Aluminum Q&A: Why Guided Bend Tests Fail and How to Fix Them

Q: I have been conducting welding procedure qualification tests in accordance with AWS D1.2, Structural Welding Code — Aluminum. I am welding 6061-T6 base metal with ER4043 filler metal. The problem I am experiencing is the failure of the guided bend tests. I am performing two root bends and two face bends, as required by the code. My two tension tests from the same test plate are passing the reduced section tension test. 

A: The guided bend test has been around for many years and is a common method of testing the integrity of welds made in many different material types. When properly used, it can be very revealing. However, with aluminum, the testing methods must be thoroughly understood for the results to be meaningful.

The guided bend test is a relatively quick and economical method of establishing the soundness of a groove weld. This test is designed to help determine whether the weldment tested contains discontinuities such as cracks, incomplete fusion, incomplete joint penetration, or severe porosity.

Because your reduced section tension tests are passing, the issue is likely related to one or more common causes of guided bend test failure. Unfortunately, failing the guided bend test is not uncommon. Passing the guided bend test during welding procedure and welder performance qualification can depend as much on how the test is performed as on the quality of the weld itself. If a guided bend test is not conducted correctly on aluminum, the test can fail regardless of how good the weld is. Problems with not passing the guided bend test are most often due to incorrect adherence to the test method instructions in the welding code. Attention to three key areas can reduce guided bend test failures.

Use the Most Appropriate Testing Jig

The AWS D1.2 very clearly states that the wrap-around bend jig (Fig. 1) is the preferred method for bend testing aluminum weldments and even provides an explanation in the code commentary for why.

Most guided bend testing of steel is conducted with a die-and-plunger arrangement, also known as the plunger-type guided bend test. The heat-affected zones of welds in aluminum alloys, particularly in heat-treatable alloys (such as 6061-T6), are significantly softer and weaker than the surrounding material. If these welds are bent around a plunger, the sample can deform sharply in the heat-affected zones and kink or break without adequately bending the weld metal, resulting in a test failure. To avoid such meaningless test failures, the wrap-around bend test fixture should always be used for testing aluminum. This testing method forces the test specimen to bend progressively around a pin or mandrel so that all portions of the weld zone achieve the same radius of curvature and, therefore, the same strain level. The plunger-type test jig may be suitable for some low-strength base materials. However, I believe the wrap-around bend jig should be used for all aluminum base metals — why take a chance?

Properly Prepare the Test Sample

It is important to consider the preparation of the guided bend test sample prior to bending. A common mistake is to leave the longitudinal corner edges of the sample square. Most codes allow up to a 1/8 in. (3 mm) radius on the corners of test specimens. For best results, samples should have a smooth surface, free of sharp notches that may cause stress concentration during bending.

Special Bending Conditions for Aluminum Base Metals and Filler Metals

AWS D1.2 stipulates special bending conditions for various base metals and filler metals. In your situation, you are working with an M23 base metal and an F23 filler metal. Test samples of base metals within the 6xxx series (M23) and any base metals welded with 4xxx series (F23) filler metals are required to be tested under either of two special bending conditions: as welded or annealed. If testing is to be conducted in the as-welded condition, the test specimen must be reduced from the standard 3/8 in. (10 mm) thickness to 1/8 in. (3 mm) before bending, then bent over a diameter of 16-1/2 thickness. If annealed before testing, the standard 3/8 in. (10 mm) specimen must be bent over a 6-2/3 thickness diameter.

The specified annealing practice in AWS D1.2 is to heat the bend specimens to 775°F (410°C), hold them at this temperature for 2–3 hours, then control cool at 50°F/h (28°C/h) down to 500°F (260°C). The rate of cooling below 500°F (260°C) is unimportant.

Some aluminum base metals have other special bending conditions. Welds made with the 2219 base metal (M24) must be annealed and bent over an 8t diameter. Welds made with 7005 base metal (M27) must be bend tested within two weeks of welding. This requirement for 7005 base metal is based on its ability to develop substantial tensile strength over time and, consequently, to suffer a reduction in ductility through natural aging.

Conclusion

Many requirements must be considered to achieve the desired results from our guided bend testing procedures. It is most important to understand the following:

• There is an optimal method for bend testing aluminum (the wrap-around bend test).
• Preparing test samples before bend testing is very important.
• There are significant differences in testing procedures depending on the base and filler metal types used.

If the correct test jig is used, test samples are prepared correctly, and test procedures specific to the base and filler metals being tested are followed, we can go a long way toward ensuring there are no questionable test results.

This article was written by Tony Anderson (ITW Welding North America) for the American Welding Society.