Page 39 - MetalForming April 2020

likelihood of hot cracking, so take care to select a weld wire with relatively low crack sensitivity when welding crack- sensitive grades.
Lastly, restrictive joint designs can contribute to hot cracking. For that reason, design weld joints wide enough to reduce the stresses that the alu- minum weld metal undergoes during the welding process.
Surface Cleaning and Preparation
The surface of aluminum sheet and plate contains an oxide layer that requires removal prior to welding. This thin, hard layer provides the material with its corrosion resistance, but it also melts at a higher temperature than the aluminum below (3700 F versus 1221 F). Because of that temperature difference, the oxide layer can cause penetration issues during welding.
To begin preparing for aluminum welding, thoroughly wipe the surface of the workpiece material with a clean cloth and a solvent, such as acetone, that cleans well and that effectively can degrease the material. When using flammable liquids such as acetone, take care to work in an area away from the weld cell to avoid the risk of fire. Never use any liquid containing chlo- rine to clean aluminum, as chlorine becomes toxic when exposed to the heat of the welding arc.
Using a stainless-steel brush or grinding wheel dedicated to the pur- pose of cleaning aluminum, brush or grind the surface to remove the oxide layer. Because the oxide layer will start forming as soon as the exposed alu- minum comes in contact with air, per- form this step as close as possible prior to welding.
Although it isn’t necessary, after remov- ing the oxide layer the operator can wipe down the surface with a clean cloth and acetone to remove any shavings.
Equipment, Consumables and Technique
When gas-metal-arc welding (GMAW ) of aluminum, the soft alu- minum welding wire is subject to defor-
mation and wire shaving during feed- ing. To prevent this, take care to use the proper equipment and consum- ables. In addition to a power source specifically designed for welding alu- minum, U-grooved drive rolls and non- metallic inlet guides for the wire feeder help to ensure good weld quality. These components provide smooth wire feed- ing and prevent wire shaving. Welding operators should set the lightest drive- roll tension that effectively will feed the wire.
Use a push-pull welding gun and a nonmetallic Teflon liner, along with con- tact tips designed specifically for weld- ing aluminum. If these contact tips are not available, use a standard copper contact tip with a bore diameter approx- imately 10 percent larger than the wire diameter, to reduce wire shaving while still providing the appropriate electrical conductivity to create a stable arc.
GMAW settings for aluminum will vary with the diameter of welding wire being used, as well as the thickness of the aluminum base material. Consult the filler-metal specification sheet for parameter settings—amperage, voltage and wire-feed speed.
To gain consistent arc characteris- tics, shield aluminum welds with argon or an argon-helium gas mix. Note that the addition of helium pro- vides greater penetration and a wider bead profile when used on thicker sections of aluminum. However, heli- um, more expensive than argon, also can create a slightly less stable arc. The needs of the application will determine whether helium’s additional cost is worthwhile compared to chang- ing the weld-joint design or making other adjustments.
Skilled welders employ specific tech- niques on aluminum. When welding with solid aluminum wire, they use a push angle that creates a wide weld bead and good tie-in at the toes of the weld. Compared to steel, aluminum’s greater thermal conductivity may require not only higher current or voltage, but also faster travel speeds. Because of this, the term “hot and fast” often is used to refer to aluminum welding.
Filler-Metal Considerations
Filler-metal selection for aluminum depends on the specific workpiece alloy and the desired properties of the fin- ished weld. Consider these perform- ance factors when selecting the filler metal:
• Crack sensitivity
• Weld strength
• Weld ductility
• Corrosion resistance
• Elevated service temperatures • Color match after anodizing
• Post-weld heat treatment
• Toughness.
Filler-metal manufacturers offer
selection charts that can help match specific aluminum alloys to the proper filler metal, based on these factors. That said, welding wires designated as Type 4043 and 5356 are among the most common options. Type 4943 filler metals also offer some unique charac- teristics that make them a desirable substitute for Type 4043 products.
Type 4043 aluminum filler metals will develop a fluid weld pool, thanks to the addition of 5-percent silicon. This improves wetting action, or flow into the joint, helping to minimize cracking and reduce the need for extra post-weld cleaning. Type 5356 filler metals contain magnesium, which increases weld toughness and strength; however, it also creates more smut (black soot) at the toes of the weld that will require post-weld cleaning.
Using Type 4943 weld wire rather than Type 4043 wire will result in higher weld strength (due to additional mag- nesium), with comparable weldability and crack resistance.
Parting Thoughts
Welding operations, when working with aluminum, can benefit from establishing some basic best practices. Start with ensuring cleanliness and proper material preparation. Then, always follow manufacturers’ recom- mendations for power-source and weld-gun setup, and consumable installation, to optimize weld quality.
And, be sure to follow recommend- ed safety instructions. MF
www.metalformingmagazine.com
MetalForming/April 2020 37
Fabrication: Welding Well