A method for forming a sheet made of an aluminium alloy into a component of complex shape, particularly a motor-vehicle component, such as the outer panel of a bonnet or a door, or the inner frame of a bonnet or a door, which comprises the following steps: - providing a mould for blow-forming said sheet made of aluminium alloy with the aid of pressurized gas, - heating said sheet made of aluminium alloy and/or said forming mould to a predetermined temperature, - arranging the sheet within the mould, closing the mould and introducing pressurized gas into a chamber of the mould with a first face of the sheet facing this chamber, so that the sheet is pushed against the surface of the mould towards which the second face of the sheet is facing, - removing the sheet formed thereby from the mould and subjecting the sheet to a heat treatment, said method being characterized in that: - the alloy constituting said sheet does not have superplasticity features, - said predetermined heating temperature is in the order of 400°-550° C, - the maximum pressure reached by the forming gas is in the order of 20-30 bars, - the time for forming the sheet within the mould is between 40 and 150 seconds, - in a first part of the forming step, the pressurized gas is fed into said chamber of the mould while leaving the peripheral portions of the sheet which are clamped within the mould free to slide with respect to the mould, so as to enable a first forming of the sheet with no substantial elongation thereof, and thereafter, in a second part of the forming step, this sliding movement is prevented and the pressurized gas is kept to be fed into said chamber, so as to press the sheet against the wall of the mould, thus causing an elongation of the sheet until it reaches its final shape. - providing a mould for blow-forming said sheet made of aluminium alloy with the aid of pressurized gas, - heating said sheet made of aluminium alloy and/or said forming mould to a predetermined temperature, - arranging the sheet within the mould, closing the mould and introducing pressurized gas into a chamber of the mould with a first face of the sheet facing this chamber, so that the sheet is pushed against the surface of the mould towards which the second face of the sheet is facing, - removing the sheet formed thereby from the mould and subjecting the sheet to a heat treatment, - the alloy constituting said sheet does not have superplasticity features, - said predetermined heating temperature is in the order of 400°-550° C, - the maximum pressure reached by the forming gas is in the order of 20-30 bars, - the time for forming the sheet within the mould is between 40 and 150 seconds, - in a first part of the forming step, the pressurized gas is fed into said chamber of the mould while leaving the peripheral portions of the sheet which are clamped within the mould free to slide with respect to the mould, so as to enable a first forming of the sheet with no substantial elongation thereof, and thereafter, in a second part of the forming step, this sliding movement is prevented and the pressurized gas is kept to be fed into said chamber, so as to press the sheet against the wall of the mould, thus causing an elongation of the sheet until it reaches its final shape.