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Alloyability of warm formed FeCrAl powder compacts
Materials Today Communications 4 (2015) 42–49
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Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm
Alloyability of warm formed FeCrAl powder compacts M.M. Rahman ∗ , A.A.A. Talib Centre for Advanced Materials (CAM), College of Engineering, Universiti Tenaga Nasional, Putrajaya Campus, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 27 April 2015 Accepted 17 May 2015 Available online 27 May 2015 Keywords: Alloyability Warm powder compaction Sample characterization
a b s t r a c t Alloyability of warm formed Fe-based powder compacts is presented here. Iron (Fe) powder ASC100.29 was mixed with other alloying elements, namely chromium (Cr), and aluminum (Al) powder for 60 min and compacted at 150 ◦ C by applying 130 kN axial loading, simultaneously from upper and lower punches to generate green compacts. The defect-free green compacts were subsequently sintered in argon gas fired furnace at 800 ◦ C for three different holding times, i.e., 30, 60, and 90 min. Sintered samples were then analyzed through XRD and SEM/EDX for their alloyability. The results from XRD and SEM/EDX such as peak, intensity, microstructure, and element spectrum were studied. The flexure stress of the sintered samples was also measured to relate with their alloyability. The results revealed that the alloyability and microstructures of sintered products were affected by sintering time. The sample sintered for 60 min showed the highest intensity of FeCrAl alloy and bending strength. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Alloy is defined as a homogenous mixture of two or more metals or a metal and a non-metal fuse together to form a new metal [1]. Alloy is proven to be harder, ductile, and more corrosion resistant material compared to individual alloying elements [2]. Due to the growing technology development in material science, most pure metals could not comply with increasing process parameters. Iron and its alloy account for about 90% of the world’s production of metals mainly because of their combination of good strength, toughness and ductility at a relatively lower cost [3]. Fe-based alloys have at least 50% iron content, such as cast iron, wrought iron, steel and stainless steel. The difference among them is the composition of other metals or non-metals and their method of fabrication [4]. Fe-based alloys are known for their outstanding capability to withstand high temperature, possess high strength, oxidation and corrosion resistance. Performance in high temperature can be observed from creep resistance analysis. Fe-based alloys have a limit of about 649 ◦ C for high-stress application [5]. The main application of these alloys are in nuclear power system, chemical and petroleum processing applications and also industrial turbines [6]. Iron, chromium, and aluminum alloy is popularly known as FeCrAl or fecralloy, a material that possess high-temperature oxidation resistance property [7]. Research activities about this alloy and
∗ Corresponding author. Tel.: +60 3 89297269; fax: +60 3 89212116. E-mail address: [email protected] (M.M. Rahman). http://dx.doi.org/10.1016/j.mtcomm.2015.05.003 2352-4928/© 2015 Elsevier Ltd. All rights reserved.
its composites have been carried out extensively during the past 30 years and found that it can resist superior temperature as high as 900–1200 ◦ C [8]. FeCrAl alloy is applicable in oxidizing atmosphere because of the presence of alumina scale that forms on the alloy’s surface at a temperature of about 1000 ◦ C [9]. Alloy is generally formed through heat treatment, cold treatment, casting, and mechanical alloying [10]. However, due to increasing demand in industries, there are changes from conventional ingot metallurgy to powder metallurgy as the later promises more advantage especially for high performance and high precision parts [11]. The demand of technologies nowadays focuses on profit, cost reduction, and environment. The process to convert raw materials into final useful products needs knowledge on the metallurgical properties and structures of materials. There are many kinds of forming techniques depend on the properties of metal [12–15], product shape-size-properties, and cost where powder metallurgy is one of the techniques [16]. Powder metallurgy especially ferrous powder has increased significantly over the last few decades especially in the area of manufacturing automotive parts. This advanced technology is growing to meet the demand in producing complex parts by altering and modifying some existing manufacturing processes. The ability of manufacturing parts without extra machining at minimum cost made this as an alternative favorable sustainable process [17]. In industries, FeCrAl alloy is formed through foundry process. The billets of pure Fe, Cr, and Al are melted in industrial furnace. The molten metal is then poured into predesigned cast/mould. The shaped products are removed from the cast/mould once they are
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Fig. 1. XRD pattern of FeCrAl alloy sintered at 800 ◦ C for 30 min.
solidified. The shaped products are required a series of further machining/metal working processes to obtain the final products. This process is time consuming hence expensive. Iron is the second most common metal in Earth’s crust, after aluminum, but because it reacts so readily with oxygen it is never mined in its pure form although meteorites are occasionally discovered that contain samples of pure iron. Pure iron is a silvery-white metal that is easy to work and shape and it is just soft enough to cut through using a knife. Pure iron can be hammered into sheets and draw it into wires. Like most metals, iron conducts electricity and heat very well and it is very easy to magnetize. Due to these several disadvantages of pure iron, it cannot be used for structural purposes as well as manufacturing of heavy duty mechanical components [18]. Therefore, pure iron must be mixed with other elements to form alloy which might have superior physical, mechanical, as well as electrical properties. One of the alloying methods is the powder metallurgy which transforms two or more metallic/non-metallic powders into a homogeneous solid without melting the powder materials [19]. The effect is the superior properties of the products formed by mixing the main powder constituent, i.e., Fe powder with other alloying elements. The introduction of warm compaction technology enabled the manufacturers to press and sinter mechanical components with minimal processing steps. Warm compaction is applicable to all ferrous material system and produces great benefits when coupled with other materials. Warm compaction process has a lot advances compared to casting. The advantages are (i) time saving, (ii) no scrap materials, (iii) no machining is required, (iv) more homogenous density distribution inside the products, and (v) higher relative density is possible to be achieved. All parameters in manufacturing
process, i.e., selection of powder, mixing, compaction, and sintering are important to produce components with favorable properties that eventually contribute to the growth of powder metallurgy in whole [20]. Fe-based alloys have the most industrial usage since raw materials of Fe are abundant and have versatile mechanical properties. However, current practices of Fe-based alloys development are either through casting or mechanical alloying where these processes are expensive and time consuming. Mechanical alloying is time consuming since the process requires high energy ball milling of the powder mass for a long time. After certain interval, the balls are also required to be washed by ethanol to avoid the sticking of powder particles at the surface of the balls which consumes more time. These processes are only capable of producing ingots of alloy or alloy in powder form, not the end products. The whole product manufacturing process becomes more time consuming hence expensive. Therefore, the objective of this paper is to present an experimental investigation of the alloyability of Fe, Cr, Al powder compacts sintered at different holding times. 2. Materials and method The experimental works consist of feedstock preparation, green compact generation, sintering in argon gas fired furnace, and sintered sample characterization. Iron powder ASC 100.29 of 20–180 m particle size range, manufactured by Höganäs AB, was used as main powder constituent. The composition of iron powder and its alloying elements were fixed throughout this research. The composition of carbon was 0.08% (as admixed lubricant), chromium was 14.0%, aluminum was 4.0%, and the balance was Fe powder.
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Fig. 2. XRD pattern of FeCrAl alloy sintered at 800 ◦ C for 60 min.
The main powder constituent and alloying elements were mixed mechanically for 60 min. The powder mass was filled into the cylindrical shape die cavity then all the die assembled together with the powder mass was heated up to 150 ◦ C and maintained for 30 min for uniform heating. Green samples were generated by applying downward and upward forces of 130 kN from upper and lower punches, simultaneously. Once the compaction load achieved the set value, the upper punch was released to its initial position while lower punch was maintained at its final position for a while for elastic recovery or spring-back of the as pressed or green compact. The green compact was subsequently ejected from the die cavity by the upward motion of the lower punch. The dimensions of the as pressed green samples were measured before storing them in properly labeled plastic container and sealed to avoid oxidation prior to sintering. Solid cylindrical shape samples of 25 mm long and 20 mm diameter were generated for the purpose of sintering. The defect-free green samples were fired in custom made sintering furnace (HT3-1400-SIC). A ceramic tube of 50 mm outer diameter, 40 mm inner diameter, and 150 mm hot length was mounted at the furnace. Since controlled atmosphere was considered throughout this study, argon gas was flown into the ceramic tube. The process could be monitored and controlled by using gas regulator where a flow rate of 100 ∼ 1000 cc/min was considered. The green samples were then sintered at 800 ◦ C, at a heating rate of 10 ◦ C/min, for three different holding times, i.e., 30 min (Sample 5), 60 min (Sample 11), and 90 min (Sample 17). The sintered samples were characterized through XRD analysis (D-8 Advance XRD Machine), SEM/EDX analysis (Hitachi S-3400N), and their flex-
ure stress was measured through three-point bending test (ASTM E290-09). 3. Results and discussion XRD analysis was conducted to detect the intensity of elements in the sample. This analysis was mainly to detect the presence of FeCrAl compound inside the sample. The findings were then compared with ‘JCPDS Database”. However, since FeCrAl alloy is not available in the database, the peaks were compared with each element’s database. SEM/EDX was then conducted to know the composition of elements in the sample. In order to observe more clearly, element’s mapping in different color was also done. The green, yellow, red, and black were to give distribution of iron, chromium, aluminum, and carbon, respectively. The XRD patterns of Samples 5, 11 and 17 are presented in Figs. 1–3. It is evident that all the samples generally contain Fe, Cr, and Al elements, thus the patterns were compared with iron, chromium and aluminum database. It can be observed from the results that the peaks of pure elements (database) are gathered in three main peaks produced by the XRD. All the three figures show the same patterns. The only difference is the intensity (counts) of the peaks, scaled from 0 to 2200 at degrees 2Â. The angle 2Â is actually related to the inter-planar spacing ‘d’ while the intensity is related to the strength of the diffractions in the sample [21]. The main peaks are formed at an angle (2Â) of about 44◦ . The intensities were also combined and presented in Fig. 4 to make better comparison of the peak and intensity of each sample. The highest peak is observed at Sample 11 which shows that it has a higher amount
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Fig. 3. XRD pattern of FeCrAl alloy sintered at 800 ◦ C for 90 min.
Fig. 4. Intensities of elements extracted from XRD patterns of samples sintered for different times.
of alloy compared to other samples. However, the counts among each sample do not vary significantly, i.e., the crystalline phase in these three samples are almost the same. The main peaks in Figs. 1–3 are believed to be FeCrAl compound, however some other small peaks are also observed. Since no comparison among these results and other databases of compounds such as Fe–Cr, Al–Cr, Fe–Al or even Al2 O3 are done, the small peaks are compared with other published results reported in [22–26]. In Fig. 1, a peak of 600 counts is observed at an angle (2Â) of 38.5◦ which shows the presence of chromium oxide (Cr2 O3 ). Another
peak of 164 counts at an angle of 65.029◦ is also observed which might either be ␣-Al2 O3 or Fe only. A peak of Al–Fe compound with 219 counts is observed at an angle of 82.317◦ . In Fig. 2, a peak of 700 counts indicating the presence of aluminum oxide, ␣-Al2 O3 is observed at the same angle, i.e., 38.5◦ . Two other peaks at 65◦ and 82.27◦ of 143 counts and 216 counts are also observed showing the presence of either iron element, aluminum oxide (␣-Al2 O3 ) or Al–Fe compound, respectively. In Fig. 3, a quite high peak of 550 counts at an angle of 38.5◦ is observed. This peak is supposed to be aluminum oxide, ␣-Al2 O3 . At 54◦ and 82.314◦ , peaks of 129 counts and 175 counts indicating the presence of Al–Fe and chromium oxide Cr2 O3 ), respectively are observed. A lot of other peaks are also observed here and there but their identities are very difficult to be confirmed. The differences in peaks are believed to be due to the sintering time which is related to the alloyability of the powder compacts. The SEM images of the sintered samples are shown in Figs. 5–7, , . In each figure, the micrograph at the upper shows the distribution of different elements by color, while at the lower is the investigated area zoomed to 300× magnification. The chemical compositions of the investigated areas of the samples are shown and compared in Table 1. It can be observed that Sample 5, sintered for 30 min (Fig. 5) contains more pores, some are round, and the others are interconnected. However, the distributions of other alloying elements such as aluminum (red) and chromium (yellow) are not equal. In the magnified view (Fig. 5b), the dark area shows the position where all the three elements, i.e., Fe, Cr, and Al exist. The surface is darker compared to the surrounding that only consists of iron and chromium, respectively. It is also observed from the micrograph that the alloy elements formed heterogeneous blend
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Table 1 Results of EDX of samples. Description of sample
Chemical composition Atomic (%)
Sample 5 Sample 11 Sample 17
Weight (%)
O
C
Al
Cr
Fe
O
C
Al
Cr
Fe
57.49 53.07 60.18
5.05 5.17 4.01
5.31 8.89 5.18
5.97 9.23 3.62
26.17 23.64 27.01
31.76 28.77 33.81
2.10 2.10 1.69
4.95 8.12 4.91
10.73 16.27 6.61
50.47 44.74 52.98
Fig. 5. SEM micrographs of sample sintered at 800 ◦ C for 30 min (a) surface topography, (b) investigated area (300× magnification).
Fig. 6. SEM micrographs of sample sintered at 800 ◦ C for 60 min (a) surface topography, (b) investigated area (300× magnification).
between aluminum and base element, i.e., iron since the structures are not comparable with any other reported research [26]. It is believed that a sintering time of 30 min is not enough to form FeCrAl alloy completely during sintering at 800 ◦ C in argon gas fired furnace. In the case of Sample 11 (Fig. 6), more equally distributed alloying elements are observed at the investigated surface (Fig. 6b). Aluminum and chromium are visible in almost all the areas. However, some aluminum elements are clustered together which indicates that the mixing of the base powder and other alloying elements during feedstock preparation might not be conducted perfectly. Less interconnected pores are visible at this area, but isolated pores are still observed here and there. The boundaries or contact regions of the particle are still visible, which are the necks among the adjacent grains believed to be formed during sintering. There is no homogenous blend of alloying elements is observed hence a lot of small pores throughout the whole investigated area is visible. Due to the existence of less interconnected pores, it is believed that the sintering is occurred completely at this condition which formed necks and eliminated interconnected pores.
Fig. 7 shows the micrograph of the sample sintered for 90 min. The concentration of aluminum is more at the upper side of investigated area while chromium is observed in almost everywhere, however the distribution of alloying elements is still clustered more at the left side of the investigated area. It is clearly observed that there is a long interconnected pore at the lower right side of the investigated area which might be due to the crack formed during sintering. The long interconnected pore might also be due to the presence of foreign particle during compaction or sintering. However, observed from a larger perspective, the structure is more homogenous than other two samples. The pores observed in the micrographs might be due to the segregation of alloying elements during powder mixing because of their density difference. They might also be formed during sintering since alloying elements such as Al and Cr have lower melting temperature compared to Fe. In a higher magnification (1000×), the boundaries of elements are not visible anymore (Fig. 8). There are about four distinguished patterns, i.e., the left side with a rough coral-like microstructure, middle-left side with a long flanged-like lamellae microstructure, upper right with smooth coral-like microstructure, and at the mid-
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Fig. 8. SEM micrograph of sample sintered at 800 ◦ C for 90 min (1000× magnification).
Fig. 7. SEM micrographs of sample sintered at 800 ◦ C for 90 min (a) surface topography, (b) investigated area (300× magnification).
dle right with an unknown structure of gray colored particle, which is believed also to be found in other areas.
The composition of elements in the samples can be observed by an EDX spectrum of each sample. The EDX analysis was conducted to observe the elements found in the sample or in other words, whether any foreign particle/element are present or not. The results of EDX (element spectrum) are presented in Figs. 9–11, , . It is emphasized that these element spectrums were generated from the entire surface area of the sample. Thus the intensity of the spectrum is the average of all elements’ intensity found in the sample. Figs. 9–11 show that all the four elements, i.e., iron, chromium, aluminum, and carbon are present with a very high amount of oxygen content. The display of double peaks of iron and chromium at 5–10 keV is due to the use of energy of the K-shell. K-shell X-ray energy consists of two lines, i.e., K␣ and K, with a ratio of 10:1, thus displaying two peaks correspond to the stated ratio. On the other hand, the single peak of iron, chromium, aluminum, and carbon at the energy range of 0–5 keV is due to the fact that at lower than 3 keV energy range, the separation among K-, L-, and M- shells is
Fig. 9. EDX element spectrum of sample sintered at 800 ◦ C for 30 min.
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Fig. 10. EDX element spectrum of sample sintered at 800 ◦ C for 60 min.
small hence not distinguishable [27]. As a comparison among these three figures (Figs. 9–11), Sample 11 shows the highest chromium, aluminum, and carbon content, while Sample 17 shows the highest iron and oxygen content. Different peaks of Cr, Al, and Fe are observed since alloyability is different due to the different processing conditions. Product with higher alloy content is found to have higher mechanical properties as depicted in Fig. 12. Fig. 12 shows the flexure stress or bending strength of samples sintered for different times. It is revealed that sample sintered for 60 min (Sample 11) obtained the highest strength. This is in line with the micrograph shown in Fig. 6b where a lot of necks among
the adjacent grains are formed during sintering. A high strength is required to create an inter-granular fracture to break the necks, hence resulted in higher bending strength. The melting point of the main powder constituent, Fe is 1538 ◦ C whereas the melting points of other alloying elements, i.e., Cr is 1907 ◦ C and Al is 660.3 ◦ C. During sintering at 880 ◦ C, for all the three holding times, i.e., 30, 60, and 90 min, portions of Al melted and created bonding agent among the powder particles. However, at a holding time of 30 min, less portions of Al might be melted hence created less bonding agent among particles which could be observed in Fig. 5 with nonhomogeneous microstructure which
Fig. 11. EDX element spectrum of sample sintered at 800 ◦ C for 90 min.
M.M. Rahman, A.A.A. Talib / Materials Today Communications 4 (2015) 42–49
Fig. 12. Bending strength of sintered products.
also resulted samples with lower bending strength (Fig. 12). When the green sample was sintered for 60 min, more Al elements were melted creating more bonding agents among the Fe and Cr. As a result, more homogeneous microstructures are observed (Fig. 6) and the sample sintered for 60 min obtained the highest bending strength (Fig. 12). Adverse result is observed when the sample was sintered for 90 min, less homogeneous microstructures together with lower bending strength were observed (Figs. 7–12). It is clear from the discussion above that suitable sintering time for FeCrAl green compacts is around 60 min which could generate stronger product with better microstructure. 4. Conclusions The results of XRD and SEM/EDX analyses on the phase identification, intensity of alloy peak, and microstructure together with element spectrum show that the alloy is present in the samples. The highest intensity of alloy is observed at the sample sintered for 60 min (Sample 11). However, based on the microstructure evaluation, sample sintered for 90 min (Sample 17) shows a homogenous structure in the investigated area. Highest bending strength is also measured at the sample sintered for 60 min. Acknowledgement The author wants to thank Ministry of Education (MOE), Malaysia for funding this research study under 20140117 FRGS grant. References [1] M.P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 5th edition, John Wiley & Sons, 2012. [2] W.D. Callister, Materials Science and Engineering, 8th edition, John Wiley & Sons, 2012.
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Contents lists available at ScienceDirect
Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm
Alloyability of warm formed FeCrAl powder compacts M.M. Rahman ∗ , A.A.A. Talib Centre for Advanced Materials (CAM), College of Engineering, Universiti Tenaga Nasional, Putrajaya Campus, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 27 April 2015 Accepted 17 May 2015 Available online 27 May 2015 Keywords: Alloyability Warm powder compaction Sample characterization
a b s t r a c t Alloyability of warm formed Fe-based powder compacts is presented here. Iron (Fe) powder ASC100.29 was mixed with other alloying elements, namely chromium (Cr), and aluminum (Al) powder for 60 min and compacted at 150 ◦ C by applying 130 kN axial loading, simultaneously from upper and lower punches to generate green compacts. The defect-free green compacts were subsequently sintered in argon gas fired furnace at 800 ◦ C for three different holding times, i.e., 30, 60, and 90 min. Sintered samples were then analyzed through XRD and SEM/EDX for their alloyability. The results from XRD and SEM/EDX such as peak, intensity, microstructure, and element spectrum were studied. The flexure stress of the sintered samples was also measured to relate with their alloyability. The results revealed that the alloyability and microstructures of sintered products were affected by sintering time. The sample sintered for 60 min showed the highest intensity of FeCrAl alloy and bending strength. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Alloy is defined as a homogenous mixture of two or more metals or a metal and a non-metal fuse together to form a new metal [1]. Alloy is proven to be harder, ductile, and more corrosion resistant material compared to individual alloying elements [2]. Due to the growing technology development in material science, most pure metals could not comply with increasing process parameters. Iron and its alloy account for about 90% of the world’s production of metals mainly because of their combination of good strength, toughness and ductility at a relatively lower cost [3]. Fe-based alloys have at least 50% iron content, such as cast iron, wrought iron, steel and stainless steel. The difference among them is the composition of other metals or non-metals and their method of fabrication [4]. Fe-based alloys are known for their outstanding capability to withstand high temperature, possess high strength, oxidation and corrosion resistance. Performance in high temperature can be observed from creep resistance analysis. Fe-based alloys have a limit of about 649 ◦ C for high-stress application [5]. The main application of these alloys are in nuclear power system, chemical and petroleum processing applications and also industrial turbines [6]. Iron, chromium, and aluminum alloy is popularly known as FeCrAl or fecralloy, a material that possess high-temperature oxidation resistance property [7]. Research activities about this alloy and
∗ Corresponding author. Tel.: +60 3 89297269; fax: +60 3 89212116. E-mail address: [email protected] (M.M. Rahman). http://dx.doi.org/10.1016/j.mtcomm.2015.05.003 2352-4928/© 2015 Elsevier Ltd. All rights reserved.
its composites have been carried out extensively during the past 30 years and found that it can resist superior temperature as high as 900–1200 ◦ C [8]. FeCrAl alloy is applicable in oxidizing atmosphere because of the presence of alumina scale that forms on the alloy’s surface at a temperature of about 1000 ◦ C [9]. Alloy is generally formed through heat treatment, cold treatment, casting, and mechanical alloying [10]. However, due to increasing demand in industries, there are changes from conventional ingot metallurgy to powder metallurgy as the later promises more advantage especially for high performance and high precision parts [11]. The demand of technologies nowadays focuses on profit, cost reduction, and environment. The process to convert raw materials into final useful products needs knowledge on the metallurgical properties and structures of materials. There are many kinds of forming techniques depend on the properties of metal [12–15], product shape-size-properties, and cost where powder metallurgy is one of the techniques [16]. Powder metallurgy especially ferrous powder has increased significantly over the last few decades especially in the area of manufacturing automotive parts. This advanced technology is growing to meet the demand in producing complex parts by altering and modifying some existing manufacturing processes. The ability of manufacturing parts without extra machining at minimum cost made this as an alternative favorable sustainable process [17]. In industries, FeCrAl alloy is formed through foundry process. The billets of pure Fe, Cr, and Al are melted in industrial furnace. The molten metal is then poured into predesigned cast/mould. The shaped products are removed from the cast/mould once they are
M.M. Rahman, A.A.A. Talib / Materials Today Communications 4 (2015) 42–49
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Fig. 1. XRD pattern of FeCrAl alloy sintered at 800 ◦ C for 30 min.
solidified. The shaped products are required a series of further machining/metal working processes to obtain the final products. This process is time consuming hence expensive. Iron is the second most common metal in Earth’s crust, after aluminum, but because it reacts so readily with oxygen it is never mined in its pure form although meteorites are occasionally discovered that contain samples of pure iron. Pure iron is a silvery-white metal that is easy to work and shape and it is just soft enough to cut through using a knife. Pure iron can be hammered into sheets and draw it into wires. Like most metals, iron conducts electricity and heat very well and it is very easy to magnetize. Due to these several disadvantages of pure iron, it cannot be used for structural purposes as well as manufacturing of heavy duty mechanical components [18]. Therefore, pure iron must be mixed with other elements to form alloy which might have superior physical, mechanical, as well as electrical properties. One of the alloying methods is the powder metallurgy which transforms two or more metallic/non-metallic powders into a homogeneous solid without melting the powder materials [19]. The effect is the superior properties of the products formed by mixing the main powder constituent, i.e., Fe powder with other alloying elements. The introduction of warm compaction technology enabled the manufacturers to press and sinter mechanical components with minimal processing steps. Warm compaction is applicable to all ferrous material system and produces great benefits when coupled with other materials. Warm compaction process has a lot advances compared to casting. The advantages are (i) time saving, (ii) no scrap materials, (iii) no machining is required, (iv) more homogenous density distribution inside the products, and (v) higher relative density is possible to be achieved. All parameters in manufacturing
process, i.e., selection of powder, mixing, compaction, and sintering are important to produce components with favorable properties that eventually contribute to the growth of powder metallurgy in whole [20]. Fe-based alloys have the most industrial usage since raw materials of Fe are abundant and have versatile mechanical properties. However, current practices of Fe-based alloys development are either through casting or mechanical alloying where these processes are expensive and time consuming. Mechanical alloying is time consuming since the process requires high energy ball milling of the powder mass for a long time. After certain interval, the balls are also required to be washed by ethanol to avoid the sticking of powder particles at the surface of the balls which consumes more time. These processes are only capable of producing ingots of alloy or alloy in powder form, not the end products. The whole product manufacturing process becomes more time consuming hence expensive. Therefore, the objective of this paper is to present an experimental investigation of the alloyability of Fe, Cr, Al powder compacts sintered at different holding times. 2. Materials and method The experimental works consist of feedstock preparation, green compact generation, sintering in argon gas fired furnace, and sintered sample characterization. Iron powder ASC 100.29 of 20–180 m particle size range, manufactured by Höganäs AB, was used as main powder constituent. The composition of iron powder and its alloying elements were fixed throughout this research. The composition of carbon was 0.08% (as admixed lubricant), chromium was 14.0%, aluminum was 4.0%, and the balance was Fe powder.
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Fig. 2. XRD pattern of FeCrAl alloy sintered at 800 ◦ C for 60 min.
The main powder constituent and alloying elements were mixed mechanically for 60 min. The powder mass was filled into the cylindrical shape die cavity then all the die assembled together with the powder mass was heated up to 150 ◦ C and maintained for 30 min for uniform heating. Green samples were generated by applying downward and upward forces of 130 kN from upper and lower punches, simultaneously. Once the compaction load achieved the set value, the upper punch was released to its initial position while lower punch was maintained at its final position for a while for elastic recovery or spring-back of the as pressed or green compact. The green compact was subsequently ejected from the die cavity by the upward motion of the lower punch. The dimensions of the as pressed green samples were measured before storing them in properly labeled plastic container and sealed to avoid oxidation prior to sintering. Solid cylindrical shape samples of 25 mm long and 20 mm diameter were generated for the purpose of sintering. The defect-free green samples were fired in custom made sintering furnace (HT3-1400-SIC). A ceramic tube of 50 mm outer diameter, 40 mm inner diameter, and 150 mm hot length was mounted at the furnace. Since controlled atmosphere was considered throughout this study, argon gas was flown into the ceramic tube. The process could be monitored and controlled by using gas regulator where a flow rate of 100 ∼ 1000 cc/min was considered. The green samples were then sintered at 800 ◦ C, at a heating rate of 10 ◦ C/min, for three different holding times, i.e., 30 min (Sample 5), 60 min (Sample 11), and 90 min (Sample 17). The sintered samples were characterized through XRD analysis (D-8 Advance XRD Machine), SEM/EDX analysis (Hitachi S-3400N), and their flex-
ure stress was measured through three-point bending test (ASTM E290-09). 3. Results and discussion XRD analysis was conducted to detect the intensity of elements in the sample. This analysis was mainly to detect the presence of FeCrAl compound inside the sample. The findings were then compared with ‘JCPDS Database”. However, since FeCrAl alloy is not available in the database, the peaks were compared with each element’s database. SEM/EDX was then conducted to know the composition of elements in the sample. In order to observe more clearly, element’s mapping in different color was also done. The green, yellow, red, and black were to give distribution of iron, chromium, aluminum, and carbon, respectively. The XRD patterns of Samples 5, 11 and 17 are presented in Figs. 1–3. It is evident that all the samples generally contain Fe, Cr, and Al elements, thus the patterns were compared with iron, chromium and aluminum database. It can be observed from the results that the peaks of pure elements (database) are gathered in three main peaks produced by the XRD. All the three figures show the same patterns. The only difference is the intensity (counts) of the peaks, scaled from 0 to 2200 at degrees 2Â. The angle 2Â is actually related to the inter-planar spacing ‘d’ while the intensity is related to the strength of the diffractions in the sample [21]. The main peaks are formed at an angle (2Â) of about 44◦ . The intensities were also combined and presented in Fig. 4 to make better comparison of the peak and intensity of each sample. The highest peak is observed at Sample 11 which shows that it has a higher amount
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Fig. 3. XRD pattern of FeCrAl alloy sintered at 800 ◦ C for 90 min.
Fig. 4. Intensities of elements extracted from XRD patterns of samples sintered for different times.
of alloy compared to other samples. However, the counts among each sample do not vary significantly, i.e., the crystalline phase in these three samples are almost the same. The main peaks in Figs. 1–3 are believed to be FeCrAl compound, however some other small peaks are also observed. Since no comparison among these results and other databases of compounds such as Fe–Cr, Al–Cr, Fe–Al or even Al2 O3 are done, the small peaks are compared with other published results reported in [22–26]. In Fig. 1, a peak of 600 counts is observed at an angle (2Â) of 38.5◦ which shows the presence of chromium oxide (Cr2 O3 ). Another
peak of 164 counts at an angle of 65.029◦ is also observed which might either be ␣-Al2 O3 or Fe only. A peak of Al–Fe compound with 219 counts is observed at an angle of 82.317◦ . In Fig. 2, a peak of 700 counts indicating the presence of aluminum oxide, ␣-Al2 O3 is observed at the same angle, i.e., 38.5◦ . Two other peaks at 65◦ and 82.27◦ of 143 counts and 216 counts are also observed showing the presence of either iron element, aluminum oxide (␣-Al2 O3 ) or Al–Fe compound, respectively. In Fig. 3, a quite high peak of 550 counts at an angle of 38.5◦ is observed. This peak is supposed to be aluminum oxide, ␣-Al2 O3 . At 54◦ and 82.314◦ , peaks of 129 counts and 175 counts indicating the presence of Al–Fe and chromium oxide Cr2 O3 ), respectively are observed. A lot of other peaks are also observed here and there but their identities are very difficult to be confirmed. The differences in peaks are believed to be due to the sintering time which is related to the alloyability of the powder compacts. The SEM images of the sintered samples are shown in Figs. 5–7, , . In each figure, the micrograph at the upper shows the distribution of different elements by color, while at the lower is the investigated area zoomed to 300× magnification. The chemical compositions of the investigated areas of the samples are shown and compared in Table 1. It can be observed that Sample 5, sintered for 30 min (Fig. 5) contains more pores, some are round, and the others are interconnected. However, the distributions of other alloying elements such as aluminum (red) and chromium (yellow) are not equal. In the magnified view (Fig. 5b), the dark area shows the position where all the three elements, i.e., Fe, Cr, and Al exist. The surface is darker compared to the surrounding that only consists of iron and chromium, respectively. It is also observed from the micrograph that the alloy elements formed heterogeneous blend
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Table 1 Results of EDX of samples. Description of sample
Chemical composition Atomic (%)
Sample 5 Sample 11 Sample 17
Weight (%)
O
C
Al
Cr
Fe
O
C
Al
Cr
Fe
57.49 53.07 60.18
5.05 5.17 4.01
5.31 8.89 5.18
5.97 9.23 3.62
26.17 23.64 27.01
31.76 28.77 33.81
2.10 2.10 1.69
4.95 8.12 4.91
10.73 16.27 6.61
50.47 44.74 52.98
Fig. 5. SEM micrographs of sample sintered at 800 ◦ C for 30 min (a) surface topography, (b) investigated area (300× magnification).
Fig. 6. SEM micrographs of sample sintered at 800 ◦ C for 60 min (a) surface topography, (b) investigated area (300× magnification).
between aluminum and base element, i.e., iron since the structures are not comparable with any other reported research [26]. It is believed that a sintering time of 30 min is not enough to form FeCrAl alloy completely during sintering at 800 ◦ C in argon gas fired furnace. In the case of Sample 11 (Fig. 6), more equally distributed alloying elements are observed at the investigated surface (Fig. 6b). Aluminum and chromium are visible in almost all the areas. However, some aluminum elements are clustered together which indicates that the mixing of the base powder and other alloying elements during feedstock preparation might not be conducted perfectly. Less interconnected pores are visible at this area, but isolated pores are still observed here and there. The boundaries or contact regions of the particle are still visible, which are the necks among the adjacent grains believed to be formed during sintering. There is no homogenous blend of alloying elements is observed hence a lot of small pores throughout the whole investigated area is visible. Due to the existence of less interconnected pores, it is believed that the sintering is occurred completely at this condition which formed necks and eliminated interconnected pores.
Fig. 7 shows the micrograph of the sample sintered for 90 min. The concentration of aluminum is more at the upper side of investigated area while chromium is observed in almost everywhere, however the distribution of alloying elements is still clustered more at the left side of the investigated area. It is clearly observed that there is a long interconnected pore at the lower right side of the investigated area which might be due to the crack formed during sintering. The long interconnected pore might also be due to the presence of foreign particle during compaction or sintering. However, observed from a larger perspective, the structure is more homogenous than other two samples. The pores observed in the micrographs might be due to the segregation of alloying elements during powder mixing because of their density difference. They might also be formed during sintering since alloying elements such as Al and Cr have lower melting temperature compared to Fe. In a higher magnification (1000×), the boundaries of elements are not visible anymore (Fig. 8). There are about four distinguished patterns, i.e., the left side with a rough coral-like microstructure, middle-left side with a long flanged-like lamellae microstructure, upper right with smooth coral-like microstructure, and at the mid-
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Fig. 8. SEM micrograph of sample sintered at 800 ◦ C for 90 min (1000× magnification).
Fig. 7. SEM micrographs of sample sintered at 800 ◦ C for 90 min (a) surface topography, (b) investigated area (300× magnification).
dle right with an unknown structure of gray colored particle, which is believed also to be found in other areas.
The composition of elements in the samples can be observed by an EDX spectrum of each sample. The EDX analysis was conducted to observe the elements found in the sample or in other words, whether any foreign particle/element are present or not. The results of EDX (element spectrum) are presented in Figs. 9–11, , . It is emphasized that these element spectrums were generated from the entire surface area of the sample. Thus the intensity of the spectrum is the average of all elements’ intensity found in the sample. Figs. 9–11 show that all the four elements, i.e., iron, chromium, aluminum, and carbon are present with a very high amount of oxygen content. The display of double peaks of iron and chromium at 5–10 keV is due to the use of energy of the K-shell. K-shell X-ray energy consists of two lines, i.e., K␣ and K, with a ratio of 10:1, thus displaying two peaks correspond to the stated ratio. On the other hand, the single peak of iron, chromium, aluminum, and carbon at the energy range of 0–5 keV is due to the fact that at lower than 3 keV energy range, the separation among K-, L-, and M- shells is
Fig. 9. EDX element spectrum of sample sintered at 800 ◦ C for 30 min.
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Fig. 10. EDX element spectrum of sample sintered at 800 ◦ C for 60 min.
small hence not distinguishable [27]. As a comparison among these three figures (Figs. 9–11), Sample 11 shows the highest chromium, aluminum, and carbon content, while Sample 17 shows the highest iron and oxygen content. Different peaks of Cr, Al, and Fe are observed since alloyability is different due to the different processing conditions. Product with higher alloy content is found to have higher mechanical properties as depicted in Fig. 12. Fig. 12 shows the flexure stress or bending strength of samples sintered for different times. It is revealed that sample sintered for 60 min (Sample 11) obtained the highest strength. This is in line with the micrograph shown in Fig. 6b where a lot of necks among
the adjacent grains are formed during sintering. A high strength is required to create an inter-granular fracture to break the necks, hence resulted in higher bending strength. The melting point of the main powder constituent, Fe is 1538 ◦ C whereas the melting points of other alloying elements, i.e., Cr is 1907 ◦ C and Al is 660.3 ◦ C. During sintering at 880 ◦ C, for all the three holding times, i.e., 30, 60, and 90 min, portions of Al melted and created bonding agent among the powder particles. However, at a holding time of 30 min, less portions of Al might be melted hence created less bonding agent among particles which could be observed in Fig. 5 with nonhomogeneous microstructure which
Fig. 11. EDX element spectrum of sample sintered at 800 ◦ C for 90 min.
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Fig. 12. Bending strength of sintered products.
also resulted samples with lower bending strength (Fig. 12). When the green sample was sintered for 60 min, more Al elements were melted creating more bonding agents among the Fe and Cr. As a result, more homogeneous microstructures are observed (Fig. 6) and the sample sintered for 60 min obtained the highest bending strength (Fig. 12). Adverse result is observed when the sample was sintered for 90 min, less homogeneous microstructures together with lower bending strength were observed (Figs. 7–12). It is clear from the discussion above that suitable sintering time for FeCrAl green compacts is around 60 min which could generate stronger product with better microstructure. 4. Conclusions The results of XRD and SEM/EDX analyses on the phase identification, intensity of alloy peak, and microstructure together with element spectrum show that the alloy is present in the samples. The highest intensity of alloy is observed at the sample sintered for 60 min (Sample 11). However, based on the microstructure evaluation, sample sintered for 90 min (Sample 17) shows a homogenous structure in the investigated area. Highest bending strength is also measured at the sample sintered for 60 min. Acknowledgement The author wants to thank Ministry of Education (MOE), Malaysia for funding this research study under 20140117 FRGS grant. References [1] M.P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, 5th edition, John Wiley & Sons, 2012. [2] W.D. Callister, Materials Science and Engineering, 8th edition, John Wiley & Sons, 2012.
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