JPS5823520A - Super-plastic molding method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to the field of material forming, and more particularly to material forming under superplastic conditions.

Under certain conditions, some materials can be plastically shaped well beyond their normal optic boundaries without rupture; this property is called superplasticity.

A typical process involves placing a sheet of material in a mold, heating the material to a concentration that exhibits superplasticity, and using a gas to apply pressure to one side of the sheet. Sufficient pressure is applied to strain the material at a strain rate that is within the superplastic range such that the material forms at the selected consistency.

This gas pressure creates a tensile stress in the face of the sheet, which causes it to stretch and form within the mold cavity.

This process was developed by O. Howard Hasilton
(Inventor of the present invention), N@il E, Pat
No. 4,181,000 to John M., Curnow, and John M. Curnow.

This process is increasingly being used to form various shapes such as titanium sheet metal structures for aircraft.

One undesirable property of many superplastic alloys is their tendency to cavitate (forming small internal cavities) during tensile deformation caused by prior art forming operations. These voids not only reduce the mechanical properties if they are present over a fairly large volume, but also limit the superplastic malleability of the material.

Unfortunately, cavitation peaks are usually at a maximum when superplasticity is at a maximum, and this problem has limited the application of prior art superplastic forming methods.

It is an object of this invention to provide a method for eliminating or minimizing cavitation during superplastic forming.

The object of this invention is to
The object of the present invention is to provide a method for erasing or reducing.

The purpose of this invention is to provide a method for superplastically forming parts using optimal strain rates without creating cavities.

According to the invention, pressure is applied to both sides of the blank either during the forming period or after forming is completed. This method of applying pressure reduces the strength of the tensile stress acting on the void nucleation site, if applied to a molding tool.
Thus, the formation of voids is prevented or their size and number reduced.

The isotropic stress component or the maximum tensile stress component acting on the void II or void nucleation location typically determines whether the void will nucleate and grow. Therefore, if these stress components can be maintained below a certain critical level, then no cavities will occur;
Or if it had grown earlier, it should be deleted upon closing. Although the strength of the critical stress for prevention and closure is different, the concept for clearing the void in both cases is the same for this invention.

Providing pressure applied on both sides of the blank results in a net hydrostatic component of reduced tension, and even a net hydrostatic component of reduced compression, while also producing an isotropic tensile stress component. A compressive isotropic stress component is added to . The same principle holds for the maximum tensile stress acting on the void or void nucleation location. The concept of these stress states is well known to those trained in the art. It is this relaxed stress state that acts on the void or void core, which is the void that occurs in superplastic forming.
effective in preventing or erasing This reduced tensile or compressive isotropic stress is obtained by applying gas pressure to both sides of the blank after it has been placed in the mold and heated to superplastic forming consistency. The pressure on the side of the blank that is fastest from the mold surface for forming is then increased in a known manner to superplastically form the material upon the mold surface. Thus, the cavity is reduced or eliminated since the forming is completed in such a way that the vacant or empty nucleation sites are exposed to reduced stress.

If pressure is applied in the amorphous bale, then the voids created are removed by plastic deformation and/or diffusion. In this second example, the blank is placed in a forming mold and heated to a superplastic forming consistency;
And next, the mold! ! It is superplastically formed in a known manner by applying gas pressure to the side of the blank furthest from the surface. After the blank is formed against the mold surface, it is held there by maintaining the forming pressure. During this holding period, due to the action of gas pressure and reaction of the mold, the void formed during the initial molding step is closed by plastic deformation and/or diffusion.

Supercooling requires materials that can exhibit effective strain rate sensitivity. This value is a function of concentration, material, microstructure, and strain evolution. Patent No. 4.181.000J! Please refer to the following.

Many superacid alloys tend to cavitate (creating internal voids) when formed in superplastic conditions.

This 1llI is caused by grain boundary sliding deformation, which is common to cavitation and superstructure. Cavitation in most super-all alloys is caused by the initiation of particles present on grain boundaries, which form voids at the interface of the grain matrix as grain boundary sliding progresses. These voids, once created, grow larger with deformation occurring, eventually connecting with the Koji voids and thus leading to breakage.

Cavitation is seen within creep rupture failures of components, and researchers have investigated the effects of cavitation under creep rupture conditions (tensile stress applied at extended periods of time) to understand the causes of these failures. was studying the nucleation and growth of voids in . The effect of hydrostatic pressure on knee formation in the creep rupture test was rPI'1iIO8OI)hl
Oal Maoazine J No. 4, page 673 (
1959) tD, Hull and Tori, E, Rj
■-e' by and rPhllosophlca
l MaOaZlnllJ No. 12, No. 5911
R, T. J (1965).

RatclH1'e and G, W, G rllll
nWOOdk: Yo') r, reported. Study 1 of these tested wire materials under creep rupture conditions and investigated the effect of hydrostatic ratio on cavitation. These tests showed that hydrostatic pressure can eliminate or reduce cavitation in wire creep rupture tests.
It has been discovered that it can be erased by molding.

FIG. 1 is a diagram of a mold used to illustrate the method of this invention. This mold has an upper part 2 and a lower part 4, at the threshold of which a blank 6 for molding is clamped. The clamping pressure can be provided by the ram 8 of the hollow press. A depleted heater 10 surrounds the mold and is used to raise the 11 degrees of the blank 6 to the forming II value required to obtain superplasticity.

Gas passages 1112.14 are formed in both the upper mold part 2 and the lower mold part 4. The lower mold part 4 is a molding machine l
116, which creates the mold cavity and defines the shape of the L&G part, such as the U-shaped channel shown in FIG. When the pressure reaches the true value of blank 6, arrow 18.2
0 through inlet 12.14.

FIG. 2 shows a pressure-time opening relationship curve 22 that can be used according to the prior art to ffl plastically form a blank, such that the gas creates a pressure on the upper surface of the blank 6 according to the curve 22. through the inlet 12 (Fig. 111). Any pressure can cause molding 111i
16; rather, gas can be vented out of the mold cavity as the blank moves downwardly into the cavity. Although parts can be molded in this way,
It may have many small voids due to capacitance.

Such cavitation is advantageous in this invention in which the cavitation is used to create a reduced isotropic tensile stress component in the blank 6 on both sides of the blank 6 during forming, and also to create a reduced compressive stress component. can be prevented by following the method according to the first embodiment. A blank of suitable material is used as the mold part 2.
6-, so that it faces the molding machine W116, it is heated with the skin to a concentration that exhibits an effective value of strain rate, sensitivity, and is held at that concentration.

Pressure is then applied to both sides of the blank by introducing gas through the inlets 12.14.

111311 shows the pressure versus time curve 24.26 for the side of the blank facing the forming machine 116 and for the opposite side of the blank. As shown at No. 311, the pressure on both sides is increased at about the same rate &t, so that an isotropic compressive stress is exerted on the blank 6.

When the isostatic compressive stress reaches a level sufficient to prevent cavitation, the pressure 2 on the bottom side of the blank 6
0 (first Ii1) is kept constant as shown by curve 24. This backpressure can be approximated using the calculations described in the article, or it can be tested running at various backpressures and then cut the part to determine if cavitation is occurring. can be determined experimentally.

The pressure 18 (FIG. 1) on the upper side of the blank is increased as shown by the forward pressure curve 1126 in FIG. The forward pressure curve 26 is shaped in accordance with the prior art techniques necessary to obtain effective values of strain rate sensitivity to form the part, as shown in FIG. However, the forward pressure curve 26 is similar to the prior art curve 22.
When compared to the back pressure curve 24, the overall pressure is pushed upward to overcome the pressure from the back pressure curve 24.

The minimum backpressure must be sufficient without creating a sufficiently reduced tensile isotropic stress component or a sufficiently increased compressive isotropic stress component acting on the void to prevent void formation. This minimum response depends on the material to be molded and the molding conditions, and it can be determined empirically. It also applies to flow and cavitation, such as the diffusion growth concept or the maximum tensile stress concept.
can be approximated using concepts that describe the mechanism.

According to the diffusion growth concept, the minimum backpressure can be calculated from the theory of void formation by cavitation. Sky-
One condition necessary for it to be stable at high temperatures is as follows.

A given isotropic tensile stress component acting on the nucleus of σ-empty or empty ■, where P≧σ−P)≧lower or P≧σ−lower11.

P - Hydrostatic pressure component acting on the nucleus of the empty S or empty ■.

γ - Surface energy in the void.

r - radius of the void;

Therefore, when the hydrostatic pressure component (resulting from back pressure and forward pressure) is increased, the stable cavity size will be increased and cavities of small dimensions will not form or will disappear. Since the dimensions of - are often related to the dimensions of the grains (or inclusions) at grain boundaries, it is important to apply hydrostatic pressure so that the dimensions of the stable voids are larger than the grains. function, thereby preventing the initiation of the deformation capitium.

111411&t, 2 X 10-4s-' nol [D
Parallel formation at f = 1ill [g and an effective forming stress of 300 psl ゛(14〈H = ff[σ - σ ]). This calculation shows that Ne1lE.

p aton and C, Howard Haulat.
US Pat. No. 4,092,181 to John J.: A sheet of 7475 aluminum alloy treated to have a grain structure suitable for ultra-high strength shapes. The estimated surface energy of this sky y
&tlooOera/cl"t's, molded 11fl[
It is 960@F. Tensile stress σ in the blank garden,
are shown by the upper curve 28, τ, and the forward and back pressures (with respect to σ, - the tensile stress in the blank image).

P, - forward pressure.

P2-back pressure.

ρ - dead weight radius of the unsupported part of the sheet.

t - Thickness of the sheet.

The lower curve 30 is the corresponding stress σ3 across the entire thickness of the blank. This is equal to back pressure P2, arrow 3
2, in this example about 1.7
No back pressure is required for the air with radius smaller than μ.

In a second embodiment of the invention, the blank is formed by prior art methods and voids are allowed to form as a result of cavitation. Once the forming is nearly complete, post-forming compressive stress is applied to create a hydrostatic pressure condition within the material and eliminate voids. After this, molding pressure closes the cavity and diffusion bonds the cavity surfaces together.

FIG. 5 shows a pressure-time relationship curve 33 according to this second embodiment. The blank is formed by applying forward gas pressure to the upper side only by prior art methods such as those already described.

However, forward pressure is applied after the part is formed (
Point 34) Not removed. Rather, a forward pressure is maintained while the molded part is in contact with the molding surface 16 which provides a post-molding pressure 36. This state is represented by the cross section of the blank 6 shown by the dotted line in Figure #11. The forward pressure shown by dotted arrow 38 creates a counter pressure from mold surface 16 shown by dotted arrow 40. Therefore, back pressure is created without the need for a second precursor gas.

Although FIG. 5 shows a constant postforming pressure 36, the pressure can be increased to shorten the time required to close the void. Optimal post-forming pressures and hold times are determined experimentally by running tests at several pressures and times and then cutting parts to determine which combinations most effectively eliminate voids. be able to.

In a third embodiment, the methods of FIGS. 3 and 5 are combined. The back pressure 24 reduces cavitation, which can be applied to the mold, and after molding, the mold pressure 26 closes any voids that may have been formed, as shown by the dotted line 38. can be maintained as follows. The backpressure 24 can be maintained during this period, or it can be reduced as the mold reaction provides the necessary backpressure. This combination of embodiments may prove most economical in applications where available equipment is unable to provide sufficient backpressure to completely eliminate cavitation during the molding operation stage.

An example of the method of the invention applied to form a rectangular box 2 inches by 6 inches by 1 inch deep is given below. This material has a fine grain structure suitable for ffl plastic forming.
0.040 inch thick processed by No. 81 74
75 aluminum alloy sheet.

[1 The material is placed in a mold and heated to 960'F. Then the forward pressure and back pressure are both 100 p
It is increased to al. This backpressure is increased at a rate calculated to produce a strain rate ε of 4-1 100 DSl&: retention sag, warpage, r2X10 s. After the part is molded, the forward pressure and back pressure are reduced to atmospheric pressure and the molded part is removed from the mold.

The material is placed in a mold and heated to 960°F. Then the forward pressure and back pressure are both 300 p
increased to sl. The backpressure is maintained at 300 pst, which is equal to the lump stress of the material at the strain rate used. The forward pressure is increased at a rate of 1° to produce a strain rate trace of 2×10 s.

After the part is formed, the forward pressure saw and back pressure are reduced to atmospheric pressure and the formed part is removed from the mold.

The parts obtained formed well, and metallographic evaluation subsequently revealed the absence of cavitation even in the tightly formed corners. Previous parts acidified under the same conditions, but without backpressure,
It showed massive cavitation.

Many variations and modifications can be made without departing from the invention. For example, forward pressure can be applied to form the blank in a conventional manner. However, before the forming is completed, sufficient backpressure can be applied to fill the voids formed in the first pressurization. Next, the backpressure is reduced and the forming continues with positive pressure applied. If it is necessary to provide sufficient isostatic compressive stress, both the forward pressure and the back pressure can be shown to be temporarily increased to close the cavity before continuing the molding operation. This backpressure can be increased and decreased periodically during molding when the most rapid clearance of voids is required for a particular application. It should therefore be clearly understood that the forms of the invention described above and illustrated in the accompanying drawings are merely illustrative and are not intended to limit the scope of the invention. be.

[Brief explanation of drawings]

FIG. 1 illustrates the superplastic forming method according to the present invention. FIG. 3 is a sectional view of a mold and a blank. FIG. 2 shows a typical time-pressure relationship for superplastically forming a U-shaped channel according to the prior art. FIG. 3 shows the relationship between time and pressure for superplastically forming a U-shaped channel using the first model A31 of the present invention. 41st! A is a pair of straight lines showing the stress state for void suppression as a function of void radius. FIG. 5 shows the relationship between time and pressure for superplastically forming a U-shaped channel according to a second embodiment of the invention. In the figure, 2 is the upper mold part, 4 is the lower mold part, and 6
10 is a blank, 10 is a heater, and 12.14 is a gas passage.

Claims (1)

[Claims] (1) Material blank! In this method, a material blank exhibiting an effective value of strain resistance and corrosion at a molding density is prepared, a mold having a molding surface is prepared, and the mold is placed facing the W#am. positioning said blank in said molded skin such that said material exhibits said effective value of strain rate sensitivity and preventing the formation of voids in said material; Applying sufficient pressure on both sides of the blank and increasing the pressure on a portion of the blank that is most different from the forming surface to strain the material at a rate that provides the effective value of the strain coupled sensitivity. , whereby the blank extends towards and is molded against the molding surface without capitation. (2) , which is applied on both sides of the blank, is the isotropic tensile stress component applied at - of the step of increasing the pressure on the part of the blank that is most different from the forming surface; 2. A method as claimed in claim 1, in which the field energy of - and r is a radius chosen to be larger than the interparticles in the material. (3) A method for removing voids from molded parts, comprising: iIm! The molded part is heated to a forming consistency and a direct pressure is applied on both sides of the molded part, thereby causing l! ! voids in molded parts
A method that includes each step in which the is removed by ■sexualization and spreading. (4) sI the material blank! A material blank exhibiting an effective value of strain rate sensitivity at a castle temperature is prepared, a mold having a molding surface is prepared, and the mold is in internal conflict with the molding surface. holding the blank at the forming density such that the material exhibits the effective value of strain rate sensitivity; and forming the material toward and against the forming surface. such that sufficient pressure is applied on the side of the blank furthest from the forming surface to strain the material at a rate that provides the effective value of strain rate sensitivity, and the blank is formed against the forming surface. the steps of applying pressure on the farthest side of the blank, whereby voids present within the material are removed by acidic deformation and diffusion. (5) For superplastically forming a material blank: Prepare a material blank that exhibits an effective value of strain rate sensitivity at the forming concentration, prepare a mold having a forming surface, and place the material blank in the mold facing the forming surface. Position the blank at the front t! holding the blank at the forming density such that the material exhibits the effective value of strain rate sensitivity; applying pressure on both sides of the blank sufficient to reduce the rate at which voids form within the material; The fastest of the blanks from the forming surface so as to strain the material at a rate that provides the effective value of strain rate sensitivity as the material stretches towards and is formed against the forming surface. the farthest side of the blank after the blank has been formed against the forming surface so as to increase the pressure on the side and compress the material in combination with the pressure and the reaction of the mold; A method comprising the steps of applying pressure on the material, thereby eliminating voids within the material. (6) A method of superplastically forming a material blank, which includes forming ll5l! Prepare a material blank that exhibits an effective value of strain rate sensitivity in , Prepare a mold having a forming surface, and perform the forming! positioning the blank in the mold opposite the mold, holding the blank in the forming WA degree such that the material exhibits the effective value of strain rate sensitivity, and the material stretches towards the forming surface. such that a forward pressure sufficient to strain the material at a rate that provides the effective value of strain rate sensitivity is applied on the side of the blank furthest from the forming surface until the material begins to elongate. , applying a back pressure equal to or less than the forming pressure to the side of the blank opposite the most different side to eliminate voids in the material, and forming the material as it hits the forming surface. reducing the back pressure while continuing to apply the forward pressure until the positive pressure is applied. (7.) The step of applying forward pressure includes increasing the forward pressure during the step of applying back pressure, and then decreasing the forward pressure while the back pressure is decreased. The method according to claim 6. 8. The method of claim 6, wherein the steps of applying and reducing backpressure are performed synchronously as the material stretches.