The influence of heating cycles during plasma metal inert gas (MIG) welding on the microstructure and corrosion properties of the AA5754 automotive alloy has been investigated. The high heat input during plasma-MIG welding results in a significant modification in the microstructure of the AA5754 alloy adjacent to the fusion boundaries. As a consequence of partial melting of the Al-Fe-Mn-(Si) intermetallics at the partially melted zone (PMZ) and segregation of the high melting point elements (particularly Fe and Mn) toward the fusion zone, severe galvanic corrosion attacks can be enhanced along the PMZ of the AA5754 weld during exposure to aqueous corrosion environments.
In the automotive industry, welding is one of the most critical elements of the body assembly process, which determines the structural integrity and quality of the vehicles being manufactured. The microstructure of the automotive aluminium wrought alloys can be significantly modified by the high heat input employed during fusion welding techniques, such as the plasma MIG welding [
The weld samples in the present work were provided by Jaguar Land Rover in the form of a plasma butt weld made of 2 mm AA5754-O panels using AA4043A filler wire “as shown in
For metallographic examination, the investigated welds were first cross-sectioned, polished using conventional grinding and polishing methods followed by etching for 15 - 20 s at ambient temperature in Keller’s reagent solution (85 ml H2O, 10 ml H2SO4 and, 5 ml HF) or 15 - 20 s of electro-etching at 20 V in Barker’s reagent (4 ml hydrofluoric acid and 200 ml H2O).
The influence of the welding cycle on the mechanical properties of the plasma welded alloys was investigated by conducting Vickers microhardness testing across the polished weld sections (parallel to the weld interface) using a Buehler microhardness tester unit. The microhardness measurements were performed using a 0.25 kg load with a 15 s dwell time.
For corrosion investigation, several 10 × 2 cm samples of the AA5754 plasma weld were cut. Further, in order to eliminate any possible interference of sample geometry with the immersion test results, and to simulate the final surface preparation (prior to painting) in the automotive industry, parts of cut AA5754 plasma welds were mechanically polished until the upper surface was completely flattened (the weld cup was removed), as schematically “shown in
Prior to the immersion test, the transverse sections of the cut samples were mechanically polished to 5 µm finish, degreased, washed with water, rinsed with acetone, and then masked with a suitable chemically resistant lacquer (lacomite solution), which was allowed to dry at room temperature for 48 h.
The localized corrosion behaviour of the AA5754 plasma welds was then assessed using a standard accelerated corrosion test in accordance to the G66 ASTM standard by a continuous immersion in in a solution containing 1.0 M NH4Cl, 0.25 M NH4NO3, 0.01 M (NH4)2C4H4O6, and 0.09 M H2O2.The immersion test was conducted at 65˚C ± 1˚C in a one litre glass vessel and the solution temperature was precisely controlled during experiments by using a thermostatically controlled water bath.
After the immersion test, samples were directly cleaned with a mixture of methanol and deionized water in an ultrasonic bath for 2 min. Samples for study by scanning electron microscopy, to reveal the morphology and severity of corrosion attack, were cross-sectioned and mechanically polished to a 1µm surface finish.
The optical micrographs of
The fusion zone of the plasma-MIG weld is the region where the parent metal is completely melted and mixed with the filler alloy material. Upon cooling, solidification of the fusion zone takes place first at the partially melted or solid grains along the fusion boundary (i.e. at the solid-liquid interface). The growth of the solidified material then proceeds toward the fusion centre (i.e. epitaxial growth opposite of the thermal gradient across the weld regions [
Next to the fusion boundary of the AA5754 plasma-MIG weld, there is a partially melted structure of about 300 µm width “Figures 3(a)-(c)”. The partial melting within this zone resulted in the formation of eutectic-phases at the grain boundaries of the parent material. The peak temperature in this region during welding is expected to exceed the equilibrium solidus or the eutectic temperature for the Al-Mg-Si eutectic constituents. However, the temperature across this region was not sufficient to fully dissolve phases containing high melting point elements (particularly those containing Fe and Mn). As a consequence, partial dissolution of Fe-rich constituents in this zone was detected “
The parent metal adjacent to the partially melted zone of the AA5754 plasma-MIG weld showed no evidence of over-heating (e.g. excessive grain growth, dissolution or coarsening of second phase particles “
The optical micrographs in Figures 5(a)-(d) show the appearance of the as-received “
Based on its severity and shape, the first type of localized corrosion attack was identified as a “knife-like attack”. Detailed investigation using scanning electron microscopy revealed that the knife-like attack was preferentially initiated and propagated more than 300 µm within specific areas just outside and parallel to the fusion zone “
The second type of attack was detected only under areas covered with loose, white scale, which suggests that the scale build up over certain areas during the immersion test somehow initiates such localized corrosion. Based
on these findings, the second type of corrosion was identified as “under deposit pitting attack”. The severity of under deposit attack is better represented in
propagation of the under deposit attack at the parent metal. Most probably, the scale deposition and, thus, the initiation of this attack are related to the local alkalinity around cathodic intermetallics at the samples surface.
Plasma-MIG welding of the AA5754 alloy using silicon-rich filler wire (AA4043A) results in the formation of a partially melted zone adjacent to the fusion boundaries. Partial melting of the Al-Fe-Mn-(Si) intermetallics at the partially melted zone, and segregation of the high melting point elements (particularly Fe and Mn) towards the fusion zone increase the open circuit potential difference between the weld and its adjacent partially melted zones. Consequently, severe galvanic corrosion attacks take place preferentially along the PMZ at each side of AA5754 weld during exposure to aqueous corrosion environments.
The authors thank EPSRC for support of the LATEST 2 Programme Grant.