Wide-spread implementation of 7xxx series aluminum alloys (AAs) in the mass production of automotive vehicles can have a broad impact on vehicle structural light-weighting and fuel efficiency improvement, while maintaining or improving structural integrity. However, there are two significant technical barriers that limit the implementation of fusion welded 7xxx series AAs, which are solidification cracking and stress corrosion cracking (SCC). The present research addresses both technical barriers through a combination of experimental testing, microstructure characterization, and computational thermodynamics simulation. First, with respect to AA 7003 base metal (BM) welded with AA 5356 filler metal (FM), cracking of the weld joint was exhibited during testing in a saline fog, where fractography confirmed the cracking type as SCC. The present research established a mechanistic understanding of the aggravated and accelerated SCC that occurs in fusion welded 7xxx series AAs, when compared to un-welded parent material. Second, a preliminary investigation into a novel solution for solidification cracking was conducted for Cu-rich AA 7075 BM.
A novel discovery in the present research is the phenomenon of a fused-overlap zone (FOZ) existing as a fusion zone layer that overlaps and fuses to the BM. FOZ may, and typically does, become enriched with solute elements that are constituents of the BM and/or FM. Moreover, large precipitates form in the solute-enriched FOZ. For instance, for AA 7003 BM welded with AA 5356 FM, a transmission electron microscopy (TEM) investigation identified the FOZ enriched with Mg and Zn and the precipitates there as T phase [(Al, Zn)49Mg32]. The FOZ was found to be a prevalent phenomenon in AA weldments, occurring regardless of joint geometry, welding heat input, welding process, or BM and FM investigated in the present research. The corrosion response of the FOZ was dependent on BM and FM compositions, which was likely a result of different precipitates formed in the FOZ that have various electrochemical properties (e.g., anodic, cathodic, or neutral to the Al matrix).
It is found that the FOZ forms as a result of elemental vaporization during welding. The vaporized elements deposit directly on the BM adjacent to the weld pool in the form of a detritus (smut). Shielding gas flow is identified as a plausible driving force for the detritus deposition. On the HAZ surface of AA 7003 BM welded with AA 5356 FM, x-ray diffraction showed that Al12Mg17, a magnesium aluminide, is the most prominent component in the detritus. A thermodynamic calculation shows that the melting temperature of magnesium aluminide (465 °C) is much lower than the HAZ temperature immediately adjacent to the weld pool; this leads to a liquid film existing atop the HAZ, which is rich in Mg. The convex weld pool subsequently wets over the BM, and mixes with the adjacent liquid film. Rapid solidification of this region occurs, trapping the solute elements and forming the solute-enriched FOZ. Subsequently, the precipitates form in the FOZ, due to the micro-segregation under a non-equilibrium solidification condition.
With respect to the SCC susceptibility of AA weldments, a band of the HAZ, extending up to 1 mm from the fusion line, was markedly more susceptible to pitting than the remainder of the HAZ. Strain maps measured by digital image correlation (DIC) indicated that geometric changes to SCC test specimens consequentially altered the stress intensity factor (KI) experienced by the susceptible HAZ region during testing. Propensity for SCC was linked to KI; particularly, if the induced KI was greater than the HAZ’s critical stress intensity factor for SCC (KISCC), SCC failure was propagated. A high KI could be introduced artificially by a sharp notch from electrical discharge machining (EDM), or naturally by intergranular corrosion in the HAZ during testing; both cases would result in SCC failures. Although KI was found to be the most dominant variable for SCC, T phase precipitates in the FOZ were linked to accelerated failures. Pitting of the T phase, anodic to the Al matrix, caused local solution acidification, and consequentially accelerated intergranular corrosion crack growth into the HAZ. Because of the accelerated natural development of KI, SCC failures were also accelerated by the presence of T phase precipitates in the FOZ. SCC resistance was promoted by the complete removal of the FOZ, but SCC was not absolutely suppressed. Additionally, a simulated paint bake thermal cycle was found to prolong the time-to-failure of tested specimens, and was hypothesized to subtly increase the HAZ’s KISCC, likely due to microstructural changes such as formation of intragranular precipitates (similar to aging). Based on the testing results, a quantitative ranking of different factors’ contribution to SCC in 7xxx AA joint is established.
Among the various options investigated, the most practical solution to SCC was found through the development of precision additive dressing (PAD), where an inert FM layer was precisely added to the location that needed corrosion resistance and protection. Cold metal transfer (CMT) or other low-heat input processes can be used for PAD. Testing of PAD specimens, even in aggressive acid testing, showed remarkable corrosion resistance of the region dressed with PAD.
Finally, preliminary attempts were made to mitigate solidification cracking by the introduction of TiO2 nanoparticles into the weld pool, but experienced marked difficulty. However, when introduced, the nanoparticles significantly refined the grain size, which was correlated with a reduction in solidification cracking susceptibility in other literature studies. A significant decrease in weld penetration was also noted with the introduction of TiO2 nanoparticles. The nanoparticles successfully dispersed throughout the entire weld pool, however were more heterogeneously distributed after long weld times. Additional work is necessary to characterize the effect of nanoparticles on solidification cracking.
In summary, the present research studied the mechanism of FOZ formation, the catalytic effect of FOZ T phase precipitates on intergranular corrosion, the SCC susceptibility of HAZ, and the grain refinement by addition of nanoparticles for solidification cracking resistance. The novel discovery and investigation of the FOZ, and its contribution to the development of SCC in welded AAs, is noted as a significant addition of knowledge to the field of welding engineering. Taken as a whole, this work significantly advanced the fundamental understanding of SCC and weldability of the fusion weldments of high-strength 7xxx series AAs.