Choosing the right aluminum filler alloy
Keganhirl > 07-10-2019, 08:30 AM
Many fabricators faced with an aluminum project often wonder what the best filler metal is for the job. The answer depends on the specific application and the filler characteristics that are most important for that application. By considering the following application factors, fabricators can determine which filler alloy to use.
Factor No. 1: Cracking Susceptibility
Aluminum cracking, commonly referred to as hot cracking, occurs as the aluminum weld solidifies. The solidifying weld’s chemistry is the single most important factor in determining cracking sensitivity. This chemistry is determined by the base metal, the filler metal, and the weld dilution. Therefore, the specified filler alloy plays a crucial role in determining cracking sensitivity.
Factor No. 2: Final Weld Strength
Both the substrate and the type of weld can affect choice of filler material and ultimately weld strength. When using aluminum, fabricators rarely obtain a weld that is as strong as the base metal. In groove welds, the tensile strength of the weld is critical as the weld is usually stressed in tension. In contrast, in lap or fillet welds, the shear strength of the weld is critical since the weld is usually stressed in shear. Different filler metals have differing tensile and shear strengths.
Factor No. 3: Weld Appearance and Aesthetics
Different alloys can provide a variety of finished-weld appearances. Some filler alloys have a greater tendency to produce a black, sooty residue on the weld surface, while others tend to be more fluid and easier to wet-in.
The 4XXX series, known as AlSi filler metals, have excellent fluidity, which gives an exceptional weld appearance and very good resistance to weld cracking. In addition, these filler metals are less prone than the 5XXX series alloys to develop porosity in the weld.
In determining the proper filler metal in respect to weld appearance, welders should also consider whether the final part will be anodized
after welding. Anodizing is an electrochemical process that produces a relatively thick coating of aluminum oxide on the surface of aluminum. This coating is very hard but porous enough to absorb organic dyes. While the final anodized coating results in a clear satin color, some alloys may impart a slight color cast to the anodizing. If desired, an organic dye can be infused into the coating later, providing a decorative wear surface on aluminum components. 5XXX series filler alloys should be used in this case to provide the best color match. If a 4XXX series filler is used, the weld will become an unattractive black color after anodizing.
Factor No. 4: Maritime Service or Corrosive Environments
In most cases, the base material the welder uses plays the biggest role in resisting corrosion. The filler metal is chosen primarily to be a match.
Factor No. 5: Elevated-temperature Service
It is a well-known phenomenon that 5XXX alloys containing more than 3 percent magnesium (Mg) can be sensitized to stress-corrosion cracking with exposure to temperatures of more than 150 degrees F for extended periods of time—as in an automotive engine cradle, for example. Other base metal alloys with less than 3 percent Mg, such as 5454, were developed for such service. Matching filler alloys, such as 5554, are available to weld these alloys.
Factor No. 6: Deformation Under Stress
Ductility is not an important filler metal characteristic for most welding applications. However, it becomes important if the weldment is to be deformed after welding by rolling or bending.