17-IJAEST-Volume-No-2-Issue-No-1-Formability-Aspects-of-aluminium-alloys(Al-4Mg)-113-118 by ISERP ISERP

RAMESH BALAKRISHNAN et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 2, Issue No. 1, 113 - 118

Formability Aspects of Aluminium Alloys(Al-4Mg) RAMESH BALAKRISHNAN

SENTHILVELAN THIYAGRAJAN

Research Scholar, Department of Mechanical Engineering Pondicherry Engineering College Pondicherry, India rameshh1980@yahoo.co.in

Professor, Department of Mechanical Engineering Pondicherry Engineering College Pondicherry, India tsenthilvelan@hotmail.com

Keywords-Formability;Barreling;Upsetting;Aluminium

alloys;FEA

I. INTRODUCTION

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In area of engineering materials, the focus is in developing new materials and processing technologies to make the products that are energy efficient, cost effective. Moreover the materials and process development are becoming increasingly challenging and demanding. Innovations bring the new materials or existing materials in different forms giving new possibilities of applications and products. In the last decade, a substantial amount of work has been carried out on aluminium alloys due to its advantage over light weight and enhanced performance which makes it suitable for many structural applications [1]. Aluminium alloys are used in advanced applications as their combination of high strength, weight reduction as the performance of the vehicle improves by rolling resistance and energy of acceleration, thus reducing the fuel consumption. Among the aluminium alloys, Al-4mg posses treasure of noted properties such as age hardenable, corrosion resistance, ductility that make them a perfect choice for different industrial applications. Such an alloy can be converted into any form or intricate shape for instance sheets or wires, since they are ductile in nature. Earlier most of the studies were aimed at determining the mechanical properties of the aluminium alloy viz., Young‟s modulus, yield point,

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elongation, tensile strength and compressive strength. Nevertheless, the other parameters of the stress–strain curves (flow curve), such as strain-hardening exponent (n) and strength coefficient (k), termed as flow properties, have not been discussed in detail. These parameters play a vital role in analyzing the plastic deformation of porous materials [2,3]. While examining the hot working processes, the flow stress of material at elevated temperatures is one of the indispensable information to gain knowledge on the deformation mechanism. Manufacturing of metal powder components in aluminum alloys is gaining momentum towards the applications in the aerospace and automotive industries.Sheet components from aluminium alloys are especially attractive. Doraivelu et al. [4] proposed a new yield function by generating the yield surfaces for various density levels in a three-dimensional spectrum by employing the principle stress space using computer graphics. A finite element technique was used [5] in the prediction of ductile fracture in axi-symmetric upsetting using continuum damage mechanics and studied the effect of process parameters, namely, friction, height to diameter ratio and hence suggested the possibility of formation of central cavity which increases with height todiameter ratio. Hence in this present investigation, behavior of k and n has been analyzed at various conditions such as change in aspect ratio, initial density, temperature

Abstract— The present work focuses on the formability studies on aluminium alloys(Al-4Mg) of sintered preforms at various working conditions such as change in Aspect ratio, Density, Temperature by conducting upsetting test. The behavior of strength coefficient (k) and strain hardening exponent (n) have been studied in detail and the inferences are validated with earlier findings. Further, the phenomenon of barreling has been studied and correlated with the results obtained from finite element analysis. The radius of curvature of the barrel obtained from experimental work concurs well with the calculated values and barreling radius determined from finite element analysis found to be closer to the experimental findings.

II.PROCESSING OF ALUMINIUM ALLOYS BY POWDER METALLURGY To fabricate Al-4Mg alloys, Powder Metallurgy (P/M) and conventional ingot metallurgy, including infiltration technique are the methods commonly employed. Among them powder metallurgy technique has major advantage over other technique due to ease of convenience to obtain homogeneous distribution of properties especially in the manufacture of high-temperature materials (refractory vessels), self – lubricating components(bearings),etc[3]. Hence, P/M has been chosen to process aluminium alloy. In this present investigation atomized aluminium powder and Mg powder of 4% was procured from M/s, The Metal Powder Company,

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RAMESH BALAKRISHNAN et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 2, Issue No. 1, 113 - 118

(σm/σ (m-1)) =(εm/ ε(m-1))n (6) Taking natural logarithm on either side ln (σm/ σ (m-1)) =ln(εm/ ε(m-1))n (7) Strain hardening exponent can be obtained as follows n= ln (σm/ σ (m-1))/ ln(εm/ ε(m-1)) (8) A. Barreling phenomenon

When a solid specimen (cylindrical billets which is prepared through Powder metallurgy route) is compressed axially between the top and bottom platen, the work piece material in contact with their surfaces undergoes heterogeneous deformation resulting in barreling of the specimen. However the barreling phenomenon that occurs during upsetting test imposes some restriction on the frictional conditions and formability properties. The existence of frictional constraints between the dies and the work piece leads to„„Barreling‟‟ of the cylinder. A conical wedge of a relatively un deformed metal is formed immediately below it while the remaining cylinder surfaces undergo high strains and bulge out in the form of a barrel as depicted in fig 1 where R=Hf2/4(Db-Dc), Db is a bulge diameter, Dc is contact diameter, Hf is the height after deformation. This demonstrates that the metal flows most easily towards the nearest free surface which is the point of least resistance, a well-known principle in plastic deformation. However, the use of lubricants reduces the degree of bulging, and under conditions of ideal lubrication bulging can be reduced to zero. In the present investigation, it has been observed that the general equations as suggested by Narayanasamy et al[6] for ascertaining the radius of barreling finds good correlation.

Madurai, India. The aluminium powder was analyzed for its purity and was found to be 99.7% pure with 0.3% insoluble impurities. Powder blend was used for the preparation of compacts of different height to diameter ratios, namely, 0.5 and 1.5 at density of 2.2g/cc and 2.6g/cc. Uni-axial compaction is widely used in metal forming industries. This technique usually involves relatively simple steps of blending powder and subsequent compression of mixture to produce compacts with stability. Hence this technique has been employed for fabrication of compact with required parameters such as density, aspect ratio. It is found that during compaction, 140KN was employed to achieve a density of 2.6g/cc for an aspect ratio of 0.5. The initial densities, initial aspect ratios were maintained by precisely controlling the mass and accurately monitoring the compacting pressure.1% admixed Zinc sterrate was used to lubricate the top punch, bottom punch, ejection rod and die. The aluminium – 4% Mg entire surfaces of the compacts were coated to avoid oxidation during sintering. The coated compacts were dried at the ambient conditions for a period of 12 h and sintered at the temperature of 530°C for a period of 60 min in an electric muffle furnace and the sintered compacts were cooled inside the furnace chamber itself till they attained room temperature [6]. III.THEORETICAL EVALUATION

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The expresion required to analyse flow properties of k and n are discused below: True stress(σ)=intanteneous load /instanteneous Area Where σ is the true stress The axial true strain (ε) is expressed as given below: ε=ln(ho/hf) (1) where ho is the initial height of P/M preform before deformation and hf is the height of the preform after deformation. The strain-hardening exponent value (n) and the strength coefficient (k) were determined employing the Power law[2] σ=kεn It is assumed that the consecutive compressive loads were specified as elsewhere [6]1, 2, 3, (m-1), m. Now the above equation can be rewritten as: σ(m-1)=(k(m-1))n (2) n σm=k(εm) (3) Subtracting Eq. [3] from Eq. [2], the following expression can be obtained (σm- σ (m-1)) =k[(εm)n- (ε(m-1))n] (4) Above equation can be rearranged as follows K=(σm- σ (m-1))/[(εm)n- (ε(m-1))n] (5) Now dividing equation (3) by (2) we get

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Figure 1 Schematic view of upsetting test

Figure 2 Flow curve at different density (h/d=1:Temp=303K)

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RAMESH BALAKRISHNAN et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 2, Issue No. 1, 113 - 118

Figure 3 Flow curve at different aspect ratio(temp=303k: density=2.2g/cc)

Figure 3 Flow curve at different temperature (h/d=1:density=2.2g/cc)

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It can be substantiated from the flow curve (true stress and true strain) that the behavior of materials could be expressed in exponential relationship in the following form [2] True stress=k (true strain)n Even though the flow curve relation is widely used for preforms the values of k and n are different from those of wrought parts when extended to sintered P/M parts. This takes place due to occurrence of persistent deformation and densification. Hence k and n could be termed as flow properties. Conducting upsetting process at different conditions suggest that strength coefficient(k) and strain exponent(n) have pronounced effect on the deformation process and it can be observed from the variation of k and n values. This is evident from the figure 2 as k value increases with higher preform density and lower in aspect ratio. whereas strain hardening exponent (n) value decreases at the above stated condition gives evidence to the findings observed by the researchers [7].During upsetting, it is observed that axial stress is compressive in nature whereas the other stresses such as hoop stress, tangential stresses are tensile [6].This occurs due to extrusion of deformed material around the periphery, in specific at the centre, it paves the way for movement material when it reaches plasticity. Hence at lower aspect ratio say 0.5, the rate of extrusion is minimal when compared to higher aspect ratio[8]. In specific, when the change in aspect ratio from 0.5 to 1.5, it is noticed that k value reduces and n value increases in power law. The decrease in k value is due to occurrence of bulk forming and adoption of mass constancy principle. On the other hand, the hike in n value takes place as the height of preform provides an opportunity to change in strain rate due to instability [10]. Subsequently, the deformation leads to the phenomenon of adiabatic temperature. Such phenomenon increase the effect of strain hardening as concluded by Rao[10]. [∂log /∂log σ]t=n (9) The k value increases with change in density from 0.5 to 1.5 is in line with the discussion made by the other researchers [6] that the presence of pores leads to less voluminous dislocation sinks and hence larger resident heat is accommodated in the pores which is ascribed to drop in k values. Hence more drop in k values are observed at lower density. On the other hand, n value decreases with increase in density as the process of hardening is absent. Confining the problem pertaining to strain hardening effect, increase in density of performs, reduces the n value as it is evident that change of geometric condition occurs during deformation [5]. This is associated with the formation of large voids when there is a collision of small pores, ascribed to the fact that the enhanced levels of

geometric work-hardening and the matrix strain hardenings have taken place in the case of perform with higher density.

IV.RESULTS AND EXPLANATION

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However, the k value increases since the absence of pores gives near theoretical density at the high density of preforms. Furthermore, increase in density gives maximum true stress in comparison with preforms with lower density in the same range of strain value. The observations in this present investigation have been extended to determine the effect of change in temperature on values of k and n. As suggested by Senthilvelan et al [3] and Rao[10] that increase in temperature decreases k value and n value as well. The decrease in k value is due to occurrence of re-crystallization process where generation of nucleation happens at particular temperature called re-crystallization temperature.This increase in the reduction in grain size serves as an obstacle for increase in k value and subsequently decrease in n value.It is due to decrease in hardness of preforms as the grain size shrinks. Also, it is suggested that process called dynamic softening that involves dynamic recovery exists and hence ample evidence suggests that strength and hardness register a drop at high temperature [1]. V.FEA CORRELATION In the present investigation, Finite Element Analysis(FEA) technique has been used for verification of barreling radius of the sintered preforms used. This investigation deals with power law which is ascertained to determine the formability parameters hence it is of Paramount importance to consider such a parameter [11] and execute the finite element analysis for verifying the results obtained during experimental investigations. For this, L.S.DYANA in ANSYS is appropriate to run the solution for this dynamic problem. Here, the tetrahedral element with 10-node (solid 92 in ANSYS library)

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RAMESH BALAKRISHNAN et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 2, Issue No. 1, 113 - 118

Table 1: Experimental results of k,n,barreling radius and barreling radius(FEA)

S.I

Conditions

T=303K; H/d=constant; D=constant T=773K; H/d=constant; D=constant T=constant; H/d=0.5; D=constant T=constant; H/d=1.5; D=constant T=constant; H/d=constant; D=2.2 g/cc T= constant; H/d=constant; D=2.6 g/cc

2 3 4 5 6 Figure 5 Sintered preforms

True stress MPa

k value MPa

n value

Barreling radius from experimental data(mm)

140

132

0.18

22.0

Barreling radius from Finite Element Analysis(mm) 21.25

115

112

0.16

22.5

22.15

145

130

0.18

21.5

22.3

113

108

0.18

21.0

22.3

125

115

0.19

21.0

22.26

130

120

0.16

21.5

22.9

of element size as suggested elsewhere [12] that reduces the number of iteration, had been chosen to preform analysis and the specimen geometry (i.e various aspect ratio) was meshed with moderate size of elements. Such that meshing process enhances the computational time and accuracy as shown in figure 6.

From the figures 7,8,9,10,11,12 finite element analysis gives stress distribution across the preform at various conditions suggest that stress distribution is maximum at the edges as there is a intersection between two areas which is perpendicular to each other[14].

Figure 6 A Typical Mesh Model

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Finite element model was generated with an aspect ratio of 1.5, 0.5 at density of 2.2g/cc&2.6g/cc as it is decided during experimental investigation. In fully dense material, the lateral spread is high as there is absence of pores such a finding is depicted in the figures 11,12 hence the barreling formation in high dense material experience the radius of about one and half times than low density preforms. Kuhn and Downey [13] proposed relationship between poison ratio‟s and density for sintered materials. υ=0.5ρa (10) a---->constant,2 for hot deformation and 1.92 for cold deformation.From the finite element analysis, it is noted that there is a good agreement between the barreled preform from experiments and finite element methods at the above explained conditions as outlined in the Table 1

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Figure 7 Specimen deformed at Low temperature (303K)

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Figure 10 Specimen deformed at High aspect ratio (1.5)

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Figure 8 Specimen deformed at High temperature (773K)

RAMESH BALAKRISHNAN et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 2, Issue No. 1, 113 - 118

Figure 9 Specimen deformed at Low Aspect ratio (0.5)

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Figure 11 Specimen deformed at Low density (2.2g/cc)

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REFERENCES

[2] [3] [4] [5] [6] Figure 12 Specimen deformed at High density (2.6g/cc)

[7] [8] [9]

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VI. CONCLUSION In this present study, flow properties such as k and n are investigated on Aluminium based alloy (Al-4Mg) and the following conclusions are arrived at: (1)As the aspect ratio increases from 0.5 to 1.5, k and n value decreases due to occurrence of bulk deformation and adoption of mass constancy principle. (2) The k value increases with increase in density from 2.2g/cc to 2.6g/cc whereas n value decreases. This is ascertained to the absence of strain hardening effect as the collision of pores causes the formation of geometric voids. (3)The flow properties such as k and n decreases with increase in temperature from 303k to 773K.This is due to the process of dynamic softening that takes place as the grain size increases. (4)The radius of curvature of the barrel obtained from experimental work concurs well with the calculated values and barreling radius determined from Finite Element Analysis found to be closer to the experimental findings.

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