Advanced Materials Research Vols 984-985 (2014) pp 124-128 ...

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Advanced Materials Research Vols 984-985 (2014) pp 124-128 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.984-985.124

Online: 2014-07-16

Development and characterization of AZ31B Mg alloy using powder metallurgy technique followed by hot extrusion R.Anish*a, M.Sivapragashb, G.Robert Singhc Department of mechanical engineering, Noorul Islam centre for higher education, Tamilnadu, India [email protected]*a,[email protected],[email protected] Keywords: Magnesium alloy, Mechanical properties, Microstructure, Hardness

Abstract: In this study AZ31B Mg alloy is developed by powder metallurgy route followed by hot extrusion. The extruded materials were characterized for microstructure, X-ray diffraction (XRD), density, porosity and mechanical properties such as tensile, compression, impact and micro hardness. The SEM analysis of fractured surface of AZ31B Mg alloy showed quasi-cleavage fracture mode. 1. Introduction The density of magnesium is 1.74g/cm3, when compared to other metals it is the lightest of all metals. The density of magnesium is two-third that of aluminium and it is also the most abundant element available in the earth crust [1]. Because of its low density magnesium based materials have been used extensively in automobile and aerospace industries [2]. Among the various grades of magnesium alloys the alloys that are named as AZ such as AZ31, AZ61, and AZ91 have been extensively used in industries [3]. In this proposed work an attempt is made to characterize AZ31B Mg alloy and to improve the mechanical properties. 2. Experimental procedure 2.1 Materials. AZ31B magnesium alloy of chemical composition (Al 3wt%, Mn 0.6 wt%, Zn 1.0 wt %) and particles size of Mg is 63µm, Al is 50µm, Zn is 45µm and Mn is 40µm were purchased from MEPCO metal powder company, India. 2.2 Processing. AZ31B Magnesium alloy was synthesized using powder metallurgy route. The powders were homogeneously mixed with the respective weight percentage of powder using suitable powder mixture set at a rotational speed of 350 rpm for 1hour. The homogenised powder mixture of AZ31B Mg alloy was then compacted at a pressure of 690MPa to form billet of 50mm diameter and 30mm height. In order to minimize reaction of composite with oxygen present in the atmosphere the compacted billets were covered using aluminium foil and sintered in a muffle furnace at 400°C in argon atmosphere for 1 hour. The sintered AZ31B Mg alloy was hot extruded at 350°c in argon atmosphere. The 150 ton vertical hydraulic press was used at an extrusion ratio of 15:1 by using colloidal graphite as lubricant. The cylindrical rod of 13mm diameter and length 400mm was obtained by hot extrusion. The extruded rods were stress relieved by cutting the rod in to suitable sizes and heated at 260°C for 15minutes in a muffle furnace with argon as protective atmosphere [4]. 2.3 Micro structure characterization. Samples of AZ31B Mg alloy were primarily polished by using sand paper grit ranging from (800-2000µm). Further it was polished by applying diamond paste of 0.2, 0.4, 0.6µm in dish polishing machine. Then finely polished samples were chemically etched using standard metallographic techniques. The etchant was prepared by mixing 50 ml of picric acid, 20 ml of glacial acetic acid, 10 ml of deionized water, and 10 ml of methanol. The etchant was then applied to the polished surface of each sample which was then thoroughly rinsed with deionized water and ethanol [5]. The samples were then examined using, atomic force microscopy to investigate the presence of porosity and reinforcement distribution.

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2.4 XRD studies. XRD analysis was performed on powder samples of AZ31B Mg alloy using an automated XPERT-PRO diffractometer. Powder samples were exposed to CuKα radiation (∆=1.5406) with a scan speed of 3.175 deg min-1. The diffraction range was 0-180ο degree with a step size of 0.10ο. The Bragg angles and values of Inter planar spacing‘d’ obtained were subsequently matched with the standard values of Mg, Al and other related phases. 2.5 Density and porosity measurement. Density and porosity measurements were performed on polished samples of AZ31B taken from extruded rods using Archimedes principle, distilled water was used as the immersion fluid. The samples were weighed using an electronic balance with an accuracy of ±0.0001g. 2.6 Micro hardness measurement. Micro hardness measurements were made on the finely polished samples. The vickers micro hardness (VHV) was measured by automatic digital micro hardness tester using a pyramidal diamond indenter with an included angle of 136ο, 25 gf Indenting load, 60µm/s Indentation rate, 20 s duel time and 15 kg major applied load with ASTM E18-94 standard. 2.7 Tensile testing.Tensile properties of the AZ31B Mg alloy were determined in accordance with ASTM standard E8-01. The tension tests were conducted on round tension test specimen of 4.74mm in diameter and 76.2mm gauge length using an automated servo hydraulic testing machine with a strain rate set at 0.300mm/min. Total of four tensile specimens were tested for each sample. Engineering stress-strain curves were plotted using the load-displacement curve. 2.8 Compression testing. Compression test of the extruded AZ31B Mg alloy was conducted in accordance with ASTM standard E9-81. The compression tests were conducted on round compression test specimen of 13mm in diameter and 25mm height using an automated computer controlled hydraulic testing machine with a cross head speed set at 0.254mm/min. 2.9 Impact testing. Izod (cantilever beam) impact test was performed on the V-notched impact test specimen which was cut and machined from these extruded rods according to ASTM E23. The round specimen is of length 75mm, diameter 11.4mm and the notch angle was 45°. 2.10 Fracture behaviour.Fractural studies were performed on the tensile, compressive, impact fractured surfaces of AZ31B Mg alloy. Fractography was accomplished utilizing a JSM- 6610LV SEM. 3. Results and Discussion 3.1 Macrostructure and Microstructure characterization. The microstructure of the extruded AZ31B Mg alloy was shown in figure 1. The sintering defects such as circumferential or radial cracks were absent on the sintered billets. No macro defects were observed on the hot extruded, stress relieved monolithic AZ31B Mg alloy and the outer surface was smooth and free of circumferential cracks. In the figure the Mg phase was distributed as block shape, separated by Mg17Al12 phase [6].

Figure1 Microstructure of AZ31B Mg alloy

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3.2 XRD.Figure2 shows the XRD spectra of extruded specimens of AZ31B Mg alloy. XRD results of the samples revealed the formation of inter metallic compound Mg17Al12 by the reaction of magnesium with aluminium. In the XRD of AZ31B Mg alloy MgO was not found this may be due to the reason that argon atmosphere has prevented reaction of magnesium with oxygen. In [7] it was reported that the formation of MgO occurs only when the magnesium composite were sintered above 773oK.

Figure 2 XRD of AZ31B Mg alloy 3.3 Density and porosity. The results of density and porosity measurements conducted on the extruded AZ31B, are shown in table1. Porosity is known to influence the mechanical properties to a great extent therefore pores are to be eliminated through post sintering process such as extrusion. Developed AZ31B Mg alloy showed no porosity. The density features revealed minimal oxidation of magnesium and absence of macro pores [8]. Table1 Showing density, porosity, impact, tensile, compression and hardness result Density, porosity and impact measurement result Material Theoretical Experimental Porosity Impact strength(joules) density density (ρex) (%) (ρth) g/cm3 g/cm3 AZ31B 1.776 1.776 0 9 Tensile and hardness measurement result Ductility Material UTS(MPa) 0.2% yield Vickers Hardness strength (%) (VHN) (MPa) 180±3

AZ31B 280±3 Compression measurement result Material UCS(MPa) 0.2%yield strength (MPa)

AZ31B

490±3

120±9

5.3±0.25

68±1

Ductility (%)

19.7±0.25

3.4 Tensile, compressive, impact and hardness behaviour. The tensile and compressive engineering stress-strain behaviour of AZ31B Mg alloy is shown in figure3. The results of tensile, compressive, impact and hardness testing were listed in table1. The increase in tensile, compressive and impact strength can be attributed to work hardening due to strain misfit, the matrix formation of intermetallic compound Mg17Al12, Orwans strengthening, reduction in grain size and well bonded Mg and Mg17Al12 [9]. Moreover the increase in tensile strength may be due to reduction in porosity formation [10].

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b) compression test

Figure 3 Engineering stress-strain curves of a) tensile test b) compression test 3.5 Fractural Characterization. SEM micrographs figure 4a, b, c, shows the tensile, compressive and impact fracture surface of AZ31B Mg alloy. In [11] it was reported that the failure of magnesium alloys is usually brittle through cleavage fracture or quasi-cleavage fracture. The crystal plane region which is (0 0 0 1) for magnesium, micro cracks develop along this plane, this fracture is characterized as cleavage fracture and some cracks which generate in some area, grow locally and finally form morphologies of pits, this is known as quasi-cleavage fracture. The tensile, compressive and impact fractured surface of AZ31B Mg alloy shows small cleavage planes, river like patterns which is known as quasi cleavage fracture.

a) Tensile test

b) compression test

c) Impact test Figure4 SEM photographs showing fractural characterization of a) tensile test b) compression test c) impact test 4. Conclusion. The powder metallurgy technique followed by hot extrusion was used to synthesize AZ31B Mg alloy. Micro structural characterization showed homogeneous distribution of particles in the Mg matrix. XRD exhibited the formation of Mg17Al12 compound. The micro hardness measurement showed increase in hardness due to stress-relieving. Porosity measurement of the samples showed no porosity. The mechanical properties such as tensile, compression and impact of the developed material showed significant improvement when compared to other processing techniques. Fractural characterization of the fractured surface showed quasi-cleavage fracture.

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Reference [1] W.L.E. Wong, M. Gupta “Development of Mg/Cu nano composites using microwave Assisted rapid sintering” Composites Science and Technology 67 (2007) 1541–1552 [2] Maung Aye Thein, L.Lu, M.O.Lai “mechanical properties of nano structured mg-5wt% Alxwt% AIN composite synthesized from Mg chips” composite structures (2006) 206-212 [3] Manel Marya, Louis Hector, Ravi Verma “Microstructural effects of AZ31B magnesium alloy on its tensile deformation and failure behaviour” material science and engineering A418(2006) 341356 [4] Stevenson “Heat treating of magnesium alloys” ASM Hand book, volume4 Heat treating [5]T.V. Padfield, “Metallography and Microstructures of Magnesium and Its Alloys,Metallography andMicrostructures”, Vol 9, ASM Handbook, ASM International, 2004, p. 801–815 [6] B.Y. Yu, C.L. Bao, H.W. song, Z. Liu and H.P. Yu “Microstructure and mechanical properties of AZ91D extuded tubes” Acta metal.Sin (Engl.lett) vol 19 no.3 pp203-208 jun,2006 [7] S. Ugandhar, M. Gupta , S.K. Sinha “Enhancing strength and ductility of Mg/SiC composites using recrystallization heat treatment” Composite Structures 72 (2006) 266–272 [8] S.F Hassan,M.Gupta “Development of a novel magnesium-copper based composite with improved mechanical properties” material research bulletin 37(2002) 377-389 [9] QianqianLi, Andreasviereckl, Christian A, Rottmair, RobertF.Singer “Improved processing of carbon nanotubes/magnesium alloy composites” composites science and Technology 69(2009) 1193-1199 [10] S.F.Hassan, M.Gupta “Effect of type of primary processing on the microstructure, CTE and mechanical properties of magnesium/alumina nanocomposites. Composite structures 72(2006) 1926 [11] M.Sivapragash, P.R.Lakshminarayanan, R.karthikeyan, M.Hanumantha, R.R.Bhatt “Hot deformation behaviour of ZE41A magnesium alloy” materials and Designs 29(2008) 860-866

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Development and Characterization of AZ31B Mg Alloy Using Powder Metallurgy Technique Followed by Hot Extrusion 10.4028/www.scientific.net/AMR.984-985.124 DOI References [1] W.L.E. Wong, M. Gupta Development of Mg/Cu nano composites using microwave Assisted rapid sintering, Composites Science and Technology 67 (2007) 1541-1552. http://dx.doi.org/10.1016/j.compscitech.2006.07.015 [7] S. Ugandhar, M. Gupta , S.K. Sinha Enhancing strength and ductility of Mg/SiC composites using recrystallization heat treatment, Composite Structures 72 (2006) 266-272. http://dx.doi.org/10.1016/j.compstruct.2004.11.010