EWM Hightec Welding designs and manufacturers arc welding equipment. It has developed a new variant of the MIG/MAG welding process known as EWM-forceArc®.
The process is similar to conventional spray transfer, but uses a shorter arc length made possible by developments in inverter technology and digital control. It's claimed that this process provides higher productivity with improved weld quality. EWM asked TWI to carry out a programme of work to validate the capability of the EWM-force Arc® process compared to conventional spray transfer.
A series of welding tests, proposed by EWM, was carried out to evaluate the EWM-forceArc®. The tests and the results obtained were detailed specifically and confidentially for the client in a subsequent report.
The approach involved making butt and fillet welds using both EWM-forceArc® and conventional spray transfer, to assess the capability of the process. Welds were then tested in accordance with EN ISO 15614-1:2004, Specification and qualification of welding procedures for metallic materials - Welding procedure test – Part 1; Arc and gas welding of steels and arc welding of nickel and nickel alloys.
Welding trials were carried out on grade S355J2 (EN10025-2:2004) steel plate of 10mm thickness for fillet welds and 20mm thickness for butt welds. Each plate was machined to 400 by 150mm and the surface ground and degreased adjacent to the weld line.
The welding consumable was a solid steel wire of type EWM SW 70S G4 (AWS E70S-6) of 1.2mm diameter and the shielding gas was argon-18%CO2 –2%O2.
All tests were carried out using an EWM AlphaQ 551 MIG/MAG power source, wire feeder and manual water-cooled welding torch supplied by EWM.
Welding parameters were measured using a calibrated Triton Electronics AMV weld monitor.
The following tests, as specified by EWM, were carried out to compare EWM-forceArc® with conventional MIG/MAG welding;
- Butt weld using spray transfer in the PA position with 20mm plate and a machined V-prep of 60°
- Butt weld using EWM-forceArc® in the PA position with 20mm plate and a machined V-prep of 40°
- Butt weld using EWM-forceArc® in the PA position with 20mm plate and a machined V-prep of 30°
- Fillet weld using spray transfer in the PB position with 10 mm plate
- Fillet weld using EWM-forceArc® in the PB position with 10 mm plate.
Butt welds were made using a backing bar and a root opening of 3.5mm. Welding conditions were chosen by the EWM welder who carried out the welding tests. TWI documented the welding procedure. Each weld was examined visually and a macro-section taken to assess bead shape and penetration in accordance with EN1321:1997. A hardness survey was also carried out in accordance with EN1043:1996. In addition the following mechanical tests were carried out on the butt welds:
- Two transverse tensile tests in accordance with EN 895:1995.
- Four transverse bend tests in accordance with EN 910:1996.
- Two sets of Charpy v-notch impact tests at -20oC in accordance with EN 875:1995.
The welding parameters showed that very similar welding conditions were used for all the welds. The wire feed speed was set at 10m/min for all the welds, apart from the butt weld with the narrow preparation of 30o, for which a wire feed speed of 11m/min was set for the first three passes. Consequently, a similar welding current of 269-318A was measured for most passes.
Differences in current are attributed to variations in contact tip to work piece distance (CTWD) rather than transfer mode. Arc voltage was also similar, being in the range 29.7 to 31.2v. This was slightly surprising, as the manufacturer's information about the process suggests that lower voltages are used with EWM-forceArc®.
However, in these trials, particularly with a narrow preparation angle, the CTWD was longer, which would lead to a reduced current and a higher voltage (due to the stick-out) for a given wire feed and voltage setting, as recorded for like-for-like passes at 10m/min wire feed speed for two of the welds.
The reduction in joint volume, by using a narrower preparation angle, is seen to provide significant benefits in the number of passes required to fill the joint, and consequently on the joint completion rate.
Two welds were completed in five passes rather than 11 required with the longer preparation angle. Total welding times were reduced in proportion to joint volume from 679s to 351s, affording approximately a doubling of productivity.
The macro-sections showed that all welds have an acceptable bead shape with full fusion into the parent material.
The fillet welds both had a mitred profile and similar leg lengths of between six and 7.5mm. The penetration for the EWM-forceArc® weld was greater than for the spray transfer giving an apparent throat thickness of 8.5mm compared to 6.2mm.
A summary of the test results was presented to the client along with detailed test certificates.
The transverse tensile tests for the butt welds were all similar, in the range 551to 559N/mm2, and met the specified minimum requirement of 470N/mm2. All three butt welds also gave acceptable transverse bend tests. There were no visible defects in the bends apart from two pores of 1mm diameter in one weld and one pore of half a millimetre diameter in another. There was a 0.25mm long tear visible on the tension face of one of the bends, but as with the pores, these defects were within the acceptable limits of EN ISO 15614-1:2004.
Charpy impact properties for the three butt welds were all above the minimum requirements and exceeded the average required value for both the weld metal and HAZ.
Similarly all hardness values for the weld metal and HAZ were within acceptable limits.
The trials demonstrated that EWM-forceArc® is capable of achieving butt welds in S355J2 plate, with a preparation angle as narrow as 30o, that meet the requirement of relevant standards. By using a narrow weld profile, with a lower joint volume, significant improvements in welding productivity were achieved.
In fillet welds, a mitred weld profile was obtained with no undercut. With EWM-forceArc®, the penetration was significantly greater than with conventional spray transfer.
Based on the welding trials carried out at TWI by an EWM welder, it was possible to draw the following conclusions.
- Butt welds made using the EWM-forceArc® welding process with narrow preparation angles of 30o and 40o met the requirements of EN ISO 15614-1:2004.
- Due to the reduced joint volume, welding times for butt welds were reduced by up to 50% compared to conventional spray transfer with a preparation angle of 60o.
- Fillet welds made with the EWM-forceArc® produced a mitred fillet with no undercut which also met the requirements of EN ISO 15614-1:2004
- The fillet weld made with EWM-forceArc® showed a greater penetration than for spray arc.
For further information read about Arc Welding at TWI or contact us.