Evidence of back diffusion reducing cracking during solidification

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• 作者：Jiangwei Liu^a ; Henrique Pinho Duarte^b ; Sindo Kou^b ; ^{kou@engr.wisc.edu" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Welding ; Solidification ; Solidification cracking ; Hot tearing ; Quenching ; Diffusion ; Aluminum alloys ; Heat transfer ; Mass transfer ; Fluid flow
• 刊名：Acta Materialia
• 出版年：2017
• 出版时间：1 January 2017
• 年：2017
• 卷：122
• 期：Complete
• 页码：47-59
• 全文大小：7481 K

文摘

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).