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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 9, Number 4 (2014) pp. 405-414 © Research India Publications http://www.ripublication.com

Experimental Study of Deformation Characteristics during Forging of Cylindrical Sintered Aluminium Preforms with

Different L/D Ratios

Jitendra Patra1, Pritam Kumar Das1, Arnab Chakraborty1, P Chandrashekhar3 and Dinakrushna Pradhan3

1 Department of Power Engineering, Jadavpur University Salt Lake Campus, Kolkata, West Bengal India, 700098

2School of Mechanical Engineering, KIIT University, Bhubaneswar, Odisha, India, 751024

3Department of Mechanical Engineering, Capital Engineering College, Bhubaneswar, Odisha, India, 752005

Abstract

The present paper aims in the experimental investigation, upper bound analysis and computer simulation of upset-forging of sintered aluminium cylindrical preforms with different L/D ratios at different die speeds. A generalized theoretical expression for average forging load and preform bulged profile based on the exponential velocity field and strain rate has been established. The upset-forging experiments are conducted using the sintered preforms and various deformation characteristics like die velocity, effective strain and effective stress distribution are measure. Finally, the computer simulation is performed using DEFORM software and the deformation behavior are critically studied. The effect of die speed, i.e. dynamic effects on average die load has been investigated using energy and load factors. During the present investigations, it is found that inertia factor, external energy supplied and forging load increases with die speed.

Introduction Modern manufacturing uses new technology which has less material istage and more strength. Sinter-forging technology is one of them which combine the advantages associated with the powder metallurgy and conventional forging by using compressed and sintered metal powder preform as starting material for the manufacture of precision engineering components virtually without scrap. In this paper we have considered sintered preforms of different length to diameter (L/D) ratios and different

406 Jitendra Patra et al

die speeds. The effect of die velocity, preform aspect ratio on energy dissipations, and die loads are investigated and the results have been verified experimentally. Finally the simulation of upset-forging are performed using FEA tool and the results of both experimental and simulation are compared. Considerable amount of work has been reported on various technological aspects of the industrial processing of metal-powder preforms by Wang et al. [1], Biner and Spitzig [2] and Kim [3] have examined the densification mechanism and density distribution during powder compaction process under different hydrostatic pressures. Some work has done in the field of forging these metal powder compacts like Hwang and Kobayashi [4] applied finite element method to analyze the compaction pressures involved during powder metal compaction process. The aim of this paper is to find the dynamic effects of sinter forging process of cylindrical aluminum component at a complex inter frictional conditions. It is expected that the present research work will be highly useful for understanding the dynamic effects during upset-forging of sintered preforms, especially at different die speeds. The Experimental Work Preparation of Closed Dies and Aluminium Preforms: Compaction Forging is done by UTM by pressing the Aluminium powder in the closed die (Die material-EN 21 Steel, 3.47% Ni) consisting of lower plunger, die and upper punch as shown in figure 1. Aluminium powder of 200 mesh size with apparent density of 1.25 gm / c.c. is used for compaction. Graphite and mineral oil is used for lubrication. The applied load is 15-20 Tone. The green compact preforms obtained after compaction are then sintered in a muffle furnace at 5000C for 4 hrs and furnace cooled. The dimensions of obtained specimens are given in table 1.

Table 1 Dimension of Aluminium metal powder preforms

Specimens Dia (mm) Length (mm) l/d ratio 1 40 10 0.25 2 40 20 0.5 3 40 30 0.75 4 30 7.5 0.25 5 30 15 0.5 6 30 22.5 0.75 7 20 5 0.25 8 20 10 0.5 9 20 15 0.75

Open Die Forging of Solid Discs & Cylinders The compacted metal powder preforms are forged using 200T hydraulic press under lubricated interfacial friction condition. The preforms are forged at room temperature under open-die forging condition using flat and parallel die platens. The various

Experimental Study of Deformation Characteristics during Forging 407

deformation characteristics like percentage height reduction, percentage change in radius, corresponding forging load and preform relative densities are measured. The height reduction and change in the radius data are further converted into axial strain and radial strain data and the corresponding Poisson’s ratio is also calculated for all the measured readings. The densification rate, i.e. change of the relative density of the preform with respect to the axial strain is also calculated for different forging loads. Theoretical Analysis For analyzing problems in conventional high speed wrought material forging the assumptions are made in this analysis are, interfacial friction due to adhesion is a function of relative density and yielding of sintered material is sensitive to the hydrostatic stresses imposed during forging Singh [5]. Figure 4(a) & (b) shows the upset-forging of cylindrical sintered preform between two perfectly flat, parallel and rigid die platens, where upper die platen is stationary and lower die platen is moving upwards with velocity ‘U’. The present theoretical analysis is performed based on ‘Upper Bound’ approach. Analysis of Average Forging Load The external power (J*) supplied by the die platen during plastic deformation of sintered cylindrical preform using upper bound approach is given below by Avitzur [6]

Viip

SVijij

o dVUadSVdVJ

21

32

* (1)

The first term on the right side denotes the rate of internal energy dissipation ‘Wi’, the second term denotes the frictional shear energy losses ‘Wf’ and the last term denotes the energy dissipation due to inertia forces ‘Wa’. Thus, the above expression becomes: J* = Wi + Wf + Wa = PU (2) Substituting values after calculation the average forging pressure (P) may be computed. Fav = Pav. Aav (4) Hence, average forging load may be computed from equation 4. Analysis for Average Bulging Radius Considering an elemental strip of thickness ‘dh’ and radius ‘rd’ with in a deforming preform the final bulged radius of the elemental strip in deforming cylindrical sintered preform under consideration is given as:

he

dzerrrH

h

ddd2112

2110

(5)


Finite Element Simulation The simulation of cold forging of cylindrical aluminium preforms is performed using Deform TM -3D software, which is based on the implicit Langrangian finite element code. The stress-strain data for the sintered aluminium preforms are performed and the corresponding stress-strain curve data is generated and added to its library as in table 2.

Table 2 Material property of sintered Aluminium preforms

Sl. No. Material Property Value 1. Yield tensile strength 6.25 MPa 2. Ultimate tensile strength 18 MPa 4. Initial density 2103 Kg/mm3 5. Stress-strain relationship MPa22.06.100

Three different velocities of 0.01, 0.5, and 1m/sec with the material flow stress obtained through basic experiments being modeled depending upon the stress-strain curve data of form with c as 100, strain coefficient n = 0.22 is considered and updated in the material library of the software. The effective coefficient of interfacial friction is considered, i.e. 0.3 for lubricated frictional conditions. The coefficient of interfacial friction are obtained after performing ring compression tests under the lubricated (graphite paste) conditions. Tetrahedral elements are used to mesh the preforms and finer meshes are generated close to the face edges of preform. The complete sinter-forging simulation is performed in 20 steps having stroke movement of the die platens in each step equal to 0.22 mm. The deformation criterion considered is based on the maximum formability of the sintered aluminium preforms, which is experimentally found to be about 45 percent. The validation of the simulation is done by comparing its results with the experimental results and is found to reasonably agree with each other, which indicated that finite element simulation represents fairly well with the present sinter-forging process. Results and discussion To illustrate the effect of deformation characteristics and die speed on the magnitude of various power dissipations, die load and strains rates involved during upset-forging of cylindrical preform, a typical case with following data has been considered; geometry dimensions as mentioned in table1 with = 0.3, 0.35 and 0.4, 00 = 0.3Pav, = 0.3 , i = 2 x 103 kg/m3, 0 = 6.25 MPa, l/d ratios = 0.25, 0.5 and 0.75 with U= 0.01, 0.5 and 1m/s respectively). Figures 5 shows the variation of inertia factor (ratio of inertia energy dissipated to total external energy supplied by the press die platens) with die velocity and it is found to increases exponentially with die velocity and preform barreling factor, indicating that inertia power dissipation is appreciable at high-speed deformation.


Also, the curves are higher, at high barreling factor indicating increase in inertial power requirement. Figures 6 shows the variation of external energy supplied by die platens with die velocity and it is found to increases exponentially with die velocity. Figures 7 show that average die load increases linearly with increase in die velocity and also, the die load requirement is higher for lower barreling factor due to higher sintered material associated with them. Figure 8 shows that both radial and axial strain rates increases exponentially with preform height reduction. The strain rates encountered during upset-forging at higher speed are considerably high, for approx at 45-percentage height reduction for die velocity of 0.01 m/s. Figure 9 shows that relative density increases with increase in height reduction and becomes comparable to that of wrought materials at the end operation. Also the preforms with higher l/d ratio become more densified because of flow of preform material is better in case of slender preforms and at lubricated conditions. This completely agrees with the work of Fischmeister [7]. Figure 10 shows that as lower die travel increases preform deforms and slightly bulges radially because of die-workpiece interfacial friction which retards its plastic flow. Figure 11 shows that the surface of the preform has been displaced maximum at the middle and the upper and lower surface the minimum maintain proportionality due to the upper stationary die and lower moving die. The maximum formability of the sintered aluminium preforms is found to be about 45 percent at room temperature under lubricated conditions during experiments, which kept as the standard for the design criterion of the upset-forging process during simulation. Figure 12 shows that the maximum stress of the order of 146 MPa is found at the peripheral region and least at the central region. The high stress concentration is found to be at the edges of the preform and the possibility of fracture is high at these points. Distribution of effective strain is shown in figure 13 and it is clearly evident that effective strain in the range of 3-4 mm/mm is found at several places on the preform surface. Figure 14 shows that the maximum velocity in the order of 644mm/s is found on the vertical surfaces which are flowing radially outwards during upset-forging process and the least velocities are found on the interfacial regions of the preform. Figure 15and 16 shows the variation of the effective stress and effective strain for l/d ratios at different velocities with the upset-forging stroke or time and is found to increase exponentially initially and then remain fairly constant at the end of the upset-forging operation. The effective stress is found to be higher for lower l/d ratio preforms, whereas the effective strain is found to highest for the lower l/d ratio preforms


Fig.1 Close dies for compaction of Aluminium powder

Fig. 2 Metal powder compacts with 40, 30 and 20 mm dia. Respectively

Fig. 3 Metal powder preforms after forging


Fig. 4 (a) Forging of axi-symetric solid discs & (b) Metal powder preforms after forging under lubricated conditions


Fig. 11 Distribution of the total displacement on the preform surface


Conclusions The relative density of preforms increases with increase in die velocity, forging load and preform height reduction are becomes comparable with corresponding wrought materials at the end of operation. The present upset-forging is characterized by high magnitude of strain rates in the order of 2000 s-1 approx. The inertia energy dissipation has been found to increase with die velocity and become comparable with other energy dissipations at higher deformation speeds. The average die load increases linearly with increase in die velocity and also, the die load requirement is higher for lower barreling factor due to higher sintered material associated with them. The FEM simulation represents fairly well with the upset-forging process so it is expected that the present research work will be highly useful for understanding the upset-forging of sintered preforms. NOMENCLATURE ro = large radius of preform H0 = initial height of preform = stress o = flow stress of preform m = hydrostatic stress I = preform initial density 0 = preform relative density U = die velocity = strain = strain rate field t = forging time U = die velocity Fav = average forging load = frictional shear stress n = constant quantity = constant function of 0

= half cone angle of preform = coefficient of friction o = specific cohesion factor = barreling factor Aav = avarage area REFERENCES [1] Wang, James C, Nadkarni, Anil V. Theoretical analysis of powder compaction

and density distribution in long parts. International Journal of Powder Metallurgy1981; 37: 371.


[2] Biner S B and Spitzig W A. Densification of iron compacts with various initial porosities under hydrostatic pressure. Acta. Metall.1990; 38:603-610.

[3] Kim H S. Densification mechanism during hot isostatic pressing of stainless steel powder compacts. International Journal of Material Processing Technology 2002; 123: 319-322.

[4] Hwang B and Kobayashi S. Application of the finite element method to powdered metal compaction processes. International Journal of Machine Tool and Manufacture 1991;31:123.

[5] Singh S Jha A K. Analysis of dynamic effects during high-speed forging of sintered performs. Journal of Materials Processing Tech. 2001;112 : 53-62.

[6] Avitzur B and Sauerwine F. Limit Analysis of Hollow Disc Forging. Journal of Engineering for Ind. 1978; 100(3): 340-346.

[7] Fischmeister H F, Aren B, Easterling K E. Deformation and densification of porous preforms in hot forging. International Journal of Powder Metallurgy 1971;14(27) :144.

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