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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



relay development.* The paper, therefore, does not purport to be a 

 thorough study of all phenomena of interest. The purpose is rather to 

 enumerate the major fundamental problems and to present first order 

 solutions. Perhaps other groups having further interest in the process 

 will carry on basic research along the lines indicated. 



BASIC PROCESS 



Basically the process consists of an electrical circuit which stores 

 energy and maintains a voltage across the two parts to be welded. This 

 is illustrated in Fig. 1 where a wire is to be welded to a small rectangular 

 block. A mechanical appendage, the "gun," holds one of these parts 

 and moves towards the other, stationary part. At a separation xo , the 

 arc initiation separation, the airgap breaks down and the arc is initiated. 

 While the arc heats the opposing surfaces, forming a thin layer of molten 

 metal on both parts, they are being brought closer together and finally 

 come into contact, extinguishing the arc. The joint now cools and the 

 weld is made. 



A properly controlled process poses two design problems. First, it is 

 necessary to select materials that are reasonably compatible and geom- 

 etries which allow each part to reach the desired temperatures. The 

 second design problem is the choice of the proper electric circuit. This 

 paper is primarily concerned with the latter problem. 



Material selection may be dictated by other considerations, but it is 

 necessary to choose materials which are capable of producing a sound 

 joint. Irregularities, such as gas pockets, possibly due to a low boiling 

 point component are to be avoided. Geometries must be chosen such that 

 in the presence of heat conduction away from the surfaces to be welded, 

 the average temperatures of both surfaces exceed their melting points 

 but stay below their boiling points. 



A proper electric circuit for percussive welding has to supply sufficient 

 energy to produce thin molten layers on l^oth parts. It is luidesirable to 



PROPELLING 

 SPRING 



(a) (b) 



Hg. 1 (a) and (W) — Percussive welding, (a) Parts to he welded, (b) Proc- 

 ess diagram. 



* A. C. Keller, A New General Purpose Relav for Telephone Switching Systems. 

 B.S.T.J., pp. 1023-1067. November, 1952. 



