Description of capacitor discharge welding process CD

    

The capacitor discharge stud welding process with tip ignition serves for welding mainly pencil-shaped metal parts from about 1 to 10 mm in diameter to thin sheets from 0.5 to about 3 mm. Distinction is made between two processes: contact welding and gap welding. The contact welding process is preferred for high-grade steel and unalloyed and zinc coated sheets (-15µ) where the welding stud is fed to the chuck and applied to the workpiece with a hand-held gun or automatic stud welding head. A compression spring in the welding gun presses the stud with welding tip as typically used in capacitor discharge stud welding against the workpiece.   

When the trigger of the welding gun is squeezed, the energy stored in the capacitors is discharged via a thyristor. The ignition tip of the stud is heated to the extent that it suddenly melts and partially evaporates. The arc occurring in the process completely melts the end of the stud and workpiece.     
After a welding time of only 1-3 ms and up to 15,000A, the studs and base metal are homogenously joined with only a very small fusion zone of a few tenths of a millimetre. This enables studs to be welded to very thin sheets without disturbing marks or discoloration being produced on the back.   

 

After a welding time of only 1-3 ms and up to 15,000A, the studs and base metal are homogenously joined with only a very small fusion zone of a few tenths of a millimetre. This enables studs to be welded to very thin sheets without disturbing marks or discoloration being produced on the back.   

 

The gap welding process differs from the contact welding process in that the stud is lifted to a preset distance or gap after squeezing the trigger and pushed against the workpiece by the force of a spring. When the ignition tip of the stud touches the workpiece, the arc is ignited. The arc duration or welding time can be varied by adjusting the gap; this is generally shorter than in the contact welding process however. The gap welding process is therefore preferred for materials with a low melting point such as aluminium or brass. The welding time is maximum 1.5ms and the fusion zone is again smaller than in the contact welding process. The workpiece surface must be clean and free of electrodeposited coatings, oil, rust or scale.    

    

 

 

Description of short cycle welding process

 





The drawn arc welding process with tip ignition serves for welding mainly pencil-shaped metal parts from 3 to 30mm in diameter to metal workpieces from a thickness of 0.8mm upwards. Distinction is made between two processes: short cycle drawn arc welding and drawn arc welding with ceramic ferrule or shielding gas.     

The short cycle drawn arc welding process is preferred for welding flange studs from 4 to 10mm in diameter to thin sheets also with electrodeposited coatings below 30µ.     

The main area of application is the automotive industry with up to 600 studs per shell where the stud is fed to the chuck of the welding gun or automatic stud welding head and the gun is supported on the workpiece with a positioning tube, stop pin or gas attachment. After squeezing the trigger, the stud is lifted from the workpiece by means of a lifting magnet in the gun and a pilot arc ignited. The main current is subsequently applied via a transformer and thyristor bridge. The arc completely melts the end of the stud and workpiece surface, whereupon the lifting magnet is de-energised after lapse of a preset welding time and a spring pushes the stud into the molten weld pool. After a welding time of 5-100ms and 500-1500A, depending on the particular application and stud diameter, the stud and base metal are homogeneously joined and have a small fusion zone of about 1/8th of the stud diameter. In contrast to the capacitor discharge welding process, no loud bang is produced here and the process reliability is higher due to the welding time which is about 10 times longer.

    

      

 

Description of drawn arc welding process DA

 

For the drawn arc welding process with ceramic ferrule, a ceramic ferrule is used as weld pool protection, which serves for forming the weld pool, stabilising the arc, shielding the atmosphere and protecting the welder against UV radiation. The ceramic ferrule is used only for a single weld and is removed after solidification of the weld pool by knocking off the stud. This process is used mainly in structural steel and mechanical engineering applications where threaded studs or shear connectors from 6 to 25mm in diameter are welded to thick steel plates or supports larger than 1/4 of the stud diameter with welding times from 100-1.000ms and 800-2.600A.   

Both processes can be extended with shielding gas or shielding gas can be used to replace the ceramic ferrule, which considerably reduces pore formation in the weld pool and produces a more homogeneous fillet type welding bead. Used as a shielding gas is a mixture of 82% argon and 18% CO2, which is supplied to the welding gun via a suitable shielding gas attachment. With welding times from 5 to 300ms and currents from 500 to 1500A, studs from 3 to 12mm in diameter can be welded to clean sheets of minimum 1/8th of the stud diameter. The welding of aluminium materials by the drawn arc process is possible only to a limited extent.

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