Evidence of back diffusion reducing cracking during solidification

Abstract

Al-Mg alloys, despite their wide freezing temperature range ΔTf, 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 TL) and the end (eutectic temperature TE). 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 TE. Since no dendrites were visible, either, and since the highest Mg content measured was well below the maximum solubility in solid Al, CSM (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 CSM), causing: 1. fraction solid fS to increase much faster with decreasing temperature T, 2. ΔTf to narrow down, and 3. dendritic grains to bond together extensively (fS ≈ 1) to resist intergranular cracking earlier (well above TE). Since |d(fS)/dT| increased, |dT/d(fS)1/2| decreased to decrease the crack susceptibility index, i.e., the maximum |dT/d(fS)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 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 (∼5086 Al) but not in Al-4.4Cu (∼2014 Al).