Method for forming superplastic and super ductile metal alloys
United States Patent 3902348
In forming a structural member from a sheet of superplastic or super ductile metal alloy material, the strain pattern to which the material is subjected in isothermal forming is modified by holding different portions of the sheet being formed to different temperatures, thus controlling the ultimate tensile strength and therefore the material thickness of such portions in the finished product.
US Patent References:
Metal flanging
Thackray - April 1926 - 1580931
Panel construction
Ensrud et al. - December 1961 - 3011602
Method for deforming metal-plastic laminates
Pohl, et al. - September 1967 - 3340714
Metal flanging
Thackray - April 1926 - 1580931
Panel construction
Ensrud et al. - December 1961 - 3011602
Method for deforming metal-plastic laminates
Pohl, et al. - September 1967 - 3340714
Application Number:
05/461028
Publication Date:
09/02/1975
Filing Date:
04/15/1974
View Patent Images:
Export Citation:
Primary Class:
International Classes:
B21D13/10; B21D22/00; B21D26/02; B21D47/00; B21D13/00; B21D26/00; B21D47/00
Field of Search:
72/342,379 29/455LM 219/7.5,10.43
Primary Examiner:
Larson, Lowell A.
Attorney, Agent or Firm:
Sokolski, Edward A.
Claims:
I claim
1. A method for forming a sheet of metal alloy into a structural member having nodal portions which project from center line portions, comprising the steps of:
2. The method of claim 1 wherein the temperatures of said sheet portions are controlled to equilibrate the strain throughout said sheet, thereby making for uniform material thickness throughout said member.
3. The method of claim 1 wherein the sheet is resistance heated by feeding an electrical current therethrough.
4. The method of claim 1 wherein the sheet is heated by inducing an electric current therein.
5. The method of claim 1 wherein the sheet is heated by subjecting said sheet to microwave energy.
6. A method for forming a structural member from a sheet of superplastic metal alloy wherein the improvement comprises the equilibrating of the workpiece strain during the forming process, comprising the steps of:
7. The method of claim 6 wherein said sheet is formed into a nodular core having nodes with end faces of a width approximately one half of that at the center line of the core, the punch member and the die member maintaining the temperatures at the nodal face portion and the core center line portions such as to provide a material tensile strength at said center line portion which is substantially one half of that at said nodal face portion.
8. The method of claim 7 wherein said material is Astrolloy and said nodal face portion is maintained at 1750°F and said center line portion is maintained at 1900°F.
9. The method of claim 7 wherein said material is zinc aluminum and said nodal face portion is maintained at a temperature of 500°F, said center line portion being maintained at a temperature of 570°F.
10. The method of claim 7 wherein said material is titanium Tt-6- 4, said nodal face portion being maintained at 1700°F, while said center line portion is maintained at 1790°F.
11. The method of claim 6 wherein the temperature at the corners of said punch member is reduced by having inserts of material with substantially higher conductivity than the remaining portions of said punch member placed at said corners and extending away from the remaining portions of said punch member into the ambient atmosphere.
12. The method of claim 6 wherein said punch and die members are heated by means of rod heaters placed therein respectively.
1. A method for forming a sheet of metal alloy into a structural member having nodal portions which project from center line portions, comprising the steps of:
2. The method of claim 1 wherein the temperatures of said sheet portions are controlled to equilibrate the strain throughout said sheet, thereby making for uniform material thickness throughout said member.
3. The method of claim 1 wherein the sheet is resistance heated by feeding an electrical current therethrough.
4. The method of claim 1 wherein the sheet is heated by inducing an electric current therein.
5. The method of claim 1 wherein the sheet is heated by subjecting said sheet to microwave energy.
6. A method for forming a structural member from a sheet of superplastic metal alloy wherein the improvement comprises the equilibrating of the workpiece strain during the forming process, comprising the steps of:
7. The method of claim 6 wherein said sheet is formed into a nodular core having nodes with end faces of a width approximately one half of that at the center line of the core, the punch member and the die member maintaining the temperatures at the nodal face portion and the core center line portions such as to provide a material tensile strength at said center line portion which is substantially one half of that at said nodal face portion.
8. The method of claim 7 wherein said material is Astrolloy and said nodal face portion is maintained at 1750°F and said center line portion is maintained at 1900°F.
9. The method of claim 7 wherein said material is zinc aluminum and said nodal face portion is maintained at a temperature of 500°F, said center line portion being maintained at a temperature of 570°F.
10. The method of claim 7 wherein said material is titanium Tt-6- 4, said nodal face portion being maintained at 1700°F, while said center line portion is maintained at 1790°F.
11. The method of claim 6 wherein the temperature at the corners of said punch member is reduced by having inserts of material with substantially higher conductivity than the remaining portions of said punch member placed at said corners and extending away from the remaining portions of said punch member into the ambient atmosphere.
12. The method of claim 6 wherein said punch and die members are heated by means of rod heaters placed therein respectively.
Description:
This invention relates to the formation of structures from superplastic or super ductile metal alloys, and more particularly to a method for controlling the strain placed on all portions of the material in such a process.
Superplastic alloys and highly ductile alloys have come into widespread use in recent years in the formation of various types of structures in view of their excellent structural properties and their adaptability to forming. Superplastic alloys technically may be described as micro duplex eutectoids having two states of grain structure. The materials in their initial fine grain condition are heated to a plastic state and formed. At the forming temperatures, the materials exhibit excellent elongation properties at low forming rates. Once formed, most of such materials are heat treated to increase grain size and improve structural properties. Examples of superplastic alloys include zinc aluminum (78% zinc, 22% aluminum), titanium T-6- 4, nickel based alloys such as Astrolloy and the highly ductile nickel alloy 304L.
A particular application for sheets of superplastic alloy material which has come into use is in the formation of nodular sandwich panel cores having tapering nodes which may be pyramidal or may be hyperbolic (as described in my U.S. Pat. Nos. 3,525,663 and 3,527,664). In forming such core structures with the techniques of the prior art, isothermal forming, it has been found that an excessive amount of material thinning takes place just below the node face due to maximum forming stresses at this point. The reason for this is that the cross-sectional area of material below the node face is substantially less than that at the core node base. Such uncontrolled non-uniformity in material thickness of the core seriously detracts from its structural properties.
The present invention overcomes this shortcoming of prior art techniques by providing means for effectively controlling ultimate tensile strength and thus strain on all portions of the workpiece during forming by holding the portions thereof being formed at predetermined temperatures as may be called for to provide a desired material thickness for each of such portions. The technique of the invention can be used to equal advantage to control temperature to make for different material thicknesses at different portions of the workpiece as well as for uniform material thickness throughout the workpiece. Applicant's inventive technique is particularly useful in solving the above mentioned problem encountered in the fabrication of nodular sandwich panel cores. Optimum structural characteristics are achieved in the end product by holding the portions of the workpiece being formed into the nodal end faces at a predetermined different temperature from the temperature at which the core node base portions are held. The temperature utilized for each material are such as to establish the necessary tensile strength of the material in each area during the forming process, this being based on the fact that the material exhibits different tensile strengths at different temperatures.
It is therefore an object of this invention to improve the structural properties of structures formed from superplastic and super ductile metal alloys by providing a method of controlling material thickness by controlling temperatures at various portions of the workpiece.
It is further object of this invention to provide means for forming structures from superplastic or super ductile metal alloys which have uniform or variable material thickness throughout, as dictated by structural requirements.
Other objects of the invention will become apparent as the description proceeds in connection with the accompanying drawings, of which:
FIG. 1 is a perspective view illustrating a first core structure having pyramidal nodes which may be formed utilizing the technique of the invention;
FIG. 2 is an elevational view in cross section illustrating one step of the technique of the invention in forming the nodes of FIG. 1;
FIG. 3 is an elevational view in cross section illustrating a further step in performing the technique of the invention in forming the nodes of FIG. 1;
FIG. 4 is an elevational view illustrating a punch particularly useful for forming core structures such as shown in FIG. 1;
FIG. 5 is a cross-sectional view taken along the plane indicated by 5--5 in FIG. 4;
FIG. 6 is a perspective view illustrating a second type of core structure which may be formed utilizing the technique of the invention;
FIG. 7 is an elevational view in cross section of the tooling which may be used for forming the core of FIG. 6; and
FIG. 8 is an elevational view in cross section showing the tooling of FIG. 7 in the process of forming the core of FIG. 6.
Briefly described, the technique of the invention basically involves controlling the temperature of the various portions of a workpiece of superplastic or super ductile alloy material as it is being formed, so as to provide an end product with predetermined material thicknesses at each of such portions. In certain situations, uniform material thickness may be desired throughout the workpiece. In such a case, the temperature must be controlled in a manner so as to equilibrate the strain over all portions of the workpiece based on the fact that the tensile strength thereof is greater at lower than at higher temperatures. Where different material thicknesses are desired in different workpiece portions, the temperature can be similarly controlled to produce a corresponding variation in tensile strength and strain to effect the desired thicknesses in the end product.
The technique of the invention as described for one of the exemplary embodiments involving the formation of a core structure with unidirectional nodes is as follows: A sheet of superplastic or super ductile alloy material is clamped against a die and a punch member driven so as to apply pressure against the material thereby extending it into the die to form core nodes. The die is held at one predetermined temperature while the punch is held at another, such that the nodal face portions and the base core portions are held at each of these temperatures respectively during the forming process, thus equilibrating the strain or differentiating the strain in a predetermined manner over the workpiece, as may be required.
The technique of the invention as described for another exemplary embodiment involving the formation of a core structure with oppositely projecting nodes is as follows: A sheet of superplastic or super ductile alloy material is placed between the sets of punches, the punch members are driven so as to apply pressure against the material, thereby extending it to form core nodes. The punches are held at one predetermined temperature while the sheet is held at another, such that the nodal face portions and the center line core portions are held at each of these temperatures respectively during the forming process, thus equilibrating the strain or differentiating the strain in a predetermined manner over the workpiece as may be required.
To better appreciate the invention, let us first examine the tensile strengths of a few superplastic alloys at different temperatures. The following table indicates the appropriate temperatures for obtaining tensile strengths having a ratio of about 2 to 1 for each one of the listed superplastic alloys.
________________________________________________________ __________________ Zinc Aluminum Ultimate Tensile Strength at 500°F 400 psi (78% Zinc " 570°F 220 psi 22% Aluminum) Titanium T-6-4 Ultimate Tensile Strength at 1700°F 720 psi " 1790°F 360 psi Astrolloy and Ultimate Tensile Strength at 1750°F 1000 psi other Similar " 1925°F 500 psi Nickel Based Alloys ____________________________________________________________ ______________
It can be seen from the above chart that for the various materials in question, the ultimate tensile strength is half or about half as great at one temperature as another. It therefore should be apparent that in situations where there is a variation in the width of the material being formed, the strain on the material in the forming process can be controlled by holding the narrower portions of the material at a predetermined lower temperature, thus a higher ultimate tensile, and the wider portions of the material at a predetermined higher temperature, thus a lower ultimate tensile, thereby controlling material thickness.
The above indicated tables are useful in demonstrating the formation of core structures having pyramidal nodes where the apices or face portions of the nodes have edges which are half as wide as the base portions thereof. In subjecting the base portions of the pyramidal nodes to one of the above indicated temperatures and the node end face portions to the other, a temperature gradient will be established between these two portions which will provide a proper temperature not only for these two portions but also for each of the points therebetween, to form the workpiece with a constant thickness.
Referring now to FIG. 1, a typical core structure which may be formed by the technique of the invention is illustrated. It is to be noted that this particular structure is shown for illustrative purposes only, and the technique of the invention may be used to equal advantage for forming cores having hyperbolic nodes, such as described in my U.S. Pat. No. 3,527,664, and other types of structures wherein there is requirement for a variation in the ultimate tensile strength of different portions of the workpiece in the forming process.
Referring now to FIGS. 2 and 3, the formation of one of the node members of the structure of FIG. 1 by the technique of the invention is illustrated. Workpiece 11, which may be a sheet of material of one of the superplastic or super ductile alloys mentioned above, is held against die member 13 by means of ring clamp 14. Heaters 15 which may be rod heaters (Cal rod) are mounted in die member 13 and are energized to heat the die to a predetermined temperature. Where the workpiece 11, for example, is of Astrolloy, this temperature could be 1925°F, or in the case of any of the other materials listed above, the higher temperature in each instance (which results in the lower tensile strength). Punch member 17 has a rectangular punch pin portion 17a and a base portion 17b. Mounted in base portion 17b are a pair of rod heaters 18 which may also be of the Calrod type. Heaters 18 are energized in a manner such as to maintain pin portion 17a at a predetermined temperature which, with the example given above for Astrolloy, may be 1750°F. In any event, the temperature of pin 17a should be that for giving a tensile strength to workpiece 11 which is substantially twice that for the temperature applied to die member 13 (as for example indicated in the table for various superplastic alloys listed above. A heat shield 18 is placed between punch member 17 and die member 13 to minimize the transfer of heat therebetween.
Referring now to FIG. 3, punch member 17 is shown in its final position in the forming operation. Throughout this operation, energy is supplied to heaters 15 and 18 so as to maintain die 13 and pin portion 17a of the punch at the predetermined temperatures for maintaining gradated tensile strength over all portions of workpiece 11 subjected to stress, such that there will be no excessive thinning at or near the face portions of the node.
Referring now to FIGS. 4 and 5, a modification of the pin portion 17a of punch member 17 is illustrated. This modification comprises the addition of inserts 17c running along the corners of pin portion 17a. Inserts 17c are of a material having a substantially higher thermal conductivity than the remaining parts of the pin portion. Corner portions 17c extend (by themselves) below the base portion 17b and out into the ambient atmosphere and thus effectively operate to radiate heat away from the corners of the nodal faces being formed. This keeps these corner sections at a reduced temperature, thereby increasing the tensile properties thereat. In view of the fact that the material is highly stressed during the forming at the corners of the punch pin, the use of inserts 17c reduces the possibility of thinning and rupture at these points.
Referring now to FIGS. 6-8, a second embodiment of the technique of the invention is illustrated. The technique as to be described for the second embodiment is suitable for use in forming sandwich panel cores having oppositely projecting nodes such as described in my U.S. Pat. No. 3,525,663. A section of such core as may be formed by the technique of the invention is illustrated in FIG. 6. In the formation of such core, it is desirable that the node portions in the region of the nodal faces be thicker than the node portions toward the center line of the core. This end result can be achieved by holding the nodal face portions at a lower temperature than the center line portions during the forming process. This end result can be achieved, as illustrated in FIGS. 7 and 8, by heating the workpiece to a predetermined higher temperature than the temperature at which the pins used to form the nodes are held. As shown in FIG. 7, prior to the forming of the core, the workpiece 30 is heated to a predetermined temperature by feeding current therethrough by means of leads 31 connected to opposite edges of the workpiece. The workpiece, as for the previous embodiment, which is of a superplastic alloy material, is thus resistance heated. Metal pins which are used for forming the core are mounted in ceramic base sections 38. While the workpiece 30 continues to be heated to the desired temperature, the pins are driven to form the nodes as shown in FIG. 8. The pins, which are at a lower temperature than the workpiece, act as heat sinks to draw heat away from the nodal portions of the workpiece which they contact, thus providing an effective temperature differential between the nodal and center line portions of the core as it is being formed. Utilizing information as to the tensile strength of the workpiece material at different temperatures, the temperature of the workpiece can thus be controlled to provide the desired material thickness at the nodal portions of the end product, as compared to that at the center line portions.
It is to be noted that various other techniques from that just described can be utilized to provide the desired temperatures at the various portions of the workpiece. The pins 36, for example, can be heated to a desired lower temperature than that to which the workpiece is heated by means of Calrod heaters. Further, the workpiece 30 can be heated by induction or microwave heating techniques. Also, the pins can be heated to the desired temperature in a furnace immediately prior to the forming process.
It is to be noted that in the interests of increased tool life, it is desirable to keep the temperature of the tooling low during the forming process. Thus, the heating of the workpiece in the manner indicated without heating the tooling to this same elevated temperature has this additional advantage.
The technique of this invention thus provides a method and the means for improving the structural properties of structural members formed from superplastic metal alloys, this end result being achieved by maintaining various portions of the superplastic metal alloy workpiece at different temperatures, thus controlling ultimate tensile, such as to equilibrate or differentiate the strain throughout such workpiece as required.
While the invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only, and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the following claims.
Superplastic alloys and highly ductile alloys have come into widespread use in recent years in the formation of various types of structures in view of their excellent structural properties and their adaptability to forming. Superplastic alloys technically may be described as micro duplex eutectoids having two states of grain structure. The materials in their initial fine grain condition are heated to a plastic state and formed. At the forming temperatures, the materials exhibit excellent elongation properties at low forming rates. Once formed, most of such materials are heat treated to increase grain size and improve structural properties. Examples of superplastic alloys include zinc aluminum (78% zinc, 22% aluminum), titanium T-6- 4, nickel based alloys such as Astrolloy and the highly ductile nickel alloy 304L.
A particular application for sheets of superplastic alloy material which has come into use is in the formation of nodular sandwich panel cores having tapering nodes which may be pyramidal or may be hyperbolic (as described in my U.S. Pat. Nos. 3,525,663 and 3,527,664). In forming such core structures with the techniques of the prior art, isothermal forming, it has been found that an excessive amount of material thinning takes place just below the node face due to maximum forming stresses at this point. The reason for this is that the cross-sectional area of material below the node face is substantially less than that at the core node base. Such uncontrolled non-uniformity in material thickness of the core seriously detracts from its structural properties.
The present invention overcomes this shortcoming of prior art techniques by providing means for effectively controlling ultimate tensile strength and thus strain on all portions of the workpiece during forming by holding the portions thereof being formed at predetermined temperatures as may be called for to provide a desired material thickness for each of such portions. The technique of the invention can be used to equal advantage to control temperature to make for different material thicknesses at different portions of the workpiece as well as for uniform material thickness throughout the workpiece. Applicant's inventive technique is particularly useful in solving the above mentioned problem encountered in the fabrication of nodular sandwich panel cores. Optimum structural characteristics are achieved in the end product by holding the portions of the workpiece being formed into the nodal end faces at a predetermined different temperature from the temperature at which the core node base portions are held. The temperature utilized for each material are such as to establish the necessary tensile strength of the material in each area during the forming process, this being based on the fact that the material exhibits different tensile strengths at different temperatures.
It is therefore an object of this invention to improve the structural properties of structures formed from superplastic and super ductile metal alloys by providing a method of controlling material thickness by controlling temperatures at various portions of the workpiece.
It is further object of this invention to provide means for forming structures from superplastic or super ductile metal alloys which have uniform or variable material thickness throughout, as dictated by structural requirements.
Other objects of the invention will become apparent as the description proceeds in connection with the accompanying drawings, of which:
FIG. 1 is a perspective view illustrating a first core structure having pyramidal nodes which may be formed utilizing the technique of the invention;
FIG. 2 is an elevational view in cross section illustrating one step of the technique of the invention in forming the nodes of FIG. 1;
FIG. 3 is an elevational view in cross section illustrating a further step in performing the technique of the invention in forming the nodes of FIG. 1;
FIG. 4 is an elevational view illustrating a punch particularly useful for forming core structures such as shown in FIG. 1;
FIG. 5 is a cross-sectional view taken along the plane indicated by 5--5 in FIG. 4;
FIG. 6 is a perspective view illustrating a second type of core structure which may be formed utilizing the technique of the invention;
FIG. 7 is an elevational view in cross section of the tooling which may be used for forming the core of FIG. 6; and
FIG. 8 is an elevational view in cross section showing the tooling of FIG. 7 in the process of forming the core of FIG. 6.
Briefly described, the technique of the invention basically involves controlling the temperature of the various portions of a workpiece of superplastic or super ductile alloy material as it is being formed, so as to provide an end product with predetermined material thicknesses at each of such portions. In certain situations, uniform material thickness may be desired throughout the workpiece. In such a case, the temperature must be controlled in a manner so as to equilibrate the strain over all portions of the workpiece based on the fact that the tensile strength thereof is greater at lower than at higher temperatures. Where different material thicknesses are desired in different workpiece portions, the temperature can be similarly controlled to produce a corresponding variation in tensile strength and strain to effect the desired thicknesses in the end product.
The technique of the invention as described for one of the exemplary embodiments involving the formation of a core structure with unidirectional nodes is as follows: A sheet of superplastic or super ductile alloy material is clamped against a die and a punch member driven so as to apply pressure against the material thereby extending it into the die to form core nodes. The die is held at one predetermined temperature while the punch is held at another, such that the nodal face portions and the base core portions are held at each of these temperatures respectively during the forming process, thus equilibrating the strain or differentiating the strain in a predetermined manner over the workpiece, as may be required.
The technique of the invention as described for another exemplary embodiment involving the formation of a core structure with oppositely projecting nodes is as follows: A sheet of superplastic or super ductile alloy material is placed between the sets of punches, the punch members are driven so as to apply pressure against the material, thereby extending it to form core nodes. The punches are held at one predetermined temperature while the sheet is held at another, such that the nodal face portions and the center line core portions are held at each of these temperatures respectively during the forming process, thus equilibrating the strain or differentiating the strain in a predetermined manner over the workpiece as may be required.
To better appreciate the invention, let us first examine the tensile strengths of a few superplastic alloys at different temperatures. The following table indicates the appropriate temperatures for obtaining tensile strengths having a ratio of about 2 to 1 for each one of the listed superplastic alloys.
________________________________________________________ __________________ Zinc Aluminum Ultimate Tensile Strength at 500°F 400 psi (78% Zinc " 570°F 220 psi 22% Aluminum) Titanium T-6-4 Ultimate Tensile Strength at 1700°F 720 psi " 1790°F 360 psi Astrolloy and Ultimate Tensile Strength at 1750°F 1000 psi other Similar " 1925°F 500 psi Nickel Based Alloys ____________________________________________________________ ______________
It can be seen from the above chart that for the various materials in question, the ultimate tensile strength is half or about half as great at one temperature as another. It therefore should be apparent that in situations where there is a variation in the width of the material being formed, the strain on the material in the forming process can be controlled by holding the narrower portions of the material at a predetermined lower temperature, thus a higher ultimate tensile, and the wider portions of the material at a predetermined higher temperature, thus a lower ultimate tensile, thereby controlling material thickness.
The above indicated tables are useful in demonstrating the formation of core structures having pyramidal nodes where the apices or face portions of the nodes have edges which are half as wide as the base portions thereof. In subjecting the base portions of the pyramidal nodes to one of the above indicated temperatures and the node end face portions to the other, a temperature gradient will be established between these two portions which will provide a proper temperature not only for these two portions but also for each of the points therebetween, to form the workpiece with a constant thickness.
Referring now to FIG. 1, a typical core structure which may be formed by the technique of the invention is illustrated. It is to be noted that this particular structure is shown for illustrative purposes only, and the technique of the invention may be used to equal advantage for forming cores having hyperbolic nodes, such as described in my U.S. Pat. No. 3,527,664, and other types of structures wherein there is requirement for a variation in the ultimate tensile strength of different portions of the workpiece in the forming process.
Referring now to FIGS. 2 and 3, the formation of one of the node members of the structure of FIG. 1 by the technique of the invention is illustrated. Workpiece 11, which may be a sheet of material of one of the superplastic or super ductile alloys mentioned above, is held against die member 13 by means of ring clamp 14. Heaters 15 which may be rod heaters (Cal rod) are mounted in die member 13 and are energized to heat the die to a predetermined temperature. Where the workpiece 11, for example, is of Astrolloy, this temperature could be 1925°F, or in the case of any of the other materials listed above, the higher temperature in each instance (which results in the lower tensile strength). Punch member 17 has a rectangular punch pin portion 17a and a base portion 17b. Mounted in base portion 17b are a pair of rod heaters 18 which may also be of the Calrod type. Heaters 18 are energized in a manner such as to maintain pin portion 17a at a predetermined temperature which, with the example given above for Astrolloy, may be 1750°F. In any event, the temperature of pin 17a should be that for giving a tensile strength to workpiece 11 which is substantially twice that for the temperature applied to die member 13 (as for example indicated in the table for various superplastic alloys listed above. A heat shield 18 is placed between punch member 17 and die member 13 to minimize the transfer of heat therebetween.
Referring now to FIG. 3, punch member 17 is shown in its final position in the forming operation. Throughout this operation, energy is supplied to heaters 15 and 18 so as to maintain die 13 and pin portion 17a of the punch at the predetermined temperatures for maintaining gradated tensile strength over all portions of workpiece 11 subjected to stress, such that there will be no excessive thinning at or near the face portions of the node.
Referring now to FIGS. 4 and 5, a modification of the pin portion 17a of punch member 17 is illustrated. This modification comprises the addition of inserts 17c running along the corners of pin portion 17a. Inserts 17c are of a material having a substantially higher thermal conductivity than the remaining parts of the pin portion. Corner portions 17c extend (by themselves) below the base portion 17b and out into the ambient atmosphere and thus effectively operate to radiate heat away from the corners of the nodal faces being formed. This keeps these corner sections at a reduced temperature, thereby increasing the tensile properties thereat. In view of the fact that the material is highly stressed during the forming at the corners of the punch pin, the use of inserts 17c reduces the possibility of thinning and rupture at these points.
Referring now to FIGS. 6-8, a second embodiment of the technique of the invention is illustrated. The technique as to be described for the second embodiment is suitable for use in forming sandwich panel cores having oppositely projecting nodes such as described in my U.S. Pat. No. 3,525,663. A section of such core as may be formed by the technique of the invention is illustrated in FIG. 6. In the formation of such core, it is desirable that the node portions in the region of the nodal faces be thicker than the node portions toward the center line of the core. This end result can be achieved by holding the nodal face portions at a lower temperature than the center line portions during the forming process. This end result can be achieved, as illustrated in FIGS. 7 and 8, by heating the workpiece to a predetermined higher temperature than the temperature at which the pins used to form the nodes are held. As shown in FIG. 7, prior to the forming of the core, the workpiece 30 is heated to a predetermined temperature by feeding current therethrough by means of leads 31 connected to opposite edges of the workpiece. The workpiece, as for the previous embodiment, which is of a superplastic alloy material, is thus resistance heated. Metal pins which are used for forming the core are mounted in ceramic base sections 38. While the workpiece 30 continues to be heated to the desired temperature, the pins are driven to form the nodes as shown in FIG. 8. The pins, which are at a lower temperature than the workpiece, act as heat sinks to draw heat away from the nodal portions of the workpiece which they contact, thus providing an effective temperature differential between the nodal and center line portions of the core as it is being formed. Utilizing information as to the tensile strength of the workpiece material at different temperatures, the temperature of the workpiece can thus be controlled to provide the desired material thickness at the nodal portions of the end product, as compared to that at the center line portions.
It is to be noted that various other techniques from that just described can be utilized to provide the desired temperatures at the various portions of the workpiece. The pins 36, for example, can be heated to a desired lower temperature than that to which the workpiece is heated by means of Calrod heaters. Further, the workpiece 30 can be heated by induction or microwave heating techniques. Also, the pins can be heated to the desired temperature in a furnace immediately prior to the forming process.
It is to be noted that in the interests of increased tool life, it is desirable to keep the temperature of the tooling low during the forming process. Thus, the heating of the workpiece in the manner indicated without heating the tooling to this same elevated temperature has this additional advantage.
The technique of this invention thus provides a method and the means for improving the structural properties of structural members formed from superplastic metal alloys, this end result being achieved by maintaining various portions of the superplastic metal alloy workpiece at different temperatures, thus controlling ultimate tensile, such as to equilibrate or differentiate the strain throughout such workpiece as required.
While the invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only, and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the following claims.