Joining aluminum to stainless steel by bonding aluminum coatings onto titanium coated stainless steel and brazing aluminum to aluminum/titanium coated stee

Huschke, E. G., Jr.

Nord, D. B.

English

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
REPLY TO 
J 
Torr USI/Scientilfic & Technical 
Attentions M i s s  Winnie M. Morgan 
FROM8 GP/Office of Assis tan t  General Counsel fo r  
Patent Matters 
SUBJECT% Announcement of NASA-Owned U. S.  Patents An STAR 
I n  accordance w i t h  the procedures agreed upon by C@e GP 
and C o d e  U S I ,  the attached NASA-owned Um Sm Patent is being 
forwarded for  abstracting and announcement i n  NASA STARo 
The following information is providedo 
U. S .  Patent NO. 8 3,473,216 
Government or  
Corporate Employee t North Amertkan Aviation, Inc, 
Supplementary Corporate 
Source ( i f  applicable) t N e  A ,  
Canoga P a r k , C a l i f  ornia 
NASA Patent C a s e  No. t ME'S-07369 
NOTE - If  this  patent covers an invention made by a corporate 
employee of a NASA Contractor, the following is applicableo 
Pursuant t o  Section 305(a) of the N onal Aeronautics and 
Space Act, the name of the Administrator 06 NASA appears on 
the first page of the patent; however, the name of the actual  
inventor (author) appears a t  the heading of Column Nom 1 of 
Yesm . No 
- the Specification, following the words * m  e m w i t h  res 
an invention ofn. e e I 
,B 
Enc Issure 
Copy of Patent G i t e  
https://ntrs.nasa.gov/search.jsp?R=19710010968 2020-03-17T02:54:17+00:00Z
ADMINIST R 
SPACE ADMINISTRATION 
METHOD OF JOINING ALUMINUM TO 
Filed May 17, 1967 
STAINLESS STEEL COMPONENT 
i TITANIUM 
1 
i 
i 
VACUUM VAPOR DEPOSITION OF ALUMINUM ALUMINUM 
ALLOY 
t 
i 
3 I 
1 ALUMINUM 
COMPONENT 
BRAZING ALUMINUM ALLOY COATING TO 
J 
z 
j 
PRODUCT ALUMINUM -STAINLESS 
STEEL JOI N T  
b 
i 
FI G. L 
STAINLESS STEEL COMPONENT 
VAPOR DEPOSITED 
T I TAN I UM 
VAPOR DEPOSITED 
ALUMINUM ALLOY 
BRAZED JOINT 13% SI, 87% 
AL BRAZING ALLOY 
' ALUMINUM. ALLOY COMPONENT 
s tent c 
x 
steel joint suitable for repeated service at high stresses 
and over a wide range of temperatures. 
Yet another object is to provide a method of coating 
a stainless sted surface so that it can be brazed or  welded 
t’ 
Other objects and advantages of the invention will be 
3,473,216 
STAINLESS STEEL 
James E. Webb, Administrator of the National Aeronau- 
tics and Space Administration, with respect to an inven- 
tion of DQUglaS B. Nord, Van Nuys, and Ernest 6. 
Wuschke. Jr.. Canoea Park. Calif. 
D OF JOINING ALUMINUM TO 
&led May 117,1967, Ser. No. 640,462 
t. ‘Cl. B23k 31 /02 
US. C1. 29-492 8 ciainls 
CT OF THE DISCLOSURE 
Aluminum is joined to stainless steel by metallurgically 
bonding a thin coating of titanium to the joining surface 
of the stainless steel component, metallurgically bonding 
a coating of aluminum or aluminum alloy to the titanium 
coating and brazing or welding the aluminum coating to 
the aluminum component. The titanium and aluminum 
coatings, which are applied and bonded to the stainless 
steel component (by a method such as vacuum vapor dep- 
osition, prevent the formation of brittle phases in the 
joint. 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 U.S.C. 2487). 
The invention relates t o  joining of dissimilar metals 
and more particularly to a method of joining aluminum 
to stainless steel. 
Joints between aluminum and stainless steel compo- 
nents are required in the fabrication of numerous prod- 
ucts which are designed to take advantage of the favor- 
able properties of both of these metals, in particular the 
light weight and low cost of aluminum and the high 
strength and superior corrosion resistance of stainless 
steel. In rocket engine assemblies aluminum-stainless 
steel joints are required for connection of aluminum valve 
components to stainless steel tubing and for connection 
of various other components. Joints for rocket engine 
applications must be capable of service under severe 
thermal and mechanical stresses. In addition to high 
strength and ductility, joints for rocket engine use should 
exhibit resistance to corrosion, capability for service 
from liquid hydrogen temperatures up to 1000” F., and 
absence of degradation in metallurgical structure after 
thermal cycling. 
Various brazing and welding methods have been avail- 
able for joining aluminum to stainless steel, but the result- 
ing joints are frequently unsuitable for rocket engine 
applications. For example, brazed joints have been formed 
by coating the steel component with aluminum or with an 
intermediate layer of silver and brazing the coated com- 
ponent to the stainless steel article. Such joints are useful 
at low temperatures and low stresses, but they fail rapidly 
at higher temperatures or upon undergoing stresses due 
to thermal cycling or vibration. Failure of these joints nor- 
mally occurs by cracking resulting from the presence of 
brittle phases in the joint such as aluminum-iron or 
aluminum-silver. In order to obtain a joint with the desired 
mechanical properties the formation of brittle phases or 
intermetallic compounds must be minimized or elimi- 
nated. 
It is therefore an object of this invention to provide a 
method of joining aluminum to stainless steel. 
Another obiect is to Drovide a method of ioining alumi- 
apparent from the following description taken in conjunc- 
tion with the accompanying drawings which illustrate em- 
bodiments thereof. 
10 In the drawings: 
FIGURE 1 is a flowsheet showing the steps to be per- 
formed in carrying out one embodiment of the invention 
and 
FIGURE 2 is an enlarged cross section through a 
15 brazed joint prepared in accordance with one form of 
the invention. 
In the present invention an aluminum component is 
joined to a stainless steel component by metallurgically 
bonding a thin coating of titanium or titanium alloy to 
20 the surface of the stainless steel component, metallurgi- 
cally bonding a coating of aluminum or aluminum alloy 
on the titanium coating, and brazing or welding the sur- 
face of the aluminum coating to the surface of the alumi- 
num component. A strong and ductile metallurgical bond 
25 is developed between the aluminum component and the 
stainless steel component to meet the severe service re- 
quirements for rocket engine applications. The formation 
of embrittling phases or intermetallic compounds at the 
joint is substantially avoided by use of the deposited 
30 titanium and aluminum coatings. No degradation of 
metallurgical structure or properties occurs in joints 
formed by this method, wen after extended exposure to 
thermal cycling and severe vibrational stresses. 
The term “stainless steel” as used herein is intended to 
35 refer to those ferrous metal alloys, containing a substan- 
tial amount of chromium or nickel and chromium which 
are commonly referred to as austenitic, martensitic or 
ferritic types. Although the present invention is broadly *, applicable to stainless steels and is not limited to a partic- 
ular type, it is particularly applicable to stainless steels 
designated as “304L” (composition in weight percent: 
nickel, 8 to 12; chromium, 18 to 20; carbon, 0.03 maxi- 
mum; manganese, 2.00 maximum; phosphorus, 0.045 
45 maximum; sulfur, 0.06, maximum; silicon, 1.00 maxi- 
mum, and the balance iron and “347” (composition in 
weight percent: nickel, 9 to 13; chromium 17 to 19; car- 
bon, 0.08 maximum; manganese, 2.00 maximum; phos- 
phorus, 0.045 maximum; sulfur, 0.06 maximurn; silicon, 
50 1.00 maximum; columbium plus tantalum equal to 10 
times carbon minimum and the balance iron). 
The composition of the aluminum component in the 
present invention can be aluminum or most of the alumi- 
num alloys used as structural materials. For the embodi- 
55 ment wherein brazing is used aluminum and any alumi- 
num alloy having a melting point lower than about 960” 
F. can be joined by the present method. For joining by 
welding this method is applicable to aluminum and all 
aluminum alloys except certain 2000 and 7000 series 
The surface of the stainless steel component to be 
joined is preferbaly cleaned by solvent degreasing prior 
to applying the titanium coating; however, no passivation 
or other chemical treatment is required. 
A thin coating of titanium or titanium alloy is metal- 
lurgically bonded to the stainless steel component at  the 
intended joining surface to serve as a metallurgical bar- 
rier to minimize formation of embrittling phases. AI- 
though the titanium barrier material can contain minor 
60 alloys such as 2024, 2025 and 7075. 
65 
num to stainjess steel-wherein the: formation 0: brittle 70 proportions of alloying elements such as tin, unalloyed 
phases at the joint is minimized. titanium is preferred. Co,mmercially pure titanium is suit- 
able for this purpose. A titanium coating thickness from Still another object is to provide an aluminum-stainless 
3,473,216 
0.000050 to 0.200 inch can be used, and 0.00065 to 
0.001 inch is preferred. 
plied by means of vacuum vapor deposition using pre- 
viously known techniques. In this method the titanium 
is placed in a water-cooled crucible, and a vacuum of 
less than one micron of mercury is established in the 
crucible chamber. A high-voltage, for example, 10 to 20 
kilovolts, electron beam is focused magnetically on the 
surface of the titanium, and the titanium is vaporized by 10 
the intense heat energy. The stainless steel substrate is 
placed above the titanium in a position such that the 
vaoorized titanium is devosited on the substrate. Prior 
The composition of the brazing alloy used as a filler 
metal in this embodiment is not critical, and any of the 
In a preferred embodiment the titanium coating is ap- alloys previously employed for brazing aluminum 
alloys can be used. A brazing alloy comprising 11  to 13 
weight percent silicon and the balance aluminum is pre- 
ferred to provide a high-strength joint. Other suitable 
brazing alloys include those having the composition, in 
weight percent: silicon, 9.3 to 10.7; copper, 3.3 to 4.7 
and the balance aluminum. 
The invention is further illustrated by the following 
example. 
EXAMPLE 
to-vaporizing the titanium, the stainless steel substrate 
is heated to a temperature of about 1400 to 1500" F. 
with the electron beam in order to achieve metallurgical 
bonding between the deposit and the substrate. For de- 
position on tubular or cylindrical-shaped substrates the 
stainless steel member is preferably rotated about its axis 
during deposition of the titaninm to ensure a uniform 
coating. Vacuum vapor deposition is advantageous in that 
a high deposition rate and high efficiency are realized. 
The titanium coating can also be applied by other meth- 
ods such as chemical vapor deposition or fused salt bath 
plating. 
The titanium-coated stainless steel component is then 
further coated with aluminum or an aluminum alloy to 
provide a surface which can be readily brazed or welded 
to the surface of the aluminum component. The alumi- 
num coating can be unalloyed aluminum or aluminum 
alloys containing a total of up to 40 weight percent al- 
loying elements such as copper, maganese, silicon and 
zerconium. Examples of suitable aluminum alloys for 
coating include those designed at "4043" (composition in 
weight per cent silicon, 5.5 to 6.0; iron, 0.8 maximum; 
copper, 0.3 maximum; manganese 0.05 maximum; mag- 
nesium, 0.05 maximum; zinc, 0.10 maximum; titanium, 
0.2 maximum; others, 0.15 maximum; and the balance 
aluminum). The aluminum coating is provided at a thick- 
ness from 0.001 to 1.0 inch, and preferably from 0.040 
to 0.070 inch. An excess thickness can be deposited on 
the surface and the surface machined off to provide the 
desired uniform thickness. The aluminum coating is pre- 
ferably applied by vacuum vapor desposition in the man- 
ner described above for the titanium coating, the sub- 
strate again being heated to a temperature of 800 to 
1100" F. prior to deposition of the aluminum. The alu- 
minum coating can also be applied by dip coating or 
metal spraying. 
The resulting double-coated stainless steel component 
can be joined to the aluminum component by welding 
or brazing. Welding can be conducted by fusion of the 
aluminum coating surface directly to the surface of the 
aluminum component or by using an aluminum or alu- 
minum alloy filler wire in the manner previously em- 
ployed for welding aluminum to aluminum. It is preferred 
to join the aluminum-coated surface to the aluminum 
component by means of brazing employing an aluminum 
alloy filler wire. For this method the coated component is 
preferably machined and positioned to provide an inter- 
facial gap of 0.001 to 0.0015 inch. The filler metal in 
the form of wire, powder-paste or sheet is placed at the 
joint and the assembly is heated to brazing temperature. 
Conventional furnace, induction, molten-salt-bath or 
torch brazing methods can be used. Molten salt bath 
with brazing is especially suitable for joining components 
of valve assemblies. In this procedure the components, 
with the filler metal preplaced at the joint, are immersed 
in a bath of a molten chemical salt flux such as chlorides 
and fluorides of sodium and potassium at a temperature 
of 960 to 11 65 F. The flux dissolves aluminum oxide a t  
the joint interfaces, and the denser brazing alloy replaces 
the molten salt and dissolved oxide in the joint. Upon 
removal from the bath the brazing alloy solidifies to 
form a metallurgical bond. 
Tubular sections of 347 stainless steel were joined to 
l5 mating 6061 aluminum valve prototype components by 
the following procedure. The stainless steel member (out- 
er diameter, 2.25 inches; inner diameter, 2.00 inches and 
length 1.5 inches) was coated with a 0.00075 inch thick 
2o layer of commercially pure titanium. The coating was ap- 
plied by vacuum vapor deposition, the stainless steel part 
being preheated to 1450+50" F. and rotated during de- 
position. A 0.060 inch thick layer of 4043 aluminum was 
then applied on the titanium coating in the same manner. 
25 The coating was then machined to an aluminum thick- 
ness of 0.040 inch to fit into the aluminum member with 
an overlap of 0.0.375+0.010 inch and an interfacial braz- 
ing gap of 0.001 to 0.0015 inch. The stainless steel part 
was then inserted in the aluminum part, and a brazing 
30 alloy comprising 87 weight percent aluminum and 13 
weight percent silicon was positioned at the interface. The 
assembly was brazed by immersion in a molten salt bath 
at a temperature of 1100 to 1115" F. End caps of 6061 
aluminum and 304L stainless steel were then welded to 
35 the aluminum and stainless steel ends, respectively, in 
order to test the brazed joint. Pressurized helium was 
then introduced through an end cap, and the joint was 
tested for leaks by means of mass spectrometry in the 
as-fabricated condition and after subjection to thermal 
40 cycling and vibration. One specimen showed no increase 
in helium leakage from an initial value of 1 . 3 8 ~ 1 0 - ~ 0  
cubic centimeters per second after being cycled 100 times 
from ambient to liquid nitrogen temperature, after being 
vibrated at resonant frequency and a force of 25 g's for 
30 minutes and after heat-treatment at 350" F. for a pe- 
45 riod of 8 hours. Hydrostatic pressure tests showed a shear 
strength at the joint of 9600 pounds per square inch. A 
second specimen also showed no increase in leakage after 
these tests, and the shear strength at the joint was over 
8800 p.s.i., failure occurring at an end cap weldment. 
50 Ultrasonic C-scans before and after testing showed es- 
sentially complete bonding at the joint, bonding being 
unaffected by the tests. The joints were sectioned and ex- 
amined metallographically. FIGURE 2 in the drawing 
55 depicts the resulting metallographic section. No evidence 
of the presence of brittle phases or intermettallic com- 
pounds was found. 
Although the invention is illustrated specifically for a 
tube-and-sleeve connection it is to be understood that 
other shapes and configurations as well as parts of other 
sizes and thicknesses can be joined by this method. 
The above example is merely illustrative and is not to 
be understood as limiting the socpe of the invention, 
which is limited only as indicated by the appended claims. 
1. The method of joining an aluminum component to 
a stainless steel component which comprises metallur- 
gically bonding a thin, uniform coating of a titanium- 
base metal on the surface of the stainless steel compo- 
nent at the intended joining surface, metallurgically bond- 
ing a coating of an aluminum-base metal on the resulting 
titanium coating and welding or brazing the resulting 
aluminum-base metal coating to said aluminum compo- 
nent. 
2. The method of claim 1 wherein said titanium-base 
65 What is claimed is: 
75 
3,473,216 
metal coating and said aluminum base-metal coating are 
metallurgically bonded by vacuum vapor deposition. 
3. The method of claim 1 wherein said titanium-base 
metal coating is 0.000050 to 0.200 inch thick. 
4. The method of claim 3 wherein said aluminum-base 
metal coating is 0.001 to 1.0 inch thick. 
5. The method of claim 4 wherein said aluminum-base 3 ~ 0 7 1 ~ ~ 9 1  /l963 Horn et -------- 29-198 x 
an aluminum brazing alloy. 3,393,446 7/1968 Hughes et al. _ _ _ _ _ _  29-198 X 
6. The method of claim 5 where the composition of 
said brazing alloy is 11 to 13 weight percent silicon and lo 
the balance aluminum. 
9. The method of claim 5 wherein the configuration of 
said stainless steel member at  the intended joining sur- 
face is tubular. 
8. The method of claim 5 wherein said stainless steel 
component is heated to 1400 to 1500" F. prior to deposi- 
tion of said titanium coating. 
References Cited 
UNITED STATES PATENTS ti 
metal coating is brazed to said aluminum member with 3,105,293 1011963 Skinner et a]. ------ 29-42 x 
F- CAMPBELL, Primary Examiner 
RONALD J. SHORE, Assistant Examiner 
U.S. C1. X.R. 
15 29-197, 198 


Method of joining aluminum to stainless steel Patent

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