Method for forming an aluminium alloy ingot in an ingot mould, the ingot mould defining a parallelepiped, such that the ingot formed extends parallel to a longitudinal axis, along a width and parallel to a transverse axis, along a thickness, the thickness being less than the width, the ingot defining a median plane extending along the mid-thickness (T/2), parallel to the longitudinal axis, the method comprising the following steps of: - preparing the liquid aluminium alloy; - casting the liquid aluminium alloy in the ingot mould along a vertical axis, the alloy being cooled, during casting, by a surface flow of a cooling liquid so as to form a solid alloy extending around a liquid alloy, referred to as a sump, said solid alloy forming a front at the interface with the sump, the front being inclined at an angle of inclination (α) to the vertical axis, the angle of inclination of the front varying along the transverse axis; - displacing the solid alloy, along the vertical axis, at a casting velocity; - during the casting, applying a travelling magnetic field, the amplitude whereof is varied periodically according to a frequency ( f), the magnetic field being generated by at least one magnetic field generator disposed at the periphery of the ingot mould, so as to apply a Lorentz force at various points of the sump, the average Lorentz force, during a period of the magnetic field, being inclined relative to the vertical axis at an angle of inclination (β), referred to as the angle of inclination of the Lorentz force, the latter varying along the transverse axis; - the travelling magnetic field propagating along a propagation axis parallel to the vertical axis; - the sump comprising a median zone, extending symmetrically on either side of the median plane, the thickness whereof corresponds to a half-thickness of the ingot; the method being characterised in that: - the frequency ( f) is less than 5 Hz; - and in that the casting velocity and the frequency ( f) are such that throughout the median zone of the sump, at the interface between the liquid alloy and the solid alloy, at the front, the angle of inclination (β) of the force is strictly less than the angle of inclination (α) of the front; the angles of inclination of the front and of the force being oriented in the same direction, such that in the median zone, the Lorentz force acts on the liquid alloy, at the interface, by pressing it against the front, and where, prior to casting, the Lorentz force applying at at least one point of the front is modelled so as to define, taking into account the thickness of the ingot mould, a frequency value (f) and/or a casting velocity value enabling an average Lorentz force to be obtained, the angle of inclination (β) whereof relative to the vertical is less than the angle, at said point, formed by the front relative to the vertical. - preparing the liquid aluminium alloy; - casting the liquid aluminium alloy in the ingot mould along a vertical axis, the alloy being cooled, during casting, by a surface flow of a cooling liquid so as to form a solid alloy extending around a liquid alloy, referred to as a sump, said solid alloy forming a front at the interface with the sump, the front being inclined at an angle of inclination (α) to the vertical axis, the angle of inclination of the front varying along the transverse axis; - displacing the solid alloy, along the vertical axis, at a casting velocity; - during the casting, applying a travelling magnetic field, the amplitude whereof is varied periodically according to a frequency ( f), the magnetic field being generated by at least one magnetic field generator disposed at the periphery of the ingot mould, so as to apply a Lorentz force at various points of the sump, the average Lorentz force, during a period of the magnetic field, being inclined relative to the vertical axis at an angle of inclination (β), referred to as the angle of inclination of the Lorentz force, the latter varying along the transverse axis; - the travelling magnetic field propagating along a propagation axis parallel to the vertical axis; - the sump comprising a median zone, extending symmetrically on either side of the median plane, the thickness whereof corresponds to a half-thickness of the ingot; - the frequency ( f) is less than 5 Hz; - and in that the casting velocity and the frequency ( f) are such that throughout the median zone of the sump, at the interface between the liquid alloy and the solid alloy, at the front, the angle of inclination (β) of the force is strictly less than the angle of inclination (α) of the front; the angles of inclination of the front and of the force being oriented in the same direction, such that in the median zone, the Lorentz force acts on the liquid alloy, at the interface, by pressing it against the front, and where, prior to casting, the Lorentz force applying at at least one point of the front is modelled so as to define, taking into account the thickness of the ingot mould, a frequency value (f) and/or a casting velocity value enabling an average Lorentz force to be obtained, the angle of inclination (β) whereof relative to the vertical is less than the angle, at said point, formed by the front relative to the vertical.