Aluminum Workshop: Tensile Testing Welding for 6061-T6

Q: I am having problems passing the tensile tests for my procedure qualification of aluminum alloy 6061-T6 on 6-inch-diameter pipe, Schedule 120.

A: Good question. I can’t give you a definite answer without a little more information. It would help to know if the tensile samples failed in the weld, at the fusion line, or in the heat-affected zone (HAZ). It also would help to know in detail how the bend samples failed.

Having said that, let me give you some possibilities. The tensile samples should fail in the HAZ. If they do, but they fail at a low strength value, I would bet that the problem is that you preheated the weld excessively. Preheat should be avoided on heat-treatable alloys. If it’s used at all, ,it must be limited to a maximum of 200 degrees F. Excessive preheat or excessive weld heat input can destroy the mechanical properties of 6061-T6 with the results you described.

If the tensile samples failed in the weld, I believe you have a weld quality problem. However, the porosity you see didn’t cause the failure. Porosity in aluminum welds, while common, is a relatively benign defect. It’s only real effect is that it reduces yield and tensile strengths by a percentage e3equal to the volume fraction of porosity. A really bad weld contains around 5 percent porosity by volume, so the yield and tensile strengths are reduced by 5 percent. I’ve never seen a reduction in strength more than this that was caused by weld porosity.

If the tensile samples fail throughout the weld, lack of fusion in the weld is the most likely cause. This should show up as a flat gray area on the fracture face of the tensile sample and should be fairly easy to see.>/p>

Any lack of fusion in the weld also will cause bend samples to fail after only a small amount of bending. However, many bend test failures also are caused by improper bend test procedures. Aluminum bend samples are machined and tested differently than steel bend test samples. You should test 6061-T6 samples at 1/8-inch thickness, not the common 3/8-in. thickness used for other materials.

Additionally, wraparound-type bend testing equipment is recommended for bend testing aluminum samples, rather than the plunger-type equipment commonly used to test steel bend samples. Schematic drawings of both types of equipment can be found in the American Welding Society’s D1.2 Structural Welding Code—Aluminum.

For more information on bend testing, read "Preparing, testing bend samples," which appeared in this column in the September/October 2002 issue.