A joining process designated weld brazing which combines resistance spot welding and brazing has been developed. Resistance spot welding is used to position and align the parts as well as to establish a suitable faying surface gap for brazing. Fabrication is then completed by capillary flow of the braze alloy into the joint. The process has been used successfully to fabricate Ti-6Al-4V titanium alloy joints using 3003 aluminum braze alloy. Test results obtained on single overlap and hat-stiffened structural specimens show that weld brazed joints are superior in tensile shear, stress rupture, fatigue, and buckling than joint fabricated by spotwelding or brazing. Another attractive feature of the process is that the brazed joints is hermetically sealed by the braze material

Arnold, W. E., Jr.

Bales, T. T.

Royster, D. M.

English

NASA Technical Reports Server

WELD-BRAZING - A NEW JOINING PROCESS 
BY 
Thomas T. Bales, Dick M. Royster, and Winfrey E. Arnold, Jr. 
NASA - Langley Research Center 
Presented a t  
Symposium on Welding, Bonding, and Fastening 
Sponsored By 
National Aeronautics and Space Administration 
George Washington Uni versi t y  
The American Society for Metals 
W i  11 iamsburg , V i  r g i n i  a 
May 30-June 1 1972 
https://ntrs.nasa.gov/search.jsp?R=19740022816 2020-03-17T15:09:51+00:00Z
WELD-BRAZING - A NEW JOINING PROCESS 
BY 
Thomas T. Bales, Dick M. Royster, and Winfrey E. Arnold, Jr. 
SUMMARY 
A jo in ing  process designated weld-brazing which combines resistance 
Resistance spot welding i s  spot welding and brazing has been developed. 
used t o  pos i t ion and a l ign  the parts as well  as t o  establ ish a sui table 
faying surface gap f o r  brazing. Fabrication i s  then completed by 
cap i l l a ry  f low o f  the braze a l l o y  i n t o  the j o i n t ,  The process has been 
used successfully t o  fabr icate Ti-6Al-4V t i tanium a l l o y  j o i n t s  using 3003 
aluminum braze a l l o y  and should be applicable t o  other metal-braze Systems. 
i 
Test resul ts  obtained on single overlap and hat-st i f fened s t ructura l  
specimens show tha t  weld-brazed j o i n t s  are superior i n  tens i le  shear, 
stress rupture, fat igue, and buck1 ing than j o i n t s  fabricated by spotwelding 
o r  brazing. Another a t t rac t i ve  feature of the process i s  t ha t  the brazed 
j o i n t  I s  hermetical ly sealed by the braze material which should el iminate 
many of the sealing problems encountered i n  r iveted o r  spotwelded 
structures. The ease o f  fabr icat ion associated wi th  the weld-brazing 
process could well  make i t  cost e f fec t i ve  on a production basis compared 
wi th  more conventional techniques i n  addi t ion t o  the improved j o i n t  
character ist ics observed. 
WELD-BRAZING - A NE# JOINING PROCESS 
BY 
Thomas T. Bales, Dick M. Royster, and Winfrey E. Arnold, Jr. 
. .  
INTRODUCTION 
Recent interest has been generated i n  the combined use of resistance 
spotwel d i  ng and epoxy bonding (we1 d-bondi ng) as an effecti ve means of 
fabricating aerospace structures. 
Materials Laboratory (ref. 1 ) ,  weld-bonding was used t o  fabricate 
aluminum a1 loy structures having mechanical properties vastly superior t o  
similar panels fabricated by conventional methods. The interest i n  weld- 
bonding as an effective manufacturing process prompted NASA-Langley t o  
investigate i t s  a p p l i c a b i l i t y  t o  some of their current and future programs. 
A study was undertaken a t  NASA-Langley t o  investigate weld-bonding as a 
potential j o i n i n g  method i n  the fabrication of titanium structures. 
However, since titanium structures are considered for use a t  temperatures 
which exceed the capability of current epoxy systems , a higher temperature 
adhesive was desired. The idea of substituting a braze a l loy  for the 
adhesive t o  extend the temperature capabi 1 i t ies  was conceived and a program 
was therefore initiated t o  evaluate the process designated as WELD-BRAZING. 
The material used i n  the study was mill annealed Ti-6A1-4V ti tanium alloy 
sheet and the braze a l loy  was 3003 aluminum. 
fabrication of single overlap specimens which were tested a t  room and 
In a study sponsored by the Air Force 
I 
The study consisted of the 
elevated temperatures , and the fabrication of hat-sti  ffened panels which 
were tested i n  compression a t  room temperature. 
348e 
2 
Weld-brazed joints were examined metallurgically t o  verify adequate 
flow of the braze and absence of'porosity. T h i s  paper reports the results 
obtained i n  this study. 
PROCESS DEVELOPMENT 
In the development of the weld-brazing process, which combined the 
use of resistance spotwel d i  ng and brazing, two primary approaches were 
investigated. One was designated the prepunched fo i l  approach (figure 1) 
and the other the capillary flow approach (figure 2) .  Both of these 
approaches were developed together and both proved t o  be successful 
j o i n i n g  methods. 
In the prepunched fo i l  approach, the fo i l  was punched (see Figure 1 )  
so t h a t  the diameter of the holes was larger t h a n  the diameter of the 
intended spotweld nugget. This was done i n  order t o  prevent excessive 
reaction of the parent material w i t h  the braze alloy i n  the vicinity of 
the spotweld. 
that expansion of the weld nugget caused the faying surface gap t o  be 
approximately 0.002-inch greater than the thickness of the braze foils 
being used. 
be greater than the overlap i n  order t o  have a sufficient volume of braze 
alloy t o  adequately f i l l  the gap resulting from welding. 
heated t o  1250°F i n  vacuum t o  accomplish the brazing cycle. 
Single overlap specimens fabricated i n  this manner showed 
I t  was therefore necessary that the width o f  the braze foil  
Specimens were 
In the capillary flow approach (see figure 2 )  the titanium strips are 
f i r s t  chemically cleaned and then joined by conventional spot-welding 
techniques. Nugget expansion during welding produced a faying surface 
1494 
t' 
3 
gap which could be varied from 0.002-inch to  0.006-inch by varying the 
welding parameters. 
the edge of the overlap and the assembly was heated t o  the brazing 
Following welding, the braze fo i l  was positioned a t  
temperature of 125OOF i n  vacuum as i n  the prepunched approach. Upon 
melting the braze alloy was drawn into the gap by capillary action. 
U1 trasoni c and metal 1 ographi c inspection of the jo in t  indicated tha t  the 
gap was f i l l e d  completely. 
Both approaches produced s a t i  sfactory jo in ts  tha t  were hermetically 
In the prepunched fo i l  approach, the positioning of the punched sealed. 
fo i l  and the welding electrodes, without the aid of precise positioning 
equipment made the process somewhat d i f f i c u l t  for application to  the 
fabri cation of structural  parts. 
where the braze would not flow into the faying surface gap and the 
prepunched f o i l  approach would be the bet ter  joining method. Weld-brazed 
jo in ts  were fabricated w i t h  re la t ive  ease by the capillary flow approach, 
and i t  was f o r  this reason tha t  this approach was used t o  fabricate the 
However, there could be certain appl i cations 
1 
test specimens used i n  the test program. 
SPECIMENS AND TEST PROCEDURES 
The weld-brazing process, using the capillary flow approach, was used 
t o  fabricate single overlap specimens and hat-stiffened panels. 
specimens were t h e n  tested t o  evaluate the mechanical properties of 
we1 d-brazed jo in ts  . The parent materi a1 used was annealed 0.050-i nch 
t h i c k  Ti  -6A1-4V titanium a1 loy and the braze a1 loy employed throughout 
These 
/’- . 
4 
the study was 0.004-inch t h i c k  3003 aluminum alloy. The Ti-6A1-4V parts 
were thoroughly cleaned before weld-brazing as outlined i n  reference 2. 
Specimens 
Si.ng1 e overl ap  specimens - The configurations of the single overl ap 
specimens are shown on figure 3. Specimens were fabricated by spotwelding, 
brazing, and weld-brazing. The overlap on a1 1 specimens was approximately 
3/8-i nch and was selected t o  insure detemi nation of j o i n t  characteri st i  cs. 
The specimen on the r i g h t  of the figure was used for determining the 
tensile shear and stress rupture properties of the joints. 
spotwelded t o  the specimens t o  prevent end failure during testing. The 
longer specimen shown i n  the figure was used for determining the fatigue 
properties of the joints. The weld spacing on the spotwelded only and 
weld-brazed specimens was 1 inch. A1 1 brazing operations were accomplished 
i n  a vacuum furnace a t  a temperature of 125OOF and a pressure o f  
End tabs  were 
3 
t 
torr for 10 minutes 
Hat-stiffened panels. - The panel configuration used i n  the program 
i s  shown on figure 4. Panels having a maximum w i d t h  t o  thickness r a t i o  
of 25 were fabricated by brazing only, riveting only, spotwelding only, 
and weld brazing. The panels fabricated by brazing only were first 
spotwelded a t  the ends and i n  the center to  eliminate f i x t u r i n g .  The 
riveted panels were fabricated using 0.16-inch diameter flush-head 
stainless steel rivets and a rivet spacing of l/e-inch. 
purposes, the spacing of the spotwelds i n  the spotwelded only and weld- 
For comparative 
brazed panels were also 1/2-inch. The weld-brazed and brazed only panels 
5 
were fabricated us ing  the same brazing parameters as for the single 
overlap specimens. 
The need for spotwelding the brazed panels, as mentioned above, 
resulted from difficulties associated w i t h  attempts t o  fabricate specimens 
by brazing alone. 
- 
Attempts t o  maintain alinement of the ha t  stringer on 
the face sheet and t o  establish the optimum gap for brazing were found t o  
be very difficult unless elaborate fixtures were used. A slight twist i n  
the stiffener dur ing  fabrication resulted i n  braze thicknesses ranging from 
0.003 t o  0.020-inch. 
brazed only panels were fabricated by spotwelding the stiffener t o  the 
Therefore, t o  eliminate the need for f i x t u r i n g ,  the 
face sheet a t  each end and i n  the center. 
along the edge of the overlap and fabrication was completed i n  the same 
The braze al loy was then placed 
manner as the weld-brazed panels. Thus, the brazed only panels were 
actual l y  fabri cated by we1 d-brazi ng us ing  . w i  dely spaced spotwel ds  . 
Test Procedures 
Tensile shear test. - The tensile shear tests of single overlap 
specimens were conducted a t  ambient temperature, 3 5 O O F  and 5 5 O O F  a t  a 
constant load rate of 2000 lb/min us ing  a 100 k i p  capacity hydraulic 
testing machine. Heating of the specimens tested a t  elevated temperature 
was accompli shed using a resi stance-wound furnace mounted i n  the testing 
machine. 
Stress-rupture tests. - Stress rupture tests were conducted using 
single overlap specimens. The specimens were loaded i n  conventional creep 
testing machines, equipped w i t h  tube furnaces, t o  various percentages of 
c 
6 
the elevated temperature static tensile shear strength. The loading was 
applied shortly after reaching test temperature and the times t o  rupture 
were obtained from d i g i t a l  elapsed-test-time indicators t h a t  stopped 
automatically when the specimens ruptured. 
Fatigue tests. - Constant-amplitude fatigue tests (R4.05) were 
conducted i n  subresonant-type axi a1 load fatigue machines (reference 3) 
Load was sensed by 
A wire strain-gage 
t o  an oscilloscope 
frequency was 1800 
a weigh-bar i n  series w i t h  the specimen and grips. 
bridge cemented t o  the weigh-bar supplied a load signal 
which was used t o  monitor cyclic loading. Operating 
cycles per minute. 
Panel tests. - the hat-sti ffened structural panels were tested i n  
compression using a 300-kip-capaci ty hydraulic testing machfne. The 
edges of the specimens were supported w i t h  knife edges positioned 1/4 
of an inch  from the edge. 
of the stiffener and face sheet and were used t o  measure local strains. 
Relative motion between the upper and lower heads of the testing machine 
was measured using linear variable differential transformers. Data were 
Foil strain gages were attached a t  the centers 
recorded every 10 seconds t o  local instabil'ity and a t  an increased rate 
of every second from local ins tab i l i ty  t o  maximum load. All tests were 
conducted a t  a load rate of 12,000 lbs/min. 
RESULTS AND DISCUSSION 
Si ngl e Over1 ap Specimens 
Tensile-shear. - The tensile shear results obtained a t  room 
temperature, 350'F and 55OOF are presented on figure 5 for specimens 
- 
7 
fabricated by spotwelding, brazing, and weld-brazing where maximum load is  
plotted against test  temperature. The data points are the average o f  
6 tests w i t h  the data spread indicated. The tensile shear properties of 
the weld-brazed specimens are shown t o  be greater t h a n  those of the 
brazed or spotwelded specimens and are approximately equal t o  the sum o f  
the values shown for the spotwelded and brazed specimens. The strength 
of the weld-brazed specimens apparently decrease 1 inearly w i t h  increasing 
temperature, and a t  550°F the specimens were capable of carrying a load 
which was 85 percent of the value obtained a t  room temperature. 
strength of the brazed specimens decreases i n  a similar fashion, and the 
spread i n  the da ta  indicates t h a t  the brazing process was much more 
difficult t o  control. A t  a temperature of 55OoF, the average strength of 
The 
the brazed specimens was equal t o  approximately 80 percent o f  the room 
temperature strength. A1 though both the weld-brazed and brazed specimens 
experienced a decrease i n  strength a t  55OoF, the decrease compared t o  
room temperature strength was proportionally less f o r  the weld-brazed 
specimens. The differences i n  the percentages noted for the weld-brazed 
specimens might  be attributed t o  the fac t  t h a t  the strength o f  the 
spotwel ds were n o t  temperature dependent. 
Stress-rupture. - The stress-rupture properties a t  3 5 O O F  and 550°F 
for single overlap specimens fabricated by spotwelding, brazing, and 
weld-brazing are shown on figure 6. A t  5 5 O O F  for a 500-hour l i fe ,  the 
weld-brazed specimens appear capable of carrying more than twice as 
much load as the brazed specimens and approximately 15 percent more load 
I?! 
L '  
4 
a 
t h a n  the spotwelded specimens. A1 though both the we1 d-brazed and brazed 
specimens exhibited substantial decreases i n  load carrying ability a t  
550°F compared t o  350°F, the decr'ease noted for the weld-brazed specimens 
was proportinally less, probably because of the contribution from the 
spotwelds. 
shear test ,  i t  would seem possible t o  ta i lar  the elevated temperature 
properties of a weld-brazed j o i n t  by varying the number of spotwelds 
present. Although the strength contribution of the braze decreases w i t h  
increasing temperature, i t  should be sufficient t o  redistribute the stress 
i n  the j o i n t  so t h a t  the stresses i n  the spotwelds are reduced. 
Based on these results and those obtained from the tensile 
a 
Fatigue. - The room temperature fatigue data obtained from the single 
overlap specimens are shown on figure 7. 
of the brazed and weld-brazed specimens are approximately equal and t h a t  
Note t h a t  the fatigue strengths 
the strengths are approximately three times as  great as the strength of 
the spotwelded specimens for a l i f e  of 200,000 cycles. 
brazed and weld-brazed specimens occurred i n  the j o i n t  for stress levels 
resulting i n  failure i n  less t h a n  about 50,000 cycles while failure of 
the gross section occurred for the specimens having a fatigue l i f e  greater 
t h a n  about 50,000 cycles. The reason for the change i n  failure mode was 
Failure of the 
attributed t o  specimen configuration. 
Hat-Sti ffened Panels 
The hat-stiffened panels fabricated by the various methods were 
tested i n  compression, and the results obtained are presented on figure 8. 
Data are shown for three panels tested for each jo in ing  process. The 
load carrying ability of the brazed and weld-brazed panels are shown t o  
be similar and substantially greater t h a n  t h a t  of the panels fabricated 
9 
by spotwelding or riveting. The maximum load of the weld-brazed panels was 
approximately 59 kips compared t o  38 kips for the riveted panel or an 
increase i n  load carrying capabili ty of 55 percent. A similar increase 
was also noted for the loads a t  which face sheet wr inkl ing  or buckling 
occurred for  the we1 d-brazed panels compared t o  the riveted specimens. 
The increase i n  load carrying a b i l i t y  of the weld-brazed and brazed panels 
is  attributed t o  an increased panel stiffeness resulting from the 
continuous bond between the stiffener and face sheet as compared t o  the 
po in t  attachment inherent w i t h  spotwelded and riveted specimens. However, 
since the specimens were of the same configuration, a structure fabricated 
by weld-brazing should provide a reasonable weight savings for buckl ing  
design consi derati ons . 
Metallographic Investigation 
Metallographic specimens were prepared us ing  conventional polishing 
I and etching techniques. Typical cross-sections of a weld-brazed j o i n t  
taken from a compression panel are shown on figure 9. The upper portion 
of the figure depicts the j o i n t  and provides evidence t h a t  the faying 
surface gap i s  adequately filled by brazing. The braze alloy which w a s  
placed adjacent t o  the flange o f  the stiffener is  shown t o  have penetrated 
completely through the j o i n t  t o  form a generous f i l l e t  between the inner 
surface of the stiffener and the face sheet. 
The photomicrograph on the lower portion of figure 9 provides further 
verification of adequate braze penetration, a t  least t o  the weld nugget.  
A good metallurgical bond i s  shown t o  exist between the braze and a l l  
adjoining t i t a n i u m  surfaces a1 though there i s  some evidence of the 
1s&= 
c 
10 
formation of titanium aluminide. 
metallic t o  the extent shown was not  detrimental t o  the properties studied. 
However, the formation of the inter- 
CONCLUDING REMARKS 
The .results obtained indicate that weld-brazing is a fabrication 
process capable of producing joints having superior ambient and elevated 
temperature properties compared to  similar joints produced by brazing only 
or  spotwelding only. Tailoring of the properties may also be possible by 
varying the extent of the overlap and the number of spotwelds. Weld- 
brazing may also eliminate many o f  the sealing problems encountered i n  
spotwelded and riveted structures because the process results i n  a 
hermetically sealed jo in t .  The ease of fabrication associated w i t h  the ' 
weld-brazing process could well make i t  cost effective on a production 
basis compared w i t h  more conventional techniques i n  addition t o  the 
improved j o i n t  characteristics observed. A1 though the process has only 
been used t o  join Ti-6A1-4V titanium alloy 3003 aluminum braze, weld- 
brazing should also be adaptable t o  any material. system where both brazing 
and spotwelding techniques are viable methods for joining. Candidate 
systems include nickel -base superalloy and refractory metal structures 
as well as the fabrication of structures using more conventional material 
such as mild steel. 
11 
,REFERENCES 
1 Fields, Dallas: Resistance Spot Weld-Adhesive Bonding Process. 
Technical Report AFML-TR-70-227, November 1970. - .  
2. Heimerl, George J.; Baucom, Robert M.; Manning, Charles R.; and 
Braski, David N.: S t a b i l i t y  o f  Four Titanium-Alloy and Four 
Stainless Steel Sheet Mater ia ls A f te r  Exposures Up t o  22,000 
Hours a t  550°F (561'K). NASA TN D-2607, 1965. 
Grover, H. J.; Hyler, W. S.; Kuhn, Paul; Landers, Charles 8.; and 
Howell, F. M.: Axial-Load Fatigue Properties o f  245-T and 
755-T A1 umi num A1 1 oy as Determined i n  Several Laboratories . 
NACA Report 1190, 1954. 
3. 
(Supersedes NACA TN 2928.) 
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NASA-Langley, IS72 


Weld-brazing - a new joining process

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