Ductility is an essential property of material for its formability. However, ductility is not something like absolute constant for a metal or alloy under all conditions. In fact, it gets modified by the process parameters that is why the same material may show different formability in different forming processes.
Ductility is measured by the strain suffered by the material before fracture. In a tensile test it may be measured by percent elongation in engineering terms, or by logarithmic strain at the fracture point. In compression test similar measures may be used. In torsion test it is measured by the strain suffered by outer layer of material of test bar before fracture.
The tensile tests show low ductility because of neck formation and consequently the negative hydrostatic pressure in the neck region promotes crack initiation and propagation. This problem is not there in compression and torsion tests which show higher ductility for the same material. Many researchers have preferred torsion test for measurement of ductility while the strength properties are related to those measured in tensile test.
Factors that Affect Ductility of Metals:
Ductility is affected by intrinsic factors like composition, grain size, cell structure etc., as well as by external factors like hydrostatic pressure, temperature, plastic deformation already suffered etc.
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Some important observations about ductility are given below:
(i) Metals with FCC and BCC crystal structure show higher ductility at high temperatures compared to those with HCP crystal structure.
(ii) Grain size has significant influence on ductility. Many alloys show super-plastic behavior when grain size is very small of the order of few microns.
(iii) Steels with higher oxygen content show low ductility.
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(iv) In some alloys impurities even in very small percentages have significant effect on ductility. Ductility of carbon steels containing sulfur impurity as small as 0.018%, drastically decreases ductility at around 1040°C. This can however be remedied if Mn content is high. In fact the ratio Mn/S is the factor which can alter ductility of carbon steels at 1040°C. With the value of this ratio at 2 the percent elongation is only 12-15% at 1040°C while with ratio of 14 it is 110 per cent.
(v) Temperature is a major factor that influences ductility and hence formability. In general it increases ductility, however, ductility may decrease at certain temperatures due to phase transformation and micro-structural changes brought about by increase in temperature. Figure 1.15 shows the effect of temperature on ductility of stainless steel. It has low ductility at 1050°C and maximum at 1350°C. Therefore it has a very narrow hot working range.
(vi) Hydrostatic pressure increases ductility. This observation was first made by Bridgeman. In torsion tests the length of specimen decreases with increase in torsion. If the specimen is subjected to an axial compressive stress in torsion test it shows higher ductility than when there is no axial stress. If a tensile axial stress is applied the ductility decreases still further.
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(vii) The temperatures for optimum ductility are also affected by concentration of alloying element. Figure 1.16 shows the temperatures for optimum ductility for different concentration of carbon in plain carbon steels.
(viii) Duplex microstructures generally lead to lower ductility. For example, low carbon steel when tested in (α + γ) range shows lower ductility.
(ix) Strain rate also influences ductility. Tests done in tension have shown that increase in strain rate increases the elongation in uniform deformation region and decreases the same in the neck region.
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For obtaining the effect of strain rate the observed values must be corrected for change in temperature, because, at high strain rates there is considerable increase in temperature of material which increases ductility. More work is needed to establish a conclusive effect of strain rate on ductility.
Theories of Fracture:
Occurrence of fracture in work material is a natural limit of formability. Therefore, it is very important to know the conditions under which fracture initiates and propagates. Several criteria have been tested. Tensile stress is regarded as a key factor. Even in forging processes wherein the material suffers predominantly compressive stresses the fracture is believed to be initiated by induced tensile stresses.
A second opinion is that fracture occurs due to accumulated strain rather than stress. Octahedral shear strain has also been suggested as a basis for fracture to occur, however, experiments show lot of scatter. Cockcroft and Latham have proposed the following criterion for occurrence of fracture in a ductile material at the given temperature and strain rate.
here ԑ̅f is effective fracture strain in torsion. The above criterion has been successfully tested by Sellars and Tegart with the help of hot torsion test data on aluminum, copper and nickel in the temperature range of 0.5 – 0.8 Tm and at strain rates of 1 and 8 s-1.
Super-Plasticity:
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It is the property of material to suffer neck free elongation of several hundred per cent. For instance, in zinc-22 Al eutectoid alloy an elongation of 2900 per cent has been achieved in the temperature range of 293-573K. In Fe-26Cr 015-6.5 Ni alloy an elongation of more than 1000 per cent may be achieved in the temperature range of 973-1293K.
In fact many metals and alloys show this property in certain temperature ranges and grain sizes. Super-plasticity may also be achieved by thermal cycling across the phase transformation temperature while the work piece is kept stretched.
The factors common to many super-plastic metals and alloys are given below:
(i) High value of strain rate exponent m, (0.3 to 0.5).
(ii) Small grain size of the order of a few micron.
(iii) Very little work hardening.
Some of the manufacturing processes used for thermoplastic materials have been successfully employed for forming metals in super-plastic condition. For example, processes such as blow forming and vacuum forming have also been used to form super-plastic alloys.
A large number of alloys show super-plastic properties but at different temperatures and grain sizes.
The different super-plastic materials may be grouped into the following three types:
(i) Low temperature alloys of Zn and Al.
(ii) Intermediate temperature alloys of Al.
(iii) High temperature alloys of nickel, some stainless steels and titanium alloys.
On experimentation with super alloy IN 718 which has application as material for turbine blades for aircrafts, Kashyap and Chaturvedi obtained maximum ductility of 485% at a strain rate of 1 x 10-4 s-1 and temperature of 1198 K. They also noted a decrease of ductility with increase in temperature. In fact, more research work is needed to established behavior of yield strength and ductility of super alloys to strain rate, strain hardening and temperature.
Anisotropy in Sheet Metal:
During cold rolling of sheet metal, the material develops anisotropy. The flow strength in thickness direction becomes different to that in the plane of sheet. For testing the anisotropy, the test specimen are cut at an angle to rolling direction. Let l1, w1 and t1 be the length, width and thickness of under formed specimen respectively and l2, w2 and t2 be the corresponding values after some plastic deformation in the tension test.
The three principal strains are as given below:
In length direction ԑl = ln (l2/l1)
In width direction ԑw = ln (w2/w1)
In the thickness direction ԑt = ln (t2/t1)
In an isotropic sheet ԑw = ԑt, however, in a cold rolled sheet ԑw ≠ ԑt. The anisotropy ratio r is defined as
For evaluation of r, test specimen are cut in three directions, i.e. 0, 45 and 90 degrees with respect to the rolling direction. The three values of r, i.e. r0, r45 and r90 are determined.
The difference (∆ r) in r values is indicator of planer anisotropy which is defined as follows:
∆ r = rmax – rmin ……… (1.19)
However, it is usually taken as an average variation, as given below, (see Fig. 1.17).
The value of ra more than unity is an indicator that the sheet is stronger in the thickness direction. This reduces thinning and neck formation in the sheet at the highly stressed locations during deep drawing and hence enhances draw ability.