CA1237484A - Laser welding of sandwich structures - Google Patents

Abstract

LASER WELDING OF SANDWICH STRUCTURES

ABSTRACT

A process is disclosed for bonding multi-layer sheet struc-ture by using controlled depth laser welding. The laser energy may be applied in stages, whereby the inner sheets are first welded together and, the outer sheets are then joined thereto.
The sheets are thereafter expanded to form the desired struc-ture. Although the laser energy may be varied to control the depth of penetration, one embodiment utilizes a shield material to be applied between two sheets to prevent the laser energy from penetrating and joining the two sheets together. Similarly, shields may be used to control the width of the laser weld.

Description

LASER WELDING OF SANDWIC~ STRUCTUR~S

BACKGROUND

The invention pertains to an improved method of welding together sheets utilizing high energy lasers, to form layered sheet structures for use in the aircraft industry.

Superplastic forming combined with diffusion bonding ~SPF/D~) is finding increased usage in sandwich structures in the aircraft industry (see U.S. Patent No. 3,927,817 entitled "Method of Making Metallic Sandwich Structures" by Hamilton, et al which supplements technology of this disclosure). Superplasticity is the capablilty of certain metals to develop unusually high elon-gations with reduced tendency towards necking, within a limited temperature and strain rate range. Dif fusion bonding is a metal-lurgical joining of similar metallic parts which are pressed together at elevated temperatures and pressures.

Many of the same alloys used in superplastic forming can also be used in diffusion bonding. When the two processes are com-bined, the temperatures and pressures for both processes are similar so that complex and expanded sandwich structures can be ~ormed in what is essentially a one-step operation.

-2- 82L7 However, SPF/DB has several limitations:
1. only those materials which are superplastic may be used, 2. the structures must be raised to high superplastic forming temperatures and pressures,

3. the considerable stretching may produce a non-uniform product, having non-uniform strength properties, and

4. certain materials cannot be readily diffusion bonded.

A novel process for fabricating sandwich structures withou~
the use of superplastic materials, is described in U.S. Patent 4,361,262 entitled "Method of Making Expanded Sandwich Struc- _ tures" and in U.S. Patent 4,588,651 entitled "~ccordion Expansion Process" both by Leonardo Israeli, both of which supplement technology presented in this disclosure. The process is essentially an unfolding process, and usually requires minimal tensile stxetching of the material during expansion, i.e. the expansion of the structure is due substantially to unfolding rather than stretching. The accordion expansion process may be used as an alternative for superplastic forming. However, 1;~3~ 8A

diffusion bonding, re~uires nigh temperdtures an~ pressures, and is limited to certain materials. Hence, an alternative bsnding process is needed that does not re4uire high ter,~peratures and pressures, that is applicable to a broad rangè of materialsJ and that can be used with accordion expansion to form sandwicl~ struc-tures.

Lasers are well suited as a manufacturing tool. ~aterial processing is currently one of the most important industrial applications of lasers. Laser welding~ which can be accomplished ' at or near atmospheric conditions, produces the highest energy concentration of all welding processes. Lasers can generate a high po~Yer density that is localized and controllable over a small area. Also, lasers allow for cost efficient energy u~iliz-ation, minimal distortion and softening in t~e surrounding inter-layer, and simplified material handling. Since lasers result in the application of considerable amounts of high eneryy in short time intervals, high speed manufacturing, accuracy~ and repeat-ability are inherent in laser applications.

W~lat is needed is a process that will utilize the many advan~
tages sf laser processing to ueld sheet structures prior to expansion to acilieve the same kind of monolithic sand~icll struc-tures produced by superplastic formirly an~ di~fusion bonding.
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It is ~lerefore a principal object of t~e present invention to provide a new method of metallursically bonding l~yered sheet structure.

It is another object of tne present invention to provide a new method of making expanded, sandwich structure~ while avoiding the high pressures and temperatures associated with diffusion bonding.

Anotner object of the present invention is to incorporate controlled depth welding inherent in laser technology into the fabrication of expandable, layered sheet structure.

It is another object of the invention to provide a new method of controlling tne depth and ~idth of a laser weld in layered sheet structure.

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The present invention involves controllin~ tl~e welding depth in layerea sheet structure prior to expansion so as to bond tnese sheets together in a pre-selected pa~tern. After three sheets have been positioned in a stack, a laser is used to weld the center core sheet to each ~ace sheet. Laser welding is the joininy of two similar surfaces Dy applying laser energy to the materials so that t~e adjoining surfaces are merged into eacil other. It is of critical importance to control the depth of the weld, so that at any one sear,l, the welded region does not pene-trate into the distant face sheet, since this wil1 interfere with the proper formation of the sandwich structure.

Althouyh it is planned that the present invention will be used primarily with metallic structures, the invention may also be a~plied to nonmetallic structures. T~le lasing energy melts the material in the laser welding area, and forms a pool of material, whic~l resolidifies thereby forming the weld.
Dissimilar materials are not norMally used because of the difficulty in controlling t~le composition of the resolidified weld.

~37~3~ 82L7 Laser weldiny is applied ~o the layered shPet siructure prior to expansion. After the surfaces are welded together, additional heat and pressure are applied as required to expand tne stack to forrn the finished structure.

The nove1 features whictl are believed to be characteristis of this new method of laser welding layered sheet structure, to-gether with furtner objects and advantages thereof, will be better understood from the followiny description in connection Witll the accompanying drawings in Whicil presently preferred embodiments of the invention are i11ustrated by way of examples.
It is to be expressly understood, however, that the drawings are for purposes of illustration and description only, and are noi intended as a definition of tlle limits of the invention.
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BRIEF DESCRIRTIOI~I OF THE I~RAWINGS
_ FI6U~E 1 depicts a detail view of the laser welded region of a three sheet structure prior to expansion.

FIGURE 2 depicts a detail view of the laser welded region of a three sheet structure using shield materials, prior to expan-sion.
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FIGURE 3 deplcts a cross-sectional view of a three-sheet ~' sandwich structure after expansion, wherein a core sheet is surrounded by two face sheets.

FIGURE 4 depicts a cross-sectional view of a fsur-sheet sand-wich structure prior to expansion.

FIGURE 5 depicts a cross-sectional view of a four-sheet sand-wich structure shown in FIGURE 4 after expansion, wilich is aligned with FIGURE 4, wherein accordion expansion has been used to form a vertical core.

FIGURE 6 depicts a cross-sectional view of a five-sheet sand-wich structure prior to expansion.

FIGURE 7 depicts a cross-sec$ional view of the same five-sheet sandwich structure shown in FIGUR~ 6 after expansion, wherein accordion expansion has been used to form an oblique core.

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`' - ' 0594~
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DETAILEU DESCRIPTI0l1 Of THE IhYENTI~N
-All drawings are exaygerated for purposes of illustration, since sheet thickliess will ordinarily ranye from 0.05 to 0.15 inches.

Referrlng now to the drawings, there is shown in FIGURE 1 a typical laser welded region 4 in a three-sheet stack prior to expanslon. The three-sheet stack includes two face sheets 6 and : ~, and core sheet 7. The laser energy penetrates through face Y
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sheet 6 and into core sheet 7, forminy laser welded region 4 .~which is surrour!ded by heat affected zone 5.
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The sheets are preferably in intimate contact, but ulllike diffusloo bondiny thiS is not required. It is suggested that spaces between the sheets 6, 7, and 8 do not exceed ten percent of the sheet thickness.

The heat affected zone 5 surrounds the laser welded region 4 and is the zone of plastic deformation of tne material in the vicinity of the joint. As a result of the nigh concentration of laser eneryy an~ the intensi~y of the enersy, laser welding is OS9~D
_g_ 3 ~ ~3~
~2L7 characterized by the fact that the thickness of tnP heat affected zone 5 is three to five tirlles smaller than the similar zone produced by other welding techniques. Althou~h zone 5 is not melted, it may be subject to transformation produced by the laser energy, The t~linness of zone 5 results in laser welds tnat are about five times lower in residual strains and transverse strains than conventional arc welds. It is believed that the thin heat affected zone 5 also contributes to tlle inlproved corrosion and .
fatigue strength of the laser ~elded regions.

Referring now to FIGURE 3 a three-sheet sandwich structure is ; shown after expansion. Laser welding is used to seam weld face .
` si~eet 13 to core sheet 12, for e~dmple at welded areas 21 and 23. Care must be exercised to control the weld depth, so that -excess ener~y will not weld face sheet 11 to welded areas 21 and 23. Similarly, welded areas 22~and 24 are for~ed by welding face sheet 11 to core sheet 12.

0~94D

~L~317~3~ ~2L7 FIGURE 4 and FIGU~E 5 are ali~ned and depict the forr.~ g of a four-sheet sandwich structure fornled by accordion expansion.
FI~URE 4 depicts the pre-expanded stack having face sheets 31 and 34, and core sileets 32 and ~3, which are select7vely cut prior to expansion at cutouts (e.g. 71, 72, and 73~. It is preferred that one core sheet is used for each l~yer of workpieces, with each sheet having the cutouts. It is furt~ler suggested that narrow slivers of sheet (not shown) be used to hold the workpieces in position within the stack, wherein the slivers rupture during the , ~
~ forming~process.~ The four-sheet structure must be formed in i stages;if laser welding is employed, since to do otherwise would ~requlre welding at each welded area through a face sheet. By - ` laser welding in stages, complex mul~isheet expanded structures ~can~be formed by ~he process of the present invention. Hence, 'I
core sheets 32 and 33 are laser welded at areas 61, 64, 66, and 6~. Then, face sneet 31 is added and controlled depth laser welded at areas 62, 65, and 67. Finally, face sheet 34 is added and controlled dépth laser welded at areas 63, 60, and 69.

05~4D

FIG~RE 5 depicts the expanded structure. The core sheets 32 and ~3 have Deen unfolded and stretched using accordion expansion to form a substantially vertical core supportiny face sheets 31 and 34, comprising core pairs 41 and 42, 43 and 44, and 45 and 45. It is estimated that only about five to ten percent stretch-ing is re4uired to make the combined pairs substantially vertical and capable of supporting heavier traverse loads.
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- ~ FlGURf 6 and FIGURE 7 are aligned and depict the forming of a ,`five-sheet sandwich structure by accordion expansion. FI~URE 6 , depicts the pre-expanded structure having face sheets 101 and ,,10~, and core`sheets 102, 103, and lU4, which are selec~ively cut prior to~expansion at~slots (e.g. 126, 1279 and 12&j. Core . I ~
~ sheets 102,'103, and 104 are positioned and controlled depth :: ~
laser welded together at seamed welded areas ~e.g. 112, 113, and 114). Then ~ace sheet 101 is added and controlled de~th laser welded (e.g. 111) to the three core sheets 102, 103, and 1 W~
Face sheet 105 is added and also contro'lled depth laser welded to the assembly.

FIGURE 7 depicts the five sheet expanded structure. The core sheets lU2, 103, and 104 have been unfolded and stretched usiny accordion expansion to form a linear but substantially oblique core (e.g. 143 and 144, and 142 and 15~) relative to an~ support-ing the face stleets 101 and 105.
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~ 3 ~7f~ 2L7 Referring now to FIGURE 2, it may be necessary to insure that the laser eneryy does not penetrate too deeply, thereby creating an unwanted wel~. This can be achieved by interlaying a s~lield 3 between the sheets 7 and 8, thereby covering the sheet 8 not to be welded. The use of shield material 3 may be further required under certain conditions because the sheets are normally extreme-ly thin (0,05 inches to 0.15 inches) necessitating precise ellergy control. Conceptually, a wide variety of shield materials may be ~utili7ed,~including plastics, thin films, or chemicals. If the ~si~ield materials used are reflective, the excess energy that penetrates the shield material can be reflected back into the covering sheet to solidify that weld. However, it is preferred -~ ~`that the shield material be a.l energy absorbing compound, tai-~;lored to the wavelength of the incident laser beam. If a C02 ~laser were used, it is believed that sulfur hexafluoride will provide significant protection for an underlying metal resulting from the very high absorbance of the laser beam energy. A thin coat of the chemical only provides temporary protection, but since laser dwell-time at any position is short, a thin coating should suffice.

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~ 3 ~7~3~ ~2L7 In addition, a shield r,laterial may be used to control the width of the laser weld. Shields 9 and l~ are placed on top of sheet 6, and are used to control tne weld width. The shields ~J
lO, and 3 may be removed a~ter the welding is completed.

In controlled deptn laser weldiny, as used in the present invention, the amount of laser power varies considerably with various materials. Surface absorption or, conversely, surface reflect1vity is be1ieved to be the primary property that deter-mines the àmoùnt of laser power needed for the controlled depth we1ding.--Ther~àl~conductivity is also important.~

~ ~A~`typical set of parameters using a continuous C02 laser `.
.with helium gas~shielding to lap weld ~OOU series aluminum sheet, each sheet having a ~llickness of about O.lO inches, would be as foll~ws:
laser ~eam wavelength - lO,600 nm laser beam power on taryet - 4~00 wa~ts travel speed - 50-~0 in/min beam spot diameter at tne work surface - 0.25 to 0.7~ rdm weld penetration - 0.14 and 0.16 inches ~ ~3~ 2L7 Althouyh a YAG laser havillg a wavelenytn of 1060 nm would be preferred to the CO2 laser~ suitable parameters for welding aluminurn sheet with the YAG laser are not currently known. This shorter wavelenyth allows for greater absorption of power at the sheet and a lower power requirement, which reduces operating costs and allows for more precise control.

Although tne relationship between tl~e laser power and the :
depth of the weld will normally have to be determined experi--mentally for each material 9 i t is believed that any gap between the sheets will have minimal effect upon laser power. Hence, that these~relat~onships will be in close correlation with ~he laser~power required for controlled depth ~elding of solid material.

OS~D

-~3~7~ 82L7 Hence, Ihere has been provided~ in accordance with the inven-tion, a metho~ of controlled dept~l welding of expandable sheet structure that fully satisfies the objectives set forth a~ove.
It is understood ~hat all terms used herein are descriptive rather than limiting. ~Ihile the invention has been described in conjunction with specific embodiments, it is evident that many alternatives,'modifications, and variations will be apparent to those skilled în the art in light of the disclosure herein.
Accordingly,'it'is intended to include all such alternatives, ~modifications,-and variations that fall within the spirit and scope of the appended claims.

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process of forming a sandwich structure which comprises:

(a) positioning at least three sheets in a stack relative to each other, said stack comprises of at least one face sheet and at least one core sheet;

(b) penetrating at selected areas of said stack to a predetermined controlled depth with a laser beam to effect welding at said selected areas, such that at said selected areas only a select number of said sheets less than all are welded together, said selected areas being in a pattern which defines the core of said sandwich structure; and (c) expanding the welded stack by applying a pressure differential to the interior and exterior of said stack.

2. The process of claim 1, wherein only two of said sheets are welded together at said selected areas.

3. The process of claim 1, wherein during said expanding step said at least one core sheet is caused to separate from said at least one face sheet except at said selected areas.

4. The process of claim 1, at least one of said core sheets and one of said face sheets possess superplastic properties, and said expanding involves superplastic forming of said sheets having superplastic properties.

5. The process of claim 1, wherein said expanding involves accordian expansion.

6. The process of claim 1, wherein there are two core sheets, and also including joining said cure sheets together at selected areas prior to said positioning step.

7. The process of claim 1, wherein there are three core sheets, and also including prior to step(a) positioning said core sheets in a first stack, and penetrating at preselected areas said first stack to a predetermined controlled depth with a laser beam to effect welding at said preselected areas only such that at said preselected areas two of said core sheets are welded together.