Edge-cracking and profile of hot-rolled aluminium slabs

Abstract

An account is given of an investigation into the effects of rolling geometry and metallurgical factors on edge-cracking of hot-rolled Al-Mg alloys: in susceptible grades of the latter, edge cracking may result in substantial scrap losses. Edge profile is secondary to metallurgical structure in causing cracking. Three types of crack, designated “segregation”, “major edge” and “massive” edge-cracks, were recognised. Initiation of cracks was usually associated with grain-boundary precipitate particles. Major- and massive-cracking were identified with sodium and hydrogen in association with lack of homogenisation of the ingot. A strongly fish-tailed edge profile may promote edge-cracking through the occurrence of tensile edge stresses. Although lack of thermal similarity restricted the agreement to a qualitative one, the profiles of slabs rolled in multi-pass schedules under geometrically similar laboratory and industrial conditions were adequately described by a fourth-order polynomial as proposed by other workers. The controlling influence of the parameter √ Rδ/ h 1 on the form of the profile was verified. In partial disagreement with other workers, it was concluded that concavity of the profile decreased with rolling temperature and slightly with reduction per pass, while lay-on temperature in the range 450°C to 550°C had no effect. From a study of the temperature dependence of the properties of a typical alloy, it is believed that the reported material independence of profile can be explained by temperature independence of deformation in the range of typical rolling temperatures rather than by independence of flow strength.