By Metal Form Magazine
Lightweight, easy to form and resistant to rust, aluminum sheet metal offers manufacturers the option of skipping the painting step when producing parts. These benefits make the material popular in a range of applications, from toolboxes and jon boats to signs and awnings.
But joining aluminum sheet—for the purposes of this article, sheet measures 0.125 in. thick or less—via gas metal arc welding (GMAW) presents some challenges, as operators must take care to avoid warpage, distortion and burnthrough. The following five tips will help operators better control heat input and improve their techniques, thus optimizing results and reducing rework.
1Use pulsed GMAW welding. The right welding process helps improve results. For example, pulsed GMAW lowers the heat input and makes it easier to weld thin materials and minimize burnthrough. This welding process alternates between a high peak current and a lower background current, which lowers overall amperage. The pulse of peak current propels the molten droplet across the arc and provides the energy to produce good fusion associated with spray transfer, while the low background current allows the weld puddle to cool.
Pulsed GMAW also allows operators to run larger-diameter wire at lower currents than needed to run a non-pulsed process, such as constant-voltage (CV) GMAW. In addition, pulsed GMAW provides the ability to better control the bead profile. A wider arc cone helps tie-in both sides of a joint or an outside corner, where a narrow arc cone helps focus the arc and provides good fusion at the root of a joint. Adjusting the arc length (voltage) and wire-feed speed for optimum performance helps eliminate excess heat input, overwelding (making welds that are larger than needed or using too much filler material) and post-weld grinding.
2Choose the best-possible filler-metal alloy. Selecting the best filler metal for welding aluminum sheet presents many factors to consider, such as the base-metal alloy, service temperature, environment, formability and more.
The two most-common aluminum filler metals, types 4043 and 5356, produce very different results. A 4043 filler metal features silicon as its main alloying element, which lowers its melting temperature and improves fluidity, helping it wet out better than a 5356 filler metal. Also, silicon expands as it solidifies, meaning that filler metals with high silicon content will experience less contraction as the weld cools. A 4047 filler metal has a slightly lower melting point and about twice as much silicon as compared to 4043, so the welds experience even less contraction/ warpage.
Common base metals compatible with 4043 and 4047 filler metals include types 3003, 3004, 6061, 6063, 5005, 5050 and 5052. When using pulsed-GMAW on material from 1⁄8 in. thick to 16-gauge, a 3⁄64-in.-thick filler metal can be used, provided the weld area has a tight fitup. For even thinner materials, try a 0.035-in. filler metal. For CV GMAW on sheet metal, we recommend wire diameters of 0.035 to 0.030 in.Note that readily available 100-percent-argon shielding gas is common for GMAW aluminum-sheet applications.
3 Take the time to ensure proper fitup. Proper, tight part fitup is essential when welding thin material such as sheet metal. Gaps between the pieces to be welded can cause warpage. Even a small gap can turn into a larger gap should a piece warp, forcing the operator to slow down to fill the larger gap, and adding unwanted heat into the weld. This can lead to bottlenecks.
In addition to ensuring no gaps in part fitup, slight overlapping of edges is recommended. A lap corner part fitup rather than an outside corner fitup offers a lesser chance of burnthrough. Another option to combat burnthrough issues: Use a copper backing bar during GMAW on sheet. This involves placing or clamping a copper bar on the back side of the welded area, which helps dissipate heat more rapidly than when relying on atmospheric cooling alone.
4 Keep travel speeds high. A rapid travel speed and a small bead during GMAW also helps minimize burnthrough, warpage and distortion. A slower travel speed means dwelling longer in an area, causing heat to build.
Operators always should employ stringer beads (not weave) and the fastest travel speed possible to maintain a good bead profile. Backstepping techniques should be avoided, as they slow travel speeds, increasing heat input and distortion. Pulsed GMAW is recommended, as it provides the ability for a rapid travel speed, especially compared to other welding processes available for aluminum sheet.5Use weld-bead sequences. Part geometry and fixturing greatly affect bead sequences. Using weld-bead sequences, such as intermittent welds, rather than a constant weld bead, can help offset distortion and warping.
To minimize burnthrough, when possible weld beads in any low heat-sinking areas first. Spreading the weld sequence around the part also helps reduce localized heating. Another option: Make short welds in one direction, starting the next weld behind the beginning of the previous bead. This helps reduce heat buildup while still allowing operators to weld in one direction.