Component residual stress control in forward rod extrusion by material flow and tribology—experiments and modeling

Abstract

The forward rod extrusion of ferritic stainless steel X6Cr17 (DIN 1.4016) is here investigated with the objectives to experimentally identify and numerically verify the effect of the lubrication system and die opening angle on residual stresses. Three lubricants – MoS2, soap and polymer – are considered whose tribological properties are characterized via double cup extrusion tests. The effect of material flow is also studied by forming in conical dies featuring three different opening angles. The extrusion experiments revealed a decrease in the near-surface tensile stresses with decreasing friction for all the considered opening angles. An opening angle of 2α = 90° led to the highest tensile residual stresses. Both an increase to 2α = 120° and a decrease to 60° resulted in reduced tensile stresses and even a shift to compressive stresses.Furthermore, a previously developed numerical model of forward rod extrusion is optimized and validated against the experimentally measured residual stresses. The effect of the spatial and temporal discretizations of the model on the predicted residual stresses is investigated. Based on the experimental and numerically verified results, the recommendation to reduce friction in forward rod extrusion is derived as a means to obtain a less detrimental to the lifecycle of extruded parts residual stress state.

Stability of forming induced residual stresses in stainless steel components under mechanical load

“…Changes in the die opening angle lead to a change in the material flow distribution during forming and therefore to a different strain and work hardening distribution in the formed shaft. With die opening angles of 60° and 120°, reduced tensile stresses are observed, which is described in more detail in [22]. In the setup used for these experiments, forming with a high die opening angle of 2α = 120° is at the process limit.…”

“…Starting from a reference variant with a die opening angle of 2α = 90°, two different specimen sets with 2α = 60° and 120° are produced. The different die opening angles lead to changes in the material flow during forming and therefore to different residual stress states in the formed parts [22]. After forming, the stresses in the formed shafts are determined by the nondestructive X-ray diffraction method.…”

Stability of forming induced residual stresses in stainless steel parts at elevated temperature

“…For a die opening angle of 2α = 90°, higher tensile stresses are present in the formed parts [15]. In Fig.…”

“…It is observed that an increase in test temperature by 100 °C leads to a mean reduction of the yield strength of 7% to 10%. In the formed parts, high compressive residual stresses are present in the core, which are in equilibrium with tensile stresses near the surface [15]. An overall reduction of the material strength by thermal mechanisms leads to a relaxation of the highest stresses due to plasticization.…”

“…One possible reason for these differences is the tribology. In [15] it was shown, that the friction influences the surface residual stress state in forward rod extruded components. A die opening angle of 2α = 120° leads to a strong deflection of the material in the forming zone of the die, resulting in high surface pressures at the die shoulder.…”