22-Mec-A4 Design and Manufacture of Machine Elements · May 2014
Question 4 of 8: Hydrostatic extrusion
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. National Examination, 07-Mec-A4 Design and Manufacture of Machine Elements, May 2014 — 3 hours, open book, any non-communicating calculator permitted. Eight questions on six pages, divided into Part A (manufacturing processes, Q1–Q4) and Part B (machine-element design, Q5–Q8). The rubric asks for three from Part A and two from Part B, five questions constituting a complete paper, all of equal value (20 % each). All eight questions are solved here, because this document is a study resource rather than an examination script.
Reference texts.
Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed. — sand casting and casting defects (Ch. 10–12), sheet-metal shearing and fine blanking (Ch. 16), bulk deformation and hydrostatic extrusion (Ch. 15), fusion welding and weld defects (Ch. 30–31).
Hibbeler, Mechanics of Materials, 10th ed. — combined loading, transverse shear and stress transformation / Mohr's circle (Ch. 7–9).
AWS D1.1 Structural Welding Code — Steel and CSA W59 — preheat, hydrogen control and minimum fillet-weld sizes (Canadian practice).
ASM Handbook Vol. 15 Casting and Vol. 6 Welding, Brazing and Soldering — hot tearing, solidification cracking and riser/chill practice.
Check: Part B is entirely figure-driven. Every number below was read from the printed figures (Figures A, B, C and S7). Two readings are worth stating explicitly so a grader can substitute a different interpretation without redoing the method: (i) in Figure A the rivet group is five rivets in the top row plus one rivet 200 mm below, the lower rivet lying on the same vertical line as the third top rivet; (ii) in Figure B the 67 500 N horizontal force acts on the centroidal axis of the section, so it produces pure tension and no additional bending.
Figure 4.1 — Hydrostatic extrusion: the ram pressurises a fluid, and the fluid — not the ram — loads the billet against the die.
(a) Description of the process
Figure S1 shows hydrostatic extrusion. The billet is placed in a thick-walled container, the container is filled with a pressurising fluid (a vegetable or mineral oil, glycerine, or for very high pressures a low-viscosity synthetic), and a ram seals the container and compresses the fluid. Unlike direct extrusion, the ram never touches the billet: the pressure in the fluid — commonly 700 MPa to about 1.4 GPa, occasionally higher — acts uniformly on every surface of the billet, and it is that pressure which forces the billet through the die orifice to form the part. Because the fluid surrounds the billet completely, the container wall exerts no friction on it; the only significant friction is at the die interface, and even there the pressurised fluid is dragged into the die land and forms a thin hydrodynamic film. The billet nose is normally machined to match the die cone so that it seals the orifice at the start of the stroke; if it did not, the fluid would simply extrude through the die instead of the metal. Once the seal is made, the pressure builds to the extrusion pressure and the billet flows. In the "augmented" or "fluid-to-fluid" variants the extrudate emerges into a second pressurised chamber, so that the metal is deformed and leaves the die while still under hydrostatic pressure.
(b) Advantages
Essentially no billet–container friction, and a hydrodynamic film at the die, so the extrusion pressure is markedly lower than in direct extrusion for the same reduction and the pressure does not fall through the stroke as the billet shortens.
Very large reduction ratios in a single pass — ratios of 20:1 and much more are routine — because the low friction leaves more of the applied energy available for the useful, homogeneous deformation.
Uniform, nearly homogeneous deformation. Without container friction there is no dead-metal zone and no shear-rich surface layer, so the "extrusion defect" (the funnel of oxidised surface metal drawn into the tail) is largely eliminated, the product is straight, and the properties are uniform across the section.
Brittle and difficult materials can be extruded. A large superimposed hydrostatic compressive stress suppresses void nucleation and growth and raises the fracture strain dramatically, so materials that would crack in direct extrusion — cast irons, molybdenum, tungsten, beryllium, tool steels, refractory metals, and metal–matrix or bimetallic composites — can be given large plastic strains without failure. This is the process's genuinely unique capability.
Good surface finish and dimensional consistency, and long, uniform lengths (coiled wire is possible), because die wear is low and the pressure is steady.
Cold working is practical, giving the strength benefit of the cold work along with dimensional accuracy, and the low-friction path means the billet need not be heated to reduce forces.
(c) Why industrial implementation has been difficult
The obstacles are all consequences of the very high fluid pressure and of the batch nature of the cycle, and together they have confined hydrostatic extrusion largely to specialist and laboratory use:
Pressure containment and sealing. Containers and seals must hold on the order of 1 GPa reliably for thousands of cycles. The container is fatigue-loaded from zero to full pressure on every stroke and must be autofrettaged or shrink-fitted in multiple layers; the ram seals must be dynamic seals working at pressures where ordinary elastomers extrude and metal seals gall. Seal life and container fatigue life dominate the running cost.
Stored energy and safety. A litre of fluid compressed to a gigapascal stores a large amount of elastic energy; a seal or container failure releases it explosively, so heavy guarding, pressure-relief provision and rigorous inspection are mandatory. The hazard, and the insurance and code implications that follow it, are a real deterrent.
Slow, batch cycle times. Each billet must be loaded, the container filled and sealed, the fluid pressurised through its own compressibility (fluids compress several percent at these pressures, so much of the ram stroke is spent merely compressing the fluid), then depressurised and drained. The result is a long cycle and a low production rate compared with continuous direct extrusion.
Billet preparation. The billet nose must be pre-machined to the die angle to make the initial seal, adding a machining operation to every billet, and the billet surface must be clean and defect-free because the fluid will otherwise penetrate surface flaws.
Process control problems. Once the billet begins to move the pressure can drop suddenly and the billet accelerates — the "stick–slip" or jerky-flow instability — which threatens dimensional control and the tooling; damping it needs careful fluid selection and back-pressure control.
Temperature limits. The fluid restricts the process largely to cold or warm working: at hot-working temperatures the pressurising fluid degrades, and the practical alternative — a viscous glass or a soft solid medium — loses the clean hydrostatic behaviour that motivates the process.
Capital cost and narrow economics. The presses and containers are expensive and dedicated, and for the common aluminium and copper shapes that make up the bulk of extrusion tonnage, conventional direct or indirect extrusion is entirely adequate and far cheaper. The process is therefore economic only where its unique capability — extruding a material that cannot otherwise be extruded — is actually needed.
