Method for filling internal casting voids

A method for filling an internal void in a metal component. After internal voids in the metal component are detected and mapped, the metal component is positioned such that an internal void is between two electrodes. The metal component is compressed by the electrodes and an electrical current is applied across the electrodes and the intervening metal component. Suitable selection of electrode compression force, electrical current, and time of application of each will collapse the internal void and cause it to migrate to at least one surface of the metal component where it will appear as a dimple in the surface. The dimple is then metallurgically filled so as to provide the desired surface characteristics and contour of the metal component. The electrical current density in areas adjacent the internal voids is higher than elsewhere in the metal component and thus causes melting of the metal in those areas. Compression of the metal component simultaneously with application of the electrical current across the electrodes enables application of a lower electrical current than would be required without the metal compression.

BACKGROUND OF THE INVENTION 
This invention relates to filling internal voids in metal components, and 
more particularly, to a method for simultaneously impressing an electrical 
current and mechanical compression to metal components containing such 
internal voids. 
Internal voids are common casting defects which normally result from metal 
shrinkage during solidification or by entrapped gases released upon 
cooling. Such internal voids (including cracks) can adversely affect the 
performance of the metal component casting. When the size and/or density 
of internal voids is less than what is judged allowable, salvage of the 
metal component is often attempted. The common method for repairing 
internal voids includes grinding out or otherwise removing portions of the 
metal component until the internal void is exposed at the ground out 
surface of the metal component. The resulting grinding cavity and internal 
void is then metallurgically filled by welding. The aforementioned repair 
technique is cumbersome, expensive, and time consuming. In some cases the 
size and/or density of the internal voids is such that the metal component 
is scrapped and recast. 
U.S. Pat. No. 4,068,111 which issued Jan. 10, 1978, illustrates a method 
for repairing casting defects by removing material from the metal 
component and providing a smooth cavity. Such method thus necessitates 
material removal from the metal component casting. 
Another commonly used technique for repairing internal voids in castings is 
hot isostatic pressing (HIP) which generally involves heating a casting in 
a furnace or other external heating means and then compressing the casting 
to collapse the internal voids. HIPing thus requires vast heat additions 
to the casting to make it conducive to compression and thus collapse of 
the internal voids. The large heating source needed in HIPing is required 
since heat externally applied to the casting section containing each void 
tends to flow laterally in the casting and thus dissipate so as to require 
an extremely large portion of the casting to be heated to the temperature 
necessary for practical compression thereof. Additionally, where the voids 
in the casting open to the surface, HIPing can cause oxidation of the 
internal void surfaces so as to preclude effective filling thereof without 
first removing such oxide. 
Patent 3,606,785 which issued Sept. 21, 1971, illustrates a typical 
compression of the subject metal components and collapse of internal voids 
by rolling metal components between rollers. Such technique is most often 
used immediately following the casting process when the metal component is 
still in a relatively hot state. Otherwise, the metal component must be 
reheated with expenditure of large amounts of heat as previously 
mentioned. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a method is provided for filling 
internal voids in metal components when the presence of such internal 
voids are detected and their position mapped. The invention generally 
comprises locating the internal void in a metal component, positioning the 
metal component between two electrodes such that the internal void is 
between the two electrodes, compressing the metal component between the 
electrodes, applying an electrical current through the electrodes and the 
intervening compressed metal component, and metallurgically filling the 
dimple which forms at the metal component surface as a result of collapse 
of the internal void. The metal component compression occurs for a 
predetermined period of time with a predetermined force and the electrical 
current application similarly occurs for a predetermined time with a 
predetermined magnitude. The time of metal compression and electrical 
current application overlap so that both occur simultaneously for at least 
a portion of the compression and current application cycle. The 
compression force and/or current magnitude may be varied during their 
times of application to provide heat treatment and minimize the energy 
needed to collapse the voids.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is concerned primarily with repairing internal voids 
in castings and other metal components. In the description which follows 
the invention will be applied to a turbine blade. It should be understood, 
however, that the invention may be applied to any metal component. 
FIG. 1 illustrates an axial flow turbine blade 10 having an integral 
platform root 12. Through the use of X-ray or other diagnostic means, the 
locations of internal voids 14 are determined and preferably mapped on the 
surface of blade 10. Conventional repair techniques, heretofore, required 
grinding the metal of the blade 10 from the blade surface down to the 
voids 14. The resultant opening in the blade was often many times the size 
of the internal void or defect volume 14. Such opening in blade 10 was 
then weld filled typically in a manual operation since the openings to the 
internal defects were not of standard size and/or shape. 
FIG. 2A illustrates a portion of blade 10 disposed between electrodes 16 
such that the portion of blade 10 having internal void 14 is directly 
between the electrodes 16. As can be seen in FIG. 2A, a current source 
such as a homopolar generator 18 is connected across the electrodes 16 
which are adapted to provide a predetermined compression force to blade 
10. By simultaneously applying a predetermined compression force with the 
electrodes 16 and conducting a current of predetermined magnitude through 
the electrodes and intervening blade 10, internal voids 14 may be 
collapsed and caused to migrate to the surface of blade 10. It is to be 
understood that under certain circumstances it may be advantageous to 
initiate application of the current prior to applying the compression 
force. 
FIG. 2B is a semischematic view of the apparatus illustrated in FIG. 2A. 
Electron paths 17 may be seen to be more dense about the edges of internal 
voids 14 so as to produce higher temperatures and cause metal melting at 
those locations. Such metal melting in combination with the compression 
force exerted by electrodes 16 facilitate void collapse and simplify 
repair of the metal component. Since the current magnitude and its time of 
application are interrelated with the magnitude of the compression force 
exerted by the electrodes 16, optimum values of each must be determined 
for specific cases. Furthermore, the magnitude of the compression force 
and current may be varied to minimize the component heat treatment and 
minimize the total energy expended in the manufacture of the component. 
Primary parameters affecting optimum values of the aforementioned 
variables include the melting point of the metal component's constituents, 
the metal component's conductivity, the metal component's compression 
strength, the section thickness where the void resides, and the effective 
internal void sizes. 
For thin sections on the order of 3/8 to 1/2 inch having voids of 
approximately 1/8 inch to 1/4 inch the compression force and current 
requirements may be met by presently existing state-of-the-art spot 
welders having compression forces of approximately 500 to 2000 pounds and 
currents of 20,000 to 40,000 amperes in 1-3 seconds. For successively 
thicker sections and smaller voids, machines having concomitantly higher 
compression force and current production capabilities are necessary. Use 
of homopolar generators capable of delivering 0.2 to 3,000,000 amperes 
within 30 milliseconds can greatly reduce the compression force necessary 
to collapse internal voids 14. 
FIG. 3 is a semi-schematic view of the portion of blade 10 previously shown 
disposed between electrodes 16. As can be seen volume 14' constitutes the 
same metal as previously surrounded internal voids 14. During application 
of the current and compression force occurred between FIG. 2B and FIG. 3, 
internal void 14 migrated to the surface 22 of blade 10 to form dimple 24. 
Dimple 24, when resident at surface 22, is easily repaired by filling with 
weld 26 and later smoothing the weld to the desired contour to match the 
surrounding surface 22. 
It will now be apparent that an improved method for repairing internal 
voids in metal components has been provided in which compression and high 
electrical current have been simultaneously applied at sections of the 
metal components where internal voids reside. Cooperative application of 
the compression force and high current facilitates collapse of the 
internal voids and causes them to migrate to the surface of the metal 
component where they may be easily filled with weld metal. One or more 
voids, depending on its size and density in the metal component, may be 
eliminated by each compression-current application. For relatively thin 
metal component sections and relatively large voids, present technology 
spot welder mechanisms may be used.