Page 36 - Australasian Paint & Panel Magazine Mar-Apr 2021

Welding
36
PAINT&PANEL MARCH / APRIL 2021
DESTRUCTIVE TESTING FAILS
BELOW ARE EXAMPLES OF PLUG WELDS WHICH HAVE NOT BEEN CLAMPED TIGHT ENOUGH ALONG WITH INCORRECT SETTINGS TO SUCCESSFULLY FUSE THE PLUG WELD HOLE IN FLANGES TOGETHER TO THE BASE PANEL. THESE WELDS CAN LOOK LIKE WELDS WE CAN BE PROUD OF. UNTIL ANOTHER COLLISION OR A RATHER BUMPY ROAD CAUSES THE WELD TO FAIL.
POROSITY WITHIN THE WELD ZONE
This plug weld has been completed through an 8mm hole in 1.5mm thick steel with an MPA rating of 340. Take into account our 1.5mm HSS is comparative to 3mm mild steel. When the destructive testing is carried out, the technician
has failed to penetrate the base material and only filled the hole - the edge of the hole is still visible due to insufficient heat to melt the edges of the hole. The molten filler material has not fused to the panels being joined. The air holes or porosity seen is from filling the plug hole too quickly and not allowing the molten filler wire to solidify correctly. But hey, it looked good from the visual inspection so that is OK, right? Wrong. A series of these plug welds on a structural part will compromise the structure.
METAL THROWOUT FROM INCORRECT CLAMPING
In this example the molten filler wire has been thrown out between the panels being joined. This will not hold anything gether. This is a result of not clamping the panel flanges tight enough when plug welding in addition to applying sufficient heat from incorrect low amps or an arc length setting too cold – which has not penetrated the base panel.
Again, visually it looks fine but has no structural strength. The issue that technicians face when plug welding panels together is the steel strength. It may look like the panels are fused together but without clamping either side of the
plug weld the thinner steel may lift off the base from the heat. Alternatively two thicker steels may have a very thin gap between the flanges. Visually, again, fine but it will have no strength. If the welding machine has been set up with amps too low to burn into the base plate, no weld puddle will be created and molten filler wire will need to find somewhere to flow and, unfortunately, this will be between the panels. This is very common on structural steels which can be up to 1500Mpa.
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NCORRECT ARC LENGTH CAUSING UNDERCUT
example highlights incorrect settings on the arc length (voltage). Here, the arc length is too hot and the molten filler wire has flowed into the weld zone excessively hot which has undercut the panel being attached. This will
ave the weld compromised around the edges of the weld.
When completing set up and destructive testing prior to welding the vehicle the plug weld should tear-out from the base
metal being welded. If the plug weld fails in the top panel then the plug weld has undercut leaving this weld with no strength. By lowering the arc length setting the welding arc will be moved slightly to create a better build up in plug weld head zone. Plug welds should not lay flat on the panel being welded, it should have a 1-2mm crown and the di-
ameter of the head should be 2mm larger than the hole being filled.
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WELD SETTINGS TOO LOW
This plug weld (below left) has been completed with setting too low to create a weld puddle in the thick base metal. The filler wire has solidified on the base plate and has not fused the plug hole edges. In this case the
technician has completed the plug welds with the same setting used to complete the thinner steel panel butt joint. The plug weld will look cold and appears to just be filling the hole, once ground down no one would be any the wiser this structure has no strength until a subsequent accident endangering the occupants.
Another issue which exists when completing plug welds is the old school method of starting the plug weld on the hole edge and
welding around the edge then finishing in the middle of the hole. That worked ok on old mild steel structures where the metal was softer and accepted the molten filler into the base metal. On the new high strength steel vehicle structures are now
made from this method will only fill the hole, look good but have zero penetration into the base metal.
The only way to start a plug weld on HSS is to penetrate the base metal first by starting in the centre of the hole with a
setting hot enough to melt the base metal then allow to fill outwards fusing the top panel to the base. Just like how a rivet joins two panels with a bigger head covering the hole.
The machine settings required to complete welds in HSS and UHSS are much higher than previous settings required for
mild steel structures. With the introduction of inverter synergic welding machines we have far greater control. The inverter synergic machine automatically combines the amps with the filler wire required which has taken away the guess work of adjusting the wireto match the amps on older transformer welding machines. The welding machines now have finer tuning
with an adjustable voltage / arc length setting. This setting moves the welding arc to a desired length from the work piece but in doing this itaffects a number of things in the weld zone.