Increase the chances of passing a CWB flat MIG (GMAW) test

“I’ve heard that the failure rate for the CWB flat MIG (GMAW) test is very high.  Are there any tips in increasing my chances to pass this test?”

Author: Jim Galloway is a Professor of Welding Engineering Technology at Conestoga College in Cambridge, Ontario.

The most common GMAW welder performance qualification test in Canada combines a flat position groove and a horizontal position fillet weld (1GF) on a single bevel groove joint with steel backing (as per CSA W47.1-19 and administered by the CWB).  This weld is normally completed using the spray transfer mode (GMAW-SP) with an argon rich shielding gas.  

Recognizing that this test has a narrow successful operating window, students at Conestoga broke this test down into its basic elements and tested welding parameters, fill sequence options, and stop/restart techniques using the aid of an arc welding robot to help to remove the ‘human elements’ from the process.  The goal was to use the robot at the relatively low energy welding parameters that an entry level welder would use, not to prove that a robot could pass the test (…it can).  From these experiments and the extensive destructive testing performed, several useful tips for welders were identified:

1. Set the arc voltage at the minimum for spray transfer – once the wire feed rate has been set, adjust the voltage to achieve spray transfer, but no more than required (with the occasional ‘crackling sound’).

2. Avoid weaving (use stringer beads) – consistent weaving can be challenging for entry level welders and can lead to lack of fusion discontinuities.

3. Use the 3-pass root option – a fillet weld size less than 8 mm (5/16 in.) is required for the first pass (see Figure 1).

4. Use a 6 mm (1/4 in.) fillet size – this will be required, combined with a similar sized 2nd root pass, to leave enough space (≥ 2 mm or 5/64 in.) for the third root pass to fuse to the backing bar.

5. Maintain a short CTTW distance – 16 mm is specified to keep the current high and aid root penetration.

6. Use a 40 – 45o work angle on the fillet weld pass – this aids penetration into the corner.

7. Use a 15o forehand (push) travel angle – this helps to flatten the bead profile and aids fusion in the subsequent passes.

8. Aim the electrode wire slightly above the corners - this aids penetration into intersections.

9. Do not fill the stop craters – while it is common practice to pause and perform a crater fill at the termination of a production weld, if the weld is to be restarted the crater must be shallow and unfilled.

10. Use a back-loop technique on the restarts (see Figure 2) – this aids weld fusion at these critical areas.

For more details on this study, including parameter settings, please refer to the paper by Shawn Runtas, Ross Gethke, and J. Galloway, from Conestoga College, published in the Fall 2022 edition of the CWBA WELD Journal.