Why do you want to take a new piece of equipment and weld it? I would suggest that this action, if taken, would invalidate any guarantee/warranty. Rodger Ker

Sorry but why didn't you get a more robust set of steps at the beginning. If you feel the rivets are "mickey mouse" then I would question whether they are fit-for-purpose but then what do I know.

Assuming you have a welder certified for aluminium, know the grade of the aluminium, have the QA processes for the consumables, do appropriate NDT on the weld afterwards and have done some sort of checking (calculations or prototype testing) that the heat-affected-zone you're now introducing to the aluminium doesn't harm the performance of the steps, then I guess it would be OK. A fatigue check wouldn't go amiss, but if they really are used only occasionally will probably not be a problem. Ten times cheaper to buy suitable kit in the first place, though. Remember that if you weld aluminium of most grades the strength around the weld drops markedly. In the case of, say, H16 temper 3103 alloy the zone around the weld is only 0.3 x 0.8 = 0.24 times the original strength. That's to BS EN 1991-1-1 - see table 3.2a. There's a good chance they are riveted because it's a grade of material that doesn't take kindly to heat (or vice versa - the manufacturer adopted a grade that doesn't like heat, so has avoided welding). To be honest, I think you're more likely to make matters worse than improve them, but you'll probably get away with it if you really want to.

If the step ladders you have purchased conform to either Class 1 or EN 131 then they will have been designed, manufactured and tested to stringent guidelines. This being the case, I can't understand why you would want to alter them and if you do and they break when in use and someone gets hurt, you and not the manufacturer, will be in big trouble. If they don't conform to either Class 1 or EN 131 then may I suggest you return them to where you got them and get some that do.