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

•A unique press for hydrostatic extrusion with back-pressure was designed and constructed.•Back-pressure enabled severe plastic deformation in one, cold deformation pass.•Cold ductility of brittle and hard to deform materials was increased by several dozen of percentage.•The materials fabricated using the HE + BP press had superior mechanical properties.•Cold deformation succeeded due to back-pressure restraining cracks initiation and propagation.

A press for hydrostatic extrusion within the extrusion pressure range up to 2 GPa with back-pressure up to 0.7 GPa 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 400 MPa to 700 MPa 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 7 mm 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 = 392 MPa in the magnesium alloy, σd0.2 = 709 MPa in molybdenum, σdM = 1140 MPa in grey cast iron, and σd0.2 = 643 MPa 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.