CA2091035A1

CA2091035A1 - Method of stamping for aluminum or aluminum alloy sheet

Info

Publication number: CA2091035A1
Application number: CA002091035A
Authority: CA
Inventors: Yoshio Okamoto; Mitsuo Hino; Shojiro Oie; Takashi Inaba; Masahiro Yanagawa; Tsuneharu Mori; Takeo Sakurai; Kazunori Kobayashi
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1992-03-06
Filing date: 1993-03-04
Publication date: 1993-09-07
Also published as: DE4307020A1; DE4307020C2; FR2688153B1; FR2688153A1

Abstract

ABSTRACT
A method of stamping for an aluminum or aluminum alloy sheet which comprises coating a liquid lubricant which becomes waxy at a cryogenic temperature to the aluminum or aluminum alloy plate, cooling the sheet and applying stamping in a cryogenic temperature region by using a die of a non-cooling structure type, by which formability is improved. A mineral series or synthetic oil series lubricant is used as the liquid lubricant.
Further, a lubricant containing paraffin as the main ingredient and having a viscosity at 40°C of less than 50 cSt or a lubricant containing an ester as the main ingredient, having a pour point from within a range -50°C to -100°C and a viscosity at 40°C of less than 50 cSt is used. In another forming method, stamping is applied by using a cooling structure type form made of a material not causing transition temperature and in which liquid nitrogen is circulated through the inside of a punch for cooling and is jetted out from upper portion thereof as a forming die, while controlling the die temperature to a range from -50°C to -196°C and controlling the aluminum or aluminum alloy sheet to a range from -50°C to -196°C, by which the formability equal with or superior to that of the steel sheet can be obtained.

Description

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Field of the Invention The present invention concerns a method of stampin~
for an aluminum or aluminum alloy sheet suitable to such application uses that ~xcellent formability is required regarding complicate shapes difficult to be formed, for example, in automobile parts, electric parts, aircraft parts and equipments.

Description of the Prior Art In the forming of automobile parts, electric parts, aircraft parts and equipments made of aluminum or aluminum alloys, there has been processing a limit in usual stamping, and in a case where the processing degree is large and severe or in a case of an article havin~ a complicate shape, stampin~ can not be applied or~ otherwise, an aimed shape has to be obtained stepwise by a pressing process divided into a plurality of st~ps. ;
In the latter case, the production cost is inevitably incxeased. ~owever, forming of articles needing severe processing degree and having complicate .; , ~
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shapes has been demanded more and more and cost down by the reduction of the number of forming steps has keenly been desired in recent years.
On the other hand, weight reduction of automobiles has been studied earnestly in order to suppress the increase of gaseous carbon dioxide in the atmospheric air with a view point of environmental problems such as global warming and destruction of ozone layers. As one o countermeasures for the weight reductionr use of aluminum or aluminum alloy sheets has been increased more instead of steel sheets that have been used mainly as the material for stamping. However, since the formability of aluminum or aluminum alloy sheets is inferior to that of the steel plates, there has been also a strong need for the improvement in view of the stampin~ process.
Regarding such requirements, there has been made an attempt for the improvement of the formability of materials by making the ingredient composition and ~`
production steps for aluminum materials appropriate, as proposed in Japanese Patent Laid-Open Sho 63-89649 in view of the materials. ~owever, since the demand for the fvxming of complicate shapes with more severe .. . .

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processing degree has become increased in recent years, improvement only for the materials is insufficient.
on the other hand, also regarding the processing technique~ Laid-Open Technical Report 89-15623 published from Nippon Hatsumei Kyokai (October 10, 1989) has proposed a forming process at a cryogenic temperature as a novel forming process. However, the above-mentioned method only discloses that aluminum or aluminum alloys are put to a cryogenic temperature and shows the effect of the forming temperature on the mechanical property and the Erichsen value of the materials, which is not yet satisfactory in view of practical situation for the improvement of actual formability.

SUMMARY OF THE INVENTION
An object of the present invention is to overcome the foregoing drawbacks in the prior art and provide a method capable of actually practicing stamping for aluminum or aluminum alloy plates, particularly, requiriny large and severe processing degree and having complicate shapes.
Another object of the present invention is to provide a method capable of actually practicing stamping for aluminum and aluminum alloy sheets requiring large ,~
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and severe processin~ degree and having complicate shapes, by using a forming die of a type not equipped with a cooling structure.
A further object of the present invention is to provide a method of stamping for aluminum or aluminum alloy sheets capable of improving the formability to an extent equal with or superior to that of steel plates, by using a forming die of a type having a cooling structure.
For overcoming the foregoing problems, the present inventor has made earnest studies on an actually practicable method of stamping for aluminum or aluminum alloy sheets as proposed above and, as a result, have accomplished the present invention based on the finding that the foregoing problem can be solved effectively by coating a specific press lubricant and controlling a forming temperature in a case of using a forming die not equipped with a cooling structure, or by ccntrolling a temperature of a forming die and a temperature of a stock material in a case of using a die equipped with a specific cooling structure.
In summary, the present invention provides a method of stamping for an aluminum or aluminum alloy plate, which comprises coating a liquid lubricant which becomes , . . . ...

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waxy at a cryogenic temperature on an aluminum or aluminum alloy plate, then cooling the sheet and conducting stamping in a cryogenic temperature region by using a die of a non-cooling structure type.
Further the present invention provides, in another aspect, a method of stamping for an aluminum or aluminum alloy plate, which comprises coating a specific lubricant, that is, either a lubricant comprising a paraffin as the main ingredient and having a viscosity at 40C of less than 50 cSt, or a lubricant containing an ester as the main ingredient, having a pour point in a range from 50 to -100C and a vlscosity at 40C of less than 50 cSt to an aluminum or aluminum alloy plate 7 and then applying stamping directly.
Further, the present invention, in a further aspect, provides a method of stamping for an aluminum or aluminum alloy plate, which comprises conducting stamping by using a die for forming made of a material not causing transient temperature and of a coolin~
structure type in which a liquid nitrogen is circulated through the inside of a punch for cooling and jetted out from the upper portion, controlling the die temperature to a range from -50C to -196C and controlling the . .
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temperature of the aluminum or aluminum alloy sheet to a range from -50C to -196C.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an explanatory view illustrating a spherical top extended die (non-cooling structure type) used in a forming test. . :
Fig. 2 is an e~planatory view illustrating a die cooling device used in a forming test.
Fig. 3 is a view illustrating the effect of forming temperature and cooling condition on the extending height in Example 5.
Fig. 4 is a view illustrating the change of the temperature sheet when it is immersed into and taken out of the liquid nitrogen in Example 3.

BRIEF DESCRIPTION OF THE INVENTION
(1) Forming by Using a Die of Non-cooling Structure Type Usually, stamping is carried out by coating a pres~
lubricant to a forming mater.ial or a forming die at a room temperature and it has been considered so fax that the lubricant is degraded when it is cooled to a cryogenic temperature lower than -40~C to deteriorate the lubricity. However, it has been found that the 3 ~

liquid lubricant actually becomes waxy to rather improve the lubricity when it is cooled to a cryo~enic temperature.
That is, it has been found that when the forming material coated with a liquid lubrisant is immersed in a liquid nitrogen and then taken out therefrom, the lubricant changes as: non-waxy state ~ waxy state ~
liquid with elapse of time, in which the lubricity is improved in the waxy state. Furthermore, it has al50 been found that stamping at a cryogenic temperature is extremely effective in view of the formability since the mechanical property of the stock material at the cryogenic temperature i5 improved more both for the strength and the elongation than those at 2 room temperature even not in the waxy state. Accordingly, in the present invention, stamping is conducted at a cryogenic temperature, preferably, in a state where the liquid lubricant becomes waxy.
The liquid lubricant is previou~ly coated to a forming material prior to stamping, that isl prior to immersion in the li~uid nitrogen. The coating method and the amount of coating may be determines in a customary manner. The liquid lubricant is coated as it is .

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As the liquid lubricant, there can be mentioned mineral oils (paraffinic or naphthenic series) or synthetic oils (ester) but the mineral oils are preferred. As the example of the former, mineral oil (liquid paraffin) can be mentioned.
Further, as other example of the liquid lubricallt, a lubricant comprising paraffin 2S the main ingredient and having a viscosity at 4~C of less than 50 cSt can be mentioned. Generally, the lubricant has higher viscosity as the temperature is lower and, when a paraffin-containing lubricant is cooled, paraffin solidifies into a waxy state to form a lubricant ;~
membrane thereby improving the lubricating effect.
Further, the higher the viscosity the better the lubricity of the lubricant. However, if the viscosity at 40C exceeds 50 cSt, though formability is preferable, the lubricant becomes solid wax at a normal temperature tending to cause degreasing failure in the degreasing step, which results in joining failure in the subsequent joining step such as adhes;on or welding.
The paraffin-rich lubricant may be composed only of the paraffin (100~ by weight) and, in addition, preferably contains more than 50% by weight of paraffin, bec~use it is advantageous in that a homogeneous i ; .:
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membrane is formed when the paraffin as the ingredient becomes waxy at a low temperature. Further, ingredients other than the paraffin, for exampler an extreme pressure agent may also be added as required, except for those ingredients which coagulate at a low temperature (frozen) and result in the destruction of the lubricant membrane (water or the like).
Further, as another examplP of the liquid lubricant, there can be mentioned a lubricant comprising an ester as the main ingredient, haviny a pour point within a range from -50~C to -100C and a viscosity at 40C of less than 50 cSt. As the temperature of the lubricant is lower, the viscosity is higher and the lubricancy is improved more. However, since the pour point is as high as 10C in mineral oils and the lubricating effect is lowered remarkably, an ester type synthesis oil capable of lowering the pour point is used as the lubricant. The ester type synthesis oil can include, for example, diester and polyol ester.
Further, it is better that the pour point is lower.
However, if it is higher than -50C, lubricity is degraded remarkably at a low temperature, whereas preparation of a lubricant with a pour point of lower than -100C i5 industrially difficult and it is not _ g _ ., . . . : .... : .
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~ 7 preferred in view of production cost as well.Accordingly, the pour point of the ester type synthetic oil is limited to a range from -50C to ~100C.
Further, in the course of cooling the oil, the highest temperature at which the oil loses fluidity is referred to as a coagulation point and a temperature higher by 2.5 than that is referred to as a pour point which is a temperature at the instance the oil loses the fluidity The reason why the viscosity of the lubricant is limited as described above is the same as that in the case of the paraffin-rich lubricant.
The ester type synthetic oil includes a case that it is composed only of the ester (100% by weight~ and in addition, preferably include a case of containing more than 70~ by weight of ester, because it is advantageous in that a homogeneous membrane is formed when the ester as the main ingredient becomes a film-like state at a lo~ temperature. Further, other ingredients than the ester, for example, an extreme pressure agent may also be added as required, except for those ingredients which coagulate at a low temperature ~fro2en) and result in the destruction of the lubricant membrane (water or the like)~ As a method of cooling the mateFial coated with the liquid lubxicant to a , . . .
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cryogenic temperature, the material is immersed in a liquid nitrogen in view of the general applicability and the cost. A measure that the liquid lubr.icant stays waxy when it is taken out after immersion is within 15 to 45 sec in a case of using the liquid n:itrogen as a coolant. Since this is more than 15 sec, the operation time is torelable from take-out to stamping, which is practically advantageous.
After cooling, the material is formed in a cryogenic temperature region, preferably, from -50C to -196C. In a case of using a lubricant having paraffin as the main ingredient and a viscosity at 40C of less than 50 cSt, or a lubricant comprising an ester as the main ingredient, having a pour point from -50C to -100C and a viscosity at 40C of less than 50 cSt, it is preferred to conduct forming within a temperature range from -50C to -150C, because the effect of improv.ing the formability is insufficient in a case of a temperature higher than -50C, whereas reduction of the formability is caused due to the degradation of the lubricant and the cost i5 increased in a case of a temperature lower than -150C.
In a case of using a lubricant having the paraffin as the main ingredient and the viscosity at 4UC of less ;
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Although the effect for improving the forming product i5 lower, the de~reasing property is better as compared with the case of conducting forming within the above-mentioned temperature range (-50C to ~15~C).
As a die not equipped with a cooling structure, a structure, for example, as shown in Fig. 1 can be mentioned. In the drawing, are shown die 1, a blank holder 2 ~nd a punch 3.

(2) Forming by Using a Die of a Cooling Structure Type The method previously proposed by the present inventor, et al is a method of conducting forming at a normal temperature although the aluminum alloy is applied with a cryogenic treatment. Accordingly, it is considered that the temperature of the sheet is elevated by the die and the atmosphere and performance at a cryogenic temperature can not be obtained~ However, lowering of the die leads to embrittling crack of the die which is not practical. In view of the above, the 1~ --: . ~, .

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present inventors have made study and experiment on the material of the die, and made a close investigation for the effect of the die temperature and the atmospheric temperature on the formability.
That is, the present inventors, et a:L
experimentally confirmed that a conventional die material (alloy tool steel SKD 11) results in embrittling fracture at a temperature lower than -100C/
whereas austenite series stainless steel and Cu Ni alloy having face-centered cubic lattice does not result in embrittling fracture even at -196C and causes no problem as the die material even in repeating pressing.
On the other hand, an aluminum alloy sheet coated with a lubricant was inserted into a die, the temperature both for the sheet and the die was lowered and the effect of temperature lowering on the formability was investiyated. As a result, it was confirmed that the formability was improved as the temperature was lowered and, in particular, a formability comparable with that of the steel sheet could be obtained at -196C.
A diel though different depending on the shape of products, basically comprises a punch and a die~
Various methods may be considered for lowering the die temperature to a ~ryogenic temperature (-196C). What :,.. .
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is essential i~ ho~ to make the entire die to the cryogenic temperature.
According to the study of the present inventors, et al, it has been found that a method of circulating a liquid nitrogen in the punch and then jetting out the liquid nitrogen over the entire die needs a shorter time for reaching the cryogenic temperature and is effective.
Specifically, as shown in Fig. 2, (1) liquid nitrogen is spirally circulated through the inside of the punch, and (2) the liquid nitrogen is jetted out from the upper portion of the die to cool the entire die. In the drawing, are shown a die cooling valve 1, a temperature control valve 2, a punch cooling valve 3, a liquid nitrogen cylinder 4, a die 5, a blank holder 6 and a punch 7. ~n the case of using the liquid nitrogen, the temperature of the liquid nitrogen for cooling the punch 7 and the entire die is controlled by the temperature control valve 2 respectively but it is of course possible to use other coolant in combination with the liquid nitrogen.
According to this cooling method, the die temperature can be controlled easily within a range from -50 to -196C. That is, it can rapidly cool the entire die, as well as make the temperature o the entire punch - .' ' ! .

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lower in view of the formability. In particular, improvement of the formability depends on the balance between the strength of the punch shoulder and the deformation strength in the dice~ in which the improvement for the strength of the former is particularly important. The punch main body can be cooled more efficiently by spirally arranging a pipe in the punch for circulating the coolant ~liquid nitrogen).
In a case of forming under such a condition, there are a mode of forming while making the temperature of the die and the temperature of the aluminum or aluminum alloy sheet identical and a mode of forming while giving a temperature slope. In each of the cases, no improvement is obtainable for the formability at a temperature exceeding -50C (elevated temperature)~
which has no significant difference with that at a normal temperature. Further, a cryogenic temperature lower than -196C (for example, -200C) can not be obtained easily by using the liquid nitrogen and it requires liquid helium, which results in a problem in view of handling and cost. Accordingly, the temperatures for the die and the aluminum or aluminum alloy sheet are controlled, respectively, to a range from -50~C to -196C.

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As in the former mode, when forming is applied~
w.ith the temperature for the aluminum or aluminum alloy sheet being identical with that for the die, the formability is improved as the temperature approaches -196C.
On the other hand, as in the latter case, when forming is conducted under a temperature slope, a further improvement is obtainable for the formability.
In order to attain this, it is ne~essary to fully open the supply of the liquid nitrogen to the inside of the punch and control the amount of the liquid nitrogen from the blank holder. The formability more excellent than that obtained in a case where the temperature of the aluminum or the aluminum alloy sheet is made identical with that of the die at -196C can be attained by controlling the temperature for the punch constant at -1~6C and the temperature for the die corresponding to the flange and the material at -5G to -100C. If the temperature corresponding to the flange is within a range from normal temperature to -50C, instantaneous cooling of the punch material (cooling from the punch) is insufficient in which no improvement can be expected for the strength of the shoulder and the formability is not improved. On the other hand, in a case of a : . , , ., . . , , . ~ . . ~ . . .. . .

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temperature lower than -100C, the deformation resistance of the Flange is increased, by which no remarkable improvement can be obtained for the formability.
It is important for the material of the die that it is free from embrittllng fracture at the cryogenic temperature and it is necessary that the die can be manufactured easily and at a low cost. With the above-mentioned view point, austenite series stainless steel and Cu~Ni alloy are suitable as the material of the die showing no abrupt change of characteristics depending on the temperature (transition temperature). As the austenite series stainless steel, SUS 304 is a typical steel species, while Colson alloy Ni-3%, can be mentioned as the Cu-Ni alloy. It will be apparent that any of materials may be used so long as it causes no embrittling fracture or cracking in the stamping at a cryogenic temp~rature.
The aluminum or the aluminum alloy sheet can be controlled to a desired temperature within the above-mentioned temperature range by an appropriate method.
Usually, a method of passing (immersing) the sheet in the liquid nitrogen or a method of jetting out a coolant :..... .

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on the sheet just before the forming process is adopted.
A particularly preferred method is as follows.
That is, after coating the aluminum or aluminum alloy sheet coil or sheet with a lubricant, it is immersed in a liquid nitrogen for more than 15 sec, the temperature of the sheet is lowered to a cryogenic temperature lower than -100C and, subsequently, the die is inserted into the cooling structure within lS sec.
After coating the lubricant, the sheet is immersed in the liquid nitrogen. It is desirable that the sheet temperature is at least lower than -100C and uniform.
If the immersion time is shorter than lS sec, such a temperature can not be ensured and, as a result, (1 improvement for the formability at a cryogenic temperature can not be obtained and (2) defect prevention due to increased strength under the cryogenic temperature can not be obtained. Accordingly, more than 15 sec of time is required as the immersion time in the liquid nitrogenO Immersion for a longer period of time gives a problem in productivity but this is effective for a further improvement of the formability Then, the material taken out of the liquid nitrogen is inserted into the die of the cooling structure shown in Figi 20 If it is inserted after a period beyond 15 sec, the . .
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sheet temperature is elevated leading to the result in (1) above. According~y, the material taken out of the liquid nitrogen is inserted within 15 sec.
The lubricant is coated prior to the cooling of the aluminum or aluminum alloy sheet foil or sheet. There is no particular restrlction on the lubricant but it should be noted that this gives an effect on the formability at the cryogenic temperature. In particular, a lubricant that crystallizes in the cryogenic temperature region due to the water content (exhibiting icy surface) is not preferred since it results in unevenness in the stamping and gives less effect for the improvement of the formability. Further, a lubricant having no fluidity at a normal temperatu-re is not preferred since this causes defects in the subsequent step. Accordingly, a lubricant that hecomes waxy or film-like state also at a cryogenic temperature and has a viscosity at 20C of less than S0 cSt is preferredO As one of themr liquid paraffin can be mentioned.
The forming material is an aluminum sheet or an aluminum alloy plate. Among them, there is no particular restriction, for example, on the material of the aluminum alloy sheet and a material of an ~ .

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appropriate ingredient system and composition may be selected depending on the required performance of the final product. For instance, Al-Mg sexies alloys containing more than 2.5 weight ~ Mg, more preferably, Al-high Mg (3 - 6~ Mg) systems are preferred in view of the formability and the strength. The reason is that My is an element giving strength and formability and they are most suitable as the material for forming having both of the characteristics together. However, if the Mg content is less than 2.5% by weight, the strength of the product after forming is insufficient. So long as Mg is incorporated by a predetermined amount, an ingredient composition that can be used for the application use of this type is obtainable and, accordingly, other alloy ingredlents can be contained properly as required.

BEST MODE CARRYING OUT THE INVENTION
Examples of the present invention are to be shown.
It will be apparent that the present invention is not limited only to these examples but various modes of practice are possible within a scope of the present invention.

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, , , -Example 1 Using JIS 5182 alloy O material of 1 mm sheet thickness as the material for forming, stamping was carried out by a spherical head extended dle shown in Fig. 1 using a mechanical press. In this case, after Goating mineral oil (liquid paraffin) as a liquid lubricant to the material, it was immersed in a liquid nitrogen for 2 min, then taken out and soon applied with stamping. The formability was evaluatedi by changing the stroke length of the punch and the result of measuring the formable height is shown in Table 1. From the table, it can be seen that the formable height was increased in the example of the present invention in which the material was immiersed in the liquid nitrogen, cooled to a cryogenic temperature then taken out and applied with stamping as compared with a conventional stamping not immersed in liquid nitrogen (comparative example).

Example 2 Using Al-Mg series alloy O material of 1 mm sheet thickness having the chemical ingredient as shown in Table 2 as the material for forming, the same stamping test as in Example 1 was conducted. Result for the measurement of the fGrmability and the hardness of forming products are shown in Table 2. In a case where the Mg content is low, strength as the forming product is insufficient.

Example 3 Using JIS 5182 alloy 0 material of l mm sheet thickness as the material for forming, a processing test was conducted while immersing the material in a liquid nitrogen and varying the time before processingO Other conditions are the same as those in Example 1. The results are shown in Table 3. In Table 3, it is recognized for the improvement of the formability when processing was applied after elapse of about 30 sec immediately after the immersion. This well corresponds to the waxy state. That is, since the waxy state ls not formed just after take-out whereas it is already changed into liquid 60 sec after take-out, by which the lubricity is reduced. The formability in each of the cases i.e., just after take out and 60 sec after take out was improved as compared with a case of no~
immerslng in the liquid nitrogen (comparative example in Example l).

, , , ~ :, Example 4 JIS 5182 alloy O material of 1 mm sheet thickness was used as the material for forming. A forming test (BHF: 4.0 Tonf. Mean velocity: 15 m/min) was conducted using the spherical head die shown in Fig. 1 and using a crank press. In this case, the material was coated with various oils shown in Table 4 were coated as the lubricant, immersed and cooled in a liquid nitrogen for two min and then served for the test. A portion of the samples was served to the test directly after coating the lubricant.
For the evaluation of the formability, the height of the forming product was gradually increased by changing the lower dead point of position of the press and it was judged depending on the forming limit height at which cracks were caused to the forming product.
Further, for the evaluation of the degreasing property, the material coated with the lubricant was degreased by using a commercially available degreasing agent (degreasing condition are shown in Table 4) r and the situation of the remaining lubricant was judged based on the water leak area ratioO The results are as shown in Table 4, and an improvement for the forming limit height was recognized in the example of the present invention - ~3 -2 ~ 3 ~

as compared with the comparative exa~ple and the degreasing property was also satisfactory. In particular, formability at low temperature (-148C) was remarkable.

Example 5 JIS 5182 alloy 0 material and steel sheet were used as the stock material, and paraffinic mineral oil having a viscosity at 40C of lO cSt was coated as a lubricant (polyethylene sheet was used for a portion of the stock material), and the stock materials were stamped at various forming temperatures. The results are shown in Fig. 3. From the figure, it can be seen that the forming limit height of the aluminum alloy is improved, in parti~ular, at a low temperature of lower than -503C
by using the lubricant according to the present invention. Further, the forming height is most improved at a stock material temperature of -196C, and 20 minutes of period was required per one stock material for cooling down to the above-mentioned temperature.

Example 6 JI5 51~2 alloy 0 material of l mm sheet thickness was used as the material for fQrming. A forming test , , :
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(BHF. 4.0 Tonf, mean velocity: 15 m/min) was conductedby using a spherical head die shown in Fig. 1 and using a crank press. In this case, various oi]s shown in ~able 5 were coated as the lubricant to the materials, which were immersed and cooled in a liquid nitrogen for 2 min and then served to the test. A portion of the samples was directly served to the test just after coating the lubricant. Evaluation for the formability and the degreasing property was made in the same manner as in Example 4 and the results are as shown in Table 5.
Improvement in the forming limit height was recognized and the degreasing property was also desirable in the example of the present invention as compared with the comparative example. Particularly, the formability at low temperature (-148C) was remarkable.

Example 7 A die made of SUS 304 and a die mechanism shown in Fig~ 2 were used. Test material of 5182-0 material (1.0 mm thickness) was coated w.ith a liquid paraffin, inserted in a die for cryogenic temperature and the formability was evaluated while varying the die temperature and the temperature for the test material.
The method of lowering the temperature was conducted by - ~5 -2 ~ 3 ~

spirally circulating a liquid nitrogen through the inside of the punch, jetting out it to the entire and to the test material. The temperature was controlled from the normal temperature to -196C and the temperature of the material in contact with the punch was measured by using a contact type thermometer. The formability was evaluated by the extruding height of a 50 mm~ spherical head punch (disposed in a 45 ton crank press). From the test results as shown in Table 6, it can be seen that the formability (extending property) was improved together with temperature lowering and it was comparable with that of the steel sheet at ~196C.

Example 8 In the test temperatures in Example 7, the die material was changed in ea~h of the tests at -100C and ~196C, and low temperature brittleness of the die was evaluated. As a result, minute cracks were recognized after forming for three times at -100C for the steel material (dice steel SKD 11) and the test was interrupted. On the other hand, no minute cracks were recognized even after 100 cycles of press tests at -196C in the case of austenite type stainless steel ~SUS

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304~ and Cu-Nl alloy (Colson alloy) and it was confirmed that such materials were effective.

Example 9 5182-0 material (1 mm thickness) was coated with a liquid paraffin, which was immersed into and taken out of a liquid nitrogen and left to examine the temperature change. From the results as shown in Fig. 4, it can be seen that the immersion time was preferably 15 sec and it was preferably inserted in a short period of time into the press under the condition of ensuring the temperature upon insertion in the die to lower than -50C while considering the productivity. Further, for ensuring a lower temperature, it is necessary to take a longer immersion time. However, if it exceeds 40 sec, it may cause time loss according to the result of this test.

Example 10 5182-0 material ~1 mm thickness) wais coated with a `
li~uid paraffin, which was immersed into and taken out of a liquid nitro~en and evaluated while varying the sheet temperature. The die temperature was constant for the punch and the dice~ and the material was inserted , . : . :- . ~:

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into the die just after the sheet temperature was reached (within 2 sec). For the formability, the limit was determined by 50 mm~ spherical head e~tendiny processing. The results are shown in Table 7. If the sheet temperature is within the present invention, the formability is satisfactory. The performance is poor as compared with the material sufficiently cooled in the die to -196C but it is remarkably excellent over the room temperature processing by the conventional method.

Example 11 5182-0 material (1 mm thickness) was coated with a liquid paraffin, which was immersed in a liquid nitrogen and then left to change the sheet temperature~ The punch temperature was set to -196C while the temperature for the flange was set to -70C and -150C, and the material was inserted into the die just after the sheet temperature was reached (within 2 sec). After about 2 sec (punch portion cooled), pressing was applied and the results are shown in Table 9. Satisfactory formability is shown in each of the cases in the method according to the present invention and it can be seen that the condition capable of obtaining formability more excellent than the case of -196C both for the sheet and ; . . ..

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-the die is the condition at a temperature on the side of the punch of -196C and at a temperature within a range of -50 to -100C for the sheet as the flanye and the die.

INDUSTRIAL APPLICABILITY
As has been described above accordin~ to the present inventionl since press processing is applied in a cryogenic temperature region, formability can ~e improved. In addition, since processing can be appliecl also in a preferred lubricating state, the formability can further be improved. Therefore, it can provide an excellent effect for forming at severe processing degree and for complicate shapes, can reduce number of forming steps, which is economical and can be put to practical use.
In particular, the formability of the aluminum alloy sheet can be improved to an extent equal with or superior to that of a steel sheet by using a die equipped with a cooling structure and controlling the temperature for the die and the sheet andl in particular, a formability superior to that of the steel sheet can be obtained by a cooling method providing a temperature slope. Therefore, it can provide an - 29 - . .

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e~cellent effect for the weight reduction of automobile parts, that is, promotion for the use of aluminum material and can contribute as a countermeasure to environmental problems such global warming.

. ~ . , ~ .

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Table 1 .. l ~toriol Lubricnnt ¦ stamping , FormabIe Note eig . Atter coating lubricont, i~mor Min~3ral sed in liquid Ex~mplo of nitrogen for tho inventiD
oil 2 min ond then 2 ~;~m stamped soon JISS182-O . .
material ., .: _ _ _ Af t~3 r coat i ng .
lubricont, Conperative . stamped cs it is 2 2 ~
. E xample .
~;-- ... _ _ .. ._ Table 2 ._ Ni~ ChemiCi9l ~(Vt%) Formable height t~rdness of . _i~l i~ . . forming product Mg S i ~?~ (~m) (~v) 1 ~ 4~2 0.11 0.21 2 ~ 9 3 2 ¦2.8 0.13 0.22 2 3 B 1 ¦ . :
_ .
3 2~3 ~11 0.22 2 O 7 3 _ 1~ ~.13 ~.2~ 2 1 _ _ Table 3 _ , i .
Tima z~ter--liquid. nitrogen ¦ Formable height t rea tmen t I . ~lo ~ e ~sec) ¦ (0~) , ._ .
Immediately after ¦ 2 ~
.. _ _ . . .
1 ~ ¦ 2 6 3efore woxy state _ . _ . _ 3 O 2 8 ~axy state 6 O 2 ~ L iqu id '~

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U~ o h ~ 1 ' ', '-, ~ ' Table 6 Formability - ~ -Test temper2ture for Note j extending formability I (C) Limit extendiDg height (mm~
! - _ Conventional ¦ Normal temperature ¦ 2 1 ~ a ¦ exa ple .Comparz'.iYe -- 3 ~) ~ 2 3 . ~ l exam?le - 1 0 0 2 ~ . 5 Exam?le of the inven--¦ - 1 3 0 ¦ 2 7 . 0 tion - 1 9 6 3 0 ~ 0 .
.
Steel sheet 3 0 0 ComparatiYe (Dormal sheet) example Table 7 I Extending Sheet Die ! property Note temperature temper2turq ~xtending ( C) ~ (C) _ I heiqht (mm) _ -- 3 ~ O ¦ 2 ~ examDl e - ~30 ~ 0 1 27~0 Example of - 1 2 0 - 1 ~ 0 2 7 ~ O the inven-_ tion - 1 7 0 - 1 ;: 0_ 2 7~ ;:
-170 -196 3 0.
_ _ Cl~nventional Normal temperature 1 2 ~ examDle At normal temperature 1 3 0 0 1 co~paratiYe for steel sheet I . IexamPle .

Table 8 , _ , temperature ~ temperature Frange temperature heigh ~ Note -- 4 0 ¦ --1 9 ~i -- 7 0 2 7 0 Icomparative ¦ _ ~ O ¦ ~ ¦ n ¦ 3 1. ~ Example of _ , the inven-- 1 2 0 _ I ¦ 2 6. 0 tion i-l7~ 1 I -l~o 7 2~3.0 .

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Claims

1. A method of stamping for an aluminum or aluminum alloy sheet, which comprises coating a liquid lubricant which becomes waxy at a cryogenic temperature to an aluminum or aluminum alloy plate, then cooling said sheet and applying a stamping in a cryogenic temperature region by using a die of a non-cooling structure type.

2. A method as defined in claim 1, wherein the cryogenic temperature region ranges from -50°C to -196°C.

3. A method as defined in claim 1, wherein the sheet material is applied with the lubricant and then immersed in and taken out of a liquid nitrogen and, subsequently, applied with stamping within 15 to 35 sec.

4. A method as defined in claim 1, wherein a mineral oil type lubricant or synthetic oil type lubricant is used as the liquid lubricant which becomes waxy at a cryogenic temperature.

5. A method as defined in claim 1, wherein the sheet is an Al-Mg series alloy sheet containing more than 2.5%
by weight of Mg.

6. A method as defined in claim 1, wherein a lubricant containing a paraffin as the main ingredient and having a viscosity at 40°C of less than 50 cSt is used and stamping is applied within a range from -50°C to -150°C

7. A method as defined in claim 1, wherein a lubricant containing an ester as the main ingredient, having a pour point within a range from -150°C to -100°C and a viscosity at 40°C of less than 50 cSt is used and stamping is applied within a range from -50°C to -150°C.

8. A method of stamping for an aluminum or aluminum alloy plate, which comprises using a lubricant containing a paraffin as the main ingredient and having a viscosity at 40°C of less than 50 cSt or a lubricant containing an ester as the main ingredient, having a pour point within a range from -50°C to -100°C and a viscosity at 40°C of less than 50 cSt, coating said lubricant to the aluminum or aluminum alloy sheet and, subsequently, applying stamping using a die of a non-cooling structure.

9. A method of stamping for an aluminum or aluminum alloy plate, which comprises applying stamping by using a die made of a material not causing transition temperature and of a cooling structure type in which liquid nitrogen is circulated through the inside of a punch for cooling and is jetted out from the upper portion thereof, controlling the die temperature to a range from -50°C to -196°C and controlling the temperature of the aluminum or aluminum alloy sheet to a range from -50°C to -196°C.

10. A method as defined in claim 9, wherein the material of the die is an austenite stainless steel or Cu-Ni alloy not causing transition temperature.

11. A method as defined in claim 9, wherein the method comprises coating a lubricant to the aluminum or aluminum alloy sheet or sheet, immersing the sheet or sheet in a liquid nitrogen for more than 15 sec, controlling the sheet temperature to a cryogenic temperature of lower than -100°C and then inserting the sheet or sheet within 15 sec into the die of the cooling structure type.

12. A method as defined in claim 9, wherein the temperature of the die and the temperature of the aluminum or aluminum alloy sheet is made to a substantially identical temperature within the above-mentioned temperature range.

13. A method as defined in claim 9, wherein the stamping is applied while lowering the temperature for the punch of the die to -196°C and the temperature for the flange and the aluminum or aluminum alloy sheet to a range from -50°C to -150°C.

14. A method as defined in claim 11, wherein a lubricant which becomes waxy at a priority temperature and has a viscosity at 20°C of less than 50 cSt is used as a lubricant to be coated.