Evidence of back diffusion reducing cracking during solidification

Abstract

Al-Mg alloys, despite their wide freezing temperature range T f , can have good resistance to cracking during solidification. To help understand why, the mushy zone of 5086 Al (~ Al-4.0Mg) 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. T 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-4.0Mg (~ 5086 Al) but not in Al-4.4Cu (~ 2014 Al).

Liquid channel segregation and morphology and their relation with hot cracking susceptibility during columnar growth in binary alloys

“…In this case, eutectics can form continuously and a so-called "continuous eutectic" can be observed. This is shown byLiu et al very recently[26] in welds of 2014 Al (an alloy with compositions close to 4.0wt%Cu). This is consistent withFig.…”

Susceptibility of ternary aluminum alloys to cracking during solidification

“…In addition to the alloy composition, the cooling rate (through the freezing temperature range T f ) and the secondary dendrite arm spacing  2 also need to be specified when using Pandat to calculate T vs. f S involving back diffusion. Liu and Kou [18] measured the cooling rate during gas-tungsten arc welding (GTAW) of 2014 Al. [18].…”

“…Liu and Kou [18] measured the cooling rate during gas-tungsten arc welding (GTAW) of 2014 Al. [18]. A much slower cooling rate of 20 o C/s was also used in the present study, which corresponds to  2 of 18 m as shown in Fig.…”

•Microstructure Evolution and Solidification Cracking in Austenitic Stainless Steel Welds

“…Liu and Kou (Refs. [21][22][23][24][25] further showed in Al alloys, backdiffusion during solidification (e.g., Mg in Al-4Mg alloy) can cause earlier bonding between dendrites and increase the resistance to cracking. Bonding between growing grains can also play a significant role in the solidification cracking of austenitic stainless steels, as will be shown subsequently.…”

“…This quenching technique was recently tried on Al alloys (Ref. 25) because liquid Sn is too warm for quenching Al. No elements in Wood's metal were detected in the Al welds.…”