Weldability of Non-Ferrous Metal Alloys Group

Magnesium

When dealing with magnesium alloys, one must always be cognizant of the possibility for corrosion. This is particularly true for weldments, due to galvanic effects arising from different microstructures and compositions. At BAM the weldability of AZ31 wrought plate has been investigated in terms of weld corrosion and weld corrosion-fatigue. In particular, attention has been paid to how variations in weld metal aluminum content may affect corrosion. Aluminum is known to improve strength, corrosion resistance, and provide grain refinement in cast magnesium alloys. Second phase particles have been identified to be β-Al₈Mg₅ in the HAZ, and β, Mg₂Si, and τ-Mg₃₂ (Al, Zn)₄₉ in the weld metal [5, 6]. Using Kelvin microprobe analysis, these particles have been shown to be cathodic with respect to the magnesium matrix, resulting in localized galvanic attack around the particles.

AZ31 weld metal microstructure showing eutectic second phase β, which increases in quantity with aluminum concentration (enlarged picture)

Potentiostatic corrosion potential measurements for different regions in the weld (base metal, heat affected zone, and weld metal) were inconclusive for predicting weld galvanic behavior (i.e. non-repeatable values arising from differences in surface condition), and thus another approach was taken. Weld immersion tests in salt water showed that pitting corrosion begins in the weld heat affected zone, moves into the weld metal, but then concentrates in the base metal. This is believed due to the formation of a protective passive film on the weld metal. Weld metal with higher aluminum content showed better corrosion resistance. Galvanic cells were constructed between AZ31 base plate and different die casting alloys, simulating weld metal with varying aluminum content. It was discovered that the base material corrodes initially, but that after a period of a few minutes to 1-2 hours, the corrosion is shifted to the cast metal (i.e. direction of corrosion current is observed to reverse) [7]. This behaviour, unlike that of the weldments, reflects an inability to form a passive film on the solidified material for reasons yet unclear.

Galvanic cell between wrought AZ31 and die cast alloys AM20, AM50, AM60, and AZ91. Negative current represents AZ31 corrosion (enlarged picture)

A limited number of fatigue and corrosion-fatigue tests have been performed on AZ31 gas-tungsten arc weldments made with AZ61 filler metal [8]. It was found that welds have significantly lower fatigue strength than the base metal when tested in air. This fatigue strength is reduced even further when tested in 3.5% salt solution. In most cases, fracture occurred in the weld metal neat the fusion line.

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