spot weld wire to wire?

Hi all

I am really struggling to find any information on spot welding wire to wire

I am trying to spot weld 0.4mm stainless steel spring wire onto itself before bending it into a spring type shape and it keeps melting the wire

I am using the following spot welder on the lowest setting: https://www.amazon.co.uk/gp/product/B09ZXRXRPQ/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I dont have any problems with 0.5mm stainless steel wire, but as soon as I use 0.4mm i have a problem

Does anyone know, what can I do to rectify this problem?

Any advice much appreciated

Thanks

legepe
Sorry if I should have posted in a different section

It may seem odd, but, your smaller wire will require MORE current to create the heat creates the weld. You're relying on the resistance of the wire to create heat
Heating power = I * I * R

The resistance of the smaller wire is about 64% of the resistance of the bigger wire (by cross-section) so you'll need about 25% more current to reach the same power. But, you'll need to apply the pulse for less time so you don't melt the wire into a puddle. You'll have to test it a bit on pulse length.

ZipZapOuch said:

It may seem odd, but, your smaller wire will require MORE current to create the heat creates the weld. You're relying on the resistance of the wire to create heat
Heating power = I * I * R

The resistance of the smaller wire is about 64% of the resistance of the bigger wire (by cross-section) so you'll need about 25% more current to reach the same power. But, you'll need to apply the pulse for less time so you don't melt the wire into a puddle. You'll have to test it a bit on pulse length.

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I tested it again on the next higher setting and it just blows the wire appart. could I be doing something wrong? Not sure what you mean by pulse.. I dont think my spot welder has it.. or it seems that its automatic?

legepe said:

I tested it again on the next higher setting and it just blows the wire appart. could I be doing something wrong? Not sure what you mean by pulse.. I dont think my spot welder has it.. or it seems that its automatic?

Click to expand...

Some spot welders can be set for the time power is supplied. It seems you have very limited control on current or time. It will be tough to get that one to do the exact wire size you want.

If the same type of wire is ok at 0.5mm but not 0.4mm, I'm guessing that a flux or other additive will help any. Products like this (below) are available but again, im not sure they'll help.

ZipZapOuch said:

Some spot welders can be set for the time power is supplied. It seems you have very limited control on current or time. It will be tough to get that one to do the exact wire size you want.

Click to expand...

Can you recommend a spot welder that will do what I need it for?

The SPot welder can supply up to 50A and 0.4mm wire of unknown coating resistance will be at least 150 microohms/mm somewhat more than Cu which from the AWG tables will fuse open in approx. 10 seconds with only 10A. That fusing energy is defined by R*I^2t = 1000R for 0.4 mm wire minimum. The contact pressure prior to weld also adds a lot to resistance but fusing open depends on Joules /mm3 of mass or mm2 depending on the gap.

Iif your wire resistance is much lower than the battery ESR ( if the length is short) with the battery limited to 50A the smaller mass of wire will melt 1st.

I don't know how the regulator lowest setting is controlled. This means the smaller wire mass heats up faster than you can remove the spot.

The smaller wire will have higher resistance axially but lower radially if clamped tight.

In any case, I think you need some kind of heat spreader to divert some of the heat Joules to act like your 0.5mm wire mass. A thin plate or 3rd wire or ? But probe stiction might be an issue.

How are you arranging the electrodes & wire to do the weld?

rjenkinsgb said:

How are you arranging the electrodes & wire to do the weld?

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I am putting wires in a crocodile clip to get first spot on wire to secure it, then adjust with pliers so that wire edges are flush and closed together on the edges overlapping, then put on a flat surface to spot weld. I am putting the electrodes directly opposite each other, this seems to be the easiest way as I am using the electrodes to control/secure the wire object at the same time as getting the weld

For such small wire, I'd probably use the side of one probe and try to lay both wires across it, then use the other probe vertically once they are set up.

You may need a larger area to quickly cool the weld and stop the heat spreading too far.
A block of copper underneath may be best? Then experiment with current and time?

rjenkinsgb said:

For such small wire, I'd probably use the side of one probe and try to lay both wires across it, then use the other probe vertically once they are set up.

You may need a larger area to quickly cool the weld and stop the heat spreading too far.
A block of copper underneath may be best? Then experiment with current and time?

Click to expand...

I was using the side of probes, or just one side of probes when in crocodile clip, and may have been working slightly better, but I think the main problem I have is my spot welder welding successfully 0.4mm wire is that I cannot control the time, or time of pulse, as this seems to be automatic. and I only seem to have any success with it set on the very lowest current setting, but for example, on one spot weld is ok'sh then the next weld just explodes the wires, or visa versa. Do I need a better spot welder and if so, can anyone recomend one.. I am on a budget with this though as this is a one off use of it

The welder is digitally controlled by levels of energy by the pulse width they call "gears"
If you have it on the minimum level you must increase the contact area to control the fusing depth and volume of material that is displaced. That means something must absorb this energy in series or parallel with your tiny 0.4mm end weld. I watched a video welding 0.1 wire to flat surface and foil to LiPo batteries.

So you must use your imagination on how to spread this energy to make a strong microweld. Modifying the internal circuit may be possible, but I don't want you to damage it trying.



Looking at the specs here the tips of the gold plated probes must be the limiting factor for resistance and the length of the wires the limiting factor the minimum inductance, L while the discharge time is limited , it also will have a longer duration with a bigger L/R. with the cables for L ~ 0.8nH/mm. The contact minimum R = 4.2V / 639 A = 6.6 milliohms. and estimated Tau = 660nH / 6.6mohm = 100 us. My guess if the minimum pulse width is well below 1 ms, one can lower resistance with larger diameter SS wire or lower current with higher resistance and inductance wire added between the probe interface while L/R might be constant but reducing current by the number of milliohms of wire added.

Tony Stewart said:

So you must use your imagination on how to spread this energy to make a strong microweld. Modifying the internal circuit may be possible, but I don't want you to damage it trying.

Click to expand...

I understand what you are saying but because the weld is to join two wires (0.4mm) that are placed together, so the only place I can displace the current is within the wires themselves. its not like I am welding the wire to a larger object ie battery, where a large amount of the energy is displaced within the larger object. I am thinking hard what to do, but really dont know what I can do to solve this?

add 10 mohms of wire to one probe? or 1ft of AWG 20 wire to socket

Use a small stainless steel washer as a buffer between the two wires (essentially welding to the washer). Or, just use a section of washer.

A block underneath plus a larger diameter probe would distribute the current through a longer area of the wire?

Another possibility would be a lever actuator handle for the top probe, with an adjustable stop screw so it cannot crush the wires together below a certain point?

If they are overlapped for welding & the stop only allows the electrode down to a single-wire thickness, than should give a reasonable joint?

Another thing to try is to fold back the end of one or both wires in the area of the weld, effectively doubling the thermal mass.