A prefabricated sheet metal wall module comprises: an upper horizontal cap member of inverted U-shaped cross-sectional configuration with a pair of parallel longitudinal dependent side flanges; a lower horizontal floor track member of U-shaped cross-sectional configuration with a pair of parallel upwardly extending longitudinal side flanges; and a series of parallel upright studs each having a pair of opposite longitudinal side flanges. The upper and lower ends of each stud telescope into the upper and lower horizontal members respectively with the side flanges of the stud in intimate surface contact with the side flanges of the horizontal members thereby providing four rectangular areas of two layers of metal. With a stud in assembled position under compression between the associated horizontal members, a power-actuated hand tool having a pair of jaws equipped with a hollow die and a cooperating punch, respectively, is applied at each juncture of a horizontal member with a stud of metal at each juncture to punch interlocking hyperboloidal crimps in the two overlapping layers resulting in prestressed joints that make the wall module rigid enough to carry to a construction site where wall modules are assembled to form a low-cost building.