Microstructure and Mechanical Properties of Gas Atomized Aluminum Alloy Powder Compact Densified by Pulsed Current Pressure Sintering Process

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(1)Materials Transactions, Vol. 43, No. 3 (2002) pp. 537 to 543 c 2002 The Japan Institute of Metals. Microstructure and Mechanical Properties of Gas Atomized Aluminum Alloy Powder Compact Densified by Pulsed Current Pressure Sintering Process Takekazu Nagae1 , Masaru Yokota2 , Masateru Nose2 , Shogo Tomida1 , Katsumasa Otera3 , Takashi Kamiya4, ∗ and Shigeoki Saji4 1. Central Research Institute, Toyama Industrial Technology Center, Takaoka 933-0981, Japan Takaoka National College, Takaoka 933-8588, Japan 3 YKK, Kurobe 938-8601, Japan 4 Faculty of Engineering, Toyama University, Toyama 930-8555, Japan 2. A Pulsed current pressure sintering (PCPS) method was employed to sinter compacts of gas atomized Al–8%Ni–2%Mn–1%Cu–0.8%Zr– 0.7%Ti–0.25%Mg alloy powder which consisted of Al–Al3 Ni eutectic and α-Al supersaturated solid solution due to high quenching rates. Results are as follows: (1) The sintered compacts had Al3 Ni precipitates on the boundaries of the initial Al particles. As those precipitates increased, the hardness of the compact decreased. Maximum value of hardness of the compact was HV200 and about 3 at% of Ni atom was excessively dissolved in this compact. (2) The hydrogen content was decreased from 60 to 20 ppm by employing the sintering schedule to remove the adsorbed gas on the particle effectively. The tensile strength of the sintered compact was increased to 500 MPa which was comparable with that of an extruded compact. (Received October 22, 2001; Accepted January 21, 2002) Keywords: pulsed current pressure sintering, aluminum alloy, supersaturated solid solution, Vickers hardness, tensile strength. 1. Introduction Gas atomization of molten alloy produces a supersaturated solid solution due to high quenching rates.1) The sintered compact of the gas atomized aluminum alloy powder has high mechanical strength due to solid solution strengthening when the microstructure of the powder is preserved after the consolidation process.2) The Al alloy powder is a hardly sinterable material because of the oxide layer on its surface.3, 4) This layer should be broken up in order to achieve high density consolidation. A highly complicated process of canning-degas-extrusiondecanning is often used to consolidate the Al alloy powder.2) In this study, we focused on Pulsed Current Pressure Sintering (we call this technique PCPS hereafter) which is commercially called Spark Plasma Sintering (SPS) or Plasma Activated Sintering (PAS). This sintering method is expected to be a new process for sintering various kinds of materials.5) Figure 1 shows the schematic illustration of PCPS apparatus. This system is a kind of solid compression sintering process which employs a low voltage (∼ 12 V) and high density pulsed current (∼ 107 Am−2 ) under uni-axial pressure (∼ 500 MPa). PCPS technique is suggested to be characterized by high density consolidation in a short time.6) We suggested that the PCPS method can densify the pure Al powder compact at lower temperature and shorter period than the conventional hot pressing method.7) The oxide layer could be broken up by the heated zone between particles caused by Joule heat due to electric current. Low sintering temperature and short sintering period are expected to cause microstructure preservation and produce Al alloy compacts with high mechanical strength. J. Qiu et al.8) sintered the atomized Al alloy powder which contains Ni, Cu, Ti, Zr and misch metal ∗ Graduate. Co., LTD.. Student, Toyama University. Present address: Soode Nagano. Fig. 1 Schematic diagram of pulse current pressure sintering apparatus.. by electro-discharge consolidation (EDC: a solid compression sintering method which employs a high voltage (∼ 35 kV) and high density pulsed current (∼ 1016 Am−2 )). The sintered compacts have high mechanical strength due to the oxide layer removal and the microstructure preservation. However, the quantitative analysis was not shown about the relation between preservation of supersaturated solid solution and the mechanical properties. Other research9, 10) on PCPS consolidation of Al alloy powder also gave no information about the quantitative analysis of supersaturated solid solution which affect the mechanical properties of the sintered compacts. Consolidated compact of atomized Al–8%Ni–2%Mn– 1%Cu–0.8%Zr–0.7%Ti–0.25%Mg alloy powder sintered by conventional extrusion process shows high mechanical properties without heat treatments because of its microstructure consist of supersaturated solid solution of α-Al and Al–Al3 Ni.

(2) 538. T. Nagae et al.. eutectic. The purpose of the present work is to clarify the relation between the mechanical properties of consolidated bulk and the amount of Ni which is excessively dissolved in Al matrix after PCPS consolidation process. Furthermore, the relation between the mechanical strength and H2 gas content which prevents joining of particles were also examined. 2. Experimental Procedure The starting material was gas atomized Al alloy powder. Table 1 shows the chemical composition and characteristics of the sample powder. The powder was filled into a cavity of hard metal mold and sintered using PCPS apparatus (produced by Sumitomo Coal Mining Co., LTD., SPS-1050). Figure 2 shows the sintering schedule in this experiment. Sintering temperature was controlled by measuring the temperature of the mold, Ts, 5 mm from the edge of the specimen using alumel-chromel thermocouple. Al alloy powder. Table 1 Chemical composition of Al alloy powder used. Ni. Mn. Cu. Zr. Ti. Mg. Al. 8.2. 2.0. 1.0. 0.8. 0.7. 0.25. Bal.. Melting point. Hardness. Mean particle size. 915 K. HV 206. 37.7 µm. Fig. 2 Schematic diagram of sintering conditions of PCPS process.. was consolidated in the mold temperature range from 580 to 830 K for 300 s heated with a rate of 0.8 K/s. Pressure was loaded by two ways of schedule modes, I and II. The mode I was a method in which the sintering pressure, Ps, was constant for the whole period of PCPS process, and the mode II was a method in which initial pressure (28 MPa) was kept at the temperature range from R.T. to approximately 600 K and raised to Ps during heating to Ts. The density of the sintered specimen was measured using Archimedes principle. Microstructures were examined by a scanning electron microscope and microanalysis was performed using an energy dispersive X-ray spectrometer. X-ray diffraction was performed on the as-received powders and sintered specimens using a diffractometer with Cu target for 2θ from 20–120◦ . Vickers microhardness was measured with 1.98 N load and 15 s holding time. The specimens for tensile test were cut using a wire electric discharge machine out of sintered compacts. Tensile tests were performed using a screw-driven testing machine at room temperature with five specimens for each sintering condition. Hydrogen content was analyzed with ten specimens for each sintering condition by thermal conductivity method. 3. Results and Discussion 3.1 Microstructure Figure 3 shows the changes in X-ray diffraction patterns of the specimen sintered by mode I with the sintering mold temperature. The peaks corresponding to Al3 Ni and η-phase except for Al were mainly observed in the atomized powder and the sintered compacts. The η-phase is an intermediate metastable phase which was observed in quenched Al–2.3–10.1 at%Ni binary alloys by A. Tonejc et al.11) New peaks which are marked by arrows in Fig. 3 were detected in the specimen sintered over 730 K. These peaks were indexed to Al3 Ni(101), (111) and (210) from lower diffraction angle,respectively. These peaks suggest the generation of a new. Fig. 3 Changes in X-ray diffraction patterns with sintering mold temperature. The specimens were sintered by mode I..

(3) Microstructure and Mechanical Properties of Gas Atomized Aluminum Alloy Powder Compact Densified by Pulsed Current Pressure Sintering Process. Fig. 4 SEM micrographs of sintered compact by PCPS process. These specimens were sintered by mode I under load of 249 MPa at (a) 630 K, (b) 680 K, (c) 730 K, (d) 780 K.. Al3 Ni phase which could not be observed at the as-atomized powder. The microstructures of the specimens which were sintered by mode I under load of 294 MPa are shown in Fig. 4. The specimen sintered at 630 K (Photo (a)) has some large pores. Above 680 K, the large pores vanished from the specimen. These microstructures showed almost the same features as the powder microstructure as the grain growth didn’t take place during the sintering process. The new precipitation phase which is indicated by arrows in Photos (c), (d) was observed. These precipitates are mainly located at the interface of the starting powder particles. They grow large at higher sintering temperature. Figure 5 shows the X-ray line profile of Ni Kα and Al Kα on the vicinity of precipitates of the compact. On the precipitation phase, intensity of Ni Kα is higher than those of the matrix phase. Judging from the result of the XRD analysis (Fig. 3), the precipitates observed is suggested to be the Al3 Ni phase. The lattice parameter of matrix phase (α-Al) is plotted against the sintering mold temperature in Fig. 6. The lattice parameter of pure Al and extruded compact also are shown in this figure. The lattice parameter of atomized alloy powder (0.40394 nm) is smaller than that of pure Al (0.40494 nm). Formation of a substitutional solid solution by dissolving Ni atom into Al matrix is considered to be the reason why the lattice parameter decreases, because the atomic diameter of Ni (0.125 nm) is smaller than that of Al (0.143 nm).12) The lattice parameter of sintered compacts didn’t change at temperature range from 630 to 670 K. The value was almost as. Fig. 5 Line profile of Ni Kα and Al Kα on the cross section of sintered compact. Sintered mold temperature, period and pressure were 780 K, 300 s, 249 MPa, respectively.. same as that of the as-atomized powder. It increased from 730 K and exceeded the value of extruded compact at 780 K. Applying the research on quenched Al–Ni binary alloy powder done by A. Tonejc et al.,11) the amount of the Ni content in the Al matrix can be estimated. The amounts of dissolved Ni in Al matrix of the sintered sample at 630, 680, 730 and 780 K were 2.9, 2.9, 2.4 and 1.1 at%, respectively. Since the equilibrium solid solubility of Ni in binary Al alloy system is 0.11 at%,13) the sintered specimens contains a supersaturated solid solution. Over 730 K sintering temperature, the amount. 539.

(4) 540. T. Nagae et al.. of Ni in the Al decreases. The Ni which could not be dissolved in the Al matrix precipitates as observed in Figs. 4 and 5. They are located at a boundary between the initial powder particles because of the high temperature zone between particles which was produced by Joule heat caused by electric current that passes through the cracks of the oxide layer on the Al particles. 3.2 Mechanical properties Figure 7 shows the effect of sintering temperatures on the relative density and Vickers hardness of sintered compacts by mode I. The relative density increased as sintering temperature was raised and it reached about 99% at lower temperature as the sintering pressure increased. In case of the specimen sintered under load of 490 MPa, the temperature to reach the density over 99% was 630 K, and in case of sintering under 98 MPa, it was 830 K which was 200 K higher than that of. Fig. 6 Lattice parameters of f.c.c solid solutions in sintered Al–8%Ni–2%Mn–1%Cu–0.8%Zr–0.7%Ti–0.25%Mg alloy plotted against sintering temperature. The specimens were sintered by mode I. (∗) This value was estimated from Ref. 11).. Fig. 7 Effect of sintering mold temperature, Ts, and pressure, Ps, on relative density and Vickers hardness of sintered specimens by mode I. The Vickers hardness was measured under load of 1.96 N for 15 s.. the former. Vickers hardness measurement was performed on these specimens. In the case of the specimen sintered under load of 98 MPa, the hardness increased as the density increased in the temperature range of 680–780 K. The hardness of the specimen sintered under load of 294 and 490 MPa and increased in the temperature range of 580 to 680 K. The progress of densification causes this increase in hardness. Maximum hardness value, HV190-200, which is higher than that of extruded compact was achieved at 680 K. Over 680 K, the hardness decreased although the specimen was densed over 99% in relative density. This drop in the hardness is considered to be concerned with the decrease of Ni content dissolved in the Al matrix. The hardness decreased from 730 K of sintering temperature, and at the same time the Ni content dissolved in the Al matrix decreased as shown in Fig. 6. The hardness of the specimen sintered at 730 K is HV180, which is the same value as extruded compact. The lattice constant of this specimen is 0.4042 nm, and is almost the same as that of extruded compact, 0.4043 nm. The Vickers hardness of the sintered specimen is plotted against the content of the Ni dissolved in the Al matrix in Fig. 8. The hardness of the specimen sintered at 294 MPa and 490 MPa is proportional to the amount of Ni dissolved in the Al matrix. These specimens were sintered at 680–780 K. This result matched with a statement in the paper of A. Tonejc et al.11) Below 780 K, the supersaturated solid solution made by dissolving of excess Ni element was preserved after PCPS process. Hardness of the specimen sintered at 830 K under load of 98 MPa deviates lower from the linearity. Network of the Al3 Ni precipitates were observed at interfaces of the initial powder particles as a result of the grain growth of the precipitates. The hardness was assumed to be decreased because of the embrittlement of interface of inicial Al alloy particles. The tensile strength of the specimen is plotted against sintering temperatures in Fig. 9. The tensile strength of the extruded specimen is also plotted to compare the mechanical property between these methods. The strength of the extruded one is 580 MPa for the longitudinal direction (LD). Fig. 8 Effect of Ni content dissolved in the Al matrix on Vickers hardness of the specimen which has sintered at 680, 730, 780 and 830 K..

(5) Microstructure and Mechanical Properties of Gas Atomized Aluminum Alloy Powder Compact Densified by Pulsed Current Pressure Sintering Process Table 2 Comparison of hydrogen content in the consolidated compacts by PCPS sintering process. Hydrogen content (ppm) mode I Ps(∗ ) R.T. ∼ Ts(∗) : Ps 294 MPa 490 MPa Extruded compact. 44.0 59.8 18.3. mode II R.T. ∼ 600 K: 30 MPa 600 K ∼ Ts: Ps 26.2 21.8 Extruded temperature: 620 K Extruded pressure: 500 MPa. (∗ )Ps: sintering pressure Ts: sintering mold temperature. Fig. 9 Tensile strength of sintered compact by PCPS sintering process. Pressure was loaded by 2 ways: mode I: Ps was loaded from room temperature. mode II: Ps was loaded above 600 K. (∗) TD: Transverse direction, LD: Longitudinal direction.. and 480 MPa for the transverse direction (TD). In the case of the PCPS specimen, maximum value of tensile strength is 410 MPa, which is achieved at the sintering temperature of 780 K. If we assume that the tensile strength changes like the Vickers hardness, the maximum tensile strength should be more than 480 MPa, which is the TD value of the extruded compact, and it should be achieved at 680 K. However, in the case of the specimen sintered under load of 490 MPa, maximum value of tensile strength, 250 MPa was marked at the temperature of 680 K, and the maximum value of 410 MPa was marked at 780 K sintered under load of 294 MPa. This tendency is not coherent with that of Vickers hardness. This discordance is considered to caused by the weakness of the joint between the initial atomized Al alloy particles. Figure 10 shows the SEM photo of the fractured surface of the specimen sintered by mode I under load of 294 MPa. Since the surface of initial Al alloy powder could be observed in the fractured surface sintered at 630 K (Photo (a)), the specimen was clearly broken at the interface between initial particles. As the sintering temperature increases, the shape of initial Al particles in the fractured surface is hard to figure out. Some tear ridge patterns appear on the surface of the specimen sintered at 730 K as indicated by the arrows in Photo (c). Photo (d) shows the transgranular fractured surface resulted from the joining between initial Al powders. These results show that the joining of Al alloy particles was strengthened with rise in sintering temperature. In order to improve the tensile strength of Al sintered material, it’s significant to strengthen the joining between particles. Al powder particles inherently exhibit a surface oxide layer because of the high reactivity of Al with oxygen. The surface oxide layer is the barrier for the joining of particles. According to Byoung-Suh14) or YoungWon Kim,15) the reactions among hydroxide, oxide and metal on the surface of aluminum particles were explained. At low temperatures (below 420 K), adsorbed water on the aluminum particle is vaporized. At the temperature range from 420 to 620 K, decomposed water vapor is the predominant. species outgassed. At high temperatures (620–770 K) water vapor reacts with Al to form additional Al2 O3 generating H2 . Thus, adsorbed water on the Al particle not only causes the Al2 O3 barrier during sintering, but also inhibits the densification by generating H2 gas resulting in the lower mechanical properties of sintered Al alloy. Then, we devised the sintering process, mode II, which is shown in Fig. 2. In this process, loading pressure is low (under 28 MPa) to eliminate the adsorbed water before the temperature raised to 600 K. Over 600 K, loading pressure was increased to the prescribed value of 294, 490 MPa. The tensile strength of the specimen sintered by process of mode II, is shown in Fig. 9. Tensile strength went over 500 MPa and was above the value of the specimen which was taken from the transverse direction of the extruded one. Table 2 shows the hydrogen content in the sintered specimens. By using mode II process, hydrogen content decreased to half of the value in the sintered specimen by mode I process. This value is equivalent to that of extruded compact which employs the degas process before extrusion. Figure 11 shows the SEM microstructure of sintered compact by mode I and II (Ps = 490 MPa). Micropores under the size of 1 µm are observed at the boundaries of initial powders in the specimen sintered by mode I, but are not observed in the specimen sintered by mode II. The extruded compact was heated at 650 K for 2 h in a vacuum before extrusion in order to avoid adsorbed water. At PCPS method, the depression of the densification due to depression of pressure at the early stage of sintering under 600 K causes the decrease of hydrogen content to an equivalent level to that of extruded compact. Improvement of tensile strength by employing mode II, can be explained by the improvement of joining of particles due to declination of the hydrogen content and declination of micro pores between the initial powder particles. 4. Conclusion The gas atomized Al alloy powder which contains Ni and other elements was consolidated by PCPS process. Results are as follows: (1) The starting Al–8%Ni–2%Mn–1%Cu–0.8%Zr– 0.7%Ti–0.25%Mg alloy powder consisted of a supersaturated solid solution and Al–Al3 Ni eutectic. The compacts consolidated over 730 K have Al3 Ni precipitates on the boundary of initial Al particles. As those precipitates increase with in-. 541.

(6) 542. T. Nagae et al.. Fig. 10 Fractured surface of tensile test sintered by mode I under load of 294 MPa at (a) 630 K, (b) 680 K, (c) 730 K, (d) 780 K. Tensile strength and elongation of each specimen were (a) 197 MPa, 0.9%, (b) 329 MPa, 1.2%, (c) 358 MPa, 1.4%, (d) 410 MPa, 1.8%.. Fig. 11 Comparison of SEM micrographs of the Al alloy compacts sintered under load of 490 MPa at 730 K for 300 s by 2 types of sintering mode. (a) mode I, (b) mode II.. creasing sintering mold temperature, the hardness of the compact decreases. Maximum value of hardness of the compact was HV200 and about 3 at% of Ni atom was excessively dissolved in this compact. (2) Relation between the tensile strength and the hydrogen content which prevents joining of Al particles were examined. The hydrogen content was decreased from 60 to 20 ppm by employing the sintering schedule to remove the adsorbed gas on the particle effectively. The tensile strength of sintered compact was increased to 500 MPa which was comparable with that of extruded compact.. Acknowledgments This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research B-2, 13555203, 2001. REFERENCES 1) 2) 3) 4). H. Jones: Aluminum 54 (1978) 274–281. J. R. Pickens: J. Mater. Sci. 16 (1981) 1437–1457. Z. A. Munir: Powder Metallurgy 24 (1981) 177–180. R. N. Lumley, T. B. Sercombe and G. B.Schaffer: Metall. Mater. Trans. A 30A (1999) 457–463. 5) O. Yanagisawa, T. Hatayama and K. Matsugi: Materia Japan 33 (1994) 1489–1496..

(7) Microstructure and Mechanical Properties of Gas Atomized Aluminum Alloy Powder Compact Densified by Pulsed Current Pressure Sintering Process 6) M. Tokita: Proc. of the International Symposium on Microwave, Plasma and Thermochemical Processing of Advanced Materials, JWRI (1997) 69–76. 7) T. Nagae, S. Tomida, M. Yokota, M. Nose, T. Kamiya and S. Saji: J. Japan Inst. Metals 65 (2001) 733–740. 8) J. Qiu, T. Shibata, C. Rock and K. Okazaki: Mater. Trans., JIM 38 (1997) 226–231. 9) T. Nagae, M. Nose and M. Yokota: J. of the Japan Society of Powder and powder Metallurgy 43 (1996) 1193–1196. 10) T. Nagae, M. Yokota and M. Nose: J. of the Japan Society of Powder and powder Metallurgy 44 (1997) 945–950.. 11) A. Tonejc, D. Rocak and A. Bonefacic: Acta Metall. 19 (1971) 311– 316. 12) Kinzoku data book 3rd edition, ed. by JIM, (Maruzen Co., LTD, 1993) p. 8. 13) Binary Alloy Phase Diagrams 2nd edition, ed. by T. B. Massalski (ASM, 1990) pp. 181–184. 14) B.-K. Suh, Y.-T. Cho and C.-H. Lee: J. of Japan Institute of Light Metals 46 (1996) 334–339. 15) Y.-W. Kim, W. M. Griffith and F. H. Froes: J. of Metals Aug. (1985) 27–33.. 543.

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