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What is the best filler alloy for 5083 base alloy?
March 26, 2014
Q. I have a situation were my customer is experiencing failures in their guided bend tests. The Base Metal is 5083 and the filler alloy is ER5556. The test plate is a 5mm thick single groove weld with a 70 degree included angle, no root gap and a 2mm nose. The weld is back gouged and welded from the second side. We are not sure if we are using the correct filler wire; our welding equipment supplier has suggested that we change to an ER5356 type wire. What is your professional advice on this issue?
A. What is the most appropriate filler alloy?
Base alloy 5083 can be successfully welded with filler alloys 5356, 5183, or 5556, any of these three filler alloys may be suitable for welding this base alloy. The reason to choose one of these filler alloys over the others is dependant on the application and service requirements of the component being welded. Used in a number of applications, base alloy 5083 aids in shipbuilding, cryogenic tanks, military vehicles, structural fabrications, just to name a few. The 5356 filler is normally only used on 5083 base alloy when there is no requirement for groove weld welding procedure qualifications, in accordance with the structural welding code; this is because the 5083 base - 5356 filler combination will typically not obtain the minimum tensile strength requirements of the code (40ksi – 275MPa) for groove weld transverse tensile strength. The 5356 filler alloy is often used on the slightly lower strength 5086 base alloy and will typically obtain its required minimum transverse tensile strength for grove welds (35ksi – 240MPa). The 5183 and the 5556 filler alloys will both typically meet the code tensile strength requirements for groove welds in 5083 base alloy. Developed specifically for welding the 5083 base alloy, the 5183 filler meets the mechanical property requirements for groove weld procedure qualification. The 5556 base alloy, with slightly higher mechanical properties than the 5183, will meet the minimum requirements for the 5083 base alloy. The 5556 filler alloy was developed for obtaining slightly higher tensile strength requirements (42ksi – 285MPa) of groove welds in 5456 base alloy.
In my opinion, the filler alloy selection is not the problem causing you to fail the guided bend tests; however, I do think you need to understand the application/welding standard requirements when selecting the most appropriate filler alloy.
Why then did the welded samples fail the guided bend test?
Start by considering the actual test itself and the differences between testing aluminum and other more common materials, such as steel, with this testing method.
Utilized for many years, the guided bend is a common method of testing the integrity of welds made in many different material types. The guided bend test is relatively quick and, is usually a comparatively economical method of establishing the soundness of a groove weld. Where properly used, it can be very revealing; however, in order for the test results to be of meaningful importance when testing aluminum, it is imperative that the testing methods used be thoroughly understood. There are various types of bend tests 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) used 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.
Reasons why a welded sample may fail a guided bend test
Discontinuities in the welded joint
The most obvious reason a welded sample may fail a guided bend test is because it has been weakened by the presence of significant discontinuities. The test helps determine whether the weldment tested contains discontinuities such as cracks, lack of fusion, inadequate penetration or severe porosity. If significant discontinuities in the weldment were present, we would expect the bend test sample to fail. If inspected after testing, it is often possible to identify the type and extent of discontinuity present in the weldment.
If this were the reason for your bend test failure, you would need to evaluate your welding procedure and make the necessary adjustments to improve the weld integrity.
Using an incorrect testing fixture
The most common way to conduct guided bend testing of welded steel samples is the use of a die and plunger arrangement, often referred to as the plunger-type guided bend test fixture. Using the plunger-type guided bend test fixture for testing aluminum is not appropriate. The heat-affected zones of welds in aluminum alloys can be significantly softer and weaker than the surrounding material. If these welds bend around a plunger, the bend sample may bend sharply in the heat-affected zones and kink and/or break without adequately bending the weld metal, resulting in a test failure. In order to avoid test failures, always use the wrap around bend test fixture 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.
Improper Sample preparation
You 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 rounded corners, a smooth surface and be free of sharp notches that may provide stress concentration during the bending operation.
With complete understanding and proper use, the guided bend test can be a very effective testing method for aluminum weldments. It is difficult to say why you are having this problem without knowing the extent of sample preparation, seeing the test samples before and after testing and knowing the testing fixture used. I hope you are able to use the information above to investigate your problem further and successfully resolve it.