Tool and process for miniature explosive joining of tubes

The invention is a tool and process to be used in explosive joining of tubes. The tool consists of an initiator 81, a tool form 82 and a ribbon explosive 25. The assembled tool 80 is a compact, storable and safe device suitable for explosive joining of small, lightweight tubes down to 0.20 inch in diameter. The invention is inserted into a tube 51 to be welded which tube has itself been inserted into either another tube 52 or a tube plate. A shim or standoff between the two surfaces to be welded is necessary. Initiation of the explosive inside the tube results in a high velocity, angular collision between the mating surfaces. This collision creates surface melts and collision bonding wherein electron-sharing linkups are formed.

TECHNICAL FIELD OF THE INVENTION 
The invention is related to the welding, joining and fabricating technology 
field. 
BACKGROUND OF THE INVENTION 
This invention relates to joining tools and processes and more particularly 
to tools and processes for explosive joining of tubes. 
Demand is increasing for highly reliable, remote, metal joining processes 
for hazardous or inaccessible operations such as nuclear reactor repairs 
or assembly of structures in space. Prior art methods, such as mechanical 
fasteners, swaging fusion welding, soldering and adhesives have been 
inadequate due to constraints to hand operations, excessive downtime of 
the mechanism, inaccessibility of the parts to be joined, insufficient 
strength, resistance to environments (particularly high temperatures) and 
other constraints. Only explosive joining offers the characteristics to 
meet the above requirements. This type of joining produces metallurgical 
bonds which are impossible to achieve by any other processes. The 
explosive joining process produces a high velocity, angular collision 
between the metal surfaces which causes interactomic, electron-sharing 
linkups to be formed. However, previous methods of explosive joining have 
had limited suitability due to the necessity of using relatively large 
amounts of explosive which results in a lack of ability to precisely 
locate the joint and probable damage to lightweight structures such as 
small tubes and thin tube plates. 
Accordingly, it is an object of the present invention to provide a means of 
explosive joining which minimizes the amount of explosive required. 
Another object of the present invention is to provide a means of explosive 
joining which allows precise location of the joint. 
Yet another object of the present invention is to provide a means of 
explosive joining which reduces damaging pressure waves, noise nuisance 
and damage to surrounding structures. 
A further object of the present invention is to provide a tool capable of 
holding an explosive joining means in contact with the internal surface of 
a tube to be joined. 
Still another object of the present invention is to provide a method of 
explosive joining which uses safe, prepackaged joining devices which are 
storable and which can be mechanized for use without human contact. 
STATEMENT OF THE INVENTION 
According to the present invention, the foregoing and additional objects 
are attained by providing a tool and process for explosive joining of 
tubes 0.20 inches in diameter and larger. The tool consists of an element, 
shaped to match the cross-section of the tube to be welded, made of a 
rigid material which can be either machined or molded. This material can 
be wood, plastic or paper or for a reusable tool can be metal such as 
steel. One or more bands of ribbon explosive are wrapped around the tool 
to form the joining charge. The external circumference of the tool is 
shaped so as to hold the ribbon explosive in contact with the inner 
surface of a tube to be welded. The tool also holds an initiation charge 
(explosive-filled tube or blasting cap). Proper standoff between the tubes 
to be joined is achieved by either shims or by machining of the contact 
surfaces. The tool assembly is then inserted within the tube to be joined 
and fired to complete the joint.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings wherein like elements are referred to by the 
same reference numeral throughout the several views and more particularly 
to FIG. 1, an explosively bonded joint, designated generally by the 
numeral 10, is shown in cross-section with the outer surface of inner tube 
11 and outer surface 12 bonded together. An unfired tool 13 of the present 
invention is shown in the welding position. 
The physical mechanism of the tool 13 and process can be best understood by 
reference to FIG. 2, wherein a sectional view depicts the operation of the 
explosive joint 10 with the surface interaction depth 23 and impact angle 
24 between plates 21 and 22 exaggerated for illustrative purposes. 
Explosive charge 25 is a high energy low-yield sheathed miniature ribbon 
explosive, such as cyclotrimethylene-trinitramine (RDX), which generates 
several million pounds of pressure per square inch on top of plate 21. 
This over pressure creates velocities 26 in plate 21 of several thousand 
feet per second. On impact with plate 22, this kinetic energy is converted 
into skin-deep (approximately 0.001 inch) melts, which are stripped from 
the surface and squeezed out in jet action 27 by the closing angle 24. 
Generally, the amount of explosive required is affected by plate material 
and thickness, and by the standoff distance between plates. Referring to 
FIG. 3, a plate standoff 31 or separation is required to achieve the high 
velocity, angular collision necessary to effect the bond. This standoff 31 
can be achieved by shimming 32, fixturing or machining. Any convenient 
shim may be used including masking tape. Alternately, a notch may be 
machined in either or both plates. The plates can be configured to present 
a parallel interface, or referring to FIG. 4, for maximum efficiency an 
inverted "V" 41 may be machined in one or both plates 21 and 22 providing 
an approximate five degree sloping surface 42. 
FIG. 5 depicts the shim 32 configured to provide proper standoff between 
inner tube 51 and outer tube 52. Minimum standoff, approximately 0.010 
inch, is required to achieve the required velocity to effect the joint. 
Maximum standoff, approximately 0.025 inch, is established to minimize 
material deformation and bending energy losses. 
Similarly, FIG. 6 shows a machining application to the tube surface. Tube 
51 is machined to provide an inverted "V" circumferentially inscribed 
around the tube. The surface of inverted "V" 61 angles at approximately a 
five degree slope 62 shown in exaggerated scale for purposes of 
illustration. The tube wall is maintained a constant thickness throughout 
the "V" section. 
Referring to FIG. 7, a cross-sectional drawing of a photographed test 
sample shows the completed joint. Inner tube 51 is bonded to outer tube 52 
at the inverted "V" 61 location which had been previously machined into 
the outer surface of inner tube 52. 
Referring to FIG. 8, a depiction of the joining tool, is shown generally by 
the numeral 80. Three components make up the completed joining tool, the 
initiator 81 which can also be an electrically initiated blasting cap, the 
tool form 82, and the low-yield sheathed miniature ribbon explosive 25. 
Initiator 81 is made up of an 0.090 inch diameter, one-inch long, 0.005 
inch wall tube with an 0.500 inch long packed explosive column that is 
located 0.300 inch from one end of the tube; a tube of 8-grains/foot mild 
detonating cord is inserted and bonded into the 0.300 inch deep cavity. 
Tool form 82 is center bored so that initiator 81 may be inserted into the 
end of the tool form. A slot 84 located at the ribbon explosive holding 
point 85 receives the end of initiator 81 such that the initiator is held 
in contact with the ribbon explosive 25. Tool form 82 also has a slight 
recess at the ribbon explosive holding point 85 so that the ribbon 
explosive 25 may be pressed flush with the tool surface. Insertion of the 
initiator 81 into tool form 82, and wrapping of ribbon explosive 25 around 
the tool form, and and completes the explosive joining tool 80. As 
packaged in this configuration, the joining tool 80 is storable with a 
shelf life of around five years. The size of the smallest completed tool 
is slightly smaller in diameter than an ordinary pencil (0.200 inch) and 
the length including the initiator can be made any convenient dimension. 
The ribbon explosive is a secondary explosive which requires an explosion 
to initiate the ribbon. Impacts, electrical shocks, heat and fire will not 
cuase the ribbon to explode. Further, the completed tool contains the 
least amount of explosive which can be used to effect a reliable joint. 
Compared to prior art the present invention uses less than ten percent of 
the explosive required for center loaded devices (cylindrical explosive 
plugs). For example, an 0.625 inch diameter tube using the center load 
method uses approximately ninety grains of explosive to form a continuous, 
sealed joint and achieve parent metal strength. This present invention 
requires only 6.5 grains to achieve the same function. Also, this tool is 
adaptable to various tube configurations by changing the shape of the tool 
form. These changes will allow joining of curved tube sections and of 
other shaped tubes such as square or triangular at any point in a tube 
length. 
OPERATION OF THE INVENTION 
The assembly and operation of the joining tool 80 is accomplished by the 
following steps: (1) Bond mild detonating fuse to initiator 81 using a 
non-reactive, non-solvent, non-vaporizing adhesive. In the present 
embodiment, a 3M adhesive "scotch cast" was used; (2) Wrap double-backed 
tape around tool form 82 at location for attaching ribbon explosive 25; 
(3) Cut ribbon explosive 25 to length to wrap around tool form 82 over the 
double-backed tape; (4) Install ribbon 25 on double-backed tape; (5) 
Insert initiator 81 into tool form 82 by inserting through slot and 
centerline hole in tool form 82; (6) Trim ends of ribbon explosive 25 with 
razor blade to produce a tight fit with the tool form 82; (7) Press ribbon 
explosive 25 into tool form 82 to achieve a flush surface with the 
initiator 81 explosive load centered on the ribbon explosive 25; (8) Use 
0.75.times.0.002 inch teflon tape to wrap around ribbon explosive 25 and 
tool form 82 or approximately 0.020 inch wall thickness heat shrink tubing 
to increase the diameter of the assembled tool to provide a snug fit to 
the inside diameter of the tube 51 to be joined; (9) Use 0.5.times.0.002 
inch teflon tape to wrap around the tool form 82 on both sides of ribbon 
explosive 25 to bring these areas up to the inside diameter of the steel 
tube 51 as well as cover the exposed portions of the initiator 81; (10) 
Insert the tube 51 to be joined to the desired depth into the outer tube 
52. Tube 51 must cover the machined area in tube 52 or shimming may be 
used; (11) Insert the assembled tool 80 into tube 51 and assure that the 
centerline of the ribbon explosive 25 is directly opposite the peak in the 
inverted "V" 41 or centered between the shims; (12) Secure assembled tool 
80 in position using any convenient means such as taping; (13) Initiate 
mild detonating fuse 81 by using of blasting cap or initiate blasting cap; 
and (14) Remove debris from tube by brush or other convenient means. 
Although specific embodiments of the invention have been described herein, 
they are to be considered as exemplary of the novel features thereof and 
are not exhaustive. There are many variations and modifications of these 
specific examples which will be readily apparent to those skilled in the 
art in light of the above teachings without departing from the scope of 
the appended claims. It is therefore understood that this invention may be 
practiced otherwise than as specifically described herein.