhibit three distinct zones under compressive loading. The first zone, where stress increases meagerly, corresponds to an elastic stage. The second zone is known as the plastic plateau stage, where a plastic deformation of the cell struts with a slow increase in the stress occurs. The third zone, where the stress increases drastically due to the densification of the collapsed cells, is related to a densification region registered at 50% strain. In general, the compressive curves show smooth behavior in the plateau regions for the foams obtained with the different additions of BaSO₄. This behavior is associated with a ductile mechanism, and the failure of ductile foam is controlled by the bending of cell walls [31]. It is desirable in practical applications that foams show a uniform collapse with a wide plateau regime.

The experimental mechanical properties obtained from the compressive stress–strain curves are summarized in

Table 2. Mechanical properties and energy absorption capacity of the Zn-22Al-2Cu alloy foams.

Sample No.	Thickening Agent (wt. %)	Foaming Agent (wt. %)	Yield Stress σ0 (N/mm2)	Average Plateau σpl (N/mm2)	Energy Absorption W (MJ m−3)
1	0.5 BaSO4	1 CaCO3	0.69	0.92	0.43
2	1.0 BaSO4	1 CaCO3	1.62	1.99	0.93
3	1.5 BaSO4	1 CaCO3	10.07	12.06	5.64

.

It is observed that when the addition of barite was increased, the yield stress and the average plateau stress were increased for the Zn-22Al-2Cu alloy foams. It has been reported [31] that the yield stress increases when the cell size and porosity decrease; a similar behavior was observed for the Zn-22Al-2Cu alloy foam produced with the barite addition. The energy absorption capacity and average plateau were increased for the highest relative density and the lowest porosity values; thus, the cell-strut thickness, as well as the strength of the Zn-22Al-2Cu alloy foams, were increased [23,31]. The Zn-22Al-2Cu alloy foams produced with barite showed lower values of energy absorption capacities for the lower BaSO₄ additions; however, the trial developed with the addition of 1.5 wt% of BaSO₄ reported a high energy absorption capacity value of 5.64 MJ m⁻³. These results match the mechanical results reported for the Zn-22Al-2Cu alloy produced with a centrifugal-replication process, where an average plateau and the yield stresses of 8.4 and 4.8 N/mm² were obtained, respectively, for a porosity of 68% [27]. The foams produced with barite additions exhibit a good combination of structure with the highest porosity values, but the lower energy absorption capacity, except for the trial, with the highest BaSO₄ addition, which presented the lowest porosity and the highest energy absorption capacity.

Although the presence of solid particles in the metal foams mainly for the highest barite content, the compressive curves showed smooth behavior, which is desirable for the foam’s mechanical properties.

4. Conclusions

Closed-cell Zn-22Al-2Cu alloy foams were successfully produced by adding barite as a thickening agent and CaCO₃ as a foaming agent. The foams produced under the proposed experimental conditions showed good foaming expansion and pore homogeneity. The Zn-22Al-2Cu alloy foams showed a typical microstructure of bright and dark regions, which correspond to the zinc-rich phase (η) and the (α) solid solution that is richer in Al, respectively, with a copper-rich (ε) phase formed in the interdendritic regions. The rate of cooling imposed by the water spraying did not show any appreciable effect on the final microstructure of the Zn-22Al-2Cu foam and the as-cast alloy, obtaining, in both cases, a typical lamellar eutectic microstructure. The highest energy absorption capacity of the Zn-22Al-2Cu foams was obtained for the additions of 1.5 wt% of BaSO₄ and 1.0 wt% of CaCO₃ (5.64 MJ m⁻³), which correspond to the highest relative density and the lowest porosity values.