Sometimes, when I try to give an answer to a question, I end up with more questions.
Is a welder the primary focus of the shot? Or is it a big welding machine with an operator?
Are the photos instructional, so you need to see the proper position of the hands, distance to the weld, etc?
I could ask a bit more, but I am assuming some of that.
If you have a bright spot compared to other zones of the image, you can do one of two things.
Lower the intensity of the bright part or pump the light on the darker zone.
As the first option is (almost) out of the question, the thing to do is to pump the light on the other zones.
Use a flash
You can have it on camera bounced somewhere, or for more pleasant and controlled lighting, remotely triggered. This option could mean having an assistant holding the flash.
An assistant can be also useful for not shooting it into the eyes of the worker, so you do not distract him.
That will automatically balance the lighting on the scene.
One obvious thing is that the welding is already lighting the worker's face, so use the flash to illuminate him as a ring light, or sidelight.
Probably to illuminate the background, to give context to the scene, to the place.
Now, we must think of the aesthetics of the photo. Do you want to see sparks as dramatic lines? you need to use a slow shutter speed. Try 1/15s - 1/2s. You probably should configure the flash as rear sync. But I think is not too important.
This again depends on the aesthetics. As in all portrait photography, you can have a wide aperture so you blur the background.
ND filter This is very important. As you are probably not using the aperture to control the light, you need to reduce it, especially with a combination of high aperture and slow speed. You need a high number ND filter (Probably 1000).
A very important step.
As you are involved in the project, you could set up a safe place to put a tripod, and determine the distance, therefore determining the focal length you need.
You need to previously determine the focus because once a filter is on, you can barely see.
This is the most important step. For safety, but also to get the results, even if you do not have a flash or ND filter.
You can instruct the welder to only deliver a super short spark, and stay in position for one instant more. That way you could use a bit longer exposition with no extra spark illuminating the scene.
You can also sync the "action" with the "camera", so you do not need to look at the spark yourself.
You can also have time to let the smoke disapear.
Remember that I said that it is almost out of the question to reduce the intensity of the light? If the photo is only for promotional purposes, you can obstruct the bright spot itself with a prop or choose a different angle.
Know your settings
It have being ages since I did that. But you should make some tests to determine the useful EV settings and combinations with the ND filter, and exposure times.
Start with very safe values for the sake of your sensor. Low aperture, and 1/125 s (thinking of implementing a normal flash). If you need faster shutter speeds, like 1/1000, think about an HSS flash. (But people have been taking photos of welding before that technology)
If you have a mirrorless camera, do not leave it with the live view on. I would prefer an optical viewfinder. But that is just me.
Remember. If you shoot in RAW, you can compensate a bit on post, lifting the dark areas. Just do not blow the white zones of the weld. You even can find details you did not notice before.