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Ablation Corrosion

OVERVIEW

Ablation casting is a newly developed casting process for Al-Si-Mg alloys that produces heat treatable cast alloys that can attain strength comparable to wrought Al-Si-Mg alloys without an elongated and anisotropic microstructure. This technology has been successfully used to produce aluminum crash nodes in the crush zones of a 2017 Acura NSX, and is being targeted for other areas to replace difficult-to-fabricate wrought Al forgings.

                            

BACKGROUND

The use of aluminum alloy castings, one the oldest manufacturing processes, is growing in the automotive industry due to the low density of aluminum and design freedom for complex components casting. Its use is limited, however, as the cast components have less strength when compared to wrought Al products. Ablation corrosion technology is being targeted for other areas to replace difficult-to-fabricate wrought Al forgings.

Although the strength of Al-Si-Mg Ablation castings have been shown to be superior to conventional castings and comparable to wrought Al alloys, its corrosion performance has not been investigated. Exposure to aggressive chloride-containing environments (acidic rainwater, deicing salts, sea spray in coastal environments, etc.) has been shown to accelerate corrosion on steels and aluminum alloys used in automotive applications. As such, it is essential to ensure that ablation cast materials are comparable to their target replacements in corrosion-related performance, including general corrosion performance, stress corrosion cracking resistance, and reductions in fatigue life and strength after corrosion has been initiated.

SCOPE OF WORK

An Al-Si-Mg alloy, with a composition similar to wrought AA6061, will be investigated as this is a likely target for component replacement in the automotive industry. Wrought 6xxx Al alloys are generally known for good resistance to pitting, exfoliation corrosion, and stress corrosion cracking (SCC).  Additionally, Ablation cast 6061 provides comparable strength to forged 6061-T6, and the absence of a rolling and fast cooling rate creates a finer and more equiaxed microstructure. 

Accelerated laboratory corrosion tests will be conducted on ablation cast 6061-T6 and compared with that of wrought 6061-T6. Corrosion performance will be characterized and documented by photographs of the samples after testing and quantification of the corrosion morphology, topography, and depth of attack by optical profilometry, scanning electron microscopy, and cross-sectional optical microscopy. The laboratory testing will be compared to samples that are exposed to typical service by placing samples for up to 3 years on The Ohio State University CABS buses. The results of type and extent of corrosion attack from the accelerated laboratory tests will be compared to the in-service type exposure to verify the correct laboratory test to predict in-service corrosion performance.

Stress corrosion cracking (SCC) is a phenomenon where fracture occurs at stresses lower than expected when tensile stresses are present in a sufficiently corrosive environment.  Generally, 6061-T6 is considered to be very resistant to SCC. Ablation cast 6061-T6 will need to match this performance to be considered an adequate replacement for wrought 6061-T6. Slow strain rate SCC testing will be conducted on ablation cast 6061-T6 in aqueous chloride solutions to ensure that SCC does not occur at loads below that of wrought 6061-T6. Additionally, because a number of current cast applications are in crash zones, the effect of corrosion on strength and fatigue life will be investigated by conducting tensile and fatigue testing on samples that have been corroded for both wrought and ablation cast 6061-T6.