Insights into the advancements of magnesium-based porous materials: A review

Abstract

Porous magnesium and its alloys have garnered significant attention due to their lightweight nature and excellent mechanical properties, making them attractive for various applications. Consequently, research in this field is expanding rapidly, utilizing a wide range of techniques to customize material properties for targeted applications. The primary objective is to develop closed-cell, open-cell, or complex 3D structures optimized for specific applications. Herein, we present a comprehensive review of the fabrication methods and properties of porous magnesium and its alloys. Fabrication techniques including liquid and powder metallurgy, provide distinct advantages in controlling pore size, morphology, and distribution. Furthermore, the review examines the properties of magnesium-based porous materials and factors influencing them, underscoring their potential for advanced applications in various industries.

Keywords

porous magnesium fabrication methods properties of Mg cellular structure pore size morphology and distribution

Introduction

Magnesium-based materials have attracted increasing interest in the field of materials science and engineering because of their outstanding combination of lightweight features, mechanical strength, and excellent corrosion resistance.¹ Magnesium, a lightweight metal, is used in a variety of engineering applications. It makes up 2.7% of the earth's crust, making it the eighth most abundant element. Magnesium has a density roughly two-thirds that of aluminum and one-quarter that of steel. This low density renders magnesium-based materials exceptionally lightweight, which is essential in industries where reducing weight is important. Despite being lightweight, magnesium exhibits remarkable mechanical properties. It has a high strength-to-weight ratio, suitable for load-bearing applications. Magnesium alloys, in particular, are recognized for their enhanced strength compared to pure magnesium. Further improvements can be achieved by incorporating reinforcing materials into magnesium to develop Mg-based composites.

When it comes to porous magnesium, it is regarded as a unique and adaptable material that has gained recognition in the field of materials science and engineering. It provides an impressive combination of qualities, including low density and high specific strength. Different methods produce varying pore structures in magnesium foams. These structures significantly influence the material's properties, making them more or less suitable for various applications. Therefore, selecting the appropriate fabrication method is essential for controlling the pore structure of porous magnesium. Currently, researchers use a variety of techniques to create porous Mg-based materials, with the main goal of precisely tailoring their properties. The resulting pores can take on a wide range of shapes, from random arrangements^2–4 to carefully controlled geometries.⁵ While some processes produce random pore structures, researchers can often adjust process parameters to control pore size, porosity, and density. Other techniques allow the creation of pores with specific shapes and sizes, such as spheres,⁶ cubes,⁷ and wire-like formations.⁸ Computer-Aided Design (CAD) is frequently used to design such complex pore architectures.⁵ Moreover, the pores in these materials can either be closed and isolated cellular structures^9,10 or open, interconnected networks,^7,8 each offering distinct benefits.

Researchers have extensively studied the properties of porous magnesium. These materials are particularly interesting because of their mechanical characteristics and ability to absorb energy. One major benefit of porous Mg is its extremely light weight, due to its pore-filled structure. However, there is a trade-off. While the lightness is advantageous, the presence of pores naturally decreases the material's mechanical strength.^6,7,11 This trade-off makes porous Mg suitable for applications where reducing weight is important, even if it means some compromise in strength. To address this limitation, researchers have explored strategies to enhance the overall properties of porous Mg. One method involves adding different materials into the porous Mg matrix,^12,13 aiming to combine its lightness with improved mechanical performance from the added elements. Additionally, the porous structure gives these materials foam-like qualities, making them highly effective energy absorbers.^12,14 This feature makes them appealing for applications where impact mitigation is vital.

This review examines methods for creating porous magnesium, detailing the unique properties this structure brings to the material. Moreover, it delves into how incorporating different materials during synthesis can enhance these properties, leading to improved functionality.

Fabrication methods of porous Mg-based materials

The fabrication of porous magnesium can be divided into liquid metallurgy and powder metallurgy. Liquid metallurgy involves melting metal in a furnace and casting it into a mold to obtain the desired shape. This process is well-suited for producing large, complex parts with homogeneous microstructures. On the other hand, powder metallurgy involves compacting metal powders into a desired shape and then sintering them at high temperatures to bond the particles together. The majority of Mg-based foams discussed in this review paper exhibit millimeter-scale pore sizes, regardless of whether they are produced via liquid metallurgy or powder metallurgy methods, highlighting the emphasis on large interconnected open-pores or closed-pore structures for enhanced mechanical and functional performance.

Liquid metallurgy

Mainly, there are two methods to fabricate porous through melt route, casting method and gas bubbling methods. The former involves pouring molten metal into a mold made from a rigid structure to produce a desired shape, whereas in the latter, melts can be foamed (bubbled) directly by blowing gas into molten metal or by addition of blowing agent to the molten metal which decompose at certain temperature and thereby evolved gases to foam the material. In this method, magnesium must be heated above its melting temperature, which can pose certain risks, primarily related to the potential for ignition and combustion. Magnesium is known for its flammability, and when exposed to hydrogen gas, which is highly flammable itself, the risks are amplified.

Casting method

One effective approach for creating these porous structures is the preform infiltration method whereby molten Mg is introduced into a preformed, rigid template, also called “pattern”, to produce a composite material. The preform is then removed from the composite, leaving behind desired interconnected pores in the magnesium-based material. The first step in this process is to fabricate the preform material itself, a rigid, three-dimensional structure made from a variety of materials. The preform is then infiltrated with molten magnesium which fills the pores of the preform. After infiltration, the molten magnesium is solidified, creating a composite structure where the Mg acts as matrix component and the preform material acts as reinforcing component. However, in order to create the foam structure, the preform material must be removed, which can be done by leaching, sublimation, etching, or thermal decomposition, leaving behind desired open, interconnected pores in the magnesium-based material. Critical parameters such as particle size, spacing, and preheating temperature strongly influence the overall pore structure of the alloy foam.¹⁵

In the preform infiltration process, insufficient pressure, holding temperature, and dwell times are crucial factors that can result in the incomplete infiltration of the sacrificial material by molten Mg, as indicated by researchers.^16,17 Yue et al.¹⁶ studied the effect of the holding temperature and infiltration pressure of porous Mg alloys while infiltrating (gravitationally) the preform structure during the casting process. They concluded that fluidity of Mg alloys increased with increasing infiltration pressure in the fixed mold and its preheated temperature, as shown in Figure 1(b). However, excessive infiltration pressure can lead to the deformation or non-uniform formation of granules, compromising the quality of the final product. Conversely, insufficient pressure results in inadequate infiltration length, as depicted in Figure 1(a), preventing the filling of the porous structure. Elevated fluidity in porous Mg-based material is commonly seen in molds with higher preheat temperatures, where the solidification process occurs slowly. However, excessively high preheating temperatures are counterproductive for energy conservation.

Numerous studies have incorporated sodium chloride (NaCl) particles^7,15 as a placeholder material due to their cost-effectiveness, high refractoriness, strength, suitable shape, and size. NaCl is chemically inert when in contact with magnesium, soluble in water, readily available, and non-toxic. However, the difficulty in completely removing sodium chloride could result in residual contamination of Mg. Alternative materials were also utilized as a space holder such as calcium chloride (CaCl₂)¹⁸ and Ti-wires.⁸

A significant body of research has explored the fabrication of porous magnesium-based materials with varying pore structures. These structures are critically dependent on the shape of the preform. Spherical preforms are a common choice, but researchers have also explored cubic⁷ and wire-shaped templates.⁸ However, some traditional methods often result in random pore architectures, making it difficult to precisely control the final pore morphology. To address this challenge, researchers have begun utilizing CAD to fabricate controlled 3D-shaped preforms.⁵ This approach allows for the creation of materials with specific porosity and well-defined pore structures. Figure 2 shows how wire-shaped preforms are prepared and incorporated in Mg.¹⁹ Table 1 shows a summary of porous Mg-based materials made by preform infiltration method.

Table 1.

Porous Mg-based materials fabricated by preform infiltration method.

Materials	Space Holder Materials	Removing Process	Pore Properties		Compressive YS (MPa)	Ref.
Materials	Space Holder Materials	Removing Process	Avg. Pore Size (μm)	Porosity (%)	Compressive YS (MPa)	Ref.
Mg-2Zn-(0.4–6)Y (wt.%)	CaCl₂	Leaching: Water		67.51	6.3-9.23	²⁰
Mg-(4–8)Gd-2Zn (wt.%)	CaCl₂	Leaching: NaOH	2380	70.85	139.4–183.2	¹⁸
AZ91 alloy	NaCl	Leaching: NaOH	200–2200	∼40–98		¹⁵
Mg–(0–6)Zn–0.3Ca (wt.%)	NaCl	Leaching: NaOH	50–500	73.4	∼3–5.2	⁷
Mg-10Zn-4Y (wt.%)	Ti-wire	Etching: hydrofluoric acid		50–60	∼70–101	⁸
Mg	Ti-beads	Etching: hydrofluoric acid	150–550	76.53–85.60	0.95–2.03	⁶

The preform infiltration method offers notable advantages as it offers the fabrication of interconnected porous structures by infiltrating molten magnesium into removable preformed patterns. This process allows precise control over pore shape, size, and distribution. A wide variety of space holder materials (e.g., NaCl, CaCl₂, and Ti-wires) can be employed, while different removal techniques (e.g., leaching or etching) help achieve the desired porosity. The method is well-suited for developing high-porosity structures with adjustable geometries. However, the process requires careful regulation of parameters such as infiltration pressure, temperature, and dwell time, as insufficient control can result in incomplete infiltration or non-uniform granules. Additionally, incomplete removal of the preform material can lead to contamination of the magnesium matrix, and the high-temperature operation presents safety challenges due to flammable nature of magnesium.

Formation of gas bubbles method

In-situ gas formation method

This method, also called melt foaming method, is used to create porous closed-cell magnesium, which relies on the expansion of gas bubbles generated from blowing agent within the molten Mg. The process starts with heating pure magnesium or magnesium alloy above its melting point in a crucible. A foaming agent or blowing agent is added to the molten magnesium, which releases and forms gas bubbles inside the molten magnesium. The size and spatial distribution of pores can be effectively controlled by adjusting the gas release rate which depends on the type of foaming agent, as well as the processing conditions. The last step is to cool and solidify the molten magnesium, preserving the gas bubbles trapped within the solid matrix and creating a porous structure.

Titanium hydride (TiH₂), zirconium hydride (ZrH₂), and calcium carbonate (CaCO₃) are commonly used as blowing agents in the production of foamed materials. These blowing agents release gas when heated, creating bubbles within the material. The research revealed a critical link between the blowing gas release rate and the resulting pore structure in the foamed material. However, studies have shown that exceeding 3 wt.% of blowing agent can lead to a highly viscous molten Mg alloy, resembling a slurry, hindering the foaming process. Conversely, insufficient blowing agent (below 3 wt.%) proves inadequate to achieve proper foaming.²¹ Consequently, most research has focused on using a blowing agent concentration of 3 wt.%.^21,22

Magnesium has an inherent ability to absorb hydrogen. According to research,²³ magnesium-based foams made by the melt foaming process with Ti-hydride used as a blowing agent exhibit foams with coarse pores. To avoid this issue, CaCO₃²² and MgCO₃^14,21,24 are used commonly for Mg foams. Yang et al.²⁵ fabricated closed-cells porous Mg using 2% of CaCO₃ as a blowing agent and 2% granular Ca as a thickening agent by varying the temperatures. The study revealed that the gas responsible for the foaming reaction between melted Mg and CaCO₃ is CO. Furthermore, the porosity of the resulting Mg foam was found to increase with higher foaming temperatures (refer to Figure 3). However, applying too much foaming temperature displays a coarse pore structure (Figure 3(d)), whereas too little foaming temperature exhibits ineffective foaming (Figure 3(a)). Additionally, Yang et al.²⁵ employed the same magnesium foaming process to produce porous magnesium alloys, AZ91 and AM60. As shown in Figure 3(e-f), their results demonstrated successful foaming and the creation of a significant amount of porosity. Other than foaming temperature, Devikar et al.²² reported that foaming time also influences pore structure, with larger pores formed with increasing foaming time.

A study²¹ investigated MgCO₃ as a blowing agent for fabricating porous Mg alloy, deviating from the traditional use of CaCO₃ powders. Their findings demonstrated that the resulting foams exhibited good expansion, low density, and a well-distributed cellular structure. Another study²⁴ investigated the use of low-cost pure dolomite (CaMg(CO₃)₂) combined with MgCO₃ acting as a dual-blowing agent. This combination resulted in a uniform and stable pore structure.

Other than foaming agent, thickening agent also plays an important role in Mg foams. The addition of thickening agents, such as calcium, to liquid magnesium enhances the stability of the magnesium foam by decreasing the surface tension and increasing the viscosity during production of Mg foams,²⁶ leading to a more uniform and well-distributed cellular structure.²¹ Having an optimum surface tension is required for foaming Mg.²⁷ Higher Ca content means lower pore size and porosity.²⁸ However, adding too much calcium will result in excessive viscosity, which will make it impossible to add a foaming agent and give the mixture a high density. It is thus 2–3% of Ca as a thickening agent are used.^14,21,22,24

Researchers have explored the addition of various elements as stabilizing agents for Mg foam. These elements include aluminum,^21,22 silicon carbide,^14,29 and beryllium.^21,22 BHOSALE et al.²¹ investigated the effect of Al content on pore formation and expansion in Mg foams. They employed varying Al amounts and observed an increase in both the number of pores and the extent of foam expansion with increasing Al content. Lu et al.¹⁴ investigated the addition of SiC particles as a stabilizing agent alongside 12 wt.% Al in Mg foams. Their findings indicated the existence of embedded different particles, located at cell edges and walls, enhanced the foam's strength but also contributed to brittleness in the cell walls. Research by²² suggests that adding beryllium (Be) promotes the formation of a protective layer at the gas-liquid interface of pores within the magnesium foam. This layer helps to minimize the loss of blowing gas, thereby enabling greater foam expansion. Moreover, it resulted in defect-free structure by enhancing the stability of the pore walls. Table 2 shows a summary of porous Mg-based materials made by melt foaming method.

Table 2.

Porous Mg-based materials fabricated by melt foaming method.

Materials	Thickening Agent Materials	Blowing Agent			Pore Properties		Compressive Yield Strength (MPa)	Ref.
Materials	Thickening Agent Materials	Materials	Stirring Temperature (°C)	Stirring Time/Speed	Pore Size (mm)	Porosity (%)	Compressive Yield Strength (MPa)	Ref.
Mg	Ca (2 wt.%)	CaCO₃ (2 wt.%)	665–750	30 s/1000 rpm	1–2.1	27.2–71	8.69–27.11	^25,30
Mg-12Al-3Ca (wt.%)		MgCO₃ + Al₂O₃	580	120 s/-	0.8	74.4	9.8	³¹
Mg-12Al-3Ca/4–8SiCp		MgCO₃ (3 wt.%) + Al₂O₃ (3 wt.%)	580	–/2000 rpm	0.8	76.4–80	8.6–13.2	^31,32
Mg-12Al-3Ca/SiC_p		MgCO₃			0.6–0.8	72.5	12	¹⁴
Mg-2Zn-2Ca (wt.%)		CaMg(CO₃)2 + MgCO₃			0.25–0.75		∼50	²⁴
AZ91	Ca (2 wt.%)	CaCO₃ (2 wt.%)	700		1.9	57.4		²⁵
AM60	Ca (2 wt.%)	CaCO₃ (2 wt.%)	700		1.8	59.1		²⁵
AZ31B/10vol.%CM	Ca (0.2 wt.%)	CaCO₃ (1.5 wt.%)	680	40 s/1200 rpm	1–2	67.37	12	³³
AZ31B/10vol.%CM (Heat treated)	Ca (0.2 wt.%)	CaCO₃ (1.5 wt.%)	680	40 s/1200 rpm	1–2	67.31–67.53	12.5–13	³³

The in-situ gas formation method is particularly useful for creating closed-cell porous magnesium by generating gas bubbles within the molten metal using blowing agents (e.g., TiH₂, ZrH₂, or CaCO₃). This method allows for control over pore size and distribution by adjusting foaming agent concentration, temperature, and foaming time. The addition of thickening and stabilizing agents (e.g., calcium, aluminum, silicon carbide, and beryllium) can further enhance foam stability and mechanical properties. Despite these benefits, the process is sensitive to the concentration of blowing agents (i.e., 3 wt.%) and higher amount can make the melt excessively viscous, while insufficient amounts result in poor foaming. Furthermore, the use of hydrogen-based agents may lead to development of coarse pore structures, and both excessive and insufficient foaming temperatures can compromise overall foam quality.

Out-situ gas formation method

The GASAR technique, also known as Directional Solidification of Metal-Gas Eutectic, presents a novel approach to produce porous metals whereby magnesium is intentionally melted in the presence of controlled hydrogen atmosphere. Hydrogen gas is introduced into the material at a predefined partial pressure after it has been heated to a molten state. The melt is then superheated to around 100 K and maintained at that temperature for approximately 30 min.³⁴ Subsequently, the hydrogen-saturated melt is poured into a mold. After achieving hydrogen saturation, directional solidification is initiated. This can be achieved by cooling the bottom of the mold while keeping the side walls uncooled, ensuring that solidification initiates at the bottom and progresses upward. To facilitate unidirectional solidification, proper insulation of the lateral walls of the mold is crucial, or they should be made from a material with high thermal capacity, such as copper,³⁴ preventing solidification from occurring at the side walls.

Porous Mg produced through the GASAR process is often referred to as lotus-type porous Mg. Researchers have primarily concentrated on examining the impact of processing variables, such as the effect of hydrogen and argon pressure. Generally, hydrogen increases porosity, while argon stabilizes nucleation and pore growth. Under unstable processing conditions where the temperature surpasses its critical value, hydrogen may escape due to bubbling or boiling. Conversely, argon can hinder bubble formation and the merging of gas pores, resulting in a consistent porous structure. The GASAR process yields tube-like pores within the Mg material. However, Figure 4 illustrates that the pore structure can exhibit variations along the height. These inconsistencies are attributed to the specific parameters during the processing. The most crucial factor is the applied gas pressure as it directly impacts both the porosity and pores sizes.³⁵

It has been demonstrated that adjusting the pressure of the hydrogen influences the porosities and averaging pore diameter of magnesium-based materials.³⁷ For instance, Hoshiyama et al.³⁷ found that maximum porosity of Mg and its alloys can be achieved by fabricating them under 0.2 MPa hydrogen pressure. Studies by Li et al.³⁴ investigated the impact of hydrogen pressure on pore morphology in Mg-Al alloys. The authors observed a gradual increase in pore diameter (across the cross-section) and improved directional growth (in the longitudinal section) with increasing pressure (0.1–0.6 MPa), leading to a more uniform pore distribution. Conversely, Zhou et al.³⁸ reported a decrease in pore diameter and porosity in Mg-Mn-Zn alloys fabricated under the same pressure range (0.1–0.6 MPa). Zhou et al.³⁸ also reported that increasing Zn content has a negative impact on porosity but gives better compressive strength values. Several studies have explored the incorporation of Ag as an alloying element to fabricate porous Mg-Ag alloys. The effect of adding silver on porous magnesium alloys exhibits similarities to that of adding zinc. While the addition of Ag enhances the compressive strength and elastic modulus of the resulting foam,³⁹ it can also lead to decreased porosity and a less uniform distribution of pore diameters.⁴⁰

Researchers also used argon alongside hydrogen to fabricate GASAR Mg.⁴⁰ Hoshiyama et al.³⁷ observed that the manufacturing technique for Mg alloys influences the resulting porous structure. For instance, in Mg alloys produced through the casting technique, pores tend to extend more frequently towards solidification at lower concentrations of alloying elements. In contrast, cylindrical pores develop in the transverse direction in the continuous zone melting technique by controlling the temperature gradient to be steeper.

Sadeghi et al.⁴¹ explored a novel method for fabricating magnesium-based foam using viscose rayon fibers (VRFs) as a hydrogen source. This Gasar method employed a casting process at atmospheric pressure. While they achieved a high porosity of up to 51%, increasing the VRF content did not lead to further porosity gains. This could be attributed to the limited solubility of hydrogen in the molten magnesium. Interestingly, the pore diameter remained relatively constant despite variations in VRF content. Table 3 shows a summary of GASAR Mg-based materials.

Table 3.

Porous Mg-based materials fabricated by GASAR method.

Materials	Gas/ Pressure (MPa)	Melting Temperature (°C)	Pore Properties		Compressive YS (MPa)	Ref.
Materials	Gas/ Pressure (MPa)	Melting Temperature (°C)	Avg. Pore Size (μm)	Porosity (%)	Compressive YS (MPa)	Ref.
Mg	Hydrogen/0.6–1	750	170–840	28–34.7	23.9	^39,42
Mg	Hydrogen-argon (60:40)/1	660–700	70–120	27		³⁶
Mg-3 wt.%Ag	Hydrogen/0.6	750	376–680	34.7		^34,39
Mg-1 wt.% Mn	Hydrogen/0.4	750		41.2		³⁸
Mg-1 Mn-(1–2)Zn (wt.%)	Hydrogen/0.1–0.6	750	537–2512	35.8–47.1		³⁸
Mg alloy/0.4 wt.%VRF		800		10		⁴¹
Mg alloy/1.41 wt.%VRF		800	1010–1060	52		⁴¹
Mg-atom%0.5–17.57Ag	Hydrogen-argon/0.2	800	200–900	30.1–18.2		⁴⁰

The out-situ gas formation method facilitates the lotus-type unidirectional pores via directional solidification in a controlled hydrogen atmosphere. This technique enables the formation of tube-like pores with adjustable porosity and diameter by varying gas pressures and thermal gradients. The combined use of hydrogen and argon can stabilize pore growth and enhance uniformity. However, process success depends on the precise control of gas composition, pressure, and temperature distribution throughout the melt. Any deviation from optimal processing conditions can lead to the loss of hydrogen or result in structural defects. Additionally, equipment complexity increases due to the need for specialized molds and insulation, and certain alloying elements such as zinc are known to adversely affect the porosity while enhancing mechanical strength.

Powder metallurgy

Powder metallurgy begins with metal powders. These powders are mixed with space holder materials. This method is called “Space Holder Method”. These space holder materials can be particles of various shapes and sizes. The space holders are well-mixed with magnesium powder to create a homogeneous blend. The subsequent steps involve compaction of the blended mixture into a green compact, creating a composite where the Mg acts as matrix component while the space holder material acts as reinforcing component. After the compaction process, the space holder materials are removed by a suitable process, that is thermal decomposition,¹¹ leaching in liquid solution,⁴³ chemical etching,⁴⁴ and sublimation.⁴⁵ leaving behind desired voids or pores in the magnesium-based material. The porosity, pore size, and distribution can be precisely controlled by adjusting the properties of the space holder material and the processing parameters, such as sintering temperature and time. Following the removal of space holder material, a sintering process is employed to consolidate the loose powder into a solid form. Some studies utilize a two-step sintering approach. The first step may aim to remove the space holder material through thermal decomposition¹¹ or eliminate impurities left behind by the space holder or any additional materials.⁴⁵

When selecting a placeholder material for a space, there are numerous options available in terms of the material type. Urea, also known as carbamide (CO(NH₂)₂), serves as a water-soluble space holder material, dissolvable in water, ethanol, and hydrofluoric acid. Additionally, it undergoes thermal decomposition at temperatures lower than the melting point of Mg. For this reason, many papers used Urea as a space holder material to fabricate Mg-based porous materials.^11,46,47

Čapek et al.⁴⁸ fabricated porous magnesium by using irregular Mg powder (75–150 μm, as shown in Figure 5(a)) and ammonium bicarbonate powder (250–500 μm, as shown in Figure 5(b)) as raw materials. According to the research findings, the porosity exhibited an increase with an elevation in ammonium bicarbonate content, as illustrated in Figure 5(c). The flexural strength exhibited a substantial decrease with an increase in porosity, as depicted in Figure 5(d).

A more recent approach involves the fabrication of a magnesium porous scaffold using titanium wire as a space holder.⁴⁴ The titanium wire can be eliminated from the magnesium matrix through an etching process using hydrofluoric acid (HF), which corrodes titanium. Alternative space-holding materials encompass naphthalene,⁴⁹ camphene,⁴⁵ sucrose,⁵⁰ and carbonate hydrogen ammonium.⁵¹ Both camphene and naphthalene undergo a sublimation process for removal from the magnesium matrix.^45,49 Sucrose, highly soluble in water, can be eliminated by immersion in water.⁵⁰ Lastly, carbonate hydrogen ammonium can be extracted from the magnesium matrix through thermal decomposition.⁵¹ A summary of porous Mg-based materials made by powder metallurgy method is provided in Table 4.

Table 4.

Porous Mg-based materials fabricated by powder metallurgy process.

Materials	Space Holder	Compact Pressure (MPa)/Time (min)	Space Holder Removal Process	Sintering Temperature (°C)/Time (h)	Pore Properties		Compressive YS (MPa)	Ref.
Materials	Materials/Content	Compact Pressure (MPa)/Time (min)	Space Holder Removal Process	Sintering Temperature (°C)/Time (h)	Avg. Pore Size (μm)	Porosity (%)	Compressive YS (MPa)	Ref.
Mg	Carbamide / 30–50 vol.%	90/2	Thermal Decomposition	250/5 and 500/2 (double sintering)	20–2400	30–50	10.1–20.48	¹¹
Mg-5Ca-3Zn-5n-HA (wt.%)	Carbamide / 30–50vol.%	90/2	Thermal Decomposition	250/5 and 500/2 (double sintering)	20–2400	30–5	11.47–23.22	¹¹
Mg-5Ca-2Zn-1.5Fe-5n-HA (wt.%)	Carbamide / 30–50vol.%	90/2	Thermal Decomposition	250/5 and 500/2 (double sintering)	20–2400	30–5	9.99–25.75	¹¹
Mg	NaCl / 60vol.%	20/-	Leaching: NaOH + ethanol	585/2	240	60	9.5	⁵²
Mg/(1–5)vol.% Al₂O₃	NaCl / 60vol.%	20/-	Leaching: NaOH + ethanol	585/2	240	60	10.6–13.8	⁵²
MgF₂-coated Mg/5vol.% Al₂O₃	NaCl / 60vol.%	20/-	Leaching: NaOH + ethanol	585/2	240	60	13.7	⁵²
Mg	Ti-wire / 50–66.66%	13.8/5	Etching: Hydro fluoride			10.39–35.29		⁴⁴
AE42 alloy	Camphene / 3.6 mL		Sublimation (for 1 week)	450/4 and 640/10 (double sintering)	12.6	51.6	∼50	⁴⁵

The powder metallurgy method, especially the space holder technique, offers great flexibility in adjusting porosity, pore size, and distribution. By mixing magnesium powder with space holder particles (such as urea, ammonium bicarbonate, NaCl, Ti-wire, or camphene), followed by compaction and sintering, porous magnesium structures can be accurately manufactured. Different removal methods (like thermal decomposition, leaching, sublimation, or etching) allow for the removal of space holders, while the sintering process consolidates the structure. This approach accommodates a wide range of pore sizes and enables control over mechanical properties. However, it is essential to ensure the complete removal of space holders such as NaCl to prevent contamination. Additionally, precise control of compaction pressure, sintering temperature, and duration is necessary to avoid defects or incomplete bonding. Some space holders, including camphene or naphthalene, require extended removal times, which can slow down the manufacturing process. Furthermore, higher porosity often results in reduced mechanical strength, calling for additional reinforcement or process optimisation.

Comparative overview of fabrication methods for open- and closed-pore porous magnesium

Open-pore magnesium

The open-pore structures are characterized by interconnected porosity. They are mostly fabricated using the preform infiltration technique and the powder metallurgy approach.

In the preform infiltration method, molten magnesium is introduced into a pre-shaped sacrificial preform made from materials such as sodium chloride (NaCl),¹⁵ calcium chloride (CaCl₂),¹⁸ or titanium wires and beads,^8,19 After solidification, the preform is removed through leaching, etching, or thermal decomposition, leaving behind a network of open and interconnected pores. The pore geometry, ranging from spherical to cubic or even wire-like, can be precisely controlled using CAD-designed preforms.⁵ This infiltration method offers a high degree of control over pore shape and distribution, making it particularly advantageous for biomedical and structural applications needing high porosity and permeability.

Another widely used method for fabricating open-pore magnesium is powder metallurgy, mainly the space holder technique. In this process, magnesium powder is well mixed with space-holding materials such as urea,^11,46 ammonium bicarbonate,⁴⁸ camphene,⁴⁵ naphthalene,⁴⁹ or titanium wire.⁴⁴ After compaction of the mixture, the space holder materials are removed using processes like thermal decomposition, leaching, or sublimation followed by sintering process. Removal of space holders leaves behind the desired porous structure, which is then consolidated through sintering. This approach allows for fine-tuning of the pore characteristics by varying the size, shape, and volume fraction of the space holders, offering a flexible and reproducible route for producing magnesium foams with well-defined interconnected pores.^44,45,48

Closed-pore magnesium

The closed–pore structures are characterized by the pores that are isolated and gas-filled. They are mainly produced through melt foaming techniques, including both in-situ and out-situ gas formation methods.

The in-situ gas formation method, also known as melt foaming, involves introducing a blowing agent such as titanium hydride (TiH₂),²¹ calcium carbonate (CaCO₃),²² or magnesium carbonate (MgCO₃) into molten magnesium.²⁵ The heating process decomposes these agents and releases gas as bubbles that become entrapped as the metal solidifies. To enhance foam stability and uniformity, a thickening agent like calcium and a stabilising agent such as aluminium, silicon carbide, or beryllium are often added.^14,21,22,29 Parameters such as foaming temperature, gas release rate, and stirring speed play critical roles in determining the final pore size and distribution.²⁵ This technique is particularly effective for creating lightweight materials with good energy absorption characteristics.

Another method for fabricating closed-pore magnesium is the GASAR (Gas Saturation) process, also known as the out-situ gas formation technique. This process involves saturating molten magnesium with hydrogen gas under controlled pressure, followed by directional solidification to trap the gas as elongated, tubular pores, commonly referred to as lotus-type structures.^36,37 The closed-pore morphology in this method is highly sensitive to variables such as gas pressure, cooling rate, and mould design.^34,35 Hydrogen gas increases pore formation, while the addition of inert gases like argon helps stabilise the pore structure and prevents pore coalescence.^36,40 The GASAR process offers a unique ability to generate directionally aligned pores, making it suitable for specialised applications requiring anisotropic mechanical or thermal properties.

The importance of properties of porous Mg-based materials and alloying

Researchers have concentrated on a variety of properties that are crucial for porous Mg-based materials, as these properties often influence one another. This section will explore the interrelationships between these properties and discuss the methods used to calculate some of them. Mechanical properties can be considered one of the most important material properties in most cases. Studies related to the compressive strengths revealed that stress-strain curves of magnesium foams have three distinct stages, as illustrated in Figure 6. The initial stage is characterized by linear deformation, where stress increases proportionally with strain until reaching the upper yield point. The second stage, known as the plateau stage, features relatively stable flow stress with minor serrations as strain increases. It is noteworthy that at the first maximum stress or limit stress, specimen deformation becomes localized, leading to cell collapse within the localized band, hence this stage is also named the ‘Collapsing Stage’.⁵³ During the plateau deformation, some curves exhibit serrations indicative of brittle fracture behaviors, while others maintain smooth curves, signifying ductile fracture behaviors.⁵⁴ The final stage, referred to as the densification stage, sees a rapid increase in stress with a slight rise in strain.

According to research, there are a number of ways to enhance the mechanical characteristics of magnesium foams, one of which is to include ceramic particles.⁵⁵ Many researchers fabricated Mg syntactic, which consists of hollow microspheres that act as both pores and reinforcements of the porous Mg.^56–58 Others have combined traditional foam fabricating methods with the Mg syntactic method, incorporating magnesium with both conventional pores and pores composed of hollow microspheres.⁵⁹ Parameters such as porosity,^6–8 pore size,⁶ the nature of the pore (i.e., closed²⁵ or open¹⁸), pore shape,⁶⁰ heat treatment,³³ the addition of alloying elements,¹⁴ and operating temperature,¹² also affect mechanical properties. Figure 6. An illustration of a typical stress-strain curve.

The compressive properties of porous magnesium-based materials are strongly influenced by the pore structure and the composition of the metal matrix. Variations in pore size, shape, and porosity level significantly alters the mechanical properties under compressive loading. For instance, larger pores and higher porosity generally reduce compressive strength due to decreased load-bearing area and stress concentration around the pore walls.^6,7 As reported by Ji et al.,⁵³ an increase in the foam density that corresponds to reduced porosity, leads to higher compressive strength and energy absorption capacity, as shown in Figure 7(a) and 7(b). Similarly, Figure 8 illustrates that finer pore sizes and lower porosity improve both compressive resistance and damping behavior.¹⁹ The pore types, open-pore and closed-pores, also play a critical role in the mechanical behavior of porous magnesium. The closed-pore foams typically exhibit higher strength owing to trapped gas pressure and more intact wall structures In contrary, open-pore foams provide better energy dissipation but at the cost of reduced strength.²⁵

In addition to the pore structure geometry, the composition of the metal matrix significantly impacts the compressive behavior. The incorporation of alloying elements such as zinc, calcium, gadolinium, yttrium, and aluminum enhances the mechanical performance by solid solution strengthening, grain refinement, and the formation of secondary phases. For example, Zn addition to pure Mg or Mg-Ca systems has been shown to increase compressive strength and elastic modulus,^7,11 while Ca contributes to pore wall stability and promotes uniform pore structures in the foams made via melt foaming.²¹ The addition of Gd and Y in Mg-Zn-based foams enhances energy absorption and yield strength by refining the microstructures and inducing long-period stacking ordered (LPSO) phases.^12,13 The moderate addition of aluminum can enhance strength but may induce brittleness in the pore walls due to the formation of intermetallics.¹⁴ Moreover, the presence of Be is found to stabilize the foam pore walls and prevent gas escape, resulting in formation of more uniform and defect-free porous structures.²²

Collectively the pore structure and compositional parameters govern the foam's load-bearing capacity, energy absorption, and failure mechanisms, underscoring the importance of a tailored design approach for specific applications.

Density also plays an important role in mechanical properties as indicated by Haibin et al.⁵³ He demonstrated that increasing density increases compressive strength and Young's modulus, as seen in Figure 7(a). Mg-based foams have energy absorption properties. The key factors for assessing the characteristics of Mg foams include energy absorption capacity (W) and ideal energy absorption efficiency (I). These properties can be determined using equations (1)¹² and (2),¹⁴ respectively.

W = \int_{0}^{ε} σ d ε

(1)

I = \frac{1}{σ_{m} ε_{m}} \int_{0}^{ε_{m}} σ d ε

(2)

where

σ

is the stress under the stain

ε

and

σ_{m}

is the stress under the strain

ε_{m}

Absorption properties can be improved by adjusting porosity or pore size, as well as by modifying density values, as illustrated in Figure 7(b). Additionally, other techniques can enhance the energy absorption behavior of porous magnesium, such as incorporating solid¹⁴ and hollow^33,59 ceramic particles, heat treatment,³³ and alloying.¹² On the other hand, porosity, alternatively known as bulk porosity (Prb), is a crucial property and it can be defined as the volume fraction of all pores within a completed Mg foam product. This can be computed using equation (5).⁴⁷

P r_{b} = \frac{V_{s} - (\frac{W}{ρ})}{V_{s}} \times 100 %

(5)

where V_s is the volume of the sample, W is the weight of the foam, and ρ is the density of Mg. Figure 8 shows the effect of both porosity and pore size on mechanical and absorption properties of Mg foams.

Researchers have extensively studied the incorporation of different alloys into Mg-based materials to achieve specific properties, such as enhanced foaming characteristics, mechanical strength, and absorption capabilities. The addition of various elements to Mg-based materials has been shown to significantly influence key parameters like porosity, pore size, compressive strength, and elastic modulus. For instance, research has highlighted that adding Ca to Mg-based materials fabricated through the melt foaming method is crucial for achieving a more uniform cell structure and improved cell size distribution.²¹ Similarly, Devikar et al.²² reported that the addition of Al and Be to Mg alloys results in better foaming properties. However, the incorporation of Al into Mg-based foams also presents challenges. Lu et al.¹⁴ found that while Al can enhance the strength of the foams by embedding phases within the cell edges and walls, it also introduces brittleness to the cell walls. On the other hand, zinc has been widely recognized for its positive impact on the mechanical properties of Mg-based porous materials. The addition of Zn in pure porous Mg has been shown to increase compressive strength and Young's modulus.¹¹ Further studies have demonstrated that increasing the Zn content in Mg-Al alloys can enhance yield strength.⁴⁷ In Mg-Mn alloys, Zn addition leads to the formation of larger pores, which, despite reducing the material's density, results in greater compressive strength.³⁸ Additionally, in the Mg-Ca system, the inclusion of Zn not only improves compressive strength but also enhances the elastic modulus.⁷

The addition of Ag has also been explored for its effects on Mg-based foams. Liu et al.³⁹ reported that Ag decreases pore diameter while increasing compressive strength and providing an excellent elastic modulus. Another study by Liu¹² found that adding Yn to porous Mg-Zn materials fabricated through the infiltration method significantly improves compressive strength and energy absorption capacity. Conversely, increasing the Gd content in the Mg-Zn system through the infiltration method refines the microstructures of the foams and promotes the formation of long-period stacking ordered phases.¹³ Furthermore, research by ZHOU et al.⁶¹ demonstrated that Mn addition in porous Mg foams fabricated by the GASAR method decreases pore size but increases surface porosity.

The low density of magnesium is a significant benefit and a key characteristic that makes it valuable in various applications. Hence, there's a necessity for achieving certain porosity and density. Figure 9 illustrates the effect of porosity on the density of various magnesium-based materials. The linear decrease in density with increasing porosity is evident. This outcome is expected, as the reduction in density is attributed to the formation of pores in magnesium. Furthermore, the graph shows that most Mg-alloys, such as AZ91, AZ31, Mg-Zn-Ca, and Mg-Ca, that have different densities than pure Mg (1.74 g/cm³) shows a similar density-porosity relationship. Moreover, if a trendline is drawn over the plots, as seen in Figure 9, the equation of the best fit line will be:

ρ = - 0.0186 \times (P r_{b}) + 1.8329

(6)

Where $ρ$ is density and Pr_b is porosity, also called bulk porosity. This equation provides the relationship between porosity and density, allowing researchers to understand how changes in porosity impact the overall density of magnesium structures. Moreover, it can also help to identify the optimal level of porosity that aligns with the desired density for a particular application. This information is crucial for tailoring magnesium-based materials where the balance between strength and density is critical.

Future perspectives

While porous magnesium offers the advantage of low density, its mechanical strength can be significantly compromised. In particular, compressive strength can plummet from 100 MPa to as low as 10 MPa when porosity reaches 50%, representing a substantial 90% reduction.⁷ This significant decrease in mechanical strength may render porous magnesium unsuitable for certain applications. To mitigate this issue, the addition of different alloying elements can be explored. Incorporating a combination of rare earth materials like Gd, Y, and Zr, along with non-rare earth metals such as Ca, Al, Zn, and Mn, can enhance the mechanical strength of porous magnesium.⁶⁸ Additionally, incorporating hollow spheres can further reduce density while simultaneously reinforcing the material.

Understanding the influence of pore distances on mechanical strength is another critical area of research. Studies suggest that a crack initiated from one pore can propagate to a neighboring pore if the distance between them is sufficiently short.⁶⁹ Studies should be conducted to determine the minimum distance required to minimize this fracture behavior for porous Mg-based materials that can occur in the ‘Collapsing Stage’ as mentioned before. Moreover, fabricating porous magnesium with precisely controlled pore sizes and distribution can be beneficial. In methods involving sacrificial materials, fabricating the sacrificial material with precision can ensure adequate spacing between pores. Leveraging CAD can further refine the fabrication process, enabling the creation of sacrificial materials with even greater precision.

Due to the unique combination of high theoretical hydrogen capacity of pure Mg, low density, and abundant magnesium resources, porous Mg-based materials offer a promising avenue for hydrogen storage. Additionally, the high surface area of porous magnesium may enhance its suitability for hydrogen storage. However, this area requires further exploration. To propel the development of these materials for hydrogen storage, several key recommendations are proposed. Firstly, researchers should investigate the relationship between pore characteristics (size and distribution) and hydrogen storage capacity. Optimizing pore structure has the potential to significantly enhance hydrogen absorption and desorption kinetics, a major hurdle for Mg-based materials. Additionally, research into developing porous Mg-based materials with other materials known for favorable hydrogen storage properties is crucial. By addressing these recommendations, researchers can unlock the potential of porous Mg-based materials for efficient hydrogen storage and other applications.

Conclusion

Porous Mg-based materials are characterized by a 3D network of interconnected or closed, isolated pores, and their fabrication method plays a crucial role in dictating the pore structure, including the porosity, pore size, number of pores, and architecture configuration.

These pore-related characteristics, strongly influenced by fabrication technology, directly impact material properties including mechanical strength, energy absorption capability, and density. Among these, porosity is particularly important due to its linear correlation with density, as evident in Figure 9.

The foam-like structure of porous Mg makes it especially useful in impact absorption applications, and its performance can be further enhanced through strategic alloying, where the choice of alloying elements helps tailor properties and compensate for mechanical strength loss.

With ever increasing industrial demand for lightweight, high-performance magnesium alloys and composites, behavior and fabrication of magnesium foams are still in infancy and require further exploration before the material can be universally adapted for engineering applications.

Footnotes

Acknowledgements

The authors would like to thank Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals for their support for this work.

Author contribution(s)

S Fida Hassan: Conceptualization; Data curation; Formal analysis; Methodology; Supervision; Writing – original draft; Writing – review & editing.

Zubair Alamin: Data curation; Formal analysis; Writing – original draft.

Amir Al-Ahmed: Conceptualization; Formal analysis; Supervision; Writing – review & editing.

Data and code availability

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

S Fida Hassan

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