Press for hydrostatic extrusion with back-pressure and the properties of thus extruded materials

Abstract

A press for hydrostatic extrusion within the extrusion pressure range up to 2GPa with back-pressure up to 0.7GPa was designed and constructed. The press is equipped with an integrated pressure intensifier, and a control and recording system which permits recording the process parameters, such as extrusion pressure, back-pressure and its stability, time and speed of the extrusion, and enables monitoring the process on-line. The double-layer high-pressure chamber and the monobloc back-pressure chamber were analyzed using the finite element method with allowance made for the self-strain-hardening effect known as autofrettage. The maximum permissible load imposed on chambers and the resulting balance pressure established in the case of the two chambers being accidentally connected were also evaluated. Several cold extrusion processes assisted with back-pressure from 400MPa to 700MPa were conducted, experimenting with low or non-ductile materials, such as the ZW3 magnesium alloy, GJL250 grey cast iron, GJS500 nodular cast iron, bismuth of 99.999% purity, and molybdenum of 99.9% purity. The bulk, non-defected products with diameters ranging from 4 to 7mm were obtained. The use of back-pressure permitted the materials to be plastically deformed during a single cold operation with the percent deformation from 36% in grey cast iron to more than 80% in Bi. Thanks to the strain-hardening due to the severe plastic deformation, the materials acquired excellent properties (YS=392MPa in the magnesium alloy, σd0.2=709MPa in molybdenum, σdM=1140MPa in grey cast iron, and σd0.2=643MPa in nodular cast iron) impossible to achieve by classical plastic deformation processes. The hardness of the materials was also increased adequately, and the refinement of their microstructure resulted in an increase of ductility. These advantageous results obtained by using the press indicate that hydrostatic extrusion with back-pressure has a great applicative potential.