Aluminations (4): Shedding Light on Aluminum Welding Issues
May 1, 2007
Is spray transfer too hot? Short circuit too cold? Then consider Pulsed MIG for aluminum. It can combat warpage, burn through, lack of puddle control, lack of fusion, spatter and poor appearance. This Aluminations explains the pulsed process and introduces a simple, affordable machine with built-in pulsing controls.
Is spray transfer too hot? Short circuit too cold? Then consider pulsed MIG for aluminum. It can combat warpage, burn through, lack of puddle control, lack of fusion, spatter and poor appearance. This aluminations explains the pulsed process and introduces a simple, affordable machine with built-in pulsing controls.
In pulsed MIG welding, technically a modified spray transfer process, the machine switches between a
high peak current and a low background current (see Fig. 1). The peak current pinches off a spray transfer droplet and propels it toward the weldment. The background current maintains the arc, but is too low for metal transfer to occur.
Pulsing can lower heat input to levels associated with short circuit transfer, yet it provides benefits associated with spray transfer. For example, pulsed welding a 1/8 in. thick section of aluminum with a 3/64 in. diameter wire requires an average of 140 amps. In this program, a 90 amp background current eliminates worries about burn through or warping, while a 350 amp pulse of peak current provides good penetration and wet out. As with spray transfer, pulsing produces little to no spatter or porosity; like short circuit transfer, it works well in thinner gauge and out-of-position applications. (See below.)
As a financial incentive, consider that non-pulsed welding 1/8 in. aluminum typically calls for .035 in. wire, which lists for $5.16/lb. Now compare this to the 3/64 in. wire in the pulsed example above; it lists for $4.85/lb. Standardizing on larger diameter wire, and using that single wire for a wide variety of thicknesses, is a common benefit of the pulsed process.
Equipment for Pulsing
Unfortunately, many aluminum fabricators hesitate to adopt pulsed MIG. Concerns about equipment complexity, operator comfort and purchase price typically hold them back. Miller's new Invision™ 354 MP and Invision 456 MP, however, overcome such concerns.
These power sources feature built-in programs for 4000 and 5000 series aluminum wire. To begin welding, select the program that matches your wire size and type. After that, they set up like a conventional MIG system: pick a wire feed speed and turn the trim control to find the correct arc length. You never have to modify pulsing parameters to maintain good welding conditions for different wire speeds or trim settings. Using this system, operators feel very comfortable switching to pulsed MIG because it's "familiar".
The Invision 354 and 456 MP provide optimized performance with Miller's standard wire feed systems for MIG welding aluminum, such as the XR™ feeder and XR-Edge™with gooseneck gun. You do not need a programmable feeder or pulsing pendant.
For more information and a free technical article on Pulsed MIG, call 1-800-4-A-MILLER (1-800-426-4553) or visit our website at www.MillerWelds.com
Questions & Answers
Q: Can pulsed MIG boost productivity?
A. For certain applications, absolutely. Compared to short circuit or spray transfer, pulsed MIG often permits using a larger diameter wire and/or faster wire feed speeds without adding excess heat. This increases travel speed and/or deposition rates. For example, one fabricator welding lap joints on thin gauge aluminum increased travel speed from 105 to 144 in./min. after switching to pulsed MIG.
Q: Does pulsed MIG create cleaner looking welds than spray transfer?
A. Yes, in aluminum with high magnesium content, such as the 5000 series. The higher current and arc temperatures of spray transfer vaporize the magnesium, causing a gray film to form on the weld surface, especially at the toes. Pulsed MIG, with its lower average current, reduces the amount of vaporization. As a secondary benefit, it also reduces fume generation.
Q: Why switch to pulsed MIG for aluminum?
A. Many shops switch for greater control and to obtain a highly uniform bead. With pulsed MIG, the weld puddle cools between pulses and freezes faster. This provides operators with better directional control over the weld bead. Also, the puddle is less likely to sag or look excessively convex when welding out-of-position. In fact, bead appearance can approach that of TIG (see photo). Shops TIG welding aluminum thicker than 1/8 in. might consider pulsed MIG as a way to increase output while satisfying appearance and quality control demands. Also, pulse welding avoids lack of fusion which can occur with the colder short circuiting transfer.
View Online Arc Shaping Videos
Frequency, or Hz, is the number of times the AC TIG arc switches between electrode negative and electrode positive in one second. Miller's Dynasty™ 300 DX inverter-based TIG power source permits adjusting output frequency from 20 to 250 Hz. Conventional TIG machines have a frequency fixed to that of the 60 Hz primary power.
Increasing the frequency narrows the shape of the arc cone and increases the arc force. This stabilizes the arc, reduces arc wandering and provides excellent directional control over the arc. On lap and T-joints, using a higher frequency lets you establish the weld puddle exactly at the root (view 200 Hz arc shaping video). This can ensure good penetration, control bead width and minimize the etched zone. With a 60 Hz output on fillet welds, the wider arc dances from plate to plate. The puddle starts at the toes of the weld and flows toward the center (view 60 Hz video). On some joints, you're almost compelled to over-weld to ensure penetration at the root.
Other Aluminations Articles
Issue 1 - See if an inverter-based AC/DC TIG machine that incorporates advanced Squarewave technology can improve your operation.
Issue 2 - How to TIG weld thin aluminum, adjust balance control and determine amperage requirements.
Issue 3 - New push-pull feeder technology improves MIG welding.
Issue 4 - Why you should consider pulsed MIG for your aluminum welding jobs.
Issue 5 - When you control the shape of an AC TIG weld bead, you control profitability.