Evidence of back diffusion reducing cracking during solidification

Al-Mg alloys, despite their wide freezing temperature range Delta T-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 fs to increase much faster with decreasing temperature T, 2. Delta T-f to narrow down, and 3. dendritic grains to bond together extensively (fs approximate to 1) to resist intergranular cracking earlier (well above TE). Since vertical bar dT/d(fs)(1/2)vertical bar increased, vertical bar dT/d(fs)(1/2)vertical bar decreased to decrease the crack susceptibility index, i.e., the maximum vertical bar dT/d(fs)(1/2)vertical bar . 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 TE and thin liquid films still separated grains at the end of the original mushy zone to allow intergranular cracking. Calculated T-(fs)(1/2) curves showed the index is reduced significantly by back diffusion in Al-4.0 Mg (similar to 5086 Al) but not in Al-4.4Cu (-2014 Al). (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.