Page 31 - MetalForming March 2014

While cracking is one of the most critical issues that can occur with aluminum weld- ments, most aluminum alloys can be successfully welded without cracking-related problems. Using the most appropriate filler metal and completing the weld with the appropriate procedure will help ensure success.
ferent aluminum alloys.
Weld-joint design is another impor-
tant consideration that affects the composition of the weld, and therefore can help prevent hot cracking. A weld joint with no bevel can result in inad- equate base-metal dilution, while a weld joint with an appropriate bevel results in adequate dilution. This means a beveled edge can increase the amount of filler metal deposited in the weld, producing a composition less likely to crack. With some alu- minum base alloys, such as the 6xxx series, this is a very important factor that can strongly influence the likeli- hood of hot cracking.
Stress Cracking
While not as common as hot crack- ing, stress cracking also can affect alu- minum welds. A primary cause of stress cracking: excessive shrinkage rates dur- ing weld solidification and cooling. Selecting a filler metal containing sili- con, when appropriate, can reduce shrinkage stresses to help avoid stress cracks.
Most cracks in aluminum welds appear in the crater (defined as an undesirable depression in the weld bead), and often go unnoticed. While crater cracks start small, if not addressed they can propagate through- out the weld and cause major failures. As a best practice, welders should take care to completely fill the crater during welding, either by using an automated crater-fill function on the welding power supply or by using some other approved method of filling the crater.
Increasing weld-travel speed can help reduce the probability of stress cracking, because it narrows the heat- affected zone and minimizes base- metal melting. Preheating also may help, by reducing residual stress levels of the base material during and after welding. This in turn will lower the probability of stress cracking.
Avoid welding aluminum that is very cold, and avoid overheating during preheating operation—a 150 F preheat temperature suffices for all aluminum alloys. Overheating some alloys, such as
How to Avoid Hot Cracking
Cracking is one of the most critical issues that can occur in aluminum weldments. Even small cracks can pre- vent welds from meeting code require- ments and can eventually lead to weld failure. Most aluminum alloys, howev- er, can be successfully welded without experiencing cracking-related prob- lems. Using the most appropriate filler metal and completing the weld with the appropriate procedure are impor- tant to success.
While there are two types of crack- ing—hot cracking and stress crack- ing—hot cracking is the more com- mon of the two experienced when welding aluminum. Hot cracking mainly is a function of weld composi- tion, which impacts how the weld metal solidifies.
Three factors can significantly influ- ence the probability for hot cracking in aluminum welding:
1) The susceptibility of the base- material composition to cracking;
2) The selection of the most appro- priate filler metal to help prevent the
formation of a crack-sensitive compo- sition; and
3) Selecting the most appropriate weld-joint design to dilute the base metal and help avoid a crack-sensitive weld composition.
Because some aluminum alloys are more susceptible to cracking than oth- ers, it’s important to select a filler metal that will result in weldmetal composi- tion with low crack sensitivity. When welding aluminum that has low crack sensitivity, always use a filler metal of similar composition. Conversely, when welding aluminum with high crack sen- sitivity, select a filler metal with a dif- ferent composition that will deposit a weld with low crack sensitivity.
Aluminum filler metals are identi- fied by a numerical American Welding Society (AWS) classification that cor- responds to the Aluminum Associa- tion registration number identifying the particular alloy composition. Not all filler metals are suitable for welding all aluminum base alloys. Reference a reputable filler-metal selection guide to make the best choice for welding dif-
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