Hot-Tear Susceptibility of Aluminum Wrought Alloys and the Effect of Grain Refining

摘要

Hot-tear resistance of aluminum wrought alloys AA1050, AA3104, and AA5182 has been determined via constrained-rod casting and two quantitative indices. Hot-tear susceptibility ranking was observed to be AA1050 < AA5182 < AA3104. Mechanisms of hot tearing were studied via microstructural investigation of hot tears. Hot tearing in AA1050 and AA5182 alloys could be characterized as interdendritic separation during solidification. Microstructural investigation of tear surfaces of AA1050 alloy showed that free-dendritic surfaces were substantially covered with solute-rich phases indicating good interdendritic feeding. The AA1050 alloy showed low susceptibility to hot tearing, and grain refining had an additional effect in reducing hot-tear susceptibility. The AA5182 alloy showed moderate susceptibility to hot tearing; tear surfaces were only partially covered with a eutectic phase, and grain refinement was very effective in eliminating hot tears in this alloy, which exhibits a large nonequilibrium freezing range. The mechanism of hot tearing in AA3104 was ductile fracture of interlocked dendrite tips in central zones deprived of interdendritic liquid film during solidification. Hot-tear mechanisms observed could seemingly be explained by Saveiko’s liquid film theory. Nonequilibrium freezing range did not explain the hot-tear susceptibility ranking of the alloys. Hot-tear sensitivity (HTS) was related to the average grain size, d (μm), of the castings as HTS ? 6 × 10?5 d 2 to 0.014d + 3, where HTS was sensitive to grain refining when the grain size was above 200 μm. Grain refining was most effective in reducing the hot-tear susceptibility in alloy AA5182, which has the largest nonequilibrium freezing range.