Metal Forming & Machining

MF C314

Effect of Hydrostatic Pressure Hydrostatic pressure affects the following properties of

metals:DuctilityYield strength or flow stressStrain hardening indexStrain hardening indexStrain rate index

Hydrostatic pressure does not lead to plastic deformation.

Hydrostatic pressure increases ductility and formability. The phenomena of increased ductility with hydrostatic

pressure is called pressure induced ductility.

Both the steel show increase in volume during plasticdeformation as well as increase in yield strength underhydrostatic pressure.

There is no effect of hydrostatic pressure on strainhardening.

A constant difference in yield strength in tension and in A constant difference in yield strength in tension and incompression.

The yield strength in compression is higher than that intension.

The difference between the two strengths is notaffected by hydrostatic pressure.

Ductility and formability Ductility is essential property of material for its

formability. Ductility is not constant for a metal or under all

conditions. It gets modified by process parameters. It gets modified by process parameters. Therefore same material may show different formability

in different forming process. Ductility is measured by the strain suffered by the

material before fracture.

Factors that affect ductility Composition of metals i.e. BCC & FCC crystal

structure. Grain size. Steel with higher oxygen content show low ductility. Presence of impurity Temperature Hydrostatic pressure Concentration of alloying element Duplex microstructure, Strain rate.

Theory of fracture Occurrence of fracture in work material is a natural

limit of formability. Conditions under which fracture initiates and

propagates.Tensile stressTensile stressCompressive stressAccumulated strain

Criteria for occurrence of fracture in a ductilematerial at the given temperature and strain rate.

The above criterion takes the following form for torsion

Super plasticity It is the property of material to suffer neck free

elongation of several hundred percent.Ex: Zinc-22 Al eutectoid alloy. Elongation =2900 percent in temperature range

973-1293K Super plasticity may also be achieved by thermal

cycling across the phase transformation temperature while workpiece is kept stretched.

The factors common to many super-plastic metals and alloys are given below. High value of strain rate exponent m (0.3-0.5) Small grain size Very little work hardening.

The different super plastic materials may grouped The different super plastic materials may grouped into the following three types. Low temperature alloys of Zn and Al. Intermediate temperature alloys of AlHigh temperature alloys of Ni, Ti, some stainless steel.

Anisotropy in sheet metals During cold rolling of sheet metal, material develops

anisotropy. The flow strength in thickness direction becomes

different to that in the plane of sheet.

For isotropic sheet

For anisotropic sheet, anisotropy ratio

17