Practical Welding Today - May/June 2009

TECHNICAL ARTICLE
Finding an ally alloy
6variables that affect variables that affect
aluminum filler alloy selection
By Tony Anderson

Editor’s Note: This article is adapted from Tony Anderson’s presentation for the Aluminum Association Conference, May 5-6, 2009, Toronto, by the American Welding Society.

Filler alloy selection for welding aluminum is an essential part of the development and qualification of a suitable weld procedure specification (WPS). Choosing the most appropriate filler alloy for welding aluminum is based on the operating conditions of the finished welded component and a number of variables.

When selecting a filler alloy for welding aluminum, it’s a good idea to compare the performance of each filler alloy against each

of the six variables: ease of welding, strength of the welded joint, ductility, corrosion resistance, sustained-temperature service, and postweld heat treatment.

No. 1: Ease of Welding

Ease of welding is often an important factor in filler alloy selection. Its significance is based on the filler/base alloy combination (chemistry) and its relative crack sensitivity.

To avoid chemistry in the weld that produces high crack sensitivity, be sure the mixture of the base alloy and filler alloy produces a low-crack-sensitivity material. Chemistry ranges that fall into the crack-sensitive category are best understood by examining the information in Figure 1,

Alloy Content vs. Crack Sensitivity

Al - Si

Relative Crack Sensitivity

Al - Cu

Al - Mg

Al - Mg Si 2

1 23 45 67

Composition of Weld - Percent Alloying Element

FIGURE 1

The crack sensitivity curve shows the relative crack sensitivity of the most common weld metal chemistries that develop during welding.

which shows the crack sensitivity curves for the most common weld metal chemistries developed during welding. Some examples are:

• The aluminum-silicon alloys (4xxx series) are seen predominantly as filler alloys and commonly contain 4. 5 percent to 13 percent silicon. Silicon in an aluminum filler/base alloy mixture of 0.5 percent to 2 percent produces a crack-sensitive weld metal composition. A weld with this chemistry has a high probability of producing hot cracking problems. Exercise care when welding a 1xxx series (pure aluminum) base alloy with a 4xxx series filler alloy to prevent a weld metal chemistry mixture within this crack-sensitive range.

• The aluminum-copper alloys (2xxx series) are heat-treatable, high-strength materials often used in specialized applications and exhibit a wide range of crack-sensitive characteristics. Some of these base alloys are not suitable for arc welding because of their sensitivity to hot cracking, but others are welded easily using the correct filler alloy and welding procedure.

• The aluminum-magnesium alloys (5xxx series) have the highest strengths of the non-heat-treatable aluminum alloys and, for this reason, are very important for structural applications. In Figure 1, a magnesium content of 0.5 to 3.0 percent in aluminum produces a crack-sensitive composition. As a rule, the aluminum-magnesium base alloys with less than 2. 5 percent magnesium content can be welded with either the 4xxx series or the 5xxx series filler alloys, depending on your weld performance requirements. The aluminum-magnesium base alloys with more than 2. 5 percent magnesium typically cannot be welded successfully with the 4xxx series filler alloys because of problems associated with decreased ductility and increased crack sensitivity.

• The aluminum-magnesium-silicon alloys (6xxx series) are heat-treatable alloys and contain about 1.0 percent Mg Si. These alloys cannot be arc welded