Why do you get "crater cracks" in aluminum and how do you prevent them?
Crater cracks happen for three reasons:
1. High thermal rate of conductivity
2. The relatively large change in volume when aluminum solidifies
3. The concave shape of the crater
Aluminum cools so fast that it doesn't provide adequate time for the weld bead to flatten or the crater to fill. The deep depression of the crater quickly freezes in a concave shape, exerting high tensile stresses on the surrounding metal. It is in this area that a crack will propagate through the weld metal. As the weld cools, crater cracking is common if proper steps are not taken to minimize the problem.
Preventing Crater Cracks in Gas Tungsten Arc Welds (i.e., TIG)
It is usually fairly easy to fill the crater in a TIG weld. Just be sure not to leave the crater concave. It should be flat or convex. You do this by decaying the welding current smoothly. While doing that, add two or three small drops of filler metal to the weld until the crater is filled.
Preventing Crater Cracks in Gas Metal Arc Welds (i.e., MIG)
Filling the crater in MIG welds is considerably more difficult because you don’t have independent control of the heat and the filler wire addition. If you just take your finger off the trigger at the end of the weld, you will get a concave crater and possibly a crack. Even if the welding power supply has a "Crater Fill" function, it is often not completely effective. The easiest way to prevent crater cracks in aluminum MIG welds is to "back step" at the end of the weld. Instead of releasing the trigger at the end of the weld, reverse the travel direction back into the already welded material. Continue to weld back over the already welded material for about ½” (12 mm) and then release the trigger to end the weld. This will make the deposit in the crater area thicker, changing its shape from concave to convex.
Customer Assistance Policy
The business of The Lincoln Electric Company is manufacturing and selling high quality welding equipment, consumables, and cutting equipment. Our challenge is to meet the needs of our customers and to exceed their expectations. On occasion, purchasers may ask Lincoln Electric for information or advice about their use of our products. Our employees respond to inquiries to the best of their ability based on information provided to them by the customers and the knowledge they may have concerning the application. Our employees, however, are not in a position to verify the information provided or to evaluate the engineering requirements for the particular weldment. Accordingly, Lincoln Electric does not warrant or guarantee or assume any liability with respect to such information or advice. Moreover, the provision of such information or advice does not create, expand, or alter any warranty on our products. Any express or implied warranty that might arise from the information or advice, including any implied warranty of merchantability or any warranty of fitness for any customers’ particular purpose is specifically disclaimed.
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