Page 39 - MetalForming May 2015

These benefits directly solve the three biggest challenges aluminum fab- ricators face: obtaining a premium bead appearance, maintaining consis- tent quality across all operators—or even throughout a single operator’s shift, and minimizing the operator skill required.
Note that modified pulsed-spray transfer differs from standard pulsed GMAW. With a conventional pulsing process, the power source produces a single waveform with four variables:
1) Peak current, which promotes weldmetal transfer across the arc;
2) Duration of peak current (pulse width), which controls average amper- age, size of the molten droplets and deposition rate;
3) Background current (often set as a percentage of peak current), which is high enough to maintain the arc but not sufficient for weldmetal transfer— this gives the weld puddle a chance to cool. Background current affects pen- etration and puddle fluidity as well; and
4) Pulses per second, which affects weld-wire burnoff rate, wire-feed speed and arc length.
Modified pulsed-spray transfer starts with a conventional pulsed wave- form and then combines it with a sec- ond waveform—or even another process —to achieve specific goals. Possible combinations include:
1) Pulse/short arc (Fig. 1), which pro- vides maximum control over heat input for welding sections as thin as 0.24 in.*, as well as for root-pass welding.
*Minimum and maximum thick- ness possibilities depend on wire diam- eter and type.
Aluminum welders strive to achieve a stacked-dime bead appearance without resorting to gun manipulation.
Fig. 2
2) Pulse/pulse (Fig. 2), for welding material 1⁄16 in. and thicker* with great control over heat input, bead profile, bead appearance and travel speed.
3) Spray arc/pulse (Fig. 3), for high- er productivity on thicker sections while still preserving the ability to weld vertical-up without any weav- ing motion, which reduces operator fatigue.
As with spray-arc GMAW of alu- minum, operators learned that they could improve pulsed-GMAW bead aesthetics by whipping the gun to cre- ate a GTAW-like bead. Unfortunately, whipping creates potential quality con- cerns, as operators quickly can go from good to poor fusion. Whipping also increases the possibility for stress ris-
ers—locations that concentrate weld fatigue and where cracks potentially start.
Conversely, the modified pulsed- spray transfer process inherently cre- ates the desired stacked-dime bead appearance, as weldmetal deposition rates consistently vary when the system switches between transfer modes. The process eliminates the variables asso- ciated with gun manipulation, pro- moting uniform penetration throughout the length of the weld and minimizing issues related to stress risers. Further, the process enables fabricators to obtain premium bead appearance with fixed and flexible automation sys- tems—particularly handy for circum- ferential welds on pontoons or for long seam welds on trailers.
Operator- and Supervisor-Friendly
Welding power supplies with com- plicated controls can alienate, intimi- date and frustrate welders, supervisors and training managers, sometimes to the point where a company fails to take advantage of the technology it has pur- chased. Early generations of pulsed- GMAW machines often fell into this category. Outside of a few canned pro- grams for the most common filler met- als, users were required to combine programming and welding expertise to fine-tune waveform variables to a specific application.
The newest generation of pulsed- GMAW machines provides much more user-friendly controls, as well as a greater breadth of factory-set pulsing programs, commonly referred to as “synergic lines.” For pulsed GMAW, a
Fig. 3
Fig. 1
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