Improving Quality of MIG Weld Starts

The Gas Saver System (GSS^TM) limits surge flow rate at the weld start while quickly providing a controlled amount of extra shielding gas to flood the weld start area. The following explains the weld quality benefits:

STARTING THE ARC

Starting a MIG arc requires the proper machine dynamic characteristics and settings.  However even with the optimum machine and settings, shielding gas must exclude air from the weld start  zone.  As a test, just shut off the shielding gas supply and try to make a weld start.  You'll see excessive spatter and a poor quality arc!  When Oxygen and particularly Nitrogen are introduced into the arc these poor arc characteristics exist.  Even a small amount of nitrogen will lead to weld porosity, even if is only below the surface.  Unfortunately users controlling gas flow directly at the wire feeder lack extra gas to quickly purge the torch nozzle and weld start area of moisture laden air.  Also many other MIG systems have a high initial gas surge at the weld start that pulls air into the gas stream.  Fortunately there is a simple, inexpensive way to fix these problem while saving 25 to over 50% shielding gas!

Nitrogen Porosity: With solid MIG wires, some Oxygen can be tolerated.  The silicon and manganese contained in the wire can combine with the Oxygen to form a silicon oxide or a manganese oxide and avoid the carbon in the weld puddle forming a CO bubble.  These oxides float to the weld surface.  However published information indicates Nitrogen at levels less than 2% in the shielding gas can cause porosity in single pass welds (Reference ESAB, former L-TEC/ Linde MIG Welding Handbook; ".. 2% Nitrogen produced porosity in single pass welds made in mild steel.." )  Unlike Oxygen, chemically combining Nitrogen into harmless compounds is not viable with solid wire. Elements like titanium can be employed but only at low levels since significant amounts cause embitterment.  Even self shielded flux cored wires have a difficult task dealing with Nitrogen, resulting in a compromise in weld performance and weld toughness.  That is why a shielding gas is used when welding with sold wire!

Remember Air contains 78% Nitrogen;  5% contamination in the shielding gas stream due to turbulence gives 3.9% Nitrogen in the gas stream.  That is sufficient to produce at least internal porosity.

Although not discussed or considered by Ludwig, since he used a chamber for his research, some shielding gas is quickly  needed at the weld start to purge air from the weld start zone, torch gas cup and that which enters the torch body and gas hose when welding is stopped.  See information on this subject covered by Stauffer  in his 1982 patent.  Preflow can be used to provide this extra gas shield but that is often an irritant to welders who will set it to zero given the chance!  In addition the initial gas surge found with most shielding gas delivery systems pulls air into the shielding gas stream for a 1 second or more.  That would require a long preflow to assure the flow was below the level creating turbulence (in fact we have found some systems where excessively high gas surge exist for over 4 seconds!)  For Robotic Welding, preflow can add to wasteful cycle time.  The best way to provide the extra gas at the weld start is to maintain the minimum pressure in the gas delivery hose needed to assume automatic flow compensation (See What Automatic Flow Compensation is All About).  That pressure is 25 psi but you must  also limit the shielding gas flow rate so that it is not significantly in the turbulent flow range.  Sounds complicated.  It's not-- we do that with our patented  GSS with no moving parts or knobs to adjust!  CHECK IT OUT.  Also see a production example where a welding engineer discarded 32 low pressure flow control systems that mounted at the wire feeders.  He reported problems encountered were partly due to insufficient extra gas delivered at the weld start (which happens when flow control is attempted at the feeder.)  He also found wide variations in flow since automatic flow compensation was not present with these low pressure devices.  CHECK OUT HIS INFORMATION.

PRODUCTION TEST RESULTS SHOW INITIAL EXTRA GAS NEEDED:

If sufficient shielding gas is not provided at the start, welders may try to compensate by using higher overall gas flow.  A bar joist manufacturer was using flow control orifices mounted at the wire feeders.  Argon/CO₂ shielding gas is supplied in a pipeline through about 15 feet of gas delivery hose.  The flow control orifice established the flow at 45 CFH.  However the welders wanted higher flow rates with some even drilling out the orifice!  The welding engineer wanted to avoid wasting shielding gas. With this flow setting arrangement where control is mounted at the feeder next to the gas solenoid  there is insufficient extra gas provided at the weld start.  This lack of extra gas prevents  to purging the weld start  area of moisture laden air.  

A test was made to check weld performance and potential shielding gas savings using two cylinders of  gas on two adjacent welders instead of their pipeline gas supply.  One was set with their standard flow control orifice system and a regulator providing a pressure that matched their pipeline, 50 psi.  The other with a regulator/flowmeter (also of a 50 psi design) using a 15  foot GSS without their flow control orifice.  Both steady state flows were set at 45 CFH.  Since welders stand side by side, it was easy to observe the weld start quality!   Instantly the welder using the GSS noticed  improved  starting.  After about an hour with observably better results the  welding engineer suggested we lower the shielding gas flow on the  welder with the GSS to 35 CFH!  The same improved weld start quality was observed and the welder was "happy."  In fact even though we lowered the steady-state flow to 35 CFH there was still about the same controlled amount of extra gas available at the start (that stored in the GSS hose when welding stopped.) The higher start gas flow rate established by the surge flow orifice in the GSS maintained the higher flow at the start.   This higher start flow rate quickly flooded and purged the weld start area of moisture laden air.  It was this air that was casing excess spatter and lack of shielding on all their other welders! After about 4 hours of observation it was obvious the spatter at the weld start was less with the GSS.  We also measured a reduced use of shielding gas of 25%.   After several months of testing to check this one system during windy days etc, this shop now has GSS's installed on all 50 welders!   Bottom Line - - "Some extra gas flow at the start is very beneficial."  In addition, after about a year of use their gas supplier called to see if their business had turned down since they were using about 30% less gas- it had not!

 Also note that any flow control device installed right at the feeder, be it an orifice flow control or a flowmeter will have the same lack of sufficient initial shielding gas to purge the weld start area.

SHIELDING QUALITY

Eliminating Oxygen and Nitrogen at the weld start to reduce the possibility to weld porosity and excess weld spatter requires good shielding.  In addition to Oxygen and Nitrogen the surrounding air contains Moisture or Water Vapor.  The Water Vapor can also be drawn into the arc where it will disassociate into Oxygen and Hydrogen.  Hydrogen can cause more than porosity problems.  Some amount can dissolve in the molten steel and will only come out when the weld cools.  Since they are very small, Hydrogen atoms it can migrate through the steel accumulating near defects, dislocations, etc. forming Hydrogen gas.  This can cause cracking.  These cracks may be in the weld itself or in the adjacent parent metal called the heat affected zone. 

At the weld start two potential problems cause excess air to enter the shielding gas zone:

Excess Flow Rate:  At the weld start, excess shielding gas is most often stored in the shielding gas delivery hose.  It is stored at the system delivery pressure which can be as high as 50 to 80 psi.  When the gas solenoid in the feeder (or torch in many Spool guns and some push-pull systems) is opened a high surge of shielding gas exits the torch.  The flow rates often exceed 5 times or more the preset flow rate.  Published data indicates this is well above the flow level that creates turbulence in the shielding gas stream*.  This turbulence pulls air into the gas stream creating poor shielding.  The turbulence takes a short time  to stabilize even after the flow returns to the preset level. 

As shown in the production example mentioned above, this excess surge flow caused internal weld start porosity in a shop doing pipe weld repair of sub arc welds.  For many of their customers ultrasonic testing of all repairs is required,  The welder quickly saw the reduced surge flow as a benefit in reducing weld start porosity.  The pipe sent in for repair and cross seam welds is not released from his work station until all weld starts are free from porosity indications.

Extra Gas is Needed at MIG Weld Starts

Certain devices such as restriction orifices and flowmeters mounted at the feeder to control flow cause inferior starts and welders often complain about their use.   

Stauffer in a 1982 patent discusses this issue.  He clearly understood the problem and designed around it using a "surge storage tank"; he states in the patent teaching, "... air leaks back into the torch and lines when welding is stopped.  The air must be quickly purged and replaced with inert gas to produce high quality welds. Also, it is critical to displace the air at the weld zone of the work piece upon initiating the weld. " 

With flow set at the feeder we find welders will increase gas flow to achieve more gas at the weld start!  But increasing the steady state flow does not sufficiently purge the weld start area!

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