Compression Molding Effect

increased tensile strength of compression-molded metal articles as compared to articles produced by other pressure-working methods, given identical conditions of thermal treatment. The tensile strength or yield point of selected samples for each method of processing are compared in the direction of deformation of the metal. The compression molding effect is sometimes also evaluated by means of the difference in strength of the compression-molded article itself in and perpendicular to the direction of deformation. The compression molding effect is exhibited by alloys of aluminum, magnesium, and other metals, mainly after annealing and natural aging. The increase in strength may be as high as 40 percent for some alloys. The compression molding effect is accompanied by a decrease in the specific elongation and a reduction of the cross section. The hardness, shear strength, and true resistance to breaking remain virtually unchanged.

Not all compression-molded articles exhibit the compression molding effect. The loss of the compression molding effect in thermally strengthened aluminum alloys is caused by the absence or reduced content of manganese, chromium, and zirconium in the alloys; prolonged homogenization of the castings; low temperature of the metal during the compression molding; very high degrees and rates of deformation; excessively long holding times during preheating for annealing; and increased temperature of artificial aging.

There are several hypotheses concerning the nature of the compression molding efffect, but almost all investigators are of the opinion that its generation requires a recrystallized structure of the metal. Thus, the compression-molding effect may be regarded as a variety of the structural hardening effect.