Title of Invention	"A PROCESS FOR PREPARATION OF IMPROVED SUPERPLASTIC FORMING OF HEMISPHERICAL DOMES AND DEEP CUPS FROM SUPERPLASTIC."
Abstract	A process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of rolling said alloy metal sheet, clamping the rolled metal alloy sheet to be superplastically formed between a flat die and a shaped die, heating the whole assembly in a furnance to the forming temperature in the range of 0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the metal sheet to be super-plastically formed, blowing a gas such as argon through a tube at a pressure for a pressure-time profile determined as herein described.

Title of Invention

"A PROCESS FOR PREPARATION OF IMPROVED SUPERPLASTIC FORMING OF HEMISPHERICAL DOMES AND DEEP CUPS FROM SUPERPLASTIC."

Abstract

A process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of rolling said alloy metal sheet, clamping the rolled metal alloy sheet to be superplastically formed between a flat die and a shaped die, heating the whole assembly in a furnance to the forming temperature in the range of 0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the metal sheet to be super-plastically formed, blowing a gas such as argon through a tube at a pressure for a pressure-time profile determined as herein described.

Full Text	This invention relates to a process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron etc. Superplastic forming is an advanced technique of forming of metallic sheets, by means of gas pressure which is very similar to soap bubble forming or molten glass blowing techniques. In this process there is no need of any matched tooling like die-punch and the process is very simple. A tubular furnace, a simple and small hydraulic press and dies are the only accessories required. The sheet or the blank is clamped between a flat die and a shaped die (hemispherical or cup-shape) by applying pressure through the rams of a hydraulic press. The entire die-blank assembly is heated to the hot working temperature (0.4 to 0.6 Tm where Tm= Melting point of the material in absolute scale). Gas pressure is then applied over the blank through a stainless steel pipe fitted to the flat die. The key to a successful (i.e puncture free) forming, however, lies in maintaining a constant strain-rate during the process. This can be achieved only by maintaining suitably designed gas pressure Vs time sequence. Superplastic forming is a relatively new process, introduced only two decades ago. Earlier superplastic blow forming was carried out by 'guess' or 'trial and error' method. In some processes, even sheets were formed under constant gas pressure, which resulted in a faulty and undesirable strain rate for superplastic forming due to which puncture free forming was not possible. The further limitation of these processes is that while hemispheres can be formed, it is not possible to form very deep cup shapes as these processes may cause premature repture and wide thickness variations which is undesirable. The rupture free superplastic forming of hemispheres and deep cups requires designing of an accurate gas pressure time sequence. A few mathematical methods for pressure design are known in the art. One such method is by Jovane. In this method, instead of trying to keep strain rate constant, which is essential for perfect superplastic forming, he allowed the strain rate to vary between upper and lower pressure - time profile which enables repeatability in superplastic forming of hemispheres and deep cups. Still further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which enables forming very deep cups with height to radius ratio of more than 2, which is extremely difficult to achieve by other presently known pressure- time designs. Yet further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which permits failure free forming of poorly superplastic materials characterised by their typically low strain rate sensitivity m According to the invention there is provided a process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of: a. rolling said alloy metal sheet, b. clamping the rolled metal alloy sheet to be superplastically formed between a flat die and a shaped die, c. heating the whole assembly in a fur nance temperature in the range of 0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the metal sheet to be super-plastically formed, characterized by, d. blowing a inert gas like argon through a tube at a pressure for a pressure-time profile determined as herein described. limits. Hence this method did not provide the precise pressure time sequence necessary for rupture-free superplastic forming. Ghosh and Hamilton considered the constant strain rate and effective strain rate criterion for "pressure-time" sequence. The shortcoming of the method proposed by these authors is that this method did not incorporate correction required for the inevitable thickness variations that take place during forming of the sheet. Another limitation of this method is that it does not consider the strain rate sensitivity m of the material. Thus, this method also has the limitation that it cannot provide accurate pressure-time profile. Thus the main limitation of the known methods is their inability to provide accurate pressure-time profile considering the thickness variations that arise in the dome during superplastic forming. This limitation of the known methods adversely affects the known superplastic forming processes in providing rupture free process . Another limitation that arises due to inaacurate pressure-timee profile is that it is very difficult to form metal into deep shaped cups or to form poorly superplastic materials characterised by their low strain rate sensitivity m having value less than the value of 0.3 (i.e. m The primary object of the present invention is to propose an improved superplastic forming process for forming of superplastic alloys like titanium alloys, aluminium alloys, magnesium alloys, zirconium alloys, iron alloys etc, based on accurate pressure -time profile. Another object of the present invention is to propose an improved superplastic forming process based on accurate pressure time profile and taking into consideration the phenomenon of thickness variation in addition to effective strain rate critereon and constant strain rate formulation. Still another object of the present invention is to propose an superplastic forming process based on accurate pressure -time profile which enables failure free (i.e without rupture or puncture) superplastic forming of hemispheres and deep cups. Further object of the present invntion is to propose an improved superplastic forming process based on accurate pressure - time profile which enables repeatability in superplastic forming of hemispheres and deep cups. Still further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which enables forming very deep cups with height to radius ratio of more than 2, which is extremely difficult to achieve by other presently known pressure- time designs. Yet further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which permits failure free forming of poorly superplastic materials characterised by their typically low strain rate sensitivity m According to the invention there is provided a process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of: a. rolling said alloy metal sheet, b. clamping the rolled metal alloy sheet to be superplastically formed between a flat die and a shaped die, c. heating the whole assembly in a furnance temperature in the range of 0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the metal sheet to be super-plastically formed., d. blowing a gas like argon through a tube at a pressure for a pressure-time profile determined as herein described. ( σ ) is the effective flow stress, ( ε ) is effective strain rate, (m) is strain rate sensitivity, (a) is die radius and (So) is initial sheet thickness. In accordanace with the present invention, the improved superplastic forming process proposed in the present invention, enables superplastic forming of hemispheres and cups. The improved process is based on accurate pressure - time profile which makes the process failure free, ie. it enables forming of hemispheres and domes without rupture or puncture. The accurate pressure time design of the process makes it possible to form very deep cups with height to radius ratio of more than 2, which are extremely difficult to achieve by conventional processes. It is usually believed that superplasticity ceases to exist when strain rate sensitivity (m) value is less than 0.3. However, the proposed process enables failure free forming of such poorly superplastic materials which are characterised by poor strain rate sensitivity(m), m Foregoing features of the presently disclosed invention will be more apparent from following description read in conjunc¬tion with the following figures, which are intended to illus¬trate an apparatus used for metal forming and two applicatio'n areas of the invention, without intending to limit any way, the scope of the invention, wherein Fig 1: shows the apparatus for superplastic forming Fig 2: shows different stages of blowing Ti-alloy hemishpere Fig 3: shows deep cup shape for Ti-alloy sheet According to the proposed process, the superplastic forming process is carried out by an apparatus shown in fig (1) , wherein, (1) is an open ended cylinderical die and (2) is a flat die. The metal sheet (3) to be superplastically formed is clamped by fixing its rims (5) and (6) between open-ended cylinderical die (l) and the flat die (2) . The flat die (2) is pressed against the cylinderical die (1) by applying pressure by a hydraulic press. The flat die (2) has a steel pipe (4) attached at its middle through which inert gas like Argon is passed to exert gas pressure on to the metal sheet (3) . The pressure of the argon gas enclosed between the flat-die (2) and the metal formed sheet (3) is monitored by a pressure gauge attached to a gas regulator or a separate pressure guage attached to the gas line. The novelty of the apparatus lies in the use of a simple hole-die for forming of hemisphere as well as dep cup. No shaped die is used, thus making the process very inexpensive. According to the process of the present invention superplastic blow forming of hemispheres and deep cups comprises of the following steps:- (a) rolling the billet of a two-phase alloy material to be superplastically-formed, by a thermo- mechanical processing treatment to obtain grain size below 10 microns (b) determining the stress values of the sheet; obtained by step (a) for 10-15 different values of strain rates ranging between 10~5 to 10~7 sec-1 by testing tensile test pieces in mechanical testing machine. (c) ploting stress Vs strain rate curve in logarithnic scale and determining maximum strain rate sensitivity as the slope of the log(stress) Vs. log(strain rate) fitted curve. (d) repeating operations (b) and (c) at 4 or 5 hot working tempe¬ ratures to be taken within the range of 0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the sheet to be formed. (e) choosing the best temperature for forming (ie. temperature where strain rate sensitivity 'm1 is maximum). (f) noting the flow stress and strain rates corresponding to maximum strain rate sensitivity m and then determining the effec¬ tive flow stress ( £ ) and effective strain rate (=) at this maximum value of m. Effective stress and strain rates are calcu¬ lated from tensile flow stress and strain by assuming Von Mises Critereon. (g) substituting the values of the parameters namely (Sr) is the effective flow stress, ( ε) is effective strain rate, (m) is strain rate sensitivity, (a) is die radius and (So) is initial sheet thickness, which accurately determines the gas pressure (P) required to be applied at any point of time (t) : (Equation Removed) where f(t)= e + e (h) clamping the metal alloy sheet to be superplastically formed between a flat die and a shaped die by applying pressure from a hydraulic press as shown in Fig.1. (j) heating the whole assembly in a tubular furnace to the requi¬red hot working temperature to be taken within the range of 0.4 to 0.6 times the melting point). (k) blowing the argon gas through a stainless steel pipe attached to the flat die. The required forming pressure is varied accor¬ding to the pressure Vs. time sequence calculated from above formula. The pressure is monitored from the guage attached to the gas regulator or alternatively a separate pressure gauge is fitted in the gas line. The total forming time for a hemisphe¬rical shape would be total strain divided by strain rate (t=0.69/£). (1) The same procedure is followed for deep cup shape also, except that beyond formation of the hemisphere i.e., after completion of step (j) mentioned above, and after time (0.69/ε ), the pressure is calculated iteratively from equation (1) upto the desired strain level of the cteep cup (e.g, l.O, 1.5, etc). The total required time would then be (t = strain/ε). EXAMPLES EXAMPLE - I Forming of a Ti-alloy hemisphere) A 2.4 mm thick sheet (So=2.4 mm) of titanium alloy (Russian designation VT-9) was rolled to obtain a fine grain size of 4 microns. Incremental strain rate test was carried out on test samples and the maximum strain rate sensitivity was 0.85 (m = 0.85). at a temperature of 1173°K. The corresponding flow stress was 7.06 MPa (σ =7.06 MPa) and the strain rate was 3.3x10 -4sec-i (ε=3 3xio4) Sec-1) . A thick walled cylindrical die as shown in fig 1 was used, so that both hemispherical shapes and cup shapes could be blown in the same die. The radius of the internal cavity was 30.3 mm (a = 30.3 mm). The die and the sheet assembly was heated to 1173 k in a furnace. The whole assembly was1 pressed tightly in a 20 Ton hydraulic press. All the above parameters were fitted in the equation for 'Pressure Vs time' (P - t) and the foming was carried out by applying gas pressure according to the sequence so obtained. The test was interruptred at 14 mins and 21 mins to monitor the progress of deformation (fig 2) . The full hemispherical shape is formed after 35 mins as predicted the formula t= 0.69/ε EXAMPLE - II Forming of deep cup from Ti-alloy A deep cup shape (cup depth = 66 mm and radius 30.3 mm) was formed in the same die as mentioned above by continuing the forming cycle mentioned above, beyond the formation of the hemisphere (for hemisphere strain 0.69) upto a larger strain of 1.48 (equivalent to 66 mm depth) . The shape as illustrated in fig 3 could be formed in 75 mins by applying pressure according to the pressure time equation, as proposed in this invention and described in step (g) of the process. EXAMPLE - III Forming of Mg-Li alloy hemisphere A Mg-ll%Li alloy was rolled to 2 mm thickness (So=2mm). The maximum strain rate sensitivity (m=0.2) was obtained at a temperature of 673k; The corresponding flow stress σ =37MPa and strain rate ε = lo-3Sec-1 as determined from the stress-strain rate curves. The same die of 30.3mm radius was used. The pressure-time cycle was then calculated from equation l by substituting the above values. The complete rupture free hemisphere could be formed even with a poor strain rate sensitivity (m=0.2). WE CLAIM; 1. A process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of: a. rolling said alloy metal sheet, b. clamping the rolled metal alloy sheet to be superplastically formed between a flat die and a shaped die, c. heating the whole assembly in a furnance temperature in the range of 0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the metal sheet to be super-plastically formed, characterized by, d. blowing a inert gas like argon through a tube at a pressure for a pressure-time profile determined as herein described. 2. A process as claimed in claim ".. wherein said heating is carried out in a tubular furnace. 3. A process as claimed in claim 1 wherein total forming time for a hemispherical shape is 0.69/ε, and then followed with pressure profile upto the desired strain rate 1.0, 1.5 etc. 4. A process as claimed in claim 1 wherein said sheet is rolled to a grain size below 10 microns. 5. A process for preparation of improved superplastic forming of hemispherical domes and deep clips from superplastic substantially as herein described and illustrated.

Full Text

This invention relates to a process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron etc.
Superplastic forming is an advanced technique of forming of metallic sheets, by means of gas pressure which is very similar to soap bubble forming or molten glass blowing techniques. In this process there is no need of any matched tooling like die-punch and the process is very simple. A tubular furnace, a simple and small hydraulic press and dies are the only accessories required. The sheet or the blank is clamped between a flat die and a shaped die (hemispherical or cup-shape) by applying pressure through the rams of a hydraulic press. The entire die-blank assembly is heated to the hot working temperature (0.4 to 0.6 Tm where Tm= Melting point of the material in absolute scale). Gas pressure is then applied over the blank through a stainless steel pipe fitted to the flat die. The key to a successful (i.e puncture free) forming, however, lies in maintaining a constant strain-rate during the process. This can be achieved only by maintaining suitably designed gas pressure Vs time sequence.
Superplastic forming is a relatively new process, introduced only two decades ago. Earlier superplastic blow forming was carried out by 'guess' or 'trial and error' method. In some processes, even sheets were formed under constant gas pressure, which resulted in a faulty and undesirable strain rate for superplastic forming due to which puncture free forming was not possible. The further limitation of these processes is that while hemispheres can be formed, it is not possible to form very deep cup shapes as these processes may cause premature repture and wide thickness variations which is undesirable. The rupture free superplastic forming of hemispheres and deep cups requires designing of an accurate gas pressure time sequence.
A few mathematical methods for pressure design are known in the art. One such method is by Jovane. In this method, instead of trying to keep strain rate constant, which is essential for perfect superplastic forming, he allowed the strain rate to vary between upper and lower

pressure - time profile which enables repeatability in superplastic forming of hemispheres and deep cups.
Still further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which enables forming very deep cups with height to radius ratio of more than 2, which is extremely difficult to achieve by other presently known pressure- time designs.
Yet further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which permits failure free forming of poorly superplastic materials characterised by their typically low strain rate sensitivity m According to the invention there is provided a process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of:
a. rolling said alloy metal sheet,
b. clamping the rolled metal alloy sheet to be superplastically formed
between a flat die and a shaped die,
c. heating the whole assembly in a fur nance temperature in the range of
0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the
metal sheet to be super-plastically formed, characterized by,
d. blowing a inert gas like argon through a tube at a pressure for a
pressure-time profile determined as herein described.

limits. Hence this method did not provide the precise pressure time sequence necessary for rupture-free superplastic forming.
Ghosh and Hamilton considered the constant strain rate and effective strain rate criterion for "pressure-time" sequence. The shortcoming of the method proposed by these authors is that this method did not incorporate correction required for the inevitable thickness variations that take place during forming of the sheet. Another
limitation of this method is that it does not consider the

strain rate sensitivity m of the material. Thus, this method also has the limitation that it cannot provide accurate pressure-time profile.
Thus the main limitation of the known methods is their inability to provide accurate pressure-time profile considering the thickness variations that arise in the dome during superplastic forming. This limitation of the known methods adversely affects the known superplastic forming processes in providing rupture free process . Another limitation that arises due to inaacurate pressure-timee profile is that it is very difficult to form metal into deep shaped cups or to form poorly superplastic materials characterised by their low strain rate sensitivity m having value less than the value of 0.3 (i.e. m The primary object of the present invention is to propose an improved superplastic forming process for forming of superplastic alloys like titanium alloys, aluminium alloys, magnesium alloys, zirconium alloys, iron alloys etc, based on accurate pressure -time profile.
Another object of the present invention is to propose an improved superplastic forming process based on accurate pressure time profile and taking into consideration the phenomenon of thickness variation in addition to effective strain rate critereon and constant strain rate formulation.
Still another object of the present invention is to propose an superplastic forming process based on accurate pressure -time profile which enables failure free (i.e without rupture or puncture) superplastic forming of hemispheres and deep cups.
Further object of the present invntion is to propose an improved superplastic forming process based on accurate

pressure - time profile which enables repeatability in superplastic forming of hemispheres and deep cups.
Still further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which enables forming very deep cups with height to radius ratio of more than 2, which is extremely difficult to achieve by other presently known pressure- time designs.
Yet further object of the present invention is to propose an improved superplastic forming process based on accurate pressure- time profile which permits failure free forming of poorly superplastic materials characterised by their typically low strain rate sensitivity m According to the invention there is provided a process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of:
a. rolling said alloy metal sheet,
b. clamping the rolled metal alloy sheet to be superplastically formed
between a flat die and a shaped die,
c. heating the whole assembly in a furnance temperature in the range of
0.4 to 0.6 Tm where Tm is the melting point in absolute scale of the
metal sheet to be super-plastically formed.,
d. blowing a gas like argon through a tube at a pressure for a pressure-time profile determined as herein described.
( σ ) is the effective flow stress, ( ε ) is effective strain rate, (m) is strain rate sensitivity, (a) is die radius and (So) is initial sheet thickness.
In accordanace with the present invention, the improved superplastic forming process proposed in the present invention, enables superplastic forming of hemispheres and cups. The improved process is based on accurate pressure - time profile which makes the process failure free, ie. it enables forming of hemispheres and domes without rupture or puncture. The accurate pressure time design of the process makes it possible to form very deep cups with height to radius ratio of more than 2, which are extremely difficult to achieve by conventional processes. It is usually believed that superplasticity ceases to exist when strain rate sensitivity (m) value is less than 0.3. However, the proposed process enables failure free forming of such poorly superplastic materials which are characterised by poor strain rate sensitivity(m), m Foregoing features of the presently disclosed invention will be more apparent from following description read in conjunc¬tion with the following figures, which are intended to illus¬trate an apparatus used for metal forming and two
applicatio'n areas of the invention, without intending to limit any way, the scope of the invention, wherein
Fig 1: shows the apparatus for superplastic forming
Fig 2: shows different stages of blowing Ti-alloy hemishpere
Fig 3: shows deep cup shape for Ti-alloy sheet
According to the proposed process, the superplastic forming process is carried out by an apparatus shown in fig (1) , wherein, (1) is an open ended cylinderical die and (2) is a flat die. The metal sheet (3) to be superplastically formed is clamped by fixing its rims (5) and (6) between open-ended cylinderical die (l) and the flat die (2) . The flat die (2) is pressed against the cylinderical die (1) by applying pressure by a hydraulic press. The flat die (2) has a steel pipe (4) attached at its middle through which inert gas like Argon is passed to exert gas pressure on to the metal sheet (3) . The pressure of the argon gas enclosed between the flat-die (2) and the metal formed sheet (3) is monitored by a pressure gauge attached to a gas regulator or a separate pressure guage attached to the gas line. The novelty of the apparatus lies in the use of a simple hole-die for forming of hemisphere as well as dep cup. No shaped die is used, thus making the process very inexpensive.
According to the process of the present invention superplastic blow forming of hemispheres and deep cups comprises of the following steps:-
(a) rolling the billet of a two-phase alloy material
to be superplastically-formed, by a thermo-
mechanical processing treatment to obtain grain
size below 10 microns
(b) determining the stress values of the sheet;
obtained by step (a) for 10-15 different values of strain
rates ranging between 10~5 to 10~7 sec-1 by testing
tensile test pieces in mechanical testing machine.
(c) ploting stress Vs strain rate curve in logarithnic
scale and determining maximum strain rate
sensitivity as the slope of the log(stress) Vs.
log(strain rate) fitted curve.
(d) repeating operations (b) and (c) at 4 or 5 hot working tempe¬
ratures to be taken within the range of 0.4 to 0.6 Tm where
Tm is the melting point in absolute scale of the sheet to be
formed.
(e) choosing the best temperature for forming (ie. temperature
where strain rate sensitivity 'm1 is maximum).
(f) noting the flow stress and strain rates corresponding to
maximum strain rate sensitivity m and then determining the effec¬
tive flow stress ( £ ) and effective strain rate (=) at this
maximum value of m. Effective stress and strain rates are calcu¬
lated from tensile flow stress and strain by assuming Von Mises
Critereon.
(g) substituting the values of the parameters namely (Sr) is
the effective flow stress, ( ε) is effective strain rate, (m)
is strain rate sensitivity, (a) is die radius and (So) is initial
sheet thickness, which accurately determines the gas pressure
(P) required to be applied at any point of time (t) :
(Equation Removed)
where f(t)= e + e
(h) clamping the metal alloy sheet to be superplastically formed between a flat die and a shaped die by applying pressure from a hydraulic press as shown in Fig.1.
(j) heating the whole assembly in a tubular furnace to the requi¬red hot working temperature to be taken within the range of 0.4 to 0.6 times the melting point).
(k) blowing the argon gas through a stainless steel pipe attached to the flat die. The required forming pressure is varied accor¬ding to the pressure Vs. time sequence calculated from above formula. The pressure is monitored from the guage attached to the gas regulator or alternatively a separate pressure gauge is fitted in the gas line. The total forming time for a hemisphe¬rical shape would be total strain divided by strain rate (t=0.69/£).
(1) The same procedure is followed for deep cup shape also, except that beyond formation of the hemisphere i.e.,
after completion of step (j) mentioned above, and after time (0.69/ε ), the pressure is calculated iteratively from equation (1) upto the desired strain level of the cteep cup (e.g, l.O, 1.5, etc). The total required time would then be (t = strain/ε).
EXAMPLES
EXAMPLE - I
Forming of a Ti-alloy hemisphere)
A 2.4 mm thick sheet (So=2.4 mm) of titanium alloy
(Russian designation VT-9) was rolled to obtain a fine
grain size of 4 microns. Incremental strain rate test
was carried out on test samples and the maximum strain
rate sensitivity was 0.85 (m = 0.85). at a temperature of
1173°K. The corresponding flow stress was 7.06 MPa
(σ =7.06 MPa) and the strain rate was 3.3x10 -4sec-i (ε=3 3xio4) Sec-1) . A thick walled cylindrical die as shown in fig 1 was used, so that both hemispherical shapes and cup shapes could be blown in the same die. The radius of the internal cavity was 30.3 mm (a = 30.3 mm). The die and the sheet assembly was heated to 1173 k in a furnace. The whole assembly was1 pressed tightly in a 20 Ton hydraulic press. All the above parameters were fitted in the equation for 'Pressure Vs time' (P - t) and the foming was carried out by applying gas pressure according to the sequence so obtained. The test was interruptred at 14 mins and 21 mins to monitor the progress of deformation (fig 2) . The full hemispherical shape is formed after 35 mins as predicted the formula t= 0.69/ε
EXAMPLE - II
Forming of deep cup from Ti-alloy
A deep cup shape (cup depth = 66 mm and radius 30.3 mm) was formed in the same die as mentioned above by continuing the forming cycle mentioned above, beyond the formation of the hemisphere (for hemisphere strain 0.69) upto a larger strain of 1.48 (equivalent to 66 mm depth) . The shape as illustrated in fig 3 could be formed in 75 mins by applying pressure according to the pressure time equation, as proposed in this invention and described in step (g) of the process.
EXAMPLE - III
Forming of Mg-Li alloy hemisphere
A Mg-ll%Li alloy was rolled to 2 mm thickness (So=2mm). The maximum strain rate sensitivity (m=0.2) was obtained at a temperature of 673k; The corresponding flow stress σ =37MPa and strain rate ε = lo-3Sec-1 as determined from the stress-strain rate curves. The same die of 30.3mm radius was used. The pressure-time cycle was then calculated from equation l by substituting the above values. The complete rupture free hemisphere could be formed even with a poor strain rate sensitivity (m=0.2).

WE CLAIM;
1. A process for preparation of improved superplastic forming of hemispherical domes and deep cups from superplastic alloy sheets of metal such as titanium, aluminium, magnesium, zirconium, iron comprising the steps of:
a. rolling said alloy metal sheet,
b. clamping the rolled metal alloy sheet to be superplastically
formed between a flat die and a shaped die,
c. heating the whole assembly in a furnance temperature in the
range of 0.4 to 0.6 Tm where Tm is the melting point in
absolute scale of the metal sheet to be super-plastically
formed, characterized by,
d. blowing a inert gas like argon through a tube at a pressure for
a pressure-time profile determined as herein described.
2. A process as claimed in claim ".. wherein said heating is carried out
in a tubular furnace.
3. A process as claimed in claim 1 wherein total forming time for a
hemispherical shape is 0.69/ε, and then followed with pressure
profile upto the desired strain rate 1.0, 1.5 etc.

4. A process as claimed in claim 1 wherein said sheet is rolled to a
grain size below 10 microns.
5. A process for preparation of improved superplastic forming of
hemispherical domes and deep clips from superplastic substantially
as herein described and illustrated.

Documents:

237-del-1997-abstract.pdf

237-del-1997-claims.pdf

237-del-1997-correspondence-others.pdf

237-del-1997-correspondence-po.pdf

237-del-1997-description (complete).pdf

237-del-1997-drawings.pdf

237-del-1997-form-1.pdf

237-del-1997-form-19.pdf

237-DEL-1997-Form-2.pdf

237-del-1997-form-3.pdf

237-del-1997-gpa.pdf

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Patent Number

214530

Indian Patent Application Number

237/DEL/1997

PG Journal Number

08/2008

Publication Date

22-Feb-2008

Grant Date

12-Feb-2008

Date of Filing

30-Jan-1997

Name of Patentee

THE CHIEF CONTROLLER, RESEARCH & DEVELOPMENT ORGANISATION, MINISTRY OF DEFENCE, GOVT. OF INDIA

Applicant Address

TECHNICAL COORDINATION DTE., B-341, SENA BHAWAN, DHQ P.O. NEW DELHI - 110011

Inventors:

#	Inventor's Name	Inventor's Address
1	DR. ABHIJIT DUTTA	SCIENTIST, DMRL, HYDERABAD -500058

PCT International Classification Number

C22K 3/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA