Abstract
The influence of anisotropic work-hardening on the component properties and process forces in cold forging is investigated. The focus is on the material behaviour exhibited after strain path reversals. The work-hardening of three steels is characterized for large monotonic strains (equivalent strains up to 1.7) and subsequent strain path reversals (accumulated strains up to 2.5). Tensile tests on specimens extracted from rods forward extruded at room temperature reveal an almost linear work-hardening for all investigated steels. The application of compressive tests on extruded material gives insights into the non-monotonic work-hardening behaviour. All previously reported anisotropic work-hardening phenomena such as the Bauschinger effect, work-hardening stagnation and permanent softening are present for all investigated steels and intensify with the pre-strain. Experimental results of 16MnCrS5 were utilized to select constitutive models of increasing complexity regarding their capability to capture anisotropic work-hardening. The best fit between experimental and numerical data was obtained by implementation of a modified Yoshida-Uemori model, which is able to capture all observed anisotropic work-hardening phenomena. The constitutive models were applied in simulations of single- and multi-stage cold forming processes, revealing the significant effect of anisotropic hardening on the predicted component properties and process forces, originating in the process-intrinsic strain path reversals as well as in strain path reversals between subsequent forming stages. Selected results were validated experimentally.
Highlights
The goals of metal forming process design have long exceeded the mere shaping of parts
The goal of this work is to quantify and model the influence of anisotropic work-hardening on process forces and component properties in cold forging with a focus on strain path reversals
For materials with a more pronounced permanent softening or even a complete loss of the work-hardening tendency after strain path reversal, the influence of anisotropic hardening on the forming forces is expected to be even more significant. The result of this investigation contribute to the three fields (i) characterization, (ii) constitutive modelling and (iii) implications for bulk metal forming in the context of strain path reversals
Summary
The goals of metal forming process design have long exceeded the mere shaping of parts. Property changes of cold forged parts due to plastic deformations have received increasing attention in the last years including the mechanical properties [1] and damage [2]. An exact incorporation of the material behaviour under complex strain paths is necessary to predict and exploit these property changes by numerical simulations. Previous research works have rarely considered anisotropic hardening in the field of cold bulk forming preventing an accurate prediction of a formed components properties and its performance. In the scope of this paper, the phrase “anisotropic work-hardening” refers to the deviation of a metal’s yield surface from an initially isotropic state, Bauschinger [3] found, that when a material is plastically deformed under tension and compressed, the yield stress under reverse loading (σf,reverse) is lower than the flow stress before unloading (σf,forward) (Fig. 1).
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