Processing and mechanical behavior of Zn–Al–Cu porous alloys

Materials Science and Engineering A 471 (2007) 28–33

Processing and mechanical behavior of Zn–Al–Cu porous alloys S.R. Casolco, G. Dominguez, D. Sandoval, J.E. Garay ∗ Department of Mechanical Engineering, University of California, Riverside, USA Received 12 December 2006; received in revised form 26 February 2007; accepted 5 March 2007

Abstract Zn–Al–Cu porous alloys were produced by the replication method using NaCl crystals as place holders. The production method utilized does not require vacuum or complicated devices, making it an inexpensive and efficient method for producing such alloys. Materials were produced with varying porosity levels (52–64%) and final pore size (2–7 mm). Compression tests and hardness measurements revealed that the mechanical properties of the materials are highly dependent on the macroscopic pore size and porosity (pore fraction). In addition the materials’ properties were tailored using simple heat treatments producing microstructural changes. Energy absorption analysis show that the energy absorbed increases with an increase in yield stress and the range is approximately from 5.9 to 8.4 MJ m−3 . © 2007 Elsevier B.V. All rights reserved. Keywords: Porous alloys; Zinc–aluminum; Replication method

1. Introduction Light porous metals (metallic foams) are receiving considerable attention due to their structural mechanical properties and also for their functional properties including sound and energy absorption, filtering abilities and heat exchange properties. The application generally depends on their macroscopic structure (pore size and overall porosity) as well as the alloy’s inherent properties (yield strength, Young’s modulus, and hardness). The ability to tailor the macroscopic structure of porous alloys as well as their microstructure is therefore quite desirable and can facilitate their application. Metallic foams based on aluminum have received particular attention [1–4] because of their extreme lightweight and relative manufacturing ease. Several approaches to their elaboration have been introduced including: eutectic solidification, gas entrapment, the inversion method and decomposition of a foaming agent [2]. If metallic foams are to find routine applications, their manufacturing efficiency (lower processing temperatures and times) needs to be increased and their production cost lowered. One of the most cost effective techniques is the salt-replication technique that offers a relatively simple and versatile way to produce open-cell foams [5–11]. The process involves pouring molten alloys into a salt bed that serves as place holders. Subsequently the salt is ∗

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dissolved leaving a porous alloy. Thus far one of the drawbacks of the salt-replication technique is that many alloys have melting temperatures high enough that reaction with the salt NaCl bed (place holders) becomes a problem, causing infiltration difficulties [12–15]. To overcome such difficulties, gas pressure or vacuum has often been used [16]. An alternative is to use an alloy with a melting temperature more conducive to the technique such as Zn–22 wt%Al–1 wt%Cu. In addition to the favorable melting temperature, the microstructure of Zn–Al–Cu eutectoid alloys can be relatively easily changed using heat treatments [17,18]. Previously, Zn–Al alloys have received considerable attention because of their superplastic properties [19,20]. Kitazono and Takiguchi recently made Zn–22 wt%Al foams using powder metallurgy in conjunction with foaming agents [10]. In this study a Zn–Al–Cu alloy with a relatively low melting point is used in conjunction with the infiltration-replication technique to produce porous alloys without the need for vacuum, gas pressure or other complications. 2. Experimental procedure The Zn–22 wt%Al–1 wt%Cu alloy was prepared by stoichiometrically mixing pieces of pure metals (ingots 99% commercial purity, RotoMetals Inc., San Francisco, CA) and melting them at 650 ◦ C in a furnace. The molten alloy was subsequently poured into steel containers (75 mm diameter, 70 mm height) contain-

S.R. Casolco et al. / Materials Science and Engineering A 471 (2007) 28–33

ing NaCl crystals. Three different NaCl crystal sizes (average 6.5 mm, 3.5 mm and 1.5 mm) were used in order to produce three different types of porous alloys (Types A, B and C, respectively). After solidification the salt was removed using hot water. The resulting porous metal alloys had an as-produced size of 65 mm diameter × 60 mm height. Compression test samples (20 mm × 20 mm × 25 mm) were cut from each of the as-produced materials (Types A, B and C) making sure that the faces were leveled and squared. In addition the Type C materials (compression test samples) were subjected to heat treatments; they were held at 350 ◦ C for 1 h and cooled in one of the following ways: (1) quenched in water (2) removed from furnace and allowed to cool outside the furnace (3) allowed to cool in furnace. These procedures represent three distinct cooling rates from high to low. The samples were compression tested using a universal testing machine a constant crosshead speed of 1 mm/min (Instron Universal Tester model 3369) at room temperature. The nominal stress was found by dividing the measured force by the initial cross-sectional area

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and nominal strain is the change in height divided by the initial height. The yield stress reported is defined as the stress of deviation from the linear elastic portion of the stress–strain curves. One surface of each of the samples was polished with SiC metallographic paper and alumina powder (down to 1 ␮m) in order to obtain micrographs and perform hardness indentations. The micrographs were obtained with a Phllips XL30-FEI scanning electron microscope using backscattered electron imaging at different magnifications. The phases in these microstructures were identified using EDS. Hardness was obtained using Vickers indentations (Instron 1200) using a 0.300 kgF for 11 s. The hardness values presented were averages of at least 10 measurements. The macroscopic pore size was determined from optical micrographs using the image analysis software ImageJ [21]. The sample density was determined geometrically by simply calculating their volume from their known external dimensions and measuring their mass. The calculations of pore volume fraction

Fig. 1. Images of the porous alloys produced with varying initial salt grain size: (a) Type A (b) Type B and (c) Type C samples (as produced and heat treated).

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S.R. Casolco et al. / Materials Science and Engineering A 471 (2007) 28–33

were then obtained as: ρ Vf = 1 − ρZn–22 wt%Al–1%Cu

(1)

where Vf is the volume fraction of the macroscopic pores, ρ the measured density of the foams Zn–Al–Cu alloy, ρZn–Al–Cu is the density of the bulk Zn–22 wt%Al–1 wt%Cu alloy. The energy absorbed, W was calculated using:  εm W= σ(ε) dε (2) 0

where εm is defined as 20% given strain and σ is the compressive stress as a function of strain ε. 20% strain was used as suggested by Lehmhus and Barnhardt [22]. Fig. 2. Percent porosity vs. pore size of the three types of samples produced. Both the pore size and overall porosity depend on the initial size of salt crystal used.

3. Results and discussion Comparison of the samples cut from the larger as-produced product confirmed that all of the Zn–Al–Cu (Types A, B and C) porous alloys produced had a relative uniform size distribution of pores and wall thickness, leading to relatively uniform porosity throughout. The range of porosity was ±2%. Fig. 1a and b shows samples made using different initial NaCl sizes (Type A, B and C). Fig. 1a represents the materials made with the coarsest and medium salt grain sizes, while Fig. 1b shows the heat treatment group derived from materials made with the finest salt grain size. A comparison of the images in Fig. 1a and b shows that the salt grain size has a considerable effect on large scale structural features: wall cell thickness, pore size and overall porosity fraction. These effects can be quantitatively appreciated in Fig. 2 showing the values obtained for measurements of the porosity and pore size of the three different sample types. The porosity ranged from ∼52 to 64%, while the pore size range was larger, from ∼2 to 7 mm. Type A has the highest pore size with average pore sizes of 6.6 mm, the pore size of Type B was 3.9 mm, and finally the Type C, samples had a pore size of 2.3 mm. In the range of salt grain size used, the pore size is proportional to the initial salt grain size i.e. larger initial salt crystal produce large final pore size. The porosity results are more complicated, with the smallest grain size producing the most porous samples, and the medium grain salt producing the least porous. The mechanical properties of the porous alloys along with the physical properties are summarized in Table 1. Fig. 3 presents the nominal stress versus nominal strain results of the all three sample types produced. In addition, it shows the effects of heat

Fig. 3. Compression stress vs. strain of the samples three types of porous alloys produced along with the heat treated Type C samples.

treatment of the Type C samples on the compression behavior. In general the samples follow the expected behavior of porous alloys. An elastic region followed by a plateau region that occurs after yielding. The length and shape of the plateau region and yield stress vary significantly among the samples. Comparison of Type A, B and the untreated Type C shows the effect of the macroscopic structure of the material. In these samples, the yield stress varies from 40.4 to 48.6 MPa. The shape of the plateau is also different—Type A and B have a flat and relatively long plateau, while the Type C alloys have a shorter plateau followed by a rather steep stress increase. The heat treated group all have a

Table 1 Summary of the properties of the porous alloys Sample

Density (g cm−3 )

Yield stress (MPa)

Young’s modulus (GPa)

Porosity (%)

Absorbed energy, W (MJ m−3 )

Hardness (HV)

Pore size (mm)

Type-B Type-A Type-C (no treatment) Type-C (water quenched) Type-C (air-cooled) Type-C (furnace cooled)

2.5 2.2 1.72 1.72 1.72 1.72

48.6 41.9 40.4 31.3 11.2 2.8

22.3 18.7 10.9 20.4 6.1 1.9

52.83 58.49 67.50 67.50 67.50 67.50

834.5 593 750 540.2 218 54.9

118 117 115 149 131 92

3.9 6.6 2.3 2.3 2.3 2.3

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Fig. 4. Dependence of yield stress on porosity results of the porous alloys.

long plateau where the stress gradually increases. Interestingly, the yield stress is dependent on porosity, which can be appreciated in Fig. 4 which shows the dependence of yield stress on porosity of the three types of porous alloys along with the heat treated samples. In this range, porosity and yield stress are inversely proportional for the as-produced samples. In addition the yield stress can be tailored at a given porosity. The heat treatments have a dramatic effect on the yield stress; it varies

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from 2.8 MPa for the furnace cooled samples to 31.3 for the aircooled samples. The stress–strain results indicate that the yield stress depends on cooling rate, with the rapidly quenched samples (water-quenched) having the highest yield stress and the intermediate cooling range (air-cooled) an intermediate yield stress and the slowest cooled samples (furnace cooled) have a correspondingly low yield stress. The modulus of the porous alloys (see Table 1) follows the same trend and is proportional to cooling rate. The differences in stress–strain behavior of the heat treated samples can be rationalized by their microstructural differences as seen in Fig. 5a–d, SEM micrographs of the Type C materials, as-produced (a), air-cooled (b), water quenched (c) and furnace cooled (d). The as-produced microstructure shown in Fig. 5a represents a completely dendritic structure. The dark zones correspond to the aluminum-rich (␣) phase, the bright zones represent the zinc rich (␩) phase. Although not explicitly shown here, the microstructures of the other as produced samples (Type A and B) exhibit a very similar dendritic structure, typical of an alloy solidified from a melt. EDS spot checks on different parts of these alloys revealed only expected Cu, Al, Zn and no Na or Cl indicating that the alloys did not significantly react with the place holders. The Zn–Al system is characterized by a very high solubility of Zn in Al-rich solid solution, ␣ and a much lower solubility of

Fig. 5. SEM micrographs of the (a) as-cast alloys and those that were cooled at different rates, (b) water quenched (c) air-cooled and (d) furnace-cooled.

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Al in Zn-rich solid solution (␩). The addition of Cu is known to increase the hardness of the materials through the intermetallic phase (CuZn3 ) existing in solid solution. The composition we chose for this study corresponds to a eutectoid region and at 350 ◦ C, the temperature we chose for heat treatment the equilibrium phase diagram [23] shows a complete ␣ solid solution. However the low temperature equilibrium structure should be ␣ + ␩. The SEM image of the water quenched sample (Fig. 5b) confirms that after heat treatment and rapid quenching, the sample is mostly solid solution (medium grey areas). The bright regions correspond to the zinc rich phase, ␩ and the darker grey regions to the ␣ solid solution richer in Al. The same three regions are seen in the air-cooled sample, Fig. 5c. Similarly to the sample water quenched sample, this aircooled sample is dominated by the ␣ solid solution. In this case, the cooling rate is lower and the Al rich regions (dark grey zones) have begun to demonstrate a lamellar structure. In contrast to the high and medium cooling rate samples (Fig. 5b and c), the furnace cooled sample results in a completely different structure (Fig. 5d). In this case, the sample has obtained a fine lamellar structure characterized by regions of fine grained ␩ and ␣ phases. Microstructural changes also affect the hardness of the porous alloys as can be seen in Fig. 6, showing hardness versus yield stress results for three different heat treatments along with the untreated samples. As expected, the two extremes in hardness correspond to the two extremes in cooling rates—the rapidly quenched samples have a high hardness while the slowly cooled furnace-cooled samples have a very low hardness. We can conclude that solid solution strengthening plays a role in the mechanical properties of the water-quenched and air-cooled samples. The sample with a fine lamellar structure has the lowest hardness value. Another possibility for the enhanced hardness is the trapping of high temperature vacancies in the rapidly quenched microstructure. Interestingly, the sample with an almost average hardness (no-treatment, dendritic structure) of the heat treated group exhibits the highest yield stress. In contrast, to the heat treated samples the as produced samples (Type A, B and untreated Type C) exhibit similar values to hardness. These similar hardnesses

Fig. 6. Hardness vs. yield stress data for the samples produced. The heat treated samples display the expected trend of proportional hardness and yield strength. The as-produced alloys have similar hardnesses indicative of similar microstructure.

Fig. 7. Comparison of yield stress vs. porosity results for alloys.

confirm that the microstructure of the as-produced samples are similar regardless of the salt grain size used. The changes in yield stress observed for these samples can be attributed to porosity differences as discussed before. Compressive yield stress for our results (Zn–22 wt%Al– 1 wt%Cu) and previous work on Zn–22 wt%Al [10] and aluminum [11] porous alloys are plotted in Fig. 7. These previous data represent results obtained using a similar replication method but on a different alloy (Al) [10] and a similar alloy but different production method. The Al porous alloys produced by Cao et al. have significantly lower porosity than we obtained. This is because pure Al is significantly more difficult to get to infiltrate the salt bed than the Zn–Al–Cu alloy we used. Despite the lower porosity the yield stress of these alloys was 10 times lower than the highest yield stress we obtained. On the other hand, the Zn–Al alloys by Kitazano and Takiguchi overlap our porosity range and have higher porosity in some cases. Their yield stress however was also significantly lower than we obtained. The increased yield stress we obtain leads to a considerably higher energy absorbed when compared to previous data as evident in Fig. 8, showing the dependence of energy absorption on porosity of the Zn–22 wt%Al and porous aluminum alloys. The

Fig. 8. Energy absorbed vs. porosity of present results along with previous work.

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significant increase in yield stress and energy absorbed is likely due to the shape of the pores produced in our samples. In most techniques used for producing porous alloys, the porosity tends to be spherical because it is caused by some form of gas. Inspection of Fig. 1 reveals that our structures are “truss-like,” and contribute to the relatively high yield stresses we observe. It is likely that the technique presented can be further optimized for a given property such as yield strength resulting in commercial application. 4. Summary Zn–Al–Cu porous alloys were fabricated by the saltreplication method using three different initial salt sizes. The important results of this study can be summarized as follows: 1. The processing method is applicable to the manufacture of Zn–Al–Cu porous alloys with porosity in the 52–64% range and pore size range was larger, from ∼2 to 7 mm. 2. The macroscopic structure (pore size and overall porosity) of the porous alloys can be easily changed by the changing the initial grain size of the salt used as a replication agent. 3. The mechanical properties of the porous alloys are highly dependent on the porosity and pore size. In our porosity range, porosity and yield stress are inversely proportional. 4. The experiments have shown that heat treatment is an easy option to tailor the mechanical properties of the Zn–Al–Cu porous alloys. The hardness, modulus, yield stress are all proportional to cooling rate and can be increased by water quenching or lowered by allowing them to cool inside a furnace. The yield stress can be varied dramatically from ∼3 to ∼40 MPa at a constant macrostructure (pore size and porosity). 5. Both the yield stresses obtained and the energy absorbed compare favorably with Zn–Al porous alloys produced using other methods. The yield stresses we obtain are about 10 times higher than Al alloys produced using a similar method.

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Acknowledgments Support of this work by UC-Mexus (University of California and CONACyT) is gratefully acknowledged. We would also like to sincerely thank M. Mundo-Ocampo for assistance with electron microscopy. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

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