Microwave Assisted Sinteringof Al-Cu-Mg-Si-Sn Alloy

Chandran Padmavathi, Anish UpadhyayaDepartment of Materials Science and Engineering, Indian Institute of Technology, Kanpur, UP, INDIA

Dinesh AgrawalMaterials Research Institute, Pennsylvania State University, University Park, USA

Received: March 11, 2011 Accepted: May 18, 2012

Journal of Microwave Power and Electromagnetic Energy, 46 (3), 2012, pp. 115-127.A Publication of the International Microwave Power Institute

ABSTRACT Microwave sintering has been a well-established technique to consolidate metal powders due to its instantaneous volumetric and rapid heating as compared to conventional heating. Al-3.8Cu-1Mg-0.8Si-0.3Sn (2712) alloy powders were compacted (200 and 400 MPa) and microwave sintered at different temperatures (570 to 630 °C) under different atmospheres (vacuum, N2, Ar and H2). Increasing sintering temperature enhanced sintered density from 91% to 98%. Sintering under vacuum at 590°C was more efficient with a densification parameter of 0.36 followed by N2, Ar and H2. Regardless of the sintering condition, phase analysis via XRD revealed the presence of only α-Al peak attributed to lesser time available for diffusion of alloying elements. In addition, microstructural inhomogeneity leading to more intergranular melt formation was observed for all sintered compacts. Contrasting to densification, sintering in N2 resulted in better corrosion resistance.

KEYWORDS: Microwave Sintering; Atmosphere; 2712 Alloy; Densification; Microstructure; XRD; Corrosion Properties.

INTRODUCTION Powder metallurgy (P/M) Al alloys were used until the introduction of camshaft bearing caps by General Motors in 1993 [Schaffer et al., 1998; Lall and Heath, 2000]. Fabrication of P/M alloys is considered to be challenging, since the starting aluminum powder is prone to spontaneous oxidation with exposure to atmosphere even at room temperature [Irmann, 1954]. The presence of thermodynamically stable oxide layer (thickness- 5 to 15 nm) prevents densification [German, 1994; Schaffer, 2001]. Further reduction of Al2O3 requires a dew point of <-140 °C or PO2 of <10-50 atm [Schaffer et al., 2001] and hence atmosphere plays an important role in sintering of aluminum alloys [Dudas, 1969; Sercombe, 1998]. For over a decade, many researchers [Schaffer et al., 2001; Martin and Castro, 2003; Sundaresan and Ramakrishnan, 1978; Kehl and Fischmeister, 1980; Schaffer et al., 2008; Min et al., 2006; Ziani and Pelletier, 1999] have consolidated P/M aluminum alloys through liquid phase sintering under dry inert atmosphere or low vacuum (10-4 torr). Nitrogen atmosphere was found to be most effective by a few researchers [Martin and Castro, 2003; Dudas and Dean, 1969; Schaffer et al., 2006]. A few others [Jha et al., 1988] showed that Ar and vacuum were more effective as compared to N2 for 6061 Al matrix composites. In addition, Youseffi

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et al., [2006] also supported that vacuum sintering resulted in higher mechanical properties under artificially aged conditions as compared to N2. Ever since [Roy et al., 1999, 2001] reported microwave sintering of metallic parts, it had been developed in to a well-established technique. They found that metal in powder form absorbs the microwaves at room temperature and gets heated very effectively and rapidly [Agrawal et al., 1999, 2000, 2004 and 2006]. Several researchers [Anklekar et al., 2005; Sethi et al., 2003; Saitou, 2006] utilized the advantages of microwave heating for powdered metals leading to better sintered density and mechanical properties as compared to other conventional techniques. Moreover, microwave sintering led to finer microstructures, reduced process time and energy; economically friendly, higher heating rates [Roy et al., 1999]. Also, various researchers [Panda et al., 2006; Agrawal et al., 2004; Prabhu et al., 2009; Agrawal, 2006; Cheng, 1991; Clark, 1996; Jain, 2006; Upadhyaya, 2007] successfully sintered pure metallic powders like Cu, Al, Ni, Mo, Co, Ti, W, WC, Sn, etc. through this microwave technique. On microwave sintering of aluminum alloys and its composites, little works [Leparoux et al., 2003; Gupta and Wong, 2005; Nawathe et al., 2009; Vaucher et al., 2008] were reported. To the best of our knowledge, there has been no literature reports available on microwave sintering of Al-Cu-Mg-Si (2712) alloys. The current work evaluates the effect of sintering conditions: temperature and atmosphere in terms of densification, microstructural changes, phase analysis and properties of 2712 alloy consolidated in a microwave furnace.

EXPERIMENTAL PROCEDURE The 2712 (Al-3.8Cu-1Mg-0.8Si-0.3Sn) alloy powder (supplier: AMPAL Inc., USA) having an average particle size of 105 µm and 1.5% acrawax binder was uniaxially compacted at 200 and 400 MPa. The as-

received powder characteristics and green density of compacts are described in detail elsewhere [Padmavathi, 2011]. To estimate the delubrication temperature, TGA analysis of 2712 green and delubricated compacts were carried out by heating them in nitrogen atmosphere up to 700 °C at a uniform heating rate of 10 °C/min. Green compact was delubed at 350 °C for 6 h and sintered at 570, 590, 610 and 630 °C for 1 h under vacuum, N2, Ar and H2. Vacuum sintering was carried out in a 6 kW and 2.45 multimode cavity (supplier: Cober electronics Inc, USA) under vacuum (10-6 torr). Similarly, sintering in other atmospheres was carried out in a 6 kW multimode microwave furnace (Supplier: Amana Radarange, model RC/20SE, USA) with gas flow rate of 80 l/min. The casket assembly for sample placement used in microwave sintering experiments is shown in Figure 1. The assembly consisted of microwave transparent insulation (mullite based made from Fiberfrax) with graphite coated SiC rods. The temperature was monitored using an infrared pyrometer attached to a DAQ computer. Emissivity value of 0.4 was used for all experiments. The power of the microwave furnace was switched off and compacts under went furnace cooling. The sintered compact was evaluated for its density and densification parameter using Archimedes density and

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Figure 1. Casket assembly used for sample placement in microwave sintering experiments.

dimensional measurements. The estimation of densification parameter was described elsewhere in detail [Padmavathi, 2011]. For microstructural analysis, sintered compact was polished using alumina and 0.02 µm colloidal silica suspensions and etched with Keller’s reagent for 30 seconds. The optical microscope (supplier: Zeiss, Germany) and scanning electron microscope in secondary electron mode along with energy dispersive spectroscopy-EDS (model: Quanta 200, supplier: FEI, Netherlands) were used for microstructural observations and quantitative chemical analysis. Phase analysis was carried out using X-Ray Diffractometer (model: RICH. SEIFERT & Co., Germany) with Cu-Kα radiation at scan rate of 3 °C/min. Electrical conductivity was measured using a digital conductivity meter (supplier: Technofour, India) and expressed in % IACS (International Annealed Copper Standard). Bulk hardness measurements as polished 2712 sintered compacts was conducted using Vickers microhardness tester (model: SHP 150, supplier: Barieiss, Germany) at an applied load of 20 g for 10 seconds. Electrochemical behavior was performed using a poteniostat (supplier: Gamry Instruments, USA). Before the potential dynamic polarization test, each sample was allowed to stabilize for about 3600 s to obtain a stable open circuit

potential (OCP). The 3.5 wt.% NaCl solution was used for potentiodynamic polarization tests in a potential range of -1300 to – 400mV at scan rate of 0.2 mVs-1. The standard three electrode configuration and corrosion rate calculation using expression are described in detail elsewhere [Padmavathi, 2007].

RESULTS AND DISCUSSION Prior to sintering experiments TGA analysis was carried out to understand delubrication behavior of as-received powder. Figure 2a represent TGA curves obtained for a green compact of as-received powder and a delubed compact. In case of as-received powder, a significant weight loss event was observed and ended up just before 350°C attributing to the burning of the acrawax lubricant. This was followed by a stable region of nearly no weight loss and a weight gain event starting from 545°C. It was hypothesized that, at this temperature, alloy undergoes phase change via eutectic reaction, and the resultant liquid reacts with N2 atmosphere to form AlN and hence weight gain. This finding was also reported in a previous literature report [Schaffer, 2008]. Figure 2b shows the XRD pattern for as pressed 2712 compact consisting of mainly α-Al and CuAl2 peaks. Also aweak peak of Si is revealed in the as pressed alloy.

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Figure 2. (a, left) TGA curves obtained for a green compact of as-received powder and a delubed compact and (b, right) XRD pattern of as pressed 2712 alloy.

DENSIFICATION BEHAVIOREffect of temperature The Al-Cu-Mg-Si-Sn alloy powder readily interacted with microwave radiation and rapidly heated up. Compared to conventional sintering, a 55% reduction in processing time was observed. Microwave sintering of aluminum alloys powders was found to be difficult due to its very good electrical conductivity and thereby lower skin depth of 1.7µm [Mishra, 2006]. They also reported interrelation between microwave interactions; skin depth and resistivity in detail [Mishra et al, 2006]. Figure 3 compares the sintered density and densification parameter of 2712 alloy pressed at 200 and 400 MPa followed by microwave sintering at different temperatures under vacuum. It was observed that increasing compaction pressure or sintering temperature resulted in enhanced densification. A sudden increase in densification with a changing in sintering

temperature from 570 to 590°C could be attributed to the initiation of the liquid phase. This hypothesis was also supported by the negative densification parameter at 570°C compared to positive densification parameter, i.e. densification, for higher temperatures. In addition, it was also understood that higher compaction pressure always resulted in inferior densification parameter attributing to higher green density of the compact. Highest sintered density of 98% and a positive densification parameter was achieved for by implementing a sintering temperature of 630°C but with little shape distortion. From the density measurement, we propose a sintering mechanism which is similar to conventional sintering as reported in literature [Schaffer et al., 2008]. During initial stages, Mg diffuses across the surface oxide layer to form a volumetrically large spinel structure resulting in shear stresses and consequently a broken oxide layer. Presence of a high vacuum assists in

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Figure 3. Effect of compaction pressure and sintering temperature of 2712 alloy sintered in microwave (a, left) sintered density and (b, right) densification parameter. All compacts were sintered in vacuum.

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Chandran Padmavathi et al., Microwave Assisted Sintering of Al-Cu-Mg-Si-Sn Alloy.

transporting these oxide inclusions to the surface of the compact. This leads to fresh Al-Al contact and also helps in forming the eutectic liquid phase for sintering [Lumley et al, 1999]. Microwave transparent nature of alumina and spinel structure did not aid any additional heating mechanism [Agrawal, 2006]. The only advantage in using microwave heating was rapid heating rate through skin effect via Eddy currents, and suceptor aided heating enhanced diffusion kinetics [Roy et al., 1999]. Figure 4 shows optical microstructures of compacts sintered at different temperatures. It was observed that higher sintering temperatures resulted in grain

coarsening. A color contrast across the grain boundaries is also seen at all temperature but not at 570°C. This was attributed to the chemical inhomogenities across the grain boundary. In the case of prealloyed powders, eutectic liquid forms at inter particle boundaries and the quantity of eutectic liquid increases with sintering temperature. However, higher cooling rates involved in microwave sintering experiments resulted in less time for the liquid to diffuse to form respective intermetallic. These optical microstructures also support our hypothesis reagarding negative densification parameter for the sintering temperature 570°C. Scanning electron micrograph in figure 5 provides

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Figure 4. Optical photomicrographs of 2712 alloy sintered in microwave vacuum furnace at (a) 570 °C, (b) 590 °C, (c) 610 °C and (b) 630 °C.

Effect of atmosphere Figure 7 shows the effect of compaction pressure and sintering atmosphere on sintered density and densification parameter of 2712 compacts sintered at 590 °C. Regardless of sintering atmosphere, compacts pressed at higher pressure, 400 MPa, resulted in higher sintered density. However, lower densification parameter for compacts pressed at 400 MPa indicates higher green density compared to 200 MPa. Higher compaction pressure results in interparticle bond formation which either closes or reduces pores. It was also reported that higher pressure leads to more dislocation density, and in turn, activates sintering [German, 1996]. However, formation of eutectic liquid might result in the swelling of the compact. Final sintered density was the combined effect of swelling due to liquid formation and densification in later stages.

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better understanding of microstructural inhomogenities. A significant amount of liquid was seen at grain boundaries which were left undiffused due to insufficient time. Elemental analysis of grain boundary region was done using EDS and given in table I. Phase change during sintering resulted in segregation of Mg and Cu along grain boundary. Higher amounts of alloying elements, Mg and Cu, at higher sintering temperatures also indicated formation of more liquid. EDS line scan was carried out to measure gradient of alloying elements across grain boundary and the same is given in figure. 6. Depletion of Al and presence of Cu at grain boundary is clearly seen. All the above observations support our hypothesis that higher amounts of liquid formation at higher sintering temperatures as well as insufficient time for diffusion lead to microstructural inhomogenities.

Figure 5. Scanning electronmicrographs of 2712 alloy sintered in microwave furnace at (a) 590°C and (b) 630°C in vacuum.

Table I. Chemical compositional (EDS) analysis of the grain boundary regions of 2712 microwave sintered compacts at 590 and 630°C in vacuum and nitrogen.

AtmosphereSintering

temperature, °C

Element content, wt %

Mg Cu Si

Vacuum590 1.6 5.9 2.2

639 2.4 23.3 -

Nitrogen 590 10.5 12.4 10.8

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Vacuum sintering yielded higher density and densification followed by N2, Argon and hydrogen atmospheres. Also, it was evident from the negative densification parameter that sintering in Ar and H2 resulted in compact swelling as opposed to nitrogen with nearly no net change in size and vacuum with a positive densification parameter. Detailed sintering mechanisms in presence of a vacuum, via Mg interaction with

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surface oxide layer, was given in the above section. Role of Mg on enhancing sinterability was aslo reported in literature [Schaffer, 2001]. In case of compacts sintered in N2, it was hypothesized that a thin layer of AlN forms on the outer surface of powder particles during sintering. This was also evident from the weight gain event in TGA analysis (figure 2a). Several researchers [Martin and Castro, 2003; Schaffer et al., 2008] reported that AlN

Figure 6. EDS line scan of 2712 alloy sintered in microwave furnace at 630°C in vacuum.

Figure 7. Effect of different sintering atmosphere on the (a, left) sintered density and (b, rigth) densification parameter of 2712 alloy pressed at 400 MPa and consolidated in microwave furnace at 590°C.

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formation enhances densification response of conventionally sintered aluminum alloys. Formation of AlN did not aid any better in densification owing to its low loss in dielectric properties [Agrawal, 2006]. However, presence of AlN on the surface restricted the diffusion of alloying species between particles. Supersolidus liquid phase sintering of prealloyed particles resulted in liquid formation, particle rearrangement and sintering aided by modified wetting characteristics due to AlN. Gas entrapment and the difference in solubility limit of Ar and H2 in eutectic liquid resulted in large amount of porosity, poor sintered density and high compact swelling. Detailed sintering mechanism for these atmospheres

has been explained well in the previous literature reports [Schaffer and Hall, 2002; Schaffer et al., 2006]. Figure 8 represents optical micrographs of compacts sintered at 590 °C under different atmospheres. Of all atmospheres, vacuum sintering resulted in the development of a well-defined grain structure with less numerous and a more rounded pore structure. Nitrogen sintered samples revealed porosity along with limited amount of melt pool formation. Whereas, sintering in Ar and H2 resulted in large amount of distributed porosity. Microstructural inhomogeneity was also higher compared to vacuum and N2. Figure 9 represents the XRD pattern of 2712 aluminum alloy compacts sintered in a microwave furnace under various sintering

Figure 8. Effect of different sintering atmosphere (a) nitrogen, (b) argon and (c) hydrogen on the optical photomicrographs of 2712 alloy pressed at 400 MPa and sintered in microwave furnace at 590 ºC.

Figure 9. XRD plots of 2712 aluminum alloy sintered in microwave furnace under different (a, left) temperature and (b, right) atmospheres.

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temperatures and atmospheres. A remarkable feature for microwave sintered compacts at different temperatures under vacuum is the marked difference in the intensity of α-Al peaks. For samples sintered at 570°C and 610°C, it is relatively weak. At 590°C, the first intense peak of α-Al prominently appears at 38.4°, however, other peaks are conspicuously absent. Very interestingly, in the case of microwave sintered compact at 630 °C, only single strong α Al peak (third intense) appears at 65.1°. To confirm these observations, XRD was performed on three compacts and observed the same results. In case of atmospheres, samples sintered under N2, Ar and H2 also showed similar XRD results, however the peak intensity was relatively very lower. Presence of only α-Al for all the sintered compacts also supports our hypothesis of insufficient time for diffusion. Literature has evidence similar reports for other metallic systems wherein microwave sintering resulted in absence of intermetallic compounds [Sethi et al., 2003, Peelamedu et al., 2004; Mondal et al., 2010].

Properties Table II compares the electrical conductivity and hardness of 2712 alloy sintered under different temperatures and atmospheres in a microwave furnace. The electrical conductivity increases with increasing sintering temperature and highest electrical conductivity (40% IACS) was achieved at 630°C. Vacuum sintering

resulted in highest electrical conductivity due to presence of only α Al solid solution and a continuous network of liquid pools which gives higher conductivity followed by nitrogen as compared to others. Ar and H2 resulted in low conductivity (25-33% IACS). However in case of nitrogen, liquid pools are surrounded by porosity which leads to inferior conductivity. In general, concentration gradient of alloying elements at grain boundaries, porosity and interparticle bonds also affect the conductivity. The highest hardness (94 HV) was obtained for a compact sintered at 570°C. A closer examination of standard deviation values associated with hardness indicates the influence of microstructural inhomogeneity and concentration gradient. Furthermore, the hardness of microwave sintered compacts decreases with increasing temperature. This was attributed to the combined effect of grain coarsening and presence of undiffused liquid phase along grain boundaries. However in the case of sintering atmosphere for compacts sintered at 590°C, hardness values were in the same range except for H2. Considering densification response, compacts sintered in vacuum and N2 were characterized for their electrochemical behavior. Figure 10 shows the effect of sintering atmosphere on open corrosion potential (OCP) and poteniodynamic polarization (PD) curves of 2712 compacts sintered at 590°C. None of the sintered compacts show pitting behavior. In regards

Table II. Effect of sintering temperature and atmosphere on electrical conductivity and hardness of 2712 alloy consolidated in microwave furnaces.

Temperature (°C) Atmopshere Electrical conductivity (%IACS)

Hardness, HV0.02

570 Vacuum 35±0.6 94±4

590 Vacuum 36±0.9 85±11

610 Vacuum 38±0.7 42±13

630 Vacuum 40±0.7 48±12

590 Nitrogen 34±0.4 103±10

590 Argon 33±0.2 99±12

590 Hydrogen 26±0.3 65±10

to OCP values, compact sintered in vacuum stabilized at lower potential, -845 mV, compared to compact sintered in N2, -774 mV. By applying Gibbs free energy rule, ΔG = -nFE, it can be said that corrosion of a compact sintered in vacuum results in lowering of free energy compared to its counterpart sintered in N2. This implies relatively noble nature of compact sintered in N2. The influence of difference in OCP was also observed in their behavior under potentiodynamic polarization. Due to the high active nature of a vacuum sintered compact, current density increases significantly in the anodic region before passivated by oxide layer. As expected, relatively noble nature of N2 sintered compact did result in sligthly increase in current density in anodic region. Regardless of their oxidation behavior and OCP, both compacts passivated at nearly same current density (Ip). Corrosion properties obtained from potentiodynamic polarization curve are given in Table III. A higher corrosion potential (Ecorr), lower

corrosion current density (Icorr) and lower corrosion rate for compacts sintered in N2 were observed. Relatively better corrosion resistance of compacts sintered in N2 could be attributed to the AlN formation at grain boundaries.

CONCLUSIONS Microwave sintering of 2712 prealloyed powder compact was successfully done at different temperatures and under different atmospheres. Important findings of this study are given below:• Alloy power compact was coupled with microwave and resulted in rapid heating rates with a 55% decrease in process time.• Increasing sintering temperature resulted in higher sintered density and densification. Highest sintered density of 98% was achieved at a temperature of 630 °C.• Increased temperature also resulted in higher eutectic liquid and larger inhomogeneous regions around grain boundaries.

Figure 10. Effect of sintering atmosphere on the (a, left) open potential (OCP) variation with time and (b, right) potentiodynamic polarization curves for 2712 alloy microwave pressed at 400 MPa and microwave sintered at 590ºC.

Table III. Effect of sintering atmosphere on corrosion parameters of 2712 alloy microwave sintered at 590 ºC.

Sintering TechniquesSintering

atmosphereEcorrmV

icorrµA/cm2

Corrosion ratex 10-3 mmpy

MicrowaveVacuum -906 0.072 0.8

Nitrogen -809 0.015 0.2

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• Among atmospheres used, vacuum was more effective in achieving better densification compared to N2, Ar and H2. • Higher solubility of gases resulted in more porosity for compacts sintered in Ar and hydrogen.• Sintered in N2 resulted in better corrosion resistance properties compared to vacuum sintered compacts. ACKNOWLEDGEMENT The authors would like to thank Mr. Jessu Joys of AMPAL Inc., Palmerton, USA for providing the 2712 aluminum alloy powders for the present study. This study was conducted under the Networked Center for Microwave Processing of Metal-Ceramic Composites funded by the Indo-US Science and Technology Forum (IUSSTF), New Delhi, India.

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