Fabrication of Porous Steels via Space Holder Technique and Their Mechanical Properties

Abstract

Porous metals, which include small pores inside metals, are promising materials due to their material and structural characteristics. Although they generally exhibit low strength because the pores behave as defects, porous metals are expected to achieve high specific strength due to their ultra-lightweight characteristic. This paper deals with a feasibility study on the fabrication of porous steels for developing unique metals with a high specific strength. Porous steels were fabricated via powder metallurgy-based space holder technique. Alloy tool steel, SKD11, and sodium chloride, NaCl, were used as a scaffold metal and spacer material, respectively. Mixed powders of SKD11 and NaCl were sintered via the spark plasma sintering technique. Each sintered compact was re-heated in an argon atmosphere to remove NaCl and densify the scaffold in the compact. Then, each compact was quenched and tempered. As a result, open-cell porous steels with porosities of 60% and 70% were successfully fabricated. The heat treatment refined the microstructure of the scaffold without changing the pore shape, porosity, etc., improving their strength property, irrespective of their porosity. Furthermore, the specific proof strength of heat-treated porous steels was comparable to that of dense pure aluminum.

 

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 71 (2022) 969–975.

1. Introduction

Porous metals consist of small pores and scaffolds of solid metals and are used for various applications as functional materials due to their material and structural properties.¹⁾ Their structural properties can be controlled by changing pore size, pore shape, and porosity. Porous metals may exhibit ultralightness, which cannot be achieved with dense metals, by structural controlling, and thus they are expected to be used as reinforcements for lightweight structures.

Various methods to fabricate porous metals have been proposed to control their microstructures: gas injection method,²⁾ space holder method,³⁾ unidirectional solidification method,⁴⁾ combustion synthesis method,⁵⁾ and gas generation method.⁶⁾ Among these, the powder metallurgy-based space holder method,^7,8) which combines the powder metallurgy and space holder methods, has attracted much attention. The method can fabricate porous metals by simultaneously sintering powders of metals and spacers and then removing the spacers from a sintered compact in some way. It has the advantage of a high degree of freedom in the shape and arrangement of pores inside; open-cell structures can be formed, which are difficult to fabricate by other methods. This method has been commonly applied to light metals with relatively low melting points, such as pure aluminum and magnesium.^9,10) It is known that it is difficult to apply the method to metals with relatively high melting points because the spacer melts and leaches out from a sintering die during sintering.¹¹⁾ However, Fujii et al.¹²⁾ demonstrated that porous titanium could be fabricated using the powder of pure titanium as a raw material, of which the melting point is relatively high (melting point: 1668°C), by using the spark plasma sintering (SPS) method because SPS would enable the sintering of metals at temperatures several hundred degrees lower than conventional methods, such as hot pressing and hot isostatic pressing.^13,14)

The strength of conventional porous metals tends to be very low because they are mainly made of light metals with low strengths and pores inside that behave as defects.¹⁵⁾ However, high specific strength applicable to load-bearing members is expected to be achieved using high-strength metals as scaffold materials. This study aimed to fabricate porous steels with various porosities using alloy tool steel (SKD11) to develop metals with high specific strength. The porous steels were fabricated by the SPS-based space holder method. The fabricated porous steels were then heat-treated to derive high strength by controlling the microstructures of an SKD11 scaffold. Finally, tensile tests were conducted to investigate the effects of the microstructures of the porous steels on their mechanical properties.

2. Experimental Procedure

2.1 Materials and specimens

Powder of alloy tool steel (SKD11), of which the average particle size is 10 µm (Daido Steel Co., Ltd.) prepared by water atomization, was used as a raw material, and powder of sodium chloride (NaCl) (FUJIFILM Wako Pure Chemical Corporation) was used as a spacer material. As-received NaCl powder has been crushed in a mortar and sized using sieves of 300 µm and 425 µm. As a result, NaCl powder with particle sizes ranging from 300 µm to 425 µm was obtained, with an average particle size of 365 µm. The microstructure of each powder is shown in Fig. 1, which was observed via scanning electron microscopy (SEM), and the chemical composition of SKD11 used is shown in Table 1.

Fig. 1

SEM images of (a) SKD11 particles and (b) NaCl particles.

Table 1 Chemical composition of SKD11 (mass%).

Figure 2 shows the temperature changes to fabricate porous steels and control their microstructures. The powders of SKD11 and NaCl were mixed so that the volume fractions of NaCl (V_NaCl) were 50%, 60%, 70%, or 80%. Note that the volume fraction, V_NaCl, is equal to the actual porosity, p_a, if an open-cell structure is formed because pores are formed by removing the spacer. Each mixed powder was filled into a graphite die so that a sintered compact had a cylindrical shape with a diameter of 15 mm and a height of 5 mm. The mixed powder was sintered in a vacuum at a pressure of 30 MPa and a temperature of 700°C for 3 h using an SPS apparatus. Then, the sintered compact was reheated in an argon atmosphere at 1300°C for 1 h to densify the steel phase and remove NaCl, and a porous steel was formed. This process is referred to as the desalting process hereafter. After that, the porous steel was quenched (1030°C, oil cooling) and tempered (180°C, 2 h holding, air cooling) to control the microstructure of the SKD11 scaffold.

Fig. 2

Procedure for fabricating porous steels and controlling their microstructures.

2.2 Microstructural investigation

To discuss the feasibility of fabricating porous steels, the dimensions and weight of each sintered compact were measured after desalination. When more than 95% of the added NaCl is removed, it is considered that a sintered compact is porous steel with an open-cell structure.¹²⁾ On the other hand, if less than 95% of the added NaCl is removed, it is considered that a sintered compact has a closed-cell structure with a non-negligible amount of NaCl remaining inside, and porous steel is not fabricated.

At each step of the procedure shown in Fig. 2, the sintered compacts were cut in half, and their cross sections were ground with emery papers and polished with diamond suspension. The polished surfaces were corroded with a ferric chloride solution, and their microstructures were observed. To investigate their microstructures in detail, diffraction profiles were measured using an X-ray diffraction (XRD) apparatus. Phase identification was performed by comparing the diffraction data in the powder diffraction database ICDD PDF-2 with the diffraction peaks obtained using CuKα characteristic X-ray (wavelength: 0.15405 nm) from a copper tube at the acceleration voltage of 40 kV and the current of 40 mA.

2.3 Mechanical properties of porous steels

The mechanical properties of the porous steels were evaluated by tensile testing, which is a basic technique to evaluate the mechanical properties of metallic materials. The dimensions of tensile specimens are shown in Fig. 3. The specimens were machined from the compacts obtained at each step in the procedure shown in Fig. 2, and their side surfaces were ground using #240 to #2000 emery papers. Grip sections were formed at both ends of each specimen using epoxy resin, and markers were attached for displacement measurement using a non-contact extensometer. Tensile tests were conducted at room temperature in air at a crosshead speed of 1 mm/min. The load P and the displacement δ between grip sections were measured during testing. After testing, nominal stress – nominal strain curves were drawn, and the mechanical properties were evaluated. Note that nominal stress is defined as the load divided by the original cross-sectional area of the specimen, and nominal strain is defined as the displacement divided by the original distance between the markers. Three specimens were tested under the same conditions to investigate the relationship between their mechanical properties and microstructures. Then, the fracture surfaces and side surfaces near the fracture sections of each specimen were observed to investigate the fracture processes of the porous steels.

Fig. 3

Dimensions of a specimen with grips made of resin.

3. Experimental Results and Discussion

3.1 Microstructural investigation of porous steels

The sintered compacts with the volume fractions of NaCl (V_NaCl) from 50% to 70% retained a cylindrical shape after desalting, while the sintered compact with V_NaCl = 80% collapsed during desalting. As the result of weight measurement at each fabrication process, a large amount of NaCl remained even after desalting in the sintered compact with V_NaCl = 50%, of which the microstructure was considered the closed-cell. Figure 4 shows the relationship between the volume fraction of NaCl in the mixed raw powders, V_NaCl, and the actual porosity, p_a, in the sintered compacts with the open-cell structure. The plots and error bars in the figure denote the average, maximum, and minimum values from measured data using two specimens. The actual porosity, p_a, roughly agreed with the volume fraction of NaCl, V_NaCl, and hence the target porosity was successfully achieved by controlling the volume fraction of the spacer, irrespective of heat treatment. Therefore, it was found that open-cell porous steels with porosities of 60% and 70% could be fabricated by this method. Note that the target porosity of porous steel, p_t, was assumed to be the volume fraction of NaCl, V_NaCl, which was somewhat different from the actual porosity, p_a, and the target porosity, p_t, and the actual porosity, p_a, are described separately in this paper.

Fig. 4

Actual porosity as a function of volume fraction of NaCl.

Figure 5 shows the microstructures of the longitudinal sections of the sintered compacts (V_NaCl or p_t = 60%) at each step of the fabrication procedure shown in Fig. 2. Figure 5(a) shows the microstructure of the sintered compact without desalting. The steel and NaCl phases were well dispersed and densely sintered. However, as seen from the enlarged view, the steel phase retained the shape of the raw powder shown in Fig. 1, and some portions were unsintered. The NaCl phase was removed in the desalting process, and the steel phase was fully densified (Fig. 5(b)). The subsequent treatment of quenching showed little change in its microstructure (Fig. 5(c)), indicating that its microstructure retained a porous metal. Figure 6 shows the change in the diameter of the NaCl phase or pore diameter in the sintered compacts with the volume fraction of NaCl, V_NaCl, or the target porosity, p_t, of 60%. Note that these data denote the constant directional diameters measured by image analysis using Image J¹⁶⁾ and were measured for 25 randomly selected NaCl phases or pores in the observed images. The bars and error bars in the figure are the average and standard deviation of the measured data, respectively. Although the dimensions of the NaCl phase after sintering were similar to those of the NaCl particles, the pores became smaller due to the removal of the NaCl phase by reheating. The subsequent heat treatment had little effect on the pore size. This trend was also true for the sintered compact with V_NaCl or p_t = 70%. Figure 7 shows the microstructures of the raw SKD11 powder and the sintered compact with the target porosity, p_t, of 60%, and Fig. 8 shows their XRD profiles. The plots above each diffraction profile are the data in ICDD PDF-2. The diffraction peaks in the XRD profiles do not necessarily match the data in ICDD PDF-2 due to the influence of residual stresses caused by sintering. Therefore, the phases are identified by selecting the peak in ICDD PDF-2 closest to the experimentally detected peak. The SKD11 powder had a rapidly solidified structure formed by water atomization. Fine spherical carbides (Cr₂Fe₁₄C) were considered dispersed in the austenite phase from Fig. 8. After desalting, agglomerated carbides were observed in the austenite phase. From the XRD profile after quenching, the peaks of ferrite and carbide (Cr₁₇Fe₆C₆) were detected, and the peaks of austenite and Cr₂Fe₁₄C that were present before desalting could not be detected. In Fig. 7(c), very fine carbides (Cr₁₇Fe₆C₆) were dispersed in the ferrite phase. The microstructural changes were not significant after tempering. Moreover, these microstructural changes occurred, irrespective of porosity. These results reveal that the combination of SPS and the space holder method can effectively fabricate porous steel with a relatively high melting point and control the steel’s microstructure by heat treatment.

Fig. 5

SEM images of (a) as-sintered compact, (b) desalted compact, and (c) quenched compact (V_NaCl or p_t = 60%).

Fig. 6

Change in NaCl diameter or pore size of the sintered compact (V_NaCl or p_t = 60%) during heat treatment processes.

Fig. 7

Microstructures of (a) a particle of as-received SKD11, (b) desalted compact, and (c) quenched compact (p_t = 60%).

Fig. 8

XRD profiles of SKD11 powder and the sintered compact (p_t = 60%).

3.2 Effect of microstructure on tensile properties

Figure 9 shows the typical stress-strain relationship of the desalted and heat-treated (quenched and tempered) porous steels (p_t = 60%). In both cases, the stress-strain relationship was linear at the beginning of loading, then exhibited large nonlinearity, and finally, the specimen was fractured. The Young’s modulus of each porous steel was calculated from the gradient of the initial stress-strain relationship. The relationship between the tensile properties and the microstructures of porous steels was evaluated.

Fig. 9

Stress-strain curves of porous steel (p_t = 60%).

Figure 10 shows the relationship between the actual porosities, p_a, and Young’s moduli of porous steels. The plots and error bars in the figure are the average and maximum/minimum values from the measured data from the three tested specimens, respectively. In the figure, Young’s modulus calculated by the effective-mean-field (EMF) theory is drawn by a solid line, which is an approach for predicting Young’s modulus of porous materials considering their porosity proposed by Tane et al.¹⁷⁾ Young’s modulus decreased with increasing actual porosity, p_a. The effect of heat treatment on Young’s modulus was unclear because the desalted and heat-treated specimens were fabricated from different manufacturing lots. The actual porosity, p_a, did not match the target porosity, p_t, even if they were the same. However, Young’s modulus predicted by the EMF theory agreed well with that of the porous steels fabricated in this study, irrespective of heat treatment. Therefore, it was concluded that Young’s modulus of the porous steels was strongly affected by their porosity and was little affected by heat treatment.

Fig. 10

Young’s modulus as a function of actual porosity.

Figure 11 shows the relationship between 0.2% proof stress, tensile strength, and actual porosity, p_a. The plots and error bars in the figure are also the average and maximum/minimum values, respectively, as described above. With an increase in the actual porosity, p_a, the 0.2% proof stresses and tensile strengths of the desalted and heat-treated porous steels decreased. The 0.2% proof stress and tensile strength of the heat-treated porous steels were also higher than those of the desalted porous steels. As shown in Figs. 7(c) and 8, it was considered that these properties were enhanced by the dispersion of fine carbides in base metal by heat treatment. That is, it was possible to improve the strength of porous steels due to microstructural control by heat treatment.

Fig. 11

0.2% proof stress and tensile strength as functions of actual porosity p_a.

Figure 12 shows SEM images of the fracture surfaces and the side surfaces near the fracture sections of porous steels (p_t = 60%). While most of the fracture surface was covered by dimples, some flaws, such as sintering defects, were also observed, irrespective of heat treatment. From the side surface observation, it was seen that a crack was initiated from a pore edge and grew to connect the neighboring pores. These results implied that the stress concentration at pore edges caused crack initiation and growth, which connected the pores and led to the fracture of porous steels.

Fig. 12

SEM images of fracture morphology. (a) Fracture surface of desalted compact, (b) fracture surface of quenched/tempered compact, and (c) side surface near the fracture part of desalted compact (p_t = 60%).

4. Discussion on Usefulness of Porous Steels

As demonstrated in Section 3.2, the heat treatment (quenching and tempering) was advantageous for strengthening the porous steels fabricated in this study. Therefore, the specific proof stress (0.2% proof stress divided by density), the specific strength (tensile strength divided by density), and the specific stiffness (Young’s modulus divided by density) of porous steels are compared with those of aluminum and its alloy, which are the representatives of light metals. Table 2 shows the tensile properties of the porous steels, pure aluminum A1060-O, and aluminum alloy A2017-T4.¹⁸⁾ The specific proof stress of the porous steels without heat treatment was almost the same, irrespective of porosity, and was approximately half that of pure aluminum. Heat treatment for the porous steels improved their specific proof stress, which is almost equal to pure aluminum. However, the specific strength of the porous steels was considerably lower than that of pure aluminum, irrespective of heat treatment. Moreover, the specific proof stress and strength of the heat-treated porous steels were approximately one-tenth or less than those of A2017-T4, which is used as various structural members. Since the specific stiffnesses of dense steel and aluminum are comparable, those of porous steels were lower due to the effect of pores inside. From the above, using the porous steels fabricated in this study as a load-bearing member was not expected as-is from the viewpoint of strength. On the other hand, as the 0.2% proof stress was improved by more than 80% by the structural control due to heat treatment, further improvement in strength might be expected by the appropriate microstructural control. This study is the first step in developing metals with an excellent specific strength. It is desirable to elucidate the fracture mechanism of porous steels and to develop some techniques to obtain appropriate microstructures.

Table 2 Tensile properties of the porous steels (SKD11) fabricated in this study, pure aluminum, and aluminum alloy.

5. Conclusions

Porous steels were fabricated to develop metallic materials with an excellent specific strength. The porous steels were heat-treated to control microstructures to improve their strength. The results obtained in this study are shown below.

1. (1)    Porous steels with 60% and 70% porosities were successfully fabricated using spark plasma sintering and the space holder method.
2. (2)    The microstructure of porous steels was refined by heat treatment (quenching and tempering) with little change in the porous structure.
3. (3)    The heat treatment could increase the 0.2% proof stress and tensile strength of the porous steels. Since the porous steels were fractured due to crack initiation near pore edges and growth connecting the pores, further strength improvement might be expected by optimal control of microstructures.

Acknowledgments

This work was supported by JGC-S Scholarship Foundation, Japan.

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