Method for welding an article and terminating the weldment within the perimeter of the article

An article is welded, as in weld repair of a defect, by positioning a weld lift-off block at a location on the surface of the article adjacent to the intended location of the end of the weldment on the surface of the article. The weld lift-off block has a wedge shape including a base contacting the surface of the article, and an upper face angled upwardly from the base from a base leading edge. A weld pool is formed on the surface of the article by directly heating the surface of the article using a heat source. The heat source is moved relative to the surface of the article and onto the upper surface of the weld lift-off block by crossing the leading edge of the wedge, without discontinuing the direct heating of the article by the heat source. The heating of the article with the heat source is discontinued only after the heat source is directly heating the upper face of the weld lift-off block, and not the article.

BACKGROUND OF THE INVENTION 
This invention relates to welding, and, more particularly, to weld repair 
wherein the weldment must be terminated within the perimeter ol the 
article being welded. 
It is not uncommon that some articles experience cracking during 
fabrication or during service. For example, aircraft engine parts may 
experience cracking during service as a result of applied strains and 
thermal strains. In addition, parts made of certain alloys, particularly 
those having low ductilities at temperatures just below the melting point, 
have a tendency to exhibit through-wall cracking during the initial 
casting fabrication. 
If the surface cracks are not overly severe, they may be repaired by 
welding. Welding as used in this context means that the cracked material 
is first removed, usually by grinding. The resulting void is then refilled 
with molten metal. Upon cooling, the molten metal solidifies so that the 
crack is filled with solid metal, termed the weldment, and thence is 
repaired. Extra "filler" metal, usually but not necessarily of the same 
composition as the base metal of the article, is supplied to the molten 
region to aid in filling the crack. 
The weld repair process generally works well along most of the region of 
the crack. However, it is sometimes found that the weld repair process 
induces further cracking of the article, particularly at locations near 
the point of termination of the weldment. These repair-induced cracks are 
usually smaller than the original crack that was to be repaired, but are 
still troublesome and must be repaired. 
A typical further repair procedure for the repair-induced cracks is to cap 
weld over these cracks with a lower-melting-temperature, weak filler 
material, and then to hot isostatically press the cap-welded region. 
Another further repair procedure is to fill the repair-induced cracks with 
a filler metal such as a powder, and then hot isostatically press the 
article. These repair procedures are suited for the repair of the small 
repair-induced cracks, but not of the original larger crack. The repair 
procedures, while operable, are expensive and may introduce material of a 
foreign composition into the article, as in the case of the cap weld 
procedure. 
There is a need for an improved approach to the weld repair of articles 
with surface cracks. The present invention fulfills this need, and further 
provides related advantages. 
SUMMARY OF THE INVENTION 
The present invention provides a welding process, usefully applied for weld 
repair, that reduces the incidence of cracking associated with the welding 
operation. In the weld repair context, there is less cracking at the 
termination point of the weldment, sometimes termed the "lift-off point", 
than in prior welding processes. The present approach may be used in a 
wide variety of welding applications, and is particularly useful in those 
where the lift-off point is within the periphery of the article. The 
welded article may be at room temperature (away from the welded region), 
or may be heated, when the weld repair is performed. The weld procedure 
may be accomplished manually or with automated equipment. 
In accordance with the invention, a method of forming a weldment that 
terminates within the perimeter of an article comprises the steps of 
furnishing an article having a surface, and positioning a weld lift-off 
block at a location on the surface of the article. The weld lift-off block 
has a wedge shape comprising a base contacting the surface of the article, 
and an upper face angled upwardly from the base from a base leading edge. 
The method further includes forming a weld pool on the surface of the 
article by directly heating the surface of the article using a heat 
source, translating the heat source relative to the surface of the article 
and onto the upper surface of the weld lift-off block by crossing the 
leading edge of the wedge, without discontinuing the direct heating of the 
article by the heat source, and thereafter discontinuing the heating of 
the article with the heat source only after the heat source is directly 
heating the upper face of the weld lift-off block, and not the article. 
In this approach, the direct heating during the welding procedure is 
terminated on the upper face of the weld lift-off block, which is separate 
from the article. Most cracks associated with the lift-off of the heat 
source are confined to the upper face of the weld lift-off block, and are 
therefore harmless because they are not in the article itself. In most 
cases, the weldment is started at a location separated from the base 
leading edge of the weld lift-off block, and the heat source is translated 
relative to the surface of the article, usually parallel to the surface, 
until it reaches the base leading edge. The heat source is then moved 
across the leading edge such that thereafter the upper face of the weld 
lift-off block is directly heated. The weld lift-off block is preferably 
made of a material that has a melting point not substantially less than 
that of the article being welded, so that it is not melted onto the 
surface of the article during the final stages of welding. The weld 
lift-off block is normally affixed to the surface of the article by a 
retainer, such as a tack-welded strip, so that it does not move during the 
welding operation. 
The present approach significantly decreases the number and size of surface 
cracks associated with the welding operation, and in man) cases there are 
no such cracks. If any such repair-induced cracks remain, either on the 
front (welded) side or the back side of the article, they may be repaired 
with known techniques suitable for repairing small cracks, such as 
powder-filler methods. The present approach may be used with a wide 
variety of heat sources, such as, for example, electric arc sources, gas 
torches, and lasers. 
Other features and advantages of the present invention will be apparent 
from the following more detailed description of the preferred embodiment, 
taken in conjunction with the accompanying drawings, which illustrate, by 
way of example, the principles of the invention. The scope of the 
invention is not, however, limited to this preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 depicts an example of a component article 20 of an aircraft gas 
turbine engine which may be repaired according to the approach of the 
invention. This article 20 is illustrated as a mixer used in an exhaust 
gas diffuser, but it may be any other operable article. 
Either during fabrication or during service of the article 20, a crack 30 
may be formed in the article. If the crack 30 cannot be repaired, it would 
be necessary to scrap the article. If the crack 30 is not overly aide, it 
may be possible to repair the article by closing the crack 30 using a 
welding procedure. 
FIG. 2 illustrates a conventional approach for the weld repair of the crack 
30. First, some material at the surface of the crack 30 is removed, 
typically by grinding, leaving a void 30a to be filled by weld repair. The 
void 30a is comparable in shape to the crack 30, but somewhat larger due 
to the removal of material. A heat source 32 is positioned adjacent to a 
surface 34 of the article 20, so that the heat source heats and melts the 
base metal of the article 20 immediately adjacent to the void 30a. The 
heat source 32 may be of any operable type, such as electric arc sources, 
gas torches, or lasers. The illustrated heat source 32 is a preferred gas 
tungsten arc source, in which an electric arc is struck between an 
electrode in the heat source and the article to be repaired. This electric 
arc locally melts the surface 34 in and around the void 30a. An inert gas 
is fed down the barrel of the heat source, emerging to surround the arc to 
prevent excessive oxidation. Optionally but preferably as shown, a 
powdered (or wire) filler metal, typically of the same composition as the 
base metal of the article being repaired, is fed through the heat source 
(or separately from the heat source) to deposit into the molten weld pool 
36. The heat source is translated along the surface 34, following the void 
30a. The base metal of the article 20 along and in the cracked region is 
progressively melted to form the weld pool 36, and filled with the filler 
metal. As the heat source 32 moves on, the weld pool 36 solidifies in its 
trail as the weldment 38 that serves as the repair for the original crack 
30. 
When the heat source 32 reaches the end of the void 30a, it is raised away 
from the surface 34 to terminate the welding portion of the repair 
procedure, giving this end point the name "lift-off region" 40. The 
lift-off region 40 is typically within a perimeter 44 of the surface 34 of 
the article being repaired. That is, the void 30a typically does not 
extend to the edge of the article 20. 
As shown in FIG. 3, new cracks, termed repair-induced cracks 42, are often 
observed in the weldment 38 and the base metal adjacent to the lift-off 
region 40. These repair-induced cracks 42 are typically smaller than the 
original crack 30 that was repaired, but are still troublesome and must be 
further repaired. The present invention is concerned with reducing the 
incidence of such weld-induced cracks, and achieving a better repair of 
the original crack 30. 
A preferred approach for practicing the present invention is illustrated in 
FIG. 4. An article to be welded, such as the article 20 having the surface 
34, is furnished, numeral 50. In the preferred application of the 
invention, the article 20 is made of a nickel-base alloy (superalloy) such 
as Rene' 108, having a nominal composition, in weight percent, of 9.4 
percent cobalt, 8.2 percent chromium, 0.5 percent molybdenum, 9.5 percent 
tungsten, 3.2 percent tantalum, 5.6 percent aluminum, 0.7 percent 
titanium, 1.5 percent hafnium, 0.1 percent carbon, 0.015 percent boron, 
balance (about 62.9 percent) nickel. The application of the invention is 
not limited to this alloy, and is more broadly applicable to a wide range 
of metallic materials, such as, for example, cobalt-base alloys, iron-base 
alloys, titanium-base alloys, or intermetallic materials such as titanium 
aluminides. The material forming the article 20 is sometimes termed the 
"base metal". 
Base metal at the surface of the crack 30 is removed, typically by 
grinding, leaving the void 30a comparable in shape to the crack 30, but 
somewhat larger due to the removal of base metal, to be filled. 
A weld lift-off block 70 is furnished and positioned at an effective 
article lift-off region 72, numeral 52. The weld lift-off block may be 
temporarily affixed to the surface 34 of the article 20 using a strip 73 
that is tack welded to the weld lift-off block 70 and to the surface 34, 
and then removed after the weld repair is completed. The weld lift-off 
block 70 is preferably made of a material having a melting point that is 
not substantially less than that of the article 20 being welded. That is, 
the melting point of the material of construction of the weld lift-off 
block is preferably equal to or greater than that of the base metal of the 
article 20. The melting point of the weld lift-off block may be slightly 
less than that of the base metal, but not more than about 100.degree. F. 
lower. The weld lift-off block 70 is also preferably made of a material 
having a composition close to that of the article being welded, so that 
there is no contamination of the article due to any incidental melting of 
the weld lift-off block during the following procedure. 
The weld lift-off block 70 and its positioning in relation to the surface 
34 and the void 30a are shown in FIGS. 5 and 6. The weld lift-off block 70 
is generally wedge shaped, and includes a base 74 contacting the surface 
34 of the article. An upper face 76 is angled upwardly from the base 74 
from a base leading edge 78. The angle A between the upper face 76 and the 
base 74 is preferably about 45 degrees, but the angle is not critical as 
long as it is sufficient to separate the direct heating effects of the 
upper face 76 from the base 74 and thence from the surface 34. A lower 
face 80 is angled upwardly from the base 74 from a base trailing edge 82 
separated from the base leading edge 78, so that there is a gap 84 between 
the lower face 80 and the surface 34 of the article 20. The gap reduces 
heat transfer from the weld lift-off block 70 to the base metal of the 
article being welded. (In other embodiments of the weld lift-off block, 
such as will be discussed in relation to FIG. 8, there is no gap 84.) 
The weld lift-off block 70 is positioned with the base leading edge 78 
immediately adjacent to the effective article lift-off region 72, and held 
in place by the tack-welded strip 73. The effective article lift-off 
region 72 is the point at which the direct heating of the heat source 32 
is removed from the article 20 being welded. By "direct" heating is meant 
the region which is immediately heated from the heat source 32 without 
substantial diffusion of the thermal energy through other, intermediate 
structure or pieces. 
A weld pool is formed on the surface of the article by direct heating of 
the article by the heat source 32, numeral 54. The weld pool is usually 
first formed at the first location at which the crack is to be repaired, 
numeral 86 in FIG. 5. Additional filler metal, preferably but not 
necessarily of the same composition as the base metal of the article 20, 
is preferably, but not necessarily, fed into the weld pool in order to 
fill the void 30a. 
During the welding operation of steps 54-58, the article 20 being welded 
may be intentionally heated by a heater other than the heat source 32 to a 
temperature greater than room temperature, or it may remain unheated, and 
at nearly room temperature, except for the heating from the heat source 
32. General heating of the article to a temperature to a moderately high 
temperature has proved effective during weld repair of some superalloys, 
particularly those which have limited ductility at temperatures near to 
their melting points. 
The heat source 32 is translated horizontally along the surface 34 relative 
to the article 20, in a relative direction of movement along the length of 
the weldment to be formed, numeral 56, as indicated by the arrow 88 in 
FIGS. 5 and 6. The heat source may be moved and the article held 
stationary, the heat source may be stationary and the article moved, or a 
combination of the two. The heat source 32 is translated along the surface 
34 of the article 20 toward the effective lift-off region 72. The 
translation of the heat source 32 is continued past the effective lift-off 
region 72 and onto the upper face 76 of the weld lift-off block 70 by 
crossing the base leading edge 78, without discontinuing the direct 
heating of first the article 20, and then the weld lift-off block 70, by 
the heat source 32. The continuous heating of the article 20, as the heat 
source 32 passes the effective lift-off region 72, avoids the production 
of most repair-induced cracks when the base metal and the filler metal, if 
any, cools and solidifies. The "effective" lift-off region is so-named 
because the heat source 32 is removed from direct heating of the surface 
34 of the article 20, without disrupting the regular lateral flow of heat, 
and while gradually decreasing the heat input to the surface 34. 
Thereafter, the heating with the heat source 32 is discontinued only after 
the heat source 32 is directly heating the upper face 76 of the weld 
lift-off block 70, and not the article 20. That is, the power to the heat 
source 32 is turned off, or the heat source 32 is lifted away, only after 
the heat source 32 has moved past the base leading edge 78 and so that its 
heat is directed onto the weld lift-off block. This location is the actual 
lift-off location 90, different from the effective lift-off location 72. 
Any repair-induced cracking in the material at the actual lift-off region 
90 is in the weld lift-off block 70, which cracking is not of concern 
because it is not in the article being welded. 
The final repaired article 20 is illustrated in FIG. 7. A relatively 
uniform solid weldment 92 extends along the region which was previously 
cracked, from the first location 86 to the effective lift-off region 72. 
The article is given optional final processing, numeral 63. Such final 
processing 60 may include removal of any artefacts of the welding, such as 
any excess material left over from the welding, and cleanup of the surface 
34. The final processing 60 may also include any post-welding heat 
treatment of the article. 
The final processing 60 may also include any required repairs of small 
cracks or other defects remaining after the weld repair. There may be a 
small number of small cracks in the weldment 92, or laterally adjacent to 
the weldment in the article 20. These small cracks may be on the front 
side of the article 20, from which the welding occurs, or the back side 
opposite the front side if the article 20 is thin. These cracks are 
typically quite small and few in number, if present at all. They may be 
repaired with known techniques suitable for filling small defects, such as 
activated diffusion clad(ling (ADC), activated diffusion healing (ADH), or 
the LPM.TM. process. See, for example, Keith A. Ellison et al., "Low Cycle 
Fatigue Properties of LPM.TM. Wide-Gap Repairs in Inconel 738", 
Superalloys 1996, Proc. of the Eighth International Symposium on 
Superalloys, Minerals, Metals & Materials Society, page 763 (1996). 
FIG. 8 illustrates another embodiment of the weld lift-off block, denoted 
by numeral 70'. This weld lift-off block 70' has a base 74', an upper face 
76', and a base leading edge 78' comparable to those of the weld lift-off 
block 70 of FIGS. 5 and 6, and those corresponding descriptions are 
incorporated here. The weld lift-off block 70' differs in that it has no 
lower face 80 and no gap 84. 
The present approach using the weld lift-off block has been comparatively 
tested with a similar approach that does not utilize the weld lift-off 
block, for the weld repair of Rene' 108 test coupons. Specimens were weld 
repaired by heating the specimen to elevated temperature, performing the 
weld repair at elevated temperature, post-weld annealing, and then 
inspecting the specimens after cooling to room temperature. FIG. 9 
summarizes the results of the tests. The temperature of the weld repair is 
indicated on the vertical axis, with a range for each test indicating the 
temperature at the beginning and at the end of the weld repair procedure. 
The post-weld annealing temperature is also indicated. The horizontal axis 
is the Crack Factor, defined as (1/number of cracks.times.maximum crack 
length).times.100. The larger the Crack Factor, the fewer the number of 
cracks and the smaller the crack size, the desired result. Tests performed 
without the use of the weld lift-off block, whose results are found on the 
left-hand side of FIG. 9, produced a relatively large number of cracks and 
large cracks. Tests performed with the use of the weld lift-off block, 
whose results are found further to the right in FIG. 9, produced a 
relatively small number of cracks and small cracks. The optimum approach, 
found in the four right-most data sets, all utilized the weld lift-off 
block and resulted in cracks having a maximum length of about 1/8 inch 
which did not penetrate through the thickness of the specimen. 
Although a particular embodiment of the invention has been described in 
detail for purposes of illustration, various modifications and 
enhancements may be made without departing from the spirit and scope of 
the invention. Accordingly, the invention is not to be limited except as 
by the appended claims.