Microstructure and mechanical properties of IF steel deformed during plane stress local torsion

Mechanical joints are inherently vulnerable to failure

because the presence of the joint hole causes a stress

concentration in the vicinity of the hole. The need for

improvement of material strength around a fastener hole

can be satisfied by severe plastic deformation (SPD) to

produce ultrafine grains. The ultrafine grained (UFG)

alloys produced by SPD processing possess higher

strengths than their coarse-grained counterparts as a result

of the reduced grain size. However, in some circumstances

such as SPD processing of Al–Zn and Al–Mg alloys the

decomposition of supersaturated solid solutions competes

with the Hall–Petch effect and leads to a more pronounced

softening of the material [1]. Another drawback of SPD

processes is that they involve bulk deformation and large

energy consumption [2]. It is therefore desirable to enhance

the global behaviour of the material by limiting improvement

of the material property by SPD to the location at

which it is needed. Localized severe plastic deformation

(LSPD) techniques, such as forward spiral extrusion [3]

and friction stir processing [4], involve lower energy consumption.

They modify the properties of materials locally

and create a gradient of grain refinement, resulting in significant

improvement in the mechanical properties of the

processed samples. However, these techniques cannot be

used for strengthening the material around fastener holes,

and thus a method for improving the strength of material

around the hole is needed.

To reinforce the mechanical properties of material

around a hole, the plane stress local torsion (PSLT) process,

which involves a plane stress axi-symmetric torsional

loading, is introduced. The PSLT takes advantage of large

shearing strains induced around the intended hole position,

through torsional deformation [5]. As a result, the material

flows plastically within a thin annular zone around the

fastener hole (AZFH). Because of the limited penetration

of the flow localization zone into the material, a major

proportion of deformation energy is consumed within the

AZFH. The PSLT therefore consumes much less energy

than do bulk grain refinement techniques.