Nondestructive radiographic tests of resistance weld

Kropp, C. J.

English

NASA Technical Reports Server

t. JAMES E. WEBB 3 94 
ADMINISTRATOR OF THE N A T I O N A L  AERONAUTICS 
A N D  S P A C E  ADMINISTRATION 
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https://ntrs.nasa.gov/search.jsp?R=19710009138 2020-03-17T02:47:58+00:00Z
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3,466,418 
DETERMINATION OF SPOT WELD QUALrrV 
James E. Webb, Administrator of the NationaJ Aeronau- 
tics and §pace Administration with respect to an in- 
vention of Charles J. Kropp, §an Diego, Calif. 
Filed July 6, 1966, Ser. No. 563,644 
Int. CI. B23k 11/10 
U,S. C1. 219-91 5 Claims 
ABSTRACT OF THE DISCLOSURE 
A non-destrpctive method of radfographically inspect- 
ing 8 resistan@ weld is disclosed. The method comprises 
surrounding the desired weld region between work pieces 
such as stainless steel with a sheet of material having a 
substaotiqlly different radiopacity such as nickel. The 
pieces are resi8tance welded to form a nugget containing 
said material and surrounded by an unfused portion of 
the sheet forming an interfacial boundary. A radiographic 
image of the weld region is then formed which includes 
,a line image of said interfacial boundary. 
ORIGIN OF THE INVENTION 
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 Aero- 
nautics and Space Act of 1958, Public Law 85-568 (72 
Stat, 435; 42 USC 4257). 
BACKGROUND OF THE INVENTION 
Field of the invention 
This invention relates generally to the welding and 
weld inspection art. More particularly, the invention re- 
lates to a novel resistance welding method which permits 
or facilitates radiographic inspection of the completed 
weld, as well as to a novel method of radiographically 
inspecting resistance welds and a resistance welded work 
product capable of radiographic weld inspection. 
Description of the prior art 
Accgrding to cQnventiona1 resistance welding practice, 
a pair of electrodes are urged against opposite sides of 
the work members to be joined and q voltage of the 
proper magnitude for resistance welding is impressed 
across the electrodes. The resulting current flow between 
the electrodes produces resistance heating of the members 
sufficiently to cause fusion of the members in their inter- 
facial region between the electrodes. Several different 
types of resistance welding methods exist. These methods 
include spot welding, seam welding, projection welding, 
flash welding, upset welding, and percussion welding. The 
present invention is intended primarily for use in spot 
welding. For this reason, the present disclosure will be 
devoted to this resistance welding method. As will appear 
from the ensuing description, however, the invention may 
be applied to other resistance welding methods. 
The fused mass of metal which constitutes the com- 
pleted resistance or spot weld is commonly referred to as 
a weld nugget. The ideal spot weld nugget is essentially 
circular in outline and solid in cross section. Many spot 
weld nuggests, however, deviate from this ideal shape 
to such an extent as tQ be unacceptable for many appli- 
cations, notably space vehicle applications. Two of the 
more common aberrated spot weld shapes, for example, 
are annular or donut and U or horseshoe shapes. An an- 
nular or donut shape spot weld has a central unfused 
region completely surrounded and enclosed by the weld 
nugget. A U or horseshoe shape weld has a central un- 
fused region which is only partially surrounded by and 
opens through one side of the weld nugget. 
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Various methods and means have been devised for in- 
specting resistance welds to determine their shape and 
quality. Some of these inspection techniques, such as 
the existing eddy-current, ultrasonic, and radiographic 
techniques, are non-destructive in nature and thus permit 
use of the welded work product after inspection. These 
non-destructive inspection methods, then, enable de- 
termination of the actual shape and quality of the resist- 
ance welds which are placed in service. Other weld in- 
spection techniques, such as those which involve seetion- 
ing and subsequent microscopic examination of the weld, 
are destructive in nature and thus do not permit use of 
the welded work product which is inspected. Aecordingly, 
such destructive weld inspection techniques provide an 
indication of the probable rather than the actual shape 
and quality of the welds which are placed in service. The 
non-destructive inspection techniques are thus most re- 
liable from the standpoint of their ability to indicate the 
shape and quality of the welds which are placed in service. 
Moreover, the non-destructive inspection procedures are 
inherently less tedious and time consuming than the de- 
structive inspection procedures. 
As is well known in the art, resistance welds in certain 
metals are ill-suited to or incapable of inspection by the 
existing non-destructive techniques. One of these metals 
is stainless steel. Thus, the only available procedure for 
reliably inspecting a resistanee weld between stainless 
steel work members involves sectioning and subsequent 
microscopic examination of the weld nugget. As just 
noted, this destructive methsd of weld inspection provides 
an indication of the probable, rather than the actual, 
shape and quality of the resistance welds which are placed 
into service and thus does not possess the high degree 
of reliability demanded in many applications, notably 
space vehicle applications, 
OBJECTS AND SUMMARY OF THE INV~NTION 
Accordingly, a definite need exists for an improved and 
more reliable non-destructive method of inspecting resist- 
ance welds in many metals, particularly stainless steel. 
It is a general object of this invention to provide such 
an inspection method, as well as a novel method of resist- 
ance welding stainless steel and other metals to permit or 
facilitate radiographic inspection of the weld and a welded 
work product capable of radiographic weld inspection. 
A more specific object of the invention is to provide a 
novel resistance welding method and a novel radiographis 
weld inspection method which involve the placement, 
between the interfaces of the work members to be joined, 
of a metal insert or sheet having substantially different 
radiopacity than the work members, which sheet is fused 
in the weld region in such a way as to define about the 
completed weld nugget an interfacial boundary edge that 
produces a visible X-ray line image conforming to the 
outline of the nugget. 
A further object of the invention is to provide a novel 
method of resistance welding stainless steel and a novel 
method of radiographically inspecting resistance welds 
between stainless steel work members, as well as a re- 
sistance welded stainless steel work product capable of 
radiographic weld inspection. 
Yet a further object of the invention is to provide a 
stainless steel resistance welding method of the 
described which produces a weld of superior strength. 
Other objects, advantages, and features of the invention 
will become readily evident as the description proceeds. 
BRIEF DESCRIPTION OF DRAWINGS 
The invention will now be described in detail by refer- 70 
ence to the attached drawings wherein: 
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FIGURE 1 is a section through a pair of work members fused portion 30 which surrounds the weld nugget. This 
about to be joined by the present resistance or spot weld- unfused portion of the sheet defines, at its juncture with 
ing method; the weld nugget, an interfacial boundary edge 32 which 
FIGURE 2 is a section through the work members follows the outline of the nugget. It has been found when 
illustrating the completed spot weld; such a weld is subjected to radiographic examination, the 
FIGURE 3a is a radiagraphic image of the spot weld interfacial boundary edge 32 appears, on the radiographic 
illustrated in FIGURE 2; image, as a dark line at the inner periphery of the light 
FIGURE 3b is a section through the spot weld in FIG- halo which is characteristic of X-ray images of stainless 
URE 2; steel resistance welds. This line image results from the 
FIGURE 4a is a radiographic image of an annular or differing radiopacities of the materials of the work mem- 
donut shaped spot weld produced by the present welding bers and insert sheet. 
method; FIGURES 3u, 4a and 5a represent actual radiographic 
FIGURE 4b is a section taken taken through the weld images of the resistance welds illustrated in FIGURES 
nugget whose radiographic image is represented in FIG- 3b, 4b, and 5b, respectively, wherein the work members 
URE 4 ~ ;  15 are stainless steel and the insert sheet is nickel. In FIG- 
FIGURE 50 is a radiographic image of a U or horse- URES 3 4  4e and 5u, the dark line images produced by 
shoe shaped spot weld produced by the Present welding the interfacial boundary edges 32 of the unfused portions 
method; of the nickel sheets 28 employed in the welds is denoted 
FIGURE 5b is a section through the weld nwget whose by the reference numeral 32a. In each figure, reference 
radiographic image is represented in FIGURE 5a; 20 numeral 34 denotes the light halo, just referred to, which 
FIGURE 6 is an enlarged section taken on lines 6-6 is characteristic of X-ray images of stainless steel resist- 
in FIGURE 4b; and ance welds. It will be observed that in each case, the 
FIGURE 7 is an enlarged section taken on line 7-7 in line image 32a follows the outline of the corresponding 
FIGURE 5b. weld nugget 18, 20, or 24, as the case may be, and thus 
Referring now to these drawings, reference numerals 25 clearly defines the nugget shape. Accordingly, the present 
10 and 12 denote a pair of work members to be resistance invention permits non-destructive radiographic inspection 
or spot welded to one another. According to conventional of resistance welds between stainless steel work members 
practice, this is accomplished by urging a pair of elec- to determine the weld shape and quality. An additional 
trodes 14 and 16 against opposite sides of the members advantage of the invention resides in the fact that fusion 
and impressing a sufficient voltage across the electrodes 30 of the stainless steel and nickel in the weld nugget pro- 
to cause fusion of the work members in their interfacial duces a welded joint of superior strength. 
region between the electrodes. The fused mass 18 of metal It is now obvious that the invention may, conceivably, 
which constitutes the completed weld is referred to as a be utilized to advantage in Some or all of the different 
weld nugget. The ideal spot weld nugget is generally cir- resistance welding methods referred to earlier. It is further 
CUlar in outline and solid in cross section, as shown in 35 obvious that the invention is not limited in application to 
FIGURE 3b. In many cases, however, spot welding pro- resistance welding of stainless steel. Thus, the invention 
duces weld nuggets having Some form of aberration or may be applied to the resistance welding of any metals by 
deformity which renders the welded joints unacceptable employing an insert sheet having a sufficiently different 
for many applications. FIGURES 4b and 6 and FIG- radiopacity than the work members to produce a readily 
URES 56 and 7 illustrate the two more common spot 40 visible radiographic image of the interfacial boundary edge 
weld aberrations, referred to earlier, which are encoun- defined by the unfused portion of the sheet. In this regard, 
tered in actual welding practice. In FIGURES 4b and 6, it should be noted that the invention may be employed to 
the deformed weld nugget 20 has a generally annular or enhance or clarify a radiographic image of a weld which 
donut shape defining an unfused region 22 at the center normally yields a relatively poorly defined image with 
of the nugget. The deformed weld nugget 24 illustrated 45 conventional radiographic inspection techniques. 
in FIGURES 5b and 7 has a generally U or horseshoe Accordingly, the invention herein described and illus- 
shape defining a central unfused region 26 which extends trated is fully capable of attaining the several objects and 
radially through one side of the nugget. advantages preliminarily set forth. 
In this disclosure, the work member 10, 12 are assumed What is claimed as new in support of Letters Patent is: 
to be stainless steel plates. As mentioned earlier, the only ' O  1. The non-destructive method of radiographically in- 
currently available procedure for inspecting resistance specting a resistance weld between two metal work mem- 
welds between stainless steel work members involves bers which when directly welded form a weld the radio- 
initial sectioning and subsequent microscopic examination graphic image of which is unsuitable for radiographic in- 
of the weld nugget. Not only is this method of weld in- 65 spection, which comprises the steps of: "" 
specton tedious and time consuming, but further, and 
more important, it results in destruction of the inspected 
weld and provides only an indication of the probable, 
rather than the actual, shape and quality of the welds 
which are placed into service. Accordingly, the inspection 60 
method has a comparatively low order of reliability. 
The present invention avoids these disadvantages of 
the existing technique of inspecting resistance welds be- 
tween stainless steel work members. According to the 
invention, an insert or sheet 28, in the form of a foil or 65 
thin plate having a substantially different radiopacity than 
the work members, is placed between the interface of the 
work members 10, 12 to be joined in such a way that the 
sheet surrounds and extends edgewise beyond the desired ,o 
weld region, as shown in FIGURE 1. When the work 
members comprise stainless steel, the preferred material 
of the insert sheet is nickel. During the actual welding 
operation, the sheet is fused only in the region of the weld. 
Accordingly, in the completed weld, the sheet has an un- 75 
placing between said work members in surrounding re- 
lation to the desired weld region a sheet of material 
having a substantially different radiopacity than the 
resistance welding said work members to one another 
within said region in such a way as to fuse said mem- 
bers and sheet within said region, thus to form a weld 
nugget surrounded by an unfused portion of said 
sheet defining an interfacial boundary edge which 
conforms to the outline of said nugget; and 
forming a radiographic image of said weld region in- 
cluding a line image of said interfacial boundary 
edge. 
2. The welding method according to claim 1 wherein: 
the material of said work members comprises stainless 
said sheet comprises a nickel foil; and 
said resistance welding step comprises spot welding said 
work members within said region. 
material of said members; I 
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steel; 


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