Title of Invention

"AN IMPROVED GAS PRESSURE SUPERPLASTIC FORMING PROCESS FOR SUPERPLASTICS FORMING OF HEMISPHERE SHAPES"

Abstract This invention relates to an improved gas pressure, super-plastic forming process for super-plastic forming of hemisphere shapes from alloy sheets of metals titanium, aluminum, magnesium, zirconium and iron wherein the process comprises of rolling the said alloy sheet, machining the said alloy sheet to required thickness, machining the said alloy sheet obtained by step (b) in such a way that while the central thickness remains the same, the thickness at the radial direction is tapered, clamping the said alloy sheet obtained from step (c ) between a flat die and a shaped die, heating the whole assembly in a furnance, blowing the argon gas through a steel pipe attached to flat die.
Full Text FIELD OF INVENTION
This invention relates to an improved process for gas-pressure super-plastic forming of hemisphere with nearly uniform thickness from super-plastic alloys of metals like titanium, aluminum, magnesium, zirconium and iron etc.
PRIOR ART
Super-plastic forming is an advanced forming technique of forming metallic sheets by gas-pressure which is very similar to soap bubble forming of molten glass blowing technique. Super-plastic forming of hemisphere and deep cup shapes are being commercially employed for producing gas bottles, pressure vessels etc. The rupture-free gas-pressure super-plastic forming of alloys requires maintenance of precise pressure Vs time sequence.
According to the conventional process of super-plastic forming the sheet or blank, to be formed into hemisphere or deep cup shapes, is clamped between a flat die and a shaped die by applying pressure through the top ram of a hydraulic press. The entire assembly is heated to the hot working temperature of the blank and gas pressure is applied over the blank through a stainless steel pipe fitted to the top die.
In the above conventional process, the forming was carried-out by gas pressure based on trial and error method or under constant gas pressure due to which rupture-free forming was not possible.
Another disadvantage of the above process lies in the undesirable gradual variation of thickness from pole to the equatorial positions of the finished product which may be as high as 60% as compared to the theoretical inevitable variation upto the extent of 22% in the finished product. These thickness variations are required to be corrected through expensive machining operations which also result in wastage of costly material.
Further limitation of the above process was the possibility of necking due to the non-uniform stress conditions prevailing at various locations.
A few mathematical methods for designing pressure-time sequence to be applied for gas-pressure super-plastic forming are known in the art. One such method is by Jovane. In this method, instead of keeping strain constant, the strain rate is allowed to vary between upper and lower limits.
The limitation of the above mathematical formulation for designing pressure-time sequence for gas pressure super-plastic forming is that it does not provide the precise pressure-time sequence for rupture free super-plastic forming.
Another method known in the art for designing pressure-time sequence for gas pressure super-plastic forming is by Ghosh and Hamilton wherein instead of keeping strain rate constant they have considered the constant strain rate and effective strain rate criterion for "pressure-time sequence".
The shortcoming of the above method is that it does not incorporate the correction required for the inevitable thickness variations that take place during the forming of the sheet.
Another shortcoming of the above method is, that it
does not consider the strain rate sensitivity (m), (m=
of the material.
Due to the above reasons this method is unable to provide accurate "pressure-time" profile for super-plastic forming of hemispheres.
The limitation of the above methods known in the art to provide accurate "pressure-time" profile was overcome by the mathematical method proposed in the co-pending patent application (No. 237/DEL/97). This mathematical formulation took into account the phenomenon of thickness variation that take place during forming of the sheet.
The above method proposed in co-pending patent application (No. 237/DEL/97), had the limitation that though this method provided rupture free forming of hemisphere by applying precise pressure-time profile determined through this method, yet the hemisphere obtained by the process had undesirable thickness variations from pole to the equator of the formed hemisphere which necessitated post-forming machining which also resulted in wastage of costly metals discarded as scrap.
OBJECT OF THE PROPOSED INVENTION
The primary object of the present invention in to propose an improved gas-pressure super-plastic forming process for super-plastic forming of hemisphere.
Another object of the present invention is to propose an improved gas-pressure super-plastic forming process for super-plastic forming of hemisphere with nearly uniform thickness from the pole to the equator of the hemisphere.
Still another object of the present invention is to
propose an improved gas-pressure super-plastic forming
process which enables rupture or puncture free forming of
hemisphere of alloys of metals like titanium, aluminum,
magnesium, zirconium and iron etc.
Yet another object of the present invention is to propose an improved gas-pressure super-plastic forming process which minimises post-forming machining operation of hemisphere formed through the process.
Further object of the present invention is to propose an improved gas-pressure super-plastic forming process which minimises wastage of costly metals as scrap.
Yet further object of the present invention is to propose an improved gas pressure super-plastic forming process which enables repeatability in super-plastic forming of hemispheres by precisely pre-determining and controlling the blank thickness at each radial angle, which will lead to uniform thickness of the hemisphere when subjected to super-plastic forming.
According to this invention there is provided an improved gas pressure, super-plastic forming process for super-plastic forming of hemisphere shapes from alloy sheets of metals titanium, aluminum, magnesium, zirconium and iron wherein the process comprises of :
(a) rolling the said alloy sheet,
(b) machining the said alloy sheet to required thickness,
(c) machining the said alloy sheet obtained by step (b) in such a way that while the central thickness remains the same, the thickness at the radial direction is tapered,
(d) The said alloy sheet obtained from step (c ) between a flat die and a shaped die,
(e) heating the whole assembly in a furnance,
(f) blowing the argon gas through a steel pipe attached to flat die.
The present invention provides an improved gas pressure super-plastic forming process for hemisphere of alloys of metals such as titanium, aluminum, magnesium, zirconium and iron etc. Whereas the process of co-pending patent application no.237/Del/97 enabled rupture-free forming of hemisphere by application of accurate "pressure—time" sequence determined by mathematical methods proposed therein, the present invention enables achievement of nearly uniform thickness of the hemisphere formed by the process, in addition to rupture free forming. In the proposed process, the thickness of the rolled sheet is determined first which is based on the required thickness of the hemisphere to be formed and the strain rate sensitivity index (m) of the blank. While central thickness of the blank is retained as equal to the thickness of roiled sheet, the thickness at each radial direction is tapered according to a mathematical formulation proposed in the present invention. The profiled blank is then blown at the temperature and pressure-time sequence as determined by the method proposed in the co-pending patent application (Mo.237/DEL/97). The process enables forming of hemispheres with nearly uniform thickness requiring little or no post-forming machining operation.
In accordance with this invention, a billet is first rolled o-f a two-phase alloy material to be super-plastically-formed, by a therma-mechanical processing treatment to obtain grain size below 10 microns.
1 he second step comprises in determining the stress values of the sheet obtained by the first step for 10-15 different values of strain rates ranging between 10-5 to 10 sec by testing tensile test pieces in mechanical testing machine.
The third step comprises in plotting stress vs strain rate curve in logarithmic scale and determining maximum strain rate sensitivity as the slope of the log(stress) Vs log(strain rate) fitted curve.
The second and third step© are repeated at 4 to 5 hot working temperatures 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.
The fourth step comprises in choosing the best temperature for forming (i.e. temperature where strain rate sensitivity m' is maximum).
The flaw stress and strain rates corresponding to maximum strain rate sensitivity m are noted and then the effective flow stress ( 6 ) and effective strain rate ( 6 ) is determined at this maximum value of (i). Effective stress and strain rates are calculated from tensile flow stress and strain by assuming Von Mises criterion.
The next step comprises in substituting the values of the parameters namely effective flow stress ( 6 > effective strain rate (  ) strain rate sensitivity ( m } die radius ( a) and initial sheet thickness ( S0 ), which accurately determines the gas pressure (P) required to be
(Equation Removed)
The thickness of the rolled sheet based upon the thickness(s) of the hemisphere to" be formed and the strain rate sensitivity index (m) of the blank material is calculated with help of the following equation
(Equation Removed)
Where , (S0)p = The sheet thickness
S = The required (IticUncss of the formed hemlspher
(Equation Removed)
The sheet obtained by the previous step is machined in such a way that the central thickness o-f the blank remains equal to the sheet thickness (S0) while the thickness at the radial direction is to be reduced in accordance with the following equation
(Equation Removed)
Where, (S0) = Vhickness o-f the blank at a
radial distance a.Coeld -from the centre
 = Angular distance from the equator ( fig.1)
a = Radius of the die (or hemisphere)
The metal alloy sheet to be super—plastically formed in clamped between a flat die and a shaped die by applying pressure from a hydraulic press.
The whole assembly is heated in a tubular furnace to the required hot working temperature to be taken within the range of 8.4 to (0.6 times the melting point.
Argon gas is blown through a stainless steel pipe attached to the flat die. The required forming pressure is varied according to the "pressure-time sequence"
calculated as per step (g). The pressure is monitored from the gauge attached to the gas regulator or alternatively a separate pressure gauge is fitted in the gas line,
DESCRIPTION OF FIGURES
The process of the present invention will be described with respect to accompanying figures wherein:-
Figure 1;— Blowing of uniform thickness hemisphere from Thickness profiled Blank.
Figure 2:- Shows nearly uniform thickness distribution in a. li-6A1-4LI Hemisphere as obtained by the process of present invention.
The present invention will now be illustrated with a working example which is intended to be a typical illustrative example and is not intended to be taken restrictively to imply any limitation on the scope of the present invention.
i7
WORKING SXAMPLE
A Titanium alloy (Ti-6AI-4V) sheet was rolled to a thickness of 2.1mm. The tensile samples were machined from this sheet and were tested at an optimum temperature of
925°C (1198k). The Strain Rate Sensitivity Index ('m') was
evaluated as 0.75 at the optimum strain rate of 10-4 sec-1
and the flow stress was 0.84 Kg/mm2 (8.24 MPa). A 50mm dia. (radius, 'a' = 25mm) was machined and a thickness profile (2.1mm at the centre, tapering to 1.66mm at the edge) was
made according to the equations (1) and (5), using 'm' valup as 0.75, required hemispherical thickness 's' as 1.05mm and die radius 'a'.as 25mm.
After forming of the hemisphere by gas pressure super-plastic blowing, the expected wall thickness of hemisphere formed should ideally be 1.05mm. However a marginal variation was noted as shown in fig.2. The super-plastically formed hemisphere showed only marginal variation in thickness, with 1.07mm as polar thickness and 1.16mm as equatorial thickness. Thus the formed hemisphere had only 8% variation in thickness. This marginal thickness increase is inevitable which is due to the fact that during super-plastic blow forming, some extra material slips from the flange of the blank, making the hemisphere slightly thicker than calculated one.



I CLAIM
1. An improved gas pressure, super-plastic forming process for super-
plastic forming of hemisphere shapes from alloy sheets of metals
titanium, aluminum, magnesium, zirconium and iron wherein the
process comprises of:
(a) rolling the said alloy sheet,
(b) machining the said alloy sheet to required thickness,
(c) machining the said alloy sheet obtained by step (b) that while the central thickness remains the same, the thickness at the radial direction is tapered,
(d) pressuring the said alloy sheet obtained from step (c) between a flat die and a shaped die,
(e) heating the whole assembly in a furnace,
(f) blowing the argon gas through a steel pipe attached to flat die.

2. An improved process as claimed in claim 1 wherein argon gas is blown at a pressure-time sequence.
3. An improved process for gas-pressure super-plastic forming in hemispheres as claimed in claim 1 wherein the step (d) pressure from a hydraulic press is applied.
4. An improved process for gas-pressure super-plastic forming of hemispheres as claimed in claim 1 wherein the heating of step (e) is carried out 0.4 to 0.6 times the melting point of said alloy sheet.

Documents:

3407-del-1998-abstract.pdf

3407-DEL-1998-Claims-(16-04-2012).pdf

3407-del-1998-claims.pdf

3407-DEL-1998-Correspondence Others-(16-04-2012).pdf

3407-del-1998-correspondence-others.pdf

3407-del-1998-correspondence-po.pdf

3407-del-1998-description (complete).pdf

3407-del-1998-drawings.pdf

3407-del-1998-form-1.pdf

3407-del-1998-form-19.pdf

3407-del-1998-form-2.pdf

3407-del-1998-form-26.pdf

3407-del-1998-form-3.pdf


Patent Number 252238
Indian Patent Application Number 3407/DEL/1998
PG Journal Number 18/2012
Publication Date 04-May-2012
Grant Date 02-May-2012
Date of Filing 16-Nov-1998
Name of Patentee THE CHIEF CONTROLLER, RESEARCH & DEVELOPMENT ORGN. MINISTRY OF DEFENCE
Applicant Address B-341, SENA BHAWAN, DHQ P.O.,NEW DELHI-110 011
Inventors:
# Inventor's Name Inventor's Address
1 DR. ABHIJIT DUTTA DMRL, HYDERABAD-500 058
PCT International Classification Number B21D 26/02
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA