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How can I put the stiffness back in my aluminum plate after welding?

March 26, 2014

Q: When I weld aluminum plate 3/16" to 1/4" for truck bodies it loses its stiffness. The same thing happens when making panels for car bodies. If I heat the panels to about 400 or 500 degrees and let it cool, it becomes very flexible and I can form the plates very easily. How can I put the stiffness back into the plates after I am done forming it?

A: Aluminum alloys are normally purchased in a specific temper. In the case of the non-heat treatable alloys, this is the -H Temper, which relates to the strain-hardened condition of the material. In the case of the heat treatable alloys, this is the -T Temper, which reflects the thermal treatment that the material has been subjected to during manufacturing. Both of these temper methods are used to impart strength to the base material. The strengthening characteristics of the -H and -T Tempers can be significantly affected by heating of the base material.

After arc welding for instance, a reduction in strength is seen as well as an increase in ductility within a localized area adjacent to the weld. This area has been heated, during the welding operation, to a sufficient temperature for an adequate amount of time to anneal or partially anneal the base material. The same effect will occur when heating an entire panel. The heating operation will start to remove some of the strengthening that was introduced by the tempering operations and it will lower the tensile strength and increase the materials ductility (formability). The higher the temperature and the longer the time at temperature, the more pronounced the effect on the material.

Unfortunately, there is not a practical way to reintroduce the stiffness into the base material of a fabricated part after it has been reduced by heating. In theory, strengthening the effected material by strain-hardening or heat treatment could be done, however, because of the procedures required to perform these operations, it is not usually an appropriate option. Strain-hardening is achieved through controlled physical deformation of the base material, which generally involves a reduction in the cross-sectional area. Heat treatment to improve mechanical properties generally involves heating to a very high temperature followed by quenching in water and then controlled reheating for a further period of time. Post weld heat treatment of aluminum alloys is used in some specialized applications, however, there are a number of major considerations such as distortion of the fabricated part as it is heated and cooled through a very wide range of temperatures, practicality of physically heating and cooling the entire component and the cost involved with this elaborate procedure.

Q: I have a customer who is having problems passing guided bend tests on 5083 base material. What filler alloy should he be using and why are the bend tests failing.

A: Base material 5083 can be welded successfully with 5356, 5183, and/or 5556. All three of these filler alloys may be suitable for welding this base material, however, choosing one of these filler alloys is dependent upon the application and service requirements of the component being welded. The 5083 base material can be used in a number of applications including shipbuilding, cryogenic tanks, military vehicles and structural fabrications. From a design standpoint, considering fillet welds, the typical transverse shear strength values of these three filler alloys are 26Ksi, 28Ksi and 30Ksi for 5356, 5183 and 5556, respectively. Considering groove weld transverse tensile strength, the 5356 filler alloy is normally only used on 5083 base material when there is no requirement for groove weld welding procedure qualifications. The 5356 filler alloy may not consistently achieve the minimum tensile strength requirements of the code (40ksi – 275MPa) for groove weld transverse tensile testing of 5083 base material. The 5183 filler, developed specifically for welding the 5083 base material, will meet the mechanical property requirements for groove weld procedure qualification. The 5556 base alloy has slightly higher mechanical properties over the 5183 and can be used to weld the slightly higher strength 5456 base material, but will also meet the minimum tensile requirements for the 5083 base material. The 5083 base material should not be welded with a 4043 or a 4047 filler alloy. It not recommended that any 5xxx series base material with more than 2.5% magnesium be welded with a 4xxx series filler alloy.

Failing a guided bend test can be due to a number of reasons:

  1. The most obvious reason is that there is some form of discontinuity in the weld, which has caused the break, usually lack of fusion. To determine the presence of any significant discontinuities that may have caused the failure, the failed sample needs to be inspected.
  2. Was the test conducted correctly? The samples may not have been prepared correctly prior to bending, or maybe they were bent over the incorrect radius. Examination of the failed samples is necessary to verify that they were prepared in accordance with the relevant specification. The test procedure should be evaluated to determine its correctness. 
  3. Which testing method was used? The wraparound guided bend test method is the preferred method of testing aluminum weldments because of the significant variations in the as welded mechanical properties of some aluminum alloys. The plunger test method is not recommended for higher strength aluminum alloys like those that you are testing. 
  4. The use of an inappropriate filler alloy such as a 4xxx series used on this base material, could result in a weld with low ductility and therefore susceptibility to failure.

Posted in Aluminum Welding , Tagged with GTAW, Heliarc, TIG

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