Aluminum foam, which belongs to ultra-light metal foams, appeared as an appealing research ground both from the industrial and scientific application standpoints. It is characterized by energy occlusion, stiffness, high density, and high level of absorption power. The paper will analyze the structural and mechanical properties of aluminum foam and its production process. In addition, it will demonstrate how aluminum foam can be applied in automobile industry, analyzing its influence on crash protection, stiffness, solid structures, sound and energy absorbing. The paper will define solid difference between aluminum foam filling automobile sections and hollow sections in regard to deformation mechanism.
The aim of the paper is to analyze the behavior of aluminum foam and the reasons why it is utilized for various purposes. The paper is also aimed at identifying aluminum foam characteristics and desirable properties in an attempt to decide on its usability. Different materials demonstrate various properties, which make them suited to different functions. This paper mainly focuses on aluminum foam and its properties, behavior, durability, and its cost. It also aims at determining its shortcomings.
Aluminum foam has served a major help in numerous settings including the sphere of automotive building industry as it can allow reducing the noise caused by them. In this sector, it is placed in the engine so that a swift sound and smoothness of it is experienced. An example of its application in the automotive industry includes the formation of the BMW engine (Srinath et al. 765) Aluminum also demonstrates good resistance in terms of reacting to chemicals. Due to the fact that aluminum foam demonstrates such property, it is used as chemical filters at high temperatures being at the same time used in liquid fuel containers in an attempt to avoid spillage in case of breakdown (Audysho et al. 394). It also appears as an excellent material in energy absorption; hence, it is preferred as filter metal or material in the energy firms because of that property. The aluminum demonstrates high thermal conductivity; thus, it can be used in the combustible gases medium to stop the spreading of the flame in case of any occurrence since they are capable of arresting the flames (Audysho et al. 394).
More application of the aluminum foam incorporates silencers, heat exchangers or cooling machines, and other structures (Kishimoto et al. 47). This proves that the material is not limited to functions only, while the unique properties enable it to fit in the majority of available applications. On a contrary to the titanium foam, aluminum foam is more frequently utilized, can be much easily found or purchased (Audysho et al. 395). Taking all these features into account, it becomes obvious that aluminum foam should be utilized instead of the titanium one. Its processing does not need much capital; hence, it has low or cheaper price (Srinath et al. 770).
Aluminum foam, which belongs to ultra-light metal foams, is an appealing research ground in terms of industrial and scientific application (Audysho et al. 394). The facts demonstrate that aluminum foam, which is known to be ultra-lights and closed-cell metal foam, can easily be utilized as lightweight, occlusion, and amortization structure in various industrial spheres. Additionally it presents tremendous potential in regard to the transportation sphere (Audysho et al. 394). Regardless the fact that several aluminum foam manufacturing methods are accessible, ultra-light aluminum foam application appears to be limited to a principally low-requiring market (Srinath et al. 770). The present accessible manufacturing procedures equip the efficient control of density via procedure parameters manipulation (Aad et al. 23). Nevertheless, none of these methods permit the relevant control of the cellular structures in the process of aluminum foam production and its formation, resulting in solid imperfections in terms of the final product’s mechanical and structural properties (Jaeger et al. 6200). These facts undoubtedly appear as a primary cause for the shortage of commercial reception of ultra-light aluminum foams in product caliber highly demanding sectors including automotive and aeronautical spheres (Audysho et al. 394). The resolution of this issue is a major challenge of the current scientific community (Kishimoto et al. 47). Thus, in order to obtain an acceptation in automotive sphere, two approaches might be followed. The first one regards the possibility to evolve new manufacturing processes or change the existing ones in order to get foams with more uniform cellular structures (Srinath et al. 770). The second one is to comprehend and quantify the thermo-physicochemical mechanisms and properties incorporated in the process of foam production so as to control the whole procedure, while eluding the appearing of various imperfections combined with structural defects (Jaeger et al. 6200).
All foam structures are believed to be both enduring and lightweight. They also demonstrate huge surface area and volume ratio (Kishimoto et al. 47). The exceptional mechanical characteristics of the aluminum foam incorporate high vigor to weight ratio and an entirely isotropic load reaction (Audysho et al. 394). Moreover, aluminum foam intercepts many properties and characteristics of its parent metal. They include corrosion resistance, vigor, electric and thermal conductance; however, the last appears at weight fraction (Srinath et al. 770). Generally speaking, there are two main fundamental aluminum foam structures. The first structure is stochastic or equally-sized demonstrating irregular shaped pores (Jaeger et al. 6200). This first form is also outlined as reticulated. On the other hand, the second form demonstrates even and stacked cells (Audysho et al. 394)
Thus, the first type of aluminum foam, which is known to be a stochastic or reticulated one, appears to be a skeletal form, which, in its turn, means that it is not metal covering on a fundamental structure, it is rather a pure metal form (also known as alloy) (Kishimoto et al. 47). The cells and connections of this type of aluminum foam generate regular, replicating, steady, and shape matrix throughout the whole piece of the material (Srinath et al. 770). This type is regarded to be inflexible having greatly perforated and penetrable structure. The second type is known to be structured and stacked cell aluminum foam. This type demonstrates equally-spaced and open pores outlined as ‘tetrakaidecahedrons’ (Jaeger et al. 6202). These pores are regarded as polyhedrons having 14 faces (8 hexagonal and 6 squares). This type can easily be associated with a 3D honeycomb (Audysho et al. 396).
The table demonstrates normal and typical characteristics of the two types of aluminum foam. These characteristics cannot be regarded as absolute material features, and can only be utilized merely for guidance (Srinath et al. 771). Therefore, it is recommended to test all of the materials and foam constituents for their conformity to a particular application (Audysho et al. 396). This foam types can be used in a wide variety of applications incorporating the absorption of sounds, granulation capturing, impact absorption, heating sinks and exchangers, chemical beds and scrubber matrix, various filters and mist liquidation of oils and waters, basic structures for a broad quantity of high vigor panels (Jaeger et al. 6202). They can also be utilized in numerous spacers and battery plates, as weight-diminishing constituents in automotive and aircraft applications, and in a form of a specific catalyst surface (Wongseedakaew and Luo 74).
Aluminum foam pertains to a category of materials outlined as cellular-rigid ones, which are known to have a sponginess reaching the level of 0.7 grades (Audysho et al. 396). Generally speaking, all metal foams unite the characteristics of their cellular materials with general characteristics of the parent metals (Wongseedakaew and Luo 74). This is the main reason why metal foams are beneficial in regard to their lightweight structures as a result of high vigor-to-weight ratio, which is combined with the constitutional and functional features including crash energy occlusion and heat and sound control (Wongseedakaew and Luo 74).. In fact, numerous metals can be foamed, including aluminum, which is known to be commercially most utilized material, resulting from its decreased density, high level of flexibility and heat conductance, and metal emulative value (Audysho et al. 396).
Generally speaking, aluminum foam is characterized by a number of structural parameters including pore size division, medium dimension, geometry and form of the pores, corpulence, decussations and imperfections in cell walls and their thickness, imperfections, and splits of the outer dense surface (Wongseedakaew and Luo 74). The structural features and characteristics are typically affected by morphological properties of the material. The majority of structural properties can be enhanced when all individual cells of the foam demonstrate even proportion, which should be combined with a spherical form (Kishimoto et al. 47). Moreover, the aluminum foam density and matrix characteristics impact the coefficient of the foam vigor. Moreover, the genuine characteristics of the foam practically appear as lesser contrary to the theoretically anticipated results due to its structural imperfections (Wongseedakaew and Luo 74). These facts demonstrate that aluminum foam requires higher pore management and diminution of structural imperfections. Aluminum foam is characterized by numerous density variation and factures, which produce a greater dispersal of the typically evaluated properties being able to limit the overall metal foams’ dependability (Jaeger et al. 6203). Aluminum foam’s structural properties demonstrate that the cellular construction of the foam obtained through the PM method reveal pores of various sizes and forms (Kishimoto et al. 47). The method demonstrates that aluminum foam is characterized by huge size allocation of cellular pores with asymmetrical cell shapes. In fact, the closed pores demonstrate majorly polyhedral and spherical geometry (Audysho et al. 403). The analysis demonstrates that spherical pores of aluminum foam with dense corpulence of cell walls are typically present in the bottom and sideward of the foam sample flanks. On the other hand, polyhedral pores, which have a subtle cell wall density, are mainly appearing at the top of the foam samples (Jaeger et al. 6203).
The allocation of the solid metal in the aluminum foam can also be regarded as uneven, resulting in greater density gradient. Therefore, aluminum foam demonstrates a wider cell diameter allocation curve. Furthermore, the cell proportion allocation of the foam is dominated by elevated numbers of small pores (Audysho et al. 403). The majority of the aluminum foam pores have a diameter smaller than 2mm (Jaeger et al. 6204). The analysis also demonstrates that aluminum foam is also characterized by essential morphological imperfections including the cracks or spherical micropores in cell walls and dense surface skin (Wongseedakaew and Luo 78). Typically, every foam cell has about 5 other cells in its vicinity. The allocation of the cell wall density results in an asymmetrical form of the foam. The maximal cell wall density accounts for approximately 500 meaning that the average density is near 70, which depends on the overall foam thickness (Audysho et al. 405). There are other structural features, which can impact the mechanical conduct of the foam incorporating massive cell material microstructure (Wongseedakaew and Luo 78). On the basis of the foam structure and on the manufacturing procedure, the foam final cellular structure can demonstrate metallic dendrites, eutectic cells, sediment, and even particles. Therefore, the aluminum foam, which has analogous density but has been created of discrepant aluminum alloys, might demonstrate absolutely different plateau stress.
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The facts demonstrate that it is highly important to analyze the conduct of metallic foams including aluminum one under various loading circumstances, especially evaluating the properties of metal foams under impact loading (Audysho et al. 405). The capability of managing and controlling the load-allocation conduct by a relevant choice of matrix material, cellular geometry, and comparative density can demonstrate that foam appears as an ideal material for energy absorbing constructions (Wongseedakaew and Luo 78). The uniaxial compressive mechanical test is typically applied in order to analyze the foam’s compressive conduct and the energy occluded (Jaeger et al. 6205). Generally speaking, the flexibility coefficient, output, and eminence vigor are the most significant mechanical characteristics and parameters (Kishimoto et al. 47). The analysis demonstrates that the stress-element curves of closed-cell aluminum foams reveal either flexible or fragile rupture on a basis of foam creation and its microstructure (Wongseedakaew and Luo 78). The compression conduct of the aluminum foams relies on numerous parameters including the aluminum foam composition, foam morphology (meaning cell size diapason), thickness incline of samples, imperfections of the foam’s cellular construction (meaning cell walls), and properties of the outer surface skin (Audysho et al. 406). The affects of thickness and structure of the foam in terms of mechanical features are solid and complex. Various mechanisms (both brittle and ductile) can appear on the basis of the material from which the foam has been created (Kishimoto et al. 47). On the other hand, the compressive features including the medium plateau stress, coefficient, the flexibility, and the energy occlusion demonstrate a tendency to rely on the foam density, in which their vigor and modulus elevate together with its increase. Generally speaking, stress-element curves of aluminum foam can be divided into three characteristic spheres (Wongseedakaew and Luo 76). The first sphere regards the linear-flexed characteristic where the load elevates with practically linearly rising of compression allocation (meaning the flexible declination of the foam’s pore walls).
This process is typically followed by plastic subsidence plateau (Audysho et al. 407). The second sphere regards the above-mentioned plastic subsidence plateau, which demonstrates practically sustainable compression load (meaning that pore walls elaborate or break at the same time when the elevating distortion does not require a load rise) (Kishimoto et al. 47). The last sphere concerns the compression of the foam, which is typically characterized by rapid elevation on the load when the cell walls crush together (Jaeger et al. 6207). The sphere also demonstrates more or less obvious plateau after an elastic loading. Such plateau stress is highly significant in order to characterize the energy occlusion conduct and appears as a good material feature for the foam’s compression effectiveness (Wongseedakaew and Luo 76). Nevertheless, this type of analysis also allows demonstrating that discrepant failure modes and mechanisms can appear during different periods of the load-displacement. Firstly, the elastic deformation appears as a result of edges curving, lengthening of the cell walls, and cornered gas pressure inside the cells (Kishimoto et al. 47). However, these affects do not cause deformation. The distortion appears as practically totally reversible in the third sphere. Thus, when the foam reaches its elastic border, the subsidence of the cells begins being caused mainly by distortion (spreading), circumvolution, and sliding of the edges and cell walls, which results in irrevocable distortion (Jaeger et al. 6207). Aluminum foams are frequently applied as filler material in lightweight constructions due to its crash resistance and high velocity affect or as thermal and acoustic isolation implements (Kishimoto et al. 47). The energy absorption capacity of the foam can be well analyzed through the stress-element compression conduct of the material (Wongseedakaew and Luo 76). Due to the fact that the foam’s material represents a permanent stress plateau, aluminum foam can occlude higher levels of energy contrary to dense aluminum foams. The majority of the occluded energy is inevitably changed to a plastic distortion energy, which appears as additional advantage of aluminum foam (Audysho et al. 407).
Aluminum foam processing typically appears through the PM method, which is usually separated into two production steps. The first step regards the generation of a specific foamable precursor (Kishimoto et al. 45). The second step concerns the generation of the actual metal foam via the process of foaming the precursor material. The first step can be regarded as the arranging of a thick strong semi-finished item outlined as a foamable precursor (Audysho et al. 408). This precursor is later acquired by condensing a specific powder blend incorporating the blowing facet and the metal itself by utilizing an accredited method (Kishimoto et al. 45). The second step encompasses the creation of the metal foam by warming the foamable precursor at higher temperatures than aluminum melting temperature. The major benefit of the demonstrated PM method is empowering the creation of the implements of metal foams with discrepant architectures (for instance, sandwich systems, stuffed profiles, and 3D complicated molded constructions) (Audysho et al. 408). All of the materials can be united in the process of the foaming step in the absence of the chemical adhesives utilization (Kishimoto et al. 45). This is the first main advantage of the method, while the second advantage concerns the fact that the augmentation of ceramic particles is not necessary, which allows avoidance of the fragile mechanical conduct deduced by these particles. Additionally, the foam parts are covered by an outside thick metal skin, which obviously enhances aluminum foam mechanical conduct equipping a perfect (almost ideal) surface finish (Kishimoto et al. 45). Nevertheless, this method also has a disadvantage in a form of high production costs majorly connected with the powder value (Wongseedakaew and Luo 77). The second disadvantage of the aluminum foam processing method concerns the difficulty of production of large volume foam parts (Jaeger et al. 6207).
The foaming processing typically appears in resistant to strains steel-enclosed moulds. The cell of the mould is supposed to have the analogous design and shape as final aluminum foam (Wongseedakaew and Luo 77). The foamable precursor is supposed to be warmed in the mould to the temperature higher than the melting point. The material demonstrates a tendency to expand and fill the whole mould cell. Following the previous procedure, the mould has to be cooled, while the foam should be extracted (Jaeger et al. 6207). The ovens utilized in the manufacturing process of the aluminum foams typically appear to be of the measuring chamber oven kind (Kishimoto et al. 45). The atmosphere, warming ratio, and temperature of the heat foaming cycle are believed to be among the most important production parameters, which have a tendency to impact the caliber and the characteristics of the resulting foam. In addition to this, the properties of the mould (material, design, and dimensions), the kind of the oven (meaning it is a batch or continuous one) should also be taken into account when analyzing the aluminum foam properties (Wongseedakaew and Luo 77). The impacts of these variables on the foaming conduct are also highly important and should be properly analyzed during the production steps (Audysho et al. 409). The foaming tests are executed by warming the precursor material up to its melting point inside the apparatus outlined as laser expandometer. Moreover, the enlargement (meaning volume) and its temperature are controlled with the help of a specific laser sensor and thermocouple (Jaeger et al. 6207). The foaming procedure is solidly controlled by temperature impacts (Kishimoto et al. 45). The enlargement curve is completely reliant on the processing circumstances, majorly on the hot stamping temperature, which is necessary to acquire the foamable precursor material together with the heating parameters appearing in the process of foaming (Jaeger et al. 6207). Despite the fact that aluminum foam production appears at an industrial stage, the procedure still has chief restrictions regarding the capability to get tailor-made cellular construction foams and to portend aluminum foam characteristics (Wongseedakaew and Luo 78). This presupposes that the production might result in cellular structured foams with discrepant sizes and shapes of pores and structural imperfections. Thus, they can demonstrate a high density gradient depending on the component size (Kishimoto et al. 45). All of these imperfections appear from the asperities in controlling the manufacturing process (Audysho et al. 409).
The present automobile sphere requires numerous advanced characteristics, including energy effective crash secure constructions, effective heat management system, quiet driving, and lowered level of vibration (Srinath et al. 765). The analysis of aluminum foam properties demonstrate that this material not merely has high level of energy and sound occlusion, but is characterized lightweight as well (Aad et al. 43). Insertion of aluminum foams into automotive constructions elevated the level of density-indemnified design indexes, incorporating curving acerbity, curving vigor, energy occlusion, bending damping, and etc (Srinath et al. 765). Despite the fact that closed cell foams are not as efficient as open cell foams in regard to the sound occlusion, any enhancement of the latter’s characteristics of closed cell foams might appear as additional bonus to their multifunctional character. The probable application spheres of aluminum foam in automobiles regards numerous areas, incorporating “crash box, pillars and frames, Rear Under Protection Device (RUPD), Side Under Protection Device (SUPD), engine mount brackets, and floor panel” (Srinath et al. 765). Moreover, the materials solution to lowering the noise vibration stiffness levels might appear as highly appealing. It is known that crash boxes are the constructions located between the affect balancer (also known as bumper) and anterior cleat of a vehicle in order to protect driver and/or passengers from the frontal crash (Wongseedakaew and Luo 74). The typical crash boxes are created of metal steel tubes, which occlude kinetic energy by enduring plastic distortion in allocated areas relying on the dimension of the affect vigor. The distortion of a crash box appears mainly in two fundamental types, meaning “concertina” and “diamond kind”, the previous regarded as preferential concerning the overall energy absorption (Srinath et al. 765).
There are also situations, in which there might appear a combined mode failure. Application of the decreased density metallic foam into the above-mentioned metal tubes elevates the level of energy occlusion features of the crash box (Srinath et al. 768). A considerable level of crush opposition of foam-incarnated tube typically attributes to the interference of the wall material into the foam topping, which generally opposes the distortion appearing in elevated deformation vigor and energy occlusion (Wongseedakaew and Luo 74). The application point of view suggests that it is beneficial to utilize steel pillar as it demonstrates a lighter unification with steel undercarriage (Aad et al. 43). Moreover, steel pillar has greater collapse vigor, when combined with the aluminum foam, and much greater collapse vigor on the contrary to the bare foam. Secondly, the interplay between the collapse of aluminum pillar and foam is typically anticipated as they demonstrate analogous collapse vigor (see fig. 1) (Srinath et al. 768).
Automobile constructions typically apply numerous thin-walled prismatic balancers, including A-pillars, B-pillars, and etc. The facts demonstrate that aluminum foam stuffed balancers have greater specific vigor and energy occlusion against collapse on the contrary to the unfilled balancers (Wongseedakaew and Luo 74). Foam inclusion can enhance resistance to biaxial curving and provoke onward destructive consuming higher energy in the process of distortion (Aad et al. 43). Aluminum foam stuffing in balancers can be performed in the whole extent or selectively at the areas where the greater level of consolidation is necessary (Srinath et al. 769). Additionally, sound and vibration resistant sides are necessary in vehicle “floors, bulkheads, engine mount brackets, etc” (Srinath et al. 769). Traditionally, the automobile industry utilizes polymer foams to perform sound occlusion. The most significant frequency diapason for sound occlusion appears “in the range of 400–4000 Hz, since higher audible frequencies get attenuated easily” (Srinath et al. 769). However, contrary to the polymer foams, aluminum foams can also be regarded as heat resistant and self-sufficient (Wongseedakaew and Luo 74).
Therefore, in order to understand whether aluminum foams are beneficial for the application in the automobile industry, the properties of closed cell aluminum foam should be analyzed in regard to their utilization in crash boxes, flexural balancers, and sound occluding panels (Aad et al. 43). Thus, the facts demonstrate that axial crushing of aluminum foam stuffed crash box reveals 2.2 times improved energy occlusion if compared with the hollow crash box (Srinath et al. 770). The distortion manner incorporated the creation of steel shell folds. Moreover, when the crush box tubes can has been filled with aluminum foam, the ‘concertina’ type distortion manner appeared (which is believed to be preferential) at the same time when the hollow steel crash box suffered from the ‘diamond’ type (Srinath et al. 770). The elevation of the energy occlusion is typically connected with the interplay affect appearing in the aluminum foam and steel shell (Srinath et al. 770). On the other hand, flexural testing of aluminum foam stuffer in aluminum section demonstrated the elevated climax load and curving moment with the rise of the rotation angle in order to obtain the maximal curving point with essential alterations in the extent of the distorted location. It demonstrates higher level of resistance of foam filled section to the overall distortion and elevated level of energy attenuation (Srinath et al. 770). Nevertheless, the acoustic amortization with the closed cell aluminum foam is regarded as efficient merely in “a very narrow frequency range of 700 to 1300 Hz” (Srinath et al. 770). Piercing of the closed cell foam enhanced the overall sound occlusion at greater diapason with attenuation climax moving to greater frequencies and creation of the derivative climaxes (Aad et al. 43). On the other hand, thinner shattered aluminum foam (meaning 8mm) demonstrated essential sound occlusion in the whole medium range of 1000 to 2500 Hz (Srinath et al. 770). Instead, thicker shattered aluminum foam (meaning 22 mm) demonstrated bad sound occlusion in frequency diapason higher than 700 Hz. At the same time, it revealed greater sound occlusion of lower than 700 Hz diapason (Srinath et al. 770). The above mentioned conduct can be attributed to the interplay of the pores in the shattered foam, where the sound energy disperses (Aad et al. 43). Moreover, aluminum foams can be easily united with polymer foams in order to obtain appropriate sound occlusion in all frequency diapasons (Srinath et al. 770).
The paper demonstrates that aluminum foam technology and particularly aluminum foam vehicle utilization has led to numerous promising small and large scale applications. The market evolvement requires the availability of the materials, which are lightweight, energy-occluding, and damping. All foam structures are believed to be both enduring and lightweight. They also demonstrate huge surface area and volume ratio. The exceptional mechanical characteristics of the aluminum foam incorporate a high vigor to weight ratio and an entirely isotropic load reaction. Moreover, aluminum foam intercepts a high quantity of properties and characteristics of its parent metal, namely the corrosion resistance, vigor, electric and thermal conductance (the last one correlates more to a fraction of the weight). Aluminum foam processing typically appears through the PM method, which is usually separated into two production steps. The first step regards the generation of a specific foamable precursor. The second step concerns the generation of the actual metal foam via the process of precursor material foaming. This method has numerous advantages. All of the materials can be united in the process of the foaming step in the absence of the chemical adhesives utilization. Secondly, the augmentation of ceramic particles is not necessary, which allows avoidance of the fragile mechanical conduct deduced by these particles. In addition to this, the foam parts are covered by an outside thick metal skin, which obviously enhances aluminum foam mechanical conduct and equips perfect (almost ideal) surface finish. The present automobile sphere requires numerous advanced characteristics including energy effective crash secure constructions, effective heat management system, quiet driving, and lowered level of vibration. The analysis of aluminum foam properties demonstrates that this material is lightweight, has high level of energy and sound occlusion. Insertion of aluminum foams into automotive constructions elevated the levels of density-indemnified design indexes, namely curving acerbity, curving vigor, energy occlusion, bending damping, etc. Regardless of all technological advancements, aluminum foam production cannot be regarded as problem-free, and it presents several challenges. The main question concerns the possibility to produce aluminum foams in series in order to obtain even cellular structure, which can assist in controlling aluminum foam architecture.