Adv Materials

8/3/2019 Adv Materials

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SUPERPLASTICITY & FORMING OF ADVANCED MATERIALS

Superplasticity is the capability to deform crystalline solids in tension to unusually large plastic

strains, often well in excess of 1000%. This phenomenon results from the ability of the material

to resist localized deformation much the same as hot glass does. As high elongations are possible,

complex contoured parts can be formed in a single press cycle often eliminating the need for

multipart fabrications. This enables the designer to capture several detail parts into a one piece

complex, formed structure. Thus materials with superplastic properties can be used to form

complex components in shapes that are very near the final dimension. Superplastic forming also

enhances design freedom, minimizes the amount of scrap produced, and reduces the need for

machining. In addition, it reduces the amount of material used, thereby lowering overall material

costs.

(i) Superplastic Forming of “commercial purity” Aluminium

Present study indicates that using optimum pressure cycle (strain rates) it is possible to

superplastically form inexpensive commercial purity aluminum (AA1100) to the same extent as

some of the commercially available superplastic aluminum alloys (AA7475) in a die with

relatively simple configuration.

Figure 1. Thickness Profiles of the Formed Cones (AA1100 vs AA7475)

1100

7475



(ii) High Strain Rate Superplasticity in Metal-Matrix Composites

P/M 6061 Al and composites with different SiC p volume fraction were studied in this work.

Tensile tests were conducted at 653K~871K and 0.1s-1. High Strain Rate Superplasticity (HSRS)

is possible and the accompanying mechanisms are identified. The SiC p

reinforcement has a

softening effect on P/M 6061 Al and composites during HSRS deformation. Mechanical analyses

show that the decrease in the strength during HSRS deformation is associated with the fine grain

size of the 6061 Al matrix. From the microstructural aspect, the decrease in the strength is due to

the occurrence of liquid phase. The formation of liquid phase results from the segregation of Mg

at grain boundaries and SiC p/Al matrix interfaces. The occurrence of liquid phase and composites

reduces the strength of P/M 6061 Al and composites during HSRS deformation, and enhances

their ability to achieve large elongation-to-failure. However, the grain size of the Al matrix

appears to play a more important role in HSRS than the reinforcement content.

Fig 2 TEM microstructure of as-extruded specimens of 6061 Al and composites.

(a).Al6061(unreinforced, 20); (b). MMC1 (10vol%SiC p, 20);

(c). MMC2 (18vol%SiC p, 20); (d). MMC3 (10vol%SiC p, 47);

(a)

1 µm1 µm

(b)

1 µm

1 µm

(c) (d)

1 µm



(iii) 2-Stage Deformation of Magnesium Alloy

Recognizing the capability of Mg-3Al-1Zn to dynamically recrystallize, the ‘Two-Stage

Deformation Method’ was proposed to further enhance superplasticity. In this method, the first

stage (Stage I) of deformation is aimed at refining the coarse microstructure through DRXutilizing the optimum DRX conditions; while the second stage (Stage II) is carried out at the

optimum conditions that have been earlier identified for superplastic deformation of coarse-

grained Mg-3Al-1Zn, i.e. 1×10-4s-1 at 400°C and 2×10-4s-1 at 450°C. Table 1 lists the results of the

elevated temperature tests performed using single and two-stage methods.

Table 1. Experimental conditions and elongation-to-failure for samples tested using single and

two-stage deformations. All samples were tested at constant strain rates.

Sample

No.

Test temperature,

°CStrain rate, s

-1Strain*, %

Elongation-

to-failure,

±5%

1 250 1×10-4 − 1402 400 1×10

-4 − 250

3 250 400 1×10-4

60 3204 450 2×10

-4 − 265

5 250 450 1×10-4

2×10-4

60 360* Strain where temperature or/and strain rate change occurred. The time required for raising the

temperature to that of Stage II was about 10 to 15 minutes.

Although Sample 1 was deformed at optimum DRX conditions and possessed a completely

recrystallized microstructure, its elongation-to-failure was limited to 140% due to cavitation

failure. Sample 2 was independently deformed at 400°C and 1×10 -4s-1 achieving 250% and

resulting in coarse-grained microstructure.

Figure 3. Specimens of the two-stage deformation tests. (a) Undeformed sample with initial

gauge length of 15mm. (b) Sample 3 failed at 320%, employing Stage I: 250°C, 1×10-4s-1 to 60%;

followed by Stage II: 400°C, 1×10-4s-1 to fracture. (c) Sample 5 fractured at 360%, using Stage I:250°C, 1×10-4s-1 to 60%; followed by Stage II: 450°C, 2×10 -4s-1. All samples failed by necking,

as indicated by the arrows.

(a) Undeformed sample

Gauge=15mm

(b) T=250°C 400°C

ε & =1×10-4s-1

f ε =320%

(c) T=250°C 450°C

ε & =1×10-4 2×10-4s-1

f ε =360%

15 mm

69 mm

63 mm



(iv) Grain Boundary Character Distribution (GBCD) on Cavitation Behaviour of

Superplastic Aluminium

The sliding capability of grain boundaries is inhomogeneous due to the anisotropic GBCD.

Cavities are nucleated at sites where high stress concentrations have been induced by GBS and

where the local stress exceeds the bonding strength of the particle to the boundary, or the

boundary to the boundary. There are interactions between the DFZ formation and the cavity

nucleation. The DFZ formation or the cavity nucleation may depend on the local stress condition

and the grain boundary bonding strength. The formation of cavity nuclei at the grain boundaries

will lower the potential of DFZ formation in their vicinity, and vice versa.

The GBCD, especially the grain boundary bonding strength at test temperature, is the key factor

controlling the formation of cavity stringers. The presence of liquid phase on the grain boundaries

significantly reduces the bonding strength. The formation of preferential cavity stringer is closely

related to the GBCD, GBS and grain boundary debonding. Based on this, a model of cavitystringer formation was proposed.

(a) (b)

(c)Fig. 4 Surface morphologies of the samples after superplastic deformation at 516°C, 10-3s-1.

(a) the T-sample strained to fracture, (b) the L-sample after fracture, (c) schematic illustration

of the configuration variation of the grains shown in figure (b). The long continuous filament

AD and CE showed that the originally contiguous Grains A and D, C and E were far

separated, while the originally contiguous grains B and C were connected via a relatively

short filament BC. Stressed horizontally.

AB C

DE

(i) Configuration of grains A, B, C, D

and E before deformation

(ii) Configuration of grains A, B, C, D

and E after deformation



(v) 3-D Modelling of Superplastic Forming Process

Finite element simulations for 3D SPF processes presented . The commercial code

ABAQUS/Standard was used as computational tool. For verification of the numerical results, the

experiment of conical bulging was carried out and the good agreement of thickness distribution

between the numerical results and the experimental results was obtained. In general the material

flow is constrained by a bead at a constant distance from the die profile in the practical forming process. So the draw in effect should be considered in finite element simulation in order to get

more accurate results.

The influence of friction depends on the bulging type. In conical bulging, small frictional

coefficient can improve the inhomogeneity of the thickness distribution. For the rectangle box

bulging, as the friction decreases, the filling ability of the sheet towards to the die corner and the

inhomogeneity of thickness distribution can also be improved, but the sheets near the polar top of

the die are thinned more seriously. In this case, it is not true that, the smaller the friction, the

better the formed part. Lubrication should be performed appropriately. A high m value can

improve the inhomogeneity of the thickness distribution, but more forming time will be needed.

Because the m value depends strongly on the strain rate of the deformation process, it is

recommended to search for an optimum strain rate such that the component can be formed as fastas possible but still under the superplastic conditions.

(a) t=183s

(b) t=659 s

Fig.5 Deformed meshes at different forming time



0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 5 10 15 20 25 30 35 40 45 50

Initial distance from center (mm)

T h i c k n e s s ( m m )

t=3500s

t=2880s

t=2280s

t=880s

Fig.6 The thickness distribution at different forming time

Acknowledgement: JT ARC 4/95 Grant from MOE & A*STAR.

Related Publications:

1. S.N.Patankar, Tan Ming Jen, “Superplastic Forming of Commercial Purity Aluminium”

(Scripta Materialia, Vol.38 (1) (1998) pp 145-148)

2. J.C.Tan, M.J.Tan, “Superplasticity in a rolled Mg-3Al-1Zn alloy by two-stage deformation

method” (Scripta Materialia, Vol. 47 (2002) pp101-106)3. X.Li, M.J.Tan, “High Strain Rate Superplasticity and deformation mechanisms of powder

metallurgy 6061 Al/SiCp composites” (Mater. Sci. & Tech., Vol. 18 (5) (2002) pp581-585)

4. C.L.Chen, M.J.Tan, "Effect of grain boundary character distribution (GBCD) on the

cavitation behaviour during superplastic deformation of Al7475" (Materials Science and

Engineering A Vol. 338 (2002) pp243-252)

5. G.Y.Li, M.J.Tan, K.M.Liew, "Three-dimensional modelling of superplastic forming and

analysis of influence of various factors on the forming processes" in (Advances in Concurrent

Engineering, Editors: Jardim-Goncalves, Roy & Steiger-Garcao, Publisher : Swets &

Zeitlinger B.V., Lisse, The Netherlands, 2002,ISBN: 90 5809 502 9, pages 297-304)

6. S.Thiruvarudchelvan, M.J.Tan, “Two Novel Techniques for Forming Regularly Spaced Deep

Recesses on Aluminium Sheet Panels” (Materials Science Forum – accepted for publication)

Tan M.J., S.Thiruvarudchelvan, and Liew K.M.

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