Finite Element Simulation of Forging of Aluminium ...

Finite Element Simulation of Forging of Aluminium Truncated Conical Sintered Preforms Y Shadangi, Non-member P Chandrasekhar, Associate Member Dr S Singh, Member The present paper deals with the three dimensional finite element simulation of sinter-forging of aluminium truncated conical preforms using DEFORM-3D software, which considers the heterogeneous deformation due to bulging of preform slant sides, composite die-workpiece interfacial friction conditions, preform densification along with compression and inertia effects. The variation of effective strain, effective stress, effective strain rate, forging load and internal energy dissipation with sinter-forging time were analyzed. The distribution of total displacement, effective stress and effective strain in form of contours on the preform surface were obtained. It was found that effective stress, effective strain, effective strain rate and internal energy dissipation increases with the sinter-forging time and die velocity. It was also noted that effective stress and strains were higher at the edges of preform. Keywords : Forging; Truncated conical; Sintered preform; Finite element simulation

NOTATIONS H0 : initial height of preform r0

: larger radius of preform

U

: die velocity

α

: half-cone angle of perform

ε

: effective strain

ρi

: preform initial density

ρ0

: preform relative density

σ0

: flow stress of preform

INTRODUCTION The technology of industrial processing of sintered powder preforms, which uses pressed and sintered metal powder preforms as starting material is used for mass production of precision engineering components at competitive rates with virtually zero scrap losses1,2. The mechanical and metallurgical properties of sinter-forged components compares favorably with those of wrought materials3,4 and finds extensive applications in automobile, aerospace and defense industries. Some of these applications are intricate engine components of automobiles, aircrafts and aerorockets5-11. The analysis during forging of sintered materials have been Y Shadangi is with the UltaTech Cement Ltd, Rawan, Raipur, Chhattisgarh; P Chandrasekhar and Dr S Singh are with the School of M echanical Engineering, KIIT University, Bhubaneswar 751 024, Orissa. This paper (modified) was received on July 21, 2011. Written discussion on the paper will be entertained till December 31, 2011.

Volume 92, September 2011

reported by different researchers from various aspects , but very minute attempt has been made so far to simulate the sinter-forging process characteristics based on finite element method12-18. The deformation of a sintered preforms during cold forging is influenced by several important factors like die-workpiece interfacial friction conditions, preform densification, bulging and yielding criterion19-21. It has been investigated and reported that during sinter-forging process, die-workpiece interfacial lubrication film is broken and conditions essential for adhesion friction are created leading to composite nature of interfacial friction conditions. The flow pattern of sintered materials during deformation suggests presence of two interfacial friction zones, ie, an inner zone with no relative movement between die and workpiece surface (sticking zone) and an outer sliding zone 22-24 . Also, compressive stresses gradually close down the inter-particle pores leading to decrease in its volume and subsequent increase in its relative density. The preform relative density almost approaches to that of the apparent density at the end of sinter-forging process and is attributed to the asymptotic increase in the real die-preform contact area2528. Thus, the yielding of sintered materials is sensitive to the compressive hydrostatic stress29-31. The vertical free surface of preform bulge out during sinter-forging process and its magnitude mainly depends on the initial relative density of preform and degree of frictional constraint at diepreform interface32,33. The present paper aims at analyzing various deformation characteristics during sinter-forging of aluminium truncated conical preform considering heterogeneous deformation due to barreling of slant sides, composite die-workpiece interfacial friction conditions and densification along with compression based on finite element simulation using DEFORM software. The die geometry was generated in DEFORM software, whereas the geometry of preforms were generated using CATIA software and later imported as STL 1

files. The basic experiments for simple compression tests were performed on sintered aluminium preforms and stressstrain curve data of the form σ = aε−b MPa was added to the material library of the software. The complete sinter-forging simulation was performed in 120 steps and the variation of effective strain, effective stress, effective strain rate, forging load and internal energy dissipation with sinter-forging time were analyzed. The distribution of effective stress and effective strain in form of color codes and contours on the preform surface were obtained. It is expected that the present research work will be useful for researchers to better understand the various characteristics of the sinter-forging process and their complex interactions. FEM SIMULATION The aluminium metal powder having physical and chemical characteristics as shown in Table 1 was used for the fabrication of the sintered preforms. The green compacts were produced by compacting the aluminium metal powder in a graphite lubricated compaction dies at recorded compacting pressure. These compacts were then sintered at about 400oC for four hours in an endothermic sand atmosphere and were finally machined and polished to the required dimensions. The material properties of the aluminium sintered preforms were measured and presented in Table 2. The simulation of cold sinter-forging of truncated conical aluminium preforms was performed using DEFORM-3D software, which is based on the implicit Lagrangian finite element code. The stress-strain data for the sintered aluminium preforms were not available in the library of the software, hence basic experiments related to simple compression tests were performed on sintered aluminium preforms and the corresponding stress-strain curve data of form σ = aε−b MPa were generated and added to its library.. Table 1 Physical and chemical characteristics of atomized aluminium metal powder Particle size, µ 118.0 88.1 65.6 48.8 36.3 27.0 17.4 13.0

Weight under, % 100.0 98.9 95.5 88.8 79.0 65.8 40.1 25.5

Chemical analysis Aluminium Iron Silicon Zinc Manganese Magnesium Apparent density Tap density

Weight, % 99.500 < 0.1700 < 0.1313 < 0.0053 < 0.0023 < 0.0016 1.25 gm / cc 1.50 gm / cc

The geometry of the forging dies were generated in DEFORM software and were modeled as rigid, parallel and flat bodies with plastic preform placed in between them. The geometry of preforms were generated using CATIA V5.0 software using part design module / workbench and data was imported to the DEFORM software in form of STL files. Three different preform dimensions with height equal to 17 mm, larger radius equal to 12 mm and half cone angle equal to 15o, 10o and 5o, respectively were considered. Two different values for effective coefficient of interfacial friction were considered, ie, 0.3 and 0.6 for lubricated and unlubricated frictional conditions respectively. The coefficient of interfacial friction were obtained after performing ring compression tests under the dry and lubricated (graphite paste) conditions. Tetrahedral elements were used to mesh the preforms and small meshes were generated close to the face edges of truncated conical preforms in order to better scope the sinter-forging process. The complete sinter-forging simulation was performed in 120 steps having stroke movement of the die platens in each step equal to 0.065 mm. The deformation criterion considered is based on the maximum formability of the sintered aluminium preforms, which was experimentally found to be about 45%. The variation of effective strain, effective stress, effective strain rate, forging load and internal energy dissipation with sinter-forging time were plotted for two different die velocities, ie, 0.001 m/s and 1.5 m/s. The distribution of total displacement, effective stress, effective strain and velocity vector in form of color codes and contours on the preform surface were obtained. To investigate these distributions over the vertical profile, preform slicing was done and the results were again plotted. The effect of die speed, ie, dynamic effects and preform shape on the various sinterforging characteristics were critically investigated. The validation of the simulation was done by comparing its results with the experimental results and was 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 Figure 1 shows the preform profile during various stages of sinter-forging. It is evident that as lower die travel increases preform deforms, its height decreases and radius increases with slight bulging. This is due to the formation of conical wedge of relatively undeformed metal immediately below the preform surface, where die-workpiece interfacial friction retards its plastic flow. To illustrate the distribution of stress at the various height sections of the preform, the preform was sliced vertically and the stress distribution in form of

Table 2 Material property of aluminium sintered preform. Serial number 1. 2. 4.

2

Material property Yield tensile strength, MPa Ultimate tensile strength, MPa Initial density, kg/mm3

Value 6.25 18 2 103

(a)

(b)

(c)

(d)

Figure 1 Sintered preform profiles during forging process

IE(I) Journal – PR

H

140

H

H H

Effective stress, MPa

120 C

H F

E

H H

G C

α = 5o α = 10o α = 15o

H

G H

C

D

G

100 80 60 40 r0 = 12 mm, H0 = 17 mm, U = 0.001 m/s, ρi = 2 × 103 kg/m 3, σ0 = 6.25 MPa

20 0 0

A = 0, B = 17.2, C = 34.3, D = 61.5, E = 68.7, F = 85.8, G = 103, H = 120, I = 137

1

2

12

4.59 3.67 2.75 1.84 0.918 Z X

0.000

Y

Figure 3 Effective strain contour profile on sliced preform

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8

8 6 4 2

r0 = 12 mm, H0 = 17 mm, U = 0.001 m/s, ρi = 2 × 103 kg/m 3, σ0 = 6.25 MPa

0 0

1

2

3

4

5

6

7

8

Sinter-forging time, s Figure 5 Variation of forging load

forging load increases almost linearly during the sinter-forging process. It is also found to be higher for smaller half-cone angles, as more tapered preforms requires lesser load for deformation. Figure 6 shows the variation of internal energy 350 Internal energy dissipation, kN-mm

Strain-effective, mm/mm

6

α = 5o α = 10o α = 15o

10

Figure 5 shows the variation of average forging load with the sinter-forging time. It is clear from the figure that average Step 120

5

Figure 4 Variation of effective stress

Forging load, kN

The distribution of effective strain is shown in Figure 3 and it is clearly evident that effective strain of the order of about 4 mm/mm to 5 mm/mm was found at several places on preform surface. The edges of preform were severely strained and the chances of fracture were maximum at those regions. Also, the central regions of sliced vertical profiles of the preforms experienced strains in the magnitude of about 0.7mm/mm to 0.9 mm/mm. Figure 4 shows the variation of effective stress with sinter-forging time for die velocity 0.001 m/s. It was found to increase exponentially initially and then remain fairly constant at the end of the sinter-forging process, as well as, found to be higher for lower die velocity and smaller half-cone angles.

4

Sinter-forging time, s

Figure 2 Effective stress contour profile on sliced preform

contours were also plotted as shown in Figure 2. It may be noted that nine tracking points were considered over the preform surface at the various node points of the meshes and the stress values were measured at these points. The maximum stress of the order of 140 MPa was found at the peripheral region and about 100 MPa in the central region of the preform. The stress concentrations were found to be at the edges of the preform and the chances of the preform fracture were maximum at these regions.

3

r0 = 12 mm, H0 = 17 mm, U = 0.001 m/s, ρi = 2 × 103 kg/m 3, σ0 = 6.25 MPa

300 250 200 150 100

α = 5o α = 10o α = 15o

50 0 0

1

2

3

4

5

6

7

8

Sinter-forging time, s Figure 6 Variation of internal energy dissipation

3

r0 = 12 mm, H0 = 17 mm, U = 0.001 m/s, ρi = 2 × 103 kg/m 3, σ0 = 6.25 MPa

conical preform approaches that of an enclosing cylinder, internal energy requirement also increases. The higher forging load and internal energy dissipations were reported for higher die velocities, which indicates that the die velocity, ie, dynamic effects appreciably, influence the present process.

– 450

Axial strain rate, εz, /s

– 400

0 0.2000 0.4000 0.6000 0.8000 1.0000 1.2000 1.4000 1.6000

– 350 – 300 – 250 – 200 – 150 – 100

"

The distribution of effective stress and effective strain over the preform surfaces showed that preform edges were severely stressed and strained and were the most probable regions of crack initialization. By slicing the preforms in vertical plane, it was also found out that central region of preform was subjected to stress and strain of about 100 MPa and 0.8, respectively.

"

The present sinter-forging process is characterized by high magnitude of strain rates, of the order of 500/ s approximately at die speed of 1.5 m/s. The relation between axial and radial strain rates, ie, Poisson's ratio was found to vary with the die velocity.

– 50 0 0

25

50

75

100

125

150

Radial strain rate, εr, /s Figure 7 Contour plot of radial and axial strain rate

dissipation with the sinter-forging time. It is clearly evident that internal energy dissipation increases as the process proceeds and is higher for smaller half-cone angles. This indicates that as the shape of truncated conical preform approaches towards that of an enclosing cylinder, internal energy requirement also increases. Figure 7 shows the contour plot of radial and axial strain rates for different die velocities during sinter-forging process. It can be seen that both radial and axial strain rates increases rapidly with the preform height reduction during the process. The axial strain rates encountered during the present process are considerably high, for example, 500/s approximately at 45% height reduction for die velocity of 1.5 m/s. The contour plot also gives the measure of the Poisson’s ratio of the sintered material, which is varying in nature due to the closing of inter-particle pores and densification of the preforms. CONCLUSIONS The present research work may be summarized into following major conclusions: "

"

"

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The maximum formability of sintered conical aluminium preforms at room temperature under lubricated interfacial frictional conditions was found to be about 45 %, which was used as the criterion for present FEM simulation. The effective stress, effective strain and forging load were found to increase during the sinter-forging process. The effective strain increased gradually initially and fairly rapidly later on during the sinterforging process, which is attributed due to the sim ultaneous compaction and compression phenomenon occurring during the process, where compaction (densification) dom inates the compression (deformation) initially and vice-versa in the final stages of the deformation. The forging load and internal energy dissipation were found to be higher for smaller half-cone angles of preform. This indicates that as the shape of truncated

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