Influence of Liquid Film Characteristics on Hot Cracking Initiation in Al-Cu Alloys at the End of Solidification

doi:10.1007/s40195-022-01449-9

Abstract

Abstract:

In this study, Al-4Cu alloy specimens with spherical grains and liquid films were obtained by isothermal reheating treatment. The hot cracking of the solidification process was determined using a modified constrained rod casting experimental apparatus, and the effect of liquid film characteristics at the end of solidification on hot cracking initiation of Al-4Cu alloys was systematically investigated by combining molecular dynamics simulations and other methods. With the extension of soaking time, the liquid fraction (liquid film fraction at the end of solidification) and grain shape factor increased with higher isothermal reheating temperatures. Additionally, the widened filling channel decreased the hot cracking initiation temperature and the critical hot cracking shrinkage stress was found to increase, thus reducing the hot cracking severity in Al-4Cu alloys. Molecular dynamics simulations revealed that with the extension of soaking time, the composition of the liquid film changed at different isothermal reheating temperatures, but the short-range structure and atomic ordering of the liquid film remained the same. The activity of the liquid film increased in equilibrium, leading to a decrease in viscosity and an increase in fluidity, which contributed to the filling behaviour. After isothermal reheating at 640 °C for 60 min, the liquid fraction reached the maximum, and the viscosity of the liquid film was the minimum. In addition, almost no hot cracks were found.

Key words: Al-4Cu alloy, Liquid film characteristics, Isothermal reheating, Molecular dynamics simulation, Hot cracking initiation mechanism

Ming Su, Xiaoguang Yuan, Chunyu Yue, Wentao Zheng, Yuxiang Wang, Jian Kang. Influence of Liquid Film Characteristics on Hot Cracking Initiation in Al-Cu Alloys at the End of Solidification[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(1): 103-117.

Figures/Tables 17

Fig. 1 DSC curve and theoretical liquid fraction versus temperature of the Al-4Cu alloy

Fig. 2 Schematic diagram of the experimental system for liquid film shrinkage stress: a isothermal reheating treatment experiment device, b overall experimental system and c size of the specimen

Fig. 3 Atomic structure evolution of liquid films after isothermal reheating treatment at 630 °C for 50 min: a 0 ps, b 100 ps, c 200 ps

Fig. 4 Microstructures and phase composition of the billet of Al-4Cu alloy: a OM, b XRD, c TEM

Fig. 5 EPMA analysis mapping results of Al-Cu alloys after isothermal reheating treatment at 630 °C for 50 min: a OM, b Al and c Cu

Fig. 6 Microstructures of Al-4Cu alloys after isothermal reheating treatment: a 620 ℃, 20 min; b 620 ℃, 40 min; c 620 ℃, 60 min; a1 630 ℃, 20 min; b1 630 ℃, 40 min; c1 630 ℃, 60 min; a2 640 ℃, 20 min; b2 640 ℃, 40 min; c2 640 ℃, 60 min

Fig. 7 Variation in the a average shape factor, b liquid fraction of Al-4Cu alloys at different temperatures for different isothermal reheating treatment time

Fig. 8 Stress-temperature-time curves after isothermal treatment: a 620 ℃, 20 min; b 620 ℃, 40 min; c 620 ℃, 60 min; a1 630 ℃, 20 min; b1 630 ℃, 40 min; c1 630 ℃, 60 min; a2 640 ℃, 20 min; b2 640 ℃, 40 min; c2 640 ℃, 60 min

Fig. 9 Variations in a the critical temperature, b critical shrinkage stress of the liquid film of hot cracking initiation vs. isothermal treatment temperature and time

Table 1 Critical solid fraction (%) after hot cracking initiation for different isothermal treatment conditions

Time (min)	10	20	30	40	50	60
620	95.87	96.38	96.60	96.66	96.73	96.78
630	96.33	96.51	96.70	96.84	96.95	97.26
640	96.76	96.91	96.95	97.43	99.60	-

Fig. 10 Microstructures and EDS analysis of hot cracking

Fig. 11 Hot cracking morphologies after isothermal reheating treatment at 630 °C for a 10 min, b 20 min, c 30 min, d 40 min, e 50 min, f 60 min

Table 2 Stoichiometry numbers of liquid film composition under different isothermal treatment conditions

Time (min)	10	20	30	40	50	60
620	Al₈₁Cu₁₉	Al₈₂Cu₁₈	Al₈₇Cu₁₃	Al₈₇Cu₁₃	Al₈₉Cu₁₁	Al₉₁Cu₉
630	Al₈₄Cu₁₆	Al₈₉Cu₁₁	Al₉₀Cu₁₀	Al₉₀Cu₁₀	Al₉₁Cu₉	Al₉₃Cu₇
640	Al₈₈Cu₁₂	Al₈₉Cu₁₁	Al₉₀Cu₁₀	Al₉₀Cu₁₀	Al₉₅Cu₅	Al₉₆Cu₄

Fig. 12 RDFs of liquid films under different isothermal treatment conditions for a 620 °C, b 630 °C, c 640 °C

Fig. 13 MSDs of liquid films under different isothermal treatment conditions for a 620 °C, b 630 °C, c 640 °C

Fig. 14 Viscosities of liquid films under different isothermal treatment conditions

Fig. 15 Schematic diagram of the hot cracking formation mechanism a feeding behaviour of thin liquid film in the narrow feeding channel, b feeding behaviour of thick liquid film in the wide feeding channel

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