iraj.in EFFECT OF SN ADDITION ON MICROSTRUCTURE ......as-cast and hot-rolled AM60 alloys....
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International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 5, Issue-7, Jul.-2017 http://iraj.in Effect of Sn Addition on Microstructure and Mechanical Properties of As-Cast and Hot-Rolled Am60 Magnesium Alloys 22 EFFECT OF SN ADDITION ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AS-CAST AND HOT-ROLLED AM60 MAGNESIUM ALLOYS 1 YAVUZ SUN, 2 HUSEYIN ZENGIN, 3 YUNUS TUREN, 4 HAYRETTIN AHLATCI, 5 EMIN KARACELEBI, 6 MEHMET UNAL 1,2,3,4,5,6 Karabuk University, Department of Metallurgy and Materials Engineering, Turkey. E-mail: [email protected] Abstract- In this study, the effect of Sn addition on microstructure and mechanical properties of AM60 magnesium alloys was investigated. Sn additions were made by 0.5 and 1 wt%. Alloys were produced by conventional gravity casting in a steel mould. A homogenization treatment at 350 °C for 24 h was performed after the casting processes. Homogenized samples were hot-rolled by 40% thickness reduction at 1 pass at 350 °C. The results showed that Sn addition decreased the grain size of both as-cast and hot-rolled AM60 alloys. Furthermore, Sn addition led to formation of Sn containing second phases. Tensile properties were significantly improved by 0.5 wt% Sn addition above which a small decrease was observed. This improvement was attributed to the refinement of microstructure and dispersion strengthening by Mg-Sn intermetallic phases. Index Terms- AM60 magnesium alloys; Sn modification; microstructure; mechanical properties I. INTRODUCTION Magnesium alloys are the lightest structural metallic material. Apart from the lightness, they have unique specific strength, good machinability and castability which make them favourable materials in automotive and aerospace industries. AZ91 and AM60 are the most commercially used magnesium alloys accounting for 90% of total use of magnesium alloys [1-3]. AM60 magnesium alloys have good combination of strength, ductility and castability. However, there is still need for improving strength, fatigue, creep resistance, corrosion resistance and wear resistance of magnesium alloys [3]. Alloying and hot forming are the main mechanisms in order to improve these properties. Alloying with rare earth elements can increase strength, corrosion and wear resistances of magnesium alloys [4-6]. It was reported that Sr and Ti addition improved the tensile and wear resistance of AM50 magnesium alloys [7]. Alloying magnesium alloys with Sn led to formation of Mg 2 Sn thermally stable, dense intermetallic compounds which is beneficial to increase strength [8-10]. Turen [8] showed that Sn addition by 0.5 wt% in AZ91 magnesium alloy improved fluidity and increased tensile strength and elongation above which these properties decreased Park et. al [9] showed that Sn additions up to 4 wt% increased the tensile strength but decreased the elongation of AZ82 alloy. Hot-rolling is an efficient way to obtain fine microstructure and to increase strength by dynamic recrystallization mechanism. In the literature, there is no study that investigates AM60 magnesium alloys with different Sn additions comparing both as-cast and hot-rolled states. Therefore, this study aims for understanding the effects of Sn additions on microstructure and mechanical properties of as-cast and hot-rolled AM60 magnesium alloys. II. EXPERIMENTAL PROCEDURE The Mg-6wt%Al-0.3wt%Mn alloys with different Sn additions (0, 0.5, 1 wt%) were produced by gravity casting. The chemical compositions of the alloys were measured by wave-length dispersion X-ray fluorescence (XRF) and listed in Table 1. High purity Mg (99.9%), Al (99.9%), Sn (99.9%) and Al-Mn master alloy were used to prepare the alloys. The alloys were melted in stainless steel crucible placed in an electric resistance furnace under controlled Ar gas flow. After holding the melt at 750 °C for 45 minutes and stirring for 15 minutes, the melt was poured into a steel mould preheated to 250 °C. A homogenization treatment at 350 °C for 24 h was performed after the casting processes. Homogenized samples were hot-rolled by 40% thickness reduction at 1 pass at 350 °C. Table I Chemical compositions of the alloys For the microstructure analysis, all the samples were mechanically ground with 240, 400, 600, 800, 1000, 1200 and 2000 grit emery papers followed by polishing with 6 m and 1 m diamond paste. The polished samples were etched with 6 gr picric acid, 5 ml glacial acetic acid, 10 ml distilled water and 100 ml ethanol. The microstructure images of the samples were taken by optical microscope and scanning electron microscope. According to EN ISO 6892-1, the tensile specimens with a gauge section of 40 mm x 9 mm x 2 mm were machined from the as-cast and hot-rolled alloys. Tensile tests were performed with a
Transcript of iraj.in EFFECT OF SN ADDITION ON MICROSTRUCTURE ......as-cast and hot-rolled AM60 alloys....
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 5, Issue-7, Jul.-2017 http://iraj.in
Effect of Sn Addition on Microstructure and Mechanical Properties of As-Cast and Hot-Rolled Am60 Magnesium Alloys
22
EFFECT OF SN ADDITION ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF AS-CAST AND HOT-ROLLED AM60
MAGNESIUM ALLOYS
1YAVUZ SUN, 2HUSEYIN ZENGIN, 3YUNUS TUREN, 4HAYRETTIN AHLATCI, 5EMIN KARACELEBI, 6MEHMET UNAL
1,2,3,4,5,6Karabuk University, Department of Metallurgy and Materials Engineering, Turkey.
E-mail: [email protected]
Abstract- In this study, the effect of Sn addition on microstructure and mechanical properties of AM60 magnesium alloys was investigated. Sn additions were made by 0.5 and 1 wt%. Alloys were produced by conventional gravity casting in a steel mould. A homogenization treatment at 350 °C for 24 h was performed after the casting processes. Homogenized samples were hot-rolled by 40% thickness reduction at 1 pass at 350 °C. The results showed that Sn addition decreased the grain size of both as-cast and hot-rolled AM60 alloys. Furthermore, Sn addition led to formation of Sn containing second phases. Tensile properties were significantly improved by 0.5 wt% Sn addition above which a small decrease was observed. This improvement was attributed to the refinement of microstructure and dispersion strengthening by Mg-Sn intermetallic phases. Index Terms- AM60 magnesium alloys; Sn modification; microstructure; mechanical properties I. INTRODUCTION Magnesium alloys are the lightest structural metallic material. Apart from the lightness, they have unique specific strength, good machinability and castability which make them favourable materials in automotive and aerospace industries. AZ91 and AM60 are the most commercially used magnesium alloys accounting for 90% of total use of magnesium alloys [1-3]. AM60 magnesium alloys have good combination of strength, ductility and castability. However, there is still need for improving strength, fatigue, creep resistance, corrosion resistance and wear resistance of magnesium alloys [3]. Alloying and hot forming are the main mechanisms in order to improve these properties. Alloying with rare earth elements can increase strength, corrosion and wear resistances of magnesium alloys [4-6]. It was reported that Sr and Ti addition improved the tensile and wear resistance of AM50 magnesium alloys [7]. Alloying magnesium alloys with Sn led to formation of Mg2Sn thermally stable, dense intermetallic compounds which is beneficial to increase strength [8-10]. Turen [8] showed that Sn addition by 0.5 wt% in AZ91 magnesium alloy improved fluidity and increased tensile strength and elongation above which these properties decreased Park et. al [9] showed that Sn additions up to 4 wt% increased the tensile strength but decreased the elongation of AZ82 alloy. Hot-rolling is an efficient way to obtain fine microstructure and to increase strength by dynamic recrystallization mechanism. In the literature, there is no study that investigates AM60 magnesium alloys with different Sn additions comparing both as-cast and hot-rolled states. Therefore, this study aims for understanding the effects of Sn additions on microstructure and mechanical properties of as-cast and hot-rolled AM60 magnesium alloys.
II. EXPERIMENTAL PROCEDURE The Mg-6wt%Al-0.3wt%Mn alloys with different Sn additions (0, 0.5, 1 wt%) were produced by gravity casting. The chemical compositions of the alloys were measured by wave-length dispersion X-ray fluorescence (XRF) and listed in Table 1. High purity Mg (99.9%), Al (99.9%), Sn (99.9%) and Al-Mn master alloy were used to prepare the alloys. The alloys were melted in stainless steel crucible placed in an electric resistance furnace under controlled Ar gas flow. After holding the melt at 750 °C for 45 minutes and stirring for 15 minutes, the melt was poured into a steel mould preheated to 250 °C. A homogenization treatment at 350 °C for 24 h was performed after the casting processes. Homogenized samples were hot-rolled by 40% thickness reduction at 1 pass at 350 °C.
Table I Chemical compositions of the alloys
For the microstructure analysis, all the samples were mechanically ground with 240, 400, 600, 800, 1000, 1200 and 2000 grit emery papers followed by polishing with 6 m and 1 m diamond paste. The polished samples were etched with 6 gr picric acid, 5 ml glacial acetic acid, 10 ml distilled water and 100 ml ethanol. The microstructure images of the samples were taken by optical microscope and scanning electron microscope. According to EN ISO 6892-1, the tensile specimens with a gauge section of 40 mm x 9 mm x 2 mm were machined from the as-cast and hot-rolled alloys. Tensile tests were performed with a
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 5, Issue-7, Jul.-2017 http://iraj.in
Effect of Sn Addition on Microstructure and Mechanical Properties of As-Cast and Hot-Rolled Am60 Magnesium Alloys
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strain rate of 0.00167 1/s at room temperature and each test condition was repeated three times. III. RESULTS AND DISCUSSION Fig. 1 shows the optical microstructures of the as-cast, homogenized and hot-rolled AM60 magnesium alloy with Sn additions of 0, 0.5 and 1 wt%. The as-cast AM60 alloy consisted of primary -Mg (white region) and -Mg17Al12 intermetallic phase (dark region). The maximum solid solubility of Al in -Mg is about 12 wt%, -Mg17Al12 eutectic phase forms during casting in Mg-Al under non-equilibrium cooling conditions [11]. The morphology of eutectic phase in Mg-Al alloy system can be affected by alloying elements and cooling conditions. In the as-cast states, Sn addition led to an alteration of -Mg17Al12 phase morphology. In Fig.2 the SEM microstructure images of the as-cast AM60 and AM60+1Sn alloy were compared with corresponding energy-dispersive spectroscopy (EDS) results. It can be seen that both -Mg solution and
second phases contained Sn concentrations. The second phases tended to change from Mg-Al eutectic phases to a mixture of Mg-Al and Mg-Sn binary compounds. In Fig.1, it can also be said that with Sn addition, as-cast and homogenized samples had slightly finer grains. Furthermore, homogenization treatment led to a dissolution of -Mg17Al12 eutectic phase into -Mg matrix but few undissolved secondary phases were still present in all the studied alloys. After hot-rolling at 350 °C, dynamically recrystallized (DRXed) grain structures were observed in all the studied alloys. However, some unrecrystallized grains were also observed in AM60 alloy constituting a bimodal distribution. In hot-rolled states, DRXed grain sizes were measured as 17.75 m, 11.20 m and 10.32 m for the AM60, AM60+0.5Sn and AM60+1Sn alloys respectively. This refinement by Sn addition was due to the presence of Mg-Sn intermetallic phases that may have pinned the grain growth during dynamic recrystallization.
Fig. 1 Optical microstructures of as-cast, homogenized and hot-rolled AM60 magnesium alloys with 0 wt%, 0.5 wt% and 1 wt% Sn
additions. Fig.3 and Fig.4 show the tensile test results of the as-cast and hot-rolled alloys respectively. In both graphs, 0.5 wt% Sn addition exhibited the best tensile properties. As-cast AM60+0.5Sn alloy showed 65 MPa yield strength and 195 MPa ultimate tensile strength allied with 5.8 % elongation. After hot-rolling, the yield strength, ultimate tensile strength and elongation increased to 170 MPa, 292 MPa and
19.8 % respectively. Since the hot-rolling process resulted in a much finer microstructure and much less dislocation density by DRX mechanism, the mechanical properties showed a remarkable improvement. It should be noted that Sn-added hot-rolled alloys showed better elongation than the AM60 alloy (9%). This improvement was because of the differences in DRXed grain size and a larger
International Journal of Mechanical And Production Engineering, ISSN: 2320-2092, Volume- 5, Issue-7, Jul.-2017 http://iraj.in
Effect of Sn Addition on Microstructure and Mechanical Properties of As-Cast and Hot-Rolled Am60 Magnesium Alloys
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bimodal grain distribution in AM60 magnesium alloy. All in all, low amount of Sn additions can enhance the mechanical properties of as-cast and hot-rolled AM60 alloys by refining the microstructure and forming Mg-Sn binary compounds that inhibits the grain growth and leads to dispersion strengthening. Further investigations, such as lower additions than 0.5 wt% of Sn and different processing routes and parameters can provide better understanding the relationship between Sn addition and the properties of AM60.
Fig. 2. SEM images of as-cast (a) AM60 and (b) AM60+1Sn with
corresponding energy-dispersive spectroscopy (EDS) results.
Fig. 3. Tensile test results of the as-cast alloys.
Fig. 4. Tensile test results of the hot-rolled alloys.
CONCLUSION AM60 alloy with 0, 0.5 and 1 wt% Sn additions were cast and hot-rolled and microstructure and mechanical properties were investigated. The following conclusions can be drawn:
The as-cast AM60 alloy consisted of primary -Mg and -Mg17Al12 eutectic phase. Sn additions led to an alteration of -Mg17Al12 phase morphology and formation of Mg-Sn intermetallic phases.
Homogenization treatment led to a dissolution of -Mg17Al12 eutectic phase into -Mg matrix but few undissolved secondary phases were still present in all the studied alloys.
After hot-rolling at 350 °C, dynamically recrystallized (DRXed) grain structures were observed in all the studied alloys. DRXed grain sizes were measures as 17.75 m, 11.20 m and 10.32 m for the AM60, AM60+0.5Sn and AM60+1Sn alloys respectively.
Mechanical properties of AM60 alloy were improved by Sn addition. 0.5 wt% Sn addition exhibited the best tensile properties in both as-cast and hot-rolled states.
ACKNOWLEDGMENTS This research is supported by the Scientific research projects (BAP) with project no. KBÜ-BAP-17-YD-211 REFERENCES
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