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Problem passing guided bend test on 6061-T6 base alloy

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Q - I am having problems passing the guided bend tests for my procedure qualification tests.  I am working to AWS D1.2 Structural Welding Code – Aluminum.  My base material is 6061-T6 and my filler alloy is ER4043.  I am using the plunger-type guided bend jig.  I am passing the reduced section tension tests on the same test specimen, and there would appear to be no relevant discontinuities in the weld and, therefore, no apparent reason for the bend tests to fail.

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.  Where properly used, it can be very revealing; however, when testing aluminum, the testing methods used must be thoroughly understood in order for the test results to be of meaningful importance.

The guided bend test is a relatively quick and, usually, a comparatively 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, lack of fusion, inadequate penetration or severe porosity.  Various types of bend tests are used to evaluate welds.  Guided bend specimens may be longitudinal or transverse to the weld axis and may be of the root bend, face bend or side bend type.  The type of bend test (root, face, or side) is determined by which surface of the weld sample (root, face, or side) is on the convex (outer) side of the bent specimen and, consequently, subjected to tension load during the testing operation.  Probably the most common combination of bend tests used for welder performance and welding procedure test samples are two transverse root bend tests and two transverse face bend tests per test plate.

BendTesting Of Aluminum Is Different Than Bend Testing Steel

Most guided bend testing of steel is conducted with the use of a die and plunger arrangement often referred to as the plunger-type guided bend test.   The plunger-type guided bend test is not recommended for testing aluminum.  The heat affected zones of welds in aluminum alloys, and particularly in the heat-treatable aluminum alloys, are significantly softer and weaker than the surrounding material.  If these welds are bent around a plunger, the bend sample usually bends sharply in the heat-affected zones and kinks and breaks without adequately bending the weld metal, resulting in a test failure.  In order 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.

There are a number of other pitfalls to avoid when bend testing aluminum.  We should be concerned about bend test sample preparation prior to bending.  A common mistake is to leave the corners of the sample square.  Most codes allow up to a 1/8 in. (3mm) radius on the corners of the test specimens.  For best results, samples should have a smooth surface, free of sharp notches that may provide stress concentration during the bending operation.

Special Bending Conditions For Aluminum Alloys

We should be aware that most codes, and certainly AWS D1.2, stipulate special bending conditions for various base alloys and filler materials.  Test samples of base alloys within the 6xxx series (M23), or other base alloys welded with the 4xxx series (F23) filler alloys, 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 is required to be reduced from the standard 3/8 in. (10mm) thickness to 1/8 in. (3mm) prior to bending, and then bent over a diameter of 16-1/2t.  If annealed prior to testing, the standard 3/8 in. (10mm) specimen is required to be bent over a 6-2/3t diameter.

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

Welds made with the 2219 base material (M24) are required to be annealed and bent over an 8t diameter.  Welds made with 7005 base material (M27) are required to be bend tested within two weeks of welding.  This requirement for 7005 materials is based on the ability of this alloy to gain substantial tensile strength over time, and, consequently, suffer a reduction in ductility through natural aging.

It is obvious that there are many requirements that need to be considered if we expect to obtain the desired results from our guided bend testing procedures.  It is most important to understand the following:

The preparation of test samples prior to bend testing is very important.

There is an optimum method of bend testing aluminum (The Wrap-Around Bend Test).  There are major differences between the testing procedures used, which are often dependent on base material, and filler alloy types being tested.

If test samples are prepared correctly and test procedures specific to the material and filler alloy being tested are used, we can go a long way in assuring that we have no questionable test results.

Guided Bend Test Sample

Guided Bend Test Samples.  Top sample shows a side bend section prior to full preparation and bending, lower sample shows a completed side bend specimen

Wrap Around Best Test Jig

The mechanism for the Wrap-Around Bend Test Fixture.  The preferred method of bend testing aluminum weldments.  The “A” dimension shown on the drawing will vary dependant on plate thickness and base alloy/filler alloy being tested.

a)Wrap Around Testing Machine

b)Basic Design of Wrap Around

Fig a) Shows a picture of a wrap around testing machine with an aluminum test sample loaded and ready to be tested.

Fig b) Is a drawing showing the basic design of a wrap around testing jig similar to the one being used in fig a)