Abstract: Process for butt welding two metal blanks(1,2) of which the first blank is of steel and the second blank is of a metal having a melting temperature lower by more than 400.degree. C. than the melting temperature of the steel. The process includes positioning the two metal blanks (1,2) so that they contact each other in the region of their lateral faces to be butted together, maintaining the two metal blanks in contact, directing onto one of the principal faces of the metal blank (1) a high energy density bean (5) which passes through the first metal blank (1) and of which the focusing point is situated at a given distance from the joint plane (3) so as to generate in the region of the joint plane (3) a temperature equal to the melting temperature of the second metal blank (2) to within 10%, and displacing the high energy density beam (5).