Evidence of back diffusion reducing cracking during solidification.

Al-Mg alloys, despite their wide freezing temperature range ΔT f , can have good resistance to cracking during solidification. To help understand why, the mushy zone of 5086 Al (∼Al-4.0 Mg) was quenched during arc welding and the cooling curve measured to locate the beginning of the original mushy zone (liquidus temperature T L ) and the end (eutectic temperature T E ). Since little eutectic was visible just slightly behind the beginning of the quenched mushy zone, little liquid was here in the original mushy zone, i.e., solidification already ended well above T E . Since no dendrites were visible, either, and since the highest Mg content measured was well below the maximum solubility in solid Al, C SM (17.5 wt% Mg), microsegregation was very mild here in the original mushy zone. These results suggest significant Mg back diffusion occurred during solidification (because of very high C SM ), causing: 1. fraction solid f S to increase much faster with decreasing temperature T , 2. ΔT f to narrow down, and 3. dendritic grains to bond together extensively ( f S ≈ 1) to resist intergranular cracking earlier (well above T E ). Since | d(f S )/dT | increased, | dT/d(f S ) 1/2 | decreased to decrease the crack susceptibility index, i.e., the maximum | dT/d(f S ) 1/2 |. All these changes reduce the crack susceptibility. For comparison, 2014 Al (∼Al-4.4Cu) was also quenched during arc welding. At the end of the quenched 2014 Al mushy zone, continuous eutectic, dendrites and microsegregation were all very clear. Thus, solidification ended at T E and thin liquid films still separated grains at the end of the original mushy zone to allow intergranular cracking. Calculated T-(f S ) 1/2 curves showed the index is reduced significantly by back diffusion in Al-4.0 Mg (∼5086 Al) but not in Al-4.4Cu (∼2014 Al). [ABSTRACT FROM AUTHOR]