Hot Tearing of Aluminum Alloys

Understanding Hot Tearing Fundamentals

Hot tearing represents the undesired formation of irregular cracks in metal castings that develop during solidification and cooling, typically while the casting remains inside the mold or die cavity. The primary cause of hot tearing stems from the development of thermally induced tensile stresses and strains in a casting as the molten metal contracts during solidification and solid-state shrinkage. This phenomenon frequently occurs at inside corners or fillets of casting geometries, where casting shrinkage becomes constrained by the relatively rigid mold cavity.

In die casting applications, the mold cavity functions as a comparatively rigid structure, usually constructed from steel, in comparison to the relatively low strength aluminum, magnesium, or zinc casting alloys at elevated temperatures. Consequently, one of the key castability attributes considered in developing new metal casting alloys involves achieving a low propensity for hot tearing.

Factors Influencing Hot Tearing Susceptibility

Beyond casting design features, multiple factors influence hot tearing, including both the casting alloy composition and solidification characteristics, as well as casting process parameters. Therefore, hot tearing tends to present greater concerns in die casting processes compared to sand casting processes, where the mold cavity typically exhibits lower strength and greater compliance to casting shrinkage.

Hot tearing cracks can be observed through Constrained Rod Casting (CRC) testing, as demonstrated in Figure 1.

Figure 1: Hot tearing at junction points of constrained rod casting

Additionally, hot tearing defects have been extensively investigated in continuous casting of steel and aluminum ingots. The rapid cooling processes generate higher thermal gradients that lead to thermal contraction in the ingots, resulting in hot tears as illustrated in Figure 2 below.

Figure 2: a) Hot tearing in aluminum ingot b) Hot tearing in extrusion billet

Evaluation Methods and Alloy Rankings

The Constrained Rod Casting (CRC) mold casting method has proven effective in evaluating and quantifying hot tearing susceptibility of aluminum alloys. Research findings indicate that the hot tearing susceptibility of wrought aluminum alloys and Al-Si binary alloys can be ranked using the Hot Tearing Susceptibility (HTS) index as follows:

For wrought aluminum alloys:

AA1050 < AA5182 < AA3104 < AA6111

For Al-Si binary alloys:

0.5wt%Si > 1.0wt%Si > 1.5wt%Si > 2.0wt%Si > 3.0wt%Si

This ranking of wrought aluminum alloys aligns well with observations in typical industrial casting practice. The ranking of binary alloys corresponds with foundry practice and previous investigations. The CRC method and HTS index successfully distinguish the hot-tearing sensitivity of alloys with and without grain refinement.

Grain Refinement Effects on Hot Tearing

Microstructural investigation of grain-refined and non-grain refined AA1050 alloy revealed an inverse relationship between the degree of grain refinement and hot tearing tendency. Experimental results demonstrate that grain refinement dramatically reduced hot tearing susceptibility in alloys exhibiting lower hot tearing tendencies, specifically AA1050, AA5182, and AA3104.

However, grain refinement proved less effective in AA6111, an alloy exhibiting higher hot tearing tendency, despite achieving fine and equiaxed grains. This limited effectiveness may be partially explained by the fact that this alloy inherently yields a fine grain size.

Research on Grain Refiner Effects

Recent work by R. Sharma, R. Tyagi, N. Yadav, and R. Goel investigated the effect of grain refiners added in different weight percentages to aluminum copper silicon alloy on hot tear formation. The research methodology involved alloy preparation followed by grain refiner addition and mold temperature variation. A permanent mold was prepared, and the molten alloy with added grain refiner was poured and allowed to solidify.

The formation of hot tears under different process parameters of aluminum copper silicon sand cast aluminum alloy was observed through enlarged views. Results indicated that grain refiner and mold temperature reduced hot tear tendency in Al-Si-Cu alloys. However, the study also revealed that grain refiner additions alone are insufficient and can produce a poisoning effect in the presence of silicon.

Key Findings on Grain Refinement and Mold Temperature Effects

The research findings regarding grain refinement and mold temperature effects can be summarized through several key observations. Hot tears consistently occurred at the junction of bottom sprue and casting bar across all test cases. Al-5Ti-1B grain refiner effectively restricts grain growth of aluminum particles and refines the grains of Al-Si-Cu alloys by providing nucleating sites in the form of TiAl3 and TiB2 particles, as confirmed through XRD analysis.

Importantly, grain refiners alone prove insufficient for optimal grain refinement. Initially, additions of 0.2, 0.4, and 0.6 weight percent grain refiners decreased hot tear tendency. However, when grain refiner addition reached 1 weight percent, hot tear tendency increased again, demonstrating the poisoning effect of grain refiners in the presence of silicon.