Hot Tearing Susceptibility of AA3000 Aluminum Alloy Containing Cu, Ti, and Zr
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HEAT exchangers are an important application of direct chill (DC) cast rolling ingots produced from AA3000 Al alloys. To optimize the material properties for these applications, copper alloying is common, as well as minor additions of titanium and zirconium. These additions are mainly made to improve the corrosion properties, which is very important for automotive heat exchangers, but the modifications have also resulted in a marked increase in hot tearing tendency, which differs from what had previously been experienced for AA3000 alloys with regular compositions. Hot tearing is a serious defect that occurs during the solidification of Al alloys in DC casting. It is an irreversible failure, which starts forming during the semi-solid stage of solidification and eventually shows up as a crack either on the surface or inside the solidified ingots. These cracks are usually visible to the naked eye, and the ingots must be scrapped to prevent failures in further processing.[1–3] Studies of hot tearing have resulted in several theories about the mechanisms of
GHADIR RAZAZ and TORBJO¨RN CARLBERG are with the Department of Natural Science, Mid Sweden University, 851 70 Sundsvall, Sweden. Contact e-mails: [email protected]; [email protected] Manuscript submitted February 7, 2019. Article published online May 21, 2019 3842—VOLUME 50A, AUGUST 2019
crack nucleation and propagation involving metallurgical and thermo-mechanical factors. The key factors for hot tearing are liquid film and inadequate eutectic feeding in the last stage of solidification, together with thermally induced stress-strain, which cannot be accommodated by the semi-solid material.[1,4–7] Hot tearing susceptibility (HTS) is highly dependent on the chemical compositions of Al alloys, and is described as a lambda curve for binary alloys. In most binary alloys, an initial increase in solute content (start of the lambda curve) enlarges the solidification interval. As a result, the alloys spend more time in a vulnerable stage for hot tearing (between 90 and 99 pct solid), and thus are prone to cracking. With further addition of solute, the hot tearing tendency decreases due to a higher eutectic fraction. Better interdendritic liquid feeding during shrinkage allows the cracks to heal (second half of lambda curve).[1,3,4] Chemical composition can also influence the hot tearing resistance of alloys by changing the morphology, precipitation, and grain size. However, contradictory results related to hot tearing have been reported. For instance, it is believed that grain refining increases the hot tearing resistance by reducing the grain size, altering the grain morphology from columnar to equiaxed, and decreasing the film thickness between the grains.[1,5,7] However, Easton et al.[6] state that refined grain size reduces the permeability of the mush and enhances the hot tearing tendency.[1,6]
METALLURGICAL AND MATERIALS TRANSACTIONS A
Besides the chemical composition, process variables such as mold and melt temperature[1] and melt quality[8] affect the HTS
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