The development of many areas of the economy, especially modern technologies related to the aerospace, nuclear and medical industries, requires the use of materials with new properties – such as lightweight aluminium, magnesium and titanium alloys, austenitic stainless steels or high-temperature creep-resistant nickel, chromium or niobium alloys. Very often these newly developed materials with specific properties are difficult to form, either as a result of their intrinsic strength or lack of ductility, or both together.
It is the increasing demand for materials that are difficult to form that has become the main reason for the search and development of new methods of forming metals and alloys.
The use of high pressure for plastic deformation was stimulated by the fact that the ductility of metals increases as the surrounding hydrostatic pressure increases. An American scientist, Percy Williams Bridgman (1882-1961) did the most distinguished work in the field of high pressure at Harvard University in the USA, for which he was awarded the Nobel Prize in 1946. He was the first to pioneer experiments in hydrostatic extrusion, a plastic processing method first named as such in 1961 by Welshman Hubert Lloyd David Pugh (1914-2005) of the NEL National Laboratory in East Kilbride, Scotland. Hydrostatic extrusion involves extruding a charge through a die using a medium compressed to high pressure, as opposed to the conventional extrusion process where a piston pushes against the material. The pressurised medium coats the entire deformable charge, exerting hydrostatic pressure on both its sides and back. Work by Bridgman and later by Pugh confirmed an approximately linear increase in ductility of steel with pressure and a faster than linear increase for other materials. In some cases, such as for zinc at critical pressure, a rapid brittle-plastic transformation is observed. Nevertheless, the main conclusion from this precursor work is that the ductility of all the metals studied increases with increasing hydrostatic pressure.

Work on hydrostatic extrusion technology was initiated in Poland by Wacek Pachla and Ludwik Styczyński of the UNIPRESS Institute of High Pressures Physics of the Polish Academy of Sciences in 1973. They intensively studied the process of hydrostatic extrusion from the theoretical point of view and carried out significant experimental work examining the relation between the extrusion pressure and the strain rate for a wide range of engineering construction materials and the resulting properties of the extruded products. They focused particular attention on the mechanical properties of extruded products and explaining the reasons for the higher strength compared to products deformed by traditional methods such as drawing, conventional extrusion, rolling or forging. Quickly, work involving the transformation of materials by hydrostatic extrusion transformed its character from pure academic curiosity to the study of a process with considerable application potential.

From the very beginning, work on hydrostatic extrusion at UNIPRESS focused on the application aspects of this technology and its commercialisation. The first practical results included the production of ultra-thin bonding wires for electronics (1986-1994), multi-core solder wires (since 1984 produced by the first so-called spin off company Cynel Unipress), bimetal wires (since 1984 produced by the first so-called spin off company Cynel Unipress). The production of ultra-thin contact wires (bonding wires) for electronics (1986-1994), multi-core soldering wires (from 1984 produced by the first so-called spin-off company Cynel Unipress), bi-metal copper-lead rods for the regeneration of bearings (1988-1989), thick-walled capillary tubes from alloyed copper for welding apparatus tips (from 1989, in the years 1993-1997 produced in STPoland, in which UNIPRESS was a shareholder) or prototype thin-walled aluminium tubes for the automotive industry (1989). Research work in the field of extrusion technology was accompanied by the development of original presses and auxiliary equipment for hot and cold hydrostatic extrusion technology. In 1986, a new 400 ton hydrostatic extrusion press operating up to 1000oC was put into service, which created new technological possibilities. Technologies of extrusion of thin-walled copper tubes (1989-1990) and brass tubes and profiles (1994) were developed and practically applied. In parallel, work was carried out on the hydrostatic extrusion of other materials, such as metals for electrical contacts, noble metals or metals based on intermetallic compounds. The experience and know-how gained in the field of hydrostatic extrusion technology enabled the founding of a second spin-off company, Hydron Unipress (1988), which, among other things, specialised in the construction of presses, including those for hydrostatic extrusion.

A significant breakthrough in the practical applicability of hydrostatic extrusion came when, for the first time in the world, UNIPRESS began to apply this method to various metals and alloys as a ‘severe plastic deformation’ method (SPD) method. This became possible thanks to the development of the method of cumulative hydrostatic extrusion, in which the total large plastic deformation is realised by multiple repeated extrusion processes with smaller deformations. Since then (early 2000), the prospects for industrial applications of extrusion have exploded. Why? Because large plastic deformation using hydrostatic extrusion enables significant fragmentation of microstructure in metals and alloys, which in consequence leads to significant increase in their strength, hardness, resistance to brittle fracture, fatigue strength and superplasticity (creep).

Since cumulative hydrostatic extrusion leading to high plastic deformation was practically implemented at UNIPRESS, the production of metals and alloys with nanocrystalline structures (nanometals) with grain sizes below 100nm has become standard. This was confirmed for commercially pure titanium (47nm), pure iron and silicon bronze (60nm), duralumin (65nm), Eurofer 97 steel (86nm) or austenitic steel and pure nickel (100nm). Other materials studied were in the ultrafine-grained range, such as copper (120nm), a copper-chromium-zirconium alloy (140nm), a nickel-aluminium intermetallic compound (300nm) or a titanium-aluminium-vanadium alloy (200-400nm). The scale of the microstructure fragmentation becomes easier to understand if it is compared to the average thickness of a human hair of 50 000nm. This means that approx. 1 000 grains of titanium or 500 grains of austenitic steel will fit into the cross-section of a hair. Such a drastic fragmentation of the microstructure leads to strength properties of hydrostatically extruded metals unachievable for their conventionally produced counterparts.

The development of modern industries requires ‘new’ materials that meet stringent design conditions in terms of strength, operating temperature and hardness. Severe plastic deformation often allows unique properties to be imparted to materials that are indicative of the creation of a ‘new’ material. This is the case with light metals, such as magnesium alloys and duralumin, produced by hydrostatic extrusion, with breaking strengths in excess of 400 MPa and 600 MPa respectively. This is also evidenced by the production by hydrostatic extrusion of pure titanium (99.99+%), free from alloying elements harmful to humans, with strength exceeding 1300 MPa, ¼ higher than the strength of titanium alloy with aluminium and vanadium used so far for implants. The three above-mentioned metals are among the lightest, commonly used structural materials, with the highest strength-to-weight ratios. Hence their great importance in areas such as transport and medical implants. Another material of significant structural importance is acid-resistant austenitic steel. Its strength has been increased by cold strengthening through hydrostatic extrusion to a range (800-1200MPa) unattainable for its microcrystalline counterparts of similar alloy composition. Bolts made from this steel are already in operation abroad. High-pressure processed pure titanium and austenitic steels are direct candidates for industrial applications. Many other structural materials processed at UNIPRESS by hydrostatic extrusion can also displace traditional materials. Eurofer 97 steel as a material for fusion apparatus parts, pure nickel 99.98% for micro-electro-mechanical systems (MEMS) or aluminium alloys of the 5XXX, 6XXX and 7XXX series for high-strength structural rivets and machine parts are arbitrary examples.