(702g) Welded Aluminum 6061: The Effect of Corrosion On Yield Strength

            Transportation vehicles contain five thousand or more spot welds, with more welds present for larger or heavier vehicles [1].  The strength and dependability of these vehicles are greatly improved with the use of spot welds [1].  Vehicles are also exposed to environments containing salt, such as de-icing salt on roads or salt in the air near saltwater bodies.  The presence of salt can then affect the undercarriage or body of these vehicles, including the welds.  The effects of corrosion on metals include mass loss and hydrogen embrittlement, both of which reduce the strength of the metal.  Hydrogen embrittlement can also cause a reduction in yield strength and the development of cracks that can result in catastrophic failure.  Unfortunately, the effects of salt on the yield strength of these spot welds are unknown. 

            Corrosion is a large problem that has led to the development of multiple ASTM standards, including immersion (G-31), salt spray (B-117), and accelerated corrosion (G-85, B-368) [2-5].  Unfortunately, the ASTM standards do not translate well to the automotive industry, as they do not contain a pollution phase, a wet phase, and a dry phase [6-8].  These specific phases are needed to replicate various conditions experienced by vehicles, including de-icing salt, mud, and condensation [6-8].  In order for any corrosion testing to be useful for the automotive industry and to adequately quantify the effects of a corrosive environment on the yield strength of welded aluminum joints, a pollution phase (acidified salt spray), a wet phase (100% humidity), and a drying phase are all necessary [7].

            Developing a corrosion model that can be coupled with a damage model to describe the interaction of corrosion and mechanical stresses is the goal of our research.  Determining how the yield strengths of three different weld production types, low, nominal, and high, are affected by a corrosive environment is a necessary part of the development of a coupled corrosion-damage model for welds.  At the Center for Advanced Vehicular Systems (CAV) at Mississippi State University (MSU), samples of aluminum alloy 6061 are being exposed to two environments, an immersion environment and a cyclical salt spray environment, using 5% sodium chloride aqueous solution and acetic acid for 240 hours.  During the 240 hours, the experiment is interrupted at various time intervals to determine how the environment is affecting the surface and the yield strength of the weld.  The surface information gathered will consist of data measuring various aspects of pit nucleation, pit growth, and pit coalescence that will be used in the development and calibration of an internal state variable model [9].  This model relates total corrosion to general corrosion, pitting corrosion, and intergranular corrosion [9] and can be coupled with a fully established damage model.  Combining these two models will allow us to accurately predict the failure of a material, in this case aluminum, due to the embrittlement effects on the mechanical properties caused by a corrosive environment. 

[1] T. Snow, “Digital Spot Welding and Car Safety”, Dassault Systemes, Published March 11, 2011; Viewed April 13, 2012.  http://perspectives.3ds.com/manufacturing/digital-spot-welding-and-car-s...

[2] “G-31, Standard Practice for Laboratory Immersion Corrosion Testing of Metals” (2008) ASTM International.

[3] “B-117, Standard Practice for Operating Salt Spray (Fog) Apparatus” (2008) ASTM International.

[4] “G-85, Standard Practice for Modified Salt Spray (Fog) Testing” (2008) ASTM International.

[5] “B-368, Standard Test Method for Copper-Accelerated Acetic Acid-Salt Spray (Fog) Testing (CASS Test)” (2008) ASTM International.

[6] K.R. Baldwin, C.J.E. Smith, (1999) “Accelerated corrosion tests for aerospace materials: current limitations and future trends”, Aircr. Eng. Aerosp. Technol. 71, 239–244.

[7] N. LeBozec, N. Blandin, D. Thierry, (2008) “Accelerated corrosion tests in the automotive industry: a comparison of the performance towards cosmetic corrosion”, Mater. Corros. 59, 889–894.

[8] G. Song, D. St. John, C. Bettles, G. Dunlop, (2005) “The corrosion performance of magnesium alloy AM-SC1 in automotive engine block applications”, JOM 57, 54–56.

[9] H.J. Martin, R.B. Alvarez, J. Danzy, M.F. Horstemeyer, P.T. Wang, (2012) “Quantification of Corrosion Pitting Under Immersion and Salt-Spray Environments on an As-Cast AM60 Magnesium Alloy”, Corrosion, In Press.

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