Structurally stronger titanium articles are produced by a weld-bonding technique comprising fastening at least two plates of titanium together using spotwelding and curing an adhesive interspersed between the spot-weld nuggets. This weld-bonding may be employed to form lap joints or to stiffen titanium metal plates

Creedon, J. F.

Vaughan, R. W.

English

NASA Technical Reports Server

United States Patent [191 
Vaughan et al. 
I 1  1 I 3,98836 B 
[451 Oct. 26, 1976 
(541 WELD-BQNDED TITANIUM STRUCTURES 
[75] Inventors: Robert W. Vaughan, Manhattan 
Beach; John F. Creedon, Santa 
Clara, both of Calif. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[73] Assignee: The United States of America as 
[22] Filed: Jan. 2, 1975 
[21] Appl. No.: 537,979 
[52] U.S. CI. ................................... 219/92; 219/1 18 
[ 581 Field of Search ............................... 2 19/92, 1 18 
1561 References Cited 
UNITED STATES PATENTS 
2,452,805 1 1/1948 Sussenback ....................... 219/92 X 
. .  [51] Int. CI. * .................... B23K 11/10; B23K 11/18 
3,337,71 1 8/1967 Garscia ................................. 219192 
3,803,081 4/1974 Lubowitz .......................... 260/37 N 
Primary Examiner-J. V. Truhe 
Assistant Examiner-David A. Tone 
Attorney, Agent, or Firm-Howard J. Osborn; Robert 
Kinberg; John R. Manning 
[571 ABSTRACT 
Structurally stronger titanium articles are produced by 
a weld-bonding technique comprising fastening at 
least two plates of titanium together using spot- 
welding and curing an adhesive interspersed between 
the spot-weld nuggets. This weld-bonding may be em- 
ployed to form lap joints or to stiffen titanium metal 
plates. 
5 Claims, 3 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19770004454 2020-03-20T07:10:33+00:00Z
U.S. Patent 
J 1  
Oct. 26, 1976 
J ---- 
3,988,561 
Fig. 1 
Fig. 2 
225,000 
(5000) 
180,000 
(4000) 
135,000 
(3000) 
90,000 
(2000) 
45,000 
(I 000) 
0 
i
I
IO0 
4 
-I-  
CYCLES (Thousands) 
Fig. 3 
4 e 
3,988,561 
tures beyond 450 K maximum. Thus, weld-bonding of 
titanium metal using a high temperature adhesive sys- 
tem are the materials currently used when the tempera- 
tures exceed 450 K. 
High temperature adhesives, i.e. 500 to 650 K, which 
could be used for the weld-bonding applications are 
polyimides, polypyrrones, polybenziamidazole, 
polyquinoxalines, polyphenylquinoxalines, or 
polyimidazoquinazolines. Several problems arise from 
the use of these adhesives in conjunction with titanium 
weld-bonding techniques. All of the high temperature 
polymers which have been considered for adhesive 
systems are condensation cured and evolve gaseous 
products during the cure. The high void content of 
these adhesives may be overcome by applying pressure 
during cure of the adhesive. However, where the work- 
piece is extremely large, application of pressure to 
collapse the voids and extrude a small portion of resin 
from the bond joint may require a very large machine 
or may be impossible. Thus, adhesives which evolve 
only a small amount of gaseous by-products upon cur- 
ing are preferred. 
Because of the poor oxidative stability of titanium in 
air, the adhesive resin used in weld-bonding must be 
applied to the titanium surface within four hours after 
cleaning of the surface. This presents a problem for the 
fabricator in that he must clean the titanium surface to 
be bonded, apply the resin adhesive, and proceed with 
the weld-bonding before the adhesive resin becomes 
oxidized from the air. Thus, several fabrication steps 
are necessary within a specified time confinement. 
US. Pat. No. 3,337,7 1 1 teaches a prior art method of 
weld-bonding two pieces of metal using a thermally, 
softenable adhesive. The patentee shunts the current 
between the metal layers to be bonded, causing the 
metal layers to heat and soften the adhesive resin. This 
method of weld-bonding is not applicable to titanium 
because of titanium oxidative instability. When tita- 
nium is welded, care is taken to prevent excessive heat- 
ing of the titanium workpiece, and this is usually done 
by using refrigerated electrodes. 
SUMMARY OF THE INVENTION 
1 L 
oxidation so that final fabrication of the workpiece is 
The invention described herein was made in the per- In order to avoid excessive material at thc point of 
formance of work under a NASA contract and is sub- the spot-weld, portions of the adhesive at the spot-weld 
ject to the provisions of Section 305 of the National 5 contact are masked from the surface. 
Aeronautics and Space Act of 1958, Public Law Weld-bonding of the titanium is accomplished by 
85-568 (72 Stat. 435; 42 U.S.C. 2457). placing the two adhesive coated surfaces together and 
applying spot-welds at the designated points. After the 
BACKGROUND OF THE INVENTION titanium plates have been joined by spot-welding, the 
Prior to the present invention, weld-bonding was 10 entire workpiece is exposed to temperatures Of approx- 
limited primarily to aluminurn alloy structures using an imately 375 to 450 K for a Period of aPProximatelY 
epoxy adhesive resin. Advancing technological require- thirty minutes. Titanium workpieces joined by this 
merits rendered weld-bonded aluminum alloy strut- weld-bonding method exhibit improved resistance to 
tures using epoxy adhesives unsuitable for tempera- fatigue failure. 
BRIEF DESCRIPTION OF THE DRAWINGS 
WELD-BONDED TITANIUM STRUCTURES not urgent. 
15 
FIG. 1 shows a top elevation view of a lap-joint hav- 
FIG. 2 is a cross-sectional side elevation taken 
20 through line 2-2 passing through the center spot-weld 
and showing the resin adhesive between the metal 
sheet; and 
FIG. 3 is a graphic comparison of the fatigue failure 
of lap-joints under various loadings at room tempera- 
DESCRIPTION OF THE PREFERRED 
’ EMBODIMENTS 
Because titanium forms an oxide coating upon expo- 
sure to air, the titanium workpiece to be weld-bonded 
must be cleaned before application of the adhesive to 
the joint surface. Preparation of the titanium involves 
degreasing with a solvent followed by grit blasting and 
immersion in a mild acid bath. The acid is removed 
then by rinsing in distilled water, and the surface is 
dried by warm air. The treated surface must be primed 
with a solution of the adhesive within 2 hours after the 
acid treatment. Solvent in the adhesive may be driven 
off and the adhesive imidized by exposure to elevated 
temperatures for 20 to 30 minutes. Where the adhesive 
is in the form of a polyimide prepolymer film, the 
cleaned surface is primed with a thin solution of adhe- 
sive, and the polyimide prepolymer film is placed on 
the primed surface. Removal of the solvent from the 
primer and subsequent imidization is effected by expo- 
sure to elevated temperatures for fifteen to thirty min- 
utes. 
Adhesives used in the present process are disclosed 
in detail in US. Pat. No. 3,803,081 to Lubowitz. The 
disclosure of this patent is incorporated herein by refer- 
ence. Finely divided aluminum powder is added to the 
varnish formulation to form a thixotropic paste which 
can be applied to the surface of the titanium prior to 
welding. In addition to the paste formation, the alumi- 
num provides a material which will expand during the 
heat-cure of the resin, and thereby apply pressure 
against the titanium surfaces. This is particularly ad- 
vantageous where a large piece of titanium is being 
spot-welded and clamping or pressing the large piece 
would be impractical. In this way, the aluminum as- 
sures good contact with the titanium surfaces and col- 
lapses any voids which may be present due to uneven 
application of the resin or other purely mechanical 
reasons. 
By an alternate method of fabrication, a glass fabric 
cloth is cut to the pattern of the weld-bond joint, omit- 
ting the areas where the spot-welds will be. The cloth is 
impregnated with a solution of the adhesive resin and is 
ing three spot-welds; 
25 ture. 
30 
3s 
40 
45 
so 
55 
In the present invention, a polyimide prepolymer is 
applied to a portion of the titanium surface immedi- 60 
ately after cleaning. The polyimide prepolymer may be 
applied in a sheet form or as a polyamide-acid which is 
later staged to the polyimide prepolymer form. Once 
the titanium surface has been coated with the poly- 
imide prepolymer, the titanium workpiece may be set 65 
aside and weld-bonded at any subsequent time. Having 
applied the polyimide prepolymer to the cleaned tita- 
nium surface, the titanium surface is protected from air 
3,988,561 
3 4 
air-dried to drive off the solvent. Next the prepreg cloth 
is heated for five minutes at 450 K to irnidize the resin 
- IROOO N (4000 Iby) 
- 12 7 mm ( 0  5-inch) to provide a reinforced film or sheet having good tack T:; :7;::;",2, 
and drape. These sheets are applied to the primed tita- cauge N~ I - 97 N/m' (14  psig) 
- 234 Nlm' ( 3 4  psig) 
- 0 3 - 5 0  
nium surface and thermally treated to imidize the 5 Gauge No 2 
primer and to adhere the film to the surface. FIG. 2 ~ ~ ~ ~ ~ t ~ ~ ~ n g  - 0.75 - 50 
shows a cross-section of the weld-bond product Weld Setting - 0 75 - 32 
Off Setting - 0 75 - 5 5  
Phase Shift Setting - 27 
Nugget Diameter - 7 6 mm (0 300-inch) 
ADHESIVE BOND ONLY 10 Current D C I ~ Y  - none 
Recompression Delay - none 
through spot-weld nugget 1 and adhesive 2. 
A polymeric adhesive composition was prepared by 
blending a polyimide precursor with a polyamide-imide 
precursor together with aluminum and fumed silica 
according to the following example. 
EXAMPLE I 
Meta-phenylene diamine in the amount of 170.0 g 
and 4,4'-diaminodiphenylsulfide in the amount of 67.9 
g were dissolved in 400 ml of dimethylformamide in a 2o Titanium alloy 6 A1 4 v  faying surfaces of lap+hear 
and a nitrogen gas bleed. A slurry of nadic anhydride or micron alumina and a water rinse. The panels were 
dride in the amount of 233.4 g dissolved in 252 ml of rinsed and dried at 339 K. paper masks were applied 
mixture while controlling the solution temperature according to the formulation in ~~~~~l~ 1, however, 
between 293 K and 298 K. After the last addition of the omitting the silica and aluminum powders, was applied 
nadic anhydride slurry, stirring was continued for ten by brush, dried in an air-circulating Oven for fifteen 
minutes, then a slurry of benzophenonetetracarboxylic ,,,inUtes at 403 K, and ,midized by thermal treatment 
acid dianhydride in 700 ml of dimethylformamide was 3o for five minutes at 450 K. paste adhesive prepared 
added until all ingredients were combined at a temper- according to the formulation in ~~~~~l~ I was splined 
ature of 293 K to 298K. The material was stirred for onto the primed surface to provide a total thickness of 
two hours and then allowed to stand for thirty minutes 0.25 mm and dried and imidized by the Same process as 
under a nitrogen purge. This polyimide resin precursor used for the primer. The specimens were assembled 
was added to an amide-acid varnish Produced by 35 and spot-welded as shown in FIGS. 1 and 2, using the 
Amoco designated as AI-1 137 and the resin mixture Same settings and machine as in the WELD ONLY 
was blended with aluminum alloy Powder and fumed method. The specimens were cured then by heating in 
silica in dimethylformamide solvent according to the an Oven to 589 K/minute followed by a sixteen hour 
following formulation. cure at 589 K. The welded-only specimens were pre- 
TABLE I 40 pared identically to the weld-bonded specimens except 
the adhesive paste was omitted. Bonded-only panels 
were prepared as discussed previously. 
Weld-bonded joints according to the present inven- 
tion were statically and dynamically tested. Generally, 
45 weld-bonded joints showed approximately the same or 
better strength when statically tested after thermal 
aging at temperatures ranging from 2 1 K to 561 K as 
before thermal aging. More significantly, however, 
were the results obtained by the fatigue test. These 
the above adhesive resin. One-half inch diameter holes Fatigue tests involving three specimens of each type 
were punched through the panels to coincide with of joining at each of the three load-levels set forth in 
primer bare spots as would be required for spot-weld- the.following table were performed in a Sonntage fixed 
ing. The panels were assembled in a bonding jig and frequency, fixed wave form, and variable load fatigue 
loaded into a cold-press. Pressure of 700 Newtons/m2 5 5  tester. All of the specimens were loaded in a clevis-pin 
(100 psig) was applied, and the press platen tempera- grip and cyclic loaded at 1800 cycles per minute 
ture was raised to 575 K at the rate of 5.56 K per min- (30Hz) at a load ratio of 0.1. Results in Table 11 which 
Ute. Panels were press cured for one hour, removed are plotted in FIG. 3 indicate a significant improvement 
from the press, and then oven cured for sixteen hours at in fatigue strength of the weld-bonded specimens over 
561 K. The resulting panels were tested statically at 60 the other two methods of joining. 
room temperature and provided an average breaking TABLE I1 
load of 19037N (4280 Ibs.). Examination of the failed 
joints showed good flow of the adhesive. 
WELD-BOND ONLY 
15 
Specimens for these tests were prepared in the fol- 
lowing manner: 
EXAMPLE I1 
round-bottomed flask fitted with a stirring apparatus 
376-endomethy1ene-1 ,2,3,6-tetrahydrophthalic anhy- 
test panels Were vapor &greased, grit blasted with 50 
immersed in solvent for fifteen minutes at 294 K, water 
to the diamine 25 for the spot-weld areas. An adhesive primer prepared was added 
Constituents Parts by Weight 
Nadic Polyimide Precursor 
50 AI-I137 (Amoco) 
Aluminum Powder - Grade 101 175 
(Aluminum Copr of America) 
Cab-0-Si1 ' I O  
(fumed silica. Cabot Corp ) 
Dimethylformamide I so 
50 
Three titanium panels were prepared and coated with 50 results are shown in FIG. 3 of the drawing. 
FATiGUE TEST RESULTS FOR WELD THROUGH PROCESS 
Cycles at Failure ( X  I O J )  
WELD ONLY 65 Load Level, "I Welded- Bondcd- Weld- 
rlr Only Only Bonded 
1 .o I O  5 5  
60 1 8  11.0 2 5  
tings: 1 3  0 5  3 0  
Spot-welding of nine titanium plates were carried out 
on a Sciaky welding machine having the following set- 
3,988,561 
5 
TABLE 11-continued 
; W 
Cycles at Failurc ( X  IO7) 
5 Load Level, nJ Weldcd- Bonded- Wcld- 
% Only Only Bonded 
5.5 60.0 14.0 
40 6 .O 81.0 14.0 
5.0 723.0 16.0 
30.0 (b) 227.0 IO 
20 33.0 2 16.0 
31.0 186.0 
a’ Pcrccnt of failure load at room temperature from static tcsls. 
No failure before completion of I million cyclcc. 
15 
We claim: 
1. A process for weld-bonding titanium comprising: 
6 
A. applying a polyimide prepolymer adhesive in a 
predetermined pattern to the surface of at least two 
titanium plates whereby areas to be spot-welded 
are void of said adhesive; 
B. bringing said plates into contact with each other; 
C. spot-welding to join said plates; and 
D. heating said adhesive to effect cure of said adhe- 
sive and form a structurally strong weld-bonded 
layer. 
2. A process according to claim 1 wherein aluminum 
3. A process according to claim I wherein fibrous 
4. A process according to claim 1 wherein said adhe- 
5. A process according to claim 1 wherein said adhc- 
powder filler is added to the polyimide adhesive. 
reinforcing material is placed between said plates. 
sive is applied as a liquid. 
sive is a solid film. * * * * *  
20 
25 
30  
35 
40 
45 
50 
55 
60 
65 


Weld-bonded titanium structures

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