Precision Welding of Leadwires and Resistor Elements, Wireforms and Glass Tube Cutting for Lighting and Electronics

+1 724.253.3161
info@cit-hadley.com

facebook
blog
youtube

ROHS Compliant Precision Welding of Leadwires
and Resistor Elements, Wireforms and Glass Tube Cutting for Lighting and Electronics

Outline

 

Contact CIT

Component InterTechnologies
2426 Perry Highway
Hadley, PA USA 16130

Tel: 724-253-3161
Fax: 724-253-3853
Email:

 

Products & Services

 

CIT Innovations

 

Company Profile

how can we help?

Resistance and Percussive Arc Welding
- page 6

2.9 Control of Parameters

Close control of voltage, capacitance, impact velocity, and limiting resistance is important to produce a sound weld.  The voltage and capacitance determine the amount of energy stored in the system, and thus, the heat generation capability of the arc.  Impact velocity determines the amount of forging energy.  The limiting resistance controls the peak discharge current.

These four parameters interact to determine the arc duration and the timing with respect to arc discharge.  The rapid approach of the work pieces serves to trigger the arc discharge.

Usually, conditions are adjusted to give the shortest arc time that will permit consistent production of welds having the desired properties.  If the work pieces are forced together too early, the arc is extinguished before the work surface of both work pieces have melted.  If the impact is delayed too long after arc initiation, the melted interfaces may solidify, not allowing the expulsion of oxides and excess molten metal.

2.9.1 Welding Current

progress of percussive weld

fig. 5

As shown at the upper right of Figure 5, peak welding current is achieved almost immediately on arc initiation (point A).  The current then decays rapidly during the arc discharge (point B).  The current increases to a secondary peak on contact of the work pieces (point C) because of the sudden drop in electrical resistance, and then tapers off to zero in an additional 3 to 5 milliseconds.

As shown at the lower right of Figure 5, voltage across the weld decreases very rapidly (point A) to a fraction of its initial open-circuit value when the arc is initiated by the close approach of the moving work piece to the stationary work piece.  The voltage then decreases less rapidly (point B) as the arc discharge continues.  The arc is extinguished on contact of the work pieces (point C).  After a typical arc time of .25 and 1.15 milliseconds, the voltage decreases almost instantly to nearly zero.

2.10 Molybdenum to Zirconium Copper

The great difference in the melting temperature of molybdenum and zirconium copper dictates that the only practical method of joining these metals is to use percussive arc welding.

Any one of the three starting methods mentioned in the arc starting section can be used to start the flow of welding current.  However, the most common methods are NIB and RF START.  Because of the low melting point of zirconium copper relative to the melting point of the molybdenum, as this weld takes place, there is a rather large amount of zirconium copper expelled from the weld junction. 

This weld splatter can cause trouble.  The problems are excess copper on the surface of the moly slug and contamination of the work area and the machine tooling that could prevent consistent welding of a sequence of parts. 

One solution is to weld in an oil bath to solidify and carry away the hot copper particles as soon as they leave the weld area.  This oil bath will also help control the weld process by forming a reduced oxygen atmosphere at the time of the weld. 

When oil is used to contain the weld splatter, NIB START is the most acceptable means of starting the weld.  The RF START method is not acceptable for use with oil because the dielectric strength of the oil will affect the timing of the high-frequency discharge and prevent consistent welding of the work pieces.

2.11 CIT Application: Tantalum Welding for Capacitors

CIT's tantalum to tantalum welding experience dates from 1979 to the present.  The CIT model 7200/1 with custom power supply was developed expressly for welding the tantalum anode riser wire to the tantalum anode.

The energy necessary to make the weld is stored in electrolytic capacitors that are charged by an adjustable DC power supply via a current-limiting resistor to a preset voltage.  The charged capacitors are then switched to the primary of a welding transformer.  The secondary of the transformer is connected to the work pieces where the weld is completed.

The resistance welding process is well suited for this type of weld because tantalum, when heated, is highly reactive to oxygen and other gases and can actually be ignited by an electrical arc.  Resistance welding produces no arc which could cause the rapid oxidation of the powdered tantalum metal slug.  It also minimizes the effects of both oxygen and
nitrogen that could cause the formation of oxides and nitrides in the weld junction and the area surrounding the weld.

The tantalum anodes are handled very carefully to prevent physical damage and contamination from oils.  They are fed from a vibratory feeder bowl through a stainless steel feed track into hardened steel injectors and pin feed units to tungsten copper welding jaws.  The tantalum riser wire is fed from the wire spool into a rotary wire straightener.  Nylon is used in the straightener dies because of the highly abrasive nature of tantalum.  After straightening, the wire is measured and fed into a tungsten carbide knife and die, cut and positioned in the tungsten copper weld jaws.  Because tantalum is so abrasive, the dust that accumulates on welding machines must be removed by daily cleaning to prevent excessive wear of moving parts.

After positioning in the weld jaws, both pieces are brought together and pressure is applied by pre-loaded spring plungers.  The capacitors are switched to the primary of the welding transformer and high current is then passed through the two work pieces, making the weld.  The current and time of the weld varies with different sizes of wire and anode density, but a typical current would be approximately 173 amps peak for about .005
seconds (5ms).  The high current for this short time produces very intense heating of the relatively high resistance interface of the two work pieces. This causes the rapid forming of a molten pool of tantalum that becomes a localized weld knot.  The welder has switch- selectable capacitor banks with a fully-adjustable DC power supply that allows fine adjustment to the weld energy.

The majority of our work has been done with cylindrical anode pellets, but we also have experience with rectangular anodes.  The range of sizes of cylindrical parts is diameters from 0.8mm to 3.5mm and lengths from 1.95mm to 7.4mm.  Typical rectangular parts are 2.30mm x 4.00mm x .75mm.  Wire diameters are .3mm to .4mm with a length of 12.7mm. 
Smaller sizes of anode pellets can possibly be welded with modifications to tooling and adjustments to the welder power supply. The powder density of these anode pellets varies, but presents no real problem welding as long as they are durable enough to withstand the
vibratory bowl feed and subsequent weld jaw chucking without damage.  CIT would test pellet density prior to accepting any order for welding equipment. Please note:  All of the welding we have done up to this time was accomplished on anodes that have been FORMED and SINTERED ONCE prior to welding.  After welding the completed anode assembly with riser wire, it undergoes a SECOND SINTERING. If you have any questions or need additional information, please contact us.

2.12 Nickel Wire to Tantalum Anode Riser Wire

Nickel Wire to Tantalum Anode Wiser Wire

This weld, shown in Figure 7, is used commonly in the fabrication of tantalum capacitors for the connection of the nickel lead wire to the tantalum anode riser wire.  Resistance welding can be used since tantalum and nickel provide a relatively high resistance point at the work piece interface.  However, the short anode riser wire, and the tantalum pentoxide coating that is on the surface of the riser wire dictate that a lap weld rather than a butt weld be made.

Using percussive welding to weld the riser wire to nickel lead wire allows the automatic feeding of the loose anodes via a vibratory bowl feeder into an automatic welding machine.  The relatively high voltages and low currents typical of percussive welding permit the welding jaws gripping the short anode riser wire to be small sized and to be made of material that exhibits a long wear life, such as tungsten or tool steel.  The high voltage of the percussive weld easily overcomes the insulating properties of the tantalum pentoxide that coats the riser wire and minimizes the effects of slightly higher resistance at the jaw clamp to riser wire interface.

Next: Resistance and Percussive Arc Welding Glossary - page 7