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Fabrication and characterization of closed-cell magnesium-based composite foams
Materials and Design 74 (2015) 36–43
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Fabrication and characterization of closed-cell magnesium-based composite foams Xingchuan Xia a,b,⇑, Junlong Feng a, Jian Ding a, Kaihong Song a, Xiaowei Chen a, Weimin Zhao a, Bo Liao a, Boyoung Hur c a b c
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China Key Lab for Micro and Nano-Scale Boron Nitride Materials of Hebei Province, China School of Nano’ Advanced Materials Science and Engineering, Gyeongsang National University of South Korea, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 November 2014 Revised 26 February 2015 Accepted 28 February 2015 Available online 3 March 2015 Keywords: Porous material Metallic composite Compressive property Metal matrix syntactic foam
a b s t r a c t Closed-cell AZ31 magnesium alloy foams with different percentages of hollow ceramic microspheres (CMs) are synthesized using modified melt foaming method. The distribution of CMs is investigated and also the effect of CMs on the foaming behaviors (specifically for porosity and pore size) and quasi-static compressive behaviors of Mg-based composite foams are characterized. The results show that CMs distribute in cell walls homogeneously and most of them are penetrated by magnesium alloy melt. In addition, the mean pore size declines with the increase of CMs percentage. Moreover, the overall porosity of the foams increases first and then decreases with the increase of CMs content, and the variation tendency is more obvious when the foaming temperature is lower (namely 680 °C). Besides, proper percentage of CMs changes the compression fracture mode of the foams from brittleness to ductility. OM/SEM/EDS/XRD detections and finite element analysis are applied to explain the reasons. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Metal matrix syntactic foam (MMSF) is a kind of special composite that consists of a metal matrix and a set of hollow, spherical particles [1]. It has been confirmed that MMSF possesses excellent mechanical properties compared with traditional metal foams, e.g. high specific strength and stiffness, good energy absorption capacity, etc. [1–4]. For these reasons, MMSF has been widely used in automotive, aerospace, military vehicles and other industrial fields [5]. Up to now, MMSF is mainly prepared by melt infiltration technique, resulting in confined product dimensions and much lower porosity than foams prepared by melt foaming method. It is because CMs are mainly used as pore generation agent or thickening agent when synthesizing MMSF [2]. Generally speaking, it is believed that the metal matrix of MMSF can be made of aluminum, steel, titanium or magnesium alloys. However, to our best knowledge, most of the researches have focused on aluminum or aluminum alloy matrix. Besides, most of the researches about magnesium metal foams are about traditional foams (without spherical particles) and it has been improved that magnesium
⇑ Corresponding author at: School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China. E-mail address: [email protected] (X. Xia). http://dx.doi.org/10.1016/j.matdes.2015.02.029 0261-3069/Ó 2015 Elsevier Ltd. All rights reserved.
metal foams have the potential to serve as structural material for regular light-weight applications. Wang et al. investigated the processing of magnesium foams fabricated by an infiltration technology. In addition, the pore structures and mechanical properties of space holder particles as well as the resultant foam were also characterized. The results showed that the foams exhibited usual stress–strain behaviors and nearly isotropic properties. Meanwhile, the yield strength of the foams increased with the decrease of sample porosity and the relative mechanical properties of foams were mostly dependent on their relative densities [6]. Osorio-Hernández et al. prepared open-cell Mg foams by replication process and the mechanical properties of the foams were investigated. The results showed that increasing the pore size, the relative density decreased, while the porosity increased, registering a minimum relative density of 0.22. Specimens with smaller pore size and lower percent porosity showed higher mechanical properties [7]. Luo et al. investigated the effect of technological parameters on preparation of Mg-based foams by melt foaming method using SiC and MgCO3 as thickening agent and foaming agent, respectively. The results showed that technological parameters may affect the preparation of the foams, resulting in the changes of the products in apparent density, porosity and structural uniformity. In addition, light weight Mg-based foams with homogeneous pores could be obtained by suitable combination
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of the technological parameters [8]. Zhang et al. produced a novel porous Mg scaffold with three-dimensional (3D) interconnected pores by the fiber deposition hot pressing technology and the effects of porosities on the microstructure and mechanical properties of the porous Mg were investigated. The results showed that the measured Young’s modulus and compressive strength of the Mg scaffold were ranged in 0.10–0.37 GPa, and 11.1–30.3 MPa, respectively [9]. Chen et al. investigated the influences of strain rate, cell size, relative density and the content of SiC on energy absorption characteristics of closed-cell Mg alloy foam by dynamic compression experiments. The results showed that cell size had significant effect on energy absorption characteristics. Additionally, the strain rate effect was more sensitive to the foams with larger cell size and the influence of relative density on energy absorption characteristics was not significant [10]. In previous research, the present authors investigated the corrosion behavior and mechanical properties of closed-cell Mg alloy foams [11,12]. It can be seen that magnesium matrix has seldom been involved in for the MMSF and further research is needed. In this paper, a new modified melt foaming method is developed to prepare magnesium matrix composite foams with different percentages of CMs. Meanwhile, the effect of CMs on the foaming behaviors and the quasi-static compressive properties are investigated. A simple 3D porous model is established and numerical simulation is applied to investigate the effect of pore size uniformity on the deformation mode of closed-cell AZ31 magnesium syntactic foams.
machining. Secondly, CMs are divided into different volumes averagely. Thirdly, magnesium sheets and CMs are stacked layer by layer in a mild steel crucible. In order to ensure the CMs uniform distributions on each magnesium sheet, a homemade wooden shovel is used. Meanwhile, a tube clamp is applied to guarantee the integrity of the CMs when put another magnesium sheet on the CMs layer. At last, the layered composite materials with the mild steel crucible are heated together to a fixed temperature. Commercial AZ31 magnesium alloy is used as matrix, 1.5 wt.% commercial available calcium granules (with diameters of 1–2.5 mm) and 2.0 wt.% CaCO3 powder (analytically pure) are selected as thickening and foaming agents, respectively. SF6 and CO2 gas mixture is used to protect the melt from being ignited or oxidized. For the details of melt foaming method please refer to [12] and detailed parameters about AZ31 magnesium alloy and the CMs are shown in Tables 1 and 2, respectively. For comparison, two types of foams with foaming temperature of 680 °C and 720 °C are prepared, respectively. It should be noted that for each type of foams the thickening temperature is identical with the foaming temperature. In addition, all of the other parameters remain unchanged (with the stirring speeds of 500 and 1000 rpm and stirring duration of 8 min and 40 s for thickening and foaming stages, respectively) except for the percentage of CMs (0, 2, 4, 8, 10 and 20 Vol.%, hereinafter refer to Vol.%). The overall porosity is measured by Archimedes principle and pore size is obtained by a scanning method [14].
2. Experimental details
2.2. Microstructure observation
2.1. Specimens preparation
Representative metallographic preparation process is applied to prepare specimens for metallographic characterization. Namely, specimens are ground through successive grades of silicon carbide abrasive papers up to 2000 grit and polished using 0.25 lm diamond polishing paste, then ultrasonic cleaned using alcohol and dried by cold flowing air. Microstructure and pore morphology are obtained by a scanning electron microscope (SEM, Hitachi S4800) equipped with energy dispersive X-ray spectrometer (EDS). Phase composition is identified by X-ray diffraction (XRD, SmartLab, Rigaku) with Cu Ka radiation.
Through previous research, it is confirmed that hollow ceramic microspheres (CMs) are hard to be introduced into magnesium alloy melt homogenously by traditional adding method, like adding thickening or foaming agents to magnesium alloy melt. It is mainly because of the reaction of SiO2, magnesium melt (Eq. (1) [13]) and O2 (which is brought in along with the addition of CMs despite the presence of protective gas and the reasons will be discussed later), resulting in the burning, coking and reunion of CMs. At last, CMs adhere to melt surface and hard to be separated through mechanical stirring.
4Mg þ SiO2 ! Mg2 Si þ 2MgO
ð1Þ
In this paper the adding method of CMs and magnesium matrix is modified (as shown in Fig. 1a). Firstly, AZ31 magnesium ingot is cut into sheets with a thickness of 5–10 mm by electro-discharging
(a)
2.3. Compression test Specimens for compression test are cut into 25 25 25 mm (length width thickness) by electro-discharging machining to avoid size effect. Uni-axial compression tests (according to GB/T 7314-2005 materials compression test standard) are performed
(b)
1cm
1cm
Fig. 1. Schematic diagram of adding CMs and magnesium alloy matrix (a); cross section morphology (b) of CMs-containing foam under foaming temperature of 680 °C and CMs percentage of 20%.
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Table 1 Composition of AZ31 (wt.%). AL
Zn
Mn
Si
Fe
Cu
Ni
Mg
2.7852
0.7925
0.5635
0.0032
0.0002
0.0003
0.0004
Bal
by using SUNS Electron Universal Material Testing Machine, with a maximum load of 300 kN. All tests are performed under displacement control, with a displacement rate of 1.5 mm/min (with initial strain rate of 0.001/s) at room temperature. Vaseline is used to minimize the friction between specimen and plates. Load and displacement are recorded using a data acquisition unit and a personal computer. All engineering stresses and engineering strains used in this paper are deduced from the recorded load–displacement data. For each parameter two specimens are compressed and the average data are used. Extrapolation method is used to determine the densification strain [15].
Pore Fig. 2. Existence forms of CMs in cell walls.
3. Results 3.1. Specimen structure Fig. 1b shows the cross-section morphology of the composite foams with 20% CMs under the foaming temperature of 680 °C. It can be seen that the pore structure is homogeneous and the pores are spherical and separated. Meanwhile, no burning, coking or reunion of CMs is observed during the whole preparation process. SEM observation (as shown in Fig. 2) is applied on the cell walls to confirm the existence forms of CMs. It is clear that CMs distribute in the cell walls uniformly and most of them maintain their original morphology [16], free of being smashed during the stirring process. In addition, more than 95% (numbers) of CMs are penetrated by magnesium melt. The above mentioned results mean that the modified melt foaming method can produce magnesium matrix composite foams successfully. 3.2. Foaming behaviors of foams with CMs In order to understand the effect of CMs on the foaming behavior of composite foams, the variation trends of pore size and porosity distributions are studied. Figs. 3 and 4 show the variation tendencies (here, ‘680-0’ in the block diagram means the foaming temperature is 680 °C and the CMs content is 0% and so on) of pore size under the foaming temperature of 680 and 720 °C, respectively. With foaming temperature of 680 °C (as shown in Fig. 3), it is clear that all of the pores are distributed between 0.5 and 4 mm while mainly between 1 and 3 mm for the foams without CMs. However, for the CMs-containing foams, the pores mainly distribute between 0.5 and 2 mm and the proportion of these pores (0.5–2 mm) goes higher along with the increase of CMs percentage. Meanwhile, pores with the diameter of 3–4 mm are almost disappeared. Similar trend happens to the foams under the foaming temperature of 720 °C (as shown in Fig. 4), though the pore size distribution range (1–6 mm) and the main distribution range (1–4 mm) are slightly different from the foams under the foaming temperature of 680 °C. The mentioned results above mean that the pore sizes of the composite foams tend to be smaller and more homogeneous with the addition of CMs.
Fig. 3. Pore sizes distribution of composite foams under foaming temperature of 680 °C with CMs percentages of 0%, 2%, 4%, 8%, 10% and 20%.
Fig. 4. Pore sizes distribution of composite foams under foaming temperature of 720 °C with CMs percentages of 0%, 2%, 4%, 8%, 10% and 20%.
Table 2 Parameters of CMs. Stacking density (g/cm3)
Size range (lm)
Wall thickness (lm)
0.42
45–150
7.5 ± 0.8
Fig. 5a and b shows the effect of CMs on the overall porosity of the composite foams with foaming temperatures of 680 and 720 °C, respectively. The red solid line shows the mean value of
X. Xia et al. / Materials and Design 74 (2015) 36–43
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Fig. 5. Effect of CMs on the porosity of foams with foaming temperatures of 680 °C (a) and 720 °C (b), it should be noted that the lines are only used to describe the trend.
the porosities obtained from the individual measurements. It is clear that in both cases the mean porosity increases first and then decreases with the increase of CMs percentage. On the whole, the porosity is higher and the variation tendency is more moderate for foams with foaming temperature of 720 °C. In addition, the porosity attains high level with CMs percentage of 2–8% under foaming temperature of 680 °C. While it requires the CMs percentage to be between 8% and 15% under foaming temperature of 720 °C. 3.3. Engineering stress–engineering strain curves
strain and some fluctuation or work hardening occurs on some curves. At last, a densification stage where the stress increases sharply with the strain increasing slightly. It should be noted that in the plateau deformation stage serrations appear on some of the curves, meaning brittle fracture behaviors. While, for the others the curves are smooth, meaning ductile fracture behaviors. It is clear that that foams without CMs show typical brittle fracture behaviors. Meanwhile, the fluctuation range for the foams with CMs of 2% is much smaller compared with foams without CMs. Continue to increase the content of CMs, namely 4% and 8%, the curves in the plateau deformation stage become very smooth (meaning ductile fracture behavior). While, if more CMs (10% and 20%) present in the composite foams, the serrations appear again, implying the brittle fracture behaviors. As shown in Fig. 7, similar results can be observed when the thickening and foaming temperature is 720 °C. All of the above mentioned results mean that with proper addition of CMs the deformation behavior of the closed-cell Mg-base foams changed from brittle fracture to ductile fracture behaviors and if excessive CMs are contained the deformation mode returns to brittle fracture again and the reasons will be discussed later.
Up to now, metal foams are mainly used in energy absorption fields and in these fields most of the components experience compressive deformation process. Thus, in the present paper, quasi-static compression test is applied to investigate the deformation process of the CMs-containing foams. Fig. 6 shows the engineering stress–engineering strain curves of the foams with different percentages of CMs and different porosities (mainly between 53% and 67%), for each percentage of foams two specimens are compressed and showed. Here, ‘8-0-59%’ in the block diagram means the specimen with the foaming temperature is 680 °C and the CMs content is 0% and porosity of 59% and so on. It is clear that in all cases the curves show typical three deformation stages as most of metal foams: First, a linear stage where stress increases almost linearly with the increase of strain until the first peak stress (defined as yield strength). Then, a plateau deformation stage where stress maintains within a certain level with the increase of
Melt foaming method with simple prepare process and high economical efficiency is the most popular way to produce closed-cell metal foams with large dimensions and uniform
Fig. 6. Quasi-static compressive engineering stress–engineering strain curves of the foams with different percentages of CMs under the foaming temperature of 680 °C.
Fig. 7. Quasi-static compressive engineering stress–engineering strain curves of the foams with different percentages of CMs under the foaming temperature of 720 °C.
4. Discussion
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structure, specially for traditional aluminum and magnesium foams. In previous research CMs was introduced into the closed-cell aluminum foam with melt foaming method to produce aluminum composite foams [16,17]. CMs were added with aluminum foil coated and it was confirmed that by this way CMs-containing aluminum foam can be successfully obtained with the CMs homogeneously distributed in the foams. As for CMs-containing magnesium composite foams, during the initial stage of the experiments similar adding method is used, unlike the aluminum composite foams, serious burning, coking and reunion of CMs is observed. This is mainly due to excess oxygen is involved onto the melt surface and chemical reaction occurs between Mg, SiO2 and O2. This is owing to that when CMs are added with aluminum foil coated, a lot of space will exist among the CMs when only one kind of CMs is used [18], resulting in involving air (O2) to the crucible. Meanwhile, unlike the thickening agent (Ca) and foaming agent (CaCO3), the stacking density of CMs is very low (as shown in Table 2). When the aluminum foil coated CMs are added longer time is need to bring the CMs into the slurry, compared with the foaming agent. Thus, due to the protective effect of aluminum foil O2 will have enough time to go down to the melt surface and chemical reaction will occur, resulting in the burning, coking and reunion of CMs on the melt surface. CMs without aluminum foil protect are also introduce into the melt and more serious burning appeared due to the involvement of gas flow. It should be noted that all the experiments are conducted under the protection of SF6 and CO2 gas mixture. In this paper, the adding method of AZ31 Mg alloy and CMs is modified as described in Section 2.1 and in this way the O2 involved in the CMs can be
Fig. 8. Phase compositions of foams with different percentages of CMs under the foaming temperature of 680 °C.
(a)
drove away before magnesium alloy melting due to the rising temperature and the protection gas mixture. Meanwhile, with the temperature increasing the reaction (Eq. (1)) will occur (which will be confirmed later) in situ, resulting in the permeation of the CMs and high bonding strength between magnesium alloy and CMs. In addition, with the temperature increasing CMs can be heated evenly to avoid being broken due to the unevenly local heating. Besides, the reaction will restrict the floatation of the CMs, resulting in homogenous distribution of CMs in the composite foams. According to the binary phase diagram of Mg–Si system [19], in the present experiment the phase composition of the as-cast foam consists of primary Mg2Si and eutectic Mg2Si + halphai-Mg phases. XRD detections are applied and the results are shown in Fig. 8. The main difference between the CMs-containing specimens and the specimen without CMs is the appearance of Mg2Si phase on the former. Meanwhile, the intensity of the MgO is increasing with the increase of CMs percentage. The above mentioned results mean the reaction between Mg and SiO2 (which is the main composition of CMs) occurred during the preparation process [20]. It is known that the intermetallic compound of Mg2Si exhibits an excellent combination of superior properties, such as high melting temperature (1085 °C), low density (1.99 103 kg/m3), high hardness, low thermal expansion coefficient and reasonably high elastic modulus [21]. All these properties mean that under the present conditions Mg2Si phase can stably exist in the composite foams which is beneficial to the macro structures and mechanical properties of foams. Furthermore, the Mg2Si phase is exceptionally stable and therefore could effectively impede grain boundary sliding at elevated temperatures, which is beneficial to mechanical properties of the composite foams [21]. It has been confirmed in Section 3.2 that with the addition of CMs the pore size becomes smaller and the uniformity of the pores increased as shown in Figs. 3 and 4. This is mainly due to the existence of Mg2Si phase. As fine Mg2Si particles can act as nucleation particles of the bubbles just as calcium particles [22]. Therefore, when there are more nucleation particles in the melt, the nascent bubbles have more choices to attach and more pores will generate, which will improve the homogeneity of the pores. Meanwhile, the total volume of the gas is assumed to be constant as the foaming agent percentage remains unchanged (2 wt.%) for all foams. Thus, the CMs-containing foams possess much smaller pore sizes and the pore size decreases with the increase of CMs percentage. In addition, as the existence of Mg2Si phases, the viscosity of the magnesium melt increases further besides the effect of calcium particles (thickening agent). As it is known, viscosity is significant for metal foams preparation [23]. Thus, when the foaming temperature is lower (680 °C) the original viscosity of the melt is higher and a small quantity of CMs (Mg2Si particles) can make the viscosity appropriate to produce higher porosity foams. When the foaming temperature is higher (720 °C) more CMs (Mg2Si particles) are needed. While, excessive CMs (Mg2Si particles) will make the melt
(b)
Fig. 9. Blocky Mg2Si phase in synthetic foams with CMs percentages of 10% (a) and 20% (b) under foaming temperature of 680 °C.
X. Xia et al. / Materials and Design 74 (2015) 36–43
1mm
D1
D2
Compression direction
(a)
41
(b)
Fig. 10. Schematic model (a) and meshing result (b) of a unit cell of closed-cell foam.
Fig. 11. Simulation results of models with local pore diameters of 2 mm (a), 3 mm (b) and 4 mm (c).
viscosity too high and the bubbles need more driving force to grow up [22]. However, during the preparation process, the parameters remain unchanged as described above and no extra driving force is available, resulting in smaller pore size. Meanwhile, the pores are hard to grow up, leading to the decrease of entire porosity (as shown in Fig. 5). As shown in Figs. 6 and 7, the addition of CMs has important effect on the compressive deformation behavior of the composite foams, namely proper percentages of CMs change the deformation mode form brittleness to ductility. In previous research Mukai et al. investigated the dynamic compressive behaviors of open-cell AZ91 magnesium alloy foam and the results showed that foams under different dynamic strain rate present typical brittle fracture behavior [24]. Yang et al. and Xu et al. studied compressive properties the closed-cell commercial pure Mg foams with different porosities and the closed-cell AZ91 magnesium alloy foams with different pore sizes, respectively. Both of the results showed typical brittle fracture behaviors [25,26]. Meanwhile, in our previous research closed-cell AZ31 magnesium alloy foams (prepared by the identical method described above) both under as-cast and heat treatment
conditions showed apparent brittleness [12,27,28]. In general, the deformation mode depends on the properties of base materials and the pore structures of foams [22]. Thus, in this experiment proper addition of CMs should have changed the properties of basic materials or the pore structure of the composite foams. It can be found from Fig. 2 that the CMs distribute homogeneously in the Mg alloy matrix and are almost completely filled by Mg alloy. A small part of the CMs are fractured at the thinnest regions or concentrating porosities on their walls because of the difference of thermal expansion coefficients between CMs particles and matrix alloy and the reaction between Mg and SiO2 (Eq. (1)). It is accessible that the content of Si element (or Mg2Si phase) increases with the increase of CMs percentage. During SEM observation the distribution of Mg2Si phase in the foams with CMs of 2%, 4% and 8% is homogeneous on the entire cross-section. While, for the other two specimens (especially for the CMs percentage of 20%), the segregation of Si element is observed and the results are shown in Fig. 9. Compared with the other foams some darker and bulky regions (as the black arrows indicated) embed in the matrix especially around CMs, which has been confirmed to be coarse Mg2Si
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[29]. According to the statistical results it is confirmed that CMs has an important effect on the pore size distributions of the composite foams (as shown in Figs. 3 and 4), which should also have a significant effect on the compressive behaviors of the foams. To clearly show the influence, a simple finite element analysis (FEM) model is built to investigate the influence of local larger pore on the compressive deformation behavior of the synthetic foams. Fig. 10a shows the plan sketch of the unit model with the local larger pore diameter of D1 = 4 mm. To simplify the calculation process all the other surrounding pores diameters are set as D2 = 2 mm and the cell wall thickness (distance between the larger pore and the surrounding pore) is assumed as 1 mm as shown in Fig. 10a. UG and Deform 3D software are applied to build up the models and simulate the quasi-static compression test at room temperature (20 °C) respectively, using AZ31 Mg alloy as raw material and with compression speed of 1.5 mm/min. Fig. 10b shows the line graph of the unit model after being meshed. In the present paper, the evolution of overall stress distribution on the unit model under the same amount of deformation is used to discuss the effect of local larger pore on the deformation behavior of the foams. Fig 11a–c shows the overall stress distribution under the identical amount of deformation for the local larger pore sizes of D2 = 2, 3 and 4 mm, respectively. It is clear that the effective stress on the foams with larger local pore size is more concentrated and propagated outward. Meanwhile, stress concentration appears on the larger pore (Fig. 11c), meaning under the identical amount of deformation the location with larger pore size will first fracture. Thus, the foams with larger pore size distribution ranges (as shown in Figs. 3 and 4) will possess reduced fracture consistency and obvious fluctuation on the engineering stress–engineering strain curves (as shown in Figs. 6 and 7). Also, it has been confirmed that the magnesium matrix composite with small particle size of Mg2Si dispersoid possesses optimal combination of ultimate tensile strength and elongation due to the pinning effect of the fine Mg2Si [30]. Under the present conditions the distribution of Mg2Si is homogenous as described above and the overall amount of Si element is restricted when CMs content is low, resulting in the small dimensions of the Mg2Si phase and this is beneficial to the improvement of the magnesium alloy composite foams elongation (or ductility). However, when more content of CMs is added, large particle size of Mg2Si phase appears (as shown in Fig. 9) and the ductility of the composite foams will decrease seriously [21,29], resulting in the brittle fracture of the foams. As described above, the deformation mode of the foams is mainly determined by two factors, the morphology of Mg2Si phase and the uniformity of the pore size. When the content of CMs is low the dimension of Mg2Si phase is small and the uniformity of the pore size is increased, which are beneficial to the ductile fracture behavior of the foams. However, when the content of CMs is high the dimension of Mg2Si phase changes to from small pieces to blocky ones and the increase of the pore size uniformity are not so obvious as the initial stage, leading to the brittle fracture behavior of the foams.
5. Conclusions A modified melt foaming method is used to produce CMs-containing magnesium matrix composite foams. The CMs distributes in the cell walls homogeneously and most of them keep the original shapes, free of being smashed by stirring. Most of CMs are permeated due to the reaction between the CMs and magnesium alloy melt. Meanwhile, due to the addition of CMs (Mg2Si particles), the number of nucleation particles of bubbles increases and the viscosity of magnesium melt is improved, resulting in smaller pore size, more homogeneous pore structure and
controllable porosity of the composite foams. Besides, the proper addition of CMs changes the foams deformation mode from brittleness to ductility due to the homogeneous distribution of Mg2Si and the improved uniformity of the pore size. However, when excessive CMs is added the Mg2Si segregates to blocky ones, leading to the brittle fracture behavior of the foams. Acknowledgements The present authors wish to thank the financial support provided by Hebei Province School Cooperation Fund Projects, Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13060), Science and Technology Project of Hebei Province (13211008D). References [1] K. Májlinger, I.N. Orbulov, Characteristic compressive properties of hybrid metal matrix syntactic foams, Mater. Sci. Eng., A 606 (2014) 248–256. [2] I.N. Orbulov, J. Ginsztler, Compressive characteristics of metal matrix syntactic foams, Compos. Part A-Appl. Sci. 43 (2012) 553–561. [3] X.F. Tao, L.P. Zhang, Y.Y. Zhao, Al matrix syntactic foam fabricated with bimodal ceramic microspheres, Mater. Des. 30 (2009) 2732–2736. [4] C.S. Karthikeyan, S. Sankaran, Kishore, Elastic behavior of plain and fibre-reinforced syntactic foams under compression, Mater. Lett. 58 (6) (2004) 995–999. [5] M. Altenaiji, Z.W. Guan, W.J. Cantwell, Y. Zhao, G.K. Schleyer, Characterisation of aluminium matrix syntactic foams under drop weight impact, Mater. Des. 59 (2014) 296–302. [6] X.F. Wang, Z.D. Li, Y.J. Huang, K. Wang, X.F. Wang, F.S. Han, Processing of magnesium foams by weakly corrosive and highly flexible space holder materials, Mater. Des. 64 (2014) 324–329. [7] J.O. Osorio-Hernández, M.A. Suarez, R. Goodall, G.A. Lara-Rodriguez, I. Alfonso, I.A. Figueroa, Manufacturing of open-cell Mg foams by replication process and mechanical properties, Mater. Des. 64 (2014) 136–141. [8] H.J. Luo, L. Zhang, Z.G. Xu, Y.S. Yang, Effect of technological parameters on preparation of Mg-based foam materials, Mater. Sci. Forum 749 (2013) 356– 360. [9] X. Zhang, X.W. Li, J.G. Li, X.D. Sun, Preparation and mechanical property of a novel 3D porous magnesium scaffold for bone tissue engineering, Mater. Sci. Eng., C 42 (2014) 362–367. [10] L. Chen, Q.M. Zhang, C.L. Liu, X.W. Zhang, R.R. Long, Energy absorption characteristics of closed-cell AZ91 magnesium alloy foam, Int. J. Nonlinear Sci. Numer. Simul. 13 (2012) 233–240. [11] X.C. Xia, X.W. Chen, W.M. Zhao, H.T. Xue, B. Liao, B.Y. Hur, et al., Corrosion behaviour of closed-cell AZ31 Mg alloy foam in NaCl aqueous solutions, Corros. Sci. 80 (2014) 247–256. [12] X.C. Xia, W.M. Zhao, Z.H. Wei, Z.G. Wang, Effects of specimen aspect ratio on the compressive properties of Mg alloy foam, Mater. Des. 42 (2012) 32–36. [13] L.X. Shi, P. Shen, D. Zhang, E.T. Dong, Q.C. Jiang, Reactive wetting in liquid magnesium/silica and magnesium/silicon systems, Appl. Surf. Sci. 274 (2013) 124–130. [14] D.H. Yang, B.Y. Hur, D.P. He, S.R. Yang, Effect of decomposition properties of titanium hydride on the foaming process and pore structures of Al alloy melt foam, Mater. Sci. Eng., A 445–446 (2007) 415–426. [15] A. Paul, U. Ramamurty, Strain rate sensitivity of a closed-cell aluminum foam, Mater. Sci. Eng., A 281 (2000) 1–7. [16] X.C. Xia, X.W. Chen, Z. Zhang, W.M. Zhao, Compressive properties of closed-cell aluminum foams with different contents of ceramic microspheres, Mater. Des. 56 (2014) 353–358. [17] D.P. Mondal, Nidhi Jha, Bilal Gull, S. Das, Anshul Badkul, Microarchitecture and compressive deformation behavior of Al-alloy (LM13)–cenosphere hybrid Al foam prepared using CaCO3 as foaming agent, Mater. Sci. Eng., A 560 (2013) 601–610. [18] X.F. Tao, L.P. Zhang, Y.Y. Zhao, Al matrix syntactic foam fabricated with bimodal ceramic microspheres, Mater. Des. 30 (2009) 2732–2736. [19] G.H. Li, H.S. Gill, R.A. Varin, Metall. Trans. A 24 (1993) 2383–2391. [20] Z.Q. Huang, S.R. Yu, Microstructure characterization on the formation of in situ Mg2Si and MgO reinforcements in AZ91D/flyash composites, J. Alloy. Compd. 509 (2011) 311–315. [21] M.E. Moussaa, M.A. Walya, A.M. El-Sheikh, Effect of high-intensity ultrasonic treatment on modification of primary Mg2Si in the hypereutectic Mg–Si alloys, J. Alloy. Compd. 577 (2013) 693–700. [22] R. Goodall, A. Mortensen, Physical Metallurgy, fifth ed., Elsevier, Amsterdam, 2014. [23] L.J. Gibson, M.F. Ashby, Cellular Solids: Structures and Properties, Cambridge University Press, Cambridge, 1997. [24] T. Mukai, H. Kanahashi, Y. Yamada, K. Shimojima, M. Mabuchi, T.G. Nieh, et al., Dynamic compressive behavior of an ultra-lightweight magnesium foam, Scr. Mater. 41 (1999) 365–371.
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[28] X.C. Xia, W.M. Zhao, X.Z. Feng, H. Feng, X. Zhang, Effect of homogenizing heat treatment on the compressive properties of closed-cell Mg alloy foams, Mater. Des. 49 (2013) 19–24. [29] G.Y. Yuan, Z.L. Liu, Q.D. Wang, W.J. Ding, Microstructure refinement of Mg–Al– Zn–Si alloys, Mater. Lett. 56 (2002) 53–58. [30] K. Kondoh, H. Oginuma, R. Tuzuki, T. Aizawa, Magnesium matrix composite with solid-state synthesized Mg2Si dispersoids, Mater. Trans. 44 (2003) 611– 618.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Fabrication and characterization of closed-cell magnesium-based composite foams Xingchuan Xia a,b,⇑, Junlong Feng a, Jian Ding a, Kaihong Song a, Xiaowei Chen a, Weimin Zhao a, Bo Liao a, Boyoung Hur c a b c
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China Key Lab for Micro and Nano-Scale Boron Nitride Materials of Hebei Province, China School of Nano’ Advanced Materials Science and Engineering, Gyeongsang National University of South Korea, Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 November 2014 Revised 26 February 2015 Accepted 28 February 2015 Available online 3 March 2015 Keywords: Porous material Metallic composite Compressive property Metal matrix syntactic foam
a b s t r a c t Closed-cell AZ31 magnesium alloy foams with different percentages of hollow ceramic microspheres (CMs) are synthesized using modified melt foaming method. The distribution of CMs is investigated and also the effect of CMs on the foaming behaviors (specifically for porosity and pore size) and quasi-static compressive behaviors of Mg-based composite foams are characterized. The results show that CMs distribute in cell walls homogeneously and most of them are penetrated by magnesium alloy melt. In addition, the mean pore size declines with the increase of CMs percentage. Moreover, the overall porosity of the foams increases first and then decreases with the increase of CMs content, and the variation tendency is more obvious when the foaming temperature is lower (namely 680 °C). Besides, proper percentage of CMs changes the compression fracture mode of the foams from brittleness to ductility. OM/SEM/EDS/XRD detections and finite element analysis are applied to explain the reasons. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Metal matrix syntactic foam (MMSF) is a kind of special composite that consists of a metal matrix and a set of hollow, spherical particles [1]. It has been confirmed that MMSF possesses excellent mechanical properties compared with traditional metal foams, e.g. high specific strength and stiffness, good energy absorption capacity, etc. [1–4]. For these reasons, MMSF has been widely used in automotive, aerospace, military vehicles and other industrial fields [5]. Up to now, MMSF is mainly prepared by melt infiltration technique, resulting in confined product dimensions and much lower porosity than foams prepared by melt foaming method. It is because CMs are mainly used as pore generation agent or thickening agent when synthesizing MMSF [2]. Generally speaking, it is believed that the metal matrix of MMSF can be made of aluminum, steel, titanium or magnesium alloys. However, to our best knowledge, most of the researches have focused on aluminum or aluminum alloy matrix. Besides, most of the researches about magnesium metal foams are about traditional foams (without spherical particles) and it has been improved that magnesium
⇑ Corresponding author at: School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China. E-mail address: [email protected] (X. Xia). http://dx.doi.org/10.1016/j.matdes.2015.02.029 0261-3069/Ó 2015 Elsevier Ltd. All rights reserved.
metal foams have the potential to serve as structural material for regular light-weight applications. Wang et al. investigated the processing of magnesium foams fabricated by an infiltration technology. In addition, the pore structures and mechanical properties of space holder particles as well as the resultant foam were also characterized. The results showed that the foams exhibited usual stress–strain behaviors and nearly isotropic properties. Meanwhile, the yield strength of the foams increased with the decrease of sample porosity and the relative mechanical properties of foams were mostly dependent on their relative densities [6]. Osorio-Hernández et al. prepared open-cell Mg foams by replication process and the mechanical properties of the foams were investigated. The results showed that increasing the pore size, the relative density decreased, while the porosity increased, registering a minimum relative density of 0.22. Specimens with smaller pore size and lower percent porosity showed higher mechanical properties [7]. Luo et al. investigated the effect of technological parameters on preparation of Mg-based foams by melt foaming method using SiC and MgCO3 as thickening agent and foaming agent, respectively. The results showed that technological parameters may affect the preparation of the foams, resulting in the changes of the products in apparent density, porosity and structural uniformity. In addition, light weight Mg-based foams with homogeneous pores could be obtained by suitable combination
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of the technological parameters [8]. Zhang et al. produced a novel porous Mg scaffold with three-dimensional (3D) interconnected pores by the fiber deposition hot pressing technology and the effects of porosities on the microstructure and mechanical properties of the porous Mg were investigated. The results showed that the measured Young’s modulus and compressive strength of the Mg scaffold were ranged in 0.10–0.37 GPa, and 11.1–30.3 MPa, respectively [9]. Chen et al. investigated the influences of strain rate, cell size, relative density and the content of SiC on energy absorption characteristics of closed-cell Mg alloy foam by dynamic compression experiments. The results showed that cell size had significant effect on energy absorption characteristics. Additionally, the strain rate effect was more sensitive to the foams with larger cell size and the influence of relative density on energy absorption characteristics was not significant [10]. In previous research, the present authors investigated the corrosion behavior and mechanical properties of closed-cell Mg alloy foams [11,12]. It can be seen that magnesium matrix has seldom been involved in for the MMSF and further research is needed. In this paper, a new modified melt foaming method is developed to prepare magnesium matrix composite foams with different percentages of CMs. Meanwhile, the effect of CMs on the foaming behaviors and the quasi-static compressive properties are investigated. A simple 3D porous model is established and numerical simulation is applied to investigate the effect of pore size uniformity on the deformation mode of closed-cell AZ31 magnesium syntactic foams.
machining. Secondly, CMs are divided into different volumes averagely. Thirdly, magnesium sheets and CMs are stacked layer by layer in a mild steel crucible. In order to ensure the CMs uniform distributions on each magnesium sheet, a homemade wooden shovel is used. Meanwhile, a tube clamp is applied to guarantee the integrity of the CMs when put another magnesium sheet on the CMs layer. At last, the layered composite materials with the mild steel crucible are heated together to a fixed temperature. Commercial AZ31 magnesium alloy is used as matrix, 1.5 wt.% commercial available calcium granules (with diameters of 1–2.5 mm) and 2.0 wt.% CaCO3 powder (analytically pure) are selected as thickening and foaming agents, respectively. SF6 and CO2 gas mixture is used to protect the melt from being ignited or oxidized. For the details of melt foaming method please refer to [12] and detailed parameters about AZ31 magnesium alloy and the CMs are shown in Tables 1 and 2, respectively. For comparison, two types of foams with foaming temperature of 680 °C and 720 °C are prepared, respectively. It should be noted that for each type of foams the thickening temperature is identical with the foaming temperature. In addition, all of the other parameters remain unchanged (with the stirring speeds of 500 and 1000 rpm and stirring duration of 8 min and 40 s for thickening and foaming stages, respectively) except for the percentage of CMs (0, 2, 4, 8, 10 and 20 Vol.%, hereinafter refer to Vol.%). The overall porosity is measured by Archimedes principle and pore size is obtained by a scanning method [14].
2. Experimental details
2.2. Microstructure observation
2.1. Specimens preparation
Representative metallographic preparation process is applied to prepare specimens for metallographic characterization. Namely, specimens are ground through successive grades of silicon carbide abrasive papers up to 2000 grit and polished using 0.25 lm diamond polishing paste, then ultrasonic cleaned using alcohol and dried by cold flowing air. Microstructure and pore morphology are obtained by a scanning electron microscope (SEM, Hitachi S4800) equipped with energy dispersive X-ray spectrometer (EDS). Phase composition is identified by X-ray diffraction (XRD, SmartLab, Rigaku) with Cu Ka radiation.
Through previous research, it is confirmed that hollow ceramic microspheres (CMs) are hard to be introduced into magnesium alloy melt homogenously by traditional adding method, like adding thickening or foaming agents to magnesium alloy melt. It is mainly because of the reaction of SiO2, magnesium melt (Eq. (1) [13]) and O2 (which is brought in along with the addition of CMs despite the presence of protective gas and the reasons will be discussed later), resulting in the burning, coking and reunion of CMs. At last, CMs adhere to melt surface and hard to be separated through mechanical stirring.
4Mg þ SiO2 ! Mg2 Si þ 2MgO
ð1Þ
In this paper the adding method of CMs and magnesium matrix is modified (as shown in Fig. 1a). Firstly, AZ31 magnesium ingot is cut into sheets with a thickness of 5–10 mm by electro-discharging
(a)
2.3. Compression test Specimens for compression test are cut into 25 25 25 mm (length width thickness) by electro-discharging machining to avoid size effect. Uni-axial compression tests (according to GB/T 7314-2005 materials compression test standard) are performed
(b)
1cm
1cm
Fig. 1. Schematic diagram of adding CMs and magnesium alloy matrix (a); cross section morphology (b) of CMs-containing foam under foaming temperature of 680 °C and CMs percentage of 20%.
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Table 1 Composition of AZ31 (wt.%). AL
Zn
Mn
Si
Fe
Cu
Ni
Mg
2.7852
0.7925
0.5635
0.0032
0.0002
0.0003
0.0004
Bal
by using SUNS Electron Universal Material Testing Machine, with a maximum load of 300 kN. All tests are performed under displacement control, with a displacement rate of 1.5 mm/min (with initial strain rate of 0.001/s) at room temperature. Vaseline is used to minimize the friction between specimen and plates. Load and displacement are recorded using a data acquisition unit and a personal computer. All engineering stresses and engineering strains used in this paper are deduced from the recorded load–displacement data. For each parameter two specimens are compressed and the average data are used. Extrapolation method is used to determine the densification strain [15].
Pore Fig. 2. Existence forms of CMs in cell walls.
3. Results 3.1. Specimen structure Fig. 1b shows the cross-section morphology of the composite foams with 20% CMs under the foaming temperature of 680 °C. It can be seen that the pore structure is homogeneous and the pores are spherical and separated. Meanwhile, no burning, coking or reunion of CMs is observed during the whole preparation process. SEM observation (as shown in Fig. 2) is applied on the cell walls to confirm the existence forms of CMs. It is clear that CMs distribute in the cell walls uniformly and most of them maintain their original morphology [16], free of being smashed during the stirring process. In addition, more than 95% (numbers) of CMs are penetrated by magnesium melt. The above mentioned results mean that the modified melt foaming method can produce magnesium matrix composite foams successfully. 3.2. Foaming behaviors of foams with CMs In order to understand the effect of CMs on the foaming behavior of composite foams, the variation trends of pore size and porosity distributions are studied. Figs. 3 and 4 show the variation tendencies (here, ‘680-0’ in the block diagram means the foaming temperature is 680 °C and the CMs content is 0% and so on) of pore size under the foaming temperature of 680 and 720 °C, respectively. With foaming temperature of 680 °C (as shown in Fig. 3), it is clear that all of the pores are distributed between 0.5 and 4 mm while mainly between 1 and 3 mm for the foams without CMs. However, for the CMs-containing foams, the pores mainly distribute between 0.5 and 2 mm and the proportion of these pores (0.5–2 mm) goes higher along with the increase of CMs percentage. Meanwhile, pores with the diameter of 3–4 mm are almost disappeared. Similar trend happens to the foams under the foaming temperature of 720 °C (as shown in Fig. 4), though the pore size distribution range (1–6 mm) and the main distribution range (1–4 mm) are slightly different from the foams under the foaming temperature of 680 °C. The mentioned results above mean that the pore sizes of the composite foams tend to be smaller and more homogeneous with the addition of CMs.
Fig. 3. Pore sizes distribution of composite foams under foaming temperature of 680 °C with CMs percentages of 0%, 2%, 4%, 8%, 10% and 20%.
Fig. 4. Pore sizes distribution of composite foams under foaming temperature of 720 °C with CMs percentages of 0%, 2%, 4%, 8%, 10% and 20%.
Table 2 Parameters of CMs. Stacking density (g/cm3)
Size range (lm)
Wall thickness (lm)
0.42
45–150
7.5 ± 0.8
Fig. 5a and b shows the effect of CMs on the overall porosity of the composite foams with foaming temperatures of 680 and 720 °C, respectively. The red solid line shows the mean value of
X. Xia et al. / Materials and Design 74 (2015) 36–43
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Fig. 5. Effect of CMs on the porosity of foams with foaming temperatures of 680 °C (a) and 720 °C (b), it should be noted that the lines are only used to describe the trend.
the porosities obtained from the individual measurements. It is clear that in both cases the mean porosity increases first and then decreases with the increase of CMs percentage. On the whole, the porosity is higher and the variation tendency is more moderate for foams with foaming temperature of 720 °C. In addition, the porosity attains high level with CMs percentage of 2–8% under foaming temperature of 680 °C. While it requires the CMs percentage to be between 8% and 15% under foaming temperature of 720 °C. 3.3. Engineering stress–engineering strain curves
strain and some fluctuation or work hardening occurs on some curves. At last, a densification stage where the stress increases sharply with the strain increasing slightly. It should be noted that in the plateau deformation stage serrations appear on some of the curves, meaning brittle fracture behaviors. While, for the others the curves are smooth, meaning ductile fracture behaviors. It is clear that that foams without CMs show typical brittle fracture behaviors. Meanwhile, the fluctuation range for the foams with CMs of 2% is much smaller compared with foams without CMs. Continue to increase the content of CMs, namely 4% and 8%, the curves in the plateau deformation stage become very smooth (meaning ductile fracture behavior). While, if more CMs (10% and 20%) present in the composite foams, the serrations appear again, implying the brittle fracture behaviors. As shown in Fig. 7, similar results can be observed when the thickening and foaming temperature is 720 °C. All of the above mentioned results mean that with proper addition of CMs the deformation behavior of the closed-cell Mg-base foams changed from brittle fracture to ductile fracture behaviors and if excessive CMs are contained the deformation mode returns to brittle fracture again and the reasons will be discussed later.
Up to now, metal foams are mainly used in energy absorption fields and in these fields most of the components experience compressive deformation process. Thus, in the present paper, quasi-static compression test is applied to investigate the deformation process of the CMs-containing foams. Fig. 6 shows the engineering stress–engineering strain curves of the foams with different percentages of CMs and different porosities (mainly between 53% and 67%), for each percentage of foams two specimens are compressed and showed. Here, ‘8-0-59%’ in the block diagram means the specimen with the foaming temperature is 680 °C and the CMs content is 0% and porosity of 59% and so on. It is clear that in all cases the curves show typical three deformation stages as most of metal foams: First, a linear stage where stress increases almost linearly with the increase of strain until the first peak stress (defined as yield strength). Then, a plateau deformation stage where stress maintains within a certain level with the increase of
Melt foaming method with simple prepare process and high economical efficiency is the most popular way to produce closed-cell metal foams with large dimensions and uniform
Fig. 6. Quasi-static compressive engineering stress–engineering strain curves of the foams with different percentages of CMs under the foaming temperature of 680 °C.
Fig. 7. Quasi-static compressive engineering stress–engineering strain curves of the foams with different percentages of CMs under the foaming temperature of 720 °C.
4. Discussion
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structure, specially for traditional aluminum and magnesium foams. In previous research CMs was introduced into the closed-cell aluminum foam with melt foaming method to produce aluminum composite foams [16,17]. CMs were added with aluminum foil coated and it was confirmed that by this way CMs-containing aluminum foam can be successfully obtained with the CMs homogeneously distributed in the foams. As for CMs-containing magnesium composite foams, during the initial stage of the experiments similar adding method is used, unlike the aluminum composite foams, serious burning, coking and reunion of CMs is observed. This is mainly due to excess oxygen is involved onto the melt surface and chemical reaction occurs between Mg, SiO2 and O2. This is owing to that when CMs are added with aluminum foil coated, a lot of space will exist among the CMs when only one kind of CMs is used [18], resulting in involving air (O2) to the crucible. Meanwhile, unlike the thickening agent (Ca) and foaming agent (CaCO3), the stacking density of CMs is very low (as shown in Table 2). When the aluminum foil coated CMs are added longer time is need to bring the CMs into the slurry, compared with the foaming agent. Thus, due to the protective effect of aluminum foil O2 will have enough time to go down to the melt surface and chemical reaction will occur, resulting in the burning, coking and reunion of CMs on the melt surface. CMs without aluminum foil protect are also introduce into the melt and more serious burning appeared due to the involvement of gas flow. It should be noted that all the experiments are conducted under the protection of SF6 and CO2 gas mixture. In this paper, the adding method of AZ31 Mg alloy and CMs is modified as described in Section 2.1 and in this way the O2 involved in the CMs can be
Fig. 8. Phase compositions of foams with different percentages of CMs under the foaming temperature of 680 °C.
(a)
drove away before magnesium alloy melting due to the rising temperature and the protection gas mixture. Meanwhile, with the temperature increasing the reaction (Eq. (1)) will occur (which will be confirmed later) in situ, resulting in the permeation of the CMs and high bonding strength between magnesium alloy and CMs. In addition, with the temperature increasing CMs can be heated evenly to avoid being broken due to the unevenly local heating. Besides, the reaction will restrict the floatation of the CMs, resulting in homogenous distribution of CMs in the composite foams. According to the binary phase diagram of Mg–Si system [19], in the present experiment the phase composition of the as-cast foam consists of primary Mg2Si and eutectic Mg2Si + halphai-Mg phases. XRD detections are applied and the results are shown in Fig. 8. The main difference between the CMs-containing specimens and the specimen without CMs is the appearance of Mg2Si phase on the former. Meanwhile, the intensity of the MgO is increasing with the increase of CMs percentage. The above mentioned results mean the reaction between Mg and SiO2 (which is the main composition of CMs) occurred during the preparation process [20]. It is known that the intermetallic compound of Mg2Si exhibits an excellent combination of superior properties, such as high melting temperature (1085 °C), low density (1.99 103 kg/m3), high hardness, low thermal expansion coefficient and reasonably high elastic modulus [21]. All these properties mean that under the present conditions Mg2Si phase can stably exist in the composite foams which is beneficial to the macro structures and mechanical properties of foams. Furthermore, the Mg2Si phase is exceptionally stable and therefore could effectively impede grain boundary sliding at elevated temperatures, which is beneficial to mechanical properties of the composite foams [21]. It has been confirmed in Section 3.2 that with the addition of CMs the pore size becomes smaller and the uniformity of the pores increased as shown in Figs. 3 and 4. This is mainly due to the existence of Mg2Si phase. As fine Mg2Si particles can act as nucleation particles of the bubbles just as calcium particles [22]. Therefore, when there are more nucleation particles in the melt, the nascent bubbles have more choices to attach and more pores will generate, which will improve the homogeneity of the pores. Meanwhile, the total volume of the gas is assumed to be constant as the foaming agent percentage remains unchanged (2 wt.%) for all foams. Thus, the CMs-containing foams possess much smaller pore sizes and the pore size decreases with the increase of CMs percentage. In addition, as the existence of Mg2Si phases, the viscosity of the magnesium melt increases further besides the effect of calcium particles (thickening agent). As it is known, viscosity is significant for metal foams preparation [23]. Thus, when the foaming temperature is lower (680 °C) the original viscosity of the melt is higher and a small quantity of CMs (Mg2Si particles) can make the viscosity appropriate to produce higher porosity foams. When the foaming temperature is higher (720 °C) more CMs (Mg2Si particles) are needed. While, excessive CMs (Mg2Si particles) will make the melt
(b)
Fig. 9. Blocky Mg2Si phase in synthetic foams with CMs percentages of 10% (a) and 20% (b) under foaming temperature of 680 °C.
X. Xia et al. / Materials and Design 74 (2015) 36–43
1mm
D1
D2
Compression direction
(a)
41
(b)
Fig. 10. Schematic model (a) and meshing result (b) of a unit cell of closed-cell foam.
Fig. 11. Simulation results of models with local pore diameters of 2 mm (a), 3 mm (b) and 4 mm (c).
viscosity too high and the bubbles need more driving force to grow up [22]. However, during the preparation process, the parameters remain unchanged as described above and no extra driving force is available, resulting in smaller pore size. Meanwhile, the pores are hard to grow up, leading to the decrease of entire porosity (as shown in Fig. 5). As shown in Figs. 6 and 7, the addition of CMs has important effect on the compressive deformation behavior of the composite foams, namely proper percentages of CMs change the deformation mode form brittleness to ductility. In previous research Mukai et al. investigated the dynamic compressive behaviors of open-cell AZ91 magnesium alloy foam and the results showed that foams under different dynamic strain rate present typical brittle fracture behavior [24]. Yang et al. and Xu et al. studied compressive properties the closed-cell commercial pure Mg foams with different porosities and the closed-cell AZ91 magnesium alloy foams with different pore sizes, respectively. Both of the results showed typical brittle fracture behaviors [25,26]. Meanwhile, in our previous research closed-cell AZ31 magnesium alloy foams (prepared by the identical method described above) both under as-cast and heat treatment
conditions showed apparent brittleness [12,27,28]. In general, the deformation mode depends on the properties of base materials and the pore structures of foams [22]. Thus, in this experiment proper addition of CMs should have changed the properties of basic materials or the pore structure of the composite foams. It can be found from Fig. 2 that the CMs distribute homogeneously in the Mg alloy matrix and are almost completely filled by Mg alloy. A small part of the CMs are fractured at the thinnest regions or concentrating porosities on their walls because of the difference of thermal expansion coefficients between CMs particles and matrix alloy and the reaction between Mg and SiO2 (Eq. (1)). It is accessible that the content of Si element (or Mg2Si phase) increases with the increase of CMs percentage. During SEM observation the distribution of Mg2Si phase in the foams with CMs of 2%, 4% and 8% is homogeneous on the entire cross-section. While, for the other two specimens (especially for the CMs percentage of 20%), the segregation of Si element is observed and the results are shown in Fig. 9. Compared with the other foams some darker and bulky regions (as the black arrows indicated) embed in the matrix especially around CMs, which has been confirmed to be coarse Mg2Si
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[29]. According to the statistical results it is confirmed that CMs has an important effect on the pore size distributions of the composite foams (as shown in Figs. 3 and 4), which should also have a significant effect on the compressive behaviors of the foams. To clearly show the influence, a simple finite element analysis (FEM) model is built to investigate the influence of local larger pore on the compressive deformation behavior of the synthetic foams. Fig. 10a shows the plan sketch of the unit model with the local larger pore diameter of D1 = 4 mm. To simplify the calculation process all the other surrounding pores diameters are set as D2 = 2 mm and the cell wall thickness (distance between the larger pore and the surrounding pore) is assumed as 1 mm as shown in Fig. 10a. UG and Deform 3D software are applied to build up the models and simulate the quasi-static compression test at room temperature (20 °C) respectively, using AZ31 Mg alloy as raw material and with compression speed of 1.5 mm/min. Fig. 10b shows the line graph of the unit model after being meshed. In the present paper, the evolution of overall stress distribution on the unit model under the same amount of deformation is used to discuss the effect of local larger pore on the deformation behavior of the foams. Fig 11a–c shows the overall stress distribution under the identical amount of deformation for the local larger pore sizes of D2 = 2, 3 and 4 mm, respectively. It is clear that the effective stress on the foams with larger local pore size is more concentrated and propagated outward. Meanwhile, stress concentration appears on the larger pore (Fig. 11c), meaning under the identical amount of deformation the location with larger pore size will first fracture. Thus, the foams with larger pore size distribution ranges (as shown in Figs. 3 and 4) will possess reduced fracture consistency and obvious fluctuation on the engineering stress–engineering strain curves (as shown in Figs. 6 and 7). Also, it has been confirmed that the magnesium matrix composite with small particle size of Mg2Si dispersoid possesses optimal combination of ultimate tensile strength and elongation due to the pinning effect of the fine Mg2Si [30]. Under the present conditions the distribution of Mg2Si is homogenous as described above and the overall amount of Si element is restricted when CMs content is low, resulting in the small dimensions of the Mg2Si phase and this is beneficial to the improvement of the magnesium alloy composite foams elongation (or ductility). However, when more content of CMs is added, large particle size of Mg2Si phase appears (as shown in Fig. 9) and the ductility of the composite foams will decrease seriously [21,29], resulting in the brittle fracture of the foams. As described above, the deformation mode of the foams is mainly determined by two factors, the morphology of Mg2Si phase and the uniformity of the pore size. When the content of CMs is low the dimension of Mg2Si phase is small and the uniformity of the pore size is increased, which are beneficial to the ductile fracture behavior of the foams. However, when the content of CMs is high the dimension of Mg2Si phase changes to from small pieces to blocky ones and the increase of the pore size uniformity are not so obvious as the initial stage, leading to the brittle fracture behavior of the foams.
5. Conclusions A modified melt foaming method is used to produce CMs-containing magnesium matrix composite foams. The CMs distributes in the cell walls homogeneously and most of them keep the original shapes, free of being smashed by stirring. Most of CMs are permeated due to the reaction between the CMs and magnesium alloy melt. Meanwhile, due to the addition of CMs (Mg2Si particles), the number of nucleation particles of bubbles increases and the viscosity of magnesium melt is improved, resulting in smaller pore size, more homogeneous pore structure and
controllable porosity of the composite foams. Besides, the proper addition of CMs changes the foams deformation mode from brittleness to ductility due to the homogeneous distribution of Mg2Si and the improved uniformity of the pore size. However, when excessive CMs is added the Mg2Si segregates to blocky ones, leading to the brittle fracture behavior of the foams. Acknowledgements The present authors wish to thank the financial support provided by Hebei Province School Cooperation Fund Projects, Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13060), Science and Technology Project of Hebei Province (13211008D). References [1] K. Májlinger, I.N. Orbulov, Characteristic compressive properties of hybrid metal matrix syntactic foams, Mater. Sci. Eng., A 606 (2014) 248–256. [2] I.N. Orbulov, J. Ginsztler, Compressive characteristics of metal matrix syntactic foams, Compos. Part A-Appl. Sci. 43 (2012) 553–561. [3] X.F. Tao, L.P. Zhang, Y.Y. Zhao, Al matrix syntactic foam fabricated with bimodal ceramic microspheres, Mater. Des. 30 (2009) 2732–2736. [4] C.S. Karthikeyan, S. Sankaran, Kishore, Elastic behavior of plain and fibre-reinforced syntactic foams under compression, Mater. Lett. 58 (6) (2004) 995–999. [5] M. Altenaiji, Z.W. Guan, W.J. Cantwell, Y. Zhao, G.K. Schleyer, Characterisation of aluminium matrix syntactic foams under drop weight impact, Mater. Des. 59 (2014) 296–302. [6] X.F. Wang, Z.D. Li, Y.J. Huang, K. Wang, X.F. Wang, F.S. Han, Processing of magnesium foams by weakly corrosive and highly flexible space holder materials, Mater. Des. 64 (2014) 324–329. [7] J.O. Osorio-Hernández, M.A. Suarez, R. Goodall, G.A. Lara-Rodriguez, I. Alfonso, I.A. Figueroa, Manufacturing of open-cell Mg foams by replication process and mechanical properties, Mater. Des. 64 (2014) 136–141. [8] H.J. Luo, L. Zhang, Z.G. Xu, Y.S. Yang, Effect of technological parameters on preparation of Mg-based foam materials, Mater. Sci. Forum 749 (2013) 356– 360. [9] X. Zhang, X.W. Li, J.G. Li, X.D. Sun, Preparation and mechanical property of a novel 3D porous magnesium scaffold for bone tissue engineering, Mater. Sci. Eng., C 42 (2014) 362–367. [10] L. Chen, Q.M. Zhang, C.L. Liu, X.W. Zhang, R.R. Long, Energy absorption characteristics of closed-cell AZ91 magnesium alloy foam, Int. J. Nonlinear Sci. Numer. Simul. 13 (2012) 233–240. [11] X.C. Xia, X.W. Chen, W.M. Zhao, H.T. Xue, B. Liao, B.Y. Hur, et al., Corrosion behaviour of closed-cell AZ31 Mg alloy foam in NaCl aqueous solutions, Corros. Sci. 80 (2014) 247–256. [12] X.C. Xia, W.M. Zhao, Z.H. Wei, Z.G. Wang, Effects of specimen aspect ratio on the compressive properties of Mg alloy foam, Mater. Des. 42 (2012) 32–36. [13] L.X. Shi, P. Shen, D. Zhang, E.T. Dong, Q.C. Jiang, Reactive wetting in liquid magnesium/silica and magnesium/silicon systems, Appl. Surf. Sci. 274 (2013) 124–130. [14] D.H. Yang, B.Y. Hur, D.P. He, S.R. Yang, Effect of decomposition properties of titanium hydride on the foaming process and pore structures of Al alloy melt foam, Mater. Sci. Eng., A 445–446 (2007) 415–426. [15] A. Paul, U. Ramamurty, Strain rate sensitivity of a closed-cell aluminum foam, Mater. Sci. Eng., A 281 (2000) 1–7. [16] X.C. Xia, X.W. Chen, Z. Zhang, W.M. Zhao, Compressive properties of closed-cell aluminum foams with different contents of ceramic microspheres, Mater. Des. 56 (2014) 353–358. [17] D.P. Mondal, Nidhi Jha, Bilal Gull, S. Das, Anshul Badkul, Microarchitecture and compressive deformation behavior of Al-alloy (LM13)–cenosphere hybrid Al foam prepared using CaCO3 as foaming agent, Mater. Sci. Eng., A 560 (2013) 601–610. [18] X.F. Tao, L.P. Zhang, Y.Y. Zhao, Al matrix syntactic foam fabricated with bimodal ceramic microspheres, Mater. Des. 30 (2009) 2732–2736. [19] G.H. Li, H.S. Gill, R.A. Varin, Metall. Trans. A 24 (1993) 2383–2391. [20] Z.Q. Huang, S.R. Yu, Microstructure characterization on the formation of in situ Mg2Si and MgO reinforcements in AZ91D/flyash composites, J. Alloy. Compd. 509 (2011) 311–315. [21] M.E. Moussaa, M.A. Walya, A.M. El-Sheikh, Effect of high-intensity ultrasonic treatment on modification of primary Mg2Si in the hypereutectic Mg–Si alloys, J. Alloy. Compd. 577 (2013) 693–700. [22] R. Goodall, A. Mortensen, Physical Metallurgy, fifth ed., Elsevier, Amsterdam, 2014. [23] L.J. Gibson, M.F. Ashby, Cellular Solids: Structures and Properties, Cambridge University Press, Cambridge, 1997. [24] T. Mukai, H. Kanahashi, Y. Yamada, K. Shimojima, M. Mabuchi, T.G. Nieh, et al., Dynamic compressive behavior of an ultra-lightweight magnesium foam, Scr. Mater. 41 (1999) 365–371.
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