Effect of casting speed on structure formation and hot tearing during direct-chill casting of Al-Cu alloys

Professor W. Kurz recently suggested Solidification Structure-Processing Maps as a useful tool for controlling commercial solidification technologies [1]. As a development of this idea we studied the effect of casting speed on the structure formation and hot tearing during direct-chill (DC) casting of binary Al-Cu alloys. Several binary alloys were cast in our laboratory scale DC casting that allowed automatically controlled change of the casting speed during casting. The casting speed was varied from 100 to 200 mm/min for a round billet 200 min in diameter. The sump depth, melt and billet temperatures in various locations, water flow rate and casting speed were measured during the casting. The microstructure of billets was analyzed by optical microscopy and computer image analysis, and hot tears were measured directly on the crack sites. Besides that a finite element simulation was performed for computing thermal, stress and strain fields in billet. In this FEM computation, an elasto-viscoplastic constitutive model valid for the whole temperature range was used. Hot tearing susceptibilities were computed using five hot tearing criteria by applying the FEM data as an input. Clear relationships between the structure parameters and hot tearing on one side and the casting speed and composition on the other side were found. The outcome of this research will be a Composition-Casting Speed-Hot Tearing process chart.